Abstract
ZnO/Bi2O3 nanofibers were synthesized by a simple electrospinning method and both the UV and visible light responsive photocatalytic properties were studied by the decolorization of RhB dye. Thermogravimetric analysis/differential thermal analysis (TGA-DTA), X-ray diffraction (XRD), scanning electron microscope (SEM), and UV-vis diffuse reflectance spectra (DRS) were employed to study the structure, morphology, and optical properties of the ZnO/Bi2O3 nanofibers, respectively. The relationship between the ZnO/Bi2O3 ratio and photocatalytic activity was also studied, and the composite with a molar ratio of 23 : 1 demonstrated the best activity under both excitations. The photocatalytic mechanisms for the composite fibers can be described as the direct photocatalysis under UV excitation and photosensitation for visible light irradiation. The enhanced photocatalytic activities can be ascribed to the effective electron-hole pairs separation that leads to the promoted photocatalytic efficiency.
1. Introduction
Owing to the discharged toxic organic pollutants and coloured wastewater, the water bodies in nature are seriously damaged, which is harmful to human health and the environment [1, 2]. Photocatalysis, an efficient and environmental friendly technique, has shown enormous potential applications for the complete decomposition of toxic chemicals in the water, which also meet with the energy problems of the 21st century [3, 4]. Many researchers have concentrated on semiconductor photocatalysts due to their adjustable band gaps for excitation and efficient photocatalytic activities on the decomposition of organic dyes. Up to now, various oxide semiconductors have been used as photocatalysts in pollutant degradation and water splitting reactions, such as TiO2, ZnO, WO3, SnO2, and CeO2 [5–7]. However, the efficiency of photocatalytic decomposition is greatly dependent on the utilization of solar energy and the recombination of photogenerated electron-hole pairs [8]. Thus, the design and fabrication of efficient photocatalysts is an essential and challenging issue. Recently, a prospective way that combining two kinds of semiconductor photocatalysts has been widely used to enlarge the light absorption and reduce the recombining of electrons and holes. Typical examples are BiOCl/BiOI [9], NiO/ZnO [10], Ag2O/TiO2 [11], Ag2O/Bi2O3 [12], and so on.
ZnO is an important semiconductor material with a band gap of 3.37 eV at room temperature and its photocatalytic activity has been widely explored and reported [13, 14]. However, the wide band gap of ZnO leads to the main absorption of UV light, which accounts for only 5% of the solar energy. Therefore, to improve the photocatalytic activity, it is necessary to modify ZnO by enlarging the light absorption and reducing the recombination of photoinduced carriers. Bismuth oxide (β-Bi2O3) is a kind of semiconductor with a band gap of 2.47 eV, which is active under visible light irradiation [15]. Studies for ZnO/Bi2O3 composite photocatalysts have been reported [16, 17]. For example, through the hydrothermal method, Balachandran and Swaminathan prepared heterostructured ZnO/Bi2O3 which showed improved activity on the photodegradation of Acid Black 1 [16]. Also Li et al. synthesized ZnO-Bi2O3 mixtures by microsphere lithography technique for the degradation of methyl orange dye [17]. However, to the best of our knowledge, there is no report on the synthesis and photocatalytic evaluation of ZnO/Bi2O3 nanofiber composites.
In this work, the ZnO/Bi2O3 nanofiber composites were fabricated through an electrospinning technique and collected as nanofibrous mats. Photodecolorization measurements of RhB dye confirmed that the composite nanofibers showed greatly improved activity under both UV and visible light irradiations. The possible photocatalytic mechanisms were also discussed.
2. Experimental
2.1. Materials
Polyvinylpyrrolidone (K30) (PVP, ), analytical pure of zinc acetate hydrate (Zn(CH3COO)2·2H2O), bismuth nitrate oxide (Bi(NO3)3·5H2O), and ethanol were purchased from Tianjin, China Chemical Co., Ltd.
2.2. Synthesis of ZnO/Bi2O3 Composite Nanofibers
The polyvinylpyrrolidone (PVP) was a kind of polymer with good physiological compatibility and low toxicity. 13 g PVP powder was added into 10 mL ethanol with vigorous stirring, forming the homogeneous PVP solution. ZnO/Bi2O3 composite nanofibers with different molar ratios of Zn2+ : Bi3+ (11 : 1 to 30 : 1) were prepared by a simple electrospinning (PVP-metal ions solutions) method. Taking the molar ratio of 11 : 1 as an example, the precursor solution was prepared by adding 0.3 g of Bi(NO3)3·5H2O into 10 mL 1 M of HNO3 to restrain the hydrolysis process and dissolving 1.5 g ZnAc2 with continuous agitating for 2 h at room temperature. Then, the PVP and the precursor solutions were mixed together and stirred for 16 h to obtain the viscous spinning composite-solution.
