Gray Ta2O5 Nanowires with Greatly Enhanced Photocatalytic

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Gray Ta2O5 Nanowires with Greatly Enhanced Photocatalytic Performance Guilian Zhu,†,‡ Tianquan Lin,† Houlei Cui,†,‡ Wenli Zhao,†,‡ Hui Zhang,*,† and Fuqiang Huang*,†,‡ †

CAS Key Laboratory of Materials for Energy Conversion and State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China ‡ Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China S Supporting Information *

ABSTRACT: Black TiO2, with enhanced solar absorption and photocatalytic activity, has gained extensive attention, inspiring us to investigate the reduction of other wide-bandgap semiconductors for improved performance. Herein, we report the preparation of gray Ta2O5 nanowires with disordered shells and abundant defects via aluminum reduction. Its water decontamination is 2.5 times faster and hydrogen production is 2.3-fold higher over pristine Ta2O5. The reduced Ta2O5 also delivers significantly enhanced photoelectrochemical performance compared with the pristine Ta2O5 nanowires, including much higher carrier concentration, easier electron−hole separation and 11 times larger photocurrent. Our results demonstrate that Ta2O5 will have great potentials in photocatalysis and solar energy utilization after proper modification. KEYWORDS: Ta2O5, nanowire, aluminum reduction, photocatalysis, PEC

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INTRODUCTION

confined to tuning its dimensions, morphologies and integrating Ta2O5 with other cocatalysts.2,5,7−9 Black TiO2, with excellent solar light absorption and photocatalytic H2 evolution activity, has attracted global interests since its first preparation via a high H2-pressure method.10 Ambient-pressure approaches were also successfully developed to prepare black TiO2 by using hydrogen as the reducing agent.11−15 Afterward, lots of follow-up studies have demonstrated that various methods, including H2 plasma treatment, 16 aluminum reduction, 17,18 and oxidation of TiH219,20 etc., are effective in preparing black, blue, or gray TiO2. Generally, surface disorders and abundant oxygen vacancies are introduced into these colored titania, which results in significantly increased photocatalytic performance. These results motivate us to explore the blackening of Ta2O5 photocatalyst to enhance its solar absorption and photocatalysis.

Tantalum oxide (Ta2O5) is one of the most important transition metal oxides because of its extraordinary physical and chemical properties, including high dielectric and refractive coefficients, excellent photoelectric performance, and good chemical stability.1−4 Besides its wide application as antireflective layer materials in optical and photovoltaic devices and dielectric materials in the electronic industry, Ta2O5 is also very promising in photcatalytic contaminant removal and hydrogen (H2) production through water splitting, for the valence band maximum (VBM) and conduction band minimum (CBM) of its bandgap straddle the redox potentials of H+/H2 and O2/ H2O.5,6 Moreover, because the CBM of Ta2O5 is more negative than many other photocatalysts such as TiO2, H2 evolution reaction happen more easily on Ta2O5. However, the band gap of Ta2O5 is so large (∼4 eV) that its solar absorption and photocatalytic application are severely restricted. Consequently, many efforts have been made to improve the solar energy conversion efficiency and photocatalytic activity of Ta2O5. However, the researches of Ta2O5 photocatalysts were mainly © XXXX American Chemical Society

Received: August 18, 2015 Accepted: December 15, 2015

A

DOI: 10.1021/acsami.5b07685 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces In this work, oxygen deficient gray Ta2O5 nanowires (NW) were synthesized through a hydrothermal process and subsequent aluminum (Al) reduction at different temperatures. The pristine Ta2O5 nanowires and the reduced samples obtained at 300, 400, and 500 °C are denoted as Ta2O5 NW, Al-300, Al-400, and A-500, respectively, hereinafter for convenience. The aluminum reduction induces substantial oxygen vacancies as well as a disordered shell into the reduced gray Ta2O5 nanowire. The gray Ta2O5 nanowires display drastically enhanced solar absorption and photocatalytic performance, exhibiting 2.4 times higher efficiency of photocatalytic water decontamination and 2.3-fold higher photocatalytic H2 production rate compared with the pristine Ta2O5. The photoelectrochemical (PEC) water splitting of Ta2O5 is first researched and the photocurrent of the reduced Ta2O5 is demonstrated to have an 11-fold increase compared with the prisitine Ta2O5.

