Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles

Journal Pre-proof Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles Mohammed Ismael

PII:

S2213-3437(20)30024-5

DOI:

https://doi.org/10.1016/j.jece.2020.103676

Reference:

JECE 103676

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

9 December 2019

Revised Date:

4 January 2020

Accepted Date:

9 January 2020

Please cite this article as: Ismael M, Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103676

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.

Manuscript

Title: Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles Author: Mohammed Ismael

Journal of Environmental Chemical Engineering

of

To appear in:

ro

Received date: Revised date:

re

-p

Accepted date:

ur na

Title of paper:

lP

MANUSCRIPT COVER PAGE FORM

Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles Corresponding Author:

Jo

Mohammed Ismael

Full Mailing Address: [email protected], [email protected] Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky Str. 9-11, 26129 Oldenburg, Germany

Enhanced photocatalytic hydrogen production and degradation of organic pollutants from Fe (III) doped TiO2 nanoparticles Mohammed Ismael1* 1

Institute of Chemistry, Technical Chemistry, Carl von Ossietzky University Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany

lP

re

-p

ro

of

Graphical abstract

ur na

Highlights  Precipitation method was used for the synthesis of Fe doped titania materials  The photocatalytic activity of Fe doped TiO2 for hydrogen production was evaluated  Their photocatalytic activity for degradation of organic pollutants was tested  0.1 mol % Fe-TiO2 shows the highest activity for both reactions  The origin of the high photoactivity of Fe doped titania was discussed in detail.

Jo

Abstract

In the present work, bare TiO2 and different molar concentrations of iron (Fe3+)-doped TiO2 photocatalysts (0.05-1 mol % Fe) were prepared by a simple precipitation method for photocatalytic hydrogen production and degradation of organic pollutants (MO and 4-CP) under light irradiation of λ ≥ 320 nm. XRD revealed the presence of anatase phase for both bare TiO2 and Fe doped titania. Based on the XPS results, Fe3+ ions are hardly seen may be attributed to the location of iron inside the titania matrix rather than their appearance at the surface. UV-vis-DRS of the doped titania demonstrated a redshift toward visible region owing to the reduced bandgap energy. N2 adsorption results showed that the doped materials have higher surface area compared to bare TiO2 due to decrease particle size. Importantly, electrochemical experiments (electrochemical impedance spectroscopy (EIS),

photocurrent (PC), and photoluminescence (PL)) revealed that higher photogenerated charge carrier separation efficiency of iron-doped titania. Results showed that the 0.1 mol % Fe doped TiO 2 exhibited the highest photocatalytic activity for both degradation and H2 evolution reactions. Furthermore, high surface area, small particle size and enhanced visible-light absorption as well as improved charge transfer and separation are believed to be responsible for the improvement of photocatalytic activity of the doped materials.

Keywords: Photocatalysis, iron-doped materials, photodegradation, hydrogen production, charge

of

separation

1. Introduction

ur na

lP

re

-p

ro

Photocatalysis technology has attracted great research interest because it offers a sustainable pathway to solve the world energy crisis and environmental problems [1-3]. In 1972, the pioneering work of Japanese scientists Fujishima and Honda for photodecomposition of water to hydrogen and oxygen on illuminated TiO2 electrode [4], numerous works have been applied on different types of photocatalysis, such as including oxides, sulfide, and oxynitride and their application for hydrogen production [5-7]. Recently, oxides semiconductors are popular photocatalyst to facilitating water splitting and degradation of organic contamination in the wastewater, this is due to their high chemical stability under most operating conditions, and simple synthesis, which it does not require vacuum facilities [8]. Titania (TiO2) as a popular n-type semiconductor photocatalyst has been extensively studied thanks to its chemical, thermal stability, nontoxicity, and low-cost [9]. However, exhibits nonsatisfactory photocatalytic efficiency due to the large bandgap values, as well as the high recombination rate of photogenerated charge carriers, limit its large scale applications in photocatalysis [10,11]. Hence, fabricating and designing doped TiO 2 materials to facilitate charge separation efficiency and improve light absorption have been suggested to enhance photocatalytic activity. Recently various modification approaches have been taken to increase the photocatalytic efficiency of TiO2, such as doping with transition metal ions (Fe, Sn, Nb) [12-14], or nonmetal elements (N, B) [15,16] and nanocomposite structure formation [17,18].

Jo

Nowadays, several transition metal ions were used as dopants, such as V, Ni, Cr, Mo, Fe, Co and Sn [19-25]. Among them, doping of Fe3+ ion in the matrix of titania is most favored owing to the similar size of Ti4+ ions, and its low bandgap energy [26,27]. In addition, Fe3+ cations can act as a sinker of photogenerated e-/h+ pairs in the titania lattice, leading to improve charge separation and as a result, enhanced the photoactivity [28]. Recently, Ismael et al [29] tested hydrogen production activity of various concentrations of Ru doped TiO2 photocatalysts. 0.1 Wt % Ru shows the highest activity assigned to the efficient charge separation and the reduced bandgap energy. Many studies were reported on iron-doped TiO2 synthesized via different methods and their applications in photocatalysis are summarized in Table 1. Table 1 Comparison of typical iron-doped TiO2 photocatalysts reported for different photocatalytic activity under different conditions

Fe doped TiO2 [30]

Synthesis method Ultrasonic assisted hydrothermal method

Photocatalytic reaction Degradation of Pnitrophenol

Porous, microsphere Fe doped TiO2 [31]

Hydrolysis hydrothermal method

Degradation of MO

Fe substituted anatase TiO2 [32]

Solution combustion method

Degradation of 4nitrophenol

UV, Solar irradiation

Quantum, nanosized Fe doped TiO2 [33]

Acid-catalyzed Sol-gel method

Transformati on of chexane into c-hexanol

150 W Xe lamp λ > 400 nm

Fe doped TiO2 [34]

Metal-ion implantation method

Degradation of 2propanol

Hydrogen production

Ag deposited Solvothermal Fe doped TiO2 method [37]

Hydrogen production

ur na

lP

Cr or Fe doped Radiofrequency TiO2 [35] magnetron sputtering + sol-gel method Fe doped TiO2 Microwavenanotubes Ag assisted chemical nanoparticles reduction [36]

Hydrogen production

Reaction conditions Visible light irradiation

-p

ro

of

UV/Vis light irradiation

re

Photocatalyst

Visible light, λ > 450 nm

Visible light

Visible light

Visible light

Jo

However, there are several methods for the preparation of Fe doped TiO2 catalysts such as the Microwave method [38], hydrothermal [39], sol-gel method [40], thermal hydrolysis [41] and wet impregnation methods [42]. Nevertheless, most of these available methods have some drawbacks, such as low production yield, use of some toxic, expensive chemicals and need special equipment. Therefore, the development of a new simple, efficient and low-cost method is still required. Precipitation method is a promising route used to prepare nanomaterial photocatalysts. The advantages of this method include small particle size of the obtained photocatalyst, simplicity, low cost, high homogeneity of the final product, low-temperature and control morphology, and large-scale synthesis of the products.

