Recent advances in MXenes supported semiconductors based photocatalysts: Properties, synthesis and photocatalytic applications

Journal Pre-proof Recent advances in MXenes supported semiconductors based photocatalysts: Properties, synthesis and photocatalytic applications Cheera Prasad, Xiaofei Yang, Qinqin Liu, Hua Tang, Aluru Rammohan, Syed Zulfiqar, Grigory V. Zyryanov, Sufaid Shah

PII:

S1226-086X(19)30641-0

DOI:

https://doi.org/10.1016/j.jiec.2019.12.003

Reference:

JIEC 4887

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

9 October 2019

Revised Date:

29 November 2019

Accepted Date:

2 December 2019

Please cite this article as: Prasad C, Yang X, Liu Q, Tang H, Rammohan A, Zulfiqar S, Zyryanov GV, Shah S, Recent advances in MXenes supported semiconductors based photocatalysts: Properties, synthesis and photocatalytic applications, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.12.003

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Recent advances in MXenes supported semiconductors based photocatalysts: Properties, synthesis and photocatalytic applications Cheera Prasad1*, Xiaofei Yang2, Qinqin Liu1, Hua Tang1*, Aluru Rammohan3, Syed Zulfiqar5, Grigory V. Zyryanov3,4, Sufaid Shah1 1

School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu Province

212013, PR China 2

College of Scienc & Institute of Materials Physics and Chemistry, Nanjing Forestry University,

3

of

Nanjing 210037, P. R. China

Department of Organic and Biomolecular Chemistry, Ural Federal University, 19 Mira,

4

ro

Yekaterinburg 620002, Russian Federation

Ural Division of the Russian Academy of Sciences, I. Ya. Postovskiy Institute of Organic

Deaprtment of Physics Abdul Wali Khan University Mardan 23200 KP Pakistan

re

5

-p

Synthesis, 22 S. Kovalevskoy Street, Yekaterinburg, Russian Federation

*Corresponding authors: [email protected] (H. Tang), [email protected] (Cheera

Jo

ur na

Graphical abstract

lP

Prasad).

1

of ro -p re lP ur na

Abstract: The families of MAX phases and their derivatives MXenes are constantly increasing

Jo

in terms of both composition and crystalline mixtures. In the last three years, numerous advances were accomplished that improved the preparation of new MAX phases with prearranged double transition metals and, as a result, the production of novel MXenes with a high structural complexity and chemical multiplicity, infrequently observed in other families of twodimensional samples. In this evaluation, hybrids of MXenes and semiconductors are efficient photocatalysts, because of their specific interface characteristics and Schottky heterojunction is capable of giving accelerated charge separation and a lower Schottky barrier for photocatalytic applications. The latest advances were proved that the MXenes supported semiconductors based 2

photocatalysts can be expected as the most advantaged and encouraged novel photocatalysts in the photocatalytic and photoelectrochemical applications. Besides, we have explained significant developments in efficient MXenes supported semiconductors based nanocomposites, including the frequent synthesis strategies and progress mechanisms, in addition to their new applications including, photodegradation of dyes, CO2 conversion, photocatalytic and photoelectrochemical water splitting reaction applications. The review was completed with a short presentation of future challenges and prospects in the progress of MXenes supported semiconductors based

of

photocatalysts. It is also believed that this review will encourage further exploration and will inaugurate new promising to increase new MXenes supported semiconductors based

Keywords: MXenes; Semiconductors; CO2 photoconversion. Contents

-p

1. Introduction

ro

photocatalysts with new and inspiring applications.

2. Basic principle of photocatalysis

re

3. Two dimensional nitrides and carbides and their innovation 4. Properties of MXenes

lP

5. Preparation of MXenes

6. Preparation of MXenes supported semiconductors based photocatalysts 6.1. Ion exchange process

ur na

6.2. Self-assembly procedure 6.3. Hydrothermal method 6.4. Sol-gel method

6.5. Solvothermal process 6.6. Calcination method

Jo

7. Applications of MXenes supported semiconductors based photocatalysts 7.1. Photodegradation of organic dyes 7.2. Photocatalytic water splitting 7.3. Photoelectrochemical water splitting 7.4 CO2 conversion 8. Conclusions and prospects Acknowledgment 3

References

1. Introduction Currently, to efficiently meet growing global energy anxiety and improve the energy emergency, there is a vital requirement for large-scale progress of advanced sustainable and renewable energies such as solar and wind power, which can provide as commercial and environmentally potential substitutes to conventional fossil fuels [1–3]. During the past few decades, many researchers have worked for the progress of effective, sustainable, visible-light

of

induced semiconductor photocatalysts to straight change solar energy into chemical energy for enhanced photocatalytic applications [4]. A variety of photocatalytic nano-structured materials

ro

have been developed for solar energy conversion owed to their unique properties like suitable band gap to exhibit a strong visible light response with a high photocatalytic presentation, nontoxicity, good thermal and chemical stability [5,6]. As well, these semiconductor photocatalysts

-p

must be comprised of earth-abundant elements, prepared easily with tailored porosity and must follow a green way.

re

To date, many metal oxides, sulfides, oxyhalides and organic semiconductor samples with effective visible light reactive were extensively explored [7–9]. Moreover, single material

lP

photocatalyst is always inadequate by the rapid recombination of photoexcited charge carriers, which outcomes in reduced quantum effectiveness and low photocatalytic performance [10]. So far, numerous investigators, such as metal or nonmetal doping, facet control, surface

ur na

sensitization and heterostructure production were fabricated to extend the photocatalytic activity and reduce the recombination of photoexcited electron/hole pairs [11–17]. Among different strategies, heterostructure photocatalysts were constructed widely to improve the separation efficiency of photogenerated charge carriers [18–21]. Since this decade, MXene has been a novel assuring sequence of the two-dimensional

Jo

sample with tremendous conductivity, hydrophilicity and mechanical characteristics [22]. Since the revolutionary work by Naguib and co-authors on the improvement of Ti3C2 MXene by wetchemical etching in hydrofluoric acid and exfoliation of Ti3AlC2, numerous kinds of MXenes manufactured through several techniques were achieved in examine [23]. Until now, to investigate on MXenes were obtained an absolute detonation of attentions because of their attractive characteristics, including higher metallic and electrical conductivities, hydrophilic character, more surface area, simply tunable structure, and higher oxidation resistance [24,25]. 4

MXenes are formed by etching their ternary layered nitride or carbide precursors and MXenes, which have a common formula of Mn+1XnTx, are composed of stacked two dimensional sheets where M symbolizes an early transition metal, A is IIIA or IVA element and X, which is C and/or N and Tx stand for surface functional groups such as O, OH and F. These terminations are adjusted by shifting the synthesis or post processing technique and they are studied to be the controllers of the electronic structure of MXenes and can bring about eminent features to develop the application fields of MXenes [26,27].

of

So far, there are above 70 MAX phases that were fabricated and, of these, over 20 types of MXenes were effectively prepared such as, Ti3C2, Ti2C, Ti3CN, Ta4C3, TiNbC, (V0.5Cr0.5)3C2, and Mo2C [28,29]. In consideration of the outstanding characteristics acquired by MXene, it was

ro

employed in an abundance of applications. Because of its excellent electronic features, there was an enormous search to seek MXene for electrochemical energy storage in batteries and super

-p

capacitors [30]. As well, MXenes was fascinated more attention, owing to its tremendous electrical conductivity, structural constancy, and hydrophilicity [31]. Due to the low Fermi level

re

of metallic MXene, compared with distinctive semiconductors, MXene is employed as a cocatalyst for semiconductor photocatalysts [32]. For instance, Ti3C2, which is mainly considered

lP

MXene, was employed as a co-catalyst to develop the photocatalytic behavior of ZnS, CdS, TiO2, ZnxCd1−xS and g-C3N4 [33,34]. Of late, MXenes, such as Nb2C, Ti3C2 and Ti2C were explored as effective co-catalysts of photocatalysts for water splitting [35,36]. If MXene has

ur na

combined with semiconductors, a Schottky barrier can be shown at the semiconductor/MXene interface [37]. The Schottky barrier will provide as the electron tank, therefore helping the partition of photogenerated charge carriers. For instance, the Ti3C2 NPs are employed as the cocatalyst to improve the photocatalytic hydrogen generation of g-C3N4 [38]. Many reviews were published on the photocatalytic application of semiconductors to

Jo

determine the energy and environmental problems [1,3,39,40]. Furthermore, this field of investigating has encouraged great efforts on the synthesis, amendment, and application of visible light reactive photocatalysts with numerous significant findings reported during the past years. Moreover, several reviews about MXenes based photocatalysts were published, but most of them focus on MXenes nanostructures, photocatalytic water splitting reactions and organic contaminants degradation. However, we recapitulate the latest progresse in the manufacture of MXenes supported semiconductors based photocatalysts with the different modification 5

methods, basic principle of photocatalysis and the applications of this series of photocatalytic organic pollutants degradation and photocatalytic and photoelectrochemical water splitting reactions and CO2 reduction have been reviewed, which are essential fields of the present study. Finally, some concluding remarks and motivating perspectives on the modern conditions and additional forecast

on

the

MXenes

supported semiconductors

based photocatalysts

interconnected explorations are displayed, which may support the insightful and important application of the MXenes supported semiconductors based photocatalysts.

of

2. Basic principle of photocatalysis Inspired by the natural photosynthesis, the direct utilization of solar energy into valuable

ro

solar fuels is a capable method for resolving energy emergency. For photocatalytic reaction, there are three major steps: (i) absorption of light by a photocatalyst to produce electron/hole pairs (ii) charge separation and transfer to the surface of the photocatalyst (iii) photoexcited

-p

electrons in the CB of the semiconductor will transfer to the co-catalyst, whereas the left over holes in the VB will oxidize H2O to oxygen as shown in Fig.1 [40]. Moreover, the generated

re

electron-hole pairs can transfer to the surface of the semiconductor and partake in the oxidation and reduction reactions. The photocatalytic reaction typically participates three major active

lP

groups: •OH, (hydroxyl radical), h+ (hole) and •O2- (superoxide radical) where •OH is the main oxidant in the photodegradation of the contaminant in the aqueous solution [12,13]. TiO2 and ZnO photocatalysts are the most used materials, however their broad band gaps cause them to be

ur na

ineffective in sunlight, because only 9.3% of solar illumination contains ultraviolet radiation, which is able of generating electron-hole pairs in these semiconductors. Even though, various approaches, including surface alteration and band gap production, were used towards the synthesis of visible-light-active TiO2 and ZnO and many accomplishments were achieved. Right now, many explore studies were carried out using semiconductors with band gap energies

Jo

appropriate for visible light absorption of the solar spectrum for photocatalytic reaction. Recently, MXene materials showed tremendous electron conductivity, large specific surface

area, and rich exposure to metal sites. Two dimensional MXene materials promises as a cocatalyst to develop the photocatalytic performance of photocatalysts through induce efficient partition of photoexcited charge carriers [36]. After irradiation, the photoexcited electrons are excited from the semiconductor. The photogenerated electrons in the conduction band of semiconductors quickly flow into MXene because of its promising electron-trapping capability, 6

getting efficient electron-hole partition. The electrons on MXene react with oxygen to generate •O2- and the h+ react with water to yield •OH radicals [34]. Both super oxides radical and hydroxyl radical react pollutant molecules and generate carbon dioxide and water molecules. Moreover, these photoexcited electrons will transfer across the Schottky barrier, and are then rapidly shuttled to the surface of the high-conductivity metallic MXene, which promotes the separation of the photogenerated charges in the semiconductor. The existence of the Schottkybarrier also hindered the photoexcited electrons migrate from MXene back to the semiconductor,

of

and therefore the alienated electrons aggregated on the texture of MXene then partake in the CO2 reduction reaction [33]. Consequently, the photocatalytic activity of the MXene/semiconductor photocatalyst was significantly improved because of the Schottky junction induced by the built-

ro

in electric field.

3. Two-dimensional nitrides and carbides and their innovation

-p

The substance can display various characteristics depending on its dimensionality [41]. Two dimensional samples contain a structure with endless lateral dimensions, however a constrained

re

thickness. They have mesmerized concentration in the past decade for their high application because of their inherent attractive chemical and physical characteristics. For instance, the

lP

mixture of higher conductivity and inplane inflexibility of GO was produced huge attention for the progress of bendable electronic strategy [42]. Investigate on two dimensional nitrides, carbonitrides and carbides initiated in 2011, with the production and partition of single layers of

ur na

Ti3C2 for the first time [23]. This work started hypothetical and experimental reports and caused an elaboration of methods of detection of several other two dimensional carbides and nitrides, making new family of samples presently eminence at approximately thirty components called MXenes [43,44].

Moreover, MXenes are prepared through preferred etching the “A” layers from the ternary

Jo

layered Mn+1AXn where M is transition metal, A is IIIA or IVA element, and X is C and/or N Fig.2 [23,45,46]. MXenes are generally regarded as Mn+1XnTx, where T is the surface termination groups, (-OH, -O and/or –F) and x is the number of terminating groups [47]. In contradiction of GO, the oxygen or hydroxyl, MXene terminations inform hydrophilicity to their texture [48]. This novel two dimensional families of samples premature on demonstrated enhancing presentations when employed for energy storage and their activity are currently discovered in a large number of applications in different fields of separation membranes, lead 7

adsorption, hydrogen storages, biosensors, lithium-ion batteries, super capacitors and sodium-ion capacitors [49-55]. Moreover, the band structure of F or OH-functional group Ti3C2 contains an obvious partition of 0.05-0.1 eV between VB and CB, noticeable its semiconducting character. Therefore, it is probable to consider that the band structure of Ti3C2 can be carefully adjusted through changeable the functional groups, which can give bendable opportunity in particular photocatalysis, if Ti3C2 was used as co-catalyst [23].

4. Properties of MXenes

of

MXenes have unique features such as, high Young modulus, greater electric and thermal conductivities and the variable band gaps are significant. Of note, the hydrophilic textures with

ro

high metallic conductivities of MXenes, differentiate themselves from the common of two dimensional samples, such as GO [56]. Finally, their characteristics and applications presentations can be adjusted by (1) composites (2) surface fictionalization and morphology

-p

modifications [29,57]. The most significant features of MXenes families are specified below. The major characteristics of the MXenes, electric and electronic features, may be adjusted by

re

functional groups modification, solid solution development. Investigationally, electric conductivities of MXenes compressed discs have analogous to GO nanosheets and superior to

lP

carbon nanotubes and RGO sample [58]. Ti3C2Tx calculated electrical conductivities varied from 850-9880 S.cm−1, owing to variation on the (1) imperfection concentration (2) d-spacing between MXenes flakes (3) delamination yield (4) surface functional groups and (5) lateral sizes induced

ur na

by each etching process. Generally, smaller hydrofluoric acid concentrations and etching periods produce MXenes with fewer defects and better lateral sizes, representation more electronic conductivity [30,59].

As the M-C and M-N are among the stronger bonds, mechanical characteristics of MXenes also fascinated enormous attention. The first simulated report was recommended elastic

Jo

constants (c11) as a minimum 2 folds higher than MAX phases [60] and other two dimensional samples, for example MoS2. Nonetheless, in spite of the c11 values 2 to 4 folds lesser when compared with graphene, 1060 GPa, their bending rigidity is larger, which points towards their employ as strengthening in nanocomposites. Furthermore, thin discs of titanium based MXenes demonstrated hydrophilic performance, with contact angle between 27 to 41° [61]. Necessary for electronic and energy-related heat dissipation devices because of their constant size

8

effectiveness, thermal expansion coefficients and thermal conductivities of MXenes are still insufficient [62]. In addition, previous reports calculate low thermal extension coefficients and greater thermal conductivities than MoS2 and phosphorene monolayer. For instance, the expected thermal conductivities of Sc2CF2, Hf2CO2, Zr2CO2 and Ti2CO2 at room temperature vary from 22 to 472 Wm-1 K-1 [63]. Investigationally, only Ti3C2Tx thermal conductivity has been estimated and therefore, other samples would be explored. Moreover, MXenes has magnetization activity

of

dissimilar from MAX phases and widespread estimations of MXenes magnetic features [64]. Numerous pure samples are determined to acquire magnetic moments, such as Zr2C, Ti3CN, Ti3N2, Ti2N, Ti4C3, Cr2C, Fe2C and Zr3C2 [65-69]. Moreover, Ti3CNTx and Ti4C3Tx turn into

ro

nonmagnetic with the functional groups, whereas Cr2CTx and Cr2NTx remain ferromagnetic at room temperature with fluorine and hydroxyl groups coupled and Mn2NTx is ferromagnetic in

-p

spite of the texture terminations [70,71].

As well, the UV light and visible absorption are significant for photovoltaic,

re

optoelectronic, transparent conductive electrode devices and photocatalytic applications. Ti3C2Tx films can absorb light in the UV–vis region from 300 to 500 nm and 5 nm thicknesses film

lP

demonstrated transmittance up to 91.2%. As well, it can show a stronger and wide absorption band at around 700–800 nm, dependent on the film thicknesses, which results in the pale greenish films color and is significant for photothermal therapy applications. Remarkable, the

ur na

transmittance values is optimized by modification of its ion intercalation and thickness [72]. For example, as, DMSO, hydrazine and urea decreased Ti3C2Tx film transmittance, Tetramethyl ammonium hydroxide improved it from 74.9% to 92%. Nevertheless, a number of optical interrelated characteristics, such as emission colors, plasmonic, non-linear optical and luminescence efficiency characteristics still require to be clarified that further broaden range of

Jo

MXenes applications [33,73].

5. Preparation of MXenes Characteristically, MXenes are manufactured via preferred etching the “A” layers from

the ternary layered Mn+1AXn where M is transition metal, A is IIIA or IVA element, and X is C and/or N [74]. MXene are generally regarded as Mn+1XnTx, where T is the surface termination groups, (-OH, -O and/or –F) and x is the number of terminating groups using HF or LiF/HCl solution Fig.3. When A is removed, the obtained sample was impulsively functionalized with O, 9

F or OH species, generating X−M−F, X−M−OH and X−M−O groups and providing MXenes a Mn+1XnTx formula. These functions are adjusted through altering the synthesis and postprocessing process. Numerous explorers were developed the higher catalytic result of MXeneTi3C2Tx derived toward the H2 storage reactions of NaAlH4 or MgH2 exposing the superior hydrogenation/dehydrogenation characteristics of MXene catalysts [75]. Typically,

multilayered

nanosheets

of

Ti3C2Tx

have

been

constructed

from

pristineTi3AlC2 employing the following process: approximately of Ti3AlC2 (3 g) sample has

of

added in HF (70 mL) in a closed Teflon bottle and stirred vigorously for 60 h at room temperature. Solids have extracted from the Teflon bottle, filtered under vacuum, and consequently rinsed with distilled water and ethanol. Lastly, the obtained product was heated in

ro

an oven at 60 °C for 6 h. The resultant product was Ti3C2 MXene nanosheets [76]. As well, multilayer MXene Ti3C2 from Ti3AlC2 MAX phase sample has been manufactured through a

-p

chemical etching technique with HF (15 wt %) solution at 40 °C for 48 h [77,78]. After that, Ti3C2 was centrifuged at 5,000 rpm and washed with distilled water 6 times to get a water

re

suspension, and kept in a refrigerator at 5°C.

Etching circumstances such as etching duration, temperature and particle size of the

lP

Ti3AlC2 (MAX) phase can affect the yield and excellence of MXene. Taking an example of Ti3C2Tx, Liu and his co-authors etched Ti3AlC2 at various temperatures, and they determined that the Al layer would be nearly removed within a range of temperatures from 0 ºC to 65 ºC [43].

ur na

Nonetheless, the exfoliation degree of Ti3C2Tx powerfully depends on the number of termination groups on the surface of Ti3C2Tx, which enhance with the increase of temperature. As well as ternary MXenes, ordered double-M 2D carbides were found by DFT techniques, and Mo2Ti2C3Tx, Cr2TiCxTx, and Mo2TiC2Tx have been effectively proposed with the help of HF etchant [79]. Herein, we recapitulate preparation of wet-chemical etching, which is by far the

Jo

most extensively employed technique to manufacture MXenes. Apart from the wet-chemical etching, MXenes were made by chemical vapor deposition and high-temperature etching [80,81]. The timeline of MXenes by different preparation techniques is shown in Fig. 4.

