Photocatalytic Performance of Ag and CuBiS2 Nanoparticle-Coated
Article pubs.acs.org/JPCC
Photocatalytic Performance of Ag and CuBiS2 Nanoparticle-Coated SiO2@TiO2 Composite Sphere under Visible and Ultraviolet Light Irradiation for Azo Dye Degradation with the Assistance of Numerous Nano p−n Diodes Hairus Abdullah† and Dong-Hau Kuo*,† †
Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ABSTRACT: Ag and CuBiS 2 nanoparticle-coated SiO 2 sphere@nano TiO2 coating (SiO2@TiO2) composite catalyst (abbreviated as SiO2/TiO2/CuBiS2/Ag) has been successfully synthesized and characterized as well as tested for photocatalysis of Acid Black 1 (AB 1) dye under visible and ultraviolet (UV) light irradiation. These as-prepared composite spheres show photocatalytic activities under visible and ultraviolet light irradiation. This work is the first report of CuBiS2 semiconductor nanoparticles used as a material for photodegradation. The data showed that the SiO2/TiO2/CuBiS2/Ag composite particles completely degraded 50 mL of 10 ppm AB 1 dye solution in only 5 min under UV light irradiation and 100 mL of 5 ppm AB 1 dye solution in 30 min under visible light irradiation. The good photocatalysis of our composite spheres is attributed to the establishment of a good p−n heterojunction interface between the p-type CuBiS2 and n-type TiO2 semiconductors with the assistance of Ag nanoparticles.
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INTRODUCTION Photocatalysis is one of the prospective techniques to overcome the energy shortage problem and global warming. It has been used widely in many applications such as elimination of gaseous 1,2 and water pollutants, 3,4 self-cleaning3−6 and antibacterial7,8 materials, and water splitting.9 However, work on enhancing the capability of photocatalysis is ongoing because photocatalysts with high activity and reactive selectivity are required for applications. Plastic, paper, textile, tanneries, chemical industries, etc. contribute their effluents to environment because of the great consumption of organic dyes. The dye pollutant in water is one kind of pollution produced by various industries in the world. The World Bank estimates that almost 20% of global industrial water pollution comes from the treatment and dyeing of textiles. The data shows that approximately 700 000 tons of dyes are produced each year, and 20% of that total is not treated.10 Due to environmental and health issues, a great deal of effort has already been dedicated to solving the problems caused by dye pollutants. Photocatalytic degradation of dye pollutants is one of the alternatives that has the ability to degrade many kinds of organic dyes and may help solve this problem.11 Nowadays, semiconductor photocatalysis has become popular because of its potential to solve environmental and energy issues effectively. In spite of many emergent technologies, the efficiency of photocatalytic reaction is still moderate and is a bottleneck for field application.12 Therefore, many attempts have been made to find desirable semiconductor © 2015 American Chemical Society
nanoparticle-based photocatalysts. TiO2 has been the most popular photocatalyst and already has been used in many applications. Because of its limited 4% absorbance under sunlight,13−15 many scientists have tried to modify it to become a visible light-activated photocatalyst.16−18 The efforts include substitutions with anions such as N, S, C, etc.19,20 and cations such as Cr, V, Fe, Mn, Co, Ni, etc.21−24 To obtain the improved photoelectrochemical properties, the composite particle architecture is crucial. When TiO2 is coupled with quantumconfined inorganic semiconductor sensitizers such as metal oxide,25,26 CdS,27 and low-band gap semiconductor to increase its ability in visible light absorbance,28−32 it becomes feasible and promising for industrial application. The concept of forming a built-in electrical field in a p−n heterojunction diode to enhance the visible light photocatalytic activity has been applied. Composite particles such as CuCr2O4/TiO2, BiVO4/CuCr2O4, BiVO4/Bi2O3, AgBr/BiPO4, Bi2S3/BiVO4, CaFe2O4/Ag3VO4, metal oxide (CuO, Co3O4, NiO)/BiVO4, Cu2O/In2O3, etc. with a p−n heterojunction structure for the efficient separation of the visible light-induced electron and hole pairs have been studied.33−40 There are few studies about CuBiS2. It was found to be a potential visible light-driven material and one of the alternative solar cell absorber materials.41,42 Balasubramanian et al. studied the structural and optical properties of CuBiS2. They found that Received: February 27, 2015 Revised: May 21, 2015 Published: June 3, 2015 13632
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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SiO2@TiO2 symbol for the core−shell structure will be replaced by SiO2/TiO2 or S/T in abbreviated format to simplify our system presentation later. Synthesis of SiO2/TiO2/CuBiS2. To coat CuBiS2 on the SiO2/TiO2 composite, 0.1 g of the as-prepared SiO2/TiO2 composite particles were first dispersed in 15 mL of oleylamine and treated in ultrasonication bath for 1 h. In the other vessel, a total mass of 0.1 g of sulfur was dissolved in oleylamine by treatment in ultrasonication bath for 2 h. The dissolving sulfur in oleylamine produced a dark red solution. The precursors of CuI and BiCl3 with the molar ratio 1:1 were prepared by dissolving 0.0115 g of CuI and 0.019 g of BiCl3 in 100 mL three-neck flask that contained 25 mL of oleylamine by heating at 130 °C in vacuum under vigorous stirring. The as-prepared SiO2/TiO2-dispersed solution was injected into the three-neck flask after 1 h of heating. The heating of reaction solution then was held for another 1 h at the same temperature. Finally, the reaction solution temperature was increased from 130 to 280 °C and held for 1 h. The dark red sulfur solution was injected drop by drop into the reaction solution when the temperature reached 230 °C. A black solution was formed immediately as the sulfur solution dropped into the reaction solution. After 1 h of heating at 280 °C, the black reaction solution was slowly cooled to room temperature under vigorous stirring. The precipitates in the solution were centrifuged and washed with hexane and alcohol three times followed by drying in a rotary evaporator. The obtained black powder was SiO2/TiO2/ CuBiS2. The total amounts of the as-prepared CuBiS2 in the black powder reached 20 wt % based on the weight amount of SiO2/TiO2 composite particles. Depositing Ag Nanoparticles on SiO2/TiO2/CuBiS2. In the final procedure, the Ag nanoparticles were coated on SiO2/ TiO2/CuBiS2 composite particles. To coat the Ag nanoparticles, 0.1 g of