HUANG Yang YAN Feng-Po CHEN Ye-Qing CHEN Da-Gui

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Subsolidus Phase Relations in the TiO2-NiO-WO3System①

HUANG Yang YAN Feng-Po CHEN Ye-Qing②CHEN Da-Gui②

(Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

The subsolidus phase relationships of the system TiO2-NiO-WO3were investigated, aiming at exploring new potential visible-light active photocatalysis systems. After the solid-state chemistry reaction in air between 1000 and 1150 ℃, X-ray diffraction (XRD) data showed that two binary compounds and three ternary-phase regions were contained in this ternary system. No new ternary compound was discovered under our experiment conditions.None of these phases in this system revealed a remarkable homogeneity range.

TiO2-NiO-WO---3 system, phase diagram, X-ray diffraction

1 INTRODUCTION

As the most promising photocatalyst, TiO2has been widely studied in hope of solving serious pollution and environmental challenges and helping ease the energy crisis based on water-splitting and photovoltaic devices[1,2]. However, some intrinsic properties of TiO2, such as the narrow light response range and a poor quantumn yield[3], limits the effective utilization of solar energy. In order to construct high visible-light active photocatalysis systems, great efforts were made on modifications of TiO2and searching for new photocatalytic materials[1,3-7].

Among all these explorations, the ternary system TiO2-NiO-WO3draws increasing attention of researchers. As a p-type semiconductor, NiO is frequently used in combination with titanates as co-catalyst. For instance, Chen[8]prepared a-n junction photocatalyst NiO/TiO2by sol-gel method which shows a higher activity in reducing Cr2O72-compared to TiO2. Also, it is reported by Sastre[5]that NiO itself shows high photocatalytic activity in the reduction of CO to CH4under visible or solar light at room temperature. Moreover, the oxides of Ni2+and Ti4+possibly form perovskite structure in an ideal chemical formula ABO3. Photocatalysts with perovskite-type structure have been extensively studied for its potential in combining with high visible-light photocatalytic activity and the great stability by structure regu- lation[9].As an example, Shu[10]obtained TiO2-coupled NiTiO3nanoparticles by copreci- pitation and found it exhibits good photocatalytic activities under visible light irradiation. As for WO3, it has a direct band-gap of 2.7 eV and has also been studied as a photocatalyst itself[11]and a co-catalyst with titanates[4,6]. For instance, Srinivasan and Miyauchi[4]reported a chemically stable WO3based thin-film for visible-light induced oxidation,revealing the TiO2layer coated on WO3plays a key role. Furthermore, some researchers begin to construct more complex ternary photocatalytic systems. Lai[12]realized the scalable one-step assembly of an inexpensive photoelectrode for water oxidation by deposition of a Ti- and Ni- conaining precursor on the nanostructured WO3. The nano WO3|TiNi electrode showed enhanced water-oxidation catalysis.

Based on the above reports, the ternary system TiO2-NiO-WO3shows a great potential in con- structing photocatalysis systems. However, the subsolidus phase relation of this ternary system is poorly reported before, which might hamper the further exploration and a better understanding of this system. Therefore, we chose to clarify the phase relations of TiO2-NiO-WO3system and hoped to explore novel photocatalysts.

1.1 TiO2-NiO system

In TiO2-NiO system, three compounds NiTiO-3[13], Ni-2.62Ti0.69O4[14]and Ni-2TiO4[15]were reported. The space groups and lattice parameters of the former two compounds are listed in Table 1. No lattice data of Ni2TiO4was provided and according to Datta and Roy[15], it could not be obtained directly by heating the constituent oxides from 650 to 1600 ℃.

The phase diagram of this system was also determined by Laqua[16], Armbruster[17]and Muan[18]. According to Laqua[16], the ilmenite-type compound NiTiO-3is the only stable binary phase from 1300 to 1600 ℃. Also, the Ni3TiO5described elsewhere was identified as a single-phased NiO-TiO2solid solution with wide ranged homogeneity. Armbruster[17]obtained stable ranges of the rutile TiO2, the ilmenite-type NiTiO3, Ni2(1+x)Ti1-xO4(≥ 0.16), a cation-excess spinel and a rocksalt structure type Ni1-2xTiO by quen- ching experiments between 1000and 1600 ℃. Muan[18]studied the system in the temperature range of 1300～1750 ℃, discovering the spinel- type phase presenting above 1430 ℃ would de- compose with decreasing the temperature to a mixture of ilemenite-type NiTiO3, periclase-type NiO and remnant spinel. Also, spinel, NiO and liquid coexist in equilibrium at 1730 ℃, so as the spinel, NiTiO3and liquid at 1613 ℃ and the rutile, NiTiO3and liquid at 1570 ℃.

1.2 TiO2-WO3 system

In TiO2-WO3system, no compound was reported. The phase diagram was studied by Chang[19]in the temperature range of 1000～1700 ℃. No intermediate compounds were found in this simple binary system.

