In recent years, a considerable interest has been focused on fabrication of heterojunctions, which are able to provide better photocatalytic activity, especially p- and n-type semiconductors, when in contact, form a junction with a space-charge region at their interfaces [1].
Our goal was to synthesize environmentally friendly double p-n heterostructure of spinel type mixed Zn-Co coatings to investigate the photoelectrochemical properties and to test as prospective materials to catalyze oxygen evolving reaction (OER).
Mixed Zn-Co oxide layers (Zn:Co 1,25:1) were formed using two-step synthesis: chemical precipitation and thermal treatment (400 – 600 °C). Firstly, 0.5 M zinc nitrate and 0.5 M cobalt nitrate solutions were mixed by adding slowly 200 mL of 1 M NaOH and under continuous stirring (Fig. 1). Precipitates were annealed at 400 – 600 °C for 2 h in air atmosphere.
Photoelectrochemical activity of prepared mixed Zn- Co oxide layers were explored in a quartz cell in phosphate-bufer solution (pH 7) using the General Electric F8W/BLB lamp, which λmax=366 nm and average power density is 1.8 mW cm-2, as a source of UV radiation. UV source was placed 2 cm from working electrode. IPCE value of a photoelectrode was calculated using the following equation [3].
$$IPCE(\%) = 100 \frac{1240 j_{\text{ph}}}{\lambda P} \tag{1}$$Where jph is photocurrent density in mA cm-2, λ is a wavelength of incident light in nanometers (here 366 nm), P is incident light intensity in mW cm-2.
Applied bias photon-to-current efficiency (ABPE) calculations were used in order to characterize photo-response efficiency of photoelectrode material under applied voltage according to the following equation [2]:
$$ABPE(\%) = 100 \frac{|j_{\text{ph}}| \cdot (1.23 - |V_{\text{appl}}|)}{I_0} \tag{2}$$Here jph is photocurrent density (mA cm-2), I0 is input intensity of incident light falling on a surface of photoelectrode (1.8 mW cm-2) and Vapp is applied potential, which is calculated according to the equation [1]:
$$V_{\text{appl}} = V_{\text{mea}} - V_{\text{oc}} \tag{3}$$Here Vmea is working electrode potential at which photocurrent was measured under illumination and Voc is working electrode potential at open circuit condition under similar conditions.