Materials Physics
Nanocrystalline TiO2 Powders Prepared by Xerogel Hydrothermal in Water: Characterization Photocatalytic Oxidation of Acetone
2020 Volume 61 Issue 9 Pages 1727-1730
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2020 Volume 61 Issue 9 Pages 1727-1730
Cu/N-codoped TiO2 was able to degrade solution under light irradiation with significantly. However, the photocatalytic activity of acetone gas-phase seldomly reported. In this research, we have developed novel method to photooxidation of acetone. The photocatalytic efficiency of different Cu/N-codoped TiO2 powders, prepared by a novel modified hydrothermal method at different temperatures, has been evaluated through the Cu/N-codoped TiO2 photo sensitized degradation of acetone. The as prepared samples were studied for their physicochemical property using XRD, SEM, UV-vis and N2 sorption analysis. Results indicated that when the temperature below 220°C, Cu/N-codoped TiO2 facilitated formation and stability of anatase and brookite phase. The sample prepared in 150°C (Ti–H2O-150) exhibited highest surface area and photo-absorption in the visible light region. The sample prepared in 150°C (Ti–H2O-150) shows highest photocatalytic activity (80.4%) with a rate constant k = 1.62 × 10−2. Cu/N-codoped TiO2 can be considered as a promising visible light-sensitized photocatalysts for removing acetone.
Fig. 3 (a) Nitrogen adsorption/desorption isotherms; (b) pore size distributions of as-prepared Cu0.6N4/TiO2 in different temperatures.
Acetone (CH3COCH3) is one of a commonly used volatile organic compound (VOC) in industries. It cause environmental pollution and damages human health. TiO2 is a well known photocatalyst for removing VOCs under UV light.1–3) However, pure TiO2 is limited due to its low quantum yield, which stems from the fast recombination of photon-induced electron–hole pairs. Metal-non-metal codopants may lead to higher photoactivity because of its ability to overcome this issue and to enhance the visible light absorption. Moreover, as a abundant metal, copper have tremendous potential and Cu2+ ions could reduce charge recombination. N could narrow the band gap and enhance the visible light absorption.4,5) Theoretical simulation work reported that Cu/N-codoped TiO2 was able to degrade solution under light irradiation significantly.6,7) However, none of the previous studies was utilized to remove the photodegradation acetone gas-phase, which is our novelty of this work.
Sol-gel and hydrothermal methods are popular to prepare the nanostructured Cu/N-codoped TiO2 that had controlled size, morphology and crystal structure.8,9) However, the process of sol-gel method produces particle agglomeration, and the growing grain can induce phase transformation. Hydrothermal method provides a simple and easy way to get controlled nanoparticles. Therefore, this study aimed to (i) synthesize Cu/N-codoped TiO2 nanoparticle by novel xerogel-hydrothermal method. (ii) the effects of hydrothermal temperature on the physicochemical property (iii) investigate the Cu/N-codoped TiO2 efficiency to remove acetone gas-phase.
Typically, 8.5 mL tetrabutyl titanate and 35 mL ethanol were mixed together to form solution A. Meanwhile, 15 mL CH3CH2OH, 1.5 mL H2O, 0.028 g copper nitrate (Cu(NO3)2·6H2O) and 0.648 g urea to form solution B. After solution A and solution B were stirred for 0.5 h, respectively, the solution B was slowly added dropwise to solution A and stirred for 1 h. This solution was stored in the dark overnight for nucleation, followed by gelation at 70°C for 12 h. Eventually, the gel was dried and annealed at 110°C. Then the xerogel was then added into the 30.0 mL distilled water, and the final solution was stirred for 0.5 h before being introduced into a 50 mL Teflon stainless steel autoclave. The autoclave was maintained at 120°C for 12 h. The product washed by water and ethanol several times and dried at 80°C in oven. The samples by hydrothermal in different temperatures were assigned as Ti–H2O-60, Ti–H2O-120, Ti–H2O-150, Ti–H2O-180, Ti–H2O-220, respectively.
2.2 Photocatalyst characterization 2.2.1 Structural and textural characterizationThe crystalline product was studied by powder XRD with a graphite-filtered Cu Kα (k = 0.154 nm) radiation. The crystalline phase of particles was determined using X-ray diffraction (Cu Kα radiation). Scherer’s equation was used to calculate the particle size of Cu/N-codoped TiO2. The structure of materials was determined with the help of scanning electron microscopy (SEM) image collected on (S-4800). The specific surface area (SBET) was determined using NOVA-2000E according to the BET method. Barrette Joynere Halenda approach was used to calculate pore size distribution of the samples using the desorption data. The optical absorption properties of photocatalysts were studied with diffused reflectance spectrophotometer (UV-3600) using BaSO4 as a standard for reflectance.