samples (3.94 eV). In comparison with the pristine Ta2O5 NWs, the visible and near-infrared light absorptions of the reduced samples are apparently improved. The solar absorption ratios of our samples are calculated according to their absorption spectra and the standard AM 1.5G solar spectrum, as shown in Table S1. It is noticed that the solar absorption of Al-400 possesses ∼40% of solar energy, much larger than the pristine Ta2O5 NW (10.7%). The solar absorption ratio increases gradually from 29.4% to 44.4% as the Al-reduction temperature rises from 300 to 500 °C, in good agreement with the color transformation. The X-ray diffraction (XRD) patterns of the white and gray Ta2O5 NW (Figure 1c) show typical features for orthorhombic Ta2O5 nanowire with preferential growth along (010).7 The similar patterns illustrate that Al reduction will not induce phase transition in Ta2O5 and the preferential growth of nanowire would be well maintained after reduction. The partial enlarged view of (010) diffraction peak (Figure 1d) exhibits a regularly increased line width and decreased peak intensity of the Al-reduced sample with the increased treatment temperature, which suggests that aluminum reduction induces considerable amount of defects and disorders in the Ta2O5 NW.17 The slight shift to higher diffraction angle of the diffraction peaks for Al-reduced samples happens to the whole pattern and can be ascribed to the influence of experiment operation. To confirm the microstructure change after Al-reduction, the Ta2O5 samples were examined by the selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM), as shown in Figure 2. The SAED

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RESULTS AND DISCUSSION The morphology of the as-synthesized Ta2O5 nanowires (NWs) was characterized by the scanning electron microscopy (SEM), as shown in Figure 1a. It is obvious that the Ta2O5

Figure 1. (a) SEM micrographs of the synthesized Ta2O5 NW, (b) optical absorption spectra and (c) XRD spectra of the pristine and aluminum reduced Ta2O5 samples, and (d) magnified image of (010) diffraction peak of the Ta2O5 XRD spectra.

NWs are relatively uniform, with diameter of about 20−40 nm and length of about 0.2−0.6 μm. The commercial Ta2O5 and amorphous Ta2O5 (a-Ta2O5) exhibit irregular morphology and aggregated spherical particles, respectively, as shown in Figure S1. The gray Ta2O5 samples were prepared by placing Ta2O5 nanowires and elemental aluminum metal powder in a twozone furnace, sample in the low temperature zone (300−500 °C) and Al powder in the 800 °C zone.17 In this way, the reaction of the thermal depletion of oxygen in Ta2O5 is accelerated because of the consumption of gaseous O2 by Al powder. As the reduction temperature increases, the color of the Ta2O5 NW converts from white to gray gradually, indicating the improved solar absorption of Ta2O5 after aluminum reduction. Figure 1b shows the optical absorption spectra of the pristine and reduced Ta2O5 NWs; the large absorption peak at ∼315 nm results from the intrinsic bandgap absorption of Ta2O5

Figure 2. (a) Selected area electron diffraction (SAED) pattern of Ta2O5 nanowires; the high resolution transmission electron microscopy (HRTEM) images of single-crystal Ta2O5 nanowire for (b) the pristine (c) Al-300 and (d) Al-400 samples.

pattern of a single Ta2O5 nanowire shows that the Ta2O5 nanowire is single-crystalline with a preferential orientation of the (010) plane, as shown in Figure 2a. Both the pristine and the Al-reduced Ta2O5 NWs show well resolved (010) plane of the orthorhombic Ta2O5 phase with the measured interplanar spacing distance of 0.39 nm. In combination with the above XRD, it is definitely demonstrated that the synthesized Ta2O5 is pure orthorhombic phase and no phase transition occurs during the Al-reduction process. Nevertheless, there exists a significant difference between the samples before and after Al reduction a specific crystalline core/disordered shell structure is clearly observed in the Al-300 and Al-400 sample (Figure 2c and 2d). B

