Preparations of TiO 2 film coated on foam nickel substrate by sol-gel processes and its photocatalytic activity for degradation of acetaldehyde

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1 Journal of Environmental Sciences 19(2007) Preparations of TiO 2 film coated on foam nickel substrate by sol-gel processes and its photocatalytic activity for degradation of acetaldehyde HU Hai, XIAO Wen-jun, YUAN Jian, SHI Jian-wei, CHEN Ming-xia, SHANG GUAN Wen-feng Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai , China. shangguan@sjtu.edu.cn Received 16 January 2006; revised 27 March 2006; accepted 6 April 2006 Abstract Anatase TiO 2 films were successfully prepared on foam nickel substrates by sol-gel technique using tetrabutyl titanate as precursor. The characteristics of the TiO 2 films were investigated by XPS, XRD, FE-SEM, TEM and UV-Vis absorption spectra. The photocatalytic activities of TiO 2 films were investigated by photocatalytic degradation reactions of gaseous acetaldehyde, an indoor pollutant, under ultraviolet light irradiation. It was found that Ni 2+ doping into TiO 2 films due to the foam nickel substrates resulted in the extension of absorption edges of TiO 2 films from UV region to visible light region. The pre-heating for foam nickel substrates resulted in the formation of NiO layer, which prevented effectively the injection of photogenerated electrons from TiO 2 films to metal nickel. The TiO 2 films displayed high photocatalytic activity for the degradation of acetaldehyde, and were enhanced by calcining the substrates and coating TiO 2 films repeatedly. The high activity was mainly attributed to the improvement of the characteristics of substrate surface and the increase of active sites on photocatalyst. Key words: photocatalyst; TiO 2 films; foam nickel; sol-gel; acetaldehyde degradation Introduction Semiconductor photocatalyst TiO 2 has attracted much attention in last decade because of its potential application in the removal of all kinds of organic and inorganic pollutants in air or water (Zhang et al., 1994; Hoffmann et al., 1995; Fox and Dulay, 1993; Tryk et al., 2000). However, difficulties in the separation of photocatalyst from suspension after reaction and the aggregation of suspended particles with ultrafine size hampered practical applications of TiO 2 powders. In order to resolve these problems, many approaches have been taken to prepare immobilized powder-type photocatalysts (Yu et al., 2002) and film-type photocatalysts (Sopyan et al., 1996a) on various supports by different methods such as magnetron sputtering (Sproul et al., 1997), chemical vapour deposition (Zhang and Griffin, 1995) and sol-gel process (Yu et al., 2000). Some highly porous materials were chosen as catalyst supports, such as ceramic foam (Richardson et al., 2003; Peng and Richardson, 2004; Buciuman and Czarnetzki, 2001), porous alumina (Hwang et al., 2001; Ding et al., 2001), porous silica (Molina et al., 2004), zeolite (Atienzar et al., 2004) and activated carbon (Lam and Hu, 2003). Recently, foam metal materials Project supported by the Special Foundation of Nanometer Technology from Shanghai Municipal Science and Technology Commission(STCSM) (No. 0552nm002). *Corresponding author. shangguan@sjtu.edu.cn. have been utilized as substrates in heterogeneous catalysis because of their uniform cellular structure with anisotropy of mechanical and gas-dynamic properties (Pestryakov et al., 2002). The highly open porous structure of the foam metals provides the photocatalytic system excellent hydrodynamics properties for gas and liquid passing, so they are expected to be utilized as substrates in air or water purification in practice, especially in the remediation of indoor air pollution mainly caused by volatile organic compounds (VOCs). It has been reported that TiO 2 powder immobilized on porous nickel substrate with polyvinyl alcohol (PVA) as binder (Liu et al., 1999; Leng et al., 2000) showed high photocatalytic activities in degradation of organic pollutants. But the organic binder might be not stable and could be oxidized by the photogenerated positive holes, resulting in the shedding of catalyst and decrease of photocatalytic activity. In the present work, anatase TiO 2 films with high photocatalytic activities were fixed in situ on foam nickel via sol-gel technique without any binder. To investigate the photocatalytic activities of the prepared TiO 2 films, gaseous acetaldehyde, a representative of indoor VOCs was used as target. All reactions were carried out under UV light irradiation. The effects of preparation procedures including calcination temperatures of foam nickel and the number of coating cycles of TiO 2 films on the characteristics and photocatalytic activities of TiO 2 films were discussed in this paper.

