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Article

Photocatalytic Overall Water Splitting by SrTiO3 with Surface Oxygen Vacancies

by
Yanfei Fan
,
Yan Liu
,
Hongyu Cui
,
Wen Wang
,
Qiaoyan Shang
,
Xifeng Shi
,
Guanwei Cui
* and
Bo Tang
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China
*
Author to whom correspondence should be addressed.
Nanomaterials 2020, 10(12), 2572; https://doi.org/10.3390/nano10122572
Submission received: 5 November 2020 / Revised: 15 December 2020 / Accepted: 17 December 2020 / Published: 21 December 2020
(This article belongs to the Section Energy and Catalysis)
Browse Figures
Versions Notes

Abstract

:
Strontium Titanate has a typical perovskite structure with advantages of low cost and photochemical stability. However, the wide bandgap and rapid recombination of electrons and holes limited its application in photocatalysis. In this work, a SrTiO3 material with surface oxygen vacancies was synthesized via carbon reduction under a high temperature. It was successfully applied for photocatalytic overall water splitting to produce clean hydrogen energy under visible light irradiation without any sacrificial reagent for the first time. The photocatalytic overall water splitting ability of the as-prepared SrTiO3-C950 is attributed to the surface oxygen vacancies that can make suitable energy levels for visible light response, improving the separation and transfer efficiency of photogenerated carriers.

1. Introduction

Photocatalytic water splitting is a promising way to produce hydrogen clean energy [1,2]. Semiconductor materials have been widely used in this field because of their excellent photoelectric response characteristics. However, limited by the inappropriate redox potential, rapid recombination of photogenerated carriers or the backward reaction of H2 and O2 to H2O, most of them can only perform the semi-reaction of water splitting to produce hydrogen or oxygen alone assisted by sacrificial agents such as methanol, sodium sulfite or silver nitrate [3,4]. Therefore, although photocatalytic water splitting has been studied for several decades, there are few reported materials that can realize overall water splitting, which still remains a challenge in the photocatalytic research field [5,6,7,8].
Strontium Titanate (SrTiO3) has typical perovskite structure with the advantages of low cost and excellent chemical stability. It has been widely used as a photocatalyst [9,10]. However, due to the wide band gap, it can only utilize ultraviolet light (about 5% of sunlight) during photocatalytic reactions. Moreover, although its band structure suits the water redox potential levels [11], SrTiO3 powders alone cannot decompose water into H2 and O2 simultaneously because of rapid recombination of photogenerated carriers or fast backward reaction. It is necessary to modify the SrTiO3 particle to obtain an active photocatalyst for water splitting. Up to now, many outstanding methods, such as doped with metals or non-metals [12,13,14,15,16], or coupled with other semiconductors [17], have been investigated to solve the abovementioned issues. Recently, a modified aluminum-doped strontium titanate (SrTiO3: Al) photocatalyst was applied to perform overall water splitting with an external quantum efficiency of up to 96 per cent, which was still performed under UV light irradiation [18].
Oxygen vacancies on a particle surface often act as active sites and play an important role in photocatalytic reactions [19]. Moreover, they can provide trapping sites for photogenerated carriers and prevent them from rapid recombination. Most notably, they can build a surface energy state that can narrow the band gap, expanding the solar absorbance range to lower energy wavelengths [20]. Thus, rational control of oxygen vacancies on the catalyst surface is extremely important for improving the photocatalytic efficiency [21,22,23]. Although there are usually many oxygen vacancies in the inner part of bulk SrTiO3, they do not contribute to the sub-band gap photoexcitation or intrinsic magnetism of SrTiO3, and are different from the surface oxygen vacancies [20]. Therefore, it is necessary to synthesize SrTiO3 materials with abundant surface oxygen vacancies to realize overall photocatalytic water splitting under visible light conditions. Carbon material is a cheap and safe reductant that can capture oxygen atoms from the oxide surface to produce reductive species or create oxygen vacancies at a high temperature [24,25]. Herein, it was found that when the pristine SrTiO3 was heated under reducing atmosphere afforded by carbon, a disordered surface layer with amounts of oxygen vacancies was obtained. The as-prepared SrTiO3 with a grey color showed overall photocatalytic water splitting ability under visible light irradiation without a sacrifice reagent.