2.2.2 Photocatalytic activityThe photocatalytic activity of the samples was studied through acetone oxidation degradation experiments under visible light irradiation. The experiments involved dispersing 0.1 g of the nanophotocatalyst in 300 mL of cylindrical glass vessel, then sealed the reactor. The acetone gas was then injected the cylindrical photoreactor. Subsequently, the acetone gas was irradiated with light supplied by 300 W Xe lamp emitting at λ = 450 nm. The concentration of acetone was evaluated using gas chromatography (GC-2014, Shimadzu, Tokyo) regularly.
Figure 1 shows the XRD spectra of sample hydrothermal in distilled water for 12 h. It is obviously observed from Fig. 1 that all the diffraction peaks were indexed to anatase and brookite phase TiO2 structure. The existence of anatase in the mixed phase catalyst is significant from the planes of (101) at 2θ values of ca. 25.3°. The presence of brookite is from its (121) diffraction peak at 2θ values of ca. 31° (JCPDS No. 21–1272). After hydrothermal treatments, Ti–H2O-60, Ti–H2O-120, Ti–H2O-150, and Ti–H2O-180 showed the mixed anatase/brookite phase. Usually, junction effect in bicrystalline with different band gap energy (anatase: 3.2 eV, brookite: 3.25–3.29 eV) is maybe believed to the leading of a remarkable photocatalytic performance.10) However, when the hydrothermal temperature increased to 220°C, only anatase was present. As the hydrothermally temperature increased, the peak intensity of anatase phase was slightly increased, indicating a crystallinity improvement. The average crystal size was calculated from Scherrer equation and the results revealed that the crystal size of the anatase and brookite phases increased from 4.9 nm to 14.6 nm and 7.3 nm to 9.1 nm (from Table 1). From Table 1, the crystal size of the anatase phases increased from 4.9 nm to 7.25 nm from 60°C to 180°C, Cu and N elament might to retard the growth of TiO2 lattice, which agreed with the results reported by Rajoriya et al.11)
XRD of the as-prepared Cu0.6N4/TiO2 in different temperatures: (a) xerogel; (b) Ti–H2O-60; (c) Ti–H2O-120; (d) Ti–H2O-150; (e) Ti–H2O-180; (f) Ti–H2O-220.
Figure 2(a), (b), (c) and (d) show the SEM images of Ti–H2O-60, Ti–H2O-120, Ti–H2O-150 and Ti–H2O-220 samples, which were used to investigate its particle size, crystallinity, and morphology. As can be seen in the morphologies of Ti–H2O-60, Ti–H2O-120 nanoparticles (Fig. 2(a) and (b)), the as-prepared samples show particle with great aggregation. The microstructure of the sample Ti–H2O-150 and Ti–H2O-220 show reduction in the agglomeration (Fig. 2(c) and (d)). The change of agglomeration is probably caused restructuring of Cu/N-codoped TiO2 from amorphous to anatase and brookite phase from 60°C to 220°C.12) With increasing hydrothermal temperature, the particles sizes and the crystallinity of the synthesized nanoparticles is increased (from Fig. 1 and Table 1), which was beneficial in the improvement of the photocatalytic performance.
SEM images of Cu0.6N4/TiO2 in different temperatures: (a) Ti–H2O-60; (b) Ti–H2O-120; (c) Ti–H2O-150; (d) Ti–H2O-220.
N2 adsorption–desorption isotherms of Ti–H2O-120, Ti–H2O-150, Ti–H2O-180 and Ti–H2O-220 of type IV with a H1 hysteresis loop (as defined by IUPAC) typical of ordered mesoporous materials in Fig. 3(a),13) indicating ink-bottle-shaped pores. As the temperature increases from 60°C to 150°C, the specific surface area and pore volume increase (from Table 2), which is attribute to the increase in the crystallinity improvement. From Table 2, with increasing the hydrothermal temperature from 150°C to 180°C, Cu/N-codoped TiO2 powders show an increase in average pore size and a decrease in the specific surface areas, pore volumes, which is attribute to the increase in the crystallite sizes particles.14) Ti–H2O-150 possesses largest surface area (260.7 m2/g) and largest pore volume (0.303 cm3/g) which can be concluded that hydrothermal of titania xerogel in 150°C with higher mesoporosity than other temperatures. Usually, a larger specific surface area, smaller crystallite size maybe influence the photocatalytic activities.15)
(a) Nitrogen adsorption/desorption isotherms; (b) pore size distributions of as-prepared Cu0.6N4/TiO2 in different temperatures.