2 No. 1 Preparations of TiO 2 film coated on foam nickel substrate by sol-gel processes and its photocatalytic activity 81 1 Experimental 1.1 Materials Precursor solution for TiO 2 films was prepared via solgel process. Tetrabutyl titante (Ti(OBu) 4, ml) and diethanolamine (DEA, 5.76 ml) were dissolved in anhydrous ethanol (70.74 ml). After vigorous stirring for 1 h at room temperature, the mixture of deionized water (1.08 ml) and anhydrous ethanol (2.0 ml) was added dropwise into the solution under stirring. The resultant precursor solution was stirred at room temperature for 2 h, and then was sealed and placed in the dark for 24 h. Finally a uniform, stable, and transparent sol of TiO 2 was obtained through above procedure. The final mole ratio of Ti(OBu) 4, DEA, H 2 O and C 2 H 5 OH was 1:1:1:10 in the sol. Foam nickel wafer (thickness of approximately 1.4 mm, porosity 95%, Ni wt%) was used as substrate. After ultrasonic treatment in ethanol to remove grease, the foam nickel was washed with distilled water and dried at room temperature, then calcinated at C for 10 min in air. TiO 2 films were prepared by a dip-coating method. The foam nickel was dipped in the precursor solution for minutes and then rotated at high speed to form a wet gel film. After drying at room temperature for 12 h, it was calcined at 550 C for 45 min in air. 1.2 Characterization The crystalline phase of the samples were characterized by an X-ray diffractometer (Bruker, D8 ADVANCE) with Cu K α radiation (λ= m). The accelerating voltage and the applied current were 40 kv and 40 ma, respectively. The surface morphology was observed using a field emission scanning electron microscope (FE-SEM, Sirion 200, Philips). The specific surface area of materials was determined by BET measurement (Quantachrome Instruments, NOVA 1000). A transmission electron microscope at 100 kv (JEOL, JEM-100CXΠ) was used to measure the particle size as follows (Shang et al., 2003): a small piece of TiO 2 film was sonic cleaned in anhydrous ethanol for 10 min. A piece of Cu net was dipped into the resultant solution and followed by a TEM analysis. The chemical states of the prepared TiO 2 films were investigated by X-ray photoelectron spectroscopy (XPS, Microlab 310F, VG SCIENTIFIC), equipped with hemispherical analyzer using Mg K α X-rays radiation ( ev). The high-resolution XPS spectra were collected at 20 ev pass energy. The binding energies of the Ti 2p and Ni 2p peaks of the samples were calibrated with respect to the C 1s peak at 285 ev from the adventitious hydrocarbon contamination. Compositions of the films were estimated from the relative area intensities of different high-resolution peaks after normalizing with respective relative sensitivity factors in the relevant software package. The light absorption characteristics of films were recorded with UV-Vis absorption spectra (TU-1901, Purkinje General Instrument), equipped with an integrating sphere accessory for diffuse reflectance. For these measurements a pressed piece of BaSO 4 was used as reference. The samples for XPS and UV-Vis absorption spectra investigation were prepared by sonic cleaning of the coated foam nickel in anhydrous ethanol for 15 min to get some exfoliated TiO 2 powder. It was then filtrated and dried for detection. 