Effective Formation of Oxygen Vacancies in Black TiO2

Aug 28, 2017 - Effective Formation of Oxygen Vacancies in Black TiO2 Nanostructures with Efficient Solar-Driven Water Splitting ... *E-mail: zlp1@zju...
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Research Article pubs.acs.org/journal/ascecg

Effective Formation of Oxygen Vacancies in Black TiO2 Nanostructures with Efficient Solar-Driven Water Splitting Hui Song,† Chenxi Li,† Zirui Lou, Zhizhen Ye, and Liping Zhu* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: Black TiO2 nanomaterials have attracted considerable attention since they usually exhibit excellent photocatalytic activities. Herein, we report the facile preparation of black TiO2 nanostructures with ultrathin hollow sphere morphology, high crystalline quality, small grain size (∼8 nm), and ultrahigh surface area (168.8 m2 g−1) through Al reduction. Electron paramagnetic resonance (EPR) spectra demonstrate the existence of oxygen vacancies in black TiO2 nanostructures, which could increase the donor density and effectively promote the separation and transportation of photogenerated electron−hole pairs. The black TiO2 nanostructures exhibit a high solar-driven hydrogen generation rate (56.7 mmol h−1 g−1) under the full spectrum of solar light, which is nearly 2.5 times than that of pristine TiO2 nanostructures and superior to those kinds of black TiO2 photocatalytic materials reported previously. KEYWORDS: Black TiO2, Hollow sphere nanostructures, Oxygen vacancies, Solar water splitting, H2 generation

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performances and improving quantum efficiency.21,22 The large specific surface area could increase the surface reaction sites and the small grain size could reduce the diffusion distances of photoinduced charge carriers, which enable the photoinduced charge carriers to separate and migrate to the surface and inhibit the fast recombination of charger carriers.23−28 Among these nanostructured TiO2 materials, TiO2 hollow nanospheres have received much attention because their mesoporous structure and large surface area could be beneficial to improve photocatalytic performances.29,30 Herein, we successfully prepared the black TiO2 nanostructures with ultrathin hollow sphere morphology, high crystalline quality, small grain size, and ultrahigh surface area. The high quality TiO2 hollow nanospheres were first prepared, then TiO2 hollow nanospheres as the precursor went through subsequent Al reduction to obtain the black TiO2 hollow nanospheres. The synthetic process is shown in Figure 1. The obtained black

INTRODUCTION Photocatalytic water-splitting into H2 is one of the most promising ways toward solving energy shortage and environmental pollution problems.1−3 TiO2, owing to its low cost, nontoxicity, chemical stability, and abundance, has been regarded as one of the most promising photocatalytic materials and widely studied for photocatalytic degradation and hydrogen generation.4,5 However, the photocatalytic efficiency of bare TiO2 for photocatalytic hydrogen production is limited by its relatively wide band gap (3.2 eV) and the rapid recombination of photoexcited electrons and holes.6 Until now, many great efforts such as heterojunction design, bandgap engineering, and exposed facet optimization have been made to achieve high photocatalytic performance TiO 2 materials.7−9 Recently, black TiO2 materials have earned much attention because of enhanced solar light absorption via formation of oxygen vacancies and Ti3+ in TiO2 or introducing disordered layers in the surface of high crystalline TiO2.10−14 Enhancing the optical absorption performances of TiO2 nanomaterials has proven to be an excellent way to promote their photocatalytic performances in the report of black TiO2 nanomaterials.15 Furthermore, oxygen vacancies and Ti3+ enhance the solar light absorption of TiO2 and promote the separation of charge carriers, thereby further improving photocatalytic activities.16 Black TiO2 photocatlytic materials can be synthesized by various methods, including hydrogen thermal treatment,10 Al reduction,17−19 Mg reduction,20 and other methods.15 The large specific surface area and small nanoparticle size (∼10 nm) play important roles in promoting photocatalytic © 2017 American Chemical Society

