Temperature-Program Assisted Synthesis of Novel Z-Scheme CuBi₂O₄/β-Bi₂O₃ Composite with Enhanced Visible Light Photocatalytic Performance.

Novel Z-Scheme CuBi₂O₄/β-Bi₂O₃ composite photocatalysts with different mass ratios and calcination temperatures were firstly synthesized by the hydrothermal method following a temperature-programmed process. The morphology, crystal structure, and light absorption properties of the as-prepared samples were systematically characterized, and the composites exhibited enhanced photocatalytic activity toward diclofenac sodium (DS) degradation compared with CuBi₂O₄ and β-Bi₂O₃ under visible light irradiation. The optimal photocatalytic efficiency of the composite, achieved at the mass ratio of CuBi₂O₄ and β-Bi₂O₃ of 1:2.25 and the calcination temperature of 600 °C is 92.17%. After the seventh recycling of the composite, the degradation of DS can still reach 82.95%. The enhanced photocatalytic activity of CuBi₂O₄/β-Bi₂O₃ is closely related to OH•, h⁺ and O₂•-, and the photocatalytic mechanism of CuBi₂O₄/β-Bi₂O₃ can be explained by the Z-Scheme theory.

1. Introduction

Diclofenac sodium (n class="Chemical">DS) is a very effective anti-inflammatory drug widely used in many countries [1,2]. Due to the poor absorbability of organisms to DS, most of the DS will be released into the environment after it is taken [1]. However, long-term exposure to a DS-polluted environment will cause serious risk to ecosystems and human health [3,4]. Therefore, effective degradation technologies for DS should be explored to control its persistent contamination.

Semiconductor photocatalytic technology has aroused widespread interest in n class="Chemical">organic pollution control [5,6,7,8] due to its advantages of rich catalysts, mild reaction conditions, fast reaction speed, and no secondary pollution. In past decades, titanium dioxide (TiO2) has attracted considerable attention for its low cost and chemical inertness [8,9,10]. However, the large band gap of TiO2, which is approximately 3.2 eV, limits its absorption of sunlight to the ultraviolet (UV) region [10,11]. Because the visible light occupies 43% of sunlight, whereas UV light occupies only 4%, the development of visible-light responsive semiconductor photocatalysts is becoming a research hotspot [12,13,14,15].

CuBi2O4 possesses a strong visible light response, excellent chemical stability, high conduction band position, and strong reducing ability [16,17,18]. However, the single n class="Chemical">CuBi2O4 exhibits poor photocatalytic performance [17]. Some related studies suggest its activity could be improved through the synergistic effect with other metal oxide semiconductors [17,19,20]. Bi2O3, a visible light-responsive semiconductor material, is regarded as a promising photocatalyst for organic pollutant degradation [21,22]. This is mainly due to that the energy band gap (in the range from 2.4 to 3.2) of Bi2O3 can be tuned by employing different synthesis techniques, such as sol gel, hydrothermal/solvothermal and solid-state decomposition methods [21,23,24]. β-Bi2O3, as one of the six different polymorphs of Bi2O3 (i.e., α-, β-, γ-, δ-, ω- and ε-polymorph), exhibits a narrower band gap with the strongest light-absorption ability than other phases [23,25,26,27]. Yet the photocatalytic activity of pure β-Bi2O3 is still far from satisfactory for the fast recombination of photogenerated charges [26,28].

In theory, CuBi2O4 coupling with β-n class="Chemical">Bi2O3 could form the hybrid composite via matching the band structure of each other with significantly enhanced photocatalytic activity. Nevertheless, the research on the temperature-programmed synthesis and photocatalytic application of a CuBi2O4/β-Bi2O3 system has not been investigated in detail. In this study, novel CuBi2O4/β-Bi2O3 composite photocatalysts with different mass ratios and calcination temperatures were firstly synthesized by the hydrothermal method following a temperature-programmed process. The as-synthesized composites were systematically characterized and their photocatalytic performance was carefully investigated by the degradation of DS under visible-light irradiation (λ > 400 nm). Moreover, the active species in the CuBi2O4/β-Bi2O3 photocatalytic system were discussed through the free radical capture experiments, and the photocatalytic mechanism of CuBi2O4/β-Bi2O3 was also put forward.

2. Materials and Methods

2.1. Materials

All reactants and solvents were analytical grade and used without further purification. Bi(NO3)3·5H2O, Cu(n class="Chemical">NO3)2·3H2O, Na2CO3, NaOH, HNO3, gluconic acid, tert-butanol (t-BuOH), para-benzoquinone (BZQ), disodium ethylenediaminetetr-aacetate (Na2-EDTA), ethanol, and diclofenac sodium were obtained from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Ultrapure water was used throughout this study.

