New route to prepare nanocrystalline TiO2 and its reaction mechanism

Materials Research Bulletin 37 (2002) 1851±1857

New route to prepare nanocrystalline TiO2 and its reaction mechanism X.F. Zhoua,*, D.B. Chua, S.W. Wanga, C.J. Linb, Z.Q. Tianb a

Institute of Organic Chemistry, Anhui Normal University, Wuhu 241000, China b Department of Chem Xiamen University, Xiamen 361005, China (Refereed) Received 21 September 2001; accepted 17 June 2002

Abstract Titanium oxide nanoparticles are prepared by electrochemical dissolution of pure titanium in a mixed acetylacetone and ethanol solution followed by direct sol±gel process of the electrolyte. Infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and TEM have been used to investigate the structure of precursor and nanocrystalline TiO2. Characterization of the electrochemical product reveals that Ti(OEt)m(acac)n was formed by anodic dissolution of titanium in a mixed acetylacetone and ethanol solution. Infrared experiments show that hydrolysis of Ti(OEt)m(acac)n precursor removes ®rst OEt groups, the chelating acac groups are still observed in the gel and can only be removed upon heating to 473 K. This study also shows that nanocrystalline TiO2 prepared by this route has a textural and thermal stability with size distribution of 5±20 nm. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Nanostructures; B. Electrochemical synthesis; B. Sol±gel chemistry; C. Raman spectroscopy; C. Infrared spectroscopy

1. Introduction It is well known that titanium oxides are useful functional materials. Nanocrystalline TiO2 is attractive for its applications in solar energy conversion, photocatalysis, pigments, electronic devices, etc. [1]. Anatase TiO2 has a higher phtocatalytic activity than rutile TiO2 [2,3]. Therefore, much attention has been focused on the preparation *

Corresponding author. Tel.: ‡86-533-3869304, ext. 8016; fax: ‡86-533-3869303. E-mail address: [email protected] (X.F. Zhou). 0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 8 6 3 - 2

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of anatase TiO2 with high textural and thermal stability [4]. A variety of methods are available for their preparation, including chemical vapor deposition (CVD) [5], sol± gel process [6], and plasma CVD (PCVD) [7]. In this paper, we describe a new route for the preparation of nanocrystalline TiO2 by electrochemical dissolution of titanium in a mixed acetylacetone and ethanol solution followed by direct sol±gel process of the electrolyte solution. We shall ®rst discuss Ti(OC2H5)m(acac)n precursor electrochemically synthesis using titanium metal as sacri®cing anode, then the direct sol±gel process of the electrolyte solution. The electrochemical reaction and hydrolysis mechanism is also studied. It shows that hydrolysis of Ti(OC2H5)m(acac)n removes ®rst OEt groups; the acac groups still remain in the gel and can only be removed upon heating to 473 K. This study also shows that nanocrystalline TiO2 prepared by this route has both textural and thermal stability with size distribution of 5±20 nm. 2. Experimental procedure Acetylacetone and ethanol were distilled and dried over molecular sieves, tetrabutylammonium bromide was dried under reduced pressure. Nickel foil (4 cm4 cm 0:005 cm) was used as cathode and pure titanium sheet (1 cm  1 cm  0:2 cm) was used as the sacri®cing anode. Acetylacetone (10 ml) was mixed in 200 ml of ethanol (0.1 g Bu4NBr was used as supporting electrolyte) and taken into the electrolysis cell. The electrochemical reaction was carried out in a cell without separating the cathode and anode spaces, the cell consists of a pyrex 200 ml glass beaker with a water jacket in a recirculating loop, which can maintain the constant solution temperature. A magnetic stirrer was used when electrolysis, direct current was obtained from WY-302 electrophoresis power supply; the potential across the electrode was adjusted as a current density of 400 A m 2 passed through the cell. The precursor and xerogel were analyzed by infrared absorption spectroscopy, respectively. Current ef®ciency was calculated by the ratio of practically obtained TiO2 powder to theoretically obtained powder. After titanium was electrochemically dissolved in the electrolyte solution for 8 h, 2 ml water diluted with 10 ml of ethanol was directly added to the electrolyte solution, the direct hydrolysis reaction occurred with the formation of the white gel. The gel was washed with ethanol and then dried in vacuum to form the yellowish xerogel. Nanocrystalline TiO2 was obtained and calcined at 473, 773, and 1193 K for 1 h, respectively. Raman spectra (Dilor, France, l ˆ 632:8 nm) and X-ray diffraction measurements (XRD, RIGAKU RINT2000) were used to identify the crystalline phase, the particle size was determined by using a transmission electron microscope with an acceleration voltage of 300 kV. 3. Results and discussion Fig. 1 shows the infrared spectrum recorded on the electrolyte solution. It shows no bands corresponding to free acetylacetonate in the electrolyte solution, but exhibits

