Comparison of the Photocatalytic Efficience for Cu and N Co-Doped TiO2 by Sol-Gel and Xerogel-Hydrothermal Methods
2018 Volume 59 Issue 5 Pages 811-815
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2018 Volume 59 Issue 5 Pages 811-815
In this study, mesoporous Cu and N doped TiO2 crystals were synthesized with Cu and N doped titania by xerogel-hydrothermal treatment and sol-gel treatment. The characteristics of Cu and N doped TiO2 were investigated with XRD, UV-vis, XPS and N2 sorption analysis. The photocatalytic activity of samples was evaluated by the photocatalytic oxidation of acetone under ultraviolet light. The photocatalytic activity of the xerogel-hydrothermal modified Cu and N doped TiO2 was considerably higher than that of the sol-gel modified Cu and N doped TiO2. This enhanced photoactivity is related to the smaller particle sizes, bicrystalline, the smaller pores and the larger specific area.
Fig. 2 TEM images of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
TiO2 photocatalytic material was a novel and intensively studied class of environmental friendly materials. TiO2 possess interesting optical, dielectric and catalytic properties, have been investigated for wide applications, including electroluminescent hybrid devices, sensing, solar energy conversion, solar cells and photocatalysis.1–5) Metal doping and non-metal doping investigated as an effective method to improve the photocatalytic activity of TiO2.6,7) Recently, the photocatalytic activity of some Cu–N co-doped TiO2 was considerably greater than that of the sample N doped TiO2 and commercially available TiO2 which is attributed to the synergistic effect of Cu–N co-doped.8,9)
Compare to the other methods, sol-gel treatment has many advantangeous to obtain nanosized Cu–N co-doped TiO2 particles. These include easy control of the doping level and simple equipment. However, the precipitates obtained by sol-gel are amorphous and require a subsequent thermal treatment that leads to crystallization. The calcination process often produces particle agglomeration, and the growing grain can induce phase transformation. The hydrothermal method is an alternative to calcination for promoting crystallization under mild temperatures. In the hydrothermal treatment, phase, size, particle morphology and crystallinity can be easily controlled by modifying some of the factors which comprise the hydrothermal conditions.10–12) The high value of larger surface area and smaller particle size were more stable obtained by hydrothermal method than those obtained by sol-gel methods.13–15)
In the present work, Cu–N co-doped TiO2 particles prepared by the sol-gel and xerogel-hydrothermal methods. Specifically, Cu0.6N4TiO2 have been synthesized, characterized and used for determining the applicability of this material to the photocatalytic oxidation acetone under ultraviolet light. The doping amount of N and Cu were 4 mass% and 0.6 mass% respectively for Cu0.6N4TiO2.
In the sol-gel method, at room temperature, a mixture solution of 1.0 mL nitric acid, 15 mL CH3CH2OH, 1.5 mL H2O, 0.028 g copper nitrate and 0.648 g urea, was dropped into the solution of 8.5 mL tetrabutyl titanate and 35 mL ethanol under magnetic. The doping amount of N and Cu were 4 mass% and 0.6 mass% respectively. After stirred for 60 min, a gel was formed, then colloid was first dried at 80°C to form xerogel and then the xerogel calcined in a muffle at 500°C for 2 h. Then the N and Cu co-doped TiO2 samples can be obtained, name as Cu0.6N4Ti-sol-gel.
2.1.2 Xerogel-hydrothermal procedureIn the xerogel-hydrothermal methods, firstly, the xerogel prepared by above mentioned sol-gel process. The xerogel was then added into the 30.0 mL distilled water, and the final suspending liquid was stirred for 0.5 h before being introduced into a 50 mL Teflon stainless steel autoclave. The autoclave was maintained at 150°C for 12 h. The product washed by water and ethanol several times and dried at 80°C in oven. The Cu and N co-doped TiO2 samples by xerogel-hydrothermal was obtained, name as Cu0.6N4Ti-xerogel-hydrothermal.
2.2 Photocatalyst characterization 2.2.1 Structural and textural characterizationMorphologies of samples were characterized using a high resolution transmission electron microscope (JEM-2100F). The specific surface area was determined using NOVA-2000E by N2 adsorption. Barrette Joynere Halenda (BJH) approach was used to calculate pore size distribution of the samples using the desorption data. UV-vis diffuse reflectance spectra (DRS) were measured on UV-3600 UV-vis apparatus.
