Facile synthesis of noble-metal free polygonal

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 1 3 1 4 5 e1 3 1 5 7

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Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation Saikumar Manchala a, Lakshmana Reddy Nagappagari b, Shankar Muthukonda Venkatakrishnan b, Vishnu Shanker a,* a

Department of Chemistry, National Institute of Technology, Warangal 506004, Telangana, India Nano Catalysis and Solar Fuels Research Laboratory, Department of Materials Science and Nanotechnology, Yogi Vemana University, Vemanapuram, Kadapa 516005, Andhra Pradesh, India

b

article info

abstract

Article history:

Designing of noble-metal free and morphologically controlled advanced photocatalysts for

Received 28 February 2018

photocatalytic water splitting using solar light is of huge interest today. In the present

Received in revised form

work, novel polygonal Zn2TiO4 (ZTO) nanostructures have been synthesized by citricacid

1 May 2018

assisted solid state method for the first time and synthesized nanostructures were char-

Accepted 7 May 2018

acterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and

Available online 1 June 2018

TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and

Keywords:

TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO

Polygonal Zn2TiO4

nanostructures have been potentially applied for solar light-driven photocatalytic

ZnO

hydrogen evaluation from water splitting and compare the photocatalytic activity with

TiO2

synthesized conventional Zn2TiO4 and commercially available TiO2, ZnO photocatalysts. A

Hydrogen evolution

high rate of 529 mmolh1g1 solar light-driven photocatalytic H2 evolution has been ach-

Noble-metal free

ieved by using a small amount (5 mg) of polygonal Zn2TiO4 nanostructures from glycerolwater solution. The enhanced photocatalytic performance of the polygonal Zn2TiO4 nanostructures compare to conventional Zn2TiO4 under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn2TiO4 nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO2, ZnO photocatalysts. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In order to meet increasing global shortage of energy, reduce environmental deterioration and help our environment, and

to prepare for the possible depletion of fossil fuel supplies in the forthcoming years, an alternative green, carbon-free and renewable energy sources must be developed. One of the most attractive possible technologies to address these problems in the world is the conversion of solar energy into chemical

* Corresponding author. E-mail address: [email protected] (V. Shanker). https://doi.org/10.1016/j.ijhydene.2018.05.035 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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energy i.e; evolution of hydrogen molecules through water splitting process, with the help of semiconductor-based photocatalysts, which is recognized as a clean and green production [1e7]. Since the first report of hydrogen evolution process by Fujishima and Honda through photo electrochemical water splitting on n-type TiO2 electrode using ultraviolet (UV) light [8], many active photocatalysts such as various oxides [9], sulphides [10], and nitrides [11] have been reported and extensively studied for heterogeneous photocatalytic water splitting for hydrogen evolution. Tailoring of morphology, size-controlled and low cost noble-metal free nanostructures have been a crucial issue in recent photocatalysis research, due to the fundamental shape and size dependence of properties (physical & chemical) and applications [12e18]. But after four decades of global research the modeling and development of morphology and sizecontrolled advanced photocatalysts with the high surface area, high efficiency, stability, low-cost, noble-metal free still remains a great challenge. Among the investigated semiconductors in the areas of solar cells [19], gas sensors [20] and photocatalysis [9] titanium dioxide (TiO2) and zinc oxide (ZnO) were recognised as a most prominent materials, because of their excellent chemical stability, mechanical stability, inexpensive, availability, non-toxic nature and wide band gap as well as their high photosensitivity [21,22]. Synthesis of ZnO-TiO2 mixed binary oxide system has been well established with following compounds namely zinc ortho titanate (Zn2TiO4), zinc ortho di titanate (Zn3Ti2O7), zinc meta titanate (ZnTiO3), zinc mesopenta titanate (Zn4Ti5Ol4), and zinc para titanate (ZnTi3O7), Zn2Ti2O6, Zn2Ti3O8, and Zn4Ti5O16 but only ZnTiO3, Zn2TiO4 and Zn2Ti3O8 are confirmed to be stable. The precursors of the Zn/Ti ratio taken and preparation method influence the phase formation of different zinc titanates at different temperatures [23e26]. Actually, zinc ternary semiconductor oxides are a significant class of materials with a molecular formula A2BO4 (A ¼ Zn, B¼ Si, Ti, Sn, Ge), widely used because of their electronic and optical properties as well as their applications in photocatalysis, gas sensor, Li-ion battery, dye-sensitized solar cells and hydrogen storage [27e30]. Among the reported zinc ternary semiconductor oxides, zinc ortho titanate (Zn2TiO4) attracted wide attention. Zn2TiO4 is an inverse spinel and the stable phase of ZnO-TiO2 system [24]. It has been investigated due to its wide range of applications in paint pigments, microwave dielectrics, catalysis as a leading regenerable catalyst for removal of sulfur from coal gasification products, dye sensitized solar cells, white light emitting diodes, gas sensors, photocatalysis and extensively for low temperature co-fired ceramics [26,31e33]. It is generally synthesized from TiO2 and ZnO by a conventional solid-state method at high temperatures like 1350 C [34] and 1200 C [35] and typically when a temperature below 1000 C is employed, prolonged heat treatment time is required [32]. According to Yasumichi Matsumoto theoretical prediction of band edge positions, Zn2TiO4 is a promising material for photo-reduction of water under UV and near-visible light by having an ideal bandgap energy, as well as the suitable band edge potentials [36]. Interestingly its band gap is similar to that of precursors (commercially available TiO2 and ZnO) used for the synthesis.

