Band gap tuning and surface modification of carbon dots for sustainable environmental remediation and photocatalytic hydrogen production – A review

Journal of Environmental Management 250 (2019) 109486

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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman

Review

Band gap tuning and surface modification of carbon dots for sustainable environmental remediation and photocatalytic hydrogen production – A review

T

Akansha Mehtaa, Amit Mishraa, Soumen Basua,∗∗, Nagaraj P. Shettib, Kakarla Raghava Reddyc,∗∗∗, Tawfik A. Salehd, Tejraj M. Aminabhavie,∗ a

School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala, 147004, India Electrochemistry and Materials Group, Department of Chemistry, K.L.E. Institute of Technology, Hubballi, 580 030, Visvesvaraya Technological University, Karnataka, India c School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW, 2006, Australia d Chemistry Department, King Fahd University of Petroleum & Minerals, B.O. Box: 346, Dhahran, 31261, Saudi Arabia e Pharmaceutical Engineering, Sonia College of Pharmacy, Dharwad, 580 002, Karnataka, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon quantum dots Band gap tuning Surface modification Photocatalysis Environmental Hydrogen production

Energy and water are the two major issues facing the modern mankind. Providing freshwater requires energy and producing energy uses water. In the present-day scenario, both these routes face growing problems and limitations. Energy crisis has risen due to the depletion of fossil fuels that cause pollution to water bodies making the water unusable for human consumption. In this regard, semiconductor nanocrystals with luminescent properties or carbon quantum dots (CQDs) are the newly developed nanomaterials whose distinctive photophysical characteristics are focusing to a new generation of robust materials and sensors for sustainable development. In this review, advances in surface and band gap modification of CQDs to improve the activity of nanomaterials will be discussed with special reference to some specific CQDs exhibiting special optical properties for water treatment/splitting applications. Recent advances on CQDs nanocomposites including their applications in photodegradation of organic pollutants, sensing of heavy metal ions in water and water splitting are discussed critically to narrate the future prospects in this field. Challenges and limitations for further improvement are covered to provide smart choices for creating sustainability of benign environment and economic benefits.

1. Introduction 1.1. General background In recent years, there has been great deal of efforts by chemists and physicists to discover novel nanomaterials for different applications (Haque et al., 2018). One of the largely moving areas of chemical research is the development of inorganic nanomaterials including metals, semiconductors and insulators in the dimension range of 1–100 nm (Haque et al., 2018). Dynamical behavior of low dimensional semiconductors has also become a crucial part of recent efforts. This has enabled intense investigations related to their novel optical, mechanical and transport phenomena. A variety of low dimensional

semiconductors, which include quantum wires, quantum wells and quantum dots are made available (Xu et al., 2018). Of these, quantum dots have gained much popularity (Chen et al., 2019) Quantum dots are the zero-dimensional structures in which electrons are delocalized along all the three spatial dimensions, leading to ‘quantum confinement effect’ (Rao et al., 2019a; Chakraborty et al., 2019). Quantum confinement occurs when the size of the nanocrystal becomes smaller compared with twice the Bohr radius, which then becomes weak, when it exceeds the Bohr radius (Bergren et al., 2016). In semiconductor nanocrystals, when a photon absorbs energy greater than that of the semiconductor band gap, it results in the generation of electron-hole pairs or excitons in which electrons and holes are bound by electrostatic attraction. The average distance between an

∗

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (S. Basu), [email protected] (K.R. Reddy), [email protected] (T.M. Aminabhavi). ∗∗

https://doi.org/10.1016/j.jenvman.2019.109486 Received 14 July 2019; Received in revised form 27 August 2019; Accepted 27 August 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.

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features, which make them highly eligible candidates for energy and environment-based applications. Moreover, such smart functional materials may be helpful in the development of clean and sustainable energy sources as well as for water and air purification-based devices. Carbon-based nanocomposites such as carbon quantum dots (CQDs) may play a significant role in this regard due to their distinctive optoelectronic properties. In recent years, these carbon-based eco-friendly materials have been widely investigated for photovoltaic applications, water splitting and their ability to degrade harmful pollutants (Sarkar et al., 2016; Reddy et al., 2019a, 2019b; Mishra et al., 2019a; Rao et al., 2019b).

electron and the hole into an exciton is of the order of Bohr radius. The optical and electrical properties of semiconductor nanocrystal then becomes dependent upon its physical dimensions when its size approaches near to that of the Bohr radius (Chakraborty et al., 2019). Luminescent semiconductor quantum dots exhibit remarkable photostability, broad absorption profiles, high quantum yields and photo bleaching stability. Controlled shape and size luminescent characteristics of quantum dots arise from the quantum confinement effect, which allow accomplishment of related materials like optical visual for equivalent investigation of dissimilar analytes. The large stokes and slender emission bands shift in the luminescence spectra of quantum dots enable efficient coupling of other fluorophores or quantum dots with the emitted light (Owen and Brus, 2017). Hence, these systems have been widely explored as the potential materials for multicolored photoluminescent probes, luminescent biological labels, markers in imaging, emitters in light emitting diodes, gain media in lasers, light harvester in photovoltaics, photocatalysis and in various environmental treatment applications (Sundheep et al., 2019; Ramachandran et al., 2019; Mallick et al., 2019).

1.3. Carbon quantum dots (CQDs) Carbon-based nanomaterials have gained much significance owing to their outstanding optical, electronic, mechanical and optical characteristics (Sarkar et al., 2016). Among them, carbon dots (CDs) have the greatest consideration due to their biocompatibility, abundance of raw materials in the nature, low toxicity, resistance to photo-bleaching and cost-effectiveness (Wang et al., 2014). With their sizes in the range of 1–10 nm, these materials are classified as polymer dots (PDs), graphene quantum dots (GQDs), and carbon quantum dots (CQDs) (Sarkar et al., 2016). CQDs are the zero-dimensional fluorescent carbon nanomaterials possessing sp2 hybridized carbon atoms with surfaced passivity for oxygen containing functional groups such as hydroxyl and carboxylic groups. In contrast to other semiconductor CQDs are chemically inert, biocompatible, can be easily functionalized and are resistant to photo-bleaching (Linehan and Doyle, 2014). Their outstanding photo-physical properties, biocompatibility and low-cost make them ideal in the fields of bioimaging (Roy et al., 2019), sensing (Shetti et al., 2019a) and photocatalysis. Researchers have reported several major developments in CQDs modifications and their applications. However, the issues based on optical and electronic properties are mainly due to the defects in CQDs, agglomeration of CQDs during the synthesis of CQDs affects the uniformity in size and the surface properties. Hitherto, approaches to precisely controlling the defects in CQDs are unavailable. Hence, it is required to put more efforts in atom-precise structural synthesis of CQDs. Also, modification of CQDs with controllable functionalization and doping of CQDs are the crucial issues. The majority of CQDs in photo-catalysis have shown restricted light-harvesting capacity, but only a few reports are available on photo-stability of CQD-based nanocomposites, thus signifying to develop further stable CQD-based photocatalytic systems for useful applications (Duarah and Karak, 2019; Ghosh et al., 2019; Mishra et al., 2019b). In biomedical field, many studies have been reported based on CQDs toxicity and their related mechanism. The C-dots are considered to be the competent nano-architectures for drug delivery and bioimaging, but their poor size and surface characteristics hinder their wider usage (Kaushik et al., 2019; Gulla et al., 2019). In order to overcome the biodegradability and toxicity problems, new fluorescent materials such as CQDs have been developed possessing better optical, biological, chemical inertness, and low toxicity compared to semiconductor quantum dots (Deng et al., 2019). This timely review summarizes surface modifications and band gap tuning of CQDs by different novel metals and metal oxides. The review will address an outlook on the change in optical properties by surface modifications of CQDs. In later parts, more focus will be placed on water treatment/splitting applications such as heavy metal ion detection, photodegradation of dyes and H2 production of CQDs. Lastly, limitations and challenges of CQDs will be discussed along with the latest published reports for a quantitative discussion.