2.2. Synthesis of CuBi2O4/β-Bi2O3

The detailed synthesis pathway of CuBi2O4/β-n class="Chemical">Bi2O3 is illustrated in Figure 1. Firstly, CuBi2O4 was prepared through a simple hydrothermal method, and the detailed experiment processes are similar to that reported by our previous paper [16]. Secondly, a core-shell structural CuBi2O4@C was synthesized, that is, 0.1 g of CuBi2O4 was ultrasonically dispersed into 70 mL of aqueous solution (with 0.3 mL gluconic acid), and then the solution was transferred into a 100-mL sealed Teflon-lined stainless steel autoclave for 4 h at 180 °C. When the autoclave was cooled naturally to room temperature, the precipitate was isolated by centrifugation and washed several times with distilled water. Thirdly, a temperature-programmed method was used to synthesize the CuBi2O4/β-Bi2O3. Then 0.1 g of CuBi2O4@C obtained above was ultrasonically dispersed into 20 mL HNO3 solution (1 mol/L) to obtain solution A; 0.39 mmol Bi(NO3)3·5H2O was completely dissolved into another 20 mL HNO3 solution (1 mol/L) under the acute agitation to obtain solution B; 2.34 mmol Na2CO3 was dissolved into 40 mL ultrapure water to obtain solution C. After solutions A and B were well mixed, solution C was added into the mixture drop by drop, and a large amount of white precipitate was produced. Then the washed precipitate by ethanol and ultrapure water was placed into a temperature-programmed furnace, and the reaction furnace was set to be heated to 600 °C within 30 min and kept at this temperature for 5 h. The CuBi2O4/β-Bi2O3 with mass ratio of 1:2.25 at different calcination temperatures of 400 °C, 600 °C and 800 °C was obtained through changing the temperature of the reaction furnace. In addition, the CuBi2O4/β-Bi2O3 with different mass ratios of 1:1, 1:2.25 and 1:4 was obtained through adjusting the added amount of Bi(NO3)3·5H2O and Na2CO3. For composition, the pure CuBi2O4 and β-Bi2O3 were also prepared through the above synthesis process, and the detailed synthesis steps can also be seen from the Figure 1.

Figure 1

The detailed synthesis pathway of CuBi2O4/β-Bi2O3.

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were examined to study the crystal structure and phase composition of the materials using the D/max 2500 PC (Rigaku, Japan) instrument with Cu Kα radiation (40 kV, 100 mA) at a rate of 4.0°/min over a 2θ range of 20°–60°. Morphologies of the samples were characterized by scanning electron microscopy (SEM, JSM-6360LV, JEOL, Tokyo, Japan). Ultraviolet-visible (UV-Vis) diffusive refln class="Chemical">ectance spectra were obtained by a Shimadzu UV-2550 spectrophotometer (Shimadzu, Kyoto, Japan) over the analysis range from 200 to 800 nm. X-ray photoelectron spectra (XPS) were recorded using the ESCALAB 250 instrument (Thermo Scientific, Waltham, MA, USA) with Al Kα radiation.

2.4. Photocatalytic Performance Experiments

The photocatalytic performance experiments were conducted in photoreaction apparatus (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, a 25 mg photocatalyst was added to 50 mL of n class="Chemical">DS solution (8 mg/L). Before irradiation, the solution was magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium. As the reaction time elapsed, the sample was taken out and filtered immediately with 0.45 µm membrane filters, the concentrations of DS and Total Organic Carbon (TOC) are measured by a high performance liquid chromatography (HPLC, Agilent 1260, Santa Clara, CA, USA) and TOC (Shimadzu TOC-VCPH, Kyoto, Japan) analyzer, respectively. In addition, repeated experiments for DS degradation were also conducted to study the stability of the as-prepared photocatalysts, and the operation processes were similar to the photocatalytic experiments. All experiments were repeated twice and the data shown in the article was averaged.

2.5. Analysis of Reactive Species

Free radical capture experiments were used to ascertain the reactive species of CuBi2O4/β-Bi2O3 photocatalytic system, and tert-butanol (t-BuOH) was chosen as the hydroxyl radical (OH•) scavenger, disodium ethylenediamine tetraacetate (EDTA-Na2) was chosen as the hole (h+) scavenger, benzoquinone (BZQ) was chosen as the superoxide radical (O2•−) scavenger. The detailed experiment processes were similar to the photocatalytic activity experiments.

