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Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
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Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties Zhen Zhang a, Xin Tan c, Tao Yu b,d,*, Lixia Jia a, Xiang Huang c,** a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China c School of Science, Tibet University, Lhasa 850000, Tibet, PR China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China b
article info
abstract
Article history:
Black TiO2 nanotube arrays were synthesized via a two-step anodization procedure by
Received 28 September 2015
controlling the duration of anodization and annealing in an argon atmosphere. Oxygen
Received in revised form
vacancies were introduced in black TiO2 nanotube arrays during the anodization and
18 December 2015
annealing process due to an insufficient oxygen atomosphere, as confirmed by EPR, XRD,
Accepted 30 December 2015
SEM, TEM, XPS, and Raman analysis. Black TiO2 nanotube arrays prepared in 6 h (denoted
Available online xxx
as Ar TNT-6h) exhibited excellent photoelectrocatalytic performance and stable photoelectrochemical performance due to a considerable amount of oxygen vacancies, which
Keywords:
ensured charge separation efficiency and strong visible light absorption. The photo-
Black TiO2 nanotube arrays
electrocatalytic activities of the samples were monitored by the decomposition of Rhoda-
Photoelectrocatalytic
mine B (RhB). Notably, the black TiO2 nanotube arrays exhibited lower charge-transfer
Oxygen vacancies
resistance in electrochemical impendence spectroscopy (EIS). The photocurrent density
Photoelectrochemical performance
of Ar TNT-6h (1.2 mA/cm2) was nearly twelve times higher than that of air-TNTs (pristine TiO2 nanotube arrays annealed in air, 0.1 mA/cm2). The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. A considerable amount of oxygen vacancies significantly improved the separation efficiency of photogenerated electrons and holes, thus enhancing the photoelectrocatalytic efficiency. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: þ86 022 23502142. ** Corresponding author. Tel.: þ86 022 23502142. E-mail addresses: [email protected] (T. Yu), [email protected] (X. Huang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.200 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Introduction Black TiO2 has attracted a considerable amount of attention since 2011 when Chen and Mao demonstrated the ability of black TiO2 to boost solar light harvesting for enhanced photocatalytic and photoelectrochemical performance [1]. Over the past several years, persistent efforts have focused on the production of colorful TiO2 with better visible light absorption via metal [2e6] and non-metal doping [7e9], as well as heating under vacuum or in a reducing atmosphere [10e12]. A variety of synthetic conditions have been employed, such as hydrogen thermal treatments, hydrogen plasma treatment, chemical reduction, electrochemical reduction, anodization and annealing, among others [13]. The properties and performance of black TiO2 nanomaterials are influenced by their synthetic conditions. Several hydrogen-based methods [10,11,14e17] have been shown to significantly increase solar absorption, resulting in the formation of lattice disorderly in black TiO2 and excellent photoelectrocatalytic performance. The mechanism of lattice disorderly was attributed to the breakage of TieO bonds on the surface of TiO2 during the process of TieH or OeH bond formation. Hydrogen insertion into the lattice results in the formation of lattice disorderly, which enhances effective light absorbtion [16]. However, this absorption enhancement is not only ineffective for visible-light photocatalysis but also dangerous and difficult to control during special hydrogenation processes such as high temperature or high pressure hydrogenation or hydrogenation in the presence of pure hydrogen [18]. As an alternative route to hydrogenation, melted aluminium reduction has been developed by Huang et al. [19e21]. Many black TiO2 materials have good visible light absorption but are unsatisfactory for visible light photocatalysis due to the short lifetime of light-excited electrons and holes. Oxygen vacancies in black TiO2 materials are responsible for charge separation and transportation. However, excessive oxygen vacancies would become electron and hole recombination centers, which negatively affect the photocatalytic performance of the material. Thus, Huang et al. attempted to add non-metallic dopants (H, N, S, I) to reduce oxygen vacancies in black TiO2-x, which gains enhanced photocatalytic activity both under the irradiation of full sunlight and under visible light [17]. In addition, Gao et al. synthesized defective TiO2-x via a facile anodization technique [22]. Moreover, the presence of oxygen vacancies, which extend photon absorbance into the visible light region, has been confirmed by electron paramagnetic resonance spectroscopy. TiO2 nanotube arrays with unidirectional architecure have been shown to enhance charge separation and transport and possess a large surface to volume ratio, which favors light capture, electron transport and close contact with the electrolyte. Basing on the aforementioned literature observations, we designed an efficient synthesis for the fabrication of the black TiO2 nanotube arrays with unique properties. To produce different TiO2 nanotube arrays, we varied the duration of the two-step anodization procedure and annealed the TiO2 nanotube arrays in an argon atmosphere (denoted as Ar-
TNTs). The signals of oxygen vacancies in the black TiO2 nanotube arrays was confirmed by Electron paramagnetic resonance (EPR). The structural, morphological, photoelectrocatalytic and photoelectrochemical performance of the Ar-TNTs nanotube arrays were investigated by a series of characterization methods, and all of the samples exhibited remarkable photoelectrochemical response in the visible light region compared to air-TNTs. Improved photoelectrochemical performance was achieved in black TiO2 nanotube arrays prepared for 6 h due to the considerable amount of oxygen vacancies, which ensured charge separation efficiency and high visible light absorption. The black TiO2 nanotube we studied in this work aimed to determine the effects of anodic oxidation time and annealing atmosphere on the formation of oxygen vacancies.
Experimental section Preparation of air-TNTs and Ar-TNTs Titanium foils were cleaned by sonicating in acetone and ethanol, followed by rinsing with deionized (DI) water and drying at 150 C in air. TiO2 nanotube arrays were synthesized via a two-step anodization on Ti foil (0.5 mm, 99.7%). An electrolyte solution containing 3 vol.% DI H2O and 0.5 vol.% NH4F in glycol was employed to prepare pristine TiO2 nanotube arrays. The first anodization was carried out at 60 V for 2 h. Subsequently, the sample was cleaned by sonicating in distilled water to strip off the initial oxide layer, and then the sample was dried in air. The second anodization was performed at 60 V for 2 h, 6 h, and 10 h, respectively. The anodization reaction was performed in a two-electrode electrochemical cell with a counter electrode made out of platinum. After the two-step anodization procedure, the pristine TiO2 nanotube arrays were cleaned with ethanol and distilled water and were dried at 120 C for approximately 30 min to completely evaporate organic material. The asanodized pristine TiO2 nanotube arrays were annealed in an argon atmosphere at 450 C for 2 h at a heating rate of 2 C/ min. In this study, the black TiO2 nanotube arrays annealed in argon atmosphere are denoted as Ar TNT-2h, Ar TNT-6h, and Ar TNT-10 h according to the duration of the second anodization procedure (2 h, 6 h and 10 h, respectively). Air-TNTs with the second anodization time of 2 h is used as a reference.