6. Preparation of MXenes supported semiconductors based photocatalysts 6.1. Ion exchange process Ion exchange was explained as the oldest scientific phenomenon known to humanity. This claim arises from descriptions that occur in the Bible and in the writings of Aristotle, 10

however the first in fact scientific allusion to ion exchange is ascribed to two English agricultural chemists in 1850. These were J. T. Way and H. S. Thompson, who separately examined the substitute of Ca in soils by Al ions. This invention was the precursor to the study of inorganic materials capable of ‘base’ exchange, and in 1858 C. H. Eichorn demonstrated that natural zeolite minerals could reversibly exchange cations. The significance of this characteristic in water softening was acknowledged by H. Gans who, at the turn of the century, patented a series of synthetic amorphous aluminosilicates for this purpose. He called them ‘permutites’, and

of

they were extensively employed to soften industrial and domestic water supplies until recent times, in addition to being used in nuclear waste treatment.

Moreover, ion exchange route involves substituting ions in an ionic particle whereas the

ro

framework residue intact, which occurs owing to revealing the parent ionic particles to innovative ions [82]. As compared to the bulk sample, the ion exchange happening on

-p

nanoparticles can be finished rapidly at eminent temperature due to the improved surface contact and lower activation difficulty to the dispersing ions. In addition, when the ion exchange

re

approach is approved, it is simple to operate the ensuing products by changing the reaction circumstances. By the way, ion exchange method was rapidly recognized as an efficient

lP

approach for post-synthetic chemical adjustment of nanoparticles. For instance, Y. Li et al. designed that the TiO2/Ti3C2 nanohybrid has prepared from MXene layered by a simultaneous oxidation and alkalization, after that ion exchange and calcination procedures, as shown in Fig.5.

ur na

To preparation of TiO2/Ti3C2 nanohybrid, 120 mg Ti3C2 MXene sheets were dissolved into a solution having 1 M NaOH (180 mL) and 30% H2O2 (3.6 mL). After that, obtained solution was put into two 100 mL Teflon-lined autoclaves and heated for 12 h at 140 ºC. The resulting product Ti3C2/Na2Ti3O7 photocatalyst was rinsed with deionized water several times and dried in a vacuum oven for 12 h at 60 ºC. Afterward, the Ti3C2/Na2Ti3O7 photocatalyst was

Jo

added in 0.1 M HCl (500 mL) solution for 24 h to substitute Na+ with H+, therefore the Ti3C2/H2Ti3O7 photocatalyst was produced. To finish, the TiO2/Ti3C2 hybrid was achieved after the thermal decomposition of the Ti3C2/H2Ti3O7 hybrid in a muffle furnace at various temperatures (300 ºC, 400 ºC and 500 ºC) for 3 h at a heating rate of 2 ºC min -1, which were indicated as TiO2/Ti3C2-300, TiO2/Ti3C2-400 and TiO2/Ti3C2-500 [83]. 6.2. Self-assembly procedure

11

Self-assembly of component molecules on the nanoscale order has existed in nature as soft materials assemble to generate cell membranes, viruses and biopolymer fibers. Of late, in laboratories, scientists and engineers have been able to manufacture nanoscale materials by a bottom–up strategy known as self-assembly. Self-assembly is a procedure in which materials at the nanoscale spontaneously arrange predefined components into ordered superstructures which can be exploited in different applications. Furthermore, self-assembly participates the procedure in which isolated samples impulsively arranged into a controlled structure to reduce the free

of

energy of the entire system. The electrostatic self-assembly is a tremendously well-liked and simplistic approach for the synthesis of heterocomposites. Because of, the gentle process circumstances, self-assembly promises narrow size allocation, in addition, to enhance

ro

management of morphology, of the resultant system.

For instance, typically, Bi(NO3)·5H2O (9.5 mg) has added into 200 mL of CH3COOH

-p

with the constant strong stirring to make the solution A. After that, 20 mL of solution A has dissolved into a specific quantity of Ti3C2 solution vigorously stirred for 2 h. Then, NaBr (4 g)

re

was added in 200 mL of distilled water with the constant stirring to create the solution B. Subsequently, 20 mL of solution B has dissolved slowly into above solution with the constant

lP

stirring for 2 h to get Ti3C2/BiOBr nanohybrid. The resultant product is rinsed with ethanol and distilled water 3 times, respectively. The obtained result is heated for 12 h at 60°C [84]. Another report, H. Zhang et al. developed that the Ti3C2 MXene/α-Fe2O3 hybrid nanocomposite has been

ur na

constructed utilizing homogeneous two dimensional α-Fe2O3 sheets and Ti3C2 MXene layered by an ultrasonic assisted self-assembly technique for photodegradation of contaminates [85]. Tao Cai et al. fabricated Ti3C2/Ag3PO4 Schottky photocatalyst and recognized that Ti3C2 can significantly improved the photocatalytic performance and strength of Ag3PO4 [86]. Y. Yang et al. found that the g-C3N4/Ti3C2 nanohybrid was made by a facile electrostatic self-assembly

Jo

technique for photocatalytic H2O2 evolution in visible light illumination [87]. Ning Liu et al. evaluated that the novel Ti3C2/g-C3N4 hybrid nanocomposite was constructed through evaporation induced self-assembly technique for ciprofloxacin photodegradation and as shown in Fig.6 [88]. Huoli Zhang and his coworkers reported that the Ti3C2 MXene/α-Fe2O3/ZnFe2O4 nanocomposite would simply achieve by ultrasonic supported self-assembly strategy for diffusing magnetic α-Fe2O3/ZnFe2O4 heterostructure on Ti3C2 MXene surface [89]. Yan Zhuang 12

et al. found that the novel 2D/1D photocatalyst of Ti3C2/TiO2 was effectively synthesized through electrostatic self-assembly method. The highest hydrogen evolution rate of Ti3C2/TiO2 photocatalyst was up to 6.979 mmol h-1 g-1, which was 3.8 folds that of pristine TiO2 NFs [34]. Also, Huang et al. developed that the Ti3C2 MXene/Bi2WO6 nanocomposite was synthesized by an electrostatic assembly technique for degradation of acetone and formaldehyde [90]. Fang and his co-author effectively prepared Ti3C2/Ag2WO4 photocatalyst and found that the presence of conductive Ti3C2 significantly improved the photocatalytic activity and corrosion resistance of

of

Ag2WO4 [91]. Lin et al. found that the 2D O-doped g-C3N4 NSs were synthesized via an annealing way, after that a 2D-2D O-doped g-C3N4/Ti3C2 MXene Schottky-junction was manufactured by an in-situ electrostatic assembly of positively charged O-doped g-C3N4 NSs and

ro

negatively charged Ti3C2 MXene [92]. 6.3. Hydrothermal method

-p

Hydrothermal method refers to the preparation of substances via chemical reactions in a sealed and heated solution above ambient pressure and temperature. The thought ‘hydrothermal’

re

initiates from earth science in the 19 century, where it implies a system of high temperatures and water pressures. Hydrothermal method suggests many benefits such as comparatively mild

lP

operating conditions, one-step synthetic process, environmental friendliness and good diffusion in solution. Furthermore, hydrothermal synthesis is low-cost in terms of the instrumentation, energy and material precursors compared with other solution preparation techniques. A typical

ur na

hydrothermal process is carried out in a conserved autoclave in high temperature and pressure, hence giving the produce of heterocomposites with high crystalline and narrow size allocation. As a result, this method allows the manufacture of MXenes supported semiconductors based photocatalysts without post performance. For example, the SrTiO3/Ti3C2 hybrid nanocomposite was manufactured through a hydrothermal procedure, as shown in Fig.7.

Jo

Typically, 1 mL of titanium isopropoxide (C12H28O4Ti) has dissolved to 50 mL of a

solution of (2:3) acetonitrile and ethanol, having NH3·H2O (0.38 g) and H2O (0.91 g) in strong stirred for 6 h. The resulting solution of TiO2 was centrifuged and several times washed with ethanol and deionized water and heated at 60 °C for 24 h. Subsequent, 0.1 g of TiO2 sample, 1:1 molar ratio of Sr(NO3)2, and 0.2 mL of (5 mg) Ti3C2 were gradually mixed into 25 mL of 2 mol/L sodium hydroxide (NaOH) solution. The obtained product after magnetic stirring for 15 min has put into a 50 mL Teflon lined stainless steel autoclave and heated for 4 h at 140 °C. The 13

resultant sample was centrifuged, systematically washed and heated at 60 °C [93]. Y. Gao et al. studied that the Ti3C2/TiO2 nanohybrid with high photocatalytic performance was effectively manufactured by a hydrothermal procedure for methyl orange dye degradation under ultraviolet light illumination [94]. Further study, Xiao Lu et al. evaluated that the Ti3C2Tx/TiO2/PANI heterostructure constructed through a hydrothermal way in addition to in situ polymerization technique [95]. W. Zhou et al. proposed that the Ti3C2/CeO2 nanohybrid was effectively manufactured through a one-step hydrothermal technique with well distributed CeO2 nanorods

of

on Ti3C2 nanosheets [96]. Tian et al. constructed that the new UiO-66-NH2/Ti3C2/TiO2 photocatalyst was logically fabricated through introduced Ti3C2Tx MXenes on water stable ZrMOFs precursors by a facile hydrothermal procedure [97].

ro

Moreover, Fang et al. developed that the ternary CdS/Ti3C2–OH/ln2S3 photocatalyst with an efficient visible-light-driven photocatalytic performance were synthesized by a facile

-p

hydrothermal technique [25]. Fang et al. also reported that the Ti3C2−OH/Bi2WO6:Yb3+, Tm3+ photocatalyst was effectively manufactured by a facile hydrothermal method [98]. Y. Li et al.

re

reported that the design motif is the grafting of in situ growth of TiO2 nanosheets on the Ti3C2 MXene and MoS2 nanosheets on the (101) facets of TiO2 nanosheets with mostly exposed high-

lP

active (001) facets by a hydrothermal technique [99]. Li et al. reported that the facile hydrothermal technique was adopted to synthesize BiOBr/Ti3C2 photocatalyst [100]. 6.4. Sol-gel technique

ur na

The first sol–gel preparation of silica was explained in 1846 by M. Ebelmen, a French chemist, who noticed that silicic esters hydrolyze gradually in the presence of moisture to provide hydrated silica. The industrial applications of the sol–gel procedure appeared in the midtwentieth century with the manufacture of coatings on glasses by Schott Glaswerke. Nonetheless, sol–gel science was proposed much later and the First International Workshop on Glasses and

Jo

Ceramics from Gels was held in 1981. Since then thousands of papers were reported and sol–gel chemistry has opened new opportunities in the field of materials science. As well, sol-gel process provides an innovative procedure for the manufacture of novel nanocomposites [101]. Sol-gel method is one of the entrenched synthesis procedures to make new metal oxide samples as well as mixed oxide nanomaterials. This system has the possible handle over the textural and surface features of the nanocomposites [102].

14

As well, high purity and homogeneity of sol–gel method is generally used to construct MXene based semiconductor photocatalysts. Initially, Ti3C2 MXene material has been synthesized in distilled water with the molarity of 0.5 mg/mL, afterward ultrasonicated for 10 min. The co-doped BFO NPs have been added in a solution of CH3COOH and ethylene glycol (C2H6O2) with a 1:1 ratio and 0.01 M molarity. The Bi1−xLaxFe1−yMnyO3 sample was ultrasonicated at 60 °C for 1 h. Subsequently, the sample of Bi1−xLaxFe1−yMnyO3 has been dissolved with Ti3C2 solutions individually for all hybrids, after that the Ti3C2

of

MXene/Bi1−xLaxFe1−yMnyO3 solutions have stirred for 2 h at 80 °C for co-precipitation process. The resultant product was rinsed with distilled water several times and stored for 3 h at 60 °C [76].

ro

6.5. Solvothermal process

The solvothermal technique is similar to the hydrothermal process, except that organic

-p

solvents, instead of water, are employed in the synthetic process. On the other hand, the reactions are called alcohothermal and glycothermal, respectively, when alcohols and glycerol are

re

employed as the reaction media. These synthetic approaches are significant for the synthesis of nanocomposites with good crystalline characteristics. Moreover, solvothermal method is one of

lP

the most typical and capable making procedures to construct MXenes supported semiconductors based photocatalysts with several morphological structures. In this technique, the autoclave is filled with organic solutions to take reaction in high temperature and pressure circumstances. For

ur na

instance, characteristically, 200 mg of WCl6 have been mixed in 100 mL of ethanol to get a yellowish solution. After that, 800 μL of acetyl acetone and various quantities of the Ti 3C2Tx nanosheets were dissolved into the above solution, subsequently sonication for 1 h. The obtained solution was transferred into the 200 mL Teflon lined stainless steel autoclave and heated at 150 ºC for 24h. After cooling, the solid material was centrifuged and rinsed with deionized water.

Jo

The obtained product was then kept at 60 °C for 12 h to dry and achieve the photocatalyst [103]. Hao and his co-workers studied efficient MXene/TiO2/NiFeCo hybrid nanocomposites

that have been effectively constructed through a facile solvothermal technique for oxygen evolution reaction [104]. Tie et al. developed that the ZnS NPs with Ti3C2 MXene sheets by ultrasonic exfoliation and solvothermal reaction toward improved photocatalytic hydrogen evolution [105]. The integration of Ti3C2 fundamentally encourages the charge transfer and enlarges the life span of photoinduced charge carriers, thus ensuing in an enlarged hydrogen 15

evolution yield of 502.6 µmol g-1 h-1 under optimal circumstances, being approximately 4 times greater than pristine ZnS (124.6 µmol g-1 h-1). Therefore, this method has shown MXene/ZnS photocatalytic as a capable candidate for H2 production to enhance the entire clean energy system and given a new insight into the further enlargement of the water splitting application of MXene based samples. 6.6. Calcination method Calcination refers to the common class of thermal procedures employed to

of

manufacture ceramic powders before dispersion. The phase composition, particle size and aggregation of ceramic powders are verified mostly during the calcination procedure, since the product of most solution preparation processes is a precursor of the designed material.

ro

Moreover, calcination is a procedure of heating a material under controlled temperature and in a controlled atmosphere. Herein, Ti3C2/TiO2 nanohybrid was attained through a simple calcination

-p

technique. Characteristically, the as-synthesized Ti3C2 nanosheets were heated for 12 h at 80 °C. Afterward, the heated Ti3C2 has heated at 350 °C, 450 °C, 550 °C and 650 °C with a heated rate

re

of 10 °C/min to get TT350, TT450, TT550 and TT650, respectively [106]. Another study, Yang Li et al. has prepared Ti3C2/TiO2 photocatalyst by primal calcination method. Characteristically,

lP

0.3 g of F–Ti3C2 was heated for 4 h at 550 °C in the air [107]. Ding and his co-workers proposed that the Ti3C2/TiO2/g-C3N4 heterostructure by ultrasonic-assisted calcination technique for organic pollutant degradation [108]. Moreover, the O/OH-terminated Ti3C2 and by-product TiO2

ur na

could perform as tremendous supporters by transferring electrons in Ti3C2/TiO2/g-C3N4 heterostructure. As a result, the maximum photocatalytic activities in the removal of aniline and Rhodamine B were increased to 5 and 1.33 folds better than that of bare g-C3N4 in visible-light illumination, respectively.

7. Applications of MXenes supported semiconductors based photocatalysts

Jo

7.1. Photodegradation of organic dyes With the fast improvement of modern industrialization and population, energy emergency

and ecological contamination have become gradually more severe trouble in the world. Since the last century, industrial growth and environmental protection have always been contradictory [109]. Nevertheless today, with the constant development of environmental consideration, people append significance to the advance and consumption of clean energy, particularly the continuous solar energy to deal with environmental problems [110-120]. These contaminants more cause 16

dangerous influences on living beings. There are 8x105 tons of dye is formed worldwide every year. Furthermore, water having organic dyes is colored in the water for a long period because of the presence of aromatic components and besides, they are very poisonous in nature [121,122]. Consequently, the action of dye contaminants water is very essential to provide the fresh water in public supplies. With the purpose of, treat the dye pollutants and remove the organic contaminates, photocatalysis is the most successful and easy technique because of the whole demolition ability of contaminates and wide compound applicability [123,124]. Nonetheless, the

of

removal of contaminating depends on the characteristics of employing photocatalyst during photocatalysis procedure. Particularly, it was studied that MXenes based semiconductors photocatalysts are the most appropriate investigators utilized in the photocatalysis procedure for

ro

the polluted water distillation [125]. Here, we have explained of this review mostly some MXenes supported semiconductor photocatalysts for removal of dye contaminates.

-p

Recently, M. Abdullah Iqbal et al. developed that the Ti3C2/Bi1−xLaxFe1−yMnyO3 hybrid nanocomposite was fabricated employing the sol-gel process for photodegradation of Congo red

re

[76]. The maximum photocatalytic performances were seen in dark conditions where Congo red photodegradation was observed Ti3C2/BLFO and Ti3C2/BLFMO-5 photodegraded the 92% and

lP

93% organic dye in dark, respectively, and additional complete (100%) photodegradation in 20 min of illumination. Under light illumination, the production of charge carriers generates •O2and •OH species. These extremely dynamic species initiate removing the contaminants present in

ur na

the aqueous solution and in turn decrease to nontoxic by molecules such as, H2O and CO2. Based on the mechanism, photoinduced Ti3C2/BLFO photocatalyst generates electron/hole pairs as in eq 1. More surfaces can perhaps have several active sites, which could enlarge the possibility of the highest interactions between the composite and contaminates. After reaction initiates, the •O2- species are generated, and in the meantime, •OH- groups are formed by the reaction of OH-

Jo

with holes. As the electron responds with the photocatalyst, the water solutions are transformed into OH-. The groups generated in the procedure, that are, •O2- and •OH being extremely reactive for the contaminates such as Congo red, remove it to the nontoxic by-products that are H2O and CO2. The mechanism of photodegradation was reported and summarized below according to the Eqs. (1) to (6).

17

of ro

Explorers were prepared to conserve the unique surface and characteristics of MXenes

-p

and to improve their behavior in photodegradation via different stabilization techniques. Wu et al. stabilized MXene beside photodegradation using a carbon nanoplating technique [126]. Novel

re

two dimensional Ti3C2 MXene/MoS2/C heterocomposite has been made via coupled MoS2 on C/Ti3C2 MXene. The synergetic effect between MoS2 and Ti3C2 material were enormously

lP

improved the structural strength of the heterocomposite, with the assist of carbon coating. What is more, the organic dye is degraded by photodegradation stimulated through the construction of Ti(OH)4 or TiO2 on the texture of MXene in UV irradiation. Nonetheless, the photodegradation

ur na

performance of MXene was inadequate through the rapid combination of charge carriers, narrow light absorption range, and restricted permanence in aqueous solution [127]. As a result, the Ti3C2Tx/TiO2 hybrid photocatalyst demonstrated a greater photodegradation of MO than Ti3C2Tx and TiO2 in UV illumination [87]. Methyl orange is about 98% photodegraded in 30 min in compared to bare Ti3C2Tx and TiO2 nanoparticles which were only removal 42% and 77%

Jo

respectively, in the same investigational circumstances. The better photocatalytic activity of the Ti3C2Tx/TiO2 hybrid photocatalyst is ascribed to a more efficient electron/hole partition and the construction of a Ti3C2Tx and TiO2 heterostructure of the nanocomposite on Ti3C2Tx or TiO2 under UV irradiation. Moreover, silver adorned Ti3C2/g-C3N4 hybrid plasmonic heterostructure with enhanced visible-light photoactivity of aniline was explored [128]. The highest photodegradation rate of aniline achieved to 81.8%, which could be ascribed to the improved visible light absorption 18

efficiency caused by surface plasmon resonance of silver nanoparticles, in addition to the efficient partition of photoexcited electron-hole pairs because of MXenes. As demonstrated in Fig.8, when the Ti3C2 MXene combined with the n-type g-C3N4, the electrons can transfer from the g-C3N4 to Ti3C2 MXene by the Schottky barrier. When the g-C3N4/Ti3C2 MXene/Ag was irradiated under visible light, free electrons in Ag was agitated to higher energy states because of surface plasmon resonance, after that those electrons migrate from silver nanoparticles to the conduction band of g-C3N4 and Ti3C2. Those photogenerated electrons respond with O2 and •OH

of

radicals are consequently produced. This report gives a new approach to the manufacture of MXene based photocatalysts.

Generally, revealing the extremely energetic facets of the photogenerated resource is

ro

measured as a great strategy to develop photocatalytic behavior. It was confirmed that (001) facets are the main dynamic in TiO2 and combination TiO2 samples with other two dimensional

-p

samples effect in the improvement of the photoactivity [129-131]. The heterocomposite comprised of (001) faceted TiO2 NPs and Ti3C2Tx sample has been manufactured and its

re

photoactivity has estimated. The extremely dynamic (001) facets of TiO2 in the heterojunction can give good efficiency photoexcited of charge carriers. As well, the electron/hole partition is

lP

extensively supported through a hole trapping effect by the interfacial Schottky junction, where Ti3C2Tx performs as a hole tank. Moreover, the synthesis of MXenes supported semiconductors

table.1.

ur na

based photocatalysts for photocatalytic degradation of organic dyes are summarized in the

Later on, Wojciechowski and his co-workers synthesized that the morphostructural characteristics of the Ti2C MXene coupled with metal oxide nanoparticles of Ag2O, TiO2, and PdO, and metal nanoparticles of Pd, Au and Ag [132]. The photocatalytic performance of the resultant sample is illustrated in the procedure of photocatalytic degradation of salicylic acid.

