Study on the Visible-Light Photocatalytic Performance and Degradation Mechanism of Diclofenac Sodium under the System of Hetero-Structural CuBi2O4/Ag3PO4 with H2O2

Two kinds of CuBi2O4/Ag3PO4 with different heterojunction structures were prepared based on the combination of hydrothermal and in-situ precipitation methods with surfactant additives (sodium citrate and sodium stearate), and their characteristics were systematically resolved by X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscope (SEM)/ High-resolution Transmission Electron Microscopy (HRTEM), UV-vis Diffuse Reflectance Spectra (DRS) and Photoluminescence (PL). Meanwhile, the photocatalytic properties of the catalysts were determined for diclofenac sodium (DS) degradation and the photocatalytic mechanism was also explored. The results indicate that both of the two kinds of CuBi2O4/Ag3PO4 exhibit higher photocatalytic efficiency, mineralization rate, and stability than that of pure CuBi2O4 or Ag3PO4. Moreover, the catalytic activity of CuBi2O4/Ag3PO4 can be further enhanced by adding H2O2. The free radical capture experiments show that in the pure CuBi2O4/Ag3PO4 photocatalytic system, the OH• and O2•− are the main species participating in DS degradation; however, in the CuBi2O4/Ag3PO4 photocatalytic system with H2O2, all OH•, h+, and O2•− take part in the DS degradation, and the contribution order is OH• > h+ > O2•−. Accordingly, the photocatalytic mechanism of CuBi2O4/Ag3PO4 could be explained by the Z-Scheme theory, while the catalysis of CuBi2O4/Ag3PO4 with H2O2 follows the heterojunction energy band theory.


Introduction
Diclofenac sodium (DS) is a typical non-steroidal anti-inflammatory drug [1][2][3]. The usage of DS in China amounts to thousands of tonnes every year. Because of the strong water solubility of DS and its poor absorption by organisms, most of the DS intake by human and animals is excreted through feces and urine in the form of parent or active metabolites, finally flowing into the aquaculture wastewater and municipal wastewater [4]. Numerous studies have indicated that only less than 20% of DS can be removed from the traditional sewage treatment plant. Therefore, the DS, which enters the environment with effluent water, not only causes a serious poisoning effect on aquatic organisms, but also poses a significant threat to other living beings and human health through food HNO 3 . Next, 20 mL of Cu(NO 3 ) 2 ·3H 2 O (0.3382 g) was added into the above solution mechanically and agitated for 30 min. Then the precipitator NaOH (1.2 M, 20 mL) was added to the reaction system drop by drop. After the solution was diluted to 70 mL, the mixture was transferred into a sealed Teflon-lined stainless steel autoclave of 100 mL and reacted at 100 • C for 24 h under autogenous pressure. The precipitates were isolated by centrifugation when the autoclave was cooled naturally to room temperature; finally, the solids were washed several times with distilled water and dried at 60 • C for 24 h.
The CuBi 2 O 4 /Ag 3 PO 4 composites with different hetero-structures were prepared through an in-situ deposition process by adding different surfactants of sodium citrate or sodium stearate into the preparation systems. The detailed preparation process is as following. An amount of 0.1 g of CuBi 2 O 4 prepared above was first dispersed in 40 mL of ultrapure water and ultrasonically treated at 100 W for 15 min, then 10 mL of sodium citrate (or sodium stearate) solution was added into the reaction system and mechanically agitated for 2 h. After that, 10 mL of AgNO 3 solution was added into the mixture. After further stirring for 30 min, 20 mL of Na 2 HPO 4 ·12H 2 O was added dropwise into the system. The precipitate was isolated and washed several times with absolute ethanol and distilled water, then dried at 60 • C overnight. According to the added surfactants of sodium citrate or sodium stearate, two kinds of CuBi 2 O 4 /Ag 3 PO 4 can be obtained, and the products can be respectively named as CuBi 2 O 4 /Ag 3 PO 4 -SC (wt:wt = 3:7) and CuBi 2 O 4 /Ag 3 PO 4 -SS (wt:wt = 1:1). As a comparison, the pure Ag 3 PO 4 with the additives of sodium citrate or sodium stearate are recorded as Ag 3 PO 4 -SC and Ag 3 PO 4 -SS, respectively.