SiO2/TiO2/CuBiS2 particles were dispersed into the mixed solution of 6 mL of DI water and 6 mL of alcohol; then, 3.15 mg of AgNO3 was added to the dispersion solution. After the treatment in ultrasonication bath for 15 min, the solution was exposed to UV irradiation for 1 h to precipitate Ag nanoparticles on SiO2 /TiO2 /CuBiS2 by reduction reaction. The solution with Ag-coated SiO2/TiO2/ CuBiS2 particles was washed three times with alcohol and dried in a rotary evaporator. The as-prepared composite particle was SiO2/TiO2/20% CuBiS2/2% Ag and is abbreviated SiO2/TiO2/ CuBiS2/Ag. Characterizations. The powder X-ray diffraction (XRD) patterns were recorded by a Bruker D2-phaser diffractometer using Cu Kα radiation (λ = 1.5418 Å). The surface morphologies were examined by transmission electron microscopy (TEM, H-7000, Hitachi) equipped with a CCD camera and field-emission scanning electron microscopy (FESEM, JSM 6500F, JEOL). The element mapping of the composite catalyst was done by scanning transmission electron microscopy (STEM, Tecnai F20 G2, Philips). The UV−vis diffuse reflectance spectra (DRS) and the photodegradation of AB 1 dye were recorded using a Jasco V-670 UV−visible−nearinfrared (UV−vis−NIR) spectrophotometer. Absorbance spectra measured by Fourier transform infrared spectroscopy (FTIR, Agilent Digilab FTS-3500) were used to identify the intermediates caused by the photoirradiation. Photodegradation of AB 1 Dye. Photodegradation experiments were conducted under visible light and ultraviolet light irradiation. The powers of the visible and ultraviolet light lamps were 150 W of incandescent halogen lamp and 450 W of
this material had a band gap of 2.19−2.62 eV and can be a visible light-driven semiconductor.41 Temple et al. also studied the geometry, electronic structure, and bonding in CuMCh2 (M = Bi, Sb; Ch = S, Se) and found that this material with poor hole mobility had an indirect band gap.42 However, CuBiS2 has not yet been applied for photocatalysis. In our previous work, Mahesh et al. successfully deposited Ag nanoparticles on SiO2 sphere@nano TiO2 layer composite particles and greatly enhanced its photocatalytic activity in the presence of ultraviolet (UV) light illumination.17 The aim in this work is to improve the previous composite catalyst as a visible light-activated material by introducing CuBiS2 nanoparticles to the catalyst. It is expected that we will obtain both UV and visible light-driven materials from our composite catalyst in this work. In this work, SiO2 spherical particles are used as a support for forming Ag and CuBiS2 nanoparticle-deposited SiO2 sphere@ nano TiO2 layer composite particles to degrade Acid Black 1 (AB 1) dye under visible and ultraviolet light irradiation by generating hole and electron pairs. These composite particles can behave as photoelectrochemical cells that have photodegradation ability when it interacts with light. In this case, TiO2 and CuBiS2 act as n-type and p-type materials, respectively. The Ag nanoparticles were coated on the surface of the composite catalyst to reduce the recombination probability of photoinduced electron and hole pairs and to enhance the capability to degrade AB 1 dye. The CuBiS2 in the SiO2/TiO2/CuBiS2/Ag composite catalyst is reported for the first time as an active photocatalyst. It is believed that CuBiS2 has an important role in the visible light absorbance; TiO2 plays an important role in UV light interaction, and the nanojunction diodes formed by the contact between CuBiS2 and TiO2 provide the enhanced pollutant adsorption and photocatalytic activity. Thus, the SiO2/TiO2/CuBiS2/Ag composite catalysts as a whole can quickly degrade the AB 1 dye under both UV and visible light irradiation. This composite catalyst is quite promising for application in eliminating gaseous and water pollutants and other photorelated reactions.
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EXPERIMENTAL METHODS Materials. Materials used in this work were commercially available without further purification. Synthesis of SiO2 Sphere Support. SiO2 spherical particles were prepared as follows: 240 mL of DI water and 160 mL of ethanol were added in sequence and treated in ultrasonication bath for 1 min. Commercial ammonium hydroxide (3.2 mL) was added to the reaction solution dropby-drop. After 30 min of stirring, 4 mL of TEOS (tetraethyl orthosilicate) was added and the mixture was stirred again for 2 h. Then, the precipitate was centrifuged and washed three times using alcohol. The washed product was dried using rotary evaporation. Finally, the SiO2 spherical particles were obtained after pyrolysis at 500 °C for 3 h. TiO2 Coating on SiO2 Spherical Particles. To coat TiO2 on SiO2 spherical particles, 0.1 g of as-pyrolyzed SiO2 spherical particles and 4 mL of isopropanol were mixed and treated in an ultrasonication bath for 15 min. To this solution was added 0.1 mL of titanium isobutoxide, and the solution was stirred for 5 min. During stirring, 20 mg of DI water was added under continuous stirring at room temperature for 4 h. The precipitate then was collected by centrifugation and washed with alcohol three times. The TiO2-coated SiO2 (SiO2@TiO2) sphere was then annealed at 450 °C for 2 h. The familiar 13633
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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The Journal of Physical Chemistry C Xenon light with cut off filter (λ > 400 nm), respectively. The halogen lamp was jacketed by a water-cooled annular quartz tube to avoid lamp heating during photodegradation experiments. The whole setup of the halogen lamp was immersed into a reactor glass containing AB 1 dye solution which was equipped with an air channel to provide enough oxygen to the dye solution during the photoreaction. The xenon light illumination was filtered to transmit light with the wavelength lower than 400 nm. In the photodegradation experiment with UV light irradiation, 10 mg of SiO2/TiO2/CuBiS2/Ag composite catalyst was dispersed in 50 mL of 10 ppm dye solution, whereas in the visible light experiment, 20 mg of SiO2/TiO2/CuBiS2/Ag composite catalyst was dispersed in 100 mL of 5 ppm dye solution. To confirm the equilibrium of adsorption and desorption between the catalyst and AB 1 dye, the as-prepared catalyst-dispersed solutions were kept in the dark and stirred for 30 min. Then the solution started to be irradiated under UV or visible light. The light-on time was set at t = 0 min, and the time of adding the catalyst into the dye solution was set at t = −30 min. The 5 mL aliquots were sampled at different irradiation time intervals, and the concentration of AB 1 dye was monitored by UV−vis absorbance intensity at 615 nm.
