Copper-Iron Oxide: A Highly Effective Photocatalyst Than TiO2 Prepared by One-Step Sparking Process

Arisara Panthawan Chiang Mai University Nidchamon Jumrus Chiang Mai University Panupong Sanmuangmoon Chiang Mai University Winai Thongpan Chiang Mai University Tewasin Kumpika Chiang Mai University Wattikon Sroila Chiang Mai University Ekkapong Kantarak Chiang Mai University Adisorn Tuantranont National Science and Technology Development Agency Pisith Singjai Chiang Mai University Wiradej Thongsuwan (  wiradej.t@cmu.ac.th ) Chiang Mai University


Introduction
Photocatalysis is a green technology for environmental puri cation, in particular the decomposition of organic pollutants [1][2][3]. Over the last decade, many researchers have been reported that n-type semiconductor materials successfully photodegraded organic pollutants based such as titanium dioxide (TiO 2 ) [4][5][6][7]. Anywise, n-type semiconductors are still limited due to their large forbidden bands, low quantum yields, and unsuitable conduction band edges [8][9]. Thus, p-type semiconductors have been developed to expand the eld of photocatalysis [10]. Copper (Cu) and iron (Fe) oxide are p-type semiconductors that can exhibit much more excellent properties in many applications [10][11][12][13].
Generally, the combinations of metal oxide can produce a novel compound which might improve their physical, chemical, optical and electrical properties, such as Cu-Fe oxides [14][15][16][17]. However, the report about Cu-Fe oxides in the eld of photocatalysis is infrequently found. In this work, we aim to synthesize novel Cu-Fe oxides composite lms by a one-step sparking process. This process has been developed in our lab which can prepare small, uniform particles, high porous lms, and determine the composite ratio [18][19][20][21][22][23][24][25][26]. Moreover, the sparking process requires neither complicated steps nor special equipment, cheap, fast, and non-toxic. Surface morphology, chemical and optical properties of the as-deposited composite lms were improved by heat treatment. The effect of heat treatment on morphology, chemical and optical properties were reported and discussed. Furthermore, the photocatalytic activity under visible light between Cu-Fe oxides and TiO 2 lms was examined and compared.

Experimental Details
The experiment was carried out using a high DC voltage of 2.0 kV applied to Fe tips (0.25 mm, purity

Results And Discussion
The effect of Cu:Fe ratio and annealing temperature on MB degradation have shown in Fig. 1a and 1b.
Moreover, the annealed TiO 2 at 500°C and 700°C which were prepared by the sparking process were used to compare MB degradation with the Cu-Fe oxide lm at 700°C against irradiation time, as shown in Fig. 1c. It is noted that the annealed Cu-Fe oxide lm at 700°C with the ratio of 2:2 has the highest degradation performance than the well-known photocatalyst such as TiO 2 . Thus, a new nding of Cu-Fe oxide which was used as photocatalyst is a strong point of this work. Comparison of the phase volumes of the annealed Cu-Fe oxide lm at 600, 700, 800 and 900°C which evaluated by XPS (data not shown) are shown in Fig. 3a. It is noted that the CuFe 2 O 4 and CuFeO 2 were increased with increasing the annealing temperature [28]. However, an exceed CuFeO 2 at the annealing temperature higher than 700°C might inhibit the photocatalytic reactions. This is due to the factors causing the thin lms to have an increased energy gap when the temperature is higher than 700°C because of the effects of % rate of the crystal structure, microstructure characteristics, and the characteristics of chemical compositions [29]. Figure 3b shows the energy band gap (E g ) of the as-deposited, the annealed Cu-Fe oxide lms at 500, 600, 700, 800 and 900°C which are 5.35 eV, 3.88 eV, 2.89 eV, 2.56 eV, 2.94 eV and 5.63 eV, respectively. This behavior can be described by an atom distancing increased with the increasing of annealing temperature [30]. Interestingly, the annealing at 700°C not only show lowest E g but also show highest photocatalytic activity (see Fig. 2b). This is because the good mixing ratio between Cu and Fe oxide phase [31].
Increasing of photocatalytic activity in the annealed Cu-Fe oxide lm at 700°C with a ratio of 2:2 can be described by Cu-Fe mixed phase mechanism, as shown in Fig. 4. The generation of photocatalytic mechanism is based on pairs of electrons (e − ) and holes (h + ) over the composites [32]. The E VB of CuO, γ-

Conclusions
A novel photocatalyst Cu-Fe oxide lms was successfully prepared by one-step sparking process. The optimum ratio of Cu:Fe and annealing temperature for MB degradation were 2:2 and 700°C. The results show the Cu:Fe ratio has direct affect to photocatalytic activity. Furthermore, the annealing temperature not only affect to the surface morphology but also affect to the E g and photocatalytic activity. A new nding of this work is the higher performance photocatalyst than TiO 2 which can be developed and used for the phtocatalytic applications in the future.     schematic diagram for photocatalytic mechanism of the optimum condition.