The objective of this work is to investigate the role of Fe doping prepared by a simple precipitation method on the average crystallite size and surface area, light absorption, bandgap, and photocatalytic activity of the synthesized photocatalyst. The synthesized materials were tested for hydrogen evolution and also for photodegradation of color MO and noncolor 4-CP. Furthermore, Investigations of the dopant concentration and charge transfer from metal dopants to TiO 2 were carried out in an attempt to enhance photocatalytic activity.

2. Experimental Materials, synthesis of photocatalysts, characterization, and photocatalytic activities for degradation of organic pollutants and hydrogen production are in the supplementary information.

of

3. Results and discussion

ro

3.1. XRD analysis

Jo

ur na

lP

re

-p

Powder X-ray was carried out to perform the crystallinity, phase structure, and the purity of the prepared photocatalysts. Fig. 1(a) shows the XRD diffraction patterns of undoped and doped titania calcined at 500 °C for 3 h. The pure TiO2 sample shows several distinct peaks at 25.1°, 38.2°, 48°, 53.1°, 55°, 62.9°, 68.9°, 70.1°, and 75.2° which can be indexed to (101), (004), (200), (105), (211), (204), (116), (220), (215) planes of anatase phase of TiO2 [43]. For doped photocatalysts, the XRD patterns show no peaks related to iron species were observed. These results are agreed with the observed peaks in Raman spectra analysis (Fig. S1). Nonetheless, roughly the same size of Ti4 + (0.74 A) and Fe3+ (0.69 A), so that iron ion can be easily introduced into the titanium lattice, It is assumed that the homogeneous distribution of iron in the titanium matrix and low iron concentration are responsible for the absence of iron peaks in the XRD patterns [44-46]. Fig. 1(b) shows The prominent anatase peak (101) slight shifts to the lower 2θ with an increase in the doping concentration attributable to the slight difference in the radii between the two metals. It is, however, different from that obtained by Ballyy et al, where the phase changes of titanian from anatase to rutile were obtained [47]. In order to investigate the influence of the particle size on the photocatalytic behavior of the photocatalyst, Debye-Scherrer equation was conducted on the anatase (101) diffraction peak to estimate the average particle size of photocatalysts [48], the calculated average crystallite sizes for undoped TiO2, (0.05 mol % Fe), (0.1 mol % Fe), (0.5 mol % Fe) and (1 mol % Fe) were 21, 19.8, 12.4, 15.5, and 17.6, respectively. This result reveals that increasing the concentration of Fe3+ ions leads to crystallite deformation and slows the growth of the TiO2 [49,50]. Furthermore, smaller particle size reduces the charge recombination, explaining the higher activity of 0.1 mol % Fe-TiO2. These results are agreed with the BET surface area (Fig. S2). This result is agreed with Miao et al [51] where irondoped materials were prepared by different synthesis methods and the immersion doping method was the best, which could be attributed to the smaller particle size obtained.

of

ro

Fig. 1 (a) XRD patterns of TiO2 and Fe doped TiO2 samples, A: anatase phase of TiO 2, and (b) effect of the amount of Fe dopant on the shift in XRD patterns due to the crystallite deformation 3.2. TEM analysis

Jo

ur na

lP

re

-p

The surface morphology and the fine structure of undoped TiO 2 and 0.1 mol % Fe-TiO2 photocatalysts were determined by TEM and HRTEM, respectively. (Fig. 2(a)) shows that the synthesized bare TiO 2 exhibits an irregular spherical shape with a diameter ranging from 20-25 nm. On the other hand, irondoped photocatalyst (Fig. 2(b)) has nearly the same morphology with smaller particle sizes in the range of 10-20 nm inconsistent with the XRD. The lattice fringe was observed in the HRTEM image of the two photocatalysts (doped and undoped), the interplanar spacing is 0.35 nm, which is very close to (101) planes of anatase phase of TiO 2, in accordance with the XRD results [52]. The selected area electron diffraction (SAED) pattern for both photocatalysts is also shown in Fig. 2(e) and (f). The five rings are assigned as (101), (004), (200), (105), (211) are matched very well with the XRD data, confirming the polycrystalline nature of the anatase TiO2 nanoparticles [53].

Fig. 2 TEM images of (a) undoped TiO2, and (b) 0.1 mol % Fe-TiO2, HRTEM of (c) undoped TiO2, and (d) 0.1 mol % Fe-TiO2, and selected area electron diffraction image for (e) undoped TiO2 and (f) 0.1 mol % Fe-TiO2 3.3. XPS analysis

Jo

ur na

lP

re

-p

ro

of

Electronic environment, surface chemical composition and oxidation states of 0.1 mol Fe-TiO2were investigated using XPS. As presented in Fig. 3(a), The survey scan XPS spectra provide the O 1s, Ti 2p, C 1s and Fe 2p peaks for 0.1 mol Fe-TiO2, confirming the existence of these elements on the surfaces of the doped photocatalyst. The C 1s peak in the survey spectrum at the binding energy of 284.8 eV arising from adventitious carbon or external carbon contamination [54]. Nevertheless, the peaks due to the iron ion in the survey spectra were hardly seen for the same reasons for the XRD. In addition, iron has also been confirmed using EDX spectra and elemental mapping (Fig. S3). Iron ion was clearly displayed and dispersed homogenously throughout the entire nanoparticle of TiO 2. Fig. 3(b) shows the XPS region for Ti 2p, two peaks at a binding energy of 464.5 (Ti 2p1/2 ) and 458.7 eV (Ti 2p3/2) are observed for the doped photocatalyst [55]. For the high-resolution XPS spectra of O 1s, there are two signals at a binding energy of 529.9 and 531.6 eV (Fig. 3(c)) is associated with the Ti-O bond or to the lattice oxygen in TiO2 and H2O [56]. Fig. 3d presents two peaks for Fe 2p, one at 710.2 eV corresponded to Fe 2p3/2 and another one located at 723.5 eV ascribed to the Fe 2p1/2 [57,58].