3. Results and Discussion

Figure 2 depicts scanning electron microscope (SEM) images of the as-prepared materials. A well-distributed micro-sphere structure (~5 μm) with smooth surface for pure n class="Chemical">CuBi2O4 is shown in Figure 2a. The transition material of CuBi2O4@C, from Figure 2b, shows increased size and relatively rough surface compared with pure CuBi2O4. Moreover, some dispersed carbon microspheres exist as well. In the formation process of CuBi2O4/β-Bi2O3, the control of temperature is an important step. As shown from Figure 2c, the products exhibit a chain-connected spherical structure when the temperature is at 400 °C, and the connection matter is the incompletely burned carbon. That is because the complete combustion temperature of carbon is higher than 500 °C [29]. Therefore, the composition of calcined products just contain CuBi2O4 and β-Bi2O3 when the temperatures are at 600 °C and 800 °C. But the rising of calcination temperature will destroy sphere-structural CuBi2O4, which can be seen from the SEM images of Figure 2d,e.

Figure 2

Scanning electron microscope (SEM) images of (a) CuBi2O4; (b) CuBi2O4@C; (c) CuBi2O4/β-Bi2O3 (1:2.25, 400 °C); (d) CuBi2O4/β-Bi2O3 (1:2.25, 600 °C); (e) CuBi2O4/β-Bi2O3 (1:2.25, 800 °C).

The purity and crystallinity of CuBi2O4 and n class="Chemical">CuBi2O4/β-Bi2O3 calcined at 600 °C were examined with XRD, as depicted in Figure 3. From the XRD pattern of CuBi2O4, the diffraction peaks can be perfectly indexed to the phase of CuBi2O4 (JCPDS No. 84-1969) [30]. In the XRD pattern of CuBi2O4/β-Bi2O3 (600 °C), except for the peaks indicating CuBi2O4, the diffraction peaks located at the 2θ values of 27.66°, 32.86°, 46.32°, 55.44°, and 57.86°, can be indexed to the (201), (220), (222), (421), and (402) crystalline planes of tetragonal β-Bi2O3 (JCPDS No. 27-0050) [22,25]. Moreover, there are no obvious carbon peaks from the XRD pattern of CuBi2O4/β-Bi2O3 (600 °C), indicating the complete combustion of carbon in this calcination temperature [29].

Figure 3

X-ray diffraction (XRD) patterns of CuBi2O4 and CuBi2O4/β-Bi2O3 (1:2.25, 600 °C).

The UV-Vis absorption spectrum of n class="Chemical">CuBi2O4, β-Bi2O3 (600 °C) and CuBi2O4/β-Bi2O3 (600 °C) are displayed in Figure 4a. Both of the materials exhibit strong absorbance in the UV and visible light regions, and the maximum absorption boundary of CuBi2O4, β-Bi2O3 (600 °C) and CuBi2O4/β-Bi2O3 (600 °C) appear at approximately 800 nm, 500 nm and 725 nm, respectively. The band gap energy (Eg) of CuBi2O4 and β-Bi2O3 (600 °C) can be determined with the classic Tauc approach by using the following equation [31]: , where α, h, v, Eg and A are the absorption coefficient, the Planck constant, the light frequency, the band gap energy, and a constant, respectively. In the equation, n is a number characteristic of the charge transition in a semiconductor, and n = 1 for a direct transition while n = 4 for an indirect transition [31]. As for CuBi2O4 and β-Bi2O3 (600 °C), n = 4 [32,33]. Therefore, their band gap energy could be elicited from the plot of light energy (αhv)2 versus energy (hv), shown in Figure 4b, which suggests that the band gap energy of CuBi2O4 and β-Bi2O3 (600 °C) are 1.72 eV and 2.70 eV, respectively, which are very close to previous literature [33,34,35]. In addition, the valence band (VB) and conduction band (CB) edge position of the semiconductors can be estimated according to the following empirical equation: where EVB and ECB are the valence band (VB) and conduction band (CB) edge potentials, respectively; X is the electronegativity of the absolute electronegativity of the constituent atoms, that is 4.59 eV for CuBi2O4 and 6.24 eV for β-Bi2O3 [25,33]; Ec is the energy of free electrons on the hydrogen scale (approximately 4.5 eV) [31]. Consequently, the EVB and ECB positions of CuBi2O4 are estimated to be 0.95 and −0.77 eV/NHE; the EVB and ECB positions of β-Bi2O3 (600 °C) are estimated to be 3.09 and 0.39 eV/NHE.

Figure 4

(a) Ultraviolet-visible (UV-Vis) diffuse reflectance spectra of CuBi2O4 and CuBi2O4/β-Bi2O3 (1:2.25, 600 °C); (b) band gap energy (Eg) of CuBi2O4 and CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) derived from the plots of (αhv)2 versus energy (hv).

