BiVO4/TiO2(N2) Nanotubes Heterojunction Photoanode for Highly Efficient Photoelectrocatalytic Applications
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We report the development of a novel visible response BiVO4/TiO2(N2) nanotubes photoanode for photoelectrocatalytic applications. The nitrogen-treated TiO2 nanotube shows a high carrier concentration rate, thus resulting in a high efficient charge transportation and low electron–hole recombination in the TiO2–BiVO4. Therefore, the BiVO4/TiO2(N2) NTs photoanode enabled with a significantly enhanced photocurrent of 2.73 mA cm−2 (at 1 V vs. Ag/AgCl) and a degradation efficiency in the oxidation of dyes under visible light. Field emission scanning electron microscopy, X-ray diffractometry, energy-dispersive X-ray spectrometer, and UV–Vis absorption spectrum were conducted to characterize the photoanode and demonstrated the presence of both metal oxides as a junction composite.
KeywordsBiVO4 TiO2(N2) nanotube Heterojunction Photoelectrocatalytic Degradation of dyes
The extreme shortage of natural resources and severe environmental problems caused by burning fossil fuels are pressing global concerns. In the past decades, many efforts were made to explore alternate energy sources. Photoelectrocatalytic (PEC) technology is widely recognized as an alternative energy source because it provides a highly efficient and eco-friendly route to produce renewable energy, and it degrades organic pollutants by the direct use of sunlight [1, 2, 3, 4]. It can be achieved using a semiconductor photoanode/liquid junction, which drives an oxidation reaction. Therefore, in most PEC cells, the overall performance is primarily determined by the photoanode. However, it is still a challenge to synthesize a photoanode material that is chemically stable and has reasonably high incident light-to-current conversion efficiency in the visible range.
In recent years, Bi3+-based complex oxides that could absorb visible light effectively and with the advantage of price beneficial have been produced as alternative energy materials [5, 6, 7, 8]. BiVO4 is a promising high efficient photoanode and photocatalysis material, with advantages of small optical band gaps (2.4 eV) and high stability, and low conduction band edges that overcome traditional photoanode materials, such as ZnO, TiO2, WO3, and Fe2O3 [9, 10, 11, 12, 13]. However, BiVO4 has the shortages of poor carrier transport properties and a substantially less efficient physical photoconversion rate .
One approach for alleviating these limitations is to use another semiconductor as support material to form a heterojunction that not only facilitates carrier transport but also enhances light absorption. Among various semiconductors, TiO2 has been intensively studied as a promising photoanode because it is stable, cost-effective, and has a negative flat band potential (∼0.2 V vs. RHE) (RHE, reversible hydrogen electrode) [14, 15, 16, 17, 18]. Recently, Xie et al.  found an unusual spatial transfer of visibly excited high-energy electrons of BiVO4 to TiO2, which indicated enhanced photoactivity in the heterojunction of BiVO4/TiO2 nanoparticles. Li et al.  demonstrated that a proper facet contact between BiVO4 and TiO2 nanoparticles was the key to improving the photoactivity of BiVO4. Recently, we studied one-dimensional (1D) nanostructured TiO2 coupled with a BiVO4 heterojunction with straight channels for electron transportation that reduced carrier diffusion lengths and improved charge collection efficiencies . However, TiO2 has an intrinsically low mobility that limits the enhancement of photoactivity of the BiVO4–TiO2 heterojunction. Therefore, increasing the carrier concentration and also the conductivity in TiO2 is crucial to constructing a BiVO4–TiO2 heterojunction for a high-performance PEC cell.
In this study, we pre-treated TiO2 nanotubes in the nitrogen gas (TiO2(N2) NTs) and then coupled them with BiVO4 to form a BiVO4/TiO2(N2) NTs heterojunction. We find that the photocurrent is increased by approximately 30 % compared to those obtained by previously reported BiVO4/TiO2 NTs heterojunction . Our PEC experiments further demonstrate the improved performance in the degradation of dyes. These results are attributed to the high carrier concentration of TiO2 NTs after annealing in a non-oxidizing atmosphere, as observed by Mott–Schottky spectra. In this case, the defects presented in the TiO2(N2) NTs increase the charge transfer kinetics, along with the reduced recombination losses due to trap filling. Thus, the charge transport between BiVO4 and TiO2 is enhanced to produce a higher photoactivity. This heterojunction provides useful insight into the design and fabrication of BiVO4-based photoanodes for potentially cost-effective and highly efficient PEC applications in large-scale applications.
2 Experimental Procedures
2.1 Preparation of BiVO4/TiO2(N2) NTs Photoanodes
TiO2 NTs were prepared by a template method in which ZnO nanowires (NWs) were transformed during a liquid-phase deposition (LPD) process. ZnO NWs were synthesized on FTO glass (2 × 2 cm2) after a hydrothermal treatment . Next, a LPD treatment was conducted by placing ZnO NW substrates in a mixed solution of 50 mm (NH4)2TiF6 and 150 mm H3BO3 for 20 min at 25 °C . After the LPD treatment, the sample was further annealed at 500 °C for 2 h in nitrogen gas, and nitrogen-treated TiO2 NTs were obtained and marked as TiO2(N2) NTs. For the fabrication of the BiVO4/TiO2(N2) NTs photoanode, a yellow precursor solutions of 300 mM Bi(NO3)3 and 300 mM NH4VO3 in 2 M HNO3 were deposited on the TiO2 NTs by spin coating . Finally, the samples were sintered at 450 °C for 2 h in room air and yielded a yellow BiVO4/TiO2(N2) NTs film. For the control, the TiO2 NTs annealed in room air were used to prepare the BiVO4/TiO2 NTs photoanodes and bare BiVO4/FTO photoanodes were also prepared using the same procedure without the TiO2 NTs substrate.
2.2 Structural Characterization
The morphologies of the samples were characterized using field emission scanning electron microscopy and a microscope equipped with an energy-dispersive X-ray spectrometer (EDX) (FEI, Sirion200) and TEM (JEM-2100F, JEOL, Japan). The crystalline phase of the samples was characterized by X-ray diffractometry (XRD) (AXS-8 Advance, Bruker, Germany). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB250 XPS measuring system with a Mg Kα X-ray source. Optical absorption measurements were conducted in a Lamda 750 UV–Vis–IR spectrophotometer using an integrating sphere.
2.3 Photoelectrochemical Measurements
The photo responses of the BiVO4/TiO2 NTs photoanode were conducted using a three-electrode system with the Ag/AgCl electrode as the reference, platinum foil as the auxiliary electrode, and the samples as the working electrode. The working electrode potential and current were controlled by an electrochemical workstation (CHI 660c, CH Instruments Inc., TX, USA). A 350-W Xe lamp was used as a simulated light source, without further description, and all experiments were conducted under visible light (light intensity, 100 mW cm−2). The electrolyte was a 0.1 M Na2SO4 solution. The linear sweep voltammograms (LSV) were conducted under chopped light irradiation. The scan rate for the linear sweep voltammetry was 10 mV s−1. Photoluminescence (PL) measurements were conducted using an OmniPL-LF325 system with a 325 nm laser at room temperature. The incident photon-to-charge conversion efficiency (IPCE) was measured by a system comprising a monochromator (Zolix, P.R. China), a 500-W xenon arc lamp, a calibrated silicon photodetector, and a power meter. Mott–Schottky (impedance) spectra were recorded in 0.2 M Na2SO4 without light at a frequency of 1 kHz and a scan rate of 10 mV s−1.
Intensity modulated photocurrent spectroscopy (IMPs) was determined using an electrochemical workstation (ZENNIUM, ZAHNER-elecktrik GmbH & Co. KG, Germany) equipped with a controlled intensity modulated photospectroscopy setup (CIMPS, PP211, ZAHNER-elecktrik GmbH & Co. KG, Germany) after a two-electrode configuration. A white light lamp (WLC02, ZAHNER-elecktrik GmbH & Co. KG, Germany) was used as the light source. The modulated light in the frequency range of 0.1 Hz–1 kHz superimposed on a steady dc light with an intensity of 60 mW cm−2 was also used as a light source.