Jo

The amendment of Ti2C MXene by Ag2O and Ag NPs resulted in an raise of the photocatalytic performance of 3%TiO2/1%Ag/Ti2C and 3%TiO2/1%Ag2O/Ti2C (97.1% and 95.8% of removed salicylic acid, respectively) in compared with the pure Ti2C MXene (91.7% of removed salicylic acid). The positive effect of the Ti2C MXene modification by PdO, Pd, and Au is not seen. Furthermore, they determined that MXene tailored by metal nanoparticles are a little more effective catalyst than this tailored by metal oxide nanoparticles. The photocatalytic degradation mechanism is shown in Fig.9. Under photocatalytic environments, the production of charge 19

carriers by visible light illumination generated •O2– and •OH groups. These extremely energetic groups initiates degrading the salicylic acid contaminate present in the water solution and in turn decrease to nontoxic by products water and carbon dioxide. As well, Xie et al. made two dimensional Ti3C2Tx/CdS hybrid photocatalyst by an electrostatic self-assembly procedure Fig. 10(A) [133]. Ti3C2Tx provides as an electron moderator to improve the electron pulling out from CdS for photocatalytic reduction reactions. With the simultaneous donation of Ti3C2Tx to in situ detain Cd2+ ions to evade their escape, the

of

photoinduced holes encouraged photocorrosion of CdS is controlled, therefore leading to the improved photocatalytic stability of CdS. Moreover, Ti3C2Tx has smaller Femi level than the conduction band of CdS, the electron life span of 0.5% Ti3C2Tx/CdS is longer than that of pure

ro

CdS. As well, Ti3C2Tx can absorb Cd2+ ions, which are generated through photocatalysis Fig.10 (B), therefore escaping Cd2+ ions melting in H2O to improve the photocatalytic stability of CdS.

-p

For itself, the double gain approach gives a theoretical method to avoid the unsteadiness and photocorrosion of CdS.

re

Compared with MXene based binary photocatalyst, the successful examination into ternary heterocomposite was developed into the distinctive in the photocatalysis field. Hou et al.

lP

made metallic Ti3C2Tx MXene/core shell In2S3/TiO2 heterojunction composite via a simplistic hydrothermal technique for the development of photodegradation of methyl orange [134]. Distinctively, the optimized sample with the preservative Ti3C2Tx quantity of 16 mg had

ur na

tremendous visible-light photocatalytic presentation towards contaminant degradation in water with a removal rate of 0.04977 min−1, which was 6.2 and 3.2 times greater than that of bare Ti3C2Tx and pristine In2S3, respectively. The promising photocatalytic activity was strongly depended on the partition and distribution of photoexcited charge carriers by a large number of charge transfer channels owing to the construction of type-II heterojunction and Schottky

Jo

junction Fig.10(C and D). Particularly, this occurrence stems from the synergy involvement among the visible light reactive In2S3, the uphill band bending of TiO2 and the significant electrical conductivity of the Ti3C2Tx. For itself, the best photocatalyst with the attendance of Ti3C2Tx quantity of 16 mg obvious the maximum photocatalytic degradation efficiency for Methyl Orange degradation and more significantly, it exceeds other In2S3 based binary composites Fig. 10(E and F). This result would be estimated to get new imaginary thought to the design of effective and stable photocatalysts for sustainable solar energy conversion. 20

On the other hand, Y. Gao et al. designed that the Ti3C2/TiO2 nanohybrid with enhanced photocatalytic performance has been effectively manufactured by a hydrothermal procedure [94]. The SEM micrographs of Ti3C2, TiO2 and Ti3C2/TiO2 nanohybrids are demonstrated in Fig.11. The surface of two dimensional Ti3C2 was decorated with nanoparticles of TiO2. The photocatalytic activity of pristine TiO2 and Ti3C2/TiO2 nanohybrid was estimated through photocatalytic degradation of methyl orange in ultraviolet light illumination. Photocatalytic degradation examinations results showed that the Ti3C2/TiO2 nanohybrid contain more enhanced

of

photoactivity than pristine TiO2 NPs. And it was recommended that the Ti3C2/TiO2 nanohybrid revealed high efficient electron/hole partition than pristine TiO2 and Ti3C2 under ultraviolet light illumination. Photocatalytic results demonstrated that the Ti3C2/TiO2 nanohybrid has excellent

ro

photocatalytic activity, which was an extremely smart feature for applications in the decontamination of contaminated water and air.

-p

In the other report, Q. Huang et al. developed that the Ti3C2/BiOBr nanohybrid has effectively made via electrostatically driven self-assembly technique [84]. Under the light

re

illumination, the Ti3C2/BiOBr nanohybrid showed tremendous photoactivity for Rhodamine B, 2,4-Di Nitro Phenol and Cr(VI), where the degradation rate of Rhodamine B, 2,4-Di Nitro

lP

Phenol and Cr(VI) up to 99.4% in 24 min, 45% in 60 min and 47.5% in 40 min, respectively. The potential photocatalytic degradation mechanism in Ti3C2/BiOBr nanohybrid was suggested, as shown in Fig.12. Under light illumination, photoexcited charge carriers are produced.

ur na

Afterward, photoexcited charge carriers are alienated, electrons are migrated to the conduction band of Ti3C2 and holes are remained on the valence band of BiOBr. Finally, e- and h+ happened reduction and oxidation reactions, respectively. The improved photocatalytic characteristic of the Ti3C2/BiOBr nanohybrid is attributed to the following causes: (i) The nanoscale close interfacial contact between Ti3C2 and BiOBr supports charge transport (ii) the surface metal Ti site on the

Jo

Ti3C2 give stronger redox activity (iii) A Schottky junction is produced at BiOBr/Ti3C2 interface, which significantly supported the partition of charge carriers because of the role of built-in electric field. This study showed the massive potential in Ti3C2 based Schottky photocatalyst in ecological remediation. The 2D MXene was synthesized via HF etching as a substrate to develop photocatalytic performance. The Ti3C2/Ag2WO4 Schottky junction has been effectively prepared for the first time through an electrostatically driven in situ growth approach [91]. The photodegradation rates 21

for sulfadimidine (SFE) and tetracycline hydrochloride (TC) were 88.6% and 62.9%, respectively. The possible photocatalytic mechanism is shown in Fig.13. Under visible-light illumination, Ag2WO4 is agitated and generated electron-hole pairs. The construction of a built-in electric field outcomes in the development of a space charge layer between the combined Ti3C2 and Ag2WO4 photocatalyst and transfers the conduction band and valence band of Ag2WO4 ″uphill” to produce a Schottky junction at the surface interface because the Fermi level (EF) of Ag2WO4 is more negative than the original Ef of oxygen ended Ti3C2. Nonetheless, this structure

of

does not influence the rapid transfer of electrons to the metalloid Ti3C2 sheets. In addition, the electrons were migrated to the surface defects of Ti3C2, ensuing in a sequence of reactions that produce •O2- radicals. Therefore, the photogenerated h+ in the Ag2WO4 valence band are

ro

powerfully oxidizing and straight respond with the antibiotics in the solution.

Later on, air pollution of volatile organic pollutants threatens human health. Increasing

-p

superior photocatalysts for volatile organic pollutants degradation therefore magnetizes much attention. Bi2WO6 demonstrates the qualities of strength, non poisonous and visible light

re

absorbance however suffers from its fast charge carriers recombination. Huang et al. found that the Ti3C2 MXene NPs were electrostatically adsorbed on Bi2WO6 nanoplates for effective

lP

electron-hole partition [90]. Fig.14 demonstrates the photocatalytic mechanism of Ti3C2 MXene/Bi2WO6. Upon visible light illumination, the photogenerated electrons in the conduction band of Bi2WO6 eagerly flow into Ti3C2 MXene due to its promising e- catching capability,

ur na

accomplishing effective electron-hole partition. The electrons on Ti3C2 MXene respond with oxygen to generate super oxide radicals (•O2-) and the holes react with water to yield •OH radicals. Both •O2- and •OH radicals attack volatile organic pollutants molecules and produce carbon dioxide and water molecules.

Recently, ternary ln2S3/CdS/Ti3C2-OH photocatalyst with an effective visible light driven

Jo

photocatalytic activity was synthesized by a facile hydrothermal synthesis technique [25]. The photodegradation experimentations confirmed that ln2S3/CdS/Ti3C2-OH photocatalyst with the more surface area of 68.2 m2g-1 showed an improved photoactivity and adsorption capacity towards Rh B in visible light illumination, which is greater than that of the pure ln2S3 and CdS, respectively. In addition, ln2S3/CdS/Ti3C2-OH photocatalyst demonstrated tremendous photodegradation efficiency for mixture dyes of Rhodamine B and Methyl Orange, and its photocatalytic capability was greater than that of ln2S3/CdS/GO nanocomposite. The 22

photodegradation mechanism for the 4-TIC nanocomposite in visible light illumination is shown in Fig.15. When 4-TIC nanocomposite was illuminated by visible light, the electrons are excited from the VB to CB of semiconductors (In2S3 and CdS). Because of the existence of Ti3C2-OH MXene in the In2S3/CdS nanocomposite, the photoexcited electrons can directly migrate from the conduction band of semiconductors to Ti3C2-OH, therefore improving the efficient partition of photoexcited charge carriers. The produced holes on the valence band of the photocatalysts with strong oxidation ability can also straight oxidize organic pollutants in visible light illumination. Furthermore, the energetic electrons that concentrated on the conduction band of

of

semiconductors can generate •O2- for photodegradation of organic contaminants because of that conduction band potential of CdS and In2S3 being more negative than the reduction potential of

ro

O2/•O2-, thus additional supporting photocatalytic degradation performance. Here in, -OH terminated Ti3C2 in the heterostructure played a vital role in improving the partition efficiency of

-p

photogenerated electron-hole pairs, and extending the electron life span to generate stronger active groups (•O2-) for photo-degradation of contaminants, which finally develops the

re

photocatalytic activity of 4-TIC photocatalyst.

More recently, Fang et al. found that the Bi2WO6:Yb3+, Tm3+/Ti3C2-OH photocatalyst

lP

was effectively prepared by a facile hydrothermal technique [98]. The as-synthesized Bi2WO6:Yb3+, Tm3+/Ti3C2-OH photocatalyst accompanied 2D/2D heterostructure with grid-like porous structure present an important improvement of photocatalytic activity towards

ur na

photodegradation of Rhodamine B in visible and NIR light illumination. Particularly, the optimized T-TB2.8/0.2-Y photocatalyst shows the greatest photoactivity for Rhodamine B degradation, in which the RhB photodegradation effectiveness was 99.8% (Vis-NIR, 30 min), 91% (Vis, 30 min), and 48% (NIR, 160 min). Moreover, the photocatalysis reaction mechanism of the Y/T-TB2.8 nanocomposites in Vis-NIR light is demonstrated in Fig.16. Under Vis-NIR

Jo

light illumination, the sensitized lanthanide ion of Yb3+ will continuously absorb NIR light and the activated lanthanide ion of Tm3+ can concurrently change NIR light into UV and visible light, then the T-TB2.8/Y heterojunction can absorb the UV–vis light directly and generate electronsholes in the conduction band and valence band, respectively. The photogenerated electrons can rapidly transport from the Bi2WO6 to the Ti3C2-OH since the CB potential of Bi2WO6 is more negative than the Fermi level (EF) of −OH-terminated Ti3C2-OH. The existence of Ti3C2-OH in

23

the photocatalyst will significantly decrease the charge carrier recombination and progress the electron and hole pair partition effectiveness. The manufacture of Ti3C2/CeO2 nanohybrid is exposed in Fig.17. Ti3C2 sheets are negatively charged as an effect of termination with functional groups such as –OH [96]. The construction approach of Ti3C2/CeO2 nanohybrid is based on the electrostatic attraction of positively charged Ce3+ ions after that in situ development of CeO2 on the nanosheets. And the oxidized Ti3C2 is generated through texture Ti layer changing to TiO2 via the hydrothermal

of

technique. The as-synthesized Ti3C2/CeO2 nanohybrid demonstrated tremendous photoactivity toward photodegradation of Rh B is compared with that of pristine Ti3C2 and CeO2, which can create this sample obtainable to be an effective, probable photocatalyst for photo dye

ro

degradation.

Moreover, Ti3C2 MXene/α-Fe2O3 hybrid nanocomposite has been constructed utilizing

-p

homogeneous two dimensional α-Fe2O3 sheets and Ti3C2 MXene layered by an ultrasonic assisted self-assembly technique [85]. Photocatalytic activity of Ti3C2 MXene/α-Fe2O3 hybrid

re

nanocomposite was estimated for the photodegradation of Rh B under visible light illumination. The obtained products showed that the two dimensional α-Fe2O3 nanosheets were better

lP

distributed and attached on the texture of Ti3C2 MXene layered to produce several heterojunction interfaces, which can get stronger visible light absorption and high charge partition efficiency. Owing to the synergy effect, Ti3C2 MXene/α-Fe2O3 hybrid nanocomposite has tremendous

ur na

photocatalytic performance and recycling constancy. In addition, the photocatalytic charge carrier migration mechanism is shown in Fig.18. The work functions of Ti3C2 layered and αFe2O3 nanosheets are 4.46 and 4.34 eV, respectively. The electron will transfer from α-Fe2O3 nanosheets to Ti3C2 layered by the intimate interface until their Fermi levels are aligned. Moreover, it is possible for photoexcited electron to migration from α-Fe2O3 to Ti3C2 layered,

Jo

achieving higher electron/hole partition efficiency. Further study, Ning Liu et al. designed Ti3C2/g-C3N4 hybrid nanocomposite has been

constructed through an evaporation-induced self-assembly technique [88]. In the Ti3C2/g-C3N4 nanohybrid, Schottky heterostructure was effectively produced between 2D Ti3C2 and g-C3N4 which is restrained the recombination of charge carriers and given enhanced photoexcited charge partition and transfer behavior. The Ti3C2/g-C3N4 photocatalyst showed noticeable red-shift to visible region with the absorption edge of 515 nm compared with g-C3N4. Under visible light 24

illumination, its photocurrent was 2.2 μA.cm-1, which was 2.75 folds greater than that of pure gC3N4. The Ti3C2/g-C3N4 photocatalyst showed improved photocatalytic ability for ciprofloxacin photodegradation than g-C3N4. The possible photocatalytic mechanism of Ti3C2/g-C3N4 photocatalyst is illustrated in Fig.19. Under visible light illumination, g-C3N4 is agitated to make charge carriers, whereas Ti3C2 can’t be stimulated and only perform as the acceptors of e- owing to the coordinated band gaps between Ti3C2 and g-C3N4. The photoexcited electron could migrate simply from CB of g-C3N4 to Ti3C2 in the great driving force reasoned by the built-in electric field. The Femi level of Ti3C2 is more negative than that of O2/•O2-, therefore the

of

dissolved O2 can be decreased by e- to produce •O2- which gives to photodegradation of ciprofloxacin. In the meantime, the photoexcited h+ is aggregated on the VB of g-C3N4 and

ro

performs as the important group for photodegradation of ciprofloxacin.

More recently, C. Peng and his co-worker studied that the Ti3+-doped TiO2 octahedrons

-p

revealing active (111) facets and Ti3C2 MXene has been prepared via hydrothermal method [135]. The (111) r-TiO2-x was stimulated to generate charge carriers in light illumination

re

advantaged from the band gap narrowing because of Ti3+ doping Fig. 20 (a). After that, the electron transport to the conduction band of rutile TiO2-x. In the meantime, the holes transport

lP

from the {111} surfaces of TiO2-x to the -OH terminated Ti3C2 nanosheets. This charge transport is determined by the dissimilarity of work function across the interfaces. The active facet of rutile TiO2 and the -OH terminated Ti3C2 have work functions about 4.2 eV, 1.8 eV respectively,

ur na

which can act as a hole tank in this nanocomposite Fig. 20(b). Remarkable, electron/hole pair transport pathway of (111) r-TiO2/Ti3C2 is unlike from the TiO2 coupled with other two dimensional samples, e.g. GO and MoS2. The spatial separation of photogenerated electron/hole pairs at the interfaces of the Ti3C2/r-TiO2-x (111) restrains the electron/hole pairs recombination, hence significantly extends electron life. In the clarification, the long life span electrons

Jo

improved on TiO2-x (111) facets can respond with the dissolved oxygen (O2) to make •O2– groups and the product react with H+ and e- to yield high reactive •OH radicals. Simultaneously, the oxidative reaction occurred between the enhanced h+ on the layered Ti3C2-OH and OH- or H2O to make •OH species. The •OH was the main ROS for the photodegradation reaction. The produced major reactive groups and the mechanism of photocatalysis are illustrated by Eqns.7 to 11 as follows:

25

of ro

7.2. Photocatalytic water splitting reactions

Owing to the fast rate of industrialization and the growing greenhouse effects, green and

-p

renewable energy are immediately required [136,137]. Solar energy is a significant source of renewable energy since its total annular solar radiation is more than 5 MJ/m2 [138], which is observed as the major energy resource in the world. Since the pioneering work on,

re

“photoelectrochemical water splits over TiO2 electrodes” by Fujishima and Honda, broad research was done for the photocatalytic oxygen evolution and hydrogen generation [139]. In

lP

contradiction of overall splitting of water, hydrogen and oxygen generation half-reactions at the expenditure of a sacrificial reagent are extensively explored by photocatalysis [140]. Right now, several photocatalytic active semiconductor samples were revealed for light energy harvesting

ur na

[141]. Regrettably, the fast recombination of photoexcited charge carriers on single semiconductor photocatalyst largely hinders its practical application in photocatalytic water splitting. Coupling MXenes and semiconductors with an appropriate band position are a hopeful approach to yield an improved photoactivity, by at the same time expanding light absorption and

Jo

encouraging charge separation [38]. However, in recent years, MXene supported semiconductors photocatalysts have been mesmerized much concentration in the H2 production and O2 evolution. More recently, Wang et al. explored the physicochemical characteristics of Ti3C2Tx

MXene combining with TiO2 for water splitting hydrogen production [36]. The transmission electron microscopy (TEM) images of Ti3C2Tx (5 wt%)/TiO2 are presented in Fig.21(a). In contrast, the TiO2 NPs are well dispersed on the Ti3C2Tx MXene flakes. Moreover, the H2 generation rates for each photocatalyst are presented in Fig.21(b). In the Ti3C2Tx/TiO2 photocatalyst, Ti3C2Tx gives a two dimensional podium for close interactions with 26

homogeneously developed TiO2 NPs and supports the partition of photoexcited electron/hole pairs. The dissimilarity of the Ti3C2Tx quantity in the nanocomposites affects the optical features and photocatalytic activity. The TiO2/Ti3C2Tx photocatalyst (5 wt%) showed a remarkable improvement of the H2 generation by photocatalytic water splitting. Fascinatingly, the TiO2 NPs are excellent distributed on the Ti3C2Tx texture without rigorous accumulation. As well as Ti3C2Tx, Nb2CTx and Ti2CTx were also used as reduction co-catalysts to composite with TiO2 and improve photocatalytic activity in hydrogen evolution. Interestingly, Nb2CTx and Ti2CTx

of

display enhanced hydrogen evolution photocatalytic activity than Ti3C2Tx when mixed with TiO2 as co-catalysts Fig.21(c). Consequently, the maximum hydrogen production activity accomplished from the materials having Nb2CTx is ascribed to the maximum work function of

ro

(4.1 eV) Nb2CTx among all of the used transition metal carbides in this work. Under light irradiation, the TiO2 photocatalyst is excited to generate electrons/holes. Because of different

-p

Fermi levels of MXene and TiO2, the photoexcited e- is migrated from the conduction band of TiO2 to MXene. In addition, with the aggregation of the negative charges in MXene and the

re

positive charges in TiO2, the conduction band and valence band of TiO2 are curved uphill Fig.21(d), causing to the construction of Schottky barrier at the MXene/TiO2 heterojunction to

lP

stop the electrons from migrating reverse to TiO2. As well, the Schottky barrier and fast electron transferring, the aggregated electron on the MXene will respond with H+ ions to generate hydrogen. This report highlights the importance of the Schottky barrier to providing as

ur na

hindrances for the transfer of electrons toward encouraged photocatalysis. Li et al. developed that the facile hydrothermal technique was adopted to make BiOBr/Ti3C2 photocatalyst [100]. The photocatalytic activity of Ti3C2/BiOBr has been explored for water splitting in visible light illumination. It was determined that Ti3C2 with tremendous conductivity, provided as a tank for photoexcited holes in the nanocomposite. In addition,