DOI: 10.1021/acsami.5b07685 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

signal, while the reduced gray Ta2O5 samples display single symmetrical signals with g value of 2.002, which is due to the superoxide radicals (O2−) adhering to the oxygen vacancies in the surface of Ta2O5.29,30 It is obvious that the intensity of the EPR signals increase regularly with the rising reduction temperature, indicating a larger oxygen vacancy density in the sample reduced at a higher annealing temperature. The O2− radicals can facilitate the oxidative reaction and stabilize the photogenerated electrons (e) and holes (h), which suggests that Al-400 sample will have a stronger oxidizing activity than Al300. The plots of magnetic field dependence of magnetization further confirm the result of EPR, as shown in Figure S4. When the Al reduction temperature increases, the magnetizatin of the reduced sample becomes stronger, in agreement with the increasing concentration of the Ta suboxides and oxygen vacancies. To track the efficiency of charge carrier separation, migration, and transfer in our samples, the measurement of photoluminescence (PL) emission was further performed since PL emission originates from the recombination process of free carriers.31 The PL emission intensity shows a trend in the following order: Ta2O5 > Al-300 > Al-400 (Figure 3d), indicating a suppressed recombination of the photogenerated carriers in the Al-reduced Ta2O5. Aluminum reduction is reported to greatly improve the conductivity and carrier density of titania and Nb2O5,17,32 which facilitates the carrier separation and transfer in these materials. Herein, the improved carrier transfer and separation in the reduced Ta2O5 sample are responsible for their drastically decreased PL intensities compared with the prisitine Ta2O5 NW. The above results demonstrate that gray Ta2O5 NW with abundant defect states and a disordered surface is constructed after Al-reduction. The abundant surface defects (surface disorder, oxygen vacancies) introduced during the reduction process are supposed to result in the darker coloration, stronger sunlight absorption, decreased bandgap and suppressed electron−hole recombination of the reduced samples, which are prerequisites for high photocatalytic activity.17 The photocatalytic performances of the reduced Ta2O5 NWs were first measured by the visible light driven decomposition of methylene blue (MB) (λ > 420 nm), as shown in Figure 4a. The Al-reduced Ta2O5 NWs show greatly enhanced photocatalytic activities compared with the original Ta2O5 and the MO decomposition time decreases with the increasing reduction temperature. After 2 h illumination, Al-400 sample degrades ∼35% MB, while the pristine only degrades ∼5% MB. We further measured methyl orange (MO) decomposition under full spectrum solar-light illumination in the presence of original and Al-reduced Ta2O5 (Figure 4b). The degradation of MO catalyzed by the reduced Ta2O5 NW is apparently enhanced in comparison with the pristine Ta2O5 NW. For the Al-400 sample, the MO concentration is degraded under 10% within 15 min. There is a regular variation in the photocatalytic activity observed in the following order: Al-400 > Al-300 > Ta2O5 NW, in accordance with the variation tendency of defects concentration and solar absorption. The photodegradation rate constants are estimated to be 0.173, 0.118, and 0.070 min−1 for Al-400, Al-300 and Ta2O5 NW according to the pseudo first-order kinetics model, respectively, as shown in Figure S5. In other words, the photodegradation rates of Al-300 and Al-400 are about 1.7 and 2.5 times faster than that of the original Ta2O5 NW, respectively. The MO degradation cycling tests demonstrate that the Al-400 sample