phases were formed because of the excess amount of sulfur added at high temperature into the reaction solution during the preparation of CuBiS2. An excess amount of sulfur was needed to compensate the possibility of losing sulfur because sulfur was easily vaporized at high temperature. The high temperature was needed to well crystallize CuBiS2. However, at these conditions of excess sulfur and high temperature, CuBiS2 suffered from phase separations of Bi2S3 and Cu3BiS3. As the ratio of phase separations was stoichiometrically balanced, the equation can be proposed as follows: Δ
3CuBiS2 → Cu3BiS3 + Bi 2S3
From the XRD phase identification, the TiO2, CuBiS2, and Ag phases had been grown on SiO2 spheres. Figure 2 shows FE-SEM images of (Figure 2a) SiO2 spheres and (Figure 2b) SiO2/TiO2 and (Figure 2c) SiO2/TiO2/ CuBiS2/Ag composite particles. FE-SEM images show that all nanoparticles are well coated on the smooth surfaces of SiO2 spheres without any aggregation outside SiO2 spheres, as shown in Figure 2a. The TiO2-coated SiO2 spheres shown in Figure 2b maintain the smooth surfaces. After Ag and CuBiS2 nanoparticles were coated on SiO2/TiO2 spheres, the composite spheres became rougher. The EDS analysis in Figure 2d confirmed that all the peaks were contributed from the compositional elements in the SiO2/TiO2/CuBiS2/Ag composite catalyst. This compositional analysis is consistent with the XRD analyses shown in Figure 1. Ag nanoparticles were added to improve photocatalytic activities by trapping the generated photoelectron and reducing the recombination rate of generated photocarriers. With the XRD analyses to identify the phase qualitative composition and the SEM images to confirm the absence of nanoparticle aggregation, we can be sure that our deposition of TiO2, CuBiS2, and Ag on SiO2 spheres is well executed. Panels a and b of Figure 3 show low-magnification and highmagnification TEM images of SiO2/TiO2/CuBiS2/Ag composite catalyst, respectively. The monodispersed SiO2@TiO2 spheres were coated by CuBiS2 and Ag nanoparticles, which turned out to form the rougher surface on the composite spheres. Originally, SiO2 spheres had a smooth surface, as shown in Figure 2a. Ag nanoparticles were black particles and uniformly distributed on spheres. From the TEM image, the Ag size was much smaller than 10 nm. The TEM images show the composite catalyst with good spherical shape and without aggregation; the TiO2 and CuBiS2 were well distributed on it, which provided a high probability of forming a p−n heterojunction between p-CuBiS2 and n-TiO2. A high-angle annular dark field (HAADF) image and element mapping are shown in Figure 4. The selected area in Figure 4a was analyzed for different elements including Si, O, Ti, Cu, Bi, S, and Ag. From the information on element mappings of Si and O (panels b and c of Figure 4, respectively), the existence and location of the SiO2 sphere were confirmed. The O and Ti mappings (panels c and d of Figure 4, respectively) indicated the TiO2 phase formed a much continuous and packed layer, as evidenced by a dense distribution of Ti at the outer surface with color gradient from the dense (surface area) to less dense area of Ti in the middle area as shown in Figure 4d. The mapping images for Cu, Bi, and S (panels e, f, and g of Figure 4, respectively) indicated the existence of CuBiS2 phase. The Ag mapping in Figure 4h indicated the Ag nanoparticles were uniformly distributed on the sphere. Because of the spherical
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RESULTS AND DISSCUSSION XRD profiles of SiO2 sphere and SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag composite spheres are shown in Figure 1. The results show that SiO2 sphere was amorphous
Figure 1. XRD patterns of SiO2, SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag.
(PDF 29-0085). The peaks of anatase TiO2 appeared after the TiO2 coating was grown on SiO2 spheres and desiccated at 450 °C. The specific TiO2 peaks were all in good agreement with the peak locations listed in the reference profile of PDF 211272. The anatase phase was well-crystallized. It has a relative small mean crystalline size of 8.9 nm, as estimated from the full width at half-maximum (fwhm) at 25.8° with (101) reflection and the Scherrer equation. After photoreduction for forming Ag, the shown peak located at 44.7° 2θ corresponded to the Ag (111), as compared with the reference of PDF 04-0783. The SiO2/TiO2/CuBiS2/Ag composite catalyst simultaneously displayed all the peaks contributing from TiO2, CuBiS2, and Ag. The observed CuBiS2 peaks agreed well with the reference profile of PDF 43-1473; however, two peaks with a small intensity were found at 22.6° and 23.1° in the pattern, which are related to Bi2S3 and Cu3BiS3, respectively. These secondary 13634
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Figure 2. FE-SEM images of (a) SiO2 spheres and (b) SiO2/TiO2 and (c) SiO2/TiO2/CuBiS2/Ag composite particles. The EDS spectrum for SiO2/ TiO2/CuBiS2/Ag composite catalyst is shown in (d).
Figure 3. (a) Low-magnification and (b) high-magnification TEM images of SiO2/TiO2/CuBiS2/Ag composite catalyst.
absorbance capability and made all the CuBiS2-containing particles have the similar visible light absorbance. The calculated band gaps of CuBiS2 were 1.33 and 2.29 eV. Our CuBiS2 had two band gaps due to the secondary phases of Bi2S3 and Cu3BiS3 with low band gaps of 1.3 eV43,44 and 1.2 eV,45,46 respectively. Because the weight ratio of CuBiS2 on SiO2 sphere was very low (about 2.92% atomic, based on EDS analysis), CuBiS2 on SiO2 had only a small intensity contribution to the visible light absorbance with the calculated optical band gap near 2.3 eV. Ag nanoparticles also contributed to the visible light absorbance because of the important surface plasmonic resonance,17 as supported by the SiO2/CuBiS2/Ag and SiO2/ TiO2/Ag data. The TiO2 coating gave the sphere absorbance
shape, most of the mapping images look similar; however, the Ti mapping could confirm the TiO2 phase deposited as a slightly dense and continuous layer on the sphere support. Figure 5 shows the UV−vis−NIR absorbance spectra for SiO2, CuBiS2, SiO2/CuBiS2 (S/CuBiS2), SiO2/CuBiS2/Ag (S/ CuBiS2/Ag), SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag). The SiO2 sphere had only a little light absorbance in the visible wavelength range. If the wavelength axis in DRS measurement is extended to the UV range, SiO2 will have a strong light absorbance at 250 nm wavelength. The calculated optical band gap showed SiO2 had a band gap value of 4.63 eV. The CuBiS2 with a little secondary phase of Bi2S3 and Cu3BiS3 had good visible light 13635
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Figure 4. (a) HAADF STEM image and elemental mapping of (b) Si, (c) O, (d) Ti, (e) Cu, (f) Bi, (g) S, and (h) Ag.