Fig. 3 (a) Survey spectra of 0.1 mol % Fe-TiO2, and high resolution XPS spectra of 0.1 mol % FeTiO2, (b) Ti 2p, (c) O 1s, and (d) Fe 2p.

3.4. U.V-vis analysis The optical properties of as-prepared TiO2 and all Fe doped TiO 2 ((0.05 mol % Fe) - (1 mol % Fe)) samples were probed by UV-vis spectrometer. As displayed in Fig. 4(a), the main absorption peak of the prepared pure TiO2 and doped photocatalysts at around 400 nm occurred due to the charge-transfer from the VB to the CB. In addition, The doping of Fe3+ on TiO2 generates an impurity energy level in

-p

ro

of

the forbidden band and narrows the bandgap, resulting in an enhancement in light absorption compared to the pure TiO2 [59]. Furthermore, It was observed that the visible light absorption was significantly enhanced with increasing the contents of iron, accompanied by changes in color from white (pure TiO2), to white-yellow (Fe doped TiO2). Moreover, the bandgap energy of TiO 2 and different Fe doped TiO2 were estimated from the Tauc plot through the Kubelka-Munk equation [60]. As presented in Fig. 4(b), the bandgap energies of TiO 2 and different Fe doped TiO 2 ((0.05 mol % Fe) - (1 mol % Fe)) samples were 3.2, 3.05, 3.0, 3.1, 2.9, 2.85, 2.75 and 2.6 eV respectively.

re

Fig. 4 (a) UV-vis spectra of TiO2 and different Fe doped TiO2 ((0.1 mol % Fe-TiO2) - (1 mol % FeTiO2)) samples, and (b) the resulting Tauc plots for bandgap determination.

lP

3.5. The electrochemical measurements (Mott Schottky plot, Photocurrent, and Electrochemical impedance measurement)

Jo

ur na

The Mott-Schottky plots were plotted to estimate the flat band potential of pure TiO2 and 0.1 mol % Fe-TiO2 are displayed in Fig. 5(a)). The two photocatalysts present the positive slope of the plot indicate that both are of n-type semiconductors [61]. Moreover, the flat band potential of pure TiO 2 and (0.1 mol % Fe-TiO2) sample were -1.1 V and -1.19 V (vs. Ag/AgCl), respectively and were equivalent to -0.65 V and -0.69 V (vs. NHE) [62,63]. It has well known that for the n-type semiconductor, the 𝐸𝑓𝑏 is located close to the bottom of the conduction band; it is usually assumed that CB is 0.1 V more negative than 𝐸𝑓𝑏 [64], resulting in CB edges at about -0.55 V and -0.59 V vs. normal hydrogen electrode (NHE), respectively. Comparing with the flat band potential of TiO2, the flat band potential of 0.1 mol % Fe-TiO2 was negatively shifted, confirming the larger accumulation of electrons in the 0.1 mol % Fe-TiO2 and reflects decreased charge recombination [65]. The VB position is accordingly calculated by adding the value of the bandgap measured from Tauc plot, the VB can be calculated according to the equation 𝐸𝑣𝑏 = 𝐸𝑐𝑏 + 𝐸𝑔 [66] resulting in about Evb = 2.65 V and 2.41 V for TiO2 and 0.1 mol % Fe-TiO2, respectively. Photocurrent (PC) is widely accepted methods used to confirm the efficiency of the synthesized photocatalysts in hindering charge carriers' recombination. It can be seen from Fig. 5b that 0.1 mol % Fe-TiO2 sample exhibited a much higher photocurrent than pure TiO2 indicating the better capability of 0.1 mol % Fe-TiO2 in transferring and separating electron-hole pairs [67], this result is consistent with the photoluminescence measurement (Fig. S4). Furthermore, EIS is also an effective method for determining the efficiency of the charge separation at the interface. Fig. 5(c) shows that the electrode

lP

re

-p

ro

of

of (0.1 mol % Fe-TiO2) has a smaller arc size that means that the electrode surface is less charging resistant [68]. These results clearly indicated that insertion iron ion as a metal dopant can improve the charge separation efficiency of the electrode. XPS is also regarded as a powerful tool for estimating the position of the valence band on the semiconductor surface. As presented in Fig. 5(d), the valence band edge position of the TiO2 and 0.1 mol % Fe-TiO2 are 2.65 and 2.4 eV, respectively, which agreed with the Mott-Schottky measurements.

ur na

Fig. 5 (a) Mott-Schottky plots measured in the dark at frequencies 100 Hz and 1 kHz, (b) Photocurrent response spectra in Na2SO4/0.1 M Na2SO3 electrolyte solution measured at pH 5.6 under white LED illumination, (c) Nyquist plot in 0.1 M Na 2SO4 at pH 5.6 vs. Ag/AgCl, and (d) VBXPS band edge position of pure TiO2 and (0.1 mol % Fe-TiO2) photocatalysts. 3.6. Photocatalytic activities of the photocatalyst toward H 2 production and organic pollutants degradation 3.6.1 Photocatalytic hydrogen production

Jo

The photocatalytic activity performance for all prepared photocatalysts was assessed for photocatalytic H2 evolution reaction under light irradiation of λ ≥ 320 nm. To obtain satisfactory activity, 10% vol. methanol as a sacrificial reagent to trap holes and 0.5 wt% Pt nanoparticle as cocatalyst and acts as an electron sinker. As can be seen in Fig. 6(a) Originally, the rate of hydrogen evolution increased by increasing the amount of Fe3+ in the first five samples (0.05-0.15% Fe) and then decreased for other samples (0.3-1% Fe). The 0.1 mol % Fe-TiO2 sample exhibits the optimal dosage of iron ion in the TiO2 matrix, where the charge separation is greatly enhanced and hydrogen evolution was at the highest (2423 μmol/h), which is higher than that mentioned in the literature (Table. 2). Nonetheless, by raising the dopant content, the likelihood of recombination of the electron/hole pairs is increased due to the two main reasons: (1) high metal ion concentration can occupy the photocatalyst's active site, which can lead to a reduction in the surface area and, as a result, photocatalytic activity decreased