The disordered shells, indicated with a white dashed line, are about 1.5 nm thick and supposed to possess looser structures than the crystalline phase. By contrast, the original Ta2O5 NW is well crystallized and its lattice fringes distinctly extend to the nanowire edges, as shown in Figure 2b. In the previously reported reduced black TiO2−x, not only surface disorder was occasionally induced but also Ti3+ (3d1) or oxygen vacancies would be created in the TiO2−x surface in some cases.18−24 These defects were combined to lead to the black coloration, improved solar absorption and enhanced photocatalysis of the reduced TiO2−x. Consequently, additional characterizations need to be performed to ensure the existence of various defects in oue reduced Ta2O5 NW. X-ray photoelectron spectroscopy (XPS) is carried out to study the surface state transition and measure the electronic valence band position of our Ta2O5 NWs. The peak location of Ta 4f XPS and O 1s XPS are almost identical for both the pristine Ta2O5 NW and Al-400 sample (Figure 3a). The Ta 4f

Figure 3. XPS (a) normalized Ta 4f and (b) valence band spectra of Ta2O5 NW and Al-400. (c) EPR spectra and (d) PL spectra of the pristine Ta2O5 NW, Al-300, and Al-400 sample.

spectra can be fitted in one set of peaks located at 26.1 and 28.0 eV (energy separation of 1.9 eV), which could be ascribed to Ta−O bonds in the stoichiometric Ta2O5.25,26 The absence of Ta signals in lower oxidation state is similar to the Al-reduced black titania, which may be due to the limited detection distance (about ten atomic layers) of XPS or the reoxidation of the surface metastable sites by certain components (O2, H2O, etc.) in air.17 The O 1s spectra of the two samples can actually be fitted with two components at 530.9 and 532.6 eV (Figure S2), which originate from the lattice oxygen component in Ta2O5 and the surface −OH groups, respectively.25−28 The valence band maximum exhibit a negligible shift from 2.70 eV of Ta2O5 to 2.68 eV of Al-400. Moreover, We confirm that barely no additional aluminum dope in or coat on our samples during the reduction process (Figure S3). The electron paramagnetic resonance (EPR), which is very effective in detecting paramagnetic species with unpaired electrons, is carried out to determine the presence of defects (Ta suboxides and/or oxygen vacancies) in our samples after Al-mediated reduction. As shown in Figure 3c, the pristine Ta2O5 exhibit very different features from the Al-reduced samples. The pristine Ta2O5 does not show apparent EPR C

DOI: 10.1021/acsami.5b07685 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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The photoelectrochemical H2 generation cell (PEC) was measured to evaluate the photocatalytic activities of our samples. Films of the pristine and reduced tantalum pentoxide were first prepared by spin-coating. Then, the photocurrents of our samples were measured in dark and under irradiation (Xe lamp) by linear sweep voltammetry at 10 mV/s between −0.6 and 0.5 V versus silver−silver chloride electrode in 1 M NaOH eletrolyte. Both of the two samples exhibit negligible dark currents in comparison with their respective photocurrents (Figure 4e), which indicates that the electrocatalytic water splitting barely occurs. It is worth noting that the photocurrent of Al-400 (0.271 mA cm−2) under illumination is about 11 times larger than that of pristine Ta2O5 (0.025 mA cm−2). The higher photocurrent density demonstrates more effective separation and transport of photogenerated carriers in Al-400. Additionally, both samples show Mott−Schottky plots with positive slopes (Figure 4f), which is typical behavior of n-type semiconductor. The free charge carrier density is inversely proportional to the slope of Mott−Schottky plot according to the following equation Nd = (2/e0εε0)[d(1/C 2)/dV ]−1

where e0, ε, ε0, Nd, and d(1/C2)/dV represent the electron charge, the dielectric constant of Ta2O5 (35 for orthorhombic Ta2O5),36,37 the permittivity of vacuum, the carrier density, and the slope of Mott−Schottky plot, respectively. It is obvous that Al-400 sample displays a smaller slope than pristine Ta2O5 NW, indicating that Al-400 has a higher free charge carrier density. The electron concentrations of Al-400 and the pristine Ta2O5 are calculated to be 1.50 × 1020 and 1.57 × 1019, respectively, eaqualling to a nearly ten times higher electron density in Al400. The increase of elctron density would lead to a significant upward shift of the Fermi level and finally induce a great band edge bending in Al-400, which can facilitate the charge separation at the interface between Ta2O5 and electrolyte. Consequently, the increased charge density and the enhanced charge separation in Al-400 are the main reasons for the significantly improved hydrogen evolution and photoelectrochemical water-splitting efficiency. Additionally, we also treat the commercial Ta2O5 powder with aluminum reduction under the same condition, and both its solar absorption and photocatalytic performance are improved significantly, as shown in Figure S7. These positive results confirm that Ta2O5 materials can be modified to be very effective in visible light absorption and photocatalysis after Al reduction.