composite catalyst has been proposed. However, most of the reported studies have focused on two-component systems because of the difficulty of controlling uniformity in the processes of adding the third nanoparticle onto two-component composite particles. Our SiO2 sphere with a size of ∼500 nm is much larger than the nanoparticles with the size smaller than 10 nm and can provide locations for multilayered nanoparticles to display the compounding advantages. Photocatalytic Degradation Tests. Figure 6 shows the changes in UV−vis spectra of AB 1 dye on UV irradiation in the
Figure 5. Comparison of UV−vis diffuse reflectance spectra for the light absorption of SiO2, CuBiS2, SiO2/CuBiS2 (S/CuBiS2), SiO2/ CuBiS2/Ag (S/CuBiS2/Ag), SiO2/TiO2(S/T), SiO2/TiO2/Ag (S/T/ Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag).
capability in the range of UV light with the calculated optical band gap of about 3.4 eV. However, after Ag nanoparticles were coated on SiO2/TiO2, the composite sphere has visible light optical band gap of 2.8 eV, as supported by the SiO2/TiO2 and SiO2/TiO2/Ag data in Figure 5. The tunable visible light band gap of TiO2 achieved by varying the Ag content in TiO2/Ag composite also has been reported by Hari et al.47 The SiO2/ TiO 2 /CuBiS 2 /Ag composite sphere combined the UV absorbance of TiO2, the visible light absorbance of CuBiS2, and the surface plasmonic resonance absorbance of Ag nanoparticles. The optical band gap of the SiO2/TiO2/ CuBiS2/Ag composite sphere was 2.9 eV as calculated from its DRS data. This composite catalyst has advantages because it utilizes more than two components. The concept of a
Figure 6. Changes in UV−vis spectra of AB 1 Dye in the presence of SiO2/TiO2/CuBiS2/Ag composite catalyst.
presence of SiO2/TiO2/CuBiS2/Ag composite catalyst. The dye degradation under UV irradiation was referred to the intensity drop of the peak located at 615 nm. After the sample was stirred for 30 min in dark conditions, there was a 55% dye adsorption and the catalyst color changed from gray to dark blue before the UV light started to irradiate the dye solution at t = 0 min. After irradiation for 10 min, the dye solution was completely discolored by composite catalyst and the catalyst color returned back to gray. The transition between dark blue and colorless for the AB 1 dye under irradiation provides us a 13636
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The Journal of Physical Chemistry C good indicator to differentiate adsorption and degradation by the photocatalyst. From the data shown in Figure 6, we can conclude that the AB 1 dye can be adsorbed by the photocatalyst in the dark conditions and that both the adsorbed dye on the photocatalyst and the dye in the solution can be degraded under UV irradiation in 10 min. The degradation time of 10 min indicates that the SiO2/TiO2/CuBiS2/Ag composite particle is a very powerful photocatalyst. The accelerated photoreaction can be attributed to the special design and the materials selected for our composite catalyst as well as the dye adsorption by the catalyst. To confirm the dye degradation, the catalyst under UV irradiation for 30 min was washed and dried for FTIR analysis, as shown in Figure 7. To easily analyze the peaks of the FTIR
Figure 8. Photocatalytic activities of SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag) composite catalysts in the presence of ultraviolet irradiation for 10 ppm AB 1 dye degradation.
the CuBiS2/Ag and SiO2/CuBiS2/Ag data, CuBiS2 is apparently little help under UV irradiation. Furthermore, the coupling effect of CuBiS2 with TiO2 apparently plays a major role in the fast photocatalysis. One major issue that needs to be clarified is the hydrophobic nature of pure CuBiS2. With the composite structure on the SiO2 sphere, the SiO2/TiO2/CuBiS2/Ag catalyst can be well dispersed in the dye solution. After the successful dye degradation under UV irradiation, the composite catalysts are also tested for visible light photocatalysis under the Halogen lamp. The difference between the UV and visible light tests was the catalyst and dye contents. As mentioned in the discussion of the experimental procedure, a greater amount of catalyst particles were added for the visible photocatalysis. The same five kinds of catalyst systems are investigated, as shown in Figure 9. Some interesting photocatalytic performances were observed. Because of the hydrophobic nature of CuBiS2, CuBiS2/Ag exhibited a total absence of any responses under visible light testing. The TiO2containing SiO2/TiO2 and SiO2/TiO2/Ag catalysts with good
Figure 7. FTIR absorbance spectra of a reused SiO2/TiO2/CuBiS2/Ag composite catalyst.
results, the SiO2/TiO2/CuBiS2/Ag powder before being used for photodegradation and pure AB 1 dye were also included in the analysis. The rinse of used composite catalyst by alcohol was used to confirm that there was no unreacted blue dye to be removed. The small peak located at 966 cm−1 confirms the Si− O−Ti bonding in the SiO2/TiO2 composite. The strong peak at 1090 cm−1 is contributed from the Si−O−Si bonding in the SiO2 major phase.17 When the SiO2/TiO2/CuBiS2/Ag catalyst before and after testing for photocatalytic experiment are compared, the peak at 1460 cm−1 is related to the peak from AB 1 dye after degradation. This peak is related to the C−C C asymmetric stretch in the aromatic ring, generated from the photodegraded intermediate products.48 These intermediate products are chemically adsorbed on catalyst particle and cannot be removed after the alcohol cleaning procedure. Although there is coverage of the intermediate products on the photocatalyst, they do not affect the photodegradation because of the nature of its monolayer adsorption and its small molecular size. To demonstrate the advantage of our specially designed SiO2/TiO2/CuBiS2/Ag composite particle, five kinds of catalyst systems including CuBiS2/Ag, SiO2/CuBiS2/Ag, SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag are investigated, as shown in Figure 8. As the dye solution was irradiated under UV light, the visible light-excitable CuBiS2-containing catalysts had inferior performance as compared with the TiO2-containing catalysts. Except for SiO2/TiO2/CuBiS2/Ag, the best performance was from SiO2/TiO2/Ag. However, SiO2/TiO2/Ag catalyst was greatly improved for photodegradation under UV irradiation after visible light-activated CuBiS2 was added. From
Figure 9. Photocatalytic activities of SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag) composite catalysts in the presence of visible light irradiation for 5 ppm AB 1 dye degradation. 13637
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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The Journal of Physical Chemistry C performance shown in Figure 8 became ineffective as visible photocatalysts. SiO2/CuBiS2/Ag showed strong adsorption at t = 0 but with very limited photocatalytic capability. SiO2/TiO2/ CuBiS2/Ag catalyst showed an 80% absorbance in the dark conditions with a dark blue color on catalyst particles, but the dye solution became colorless and the catalyst became its own color of gray after 30 min under the visible light irradiation. The color change informs us that the photocatalysis is executed at two steps. The first step is the dye adsorption on the catalyst. The second step is the photoreaction to degrade the dye. With the contact between dye and catalyst after strong adsorption, the photodegradation between dye and catalyst can be accelerated. Reusability of the composite photocatalyst is to repeatedly test the photocatalytic capability of the used composite catalyst in degrading the fresh AB 1 dye. The reusability testing is conducted for four times in the same experimental conditions. After degradation was completed in each run, the catalyst was taken out and washed with deionized water several times followed by methanol cleaning. The catalyst was dried in a vacuum oven at 80 °C for 12 h and used for the next run. An additional three runs were carried out with the used SiO2/ TiO2/CuBiS2/Ag catalyst for ultraviolet and visible-light photocatalytic experiments, as shown in Figures 10 and 11,
Figure 11. Four cycles (a, b, c, d) of SiO2/TiO2/CuBiS2/Ag composite catalyst reusability for the photodegradation experiments under visible light irradiation.