2.4 Organics Compounds Degradation
The PEC degradation of the methylene blue (MB) experiment was conducted under the following conditions: visible light irradiation (100 mW cm−2), vigorous stirring, 1.0 V (vs. Ag/AgCl) of electric bias, pH 7, and 0.1 M sodium sulfate as the supporting electrolyte. Before degradation test, the nitrogen was bubbled to remove oxygen from the solution. The initial concentration of MB solution was 10 mg L−1 and the reaction solution was 20 mL during the experiment. The degradation rates of the dyes were analyzed with an UV–Vis spectrophotometer (UV2102 PCS, UNICO, Shanghai).
3 Results and Discussion
Incident photon-to-current efficiency was measured in order to ascertain the light conversion efficiency of heterojunction of the BiVO4/TiO2(N2) NTs and was compared to the BiVO4/TiO2 NTs, BiVO4, and TiO2 in Fig. 6b. Due to a large band gap, both the TiO2 NTs and TiO2(N2) NTs had low efficiencies below 400 nm, although the TiO2(N2) NTs exhibited better performances. The IPCE of BiVO4 was comparatively at ~20 % at 410 nm, whereas heterojunction BiVO4/TiO2 NTs had a higher IPCE at nearly 28 % at 410 nm. Comparably, the IPCE of BiVO4/TiO2(N2) NTs further increased to 44 % at 410 nm, which was more than 100 % higher than the IPCE of bare BiVO4. This again suggests that the rectifying electron transfer from BiVO4 to TiO2 likely inhibits the fast recombination and increases the solar energy conversion efficiency of the junction. The IPCE was nearly zero at 550 nm, which is consistent with the optical absorption of the samples.
To further confirm enhanced charge transfers between BiVO4 and the TiO2(N2) NTs in the heterojunction material, the transit time (τ d) of the majority carriers in the BiVO4/TiO2 NTs electrode and the BiVO4/TiO2(N2) NTs electrode was measured by IMPS, respectively. The transit time τ d was the average time that the photogenerated charges took to transfer to the back contact, and were estimated from the equation τ d = (2π·f min (IMPS))−1, where f min is the frequency at the minimal value in the IMPS plot. The transit time reflects the recombination probability of the photogenerated electrons and holes in the photoelectrode . Figure 8b shows the IMPS plots of the BiVO4/TiO2 NTs electrode and the BiVO4/TiO2(N2) NTs electrode, respectively. According to the previous equation, the transit time τ d for the BiVO4/TiO2 NTs was 11.9, and 3.82 ms for BiVO4/TiO2(N2) NTs electrode, which indicated that the transport speed of the majority of photogenerated charges in the BiVO4/TiO2(N2) NTs electrode was three times faster than that of the BiVO4/TiO2 electrode. In other words, the BiVO4/TiO2(N2) NTs heterojunction could facilitate the majority of the photogenerated charges transported to the counter electrode and likewise, the transport of photogenerated electrons to the electrolyte is enhanced.
The transportation of electrons between the two materials was also certified by PL measurement as shown in Fig. 8c. We observed strong emission from bare TiO2 NTs and BiVO4, whereas the BiVO4/TiO2 heterojunction resulted in a near 90 % reduction in the emission intensity. The obvious quenching of luminescence of BiVO4 is characteristic of charge transfer between the BiVO4 and TiO2 NTs, implying a strong indication of the efficient reduction in recombination of charge carriers in the 1D heterojunction material. In consequence, the separation efficiency of photogenerated electron–hole pairs in BiVO4/TiO2(N2) NTs heterojunction could be improved.
A visible light response BiVO4/TiO2(N2) NTs photoelectrode was fabricated for photoelectrochemical (PEC) organic degradation. Mott–Schottky plots and IMPS demonstrated the increased carrier concentration in the TiO2(N2) NTs, which enhanced electron transfers between BiVO4 and TiO2. A photoelectrochemical measurement confirmed that the photocurrent was increased approximately 100 % using the heterojunction when compared to bare BiVO4 under 100 mW cm−2 visible light illumination. Due to its excellent photoactivity and stability, the BiVO4/TiO2(N2) NTs show a promising future in PEC applications.
The authors would like to acknowledge the National Nature Science Foundation of China (21507085, 21576162) and Shanghai Sailing Program of China (14YF1401500) for financial support.
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