In the present work, an alternate facile synthesis method has been employed such as the citric acid assisted solid state reaction method for the synthesis of novel polygonal Zn2TiO4 nanoparticles at 1000 C for 2 h and we have carried out a comparative study of photocatalytic properties of polygonal Zn2TiO4 with conventional Zn2TiO4, and commercially available TiO2 and ZnO photocatalysts under solar light irradiation for hydrogen evolution through water splitting. Recently, Latesh Nikam et al. reported the synthesis of nanostructured Zn2TiO4, Ag-doped Zn2TiO4 and Co-doped Zn2TiO4 by a combustion method and applied for photocatalytic hydrogen evolution from H2S splitting [24]. In the case of pure Zn2TiO4 and Ag-doped Zn2TiO4, the hydrogen evolution observed is 633 and 2784 mmolh1g1 respectively with 100 mg of catalyst that is very high amount of catalyst used for the H2 evolution compared to our present study i.e; 529 mmolh1g1 with 5 mg of polygonal Zn2TiO4 photocatalyst. However, to the best of our knowledge from the literature, there has been no report on the synthesis of polygonal Zn2TiO4 nanostructures and its photocatalytic application for H2 evolution from water splitting. Here we also presented the comparison study with conventional Zn2TiO4, TiO2 and ZnO.

Experimental section Materials TiO2 (Sigma Aldrich, 99%), ZnO (Finar chemicals Ltd., 99%) and Citric acid (Finar chemicals Ltd., 99.5%) have been used as received. Double distilled (DD) water was used in all experiments and all other reagents are at least of analytic reagent grade and used without further purification.

Synthesis In a typical procedure, polygonal Zn2TiO4 nanostructures have been prepared by citric acid assisted solid-state method. ZnO (0.732 g), TiO2 (1.278 g) powders were taken and mix with citric acid (0.576 g). The mixture was well ground by using mortar and pestle and transfer to alumina crucible. The crucible was placed in a programmable tubular furnace for 2 h and calcined at 1000 C with slow heating rate (2 C/min), a white colour powder was obtained. In this work, we believed that the chelating agent, citric acid binds through its carboxylic end and make a complex with the oxides of metals and while raising the calcination temperature forms a carbonaceous amorphous liquid. It served as a fuel for uniform distribution of both the metal oxides. Substantial distribution of the carbonaceous amorphous liquid helps in the formation of a nanocrystalline product of polygonal Zn2TiO4 by avoids agglomeration as well as furnishes a more number of satisfactorily-separated nucleation sites. Conventional Zn2TiO4 was synthesized without the addition of citric acid under similar conditions. Furthermore, the synthesis procedure for the formation of conventional and polygonal Zn2TiO4 nanostructures was shown in Scheme 1 and synthesized nanostructures were named as conventional ZTO and polygonal ZTO.


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Scheme 1 e Schematic representation of the synthesis processes of conventional and polygonal Zn2TiO4 nanostructures.