2. Materials and Methods

2.1. Materials

Ti(C4H9O)4,Sr(NO3)2, KOH, ammonia, NaClO4, terpinol, naphthol, polyvinyl alcohol (PVA), ethanol and active carbon power were purchased from Sinopharm Chemical Reagent Company. All the chemicals were of AR grade. Ultrapure water used in the experiment was Wahaha pure water.

2.2. Synthesis of SrTiO3-C950

SrTiO3 was synthesized by a hydrothermal method [26]. Typically, Ti(C4H9O)4 (2.72 g) and ethanol (70.00 mL) were mixed homogeneously by stirring at room temperature. Then, ammonia solution (2.50 mL of ammonia diluted with 30.00 mL of ethanol) was added to the mixed solution with stirring for one hour. The obtained precipitate was washed with distilled water several times and dispersed in 80.00 mL distilled water with vigorous stirring. Then, 5.08 g Sr(NO3)2, 2.24 g KOH, and 0.32 g polyvinyl alcohol (PVA) were added to the mixture solution. Finally, the obtained suspension was transferred into a stainless-steel Teflon-lined autoclave and heated at 180 °C for 12 h. The obtained solid product was washed with diluted nitric acid, distilled water and ethanol several times in turn and then dried at 80 °C for 4 h. Finally, the as-prepared SrTiO3 (0.50 g) and carbon (0.08 g) were well mixed. The mixture was calcined at 950 °C for 3 h with a heating rate of 5 °C/min under N2 atmosphere, and then naturally cooled at room temperature. The as-prepared sample was denoted as SrTiO3-C950. The mixture of SrTiO3 (0.50 g) and carbon (0.08 g) calcined at 300 °C for 3 h with a heating rate of 5 °C/min under N2 atmosphere, denoted as SrTiO3-C300, was used as reference samples.

2.3. Electrochemical Measurements

The photocurrent was measured by an electrochemical analyzer (CHI660D Instruments, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) with a standard three-electrode system. The electrolyte was a 0.10 M NaClO4 aqueous solution. The working electrode was prepared as follows. The slurry was obtained by grinding a mixture of the sample (0.05 g) and terpinol (0.10 g). Then, the slurry was coated onto a 4.00 cm × 1.00 cm indium tin oxide-coated glass (ITO glass) electrode by the doctor blade technique, dried in an oven, and calcined at 290 °C for 30 min under Ar conditions. The as-prepared sample, Pt sheet and saturated calomel electrode were used as the working electrode, counter electrode and reference electrode, respectively. Prior to the photocurrent measurements, the electrolyte (0.10 M NaClO4, pH = 6.56) was purged with Ar for 30 min.
The electrochemical impedance test was also carried out with a standard three-electrode system. The glassy carbon electrode deposited with the as-prepared samples was used as the working electrode. Pt sheet and saturated calomel electrode was used as the counter electrode and the reference electrode, respectively. The working electrode was prepared as follows: 4.00 mg the as-prepared sample was dispersed in 1.00 mL mixture of water and isopropanol with the volume ratio of 3:1, then 40.00 μL naphthol was added into the mixture and ultrasonicated for 30 min. The obtained slurry was dropped onto the glassy carbon electrode, and then dried naturally at room temperature. Finally, the prepared working electrode, counter electrode and reference electrode were immersed in a deionized water solution for testing.