1.3 Photocatalytic reactions The photocatalytic activities of TiO 2 films were evaluated by the photocatalytic degradation of gaseous acetaldehyde. The TiO 2 films loaded on foam nickel substrate (φ= 140 mm) was put on the bottom of a cylindrical reactor, which was provided with a quartz window of approximately cm 2 and a volume of 1000 cm 3. The bare foam nickel and calcined foam nickel without photocatalyst loading was used for contrast tests. Gaseous acetaldehyde (from GuangMing Special Gas Co. Ltd., Dalian, China) was injected into the reactor. The initial concentration of acetaldehyde in the reactor was approximately 100 ppmv (parts per million by volume). The irradiation was started after equilibrium between gaseous and adsorbed acetaldehyde was reached. TiO 2 films were irradiated through the quartz window by a bactericidal light (main wavelength nm, 15 W). The vertical distance between light source and photocatalyst was 25 cm and the radiation power on the surface of TiO 2 films was 400 µw/cm 2. The concentration of gaseous acetaldehyde as a function of irradiation time was measured by a gas chromatograph equipped with a flame ionization detector (FID). 2 Results and discussion 2.1 Morphology and structure of materials Fig.1 shows that the XRD patterns of bare foam nickel and foam nickel calcinated at various temperatures ranging from 400 to 700 C. When calcination temperature was higher than 550 C, the diffraction peaks of nickel oxide (NiO) were detected obviously, and the diffraction peak Fig. 1 XRD patterns of foam nickel calcined at different temperatures: (a) bare foam nickel; (b) 400 C; (c) 450 C; (d) 500 C; (e) 550 C; (f) 600 C; (g) 650 C; (h) 700 C. Symbol: NiO, Ni.

3 82 HU Hai et al. intensity of nickel oxide increased with increasing calcination temperature. This indicates that the surface of foam nickel has been partly oxidized into nickel oxide after heating treatment. The UV-Vis absorption spectra (Fig.2) show that the absorption edges of TiO2 films coated on foam nickel extended to visible light region (approximately 520 nm), while pure TiO2 (anatase) is about 385 nm. In order to investigate the reason of the enhanced absorption for visible light, XPS measurement was used for analyzing the chemical states of elements of the TiO2 films. Fig.3a shows Fig. 2 UV-Vis absorption spectra of pure TiO2 (a) and TiO2 films loaded on foam nickel substrates with pre-heating at 550 C (b) and without preheating (c). Vol. 19 the peaks at 458.9±0.2 ev derived from TiO2 films loaded on foam nickel substrates without and with pre-heating at 550 C respectively, being attributable to the binding energy of Ti 2p3/2 in TiO2 films (Saha and Tompkins, 1992). Fig.3b shows the peak at ev, which is in accordance with the binding energy of Ni 2p3/2. This is attributed to the Ni2+ ion diffused from Ni substrate into the TiO2 films. Based on the XPS analysis, the relative amounts of doped Ni in TiO2 films loaded on foam nickel without and with calcination at 550 C are 1.23% and 0.72% respectively, which implied that the formation of NiO due to calcinations inhibited the diffusion of Ni2+ ion from foam nickel substrates into TiO2 films. Thus, it can be concluded that Ni2+ doping resulted in the extension of absorption edges of TiO2 films from UV region to visible light region. The FE-SEM images of foam nickel before and after calcination at 550 C are shown in Fig.4, presenting that the original smooth surface of foam nickel became rough and squama-like after heating. The rough surface provides more area for the loading of TiO2 films, which is crucial for photocatalytic reaction. The BET measurement showed that the specific surface area of foam nickel increased to 0.24 m2 /g from 0.12 m2 /g due to calcination at 550 C. The FE-SEM images of TiO2 films loaded on foam nickel are shown in Fig.5. XRD patterns of TiO2 films loaded on foam nickel with various coating cycles are showed in Fig.6. When TiO2 films were coated with one cycle, Fig. 3 XPS spectra of Ti 2p (a) and Ni 2p (b) for TiO2 films loaded on foam nickel substrates (a) without pre-heating and (b) with pre-heating at 550 C. Fig. 4 FE-SEM images of bare foam nickel(a) and foam nickel(b) calcined at 550 C.