Photoelectrocatalytic degradation and photoelectrochemical test The photoelectrocatalytic activities of the samples were investigated by detecting the degradation of Rhodamine B (RhB) in an aqueous solution using a 300 W Xe lamp (HSX-F/ UV300, NBeT) as a solar light source with a light intensity of 100 mW cm2. Samples with an effective area of 1 cm 2 cm were used in the decomposition experiments. In particular, 20 mL of a solution containing 4 mg/L of RhB was employed at an external bias of 0.3 V, and 20 mL of 0.1 mol/L NaCl was used as the supporting electrolyte (pH ¼ 5). Prior to the photoelectrocatalytic reaction, the solution was slowly purged with
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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N2 to remove O2. The mixture was stirred for 30 min in the dark and was cooled using a water circulating jacket. A 4-mL aliqout of the mixture was collected every 15 min for UVevis spectrophotometer analysis. The photoelectrochemical properties of the samples were tested using a CHI660E (Shanghai, Chenhua, China) electrochemical workstation with a three-electrode system (counter electrode: Pt, reference electrode: Ag/AgCl, working electrode: the sample). The supporting electrolyte was 40 mL of 1 mol/L NaCl, and AM 1.5 (WXS-80C-3 AM 1.5G, 100 mW cm2) solar simulation was used as the illumination source.
Sample characterization The morphology of the black TiO2 nanotube arrays was investigated using a field emission scanning microscopy electron microscope (FE-SEM, S-4800, Japan) and a transmission electron microscope (TEM, JEM-2100F, Japan). The crystalline structure was determined by X-ray diffraction (XRD, Bruker D8-Focus diffractometer with monochromatized CuKa radiation, l ¼ 1.5418 A, Germany). Raman spectra were collected by a confocal system with an excitation wavelength of 530 nm (Bruker, RENISHAW). X-ray photoelectron spectroscopy spectra (XPS) was measured by Kratos Axis Ultra. Electron paramagnetic resonance (EPR) spectra were collected by a Bruker A300 system at a frequency of 9.67 GHz (Germany), and UVevis diffuse reflectance spectra were recorded on a UVevisible spectrophotometer (UV-2700, Shimadzu, Japan) and converted into absorption spectra via the KubelkaeMunk transformation.
Results and discussion Morphology and phase structure As shown in Fig. 1a, the XRD diagram indicated that the samples annealed in either argon or air gave the typical diffraction patterns of a main anatase structure. The presence of strong diffraction peaks indicated that all of the samples were highly crystalline. In the XRD patterns, air-TNTs and ArTNTs showed several diffraction peaks at approximately 2q ¼ 25.3 , 38.3 and 48.1 , which were well indexed to the {101} {004} {200} facets of anatase TiO2 (JCPDS no. 65-5714), respectively. As shown in Fig. 1b, the presence of oxygen vacancies in Ar-TNTs may be attributed to oxide formation mechanisms. Namely, a dynamic process involving oxidation at the oxide/metal interface and chemical dissolution at the oxide/ electrolyte/interface had occurred during the anodization process, and the length of the TiO2 nanotube arrays increased with increasing the anodization time. As the only oxygen source, water molecules were hindered by the large draw ratio of the nanotube arrays, which grew along the surface. Oxygen vacancies and Ti3þ ions were produced simultaneously at the oxide/metal interface due to the insufficient supply of oxygen. Meanwhile, generated Ti3þ ions were consumed by F ions, which were attracted to the anode by the electrical field [22]. Because Ti3þ ions and F ions were more active and vulnerable to chemical dissolution, TiO2 nanotube arrays with rich oxygen vacancies formed during the anodization process.
Fig. 1 e (a) XRD spectra of air-TNTs and Ar-TNTs. (b) The formation of oxygen vacancies during the anodization process.
Fig. 2a, b and c shows the top, bottom and cross-sectional view of FE-SEM images of Ar TNT-6h, respectively. It can be seen from Fig. 2a, b and c that the black nanotube arrays were uniform, clean, and regular, and they exhibited a homogeneous structure with an average pore size of approximately 95e105 nm and a wall thickness of 20e25 nm. Fig. 2c indicated that nanotube arrays with double-shelled tubes after annealing in argon were well-aligned and perpendicular to the underlying substrate. The morphology and structure of ArTNTs and air-TNTs were further examined by TEM (Fig. 2def). Fig. 2d shows TiO2 nanotube arrays with a disorderly surface, in accordance with the FE-SEM analysis. A lattice space of 0.35 nm was determined by TEM analysis (Fig. 2eef) and was identified as the {101} facet of anatase TiO2. These results showed that air-TNTs and Ar-TNTs contained pure anatase phase, which is consistent with the XRD results. Compared to air-TNTs, Ar-TNTs contained an outer disorderly surface layer, as indicated by the red arrows. Raman scattering analysis was performed to investigate changes in the chemical coordination structure of black TiO2 nanotube arrays. As shown in Fig. 3a, five Raman active modes, A1g þ B1g þ 3Eg, were detected at 144 cm1 (Eg), 197 cm1 (Eg), 394.2 cm1 (B1g), 515.2 cm1 (A1g), and 636.4 cm1 (Eg), which were consistent with anatase [18]. The 144 cm1 peak was assigned to the external vibration of the TieO bond. As shown in Fig. 3b, the frequency of the strongest Eg mode in Ar-TNTs expressed a blue shift to 152.1 cm1. Compared to air-TNTs, the broadened full width of half maximum peak of Ar-TNTs was due to an increase in the
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 2 e FE-SEM images of Ar TNT-6h: (a) Top view, (b) bottom view, (c) cross-sectional view, (d) side view TEM of Ar-TNTs, (e) TEM image of air-TNTs, and (f) TEM image of Ar TNT-6h.
Fig. 3 e Raman spectra of air-TNTs and Ar-TNTs: (a) and (b). (c) UVevisible absorption spectra of air-TNTs and Ar-TNTs. (d) Plots of (ahv)1/2 versus photon energy (hv). Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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amount of oxygen vacancies. The blue shift and broadening of Eg confirmed the presence of defects. In addition, the Raman spectra response of Ar-TNTs displayed a remarkable weakening compared to that of air-TNTs due to the low crystalline of the material. These results also revealed that the defects were generated in Ar-TNTs after annealing at 450 C. As shown in Fig. 3c, the absorption edges of Ar-TNTs were apparently smaller than that of air-TNTs. Based on the Butler equation of ahv ¼ A (hv-Eg)2, the band gaps of TiO2 can be deduced (a is the absorption coefficient, hv is the incident photon energy, A is the proportionality constant, and Eg is the band gap [23]). By plotting (ahv)1/2 versus hv from the absorption spectra, extrapolating the linear portion to zero on an abscissas axis and using the intersections of the straight lines, band gaps of 3.02 eV, 2.64 eV, 2.60 eV and 2.50 eV were obtained for air-TNTs, Ar TNT-10 h, Ar TNT-2h, and Ar TNT-6h, respectively (Fig. 3d).