1.2. Need for sustainable development Clean environment, water and energy are the basic needs for human survival and economic development (Gopinath et al., 2019). However, the rise in demand for various products has led the manufacturers to indulge in risky, but profitable production modes, leading to long-term environmental threats (Zandi et al., 2019). Additionally, utilization of energy through fossil fuels such as petroleum, natural gas and coal has led to pollution of air and water bodies (BusaidiBaawain et al., 2019). The limited availability of fossil fuels and their increasing year-wise unregulated consumption may lead to energy crises of the future. Since the Kyoto protocol in 2005, these issues have emerged in the global scenario in a highly debatable manner (Chen and Chen, 2002). Formulation and implementation of an integrated set of policies addressing the energy and environmental concerns simultaneously have met with several challenges in a developed nation such as the United States of America (Greening and Bernow, 2004). However, the present policies and technologies are ineffective in the realization of long-term goals (Andrews and Govil, 1995). In order to tackle fossil fuel-based energy crisis and environmental pollution, there is a greater need for greener and sustainable source, which can resolve both the energy and the environment related issues. Nanotechnology can be a highly promising field for development of sustainability. It is an emerging field, which could contribute for the development of smarter materials capable of both generating energy and degrading the environmental toxic pollutants. It deals with designing and manipulation of materials at the molecular scale. Rapid development of novel nanomaterials can create options regarding new product innovation and high performance applications (Jyothi et al., 2019). Fabrication of such novel functional materials with tunable sizes, shapes, crystallinity, porosity and structures are of great importance for novel innovations in sustainable energy technologies (RoyDas et al., 2019). It also allows the fabrication of materials having specific functionalities capable of recognizing a particular pollutant in a mixture (Dharupaneedi et al., 2019). Thus, tremendous progress in nanotechnology may lead to the basic understanding of physics at the nanoscale in order to control the system properties and searching for new materials for energy and environmental applications (Chung et al., 2012). Low energy solution-based synthesis of nanomaterials would allow their incorporation into the devices (Bera et al., 2019). Among the nanomaterials, photoactive metal oxide nanoparticles, quantum dots and carbon-based nanomaterials are the potential candidature since they rely upon the sunlight, which itself is a clean and sustainable energy source. Quantum confinement effect in nanomaterials such as quantum dots has given rise to many of the fascinating optoelectronic 2

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2. Optical properties, band gap tuning and surface modification of CQDs

they have the prominent UV–vis photocatalytic ability. However, CQDs and Ag NPs provide electron-reservoirs, which arouse the parting of photogenerated electron-holes by increasing the quantum effectiveness of g-C3N4. Some important reports on band gap tuning of CQDs by different catalysts are summarised in Table 1.

The electronic properties of semiconductors suggest that as one goes down in the periodic table, the band gap of the solid decreases. Solid solutions of most of these semiconductors can be made and the band gap of the subsequent solution will be in between the last two members. For instance, germanium phosphate (GaP) has a band gap of 2.3 eV (~540 nm), gallium arsenide (GaAs) has a band gap of 1.4 eV (~890 nm), and gallium phosphorus arsenide (GaPxAs1–x) has a band gap energy that depends almost linearly on x (10) (Pandit et al., 2019; Ellis, 1993; Brus, 1984). Since over the years, carbon dots are not commercialized fully, and hence, additional research is needed on their synthesis and characterization. Like most of the nanomaterials, bottomup and top-down approaches have been the two major routines for the synthesis of CQDs (Gu et al., 2019). In top-down looms, the bigger carbon structures break down to slighter CQDs, which include electrochemical oxidation (Shetti et al., 2019b, 2019c; Shikandar et al., 2019), laser ablation technique and arc discharge. Whereas, bottom-up method includes combustion/hydrothermal/thermal, ultrasonic/microwave, solution chemistry methods in which CQDs are fabricated via molecular precursors (Jing et al., 2019; Muthusankar et al., 2019). Innumerable review articles have been dealt with the synthesis and characterization of CQDs, but very less focus has been made on the band gap tuning of CQDs by different novel metals and metal oxides such as TiO2 and MnO2.

2.2. Surface modification of CQDs The CQDs with splendid photophysical and fluorescent properties can provide photo-induced electron donors or acceptors. Also, they are capable of multifunctional modules in photocatalytic design (Liu et al., 2013a). The grouping of these fluorescent probes (CQDs) and other functional materials in one micro matrix has been widely exploited in the construction of ideal sensors for water treatment (Shorie et al., 2019). Depending upon the photo-induced electron-stimulate, PL, with electron basin characteristics of CQDs/semiconductor composites, CQDs, (CQDs/TiO2 (Yu et al., 2014a), CQDs/SiO2 (Han et al., 2013), CQDs/Fe2O3 (Zhang et al., 2011), CQDs/Cu2O (Li et al., 2012), CQDs/ Ag3PO4 (Zhang et al., 2012a), CQDs/MnO2 (Garg et al., 2018)) could modify the surface of CQDs, resulting in superior activity and stability under the visible light. The highly water dispersible superparamagnetic Fe3O4@mTiO2 pom-pom balls deposited by CQD were synthesized by hydrothermal technique by Das et al. (2016). In Fig. 2, it is shown that the CQDs were able to increase the light absorption of mTiO2 from UV to the visible region. The composite catalyst improved visible light photocatalytic activity was verified for the degradation of common pollutants like pesticides, organic dyes, antibiotics, and phenolic compounds, taking one molecule from each category as a model. The structures of C-dots deposited mesoporous superparamagnetic Fe3O4@mTiO2 remains unchanged even after five catalytic cycles. Xie et al. (2014a) reported a silicon nanowire (SiNW) array/CQD core shell heterojunction photovoltaic device by directly coating Agassisted chemical-etched SiNW arrays with CQDs. The heterojunction with a barrier height of 0.75 eV revealed outstanding behaviour in dark with a power conversion efficiency (PCE) as high as 9.10% under AM 1.5G irradiation (air mass coefficient). Such high power conversion efficiency comes from the superior optical absorption as well as the optimized carrier transfer and collection capability. A recent review by Kuvarega et al. (Kuvarega and Mamba, 2017) described some important aspects of designing nanocarbon-TiO2 composite materials for water treatment and energy applications as superior materials with higher performance. These nanocarbons offer a platform for efficient dispersion of semiconductor, preventing the agglomeration and also providing a hierarchical structure for light harvesting and eventually easy access for gas/liquid-phase components in photocatalytic reactions. The carbon nanodots may act as efficient solid-state sensitizers to promote visible light absorption. Carbon may also dope the TiO2 semiconductor, inducing a shift in the band edge toward the visible region. The interaction of carbon with semiconductor induced a modification of the intrinsic properties of semiconductor particles (band gap, charge carrier density, lifetime of charge separation, non-radiative paths, etc.) as well as surface properties. CQDs have shown excellent electronic conductivity, but defects and other aspects may greatly reduce the conductivity. The CQDs in CQDsemiconductor hybrids could act as sink for electrons, enhancing the lifetime of charge separation. Fermi levels of CQDs are generally below the conduction band minimum of most of the semiconductors. However, depending upon the specific characteristics of nanocarbons, the Fermi level could also lie above that of TiO2. In any case, due to their thermal conductivity, CQDs would maintain a more uniform temperature of the semiconductor-nanocarbon hybrid upon irradiation. The CQDs may, therefore, provide an optimal nano-architecture in the photoanode for light absorption that make them to be sustainable materials in the treatment of wastewater. Despite these accomplishments, further work needs to be done in