Characterisation Powder X-ray diffraction (PXRD) measurements have been performed by using a PAN Analytical Advance X-ray diffractometer using Ni-filtered Cu Ka (l ¼ 1.5406 Å) radiation in a 2q scan range between 10 and 90 . The surface morphology and chemical composition of the photocatalysts have been recorded by field emission scanning electron microscopy (FESEM, JEM-2100F, JEOL, Japan) coupled with an energy dispersive Xray (EDX) spectrometry. Transmission electron microscopy (TEM) and High resolution TEM (HRTEM) images have been taken by using a JEOL JEM-2100 transmission electron microscope at an acceleration voltage of 200 kV. Selected area EDS elemental mapping images have been taken by using an Oxford instruments, INCAx-act scanning electron microscope. The real compositions of Zn and Ti in the samples have been measured by Microwave plasma atomic emission spectrometry (MP-AES) by using Agilent technologies-4210 microwave plasma atomic emission spectrometer. Nitrogen adsorptiondesorption studies have been carried out by using a Quantachrome NOVA 3000e. Brunauer-Emmett-Teller (BET) surface areas have been determined over the relative pressure range 0.02e1.0. The optical properties of the photocatalysts have been obtained by using Shimadzu UV-3600 visible diffuse reflectance spectrophotometer (UVeVis DRS) with BaSO4 as the reference material. Fourier transform infrared (FT-IR) spectra have been recorded from 4000 to 400 cm1 on a PerkinElmer Spectrum 100 FT-IR spectrophotometer in transmission mode using the KBr pellet. Photo luminescence (PL) spectra have been measured at room temperature on a Jasco FP-8500 spectrofluorometer, using a xenon lamp as the excitation source and 300 nm as the excitation wavelength.

Photocatalytic activity The photocatalytic H2 evolution reactions have been carried out in a 180 mL air tight rubber septum sealed quartz reactor under solar light irradiation at ambient temperature and atmospheric pressure. The solar light intensity was observed using digital lux meter (TES 1332A) for every hour between

11:00 a.m. and 15:00 p.m. in the city of Kadapa, Andhra Pradesh State, INDIA. The average light intensity measured value was approximately 8.5 104 lx during the course of light reactions. In a typical process, photocatalyst powders were suspended into 5 vol % aqueous glycerol mixtures at a concentration of 50 mgL1. Here glycerol was used as a hole scavenger. Dark experiments have been carried out to ensure uniform dispersion for at-least 30 min. The reaction suspension was evacuated and purged with high pure N2 gas for atleast 30 min in order to remove dissolved gases present in the homogeneous system. The evacuated reactor was irradiated to natural solar light under magnetic stirring and the gaseous sample was collected with an air tight syringe and analyzed at regular time intervals. The volume of H2 gas evolved via photocatalytic water splitting was quantified using an offline gas chromatography (Shimadzu GC-2014 with Molecular Sieve/5A) equipped with thermal conductivity detector (TCD), at 70 C and N2 as a carrier gas.

Results and discussion Characterisation of as-synthesised photocatalysts First Powder X-Ray diffraction (PXRD) studies have been carried out in order to investigate the crystallographic structure and phase purity of as-synthesized conventional and polygonal Zn2TiO4 nanostructures have been shown in Fig. 1. It shows that the presence of spinel structure of Zn2TiO4. The results indicate that the diffraction peaks due to conventional and polygonal Zn2TiO4 were corresponds to both cubic phase (JCPDS Card No. 86-0154), as well as tetragonal phase (JCPDS card No. 86-0158). The XRD studies further supported by HRTEM and SAED images. FESEM and TEM have been carried out subsequently to investigate the morphology of the resultant Zn2TiO4 nanostructures. FE-SEM images (Fig. 2aed) revealed that the synthesized conventional Zn2TiO4 were not shown any regular morphology and the particles appeared to be agglomerated without using any structure directing agent. Whereas, while

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Fig. 1 e PXRD patterns of as-synthesized conventional and polygonal Zn2TiO4.