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The chemical states of air-TNTs and Ar-TNTs were determined by X-ray photoelectron spectroscopy (XPS). Fig. 4a depicted the two Ti 2p peaks centered at 458.6 eV and 464.4 eV from both air-TNTs and Ar-TNTs, which were characteristic of Ti4þ in TiO2 [24]. Significant differences between air-TNTs and Ar-TNTs were not observed in the Ti 2p XPS spectra. Obvious Ti3þ signals were not observed in Fig. 4a, because Ti3þ played an important role in the appearance of color instead of presenting in the crystal lattice. Fig. 4b shows the valence band maxima of air-TNTs and Ar-TNTs. Compared to Ar-TNTs, the band edge position of air-TNTs at 2.77 eV exhibited much weaker oxidation, which may be due to the presence of defects and disorderly on the Ar-TNTs surface. In addition, three different band edge positions were observed from 2.84 eV to 2.99 eV of Ar-TNTs, which were attributed to the duration of anodic oxidation in the synthesis of TiO2 nanotube arrays. The O 1s XPS spectra of air-TNTs and Ar-TNTs are exhibited in
Fig. 4 e XPS spectra of air-TNTs and Ar-TNTs. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 4cef. Each XPS spectra of O 1s could be separated into two symmetric peaks after Gaussian curve fitting, which were located at 530.3 eV and 532.3 eV [21]. The peaks at 530.3 eV were assigned to the crystal lattice of TieO bonds in both airTNTs and Ar-TNTs, while the peak at 532.3 corresponded to the adsorbed O2 [25]. The intensity of absorbed O2 in Ar-TNTs was significantly larger than that of air-TNTs, which was attributed to the presence of oxygen vacancies generated by adsorbed oxygen. Meanwhile, the intensity of the absorbed O2 in Ar TNT-2h, 6 h, and 10 h increased with increasing in the number of oxygen vacancies, which grew as the anodic oxidation time was extended. To detect oxygen vacancies, electron paramagnetic resonance (EPR) was conducted at room temperature and low temperature (at 123 K), respectively. Fig. 5a shows the EPR data obtained at room temperature. Ar TNT-2h, Ar TNT-6h, and Ar TNT-10 h all presented strong signals at g ¼ 2.003, which was characteristic of oxygen vacancies (V0) [12], while no signals of V0 were detected in air-TNTs. The signals of V0 remained strong at 123 K (Fig. 5b) and gradually became clear
as the duration of anodic oxidation increased. Oxygen vacancies were observed because the Ti4þ localized environment changed to an insufficient oxygen atmosphere during the anodic oxidation and annealing process under argon. The generation of oxygen vacancies formed during the annealing process is shown in Fig. 5c. When the oxygen content was low, the balance moved to the right, and oxygen vacancies were created. According to the law of charge conservation, one Ti3þ ion or two Ti2þ ions were present. However, signals relating to Ti3þ ions were not observed in the EPR spectra because Ti3þ played an important role in the observation of a black color.
Photoelectrocatalytic performance and photoelectrochemical properties The photoelectrocatalytic activity of air-TNTs and Ar-TNTs was monitored by determining the decomposition of RhB (Rhodamine B) at an external bias of 0.3 V vs. AgCl under visible light (l ¼ 420 nm). Fig. 6a shows the degradation curve
Fig. 5 e EPR spectra of air-TNTs and Ar-TNTs (a) at room temperature (b) and 123 K. (c) A schematic depiction of the formation of oxygen vacancies in the preparation of Ar-TNTs. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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of air-TNTs and Ar-TNTs. The photoelectrocatalytic activity of the samples decreased according to the following order: Ar TNT-6h, Ar TNT-2h, Ar TNT-10 h and air-TNTs. Approximately 85% of RhB was degraded by Ar TNT-6h after 1.5 h Fig. 6a revealed that benzoquinone (BQ) suppressed the degradation of RhB. BQ was an effective O2C scavenger, which suggested that oxygen played an important role in the decomposition of RhB. The stability of a photocatalyst should be considered an important factor in practical applications. Thus, catalysts must be evaluated. Fig. 6b displays the cycling tests (photoelectrocatalytic degradation of RhB solution) for Ar TNT-6h. The degradation efficiency of RhB over Ar TNT-6h decreased slightly from 85% to 81% after five cycling experiments, indicating that Ar TNT-6h exhibited excellent photostability, which is important for practical applications. As shown in Fig. 6c, the band gap changed from 3.05 eV to 2.50 eV after the samples were annealed in an argon atmosphere, which coincides with the degradation process of a RhB solution. Moreover, a small external bias was also beneficial in driving electron transfer, which reduced the recombination of photoelectron-hole pairs and facilitated rapid degradation of RhB. The photoelectrochemical performance of black TiO2 nanotube arrays was further characterized using MotteSchottky (MS) plots. The flat band potential represented the apparent Fermi level of a semiconductor in equilibrium with a
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redox couple. Thereby, the influence of the annealing atmosphere and duration of anodic oxidation on the interface structure of black nanotube arrays was evaluated by shifting the zero current potential. The slope of all of the samples was positive, as the MS plots shown in Fig. 7a. The slopes of the linear parts of the curves in the MS plots were positive, which is indicative of n-type semiconductors [12]. This analysis provided insight into the carrier density and location of the flat band potential of the samples. As the flat band potential shifted to a more negative value, greater band bending would occur, facilitating improved photo effects with respect to the electrolyte reorganization energy. However, the flat band potential dramatically decreased due to the density of additional oxygen vacancies. Fig. 7b shows that Ar TNT-6h exhibited a smaller radius than the other samples. Thus, the charge in Ar TNT-6h would be quickly transferred to the reactant through the solid/liquid interface and consumed by the chemical reaction, which would inhibit charge recombination. To investigate the photoelectrochemical properties of airTNTs and Ar-TNTs, linear sweep voltammograms were recorded in the dark and under simulated AM 1.5 illuminations conditions at a scan rate of 50 mv/s from 1.0 V to 0.8 V vs. Ag/AgCl (1 M NaCl electrolyte, pH ¼ 7). Fig. 8aeb shows the linear sweep voltammograms collected from air-TNTs and ArTNTs. Fig. 8a shows that the photocurrent in the dark was less
Fig. 6 e (a) Photoelectric properties of RhB over the samples under visible light irradiation. (b) Cycling tests of solar driven photoelectric activity of Ar TNT-6h at pH ¼ 5. (c) The degradation mechanism of RhB at an external bias of 0.3 V. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 7 e (a) MotteSchottky plots of air-TNTs and Ar-TNTs collected at a frequency of 100 Hz in the dark and (b) EIS Nyquist plots of air-TNTs and Ar-TNTs collected at 0.5 V vs. Ag/AgCl in 0.1 mol NaCl at frequencies of 100 Hze105 Hz in the dark.