Figure 4. (a) Visible-light driven photodegradation of methyl blue (MB) and (b) full-spectrum solar-driven photodegradation of aqueous methylene orange (MO) in the presence of the original and Alreduced Ta2O5. (c) Cycling tests of solar-driven MO photodegradation over the Al-400 sample. (d) Photocatalytic hydrogen production in aqueous solution containing 20% methanol over the pristine Ta2O5 NW, Al-300, and Al-400 sample under full-spectrum light irradiation. (e) Linear sweep voltammograms collected with an AM 1.5 solar spectrum simulator using three electrodes setup at a scan rate of 10 mV/s in 1 M NaOH electrolyte (pH ∼ 13.6) (P25 or Al400, Pt, and Ag/AgCl act as working, counter, and reference electrode separately. (f) Mott−Schottky plots collected at a frequency of 1 kHz in dark.

has very stable activity in five photodegradation cycles (Figure 4c). However, the photocatalytic activity of the reduced Ta2O5 would be impaired when the Al-reduction temperature increases to 500 °C in spite of the improved solar absorption and decreased e-h recombination rate, as shown in Figure S6. The reason for the deterioration in photocatalytic activity of Al500 is supposed to be the serious particle aggregation of the Ta2O5 nanowires because of the excessive calcination temperature, as shown in Figure S6c. We also evaluated the photocatalytic performances of Ta2O5 by measuring the water-splitting hydrogen production, as shown in Figure 4b. The hydrogen generation of our Ta2O5 sample is greatly improved after aluminum reduction. The Al300 and Al-400 sample exhibit hydrogen generation rates of 1.32 and 2.05 mmol h−1 g−1, which are about 1.5 and 2.3 times higher than that of the pristine Ta2O5 (0.88 mmol h−1 g−1). The hydrogen production over Al-400 is comparable to or even better than many reported Ta2O5 materials.33−35 Al-400 shows the highest photocatalytic activity in both MO degradation and H2 generation because of its high solar absorption and low e−h recombination rate.

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CONCLUSIONS In conclusion, gray Ta2O5 nanowires with significantly improved photocatalytic performance are successfully prepared through aluminum reduction in a two-temperature-zone furnace. The introduction of surface disorder and oxygen vacancies renders the reduced Ta2O5 with an enhanced solar light absorption and a narrowing bandgap. The photocatalytic contamination removal and hydrogen production activities of our reduced Ta2O5 NWs under both visible light and solar light irradiation are superior to that of the pristine Ta2O5. Additionally, the electron density and photoelectrochemical water-splitting property have been greatly improved after aluminum reduction as well. We have demonstrated that Ta2O5 can be modified to be very active in photocatalytic removal of organic pollutants and hydrogen production and it D

DOI: 10.1021/acsami.5b07685 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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could be a promising photocatalytic PEC photoanode material through aluminum reduction. The positive results motivate us to treat other semiconductor oxides with aluminum-mediated reduction for improved photoelectric properties.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07685. Sample characterization methods and SEM micrographs of commercial and amorphous Ta2O5, calculated solar absorption, X-ray photoelectron wide-spectrum survey spectra, Al 2p, O 1s spectra, and magnetic field dependence of magnetization of pristine Ta2O5 NW and Al-reduced samples, optical absorption spectra, PL spectra, SEM micrographs, and photocatalytic MO degradation activity of pristine Ta2O5 NW and 500 °C reduced samples, and solar absorption and MO photodegradation of commercial Ta2O5 before and after Alreduction (PDF)

