SrTiO3 tubular structures co-doped with Cr and Ta were synthesized through a combination of solvothermal-hydrothermal processes. X-ray photoelectron spectroscopy (XPS) measurements of the oxidation state of Cr ions reveal that the formation of Cr6+ ions, which would serve as the non-radiative recombination centers for photogenerated electrons and holes, was suppressed without the process of high temperature hydrogen reduction. Compared to similar co-doped materials synthesized by solid-state reaction, (Cr, Ta) co-doped SrTiO3 tubular structures have significantly higher photocatalytic activity for hydrogen evolution as measured in an aqueous methanol solution under visible light irradiation.
©2012 Optical Society of America
Solar-driven water splitting for hydrogen production using semiconductor-based photocatalysts has attracted a significant amount of attention from both the fundamental science point of view and the potential as a clean energy solution. Since Fujishima and Honda’s pioneering work in 1972 , different classes of semiconductor materials, such as metal oxides as best represented by TiO2, have been developed and evaluated as potential candidate photocatalysts for high-efficiency solar-driven hydrogen conversion [2,3]. Considering that UV light consists of only a small portion (~4%) of the solar spectrum, the energy conversion efficiency for solar-hydrogen water splitting using TiO2 as the photocatalyst is typically <1%, which is limited to largely by TiO2’s wide band gap (~3.0 eV) . To enhance light absorption in the visible range of the solar spectrum, TiO2 has been disorder engineered  or doped by various metal or nonmetal ions that yield narrowed band gaps . On the other hand, TiO2 often underwent nanostructure design to create a hierarchical assembly, such as sphere-in-sphere structure [7,8] and spiny mesoporous tubular structure , aimed for more efficient light harvesting through multiple optical scattering or reflection.
A variation of Ti-based metal oxide, SrTiO3, which has a band gap of ~3.2 eV, is able to split water into H2 and O2 under UV irradiation. Visible light response has been reported when SrTiO3 was doped or co-doped with transition metal ions, such as Rh3+, Pt4+, Cr3+, Ni2+, and Sb5+ [10–15]. In general, charge balance is important for doped or co-doped SrTiO3 as a photocatalyst to reduce the chance of electron-hole recombination. It was suggested that co-doping SrTiO3 with Ta5+ and Cr3+ ions might benefit as the formation of Cr6+ ions and oxygen defects that would act as non-radiative recombination centers for photogenerated charges should be suppressed . Suppression of non-radiative recombination usually results in high photocatalytic activities. However, (Cr3+, Ta5+) co-doped SrTiO3 synthesized through solid-state reactions had a induction period for photocatalytic H2 production, even after high temperature reduction under H2 environment, indicating the existence of Cr6+ ions in the material . Alternatively, metal oxide-based photocatalysts synthesized by hydrothermal and sol-gel methods usually exhibit higher photocatalytic activities, presumably owing to factors such as better crystallinity, larger surface area, and charge-transfer favored nanostructure [2,3,17–19].
In the present study, (Cr, Ta) co-doped SrTiO3 tubular structures were developed through a two-step solvothermal-hydrothermal process. The materials were investigated for their capacity of photocatalytic hydrogen production under visible light. The oxidation states of Cr ions and their effects on photocatalytic H2 production were discussed, which were compared to similar co-doped materials synthesized through solid-state reaction.
2.1 Synthesis of (Cr, Ta) co-doped SrTiO3 tubular structures
In the first step, tubular (Cr, Ta) co-doped TiO2 was prepared by a solvothermal approach . Typically, approximately 4.6 g of TiOSO4·xH2SO4·xH2O, as the Ti source, together with 0.31 g of Cr(NO3)3·9H2O and 0.28 g of TaCl5, with an elemental ratio of Ti: Cr: Ta = 0.96: 0.04: 0.04, were dispersed in a mixed solution of absolute ethanol (40.0 g), ethyl ether (16.62 g) and ethylene glycerol (27.01 g) under stirring. The resulted green dispersion was transferred into a Teflon autoclave followed by solvothermal treatment at 110 °C for 48 h. After filtering and washing with water, the dried precipitation was annealed in air at 550 °C or 900 °C with ramping rate of 1 °C/min to obtain crystallized (Cr, Ta) co-doped TiO2 that have a tubular structure, denoted as T-550 and T-900, respectively.
In the second step, T-550 and T-900 were hydrothermally reacted with excessive Sr(OH)2 at 250 °C for 12–48 h to synthesize the final tubular (Cr, Ta) co-doped SrTiO3 products (Sr: Ti = 10: 1). Process parameters for samples denoted as ST-01, ST-02 and ST-03 are given in Table 1 .