2.1. Band gap tuning of CQDs The interaction of metal and ligands has been the driving force to develop nanoparticles (NPs) for sensor development (Shetti et al., 2019d). The metal-based NPs have been popular for their quenching effect, thus known as exceptional fluorescence quenchers and have been scrupulously surveyed on diverse fluorophores. In addition, these NPs contain innumerable applications in areas of catalysis and sensing (Ratnayake et al., 2019; Lu and Zhou, 2019; Kalaiyarasan and Joseph, 2019). Fluorescent quenching by metal-based NPs is a kind of energy transfer process, which can take place among the NPs and fluorophores. The composite fabrication of metal-based NPs amid other photoactive materials has been the testimony of proficient NPs in several advanced biomedical applications (Kaliayarasan et al., 2019). Commonly, such nano-sensors are modified with functional groups that can selectively attach analyte, resulting in 3D, 2D, and 1D particle bridge assemblies. Also, ligand exchange reactions, such as replacement of surface stabilizing groups through analytes via higher affinity for NP surface can potentially be utilized for biological detection procedures, if a particular marker of concern leads to crosslinking of interparticle (Shi et al., 2019). Recently, Mehta et al. (2017) synthesized spherical CQDs by microwave-assisted procedure to fabricate the core shell Au@CQDs composite at room temperature. Their optical properties revealed the band gap energy for CQDs was 2.78 eV, which was decreased to 2.68 eV for Au@CQDs. The synthetic mechanism for the formation of CQDs and Au@CQDs is shown in Fig. 1. Au@CQDs are the suitable photocatalysts, which remain stable upon contact with water. Mandani et al. (2015) successfully synthesized the core shell Au@CQDs composites for the detection of various amino acids and biomolecules that showed excellent results compared to other methods. Liu et al. (2017a) fabricated a facile Ag/CQDs nanocomposite-based dual signal probe for the detection of Hg2+. The proposed scheme shows a prospective for Hg2+ detection in real water samples. Very recently, Qin et al. (Qin and Zeng, 2017) synthesized a mixture of plasmonic Ag NPs possessing upconverted photoluminescent (PL) characteristics from CQDs assist graphitic carbon nitride (g-C3N4) to advance photocatalytic H2 evolution reaction. Such novel composites bind near-infrared light to prompt H2 evolution in aqueous solution and 3

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Fig. 1. Graphical mechanism for the formation of fluorescent CQDs and Au@CQDs nanocatalyst. Reproduced from Ref (Mehta et al., 2017). with permission from RSC.

Table 1 Band gap tuning and surface modification of CQDs by different analytes. Analyte

Advantages

Application

Reference

Core shell Au@CQDs Core shell silicon nanowire (SiNW) array/ carbon quantum dot (CQD) core–shell Carbon nitride nanosheets (CNNS)/CQDs composites Carbon quantum dot/Silver nanoparticle/ polyoxometalate composites Glutathione-capped quantum Dots (QDs)

Band gap decreased from 2.78 eV to 2.68 eV Heterojunction with a barrier height of 0.75 eV exhibited excellent rectifying behaviour Higher photocatalytic efficiency than pure g-C3N4.

Photocatalytic H2 production High-performance optoelectronic devices

Mehta et al. (2017) Xie et al. (2014a)

Photocatalytic H2 production

Li et al. (2016a)

Surface plasmon resonance of Ag improves the solarenergy conversion efficiency Efficient surface and photophysical properties of the QDs Good luminescence, uniform size, excellent stability, and excitation-dependent photoluminescence (PL) property Specific quenching of the QDs by the ions enabled us to use Hg2+ and Ag+ ions as inputs that activate logic gates Water-soluble and highly stable in aqueous solution.

Overall water splitting in visible light.

Liu et al. (2014a)

Pb2+ ion detection in water.

Mohamed Ali et al. (2007) Chandra et al. (2017)

Nitrogen (N) and sulphur (S) doped carbon dots (NSCQDs) T-rich- or C-rich-modified QDs

CdS QDs capped with l-cysteine TiO2/CQDs and SiO2/CQDs complex system CQDs/Bi2WO6 hybrid

Carbon quantum dot (CQD) deposited Fe3O4@mTiO2 nano-Pom-Pom Balls

Efficient usage of the full spectrum of sunlight Interfacial transfer of photogenerated electrons from Bi2WO6 to CQDs, leading to effective charge separation of Bi2WO6. Expanding the light absorption of mTiO2 from the UV to visible region.

CdS-Bi2WO6/CQDs

Stronger absorption in the visible light region

Hg

2+

ion detection in living cells.

Selective analysis of Hg2+ or Ag+ ions in water.

Freeman et al. (2009)

Detection of heavy and transition metal (HTM) ions in aqueous solution. Photocatalytic degradation of methyl blue dye Photocatalytic degradation of organic pollutant.

Chen et al. (2008)

Photodegradation for ciprofloxacin, methylene blue, quinalphos, and 4-nitrophenol under visible light Photodegradation of methyl orange

Das et al. (2016)

Li et al. (2010) Di et al. (2015)

Ge and Liu (2011)

wavelength of CQDs (Muthusankar et al., 2019). Absorption features of CQDs also depend upon their preparation methods as these are relatively quite efficient in the absorption of long wavelengths (Huang et al., 2019).

this area to tackle the issues such as improvement of facile synthesis methods and enhancement of the interfacial interaction between the CQDs and the supported materials. However, the understanding of these phenomena still warrants additional investigations before applying these materials to large-scale commercialization.

2.3.2. Photoluminescence CQDs also consist of unique tunable photoluminescence ranging from deep UV to NIR arising from quantum confinement effects. Due to the presence of emissive traps on their surface, the PL quantum yield of CQDs is low (< 10%), which necessitates the requirement of surface passivation layer onto the CQDs to intensify the PL brightness. It has also been observed that upon increasing the excitation wavelength leads to the red shift in PL emission wavelength of CQDs along with reduced emission intensity (Omer et al., 2019). Most CQDs emit in the range of 420–450 nm, which is also red-shifted with increasing size of the CQDs. However, it is blue-shifted upon doping CQDs with nitrogen

2.3. Optical properties of CQDs 2.3.1. Optical absorption The presence of functional groups onto the surface of CQDs imparts hydrophilicity and increases their ability for functionalization with various other organic, biological and polymeric molecules. CQDs actively harvest photons in short wavelength region due to π-π* transitions of C]C bonds. Also, CQDs have strong absorption in the UV region (260–320 nm) having tail extending up to visible range. Also, different surface functional groups tend to affect the absorption 4

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synthesizing CQDs from the thermal treatment of sustainable hair. These CQDs were used for the treatment of Hg2+ removal from water at a concentration as low as 10 nM Hg2+ that can be successfully detected. A similar approach was utilized for the detection of Be2+ by Li et al. (2014a), where the detection capability was still good at concentration of Be2+ ˂ 0.1 mM, while the calculated detection limit was 23.3 mM as per the Stern-Volmer equation shown in Fig. 3. The system was also detected for a variety of metal ions and different quenching degrees were observed. Even though Cu2+ displayed a relatively strong quenching effect, but the most promising results were demonstrated for Be2+ and Fe3+ ions (Fig. 3 (b)). Kumar et al. (2017) proposed a green, economical, and one-step process of synthesizing CQDs by a hydrothermal reaction. These CQDs were useful for the detection of trace Pb2+ ions in real water samples. Compared to fluorescence method for lead detection, this strategy showed several advantages such as simple, label-free, accurate and rapid analysis. The CQDs showed a high selectivity for Pb2+ ions with a limit of detection of 0.59 nM. They also demonstrated the practical feasibility of this approach for the detection of Pb2+ ions in cancer celllines as well as real water samples. Further, the authors determined sensitivity for other metal ions such as Cu2+, Mg2+, K+, Ca2+, Ni2+, Pb2+, Co2+, Hg2+, Cd2+, Na+, Sn2+ and Al3+, but the obtained CQDs were selectively good for the detection of Pb2+ ions. Majority of the previously reported CQD-based sensors for the detection of metal ions are summarised in Table 2.