4 No. 1 Preparations of TiO2 film coated on foam nickel substrate by sol-gel processes and its photocatalytic activity 83 Fig. 5 FE-SEM images of TiO2 films loaded on foam nickel calcined at 550 C with (a) one coating cycle and (b) two coating cycles. one, two and three cycles are 5.0 wt%, 8.0 wt% and 12.0 wt%, respectively. Accordingly, the specific surface areas are approximately 0.7 m2 /g for one cycle, 0.9 m2 /g for two and three cycles. Fig.7 shows TEM images of the TiO2 films flaked off from foam nickel substrates with various TiO2 coating cycles. It is found that the TiO2 films are made up of spherical particles, and the particle sizes of TiO2 films coated with 1 to 3 cycles are 15 nm, 18 nm and 25 nm, respectively. The particle size becomes larger with increasing coating cycles. This indicates that the repeated calcination treatment for TiO2 films results in growth and aggregation of the particles. 2.2 Photocatalytic degradation of acetaldehyde Fig. 6 XRD patterns of TiO2 films on foam nickel calcined at 550 C with various coating cycles of (a) no coating cycle (blank sample); (b) one coating cycle; (c) two coating cycles; (d) three coating cycles. Symbol: anatase TiO2, NiO, F Ni. no any diffraction peak is obviously examined, except for the peaks of Ni and NiO generated from foam nickel and its calcination at high temperature. When coating with two and three cycles, however, diffraction peaks of TiO2 (anatase) can be observed clearly. In the case when TiO2 films were coated with one cycle, the quantity of TiO2 was not enough to be detected by XRD due to the low coverage of the TiO2 films on foam nickel. The quality of loaded TiO2 increased with increasing coating cycles, and the incremental weight ratios for foam nickel coating with The photocatalytic degradation of acetaldehyde was carried out in the reactor described above. The equilibrium between gaseous and adsorbed acetaldehyde was reached in about 90 min after injecting the acetaldehyde gas into the reactor, and the initial equilibrium concentration of gaseous acetaldehyde for irradiation was about 70 ppmv. The degradation ratio of gaseous acetaldehyde was calculated as follows: Rt = Ci Ct Ci (1) where Rt is the degradation ratio of gaseous acetaldehyde, Ci is the initial equilibrium concentration of the photocatalytic reaction, and Ct is the concentration measured by a gas chromatograph during the reaction. Fig.8 shows the photocatalytic activity for acetaldehyde degradation on foam nickel with and without loading Fig. 7 TEM images of TiO2 films loaded on foam nickel calcined at 550 C with (a) one coating cycle, (b) two coating cycles and (c) three coating cycles.

5 84 HU Hai et al. Vol. 19 Fig. 8 Gaseous acetaldehyde degradation rates of TiO 2 films coated one time on foam nickel substrates without calcination and calcined at various temperatures, respectively. TiO 2. The photocatalytic reaction did not proceed on bare foam nickel without TiO 2 loading under UV irradiation. However, the activity on TiO 2 films coated on foam nickel was examined, and was dependent on the calcination temperature of foam nickel substrate. The degradation rate of acetaldehyde was significantly enhanced with increasing the calcination temperatures during C, and kept a same level over 550 C, while the activity was not increased obviously under calcination at 400 C. Basing on the fact that nickel oxide (NiO) were formed obviously when the foam nickel was calcined over 550 C (Fig.1), it can be concluded that the effect of calcination temperature on the activity is attributed to the formation of NiO on the surface of foam nickel. The photocatalytic activity of TiO 2 films coated on NiO is higher than that coated on metal nickel, because the active metal is prone to react with photogenerated