Self-organized vanadium and nitrogen co-doped titania nanotube arrays with enhanced photocatalytic reduction of CO2 into CH4
© Lu et al.; licensee Springer. 2014
Received: 26 March 2014
Accepted: 21 May 2014
Published: 29 May 2014
Self-organized V-N co-doped TiO2 nanotube arrays (TNAs) with various doping amount were synthesized by anodizing in association with hydrothermal treatment. Impacts of V-N co-doping on the morphologies, phase structures, and photoelectrochemical properties of the TNAs films were thoroughly investigated. The co-doped TiO2 photocatalysts show remarkably enhanced photocatalytic activity for the CO2 photoreduction to methane under ultraviolet illumination. The mechanism of the enhanced photocatalytic activity is discussed in detail.
Greenhouse gases such as CO2 and chlorofluorocarbon (CFCs) are the primary causes of global warming. The atmospheric concentration of CO2 has steadily increased owing to human activity, and this accelerates the greenhouse effect. The photocatalytic reduction of CO2 is a promising technical solution since it uses readily available sunlight to convert CO2 into valuable chemicals, such as methanol or methane, in a carbon friendly manner.
TiO2 is a popular catalyst for photoreduction of CO2 owing to the advantages of earth abundance, low toxicity, and chemical stability. Yet it has so far yielded only low carbon dioxide conversion rates despite using ultraviolet illumination for band gap excitations. While the intrinsic idea of photocatalytic conversion of carbon dioxide and water (vapor) into hydrocarbon fuels is appealing, the process has historically suffered from low conversion rates. Numerous studies have been reported on how to increase the photoreduction activity of TiO2 using transition metal-doped and/or modified TiO2. Transition metal doping has been applied not only to modify the photoactivity of TiO2 but also to influence the product selectivity. For example, mesoporous silica-supported Cu/TiO2 nanocomposites showed significantly enhanced CO2 photoreduction rates due to the synergistic combination of Cu deposition and high surface area SiO2 support. Dispersing Ce-TiO2 nanoparticles on mesoporous SBA-15 support was reported to further enhance both CO and CH4 production due to the modification of TiO2 with Ce significantly stabilized the TiO2 anatase phase and increased the specific surface area. However, increasing the content of metal dopant does not always lead to better photocatalytic activity. The promotion of the recombination efficiency of the electron-hole pairs may be due to excessively doped transition metal.
Besides, nonmetal-doped TiO2 have been used as visible light-responsive photocatalysts for CO2 photoreduction. Significant enhancement of CO2 photoreduction to CO had been reported for I-doped TiO2 due to the extension of TiO2 absorption spectra to the visible light region by I doping. Enhanced visible light-responsive activity for CO2 photoreduction was obtained over mesoporous N-doped TiO2 with noble metal loading. Nitrogen doping into TiO2 matrix is more beneficial from the viewpoint of its comparable atomic size with oxygen, small ionization energy, metastable center formation and stability. However, a main drawback of N doping is that only relatively low concentrations of N dopants can be implanted in TiO2.
In order to overcome the abovementioned limitations, modified TiO2 by means of nonmetal and metal co-doping was investigated as an effective method to improve the photocatalytic activity. Among the current research of single ion doping into anatase TiO2, N-doping and V-doping are noteworthy. Firstly, both elements are close neighbors of the elements they replace in the periodic table. They also share certain similar physical and chemical characteristics with the replaced elements. Secondly, impurity states of N dopants act as shallow acceptor levels, while those of V dopants act as shallow donor levels. This result in less recombination centers in the forbidden band of TiO2 and thus prolongs the lifetime of photoexcited carriers. So the co-doping of V and N into the TiO2 lattice is of particular significance. Recently, V and N co-doped TiO2 nanocatalysts showed enhanced photocatalytic activities for the degradation of methylene blue compared with mono-doped TiO2. Wang et al. synthesized V-N co-doped TiO2 nanocatalysts using a novel two-phase hydrothermal method applied in hazardous PCP-Na decomposition. Theoretical and simulation work also found that N-V co-doping could broaden the absorption spectrum of anatase TiO2 to the visible light region and increase its quantum efficiency. However, the effect of V, N co-dopant in TiO2 on the efficiency of CO2 photocatalytic reduction has not been studied yet. In the present work, we made efforts to improve photocatalytic carbon dioxide conversion rates by the following strategies: (1) employ high surface area titania nanotube arrays, with vectorial charge transfer, and long-term stability to photo and chemical corrosion; and (2) modify the titania to enhance the separation of electron-hole pairs by incorporating nitrogen and vanadium. This article reports the synthesis, morphologies, phase structures, and photoelectrochemical of self-organized V, N co-doped TiO2 nanotube arrays as well as the effect of V and N co-doping on photocatalytic reduction performance of CO2 into CH4.
Fabrication of V, N co-doped TiO2 nanotube arrays
V, N co-doped TiO2 nanotube arrays (TNAs) were fabricated by a combination of electrochemical anodization and hydrothermal reaction. Firstly, highly ordered TNAs were fabricated on a Ti substrate in a mixed electrolyte solution of ethylene glycol containing NH4F and deionized water by a two-step electrochemical anodic oxidation process according to our previous reports. Interstitial nitrogen species were formed in the TNAs due to the electrolyte containing NH4F. Then, the amorphous TNAs were annealed at 500°C for 3 h with a heating rate of 10°C/min in air ambience to obtain crystalline phase. We denoted these single N-doped TNAs samples as N-TiO2.