Fig. 2. Proposed mechanism for Fe3O4@mTiO2@20%C-dot nanospheres photocatalyzed dye degradation under visible light. Reproduced from Ref (Das et al., 2016). with permission from ACS.

and sulphur. Moreover, it depends upon the precursor during preparation, elemental doping, presence of unique surface sites, and emission wavelengths can be prepared via special starting materials, heteroatom doping, and creation of unique surface active sites. These can exhibit multiphoton up-conversion, which can be controlled by varying the size, shape, heteroatom doping, surface and edge modification, quantum confinement effects, size and edge effects (Wu et al., 2019; Cheng et al., 2019).

3.1. Detection mechanism Generally, four types of fluorescence response approaches have been highlighted: fluorescence turn-on, fluorescence turn-off, ratiometric response and fluorescence resonance energy transfer (FRET). Majority of CQDs-based sensors are based on the fluorescence turn-off/ on strategy, but the real fluorescence quenching mechanism tempted by metal ions has not yet been accurately elucidated. Majority of the fluorescence quenching mechanisms are mostly credited to the charge, electron or energy transfers that are the outcome from the discerning interfacing between CQDs and metal ions. This could be due to the function of the group present onto the surface of CQDs such as amino hydroxyl and carboxyl groups. The functional groups interact selectively with metal species suggesting the formation of a complex with metal ions. The complex might alter the electronic structure of CQDs and change the allocation of excitons that hasten the non-radiative recombination of exciton through efficient energy transfer or electron charge (Chen et al., 2013). Additionally, the inner filter effect may be the other cause for fluorescence quenching by the metal ions. The inner filter effect is due to the absorption of excitation and/or emitted light by the absorbers in the detection system when the absorption spectrum of the absorber overlap with the fluorescence excitation or emission spectra of CQDs. The inner filter effect has enhanced the sensitivity compared to other mechanisms because the changes in absorbance of sensors can transform exponentially into fluorescence intensity changes (Gu et al., 2017). Considering all these reports, one can expect CQDs are the more versatile tools for sensing heavy metal ions from water samples.

2.3.3. Photoluminescence up-conversion in CQDs The up-conversion phenomenon arises by the emission of a photon of higher frequency when two or more photons of lower frequency are absorbed by the CQDs. Emission of a visible photon by absorption of multiple infra-red photons is a typical example. As a process, involving multiple photons intensity of up-conversion fluorescence depends upon the excitation wavelength intensity as (Iex)n, where n denotes the number of photons (Zhou et al., 2019). Three mechanisms responsible for up-conversion fluorescence are energy transfer up-conversion, excited state absorption and photon avalanche (Matsuura, 2002; Kong et al., 2019). The up-conversion fluorescence is difficult to observe in a commercially available fluorescence spectrophotometer due to incoherency and low intensity of xenon lamp, which is used as a common excitation source due to which its efficiency is very low (Zhou et al., 2019). 3. CQDs as sensors for heavy metal removal from water The toxic emerging effluents in the form of heavy metals are a major concern decreasing the quality of water, due to ever-growing population, commercialization, ecological degradation and harmful domestic/ industrial uncontrolled discharges (Reddy et al., 2015). Among the diverse heavy metal ions, mercury (Hg), iron (Fe) and copper (Cu) are forbidden in electronic equipment by the European Union's Restriction on Hazardous Substances (RoHS) directive owing to their harmful nature (Rajamani and Rajendrakumar, 2019). Currently, the most common technique used for the removal of ions is optical detection using colorimetric and fluorescence changes. The major benefit of utilizing fluorescent materials is their ease and lower limit of detection. Recently, Huang et al. (2017a) used nitrogen-doped CQDs for the detection of mercury ions, L-cysteine (L-Cys) and iodide ions. Fluorescence of nitrogen-doped CQDs could quench the sensitively and selectively by the addition of Hg 2+ with a detection limit of 83.5 nM. When L-Cys was added to N-CQDs-Hg2+composite, fluorescence was recovered effectively. Guo et al. (2016) reported a low-cost approach for

4. CQDs for degradation of dyes and organic pollutants Due to strong blue photoluminescence and optical absorption in UV and near visible regions, CQDs are the eligible candidates for photocatalytic applications (Ramachandran et al., 2019; Mehta et al., 2017; Qin and Zeng, 2017). For the first time, photocatalytic activity of CQDs was reported in 2010 when CQDs/TiO2 and CQDs/SiO2 degraded methylene blue (MB) dye within 20 min after illumination with 300 W halogen lamp (Li et al., 2010). The minimum degradation of MB dye in the absence of CQDs confirmed their participation in dye degradation and their interaction with TiO2 or SiO2. Two possible mechanisms were 5

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Fig. 3. (a) Typical PL quenching of CQDs in the presence of Be2+ions; (b) Typical PL quenching of CQDs in the presence of Fe3+ ions; both the insets are concentration dependent fluorescence response. (c) Performance of sensors based on prepared carbon dots: comparison of fluorescence intensities in the absence and presence of different metal ions. Reproduced from Ref (Pollnau et al., 2000). with permission from Elsevier.

proposed; firstly emission of high frequency photons due to photoluminescence up-conversion in CQDs must have photoexcited TiO2 or SiO2. Secondly, band position of CQDs, which must have enabled electron transfer from the excited TiO2 or SiO2. The photoluminescence up-conversion in CQDs can be a highly attractive phenomenon regarding the fabrication of smart visible light-active photocatalyst. However, till date there are no results stating the role of up-conversion in enhancing the photocatalytic activity by the CQDs as discussed in many CQDs-based photocatalytic systems (Yu et al., 2014b). The low efficiency of up-conversion effect as stated above could indicate its negligible influence on the photoactivity compared to other factors. It was proposed (Huang et al., 2017b) that CQDs tend to form “dyade” like structure on semiconductor surface such as ZnO providing

access to photogenerated charge transfer in order to hinder the electron-hole recombination. ZnFe2O4 displayed enhanced photoactivity upon loading it with CQDs in which CQDs were believed to act as electron reservoir and transporter, thus acting as an efficient energy transfer component (Huang et al., 2017b). The CQDs tend to provide more absorption sites, leading to more anchoring of Fe(III) species upon Fe(III)/Fe-doped g-C3N4 nanocomposite, which accounts for its high photocatalytic activity towards MO and phenol degradation (Liu et al., 2017b). Herein, most of the CQD-based composites reported till date for the degradation of dyes and organic pollutants are summarised in Table 3. Recently Ebrahimi et al. (2016) fabricated graphene CQDs for solardriven photocatalytic degradation of methyl blue dye. Authors 6