1.3 NiO-WO3 system

In the NiO-WO3system, only one compound NiWO4was reported and its space group and lattice parameters are shown in Table 1. The phase diagam was studied by Jacob[20], showing NiWO4is the only binary oxide in this system. The space group and lattice parameters of this only compound are shown in Table 1.

Table 1. Crystallographic Data of Compounds for the TiO2-NiO-WO3

1.4 TiO2-NiO-WO3 system

In the TiO2-NiO-WO3system, no ternary com- pound was reported.

2 EXPERIMENTAL

A series of samples with different compositions of TiO2-NiO-WO3ternary systems were all pre- pared by solid-state chemistry reaction in air. The purity of the starting materials TiO2, NiO and WO3(Sinopharm Chemical Reagent Co. Ltd.) is higher than 99.9%. The raw powders with certain chemical compositions were mixed thoroughly, ground in an agate mortar, and pressed into pellets with diameter of 10 mm and thickness of 1～2 mm at a pressure around 108Pa. Then the pellets were sintered for about 72～96 h between 1000and 1150 ℃according to different compositions and quenched at atmosphere. The above process should repeat several times until the X-ray pattern of the specimen showed no change upon successive heat treatment, which represented the equilibrium was achie--ved.

Phase identification of the samples was carried out on a PANalytical X’Pert Pro diffractometer with Curadiation (45 kV×40 mA) using continuous mode at a rate of 2=4 °/min for routine phase identification.

3 RESULTS AND DISCUSSION

3.1 Binary systems

The phase relations of the three binary systems, TiO2-NiO, TiO2-WO3and NiO-WO3, were inves- tigated. The solid solubility ranges of all single-phases in this system were determined by X-ray diffraction pattern using the phase- disap- pearing method and comparing the shift of the X-ray diffraction pattern of the samples near the compositions of the binary phases. The results showed that the diffraction patterns did not shift and the second phase could easily be detected when the composition of the samples deviated from its single-phases region by 1.0at.%. Therefore, none of these phases in this system revealed a remarkable homogeneity range. The relative compositions of the samples are shown in Table 2.

Table 2. List of Phase Identification for Samples with Different Composition in the System TiO2-NiO-WO3

3.1.1 TiO2-NiO system

In the TiO2-NiO system, only one compound NiTiO3was observed in our work which is in accordance with Lerch[13]and Laqua.[16]. The sample was sintered at 1150 ℃ for 72～96 h in air. The results show that NiTiO3belongs to rhombohedral with space group3. Not any other reported compounds were observed in this work. Considering all the other compounds reported were obtained in a temperature zone (above 1300 ℃) much higher than our condition (1000～1150 ℃), the result is in good agreement with the previous research[16-18].

3.1.2 TiO2-WO3system

In the TiO2-WO3system, all the samples were sintered at 1000 ℃ for 72～96 h. No new compound was found in our work. The result is consistent with the previous reports[19].

3.1.3 NiO-WO3system

In the NiO-WO3system, only one compound NiWO4was observed by sintering at 1000 ℃ in our work, agreeing with the results from Jacob[20]. Compound NiWO4belongs to a monoclinic system with space group2/. The results agree with the previous research by Weitzel[21].

3.2 TiO2-NiO-WO3 ternary systems

3.2.1 Ternary compound

No ternary compound in the TiO2-NiO-WO3system was obtained in this work.

3.2.2 Subsolidus phase relations of the TiO2-NiO-WO3system

Based on the results of XRD phase identification of 30 samples, the subsolidus phase relation of the TiO2-NiO-WO3system is shown in Fig. 1. Two binary compounds: NiTiO3and NiWO4, exist in the TiO2-NiO-WO3system without ternary compound. The subsolidus phase relation of the ternary system consisting of two tie lines, NiWO4-TiO2and NiWO4-NiTiO3, is divided into three 3-phase regions: (I) NiO-NiTiO3-NiWO4, (II) TiO2- NiTiO3-NiWO4and (III) TiO2-WO3-NiWO4.

Fig. 1. Subsolidus phase relations of the TiO2-NiO-WO3system. (●) Single-phase samples, (■) two-phase samples and (△) three-phase samples

4 CONCLUSION

The subsolidus phase relation of the TiO2- NiO-WO3ternary system was determined. There are two binary compounds and three ternary-phase regions in this system. No ternary compound was found in this system.

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13 January 2014;

9 April 2014

① This work was supported by the National Natural Science Foundation of China (51302261, 51302262, 21103191 and 21203198), the Natural Science Foundation of Fujian province (2013J05037, 2011J01335 and 2012J05033), and the NNSF Outstanding Youth Fund (21125730)

. E-mail: yqchen@fjirsm.ac.cn and dgchen@fjirsm.ac.cn