Fig. 8 e The photoelectrochemical properties were detected using a three-electrode system (counter electrode: Pt, reference electrode: Ag/AgCl, working electrode: the sample). (a) Linear sweep voltammograms collected in the dark. (b) Linear sweep voltammograms collected under AM 1.5 solar simulation. (c) Transient photocurrent responses of air-TNTs and Ar-TNTs at 0.5 V vs. Ag/AgCl.
than 0.5 mA/cm2 for all of the samples, except for Ar TNT-6h (1.3 mA/cm2). Moreover, the onset potential of Ar TNT-6h was lower at 0.83 eV vs. Ag/AgCl compared to the other samples. The lower potential confirmed the enhanced photoresponse because the onset potential represented the contribution of light toward the minimum voltage required for the watersplitting potential (1.2 V) [2]. Fig. 8b shows that the photocurrent density under simulated AM 1.5 illuminations of Ar TNT-6h (2 mA/cm2) was significantly higher than that of Ar TNT-2h, Ar TNT-10 h and air-TNTs. The line graph clearly shows that the photocurrent density increased in the
following order: air-TNTs, Ar TNT-10 h, Ar TNT-2h and Ar TNT-6h. Compared to the photocurrent density in the dark, Ar-TNTs nanotube arrays became significant relative to the photocurrent. The greatly enhanced electrical conductivity of the Ar-TNTs due to the increased carrier density, coupled with the massive oxygen vacancies in Ar-TNTs, contributed to the distinct increase in the dark current. The photocurrent induced by AM 1.5 illuminations of Ar TNT-6h (2.0 mA/cm2) at 0.5 V vs. Ag/AgCl was approximately two times larger than that of Ar TNT-10 h. This enhancement was attributed to the improved visible light absorption of Ar TNT-6h, which was
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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mainly caused by an increase in the amount of oxygen vacancy donor sites and promoted the overall photoelectrochemical performance of Ar TNT-6h. However, when too many oxygen vacancies were present, the vacancies would act as electron and hole recombination sites, which reduced the photoelectrochemical properties of Ar TNT-10 h. The amperometric I-t curve was introduced to compare to the photo responses of Ar-TNTs and air-TNTs under simulated 1.5 M illuminations with several 20-s light on/off cycles at 0.5 V. The bar chart shows the transient photocurrent response vs. time (Fig. 8c). All of the samples exhibited good stability. Without illumination, the current values were nearly zero, while the photocurrent rapidly increased to a steadystate value upon illumination. This behavior was reproducible for several on/off cycles, and an identical photocurrent and dark current was observed. The photocurrent density of Ar TNT-6h (1.2 mA/cm2) was twice as high as that of Ar TNT10 h (0.6 mA/cm2). The photocurrent density of air-TNTs was significantly lower than that of Ar-TNTs, which was attributed to the presence of oxygen vacancies, improved charge separation and transmission.
Conclusions Ar-TNTs nanotube arrays were successfully synthesized by a two-step anodization procedure followed by annealing under an argon atmosphere. The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. The optimal anodic oxidation time of black TiO2 nanotube arrays annealed in an argon atmosphere was determined. Compared to airTNTs, all of the Ar-TNTs exhibited excellent photoelectrochemical and photoelectrocatalytic properties. Specifically, Ar TNT-6h displayed the best photoelectrochemical and photoelectrocatalytic properties among the resulting ArTNTs, because the formation of a considerable amount of oxygen vacancies in an oxygen-deprived atmosphere improved charge transport and separation. Meanwhile, ArTNTs performed better photoelectrocatalytic properties in monochromatic light (l ¼ 420 nm) than that of air-TNTs due to the presence of oxygen vacancies, which improved the carrier density. Moreover, the formation of oxygen vacancies and the photoelectrocatalytic mechanism was proposed based on the preparation of black TiO2 nanotube arrays and the dye degradation experiments. We hope the results of the present study will provide new ideas on black TiO2 nanotube arrays in the future.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21406164, and 21466035), the National Key Basic Research and Development Program of China (973 program, No. 2014CB239300, and 2012CB720100), Research Fund for the Doctoral Program of Higher Education of China (No. 20110032110037, 20130032120019).
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[1] Chen XB, Liu L, Yu PY, Mao SS. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 2011;331:746e50. [2] Liu Q, Ding D, Ning C, Wang X. Black Ni-doped TiO2 photoanodes for high-efficiency photoelectrochemical water-splitting. Int J Hydrogen Energy 2015;40:2107e14. [3] Tang CN, Hou WQ, Liu EZ, Hu XY, Fan J. CeF3/TiO2 composite as a novel visible-light-driven photocatalyst based on upconversion emission and its application for photocatalytic reduction of CO2. J Lumines 2014;154:305e9. [4] Pan L, Wang S, Zou J-J, Huang Z-F, Wang L, Zhang X. Ti3þdefected and V-doped TiO2 quantum dots loaded on MCM41. Chem Commun 2014;50:988e90. [5] Inturi SNR, Boningari T, Suidan M, Smirniotis PG. Flame aerosol synthesized Cr incorporated TiO2 for visible light photodegradation of gas phase acetonitrile. J Phys Chem C 2014;118:231e42. [6] Fujiwara K, Deligiannakis Y, Skoutelis CG, Pratsinis SE. Visible-light active black TiO2-Ag/TiOx particles. Appl Catal B Environ 2014;154e155:9e15. [7] Cong Y, Li X, Dong Z, Yuan G, Cui Z, Zhang J. Core shell structured carbon black@TiO2 microsphere with enhanced visible light photocatalytic activity. Mater Lett 2015;138:200e3. [8] Meng X, Yao J, Liu F, He H, Zhou M, Xiao P, et al. Preparation of SnO2@C-doping TiO2 nanotube arrays and its electrochemical and photoelectrochemical properties. J Alloys Compd 2013;552:392e7. [9] Zhou Y, Liu Y, Liu P, Zhang W, Xing M, Zhang J. A facile approach to further improve the substitution of nitrogen into reduced TiO2x with an enhanced photocatalytic activity. Appl Catal B Environ 2015;170e171:66e73. [10] Amano F, Nakata M. High-temperature calcination and hydrogen reduction of rutile TiO2: a method to improve the photocatalytic activity for water oxidation. Appl Catal B Environ 2014;158e159:202e8. [11] Liu L, Yu PY, Chen X, Mao SS, Shen DZ. Hydrogenation and disorder in engineered black TiO2. Phys Rev Lett 2013;111. [12] Wang S, Pan L, Song J-J, Mi W, Zou JJ, Wang L, et al. Titaniumdefected undoped anatase TiO2 with p-type conductivity, room-temperature ferromagnetism, and remarkable photocatalytic performance. J Am Chem Soc 2015;137:2975e83. [13] Chen X, Liu L, Huang F. Black titanium dioxide (TiO2) nanomaterials. Chem Soc Rev 2015;44:1861e85. [14] Ramchiary A, Samdarshi SK. High visible light activity of hydrogenated structure-engineered mixed phase titania photocatalyst. Chem Phys Lett 2014;597:63e8. [15] Wang Z, Yang C, Lin T, Yin H, Chen P, Wan D, et al. H-doped black titania with very high solar absorption and excellent photocatalysis enhanced by localized surface plasmon resonance. Adv Funct Mater 2013;23:5444e50. [16] Hu YH. A highly efficient photocatalystdhydrogenated black TiO2 for the photocatalytic splitting of water. Angew Chem Int Ed 2012;51:12410e2. [17] Wang G, Wang H, Ling Y, Tang Y, Yang X, Fitzmorris RC, et al. Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting. Nano Lett 2011;11:3026e33. [18] Wang H, Lin T, Zhu G, Yin H, Lu¨ X, Li Y, et al. Colored titania nanocrystals and excellent photocatalysis for water cleaning. Catal Commun 2015;60:55e9. [19] Cui H, Zhao W, Yang C, Yin H, Lin T, Shan Y, et al. Black TiO2 nanotube arrays for high-efficiency photoelectrochemical water-splitting. J Mater Chem A 2014;2:8612e6.