citric acid was used as a structure directing agent polygonal Zn2TiO4 were obtained and shown different shapes of morphologies, which further confirmed by TEM. TEM images (Fig. 3aed) were shown triangular, cube, hexagonal and spherical shape morphologies with smooth edges and a smooth surface. Compare to conventional synthesis, new methodology lead to the development of smooth surfaces of individual polygonal Zn2TiO4 particles with dimensions 50 nme200 nm consistent with the FESEM and TEM images. This indicates the citric acid plays a major role in order to influence the formation of polygonal Zn2TiO4 nanostructures. HRTEM and SAED images of the polygonal Zn2TiO4 nanostructures were shown in Fig. 3eef. Lattice fringes are good agreement with the (211) plane (dspacing ¼ 0.255 nm) and (400) plane (dspacing ¼ 0.150 nm) of tetragonal phase of polygonal Zn2TiO4 in the PXRD pattern. Energy-dispersive X-ray spectroscopic (EDX) studies have been carried out to determine the chemical composition of assynthesized conventional and polygonal Zn2TiO4 nanostructures and shown in Fig. 4aeb, which shows that Zn, Ti and O elements can be observed in both the samples. Selected area EDX elemental mapping has been carried out in order to get further information on the distribution of elements in the

Fig. 2 e FESEM images of conventional (a, b) and polygonal (c, d) Zn2TiO4 nanostructures.


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Fig. 3 e TEM (aed) images of polygonal Zn2TiO4 nanostructures HRTEM (e,f) and SAED (g) images of polygonal Zn2TiO4 nanostructures.

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Fig. 4 e EDX of as-synthesized conventional (a) and polygonal (b) Zn2TiO4 nanostructures.

sample and shown in Fig. 5, which shows the even distribution of Zn, Ti and O elements in the polygonal Zn2TiO4 nanostructures. MP-AES has been performed to know the actual composition of as-synthesized conventional Zn2TiO4 and polygonal Zn2TiO4 nanostructures and the results shown in Table 1. The results show that the real composition of conventional Zn2TiO4 and polygonal Zn2TiO4 are Zn1.99Ti1.01O4 and Zn1.97Ti1.03O4, respectively. N2 adsorptionedesorption measurements have been performed to know the BET specific surface area of assynthesized polygonal Zn2TiO4 and conventional Zn2TiO4 nanostructures. According to classifications of IUPAC, the N2 adsorption-desorption isotherms of conventional and polygonal Zn2TiO4 nanostructures exhibited the type-III isotherm with H3-type hysteresis loop which is related to large mesopores or macro-pores surrounded via a matrix of smaller pores. As can be seen from Fig. 6, the BET specific surface area of polygonal Zn2TiO4 is 3.495 m2g1 which is much higher than

that of conventional Zn2TiO4 (0.740 m2g1). Comparatively, a large specific surface area of polygonal Zn2TiO4 (five-fold to the conventional Zn2TiO4) was utilized for the superior adsorption as well as provides a number of surface reactive sites for the photocatalytic process. Therefore, it leads to the improvement of the photocatalytic efficiency/rate. The UV-Vis DRS of as-synthesized conventional and polygonal Zn2TiO4 nanostructures have been shown in Fig. 7a. Conventional and polygonal Zn2TiO4 were shown two absorption edges in the optical spectrum which indicate the existence of Zn2TiO4 in two phases i.e; cubic and tetragonal. The associated optical band gaps were calculated using the following Tauc equation (1) [37]. 1=2

ðahwÞ

¼ B hw Eg

(1)

Here, a is the absorption coefficient, B is band tailing parameter, Eg is band gap of the semiconductor, and hw is the energy of the photons from Einstein's equation [38]. According

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Table 1 e MP-AES analysis results of as-synthesized conventional and polygonal Zn2TiO4. S.No.