[69]. (2) heavy doping results in a dense distribution of iron on the TiO2 surface, which can block the catalyst surface from light irradiation (shielding effect), delay the activation of TiO 2 by produced e-/h+ pairs and thus decrease the output of H2 [70,71]. Table 2 Comparison of iron-doped TiO2 photocatalysts reported for photocatalytic hydrogen activity under different conditions

Parameters

Ref

12.5 μmol/h

44

15.5 mmol/h

35

-p

Aqueous CH3OH, 300 W Xe lamp RF magnetron TiO2 (anode), Pt Fe-ion-doped TiO2 thin films sputtering, sol- mesh (cathode), gel and aqueous electrolytes, a 250-W tungsten halogen lamp 3+ lamp ordered Fe - electrochemical Xenon doped TiO2 anodic oxidation equipped with a 400-nm cut-off nanotube optical filter, arrays ethanol visible-light (Si,Fe)-codoped sol-gelsolvothermal irradiation, pure TiO2 method water solid-state Solar light, Fe-loaded approach glycerol TiO2/ g-C3N4 composite 300 W Xe lamp, Fe-loaded TiO2 Hydrothermal method methanol ball-flowers

H2 production

of

Fe3+-doped TiO2

Synthesis method Hydrothermal method

ro

Photocatalyst

ur na

lP

re

1.35 μmol/(cm2 72 h)

600 mmol/g

73

2820 μmol h-1 g- 74 1 cat 697 μmol/g

75

3.6.2 Photocatalytic degradation of organic pollutants (MO and 4-CP)

Jo

The photocatalytic activity of TiO2 and different Fe doped TiO2 was tested for photodegradation of MO and 4-CP under λ ≥ 320 is shown in Fig. 6(b). The self-degradation or pyrolysis of both 4-CP and MO can be negligible in the absence of the photocatalyst (Fig. 6 b&c). After 4 h of light irradiation, 30% of 4-CP and 45% of MO removal is achieved over the TiO2 sample (Fig. 6 b&c), indicating its moderate photocatalytic activity. Furthermore, the highest degradation rate of 4-CP (around 65%) and MO (around 95%) is obtained over 0.1 mol % Fe-TiO2. Fig. 6(d) shows the UV-vis spectra for the degradation of MO under light irradiation of λ ≥ 320 nm on 0.1 mol % Fe-TiO2, the result indicated that the concentration of MO under light irradiation decreased and there was no corresponding increase in the U.V region with increasing irradiation time, which ascribed to the fact that most of the aromatic structure was completely destroyed [76,77].

of ro -p

re

Fig. 6 (a) Photocatalytic H2 production using 500 W Hg lamp, (b) Photodegradation of 4-CP (c) and MO, over TiO2 and various Fe doped samples using 150 W Xe lamp, and (d) Absorption spectra of MO under light irradiation of λ ≥ 320 nm over 0.1 mol % Fe-TiO2 3.7. Effect of Catalyst Concentration and pH

Jo

ur na

lP

The effect of catalyst concentration on the photocatalytic hydrogen activity over 0.1 mol % Fe-TiO2 photocatalyst was shown in Fig. 7(a). Experiments performed with different concentrations showed that photocatalytic hydrogen production efficiency increases with an increase in 0.1 mol % Fe-TiO2 concentration up to 0.03g/L and is then decreased. There are two reasons explaining this behavior: (1) high concentration of catalyst loading can prevent light penetration, thereby delaying the activation of the photocatalyst (2) higher dose of photocatalyst may restrict the quantity of Pt nanoparticle, resulting in decreased the hydrogen evolution. Furthermore, In order to interpret the pH effect on the efficiency of the dye photodegradation process on 0.1 mol % Fe-TiO2 catalyst, the experiments were conducted at various pH ranging from 6 to 12. As shown in Fig 7(b), the degradation rate of MO increased and reached a maximum at pH 10 and then decreased to pH 11&12. At a pH greater than 10, the decrease in degradation efficiency may be correlated with the ionization state of catalyst surface (forming of TiOH2+ or TiO- is highly pH-dependent). Thus, at higher pH, the photocatalyst surface becomes negatively charged (TiO-), and the adsorption with the negative charge dye (MO-) does not occur due to the electrostatic repulsion between the same charge molecules, resulting in decreasing the photocatalytic degradation activity.

of

ro

Fig. 7 (a) Effect of 0.1 mol % Fe-TiO2 concentration on photocatalytic degradation of MO at irradiation time 4 h, (b) Effect of pH on photocatalytic degradation of MO at irradiation time 4 h 3.8. Stability of the catalyst and total organic carbon experiment

Jo

ur na

lP

re

-p

Fig. 8 (a) appears no self-evident changes from the XRD patterns of the reused photocatalyst. The result demonstrated that the 0.1 mol % Fe-TiO2 was a stable photocatalyst. Hence, the 0.1 mol % FeTiO2 had favorable stability and reusability during photocatalytic experiments. Furthermore, the mineralization of methyl orange was studied in terms of percentage removal of TOC with iron-doped TiO2. Within 4 h of visible light irradiation, the TOC removal of the methyl orange by 0.1 mol % FeTiO2 as a photocatalyst is 60% (Fig. 8(b)), which is consistent with the tendency shown in Fig. 6(c).

Fig. 8 (a) XRD patterns for reused 0.1 mol % Fe-TiO2 photocatalyst after 5 h photocatalytic reaction, (b) Total organic carbon (TOC) during photocatalytic degradation reaction of MO by 0.1 mol % FeTiO2 catalyst. 3.9. Proposed photocatalytic mechanisms 3.9.1 Photocatalytic hydrogen evolution mechanism Fig. 9a shows the schematic diagram of the photocatalytic reaction taking place on the surface of Fe (III) doped TiO2. Upon light irradiation on the surface of Fe doped TiO2, electrons and holes are generated [78]. Iron dopants act as a trapping center for photogenerated electrons and hole pairs and

of

may promote their lifetime [79]. Thus, the separation rate enhanced and, as a result, the photocatalytic hydrogen evolution improved. Moreover, the creation of impurity energy levels in the forbidden band of titania narrows its bandgap energy and make titania responds to the visible light [80]. Thus, Fe3+ traps electrons to form Fe2+ and trap holes to form Fe4+ ions respectively, due to the instability of the two iron species (Fe2+ and Fe4+) [31], the trapped charges can be easily released back to form stable Fe3+ ions. The released electrons react with H 2O or H+ to form hydrogen. The holes in the TiO 2 valence band have enough oxidizing power to convert CH3OH to CO2 and H2O. Also, the photogenerated electrons can transfer and accumulate on the Pt nanoparticles thanks to the formation of a Schottky barrier at the interface between Pt nanoparticles and titania. These platinum nanoparticles can act as a second site for hydrogen production and can reduce H2O or H+ to produce H2 [81]. 3.9.2 Photocatalytic degradation mechanism

