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

Abstract: ZnGa 2 O 4 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 ZnGa 2 O 4 powder prepared by traditional coprecipitation, but disappears in ZnGa 2 O 4 nano crystal prepared by spraying coprecipitation. SEM and TEM were used to analysis the structural characteristics of ZnGa 2 O 4 nano crystals. The gas sensitive characteristics of ZnGa 2 O 4 nano crystals are reported. Keywords: Coprecipitation, Nano crystals, Spray, Gas sensitive. Introduction ZnO and Ga 2 O 3 have been used as gas sensors for decades due to their high sensitivity. Although the ZnO-Ga 2 O 3 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 [1-5], with the materials recently investigated for application to vacuum fluorescent displays (VFDs) [6]. Bulk single crystals of ZnGa


Introduction
ZnO and Ga 2 O 3 have been used as gas sensors for decades due to their high sensitivity.Although the ZnO-Ga 2 O 3 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 ZnGa 2 O 4 spinel, with an edge length of up to 10 mm, have been successfully synthesized by slowly cooling a PbF 2 -free, PbO-B 2 O 3 [6].Satyanarayna and Reddy have prepared ZnGa 2 O 4 powders [7] via a thick film preparation technique using a paste composed of ZnGa 2 O 4 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 ZnGa 2 O 4 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 ZnGa 2 O 4 powder prepared by traditional coprecipitation, but disappears in ZnGa 2 O 4 nano crystals prepared by spraying coprecipitation.The gas sensitive characteristics of ZnGa 2 O 4 nano crystal were tested.

Experimental
Ga 2 (SO 4 ) 3 ⋅18H 2 O and ZnCl 2 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 Zn 2+ and Ga 2+ ) 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.ZnGa 2 O 4 powder is used to fabricate a gas sensor in the following way.A total of 2.5wt.%polyvinyl 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 .According to the report of Nakatani et al. [8], the small grain size plays an important role in gassensing properties.Therefore, the thick film sensors based on ZnGa 2 O 4 prepared by spray coprecipitation, which have a larger specific surface area, will have excellent gas sensitivity.

Results and Discussion
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.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 O 2 -, 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 CH 4 and CO, it can be inferred that the conductance of nanocrystalline ZnGa 2 O 4 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
ZnGa 2 O 4 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 ZnGa 2 O 4 powder prepared by traditional coprecipitation, they disappear in ZnGa 2 O 4 nano crystals prepared by spray coprecipitation.SEM and TEM were used to analyze the structural characteristics of ZnGa 2 O 4 nano crystals.The spray coprecipitation fabrication route was presented, and the gas sensitive characteristics of ZnGa 2 O 4 reported.

Figure 3 .
Figure 3. SEM and TEM image of ZnGa2O4 powder.

Figure 6 .
Figure 6.Resistance stability of ZnGa 2 O 4 gas sensor measured in air.

Fig. 7
Fig.7shows relationships between gas sensitivity and working temperature of a ZnGa 2 O 4 gas sensor exposed to a 500ppm LPG atmosphere.Maximum sensitivity to liquid petroleum gas (LPG) is achieved at an operating temperature of 410°C.

Fig. 9
Fig.9shows the time dependent response of a ZnGa 2 O 4 gas sensor to LPG.The response time is on the order of a few seconds, with a recovery time of approximately 60s.

Figure 9 .
Figure 9. Response of a ZnGa 2 O 4 gas sensor to a nitrogen/ 500 ppm CO/nitrogen environment.