not adsorb the dye after being stirred for 30 min in the dark conditions, and it still could perform the photodegradation in 20 min. This example indicates that the photocatalytic capability mainly originates from the composite particle itself with only a small part contributed from the adsorption. In the reusability testing under visible light irradiation (Figure 11), the catalyst adsorbed 78% (Figure 11a), 56% (Figure 11b), 10% (Figure 11c), and 8% (Figure 11d) dye after the sequential tests. The required durations to reach the 99%, 97%, 94%, and 96% degradation were 30, 40, 45, and 45 min, respectively. Results similar to those of the UV tests were obtained. After the third run (Figure 11c), the performance of the catalyst was the same as that of fourth run and were stable. From the reusability tests, the composite catalyst can have a long-term usage for photodegradation with performance for the tests under UV irradiation slightly superior to that under visible light. The surface diffusion-controlled reaction kinetics influenced much more in the UV reusability experiments in Figure 10. The high energy of the UV irradiation resulted in a faster photoreaction; therefore, the reaction rate was limited by diffusion of dye molecules on the surface of the catalyst. The dye molecules will be degraded more quickly as they interact with active sites of the catalyst. Therefore, the total photoreaction rate under UV illumination depends on the slower step of surface diffusion (rate-determining step). The characteristic diffusion length (diffusion distance, x) is related to the characteristic diffusion time (t) through x = (2Dt)1/2, where D is the diffusion constant of the diffusive species. Figure 10 shows the characteristic diffusion time in which the reaction rate is controlled by dye diffusion during the photodegradation. The second, third, and fourth reusability experiments under UV light illumination also experienced a reaction rate controlled by diffusion. Although the adsorption capability of the composite catalyst became lower in the next reusability experiment, the photoreaction on the composite surface was still strong enough to degrade the dye. The higher photon energy interaction that induced many more electron and hole pairs caused the reaction step did not limit the photodegradation rate in Figure 10. Therefore, the UV photodegradation is limited by mass transport of dye molecules via a diffusion mechanism. The photoreaction-controlled kinetics in the visible light reusability experiments in Figure 11 was dominant, which was
Figure 10. Four cycles (a, b, c, d) of SiO2/TiO2/CuBiS2/Ag composite catalyst reusability for photodegradation experiments under ultraviolet irradiation.
respectively. The consecutive tests of reusability under UV irradiation revealed the dye adsorption capability of catalyst decreased from the first run to the fourth run as shown in Figure 10a−d. The catalyst adsorbed 55% (Figure 10a), 15% (Figure 10b), 3% (Figure 10c), and 8% (Figure 10d) dye at t = 0 min after the sequential tests. this result indicates that after the chemical adsorption at the first run, the catalyst surface was covered by the intermediate compounds and was not available for further adsorption at the following reusability tests. The required durations to reach the 99%, 97%, 94%, and 97% degradation of AB 1 dye were 5, 15, 20, and 20 min for the subsequent reusability tests in panels a, b, c, and d of Figure 10, respectively. After the continuous four runs of reusability, the catalyst remained active and could degrade the AB 1 dye in 20 min under the UV irradiation. As a specific example for the third run shown in Figure 10c, we found that the catalyst did 13638
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Figure 12. Photodegradation mechanism of the SiO2/TiO2/CuBiS2/Ag composite catalyst.
related to the lower photon energy in the visible light. However, the first run (Figure 11a) in the visible light reusability experiment showed the surface diffusion-controlled kinetics. In the first run, the fresh composite catalyst surface was uncovered by an intermediate dye layer; therefore, the photoreaction rate was faster than the dye diffusion rate. After the first run, the composite catalyst was covered by a thin layer of intermediate dye that made the adsorption unavailable. The covered composite surface caused the photocarriers to take more time to react with the dye molecules because of the limiting photon energy of visible light. Consequently, the curve of degradation percentage was linearly proportional to the degradation time, as shown in Figure 11b−d. Under these conditions, photoreaction is the determining step for overall reaction rate. As we explained with the data in Figures 8 and 9, the coupling effect of CuBiS2 with TiO2 apparently plays a major role in photodegradation. The coupling effect between pCuBiS2 and n-TiO2 and its response to the photoenergy can be interpreted as the p−n solar cell behavior. Each contact between p-CuBiS2 and n-TiO2 behaves as a p−n heterojunction diode with a built-in electric field. The schematic presentation of the photocatalytic mechanism proposed in this work is shown in Figure 12. After charge separation, the photogenerated electron will induce water in solution to form a hydroxyl radical and the dissolved oxygen to form an oxygen radical followed by the dye mineralization reaction.48 Each composite catalyst can be viewed as many nano p−n diodes. Without the light illumination, the built-in electric field (electrostatic force) can adsorb the molecules with permanent or induced dipoles and adsorb the ionic species.49,50 Once the nano p−n diodes are illuminated, the photogenerated electron−hole pairs will be separated by the built-in electric field with the photoinduced hole drifting to the p-type end for the oxidation reaction and the photoinduced electron drifting to the n-type end for the reduction reaction. The difficulty in the separation of photoinduced charges can be diminished. However, the built-in electric field at the p−n hetero junction needs to have a good p−n interface to establish the alignment of Fermi levels for both p-CuBiS2 and n-TiO2. As our design with the outer p-CuBiS2 nanoparticles covering on n-TiO2, the oxidation reaction with the dye solution can be dominate over the reduction reaction.