Jo

ur na

lP

re

-p

ro

Photogenerated electrons and holes are generated due to the light absorption by the photocatalyst. The iron as electron and hole trap has the same effects as in the photocatalytic hydrogen production mechanism. Thus, the generated electrons are more negative (-0.59 V, estimated by Mott-Schottky plot, see section 3.5.) than the potential for the reaction (O2/.O2-, -0.046 V vs NHE) [82]. Thus they are good reductants and could capture the surface chemisorbed oxygen to superoxide radicals. Meanwhile, the holes in the valence band of Fe doped TiO2 are good oxidants and they have the ability to reduce hydroxyl anion to hydroxyl radical because the conduction band position is more positive (2.41 V) than the redox potential of.OH/-OH (1.99 V vs NHE) [83]. Furthermore, numerous holes stored on the VB of Fe doped TiO2 will oxidize MO/4-CP directly (Fig. 9b).

Fig. 9 (a) Proposed schematic of photocatalytic water splitting for hydrogen production in Fe doped TiO2 prepared by precipitation method, and (b) setup used. (c) Schematic diagram of the mechanism of photocatalytic degradation reaction, and (d) the setup used.

4. Conclusions

ur na

Author Contribution

lP

re

-p

ro

of

Bare TiO2 and Fe-doped TiO2 nanoparticles with the different mole ratios of iron were successfully fabricated by the facile, low-cost precipitation method. The XRD results revealed the presence of anatase phase in both undoped and doped titania. XPS results illustrated that Fe3+ found in the matrix of titania and not found on the surface that inconsistent with Raman spectra and XRD results. The N2 adsorption/desorption isotherms presented that the highest surface area was obtained for the doped materials with the optimum iron dopant that fully agreed with the particle size estimated by the XRD. Electrochemical experiments (PC and EIS) showed efficient charge transfer and separation of doped materials compared to bare TiO2. Furthermore, the flat band potential of the doped material estimated by the Mott-Schottky analysts is more negative than the bare TiO2. UV-vis-DRS spectra displayed a redshift owing to the reduced bandgap energy with the increase of iron dopant concentration. The photocatalytic properties of photocatalysts were investigated for hydrogen production using methanol as a sacrificial reagent and Pt as a cocatalyst and for photodegradation of MO and 4-CP under light irradiation of λ ≥ 320 nm. The optimal iron content in the doped titania is 0.1 mol % Fe shows the highest hydrogen evolution rate (2423 μmol/h) and degradation efficiency (65% for 4-CP, and 95% for MO), this enhancement is due to (1) a decrease of bandgap energy, (2) large surface area and smaller particle size, and (3) efficient charge transfer and separation. The precipitation method as an easy, low cost, can be applied to dope and stabilize transition metal dopants in the titania matrix, which may open the alternative way for the synthesis of efficient photocatalysts for different photocatalytic applications.

Mohammed Ismael (as the only and corresponding author in the manuscript) performed all things, including the catalyst preparation, structural, morphological, electrochemical characterization (XRD, BET, DRS, XPS, and SEM), and photocatalytic test experiments (Degradation and hydrogen production experiments). Mohammed Ismael analyzes the results and wrote the whole manuscript.

Jo

Conflicts of interest

There are no conflicts to declare.

Acknowledgments We thank the Phoenix Scholarship program (PX14DF0164) for a Ph.D. scholarship for Mohammed Ismael. I acknowledge Professor Wark, a group leader for a productive discussion.

References [1] A. Ismail, D. W. Bahnemann, Photochemical splitting of water for hydrogen production by photocatalysis: A review, Sol. Energy Mater. Sol. Cells, 128 (2014) 85-101. [2] M. Pelaez, N. T. Nolan, S. C. Pillai, M. K. Seery, P. Falaras, A. G. Kontos, P. S. M. Dunlop, J. W. J. Hamilton, J. A. Byrne, K. Ó. Sea, M. H. Entezari, D. D. Dionysiou, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B: Environ, 125 (2012) 331-349.

of

[3] G. H. Liu, N. Hoivik, K. Y. Wang, H. Jakobsen, Engineering TiO2 nanomaterials for CO2 conversion/solar fuels, Sol. Energy Mater. Sol. Cells, 105 (2012) 53-68.

ro

[4] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 238 (1972) 37-38. [5] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature, 414 (2001) 625-627.

re

-p

[6] I. Tsuji, H. Kato, A. Kudo, Chem. Mater, Photocatalytic Hydrogen Evolution on ZnS-CuInS2AgInS2 Solid Solution Photocatalysts with Wide Visible Light Absorption Bands, 18 (2006) 19691975.

lP

[7 ] K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN: ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting, J. Am. Chem. Soc, 127 (2005) 8286-8287. [8] V. M. Aroutiounian, V. M. Arakelyan, G. E. Shahnazaryan, Metal oxide photoelectrodes for hydrogen generation using solar radiation-driven water splitting, Sol Energy, 78 (2005) 581-592.

ur na

[9] C. Kim, M. Choi, J. Jang, Nitrogen-doped SiO2/TiO2 core/shell nanoparticles as highly efficient visible light photocatalyst, Catal. Commun, 11 (2010) 378-382. [10] H. Li, X. Duan, G. Liu, X. Liu, Photochemical synthesis and characterization of Ag/TiO 2 nanotube compositesJ. Mater. Sci, 43 (2008) 1669-1676.