Jo

photoexcited electrons on the CB of BiOBr were blocked from being migrated to Ti3C2, as the work function of Ti3C2 was more negative compared to the conduction band potential of BiOBr. The possible photocatalytic mechanism of improved photocatalytic performance for Ti3C2/BiOBr was proposed and shown in Fig.22. The migration of photoexcited electrons on the CB of BiOBr to Ti3C2 was reserved at the interface. Nonetheless, the band bending taken place at the depletion layer when Ti3C2 was coupled with BiOBr, and electron (e-) at the interface with more negative potentials were more capable of reducing water for the production of hydrogen. 27

As for the photoexcited h+ on the VB of BiOBr, they were free to transport from BiOBr to the CB of Ti3C2. As e- will not transport from BiOBr to Ti3C2, the recombination of photoexcited electron-hole pairs on Ti3C2 can be disallowed. This trend assisted the partition of photoexcited electron-hole pairs and encouraged the photoactivity in water splitting. Construction of the Schottky junction is measured to be a suitable way to enhance the spatial charge partition and migrate of the photocatalytic system. Lin et al. reported that the 2D O-doped g-C3N4 NSs have been synthesized via an annealing way, after that a 2D/2D O-doped

of

g-C3N4/Ti3C2 MXene Schottky-junction was employing an in-situ electrostatic assembly of positively charged O-doped g-C3N4 NSs and negatively charged Ti3C2 MXene [92]. The preparation procedure of the O-doped g-C3N4/Ti3C2 MXene Schottky-junction is illustrated in

ro

Fig.23. The as-synthesized O-doped g-C3N4/Ti3C2 MXene Schottky-junction showed approximately 2 folds of improved H2 production (25124 μmol/g/h) in comparison with pure O-

-p

doped g-C3N4 (13745 μmol/g/h) and g-C3N4/Ti3C2 MXene (15573 μmol/g/h). The MXene has a lower work function than that of the HCN, thus, the electrons tend to migrate from the MXene to

re

the HCN, leading to the energy bands of HCN bend upward to produce Schottky-junction with the MXene upon contact Fig.24. Under visible light illumination, the electrons in the conduction

lP

band of the HCN tend to skip the Schottky-junction and migrate to the MXene NSs owing to the close interfacial contact of 2D MXene/2D HCN in addition to the presence of the built-in electric field, leading to the band alignment between these two photocatalysts. The electrons aggregated

ur na

on the MXene nanosheets with plentiful active sites respond with the adsorbed water molecules to produce hydrogen. As well that, the construction of the Schottky-junction between MXene and HCN avoids the electrons transfer from the MXene back to the HCN, therefore encourage the partition effectiveness of electron/hole pair and enhanced the hydrogen production reaction. Later on, Zhuang et al. found that the novel 2D/1D photocatalyst of Ti3C2/TiO2 was

Jo

effectively synthesized through the electrostatic self-assembly method, as shown in Fig.25. [34]. The highest hydrogen generation rate of Ti3C2/TiO2 photocatalyst was up to 6.979 mmol h-1 g-1, which was 3.8 folds that of pristine TiO2 nanofibers. This development in photocatalytic hydrogen evolution of Ti3C2/TiO2 photocatalyst was derived from the heterointerface between TiO2 nanofibers and Ti3C2 sheets. The photocatalytic hydrogen evolution mechanism of Ti3C2/TiO2 photocatalyst was proposed as demonstrated in Fig.26. The TiO2 nanofibers produce a close contact interface with Ti3C2 nanosheets. Under the visible light irradiation, electrons in 28

the valence band of TiO2 are excited into the conduction band of TiO2 nanofibers. After that, the photoexcited electrons rapidly transfer from the contact interface to the Ti 3C2 sheets, and a large number of photoexcited holes can stay in the TiO2 nanofibers, which significantly encourages the partition of photoexcited electron-hole pairs and extends the life span of photoexcited electronhole pairs. Subsequently, the photoexcited electrons can aggregate on the Pt particles through a percolation mechanism, and the water molecules adsorbed on the surface of Pt particles will be reduced to H2 by photoexcited electrons.

of

As well, Su and his co-workers fabricated C/Nb2C MXene/Nb2O5 photocatalyst via oxidizing the Nb2CTx surface at different durations to produce Nb2O5 layer by employing CO2 as the mild oxidant [35]. Dissimilar oxidation times were a marked manipulate on the

ro

photocatalytic performance. With the optimized oxidation time of 1 hr, the photocatalytic performance of NCN-1.0 toward hydrogen evolution reaction is 4 folds greater than that of pure

-p

Nb2O5, which is initiated from the friendly interfacial junction between conductive Nb2C and Nb2O5 for excellent e-/h+ partition. This emphasizes the overview of MXene as feasible co-

re

catalysts for solar to chemical energy translation. The preparation of MXenes supported semiconductors based photocatalysts for photocatalytic water splitting reactions are recapitulated

lP

in the table.2.

Compared to metal oxide composites, coupling transition metal chalcogenides with MXene has also developed into an exciting field of photocatalysis study for energy adaptation.

ur na

Qiao and his co-workers found that the Ti3C2 NPs are realistically incorporated with CdS by a hydrothermal approach to encourage an efficient visible light photocatalytic H2 generation activity of 14,342 µmol/hg and an apparent quantum efficiency of 40.1% at 420 nm [32]. They determined that this remarkable activity is ascribed to the synergetic effect of the extremely efficient charge partition and transfer from CdS to Ti3C2 nanoparticles and the fast hydrogen

Jo

production on many –O terminations present on Ti3C2 nanoparticles. Another study, coupling of MXene with g-C3N4 hybrid nanocomposite was observed a regeneration of attention in the power of renewable energy generation. Another significant study, Shao et al. has designed gC3N4/Ti2C heterocomposite by thermal annealing of melamine with 2D Ti2C for significant hydrogen generation performance [142]. The optimized 0.4 wt% of Ti2C provides a higher hydrogen generation rate of 950 mmol h-1 g-1 with AQY of 4.3% at 420 nm. The improved

29

photocatalytic activity stems from the efficient charge migration and partition because of the presence of the Schottky barrier to decrease H+ to hydrogen. Co-catalyst loading gives an efficient method to improve the effectiveness of photocatalysts for solar H2 generation. X. An and co-authors effectively studied that the simultaneous mixing of Pt and MXene Ti3C2 as co-catalysts can successfully improve the photocatalytic activity of g-C3N4 for H2 evolution [143]. Owing to the synergetic effects between Ti3C2 and Pt NPs, 3 % Ti3C2/2% Pt/g-C3N4 demonstrated a widely enhanced H2 evolution rate,

of

which was greater than the Ti3C2/g-C3N4 and Pt/g-C3N4 nanohybrids. The enhanced performance of the dual co-catalyst tailored photocatalyst was attributed to the helped partition of photoexcited electron/hole pairs and the effective transport of e- from the CB of g-C3N4 to

ro

metallic co-catalysts. This work is encouraged continuing concentration in using Ti3C2 MXene in addition to other co-catalysts to progress the activity of photocatalysts for H2 evolution.

-p

In another study, Yang Li and colleagues found that the TiO2/Ti3C2 MXene photocatalyst was effectively prepared by an easy calcination method of F-terminated Ti3C2 MXene [107]. The

re

as-synthesized TiO2/Ti3C2 MXene hybrid nanocomposite has a two dimensional multilayer structure like MXene Ti3C2 and TiO2 showed a truncated octahedral bipyramidal structure with

lP

showing (001) facets under the involvement of fluorine ions. In addition, Fig.27 shows the possible mechanism of TiO2/Ti3C2 MXene photocatalyst for photocatalytic hydrogen production. Under visible light illumination, the (001) and (101) facets of TiO2 can be excited to generate

ur na

photoexcited electron-hole pairs. The surface band gap of (001) facets is uneven with that of (101) facets owing to the dissimilar atomic display on the surface of each facet. Consequently, the construction of the surface heterostructure between (001) and (101) facets can help the photoexcited electron to migrate from the (001) to (101) facets owing to the staggering band gap. This effect guided to the aggregation of photoexcited e-/h+ on the (101) and (001) facets,

Jo

respectively. Furthermore, the photoexcited electron can migrate from TiO2 to Ti3C2 owing to the high conductivity of Ti3C2. The remaining Ti3C2 can perform as a co-catalyst to improve the photocatalytic hydrogen evolution performance by detaining photoexcited electron from TiO2 owing to its electron tank characteristic and appropriate Fermi level. To increase electronic and photocatalytic properties, Hou Wang et al. developed that the Ti3C2(O, OH)x/Zn2In2S5 photocatalyst was reasonably made using Ti3C2(O, OH)x as a 2D stage for in situ development of flower-like Zn2In2S5 sample in an aerobically hydrothermal 30

circumstance [144]. The Ti3C2 (O, OH)x was approved as a two dimensional stage to produce in situ hierarchically flower-like Zn2In2S5 sample through a facile hydrothermal technique under anaerobic circumstance, as shown in Fig.28. Under visible light, the Ti3C2(O,OH)x/Zn2In2S5 photocatalyst with the Ti3C2(O, OH)x amount of 1.5% have H2 production yields of 12,983.8 μmol g-1, which is extensively enhanced than that of bare Zn2In2S5. The obvious quantum efficiency achieved 8.96% at 420 nm. Under light irradiation, the photoexcited electron of Zn2In2S5 was excited from the valence band to the conduction band to make the charge carriers,

of

escorted by the transfer from the conduction band of Zn2In2S5 to Ti3C2 (O, OH)x by the heterostructure interface, as shown in Fig.29. Photogenerated electrons were accumulated on the Ti3C2 (O, OH)x since the conduction band potential of Zn2In2S5 was more negative than the

ro

Fermi level (EF) of Ti3C2(O, OH)x. The in-plane electrical conductivity of Ti3C2 (O, OH)x was as a minimum one order of magnitude superior to that vertical to the basal plane. The immobilized

-p

oxygen responded with the aggregated electron at the in-plane of Ti3C2Tx to produce the super oxide radicals owing to high negative potentials than that of the oxygen reduction to super oxide

re

radical. In the meantime, the super oxide radicals could be decreased to produce the hydroxyl radicals, whereas the residual h+ can straight contribute to the photodegradation reaction of the

lP

contaminant. This work is anticipated to be precious towards the improvement of hierarchical MXene based nanohybrid photocatalyst for solar consumption and ecological remediation. Another important study, Pan Tian et al. assembled that the new UiO-66-NH2/Ti3C2/TiO2

ur na

heterojunction was logically fabricated through introduced Ti3C2Tx MXenes on water stable ZrMOFs precursors by a facile hydrothermal procedure [97]. Moreover, UiO-66-NH2 was covered on the texture of annealed Ti3C2Tx by electrostatic adsorption by a one-step hydrothermal technique, as shown in Fig.30. This improvement of hydrogen evolution reaction activity provoked attention in separating the definite photocatalytic procedure of UiO-66-

Jo

NH2/Ti3C2/TiO2. As suggested in Fig.31, 3 kinds of contact interfaces in the UiO-66NH2/Ti3C2/TiO2 photocatalyst are illustrated. Under the light irradiation, UiO-66-NH2 has excited to produce photoexcited charge carriers. The positions of the conduction band for TiO2 and UiO-66-NH2 are -0.1 eV and -0.26 respectively. Whereas the Ti3C2 given encouraging Fermi level and provided as an electron tank, therefore the photoexcited electron at the conduction band of UiO-66-NH2 were transferred straight (pathway I) or through TiO2 channels indirectly (pathway III) to Ti3C2 and spatially hindered the recombination of photoexcited electron/hole 31

pairs [145]. As well, the photoexcited e- at the conduction band of TiO2 can also individually migrate to Ti3C2 owing to the TiO2/Ti3C2 interface (pathway II). Subsequently, the photoexcited electrons on the showing active sites of Ti3C2 will then drive the H2O to generate H2. The synergistic effects of Schttoky junctions among the UiO-66-NH2/Ti3C2/TiO2, TiO2/Ti3C2 and UiO-66-NH2/Ti3C2 interfaces successfully assisted the partition and migration of photoexcited electrons which could permit the aggregation of photogenerated electrons on the texture of Ti3C2, causing the higher H2 evolution reaction activity.

of

Further study, Yujie Li et al. developed that the TiO2 sheets are in situ grown-ups on extremely Ti3C2 MXene and then MoS2 nanosheets are loaded on the (101) facets of TiO2 nanosheets with mostly showing high active (001) facets by a two-step hydrothermal technique

ro

[99]. Moreover, Fig.32 illustrates the preparation approach of the heterostructure of the MoS2/Ti3C2/TiO2 nanohybrid. The possible photocatalytic mechanism for hydrogen production MoS2/Ti3C2/TiO2

nanohybrid

is

suggested

and

demonstrated

-p

with

in

Fig.33.

For

MoS2/Ti3C2/TiO2 nanohybrid, under light illumination, the photogenerated charge carriers are

re

produced in the VB and CB of TiO2, respectively. The coexposed (101) and (001) facets of TiO2 may produce a surface heterostructure within single TiO2. The electrons transfer from the (001)

lP

facets of TiO2 to (101) facets and Ti3C2 owing to the tremendous electron conductivity of Ti3C2. Therefore, an electron rich location is attained on the planar surface of Ti3C2 and MoS2, on which the water is decreased to generate hydrogen.

ur na

Later on, Yujie and colleagues found that the in situ development approach for prepared TiO2/Ti3C2 nanohybrid through simultaneous oxidation and alkalization, after that ion exchange and calcination procedures of Ti3C2 MXene as photocatalysts for hydrogen and oxygen generation from water splitting reaction [83]. Furthermore, close contact between TiO2 and Ti3C2 MXene produces the synergetic effect and Schottky junction, attractive the charge partition and

Jo

efficiently reduced recombination, important to more electrons involves in the photocatalytic reduction for hydrogen production and more holes involves in photocatalytic oxidation for oxygen generation. The charge migration mechanism of photocatalytic overall water splitting on TiO2/Ti3C2 nanohybrid is shown in Fig. 34(a). Under light illumination, the TiO2 is excited to generate charge carriers. The photogenerated electrons on the CB of TiO2 can further quickly transfer to Ti3C2, remaining the holes on the VB of TiO2, owing to the excellent metallic features of Ti3C2. Besides, with the aggregation of the negative charge in Ti3C2 and the positive charges 32

in TiO2, a space charge layer is produced, and then the conduction band and valence band of TiO2 are curved uphill. As a result, for TiO2/Ti3C2 nanohybrid, electrons were migrated from TiO2 to Ti3C2 and then further quickly transferred to the Ti3C2 MXene texture owing to its tremendous electronic conductivity. And the H+ in the H2O was effectively decreased to generate hydrogen through the photogenerated electrons on Ti3C2 MXene due to its admirable hydrogen evolution ability. Eventually, the significant hydrogen evolution reaction performance of Ti 3C2 MXene and the electrons aggregation on Ti3C2 MXene encourage the reduction of the proton

of

into hydrogen by photogenerated electrons [146,147]. Meanwhile, the holes in the valence band

-p

ro

of TiO2 can respond with the water to generate oxygen. The Eqns are as follows;

re

Consequently, the photogenerated charge carriers could be efficiently alienated and migrated in the company of co-catalyst Ti3C2 MXene, which guides to extremely enhanced photocatalytic

lP

activity. Nonetheless, as revealed in Fig.34(b), pristine TiO2 is agitated to produce electrons/holes under solar light illumination. The electron/hole will rapidly recombine without the co-catalysts, which provoked the low photocatalytic activity.

ur na

7.3. Photoelectrochemical water splitting reactions Photoelectrochemical water splitting can straight translate solar energy into renewable chemical fuels like hydrogen from H2O or methanol from CO2, signifying a fascinating solution to the energy problems [148-150]. Since the n-type semiconductor TiO2 was initially

Jo

implemented as a photoanode for photoelectrochemical water oxidation [151], numerous efforts were observed on investigating earth plentiful, effective and constant photoelectrode samples over the past decades [152]. A perfect photoelectrochemical water splitting cell will necessitate a photocathode (p-type semiconductor) for water reduction and a photoanode (n-type semiconductor) for water oxidation. The photoelectrochemical and electrochemical water splitting is extremely expected for the sustainable fabrication of H2 and O2. The OER, as one of the two half-reactions splitting of water, involving complex electron and ion migration that typically guiding to lethargic kinetics and poor energy exchange effectiveness. Established 33

expensive metal based oxides such as RuO2 and IrO2 are among the most dynamic electrocatalysts towards the oxygen evolution reactions. Nonetheless, their shortage and associated high cost have inadequate mass manufacture and broad applications. Moreover, MXenes with a more surface areas, high surface hydrophilicity, and surface activities were fascinated applications as catalyst support. As one of the extensively reported cases, MXenes were measured for water electrolysis applications in which hydrogen and oxygen generation are produced by a HER at the cathode and an OER at the anode. Even though, Pt is

of

one of the most effective photocatalysts for the splitting of water, it has disadvantages owing to its expensive and low accessibility. MXenes are superior charge migration capabilities. It was revealed that O-terminated MXenes such as Ti2CO2 and W2CO2 have Gibbs free energies close

ro

to 0 eV for H2 adsorption [153,154], appropriate for hydrogen evolution reactions. Investigational confirmation of the hydrogen evolution reaction activity of Ti and Mo based

-p

MXenes proposed that Mo2CTx and Ti3C2Tx show an excellent hydrogen evolution reaction activity for producing H2 from H2O and ammonia borane, respectively [155-157]. The

re

photocatalytic characteristics of MXenes are enhanced by the introduced of metal elements such as iron. This is because less electronegative atoms such as iron may migrate high charge to the

lP

oxygen element of H2O; therefore, the O–H bonds of H2O are diluted, leading to an enhanced hydrogen evolution reaction activity.

Additionally, the heterostructure of g-C3N4/Ti3C2 develops the activity for the oxygen

ur na

evolution reaction as compared to the case of the pure MXene due to the charge migration between Ti elements and g-C3N4 helps the electron transfer for oxygen generation [158,38]. One more report was exposed that Mo2CO2 texture can be appropriate substrates for palladium atom attaching to construct oxygen from CO molecules [159]. Valuable metal oxides based such as IrO2 and RuO2 are conventional and extremely energetic toward oxygen evolution reaction,

Jo

however the useful applications are disadvantaged by their shortage and high price. In 2016, by manufacturing dissimilar two dimensional samples, Ma et al. synthesized unconnected gC3N4/MXene films by uniform gathering of g-C3N4 and Ti3C2 in catalyzing oxygen evolution reaction in basic nature [160]. By acting as a bendable oxygen electrode, the hierarchically porous films obvious high performance and stability by enchanting the qualities of the Ti-Nx motifs as electrocatalytic active sites for encouraging a new investigate increase in oxygen electrochemistry. 34

As well as two dimensional g-C3N4, a new electrocatalyst, coupled of two dimensional MOFs and Ti3C2Tx, was newly fabricated through an inter diffusion reaction approach toward tremendous electrocatalytic oxygen evolution reaction performance [161]. The cobalt 1,4benzenedicarboxylate (CoBDC)/Ti3C2Tx hybrid electrode had start potential of 1.51 V versus the reversible hydrogen electrode at a current density of 0.1 mA cm-2. The resultant composite is tested in the O2 generation reaction and accomplished a current density of 10 mA cm−2 at a potential of 1.64 V vs reversible hydrogen electrode and a Tafel slope of 48.2 mV dec−1 in 0.1 M

of

KOH. These outcomes better those achieved by the standard IrO2-based sample and are analogous with or even better than those accomplished by the earlier demonstrated state-of-theart transition-metal-based material. Moreover, this work proposed that the gathering of attractive

ro

functional characteristics is accomplished by coupling unusual 2D materials for highperformance electrocatalysis.