Figure 1. Schematic illustration of the formation of TiO 2 nanostructures and black TiO2 nanostructures. Received: June 4, 2017 Revised: July 23, 2017 Published: August 28, 2017 8982

DOI: 10.1021/acssuschemeng.7b01774 ACS Sustainable Chem. Eng. 2017, 5, 8982−8987

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ACS Sustainable Chemistry & Engineering

efficiency could be calculated through equation: QE = (N × 2/M) × 100, M and N are the number of incident photons and the number of generated hydrogen molecules, respectively. Photoelectrochemical Measurements. The photoelectrochemical measurements were carried out in a conventional three-electrode cell using a Pt plate and an Ag/AgCl electrode as the counter electrode and reference electrode, respectively. A 0.5 M Na2SO4 aqueous solution without additive was used as the electrolyte. The samples was prepared on indium tin oxide (ITO) glass as working electrode. Briefly, the sample (6 mg) was added into1 mL of dimethylformamide (DMF). After ultrasonicated for 0.5 h, the sample was spread on indium tin oxide glass and the sample area on the working electrode was 1.0 cm2. Then the working electrode was dried at 60 °C and further annealed in N2 atmosphere at 300 °C for 2 h.

TiO2 hollow nanospheres have a high specific surface area of 168.8 m2 g−1 and small nanoparticles size (∼8 nm), and black TiO2 nanostructure is derived from the formation of oxygen vacancies, which could greatly increase the donor density and effectively promote the separation and migration of photoinduced charge carriers, thereby exhibiting excellent photocatalytic hydrogen generation performance. The photocatalytic hydrogen generation rate of black TiO2 nanostructures is up to 56.7 mmol h−1 g−1 under the full spectrum of solar light, which is nearly 2.5 times that of TiO2 nanostructures and superior to those kinds of black TiO2 photocatalytic materials reported previously.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The crystalline structure of samples were determined by XRD analysis (Figure 2a). Mainly, the anatase phase of TiO2

Preparation of Black TiO 2 Nanostructures. First, TiO2 nanostructures were prepared according to our previous report.31 Briefly, the synthesis of carbon spheres is based on the method reported by Li et al.32 1 mmol portion of tetrabutyl titanate (99% purity) was dissolved in 100 mL of anhydrous ethanol. Then, asprepared carbon spheres (500 mg) was added to the above solution. After ultrasonication for 0.5 h, the as-obtained mixture solution was stirred at ambient condition for 6 h. After that, the product was collected by centrifugation, rinsed with ethanol three times, and dried at 90 °C for 6 h. Then, to remove all the carbon spheres and other hydrocarbons, the samples were annealed at 400 or 500 °C in air for 4 h. Finally, the TiO2 hollow nanospheres were obtained. For the preparation of black TiO2 hollow nanospheres, an aluminum reaction was performed in an evacuated two-zone furnace. The obtained TiO2 hollow nanospheres were placed in the low temperature zone (500 °C), and a certain amount of aluminum powder was placed in 800 °C. After annealing for 6 h and cooling down to room temperature, the black TiO2 nanostructures were obtained. To obtain the Pt-loaded black TiO2 nanostructures, 20 mg of black TiO2 hollow nanospheres was impregnated into 5 mL 10 vol % chloroplatinic acid solution. After drying at 70 °C, the samples were reduced by annealing in an H2 atmosphere at 200 °C for 2 h, obtaining 1 wt % Pt-loaded black TiO2 nanostructures. Material Characterization. The crystallographic structures of asprepared samples were investigated by X-ray diffraction utilizing a Cu Kα line with 40 kV of voltage and 40 mA of tube current. Field emission scanning electron microscopy (FESEM, Hitachi S-4800) was used to investigate the morphologies of the samples. The morphology and crystal structure of the as-prepared samples were characterized by high-resolution TEM. A Thermo ESCALAB-250 spectrometer utilizing a monochromatic Al Kα radiation source were employed to characterize the XPS spectra. A UNICO UV-2102 ultraviolet−visible spectrophotometer was employed to investigate the UV−vis absorption. Photoluminescence spectra were investigated by a He− Cd laser of a 325 nm line. Photocatalytic Hydrogen Production. The hydrogen generation characterizations were measured using Labsolar-II system (Beijing Perfectlight Technology Co., Ltd.). Ar gas was employed as the carrier gas. A 300 W Xe-lamp was utilized as the light source. A 100 mL aqueous solution with 10 vol % methanol as a sacrificial reagent was prepared for the photocatalytic H2 production reactions tested. Samples (20 mg) were added into the above solution, and the water bath cycle system was used to keep the mixed solution at room temperature. The produced gases were analyzed by a GC (gas chromatography). To make sure the same content of sacrificial reagent (methanol) in the solution, the samples were recollected by centrifugation process after 3 h of H2 generation experiments, and then identical 100 mL methanol (10%) aqueous solution and the recollected samples were added into the container to perform the cycle measurements. Quantum Efficiency. In order to measure the quantum efficiency (QE) in the hydrogen production, a band-pass filter (λ = 365 nm) was used in the photocatalytic reaction experiment. The quantum

