The Preparation of ZnGa₂O₄ Nano Crystals by Spray Coprecipitation and Its Gas Sensitive Characteristics

Abstract

ZnGa₂O₄ nano crystals were prepared by an improved coprecipitation method, which we call ‘spray coprecipitation’. XRD results shows the resulting crystal size using the new method is under 10nm, whereas the powder prepared by ordinary coprecipitation is about 30nm. XRD results also shows ZnO peaks exists in ZnGa₂O₄ powder prepared by traditional coprecipitation, but disappears in ZnGa₂O₄ nano crystal prepared by spraying coprecipitation. SEM and TEM were used to analysis the structural characteristics of ZnGa₂O₄ nano crystals. The gas sensitive characteristics of ZnGa₂O₄ nano crystals are reported.

Introduction

ZnO and Ga₂O₃ have been used as gas sensors for decades due to their high sensitivity. Although the ZnO-Ga₂O₃ system offers a great potential for sensing applications there is a relative dearth of literature on the subject. Recent publications report the preparation of the films of spinel zinc gallate [1,2,3,4,5], with the materials recently investigated for application to vacuum fluorescent displays (VFDs) [6]. Bulk single crystals of ZnGa₂O₄ spinel, with an edge length of up to 10 mm, have been successfully synthesized by slowly cooling a PbF₂-free, PbO-B₂O₃ [6]. Satyanarayna and Reddy have prepared ZnGa₂O₄ powders [7] via a thick film preparation technique using a paste composed of ZnGa₂O₄ powder and a polyvinyl alcohol solution; the resulting films were then investigated establishing a relationship between method of synthesis, crystal structure and the gas sensing properties.

In this paper ZnGa₂O₄ nano crystals were prepared by an improved spray coprecipitation method. XRD results shows the crystal size is under 10nm, whereas the powder prepared by ordinary coprecipitation is about 30nm. XRD results also shows ZnO peaks exists in ZnGa₂O₄ powder prepared by traditional coprecipitation, but disappears in ZnGa₂O₄ nano crystals prepared by spraying coprecipitation. The gas sensitive characteristics of ZnGa₂O₄ nano crystal were tested.

Experimental

Ga₂(SO₄)₃·18H₂O and ZnCl₂ were taken in a Zn:Ga=1:2 mole ratio and dissolved in de-ionized water. The mixture was coprecipitated with urea under constant stirring. The precipitate was washed several times. Finally the mixture was dried, then calcined at 750°C for 6 hours.

In order to reduce particles size and improve size distribution, we developed a spraying coprecipitation method. Fig.1 shows a schematic drawing of the spraying coprecipitation apparatus. In the scheme, compressed air drives reactants (ammonia and reagent containing Zn²⁺ and Ga²⁺) quickly through the pipeline. Passed through respective flowmeters, the reactants mix and react in the tee junction. The resulting mixture in then sprayed into a beaker. Typical experimental settings in spraying coprecipitation method are as follows: diameter of pipe 4mm, flow rate 10m/s.

Figure 1. Scheme of spraying coprecipitation.

Figure 1. Scheme of spraying coprecipitation.

ZnGa₂O₄ powder is used to fabricate a gas sensor in the following way. A total of 2.5wt.% poly-vinyl alcohol(PVA) was used as a binder to form a paste and the materials were then coated onto aluminum tube substrates provide with platinum wire electrodes for electrical contacts. Finally the sensor is sintered at 650°C for 2 hours to make it rigid and impart ceramic properties. The resistance of sensor element was measured in the presence and absence of test gases. The sensitivity, S is defined as the ratio of resistance of the sensor in air, R_a, to resistance of the sensor in the presence of gases, R_g.

Results and Discussion

Fig.2 is XRD pattern of the resulting powder. The powder is spinel structure ZnGa₂O₄, JCPDS file 3-1155. The ZnGa₂O₄ in Fig.2(a) is prepared by coprecipitation, and Fig.2(b) by spray coprecipitation.

Figure 2. XRD pattern of ZnGa₂O₄ powder.

Figure 2. XRD pattern of ZnGa₂O₄ powder.

In Fig.2(a) ZnO Peaks can be found, while no Ga₂O₃ peaks exist, which means a small quantity of ZnO exists outside the crystal lattice of ZnGa₂O₄. While in Fig.2(b) no ZnO peaks exists, all Zn²⁺ are in ZnGa₂O₄ crystals. Peaks in Fig.2(b) are wider than those in Fig.2(a); the crystallite sizes were evaluated from the line width. In Fig.2(a) the average size of ZnGa₂O₄ particles was 30nm while in Fig.2(b) the average size was less than 10nm. These results demonstrate that spray coprecipitation produces smaller particle sizes and purer nano powders.