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[20] Lin T, Yang C, Wang Z, Yin H, Lu X, Huang F, et al. Effective nonmetal incorporation in black titania with enhanced solar energy utilization. Energy Environ Sci 2014;7:967e72. [21] Wang Z, Yang CY, Lin TQ, Yin H, Chen P, Wan DY, et al. Visible-light photocatalytic, solar thermal and photoelectrochemical properties of aluminium-reduced black titania. Energy Environ Sci 2013;6:3007e14. [22] Dong J, Han J, Liu Y, Nakajima A, Matsushita S, Wei S, et al. Defective black TiO2 synthesized via anodization for visiblelight photocatalysis. ACS Appl Mater Interfaces 2014;6:1385e8.
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Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties Zhen Zhang a, Xin Tan c, Tao Yu b,d,*, Lixia Jia a, Xiang Huang c,** a
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China c School of Science, Tibet University, Lhasa 850000, Tibet, PR China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China b
article info
abstract
Article history:
Black TiO2 nanotube arrays were synthesized via a two-step anodization procedure by
Received 28 September 2015
controlling the duration of anodization and annealing in an argon atmosphere. Oxygen
Received in revised form
vacancies were introduced in black TiO2 nanotube arrays during the anodization and
18 December 2015
annealing process due to an insufficient oxygen atomosphere, as confirmed by EPR, XRD,
Accepted 30 December 2015
SEM, TEM, XPS, and Raman analysis. Black TiO2 nanotube arrays prepared in 6 h (denoted
Available online xxx
as Ar TNT-6h) exhibited excellent photoelectrocatalytic performance and stable photoelectrochemical performance due to a considerable amount of oxygen vacancies, which
Keywords:
ensured charge separation efficiency and strong visible light absorption. The photo-
Black TiO2 nanotube arrays
electrocatalytic activities of the samples were monitored by the decomposition of Rhoda-
Photoelectrocatalytic
mine B (RhB). Notably, the black TiO2 nanotube arrays exhibited lower charge-transfer
Oxygen vacancies
resistance in electrochemical impendence spectroscopy (EIS). The photocurrent density
Photoelectrochemical performance
of Ar TNT-6h (1.2 mA/cm2) was nearly twelve times higher than that of air-TNTs (pristine TiO2 nanotube arrays annealed in air, 0.1 mA/cm2). The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. A considerable amount of oxygen vacancies significantly improved the separation efficiency of photogenerated electrons and holes, thus enhancing the photoelectrocatalytic efficiency. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: þ86 022 23502142. ** Corresponding author. Tel.: þ86 022 23502142. E-mail addresses: [email protected] (T. Yu), [email protected] (X. Huang). http://dx.doi.org/10.1016/j.ijhydene.2015.12.200 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Introduction Black TiO2 has attracted a considerable amount of attention since 2011 when Chen and Mao demonstrated the ability of black TiO2 to boost solar light harvesting for enhanced photocatalytic and photoelectrochemical performance [1]. Over the past several years, persistent efforts have focused on the production of colorful TiO2 with better visible light absorption via metal [2e6] and non-metal doping [7e9], as well as heating under vacuum or in a reducing atmosphere [10e12]. A variety of synthetic conditions have been employed, such as hydrogen thermal treatments, hydrogen plasma treatment, chemical reduction, electrochemical reduction, anodization and annealing, among others [13]. The properties and performance of black TiO2 nanomaterials are influenced by their synthetic conditions. Several hydrogen-based methods [10,11,14e17] have been shown to significantly increase solar absorption, resulting in the formation of lattice disorderly in black TiO2 and excellent photoelectrocatalytic performance. The mechanism of lattice disorderly was attributed to the breakage of TieO bonds on the surface of TiO2 during the process of TieH or OeH bond formation. Hydrogen insertion into the lattice results in the formation of lattice disorderly, which enhances effective light absorbtion [16]. However, this absorption enhancement is not only ineffective for visible-light photocatalysis but also dangerous and difficult to control during special hydrogenation processes such as high temperature or high pressure hydrogenation or hydrogenation in the presence of pure hydrogen [18]. As an alternative route to hydrogenation, melted aluminium reduction has been developed by Huang et al. [19e21]. Many black TiO2 materials have good visible light absorption but are unsatisfactory for visible light photocatalysis due to the short lifetime of light-excited electrons and holes. Oxygen vacancies in black TiO2 materials are responsible for charge separation and transportation. However, excessive oxygen vacancies would become electron and hole recombination centers, which negatively affect the photocatalytic performance of the material. Thus, Huang et al. attempted to add non-metallic dopants (H, N, S, I) to reduce oxygen vacancies in black TiO2-x, which gains enhanced photocatalytic activity both under the irradiation of full sunlight and under visible light [17]. In addition, Gao et al. synthesized defective TiO2-x via a facile anodization technique [22]. Moreover, the presence of oxygen vacancies, which extend photon absorbance into the visible light region, has been confirmed by electron paramagnetic resonance spectroscopy. TiO2 nanotube arrays with unidirectional architecure have been shown to enhance charge separation and transport and possess a large surface to volume ratio, which favors light capture, electron transport and close contact with the electrolyte. Basing on the aforementioned literature observations, we designed an efficient synthesis for the fabrication of the black TiO2 nanotube arrays with unique properties. To produce different TiO2 nanotube arrays, we varied the duration of the two-step anodization procedure and annealed the TiO2 nanotube arrays in an argon atmosphere (denoted as Ar-
TNTs). The signals of oxygen vacancies in the black TiO2 nanotube arrays was confirmed by Electron paramagnetic resonance (EPR). The structural, morphological, photoelectrocatalytic and photoelectrochemical performance of the Ar-TNTs nanotube arrays were investigated by a series of characterization methods, and all of the samples exhibited remarkable photoelectrochemical response in the visible light region compared to air-TNTs. Improved photoelectrochemical performance was achieved in black TiO2 nanotube arrays prepared for 6 h due to the considerable amount of oxygen vacancies, which ensured charge separation efficiency and high visible light absorption. The black TiO2 nanotube we studied in this work aimed to determine the effects of anodic oxidation time and annealing atmosphere on the formation of oxygen vacancies.