electrons and debases the photocatalytic activity of TiO 2 films (Shang et al., 2003). Thus, it seems that the formation of NiO layer on the foam nickel due to pre-heating prevented effectively the injection of photogenerated electrons from TiO 2 films to metal nickel to some extent, giving rise to the increase in photocatalytic activity. Fig.9 shows that the coating cycle dependence on the photocatalytic activity. The enhancement in activity is obvious by repeating 2nd coating cycle, and is slight by repeating 3rd coating cycle. This is in accordance with the change of specific surface area of the catalyst. It has been indicated above that the 2nd coating cycle increased the specific surface area from 0.7 to 0.9 m 2 /g, while the 3rd coating cycle did not improve the specific surface area. In order to evaluate the stability of photocatalytic activity, consecutive photocatalytic reactions for acetaldehyde degradation were carried out. As shown in Fig.10, the photocatalytic activities of TiO 2 films decreased gradually in subsequent runs. However, complete reactivation was achieved by heating the TiO 2 films at 300 C for 1 h. As have known, the basic process of photocatalysis for TiO 2 semiconductor consists of UV light exciting an electron from the valence band (VB) to the conduction band (CB) of TiO 2 creating a hole in the valence band. The Fig. 9 Gaseous acetaldehyde degradation rates of TiO 2 films coated for various cycles on foam nickel calcined at 550 C. (a) no TiO 2 coating; (b) one coating cycle; (c) two coating cycles; (d) three coating cycles. electron-hole pairs, after migrating to the surface, can in turn be trapped by surface-adsorbed molecules at different sites, leading to oxidation and reduction processes. Under the experimental conditions (high acetaldehyde concentrations, weak UV illumination intensity and ambient air), the number of acetaldehyde molecules on the TiO 2 surface is much larger than the number of incident photons, most of the adsorbed acetaldehyde molecules should be oxidized to acetic acid, followed by oxidized into carbon dioxide and water with further illumination (Sopyan et al., 1996b). The oxidative photodegradation process of acetaldehyde on TiO 2 in principle is described as follows: O 2 + e - CB O - 2 (2) H 2 O + 2h + OH + 2H + (3) CH 3 CHO + H 2 O + 2h + CH 3 COOH + 2H + (4) 2CH 3 CHO + O 2 + e - CB 2CH 3COOH + e - (5) CH 3 COOH + 2H 2 O + 8h + 2CO 2 + 8H + (6) CH 3 COOH + 2O 2 2CO 2 + 2H 2 O (7) Basing on the mechanism above, the deactivation in subsequent reaction runs might result from the adsorption of reactant and intermediate (for example CH 3 COOH) on Fig. 10 Gaseous acetaldehyde degradation rates of TiO 2 film coated for two cycles on foam nickel calcinated at 550 C with consecutive runs. (a) 1st run; (b) 2nd run; (c) 3rd run; (d) 4th run; (e) 5th run; (f) after heating at 300 C for 1 h.

6 No. 1 Preparations of TiO 2 film coated on foam nickel substrate by sol-gel processes and its photocatalytic activity 85 the active sites of photocatalyst. The adsorbed reactant and intermediate were desorbed after the heating treatment, thus the photocatalytic activity was recovered. 3 Conclusions Anatase TiO 2 films were successfully prepared on foam nickel substrates by sol-gel technique. Ni 2+ doping due to the foam nickel substrates resulted in the extension of absorption edges of TiO 2 films from UV region to visible light region (λ<520 nm). The TiO 2 films displayed high activities in the photocatalytic degradation reactions of gaseous acetaldehyde under ultraviolet irradiation. The photocatalytic activity of the TiO 2 films was affected by the preparation procedure, and was enhanced by calcinating the substrates and coating TiO 2 films repeatedly. The calcination at high temperatures