-p

More recently, N. Hao et al. reported that the NiFeCo LDH/TiO2/Ti3C2Tx hybrid nanocomposite has been fabricated by development of NiFeCo LDH sample on the surface of

re

Ti3C2Tx sheets via solvothermally method, during which TiO2 NPs were at the same time produced [104]. In this study, the heterostructure of NiFeCo LDH/TiO2/Ti3C2Tx hybrid

lP

nanocomposite exhibited tremendous electrocatalytic performance toward oxygen evolution reaction and attained a current density of 10 mA cm-2 in 0.1M KOH at 1.55 V vs. RHE, which is among the preeminent study oxygen evolution reaction photocatalysts. As well, the

ur na

heterostructure was mixed with BiVO4 for photoelectrochemical O2 evolution reactions and showed the much improved performance in comparison with the pristine BiVO4. The excellent performance for both the O2 evolution reaction and photoelectrochemical O2 evolution reaction effected from a synergistic effect between molecules. Cu2O was predicted as an efficient photocathode sample for photoelectrochemical water

Jo

reduction owing to its plenty, scalability and non-hazardous. The O2 vacancies improved Ti3C2TX flakes are attained by reducing Ti3C2TX with H2/Ar gas (denoted as H:Ti3C2TX). The H:Ti3C2TX/Cu2O, Cu2O/Ti3C2TX in addition to Au/Cu2O nanocomposite photocathodes are constructed by a facile dip-coating technique [162]. The manufacture way is demonstrated in Fig.35. The results proposed that the Ti3C2TX was high efficient than noble metal Au for enhanced the photoelectrochemical activity of Cu2O photocathode. The higher activity of H: Cu2O/Ti3C2TX is ascribed to improved O2 vacancies in the H:Ti3C2TX, which can develop the 35

conductivity, light-harvesting effectiveness and charge-transfer capability of the constructed photocathode. The present arrangement displays a 1.6 fold greater solar to hydrogen conversion effectiveness of 0.55% than the conservative propose with a solar to the hydrogen conversion efficiency of 0.34%. More important study, Dejian Yan et al. reported that the BiVO4/Ti3C2TX nanohybrid on the FTO substrate as a photoanode for photoelectrochemical water oxidation [163]. The manufacture path was demonstrated in Fig.36. A facile re-annealing behavior of pure FTO/BiVO4 photoanode in argon can remarkably enhance the photocurrent density from 2.1 mA

of

cm-2 up to 2.95 mA cm-2. Investigationally, they obviously clarified that by re-annealing, the interfacial FTO/BiVO4 charge recombination could be efficiently repressed owing to the

ro

enhanced contact between the FTO and BiVO4 film. The coating of thin Ti3C2TX flakes onto the BiVO4 film would additional enlarges the photocurrent density to 3.45 mA cm-2, showing a

probable

mechanism

of

the

charge

-p

photoconversion effectiveness of 0.78% and a surface charge partition efficiency of 73%. The migration

procedure

concerning its

improved

re

photoelectrochemical performance of BiVO4/Ti3C2TX photoanode is suggested and shown in Fig.37, initially BiVO4 can be agitated upon light irradiation and produce the charge carriers. In

lP

this work, the thin Ti3C2TX can enhance the visible light absorption. Consequently, the photoexcited e- will migrate from the BiVO4 film to the FTO substrate. The excellent contact between the FTO and BiVO4 film by the re-annealing behavior in argon can speed up

ur na

photoexcited e- migration and restrain interfacial charge recombination. In the meantime, the photoexcited h+ migrate to the interface of Ti3C2TX and electrolyte. After being migrated to Ti3C2TX, the photoexcited holes are additional quickly transferred to its texture, because of the superb metallic conductivity. As well, the Ti3C2TX as co-catalysts can help the charge migration of the photoexcited electron/hole pairs. Consequently, the novel BiVO4/Ti3C2TX hybrid

Jo

nanocomposite photoanode can get considerable improvement of photoelectrochemical behavior.

7.4. CO2 photoreductions Photocatalytic reduction of CO2 to worth added chemicals is enormously significant for

that not only decreases the level of the greenhouse gas but also facilitates meet the stipulate for the renewable fuels. In the late 1970s, Halmann and Inoue et al. initial studied the photoconversion of CO2 [150,164]. Artificial photosynthesis over semiconductor photocatalysts for the conversion of CO2 into different chemicals are an extremely required after reaction to 36

complete energy demands and transfers climate changes. As the generation of hydrocarbon fuels from CO2 was measured best solution to concurrently resolve the problems with the environment and energy. Consequently, numerous explorers were efforts different semiconductor sample for the photocatalytic reduction of CO2. The CO2 is transformed into CH3OH, CO, HCHO, HCOOH, and CH4 during the photocatalytic routes by using semiconductors GaN, WO3, TiO2, Fe2O3, ZnO and g-C3N4 were studied and widely explored. Numerous latest works were shown it is probable in CO2 reduction reactions, demonstrating that the admirable electrical conductivity, greater

of

structural constancy and convenient surface anchoring of functional groups could provide Ti3C2 to be extremely enhancing and characteristic noble metal free co-catalyst in photoreduction of CO2 [165-167]. Mainly, the -OH groups can provide plentiful basic sites, which may advantage

ro

for adsorption and activation of the acidic CO2 molecules. Therefore, important improvement in the photoconversion of CO2 could be expected. Here, MXene supported semiconductors

carriers without shifting their inventive redox ability.

-p

photocatalysts can significantly develop the separation effectiveness of photoinduced charge

re

A few investigators reported nanocomposites of MXene with semiconductors for photocatalytic applications of CO2 reduction. Ti3C2 MXene, which shows tremendous optical

lP

properties and electronic conductivity, was recognized as a promising noble metal free the cocatalyst for the improvement of effective photocatalysts for CO2 reduction. Shen et al. reported that the Ti3C2 MXene/CeO2 photocatalyst was synthesized through an in-situ growth of the cube

ur na

like CeO2 employing ultrathin Ti3C2-MXene NSs as a 2D podium by a hydrothermal way [168]. Under light irradiation, the Ti3C2 MXene/CeO2 photocatalyst with the optimal ratio of Ti3C2 MXene showed improved activity in the photocatalytic CO2 conversion, with activity 1.5 folds higher than that of pure CeO2. The enhanced performance of Ti3C2 MXene/CeO2 photocatalyst was ascribed to the Schottky junction induced by the built-in electric field between Ti3C2 MXene

Jo

and CeO2, which drives the photoexcited e- from CeO2 to Ti3C2 MXene and accelerates the separation of the electrons-holes. The electrons migration in the Ti3C2 MXene/CeO2 photocatalyst will be further confirmed by the band alignments of the CeO2 and Ti3C2-MXene are depicted in Fig.38. The work functions of Ti3C2-MXene and CeO2 were reported to be 5.78 eV and 4.69 eV, respectively, and when they combined with each other, the electrons can transport from Ti3C2-MXene to CeO2, leading the CB of CeO2 to bend upward and form a Schottky junction with Ti3C2 MXene. After irradiation, the photoexcited electrons are agitated 37

from CeO2. These photoexcited electrons will transfer across the Schottky barrier, and are then rapidly shuttled to the surface of the high-conductivity metallic Ti3C2-MXene, which promotes the separation of the photogenerated charges in CeO2. The existence of the Schottky-barrier also hindered the photoexcited electrons migrate from Ti3C2 MXene back to CeO2, and therefore the alienated electrons aggregated on the texture of Ti3C2 MXene then partake in the CO2 reduction reaction. Consequently, the photocatalytic activity of the Ti3C2-MXene/CeO2 photocatalyst was significantly improved because of the Schottky junction induced by the built-in electric field.

of

Moreover, J. Low et al. evaluated that the TiO2 NPs in situ developed on extremely MXene Ti3C2 by calcination technique [169]. The as-synthesized nanohybrid distinctive rice crust-like arrangement was attained by the homogeneous dispersion of TiO2 NPs on Ti3C2. This

ro

optimal Ti3C2/TiO2 nanohybrid demonstrated a 3.7 folds greater photoreduction of CO2 activity for methane (0.22 µmol h-1) generation than profitable TiO2. The photocatalytic improvement

-p

mechanism of Ti3C2/TiO2 nanohybrid for CO2 photoconversion is suggested and shown in Fig.39. Initially, more surface areas bouncing from fascinating structures must get the credit.

re

This is not only guaranteed plentiful surface reactive sites, but also encourages CO2 adsorption capability. After that, the single sample gives to the extensive activity. The light adsorption of

lP

the nanohybrid is improved by the black color character of Ti3C2, making photothermal effect. Subsequently, in situ development on Ti3C2 produces heterogeneous interfaces that can adapt resident material characteristics. Furthermore, the aggregation of the photoexcited electron on

ur na

Ti3C2 is constructive for the multi-electron CO2 photoreduction reaction. Even though, the combinations of the Ti3C2 with TiO2 nanoparticles are advantageous for photoconversion of CO2. A surplus of Ti3C2 guides to the light shielding effect. Comprehensively, Ti3C2 will participate with TiO2 nanoparticles to absorb the incident H+, extensively restraining the photoreduction effectiveness of the TiO2 nanoparticles. Consequently, to manage the ratio of

Jo

TiO2 nanoparticles to Ti3C2 is vital to attain optimal photocatalytic activity of the nanohybrid. This technique is anticipated to be comprehensive to make other metal oxides/MXene nanohybrids.

More recently, Shaowen Cao et al. developed that the Bi2WO6/Ti3C2 2D/2D heterojunction is effectively synthesized through in situ development of Bi2WO6 sheets on the texture of Ti3C2 nanosheets [170]. The manufacturing procedure of the Bi2WO6/Ti3C2 2D/2D heterostructure photocatalyst is demonstrated in 38

Fig.40. The Bi2WO6/Ti3C2 2D/2D

heterojunction photocatalyst with numerous atomic layers are discovered to have extremely improved behavior toward photoreduction of CO2, with a 4.6 folds total yield of methane and methanol as compared to that resultant on pure Bi2WO6 sheets. Due to the 2D/2D heterojunction of ultrathin Bi2WO6/Ti3C2 nanosheets have more interface contact area and relatively short charge transfer distance. Moreover, the photocatalytic procedure over ultrathin Bi2WO6/Ti3C2 2D/2D heterojunction can be demonstrated as Fig41(a). The photogenerated electrons are excited and jump from the valence band to the conduction band of Bi2WO6 in light illumination. Because

of

the conduction band potential of Bi2WO6 is more negative than the EF of Ti3C2 with terminal -O, the photoexcited electron can then migrate from Bi2WO6 to Ti3C2 by heterostructure nanosheets [171,172]. Consequently, the photogenerated electrons aggregated on the texture of Ti3C2 can

ro

respond with the adsorbed CO2. As shown in Fig 41(b), owing to the unique heterojunction with numerous atomic layers, photogenerated electrons can rapidly migrate from the bulk of Bi2WO6

-p

to the heterostructure interface, after that to the texture of Ti3C2. Therefore, the photoreduction of CO2 effectiveness can be significantly enhanced. The synthesis of MXenes supported

re

semiconductors based photocatalysts for photocatalytic CO2 reduction reactions are summarized in table.3.

lP

Later on, Minheng Ye et al. found that the Ti3C2 MXene as an extremely capable noble metal free co-catalyst coupled with profitable titania (P25) for photoreduction of CO2 [33]. Moreover, surface alkalinization of Ti3C2 noticeably improves the performance, the generation

ur na

rates of CH4 (16.61 μ molg-1 h-1) and CO (11.74 μ molg-1 h-1) are 277 and 3 times those of pure P25, respectively. The extensively improved performance is ascribed to the better electrical conductivity and charge carrier partition capability, in addition to the plentiful CO2 adsorption and reactive sites of the surface alkalinized Ti3C2 MXene. Furthermore, the mechanism showing the excellent performance and high selectivity for methane of P25/Ti3C2-OH are suggested in

Jo

Fig.42. Upon light illumination, electrons are excited from the valence band to the conduction band of P25, remaining holes at the valence band. As the Fermi level (EF) of Ti3C2-OH is smaller than that of P25, the electron transfer from the conduction band of P25 to Ti3C2-OH, so electrons/holes are alienated efficiently. Due to the superb electron conductivity of Ti3C2-OH, electron well-off surroundings are attained on the planar surface of Ti3C2-OH, on which the adsorbed CO2 molecules are condensed with electrons into carbon monoxide and methane. At this time, the plentiful -OH functions on Ti3C2-OH textures are active sites for adsorption and 39

activation of CO2 molecules. The enrichment of electrons, strong interaction between the Ti3C2OH co-catalyst and CO2 molecules, all help the multiple proton/electron migration, thus the photoreduction of CO2 to the methane is improved extensively. We consider that this report could shed a light on increasing extremely effective noble metal free co-catalysts for artificial photosynthesis.

8. Conclusions and prospects In summary, this review highlighted the current study progress related to MXene supported

of

semiconductors photocatalysts for different applications in energy conversion and environmental decontamination, including photocatalytic degradation of organic contaminants, photocatalytic

ro

and photoelectrochemical water splitting reactions and photocatalytic CO2 reduction. Also, this review article recapitulated the different synthesis techniques such as solvothermal process, hydrothermal procedure, ion exchange method and self-assembly procedure were employed to

-p

synthesize the MXene supported semiconductors photocatalysts. Herein, the MXene acts as a pathway transferring photoexcited electrons to improve the separation efficiency of charge

re

carriers. Schottky junctions formed by interfacial interaction between MXene and semiconductor also significantly accelerate photoinduced charge separation, in the meantime semiconductor

lP

gives electrons and holes by absorbing light. Moreover, the 2D MXene structure can enlarge the specific surface area, enable the solvent to access the reactive sites, enhance the light-harvesting capability, and shorten the diffusion paths for photoexcited electrons and holes. Despite the

ur na

absence of noble metals (Pt, Au, Ru2O, etc.), these unique characteristics of our designed MXene/semiconductor based photocatalyst make them present a significantly enhanced efficiency of photocatalytic and photoelectrochemical water splitting and CO2 reduction. So far, despite the important development in MXene supported semiconductors photocatalysts procedures, there are various impediments and problems to be solved for increasing effective

Jo

MXene supported semiconductors photocatalysts system and investigating the basic mechanism for improved photocatalytic performance. The present review article can be summarized in the following prospects: 1. Even though the probable applications of MXene supported semiconductors photocatalysts were expanded, the associations between the MXene structure and its photocatalytic performance in a detailed reaction are still not very understandable.

40

2. Increasing more novel and efficient amendment approaches to develop the photocatalytic performance and strength of MXene supported semiconductors photoheterojunction. 3. Performing more comprehensively hypothetical and computational analysis to investigate the inter correlations among the MXene supported semiconductors photo heterojunctions microstructures, compositions and photocatalytic characteristics. 4. Hypothetical modeling is required to recognize the physicochemical features of MXene supported semiconductors photoheterojunctions and to clarify the correlation of

of

nanostructure, catalytic kinetics and shape selectivity.

ro

Declaration of interests

The authors are grateful to financial support from National Natural Science Foundation of

-p

China (51672113, 21975110 and 21972058).

re

Acknowledgment

The authors are grateful to financial support from National Natural Science Foundation of

Jo

ur na

lP

China (51672113, 21975110 and 21972058).

41

References 1. A. Meng, L. Zhang, B. Cheng, J. Yu, Dual Cocatalysts in TiO2 Photocatalysis, Adv. Mater. 31 (2019) 1807660. 2. D.S. Li, H.C. Wang, H. Tang, X.F. Yang, Q.Q. Liu, Remarkable enhancement in solar oxygen evolution from MoSe2/Ag3PO4 heterojunction photocatalyst via in situ constructing interfacial contact, ACS Sustain. Chem. Eng. 7 (2019) 8466-8474. 3. Q. Xu, L. Zhang, J. Yu, S. Wageh, A.A. Al-Ghamdi, M. Jaroniec, Direct Z-scheme

of

photocatalysts: Principles, synthesis, and applications, Mater. Today. 21 (2018) 10421063.

ro

4. Y. Fu, Z. Li, Q. Liu, X. Yang, H. Tang, Construction of carbon nitride and MoS2 quantum dot 2D/0D hybrid photocatalyst: Direct Z-scheme mechanism for improved photocatalytic activity, Chinese J. Catal. 38 (2017) 2160–2170.

-p

5. W. Liu, J. Shen, X. Yang, Q. Liu, H. Tang, Dual Z-scheme g-C3N4/Ag3PO4/Ag2MoO4

Surf. Sci. 456 (2018) 369–378.

re

ternary composite photocatalyst for solar oxygen evolution from water splitting, Appl.

6. T. Xiong, H. Wang, Y. Zhou, Y. Sun, W. Cen, H. Huang, Y. Zhang, F. Dong, KCl-

lP

mediated dual electronic channels in layered g-C3N4 for enhanced visible light photocatalytic NO removal, Nanoscale. 10 (2018) 8066-8074. 7. H. Tang, S. Chang, G. Tang, W. Liang, AgBr and g-C3N4 co-modified Ag2CO3 A

novel

multi-heterostructured

ur na

photocatalyst:

photocatalyst

with

enhanced

photocatalytic activity, Appl. Surf. Sci. 391 (2017) 440–448. 8. Z. Wu, X. Yuan, J. Zhang, H. Wang, L. Jiang, G. Zeng, Photocatalytic decontamination of wastewater containing organic dyes by metal-organic frameworks and their derivatives, ChemCatChem. 9(1) (2016) 41–64.

Jo

9. X. Yuan, H. Wang, Y. Wu, X. Chen, G. Zeng, L. Leng, C. Zhang, A novel SnS 2– MgFe2O4/reduced graphene oxide flower-like photocatalyst: solvothermal synthesis, characterization and improved visible-light photocatalytic activity, Catal. Commun. 61 (2015) 62–66. 10. F. Le Formal, S.R. Pendlebury, M. Cornuz, S.D. Tilley, M. Gra, J.R. Durrant, Back electron-hole recombination in hematite photoanodes for water splitting, J. Am. Chem. Soc. 136 (2014) 2564–2574. 42

11. D.J. Martin, N. Umezawa, X. Chen, J. Ye, J. Tang, Facet engineered Ag3PO4 for efficient water photooxidation, Energy Environ. Sci. 6 (2013) 3380. 12. W.K. Jo, T.S. Natarajan, Influence of TiO2 morphology on the photocatalytic efficiency of direct Z-scheme g-C3N4/TiO2 photocatalysts for isoniazid degradation, Chem. Eng. J. 281 (2015) 549–565. 13. Y.P. Yuan, L.S. Yin, S.W. Cao, G.S. Xu, C.H. Li, C. Xue, Improving photocatalytic hydrogen production of metal-organic framework UiO-66 octahedrons by dyes dye

of

sensitization, Appl. Catal. B Environ. 168–169 (168) (2015) 572–576. 14. X. Li, W. Zhang, J. Li, G. Jiang, Y. Zhou, S. Cheng Lee, F. Dong, Transformation pathway and toxic intermediates inhibition of photocatalytic NO removal on designed Bi

ro

metal@defective Bi2O2SiO3, Appl. Catal. B Environ. 241 (2019) 187–195.

15. J. Liao, W. Cui, J. Li, J. Sheng, H. Wang, X. Dong, P. Chen, G. Jiang, Z. Wang, F.

-p

Dong, Nitrogen defect structure and NO+ intermediate promoted photocatalytic NO removal on H2 treated g-C3N4, Chem. Eng. J. 379 (2020) 122282.

re

16. L. Chen, L. Tian, X. Zhao, Z. Hu, J. Fan, K. Lv, SPR effect of Au nanoparticles on the visible photocatalytic RhB degradation and NO oxidation over TiO2 hollow nanoboxes,

lP

Arab. J. Chem. https://doi.org/10.1016/j.arabjc.2019.08.011. 17. B. Yang, K. Lv, Q. Li, J. Fan, M. Li, Photosensitization of Bi2O2CO3 nanoplates with amorphous Bi2S3 to improve the visible photoreactivity towards NO oxidation, Appl.

ur na

Sur. Sci. 495 (2019) 143561.

18. D. Yin, L. Zhang, X. Cao, Z. Chen, J. Tang, Y. Liu, T. Zhang, M. Wu, Preparation of a novel nanocomposite NaLuF4: Gd, Yb, Tm/SiO2/Ag/TiO2 with high photocatalytic activity driven by simulated solar light, Dalt. Trans. 45 (2016) 1467–1475. 19. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor

Jo

heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244.

20. M. Reza Gholipour, C.T. Dinh, F. Béland, T.O. Do, Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting, Nanoscale. 7 (2015) 8187–8208.

43

21. K. Zhang, Y. Liu, J. Deng, S. Xie, H. Lin, X. Zhao, J. Yang, Z. Han, H. Dai, Fe2O3/3DOM BiVO4, high-performance photocatalysts for the visible light-driven degradation of 4-nitrophenol, Appl. Catal. B Environ. 202 (2017) 569–579. 22. M. Khazaei, M. Arai, T. Sasaki, C. Chung, N.S. Venkataramanan, M. Estili, Y. Sakka, Y. Kawazoe, Novel electronic and magnetic properties of two-dimensional transition metal carbides and nitrides, Adv. Funct. Mater. 23 (2013) 2185−2192. 23. M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi,

of

M.W. Barsoum, Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23 (2011) 4248−4253. 24. P. Simon, Two-dimensional

MXene

with

controlled interlayer

spacing for

ro

electrochemical energy storage, ACS Nano. 11 (2017) 2393−2396.