2.4. Photocatalytic Water Splitting

The photocatalytic water splitting reaction was performed using an XPA-7 photocatalytic reaction instrument (Xujiang Electromechanical Plant, Nanjing, China). In a typical process, 0.40 g SrTiO3-C950, 0.20 g La2O3 and 10.00 mL water were mixed in a 20 mL quartz bottle at room temperature. The bottle was sealed with a silicone rubber septum. The suspension was thoroughly deaerated and purged with N2 for 30 min before the photocatalysis experiment. Then, the suspension was irradiated by a 1000 W Xe lamp with or without light cutoff filters (λ > 420 nm) under ambient conditions and magnetic stirring for a certain time. The gaseous production was analyzed by gas chromatography (FULI 9750, TCD, argon as the carrier gas, and 5 Å molecular sieve column).
The solar-to-hydrogen (STH) conversion efficiency was evaluated by using a 1000 W Xe lamp as the light source and 0.40 g catalyst in water. After irradiation for 48 h, an average of 360 µmol H2 was obtained, which corresponds to 93 J free energy. The incident light power was 162 mW/cm2, which was measured by a handheld Optical Power Meter (Newport 1916-R). The Light exposure area of the quartz bottle is about 3.84 cm2. Therefore, the total incident energy over the quartz bottle was about 107,495 J in 48 h. A solar-to-hydrogen efficiency of 0.09% was obtained, assuming all incident light was absorbed by optically thick nanoparticle suspension, which was obtained by the following calculation formula: Solar-to-hydrogen efficiency = Output energy of hydrogen generated/Energy of incident light = 93/107495 = 0.09% [1,27].