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Table 2 CQD-based sensors for the detection of metal ions. CQD as sensors N-doped CQDs CQDs from candle root Ge-doped CQDs N and S co-doped CQDs CQDs from coconut milk Amino-modified CQDs CQDs from citric acid N,S,P-co-doped CQDs N,S-co-doped CQDs CQDs from apple juice CQDs/Si NPs CdSe/ZnS@CQDs CQDs CdTe–CQDs Si-CQDs Hydroxyl coated CQDs Luminescent carbon dots CdSe quantum dots CD- graphene oxide N,S-co-doped CQDs B-doped CQDs Amino acid-derived CQDs CQDs from citric acid CQDs from ethanol S-doped CQDs CQDs from banana CQDs from o-phenylenediamine

Metal ion 2+

Hg Hg2+ Hg2+ Hg2+ Hg2+ Hg2+ Hg2+ Hg2+ Hg2+ Hg2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cr3+, Al3+ and Fe3+ Be2+ Ba2+ K+ Fe3+ Fe3+ Fe3+ Fe3+ Fe3+ Fe3+ Fe3+ Fe3+

Linear range

Detection limit

Ref

0–0.05 μM 0–10 μM 0.2–3 μM 2–40 μM 10–100 nM 0.1–1.2 μM 1–700 nM 1–70 mM 2–200 nM 5–100 nM 0–3 μM 1–100 μM 0–10 μM 0–100 nM 0.833–833 μM 1.0–25.0 μM – 1.0 × 10−7 -1.2 × 10−7 mol L−1 0.05–10.0 mM 0–500 mM 0–16 μM 6–250 mg L−1 2–50 μM 1–80 mM 1–500 μM 2–16 μM 0–100 nM

2.91 nM 10 nM 75 nM 2 μM 16.5 nM 20 nM 0.57 nM 180 nM 2 nM 2.3 nM 35.2 nM 1 μM 23 nM 0.36 nM 0.3 μM 60 nM 23 μM 4.2 × 10−9 mol L−1 10 μM 0.2 μM 242 nM 3 mg L−1 1.3 μM 0.04 μM 0.1 μM 211 nM 16.1 nM

Zhang et al. (2015a) Li et al. (2011) Li et al. (2015) Roshni and Ottoor (2015) Gao et al. (2016) Zhou et al. (2015a) Wang et al. (2016a) Yang et al. (2014) Yue et al. (2014) Liu et al. (2014b) Zhu et al. (2012) Zong et al. (2014a) Wang et al. (2016b) Zong et al. (2014b) Liu et al. (2011) Jia et al. (2014) Mahmoud (2012) Zhang et al. (2015b) Sun et al. (2016) Wang et al. (2016c) Karfa et al. (2015) Zhou et al. (2015b) Miao et al. (2015) Xu et al. (2015) Vikneswaran et al. (2014) Wang et al. (2017)

valence band of the oxide. Also, CQDs might operate as a basin for trapping the emitted electrons from the metal oxide to control the recombination of hole-electron in metal oxide. The electron in basin reacts with the oxygen to fabricate O2.- radicals, while holes generate OH•, after reacting with water. Overall, the produced radicals are responsible for the degradation of dye molecules. The functional groups containing CQDs might promote superior solubility in water, creating the CQDs for increasing dye degradation in wastewater, justifying the use of catalytic effort of biowaste mitigation.

described the mechanism of charge carrier generation, transport, and separation using different scavengers to probe the potential reaction pathway following the direct Z-scheme approach. The dye was efficiently degraded by attacking the reactive hydroxyl radicals and superoxide onto the surface of CQD. Graphical representation of Z-scheme is shown in Fig. 4. Prasannan et al. (Prasannan and Imae, 2013) reported the synthesis of CQDs from the orange waste peels and utilized the composite of CQDs/ZnO as a photocatalyst for the degradation of naphthol blueblack azo dye under the UV irradiation. A schematic illustration of the degradation mechanism of CQDs/ZnO is shown in Fig. 5. When the composite is exposed to UV-light, electrons in the valence band of metal oxide are excited into the conduction band, and the excited electrons transfer to the CQDs together with the generation of holes in the

Table 3 CQDs composites reported till date for the degradation of dyes and organic pollutants. Composite

Pollutant

Light source

Reference

CQDs/TiO2 CdS QDs/chitosan composite films CQD deposited Fe3O4@mTiO2 pom-pom balls CdS QDs/C3N4 TiO2/CQDs and SiO2/CQDs

Methyl blue photodegradation Methyl orange photodegradation Ciprofloxacin, Methylene blue, Quinalphos, and 4-nitrophenol photodegradation Rhodamine B photodegradation Methylene blue photodegradation

Visible light/sunlight Visible light Visible light

Sun et al. (2014) Jiang et al. (2012) Das et al. (2016) Fan et al. (2016) Li et al. (2010)

ZnO/CQDs CQDs/Fe-doped g-C3N4 CQDs/Ag/Ag3PO4

Benzene and methanol photodegradation Methyl orange and phenol photodegradation Methyl orange Photodecomposition Methyl blue photodegradation Methyl orange and Methyl red photodegradation Naphthol blue-black azo dye

Visible light 300 W halogen lamp at λmax = 605 nm Visible light Visible light Visible light Visible light at λmax = 605 nm Sunlight UV-irradiation

Li et al. (2012) Safavi et al. (2012) Prasannan and Imae (2013) Yu and Kwak (2012) Ma et al. (2012) Liu et al. (2013b) Mansur et al. (2014)

CQDs/Cu2O Carbon nanodots Carbon nanodots from orange peels CQD-embedded mesoporous α-Fe2O3 N-doped carbon dots Water-soluble graphitized carbon dots ZnS quantum dots/chitosan CQDs/hydrogenated TiO2

Methyl blue photodegradation Methyl orange photodegradation Rhodamine B photodegradation Methyl orange and Methyl red photodegradation

Visible light Visible light 500 W high pressure Hg lamp UV-irradiation

Methyl orange photodegradation

UV–visible, near-infrared irradiation

7

Yu et al. (2012) Liu et al. (2017b) Zhang et al. (2012b)

Tian et al. (2015)

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A. Mehta, et al.

Fig. 4. Schematic illustration of charge transfer process for graphene CQDs/ZnO nanowires under solar light irradiation. Reproduced from Ref (Ebrahimi et al., 2016). with permission from ACS.

semiconductor, which is an essential factor in charge separation, since the generation and recombination of excited charge carriers happen in micro to femto-seconds time regime. Hence, high photon efficiency can only be achieved by the suppression of charge recombination both in bulk and surface of the photocatalyst (Kaliayarasan et al., 2019). TiO2 has been widely investigated as a benchmark photocatalyst for such a sole purpose. The conduction and valence band positions of TiO2 are such that the conduction band is slightly negative than the reduction potential of H+ to H2. However, due to its wide band gap (3.2 eV), it is active only in UV region of sunlight, which comprises of just 5% of total solar spectrum. Hence, fabrication of a photocatalyst having an optimum band gap and which can harvest the visible and NIR light has become the prime objective nowadays. Development of CQDs-based photocatalyst for solar water splitting has gained immense attention in recent years. Up and down converted photoluminescence, electron accepting and transport properties of CQDs make them near infra red (NIR) active hybrid photocatalysts due to which these are highly promising in harvesting visible and NIR light (Tian et al., 2015). Some representative reported data for water splitting by CQDs are summarised in Table 4.