As a reference, (Cr, Ta) co-doped SrTiO3 was also synthesized through a solid-state reaction followed by H2 reduction . Stoichiometric mixture of SrCO3, TiO2, Cr2O3 and Ta2O5 was calcined at 1050 °C for 20 h in air. The ramping rate was 1 °C/min. Then H2 reduction treatment at 500 °C was performed on the oxide precursor in order to obtain (Cr, Ta) co-doped SrTiO3, which was denoted as ST-SSR.
X-ray diffraction (XRD) patterns of the samples were obtained from a PANalytical X’pert MPD Pro diffractometer using Ni-filtered Cu Kα irradiation (1.5406 Å). The patterns of powder X-ray diffraction (PXRD) in the 2θ range from 10.0 to 80.0° were collected (40 kV, 40 mA; real-time multiple strip (RTMS) detector, X'Celerator) with a scan step size of 0.0334° and counting time of 19.685 s. A divergence slit of 1°, antiscatter slit of 2°, and a 0.04 radian Soller slit were used in the incident beam path, whereas a 6.6 mm antiscatter slit and a 0.04 radian Soller slit were used in the diffracted beam path. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD) with monochromatic Al Kα radiation (hν = 1486.69 eV) and with a concentric hemispherical analyzer working at 15 kV and 8 mA, and with the pressure of sample analysis chamber under high vacuum (< 3×10−9 Torr). Both survey-scan (pass energy 160 eV, step size 1000 meV) and high-resolution (pass energy 40 eV, step size 50 meV) spectra of X-ray photoelectron spectroscopy (XPS) were obtained with an analysis area of 300×700 μm and with charge corrected to C 1s line of adventitious carbon set to 284.8 eV. UV-Vis absorption spectra were recorded by a Hitachi U-4100 UV–vis–near-IR spectrometer under diffuse reflectance (DR) mode with BaSO4 as the reference. Sample morphology was examined by a JEOL JSM-6700FE scanning electron microscope with accelerating voltage of 15 kV. The Brunauer–Emmett–Teller (BET) surface areas of samples were deduced from N2 adsorption-desorption isotherms at 77 K. The isotherms were determined by using an Accelerated Surface Area and Porosimetry Analyzer (ASAP 2020, Micromeritics) after degassing the samples at 120 °C for 6 h.
2.3 Evaluation of photocatalytic activity
Photocatalytic hydrogen evolution was performed in a side irradiation Pyrex cell. A 300-W Xe lamp was used as the light source, and the UV portion of the light was removed by a cut-off filter (λ > 420 nm). Hydrogen gas was analyzed by an online thermal conductivity detector (TCD) gas chromatograph (NaX zeolite column, nitrogen as a carrier gas). In all experiments, an amount of 0.30 g photocatalysts was thoroughly suspended, using a magnetic stirrer with constant rotational velocity, into a CH3OH aqueous solution (18.5 vol%, 270 mL) in the Pyrex cell. Nitrogen gas was purged through the cell before reaction in order to remove oxygen. 0.5 wt% Pt as cocatalyst for the promotion of hydrogen evolution was photo-deposited in situ on the photocatalyst from a H2PtCl6·6H2O precursor. The temperature for all photocatalytic reactions was kept at 20 °C. Control experiments showed no appreciable H2 evolution without irradiation or photocatalysts. All the photocatalyts cannot produce hydrogen in the absence of methanol as sacrificial reagent.
3. Results and discussion
Figure 1 shows SEM images of crystallized tubular (Cr, Ta) co-doped TiO2, which are precursors of the final SrTiO3 products. It is noted that annealing of (Cr, Ta) co-doped TiO2 (T-550 and T-900) would keep the morphology of a tubular microstructure, suggesting good thermal stability. Figure 1b revealed that the tubular structure has a rough surface consisting of spiny complex architecture for the sample annealed at 550 °C (T-550). While annealed at 900 °C, T-900 sample maintained the hollow tube structure, but with the surface spiny architecture destructed and coalesced (Fig. 1d).
Figure 2 shows SEM images of the final tubular (Cr, Ta) co-doped SrTiO3 materials. As seen in Figs. 2a-2d, ST-01 and ST-02 samples exhibit similar morphology, which keeps the tubular structure like the tubular T-550 template. The walls of the tubes were not characterized by spiny surface, but self-assembled nanoparticles with diameters of 40–60 nm (Figs. 2b and 2d). The ST-03 sample that was hydrothermally templated from T-900 also keeps the tubular structure (Fig. 2e), but with the wall of the tubes comprised of uniform cubes with side length of approximately 150 nm (Fig. 2f).