For electrospinning, the resulting glutinous hybrid solution was breathed into the electrospinning apparatus by a vitreous syringe. The spinneret needle with an outward diameter of 1 mm was placed 10 cm away from the collector. A 20 kV voltage was applied between the collector and the needle tip. As obtained fibrous composite on the collector was dried at 60°C for 2 h and then collected. To acquire the composite nanofibers, the precursor fibers were calcined in air at 550°C for 2 h with a heating rate of 2°C min−1. The obtained sample with a Zn2+ : Bi3+ molar ratio of 11 : 1 was marked as Z11. Similarly, different composite samples prepared in the different contents of Zn2+ and Bi3+ for 15 : 1, 23 : 1, 25 : 1, and 30 : 1 were labeled as Z15, Z23, Z25, and Z30, respectively.
2.3. Characterization
The morphology of the electrospun composite fibers was characterized by a field emission scanning electron microscope (FE-SEM, Hitachi S-4800). The thermal decomposition process of the precursors was investigated by thermogravimetric analysis/differential thermal analysis (TGA-DTA) using a TA SDT 2960 instrument. It was performed in air from 40 to 1000°C with a heating rate of 10°C min−1 and flow rate of 100 mL min−1. X-ray diffraction (XRD) measurement was carried out using a Rigaku D/max-2600/PC spectrometer with Cu Kα radiation in the 2θ range of 30–80°. The UV-Vis diffuse reflectance spectra (DRS) were obtained by a Shimadzu UV-2550 spectrophotometer and the wavelength range was changed from 220 nm to 700 nm.
Photocatalytic decomposition of the RhB aqueous solution (1.0 × 10−5 M, 60 mL) was performed for activity evaluation. For each sample, 60 mg catalyst was magnetically agitated for 30 min in dark to allow equilibrated adsorption-desorption of RhB on the photocatalyst surface. Then the suspension solution was irradiated in a photoreaction apparatus, and a 300 W Xe lamp was used as the light source with a filter (>420 nm). As for the UV-light excited photocatalytic process, band-pass filter (centered at 350 nm) was used to obtain the UV light. At given intervals of illumination, 4 mL of the reaction solution with catalysts was taken out during the whole process. The suspensions were centrifuged and then analyzed by a Perkin-Elmer Lambda-35 UV-Vis spectrometer.
3. Results and Discussion
In order to determine the suitable annealing temperature to get ZnO/Bi2O3 composite nanofibers, the thermal behavior of the precursor was tested. Figure 1 showed the typical results of synchronous TG and DTA analyses for the hybrid precursor of Z23. The differential thermal analysis (DTA) curve showed broad exothermic peaks below 300°C, which could be associated with the loss of moisture and trapped solvent ethanol and carbon dioxide [10]. A weight loss of ~20% was observed at the corresponding temperature range in TG curve. Further, the pyrolysis of PVP by a dehydration on the polymer side chain and the disintegration of zinc acetate and bismuth nitrate occurred between 300°C and 500°C, accompanied by a continuous weight loss of ~70% [18, 19]. The exothermic peaks at about 363, 463, and 500°C in the DTA curve were attributed to the decomposition of zinc acetate, bismuth nitrate oxide, and the main chain of PVP, respectively. There was no obvious weight loss after 500°C, which verified that the precursor was completely decomposed and ZnO/Bi2O3 composite formed. Therefore, the calcination temperature for precursor fibers was selected as 550°C in order to obtain the composite fibers.
In order to obtain the microstructure information, SEM observations of the precursor and calcined nanofibers were carried out and the images are given in Figure 2. Before the annealing treatment, the morphology of as-spun composite nanofibers Z11 and Z23 in Figures 2(a) and 2(b) showed that the diameters of nanofibers were in the range of 700–900 nm with a relatively smooth surface. After annealing, the nanofibers maintained the continuous fiber structure, whilst the diameter of composite nanofibers of Z11 and Z23 (Figures 2(c), and 2(d)) was reduced to 100–200 nm. High-magnified image of sample Z23 (Figure 2(e)) showed that the surface of the annealed nanofibers turned to be rough and porous. The energy-dispersive X-ray fluorescence (EDX) spectrum (Figure 2(f)) taken from Z23 confirmed the existence of C, O, Bi, and Zn elements in the nanofibers. The EDX spectrum further indicated that the composite nanofibers were fabricated.
(a)
(b)
(c)
(d)
(e)
(f)
XRD patterns of the calcined nanofibers are shown in Figure 3. The crystal planes of (100), (002), (101), (102), (110), (103), (200), (112), and (201) from the ZnO wurtzite (JCPDS 36-1451) were observed in the patterns for all samples and the characteristic crystalline peaks of ZnO showed no obvious change in theses samples. ZnO also presented good crystallinity in the composite nanofibers, as confirmed by the sharp diffraction peaks. The diffraction peaks of β-Bi2O3 (JCPDS 27-0050) were also observed in each sample and slightly varied due to the different contents of Bi2O3. It showed that the intensity of diffraction peaks of β-Bi2O3 turned to be stronger with increase content in the composite. No characteristic peaks of impurities such as ZnO2, BiO2, and Bi4O7 were discovered, suggesting that the nanofibers composed of ZnO and Bi2O3 phases.
The DRS spectra of the ZnO/Bi2O3 nanofibers were shown in Figure 4. It can be seen that all the composites exhibited the similar absorption edge below 400 nm, while an obvious visible light absorption band around 400–550 nm was observed and striking red shifts with the elevating content of β-Bi2O3 were observed for the composites, due to the narrow band gap of β-Bi2O3 (~2.47 eV) compared with that of ZnO (3.37 eV). In sequence, the ability of light absorption in the visible range for the composites catalysts is as follows: ZnO < Z30 < Z25 < Z11 < Z15 < Z23.
The photocatalytic activities of the ZnO/Bi2O3 nanofibers were demonstrated for the degradation of organic pollutants Rhodamine B (RhB) irradiated with both UV and visible light (Figures 5(a), and 5(b)), respectively. The characteristics of Rhodamine B were provided in Table 1. The photodegradation process was measured by monitoring the absorbing band of RhB at 554 nm. The photodegradation efficiency is defined as the , where (1.0 × 10−5 M) is the initial concentration and is the concentration of RhB at intervals. Under visible illumination (Figure 5(a)), the time required to decolorize 85% of the RhB was 180 min by using the Z23 composite nanofibers catalyst, which was much shorter than the other nanofibers in our experiment. For comparison, the direct photolysis of RhB without catalyst was tested and only 10% of organic dye was degraded at 180 min. The results illustrated that the ZnO/Bi2O3 composite nanofibers presented good visible light responsive activity for the photodegradation of RhB. Since ZnO presents no absorption of visible light from DRS, there is no photodegradation of RhB on ZnO under visible light excitation. Moreover, the photocatalytic activities of ZnO/Bi2O3 nanofibers under UV light were shown in Figure 5(b). The Z23 photocatalyst also exhibited the highest photocatalytic activity and nearly 95% of RhB was decomposed after 90 min under UV-light irradiation. Typical evolution of RhB absorption spectra for Z23 nanofiber under UV light was inserted in Figure 5(b). The maximum absorption spectra of RhB with the characteristic adsorption peak at 554 nm shifted to 490 nm and decreased obviously with the increase in the exposed duration in approximately 90 min. The formation of the intermediates leads to the hypsochromic shift of the maximum absorption band [20]. This also confirmed that the decomposition of RhB took place by the photocatalysis of the nanofibers.
(a)
(b)
To better interpret the mechanisms of the enhanced photocatalytic activity for ZnO/Bi2O3 composites, a schematic band structure was given in Figure 6. It is well known that ZnO can only be photoactivated by the ultraviolet excitation, while can absorb visible light with wavelength <500 nm. In our photocatalytic experiments, both UV and visible light induced photocatalysis process were observed in the degradation of RhB over the ZnO/Bi2O3 composite nanofibers and the Z23 composite exhibited the highest photocatalytic activity. The possible photocatalytic process can be described as follows:
(a)
(b)
Under the UV light excitation, photoinduced electrons and holes can be generated in both of the ZnO and Bi2O3, as demonstrated in (1). Subsequently, the electrons from the conduction band (CB) of ZnO can transfer to the CB of Bi2O3 due to the energy matching of CB in the composite, which also greatly inhibited the recombination of photoexcited carriers. The efficient separation between the electrons and holes at the interface of the two semiconductors promoted the photocatalytic performance of the ZnO/Bi2O3 nanofibers, as shown in Figure 6(a).
When being under the visible light excitation, only Bi2O3 in the composite can absorb the light above 420 nm to generate photoinduced carriers, indicating that there are other reasons for the photocatalytic degradation of RhB, as shown in Figure 6(b). The RhB dye used in this study can also be excited by the visible light with photogenerated electrons from the highest occupied molecular orbital (HOMO) up to the lowest unoccupied molecular orbital (LUMO), producing the singlet and triplet excited states (2). Electrons from the RhB dye would be injected to the CB of ZnO, and subsequently the excited electrons on ZnO transferred to the CB of Bi2O3 [17]. The photogenerated electrons and holes transferred to the surface and reacted with the adsorbed reactants in an analogous process with the UV-light photocatalytic reaction. The photoinduced holes (trapped by H2O) produced hydroxyl radical species and the photoinduced electrons (trapped by O2 and H2O) produced hydroxyl radical species , which are extremely strong oxidants for the degradation of organic chemicals (3) to (7) [17, 21]. So based on the discussion above, the mechanisms for the UV and visible light photocatalytic processes were considered as direct photocatalysis and photosensitization, respectively.
4. Conclusions
In summary, ZnO/Bi2O3 composites nanofibers with diameters of 100–200 nm were synthesized using an electrospinning method and their UV and visible light responsive photocatalysis activities were evaluated. Our experimental results showed that the photodecomposition rate of RhB under both UV and visible light by the Z23 was much higher than other nanofibers within the same duration. The improvements of photocatalytic activity were ascribed to the effective separation of photoinduced electrons and hole, which presented in the direct photocatalysis and photosensitization process for the UV and visible light excitation photocatalytic measurements.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The work was sponsored by the National Science Foundation of China (51102069). This work was also partly supported by foundation from Heilongjiang Provincial Committee of Education (1253G036, JG2012010370) and Open Project from Key Laboratory for Photonic and Electronic Bandgap Materials of Ministry of Education (PEBM 201209).