The crystalline phase of particles were determined by X-ray diffraction (Cu Kα radiation). The crystallite sizes of the composite catalysts were calculated using the Scherrer equation. For the samples, the weight fraction of every component can be calculated from (eq. (1) and eq. (2)).
| \begin{equation} W_{A} = \frac{k_{A}A_{A}}{k_{A}A_{A} + k_{A}A_{B}} \end{equation} | (1) |
| \begin{equation} W_{B} = \frac{k_{A}A_{B}}{k_{A}A_{A} + k_{A}A_{B}} \end{equation} | (2) |
Where WA and WB separately represent the weight fraction of anatase and brookite, respectively, coefficients kA and kB are 0.886 and 2.721, respectively, and IA and IB denote the integrated intensity of anatase (101) peak and brookite (121) peak, respectively. Where AA and AB represent the integrated intensity of the anatase (101) peak (2θ = 25.28°) and the brookite (121) peak (2θ = 30.81°), respectively.16)
2.2.2 Photocatalytic activityPhotocatalytic oxidation of acetone was performed in a 300 mL cylindrical glass vessel with a 300 W Xe lamp as light source. The sample powder were placed at the bottom center of the reactor, the gas valves on both sides of reactor were closed to seal the reactor. The acetone gas was then injected the cylindrical photoreactor. Then the reactor was kept in dark for 30 min to reach the adsorption equilibrium. After that, the ultraviolet light source was turned on and the inside temperature of the reactor was kept with circulation water. The acetone concentration was evaluated using gas chromatography (GC-2014, Shimadzu, Tokyo) regularly.
The two types of Cu0.6N4TiO2 powders synthesized were examined by X-ray diffraction, as shown in Fig. 1. The XRD pattern of Cu0.6N4Ti-xerogel-hydrothermal sample revealed the anatase (101) phase and emerging peak of brookite (121), indicating the prepared material was a mixture of anatase and brookite (95.7% anatase and 4.3% brookite). In the sol-gel method, anatase is the only phase of the Cu0.6N4Ti-sol-gel sample. The peak intensity of the xerogel-hydrothermal-modified Cu0.6N4TiO2 sample was slightly smaller and broader than sol-gel-modified Cu0.6N4TiO2 sample. The calculated crystalline domain sizes were 9.6 and 5.7 nm (for anatase) for the sol-gel and xerogel-hydrothermal modified Cu0.6N4TiO2 samples, respectively. The particle size of sample sol-gel is bigger than that of xerogel-hydrothermal because calcination process often produces grain growing.
XRD of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
The morphologies of the particles measured by transmission electron microscopy are given in Fig. 2(a) and (b). A relatively narrow size distribution of monodispersed nano-particles with spheroid morphology was revealed by the TEM analysis. Mean particle diameters of 9–11 nm and 4–7 nm were observed for the sol-gel and xerogel-hydrothermal modified for Cu0.6N4TiO2 nanoparicles. The particles of the xerogel-hydrothermal-modified Cu0.6N4TiO2 were slightly smaller than those of the sol-gel modified Cu0.6N4TiO2 sample. This is in agreement with the XRD patterns.
TEM images of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
The mesostructure characteristics of the as-prepared catalysts were also studied by nitrogen sorption measurements. The isotherms of materials of Cu0.6N4Ti-sol-gel and Cu0.6N4Ti-xerogel-hydrothermal in Fig. 3(A) exhibit type-IV behavior, which is characteristic of mesoporous materials based on the IUPAC classification.17) The type-IV isotherm of sol-gel and xerogel-hydrothermal modified Cu0.6N4TiO2 in Fig. 3(A) along with a H2 hysteresis loop, indicating ink-bottle-shaped pores. Table 1 also shows quantitative details on BET surface area, pore volume and average pore size of sol-gel and xerogel-hydrothermal modified Cu0.6N4/TiO2. It can be seen that xerogel-hydrothermal modified Cu0.6N4/TiO2 exhibited larger BET surface area, smaller pore volume. The bigger pores of Cu0.6N4Ti-sol-gel is due to bigger crystallites aggregation by sol-gel treatment. The smaller pores and the larger specific surface area of the Cu0.6N4Ti-xerogel-hydrothermal sample are favorable to the photocatalytic degradation.18) The smaller pores and the larger specific surface area of Cu0.6N4Ti-xerogel-hydrothermal sample are due to hydrothermal improving particle dispersibility. The pore size distribution curves (Fig. 3(B)) show significant differences exist in pore structures. There was only one narrow pore size distribution could be observed in Cu0.6N4Ti-xerogel-hydrothermal, whereas double pore size distribution curves found in Cu0.6N4Ti-sol-gel, one narrow peak in the range of 4–6 nm and one broad distribution in the range of 6–10 nm. The hierarchical structure is favorable for dye molecules to access the pores.19) Cu0.6N4Ti-xerogel-hydrothermal sample exhibits narrow pore size distribution behavior, because of its smaller average pore size.