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CONCLUSIONS
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AUTHOR INFORMATION
20% CuBiS2 and 2% Ag nanoparticles-coated SiO2 sphere@ nano TiO2 layer (SiO2@TiO2) composite catalyst, abbreviated as SiO2/TiO2/CuBiS2/Ag, was fabricated by a facile method and characterized. The diffraction peaks identified by XRD were contributed from TiO2, CuBiS2, and Ag with a small intensity of Bi2S3 and Cu3BiS3 as secondary phases. TEM and STEM images showed uniform deposition of TiO2, CuBiS2, and Ag on SiO2 spheres. The DRS UV−vis spectroscopy showed that the composite catalyst has an absorbance in the wavelength range of UV and visible light. As-prepared composites of SiO2/TiO2/CuBiS2/Ag showed excellent photocatalytic activities under UV and visible light irradiation. The Ag nanoparticle was used to accelerate and facilitate the charge separation of the photoinduced electron and hole pairs. The design concept of introducing CuBiS2 onto our composite catalyst is novel and leads to greatly improved visible light photocatalysis. Without coupling with the TiO2, CuBiS2 nanoparticle itself was hydrophobic in nature and totally could not work in dye degradation, but it showed good performance after coupling with TiO2 nanoparticles to form a heterojunction diode with a built-in electric field. As a result, millions of nano p−n diodes on composite catalysts exist in a solution state to interact with dye through the electric field and to enhance photodegradation. Owing to our efficient composite photocatalyst under visible and UV light irradiation, these results show promise for applications in environmental cleaning and other photoinduced reactions.
Corresponding Author
*Fax: +011-886-2-27303291. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by Ministry of Science and Technology of the Republic of China under Grant MOST 103-2218-E-011-015 and by the National Taiwan University of Science and Technology through Grant 103H451201. 13639
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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Photocatalytic Performance of Ag and CuBiS2 Nanoparticle-Coated SiO2@TiO2 Composite Sphere under Visible and Ultraviolet Light Irradiation for Azo Dye Degradation with the Assistance of Numerous Nano p−n Diodes Hairus Abdullah† and Dong-Hau Kuo*,† †
Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ABSTRACT: Ag and CuBiS 2 nanoparticle-coated SiO 2 sphere@nano TiO2 coating (SiO2@TiO2) composite catalyst (abbreviated as SiO2/TiO2/CuBiS2/Ag) has been successfully synthesized and characterized as well as tested for photocatalysis of Acid Black 1 (AB 1) dye under visible and ultraviolet (UV) light irradiation. These as-prepared composite spheres show photocatalytic activities under visible and ultraviolet light irradiation. This work is the first report of CuBiS2 semiconductor nanoparticles used as a material for photodegradation. The data showed that the SiO2/TiO2/CuBiS2/Ag composite particles completely degraded 50 mL of 10 ppm AB 1 dye solution in only 5 min under UV light irradiation and 100 mL of 5 ppm AB 1 dye solution in 30 min under visible light irradiation. The good photocatalysis of our composite spheres is attributed to the establishment of a good p−n heterojunction interface between the p-type CuBiS2 and n-type TiO2 semiconductors with the assistance of Ag nanoparticles.
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INTRODUCTION Photocatalysis is one of the prospective techniques to overcome the energy shortage problem and global warming. It has been used widely in many applications such as elimination of gaseous 1,2 and water pollutants, 3,4 self-cleaning3−6 and antibacterial7,8 materials, and water splitting.9 However, work on enhancing the capability of photocatalysis is ongoing because photocatalysts with high activity and reactive selectivity are required for applications. Plastic, paper, textile, tanneries, chemical industries, etc. contribute their effluents to environment because of the great consumption of organic dyes. The dye pollutant in water is one kind of pollution produced by various industries in the world. The World Bank estimates that almost 20% of global industrial water pollution comes from the treatment and dyeing of textiles. The data shows that approximately 700 000 tons of dyes are produced each year, and 20% of that total is not treated.10 Due to environmental and health issues, a great deal of effort has already been dedicated to solving the problems caused by dye pollutants. Photocatalytic degradation of dye pollutants is one of the alternatives that has the ability to degrade many kinds of organic dyes and may help solve this problem.11 Nowadays, semiconductor photocatalysis has become popular because of its potential to solve environmental and energy issues effectively. In spite of many emergent technologies, the efficiency of photocatalytic reaction is still moderate and is a bottleneck for field application.12 Therefore, many attempts have been made to find desirable semiconductor © 2015 American Chemical Society
nanoparticle-based photocatalysts. TiO2 has been the most popular photocatalyst and already has been used in many applications. Because of its limited 4% absorbance under sunlight,13−15 many scientists have tried to modify it to become a visible light-activated photocatalyst.16−18 The efforts include substitutions with anions such as N, S, C, etc.19,20 and cations such as Cr, V, Fe, Mn, Co, Ni, etc.21−24 To obtain the improved photoelectrochemical properties, the composite particle architecture is crucial. When TiO2 is coupled with quantumconfined inorganic semiconductor sensitizers such as metal oxide,25,26 CdS,27 and low-band gap semiconductor to increase its ability in visible light absorbance,28−32 it becomes feasible and promising for industrial application. The concept of forming a built-in electrical field in a p−n heterojunction diode to enhance the visible light photocatalytic activity has been applied. Composite particles such as CuCr2O4/TiO2, BiVO4/CuCr2O4, BiVO4/Bi2O3, AgBr/BiPO4, Bi2S3/BiVO4, CaFe2O4/Ag3VO4, metal oxide (CuO, Co3O4, NiO)/BiVO4, Cu2O/In2O3, etc. with a p−n heterojunction structure for the efficient separation of the visible light-induced electron and hole pairs have been studied.33−40 There are few studies about CuBiS2. It was found to be a potential visible light-driven material and one of the alternative solar cell absorber materials.41,42 Balasubramanian et al. studied the structural and optical properties of CuBiS2. They found that Received: February 27, 2015 Revised: May 21, 2015 Published: June 3, 2015 13632