The degradation of n class="Chemical">DS in different photocatalytic systems was evaluated under visible light irradiation, and the results are described inFigure 5. It can be seen from Figure 5a, the degradation efficiency of DS in the blank irradiation system is 38.21% in 240 min reaction while the degradation efficiency of DS is 44.48% and 65.85% with the addition of pure β-Bi2O3 and CuBi2O4. As for the system of mechanically mixed CuBi2O4 and β-Bi2O3 with the mass ratio of 1:2.25, 70.11% of DS can be degraded in the same condition. Figure 5c shows the photodegradation efficiency of DS in the system of composites CuBi2O4/β-Bi2O3 with different mass ratios. The adsorption capacity of composites to DS is less than 10%, suggesting the removal of DS is closely related to photodegradation and photocatalysis. The degradation efficiency of DS is enhanced with increasing β-Bi2O3 in the composite as the mass ratio of CuBi2O4 and β-Bi2O3 changes from 1:0.5 to 1:2.25, but will decrease with a further increase of the content of β-Bi2O3, i.e., the mass ratio of 1:4. Figure 5e shows the degradation efficiency of DS using CuBi2O4/β-Bi2O3 calcined at different temperatures. It can be seen that the degradation efficiencies of DS are 84.37%, 92.17% and 76.80%, respectively, and the composites calcined at 400 °C, 600 °C and 800 °C. Figure 5b,d,f describe the degradation rate curves of DS in different photocatalytic systems which derive from the ln(C0/C) versus irradiation time and the values of degradation rates are listed in Table 1. The maximum degradation rate of DS can be obtained in the system of composite CuBi2O4/β-Bi2O3 with the mass ratio of 1:2.25 and the calcination temperature of 600 °C, which is 0.0099 min−1.

Figure 5

Diclofenac sodium (DS) degradation efficiency in the photocatalytic system of (a) blank irradiation, CuBi2O4, β-Bi2O3 and CuBi2O4 + β-Bi2O3; (c) CuBi2O4/β-Bi2O3 with different mass ratios; (e) CuBi2O4/β-Bi2O3 with different calcination temperatures; (b,d,f) plots of ln(C0/C) versus irradiation time over different photocatalytic systems.

Table 1

Kinetic analysis of DS degradation in different photocatalyst systems.

Photocatalytic System	Kapp (min−1)	R2
Blank irradiation	0.0027	0.92
β-Bi2O3	0.0030	0.90
CuBi2O4	0.0040	0.94
Mechanically mixed with CuBi2O4 and β-Bi2O3	00048	0.94
CuBi2O4/β-Bi2O3 (1:0.5, 600 °C)	0.0055	0.92
CuBi2O4/β-Bi2O3 (1:1, 600 °C)	0.0076	0.91
CuBi2O4/β-Bi2O3 (1:2.25, 600 °C)	0.0099	0.90
CuBi2O4/β-Bi2O3 (1:4, 600 °C)	0.0059	0.96
CuBi2O4/β-Bi2O3 (1:2.25, 400 °C)	0.0065	0.95
CuBi2O4/β-Bi2O3 (1:2.25, 800 °C)	0.0060	0.95

To further evaluate the degradation efficiency of n class="Chemical">DS in the as-prepared catalyst’s system, the removal efficiencies of Total Organic Carbon (TOC) are also explored and the results are shown in Figure 6. In the blank irradiation system, only 18.01% of TOC can be removed. And in the pure β-Bi2O3 and CuBi2O4 photocatalytic systems, the TOC removal efficiencies are 22.52% and 30.59%, respectively. When CuBi2O4 and β-Bi2O3 with the mass ratio of 1:2.25 are mechanically mixed to add into the system, 35.96% of TOC are removed. But for the composites CuBi2O4/β-Bi2O3 with different mass ratios calcinated at various temperatures, the removal efficiencies of TOC can be greatly improved, and the maximum value reaches 57.37%, which is achieved in the photocatalytic system of CuBi2O4/β-Bi2O3 with the mass ratio of 1:2.25 and the calcination temperature of 600 °C.

Figure 6

Total Organic Carbon (TOC) removal efficiency of the DS photodegradation solutions in different photocatalytic systems.

