Innovative Construction of SrTiO3 Nanoparticle/ZnO Nanorod Heterojunction Photocatalyst for Hydrogen Production Under Simulated Sunlight Radiation

INTRODUCTION
Nowadays, Because of the increasing global energy demand, there is a serious need for the development of new energy resources, however, considering of the environmental pollution issues, the clean and renewable energy sources is the best choice [1]. Among the clean and sustainable energy sources, Hydrogen energy recognized as a promising one because of its characteristics of less environmental pollution, convenient storage and  high energy density [2, 3]. There are many methods for hydrogen production such as methane steam reforming (MSR) reaction, electrolysis of water, and gasification of coal, however, photocatalytic water splitting by semiconductor photocatalysts is cost-effective, highly efficient, clean and environmentally friendly technique [2, 4].
ZnO semiconductor has been extensively used for photocatalytic Hydrogen production from water splitting because of its excellent features such as environmental friendliness, good stability, non-toxicity,  and low cost [5, 6]. However, ZnO has a poor photocatalytic Hydrogen production activity due to the low visible light absorption, and fast charge carriers recombination rate [7, 8]. In this regards, various strategies have been suggested for improvement of its photocatalytic performance, such as doping with other elements [9], compositing with carbon nanostructures [10, 11], surface engineering [12], and hybridization with other semiconductor photocatalysts in heterojunction nanocomposites [13-15]. Among these methods, construction of the heterojunction photocatalysts are promising one because it could efficiently separate photo-induced electron/hole pairs [16]. Different heterojunction nanocomposites of ZnO were synthesized such as Bi2O3/ZnO [12], ZnO/BiVO4 [17], WO3/ZnO [18], ZnO/ZnS [19], and ZnO/AgBr [20]. 
Strontium titanate (SrTiO3) has a cubic perovskite structure of ABO3, with Sr2+ and Ti4+ in the A and B sites, respectively [21]. This n-type semiconductor with band gap of ~3.2 eV has been widely used as an efficient photocatalyst in a wide range of photocatalytic applications due to its suitable valence and conduction band position, good structural and thermal stability, resistance to photo-corrosion and excellent photocatalytic activity [22, 23]. However, because of its high band gap and fast charge carriers recombination rate, this semiconductor has low photocatalytic efficiency under sunlight irradiation [24]. For improvement of its photocatalytic performance this semiconductor is used as a cocatalyst with other semiconductors in a heterojunction forms and different heterojunction nanocomposites of SrTiO3 were synthesized such as WO3/SrTiO3 [25], g-C3N4/SrTiO3 [26], SrTiO3/Bi2S3 [27], and SrTiO3/BiOBr [28].
Inspired from the above discussions, in current study, ZnO/SrTiO3 heterojunction nanocomposite was synthesized from new method by compositing of ZnO nanorods, and SrTiO3 nanoparticles through hydrothermal technique and was applied for photocatalytic Hydrogen production under simulated sunlight irradiation. Due to the many advantages of hydrothermal method such as high purity and homogeneity of product, narrow particles distribution, high yield, cost-effectiveness, and ease of morphology controlling [29], this technique was used for preparation of the nanocomposite. The as-prepared composite was fully characterized by XRD, FESEM, DRS, PL, and Mott-Schottky analysis. Furthermore, based on the optical, photoelectrochemical and photocatalytic activity tests results, a possible charge transfer mechanism was proposed. 

MATERIALS AND METHODS
Materials
Zn(NO3)2.6H2O, 25% ammonia solution, Na2CO3, ethanol, ethylene glycol, Titanium butoxide, Sr(NO3)2 and KOH were purchased in analytical grade from Merck, Germany, and were used as raw materials without any purification. 

Synthesis of ZnO nanorods
ZnO nanorods were prepared by hydrothermal method. Briefly, 5 mmol of Zn(NO3)2.6H2O was dissolved in 50 ml ethylene glycol, the final solution was obtained by adding 3 ml of 25% ammonia solution and 5 mmol of Na2CO3. Then the resulted mixture was subjected to hydrothermal process in a 75 ml Teflon lined stainless autoclave and maintained at 120 ℃ for 10 hours. Preparation in nonaqueous solvent and presence of Na2CO3 may induce preferred growth in one direction and lead to the formation of nanorod morphology during the hydrothermal method [30]. The obtained ZnO nanorods was separated by centrifuging and washed several times with distilled water and ethanol and dried at 60 ℃. 

Synthesis of SrTiO3 nanoparticles
Hydrothermal technique was used for preparation of SrTiO3 nanoparticles. For this purpose, 3.5 ml of Titanium butoxide was dissolved in 25 ml of Butanol  then under magnetic stirring 2.1 g of Sr(NO3)2 was added, After addition of  25 ml of 2 M KOH solution, the final solution was transferred into a 75 ml Teflon lined stainless autoclave vessel and maintained at 200 °C for 12h. The final precipitates were immediately separated-out by centrifugation, washed several times with distilled water and ethanol, and dried at 60 °C.

Synthesis of ZnO/SrTiO3 heterojunction nanocomposite
For synthesis of ZnO/SrTiO3 heterojunction nanocomposite, containing 35% (w/w) ZnO and 65% (w/w)  SrTiO3, in a typical process, 1 g ZnO nanorods were fully dispersed in 25 ml of Butanol  by probe ultrasonication, then 3.5 ml of Titanium butoxide and 2.1 g of Sr(NO3)2 were disolved in above suspension by magnetic stirring. After addition of 25 ml of 2 M KOH solution, the final suspension was poured into a 75 ml Teflon lined stainless autoclave and maintained at 200 °C for 12h.  The resulted nanocomposite were separated-out by centrifugation, washed several times with distilled water and ethanol, and dried at 60 °C. 

Characterizations
The crystal characteristics of the obtained photocatalysts were analyzed by X-ray diffraction (XRD) on Philips X’ Pert MPD with Cu Kα radiation (λ= 0.15406 nm) in 2θ range from 10° to 80°. MIRA3 TESCAN field emission scanning electron microscopy (FESEM) was applied to investigate the morphology and particle size of the photocatalyst samples. Diffuse reflectance spectroscopy (DRS) in the region of 200 to 800 nm was performed by means of a Shimadzu UV-2550 UV–vis spectrophotometer. Varian Cary-Eclipse 500 fluorescence spectrometer was used to obtain the photoluminescence (PL) spectra of samples at excitation wavelength of 300 nm. Photo-electrochemical characteristics of the samples were assessed using a Gamry potentiostat in a conventional three electrode system of Pt foil (counter electrode), Ag/AgCl (reference electrode), and the prepared samples as working electrode under 570W Xenon lamp as the simulated sunlight source. 

Photocatalytic activity
The photocatalytic H2 production tests were conducted in a quartz reactor. In a typical process, 100 mg of the prepared photocatalyst samples were fully dispersed by ultrasonication in 100 ml deionized water containing TEOA as sacrificial agents, then this suspension was deoxygenated by nitrogen gas purging for 20 min. The resulted suspension was stirred under dark condition for 1 h to reach an adsorption–desorption equilibrium, and afterward was irradiated with a 570W Xenon lamp (as a simulated solar light source) at 25°C. The yield of hydrogen was measured by Shimadzu GC-2014 gas chromatograph by using N2 gas as carrier gas with a TCD detector. 

RESULTS AND DISCUSSION
XRD 
The XRD patterns of the prepared samples were illustrated in Fig. 1. For ZnO sample, the main peaks of (100), (002), (101), (102), (110), (103), (200), (112), and (201) are present at 2θ of 31.7º, 34.3º, 36.2º, 47.6 º, 56.4º, 62.9º, 66.3º, 67.9º, and 69.1º, respectively,  which well matched with wurtzite structure of ZnO (JCPDS # 36-1451) [31]. In XRD pattern of SrTiO3, the diffraction peaks positioned at 2θ of 22.2, 32.3, 39.9, 46.2, 57.6, and 67.7° can be indexed to the (100), (110), (111), (200), (211), and (220) diffraction planes of the cubic perovskite SrTiO3 with JCPDS card no. 35–0734 [36]. In the diffraction patterns of ZnO/SrTiO3 heterojunction sample, the characteristic diffraction peaks of both SrTiO3 and ZnO are present, denoting that the nanocomposite sample are successfully synthesized. The broadening of the diffraction peaks indicates nanostructure nature of the prepared samples.  

FE-SEM
The FE-SEM images were taken from the ZnO/SrTiO3 sample to characterize its morphology and particle size. The results of FE-SEM images for this sample is given in Fig. 2 (A). As seen in this image, this sample contains the ZnO nanorods with an approximate diameter of 50 nm and an approximate length of 600 nm, and SrTiO3 nanoparticles with size of about 30 nm. Furthermore, the well distribution of the SrTiO3 nanoparticles on the ZnO nanorods is clearly observed in this image. 
To verify the presence of SrTiO3 and ZnO compounds in the ZnO/SrTiO3 heterojunction nanocomposite, EDS analysis were carried out on this sample. Fig. 2 (B) shows the EDS spectrum of the ZnO/SrTiO3 sample. In the EDS spectrum, peaks corresponding to Ti, Sr, O and Zn can be obviously observed, suggesting the coexistence of SrTiO3 and ZnO in the ZnO/SrTiO3 sample, which demonstrates successfully synthesis of the ZnO/SrTiO3 heterojunction nanocomposite. 

DRS
In order to evaluate the photocatalytic performance of a photocatalyst, its optical behavior must be examined. To study the photo-response characteristics of the prepared samples, the light absorption spectra of the prepared samples were tested by UV-Vis diffuse reflectance spectroscopy (UV-DRS), and the relevant results are shown in Fig. 3. As it is clearly seen in Fig. 3(A), the absorption edges of the SrTiO3, ZnO, and ZnO/SrTiO3 samples are found to be around 380, 410, and 410 nm, respectively. As can be seen, SrTiO3 nanoparticles mainly absorb the ultraviolet light. The presence of ZnO in the structure of SrTiO3 shifts the SrTiO3 absorption edge towards the visible light region, which can improve the photocatalytic performance of the nanocomposite sample under solar light radiation. In order to study this effect more precisely, the band gap energy of the samples was examined based on Tauc formula [32]. As shown in (Fig. 3(B)) the band gap energies of the SrTiO3, ZnO, and ZnO/SrTiO3 samples are 3.4, 3.1, and 3.1 eV, respectively. Therefore heterojunction formation between SrTiO3 and ZnO, remarkably decreases the band gap energy of SrTiO3, which could results in an improvement of photocatalytic activity under solar light irradiation.

Photoluminescence (PL)
The separation of charge carriers, i.e. photoinduced electrons and holes, is one of the effective factors on the photocatalytic performance of a photocatalyst sample (PL) spectroscopy can be used to study the effect heterojunction formation between ZnO and SrTiO3 semiconductors on the separation and transportation of charge carriers in the ZnO/SrTiO3 heterojunction sample. In this case, any decrease in the PL intensity indicates a decrease in the electron-hole recombination which can results in the improvement of the photocatalyst performance [33]. As can be seen in Fig. 4, the PL intensity of the ZnO/SrTiO3 nanocomposite is remarkably lower than that of the SrTiO3 and ZnO samples, so it can be concluded that heterojunction formation between ZnO and SrTiO3 effectively reduced the electron-hole recombination. Therefore, the ZnO/SrTiO3 sample could has the improved photocatalytic activity due to the diminished charge carriers recombination rate.

Mott-Schottky 
To determine the conduction and valance band energies of the ZnO and SrTiO3 samples, Mott-Schottky tests were conducted, as depicted in (Fig. 5). The Mott-Schottky curves of ZnO and SrTiO3 samples have positive slopes, reflecting that these samples are n-type semiconductors [34]. The flat band potentials (EFB) for pure ZnO and SrTiO3 were found to be -0.4 and -0.9 V versus Ag/AgCl reference electrode (-0.2 and -0.7 V relative to NHE), respectively. It is generally documented that the conduction band potential (EC) in n-type semiconductors is located ~0.1 eV lower than EFB, and the potential of valance band (EV) of p-type semiconductors is approximately 0.1 V higher than EFB [35]. In this regard, EC of ZnO and SrTiO3 samples are calculated around -0.3 and -0.8 eV vs. NHE, respectively. Further, EV of ZnO and SrTiO3 samples are estimated through the equation EV=Eg + EC, therefore EV of these samples are 2.8 and 2.6 eV vs. NHE, respectively.  

Photocatalytic performance
The photocatalytic efficiencies of the prepared photocatalyst samples were examined by measuring the photocatalytic hydrogen production under simulated sunlight irradiation. As shown in Fig. 6. The photocatalytic performance of the ZnO/SrTiO3 heterojunction nanocomposite photocatalyst is about 10 times and three time higher than that of the pure ZnO and SrTiO3 samples, respectively, and the ZnO/SrTiO3 nanocomposite could produce 25 mmol hydrogen molecule during 3 hours simulated sunlight irradiation. The significantly improved photocatalytic performance of the ZnO/SrTiO3 sample can be attributed to the decreasing of the charge carriers recombination rate and improvement of the visible light absorbance harvesting.
For investigation the stability of the ZnO/SrTiO3 heterojunction nanocomposite during the hydrogen production reaction condition, recycling test was done on this sample. As shown in Fig. 7, the ZnO/SrTiO3 sample has reasonable stability during the photocatalytic hydrogen production reaction and maintains 92% of its initial activity after five successive cycles. Therefore this heterojunction could be recycled and reused many times for photocatalytic hydrogen evolution without significant reduction in its activity. 
Plausible type II charge transfer pathways for the photocatalytic Hydrogen production activity of the ZnO/SrTiO3 heterojunction are thoroughly discussed in Fig.8. In type II mechanism, during the simulated sunlight irradiation of the heterojunction photocatalyst, the electrons on conduction band of SrTiO3 nanoparticles migrate to the conduction band of ZnO nanorods, on the other hand, the photoinduced holes on the valence band of ZnO nanorods migrate to the valence band SrTiO3 nanoparticles [11]. In this regard, the charge carriers are efficiently separated, which results in the production of more photoinduced electrons for reduction of H+ to H2. 

CONCLUSION
In summary, a novel ZnO nanorod/SrTiO3 nanoparticles heterojunction photocatalyst was synthesized from the combination of ZnO nanorod and SrTiO3 nanoparticles through innovative hydrothermal technique and was applied for first time for photocatalytic hydrogen production under simulated sunlight irradiation. According to the obtained results, the highest photocatalytic efficiency was obtained for the ZnO/SrTiO3 heterojunction sample which is attributed to the decreasing of the charge carriers recombination rate, and enhanced visible light harvesting. Moreover, based on the Mott-Schottky calculations, a type II charge transfer pathway was suggested for the enhancement of the photocatalytic performance on the prepared heterojunction nanocomposite. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.