25. H. Fang, Y. Pan, M. Yin, L. Xu, Y. Zhu, C. Pan, Facile synthesis of ternary Ti3C2–

-p

OH/ln2S3/CdS composite with efficient adsorption and photocatalytic performance towards organic dyes, J. Sol. Stat. Chem. 280 (2019) 120981.

re

26. Z. Ling, C.E. Ren, M.Q. Zhao, J. Yang, J.M. Giammarco, J. Qiu, M.W. Barsoum, Y. Gogotsi, Flexible and Conductive MXene films and nanocomposites with high

lP

capacitance, Proc. Natl. Acad. Sci. U. S. A. 111 (2014) 16676−16681. 27. S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, T.F. Heinz, S.S. Hong, J. Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V.V. Plashnitsa, Ruoff, S.

ur na

R.D. Robinson, R.S.

Salahuddin, J.

Shan,

L. Shi, M.G.

Spencer,

M. Terrones, W. Windl, J.E. Goldberger, Progress, challenges, and opportunities in two-dimensional materials beyond graphene, ACS Nano. 7 (2013) 2898−2926. 28. H. Tang, Q. Hu, M. Zheng, Y. Chi, X. Qin, H. Pang, Q. Xu, MXene–2D layered electrode materials for energy storage, Prog. Nat. Sci. Mater. Int. 28 (2018) 133–147.

Jo

29. R. Mantovani Ronchi, J. Teodoro Arantes, S. Ferreira Santos, Synthesis, structure, properties and applications of MXenes: Current status and perspectives, Ceramics International. 45 (2019) 18167-18188.

30. C. Zhang, V. Nicolosi, Graphene and MXene-based transparent conductive electrodes and supercapacitors, Ener. Stor. Mater. 16 (2019) 102–125. 31. B. Lyu, M. Kim, H. Jing, J.Kang, C. Qian, S. Lee, J.H. Cho, Large-area MXene electrode array for flexible electronics, ACS Nano. 13 (2019) 11392−11400. 44

32. J. Ran, G. Gao, F.T. Li, T.Y. Ma, A. Du, S.Z. Qiao, Ti3C2 MXene co-catalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production, Nat. Commun. 8 (2017) 13907. 33. M. Ye, X. Wang, E. Liu, J. Ye, D. Wang, Boosting the photocatalytic activity of P25 for carbon dioxide reduction by using a surface-alkalinized titanium carbide MXene as cocatalyst, ChemSusChem. 11 (2018) 1606−1611. 34. Y. Zhuang, Y. Liu, X. Meng, Fabrication of TiO2 nanofibers/MXene Ti3C2

of

nanocomposites for photocatalytic H2 evolution by electrostatic self-assembly, Appl. Surf. Sci. 496 (2019) 143647.

35. T. Su, R. Peng, Z.D. Hood, M. Naguib, I.N. Ivanov, J.K. Keum, Z. Qin, Z. Guo, Z. Wu,

ro

One‐step synthesis of Nb2O5/C/Nb2C (MXene) composites and their use as photocatalysts for hydrogen evolution, ChemSusChem. 11 (2018) 688–699.

-p

36. H. Wang, R. Peng, Z.D. Hood, M. Naguib, S.P. Adhikari and Z. Wu, Titania composites with 2 D transition metal carbides as photocatalysts for hydrogen production under

re

visible‐light irradiation, ChemSusChem. 9 (2016) 1490–1497.

37. X. Yu, W. Yin, T. Wang, Y. Zhang, Decorating g-C3N4 nanosheets with Ti3C2 MXene

lP

nanoparticles for efficient oxygen reduction reaction, Langmuir. 35 (2019) 2909−2916. 38. Y. Sun, D. Jin, Y. Sun, X. Meng, Y. Gao, Y. Dall’Agnese, G. Chen, X.F. Wang, gC3N4/Ti3C2Tx (MXenes) composite with oxidized surface groups for efficient photocatalytic

ur na

hydrogen evolution, J. Mater. Chem. A. 6 (2018) 9124–9131. 39. Ch. Prasad, H. Tang, Q. Liu, S. Zulfiqar, S. Shah, I. Bahadur, An overview of semiconductors/layered

double

hydroxides

composites:

properties,

synthesis,

photocatalytic and photoelectrochemical applications, J. Mol. Liq. 289 (2019) 111114. 40. Ch. Prasad, H. Tang, I. Bahadur, Graphitic carbon nitride based ternary nanocomposites:

Jo

From synthesis to their applications in photocatalysis: A recent review, J. Mol. Liq. 281 (2019) 634–654.

41. A.K.N. Geim, The rise of graphene, Nat. Mater. 6 (2007) 1–14. 42. K.S. Novoselov et al. A roadmap for graphene, Nature. 490 (2012) 192–200. 43. Y. Liu, X.F. Zhang, S.L. Dong, Z.Y. Ye, Y.D. Wei, Synthesis and tribological property of Ti3C2T(X) nanosheets, J. Mater. Sci. 52 (2017) 2200–2209.

45

44. L. Verger, C. Xu, V. Natu, H.M. Cheng, W. Ren, M.W. Barsoum, Overview of the synthesis of MXenes and other ultrathin 2D transition metal carbides and nitrides, Curr. pin. Solid State Mater. Sci. 23 (2019) 149-163. 45. J. Li, R. Qin, L. Yan, Z. Chi, Z. Yu, N. Li, M. Hu, H. Chen, G. Shan, Plasmonic light illumination creates a channel to achieve fast degradation of Ti3C2Tx nanosheets, Inorg. Chem. 58 (2019) 7285−7294. 46. M. Naguib, J. Come, B. Dyatkin, V. Presser, P.L. Taberna, P. Simon, M.W. Barsoum,

of

Y. Gogotsi, MXene: a promising transition metal carbide anode for lithium-ion batteries, Electrochem. Commun. 16 (2012) 61–64.

47. J. Pang, R.G. Mendes, A. Bachmatiuk, L Zhao, H.Q. Ta, T. Gemming, H. Liu, Z. Liu,

ro

M.H. Rummeli, Applications of 2D MXenes in energy conversion and storage systems, Chem. Soc. Rev. 48 (2019) 72–133.

-p

48. X. Xiao, H. Wang, P. Urbankowski, Y. Gogotsi, Topochemical synthesis of 2D materials, Chem. Soc. Rev. 47 (2018) 8744–8765.

re

49. Q. Hu, D. Sun, Q. Wu, H. Wang, L. Wang, B. Liu, A. Zhou and J. He, MXene: a new family of promising hydrogen storage medium, J. Physic. Chem. A. 117 (2013) 14253-

lP

14260.

50. Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu, Y. Tian, Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide, J.

ur na

Am. Chem. Soc. 136 (2014) 4113-4116.

51. C.E. Ren, K.B. Hatzell, M. Alhabeb, Z. Ling, K.A. Mahmoud, Y. Gogotsi, Charge- and size-selective ion sieving through Ti3C2Tx MXene membranes, J. Physic.Chem. Letter. 6 (2015) 4026-4031.

52. H. Liu, C. Duan, C. Yang, W. Shen, F. Wang, Z. Zhu, A novel nitrite biosensor based on

Jo

the direct electrochemistry of hemoglobin immobilized on MXene-Ti3C2, Sens. Actua B: Chem. 218 (2015) 60-66.

53. X. Wang, S. Kajiyama, H. Iinuma, E. Hosono, S. Oro, I. Moriguchi, M. Okubo, A. Yamada, Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors, Nat. com. 6 (2015) 6544.

46

54. S.J. Kim, M. Naguib, M. Zhao, C. Zhang, H.-T. Jung, M. W. Barsoum, Y. Gogotsi, High mass loading, binder-free MXene anodes for high areal capacity Li-ion batteries, Electrochimica Acta. 163 (2015) 246-251. 55. S.Y. Lin, X. Zhang, Two-dimensional titanium carbide electrode with large mass loading for super capacitor, J. Power Sources. 294 (2015) 354-359. 56. M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi, Guidelines for synthesis and processing of 2D titanium carbide (Ti3C2Tx MXene),

of

Chem. Mater. 29 (2017) 7633–7644. 57. B. Anasori, M.R. Lukatskaya, Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater. 2 (2017) 16098.

ro

58. M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, 25th anniversary article: MXenes: a new family of two-dimensional materials, Adv. Mater. 26 (2014) 992–1005.

-p

59. H. Wang, Y. Wu, J. Zhang, G. Li, H. Huang, X. Zhang, Q. Jiang, Enhancement of the electrical properties of MXene Ti3C2 nanosheets by post-treatments of alkalization and

re

calcination, Mater. Lett. 160 (2015) 537–540.

60. M. Kurtoglu, M. Naguib, Y. Gogotsi, M.W. Barsoum, First principles study of two

lP

dimensional early transition metal carbides, MRS Commun. 2 (2012) 133–137. 61. M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi, M.W. Barsoum, Two-dimensional transition metal carbides, ACS Nano. 6 (2012) 1322–1331.

ur na

62. H. Wang, Y. Wu, X. Yuan, G. Zeng, J. Zhou, X. Wang, J.W. Chew, Clay-inspired MXene-based electrochemical devices and photo-electrocatalyst: state-of-the-art progresses and challenges, Adv. Mater. 30 (12) (2017) 1704561. 63. X.H. Zha, J. Zhou, Y. Zhou, Q. Huang, J. He, J.S. Francisco, K. Luo, S. Du, Promising electron mobility and high thermal conductivity in Sc2CT2 (T = F, OH) MXenes,

Jo

Nanoscale. 8 (2016) 6110–6117.

64. M. Khazaei, A. Ranjbar, K. Esfarjani, D. Bogdanovski, R. Dronskowski, S. Yunoki, Insights into exfoliation possibility of MAX phases to MXenes, Phys. Chem. Chem. Phys. 20 (2018) 8579–8592. 65. Q. Tang, Z. Zhou, P. Shen, Are MXenes promising anode materials for Li ion batteries? computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X = F, OH) monolayer, J. Am. Chem. Soc. 134 (2012) 16909–16916. 47

66. Q. Meng, J. Ma, Y. Zhang, Z. Li, A. Hu, J.J. Kai, J. Fan, Theoretical investigation of zirconium carbide MXenes as prospective high capacity anode materials for Na-ion batteries, J. Mater. Chem. A. 6 (2018) 13652–13660. 67. G. Gao, G. Ding, J. Li, K. Yao, M. Wu, M. Qian, Monolayer MXenes: promising half metals and spin gapless semiconductors, Nanoscale. 8 (2016) 8986–8994. 68. F. Wu, K. Luo, C. Huang, W. Wu, P. Meng, Y. Liu, E. Kan, Theoretical understanding of magnetic and electronic structures of Ti3C2 monolayer and its derivatives, Solid State

of

Commun. 222 (2015) 9–13. 69. Y. Yue, Fe2C monolayer: an intrinsic ferromagnetic MXene, J. Magn. Magn. Mater. 434 (2017) 164–168.

ro

70. I.R. Shein, A.L. Ivanovskii, Graphene-like nanocarbides and nanonitrides of d metals (MXenes): synthesis, properties and simulation, Micro & Nano Lett. 8 (2013) 59–62.

-p

71. H. Kumar, N.C. Frey, L. Dong, B. Anasori, Y. Gogotsi, V.B. Shenoy, Tunable magnetism and transport properties in nitride MXenes, ACS Nano. 11 (2017) 7648–

re

7655.

72. K. Hantanasirisakul, M.Q. Zhao, P. Urbankowski, J. Halim, B. Anasori, S. Kota, C.E.

lP

Ren, M.W. Barsoum, Y. Gogotsi, Fabrication of Ti3C2Tx MXene transparent thin films with tunable optoelectronic properties, Adv. Electron. Mater. 2(6) (2016) 1600050– 1600057.

ur na

73. K. Huang, Z. Li, J. Lin, G. Han, P. Huang, Two-dimensional transition metal carbides and nitrides (MXenes) for biomedical applications, Chem. Soc. Rev. 47 (2018) 5109– 5124.

74. D. Magne, V. Mauchamp, S. Celerier, P. Chartier, T. Cabioc’h, Site-projected electronic structure of two-dimensional Ti3C2MXene: the role of the surface functionalization

Jo

groups, Phys.Chem. Chem. Phys. 18 (2016) 30946−30953.

75. Y. Wen, T.E. Rufford, X. Chen, N. Li, M. Lyu, L. Dai, L. Wang, Nitrogen-doped Ti3C2Tx MXene electrodes for high performance super capacitors, Nano Energy. 38 (2017) 368−376. 76. M. Abdullah Iqbal, S. Irfan Ali et al., La- and Mn-codoped bismuth ferrite/Ti3C2 MXene composites for efficient photocatalytic degradation of congo red dye, ACS Omega. 4 (2019) 8661-8668. 48

77. M. Nagui, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M. W. Barsoum, Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23 (20114) 248−4253. 78. L. Wang, W. Tao, L. Yuan, Z. Liu, Q. Huang, Z. Chai, J.K. Gibson, W. Shi, Rational control of the interlayer space inside two-dimensional titanium carbides for highly efficient uranium removal and imprisonment, Chem. Commun. 53 (2017) 12084−12087. 79. B. Anasori, Y. Xie, M. Beidaghi, J. Lu, B.C. Hosler, L. Hultman, P.R.C. Kent, Y.

of

Gogotsi, M.W. Barsoum, Two dimensional, ordered, double transition metals carbides (MXenes), ACS Nano. 9 (2015) 9507–9516.

80. P. Urbankowski, B. Anasori, T. Makaryan, D. Er, S. Kota, P.L. Walsh, M. Zhao, V.B.

Ti4N3 (MXene), Nanoscale. 8 (2016) 11385–11391.

ro

Shenoy, M.W. Barsoum, Y. Gogotsi, Synthesis of two-dimensional titanium nitride

-p

81. J. Peng, X. Chen, W. Ong, X. Zhao, N. Li, Surface and heterointerface engineering of 2D MXenes and their nanocomposites: insights into electro- and photocatalysis, Chem.

re

5 (2019) 18–50.

82. J.B. Rivest, P.K. Jain, Cation exchange on the nanoscale: an emerging technique for new

89–96.

lP

material synthesis, device fabrication, and chemical sensing, Chem. Soc. Rev. 42 (2013)

83. Y. Li, X. Deng, J. Tian, Z. Liang, H. Cui, Ti3C2 MXene-derived Ti3C2/TiO2 nanoflowers

ur na

for noble-metal-free photocatalytic overall water splitting, Appl. Mater. Today. 13 (2018) 217-227.

84. Q. Huang, Y. Liu, T. Cai, X. Xia, Simultaneous removal of heavy metal ions and organic pollutant by BiOBr/Ti3C2 nanocomposite, J. Photochem. Photobiol. A: Chem. 375 (2019) 201–20.

Jo

85. H. Zhang, M. Li, J. Cao, Q. Tang, P. Kang, C. Zhu, M. Ma, 2D α-Fe2O3 doped Ti3C2 MXene composite with enhanced visible light photocatalytic activity for degradation of Rhodamine B, Ceramics International. 44 (2018) 19958–19962.

86. T. Cai, L. Wang, Y. Liu, S. Zhang, W. Dong, H. Chen, X. Yi, J.Yuan, X. Xia, C. Liu, S. Luo, Ag3PO4/Ti3C2 MXene interface materials as a Schottky catalyst with enhanced photocatalytic activities and anti-photocorrosion performance, Appl. Catal. B Environ. 239 (2018) 545–554. 49

87. Y. Yang, Z. Zeng, G. Zeng, D. Huang, R. Xiao, C. Zhang, C. Zhoua, W. Xiong, W. Wang, M. Cheng, W. Xue, H. Guo, X. Tang, D. He, Ti3C2 Mxene/porous g-C3N4 interfacial Schottky junction for boosting spatial charge separation in photocatalytic H2O2 production, Appl. Catal. B Environ. 258 (2019) 117956. 88. N. Liu, N. Lu, Y. Su, P. Wang, X. Quan, Fabrication of Ti3C2/g-C3N4 composite and its visible-light photocatalytic capability for ciprofloxacin degradation, Sep. Purif. Tech. 211 (2019) 782–789.

of

89. H. Zhang, M. Li, C. Zhu, Q. Tang, P. Kang, J. Cao, Preparation of magnetic αFe2O3/ZnFe2O4/Ti3C2 MXene with excellentphotocatalytic performance, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.236.

ro

90. G. Huang, S. Li, L. Liu, L. Zhu, Q. Wang, Ti3C2 MXene-Modified Bi2WO6 nanoplates for efficient photodegradation of volatile organic compounds, Appl. Surf. Sci. (2019),

-p

doi: https://doi.org/10.1016/j.apsusc.2019.144183.

91. Y. Fang, Y.Cao, Q. Chen, Synthesis of an Ag2WO4/Ti3C2 Schottky composite by

re

electrostatic traction and its photocatalytic activity, Ceramics International. 45 (2019) 22298–22307.

lP

92. P. Lin, J. Shen, X. Yu, Q. Liu, D. Li, H. Tang, Construction of Ti3C2 MXene/O-doped gC3N4 2D/2D Schottky-junction for enhanced photocatalytic hydrogen evolution, Ceramics International. 45 (2019) 24656–24663.

ur na

93. H. Deng, Z. Li, L. Wang, L. Yuan, J. Lan, Z. Chang, Z. Chai, W. Shi, Nanolayered Ti3C2 and SrTiO3 composites for photocatalytic reduction and removal of uranium(VI), ACS Appl. Nano Mater. 2 (2019) 2283-2294. 94. Y. Gao, L. Wang, A. Zhou, Z. Li, J. Chen, H. Bala, Q. Hu, X. Cao, Hydrothermal synthesis of TiO2/Ti3C2 nanocomposites with enhanced photocatalytic activity, Mater.

Jo

Lett. 150 (2015) 62–64.

95. X. Lu, J. Zhu, W. Wu, B. Zhang, Hierarchical architecture of PANI@TiO2/Ti3C2Tx ternary composite electrode for enhanced electrochemical performance, Electrochimica Acta. 228 (2017) 282-289. 96. W. Zhou, J. Zhu, F. Wang, M. Cao, T. Zhao, One-step synthesis of Ceria/Ti3C2 nanocomposites with enhanced photocatalytic activity, Mater. Lett. 206 (2017) 237–240.

50

97. P. Tian, X. He, L. Zhao, W. Li, W. Fang, H. Chen, F. Zhang, Z. Huang, H. Wang, Enhanced charge transfer for efficient photocatalytic H2 evolution over UiO-66-NH2 with annealed Ti3C2Tx MXenes, Int. J. Hydrogen Energy. 44 (2019) 788-800. 98. H. Fang, Y. Pan, M. Yin, C. Pan, Enhanced photocatalytic activity and mechanism of Ti3C2−OH/Bi2WO6:Yb3+, Tm3+ towards degradation of RhB under visible and near infrared light irradiation, Mater. Rese. Bull. 121 (2020) 110618. 99. Y. Li, Z. Yin, G. Ji, Z. Liang, Y. Xue, Y. Guo, J. Tian, X. Wang, H. Cui, 2D/2D/2D

of

heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity, Appl. Catal. B: Environ. 246 (2019) 12–20.

ro

100. Z. Li, H. Zhang, L. Wang, M. Xiangchao, J. Shi, C. Qi, Z. Zhang, L. Feng, C. Li, 2D/2D BiOBr/Ti3C2 heterojunction with dual applications in both water detoxification

-p

and water splitting, J. Photochem. Photobiol. Chem. 386 (2020) 112099. 101. C. Su, B.Y. Hong, C.M. Tseng, Sol–gel preparation and photocatalysis of titanium

re

dioxide, Catal. Today. 96 (2004) 119–126.

102. A. Rozmysłowska-Wojciechowska, E. Karwowska, S. Pozniak, T. Wojciechowski, L.

lP

Chlubny, A. Olszyna, W. Ziemkowska, A. M. Jastrzebska, Influence of modification of Ti3C2 MXene with ceramic oxide and noble metal nanoparticles on its antimicrobial properties and ecotoxicity towards selected algae and higher plants, RSC Adv. 9 (2019)

ur na

4092.

103. S. Sun, M. Wang, X. Chang, Y. Jiang, D. Zhang, D. Wang, Y. Zhang, Y. Lei, W18O49/Ti3C2Tx Mxene nanocomposites for highly sensitive acetone gas sensor with low detection limit, Sens. Actua. Chem. 304 (2020) 127274. 104. N. Hao, Y. Wei, J. Wang, Z. Wang, Z. Zhu, S. Zhao, M. Han and X. Huang, In situ

Jo

hybridization of an MXene/TiO2/NiFeCo layered double hydroxide composite for electrochemical and photoelectrochemical oxygen evolution, RSC Adv. 8 (2018) 20576.