Fig. 5. Schematic mechanism for the photocatalytic degradation of dyes on CQDs/ZnO. Reproduced from Ref (Prasannan and Imae, 2013). with permission from ACS.

5. CQDs for water splitting (hydrogen production) 5.1. Carbon quantum dots for H2 generation

5.2. CQDs as active materials for PEC H2 generation

Hydrogen obtained from photocatalytic water splitting using sunlight can be highly clean and is a sustainable energy source compared to conventional fossil fuels. In the photocatalytic water splitting process, electrons and holes generated via photonic excitation act as reducing and oxidizing agents for producing H2 and O2. The separation of O2 and H2 gases after water spitting is a huge challenge to prevent the back reaction of water formation (Saravanan et al., 2019). As an uphill reaction (ΔG > 0), splitting of water into O2 and H2 requires 1.23 eV (237 kJ/mol) of energy (Reddy et al., 1016a). Therefore, the band gap of a photocatalyst should be such that the conduction band should be more negative than that of the redox potential of H+/H2 (0 V vs. NHE) and the valence band should be more positive than the redox potential of O2/H2O (1.23 V vs. NHE). Hence, the minimum band gap requirement for photocatalytic water splitting is 1.23 eV, which is equivalent to the wavelength of 1100 nm. Water splitting consists of two half reactions; one being O2 evolution reaction (OER) and the other is H2 evolution reaction (HER) (Fang et al., 2019). The photocatalytic water splitting on the surface of a photocatalyst involves: (i) generation of electrons and holes inside the semiconductor by band gap excitation, (ii) separation of electron-hole pairs and their migration towards the surface of semiconductor, and (iii) trapping of photogenerated electrons and holes by the surface active sites and their subsequent consumption in H2 and H2 evolution reactions (Reddy et al., 1016b). The efficiency of photocatalytic water splitting is highly dependent upon the electronic structure of the

The H2 generation from photoelectrochemical (PEC) method involves photoanode, which is an n-type semiconductor and the counter electrode in a device called photoelectrochemical cell. Hydrogen is produced under thermodynamically unfavorable circumstances onto the electrode surface (Yu et al., 2014b; Luo et al., 2014). The photogenerated electrons are then transported through an external circuit to counter electrode where H2 is produced onto its surface. At the same time, holes in the photoanode interact with the sacrificial agents present as electrolyte to complete the redox cycle (Yu et al., 2014b). Nowadays, CQDs-based photoanodes have gained much attention when combined with other semiconductors to fabricate hybrid photoanodes, especially TiO2. Other semiconductors have also been combined with CQDs such as ZnO, CdS and Si. The introduction of less toxic CODs enhanced the PEC activity of TiO2 photoanodes. The photocurrent doubled under full spectrum and became five-fold under the visible light illumination. Light absorption in these electrodes increased with CQDs content (Xie et al., 2014b). CQDs when anchored upon nanocrystalline TiO2 were found to be the highly active sensitizers for photoelectrochemical (PEC) H2 production. The high activity of CQD/ TiO2 was ascribed to high conductivity of CQDs, resulting in a high photocurrent of 1.2 mA cm−2 at 0 V vs Ag/AgCl higher than the pristine TiO2 (Liu et al., 2017a). CQDs were electrodeposited upon TiO2 nanotubes (NTs), photoelectrochemical (PEC) and sensitization mechanism of CQDs was investigated. It was found that CQDs tend to broaden the photoresponse range of TiO2 NTs up to visible and NIR. The photocurrent density of CQDs/TiO2 NTs photoanode was recorded four-times 8

Journal of Environmental Management 250 (2019) 109486 Yeh et al. (2015) Lei et al. (2017) Lu et al. (2016) AQY = 12.8% AQE = 4.2% N.A.

N.A. AQY = 16.8%

higher than the pristine TiO2 NTs photoanode at 0 V vs Ag/AgCl. The generation rate of H2 was 14 μmol L−1 and photoanode was also found to be stable for a long-term application generating the H2 with 100% Faradaic efficiency (Zhang et al., 2013a). The PEC characteristics of CQDs when deposited onto TiO2 nanorod arrays (TNRAs) depended upon the CQD loading and TNRA length. The incident photon to current conversion efficiency (IPCE) of TNRA/CQDs nanocomposite was in the range of 1.2–3.4%, depending upon the TNRA and CQD ratio. It was found from the impedance spectroscopy that CQDs are responsible for improving the charge transfer between electrode and electrolyte (Bian et al., 2014a). There have been some reports depicting the good conductivity of CQDs (Sundheep et al., 2019). These have played an important role in the vectorial charge transfer of photogenerated electrons in Pt/CQS/ZnIn2S4 composite photocatalyst. The presence of both CQDs and PtNPs not only lead to modified crystallanity, but also increased the light absorption intensity of the resulting nanocomposite photocatalyst (Li et al., 2014b). The authors have also reported the synthesis of Au@CQDs for water splitting under sunlight and explored the full mechanism responsible for water splitting process as shown in Fig. 6. The adsorption of visible light occurs at Au nanoparticle surface because of the SPR effect, since the absorbed photons are separated efficiently into electrons and holes. These free electrons in the intermediate energy levels act as a “color center”, which can be stabilized transiently and promoting further shuttling of photoexcited electrons to the LUMO of CQDs in the visible regions. The photoexcited electrons in LUMO of CQDs are thermodynamically feasible for constructive water splitting. In view of their attractive properties, CQDs and Au@CQDs would expect to realise the efficient usage of the full spectrum of sunlight. Table 5 represents a summary of the latest data on H2 generation from water using the PEC method.

10 vol% TEOA 0.5 M Na2S and 0.5 M Na2SO3 No sacrificial agent

4.9 μmol h −1 95.4 μmol·h-1 2.5 μmol·h-1

300 W Xe-lamp 300 W Xe-lamp intensity 45 mW cm−2 300 W Xe-lamp A 300-W Xe 300 W Xe-lamp 20 vol% methanol 10 vol% TEOA

340.9 μmol h −1 g -1 10.8 μmol h −1

300 W Xe-lamp 10 vol% Triethanolamine

Fig. 6. Proposed reaction mechanism for visible-light/sunlight-driven water splitting on CQDs and Au@CQDs. Reproduced from Ref (Mehta et al., 2017). with permission from RSC.

N.A. 100 ml aqueous solution 50 mg in 25 ml water Intact-nitrogen doped GQDs Cds-GQDs CQDs/CdS

In photocatalytic H2 production, photocatalyst is simply irradiated while being dispersed in an aqueous solution. This makes it more advantageous when compared to PEC-based H2 generation process, which requires an external circuit and voltage bias to generate H2. Hence, it is a simple and low cost process. Photocatalytic activity for H2 generation is strongly dependent on the band gap characteristics of the semiconducting composite. CQDs demonstrate exceptional and tunable optical properties of absorbance and PL after specific surface modification. Moreover, photoinduced CQDs are both excellent electron donors and acceptors, resulting in efficient separation of electrons and holes. Therefore, CQDs can serve as versatile components in photocatalyst H2 production design, such as electron mediators, photosensitizers, spectral converter and sole photocatalyst. Essentially, these multiple effects occur concurrently in many cases. The second key characteristics for H2 production is based upon the defects of CQDs, which have significant influence on their optical and electronic properties. However,

N.A.- Not available.

50 mg in 200 ml water N.A.