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Fig. 1. Infrared spectrum of electrolyte solution.

absorption corresponding to the hydroxyl groups of EtOH. We believe the acetylacetone in the solution reacted entirely with the titanium, but EtOH was not, since the chelating acac groups are stronger, complexing ligands compared with the ethoxyl groups. The infrared spectrum also exhibits sharp bands around 2958±2932± 2870 cm 1, and 1462±1382 cm 1 corresponding, respectively, to the stretching and banding vibration of the aliphatic CH2 and CH3 groups [8]. It is more interesting, however, to note a set of two bands at 1590 and 1527 cm 1, which could be due to acetylacetonate chelating ligands [9]. A series of bands around 1124±1097± 1038 cm 1 can be seen in the ®gure corresponding to Ti±O±C vibrations of ethoxyl groups directly bonded to titanium. The low energy side of the spectrum exhibits only a broad signal that should be due to the envelope of the phonon bands of Ti±O±Ti bond of a titanium oxide network [10]. The infrared spectrum reveals that Ti(OEt)m(acac)n was synthesized by anodic dissolution of titanium in a mixed acetylacetone and ethanol solution. Electrochemical dissolution of titanium in the cell started at room temperature with the evolution of hydrogen at cathode, the solution was warmed up during the process and maintained at a constant temperature by the recirculating water. Tetrabutylammonium bromide was selected as the best supporting electrolyte. With this substance, the electrochemical dissolution of titanium may run for a long time. It was found that black deposits formed on the anode surface or ¯aked off into the solution during the process of electrochemical dissolution. This material was identi®ed as a-Ti by XRD as also found by Burstein and Whillock [11]. The dissolution occurred more rapidly after titanium has been electrochemically activated. The mixed solution turned yellow within 2 h, but quickly changed to green when argon gas was bubbled into the mixture, or the oxygen dissolved in the solution was run out. We think that the green TiIII±IV derivatives [12] are ®rst formed on the anode surface, their subsequent oxidation by dry air leads to the formation of soluble TiIV(OEt)m(acac)n. On the basis of surface Raman spectroscopy investigation of electrochemical synthesis of

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titanium alkoxides [13] and electrochemical synthesis of other metal alkoxides [14± 16], the possible electrochemical reaction may be written as follows: Anode : Cathode :

Ti ‡ EtOH ‡ Hacac ‡ Br ! TiIII IV …OEt†x …acac†y Brz TiIII IV …OEt†x …acac†y Brz ‡ O2 ! TiIV …OEt†x …acac†y Brz TiIV …OEt†x …acac†y Brz ‡Hacac‡EtOH ! Ti…OEt†m …acac†n ‡Br ‡H2