Figure 2. (a) XRD pattern of TiO2 nanostructures and black TiO2 nanostructures, (b) SEM image, and (c) TEM image of black TiO2 nanostructures. (d) Nitrogen adsorption−desorption isotherms of TiO2 nanostructures and black TiO2 nanostructures. (e) Absorption spectra and (f) Raman spectra of TiO2 nanostructures and black TiO2 nanostructures.

nanostructures annealed at 500 °C with a little rutile phase could be observed. After Al reduction at 500 °C, the diffraction peaks of Al-reduced TiO2 nanostructures showed almost no change, indicating that TiO2 nanostructures had structural stability. In order to gain a better understanding of the phase change of TiO2 nanostructures, the TiO2 nanostructures annealing the Ti-precursor with carbon sphere in air at 400 °C for 4 h were obtained. Then, the TiO2 nanostructures were annealed at 400, 500, and 600 °C under Al reduction process for 6 h, respectively. The rutile peaks appeared when the reduction temperature increased to 500 °C and further increased with the increase of reduction temperature, indicating 8983

DOI: 10.1021/acssuschemeng.7b01774 ACS Sustainable Chem. Eng. 2017, 5, 8982−8987

Research Article

ACS Sustainable Chemistry & Engineering that anatase changed to rutile when the temperature was 500 °C, as shown in Figure S1. The microstructure and morphology of TiO2 nanostructures and black TiO2 nanostructures were investigated by SEM, TEM, and HRTEM. The SEM image (Figure S2) shows that the as-prepared TiO2 nanostructures own hollow nanosphere morphology and diameters are ranged from 100 to 400 nm. The TEM image (Figure S3) further indicates that TiO2 have hollow spheres nanostructures with ultrathin shells (∼10 nm), and there are abundant mesopores on the surface of hollow nanospheres, which could be helpful for improving photocatalytic performance. From the HRTEM image (Figure S4), TiO2 nanostructures have high crystalline quality and the average grain size of TiO2 nanostructures is about 8 nm. After Al reduction, the black TiO2 nanostructures have similar morphology and microstructure compared to TiO2 nanostructures, as are shown in Figure 2b and c. Nitrogen adsorption−desorption isotherms of TiO2 nanostructures and black TiO2 nanostructures were examined. Figure 2d shows that both TiO2 nanostructures and black TiO2 nanostructures have an obvious hysteresis loop, suggesting that ample pores exist in the nanostructures. The BET surface area of TiO2 nanostructures and black TiO2 nanostructures is 171.5 and 168.8 m2 g−1, respectively, which means that Al reduction does not change the grain size and structure of black TiO2 nanostructures. The high specific surface area mesoporous nanostructures are beneficial to improve photocatalytic activities through providing numerous catalytic surface sites. Figure 2e exhibits the UV−visible absorption spectra of samples. For the black TiO2 nanostructures, the noticeably large absorption tail in the visible and even infrared light region could be observed. The extended absorption is well consistent with the change of sample color from white to black (Figure S5), thus indicating that black TiO2 nanostructures contain massive oxygen vacancies.33 Furthermore, the black TiO2 nanostructures have good thermal stability and still stay black even when annealed at 200 °C in air for 4 h (Figure S5). The absorption edge of black TiO2 nanostructures is similar to that of TiO2 nanostructures (382 nm), meaning that their band gap is about 3.25 eV. These results show that oxygen vacancies induced visible light absorption could be originated from intermediate energy level between the conduction band (CB) and valence band (VB) of TiO2,34 instead of a change of the position of CB and/orVB. Theoretical researches also revealed that