SEM and TEM images of ZnGa₂O₄ powder are shown in Fig.3. Fig.4 is an electron diffraction image of ZnGa₂O₄. Comparing Fig3(a) and Fig.3(b), It can be found that the ZnGa₂O₄ particles prepared by spray coprecipitation are smaller and more uniform. The particle size in Fig3(a) is about 20nm, while in Fig.3(b) 10nm. In Fig.3(c) there are many particles less than 5nm. In Fig.4 the ZnGa₂O₄ powder is crystalline.

Figure 3. SEM and TEM image of ZnGa2O4 powder.

Figure 3. SEM and TEM image of ZnGa2O4 powder.

Figure 4. Electron diffraction image of ZnGa₂O₄.

Figure 4. Electron diffraction image of ZnGa₂O₄.

According to the report of Nakatani et al. [8], the small grain size plays an important role in gas-sensing properties. Therefore, the thick film sensors based on ZnGa₂O₄ prepared by spray coprecipitation, which have a larger specific surface area, will have excellent gas sensitivity.

The adsorption activation energy of surface oxygen can be determined from the slope of the conductance-temperature line seen in Fig.5, which is 1.02eV.

Figure 5. Conductance-temperature relationship of ZnGa₂O₄ gas sensor.

Figure 5. Conductance-temperature relationship of ZnGa₂O₄ gas sensor.

Fig.6 shows the change in electrical resistance with time; as indicated by the figure ZnGa₂O₄ gas sensors have excellent stability.

Figure 6. Resistance stability of ZnGa₂O₄ gas sensor measured in air.

Figure 6. Resistance stability of ZnGa₂O₄ gas sensor measured in air.

Fig.7 shows relationships between gas sensitivity and working temperature of a ZnGa₂O₄ gas sensor exposed to a 500ppm LPG atmosphere. Maximum sensitivity to liquid petroleum gas (LPG) is achieved at an operating temperature of 410°C.

Figure 7. Relationships between gas sensitivity and working temperature of ZnGa₂O₄ gas sensor (500ppmLPG).

Figure 7. Relationships between gas sensitivity and working temperature of ZnGa₂O₄ gas sensor (500ppmLPG).

Figure 8. Sensitivity of ZnGa₂O₄ gas sensor to different gases.

Figure 8. Sensitivity of ZnGa₂O₄ gas sensor to different gases.

Fig.8 shows change in sensitivity of a ZnGa₂O₄ gas sensor to different gases. Obviously, the element is more sensitive to LPG than to CO, C₂H₅OH and CH₄. When LPG concentration is 500ppm, the sensitivity to LPG already reaches 10, while sensitivity to CO, C₂H₅OH and CH₄ are only 1.6, 2.0 and 1.4 respectively.

Fig.9 shows the time dependent response of a ZnGa₂O₄ gas sensor to LPG. The response time is on the order of a few seconds, with a recovery time of approximately 60s.

Figure 9. Response of a ZnGa₂O₄ gas sensor to a nitrogen/ 500 ppm CO/nitrogen environment.

Figure 9. Response of a ZnGa₂O₄ gas sensor to a nitrogen/ 500 ppm CO/nitrogen environment.

Per sensing mechanism, as highlighted by works on metal oxides [9,10,11,12,13], the occurrence of different types of ionosorbed oxygen species like O₂^-, O^-, or O^2- is a function of temperature and prevailing atmospheric conditions. Electron exchange between the test gas and the oxide surface upon adsorption is suggested, i.e. a surface charge layer is formed when the test gas is adsorbed at the oxide surface, so that the bulk charge is generated and the energy band changes near its surface. For gases such as CH₄ and CO, it can be inferred that the conductance of nanocrystalline ZnGa₂O₄ elements will change greatly when reducing gases are allowed to react with the surface-adsorbed species releasing electrons. Consequently, the selectivity and sensitivity of the sensors can be controlled via modification of the conditions at which the reactions take place on the surface [14,15,16].

Conclusions

ZnGa₂O₄ nano crystals were prepared by spray coprecipitation. XRD results show the crystal size is under 10nm, whereas the powder prepared by ordinary coprecipitation is about 30nm. XRD results show that while ZnO peaks exist in ZnGa₂O₄ powder prepared by traditional coprecipitation, they disappear in ZnGa₂O₄ nano crystals prepared by spray coprecipitation. SEM and TEM were used to analyze the structural characteristics of ZnGa₂O₄ nano crystals. The spray coprecipitation fabrication route was presented, and the gas sensitive characteristics of ZnGa₂O₄ reported.