where x ‡ y ‡ z ˆ m ‡ n ˆ 4. Fig. 2 shows the infrared spectrum of the obtained xerogel. A set of two bands at 1598 and 1526 cm 1 corresponding to acac groups increased in intensity, while the bands around 1462 and 1097 cm 1 corresponding to the OEt groups disappeared, and the bands around 2958±2932±2870 cm 1 also decreased in intensity. This shows that the OEt groups had been removed during the hydrolysis process. These results also suggest that the molecular compound of TiO(acac)2 was formed [17]. The electrochemical formation of Ti(OEt)m(acac)n overcomes the disadvantages of sol±gel process of titanium ethoxide, the gelation rates decrease upon complexation, and the hydrolysis process can be controlled to avoid precipitation. Stronger complexation acac ligands cannot be hydrolyzed easily and condensation does not go any further, so the acac groups still remain in the monolithic xerogel, and can only be removed upon heating to 473 K (see Fig. 3). Since complexing acac ligands behave as termination reagents, the precipitation can be avoided and monodispersed colloids can be obtained [18]. Fig. 3 shows the XRD patterns of nanocrystalline TiO2. It shows that some anatase phase TiO2 had been formed after hydrolysis, but entirely turned into anatase phase when calcined at 473 K. It is clear that all diffraction peaks can be attributed to anatase TiO2 [19], indicating a very high purity of the powder in all cases. The shape of the diffraction peaks, suggests that titanium oxide consists of very ®ne particles and the particle size increases with an increase in temperature. The crystallite sizes of samples calcined at 473 K, which were calculated from the Scherer equation were less

Fig. 2. Infrared spectrum of yellowish xerogel upon heating for 12 h in vacuum.

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Fig. 3. XRD patterns of TiO2 powder calcined at different temperature: (a) yellowish xerogel, (b) calcined at 473 K for 1 h, (c) calcined at 773 K for 1 h, and (d) calcined at 1193 K for 1 h.

than 5 nm in all cases. The sizes of the particles were increased to 10 and 20 nm, respectively, after calcination at 723 and 1193 K for 1 h. Fig. 3 also shows that TiO2 nanoparticles are stable in the antanse structure in the temperature range from 473 to 1193 K, so this route can be applied to preparing nanocrystalline TiO2 photocatalysts. The current ef®ciency of TiO2, prepared by this method, which was calculated by the ratio of obtained TiO2 powder calcined at 1193 K for 4 h to the theoretically obtained powder, was 92%. This value is higher than that of electrochemical dissolution of titanium in ethanol. Further investigation is in progress. Fig. 4 shows the Raman spectrum of nano-TiO2 prepared at different annealing temperatures. The spectra varies systematically with the grain size. It deteriorates with spectroscopic lines broadening and merging, line intensity decreasing and position shifting with decreasing annealing temperature. The Raman spectra of the xerogel (a) show the vibrations of organic groups and anatase phase of TiO2. The organic groups completely disappeared after calcination at 773 K (b) for 1 h.

Fig. 4. Raman spectrum of TiO2 powder calcined at different temperature: (a) calcined at 473 K for 1 h, (b) calcined at 773 K for 1 h, (c) calcined at 1193 K for 1 h.

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Fig. 5. TEM micrographs of TiO2 calcined at 773 K for 1 h.

There are no spectroscopic lines corresponding to rutile TiO2 [20] after nanocrystalline TiO2 was calcined at 1193 K for 1 h. It also shows that nano-TiO2 prepared by this route has a stable antanse structure in temperature range from 298 to 1193 K. TEM micrographs of the nanocrystalline TiO2 are shown in Figs. 5 and 6. The particles did not aggregate, particle size was very small, and the mean diameter of the nanoparticles is 10 nm at 773 K and 20 nm at 1193 K. The mean particle size obtained from TEM micrography was similar to the crystalline size from the XRD patterns.

Fig. 6. TEM micrographs of TiO2 calcined at 1193 K for 1 h.

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4. Conclusions Electrochemical dissolution of pure titanium in a mixed acetylacetone and ethanol solution and direct sol±gel process of the electrolyte solution can produce nanocrystalline TiO2 with a stable antanse structure in temperature range from 473 to 1193 K. The mean diameter of the nanoparticles is 10 nm at 773 K and 20 nm at 1193 K. The precursor of Ti(OEt)m(acac)n has been electrochemically synthesized by anodic dissolution of titanium in a mixed acetylacetone and ethanol solution, hydrolysis of this molecular precursor removes ®rst OEt groups; the chelating acac groups still remain in the gel and can only be removed upon heating to 473 K. This process is a promising technique for the synthesis of nanocrystalline TiO2. Acknowledgments This work was supported by a Grant-in-aid for Scienti®c Research from Anhui province an Anhui Education Burea. Nos.: 00046112, 2001kj116. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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