an intermediate energy level below the CB could be generated by oxygen vacancies.35 The formation of oxygen vacancies could be further demonstrated by Raman spectroscopy spectra (Figure 2f). For black TiO2 nanostructures, the peak at 144 cm−1 exhibits obvious blue shifting and broadening compared to TiO2 nanostructures. This phenomenon was reported in the previous studies36,37 and ascribed to oxygen deficiency and nonstoichiometry at the surface of black TiO2 nanostructures, demonstrating that the original symmetry of TiO2 nanostructures lattice was destroyed after Al reduction. As demonstrated previously, the existence of Ti3+ and/or oxygen vacancies in the surface layer of black titanium dioxide nanostructures results in the enhancement of solar light absorption. The surface chemical compositions and the valence states of TiO2 nanostructures and black TiO2 nanostructures were characterized by X-ray photoelectron spectroscopy (XPS) analysis, and the results were shown in Figures 3a−c and S5−7. The Ti 2p XPS spectra of TiO2 nanostructures and black TiO2 nanostructures are almost the same without Ar+ sputtering, and the peaks at 464.3 and 458.5 eV are ascribed to the 2p1/2 and

Figure 3. (a and b) XPS spectra of black TiO2 nanostructures for Ti 2p spectrum after Ar+ sputtering the surface layer for 4 min and O 1s spectrum. (c) Valence band spectra and (d) EPR spectra of TiO2 nanostructures and black TiO2 nanostructures.

2p3/2 peaks of Ti4+ (Figure S6).20 Considering the fact that surface Ti3+ are easily oxidized by certain components (like O2 and H2O), Ar+ was used to sputter the surface layer of catalysts. After Ar+ sputtering the surface layer for 4 min (Figure 3a), extra small peaks located at 463.3 and 457.7 eV in the black TiO2 nanostructures are appeared and could be attributed to the 2p1/2 and 2p3/2 peaks of Ti3+compared with the TiO2 nanostructures. The existing Ti3+ signals indicate that oxygen vacancies are generated in the black TiO2 nanostructures during the reduction process. The O 1s XPS spectra (Figure 3b) of TiO2 nanostructures and black TiO2 nanostructures could be fitted into two distinct peaks (529.8 and 531.2 eV). The main peak (centering at 529.8 eV) is ascribed to the Ti−O bonds (crystal lattice oxygen) in TiO2. The peak locating at 531.2 eV is attributed to the adsorbed hydroxyl (−OH) at the surface of TiO2.20,38 The full XPS spectrum (Figure S7) and the Al 2p XPS spectrum of black TiO2 nanostructures (Figure S8) indicate that no impurities like Al exists in black TiO2 nanostructures after Al reduction. The energy level valence band maximum (VBM) of TiO2 nanostructures and black TiO2 nanostructures is also identical and about 3.02 eV (Figure 3c). As shown in Figure 3d, the existence of oxygen vacancies in black TiO2 nanostructures could be also proved by electron paramagnetic resonance (EPR). TiO2 nanostructures have almost completely Ti4+ (3d0) and exhibit an extremely weak EPR signal (centering at g = 2.002), which can be ascribed to the surface absorption of O2 in air. However, a strong EPR signal (centering at g = 2.002) could be detected, which can be ascribed to oxygen vacancies.39 The photocatalytic hydrogen generation performances of TiO2 nanostructures and black TiO2 nanostructures were evaluated under the full spectrum of solar light (Figure 4a). All the samples were loaded with 1 wt % platinum (Pt) as cocatalyst. Black TiO2 nanostructures exhibit a much high photocatalytic performance with a hydrogen generation rate of 56.7 mmol h−1 g−1. The hydrogen generation rate is almost 2.5 times than that of TiO2 nanostructures (22.9 mmol h−1 g−1). The QE (quantum efficiency) of black TiO2 nanostructures and TiO2 nanostructures was estimated to be 90.6 and 35.2% at 365 8984