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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SiO2@TiO2 symbol for the core−shell structure will be replaced by SiO2/TiO2 or S/T in abbreviated format to simplify our system presentation later. Synthesis of SiO2/TiO2/CuBiS2. To coat CuBiS2 on the SiO2/TiO2 composite, 0.1 g of the as-prepared SiO2/TiO2 composite particles were first dispersed in 15 mL of oleylamine and treated in ultrasonication bath for 1 h. In the other vessel, a total mass of 0.1 g of sulfur was dissolved in oleylamine by treatment in ultrasonication bath for 2 h. The dissolving sulfur in oleylamine produced a dark red solution. The precursors of CuI and BiCl3 with the molar ratio 1:1 were prepared by dissolving 0.0115 g of CuI and 0.019 g of BiCl3 in 100 mL three-neck flask that contained 25 mL of oleylamine by heating at 130 °C in vacuum under vigorous stirring. The as-prepared SiO2/TiO2-dispersed solution was injected into the three-neck flask after 1 h of heating. The heating of reaction solution then was held for another 1 h at the same temperature. Finally, the reaction solution temperature was increased from 130 to 280 °C and held for 1 h. The dark red sulfur solution was injected drop by drop into the reaction solution when the temperature reached 230 °C. A black solution was formed immediately as the sulfur solution dropped into the reaction solution. After 1 h of heating at 280 °C, the black reaction solution was slowly cooled to room temperature under vigorous stirring. The precipitates in the solution were centrifuged and washed with hexane and alcohol three times followed by drying in a rotary evaporator. The obtained black powder was SiO2/TiO2/ CuBiS2. The total amounts of the as-prepared CuBiS2 in the black powder reached 20 wt % based on the weight amount of SiO2/TiO2 composite particles. Depositing Ag Nanoparticles on SiO2/TiO2/CuBiS2. In the final procedure, the Ag nanoparticles were coated on SiO2/ TiO2/CuBiS2 composite particles. To coat the Ag nanoparticles, 0.1 g of SiO2/TiO2/CuBiS2 particles were dispersed into the mixed solution of 6 mL of DI water and 6 mL of alcohol; then, 3.15 mg of AgNO3 was added to the dispersion solution. After the treatment in ultrasonication bath for 15 min, the solution was exposed to UV irradiation for 1 h to precipitate Ag nanoparticles on SiO2 /TiO2 /CuBiS2 by reduction reaction. The solution with Ag-coated SiO2/TiO2/ CuBiS2 particles was washed three times with alcohol and dried in a rotary evaporator. The as-prepared composite particle was SiO2/TiO2/20% CuBiS2/2% Ag and is abbreviated SiO2/TiO2/ CuBiS2/Ag. Characterizations. The powder X-ray diffraction (XRD) patterns were recorded by a Bruker D2-phaser diffractometer using Cu Kα radiation (λ = 1.5418 Å). The surface morphologies were examined by transmission electron microscopy (TEM, H-7000, Hitachi) equipped with a CCD camera and field-emission scanning electron microscopy (FESEM, JSM 6500F, JEOL). The element mapping of the composite catalyst was done by scanning transmission electron microscopy (STEM, Tecnai F20 G2, Philips). The UV−vis diffuse reflectance spectra (DRS) and the photodegradation of AB 1 dye were recorded using a Jasco V-670 UV−visible−nearinfrared (UV−vis−NIR) spectrophotometer. Absorbance spectra measured by Fourier transform infrared spectroscopy (FTIR, Agilent Digilab FTS-3500) were used to identify the intermediates caused by the photoirradiation. Photodegradation of AB 1 Dye. Photodegradation experiments were conducted under visible light and ultraviolet light irradiation. The powers of the visible and ultraviolet light lamps were 150 W of incandescent halogen lamp and 450 W of
this material had a band gap of 2.19−2.62 eV and can be a visible light-driven semiconductor.41 Temple et al. also studied the geometry, electronic structure, and bonding in CuMCh2 (M = Bi, Sb; Ch = S, Se) and found that this material with poor hole mobility had an indirect band gap.42 However, CuBiS2 has not yet been applied for photocatalysis. In our previous work, Mahesh et al. successfully deposited Ag nanoparticles on SiO2 sphere@nano TiO2 layer composite particles and greatly enhanced its photocatalytic activity in the presence of ultraviolet (UV) light illumination.17 The aim in this work is to improve the previous composite catalyst as a visible light-activated material by introducing CuBiS2 nanoparticles to the catalyst. It is expected that we will obtain both UV and visible light-driven materials from our composite catalyst in this work. In this work, SiO2 spherical particles are used as a support for forming Ag and CuBiS2 nanoparticle-deposited SiO2 sphere@ nano TiO2 layer composite particles to degrade Acid Black 1 (AB 1) dye under visible and ultraviolet light irradiation by generating hole and electron pairs. These composite particles can behave as photoelectrochemical cells that have photodegradation ability when it interacts with light. In this case, TiO2 and CuBiS2 act as n-type and p-type materials, respectively. The Ag nanoparticles were coated on the surface of the composite catalyst to reduce the recombination probability of photoinduced electron and hole pairs and to enhance the capability to degrade AB 1 dye. The CuBiS2 in the SiO2/TiO2/CuBiS2/Ag composite catalyst is reported for the first time as an active photocatalyst. It is believed that CuBiS2 has an important role in the visible light absorbance; TiO2 plays an important role in UV light interaction, and the nanojunction diodes formed by the contact between CuBiS2 and TiO2 provide the enhanced pollutant adsorption and photocatalytic activity. Thus, the SiO2/TiO2/CuBiS2/Ag composite catalysts as a whole can quickly degrade the AB 1 dye under both UV and visible light irradiation. This composite catalyst is quite promising for application in eliminating gaseous and water pollutants and other photorelated reactions.