105. L. Tie, S. Yang, C. Yu, H. Chen, Y. Liu, S. Dong, J. Sun, J. Sun, In situ decoration of ZnS nanoparticles with Ti3C2 MXene nanosheets for efficient photocatalytic hydrogen evolution, J. Coll. Inter. Sci. 545 (2019) 63–70. 106. J. Low, L. Zhang, T. Tong, B. Shen, J. Yu, TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity, J. Catal. 361 (2018) 255–266. 51

107. Y. Li, D. Zhang, X. Feng, Y. Liao, Q. Wen and Q. Xiang, Truncated octahedral bipyramidal TiO2/MXene Ti3C2 hybrids with enhanced photocatalytic H2 production activity, Nanoscale Adv. 1 (2019) 1812. 108. X. Ding, Y. Li, C. Li, W. Wang, L. Wang, L. Feng, D. Han, 2D visible-light-driven Ti3C2/TiO2/g-C3N4 ternary heterostructure for high photocatalytic activity, J Mater Sci. 54 (2019) 9385–9396. 109. K.M. Zhang, Z.G. Wen, Review and challenges of policies of environmental protection

of

and sustainable development in China, J. Environ. Manage. 88 (2008) 1249–1261. 110. Y. Guo, Y.H. Ao, P.F. Wang, C. Wang, Mediator-free direct dual-Z-scheme Bi2S3/BiVO4/MgIn2S4 composite photocatalysts with enhanced visible-light-driven

ro

performance towards carbamazepine degradation, Appl. Catal. B Environ. 254 (2019) 479-490.

-p

111. Ch. Prasad S. Karlapudi, G. Sellola, V. Ponneri, A facile green synthesis of spherical Fe3O4 magnetic nanoparticles and their effect on degradation of methylene blue in

re

aqueous solution, J. Mol. Liq. 221 (2016) 993–998.

112. Ch. Prasad S. Karlapudi, G. Sellola, P. Venkateswarlu, Bio inspired green synthesis of

lP

Ni/Fe3O4 magnetic nanoparticles using Moringa Oleifera leaves extract: A magnetically recoverable catalyst for organic dye degradation in aqueous solution, J. Alloy. Com. 700 (2017) 252-258.

ur na

113. Ch. Prasad, G. Yuvaraja, P. Venkateswarlu, Biogenic synthesis of Fe3O4 magnetic nanoparticles using pisum sativum peels extract and its effect on magnetic and methyl orange dye degradation studies, J. Magnet. Magnet. Mater. 424 (2017) 376–381. 114. Ch. Prasad, S. Karlapudi, P. Venkateswarlu, I. Bahadur, S. Kumar, Green arbitrated synthesis of Fe3O4 magnetic nanoparticles with nanorod structure from pomegranate

Jo

leaves and Congo red dye degradation studies for water treatment, J. Mol. Liq. 240 (2017) 322–328.

115. Ch. Prasad, S. Karlapudi, C. Narasimha Rao, P. Venkateswarlu, I. Bahadur, A highly resourceful magnetically separable magnetic nanoparticles from aqueous peel extract of Bottle gourds for organic dyes degradation, J. Mol. Liq. 243 (2017) 611–615. 116. Ch. Prasad, P. Murthy, R. HariKrishna, R.S. Rao, V. Suneetha, P. Venkateswarlu, Bioinspired

green

synthesis

of

RGO/Fe3O4 magnetic 52

nanoparticles

using

murrayakoenigii leaves extract and its application for removal of Pb(II) from aqueous solution, J. Environ. Chem. Eng. 5 (2017) 4374-4380. 117. Ch. Prasad, P. Venkateswarlu, Soybean seeds extract based green synthesis of silver nanoparticles, Ind. J. Advan. Chem. Sci. 2(3) (2014) 208-211. 118. V. Sada B. Natesh Kumar, Ch. Prasad, P. Venkateswarlu, N.V.V. Jyothi, Bio-inspired green synthesis of Fe3O4 spherical magnetic nanoparticles using syzygium cumini seed extract, Physica B. 449 (2014) 67–71.

of

119. Ch. Prasad, H. Tang, W. Liu, Magnetic Fe3O4 based layered double hydroxides (LDHs) nanocomposites (Fe3O4/LDHs): recent review of progress in synthesis, properties and applications, J. Nano. Chem. 8 (2018) 393–412.

ro

120. S.H. Kumar, Ch. Prasad, S. Venkateswarlu, P. Venkateswarlu, N. V. V. Jyothi, Green synthesis of silver nanoparticles using aqueous solution of syzygium cumini flowering

-p

extract and its antimicrobial activity, Ind. J. Advan. Chem. Sci. 3(4) (2015) 299-303. 121. P. Kaur, V. Sangal, J. Kushwaha, Parametric study of electro-Fenton treatment for real

re

textile wastewater, disposal study and its cost analysis, Int. J. Environ. Sci. Technol. 16 (2019) 801-810.

lP

122. W. Lemlikchi, S. Khaldi, M. Mecherri, H. Lounici, N. Drouiche, Degradation of disperse red 167 azo dye by bipolar electrocoagulation, Sep. Sci. Technol. 47 (2012) 1682.

ur na

123. Z. Lu, J. Peng, M. Song, Y. Liu, X. Liu, P. Huo, H. Dong, S. Yuan, Z. Ma, S. Han. Improved

recyclability

and

selectivity

of

environment-friendly

MFA-based

heterojunction imprinted photocatalyst for secondary pollution free tetracycline. Chem. Eng. J. 360 (2019) 1262–1276.

124. Y. Xue, P.F. Wang, C. Wang, Y.H. Ao, Efficient degradation of atrazine by

Jo

BiOBr/UiO-66 composite photocatalyst under visible light irradiation: Environmental factors, mechanisms and degradation pathways, Chemosphere. 203 (2018) 497-505.

125. C. Peng, X. Yang, Y. Li, H. Yu, H. Wang and F. Peng, Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity, ACSAppl. Mater. Interfaces. 8 (2016) 6051–6060.

53

126. X. Wu, Z. Wang, M. Yu, L. Xiu, J. Qiu, Stabilizing the MXenes by carbon nanoplating for developing hierarchical nanohybrids with efficient lithium storage and hydrogen evolution capability, Adv. Mater. 29 (24) (2017) 1607017. 127. H. Wang, Y. Wu, X. Yuan, G. Zheng, J. Zhou, X. Wang, J.W. Chew, Clay-inspired Mxene-based electrochemical devices and photo-electrocatalyst: state-of-the-art progresses and challenges, Adv. Mater. 30 (12) (2018) 1704561. 128. X. Ding, C. Li, L. Wang, L. Feng, D. Han, W. Wang, Fabrication of hierarchical g-

of

C3N4/MXene-AgNPs nanocomposites with enhanced photocatalytic performances, Mater. Lett. 247 (2019) 174–177.

129. J. Zhang, J. Xi, Z. Ji, Mo+N Codoped TiO2 sheets with dominant {001} facets for

ro

enhancing visible-light photocatalytic activity, J. Mater. Chem. 22 (2012) 17700–17708. 130. L. Gu, J. Wang, H. Cheng, Y. Zhao, L. Liu, X. Han, One-step preparation of graphene-

-p

supported anatase TiO2 with exposed {001} facets and mechanism of enhanced photocatalytic properties, ACS Appl. Mater. Interfaces. 5 (8) (2013) 3085.

re

131. Y.J. Yuan, Z.J. Ye, H.W. Lu, B. Hu, Y.H. Li, D.Q. Chen, J.S. Zhong, Z.T. Yu, Z.G. Zou, Constructing anatase TiO2 nanosheets with exposed (001) facets/layered

Catal. 6 (2) (2016) 532. 132. T.

Wojciechowski,

lP

MoS2 two-dimensional nanojunctions for enhanced solar hydrogen generation, ACS A.

Rozmysłowska-Wojciechowska,

G.

Matyszczak,

M.

ur na

Wrzecionek, A. Olszyna, A. Peter, A. Mihaly-Cozmuta, C. Nicula, L. Mihaly-Cozmuta, S. Podsiadło, D. Basiak, W. Ziemkowska, A. Jastrzębska, Ti2C MXene modified with ceramic oxide and noble metal nanoparticles: synthesis, morphostructural properties and high photocatalytic activity, Inorg. Chem. 58 (2019) 7602−7614. 133. X. Xie, N. Zhang, Z.R. Tang, M. Anpo, Y.J. Xu, Ti3C2Tx MXene as a janus co-catalyst

Jo

for concurrent promoted photoactivity and inhibited photocorrosion, Appl. Catal. B. Environ. 237 (2018) 43–49.

134. H. Wang, Y. Wu, T. Xiao, X. Yuan, G. Zeng, W. Tu, S. Wu, H.Y. Lee, Y.Z. Tan, J.W. Chew, Formation of quasi-core-shell In2S3/anatase TiO2@metallic Ti3C2Tx hybrids with favorable

charge

transfer

channels

for

excellent

performance, Appl. Catal. B. Environ. 233 (2018) 213–225.

54

visible-light-photocatalytic

135. C. Peng, H. Wang, H. Yu, F. Peng, (111) TiO2-x/Ti3C2: Synergy of active facets, interfacial charge transfer and Ti3+ doping for enhances photocatalytic activity, Mater. Res. Bull. 89 (2017) 16–25. 136. J.F. Wang, J. Chen, P.F. Wang, J. Hou, C. Wang, Y.H. Ao, Robust photocatalytic hydrogen evolution over amorphous ruthenium phosphide quantum dots modified gC3N4 nanosheet, Appl. Catal. B Environ. 239 (2018) 578-585. 137. S. Lin, H.Y. Wang, X.N. Zhang, D. Wang, D. Zu, J.N. Song, Z.L. Liu, Y. Huang, K.

of

Huang, N. Tao, Z.W. Li, X.P. Bai, B. Li, M. Lei, Z.F. Yu, Hui Wu, Direct spray-coating of highly robust and transparent Ag nanowires for energy saving windows, Nano Energy. 62 (2019) 111-116.

ro

138. L. Schlapbach, A. Zuttel, Hydrogen-storage materials for mobile applications, Nature. 414 (2001) 353–358.

-p

139. A. Fujishima, K. Honda. Electrochemical photolysis of water at a semiconductor electrode, Nature. 238 (1972) 37-38.

re

140. X. Cui, L. Tian, X. Xian, H. Tang, X. Yang, Solar photocatalytic water oxidation over Ag3PO4/g-C3N4 composite materials mediated by metallic Ag and graphene, Appl. Surf.

lP

Sci. 430 (2018) 108-115.

141. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water

ur na

under visible light, Nat. Mater. 8 (2009) 76-80. 142. M. Shao, Y. Shao, J. Chai, Y. Qu, M. Yang, Z. Wang, M. Yang, W.F. Ip, C.T. Kwok, X. Shi, Z. Liu, S. Wang, X. Wang, H. Pan, Synergistic effect of 2D Ti2C and g-C3N4 for efficient photocatalytic hydrogen production, J. Mater. Chem. A. 5 (2017) 16748– 16756.

Jo

143. X. An, W. Wang, J. Wang, H. Duan, J. Shi, X. Yu, The synergetic effect of Ti3C2 MXene and Pt as co-catalysts for highly efficient photocatalytic hydrogen evolution over g-C3N4, Phys. Chem. Chem. Phys. 20 (2018) 11405-11411.

144. H. Wang, Y. Sun, Y. Wu, W. Tu, S. Wu, X. Yuan, G. Zeng, Z.J. Xu, S. Li, J.W. Chew, Electrical promotion of spatially photoinduced charge separation via interfacial-built-in quasi-alloying effect in hierarchical Zn2In2S5/Ti3C2(O, OH)x hybrids toward efficient

55

photocatalytic hydrogen evolution and environmental remediation, Appl. Catal. B Environ. 245 (2019) 290–301. 145. S. Ye, M. Yan, X. Tan, J. Liang, G. Zeng, H. Wu, B. Song, C. Zhou, Y. Yang, H. Wang, Facile assembled biochar-based nanocomposite with improved graphitization for efficient photocatalytic activity driven by visible light, Appl. Catal. B. Environ. 250 (2019) 78–88. 146. M.Y. Zhang, J.Q. Qin, S. Rajendran, X.Y. Zhang, R.P. Liu, Heterostructured with

enhanced

visible‐light

photocatalytic

of

d‐Ti3C2/TiO2/g‐C3N4 nanocomposites

hydrogen production activity, ChemSusChem. 11 (2018) 4226–4236.

147. C. Peng, P. Wei, X.Y. Li, Y.P. Liu, Y.H. Cao, H.J. Wang, H. Yu, F. Peng, L.Y. Zhang,

ro

B. S. Zhang, K.L. Lv, High efficiency photocatalytic hydrogen production over ternary Cu/TiO2/Ti3C2Tx enabled by low-work-function 2D titanium carbide, Nano Energy. 53

-p

(2018) 97–107.

148. M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S.

re

Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446-6473. 149. D. Kang, T.W. Kim, S.R. Kubota, A.C. Cardiel, H.G. Cha, K.S. Choi, Electrochemical

lP

synthesis of photoelectrodes and catalysts for use in solar water splitting, Chem. Rev. 115 (2015) 12839-12887.

150. M. Halmann, Photoelectrochemical reduction of aqueous carbon dioxide on p-type

ur na

gallium phosphide in liquid junction solar cells, Nature. 275 (1978) 115-116. 151. C. Wang, C. Chou, S. Yi, H. Chen, Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting, Inter. J. hydr. ener. 44 (2019) 28685 -28697. 152. J.M. Buriak, C. Toro, K.S. Choi, Chemistry of materials for water splitting reactions,

Jo

Chem. Mater. 30 (2018) 7325-7327.

153. C. Ling, L. Shi, Y. Ouyang, J. Wang, Searching for highly active catalysts for hydrogen evolution reaction based on O-terminated MXenes through a simple descriptor, Chem. Mat. 28 (2016) 9026–9032. 154. G. Gao, A.P. O’Mullane, A. Du, 2D MXenes: A new family of promising catalysts for the hydrogen evolution reaction, ACS Catal. 7 (2016) 494–500.

56

155. N.K. Chaudhari, H. Jin, B. Kim, D.S. Baek, S.H. Joo, K. Lee, MXene: an emerging two-dimensional material for future energy conversion and storage applications, J. Mater. Chem. A. 5 (2017) 24579. 156. J.C. Lei, X. Zhang, Z. Zhou, Recent advances in MXene: preparation, properties, and applications, Front. Phys. 10 (2015) 276–286. 157. Z.W. Seh, K.D. Fredrickson, B. Anasori, J. Kibsgaard, A.L. Strickler, M.R. Lukatskaya, Y. Gogotsi, T.F. Jaramillo, A. Vojvodic, Two-dimensional molybdenum

of

carbide (MXene) as an efficient electrocatalyst for hydrogen evolution, ACS Energy Lett. 1 (2016) 589–594.

158. X. Yang, L. Tian, X. Zhao, H. Tang, Q. Liu, G. Li, Interfacial optimization of g-C3N4

ro

based Z-scheme heterojunction toward synergistic enhancement of solar-driven photocatalytic oxygen evolution, Appl. Catal. B Environ. 244 (2019) 240–249.

-p

159. C. Cheng, X. Zhang, M. Wang, S. Wang, Z. Yang, Single Pd atomic catalyst on Mo2CO2 monolayer (MXene): unusual activity for CO oxidation by trimolecular Eley–

re

rideal mechanism, Phys. Chem. Chem. Phys. 20 (2018) 3504–3513. 160. T. Ma, J. Cao, M. Jaroniec, S, Qiao, Interacting carbon nitride and titanium carbide

1138–1142.

lP

nanosheets for high-performance oxygen evolution, Angew. Chem. Int. Ed. 55 (2016)

161. L. Zhao, B. Dong, S. Li, L. Zhou, L. Lai, Z. Wang, S. Zhao, M. Han, K. Gao, M. Lu,

ur na

Inter-diffusion reaction assisted hybridization of two-dimensional metal-organic frameworks and Ti3C2Tx nanosheets for electrocatalytic oxygen evolution, ACS Nano. 11 (2017) 5800–5807.

162. X. Fu, H. Chang, Z. Shang, P. Liu, J. Liu, H. Luo, Three-dimensional Cu2O nanorods modified by hydrogen treated Ti3C2TX MXene with enriched oxygen vacancies as a

Jo

photocathode and a tandem cell for unassisted solar water splitting, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122001.

163. D. Yan, X. Fu, Z. Shang, J. Liua, H. Luo, A BiVO4 film photoanode with re-annealing treatment and 2D thin Ti3C2TX flakes decoration for enhanced photoelectrochemical water oxidation, Chem. Eng. J. 361 (2019) 853–861. 164. T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature. 277 (1979) 637–638. 57

165. T.Y. Ma, J.L. Cao, M. Jaroniec, S.Z. Qiao, Interacting carbon nitride and titanium carbide nanosheets for high‐performance oxygen evolution, Angew. Chem. Int. Ed. 55 (3) (2015) 1138−1142. 166. M.R. Lukatskaya, O. Mashtalir, C.E. Ren, Y. Dall’Agnese, P. Rozier, P.L. Taberna, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide, Science. 341 (2013) 1502-1505. 167. Y. Wen, T.E. Rufford, X. Chen, N. Li, M. Lyu, L. Dai, L. Wang, Nitrogen-doped

of

Ti3C2Tx MXene electrodes for high-performance supercapacitors, Nano Energy. 38 (2017) 368-376.

168. J. Shen, J. Shen, W. Zhang, X. Yu, H. Tang, M. Zhang, Zulfiqar, Q. Liu, Built-in

ro

electric field induced CeO2/Ti3C2 MXene Schottky-junction for coupled photocatalytic tetracycline degradation and CO2 reduction, Ceramics International. 45 (2019) 24146–

-p

24153.

169. J. Low, L. Zhang, T. Tong, B. Shen, J. Yu, TiO2/MXene Ti3C2 composite with

re

excellent photocatalytic CO2 reduction activity, J. Catal. 361 (2018) 255-266. 170. S. Cao, B. Shen, T. Tong, J. Fu, and J. Yu, 2D/2D Heterojunction of Ultrathin

lP

MXene/Bi2WO6 Nanosheets for Improved Photocatalytic CO2 Reduction, Adv. Funct. Mater. 28 (2018) 1800136.

171. M. Jakob, H. Levanon, P.V. Kamat, Charge Distribution between UV-irradiated

ur na

TiO2 and gold nanoparticles: determination of shift in the fermi level, Nano Lett. 3 (2003) 353.

172. J.X. Low, J.G. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv.Mater. 29 (2017) 1601694. 173. C. Peng, X. Yang, Y. Li, H. Yu, H. Wang, F. Peng, Hybrids of two-dimensional Ti3C2

Jo

and TiO2 exposing {001} facets toward enhanced photocatalytic activity, ACS Appl. Mater. Interfaces. 8 (2016) 6051−6060.

174. H. Zhang, M. Li, C. Zhu, Q. Tang, P. Kang, J. Cao, Preparation of magnetic αFe2O3/ZnFe2O4/Ti3C2 MXene with excellent photocatalytic performance, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.236. 175. R. Chen, P.F. Wang, J. Chen, C. Wang, Y.H. Ao, Synergetic effect of MoS 2 and MXene on the enhanced H2 evolution performance of CdS under visible light 58

irradiation, App. Surf. Sci. 473 (2019) 11-19. 176. R. Xiao, C. Zhao, Z. Zou, Z. Chen, L. Tian, H. Xu, H. Tang, Q. Liu, Z. Lin, X. Yang, In situ fabrication of 1D CdS nanorod/2D Ti3C2 MXene nanosheet Schottky heterojunction toward enhanced photocatalytic hydrogen evolution, Appl. Catal.B:

Jo

ur na

lP

re

-p

ro

of

Environ.(2019), doi: https://doi.org/10.1016/j.apcatb.2019.118382.

59

of ro -p

Jo

ur na

lP

re

Fig.1. The essential mechanism of MXenes supported semiconductors based photocatalysis

60

ur na

lP

re

-p

ro

of

Fig.2. Chemical Formula and Structure of two dimensional Carbides and Nitrides (MXenes) acquired experimentally.

Jo

Fig.3. Diagram illustration of the Eetching procedure of Ti3AlC2 (MAX) to Ti3C2Tx (MXene). Reproduced from Ref. [74].

61

of ro -p

Jo

ur na

lP

re

Fig.4. Timeline of MXenes Summary of etching methods of MAX precursors to produce MXenes since the first discovery of Ti3C2 by Gogotsi’s group in 2011 Reproduced from Ref. [81].

62

re

-p

ro

of

Fig.5. Representation diagram of the synthesis of TiO2/Ti3C2 nanohybrid. Reproduced from Ref. [83].