A sharp pore size distribution at a mean value of 10–20 nm was calculated from the adsorption branch on the basis of BJH model for Ti–H2O-120, Ti–H2O-150 and Ti–H2O-180 in Fig. 3(b). However, a broad pore-size distribution was achieved for sample (Ti–H2O-220), which could be because of a higher vapour pressure of water inside the pores during the hydrothermal process.16,17) The average pore diameters are concretely noted for each sample in Table 2.
The diffuse reflectance spectra of samples with different hydrothermal temperatures were depicted in Fig. 4. For the Ti–H2O-60 sample, the absorption edge is only slightly shifted as compared to other samples. The band gap (Eg) can be estimated by the Kubelka-Munk function. The Eg values are calculated to be 2.65 eV, 2.92 eV, 3.0 eV, 2.9 eV and 2.95 eV for Ti–H2O-60, Ti–H2O-120, Ti–H2O-150, Ti–H2O-180 and Ti–H2O-220. Considering the main reported band gap of TiO2 (3.2 eV), the calculated values in all of the products show a significant band gap narrowing due to co-doped Cu and N element. It has been shown that increasing in the crystalline size of the nanoparticles will result in a band gap narrowing.18) In according to crystalline size of Table 1, the differences in band gap values between Ti–H2O-60 and other samples are attributed to the crystalline size of the products. Among all of Cu/N-codoped TiO2 extend the absorption ranges to the visible light and might enhance the photocatalytic activity.
(a) UV-Vis diffuse reflection spectra of Ti–H2O-60, Ti–H2O-120, Ti–H2O-150, Ti–H2O-180 and Ti–H2O-220; (b) (ahv)1/2 vs. photon energy.
The photocatalytic activity of the resulting products with different hydrothermal temperatures by analyzing the photocatalytic of acetone under visible light for 90 min in Fig. 5. As displayed in Fig. 5(a), the degradation ratios of acetone over Ti–H2O-60, Ti–H2O-120, Ti–H2O-150, Ti–H2O-180 and Ti–H2O-220 were 71.1%, 76.4%, 80.4%, 61.1% and 54.8%, respectively. The Cu/N-codoped TiO2 sample prepared by 150°C (Ti–H2O-150) shows a better photocatalytic activity with a rate constant k = 1.62 × 10−2, exceeding that of Ti–H2O-60 and Ti–H2O-120 by a factor of 1.21 and 1.33. With increasing hydrothermal temperature, the rate constant decrease to 8.79 × 10−3. As expected, Ti–H2O-150 was the best for acetone photooxidation about 80.4% because of higher crystallinity (from XRD result in Fig. 1) and highest surface area (from BET result in Fig. 3 and Table 2). Although a higher hydrothermal temperature results in greater crystallinity, which should improve photocatalytic performance. It was slightly decreased when further increasing the temperature from 180°C to 220°C because of a decrease in surface area. The special surface areas, particle size and the bicrystalline must be considered in comparison the photocatalytic activity among these samples.19) Compare with TiO2 nanopaiticle under ultraviolet light reported by Wang (rate constant k = 5.99 × 10−3),20) the Cu/N-codoped TiO2 display highly photocatalytic performance (rate constant k = 1.62 × 10−2) on photooxidation of acetone under visible light.
(a) Photocatalytic oxidation of acetone over Cu0.6N4/TiO2 synthesized with different temperatures under visible light. (b) ln(C0/C) as a function of visible irradiation time in the presence of the Cu0.6N4/TiO2 synthesized with different temperatures.
In summary, Cu/N-codoped TiO2 was prepared by the novel xerogel-hydrothermal method, with varied hydrothermal temperatures. The structural, morphological and mesoporosity properties of Cu/N-codoped TiO2 were investigated by techniques were used such as XRD, SEM, BET etc. The prepared Cu/N-codoped TiO2 samples showed efficiency for photocatalytic oxidation of acetone. The sample as-prepared in 150°C exhibited highest photocatalytic activities of acetone than that of the sample prepared in other temperatures under visible light. The enhanced photocatalytic activity is because: (i) largest specific surface area (260.7 m2/g), (ii) mixed crystal phases of anatase and brookite phase; (iii) good optical properties due to Cu/N codoped. This study may become the promising photodegradation of acetone for industrial applications.
This work was supported by the National Key Research and Development Program of China (No. 2017YFB0602901) and Tianjin Science and Technology Commissioner project (No. 18JCTPJC61000).