Experimental section Preparation of air-TNTs and Ar-TNTs Titanium foils were cleaned by sonicating in acetone and ethanol, followed by rinsing with deionized (DI) water and drying at 150 C in air. TiO2 nanotube arrays were synthesized via a two-step anodization on Ti foil (0.5 mm, 99.7%). An electrolyte solution containing 3 vol.% DI H2O and 0.5 vol.% NH4F in glycol was employed to prepare pristine TiO2 nanotube arrays. The first anodization was carried out at 60 V for 2 h. Subsequently, the sample was cleaned by sonicating in distilled water to strip off the initial oxide layer, and then the sample was dried in air. The second anodization was performed at 60 V for 2 h, 6 h, and 10 h, respectively. The anodization reaction was performed in a two-electrode electrochemical cell with a counter electrode made out of platinum. After the two-step anodization procedure, the pristine TiO2 nanotube arrays were cleaned with ethanol and distilled water and were dried at 120 C for approximately 30 min to completely evaporate organic material. The asanodized pristine TiO2 nanotube arrays were annealed in an argon atmosphere at 450 C for 2 h at a heating rate of 2 C/ min. In this study, the black TiO2 nanotube arrays annealed in argon atmosphere are denoted as Ar TNT-2h, Ar TNT-6h, and Ar TNT-10 h according to the duration of the second anodization procedure (2 h, 6 h and 10 h, respectively). Air-TNTs with the second anodization time of 2 h is used as a reference.
Photoelectrocatalytic degradation and photoelectrochemical test The photoelectrocatalytic activities of the samples were investigated by detecting the degradation of Rhodamine B (RhB) in an aqueous solution using a 300 W Xe lamp (HSX-F/ UV300, NBeT) as a solar light source with a light intensity of 100 mW cm2. Samples with an effective area of 1 cm 2 cm were used in the decomposition experiments. In particular, 20 mL of a solution containing 4 mg/L of RhB was employed at an external bias of 0.3 V, and 20 mL of 0.1 mol/L NaCl was used as the supporting electrolyte (pH ¼ 5). Prior to the photoelectrocatalytic reaction, the solution was slowly purged with
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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N2 to remove O2. The mixture was stirred for 30 min in the dark and was cooled using a water circulating jacket. A 4-mL aliqout of the mixture was collected every 15 min for UVevis spectrophotometer analysis. The photoelectrochemical properties of the samples were tested using a CHI660E (Shanghai, Chenhua, China) electrochemical workstation with a three-electrode system (counter electrode: Pt, reference electrode: Ag/AgCl, working electrode: the sample). The supporting electrolyte was 40 mL of 1 mol/L NaCl, and AM 1.5 (WXS-80C-3 AM 1.5G, 100 mW cm2) solar simulation was used as the illumination source.
Sample characterization The morphology of the black TiO2 nanotube arrays was investigated using a field emission scanning microscopy electron microscope (FE-SEM, S-4800, Japan) and a transmission electron microscope (TEM, JEM-2100F, Japan). The crystalline structure was determined by X-ray diffraction (XRD, Bruker D8-Focus diffractometer with monochromatized CuKa radiation, l ¼ 1.5418 A, Germany). Raman spectra were collected by a confocal system with an excitation wavelength of 530 nm (Bruker, RENISHAW). X-ray photoelectron spectroscopy spectra (XPS) was measured by Kratos Axis Ultra. Electron paramagnetic resonance (EPR) spectra were collected by a Bruker A300 system at a frequency of 9.67 GHz (Germany), and UVevis diffuse reflectance spectra were recorded on a UVevisible spectrophotometer (UV-2700, Shimadzu, Japan) and converted into absorption spectra via the KubelkaeMunk transformation.
Results and discussion Morphology and phase structure As shown in Fig. 1a, the XRD diagram indicated that the samples annealed in either argon or air gave the typical diffraction patterns of a main anatase structure. The presence of strong diffraction peaks indicated that all of the samples were highly crystalline. In the XRD patterns, air-TNTs and ArTNTs showed several diffraction peaks at approximately 2q ¼ 25.3 , 38.3 and 48.1 , which were well indexed to the {101} {004} {200} facets of anatase TiO2 (JCPDS no. 65-5714), respectively. As shown in Fig. 1b, the presence of oxygen vacancies in Ar-TNTs may be attributed to oxide formation mechanisms. Namely, a dynamic process involving oxidation at the oxide/metal interface and chemical dissolution at the oxide/ electrolyte/interface had occurred during the anodization process, and the length of the TiO2 nanotube arrays increased with increasing the anodization time. As the only oxygen source, water molecules were hindered by the large draw ratio of the nanotube arrays, which grew along the surface. Oxygen vacancies and Ti3þ ions were produced simultaneously at the oxide/metal interface due to the insufficient supply of oxygen. Meanwhile, generated Ti3þ ions were consumed by F ions, which were attracted to the anode by the electrical field [22]. Because Ti3þ ions and F ions were more active and vulnerable to chemical dissolution, TiO2 nanotube arrays with rich oxygen vacancies formed during the anodization process.
Fig. 1 e (a) XRD spectra of air-TNTs and Ar-TNTs. (b) The formation of oxygen vacancies during the anodization process.
