Insitu Surface Decoration of Titanium Nano-Substrate by TiO2-WO3

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Kinetics, Catalysis, and Reaction Engineering

Insitu Surface Decoration of Titanium Nano-Substrate by TiO2-WO3 Composite Athira Krishnan, and Sheik Muhammadhu Aboobakar Shibli Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03692 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 11, 2018

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Industrial & Engineering Chemistry Research

Insitu Surface Decoration of Titanium Nano-Substrate by TiO2-WO3 Composite *

1

Athira Krishnan1, Sheik Muhammadhu Aboobakar Shibli2

Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala-690 525, India. 2

Department of Chemistry, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala-695 581, India.

Key Words: Core-Shell composite, Photocatalysis, Nanocomposite, Water splitting reaction, Active surface area.

ABSTRACT: An efficient TiO2 core/TiO2-WO3 shell structured nano composite is successfully synthesized by a method of acid precipitation followed by thermal decomposition. The metal weight ratio of the TiO2-WO3 is optimized in order to achieve visible light absorption. We made the mixed oxide composite on titanium metal turnings in order to enhance active surface area available for water splitting reaction. The novelty of the present work mainly lay on the utterly different approach that we adopt for the tuning of the composite with titanium turnings as the major material. The structural and morphological changes in the mixed oxide on the turnings are characterized in detail based on XRD, FTIR, TEM, SEM-EDAX and Raman spectroscopy. The enhanced surface properties are evaluated based on BET adsorption. The band edge position, band gap and the range of light absorption are envisaged by UV-Visible spectroscopy. Photocatalytic activity of the composite is evaluated under visible light irradiation. The volume of hydrogen gas evolved during water splitting reaction is quantified after confirming the purity of the evolved hydrogen by GC. The mechanism of the enhanced water splitting process by the fabricated photocatalytic system is then

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depicted. An appreciable quantity of pure hydrogen is produced by the catalyst composite, the stability and reproducibility of which is further ascertained by different experiments.

1. Introduction The enhanced depletion of fossil fuels, increased rate of CO2 emission, abnormal seasonal changes and global warming emerge the interest in using hydrogen as a fuel for future1. The priorities that made hydrogen as an energy carrier involves its abundance on earth, high energy density and on combustion it produces energy along with water as the byproduct

2,3

. There exist a large number of

methods for hydrogen generation that involves; steam reforming; biomass gasification; coal gasification; microbial biomass conversion; biomass derived liquid reforming; water electrolysis etc 4 -7

. If it is possible to derive hydrogen from water using a clean and renewable energy source such as

solar light, can resolve our energy and environmental problems. The significance of solar energy lies in its abundant availability, accessibility and constant distribution. Water splitting reaction is accompanied by a positive change in Gibbs free energy value of 237 kJ/mol; therefore it is desirable to overcome an energy barrier of 1.23 eV in order to achieve water splitting reaction. A photocatalyst with a band gap in between 1.23 eV and 3 eV and with a suitable band edge position can efficiently achieve water splitting under visible light irradiation

8,9

. Photocatalyst assisted water

splitting is the simplest method that can be exploited effectively for economical and large scale hydrogen production. TiO2 has long been used for photocatalytic applications due to its abundant availability, low cost, chemical and mechanical stability, non-toxic behavior and strong oxidizing nature. Although the band edge position of TiO2 is suitable to achieve water splitting, its wide band gap makes it inefficient to carry out water splitting reaction under visible light irradiation10. Researchers still strive to achieve feasible solar hydrogen production practicably by means of exploring different methods to tune the band gap and its position. One of the successful approaches is to reduce the band gap by doping with metal or metal oxides. As a low band gap material, WO3 can be explored to harvest visible light12. Synergistic combination of TiO2 and WO3 has recently been studied ACS Paragon Plus Environment

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extensively to enhance mutual reactivity and stability. Humidity sensing application of TiO2-WO3 composite has also been reported

11,12

. There are several other reports regarding the application of

WO3 doped TiO2 composite for dye degradation, electrocatalysis and sensing processes13,14,15. However, enhancement of synergistic photocatalytic activity of TiO2-WO3 composite for photocatalytic water splitting reaction has not yet been explored. The present work had the objective to make use of a suitable band structure of TiO 2 in conjunction with the narrow band gap of WO3 in order to achieve photocatalytic water splitting reaction under visible light irradiation. The hydrophilic nature of the composite was an added advantage for water splitting application12. Furthermore, we successfully tuned TiO2-WO3 composite in an extremely new path way by exploiting sieved scrap titanium turnings as the major material which caused enhancement of active surface area available for photocatalytic water splitting reaction. Finally, a highly surface and photocatalytic active core/shell structured TiO2/TiO2-WO3 nanocomposite was tuned for efficient water splitting reaction.

2. Materials and Methods 2.1. Fabrication of photocatalyst TiO2-WO3 mixed oxide composite was synthesized by acid precipitation followed by thermal decomposition16,17. Sodium tungstate dihydrate (Na2WO4.2H2O) was the precursor. The requisite amount of tungstate precursor was accurately weighed and made up to the required volume. To the solution 6 N HCl was added till a white amorphous precipitate was formed. TiO2 powder (CDH Analytical lab, 99.5% assay) was then added to the solution in different quantities in order to prepare composite with different metal weight ratios (1:1, 1:2, 2:1 Ti/W), stirred well and dried. The resulting yellow powder was ground in a mortar and then dried in an oven at 120 C for 1 hour and then calcined at 450 C for 2 hours. Mixed oxide decorated titanium nanosubstrate synthesis was achieved by a similar procedure as described above. In this procedure, after the addition of TiO2 powder, the solution was subjected to heating for 30 minutes. Then the required quantity of titanium metal powder sieved from the ACS Paragon Plus Environment