Jo

ur na

lP

Fig.6. Schematic illustration for the preparation procedure of CNTC composite by a combined HF etching and ultrasonic dispersion and evaporation induced self-assembly approach. Reproduced from Ref. [88].

Fig.7. Preparation drawing of SrTiO3/Ti3C2. Reproduced from Ref. [93]. 63

of

Jo

ur na

lP

re

-p

ro

Fig.8. Schematic illustration of the charge transfer and separation in the CN/M Ag composites for aniline degradation. Reproduced from Ref. [128].

Fig.9. Photocatalytic mechanism of a charge transfer in the Ti2C MXene tailored by metallic nanoparticles (metal =M = Ag, Pd, Au) and metal oxides. In the white field, the recombination of electrons and M+ ions is showed. Reproduced from Ref. [132].

64

of ro -p

Jo

ur na

lP

re

Fig.10. (A) Representation of CdS/Ti3C2Tx photocatalyst for improved photocatalytic activity and photostability and (B) adsorption of Cd2+ over CdS nanosheets and Ti3C2Tx MXene in the dark, (C) Charge transfer and (D) separation and reaction mechanism for the methyl orange photodegradation in the In2S3/anatase TiO2@metallic Ti3C2Tx (InTi) system in visible light and (E) photodegradation of methyl orange over other In2S3-based binary nanocomposite and (F) In2S3, InTi composites, and Ti3C2Tx. Reproduced from Ref. [133,134].

65

of ro -p re

Jo

ur na

lP

Fig.11. SEM micrographs of (a) Ti3C2 (b) Ti3C2 180 ºC (c)TiO2, (d) 0.001 mol Ti3C2/TiO2, (e) 0.002 mol Ti3C2/TiO2, (f) 0.003 mol Ti3C2/TiO2, (g) 0.004 mol Ti3C2/TiO2, respectively.The insets in micrographs demonstrated high magnification SEM. Reproduced from Ref. [94].

Fig.12. Schematic diagram for the photodegradation and photoreduction of BiOBr/Ti3C2. Reproduced from Ref. [84].

66

of ro -p

Jo

ur na

lP

re

Fig.13. Schematic of the mechanism of TC and SFE photodegradation over the Ti3C2/Ag2WO4 Schottky catalyst (a) and the Ti3C2/Ag2WO4 interfacial photocarrier transfer process (b). Reproduced from Ref. [91].

Fig.14. The photocatalytic degradation mechanism of BT4. Reproduced from Ref. [90].

67

of

lP

re

-p

ro

Fig.15. Photocatalytic mechanisms of the CdS/Ti3C2-OH/In2S3 nanocomposites. Reproduced from Ref. [25].

Fig.16. Photocatalyzed mechanism of the Y/T-TB2.8 nanocomposite. Reproduced from Ref.

Jo

ur na

[98].

Fig.17. The Schematic diagram for preparation of the Ti3C2/CeO2 nanocomposites. Reproduced from Ref. [96].

68

of ro

Jo

ur na

lP

re

-p

Fig.18. The schematic diagram for the charge carrier transfer mechanism for g-C3N4/α-Fe2O3 hybrid nanocomposite. Reproduced from Ref. [85].

Fig.19. The schematic diagram of the charge transfer and partition in the CNTC photocatalyst for organic contaminants oxidation with h+ and •O2- to oxidation products in visible light illumination. Reproduced from Ref. [88].

69

of

Jo

ur na

lP

re

-p

ro

Fig.20. (a) The photoexcited charge carrier transport procedure over Ti3C2/(111) r-TiO2-x. (b) representation band alignments and charge flows at Ti3C2/ 111) r-TiO2-x interfaces. Reproduced from Ref. [135].

Fig.21. (a) TEM micrographs of Ti3C2Tx (5 wt%)/TiO2 (b) Photocatalytic H2 generation rates of pristine TiO2, Ti3C2Tx (5 wt%)/TiO2, Ti3C2Tx (10 wt%)/TiO2, Ti3C2Tx (25 wt%)/TiO2), Ti3C2Tx (50 wt%)/TiO2, pristine Ti3C2Tx, and a physically mixed Ti3C2Tx/TiO2 sample with 5 wt% Ti3C2Tx loading, (c) Photocatalytic H2 generation rate of the samples with dissimilar metal

70

-p

ro

of

carbide co-catalysts [Ti3C2Tx (5 wt%)/TiO2, Ti2CTx (5 wt%)/TiO2, and Nb2CTx (5 wt%)/TiO2], (d) Formation of Schottky Barrier at the TiO2/MXene interface. Reproduced from Ref. [36].

Jo

ur na

lP

re

Fig.22. Schematic band alignments and charge flows at BiOBr/Ti3C2 interfaces (vs. NHE). Reproduced from Ref. [100].

Fig.23. The synthetic process of the 2D/2D MX/HCN Schottky-junction. Reproduced from Ref. [92].

71

of ro -p

Jo

ur na

lP

re

Fig.24. The mechanism of photocatalytic H2 evolution of MX1/HCN Schottky-junction. Reproduced from Ref. [92].

Fig.25. Schematic design for the construction of Ti3C2/TiO2 photocatalyst. Reproduced from Ref. [34].

72

of ro

Jo

ur na

lP

re

-p

Fig.26. Schematic mechanism for hydrogen evolution over Ti3C2/TiO2. Reproduced from Ref. [34].

Fig.27. The possible photocatalytic mechanism of Ti3C2/TiO2 truncated octahedral bipyramidal photocatalyst under light illumination. Reproduced from Ref. [107].

73

of ro

Jo

ur na

lP

re

-p

Fig.28. Schematic design of the synthesis process of Zn2In2S5/Ti3C2(O, OH)x nancomposite for H2 evolution from water splitting and contaminant removal. Reproduced from Ref. [144].

Fig.29. Proposed photocatalytic mechanism for H2 production and contaminant elimination by the Zn2In2S5/Ti3C2 (O, OH)x photocatalyst in visible light irradiation. Reproduced from Ref. [144]. 74

of ro -p

Jo

ur na

lP

re

Fig.30. Schematic design demonstrating probable reaction way for the production of UiO-66NH2/Ti3C2/TiO2. Reproduced from Ref. [97].

Fig.31. Schematic diagram of the charge-transfer ways for UiO-66-NH2/Ti3C2/TiO2. Reproduced from Ref. [97]. 75

of ro -p

Jo

ur na

lP

re

Fig.32. Schematic drawing of the preparation of TiO2/MoS2/Ti3C2 nanohybrid. Reproduced from Ref. [99].

Fig.33. Schematic drawing of the photocatalytic process and charge transfer mechanism in theTiO2/MoS2/Ti3C2 composites under solar light irradiation. Reproduced from Ref. [99].

76

of ro

Jo

ur na

lP

re

-p

Fig.34. Schematic photocatalytic mechanism for (a) TiO2/Ti3C2 nanohybrid and (b) TiO2 nanobelts under solar light illumination. Reproduced from Ref. [83].

Fig.35. Schematic diagram of the preparation process of the H: Cu2O/Ti3C2TX. Reproduced from Ref. [162].

77

of ro

ur na

lP

re

-p

Fig.36. Schematic design of the synthetic procedure of the D-Ti3C2TX and BiVO4/Ti3C2TX electrodes. Reproduced from Ref. [163].

Jo

Fig.37. The proposed charge transfer mechanism of the Ti3C2TX/BiVO4 photoanodes. Reproduced from Ref. [163].

78

of ro

ur na

lP

re

-p

Fig.38. Enhancement mechanism of photocatalytic CO2 reduction activity of CeO2/Ti3C2 MXene with built-in electric field induced Schottky-junction. Reproduced from Ref. [168].

Jo

Fig.39. Schematic diagram for the improvement mechanism of photoreduction of CO2 on Ti3C2/TiO2 hybrid nanocomposite. Reproduced from Ref. [169].

79

of ro -p

Jo

ur na

lP

re

Fig.40. The schematic diagram of the preparation procedure of 2D/2D heterocomposite of Ti3C2/Bi2WO6 nanocomposit. Reproduced from Ref. [170].

Fig.41. (a) Energy level structure illustration of Bi2WO6 and Ti3C2 (b) Photogenerated electron transport procedure at the interface of the hybrids. Reproduced from Ref. [170].

80

of ro -p

Jo

ur na

lP

re

Fig.42. Illustration of the photocatalytic CO2 conversion mechanism of P25/TC-OH in illumination, due to the mixed phase of P25. Reproduced from Ref. [33].

81

Table.1. MXenes supported semiconductors based photocatalysts for photodegradation of organic dyes Methods

Application

Light source

Photocatalytic activity

Refere nces

Hydrotherm al procedure

RhB and Methyl Orange

300W Xe lamp

[25]

Mn-codoped bismuth ferrite/Ti3C2 nanohybrids BiOBr/Ti3C2 nanocomposite

Ti3C2

Double solvent sol−gel method selfassembly method

Congo Red

300 W xenon lamp

CdS/Ti3C2-OH/ln2S3 photocatalyst with a large surface area of 68.2 m2/g showed an improved photocatalytic activity and adsorption capacity towards Rh B in visible light illumination 93% of the Congo Red photodegradation

Removal effectiveness of RhB, 2,4dinitrophenol and Cr(VI).

UV–vis irradiation

a-Fe2O3/Ti3C2

Ti3C2

Facile ultrasonic assisted selfassembly method

Photodegradatio n Rhodamine B

Ag3PO4/Ti3C2

Ti3C2

Electrostatic ally driven selfassembly method

Methyl Orange 2,4-Dinitro Phenol, Tetracycline Hydrochloride, Thiamphenicol

300W Xe lamp

Jo

ro

g-C3N4/Ti3C2

Ti3C2

selfassembly method

ciprofloxacin (CIP) degradation

500W Xenon light

Bi2WO6/Ti3C2 MXene

Ti3C2 MXene

electrostatic assembly method

photodegradatio n of HCHO and CH3COCH3

300 W xenon lamp

82

The removal apparent rate constant of RhB, 2,4dinitrophenol and Cr(VI) with Ti3C2/BiOBr were 1.2, 1.3, 6 folds than that of pure BiOBr. the degradation rate of aFe2O3/Ti3C2-1, aFe2O3/Ti3C2-2 and aFe2O3/Ti3C2-3 is 91%, 98% and 89%, respectively The apparent rate constant of 2,4-dinitrophenol removal with Ti3C2/Ag3PO4 is 2.5 folds than that of RGO/Ag3PO4 and 10 folds than that of Ag3PO4 and tetracycline hydrochloride still maintained 68.4% after 8 cycles, whereas RGO/Ag3PO4 and Ag3PO4 only maintained 36.2% and 7.8%. The pseudo first order kinetic constant of ciprofloxacin degradation on g-C3N4/Ti3C2 was 2.2 folds higher than that of pure g-C3N4 An Optimized Bi2WO6/Ti3C2 MXene photocatalyst showed 2 folds and 6.6 folds greater photocatalytic activity

-p 500W Xe lamp

re

Ti3C2

ur na

CdS/Ti3C2OH/ln2S3 photocatalyst

of

Types of Mxenes Ti3C2– OH

lP

Photocatalyst s

[76]

[84]

[85]

[86

[88]

[90]

Sulfadimidine and Tetracycline hydrochloride

300-W Xe lamp

TiO2/Ti3C2 nanocomposite s CeO2/Ti3C2 nanocomposite s

Ti3C2

photodegradatio n of MO

175W mercury lamp

Rhodamine B

500W Hg lamp

Ti3C2−OH/Bi2 WO6:Yb3+, Tm3+

Ti3C2−O H

Hydrotherm al technique

RhB degradation

300W Xe lamp

TiO2/Ti3C2/gC3N4

Ti3C2

ultrasonicassisted calcination method

lP

re

Ti3C2

aniline and RhB

ur na Ti2C MXene

Methyl orange was rapidly removed about 98% in 30 min. The degradation effectiveness of CeO2/Ti3C2 hybrid was 75%, which was 3 folds of that of CeO2 (24%) and 1.2 folds than that of Ti3C2 (63%). the 0.2-Y/T-TB2.8 photocatalyst shows the best photocatalytic activity for RhB degradation, in which the RhB degradation efficiency was 99.8% (Vis-NIR, 30 min), 91% (Vis, 30 min), and 48% (NIR, 160 min). the photodegradation of aniline and Rhodamine B were increased to 5 and 1.33 folds greater than that of pure g-C3N4 Ti2C@3%TiO2@1% Ag: (97.1%), Ti2C@3%TiO2@1%Ag2O :(95.8%) and Ti2C@3%TiO2@1%Pd : (88.7%), Ti2C@3%TiO2@1%PdO: (86.1%).

[94]

removal rate of 0.04977 min−1, which is 3.2 and 6.2 times greater than that of In2S3 and Ti3C2Tx distinct Methylene blue

[134]

[96]

[98]

300-W Xe lamp

--

Photodegradatio n of salicylic acid

Hg lamp (150 W)

[108]

[132]

Jo

Ti2C MXene hybrids with metal oxide NPs and metal NPs

[91]

of

electrostatic ally driven selfsustaining deposition strategy hydrotherma l process hydrotherma l method

ro

Ti3C2

-p

Ag2WO4/Ti3C2

towards the photodegradation of HCHO and CH3COCH3 in comparison with Bi2WO6 The photodegradation rates for tetracycline hydrochloride and sulfadimidine were 62.9% and 88.6%

In2S3/anatase TiO2/metallic Ti3C2Tx photocatalyst

Ti3C2Tx

hydrotherma l method

Methyl orange

300W Xenon lamp

(111) r-TiO2-

Ti3C2

hydrotherma

Methyl orange

500 W Xe

83

[135]

x/Ti3C2

l treatment

and Methylene blue

lamp

Ti3C2

hydrotherma l conditions

tetracycline hydrochloride

Visible light.

(001)TiO2/Ti3 C2

Ti3C2

hydrotherma l

methyl orange

300 W mercury lamp

α-Fe2O3 /ZnFe2O4/Ti3C 2 MXene photocatalyst

Ti3C2 MXene

ultrasonic assisted selfassembly approach

Rhodamine B

[144]

[173]

-p

ro

of

Zn2In2S5/Ti3C2

removal of 30%, 75% and 39% at 150 min were examined over the material tested by hydrazine hydrate time for 12, 20 and 18 h. Tetracycline degradation rate is 1.25 folds greater than that of pristine Zn2In2S5 The photoactivity of Ti3C2/(001)TiO2 12h for methyl orange removal is well maintained after 4 runs with 4.9% reduce of methyl orange removal from 97.4% to 92.5%, signifying that Ti3C2/(001)TiO2 nanocomposite is employed as a strong photocatalyst for the photocatalytic degradation development 10 wt% α-Fe2O3 / ZnFe2O4/Ti3C2 MXene has the optimal photocatalytic activity owing to high distribution of α-Fe2O3/ZnFe2O4 on Ti3C2 MXene

Jo

ur na

lP

re

300 W Xe lamp

84

[174]

Table.2. MXenes supported semiconductors based photocatalysts for photocatalytic water splitting reactions Applicatio n

Light source

Sacrificial reagent

Photocatalytic activity

Refer ences

electrostatic selfassembly technique

Hydrogen production

300W Xelamp

10% methanol solution

[34]

ion exchange and calcination processes

H2 and O2 evolution

300 W Xe arc lamp

methanol solution

electrostatic assembly

hydrogen production

300W lamp

The highest hydrogen evolution rate of Ti3C2/TiO2 photocatalyst is up to 6.979 mmol h-1 g-1, which was 3.8 folds that of pristine TiO2 nanofibers The Ti3C2/TiO2-500 showed a dramatic improvement in H2 and O2 production without the use of any (noble metal) cocatalyst from photocatalytic water splitting as compared to other catalysts, The as-synthesized Ti3C2 MXene/O-doped g-C3N4 Schottkyjunctionshowed about 2 folds improved H2 production (25124 μmol/g/h) in comparison to pure Odoped g-C3N4 (13745 μmol/g/h) and Ti3C2 MXene/pure C3N4 (15573 μmol/g/h). UiO-66-NH2 /Ti3C2 /TiO2 showed extensively enhancement in photocatalytic hydrogen activity: 1980 µmolh-1. g-1 than the pure UiO66-NH2 Ti3C2/TiO2/MoS2 nanocomposites shows high photocatalytic hydrogen production activity with a rate as high as 6425.297 μmol h−1 g−1 for the material with 15 wt% MoS2 loading, which was

[99]

Ti3C2 MXen e

Ti3C2/TiO2/ UiO-66NH2

Xe

Jo

Ti3C2

Ti3C2/TiO2/ MoS2 composite

Ti3C2

hydrotherma l process

hydrogen production

300 W Xe lamp

two-step hydrotherma l method

H2 evolution

300W Xe arc lamp

85

[83]

ro

Ti3C2 MXene/Og-C3N4

-p

Ti3C2

triethanola mine solu

lP

Ti3C2/TiO2 nanoflower s

of

Methods

ur na

TiO2/Ti3C2

Types of Mxene s Ti3C2

re

Photocatal ysts

50 mL of 0.1 M Na2S and 0.1MNa2S O3 aqueous solution as sacrificial electron donor. TEOA

[92]

[97]

hydrotherma l method

H2 production

300W lamp

Xe

methanol

ZnS/Ti3C2

Ti3C2

ultrasonic exfoliation and solvotherma l reaction

H2 production

300 W Xe lamp

lactic acid

MXene Ti3C2/ TiO2 hybrids

MXen e Ti3C2

calcination

H2 production

350 W Xe arc lamp

10% of Glycerinu m aqueous solution

g-C3N4 /Ti3C2/Pt

Ti3C2/ Pt

--

hydrogen production

300W xenon lamp

CdS-MoS2MXene

Ti3C2

hydrotherma l method

[105]

Ti3C2 and Pt cotailored composite showed higher photocatalytic H2 evolution activity of 5.1 mmol h−1 g-1. compared to Ti3C2/g-C3N4 and Pt/g-C3N4, 3 and 5 times Hydrogen evolution rate: 9679 μmol⋅g−1 ⋅h−1

[143]

The schottky-based photocatalyst is 7 fold more active in the irradiated H2 production reaction than pure CdS nanorods

[176]

-p triethanola mine (TEOA)

re lP hydrogen production

ur na 2D Ti3C2 MXen e

Jo

1D CdS/2D Ti3C2 MXene

[100]

H2 production yield of 502.6 µmol g-1 h-1 under optimal conditions, being almost 4times greater than pristine ZnS (124.6 µmol g-1 h-1) The improved HER should be ascribed to the (001)– (101) surface heterojunction in the TF samples, as well as the electron reservoir characteristic of Ti3C2

of

Ti3C2

[107]

ro

Ti3C2/BiO Br photocataly st

greater than Ti3C2/TiO2 nanocomposites and TiO2 sample H2 generation rate BiOBr :2.07 mM/g and Ti3C2/BiOBr: 8.04 mM/g

electrostatic allydriven assembly and hydrotherma l

hydrogen production

300 W xenon lamp

300 W Xe lamp

86

0.25 M sodium sulfide and 0.35 M sodium sulfite 50 mL of 10 wt % lactic acid sol

[175]

Table.3. MXenes supported semiconductors based photocatalysts for photocatalytic CO2 reduction reactions

TiO2/Ti3C2 composite

Types of Mxene s Ti3C2

Methods

Applications

Light source

Photocatalytic activity

Refer ences

Simple Calcination method

Photocatalytic CO2 reduction

300W simulated solar Xe arc lamp

Ti3C2/TiO2 sample showed a 3.7 folds greater photocatalytic CO2 conversion activity for CH4 evolution :0.22 µmol h-1 than commercial TiO2 (P25) The Ti3C2 MXene/CeO2 photocatalyst with the optimal ratio of Ti3C2 MXene showed improved activity in CO2 reduction, with activity 1.5 folds higher than that of pure CeO2

[106]

The total yield of CH4 and CH3OH obtained on the optimal Ti3C2 /Bi2WO6 hybrid is 4.6 folds that obtained on pure Bi2WO6 The production rates of CO:11.74 μmol g−1 h−1 and CH4 :16.61 μmol g−1 h−1) are 3‐ and 277‐times higher than those of bare P25

[170]

Ti3C2 MXen e

hydrotherm al route

Photocatalytic CO2 reduction

350W Xenon lamp

Ti3C2/Bi2WO6

Ti3C2

Hydrother mal treatment

Photocatalytic CO2 reduction

Xe lamp

P25/Ti3C2-OH

Ti3C2

Mechanical mixing method

Photocatalytic CO2 reduction

[168]

re

-p

ro

Ti3C2 MXene/CeO2

of

Photocatalysts

Jo

ur na

lP

300 W Xe lamp

87

[33]