for foam nickel substrates not only improved the characteristics of substrate surface, but also resulted in the formation of NiO crystalline phase on the surface of foam nickel, which prevented effectively the injection of photogenerated electrons from TiO 2 films to metal nickel to some extent, giving rise to the increase in photocatalytic activity. References Atienzar P, Corma A, Garcia H et al., Diffuse reflectance laser flash photolysis study of titanium-containing zeolites[j]. Chem Mater, 16: Buciuman F C, Czarnetzki B K, Preparation and characterization of ceramic foam supported nanocrystalline zeolite catalysts[j]. Catal Today, 69: Ding Z, Hu X J, Yue P L et al., Synthesis of anatase TiO 2 supported on porous solids by chemical vapor deposition[j]. Catal Today, 68: Fox M A, Dulay M T, Heterogeneous photocatalysis[j]. Chem Rev, 93: Hoffmann M R, Martin S T, Choi W et al., Environmental applications of semiconductor photocatalysis[j]. Chem Rev, 95: Hwang K S, Zhu H Y, Lu G Q, New nickel catalysts supported on highly porous alumina intercalated laponite for methane reforming with CO 2 [J]. Catal Today, 68: Lam F L Y, Hu X J, A new system design for the preparation of copper/activated carbon catalyst by metalorganic chemical vapor deposition method[j]. Chem Eng Sci, 58: Leng W H, Liu H, Cheng S A et al., Kinetics of photocatalytic degradation of aniline in water over TiO 2 supported on porous nickel[j]. J Photoch Photobio A: Chemistry, 131: Liu H, Cheng S A, Zhang J Q et al., Titanium dioxide as photocatalyst on porous nickel adsorption and the photocatalytic degradation of sulfosalicylic acid[j]. Chemosphere, 38: Molina A I, Robles J M, Garcła P B et al., Nickel supported on porous silica as catalysts for the gas-phase hydrogenation of acetonitrile[j]. J Catal, 225: Peng Y, Richardson J T, Properties of ceramic foam catalyst supports: one-dimensional and two-dimensional heat transfer correlations[j]. Appl Catal A-Gen, 266: Pestryakov A N, Lunin V V, Devochkin A N et al., Selective oxidation of alcohols over foam-metal catalysts[j]. Appl Catal A-Gen, 227: Richardson J T, Garrait M, Hung J K, Carbon dioxide reforming with Rh and Pt-Re catalysts dispersed on ceramic foam supports[j]. Appl Catal A-Gen, 255: Saha N C, Tompkins H G, Titanium nitride oxidation chemistry: An X-ray photoelectron spectroscopy study[j]. J Appl Phys, 72: Shang J, Li W, Zhu Y F, Structure and photocatalytic characteristics of TiO 2 film photocatalyst coated on stainless steel webnet[j]. J Mol Catal A-Chem, 202: Sopyan I, Watanabe M, Murasawa S et al., 1996a. A film-type photocatalyst incorporating highly active TiO 2 powder and fluororesin binder: photocatalytic activity and long-term stability[j]. J Electroanal Chem, 415: Sopyan I, Watanabe M, Murasawa S et al., 1996b. An efficient TiO 2 thin-film photocatalyst photocatalytic properties in gas-phase acetaldehyde degradation[j]. J Photoch Photobio A: Chemistry, 98: Sproul W D, Graham M E, Wong M S et al., Reactive d.c. magnetron sputtering of the oxides of Ti, Zr, and Hf[J]. Surf Coat Tech, 89: Tryk D A, Fujishima A, Honda K, Recent topics in photoelectrochemistry: achievements and future prospects[j]. Electrochim Acta, 45: Yu J G, Zhao X J, Zhao Q N Effect of surface structure on photocatalytic activity of TiO 2 thin films prepared by solgel method[j]. Thin Solid Films, 379: Yu J C, Yu J G, Zhang L Z et al., Enhancing effects of water content and ultrasonic irradiation on the photocatalytic activity of nano-sized TiO 2 powders[j]. J Photoch Photobio A: Chemistry, 148: Zhang Q M, Griffin G L, Gas-phase kinetics for TiO 2 CVD: Hot-wall reactor results[j]. Thin Solid Films, 263: Zhang Y, Crittenden J C, Hand D W et al., Fixedbed photocatalysts for solar decontamination of water[j]. Environ Sci Technol, 28: