ZnO nanopowders used in the present work were purchased from Sigma-Aldrich Co. (specific surface area: 15 - 25 m²/g, mean particle size: 50 - 70 nm). The ZnO nanopowders were put into 100 mL of NaBH₄ solution (5 mg/mL), and then 100 mL of metal solution with a given concentration was slowly added into the ZnO suspension dropwise. The ZnO suspension was centrifuged at 4000 rpm for 5 min. After the supernatant solution was removed by the pipette, the metal-doped ZnO was dried in a vacuum drying oven (AS ONE Corp., AVO-200NB) for more than 24 hours.

BPA (10 μg/mL) aqueous solutions were prepared with ultrapure water. Laboratory ultrapure water was obtained from an ultrapure Advantac RDF 260 water system with high purity ion exchanged water (Advantec MFS Inc., Tokyo, Japan) resulting in a resistivity > 18 MΩ∙cm. Then, a 30 mL aqueous solution containing 10 μg/mL (0.044 μM) BPA was put into a Pyrex glass reaction vessel (30 mL capacity). The photocatalyst powder was added into the solution to produce a 1.67 mg/mL of suspension concentration. The experimental conditions for the degradation treatment were shown in Table 1.

A xenonlamp (500 W, UXL-500D-O) was applied as light source, which was positioned on the side of photoreactor. The IR cutoff filter (>750 nm, HA-50, HOYA) was used for the ultraviolet and visible light illumination. The light intensity was measured by a UV radio meter with a sensor of 320 - 410 nm wavelengths (UVR-400, Iuchi Co., Osaka, Japan), and the value was 0.055 mW/cm². The photocatalysts were continuously dispersed in the aqueous solution by a magnetic stirrer during the irradiation.

After the illumination, the photocatalyst was separated by centrifugation, and the BPA concentration in the solution was determined by high-performance liquid chromatograph with TSK gel ODS-100V column (Tosoh Corp.) The elution was monitored at 276 nm. The eluent used was a mixed solvent of acetonitrile and water (1:1, v/v). The flow rate of the mobile phase was 1.5 mL/min.

First, the effect of metal doping into the photocatalyst was investigated on the photocatalytic degradation of BPA in water with ZnO powders. The Au, Ag, and Cu-doped ZnO were evaluated in the present work. The results are shown in Figure 1. The degradation efficiencies with gold and copper-doped ZnO were worse compared with those obtained with bare ZnO. However, the silver-deposited ZnO was very effective for the improvement of photocatalytic degradation of BPA in aqueous solution. BPA was completely degraded by the photocatalytic degradation with bare ZnO on 60 min of irradiation time; on the other hand, the BPA disappeared on the 45 min irradiation.

Since it was found that the silver deposition into ZnO photocatalyst was very effective for improvement of photocatalytic degradation of BPA in aqueous solution, the silver doping concentration with 1 wt% to 5 wt% was evaluated for the BPA degradation.

The photocatalytic degradation of various organic compounds by means of illuminated ZnO can be estimated to follow a pseudo first-order kinetic law, according to the equation,

where C_subs is the BPA concentration and k_obs is the observed first-order rate constant. It was concluded in the previous work that the primary photocatalytic decomposition reaction follows a pseudo first-order kinetic laws. Therefore, in order to estimate the pseudo first-order rate constant, Ln (C/C₀) was plotted as a function of the illumination time. Figure 2 shows the comparison between bare ZnO and silver deposited-ZnO (3 wt% and 4 wt%) The value k_obs in the case of photocatalyst with bare ZnO was 0.044 min⁻¹. However, the value k_obs with silver-deposited ZnO was 0.058 and 0.047 min⁻¹ for 3 wt% and 4 wt% doping amount, respectively. Therefore, the pseudo first-order rate constant using 3 wt% Ag/ZnO was 1.3 times larger relative to bare ZnO. The first- order rate constant with 4 wt% Ag/ZnO was worse relative to that with 3 wt% Ag because the photocatalytic activity became poor in the large coverage of silver on the ZnO surface.

In the semiconductor Ag/TiO₂ photocatalyst with a band gap E_g, upon the illumination with radiation having an energy greater relative or equal to the band gap E_g, the promotion of an electron (e⁻) from the valance band (VB)

to the conduction band (CB) takes place. Concomitantly, the formation of a positive hole (h⁺) in the VB occurs. Photogenerated electrons and holes can either undergo undesired recombination/or migrate to the surface of the system, where they can initiate reactions with adsorbed species. Whereas holes in the VB are powerful oxidizing species that can produce hydroxyl radicals (•OH) from the reaction with H₂O, photogenerated electrons in the CB are involved in the formation of •OOH. These oxidizing species may attack the pollutant BPA in the aqueous solution. The presence of the Schottky barrier can decrease the recombination of photogenerated electron-hole pairs consequently prolong their lifetime, and greatly enhance the photocatalytic activity of ZnO. Silver loading can result in stronger Schottky barrier effect, and therefore shows better photocatalytic activity of ZnO, as illustrated in Figure 3 [9] .