EXPERIMENTAL SECTION

Preparation of Ta2O5 Nanowires. The detailed synthetic process for the Ta2O5 nanowires is similarly to reference.7 A typical synthesis process is briefly discussed in the following part. First, 3.0 g of commercial Ta2O5 powder was added to 30 mL of hydrofluoric acid solution in Teflon-lined autoclave and reacted at 120 °C for 12 h to obtain a Ta-fluoro complex solution. Second, after it was cooled to room temperature, 20 mL of ammonia was added by dropwise to get white amorphous precipitates (a-Ta2O5). These precipitates were rinsed with deionized water to remove NH4+ and F− ions. Third, 0.2 g of fresh precipitate was dissolved in a solution containing 25 mL of hydrogen peroxide and 5 mL of ammonia. Finally, Ta2O5 nanowires were obtained after reacted at 240 °C for 24 h and washed thoroughly with deionized water and ethanol. Preparation of Gray Ta2O5 Nanowires. The gray Ta2O5 sample was prepared through reducing by melted aluminum in a two-zone evacuated furnace. The aluminum powders and Ta2O5 nanowires were separately placed at 800 °C and 300−500 °C for 4 h under a pressure of 5 Pa. Photocatalytic Methyl Orange (MO) Degradation. The photocatalytic performance of the Ta2O5 nanowires were estimated by measuring the photodegradation of MO aqueous solution (10 mg/ L) under illumination by a 300 W iodine gallium lamp. The experiment was performed as follows: 100 mg of Ta2O5 nanowires were dispersed in a Pyrex glass reactor containing 100 mL of MO solution. After magnetically stirred in dark for 30 min, the suspension was irradiated and about 5 mL of the suspension was withdrawn at given time intervals, which was centrifuged and measured by tracking the typical absorption peak at 464 nm with the Hitachi U-3010 UV− vis spectrophotometer to evaluate the MO concentration. Photocatalytic Methylene Blue (MB) Degradation. The visible photocatalytic performance of Ta2O5 NWs were evaluated by measuring the photodegradation of MB aqueous solution (10 mg/L) under illumination by a 300W Xe lamp with a 420 nm filter. After magnetically stirred in dark for 30 min, the suspension was irradiated and about 5 mL of the suspension was withdrawn at given time intervals, which was centrifuged and measured by tracking the typical absorption peak at 664 nm with the Hitachi U-3010 UV−vis spectrophotometer to evaluate the MB concentration. Hydrogen Production. The photocatalytic hydrogen production was carried out in a top-irradiation Pyrex reactor. Ta2O5 powder (100 mg) was dispersed in a 200 mL of aqueous solution with 20% methanol as the sacrificial agent. After it was loaded with 0.5 wt % Pt, the mixture was pursed by pure N2 for 10 min and illuminated by a 300 W Xe lamp. The H2 evolution amount was measured using a gas chromatographer (Shanghai, GC-7900, TCD, N2 carrier). The photocatalytic activities were compared by the average H2 evolution rate in the first 5 h. Photoelectrochemical Cell (PEC). PEC performance were measured in a conventional three-electrode electrochemical workstation (CHI600B, CH Instruments). The pristine and reduced tantalum pentoxide films were first prepared by spin-coating and used as working electrodes. The counter and reference electrodes are Pt wire and KCl-saturated Ag/AgCl electrode, respectively. Then, the photocurrents of Ta2O5 films were measured in dark and under simulated solar irradiation (150 W Xe lamp) by measuring a set of linear sweeps at a scan rate of 10 mV/s between −0.6 and 0.5 V vs Ag/ AgCl for full spectrum in 1 M NaOH eletrolyte, which is used to stabilize the films. Mott−Schottky plots were measured through electrical impedance-potential tests at a frequency of 100 Hz in dark.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written with contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was financially supported by the National Natural Science Foundation of China and Science and Technology Commission of Shanghai. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would greatly thank NSF of China (Grants 91122034, 51125006, 61376056, and 51402336) and Science and Technology Commission of Shanghai (Grants 13JC1405700 and 14YF1406500).

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DOI: 10.1021/acsami.5b07685 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX