Synthesis and photocatalytic properties of InVO₄ sol containing nanocrystals by mild hydrothermal processing

FANG Hai-bo(方海波), XU Ming-xia(徐明霞), GE Lei(戈  磊), HE Zhi-yuan(何智远)

School of Materials Science and Engineering, Key Laboratory for Advanced Ceramics and
Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The precursor precipitation of InVO₄ was synthesized by co-precipitation using indium trichloride (InCl₃), ammonium metavanadate (NH₄VO₃) and ammonia (NH₃?H₂O) as raw materials. The InVO₄ sols with orthorhombic phase were obtained by hydrothermal treatment (the precursor precipitation solution at 423 K, for 4 h). The precursor and sol of InVO₄ were characterized by X-ray diffraction (XRD), Fourier Transform Infra-red spectra (FT-IR), scanning electron microscopy (SEM) measurements. The XRD patterns indicate that the InVO₄ precursor is amorphous phase, InVO₄ sol contains orthorhombic InVO₄ nanocrystals. The results also reveal that the pH value of the reaction mixture and reaction temperature play important roles to the target phase. InVO₄-TiO₂ thin films on glass slides were prepared by the dip-coating method from the composite sol. The photocatalytic properties of the InVO₄-TiO₂ thin films were investigated by the photocatalytic degradation of methyl orange solution. The results indicate that it has better photocatalytic activities than pure TiO₂ thin films or pure InVO₄ thin films with UV light.

Key words: InVO₄ nanocrystals; mild hydrothermal processing; co-precipitation; photocatalytic properties

1 Introduction

As a well-known photocatalyst, TiO₂ is one of the most popular and promising materials because it is stable in various solvents under photoirradiation, available commercially, and easy to prepare in the laboratory [1, 2]. However, the band gap of TiO₂ is so wide that it only responds to the ultra violet (UV) light, which occupies only about 4% of the solar spectrum. Therefore, there is an urgent need to develop new photocatalysts with higher photocatalytic activities. In recent years, some papers were published on novel photocatalysts active under visible light irradiation, including ABO₄(A=Bi, In, B=V, Nb, Ta)[3-6]，Bi₂MNbO₇[7], CaIn₂O₄[8], etc. The new type of semiconductor InVO₄ has been developed by YE et al [4], which was used as water splitting photocatalysis under visible irradiation. Several methods have been developed to synthesize InVO₄, including the solid-state reaction between In₂O₃ and V₂O₅ [4], ‘chimie douce’method[9], sol-gel approach[10,11] and precipitation of the aqueous solution of InCl₃ and NH₄VO₃[12], etc. Nevertheless, in orden to obtain crystalline InVO₄, all of these ways needed heat treatment process with high temperature, which consume more energy and meet the difficulty in controlling the component, homogeneity and crystal size of the products [13]. XIAO et al [13] and CHEN et al [14] have synthesized nano-crystalline InVO₄ powders by hydrothermal approach. However, there were two disadvantages: firstly, the agglomeration of InVO₄ nanoparticles was obvious; secondly, the organic additives in the experiment were expensive and not friendly to environment.

In this study, the InVO₄ sol was prepared from a mild hydrothermal method without organic additives at low temperature of 423 K, which was much lower than the solid-state reaction. The photocatalytic activity of the composite thin films was evaluated by the photocatalytic decoloration of methyl orange (MO) aqueous solution.

2 Experimental

The starting materials for the synthesis of InVO₄ sol were indium trichloride (InCl₃, AR), ammonium metavanadate (NH₄VO₃, AR) and ammonia (NH₃?H₂O, 3 mol/L). In a typical procedure, InCl₃ was first dissolved with deionized water, then, precipitated by adding NH₄VO₃ solution resulting in the formation of yellow deposit with continual magnetic stirring until the stoichiometric proportion was 1. Under the vigorous stirring, the pH value of the mixture was adjusted to about 7 with ammonia solution. After centrifuging the precipitate, appropriate distilled water was added to disperse the precipitate homogeneously. InVO₄ sol containing nanocrystals was obtained by reacting in a Teflon-lined autoclave, which was used to prepare composite sol (InVO₄-TiO₂). The TiO₂ sol had been prepared by GE et al [15]. Glass slides were used as substrates. Before the deposition, substrates were ultrasonically cleaned in acetone and absolute ethanol for 10 min, respectively. Finally, they were thoroughly rinsed with distilled water. InVO₄-TiO₂ thin films were deposited on substrates by a dip-coating process at room temperature. The layers could be thickened by means of consecutive dip-coating processes. Afterward, the films were calcined to 773 K at 6 K/min, and then calcined for 30 min in an air flow oven.

The precursor and sol of InVO₄ were characterized by FT-IR (BIO-RAD FTS-3000), XRD (Rigaka D/max 2 500 v/pc, Cu K_α, 40 kV and 40 mA) and SEM (JEOL JSF-6700F, FE-SEM, 10kV) measurements, respectively. The photocatalytic activities of the composite films were evaluated from an analysis of the degradation of MO solution under UV illumination. Two pieces of 25 mm×75 mm glass slides coated with composite films were settled in a watch glass containing 50 mL MO solution (10 mg/L) and exposed to UV-irradiation. Two UV-light lamps were used to provide the UV light-source. The distance between the films and the light source was 15 cm. UV absorption spectra of samples were measured every 20 min with a UV-vis spectrophotometer (7 230 G, Shanghai, China). The changes in concentrations of MO were estimated from the changes in absorbance of the absorption maximum at 500 nm.

Fig.1 shows the FT-IR spectra of the dried InVO₄ sol. The absorption bands of the dried powders at 406.7 cm^-1 and 434.1 cm^-1 are ascribed to V—O—V deformations mode. The 462.4 cm^-1 band is ascribed to the V—O—V scissoring vibration. The bands of the samples at 748.7, 775.4 and 909.1 cm^-1 originate from the V—O—In stretching vibration mode. The band of 951.0 cm^-1 is assigned to the terminal V—O stretching.

The influence of the pH value and reaction temperature on the formation of InVO₄ crystalline phase has been investigated by XRD in Fig.2 and Fig.3, respectively. The XRD patterns of the InVO₄ samples

Fig.1 FT-IR spectra of dried InVO₄ sol (pH=7, T=423 K, 4 h)

Fig.2 XRD patterns of samples (T=423 K, 4 h): (a) pH=7; (b) pH=6; (c) pH=8

Fig.3 XRD patterns of samples (pH=7, 4 h): (a) T=298 K; (b) T=373 K; (c) T=423 K

synthesized at different pH values are shown in Fig.2. All the reflection peaks in Fig.2(a) could be indexed to the orthorhombic phase of InVO₄ (JCPDF card 48-0898), no other impurities, such as In(OH)₃, is detected in the samples. It indicates that the InVO₄ samples prepared from hydrothermal method are absolutely orthorhombic phase. When pH value is less than or more than 7, In(OH)₃ gradually becomes the main crystalline phase in the sol. In Fig.2(b), the main phase is orthorhombic InVO₄, together with a spot of In(OH)₃  detected as parent peaks at around 2θ=22.36? and 31.89?. When the pH value reaches to 8 (Fig.2(c)), the intensity of the peaks indexed as InVO₄ decreases. The majority of crystalline phase is In(OH)₃. From Fig.3(a), it can be seen that the precursor precipitation of InVO₄ is amorphous. Upon increasing the synthetic temperature, no peaks of orthorhombic InVO₄ appear in the XRD pattern, the crystalline phase is In(OH)₃ (Fig.3(b)). We obtain the X-ray signature of the orthorhombic InVO₄ phase at 423 K (Fig.3(c)). From Ref. [11], the InVO₄ with the pure orthorhombic phase was obtained above 873 K. In the present work, it gives a new method to prepare the orthorhombic phase at a low temperature.

The morphologies of the InVO₄ sol and as-prepared InVO₄-TiO₂ composite films are shown in Fig.4. All the InVO₄ sols are obtained at pH=7. From Fig.4(a), we can clearly see the round, homogenous and fairly small crystals in the sols. The average grain size is 100 nm. The size of sphere particles grows with increasing reaction temperature (Fig.4(b)). Fig.4(b) also shows that the grain size distribution is wider with higher temperature, such as the larger particles reach to 200 nm in diameters. When extending the reaction time, it is found that the sols contain regular polygonal crystals of about 400-500 nm in size as shown in Fig.4(c). Fig.4(d) shows that the InVO₄-TiO₂ composite thin films are compact, uniform and no cracks, and abnormal large particles are observed. The composite films consist of rice-bodies crystals, which are 100 nm in length and 50 nm in width.

Photocatalytic performance of the InVO₄-TiO₂ composite thin films was assessed by investigating photodegradation effect of methyl orange(MO)under UV illumination, as shown in Fig.5. For comparison, the decoloration of pure TiO₂ and InVO₄ films are given as well. Temporal changes in the concentration of MO are monitored by examining the variations in maximal absorption in UV-vis spectra at 500 nm. From Fig.5, it can be seen that the decoloration of MO increases with extending of the irradiation time. And the results show that the InVO₄-TiO₂ composite thin films exhibit an extremely high activity (Fig.5(c)); almost 100% of methyl orange is degraded in 80 min. This is attributed to the efficient separation of e^--h⁺ pairs in the excited state, where the InVO₄ and TiO₂ are coupled; and electrons, which are promoted from conduction band of the InVO₄ under UV light, are easily injected into the conduction band of TiO₂; whereas holes in the valence band of TiO₂ are transferred into the InVO₄, which could product —OH radical on the surface of TiO₂, leading to the higher activity[16,17]. At the same time, the photocatalytic activity of the TiO₂ (Fig.6(a)) and InVO₄ (Fig.6(b)) shows that the catalytic activities of them are low, because the photo-generated electron–holes are de-coupled easily on the surface of InVO₄ and the electrons are not easily excited in pure TiO₂ under these conditions.

Fig.4 FE-SEM micrographs of as-prepared samples (pH=7): (a) InVO₄ sol, 423 K, 4 h; (b) InVO₄ sol, 453 K, 4 h; (c) InVO₄ sol, 423 K, 6h; (d) InVO₄-TiO₂ composite thin films, using (a) InVO₄ sol

Fig.5 Decoloration rate curves of methyl orange under UV light irradiation: (a) TiO₂ thin films; (b) InVO₄ thin films; (c) InVO₄-TiO₂ composite thin films

4 Conclusions

InVO₄ sol containing pure orthorhombic phase was synthesized by hydrothermal method at low temperature. The optimal conditions for the synthesis are: pH=7, T=423 K, reaction time: 3-6 h. It is found that the sols consist of the round, homogenous nanocrystals. The average grain size is 100 nm. The photocatalytic exper-

iment exhibits that the InVO₄-TiO₂ composite films have the highest photocatalytic activity among the three thin films with UV light.

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(Edited by HE Xue-feng)

Foundation item: Project (20030056001) supported by the Doctor Foundation of Ministry of Education of China

Corresponding author: XU Ming-xia; Tel.: +86-22-27890489; Fax: +86-22-27404724; E-mail: xumingxia@tju.edu.cn