3. Results and Discussion

Irregular spherical SrTiO3 particles with diameters of 100–300 nm were synthesized by a hydrothermal method (Figure 1a) [26]. The carbon reduction process was performed under different calcination temperatures and retention time. It was found that the samples obtained under calcination temperature of 950 °C and a retention time of 3 h under N2 atmosphere (marked as SrTiO3-C950) and showed the highest photocatalytic activity. Compared with uncalcined SrTiO3 particles, the as-prepared SrTiO3-C950 particles showed larger particle size (Figure 1b) because of the Ostwald ripening under a high temperature, which was further determined by a laser particle size analyzer (Figure S1). The as-prepared SrTiO3 calcined at 950 °C for 3 h with a heating rate of 5 °C/min under air atmosphere was used as a reference sample. As shown in Figure 1c, the X-ray diffraction (XRD) peaks at 22.78, 32.42, 39.98, 46.48, 52.35, 57.79, 67.80, 72.54 and 77.18 can be attributed to the Miller indices of (100), (110), (111), (200), (210), (211), (220), (300) and (310), which can be indexed to the standard cubic perovskite structure (space group: Pm3m) of SrTiO3 (JPCDS No. 35–734) without any crystalline by-product. There is no obvious difference in XRD patterns of SrTiO3-C950 and the reference SrTiO3. This result indicates that the overall crystal structure of SrTiO3 is not fundamentally changed under high temperature reduction treatment. No obvious diffraction peaks of graphite carbon are observed due to its amorphous phase or low residue.
As shown in Figure 2b, a highly disordered layer with a thickness of 2.0 nm was formed on the surface of SrTiO3 after carbonization. It is ascribed to the presence of a large number of oxygen vacancies on the surface of SrTiO3 particles caused by the reduction treatment under a high temperature [28], which is further confirmed by EPR and X-ray photoelectron spectroscopy (XPS) analysis. The produced surface oxygen vacancies will act as an electron-trapping center, showing EPR signals. Therefore, EPR is one of the most powerful methods to identify the presence of oxygen vacancies in the solid materials [29]. The low-field signal with g-factor (g is the spectroscopic splitting factor in the case of a free electron) close to the free-electron value (g = 2.0023) is generally attributed to an unpaired electron trapped on an oxygen vacancy site. If the g-value of the light-induced EPR signal is smaller than that of a free electron, 2.0023, the signal is due to the trapped electron. Herein, as shown in Figure 2c, the EPR signal of SrTiO3-C950 with a g-value of 1.97 is due to the electrons captured by the oxygen vacancies. As a comparison, there are no EPR signals at the same position observed for SrTiO3.
The chemical states of Sr, Ti, O and C in SrTiO3 and SrTiO3-C950 were determined by XPS spectra (Figure S2). As shown in Figure 3a–d, compared with SrTiO3, SrTiO3-C950 shows distinctly different XPS peaks. The Sr2+ 3d5/2 peak centered at 135.58 eV, 136.88 eV and the O1s peak centered at 530.87 eV ascribed to the SrO crystal phase and are observed on SrTiO3-C950 (Figure 3a,b) [30]. The as-prepared SrTiO3-C950 samples showed the same two Ti2p peaks centered at 458.04 eV (Peak 1) and 463.64 eV (Peak 2) with the referenced SrTiO3. However, three reduced titanium ion XPS peaks centered at 457.18 eV (Peak 3), 460.3 eV (Peak 4) and 465.48 eV (Peak 5) were observed for SrTiO3-C950, which could be ascribed to the low valence state titanium of nonstoichiometric TiO2-x (0 < X < 2) species, mainly including Ti2+ 2p1/2 of TiO (Figure 3c) [31,32], which are quite different from the titanium ion XPS peaks of the SrTiO3. The new appearance of SrO and reduced Ti species indicates that there is a large number of oxygen vacancies existing in the surface layer of SrTiO3-C950 [33]. Meanwhile, it preliminary indicates that the surface of SrTiO3-C950 may be mainly composed of SrO and TiO species layers [34]. The SrO surface layers of SrTiO3 was thought to have higher water adsorption dissociation ability [35], which may be more helpful for the photocatalytic water splitting. Because of the existence of carbon standard materials for the XPS test, both the pristine SrTiO3 and SrTiO3-C950 show obvious carbon peaks (Figure 3d). However, there are significant differences between them on the XPS C1s peaks. For the SrTiO3-C950, a C1s peak centered at 285.67 eV ascribed to β-carbon connected with the oxygenic groups on the alkyl chain is believed to originate from carbon reductant [36]. It indicates that there is still a small amount of residual carbon on the surface of strontium titanate. There are no C1s peaks attributed to TiC observed for SrTiO3-C950 and pristine SrTiO3 [37], indicating that carbon atoms are not doped into a strontium titanate lattice during the reduction process.