Samples

MP-AES (Wt %) Zn: Ti (mol %) Zn

Ti

1

Polygonal Zn2TiO4

53.2

19.96

2

Conventional Zn2TiO4

52.36

20.7

2:1.02 Zn1.99Ti1.01O4 2:1.08 Zn1.95Ti1.05O4

to Tauc plots have been shown in Fig. 7b the cut off observed at 413 nm is attributed to tetragonal phase of conventional and polygonal Zn2TiO4 with the corresponding band gap found to be 3 eV. The FT-IR spectra of as-synthesized conventional and polygonal Zn2TiO4 nanostructures have been shown in Fig. 8. It shows that the broad band has been observed at 36003100 cm1 was originated from stretching vibrations of hydroxyl groups of chemisorbed and/or physisorbed water molecules. The 1650 cm1 band arises from bending vibrations of hydroxyl groups. The C]O stretching vibrations were expressed at 1568 cm1. In the spectra, the bands observed in the region 1000-400 cm1 due to metal-oxygen linkages. A broad and weak band around 713 cm1 corresponding to the ZnOn polyhedron and the broad band around ~594 and one more weak and small band at ~435 cm1 corresponding to the stretching vibrations in the octahedral TiO6 group which appears in all kinds of zinc titanates [39]. Photoluminescence (PL) spectroscopic measurements of synthesized photocatalysts have been subsequently used to examine the migration, transfer, and recombination processes of the photoexcited charge carriers and shown in Fig. 9. The emission spectra revealed a strong emission peak at 345 nm, following broad emission peak at 420 nm and a weak emission peak at 468 nm for both conventional and polygonal Zn2TiO4. The emission peaks at 345 and 420 nm corresponding to the band to band transitions. These were mentioned as NBE (near band edge) emissions which are closed to the absorption noticed at 340 and 418 nm in absorption spectra [40]. The emission peak at 468 nm may be due to the presence of defects like Zn2þ and O2 vacancies [24]. However, the emission peak intensities of the polygonal Zn2TiO4 is smaller, indicating the slower recombination rate of photo excited charge carriers compares to conventional Zn2TiO4. Due to the high surface area: volume ratio of the polygonal Zn2TiO4, the lower recombination rate was tentatively confirmed to surface charge trapping of photoexcited charge carriers. Therefore, it leads to the improvement of the photocatalytic efficiency.

Photocatalytic hydrogen evolution

Fig. 5 e EDX elemental mapping images of polygonal Zn2TiO4 nanostructures.

Furthermore, to confirm the H2 evolution from photocatalytic water splitting, the H2-evolution potentials of all the synthesized photocatalysts have been inspected under solar light irradiation in an aqueous solution containing glycerol (5 vol %) as the sacrificial hole scavenger. For comparison commercially available TiO2 and ZnO photocatalyst samples have been also tested for H2 evolution from photocatalytic water

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Fig. 6 e Nitrogen adsorption-desorption studies of as-synthesized conventional (a) and polygonal (b) Zn2TiO4 nanostructures.

Fig. 7 e UV-Vis-DRS absorption plots (a) and Tauc plot (b) of as-synthesized conventional and polygonal Zn2TiO4 nanostructures.

Fig. 8 e FT-IR spectra of as-synthesized conventional and polygonal Zn2TiO4 nanostructures.

Fig. 9 e Room temperature PL Spectra of as-synthesised conventional and polygonal Zn2TiO4 nanostructures.


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splitting under similar conditions. No obvious H2 evolution was noticed over all photocatalysts in the control experiments (without photocatalyst), demonstrated that the H2 was truly evaluated from the photocatalytic process. Table 2 shows the results of the amount of H2 evolved under solar light irradiation as well as catalyst amount of the synthesized conventional, polygonal Zn2TiO4 and commercially available TiO2, ZnO photocatalyst samples. Fig. 10a shows the analogical studies of photocatalytic H2 evolution for synthesized conventional, polygonal Zn2TiO4 and commercially available TiO2, ZnO photocatalysts under solar light irradiation and Fig. 10b shows the analogical studies of photocatalytic H2 evolution for synthesized conventional and polygonal Zn2TiO4 photocatalysts during 4 h solar light irradiation. Among all the photocatalyst samples, polygonal Zn2TiO4 photocatalyst was found to be excellent photocatalyst as compared to other samples. The polygonal Zn2TiO4 showed H2 evaluation as high as 529 mmolh1g1, which is higher than that of conventional Zn2TiO4 (467 mmolh1g1) under solar light irradiation and there is no H2 evolution for commercially available TiO2, ZnO. This indicates that the Zn2TiO4 seems to be a polygonal morphology which could lead to the proper bandgap and also band edge potential for H2 evolution over other photocatalyst samples. The high photocatalytic activity of polygonal Zn2TiO4 compare to conventional Zn2TiO4 photocatalyst was supported by BET and PL studies. Because of the introduction of polygonal morphology, (1) the BET surface area increases, which leads to high surface active sites for photocatalytic reaction and (2) the PL intensity decreases, which leads to slower recombination of charge carriers. Finally, higher surface area: volume ratio and slower recombination rate of the polygonal Zn2TiO4 was lead to high photocatalytic H2 evolution compared to conventional Zn2TiO4. Four typical time courses of H2 evolution under solar light irradiation from two different photocatalysts were shown in Fig. 10c. As noticed in 8c, the rates of hydrogen evolution by two different photocatalysts displayed a sharp linear increase during the entire photocatalytic reactions, clearly gives the information about stability and reproducibility of photocatalytic H2 evolved over synthesized photocatalysts. Fig. 11 shows band gap and band edge potentials of Zn2TiO4, TiO2 and ZnO photocatalysts reported by Yasumichi Matsumoto [36]. Although the three photocatalysts approximately have the same bandgap energy, the conduction band potential of Zn2TiO4 is located at the more negative position (ECB ¼ 0.9 Vs NHE) than that of the TiO2 (ECB ¼ 0.1 Vs NHE) and ZnO (ECB ¼ 0.1 Vs NHE). That is