Jo

[11] J. C. Colmenares, M. A. Aramendia, A. Marinas, J. M. Marinas, F. J. Urbano, Synthesis, characterization and photocatalytic activity of different metal-doped titania systems, Appl. Catal. A, 306 (2006) 120-127. [12] H. X. Wang, G-J. Li, H. Kamiyama, Y. Moriyoshi, T. Ishigaki, Wavelength-Sensitive Photocatalytic Degradation of Methyl Orange in Aqueous Suspension over Iron(III)-doped TiO2 Nanopowders under UV and Visible Light Irradiation, J Phys. Chem B, 110 (2006) 6804-6809. [13] J. Chattopadhyay, H. R. Kim, S. B. Moon, D. Park, Performance of tin doped titania hollow spheres as electrocatalysts for hydrogen and oxygen production in water electrolysis, Int J Hydrog Energy, 33 (2008) 3270-3280.

[14] A. V. Emeline, Y. Furubayashi, X. Zhang, M. Jin, T. Murakami, A. Fujishima, Photoelectrochemical Behavior of Nb-Doped TiO2 Electrodes, J Phys. Chem B, 109 (2005) 2444124444. [15] S. Z. Chen, P. Y. Zhang, D. M. Zhuang, W. P. Zhu, Investigation of nitrogen doped TiO 2 photocatalytic films prepared by reactive magnetron sputtering, Catal. Commun, 5 (2004) 677-680. [16] F. Dong, W. R. Zhao, Z. B. Wu, Characterization and photocatalytic activities of C, N and S codoped TiO2 with 1D nanostructure prepared by the nano-confinement effect, Nanotechnology, 19 (2008) 365607-365616.

of

[17] S. K. Poznyak, D. Talapin, A. Kulak, Structural, Optical, and Photoelectrochemical Properties of Nanocrystalline TiO2-In2O3 Composite Solids and Films Prepared by Sol-Gel Method, J. Phys. Chem. B, 105 (2001) 4816-4823.

ro

[18] Y. Liu, F. Xin, F. Wang, S. Luo, X. Yin, Synthesis, characterization, and activities of visible light-driven Bi2O3-TiO2 composite photocatalysts, J. Alloys Compd, 498 (2010) 179-184.

-p

[19] S-m. Chang, W-szu. Liu, Surface doping is more beneficial than bulk doping to the photocatalytic activity of vanadium-doped TiO2, Appl. Catal. B: Environ, 101 (2011) 333-342.

re

[20] D. Jing, Y. Zhang, L. Guo, Study on the synthesis of Ni doped mesoporous TiO2 and its photocatalytic activity for hydrogen evolution in aqueous methanol solution, Chem. Phys. Lett. 415 (2005) 74-78.

lP

[21] Y-H. Peng, G-F. Huang, W-Q. Huang, Visible-light absorption and photocatalytic activity of Crdoped TiO2 nanocrystal films, Adv. Powder. Technol, 23 (2012) 8-12.

ur na

[22] S. Wang, L. N. Bai, H. M. Sun, Q. Jiang, J. S. Lian, Structure and photocatalytic property of Modoped TiO2 nanoparticles, Powder. Technol, 244 (2013) 9-15. [23] D. H. Kim, H. S. Hong, S. J. Kim, J. S. Song, K. S. Lee, Photocatalytic behaviors and structural characterization of nanocrystalline Fe-doped TiO2 synthesized by mechanical alloying, J. Alloys Compd, 375 (2004) 259-264. [24] D. Chen, Z. Jiang, J. Geng, Q. Wang, D. Yang, Carbon and Nitrogen Co-doped TiO2 with Enhanced Visible-Light Photocatalytic Activity, Ind. Eng. Chem. Res. 46 (2004) 2741-2746.

Jo

[25] J. Liqiang, F. Honggang, W. Baiqi, W. Dejun, X. Baifu, L. Shudan, S. Jiazhong, Effects of Sn dopant on the photoinduced charge property and photocatalytic activity of TiO 2 nanoparticles, Appl. Catal. B: Environ, 62 (2006) 282-291. [26] Y. Zhang, Y. Shen, F. Gu, M. Wu, Y. Xie, Influence of Fe ions in characteristics and optical properties of mesoporous titanium oxide thin films, Appl. Surf. Sci, 256(1) (2009) 85-89. [27] K. Vijayalakshmi, S. D. Jereil, Influence of Fe catalytic doping on the properties of TiO2 nanoparticles synthesized by microwave method, J Mater Sci: Mater Electron, 25 (2014) 5089-5094.

[28] X. Zhang, L. Lei, One step preparation of visible-light responsive Fe-TiO2 coating photocatalysts by MOCVD, Mater. Lett, 62 (2008) 895-897. [29] M. Ismael, High effective ruthenium-doped TiO2 nanoparticles photocatalyst for visible-lightdriven photocatalytic hydrogen production, New J. Chem, 43 (2019) 9596. [30] S. Sood, A. Umar, S. K. Mehta, S. K. Kansal, Highly effective Fe-doped TiO2 nanoparticles photocatalysts for visible-light driven photocatalytic degradation of toxic organic compounds, J. Colloid. Interface. Sci, 450 (2015) 213-223.

of

[31] T. Tong, J. Zhang, B. Tian, F. Chen, D. He, Preparation of Fe3+-doped TiO2 catalysts by controlled hydrolysis of titanium alkoxide and study on their photocatalytic activity for methyl orange degradation, J Hazard Mater, 155 (2008) 572-579.

ro

[32] K. Nagaveni, M. S. Hegde, G. Madras, Structure and Photocatalytic Activity of Ti1-xMxO2±δ (M = W, V, Ce, Zr, Fe, and Cu) Synthesized by Solution Combustion Method, J Phys. Chem B, 108 (2004) 20204-20212.

-p

[33] X. Li, P. L. Yue, C. Kutal, Synthesis and photocatalytic oxidation properties of iron doped titanium dioxide nanosemiconductor particles, New J Chem, 27 (2003) 1264-1269.

re

[34] H. Yamashita, M. Harada, J. Misaka, M. Takeuchi, B. Neppolian, M. Anpo, Photocatalytic degradation of organic compounds diluted in water using visible light-responsive metal ion-implanted TiO2 catalysts: Fe ion-implanted TiO2, Catal Today, 84 (2003) 191-196.

lP

[35] R. Dholam, N. Patel, M. Adami, A. Miotello, Hydrogen production by photocatalytic watersplitting using Cr-or Fe-doped TiO2 composite thin films photocatalyst, Int. J. Hydrog. Energy, 34 (2009) 5337-5346.

ur na

[36] X. Fan, J. Fan, X. Hu, E. Liu, L. Kang, C. Tang, Y. Ma, H. Wu, Y. Li, Preparation and characterization of Ag deposited and Fe doped TiO2 nanotube arrays for photocatalytic hydrogen production by water splitting, Ceramics Int, 40 (2014) 15907-15917. [37] T. Sun, E. Liu, J. Fan, X. Hu, F. Wu, W. Hou, Y. Yang, L. Kang, 2013, High photocatalytic activity of hydrogen production from water over Fe doped and Ag deposited anatase TiO 2 catalyst synthesized by solvothermal method, Chem. Eng. J, 228 (2013) 896-906.

Jo

[38] Y. Gao, Y. Masuda, W.-S. Seo, H. Ohta, K. Koumoto, TiO2 nanoparticles prepared using an aqueous peroxotitanate solution, Ceram. Int. 30 (2004) 1365-1368. [39] J. Zhu, W. Zheng, B. He, J. Zhang, M. Anpo, Characterization of Fe-TiO2 photocatalysts synthesized by hydrothermal method and their photocatalytic reactivity for photodegradation of XRG dye diluted in water, J. Mol. Catal. A, 216 (2004) 35-43. [40] Y. H. Zhang, A. Reller, Nanocrystalline iron-doped mesoporous titania and its phase transition, J. Mater. Chem, 11 (2011) 2537-2541.

[41] M. Hirano, T. Joji, M. Inagaki, H. Iwata, Direct formation of iron (III)-doped titanium oxide (anatase) by thermal hydrolysis and its structural property, J. Am. Ceram. Soc, 87 (2004) 13651368. [42] J. A. Navio, G. Colon, M. I. Litter, G. N. Bianco, Synthesis, characterization and photocatalytic properties of iron-doped titania semiconductors prepared from TiO2 and iron (III) acetylacetonate, J. Mol. Catal. A, 106 (1996) 267-276. [43] M. Hussain, M. Ahmad, A. Nisar, H. Sun, S. Karim, M. Khan, S. D. Khan, M. Iqbal, S. Z. Hussain, Enhanced photocatalytic and electrochemical properties of Au nanoparticles supported TiO 2 microspheres, New. J. Chem, 38 (2014) 1424-1432.

of

[44] M. A. Khan, S. I. Woo, O.-B. Yang, Hydrothermally stabilized Fe (III) doped titania active under visible light for water splitting reaction, Int J Hydrog Energy, 33 (2008) 5345-5351.

-p

ro

[45] C. Y. Wang, C. Bottcher, D. W. Bahnemann, J. K. Dohrmann, A comparative study of nanometer sized Fe (III)-doped TiO2 photocatalysts: synthesis, characterization and activity, J. Mater. Chem, 13 (2003) 2322-2329. [46] M. H. Zhou, J. G. Yu, B. Cheng, Effects of Fe-doping on the photocatalytic activity of mesoporous TiO2 powders prepared by an ultrasonic method, J. Hazard. Mater, 137 (2006)1838-1847.

re

[47] A. R. Ballyy, E. N. Korobeinikova, P. E. Schmid, F. Levy, F. Bussy, Structural and electrical properties of Fe-doped thin films, J. Phys. D Appl. Phys, 3 (1998) 1149-1154.

lP

[48] W. Ho, J. C.Yu, S. Lee, Low-temperature hydrothermal synthesis of S-doped TiO2 with visible light photocatalytic activity, J. Solid State Chem, 179 (2006) 1171-1176.

ur na

[49] K. Naeem, F. Ouyang, Preparation of Fe3+-doped TiO2 nanoparticles and its photocatalytic activity under UV light, Physica B, 405 (2010) 221-226. [50] W. Choi, A. Termin, M. R. Hoffmann, The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics, J. Phys. Chem, 98 (1994) 13669-13679. [51] S. Miao, Y. Lin-Hai, X. Zhe-De, Photocatalytic Activity of Iron Doping TiO2 Prepared by Several Methods, J.Acta Phys. Chim. Sin, 17(03) (2001) 282-285.

Jo

[52] S. K. Das, M. K. Bhunia, A. Bhaumik, Self-assembled TiO2 nanoparticles: mesoporosity, optical and catalytic properties, Dalton Trans, 39 (2010) 4382-4390. [53] P. Pongwan, B. Inceesungvorn, K. Wetchakun, S. Phanichphant, N. Wetchakun, Synthesis and characterization of a magnetically separable CoFe2O4/TiO2 nanocomposite for the photomineralization of formic acid,” Ferroelectrics, 16 (2013) 144-151. [54] J. Mao, T. Y. Peng, X. H. Zhang, K. Li, L. Q. Ye, L. Zan, Effect of graphitic carbon nitride microstructures on the activity and selectivity of photocatalytic CO2 reduction under visible light, Catal. Sci. Technol, 3 (2013) 1253-1260.

[55] R. A. Rather, S. Singh, B. Pal, A C3N4 surface passivated highly photoactive Au-TiO2 tubular nanostructure for the efficient H 2 production from water under sunlight irradiation, Appl. Catal B : Environ, 213(c) (2017) 9-17. [56] M. Zhang, X. Yu, D. Lu, J. Yang, Facile synthesis and enhanced visible light photocatalytic activity of N and Zr co-doped TiO2 nanostructures from nanotubular titanic acid precursors, Nanoscale Res. Lett, 8 (2013) 543. [57] M. Zhou, J. Yu, B. Cheng, Effects of Fe-doping on the photocatalytic activity of mesoporous TiO2 powders prepared by an ultrasonic method, J. Hazard. Mater. B, 137 (2006) 1838-1847.

of

[58] Z. Yang, Y. Huang, B. Dong, H.L. Li, Controlled synthesis of highly ordered LaFeO3 nanowires using a citrate-based sol-gel route, Mater Res Bull, 41 (2006) 274-281.

ro

[59] X. H. Wang, J-G Li, H. Kamiyama, M. Katada, N. Ohashi, Pyrogenic Iron(III)-Doped TiO2 Nanopowders Synthesized in RF Thermal Plasma:  Phase Formation, Defect Structure, Band Gap, and Magnetic Properties, J. Am. Chem Soc, 127 (2005) 10982-10990.