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titanium turnings (Purchased from KMML, assay>99.9%) using (250 mm) sieving machine was added. The solution was then evaporated and dried. The resulting yellow powder was then ground and placed in an oven at 120 C for 1 hour and calcined at 450 C for 2 hours. It was predicted that the present synthesis process could yield a metal oxide assisted growth of TiO2-WO3 composite as encrypted in Figure 1. Figure 1. Fabrication of TiO2 core/TiO2-WO3 shell composite. 2.2. Characterization of the photocatalyst Crystallinity and Phase identification of the prepared mixed oxide composite was achieved by Xray Diffraction (XRD) studies by using a Philip’s X’pert-MPD X-ray diffractometer with Cu Kα radiation (λ = 1.540 A). The average crystallite size of the composite was calculated using the Debye- Scherrer equation, D = 0.9λ / β cosθ, where D is the crystallite size; λ, the wavelength; β, the full width half maximum (FWHM) (in radians) and θ (degree), the Bragg’s angle 17,18. A PerkinElmer spectrophotometer was used to record FT-IR spectra and the types of bonds and the functional groups were characterized. The spectrum was recorded in the range limit between 4000-400 cm-1. Raman spectroscopy (Lab RAM HR-800 Spectrometer) was used to interpret vibrational, rotational and the low frequency modes in the system. It can provide information on the crystalline phase and orientation of the composite. Photoluminescence (PL) spectra were recorded at room temperature using a Hitachi FL 700 fluorescence spectrophotometer, to predict the life time of electron-hole pair generated in the valance band and conduction band respectively and thereby ensured the availability of surface electrons for water reduction. UV-visible spectrophotometer was used to identify the characteristic wavelength of absorption, band structure, band gap and thereby exploited to predict the photocatalytic activity of the composite. Both absorption and reflectance spectra were recorded over a wavelength range from 200-800 nm using Schimadzu UV 2450 spectrometer. The band gap was calculated from Tauc plot [hν vs. (αhν)2] obtained from Kubelka-Munk calculation. As the splitting of water occurs only at certain potential it was important to calculate the band edge positions. Band gap was employed to locate the position of the valance band and conduction band, 1

i.e., ECB = X − Ec − 2 Eg and 𝐸𝑉𝐵 = 𝐸𝐶𝐵 + 𝐸𝑔 . ACS Paragon Plus Environment

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Morphology of the composite was analyzed based on Scanning Electron Microscopy (SEM), using JOEL J SEM 840. An Energy Dispersive Spectrophotometer (EDS) attached to the SEM instrument was used to analyze the elemental composition of the composite. The structure and composition of the catalyst was further confirmed using HRTEM imaging (TEM; HITACHI H7650). An electron beam in the range of 10-20 KeV was used for the detection. Conductivity of the composite was determined by the electrochemical impedance spectroscopic technique using Electrochemical Workstation (SP200 model). The change in surface area of the composite during in situ decoration by titanium nano substrate was analyzed using a surface area analyser (BET: Autosorb ASIQMU010000-6). 2.3. Evaluation of photocatalytic activity Photocatalytic water splitting for hydrogen generation was performed in a 50 mL glass reactor of diameter 3 cm and length 6.2 cm at 25 C using a Solar Simulator (solar light line A1, Science Tech, made in Canada) (Class A spectrum). The lamp is adjusted to a distance of 12.5 cm from the solution surface. In a typical procedure, required quantities of mixed oxide composite was dispersed directly in 20 mL water with a constant stirring of 900 rpm throughout the experiment and simulated by a Solar Simulator of intensity equal to one sun (1000 W /m2). The gas evolved during photocatalytic water splitting reaction was collected in a syringe and injected to a sampling valve connected to back inlet of a Gas Chromatograph [G 4350 AC Agilent 7820 AGC System (CN15352024)] equipped with a molecular sieve column with a film thickness of 30 μ m and a Thermal conductivity detector was used for the analysis of hydrogen. Nitrogen (high purity 99.99%) was used as carrier gas and the Agilent Open Lab CDS software was used to identify the individual peak in the sample., where the data obtained for the gas evolved using the photocatalyst was compared with that of standard (pure hydrogen). The apparent quantum yield for hydrogen evolution reaction was calculated by the following equation; 𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑞𝑢𝑎𝑛𝑡𝑢𝑚 𝑦𝑖𝑒𝑙𝑑 =

2𝑛𝐻2 𝑁𝐴 ℎ𝑐 𝐼λSt

× 100

Where 𝑛𝐻2 , the number of moles of hydrogen; NA, is Avogadro number (6.022×1023 mol-1); h represented Planck constant (6.626×10-34 J S) and c stands for velocity of light (3×108 m s-1). I, the ACS Paragon Plus Environment


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experimental intensity of irradiation (8.6 mW cm-2); λ, the wavelength of light absorbed by the catalyst; S, represented the area of irradiation (cm2) and t, the time of irradiation. In order to verify the stability and long term use, the photocatalyst used in the experiment was filtered, dried in air oven and was re-used for repeated cycles. An optimized amount of the catalyst was then used along with electron donors inorder to study its hole-scavenging action for water splitting reaction. Aniline hydrogen chloride, diethyl amine hydrogen chloride and pyrrole were used as hole-scavengers in the present work.

3. Results and Discussions 3.1. Structural characterization of the photocatalyst 3.1.1. Crystallinity analysis and phase identification of the catalyst The crystallinity and particle size of the catalyst play important role in its photocatalytic activity. The X-ray diffraction pattern shown in Figure 2 of the TiO2-WO3 composite and the core/shell structured composite envisaged not only the position of diffraction peaks but also their relative intensities. The diffraction peaks obtained for the TiO2-WO3 and turnings supported TiO2-WO3 were found approximately at similar 2θ values. Absence of any additional peaks in the case of the turnings modified mixed oxide compared to the bare composite attributed the presence of similar crystalline phases in both sample and standard. This suggested that the metal turnings added in the mixed oxide synthesis during insitu decoration process might be converted to its TiO2 form. The distinctive diffraction peaks observed at 26.2, 36, 45.7, 54.3, 55.6 and 69.8 indicated the presence of TiO2 in anatase phase. The diffraction peaks obtained was in line with the standard values [JCPDS No.21-1272]. The peaks of anatase TiO2 was due to the X- ray diffraction at (101), (004), (200), (105), (211) and (204) crystallographic planes10,19,20. The peaks identified at 23.2, 31.7, 44 matched well with (002), (020) and (132) planes of orthorhombic WO3 whereas, the peaks observed at 24.2, 27.4, 28.6, 33.9 and 41.5 revealed monoclinic WO3. The diffraction peaks obtained for the orthorhombic and monoclinic phases of WO3 were found in agreement with the standard data [JCPDS No.084-0886 and JCPDS No.043-1035 respectively]. Thus it was confirmed that the composites contained pure anatase TiO2 along with mixed orthorhombic and monoclinic ACS Paragon Plus Environment