The photocatalytic activity of SrTiO3-C950 and pristine SrTiO3 was evaluated by photocatalytic water splitting under Xe lamp light irradiation with or without light cutoff filters (λ > 420 nm) (Figure 4). Compared with pristine SrTiO3, the as-prepared SrTiO3-C950 shows an obvious photocatalytic water splitting capability with a solar-to-hydrogen (STH) conversion efficiency of 0.09% without any sacrificial reagent. Under the optimal photocatalytic reaction conditions, the average H2 evolution rate and O2 evolution rate of SrTiO3-C950 is 20.6 umol/g×h and 7.5 umol/g∙h under Xe lamp light irradiation, respectively (Figure 4a). Under the visible light irradiation, the average H2 evolution rate and O2 evolution rate of SrTiO3-C950 are 2.3 µmol/g×h and 1.0 µmol/g∙h, respectively (Figure 4b). There is no noticeable difference in the photocatalytic activity of SrTiO3-C950 under different pH conditions. In this work, the evolution rates ratio of O2 and H2 are 0.7:2, which is lower than that of the theoretical stoichiometrically ratio 1:2 of water splitting. This phenomenon has been observed in many water splitting systems, which is due to the produced O2 or intermediate oxygen species being readily absorbed by the metal elements to form steady peroxide complexes [38,39].
Although pristine SrTiO3 has suitable energy band structure for the water redox potential levels [11], it cannot decompose water into H2 and O2 simultaneously because of rapid recombination of photogenerated carriers or fast backward reaction. According to the previous reports, it is proposed that the residual carbon on the surface of SrTiO3-C950 is favorable for improving the photocatalytic efficiency [40]. However, a referenced SrTiO3 modified with carbon calcinated under 300 °C temperatures without reduction treatment, showed no photoactivity on overall water splitting. Therefore, herein, the photocatalytic overall water splitting ability of the as-prepared SrTiO3-C950 was suggested to be owed to the surface oxygen vacancies that can make suitable energy levels for visible light response and improve the separation and transfer efficiency of photogenerated carriers [41].
As shown in Figure 5a, compared with SrTiO3, the absorption wavelength of SrTiO3-C950 shows an obvious red shift to the visible light region. Estimated from the UV-V is the DRS absorption tail, the band gap of SrTiO3-C950 is 2.92 eV, which is significantly smaller than that of SrTiO3 with 3.10 eV. It is due to the formation of the mid-gap states between the valence band and the conduction band, which is attributed to surface oxygen vacancies [19]. These mid-gap states can form a continuum extending to and overlapping with the conduction band edges in a result of narrowing the band gap and expanding the solar absorbance range to lower energy waves. As shown in Figure 5b, obtained from the XPS valence band spectrum (VBXPS), the valence band edge of pristine SrTiO3 is localized at 1.94 eV below the Fermi level. Then, it is calculated that the conduction band minimum would localize at about −1.16 eV. The valence band edge of SrTiO3-C950 expands to 1.82 eV. In the same way, combined with the band gap of 2.92 eV from optical measurements, the conduction band minimum edge of SrTiO3-C950 expands to −1.10 eV. Therefore, the formed energy band structure of SrTiO3-C950 is suitable for photocatalytic water splitting under visible light irradiation.
In addition, because of the presence of free electrons bounded loosely in the oxygen vacancies [42,43,44], the surface electric conductivity of SrTiO3-C950 will be improved, as a result of improving the carriers’ transfer efficiency, which is also helpful for improving the photocatalytic activity. The electrochemical impedance spectroscopy (Nyquist plot) was used to determine the charge transfer ability. The pristine SrTiO3 and SrTiO3-C950 all show a classical semicircular Nyquist diagram (Figure 5c). The arc radius of SrTiO3-C950 is obviously smaller than that of SrTiO3, which indicates that the photogenerated charges in SrTiO3-C950 would suffer less resistance than that of SrTiO3 and obtain a higher transfer rate. The photocurrent density of SrTiO3-C950 was much higher than that of pristine SrTiO3 under Xe lamp light irradiation (Figure 5d), which indicates that the charge separation in SrTiO3-C950 has been enhanced remarkably. Herein, the photocurrent density of SrTiO3-C950 shows a slow decrease with the following cycles, which may be due to the destruction of surface oxygen vacancies under the presence of external electric fields during a photocurrent test. It is proposed that, besides the aforementioned two factors, the higher charge separation efficiency is ascribed to the temporary storage capacity of surface oxygen vacancies for photogenerated carriers [21,45]. Moreover, according to the previous reports, the formed surface oxygen vacancies and SrO species are proposed to be more favorable for the adsorption and decomposition of water molecules [30].