Fig. 2a, b and c shows the top, bottom and cross-sectional view of FE-SEM images of Ar TNT-6h, respectively. It can be seen from Fig. 2a, b and c that the black nanotube arrays were uniform, clean, and regular, and they exhibited a homogeneous structure with an average pore size of approximately 95e105 nm and a wall thickness of 20e25 nm. Fig. 2c indicated that nanotube arrays with double-shelled tubes after annealing in argon were well-aligned and perpendicular to the underlying substrate. The morphology and structure of ArTNTs and air-TNTs were further examined by TEM (Fig. 2def). Fig. 2d shows TiO2 nanotube arrays with a disorderly surface, in accordance with the FE-SEM analysis. A lattice space of 0.35 nm was determined by TEM analysis (Fig. 2eef) and was identified as the {101} facet of anatase TiO2. These results showed that air-TNTs and Ar-TNTs contained pure anatase phase, which is consistent with the XRD results. Compared to air-TNTs, Ar-TNTs contained an outer disorderly surface layer, as indicated by the red arrows. Raman scattering analysis was performed to investigate changes in the chemical coordination structure of black TiO2 nanotube arrays. As shown in Fig. 3a, five Raman active modes, A1g þ B1g þ 3Eg, were detected at 144 cm1 (Eg), 197 cm1 (Eg), 394.2 cm1 (B1g), 515.2 cm1 (A1g), and 636.4 cm1 (Eg), which were consistent with anatase [18]. The 144 cm1 peak was assigned to the external vibration of the TieO bond. As shown in Fig. 3b, the frequency of the strongest Eg mode in Ar-TNTs expressed a blue shift to 152.1 cm1. Compared to air-TNTs, the broadened full width of half maximum peak of Ar-TNTs was due to an increase in the
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 2 e FE-SEM images of Ar TNT-6h: (a) Top view, (b) bottom view, (c) cross-sectional view, (d) side view TEM of Ar-TNTs, (e) TEM image of air-TNTs, and (f) TEM image of Ar TNT-6h.
Fig. 3 e Raman spectra of air-TNTs and Ar-TNTs: (a) and (b). (c) UVevisible absorption spectra of air-TNTs and Ar-TNTs. (d) Plots of (ahv)1/2 versus photon energy (hv). Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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amount of oxygen vacancies. The blue shift and broadening of Eg confirmed the presence of defects. In addition, the Raman spectra response of Ar-TNTs displayed a remarkable weakening compared to that of air-TNTs due to the low crystalline of the material. These results also revealed that the defects were generated in Ar-TNTs after annealing at 450 C. As shown in Fig. 3c, the absorption edges of Ar-TNTs were apparently smaller than that of air-TNTs. Based on the Butler equation of ahv ¼ A (hv-Eg)2, the band gaps of TiO2 can be deduced (a is the absorption coefficient, hv is the incident photon energy, A is the proportionality constant, and Eg is the band gap [23]). By plotting (ahv)1/2 versus hv from the absorption spectra, extrapolating the linear portion to zero on an abscissas axis and using the intersections of the straight lines, band gaps of 3.02 eV, 2.64 eV, 2.60 eV and 2.50 eV were obtained for air-TNTs, Ar TNT-10 h, Ar TNT-2h, and Ar TNT-6h, respectively (Fig. 3d).
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The chemical states of air-TNTs and Ar-TNTs were determined by X-ray photoelectron spectroscopy (XPS). Fig. 4a depicted the two Ti 2p peaks centered at 458.6 eV and 464.4 eV from both air-TNTs and Ar-TNTs, which were characteristic of Ti4þ in TiO2 [24]. Significant differences between air-TNTs and Ar-TNTs were not observed in the Ti 2p XPS spectra. Obvious Ti3þ signals were not observed in Fig. 4a, because Ti3þ played an important role in the appearance of color instead of presenting in the crystal lattice. Fig. 4b shows the valence band maxima of air-TNTs and Ar-TNTs. Compared to Ar-TNTs, the band edge position of air-TNTs at 2.77 eV exhibited much weaker oxidation, which may be due to the presence of defects and disorderly on the Ar-TNTs surface. In addition, three different band edge positions were observed from 2.84 eV to 2.99 eV of Ar-TNTs, which were attributed to the duration of anodic oxidation in the synthesis of TiO2 nanotube arrays. The O 1s XPS spectra of air-TNTs and Ar-TNTs are exhibited in
Fig. 4 e XPS spectra of air-TNTs and Ar-TNTs. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 4cef. Each XPS spectra of O 1s could be separated into two symmetric peaks after Gaussian curve fitting, which were located at 530.3 eV and 532.3 eV [21]. The peaks at 530.3 eV were assigned to the crystal lattice of TieO bonds in both airTNTs and Ar-TNTs, while the peak at 532.3 corresponded to the adsorbed O2 [25]. The intensity of absorbed O2 in Ar-TNTs was significantly larger than that of air-TNTs, which was attributed to the presence of oxygen vacancies generated by adsorbed oxygen. Meanwhile, the intensity of the absorbed O2 in Ar TNT-2h, 6 h, and 10 h increased with increasing in the number of oxygen vacancies, which grew as the anodic oxidation time was extended. To detect oxygen vacancies, electron paramagnetic resonance (EPR) was conducted at room temperature and low temperature (at 123 K), respectively. Fig. 5a shows the EPR data obtained at room temperature. Ar TNT-2h, Ar TNT-6h, and Ar TNT-10 h all presented strong signals at g ¼ 2.003, which was characteristic of oxygen vacancies (V0) [12], while no signals of V0 were detected in air-TNTs. The signals of V0 remained strong at 123 K (Fig. 5b) and gradually became clear
as the duration of anodic oxidation increased. Oxygen vacancies were observed because the Ti4þ localized environment changed to an insufficient oxygen atmosphere during the anodic oxidation and annealing process under argon. The generation of oxygen vacancies formed during the annealing process is shown in Fig. 5c. When the oxygen content was low, the balance moved to the right, and oxygen vacancies were created. According to the law of charge conservation, one Ti3þ ion or two Ti2þ ions were present. However, signals relating to Ti3þ ions were not observed in the EPR spectra because Ti3þ played an important role in the observation of a black color.