5.3. Role of CQDs as active materials for H2 generation

g-C3N4/graphene quantum dots (GQDs) NGQDs-ZnNb2O6/g-C3N4 NGQDs-GO

68 μg ml−1 of CQDs and 6 μg ml−1 of NiNPs in total 10 ml solution 80 mL of aqueous solution PVP-coated CQDs and NiNPs

Ag/CQD/C3N4

Amino-conjugated CQDs

Cu/CQDs

2.18 mmol/g/h

470 nm LED lights

Triethalonamine (water: triethalonamine 7 :1) N.A.

Quantum efficiency = 5.25%

Virca et al. (2017) Zou et al. (2016a) Yan et al. (2017) Du et al. (2015) 6% Quantum Yield

626.93 μmol/g/h 300 W Xe lamp

0.43M Na2S and 0.5M Na2SO3

330 mmols H2/g CQD

Qin and Zeng (2017)

27.31 μmol/g/h 300 W Xe lamp

PEG

CQDs in 56 ml water 2 ml of 0.01 g l containing 1 g PEG 50 mg in 100 ml containing 0.43M Na2S and 0.5M Na2SO3 50 mg in 70 ml water triethalonamine solution

STH = 0.37%

– 300 W Xe lamp

Methanol (water: methanol-1:1) Au/CQDs

−1

1 ml Au/CQDs in water methanol solution

56 under sunlight and 50 under CFL light 64 mmol/g/h

Apparent quantum efficiency 4.81%

Hou et al. (2015)

12.2%(Apparent quantum efficiency) STH = 1.89% 496.5 μmol/g/h

300 W Xe lamp and 250W IR lamp 65W CFl and Sunlight Methanol (water: methanol-9:1) 0.3 g in 200 ml solution CQDs/γTaON

H2 Yield Light source Sacrificial agent Catalytic amount Photocatalyst

Table 4 A recent summary of reports on H2 production by CQDs.

Mehta et al. (2017) Zhang et al. (2017a) Xu et al. (2016)

Ref Efficiency

A. Mehta, et al.

9

10

AM 1.5G (100 mW cm−2)

CQDs/TiO2 NWs

N.A. – Not available.

CQDs-CdS/TiO2 NTs CQDs/BiVO4

N-CQD/ZnO NWs

CQDs-C3N4/TiO2 CQD/ZnO NWs

100 mW cm solar simulator Xe lamp equipped with an AM 1.5G filter, and the light power intensity was uniformly calibrated to 100 mW cm2 300W Xe-lampwith an AM 1.5 G filter (Newport) and a UV cut-off filter (λ > 420 nm) 300W Xe-lamp Xe lamp equipped with an AM 1.5G filter

N.A. photocurrent density of 5.99 mA cm−2 at 1.23 V vs. RHE

2.2 mA cm at 0 V vs Ag/AgCl 250 mmol h1 cm1 at 0 V vs. Ag/AgCl ~1.6 mA/Cm2 photocurrent at 1.8 V vs Ag/AgCl ~0.6 mA/Cm2 photocurrent photocurrent at 1 V vs Ag/AgCl

300W Xe-lamp (λ > 750 nm)

CQD/CdS-TiO2

−2

4 mA cm−2 at 60V vs SCE and 150 mmol cm−2 h−1 H2 production rate at E aap = 408 mV (for tube length = 13.5 μm) 12.1 μA cm−2 at 0V vs Ag/AgCl 34 ± 5 μlh−1 cm−2 hydrogen generation rate 160 μA cm−2 at 0 V vs Ag/AgCl

AM 1.5G (100 mW cm−2)

CQD/TiO2 NTs

−2

1.05 mA cm−2 at 0 V vs Ag/AgCl 0.16 mA cm−2 at 1 V vs Ag/AgCl ~2.5 mA cm−2 at 0–1.35 V vs RHE

CQD-sensitized TiO2

AM 1.5G (100 mW cm−2) AM 1.5G (100 mW cm−2) 300W Xe-lamp

at 0.4 V vs Ag/AgCl

CQD/Cu2S CQD/ZnO:Ga/ZnO CQD/BiVO4

at 0 V and 8.5 mA cm

12 mA cm−2 with photocurrent decay time of 1.75 s

0.6 mA cm

−2

−0.4 to 0.4 V 100 mW cm visible lamp slow scan rate of 0.05mVsec-1 150 W visible lamp (wavelength 400–800 nm)

−2

100 mW cm−2 visible lamp −2

~0.35 mA cm−2 at ~0.8 V vs Ag/AgCl

100 mW/cm−2 simulated light −0.9 to 0.5 V and 100 mW cm−2 visible lamp

CQD-sensitized TiO2 CQD-sensitized TiO2 NTs

CQD-sensitized TiO2 NR arrays CQD-sensitized TiO2

0.05 mA per cm photocurrent and 22 μl h H2 production rate at 0.4 V 1.2 mA per cm−2 at 0 V vs Ag/AgCl 1 mA cm−2 at 0 V, 14 μ mol h−1 H2 production rate

−0.1–0.6 V and λ > 400 nm irradiation

−1

CQD-sensitized TiO2

−2

Output

Input

Photocatalyst

Table 5 Summary of the latest works carried out for H2 generation from water using the PEC method.

N.A. IPCE = 2.29%

IPCE = 0.86%

1.45% STH Photoconversion efficiency = 0.36%

N.A.

0.148 N.A. Photo conversion efficiency 0.35% at 0.8 V vs RHE 2.47% at E aap = 408 mV (for tube length = 13.5 μm) N.A.

N.A.

N.A.

(IPCE)1.2–3.4%

N.A. Faradaic efficiency 100%

N.A.

Efficiency

Yu et al. (2013b) Zou et al. (2016b)

Xu et al. (2016)

Dubey et al. (2017) Zhang et al. (2013b) Qin and Zeng (2017) Zou et al. (2016a) Liu et al. (2014c)

Zhang et al. (2015c) Li et al. (2016b) Xiao et al. (2017) Nan et al. (2015)

Sang et al. (2017)

Yu et al. (2013a) Bian et al. (2014b) Bian et al. (2014c)

Xie et al. (2014c)

Ref

A. Mehta, et al.

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limitations: Even though a broad range of synthetic scheme has been developed for synthesizing CQDs, still there is a need for profound indulgent of relationship between the method of synthesis and opto-electronics as well as photoelectrochemical parameters of CQDs. The understanding between these factors is required for an improved design of CQDs-based PEC H2 production. Bottom-up processes have been extensively used for the fabrication of CQDs due to their simplicity; however, some bottomup approaches generate CQDs with wide size distributions and irregular nanostructures. This renders CQDs useless for photocatalytic activity, photocatalytic and PEC water splitting. Some top-down strategies have also been employed for the synthesis of CQDs with regular structure and uniform size distribution, but their yield is too low and these often require harsh conditions hindering their use for practical purposes. CQDs tend to provide more efficient solutions to metal-free photocatalysts for H2 production, but most of these require noble metal cocatalysts. In addition, modification of CQDs with an optimum functional group can generate a great amount of active sites required for enhanced photocatalytic activity, photocatalytic and PEC water splitting. Low efficiency of CQDs in water splitting is a huge challenge for which band gap engineering of CQDs could be an appropriate approach. In any case, further investigations are needed to understand the relationship between the structure and electronic energy levels, surface adsorption of reactive molecules, energy transmit processes, cause of diverse doping of ions on the distribution of charge, function of surface functional group in photocatalytic activity, photocatalytic and PEC water splitting.