4. Conclusions

In summary, a SrTiO3 material with surface oxygen vacancies was synthesized by carbon reduction under a high temperature. The as-prepared SrTiO3-C950 was successfully applied in photocatalytic water splitting research under visible light irradiation without any sacrificial reagent for the first time. The photocatalytic overall water splitting ability of the as-prepared SrTiO3-C950 is attributed to the surface oxygen vacancies that can make suitable energy levels for a visible light response, improve the separation and transfer efficiency of photogenerated carriers. In addition, according to the previous reports, it is proposed that the formed surface oxygen and SrO species is proposed to be more favorable for the adsorption and decomposition of water molecular. This work provided a strategy to design efficient photocatalysts by constructing oxygen vacancies.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/10/12/2572/s1. Figure S1: Particle size distribution of SrTiO3 before (a) and after (b) carbon reduction process under 950 °C. Figure S2: Survey XPS spectra of SrTiO3 (a) and SrTiO3-C950 (b).

Author Contributions

Conceptualization, G.C.; methodology, Y.L. and W.W.; validation, Y.F. and X.S.; investigation, Y.L.; writing—original draft preparation, H.C. and Q.S.; writing—review and editing, G.C.; supervision, B.T.; project administration, B.T.; funding acquisition, B.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (21927811, 21575082, 21535004, 91753111, and 21976110) and the Key Research and Development Program of Shandong Province (2018YFJH0502), Development plan of science and technology for Shandong Province of China (2013GGX10706), and A Project of Shandong Province Higher Educational Science and Technology Program (J13LD06).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Morphology and Structure characterizations of SrTiO3. (a), Scanning Electron Microscope (SEM) image of SrTiO3; (b), SEM of SrTiO3-C950; (c), XRD of SrTiO3 and SrTiO3-C950.
Figure 1. Morphology and Structure characterizations of SrTiO3. (a), Scanning Electron Microscope (SEM) image of SrTiO3; (b), SEM of SrTiO3-C950; (c), XRD of SrTiO3 and SrTiO3-C950.
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Figure 2. Transmission Electron Microscope (TEM) image (a), High Resolution Transmission Electron Microscope (HRTEM) image (b) and Electron Paramagnetic Resonance (EPR) image (c) of SrTiO3-C950.
Figure 2. Transmission Electron Microscope (TEM) image (a), High Resolution Transmission Electron Microscope (HRTEM) image (b) and Electron Paramagnetic Resonance (EPR) image (c) of SrTiO3-C950.
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Figure 3. XPS spectra of Sr3d (a), O1s (b), Ti2p (c) and C1s (d) of SrTiO3 and SrTiO3-C950.
Figure 3. XPS spectra of Sr3d (a), O1s (b), Ti2p (c) and C1s (d) of SrTiO3 and SrTiO3-C950.
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Figure 4. Photocatalytic activity of SrTiO3-C950 under full-spectrum light irradiation (a) or visible light irradiation (b) without sacrificial reagent. No photocatalytic water splitting activity was observed for pristine SrTiO3 and SrTiO3-C300 under the same conditions.
Figure 4. Photocatalytic activity of SrTiO3-C950 under full-spectrum light irradiation (a) or visible light irradiation (b) without sacrificial reagent. No photocatalytic water splitting activity was observed for pristine SrTiO3 and SrTiO3-C300 under the same conditions.
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Figure 5. Ultraviolet–visible diffuse reflectance spectrum (UV-Vis DRS) (a), XPS Valence Band Spectrum and Energy band structure of SrTiO3 (left) and SrTiO3-C950 (right) (b), Electrochemical impedance spectroscopy (c) and Photocurrent density (d) of SrTiO3-C950 and SrTiO3.
Figure 5. Ultraviolet–visible diffuse reflectance spectrum (UV-Vis DRS) (a), XPS Valence Band Spectrum and Energy band structure of SrTiO3 (left) and SrTiO3-C950 (right) (b), Electrochemical impedance spectroscopy (c) and Photocurrent density (d) of SrTiO3-C950 and SrTiO3.
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Fan, Y.; Liu, Y.; Cui, H.; Wang, W.; Shang, Q.; Shi, X.; Cui, G.; Tang, B. Photocatalytic Overall Water Splitting by SrTiO3 with Surface Oxygen Vacancies. Nanomaterials 2020, 10, 2572. https://doi.org/10.3390/nano10122572

AMA Style

Fan Y, Liu Y, Cui H, Wang W, Shang Q, Shi X, Cui G, Tang B. Photocatalytic Overall Water Splitting by SrTiO3 with Surface Oxygen Vacancies. Nanomaterials. 2020; 10(12):2572. https://doi.org/10.3390/nano10122572

Chicago/Turabian Style

Fan, Yanfei, Yan Liu, Hongyu Cui, Wen Wang, Qiaoyan Shang, Xifeng Shi, Guanwei Cui, and Bo Tang. 2020. "Photocatalytic Overall Water Splitting by SrTiO3 with Surface Oxygen Vacancies" Nanomaterials 10, no. 12: 2572. https://doi.org/10.3390/nano10122572

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