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WO321,16. The average particle size of the core/shell structured composite was calculated to be 12 nm. Figure 2. The XRD patterns of the as-synthesized composites. A) TiO2 B) TiO2-WO3 C) TiO2/ TiO2-WO3. 3.1.2. Reiteration of crystallite phase by Raman spectroscopy The structural data and crystallite phase of the mixed oxide was confirmed by the spectral response of the composite towards Raman spectroscopy. Both TiO2-WO3 and the turnings incorporated mixed oxide shown similar wave number in the wide spectral zone. Raman peaks observed at low wave numbers of 270 cm-1 and 327 cm-1 were due to the bending vibrations of W-OW bonds. The higher wavenumber peak at 713 cm-1 was due to O-W-O vibration and 805 cm-1 corresponded to crystalline WO3 stretching vibrations of the bridging oxygen of W-O-W. These two peaks clearly attributed monoclinic structure of WO310. Ti metal incorporation in the composites was revealed from the decreased peak intensity in the Raman spectrum of the metal modified composite compared to mixed oxide composite. The peaks at 147 cm-1, 397.5 cm-1, 517.5 cm-1, 639 cm-1 originated from Eg, B1g, A1g (B1g) and Eg modes of anatase phase of TiO 2. In TiO2-WO3 the band observed at 965 cm-1 could be assigned to the W=O stretching vibration. The peaks at 805 ± 5 cm-1 indicated the presence of O-W-O unit. Thus the presence of orthorhombic and monoclinic WO3 in the mixed oxide was further confirmed19,21-23. A slight shift in peak intensity of crystalline TiO2 and WO3 in the mixed oxide, in comparison to pure oxide, revealed the existence of efficient interaction within the mixed oxide composite. The intense and less broad peak of the composite in the spectra again confirmed the fine crystalline nature of the composite in support to XRD studies. Raman spectra were recorded as shown in Figure 3. Figure 3. Raman spectra of A) TiO2-WO3 B) TiO2/TiO2-WO3. FT-IR analysis was performed to identify the structure and bonding nature in the synthesized photocatalysts24. The vibrational bands reiterated the presence of anatase TiO2, orthorhombic & monoclinic WO3 in the composite25,26. Figure S1 of Supplementary Information revealed the vibrational characteristics of the fabricated composites. 3.1.3. Evaluation of structural and compositional changes in the composite during band gap tuning ACS Paragon Plus Environment


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The morphology of the synthesized TiO2-WO3 composite and turnings modified TiO2-WO3 mixed oxide particles were observed from the SEM images (Figure 4). The size of the particle was estimated approximately as 18 nm and 10 nm for the mixed oxide and composite modified metal substrate respectively. Nanoflakes shaped agglomerate was observed in the SEM images. The mean hydrodynamic diameter of 80 nm determined by DLS in the case of the nanoparticles dispersion in water was found to be in accordance with the agglomerate size observed by SEM. The percentage chemical composition of each element in the photo catalyst was confirmed by an energy dispersive spectrometer attached to SEM instrument, as is embodied in Figure 5. The exact percentage composition of each element in the composite is enclosed in Table S1. The peak intensity of Titanium and oxygen in EDS pattern of TiO2-WO3 composite seemed alike to the composite modified nano substrate. This gave insight on the role of turnings that it acted as an efficient support to the growth of TiO2-WO3 on its surface which in turn attributed the TiO2 core/ TiO2-WO3 shell type composite formation10,23. Figure 4. SEM images of A) TiO2-WO3 B) TiO2/ TiO2-WO3. Figure 5. Energy dispersive spectra of A) TiO2-WO3 B) TiO2/ TiO2-WO3. Elemental distribution in the composite was evaluated by EDS elemental mapping. Even distribution of Ti, W and O in the composite is corroborated in Figure 6. Figure 6. EDS elemental mapping images of the elements in the core/shell structured TiO2/TiO2WO3 nano composites. A) TiO2/TiO2-WO3 B) Ti Kα C) W Lα D) O Kα. The composite was analyzed by HRTEM to confirm about the core/shell geometry of the composite. Figure 7 illustrate the core/shell appearance of the composite in good agreement with the results of other characterizations that all are reported in the manuscript. The images identified the average particle size of the composite to be 8-10 nm, as corroborated with SEM results. The composition of core as well as shell structure of the composite embedded is shown in Table 2. Figure 7. HRTEM image of core/shell structured TiO2/TiO2-WO3 composite. Table 2. Elemental composition of dark (core) and light (shell) regions in HRTEM. ACS Paragon Plus Environment

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3.1.4. Evaluation of change in specific surface area of the composites by adsorption-desorption isotherm. The amelioration in specific surface area achieved by the composite involved in nano titanium substrate modification was estimated by BET adsorption by N2 adsorption/ desorption method. Specific surface area of TiO2-WO3 and TiO2/TiO2-WO3 were found to be 89 m2/g and 126 m2/g respectively, as depicted in Figure 8. BET adsorption study of TiO2 is included as Figure S2 of Supplementary Information. The enhanced specific surface area for the turnings modified composite revealed the presence of high concentration of active sites on the catalyst surface which facilated enhanced rate of water splitting on the surface. The increased surface area attained by the composite decorated titanium nanosubstrate in comparison to the bare composite also paid attention to a mechanism that involve the growth of TiO2-WO3 composite on TiO2 surface. That is to say that the core/shell structure of the composite proposed by SEM-EDS and HRTEM images was further confirmed by BET adsorption results. Figure 8. BET adsorption isotherm of the prepared photocatalysts. A) TiO2-WO3 B) TiO2 /TiO2WO3. 3.2. Optical properties of synthesized photocatalysts 3.2.1 Examining the changes in absorptive behavior involved in band gap tuning of the composite The light absorption properties of the prepared samples were evaluated by UV-Visible absorption spectroscopy carried out in the wavelength region 200-800 nm. As per the absorption spectrum, as shown in Figure 9, the TiO2 powder exhibited an absorption in the wavelength of 385 nm. But in the case of TiO2-WO3 composite, the absorption wavelength shifted to 420 nm. This may be due to some hybridization that can occur between the 2p orbital of O in TiO2 and 4f/5d orbital of the dopant in the composite. The absorbance further shifted to a higher wavelength region in the case of the core/shell structured composite formed by the mixed oxide with titanium turnings, confirming its enhanced photocatalytic activity. The band gap and band position of the fabricated photocatalyst were retrieved from the collected data of diffused reflectance spectroscopy. The Tauc plot (Figure 10) was drawn by the Kubelka-munk transformation of UV-Visible spectral data. The bandgap energy (E=hʋ=hc/λ) was ACS Paragon Plus Environment


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plotted against a function of reflectance (F(R) = [(1-R)2/ 2R  hʋ]1/2). The linear portion of the plot extended to the X-axis to find out the band gap of the material. The band gap of TiO2 decreased from 3.11 eV to 2.93 eV during its composite preparation with WO327,28,29. The band gap value further declined to 2.74 eV in the case of the titanium turnings supported mixed oxide nanocomposite, revealing its activity in visible region. Figure 9. UV-visible absorption spectra of the composites. A) TiO2 B) TiO2-WO3 C) TiO2/ TiO2WO3. Figure 10. Tauc plot for the evaluation of band gap of the prepared photocatalyst: a) TiO2/TiO2-WO3 b) TiO2-WO3 c) TiO2. 3.2.2. Analysis of electron-hole recombination rate by PL emission spectra Since photo catalytic water splitting is a surface reaction, the surface availability of photo induced charge carriers play a crucial role during hydrogen generation and hence the recombination of photo generated electrons and holes usually limit the water splitting reaction. Photoluminescence emission spectra were recorded at an excitation wavelength of 390 nm to investigate the electronhole recombination rate in the mixed oxide catalysts during photocatalytic reaction. The strong emission peak observed in the case of TiO2, attributed a greater fluorescence emission. But the reduced peak intensity observed in the case TiO2-WO3 revealed a greater charge separation. The results from Figure 11 revealed that the lowest peak intensity in the PL emission spectrum was obtained due to TiO2/TiO2-WO3 composite. As per few reports, the lower the PL intensity the greater will be the surface availability for charge carriers and hence greater will be the catalytic activity30,31. Figure 11. The PL emission spectra. a) TiO2 b) TiO2-WO3 c) TiO2/TiO2-WO3. 3.2.3. Confirmation of surface availability of electrons As per literature, any defects present within the composite could act as trap centers for electrons and thereby the fluorescence emission intensity may get reduced32,33. Hence, the reason for diminishing in the PL intensity was not due to the presence of defects but predominantly due to the ACS Paragon Plus Environment

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fall in transition of electrons from conduction band to valance band. Therefore the surface availability of photo separated electron for water splitting reaction was further evaluated and confirmed from the conductivity parameters of the photocatalytic materials analyzed by electrochemical impedance spectroscopic technique. The low value of Rct revealed the existence of enhanced electron transfer at the catalyst-electrolyte interface, which in turn facilitated the migration of photogenerated electron to the catalytic surface to enable H2 generation at an enhanced rate34,35. Figure 12. Electrochemical Impedance Spectra: a) TiO2-WO3 b) TiO2/ TiO2-WO3. The visible light absorptive band gap, the lowest recombination of the photo excited electrons and holes, the surface availability of photo separated charges, suitably located valance and conduction band positions and the enhanced active surface area altogether confirmed about the appropriateness of the fabricated material towards efficient water splitting reaction. 3.3. Evaluation of photocatalytic performance of the composite towards water splitting HER reaction 3.3.1. Optimization of catalyst weight ratio and their loading amount for water splitting reaction TiO2-WO3 and titanium nanosubstrate decorated with mixed oxides in different metal weight ratios, were analyzed to assess their efficiency during photocatalytic water splitting reaction leading to hydrogen generation. The catalyst was dispersed in water in a glass reactor and solar light, equivalent to one sun, was irradiated using a solar simulator at 25 C for a period of 2 h. The data related to photocatalytic performance of the fabricated photo catalysts towards the water splitting reaction in terms of amount of hydrogen generation under visible light irradiation is shown in Table.1. An amount of 1g in 20 mL water was identified as the optimum loading quantity for photocatalytic splitting in 2 h. Hence, all the remaining studies were carried out with a catalytic loading of 1 g in 20 mL distilled water. 3.3.2. Confirmation of the role of solar light in catalyst triggered hydrogen generation The performance of the catalyst with the irradiation time was evaluated by different parameters, as shown in Figure 13. The effective water splitting reaction initiated only after 5 minutes since the irradiation started. This could be considered as the activation period required for ACS Paragon Plus Environment


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the catalyst. Once the catalyst started its action, the rate of water splitting varied directly with irradiation time. Maximum yield for hydrogen was obtained within the first 40 minutes. A volume of 8.3 mL was obtained with TiO2-WO3 while the turnings supported core/shell structured nano composite yielded 13.2 mL in the first 30 minutes. Then the activity got gradually decreased. This might be due to the deactivation of the catalytic surface and surface reactions. The total gaseous products produced by TiO2-WO3 photocatalyst in 1 hour, was about 10.7 mL while TiO2/TiO2-WO3 produced a volume of 17.1 mL hydrogen under similar conditions. Even though Ti/W mixed oxide composite followed a similar trend in photocatalytic action with the turnings supported composites, their total hydrogen generation and the volume of hydrogen delivered after different time intervals, were found to vary significantly. Table.1. Variation in hydrogen generation with catalyst loading Figure13. The trend of variation of hydrogen generation as a function of irradiation time. A) TiO2/TiO2-WO3 B) TiO2-WO3. 3.3.3. Evaluating the role of hole scavengers in photocatalytic water splitting reaction As the surface availability of charge carriers was prominent in water splitting reaction, we used some electron donors along with the catalyst during the experiment. The electron doners were expected to scavenge the holes that could decrease recombination of the charge carriers. Although less in numbers, it can contribute electrons for water reduction. In the present work, we used Pyrrole, aniline hydrogen chloride and diethyl amine hydrogen chloride to evaluate the effect of sacrificial donors towards the water splitting reaction. The bases were expected to donate electrons and they could act as hole arresters. This could mitigate the chances for electron–hole recombination. The results of the study are shown in Figure14. Among all the used sacrificial reagents, diethyl amine hydrogen chloride showed better activity than aniline hydrogen chloride and pyrrole. The superior catalytic behavior of TiO2 core/ TiO2-WO3 shell structured composite in presence of diethyl amine hydrogen chloride reagent could be due to greater hole scavenging action of diethyl amine due to its basic character. The TiO2/TiO2-WO3 catalyst of 1 g in 20 mL water in presence of 0.1 g diethyl amine hydrochloride proton source retrieved a relatively higher amount of hydrogen production of ACS Paragon Plus Environment

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about 19.8 mL with an apparent quantum yield of 20.3%. Figure S3 represent a comparison of the quantum yield among all the prepared photocatalysts. The efficiency of electron sources in water splitting followed the order: Diethyl amine hydrogen chloride > Aniline hydrogen chloride > Pyrrole. The present study unfolded that the scavenging action of the used sensitizers was in accordance with the basic character. Figure 14. The trend of variation in volume of hydrogen produced by TiO 2/TiO2-WO3 photocatalyst with different sensitizers: A) Aniline hydrogen chloride B) Pyrrole C) Diethyl amine hydrogen chloride D) In the absence of sensitizers. 3.3.4. Reusability analysis of the catalyst Therefore the reusability of the optimized catalyst was evaluated with different cyclic runs. The catalyst used in one photocatalytic evaluation process was recovered, dried and then reused for the next cycles. The fabricated photo catalysts possessed good catalytic activity even in repeated turns as indicated by the results of evaluation, as shown in Figure 15. Figure 15. The performance as a function of reusability of the core/shell structured composite during water splitting reaction in presence of diethyl amine: A) First run B) Second run C) Third run D) Fourth run E) Fifth run F) Sixth run. A comparison of the activity parameters of the fabricated catalytic system with other recent reports of similar studies is presented in Table S2, in order to establish the performance of the proposed catalyst. 3.3.5. Purity confirmation of the evolved gas GC analysis was carried out to evaluate the purity of evolved hydrogen. The hydrogen produced by the turnings incorporated TiO2-WO3 composite, from water with diethyl amine hydrogen chloride as sacrificial reagent, was compared with that of pure hydrogen in terms of area % (purity), as represented in Figure 16. Hydrogen generated by the photocatalyst possessed a purity of 99.1 %. The gas produced by the fabricated photocatalyst could be distinguished by their retention time. The retention time of the gas evolved from the mixed oxide decorated composite catalyzed hydrogen evolution reaction was 2.8036 minutes, which was found in good agreement with the retention time corresponding to ultrapure hydrogen at 2.7936 minutes.



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Figure 16. Purity analysis of the gas by chromatography. a) H2 gas evolved by TiO2/TiO2WO3/diethyl amine and b) Pure hydrogen. 3.3. Proposed mechanism of photocatalysis Photocatalytic water splitting reaction was largely limited by wider band gap, in appropriate band structure and unavailability of surface charge carriers due to electron-hole pair recombination. In the present case, it was attempted to establish a synergistic interaction between oxides of titanium and tungsten. An Z-scheme mechanism which is commonly adopted by many of the researchers to explain synergistic interaction within the composite for catalysis generally fail to elucidate the action of proposed photocatalyst, as it favours accumulation of electron in the conduction band of WO3, whose band position is not suitable for water splitting reaction36,37,38. An efficient separation of photo induced charge carriers could be achieved by designing a core/shell structured composite. A core/shell geometry, as in the present case, enabled efficient interaction between core and shell components (TiO2 and TiO2-WO3) and there by curtailed recombination of photo generated electrons and holes. Usually there are mainly three important reasons that highlight the importance of core-shell structured composite39,40; 1. The interaction between core and shell affects the charge transfer by altering the band structure. 2. The core acts as a support to the growth of shell component and subsequently leads to enhanced surface area, increased number of active surface sites, porosity etc. 3. Synergism between the core and shell enables to achieve combined characteristics of both core and shell that make it suitable applications. In the present core-shell structured composite, WO3 could be seemed to be an active solar light harvester due to the narrow band gap it had. But actually the recombination of charge carriers made it unsuitable for visible light driven water splitting process. The shell, in addition to WO 3, had anatase TiO2 with large band gap but at the favourable band position. The co presence of WO3 caused lowering of the band gap of TiO2 as detailed below:


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Both TiO2 and WO3 are normally n-type semiconductors. The Fermi level of these semi conductors situates them close to their conduction band. The electrons present in the conduction band of TiO2 can jump into the conduction band of the WO3 until the conduction band level of the latter raises to an equilibrium state where the Fermi level of these two metal oxides coincides. The coupled heterojunction in the shell structure can facilitate separation of the excitons41-46. An enhanced separation of charge carriers can be enabled with the anatase TiO2 present in the core structure of the catalyst40. Subsequently the electrons can migrate to the surface of the photocatalyst and assist splitting of water leading to hydrogen generation. The optimized catalyst exhibited an enhanced catalytic activity with the sensitizers having basic character. The amine based sensitizers functioned as hole arresters and thereby increased the surface availability for the electrons for water reduction. They could also contribute H+ ions in small quantities to hydrogen evolution. The proposed scheme for the catalytic action of the mixed oxide composite decorated titanium nano substrate towards water splitting reaction can be sketched as shown in Figure 17. Figure 17. The mechanism of water splitting by TiO2/TiO2-WO3 photocatalyst.

4. Conclusions: In the present study, a new design pattern for enhancement of surface area of TiO2-WO3 composite due to the incorporation of Ti metal turnings was explored. The titanium turnings incorporated during the process of insitu decoration, converted to TiO2 and acted as a core for the deposition of Ti/W mixed oxide composite. Thus fabrication of highly surface active TiO2core/TiO2-WO3shell structured composite was achieved. The catalytically active crystalline phases in the modified composite were tuned to be anatase phase of TiO2, orthorhombic as well as monoclinic phases of WO3. Separation of charge carriers facilitated by the coupled heterojunction in the shell structure was achieved by the interaction of the Fermi levels of the shell components. The core TiO2 could also enable the modified composite for enhanced charge separation. The subsequent migration of electrons to the surface of the catalyst could be suppressed to achieve reduction of water. The activity of the catalyst was in direct relation to the turnings incorporation. The photocatalyst with optimized quantity could yield 19.8 mL hydrogen for a period of 1 hour in the ACS Paragon Plus Environment


presence of diethyl amine hydrogen chloride. In short, the present report foresees potential application of the present method of using titanium metal turnings for fabrication of surface and photoctalytically active composites for wide range of surface related applications.

Figure1. Fabrication of TiO2 core/TiO2-WO3 shell composite.

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

C B A 10

20

30

40

50

60

70

80

2 (degrees) Figure 2. The XRD patterns of the as-synthesized composites. A) TiO2 B) TiO2-WO3 C) TiO2/ TiO2-WO3.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u)

Page 17 of 34

A B 300

600

900

1200 -1

Raman shift (cm ) Figure 3. Raman spectra of A) TiO2-WO3 B) TiO2/TiO2-WO3.

Figure 4. SEM images of A) TiO2-WO3 B) TiO2/ TiO2-WO3.


1500


Figure 5. Energy dispersive spectra of A) TiO2-WO3 B) TiO2/ TiO2-WO3.


Page 18 of 34

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Figure 6. EDS elemental mapping images of the elements in the core/shell structured TiO2/TiO2WO3 nano composites. A) TiO2/TiO2-WO3 B) Ti Kα C) W Lα D) O Kα.

Figure 7. HRTEM image of core/shell structured TiO2/TiO2-WO3 composite.

Figure 8. BET adsorption isotherm of the prepared photocatalysts. A) TiO2-WO3 B) TiO2 /TiO2WO3. ACS Paragon Plus Environment


Page 20 of 34

Figure 9. UV-Visible absorption spectra of the composites. A) TiO2 B) TiO2-WO3 C) TiO2/ TiO2WO3.

Figure 10. Tauc plot for the evaluation of band gap of the prepared photocatalyst: a) TiO2/TiO2-WO3 b) TiO2-WO3 c) TiO2. ACS Paragon Plus Environment

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Figure 11. The PL emission spectra. a) TiO2 b) TiO2-WO3 c) TiO2/TiO2-WO3.

Figure 12. Electrochemical Impedance Spectra: a) TiO2-WO3 b) TiO2/ TiO2-WO3.



Page 22 of 34

Figure 13. The trend of variation of hydrogen production as a function of irradiation time. A) TiO2/TiO2-WO3 B) TiO2-WO3.


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Figure 14. The trend of variation in volume of hydrogen produced by TiO 2/TiO2-WO3 photocatalyst with different sensitizers: A) Aniline hydrogen chloride B) Pyrrole C) Diethyl amine hydrogen chloride D) In the absence of sensitizers.

Figure 15. The performance as a function of reusability of the core/shell structured composite during water splitting reaction in presence of diethyl amine: A) First run B) Second run C) Third run D) Fourth run E) Fifth run F) Sixth run.



Page 24 of 34

Figure16. Purity analysis of gas by chromatography. a) H2 gas evolved by TiO2/TiO2-WO3/diethyl amine b) Pure hydrogen.


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e- + H2O e- e-

-1

CB 0 1

H2

e- e-

Ef Ef

TiO2

WO3

2

+ + VB

TiO2

3.11 eV

2.75 eV

3

ee- e- Charge separation e- e- e-

Core

eShell

2H+ +2e-

H2

Figure 17. The mechanism of water splitting by TiO2/TiO2-WO3 photocatalyst.


25


Page 26 of 34

Table.1. Variation in hydrogen production with catalyst loading. Composite Catalyst

Amount of catalyst (g)

Amount of hydrogen production (mL)

1:2 TiO2-WO3

0.5

3.2

1:2 TiO2-WO3

1

10.8

1:2 TiO2-WO3

1.5

4.5

1:1 TiO2-WO3

1

4.8

2:1 TiO2-WO3

1

2.5

TiO2-WO3 (1:2) /TiO2 (0.025 g Ti turnings)

1

10.5

TiO2-WO3 (1:1)/TiO2 (0.05 g Ti turnings)

1

12.8


1

17.1

1

10.4



26

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Elemental composition of dark (core) and light (shell) regions in HRTEM.

Shell

Core

Elements

Weight %

Atomic weight %

O

24.09

26.28

Ti

22.62

39.83

W

53.29

33.9

Elements

Weight %

Atomic weight %

O

26.32

35.49

Ti

73.68

64.51

ASSOCIATED CONTENT Vibrational spectrum of the composites, BET adsorption isotherm for TiO2, A graph showing quantum efficiency of composites, Table incorporating EDS elemental composition data of composites as well as, enclosing comparison of the fabricated catalyst with recently reported catalysts, were included as supplementary information. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The paper included contributions of all authors. The final version of the manuscript was approved by all authors. Notes


27


Page 28 of 34

The authors did not receive financial support from any funding sources. ACKNOWLEDGMENT

We acknowledge Prof& Head, department of chemistry, University of Kerala for supporting us by providing lab and technical facilities. REFERENCES [1] Chen, X.; Shan, W.; Guaan. Hydrogen Production from Water Splitting on CdS-based Photo catalysts

using Solar Light. Front Energy 2013, 7, 111–118.

[2] Jaksic, M.M. Advances in Electro Catalysis for Hydrogen Evolution in the Light of the Brewer-Engel Valance Bond Theory. Int J Hydrogen Energy 1987, 12, 727-752. [3] Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nano Particles Grown on Graphene: an Advanced Catalyst for Hydrogen Evolution Reaction. J Am Chem Soc 2011, 133, 7296-7299. [4] Safizadeh, F.; Ghali, E.; Houlachi, G.

Electrocatalysis Developments for Hydrogen

Evolution Reaction in Alkaline Solutions-A Review. Int J Hydrogen Energy 2015, 40, 256-274. [5] Zheng, Z.; Li, N.; Wang, C.Q.; Li, D.Y.; Zhu, Y. M.; Wu, G. Ni-CeO2 Composite Cathode Material for Hydrogen Evolution Reaction in Alkaline Electrolyte. Int J Hydrogen Energy 2012, 37, 13921-13932. [6] Kudo, A.; Miseki, Y. Heterogenous Photo Catalyst Materials for Water Splitting. Chem Soc Rev 2009, 38, 253-278.


28

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[7] Natali, M.; Luisa, A.; Lengo, E.; Scandola, F. Efficient Photo Catalytic Hydrogen Generation from Water by a Cationic Cobalt (II) Porphyrin. Chem Commun 2014, 50, 18421844. [8] Xiaobo, C.; Shaohua, S.; Liejin, G.; Samuel, S. M. Semiconductor-based Photo Catalytic Hydrogen Generation. Chem Rev 2009, 110, 6503-6570. [9] Maeda, K.; Domen, K. New Non-oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J Phys Cheam C 2007, 111, 7851-7861. [10] Chen, Z.; Zhao, J.; Yang, Z.; Ye, Q.; Huang, K.; Hou, C.; Zhao, Z.; You, J.; Yongjin Li. Fabrication of TiO2/WO3 Composite Nano Fibers by Electro Spinning and Photo Catalystic Performance of the Resultant Fabrics. Ind Eng Chem Re 2016, 1, 80-85. [11] Faia, P. M.; Jesus, E, L.; Loura, C. S. TiO2:WO3 Composite Humidity Sensors Doped with ZnO and CuO Investigated by Impedance Spectroscopy. Sens Actuators B 2014, 203, 340– 348. [12] Kumar, R.; Yadav, B.C. Fabrication of Polyaniline (PANI)—Tungsten Oxide (WO3) Composite for Humidity Sensing Application. Inorg Organomet Polym 2016, 26, 1421–1427. [13] Chai, S.; Kim, Y.; Lee, W. Photocatalytic WO3/TiO2 Nanoparticles Working under Visible Light. J Electroceram 2006, 17, 909–912. [14] Wan, S.; Ou, M.; Zhong, Q.; Zhang, S.; Song, F. Comparison of Structural, Mechanical and Corrosion Properties of (Ti0.68W0.32)Ox and (Ti0.41W0.59)Ox Thin Films, Deposited on TiAlV Surface by Electron Beam Evaporation. Surf Coat Technol 2016, 307, 596–602.


29


Page 30 of 34

[15] Kumar, S.G.; Rao, K.S.R.K. Comparison of Modification Strategies towards Enhanced Charge Carrier Separation and Photocatalytic Degradation Activity of Metal Oxide Semiconductors (TiO2, WO3 and ZnO). Appl Surf Sci 2017, 391, 124–148. [16] Shibli, S. M. A.; Anupama, V. R.; Arun, P. S.; Jineesh, P.; Suji, L. Synthesis and Development of Nano WO3 Catalyst Incorporated Ni-P Coating for Electrocatalytic Hydrogen Evolution Reaction. Int J Hydrogen Energy 2016, 41, 10090-10102. [17] Shibli, S. M. A.; Dilmon, V. S. Development of TiO2- Supported Nano-RuO2 Incorporated Catalytic Nickel Coating for Hydrogen Evolution Reaction. Int J Hydrogen Energy 2008, 33, 1104-1111. [18] Xu, F.; Zhang, L.; Cheng, B.; Yu, J. Direct Z-Scheme TiO2/NiS Core–Shell Hybrid Nanofibers with Enhanced Photocatalytic H2-Production Activity. ACS Sustainable Chem. Eng., 2018, 6, 12291-12298. [19] Kenahakis, G.; Vamvakaki, M.; Katsarakis, N. TiO2-WO3 Photoactive Bilayer in the UVVis Light Region, Appl Phy A 2017, 123, 231. [20] Liu, Y.; Li, J. Enhancement of Photoelectrochemical Performance of WO3 Vertical Arrangement Film for Solar Water Splitting by Gadolinium Doping, J Phy Chem C 2015, 119, 14834-14842. [21] Katarzyna, A.; Mauke, M. Flame Made WO3/CeO2-TiO2 Catalyst for Selective Catalyst Reduction of NOx by NH3. ACS Catal 2015, 5, 5657-5672. [22] Leghari, S. A. K.; Sajjad, S.; Zhang, J. Large Meso Porous Micro-Spheres of WO3/TiO2 Composite with Enhanced Visible Light Photo Activity. RSC Advances 2013, 3, 15354-15361


30

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[23] Abazari, R.; Mahjoub, A. R.; Sagatforoush, L. A.; Sanati, S. Characterization and Optical Properties of Spherical WO3 Nano Particles Synthesized via the Reverse Micro Emulsion Process and their Photo Catalytic Behaviour. Mater Lett 2014, 133, 208-211. [24] Wang, Li.; Mak, Y. Visible Near-Infrared Light Driven Oxygen Evolution Reaction with Noble Metal Free WO2/WO3 Hybrid Nanorods. Langmuir 2016, 32, 13046-13053. [25] Sataei, E.; Mohebbi, S. Photocatalytic Activity of Nanohybrid CoTCPP @ TiO2-WO3 in Aerobic Oxidation of Alcohols under Visible Light. J Mater Chem A 2016, 4, 3933-3946. [26] Santos L, Neto J. P.; Crespo, A.; Nuner, D.; Costa, N.; Foseca, I. M, et al. WO3 Nano Particles- Base Conformable pH Sensor. Appl Mater Interfaces 2014; 6, 12226-34. [27] Wang, S.; Zhang, X.; Cheng, G.; Jiang, X.; Li, Y.; Huang, Y.; Du. Study on Electronic Transport Properties of WO3/TiO2 Nano Crystalline Thin Films by Photo Assisted Conductive Atomic Force Microscopy. Chem Phys Lett 2005, 405, 63–67. [28] Waterhouse, G. I. N.; Wahab, A. K.; M. Al-Oufi, Jovic, V.; Anjum, D. H.; D. SunWaterhouse, Llorca, J.; Idriss, H. Hydrogen Production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2. Sci Rep 2013, 3, 2849. [29] Thimsen, E.; Biswas, S.; Cynthia S. L.; Biswas, P. Predicting the Band Structure of Mixed Transition Metal Oxides: Theory and Experiment. J Phys Chem C 2009, 113, 2014–2021. [30] Vasilaki, E.; Vernardou, D.; Kenanakis,G.; Vamvakaki, M.; Katsarakis, N. TiO2-WO3 Photoactive Bilayers in the UV-Vis Light Region. Appl phy A 2017,123, 231.