X-ray diffraction (XRD) was used to characterize the structures of the crystallized tubular (Cr, Ta) co-doped TiO2 (T-550 and T-900). As shown in Fig. 3 , XRD patterns of T-550 and T-900 could be indexed to anatase and rutile TiO2, respectively. The broadening of the reflections in the XRD pattern of T-550 indicated its poor crystallinity with small crystallites on the nanometer scale, while the intense XRD peaks of T-900 implied its good crystallinity and large crystallites, which are resulted from higher annealing temperature. The crystallite sizes of T-550 and T-900 calculated from the peak at ca. 25.2° and 27.5° using the Scherrer formula are 9.6 nm and 27.1 nm, respectively. The structure of the final tubular (Cr, Ta) co-doped SrTiO3 were also characterized by XRD as shown in Fig. 3. ST-01 and ST-02 have similar XRD profiles that could be assigned to the pure SiTiO3 phase. No other phases like unreacted anatase TiO2 were observed. By contrast, ST-03 possesses unreacted rutile TiO2 together with the SrTiO3 product, although it experienced a longer hydrothermal reaction process than ST-01 (24 h vs. 12 h). By using the normalized RIR (Reference Intensity Ratio) method, the percentages of rutile (JCPDS No. 01-076-0318, RIR = 3.51) and SrTiO3 (JCPDS No. 01-079-0174, RIR = 8.15) in ST-03 were determined to be 18.1 and 81.9 wt%, respectively. This indicated that anatase TiO2 should be easier to react with Sr(OH)2 to form SrTiO3 in hydrothermal condition as compared to rutile TiO2. Additionally, one cannot find other oxides such as Cr2O3 and Ta2O5.
Optical properties of tubular (Cr, Ta) co-doped SrTiO3 products (ST-01, ST-02, and ST-03) were characterized with UV-Vis spectra (Fig. 4 ). ST-01 and ST-02 prepared from T-550 possess similar absorption bands in the visible region, with onsets around 520 nm. The shapes of UV-Vis spectra were characteristics of metal doping, indicating discontinuous levels formed by the dopants in the forbidden band . The absorption bands of ST-01 and ST-02 in the visible light region can be attributed to a Cr3+ to Ti4+ charge-transfer transition, which agrees well with the absorption spectra of SrTiO3 doped with Cr3+ ions  or co-doped with Cr3+ and Ta5+ . ST-03 prepared from T-900 also shows an absorption band in the visible region due to Cr3+ to Ti4+ charge-transfer transition, but with onset red-shifted to around 550 nm when compared to ST-01 and ST-02. Such a change in the absorption spectra implies the existence of Cr6+ ions in ST-03 resulted from high temperature annealing of tubular (Cr, Ta) co-doped TiO2 precursor at 900 °C. As shown in the inset of Fig. 4, the color of ST-01 and ST-02 was gray-green whereas the color of ST-03 was yellow. The color difference gave an additional clue to the different oxidation states of Cr ions in ST-01, ST-02 and ST-03.
Figure 4 also shows the UV-Vis spectra of ST-SSR as the reference, which was prepared by a solid-state reaction approach followed by H2 reduction. ST-SSR had broad absorption bands in the region of 550–700 nm, which is quite similar to the (Cr, Ta) co-doped SrTiO3 samples reported in literature . Though it was suggested that Cr6+ involved in SrTiO3 could be suppressed by Ta5+ co-doping or reduced to Cr3+ ions by H2 reduction, there are Cr6+ ions in ST-SSR as deduced from its broader absorption bands in visible region when compared to those of ST-01, ST-02 and ST-03. Additionally, short wavelength absorption of ST-01, ST-02 and ST-03 ascribed to the band-to-band transition of SrTiO3 were enhanced due to the possibility of multiple reflections of trapped light in the tubular structure [8,9,22].
X-ray photoelectron spectroscopy (XPS) measurements were carried out to examine the oxidation states of Cr ions in the tubular (Cr, Ta) co-doped SrTiO3 (ST-01, ST-02, and ST-03) samples. As shown in Fig. 5 , for ST-01 and ST-02 samples the peaks for Cr 2p1/2 and Cr 2p3/2 were obtained at about 586.8 eV and 577.1 eV, respectively, which could be assigned to the Cr3+ ions  in ST-01 and ST-02. No other XPS peak for Cr6+ was observed. In contrast, in the case of ST-03, the majority of Cr ions were Cr3+, but a small amount of Cr6+ seems to exist, as detected by the XPS peak at about 580.9 eV. As these tubular (Cr, Ta) co-doped SrTiO3 (ST-01, ST-02, and ST-03) samples were synthesized from tubular (Cr, Ta) co-doped TiO2 precursor, the doped Cr3+ (or Cr6+) and Ta5+ ions should locate at the Ti sites.
Figure 6 shows measurements of photocatalytic H2 evolution from an aqueous methanol solution over the tubular (Cr, Ta) co-doped SrTiO3 (ST-01, ST-02, and ST-03) samples and the reference (ST-SSR) under visible light irradiation (λ > 420 nm). All co-doped SrTiO3 photocatalysts, including ST-SSR, are able to produce H2. Considering the photophysical properties revealed by the UV-Vis and XPS spectra, it could be determined that the ability of these (Cr, Ta) co-doped SrTiO3 catalysts for visible-light-driven photocatalytic H2 evolution arose from the doping of Cr3+ ions. Compared with ST-01, ST-02 showed much higher H2 evolution rate. This could be the result of fewer defects in ST-02 synthesized via a longer hydrothermal process (48 h vs. 12 h), as the similar morphology (Fig. 2) and visible light absorption ability (Fig. 4) would not lead to great difference between the photocatalytic activities of ST-01 and ST-02. In the case of ST-03, the photocatalytic activity for H2 production was relatively low, even though the visible light response was better than those of ST-1 or ST-02. This could be attributed to the existence of the rutile phase in ST-03, of which the ability for H2 evolution was low. It is known that rutile TiO2 possesses lower conduction band level than SrTiO3, thus the photoexcited electrons would transfer from the conduction band of SrTiO3 to that of rutile TiO2, leading to weaker driving force for proton reduction. This is also the reason why (Sb, Cr) co-doped SrTiO3 was reported to display higher activity for H2 production than (Sb, Cr) co-doped TiO2 under visible light . Moreover, both ST-01 (19.5 m2/g) and ST-02 (19.1 m2/g) had much higher surface area than ST-03 (2.8 m2/g), as obtained from BET analysis. The high surface areas could enrich the reactive sites, and also enhance the adsorption of reactants, thereby accelerate the photocatalytic redox reactions for hydrogen production. This may be the other reason why ST-03 showed much lower photocatalytic activity than ST-01 and ST-02.
As the reference, ST-SSR exhibited the lowest photocatalytic activity. It’s reasonable to assume that the hydrothermal method is able to produce photocatalysts with good crystallinity as well as high surface areas that exhibit higher photocatalytic activity for water splitting than those synthesized by solid-state reaction [24,25]. The more efficient light harvesting of the tubular structure due to multi-scattering effect may be another possible reason for the higher photocatalytic activities of hydrothermally (vs. solid-state reaction) synthesized materials . By checking into the initial stage (~1 h) of photocatalytic reaction, ST-03 showed very low activity for H2 evolution. After this stage, a considerable H2 evolution rate was obtained. It was assumed that the Cr6+ ions had been reduced to the Cr3+ ions by photogenerated electrons during this induction period . In contrast, there was no induction period for photocatalytic H2 production over ST-01 or ST-02, owing to the absence of Cr6+ ions as revealed by XPS measurements. The longer induction period (~3 h) of ST-SSR indicated the presence of larger amount of Cr6+ ions, even after H2 reduction.
(Cr, Ta) co-doped SrTiO3 tubular structures were fabricated by a solvothermal-hydrothermal two-step process. It was found that the tubular (Cr, Ta) co-doped SrTiO3 synthesized using anatase tubular (Cr, Ta) co-doped TiO2 as the precursor showed higher photocatalytic activity for hydrogen production than that synthesized from rutile precursor, in which the unreacted rutile TiO2 with lower conduction band level than SrTiO3 led to weaker driving force for H2 production. XPS measurements revealed the formation of Cr6+ ions, which would work as the charge recombination centers, could be avoided using the solvothermal-hydrothermal two-step process. The resulted tubular (Cr, Ta) co-doped SrTiO3 exhibited much higher photocatalytic H2 production activities without an induction period when compared to that synthesized by the solid-state reaction approach.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51102194 and 51121092), Natural Science Foundation of Shaanxi Province (No. 2011JQ7017), Doctoral Program of the Ministry of Education (No. 20110201120040) and National Basic Research Program of China (No. 2009CB220000). J. Shi thanks Prof. J. Ye from National Institute for Materials Science (NIMS), Japan for the help with photocatalytic activity evaluation. S. Shen was supported by “the Fundamental Research Funds for the Central University” from Xi’an Jiaotong University, China. Additional support was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy.
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