Characterization
Powder X-ray diffraction (XRD, D/MAX-2500/PC) was used to examine the crystalline phase of the products, and each sample was scanned through a 2θ range of 10-90 • at a rate of 4 • /min. The nitrogen adsorption-desorption isotherms of the photocatalysts were obtained using an NOVA-2200e volumetric analyzer (Quantachrome, Boynton Beach, FL, USA), and the surface areas of the samples estimated by the BET model. The morphology of the samples was obtained by scanning electron microscopy (SEM, JSM-6610LV, JOEL, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai G 2 F20 S-TWIN, FEI, Hillsboro, OR, USA). The ultraviolet-visible diffuse reflectance spectra were obtained using a scanning UV-Vis spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) within a range of 200-800 nm. Additionally, the X-ray photoelectron spectra (XPS, ESCALAB 250, Thermo Scientific, Waltham, MA, USA) were recorded with Al Kα radiation to investigate the content of Ag 0 in the fresh and reused catalysts. The photoluminescence (PL) spectroscopy was performed using a florescence spectrophotometer (FP-6500, JASCO, Oklahoma City, OK, USA).

Photocatalytic Performance Measurement
The photocatalytic activity of the as-prepared materials was evaluated for DS degradation, and the reaction apparatus was a photocatalytic reactor (BL-GHX-V, Bilang Biological Science and Technology Co., Ltd., Xi'an, China) using a 300 W Xe lamp with an ultraviolet cutoff filter (providing visible light ≥400 nm) as the light source. In each experiment, 30 mg of photocatalyst was added to a 50 mL DS solution at an initial concentration of 15 mg/L. Prior to illumination, the solution was magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium between the DS and photocatalysts. Then, the solution was exposed to Xe lamp irradiation. At a given time interval of irradiation, one reaction tube was taken out and magnetically separated to remove the catalyst. Finally, the supernatant was withdrawn and filtered with 0.45 µm membrane filters. The concentration of DS and TOC were measured by high efficiency liquid chromatography (HPLC, Agilent 1260, Santa Clara, CA, USA) and a TOC (Shimadzu TOC-V CPH , Kyoto, Japan) analyzer, respectively. To determine the effect of H 2 O 2 on the catalytic performance of CuBi 2 O 4 /Ag 3 PO 4 , different concentrations of H 2 O 2 were added to the reaction system in the early stage, while the other experimental steps were the same as for the photocatalytic activity. In addition, the repeated experiments for DS degradation were also conducted to study the stability of the as-prepared photocatalysts, and the operation processes were also similar to the photocatalytic experiments.

Analysis of Reactive Species
Free radical capture experiments were used to ascertain the reactive species for DS photodegradation, and tert-butanol (t-BuOH) was chose as the hydroxyl radical (OH • ) scavenger, disodium ethylenediamine tetra-acetate (EDTA-Na 2 ) was chose as the hole (h + ) scavenger, benzoquinone (BZQ) was chose as the superoxide radical (O 2 •− ) scavenger. The detailed free radical capture experiment processes were similar to the photocatalytic activity experiments.   The UV-Vis absorption spectra of the as-synthesized samples are shown in Figure 3a. Both pure Ag 3 PO 4 -SC and Ag 3 PO 4 -SS can absorb visible light with wavelengths higher than 500 nm, and pure CuBi 2 O 4 shows the largest absorbing boundary higher than 800 nm. When the heterojunction structure forms between Ag 3 PO 4 and CuBi 2 O 4 , the composites CuBi 2 O 4 /Ag 3 PO 4 -SC and CuBi 2 O 4 /Ag 3 PO 4 -SS also exhibit intense absorption bands in the visible-light region. According to the Kubelka-Munk function [27] and the plot of (αhv) 2 vs. hv (shown in Figure 3b), the band gaps (Eg) of CuBi 2 O 4 , Ag 3 PO 4 -SC, and Ag 3 PO 4 -SS can be estimated as 1.72 eV, 2.38 eV, and 2.42 eV, respectively. Besides, the band-edge potentials of the conduction band (E CB ) and valence band (E VB ) can be designated as [28]:

Characterizations
and where X is the geometric mean of the electronegativity of the constituent atoms (5.96 eV for Ag 3 PO 4 , and 4.59 eV for CuBi 2 O 4 [28,29]), and E c is the energy of the free electrons on the hydrogen scale (approximately 4.5 eV) [28].

Photocatalytic Activity for DS Degradation
To evaluate the photocatalytic activity of the as-prepared catalysts, DS was selected as the target pollutant. Figure 4a shows the degradation efficiency of DS in different photocatalyst systems.
In the system without any other catalysts, the degradation efficiency of DS is 36.87% under 300 min of visible light irradiation. As for the prepared catalysts, their adsorption efficiencies in the dark for DS are 1.76% (CuBi 2 O 4 ), 0.30% (Ag 3 PO 4 -SC), 0.77% (Ag 3 PO 4 -SS), 3.36% (CuBi 2 O 4 /Ag 3 PO 4 -SC), and 6.98% (CuBi 2 O 4 /Ag 3 PO 4 -SS), respectively. This adsorption rule of catalysts is consistent with their specific surface area characteristics, whose data are displayed in Table 1; the specific surface area is an important parameter to evaluate the active adsorption sites of materials. Thus, the relatively lower adsorption efficiencies of as-prepared catalysts toward DS suggests the final removal rate of DS in the catalysts' system is mainly due to the photocatalysis.