Photoelectrocatalytic performance and photoelectrochemical properties The photoelectrocatalytic activity of air-TNTs and Ar-TNTs was monitored by determining the decomposition of RhB (Rhodamine B) at an external bias of 0.3 V vs. AgCl under visible light (l ¼ 420 nm). Fig. 6a shows the degradation curve
Fig. 5 e EPR spectra of air-TNTs and Ar-TNTs (a) at room temperature (b) and 123 K. (c) A schematic depiction of the formation of oxygen vacancies in the preparation of Ar-TNTs. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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of air-TNTs and Ar-TNTs. The photoelectrocatalytic activity of the samples decreased according to the following order: Ar TNT-6h, Ar TNT-2h, Ar TNT-10 h and air-TNTs. Approximately 85% of RhB was degraded by Ar TNT-6h after 1.5 h Fig. 6a revealed that benzoquinone (BQ) suppressed the degradation of RhB. BQ was an effective O2C scavenger, which suggested that oxygen played an important role in the decomposition of RhB. The stability of a photocatalyst should be considered an important factor in practical applications. Thus, catalysts must be evaluated. Fig. 6b displays the cycling tests (photoelectrocatalytic degradation of RhB solution) for Ar TNT-6h. The degradation efficiency of RhB over Ar TNT-6h decreased slightly from 85% to 81% after five cycling experiments, indicating that Ar TNT-6h exhibited excellent photostability, which is important for practical applications. As shown in Fig. 6c, the band gap changed from 3.05 eV to 2.50 eV after the samples were annealed in an argon atmosphere, which coincides with the degradation process of a RhB solution. Moreover, a small external bias was also beneficial in driving electron transfer, which reduced the recombination of photoelectron-hole pairs and facilitated rapid degradation of RhB. The photoelectrochemical performance of black TiO2 nanotube arrays was further characterized using MotteSchottky (MS) plots. The flat band potential represented the apparent Fermi level of a semiconductor in equilibrium with a
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redox couple. Thereby, the influence of the annealing atmosphere and duration of anodic oxidation on the interface structure of black nanotube arrays was evaluated by shifting the zero current potential. The slope of all of the samples was positive, as the MS plots shown in Fig. 7a. The slopes of the linear parts of the curves in the MS plots were positive, which is indicative of n-type semiconductors [12]. This analysis provided insight into the carrier density and location of the flat band potential of the samples. As the flat band potential shifted to a more negative value, greater band bending would occur, facilitating improved photo effects with respect to the electrolyte reorganization energy. However, the flat band potential dramatically decreased due to the density of additional oxygen vacancies. Fig. 7b shows that Ar TNT-6h exhibited a smaller radius than the other samples. Thus, the charge in Ar TNT-6h would be quickly transferred to the reactant through the solid/liquid interface and consumed by the chemical reaction, which would inhibit charge recombination. To investigate the photoelectrochemical properties of airTNTs and Ar-TNTs, linear sweep voltammograms were recorded in the dark and under simulated AM 1.5 illuminations conditions at a scan rate of 50 mv/s from 1.0 V to 0.8 V vs. Ag/AgCl (1 M NaCl electrolyte, pH ¼ 7). Fig. 8aeb shows the linear sweep voltammograms collected from air-TNTs and ArTNTs. Fig. 8a shows that the photocurrent in the dark was less
Fig. 6 e (a) Photoelectric properties of RhB over the samples under visible light irradiation. (b) Cycling tests of solar driven photoelectric activity of Ar TNT-6h at pH ¼ 5. (c) The degradation mechanism of RhB at an external bias of 0.3 V. Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
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Fig. 7 e (a) MotteSchottky plots of air-TNTs and Ar-TNTs collected at a frequency of 100 Hz in the dark and (b) EIS Nyquist plots of air-TNTs and Ar-TNTs collected at 0.5 V vs. Ag/AgCl in 0.1 mol NaCl at frequencies of 100 Hze105 Hz in the dark.
Fig. 8 e The photoelectrochemical properties were detected using a three-electrode system (counter electrode: Pt, reference electrode: Ag/AgCl, working electrode: the sample). (a) Linear sweep voltammograms collected in the dark. (b) Linear sweep voltammograms collected under AM 1.5 solar simulation. (c) Transient photocurrent responses of air-TNTs and Ar-TNTs at 0.5 V vs. Ag/AgCl.
than 0.5 mA/cm2 for all of the samples, except for Ar TNT-6h (1.3 mA/cm2). Moreover, the onset potential of Ar TNT-6h was lower at 0.83 eV vs. Ag/AgCl compared to the other samples. The lower potential confirmed the enhanced photoresponse because the onset potential represented the contribution of light toward the minimum voltage required for the watersplitting potential (1.2 V) [2]. Fig. 8b shows that the photocurrent density under simulated AM 1.5 illuminations of Ar TNT-6h (2 mA/cm2) was significantly higher than that of Ar TNT-2h, Ar TNT-10 h and air-TNTs. The line graph clearly shows that the photocurrent density increased in the
following order: air-TNTs, Ar TNT-10 h, Ar TNT-2h and Ar TNT-6h. Compared to the photocurrent density in the dark, Ar-TNTs nanotube arrays became significant relative to the photocurrent. The greatly enhanced electrical conductivity of the Ar-TNTs due to the increased carrier density, coupled with the massive oxygen vacancies in Ar-TNTs, contributed to the distinct increase in the dark current. The photocurrent induced by AM 1.5 illuminations of Ar TNT-6h (2.0 mA/cm2) at 0.5 V vs. Ag/AgCl was approximately two times larger than that of Ar TNT-10 h. This enhancement was attributed to the improved visible light absorption of Ar TNT-6h, which was
Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
mainly caused by an increase in the amount of oxygen vacancy donor sites and promoted the overall photoelectrochemical performance of Ar TNT-6h. However, when too many oxygen vacancies were present, the vacancies would act as electron and hole recombination sites, which reduced the photoelectrochemical properties of Ar TNT-10 h. The amperometric I-t curve was introduced to compare to the photo responses of Ar-TNTs and air-TNTs under simulated 1.5 M illuminations with several 20-s light on/off cycles at 0.5 V. The bar chart shows the transient photocurrent response vs. time (Fig. 8c). All of the samples exhibited good stability. Without illumination, the current values were nearly zero, while the photocurrent rapidly increased to a steadystate value upon illumination. This behavior was reproducible for several on/off cycles, and an identical photocurrent and dark current was observed. The photocurrent density of Ar TNT-6h (1.2 mA/cm2) was twice as high as that of Ar TNT10 h (0.6 mA/cm2). The photocurrent density of air-TNTs was significantly lower than that of Ar-TNTs, which was attributed to the presence of oxygen vacancies, improved charge separation and transmission.
Conclusions Ar-TNTs nanotube arrays were successfully synthesized by a two-step anodization procedure followed by annealing under an argon atmosphere. The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. The optimal anodic oxidation time of black TiO2 nanotube arrays annealed in an argon atmosphere was determined. Compared to airTNTs, all of the Ar-TNTs exhibited excellent photoelectrochemical and photoelectrocatalytic properties. Specifically, Ar TNT-6h displayed the best photoelectrochemical and photoelectrocatalytic properties among the resulting ArTNTs, because the formation of a considerable amount of oxygen vacancies in an oxygen-deprived atmosphere improved charge transport and separation. Meanwhile, ArTNTs performed better photoelectrocatalytic properties in monochromatic light (l ¼ 420 nm) than that of air-TNTs due to the presence of oxygen vacancies, which improved the carrier density. Moreover, the formation of oxygen vacancies and the photoelectrocatalytic mechanism was proposed based on the preparation of black TiO2 nanotube arrays and the dye degradation experiments. We hope the results of the present study will provide new ideas on black TiO2 nanotube arrays in the future.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21406164, and 21466035), the National Key Basic Research and Development Program of China (973 program, No. 2014CB239300, and 2012CB720100), Research Fund for the Doctoral Program of Higher Education of China (No. 20110032110037, 20130032120019).
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Please cite this article in press as: Zhang Z, et al., Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2015.12.200