31


Page 32 of 34

[31] Shehata, N.; Meehan, K.; Hudait, M.; Jain, N. Control of Oxygen Vacancies and Ce3+ Concentrations in Doped Ceria Nanoparticles via the Selection of Lanthanide Element. J Nano part Res 2012, 14, 1173. [32] Selvalakshmi, T.; Sellaiyan, S.; Uedonob, A.; Bose, A.C. Investigation of Defect Related Photoluminescence Property of Multicolour Emitting Gd2O3:Dy3+ Phosphor. RSC Adv 2014, 4, 34257. [33] Li, H.; Wang, Y. Effect of Oxygen Vacancies on the Reduction of Eu3+ in Mg3Ca3(PO4)4 in Air Atmosphere. Inorg Chem 2017, 56, 10396–10403. [34] Dong, P.; Yang, B.; Liu, C.; Xu, F.; Xi, X.; Hou, G.; Shao, R. Highly Enhanced Photocatalytic Activity of WO3 Thin Films Loaded with Pt–Ag Bimetallic Alloy Nanoparticles. RSC Adv 2017, 7, 947. [35] Wang, Y.; Zhao, S.; Zhang, Y.; Fang, J.; Chen, W.; Yuan, S.; Zhou, Y. Facile Synthesis of Self-Assembled g-C3N4 with Abundant Nitrogen Defects for Photocatalytic Hydrogen Evolution. ACS Sustainable Chem Eng 2018, 6, 10200−10210. [36] Bian, J.; Qu, Y.; Fazal, R.; Li, X.; Sun, N.; Jing, L. Accepting Excited High-EnergyLevel Electrons and Catalyzing H2 Evolution of Dual-Functional Ag-TiO2 Modifier for Promoting Visible-Light Photocatalytic Activities of Nanosized Oxides. J Phys Chem C 2016, 120, 11831-11836. [37] Pan, L.; Zhang, J.; Jia, X.; Ma, Y.H.; Zhang, X.; Wang,L.; Zou, J.J. Highly Efficient ZScheme WO3−x Quantum Dots/TiO2 for Photocatalytic Hydrogen Generation. Chin J Catal 2017, 38, 253–259.


32

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[38] He, K.; Xie, J.; Luo, X.; Wen, J.; Ma, S.; Li, X.; Fang,Y.; Zhang, X. Enhanced Visible Light Photocatalytic H2 Production Over Z-Scheme g-C3N4 Nanosheets/WO3 Nanorods Nanocomposites Loaded with Ni(OH)x Cocatalysts. Chin J Catal 2017, 38, 240–252. [39] Khanchandani, S.; Kumar, S.; Ganguli, A.K. Comparative Study of TiO2/CuS Core/Shell and Composite Nanostructures for Efficient Visible Light Photocatalysis. ACS Sustainable Chem. Eng 2016, 4, 1487-1499. [40] Gawande, M.B.; Goswami, A.; Asefa, T.; Guo, H.; Biradar, A.V.; Peng, D.; Zboril, R.; Varma, S.R. Core–Shell Nanoparticles: Synthesis and Applications in Catalysis and Electrocatalysis. Chem Soc Rev 2015, 44, 7540-7590. [41] Li, L.; Zhao, X.; Pan, D.; Li, G. Nanotube Array‐Like WO3/W Photoanode Fabricated by Electrochemical Anodization for Photoelectrocatalytic Overall Water Splitting. Chin J Catal 2017 38, 2132–2140. [42] Guzman, F.; Chuang, S. S. C.; Yang, C. Role of Methanol Sacrificing Reagent in the Photocatalytic Evolution of Hydrogen. Ind Eng Chem Res 2013, 52, 61−65. [43] Zhao, X.; Wu, P.; Lu, D.; Fang, P.; Liu, M.; Qian, Z. Synergistic Effect of Fe2O3 and CeO2 Co-Modified Titanate Nanosheet Heterojunction on Enhanced Photocatalytic Degradation. J Nanopart Res 2017, 19, 325. [44]

Shekofteh-Gohari,

M.;

Habibi-Yangjeh,

A.

Novel

Magnetically

Separable

ZnO/AgBr/Fe3O4/Ag3VO4 Nanocomposites with Tandem n–n Heterojunctions as Highly Efficient Visible-Light-Driven Photocatalysts. RSC Adv 2016, 6, 2402-2413.


33


Page 34 of 34

[45] Yu, T.; Liu, L.; Yang, F. Heterojunction between Anodic TiO2/g‐C3N4 and Cathodic WO3/W Nano‐Catalysts for Coupled Pollutant Removal in a Self‐Biased System. Chin J Catal 2017, 38, 270–277. [46] Manchala, S.; Nagappagari, L.R.; Venkatakrishnan, S.M.; Shanker, V. Facile Synthesis of Noble-Metal free Polygonal Zn2TiO4 Nanostructures for Highly Efficient Photocatalytic Hydrogen Evolution Solar Light Irradiation. Int J Hydrogen Energy 2018, 1 -1 3. Graphical Abstract


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Insitu Surface Decoration of Titanium Nano-Substrate by TiO2-WO3

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