Photocatalytic Stability of the Catalysts
The stability of a photocatalyst is an important index to evaluate the value of its research value and actual utilization [30]. Therefore, we investigated the photocatalytic stability of CuBi 2   The instability of the catalysts is mainly caused by the reduction of Ag + from the lattice of Ag 3 PO 4 to generate Ag 0 during the reaction process. In order to analyze the photocatalytic stability of the as-prepared catalysts from the perspective of structure itself, XPS analysis of Ag3d for the fresh and reused catalysts was performed and the results are shown in Figure 6. It can be seen that, except for the Ag + peaks (374.17 eV and 368.16 eV) in the reused catalysts of Ag 3 PO 4 -SC and CuBi 2 O 4 /Ag 3 PO 4 -SC, peaks indicating Ag 0 (374.40 eV and 368.32 eV) [31] are also observed, and the proportion of the peaks relative to Ag 0 occupy 11.38% and 9.52% of the total fitting peak area in the two catalysts. However, no obvious peaks relative to Ag 0 exist in the catalyst of CuBi 2 O 4 /Ag 3 PO 4 -SC when 1.06 mM H 2 O 2 was added into the reaction system. These results imply the enhanced stability of CuBi 2 O 4 /Ag 3 PO 4 in comparison to pure Ag 3 PO 4 , and again the further promotion of H 2 O 2 for the composite's stability.

Photocatalytic Mechanism Discussion
Photoluminescence (PL) emission spectroscopy is important for understanding the photocatalytic mechanism of a catalyst by analyzing the migration and separation efficiency of photogenerated charge carriers in the material. Also, lower PL emission intensity implies a lower electron-hole recombination rate and corresponds to higher photocatalytic activity. Thus in this study, the PL emission spectra of the as-prepared catalysts were recorded under excitation at 500 nm, and the results are shown in Figure 7a. Compared with the pure CuBi 2 O 4 , Ag 3 PO 4 -SC, and Ag 3 PO 4 -SS, the composites CuBi 2 O 4 /Ag 3 PO 4 -SC and CuBi 2 O 4 /Ag 3 PO 4 -SS exhibit decreased emission intensity, suggesting the formation of the heterojunction between CuBi 2 O 4 and Ag 3 PO 4 decreases the recombination rate of electron-hole pairs. Moreover, the PL emission intensity rule of these catalysts is seriously coincident with that of the photocatalytic activity of the as-prepared materials.
Active species are important participants in photochemical reactions and they are also the key to discuss the photocatalytic mechanism. Thus, the free radical capture experiments were used to investigate whether OH • , h + , and O 2 •− are involved in the DS photodegradation process as well as their contribution order, while tert-butanol (t-BuOH), disodium ethylenediaminetetra-acetate (EDTA-Na 2 ), benzoquinone (BZQ) were chosen as the hydroxyl radical (OH • ), hole (h + ), superoxide radical (O 2 •− ) scavengers, respectively. The experiment results are shown in Figure 7b.   [32]. The results of the free radical capture experiment show that the contribution of h + to DS degradation is small in the photocatalytic system using CuBi 2 O 4 /Ag 3 PO 4 as catalyst, indicating that h + is mainly involved in the production of OH • and not involved in the degradation of DS. Therefore, the photocatalytic mechanism of CuBi 2 O 4 /Ag 3 PO 4 is consistent with the Z-scheme theory described by Yu et al. [17,33] But as can be seen from Figure 8b, the photocatalytic mechanism of CuBi 2 O 4 /Ag 3 PO 4 in the presence of H 2 O 2 is more suitable to be explained by the heterojunction band theory [34].

Conclusions
In this paper, two kinds of hetero-structural composite photocatalysts CuBi 2 O 4 /Ag 3 PO 4 were prepared using sodium citrate or sodium stearate as additives. Under the same conditions, both of the two kinds of CuBi 2 O 4 /Ag 3 PO 4 composites showed much improved excellent photocatalytic activity and stability than the single Ag 3  conforms to the Z-scheme theory; while the mechanism changes to heterojunction band theory after the addition of H 2 O 2 .