approaches and techniques that are capable of precisely manipulating the extent of defects in CQDs are still unavailable. Thus, more efforts are needed to be devoted to well-defined and atom-precise structural synthesis of CQDs. Lastly, the up-conversion photoluminescence property of CQDs has been initiated to co-operate a significant role in enhancing the wide spectrum absorption and photo-induced charge transmit of g-C3N4 for H2 generation together with surface plasmon resonance (SPR) phenomenon of Ag NPs. Both CQDs and Ag NPs behave as an electron reservoir improving the severance of electrons and holes, quantum effectiveness of g-C3N4 and enabling their activity in the NIR region (Qin and Zeng, 2017). CQDs trap electrons generated from Cu NPs, resulting in the suppression of hole-electron recombination. The SPR cause CuNPs broadens the photoactivity spectrum, which is accomplished under monochromatic light evolution at 900, 800 and 700 nm. The synergistic effect of CQDs has also been reported on the overall water splitting capability of CQD/Ag/Ag3PW12O40 nanocomposite photocatalyst (Zhang et al., 2017b). CQDs not only enhanced the electron transfer, but also protected the dissolution of Ag3PW12O40 in water (Liu et al., 2014a). CQDs have also been found to photosensitize molecular H2 evolution from nickel bis(diphosphine) hybrid catalyst having earth plentiful metals with photo-activity in aqueous media. This has widely demonstrated the potential capability of CQDs to attract visible and UV light to directly transport photo-excited electrons to the solution base catalysts and acceptors. 5.4. Reaction setup for photocatalytic H2 production and degradation of organic pollutants

7. Conclusions and future outlook Reactions are performed in a test tube containing water with a bare or metal-loaded CQD catalyst of different shapes and sizes, and irradiated with UV/visible light source under constant magnetic stirring for different time intervals. The reaction samples are analyzed by GC. A 250 W halogen (tungsten) lamp serves as the light source to provide visible light within the range of 400–1100 nm using 125 W Hg arc lamp as the UV light source. The source set up was extended with a thin fiber probe of length 1 m and a diameter of 1.4 cm terminated with an attachment of a lens (0.6 cm dia) for the passage of light. The gas tight Pyrex test tube (20 cm length and 2.2 cm dia) containing water and photocatalyst was irradiated along with the 1D under continuous stirring (Fig. 7).

Majority of reviews published in this area have focused on the synthetic methods of CQDs and characterization, but very few of them have discussed about water and energy applications. In this context, diverse water treatment and splitting features of CQDs-based materials are discussed here. Since the discovery of carbon-based materials, particularly graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO) have inspired intensive research efforts for a range of disciplines comprising chemistry, material science, physics, and nanotechnology. However, researchers realized many limitations of graphene, such as zero band-gap and low absorptivity. Surface modification, doping as well as reducing lateral dimensions of carbon materials into nanoribbon and/or quantum dots (QDs) are considered the leading approaches to deal with its band-gap phenomena. Moreover, CQDs possess a large number of extraordinary properties in comparison to other toxic heavy metal quantum dots and

6. Limitations and challenges Despite numerous features, carbon quantum dots suffer some

Fig. 7. Reaction setup for photocatalytic water treatment/splitting applications. 11

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contrary to other carbon nanostructures; CQDs can be synthesized and functionalized quite fast and easily. Combined with the exceptional optical features of CQDs and electrical characteristics of sp2 hybridized carbon, makes them discernible from the conventional semiconductor quantum dots as well as other carbon materials. Herein, we briefly discussed three main applications of CQDs: the first one is exclusion of heavy metal ions through water by utilizing its unique sensing abilities. In this technique, photo-physical properties of CQDs such as their fluorescence enable the development of highly sensitive sensing platform. Remaining are the photocatalytic degradation and water splitting applications, which make the prefect utilization of CQDs electron donor-acceptor properties under light illumination. The role played by CQDs in charge transfer mechanism is extremely profitable for the development of CQDs-based photocatalytic materials. The proposed mechanism and general reaction setup for these applications have also been briefly discussed in this review. Considering the future prospective of CQDs, they can play a significant role in addressing the environmental water pollution issues and solar energy conversion. But there is a need for deeper understanding of the surface activity relationships in CQDs-based systems, which is also highlighted in this review. Improvement in the molecular design of CQDs is also necessary for improving water treatment/splitting activates. Conflict of interests The authors declare no intellectual as well as financial conflict of interests. Acknowledgments Authors acknowledge the support from King Fahd University of Petroleum and Minerals (KFUPM) [Grant # DF181006]. References Andrews, C.J., Govil, S., 1995. Becoming proactive about environmental risks: regulatory reform and risk management in the US electricity sector. Energy Policy 23, 885–892. Bera, S., Ghosh, D., Dutta, A., Bhattacharya, S., Chakraborty, S., Pradhan, N., 2019. Limiting heterovalent B-site doping in CsPbI3 nanocrystals: phase and optical stability. ACS Energy Lett. 4, 1364–1369. Bergren, M.R., Palomaki, P.K., Neale, N.R., Furtak, T.E., Beard, M.C., 2016. Size-dependent exciton formation dynamics in colloidal silicon quantum dots. ACS Nano 10, 2316–2323. Bian, J., Huang, C., Wang, L., Hung, T., Daoud, W.A., Zhang, R., 2014a. Carbon dot loading and TiO2 nanorod length dependence of photoelectrochemical properties in carbon dot/TiO2 nanorod array nanocomposites. ACS Appl. Mater. Interfaces 6, 4883–4890. Bian, J., Huang, C., Wang, L., Hung, T., Daoud, W.A., Zhang, R., 2014b. Carbon dot loading and TiO2 nanorod length dependence of photoelectrochemical properties in carbon dot/TiO2 nanorod array nanocomposites. ACS Appl. Mater. Interfaces 6, 4883–4890. Bian, J., Huang, C., Wang, L., Hung, T., Daoud, W.A., Zhang, R., 2014c. Carbon dot loading and TiO2 nanorod length dependence of photoelectrochemical properties in carbon dot/TiO2 nanorod array nanocomposites. ACS Appl. Mater. Interfaces 6, 4883–4890. Brus, L.E., 1984. Electron–electron and electron hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state. J. Chem. Phys. 80, 4403–4409. Busaidi, Baawain, M., Sana, A., Ebrahimi, A., Omidvarborna, H., 2019. Sustainable riskbased analysis towards remediation of an aquifer impacted by crude oil spills. J. Environ. Manag. 247, 333–341. Chakraborty, I.N., Roy, S., Devatha, G., Rao, A., Pillai, P.P., 2019. InP/ZnS Quantum dots as efficient visible-light photocatalysts for redox and carbon–carbon coupling reactions. Chem. Mater. 317, 2258–2262. Chandra, S., Chowdhuri, A.R., Mahto, T.K., Laha, D., Sahu, S.K., 2017. Sulphur and nitrogen doped carbon dots: a facile synthetic strategy for multicolour bioimaging, tiopronin sensing, and Hg2+ ion detection. Nano-Struct. Nano-Objects 12, 10–18. Chen, D.-H., Chen, C.-J., 2002. Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions. J. Mater. Chem. 12, 1557–1562. Chen, J., Zheng, A., Gao, Y., He, C., Wu, G., Chen, Y., et al., 2008. Functionalized CdS quantum dots-based luminescence probe for detection of heavy and transition metal ions in aqueous solution. Spectrochim. Acta A Mol. Biomol. Spectrosc. 69 (3), 1044–1052. Chen, G., Song, F., Xiong, X., Peng, X., 2013. Fluorescent nanosensors based on

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