Table 2 e Rate of H2 evolution for synthesized Polygonal Zn2TiO4, Conventional Zn2TiO4, TiO2 and ZnO under solar light irradiation. S.No. 1 2 3 4

Photocatalyst Polygonal Zn2TiO4 Conventional Zn2TiO4 TiO2 ZnO

Amount Rate of H2 (mg) Evolution mmolh1g1 5 5 5 5

529 467 No Activity No Activity

Fig. 10 e a. Analogical studies of photocatalytic H2 evolution under solar light irradiation for TiO2 (A), ZnO (B), synthesized conventional (C) and Polygonal (D) Zn2TiO4 photocatalysts. b. Analogical studies of photocatalytic H2 evolution during 4 h under solar light irradiation for synthesized conventional and polygonal Zn2TiO4 photocatalysts. c. H2 evolution rates of conventional and polygonal Zn2TiO4 photocatalysts under solar light irradiation.

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[17]. Therefore, polygonal Zn2TiO4 is a potential candidate for photocatalytic water splitting process. Amongst the zinc titanates reported, polygonal Zn2TiO4 has been established as an excellent photocatalyst. The utmost H2 evolution (529 mmolh1g1 per 5 mg of photocatalyst) was obtained under solar light irradiation, which was much higher than H2 produced using Zn2TiO4, Rh doped Zn2TiO4 [24,41], ZnTiO3-TiO2 [42], TiO2-Zn2Ti3O8 [43], TiO2-ZnO [44] shown in Table 3 and further these results are much better than other photocatalysts i.e Hierarchical Cu2O nanocubes [16], g-C3N4 sheets [45], MoS2 quantum dots [46], CdS nanoparticles [47], ZrO2 [48], In2S3 [49], Anatase and Rutile TiO2 [50], CdS nanoparticles [51], TiO2 and Bi2O3 [52], g-C3N4 and WO3 [53], TiO2 [54], P25 (TiO2) [55], g-C3N4/Ag [56] shown in Table 4 so far reported. Fig. 11 e The Bandgap and band edge positions of Zn2TiO4, TiO2 and ZnO photocatalysts.

in the case of Zn2TiO4 an electron which is excited from valence band edge to the conduction band edge has a higher negative potential suited to reduce a hydrogen ion (Hþ), and a hole also has a lower reduction potential which is present in the valence band for decomposition of glycerol to Hþ and CO2. The figure clearly consistent with this fact for polygonal Zn2TiO4 and further compares it with the case of TiO2 and ZnO

Photocatalytic mechanism of H2 evolution from water splitting The photocatalytic mechanism of H2 evolution by water splitting under a solar light irradiation has been mentioned by many researchers [16,45]. Fig. 12 is shown the proposed mechanism for photocatalytic hydrogen evolution from a water-glycerol solution of polygonal Zn2TiO4 under solar light irradiation. When the reaction suspension is exposed to solar light, the semiconductor photocatalyst produces electrons at conduction þ band edge (e CB ) and holes at valence band edge (hVB ). The comprehensive photocatalytic mechanism is specified below:

Table 3 e Reported rate of H2 evolution for zinc titanate families. S.No. 1 2 3 4 5 6 7 8

Photocatalyst

Amount (mg)

Rate of H2 Evolution (mmolh1g1)

Reference

Polygonal Zn2TiO4 Conventional Zn2TiO4 Zn2TiO4 Zn2TiO4 Rh doped Zn2TiO4 ZnTiO3-TiO2 TiO2-Zn2Ti3O8 TiO2-ZnO