DOI: 10.1021/acssuschemeng.7b01774 ACS Sustainable Chem. Eng. 2017, 5, 8982−8987

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Figure 4. Photocatalytic hydrogen generation of TiO2 nanostructures and black TiO2 nanostructures as a function of time with 20 mg photocatalysts loading 1 wt % Pt under the full spectrum of solar light.

Figure 6. Energy band structure of TiO2 nanostructures and black TiO2 nanostructures. Eo and Eo′ are considered as the energy levels of the oxygen vacancies located at 0.73 and 1.18 eV below the CB of TiO2. Arrows are the possible electron transitions within the different energy levels in TiO2 nanostructures and black TiO2 nanostructures.

nm, respectively. Therefore, the photocatalytic performance of TiO2 can be indeed enhanced by Al reduction treatment. A comparison between TiO2-based photocatalytic materials for hydrogen production activities is summarized in Table S1 (in the Supporting Information). This obviously suggests that black TiO2 nanostructures exceeds the other related materials reported earlier, indicating extremely high photocatalytic hydrogen production activity under full spectrum of solar light irradiation. The stability of photocatalytic activity of black TiO2 nanostructures over a period of time under full spectrum of solar light irradiation were investigated. Figure 5 clearly

nanostructures and black TiO2 nanostructures. The strong UV absorption of TiO2 results from the electronic transition from VB to CB. Al reduction could create electronic state bands of oxygen vacancies in TiO2 nanostructures. The electron transitions from VB to the oxygen vacancies levels and/or from the oxygen vacancies to VB result in the visible and infrared light absorption. But the photogenerated electrons located at the energy level of oxygen vacancies are unable to participate in hydrogen production (H+ → H2) because the energy levels of photogenerated electrons are below the H2O/ H2 reduction potential, which may be the reason why black TiO2 nanostructures did not show any photocatalytic activities under visible light illumination. Therefore, we think that the superior photocatalytic hydrogen generation of black TiO2 nanostructures under full solar light could be ascribed to the existence of oxygen vacancies instead of the enhancement of solar light absorption. The oxygen vacancies can be considered as an electron donor for TiO2 and the increment of donor density efficiently separate and transport the photoexcited electron−hole pairs.34 The photoluminescent (PL) spectra is utilized to examine the separation efficiency of photogenerated charge carriers in photocatalysts.41 Thus, PL behavior could be beneficial to study the separation efficiency of photogenerated charge carriers in TiO2 nanostructures and black TiO2 nanostructures. Figure 7a show the PL spectra of TiO2 nanostructures and black TiO2 nanostructures with 325 nm excitation wavelength. TiO2 nanostructures exhibit clear PL emission signal. But, the PL intensity of black TiO2 nanostructures significantly decrease compared with TiO2 nanostructures, meaning that the effective separation and migration of photogenerated charge carriers or significant suppression of the recombination of the photogenerated charge carriers are achieved, and the lifetime of charge carriers are effectively lengthened in black TiO2 nanostructures,23 thereby leading to the higher photocatalytic hydrogen production rate than TiO2 nanostructures. Transient photocurrent response experiments are investigated to demonstrate the enhancement in separation and transportation efficiency of charge carriers in black TiO2 nanostructures. As shown in Figure 7b, the nearly same photocurrents were achieved when full solar light was consecutively turned on and off. But black TiO2 nanostructures exhibited a much high photocurrent density (18 μA/cm3), which is nearly 2.3 times than that of TiO2 nanostructures (8 μA/cm3), revealing that