Figure 7a describes the photodegradation efficiency of n class="Chemical">DS under different recycling runs using CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) as the catalyst. When the catalyst was repeated for seven times, the degradation efficiency of DS is 82.95%. Figure 7b shows the degradation rate curves of DS over CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) under different recycling runs, and the values of rate constants are described in the illustration. It can be seen that the degradation rate of DS decreases from 0.0099 min−1 to 0.0071 min−1 after the seventh recycle. To further understand the photocatalytic stability of the as-prepared CuBi2O4/β-Bi2O3, XRD and Bi 4f XPS spectrums of the recycled composites are conducted seven times and the results are shown in Figure 8. Compared with the XRD spectra of the fresh and reused CuBi2O4/β-Bi2O3 in Figure 8a, there are no obvious changes for the main peaks. But from the Figure 8b, a new peak occur in the composite after recycling expect for the two main peaks existing in both of the two samples. According to reports, the main 4f 7/2 peak at 158.32 eV for a fresh sample and 158.30 eV for a reused sample, and the other main 4f 5/2 peak at 163.60 eV for a fresh sample and 163.49 eV for a reused sample, correspond to the Bi3+ oxidation state [36,37], which are in accordance to the presence of either CuBi2O4 or β-Bi2O3 phases. The extra peak at 156.63 eV in the reused sample is ascribed to Bi metal [38], indicating that some Bi3+ in the substance was reduced to be Bi metal after the reaction.

Figure 7

(a) Photodegradation efficiency of DS; (b) plots of ln(C0/C) versus irradiation time over CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) under different recycling runs. The illustration in (b) shows the degradation rate constants of DS under different recycling runs.

Figure 8

(a) XRD pattern; (b) Bi 4f X-ray photoelectron spectrum (XPS) of the composite catalyst CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) before and after reaction for seventh times.

Moreover, the free radical capture experiments used to investigate the active species involved in the n class="Chemical">DS degradation with the CuBi2O4/β-Bi2O3 (1:2.25, 600 °C) photocatalyst are explored and the results are shown in Figure 9. The degradation efficiency of DS decreases to 41.74% when 1 mM EDTA-Na2 was added, and the DS degradation was completely inhibited with the addition of 2 mM EDTA-Na2. But the degradation efficiency of DS decreases to 54.98% and 27.82% when 2 mM t-BuOH or BZQ was added, respectively. The results suggest that the enhanced photocatalytic activity of CuBi2O4/β-Bi2O3 is closely related to OH•, h+ and O2•−, and the contribution order is h+ > O2•− > OH•.

Figure 9

Photodegradation efficiency of DS under different scavengers in the system of CuBi2O4/β-Bi2O3 (1:2.25, 600 °C).

As for the photocatalytic mechanisms of a semiconductor–semiconductor composite catalyst, the heterojunction energy band theory and Z-scheme theory were of concern [8]. Based on the analysis above, a possible Z-scheme photocatalytic mn class="Chemical">echanism was put forward to explain the enhanced photocatalytic activity of the CuBi2O4/β-Bi2O3 composite, which is illustrated in Figure 10. That is, both CuBi2O4 and β-Bi2O3 could generate the photoinduced electron-hole pairs under visible-light illumination owing to their good light absorption abilities. Then, the formed Bi metal becomes the recombination center of the photogenerated electron from CB of β-Bi2O3 and the holes from the VB of the CuBi2O4 in the photocatalytic reaction process, leading to the improved charge separation. Besides, the dissolved O2 in the solution can be captured by the photogenerated electrons in the CB of CuBi2O4 to form O2•− due to the more negative CB level of CuBi2O4 than the potential of O2/O2•− (E(O2/O2•−) = 0.13 eV (vs. NHE) [39]; while the photogenerated holes in the VB of β-Bi2O3 can lead the H2O/OH− to be oxidized to OH• for the higher energy of holes in the VB of β-Bi2O3 than the potential of OH−/OH• E(OH−/OH•) = 1.99 eV (vs. NHE) [39]. Subsequently, the holes in the VB of β-Bi2O3, the formed OH• and O2•− participate in the DS photodegradation.

Figure 10

Supposed photocatalytic mechanism of CuBi2O4/β-Bi2O3 by the Z-scheme theory.

4. Conclusions

A combined hydrothermal and temperature-programmed method was employed to synthesize the CuBi2O4/β-n class="Chemical">Bi2O3 composite photocatalysts. The properties of the as-prepared materials were systematically characterized by SEM, XRD, XPS and UV-Vis. Moreover, the composites exhibit enhanced photocatalytic performance toward DS degradation under visible light irradiation, and the optimal photodegradation efficiency of DS is achieved in the catalytic system of CuBi2O4/β-Bi2O3 with the mass ratio of 1:2.25 and the calcination temperature of 600 °C. Besides, the active species in the CuBi2O4/β-Bi2O3 photocatalytic system were discussed through the free radical capture experiments, and the photocatalytic mechanism of CuBi2O4/β-Bi2O3 was also put forward.