(A) Nitrogen adsorption/desorption isotherms of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal and (B) pore size distributions of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
Figure 4 shows the UV-vis diffuse reflectance spectra of sol-gel and xerogel-hydrothermal modified Cu0.6N4/TiO2. For the sol-gel modified Cu0.6N4/TiO2 sample, the absorption edge is only slightly red-shifted as compared to xerogel-hydrothermal modified Cu0.6N4/TiO2. Kubelka-Munk function was used to estimate the bandbgap energy of all samples. The calculated results showed that the band gap of sol-gel and xerogel-hydrothermal modified Cu0.6N4/TiO2 are 2.9 eV and 2.98 eV. It is believed that the sol-gel-modification reduced the band gap of the Cu0.6N4/TiO2 semiconductor significantly, which resulted in better photoactivity. However, the xerogel-hydrothermal modified Cu0.6N4/TiO2 have bicrystalline structure. Usually, junction effect (i.e., interfacial charge transfer) in bicrystalline with different band gap energy (anatase: 3.2 eV, brookite: 3.25–3.29 eV) is may be believed to the leading of a remarkable photocatalytic performance.20)
UV-vis diffuse reflection spectra of as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
In order to verify the doping of N and Cu species into nanosized TiO2, XPS measurements were conducted. Figure 5(A) shows the XPS spectra of sol-gel and xerogel-hydrothermal modified Cu0.6N4/TiO2. There are five main peaks for Cu0.6N4Ti-sol-gel and Cu0.6N4Ti-xerogel-hydrothermal. The first two peaks located at ca. 458.26 and 464.5 eV can be assigned as the Ti 2p3/2 and Ti 2p1/2 of TiO2. The second two peaks located at ca. 929 eV and 934.32 eV can be assigned as the Cu 2p3/2 and Cu 2p1/2. Another peak at 529.7 eV is attributable to O1s. The six weak peak at 397.2 eV can be ascribed to N 1s. Obviously, the XPS results proved that the N and Cu species have been doped into TiO2. The high resolution XPS spectra of the binding energies for N1s of Cu0.6N4Ti-sol-gel and Cu0.6N4Ti-xerogel-hydrothermal are presented in Fig. 5(B) and (C). As shown in Fig. 5(B) and Fig. 5(C), the peak at 400.23 eV and 400.41 eV were attributed to molecularly adsorbed N species on the surface of the nitrogen modified titanium dioxide Nanoparticles. As shown in Fig. 5(C), the peak at 396.66 eV was attributed to substitutional N species in the Ti–O–N structure for Cu0.6N4Ti-xerogel-hydrothermal.21) It was likely that the chemisorbed nitrogen did not contribute to catalytic activity. So the xerogel-hydrothermal modified Cu0.6N4/TiO2 have higher photoactivity.
(A) X-ray photoelectron spectra of the as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4 Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal (B) N1s photoelectron spectrum of Cu0.6N4Ti-sol-gel (C) N1s photoelectron spectrum of Cu0.6N4Ti-xerogel-hydrothermal.
Acetone is a common chemical used in industrial. Photocatalytic oxidation of acetone was employed as a probe reaction to investigate the activities of as-synthesized Cu0.6N4/TiO2 under the irradiation of ultraviolet light. As shown in Fig. 6, xerogel-hydrothermal modified Cu0.6N4/TiO2 exhibits excellent photocatalytic activity, and can degrade 90% acetone within 50 min, whereas 85% of acetone is degraded with sol-gel-modified Cu0.6N4/TiO2 within 70 min. The band gap narrowing and double pore size distribution of sol-gel-modified Cu0.6N4/TiO2 facilitate photocatalytic performance. However, Cu0.6N4Ti-xerogel-hydrothermal exhibited relatively higher photocatalytic performance than Cu0.6N4Ti-sol-gel due to smaller particle sizes, bicrystalline structure, the smaller pores and the larger specific surface.
Photocatalytic oxidation of acetone under ultraviolet light for as-prepared Cu0.6N4/TiO2 by different methods: (a) Cu0.6N4Ti-sol-gel (b) Cu0.6N4Ti-xerogel-hydrothermal.
In summary, Cu and N co-doped TiO2 were successfully prepared by sol-gel and xerogel-hydrothermal methods. Specifically, the Cu0.6N4/TiO2 particle sizes observed in the transmission electron microscopy images were smaller in the xerogel-hydrothermal modified than in the sol-gel modified. The absorption band of the Cu0.6N4/TiO2 by sol-gel treatment was slightly stronger and more red-shifted than the xerogel-hydrothermal modified Cu0.6N4/TiO2. The xerogel-hydrothermal modified Cu0.6N4/TiO2 showed better photocatalytic activity than that of sol-gel modified Cu0.6N4/TiO2 in terms of oxidation of acetone. From the XRD, UV-vis, TEM and BET result, the enhanced activity should be attributed to the smaller particle sizes, bicrystalline structure, the smaller pores and the larger specific area.
This work was supported by *National Natural Science Foundation of China* (grant 21406164, 21466035) and* Tianjin Research Program of Application Foundation and Advanced Technology* (15JCQNJC09100).