V, N co-doped TNAs were prepared by a hydrothermal process. As-prepared N-TiO2 samples were immersed in Teflon-lined autoclaves (120 mL, Parr Instrument, Moline, IL, USA) containing approximately 60 mL of NH4VO3 aqueous solution (with different concentration 0.5, 1, 3, and 5 wt.%) as the source of both V and N. All samples were hydrothermally treated at 180°C for 5 h and then naturally cooled down to room temperature. Finally, all samples were rinsed with deionized water and dried under high purityN2 stream. The corresponding samples (0.5%, 1%, 3%, and 5%) were labeled as VN0.5, VN1, VN3, and VN5. For control experiment, sample denoted as VN0 was prepared by the previously mentioned hydrothermal process in 60 mL pure water without NH4VO3 addition.
Surface morphologies of all samples were observed with field emission scanning electron microscope (FESEM, JEOL JSM-7001 F, Akishima-shi, Japan) at an accelerating voltage of 15 kV. Phase structures of the photocatalysts were analyzed by X-ray diffraction (XRD) analysis on an X'Pert Philips (Amsterdam, The Netherlands) diffractometer (Cu Kα radiation, 2θ range, 10° to 90°; step size, 0.08°). Chemical state and surface composition of the samples were obtained with an Axis Ultra X-ray photoelectron spectroscope (XPS, Kratos, Manchester, UK; a monochromatic Al source operating at 210 W with a pass energy of 20 eV and a step of 0.1 eV was used). All binding energies (BE) were referenced to the C 1 s peak at 284.8 eV of the surface adventitious carbon. UV-vis diffused reflectance spectra of N-TiO2 and V, N-co-doped TiO2 nanotube arrays were obtained using a UV-vis spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan).
Photoelectrochemical experiments were monitored by an electrochemical workstation (IM6ex, Zahner, Germany). V, N co-doped TNAs (an active area of 4 cm2) and platinum foil electrode were used as working electrode and counter electrode, and saturated calomel electrode (SCE) acted as reference electrode, respectively. 1 M KOH aqueous solution was used as the supporting electrolyte and purged with N2 for 20 min before measurement to remove the dissolved oxygen. A 300-W Hg lamp was used as the light source. Photocurrent measurements were carried out under UV-vis irradiation at an applied bias voltage of 0.4 V (vs. SCE) in ambient conditions at room temperature.
Photocatalytic reduction of CO2
Photocatalytic reduction of CO2 was performed in a 358-mL cylindrical glass vessel containing 20 mL 0.1 mol/L KHCO3 solution with a 300-W Hg lamp fixed parallel to the glass reactor as light source. TNAs films were placed in the center of the reactor before sealing the reactor. Prior to reduction experiment under irradiation, ultra-pure gaseous CO2 and water vapor were flowed through the reactor for 2 h to reach adsorption equilibrium within the reactor. Each experiment was followed for 6 h. The analysis of CH4 was online conducted with a gas chromatography (GC).
Results and discussion
Figure 3c shows the high-resolution XPS spectra and corresponding fitted XPS for the N 1 s region of N-TiO2, VN0, and VN3. A broad peak extending from 397 to 403 eV is observed for all samples. The center of the N 1 s peak locates at ca. 399.7, 399.6, and 399.4 eV for N-TiO2, VN0, and VN3 samples, respectively. These three peaks are higher than that of typical binding energy of N 1 s (396.9 eV) in TiN, indicating that the N atoms in all samples interact strongly with O atoms. The binding energies of 399.7, 399.6, and 399.4 eV here are attributed to the oxidized nitrogen similar to NO x species, which means Ti-N-O linkage possibly formed on the surface of N-TiO2, VN0, and VN3 samples[21–23]. The concentrations of V and N in VN3 derived from XPS analysis were 3.38% and 4.21% (at.%), respectively. The molar ratios of N/Ti on the surface of N-TiO2 and VN3 were 2.89% and 14.04%, respectively, indicating obvious increase of N doping content by hydrothermal treatment with ammonium metavanadate. As shown in Figure 3d, the peaks appearing at 516.3, 516.9, 523.8, and 524.4 eV could be attributed to 2p3/2 of V4+, 2p3/2 of V5+, 2p1/2 of V4+, and 2p1/2 of V5+[24, 25]. It was established that the V4+ and V5+ions were successfully incorporated into the crystal lattice of anatase TiO2 and substituted for Ti4+ ions.
UV-vis DRS spectra analysis
Photocatalytic reduction performance
Photocatalytic reduction mechanism
Superabundant V and N could result in a decrease of photoreduction activity for increasing recombination centers of electrons and holes.
V-N co-doped TiO2 nanotube arrays have been fabricated by a simple two-step method. V and N co-doped TiO2 photocatalysts exhibit fine tubular structures after hydrothermal co-doping process. XPS data reveal that N is found in the forms of Ti-N-O and V incorporates into the TiO2 lattice in V-N co-doped TNAs. V and N co-doping result in remarkably enhanced activity for CO2 photoreduction to CH4 due to the effective separation of electron-hole pairs. Meanwhile, the unique structure of co-doped TiO2 nanotube arrays promoted the electron transfer and the substance diffusion.
The authors thank the National Natural Science Foundation of China (no.21203054) and Program for Changjiang Scholars and Innovation Research Team in University (no. PCS IRT1126).
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