Figure 5. Cycling tests of photocatalytic hydrogen production of black TiO2 nanostructures. Cycling tests were tested in a 7-day period, with 70 h of overall light irradiation time.

exhibits that the nearly consistent photocatalytic H2 generation over 70 h is observed. The morphology structure of black TiO2 nanostructures show no obvious change after 70 h of photocatalytic reaction (Figure S9). The above results reveal that black TiO2 nanostructures own high photocatalytic performance and stability for the photocatalytic hydrogen production. Unexpectedly, although the photoresponse of black TiO2 nanostructures increases from ultraviolet to visible light, black TiO2 nanostructures did not show any photocatalytic activities under visible light (λ > 420 nm) irradiation. Considering the hydrogen treated TiO2 nanomaterials reported by Mao et al., the change of color from black to white was ascribed to an obvious shift (2.18 eV) of VB position.10 However, our work showed that there is no shift of valence band maximum (VBM) for black TiO2 nanostructures, which means that VBM has no effect on the black color. Therefore, the black color may be ascribed to the existence of defect states (i.e., oxygen vacancies) between CB and VB in TiO2 nanostructures after Al reduction.38 The energy level position of oxygen vacancies is supposed to be from 0.75 to 1.18 eV below CB of TiO2.40 Figure 6 shows a simplified energy band structure of TiO2 8985

DOI: 10.1021/acssuschemeng.7b01774 ACS Sustainable Chem. Eng. 2017, 5, 8982−8987

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ACS Sustainable Chemistry & Engineering

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S5). XRD patterns (Figure S5). Light intensity and wavelength range (Figure S6). XPS spectra (Figure S6− 8). Photocatalytic activities (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liping Zhu: 0000-0003-2592-8043 Author Contributions †

H.S. and C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51572239, 51372224, and 91333203), Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), and Major Project of Zhejiang Natural Science Foundation of China No. D180E020002.

Figure 7. (a) PL spectra of TiO2 nanostructures and black TiO2 nanostructures. (b) Photocurrent response of TiO2 nanostructures and black TiO2 nanostructures in 0.5 M Na2SO4 aqueous solution with off−on full solar light irradiation at 0.6 V vs NHE.

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black TiO2 nanostructures remarkably improved the electron mobility by suppressing the recombination of charge carriers. Therefore, oxygen vacancies in black TiO2 nanostructures could enhance charge carrier separation and migration, which can be considered as the main reasons for the improved photocatalytic H2 generation under full solar light.

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CONCLUSIONS In summary, the TiO 2 nanostructures and black TiO2 nanostructures with ultrathin hollow sphere morphology, high crystalline quality, small grain size, and ultrahigh surface area are successfully synthesized. The oxygen vacancies formed in black TiO2 nanostructures after Al reduction could increase donor density and accelerate the separation and transportation of the photogenerated charge carriers. Therefore, black TiO2 nanostructures exhibit excellent photocatalytic hydrogen generation rate (56.7 mmol h−1 g−1) under the full spectrum of solar light, which is nearly 2.5 times than that of TiO2 nanostructures and superior to those of black TiO2 materials reported previously. Although the photocatalytic hydrogen rate could be efficiently enhanced under full solar light, black TiO2 nanostructures did not exhibit photocatalytic performance under visible light. Coupling visible light photocatalysts (such as CdS and TaON) with black TiO2 nanostructures to form heterojunction and/or loading plasmonic Au and Ag nanoparticles on black TiO2 nanostructures may be effective methods to improve visible light photocatalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01774. Additional experimental data and results including XRD pattern (Figure S1). SEM, TEM, HRTEM images (Figures S2−4 and 9). Images of photocatalysts (Figure 8986

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DOI: 10.1021/acssuschemeng.7b01774 ACS Sustainable Chem. Eng. 2017, 5, 8982−8987