-p

[60] R. Marchall, J. Soldat, M. Wark, Enhanced photocatalytic hydrogen generation from barium tantalate composites, Photochem. Photobiol. Sci, 12 (2013) 671-677.

re

[61] D. Kong. Langmuir, The influence of fluoride on the physicochemical properties of anodic oxide films formed on titanium surfaces, 2 (2008) 5324-5331.

lP

[62] H. G. Huang, D. Z. Li, Q. Lin, W. J. Zhang, Y. Shao, Y. B. Chen, M. Sun, X. Z. Fu, Efficient Degradation of Benzene over LaVO 4/TiO2 Nanocrystalline Heterojunction Photocatalyst under Visible Light Irradiation, Environ. Sci. Technol, 43 (2009) 4164-4168.

ur na

[63] A. P. Singh, S. Kumari, R. Shrivastav, S. Dass, V. R. Satsangi, Iron doped nanostructured TiO 2 for photoelectrochemical generation of hydrogen, Int. J. Hydrog Energy, 32 (2008) 5363-5368. [64] X. M. Song, J. M. Wu, M. Z. Tang, B. Qi, M. Yan, Enhanced photoelectrochemical response of a composite titania thin film with single-crystalline rutile nanorods embedded in anatase aggregates, J. Phys. Chem. C, 112 (2008) 19484-19492. [65] H. Katsumata, Y. Tachi, T. Suzuki, S. Kaneco, Z-scheme photocatalytic hydrogen production over WO3/g-C3N4 composite photocatalysts, RSC Adv, 4 (2014) 21405-21409.

Jo

[66] J. Lim, D. M-Satocaa, J. Jang, S. Lee, W. Choi, Visible light photocatalysis of fullerolcomplexed TiO2 enhanced by Nb doping, Appl. Catal. B: Environ, 152-153 (2014) 233-240. [67] L- Yu, X. Zhang, G. Li, Y. Cao, Y. Shao, D. Li, Highly efficient Bi2O2CO3/BiOCl photocatalyst based on heterojunction with enhanced dye-sensitization under visible light, Appl. Catal. B Environ, 187 (2016) 301-309. [68] Y. He, J. Cai, L. Zhang, X. Wang, H. Lin, B. Teng, L. Zhao, W. Weng, H. Wan, M. Fan, Comparing Two New Composite Photocatalysts, t-LaVO4/gC3N4 and m-LaVO4/gC3N4, for Their Structures and Performances, Ind. Eng. Chem. Res, 53 (2014) 5905-5915.

[69] J. Wang, R. Li, L. Zhang, Y. Xie, Z. Jiang, R. Xu, Z. Xing, X. Zhang, Russ, 2010, Sol-gel preparation of Fe-doped mixed crystal TiO2 powder and its photocatalytic activity for degradation of azo fuchsine under visible light irradiation, J. Inorg. Chem. 55(5) (2010) 692-699. [70] T. Sakata, K. Hashimoto, T. Kawai, J. Phys. Chem. Catalytic properties of ruthenium oxide on ntype semiconductors under illumination, 88 (1984) 5214-5221. [71] S. I. Mogal, M. Mishra1, V. G. Gandhi, R. J. Tayade, Metal doped titanium dioxide: synthesis and effect of metal ions on physico-chemical and photocatalytic properties, Materials Science Forum, 734 (2013) 364-378.

of

[72] X. Fan, Jun Fan, X. Hu, E. Liu, L. Kang, C. Tang, Y. Ma, H. Wu, Y. Li, Preparation and characterization of Ag deposited and Fe doped TiO2 nanotube arrays for photocatalytic hydrogen production by water splitting, Ceramic Int, 40 (2014) 15907-15917.

-p

ro

[73] Y. Lin, Z. Jiang, C. Zhu, X. Hu, H. Zhu, X. Zhang, J. Fan, S. H. Lin, The optical absorption and hydrogen production by water splitting of (Si,Fe)-codoped anatase TiO2 photocatalyst, Int J Hydrog Energy, 38 (2013) 5209-5214.

re

[74] G. Sadanandam, L. Zhang, and M. S. Scurrell, Enhanced photocatalytic hydrogen formation over Fe-loaded TiO2 and g-C3N4 composites from mixed glycerol and water by solar irradiation, J Renew Sust Energy 10 (2018) 034703.

lP

[75] F. Wang, T. Shen, Z. Fu, Y. Lu, and C. Chen, Enhanced photocatalytic water-splitting performance using Fe-doped hierarchical TiO2 ball-flowers, Nanotechnology, 29 (2017) No 3. [76] H. P. Li, J. Y. Liu, W. G. Hou, N. Du, R. J. Zhang and X. T. Tao, Synthesis and characterization of g-C3N4/Bi2MoO6 heterojunctions with enhanced visible light photocatalytic activity, Appl. Catal. B, 160-161 (2014) 89-97.

ur na

[77] Y. Tian, B. B. Chang, J. L. Lu, J. Fu, F. N. Xi and X. P. Dong, Hydrothermal Synthesis of Graphitic Carbon Nitride–Bi2WO6 Heterojunctions with Enhanced Visible Light Photocatalytic Activities, ACS Appl. Mater. Interface, 5 (2013) 7079-7085. [78] S. Weihua, Z. Zheng, A. Rami, Z. Tao, H. Desheng, Degradation and detoxification of aqueous nitrophenol solutions by electron beam irradiation, Radiat. Phys. Chem, 65 (2002) 559-563.

Jo

[79] K. Parida, D. Prakasini, Photo-oxidation of 4-nitrophenol in aqueous suspensions, catalysed by titania intercalated zirconium phosphate (ZrP) and titanium phosphate (TiP), J. Photochem. Photobiol. A, 163 (2004) 561-567. [80] B. Tryba, Increase of the Photocatalytic Activity of TiO2 by Carbon and Iron Modifications, Int J Photoenergy, 2008 (2007) 1-15. [81] Q. Xiang, J. Yu, M. Jaroniec, Preparation and Enhanced Visible-Light Photocatalytic H2Production Activity of Graphene/C3N4 Composites, J. Phys. Chem. C. 115 (2011) 7355-7363.

[82] D. F. Wang, T. Kako, J. H. Ye, Efficient Photocatalytic Decomposition of Acetaldehyde over a Solid-Solution Perovskite (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under Visible-Light IrradiationJ. Am. Chem. Soc, 130 (2008) 2724-2725.

Jo

ur na

lP

re

-p

ro

of

[83] L-.Y. Chen, W-.D Zhang, A Simple Strategy for the Preparation of g-C3N4/SnO2 Nanocomposite Photocatalysts, Sci. Adv. Mater, 6 (2014) 1091-1098.