5 5 100 100 100 50 0.1 500

529 467 633 (From H2S splitting) 13.07 58.05 6.8 10.16 203

Present work Present work [22] [36] [36] [37] [38] [28]

Table 4 e Recent report of rate of H2 evolution using various photocatalysts. S.No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Photocatalyst

Amount (mg)

Rate of H2 Evolution (mmolh1g1)

Reference

Polygonal Zn2TiO4 Conventional Zn2TiO4 Hierarchical Cu2O nanocubes g-C3N4 sheets MoS2 quantum dots CdS nanoparticles ZrO2 b-In2S3 Anatase and Rutile TiO2 CdS nanoparticles TiO2 and Bi2O3 g-C3N4 and WO3 TiO2 P25 (TiO2) g-C3N4/Ag

5 5 25 50 4 100 100 50 50 50 50 50 100 50 50

529 (Without co-catalyst) 467 (Without co-catalyst) 15 (0.6 wt% Pt) 108.5 (0.5 wt% Pt) 200 (Without co-catalyst) 0.534 (Without co-catalyst) 203 (Without co-catalyst) 91 (1 wt% Pt) 37 and 5 (0.6 wt% Pt) 172 (Without co-catalyst) 12.6 and 0 (Without co-catalyst) 440 and traces (0.1 ml of H2PtCl6 (1 g/100 ml)) 10.3 (Without co-catalyst) 172 (Without co-catalyst) 3.89 (Without co-catalyst)

Present work Present work [16] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50]


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morphology has been confirmed by TEM studies. The synthesized polygonal Zn2TiO4 nanostructures showed outstanding photocatalytic properties for the H2 evolution from the water splitting under irradiation of solar light compare to conventional Zn2TiO4 and commercially available TiO2, ZnO. The high photocatalytic activity for Zn2TiO4 nanostructures compare to commercially available TiO2, ZnO has been explained by more negative conduction band potential (ECB ¼ 0.9 Vs NHE). For the H2 evolution from the water splitting under irradiation of solar light, polygonal Zn2TiO4 nanostructures showed 1.2 times higher photocatalytic activity than conventional Zn2TiO4 and exhibited outstanding stability. The enhanced photocatalytic ability originated from its polygonal morphology increase their surface area and more attainable active sites on the surface and decrease in the PL intensity compared to conventional Zn2TiO4. The present work thus provides an insight into the design and development of noble metal free and cost-effective nanostructured semiconductor photocatalysts for diverse photocatalytic applications like dye degradation, CO2 reduction, and sensing applications. Fig. 12 e The proposed mechanism of H2 evolution from water splitting under solar light irradiation over polygonal Zn2TiO4 nanostructures.

Acknowledgments

ZTO þ hwðSun lightÞ/ZTO hþ VB þ eCB /1½Excitation process

Author thankful to the MHRD, Govt. of India for providing fellowship and also thanks to SAIF, NEHU (North-Eastern Hill University), Shillong for providing TEM facility.

. þ ZTO hþ ZTO eCB VB þ eCB /ZTO hVB /2½Charge transfer process þ ZTO hþ VB þ Glycerol þ H2 O/ZTO þ H þ Oxidized Intermediates/3½Holes trapping þ ZTO hþ VB þ Oxidized Intermediates/ZTO þ 14H þ 3CO2 /4 ZTO eCB þ 14Hþ /ZTO þ 7H2 /5½Hydrogen evolution process The overall reaction is given below: ZTO þ Glycerol þ 3H2 O þ hwðSun lightÞ/ZTO þ 7H2 þ 3CO2 In a nutshell, the Zn2TiO4 nanostructures were observed to be excellent oxide photocatalysts for hydrogen production from water splitting, which have high reduction potential and lower oxidation potential. It is noteworthy that the polygonal Zn2TiO4 nanostructures have been prepared for the first time. Furthermore, the hydrogen evolution from water splitting using polygonal Zn2TiO4 nanostructures under natural solar light is hitherto unattempted.

Conclusions In summary, phase pure and novel Zn2TiO4 nanostructures with polygonal morphology have been successfully synthesized by a facile solid state method employing citric acid as a structure direct agent. This solid-state method avoids the prolonged reaction times or several steps. The polygonal

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