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EXPERIMENTAL METHODS Materials. Materials used in this work were commercially available without further purification. Synthesis of SiO2 Sphere Support. SiO2 spherical particles were prepared as follows: 240 mL of DI water and 160 mL of ethanol were added in sequence and treated in ultrasonication bath for 1 min. Commercial ammonium hydroxide (3.2 mL) was added to the reaction solution dropby-drop. After 30 min of stirring, 4 mL of TEOS (tetraethyl orthosilicate) was added and the mixture was stirred again for 2 h. Then, the precipitate was centrifuged and washed three times using alcohol. The washed product was dried using rotary evaporation. Finally, the SiO2 spherical particles were obtained after pyrolysis at 500 °C for 3 h. TiO2 Coating on SiO2 Spherical Particles. To coat TiO2 on SiO2 spherical particles, 0.1 g of as-pyrolyzed SiO2 spherical particles and 4 mL of isopropanol were mixed and treated in an ultrasonication bath for 15 min. To this solution was added 0.1 mL of titanium isobutoxide, and the solution was stirred for 5 min. During stirring, 20 mg of DI water was added under continuous stirring at room temperature for 4 h. The precipitate then was collected by centrifugation and washed with alcohol three times. The TiO2-coated SiO2 (SiO2@TiO2) sphere was then annealed at 450 °C for 2 h. The familiar 13633
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The Journal of Physical Chemistry C Xenon light with cut off filter (λ > 400 nm), respectively. The halogen lamp was jacketed by a water-cooled annular quartz tube to avoid lamp heating during photodegradation experiments. The whole setup of the halogen lamp was immersed into a reactor glass containing AB 1 dye solution which was equipped with an air channel to provide enough oxygen to the dye solution during the photoreaction. The xenon light illumination was filtered to transmit light with the wavelength lower than 400 nm. In the photodegradation experiment with UV light irradiation, 10 mg of SiO2/TiO2/CuBiS2/Ag composite catalyst was dispersed in 50 mL of 10 ppm dye solution, whereas in the visible light experiment, 20 mg of SiO2/TiO2/CuBiS2/Ag composite catalyst was dispersed in 100 mL of 5 ppm dye solution. To confirm the equilibrium of adsorption and desorption between the catalyst and AB 1 dye, the as-prepared catalyst-dispersed solutions were kept in the dark and stirred for 30 min. Then the solution started to be irradiated under UV or visible light. The light-on time was set at t = 0 min, and the time of adding the catalyst into the dye solution was set at t = −30 min. The 5 mL aliquots were sampled at different irradiation time intervals, and the concentration of AB 1 dye was monitored by UV−vis absorbance intensity at 615 nm.
phases were formed because of the excess amount of sulfur added at high temperature into the reaction solution during the preparation of CuBiS2. An excess amount of sulfur was needed to compensate the possibility of losing sulfur because sulfur was easily vaporized at high temperature. The high temperature was needed to well crystallize CuBiS2. However, at these conditions of excess sulfur and high temperature, CuBiS2 suffered from phase separations of Bi2S3 and Cu3BiS3. As the ratio of phase separations was stoichiometrically balanced, the equation can be proposed as follows: Δ
3CuBiS2 → Cu3BiS3 + Bi 2S3
From the XRD phase identification, the TiO2, CuBiS2, and Ag phases had been grown on SiO2 spheres. Figure 2 shows FE-SEM images of (Figure 2a) SiO2 spheres and (Figure 2b) SiO2/TiO2 and (Figure 2c) SiO2/TiO2/ CuBiS2/Ag composite particles. FE-SEM images show that all nanoparticles are well coated on the smooth surfaces of SiO2 spheres without any aggregation outside SiO2 spheres, as shown in Figure 2a. The TiO2-coated SiO2 spheres shown in Figure 2b maintain the smooth surfaces. After Ag and CuBiS2 nanoparticles were coated on SiO2/TiO2 spheres, the composite spheres became rougher. The EDS analysis in Figure 2d confirmed that all the peaks were contributed from the compositional elements in the SiO2/TiO2/CuBiS2/Ag composite catalyst. This compositional analysis is consistent with the XRD analyses shown in Figure 1. Ag nanoparticles were added to improve photocatalytic activities by trapping the generated photoelectron and reducing the recombination rate of generated photocarriers. With the XRD analyses to identify the phase qualitative composition and the SEM images to confirm the absence of nanoparticle aggregation, we can be sure that our deposition of TiO2, CuBiS2, and Ag on SiO2 spheres is well executed. Panels a and b of Figure 3 show low-magnification and highmagnification TEM images of SiO2/TiO2/CuBiS2/Ag composite catalyst, respectively. The monodispersed SiO2@TiO2 spheres were coated by CuBiS2 and Ag nanoparticles, which turned out to form the rougher surface on the composite spheres. Originally, SiO2 spheres had a smooth surface, as shown in Figure 2a. Ag nanoparticles were black particles and uniformly distributed on spheres. From the TEM image, the Ag size was much smaller than 10 nm. The TEM images show the composite catalyst with good spherical shape and without aggregation; the TiO2 and CuBiS2 were well distributed on it, which provided a high probability of forming a p−n heterojunction between p-CuBiS2 and n-TiO2. A high-angle annular dark field (HAADF) image and element mapping are shown in Figure 4. The selected area in Figure 4a was analyzed for different elements including Si, O, Ti, Cu, Bi, S, and Ag. From the information on element mappings of Si and O (panels b and c of Figure 4, respectively), the existence and location of the SiO2 sphere were confirmed. The O and Ti mappings (panels c and d of Figure 4, respectively) indicated the TiO2 phase formed a much continuous and packed layer, as evidenced by a dense distribution of Ti at the outer surface with color gradient from the dense (surface area) to less dense area of Ti in the middle area as shown in Figure 4d. The mapping images for Cu, Bi, and S (panels e, f, and g of Figure 4, respectively) indicated the existence of CuBiS2 phase. The Ag mapping in Figure 4h indicated the Ag nanoparticles were uniformly distributed on the sphere. Because of the spherical
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RESULTS AND DISSCUSSION XRD profiles of SiO2 sphere and SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag composite spheres are shown in Figure 1. The results show that SiO2 sphere was amorphous
Figure 1. XRD patterns of SiO2, SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag.
(PDF 29-0085). The peaks of anatase TiO2 appeared after the TiO2 coating was grown on SiO2 spheres and desiccated at 450 °C. The specific TiO2 peaks were all in good agreement with the peak locations listed in the reference profile of PDF 211272. The anatase phase was well-crystallized. It has a relative small mean crystalline size of 8.9 nm, as estimated from the full width at half-maximum (fwhm) at 25.8° with (101) reflection and the Scherrer equation. After photoreduction for forming Ag, the shown peak located at 44.7° 2θ corresponded to the Ag (111), as compared with the reference of PDF 04-0783. The SiO2/TiO2/CuBiS2/Ag composite catalyst simultaneously displayed all the peaks contributing from TiO2, CuBiS2, and Ag. The observed CuBiS2 peaks agreed well with the reference profile of PDF 43-1473; however, two peaks with a small intensity were found at 22.6° and 23.1° in the pattern, which are related to Bi2S3 and Cu3BiS3, respectively. These secondary 13634
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Figure 2. FE-SEM images of (a) SiO2 spheres and (b) SiO2/TiO2 and (c) SiO2/TiO2/CuBiS2/Ag composite particles. The EDS spectrum for SiO2/ TiO2/CuBiS2/Ag composite catalyst is shown in (d).
Figure 3. (a) Low-magnification and (b) high-magnification TEM images of SiO2/TiO2/CuBiS2/Ag composite catalyst.
absorbance capability and made all the CuBiS2-containing particles have the similar visible light absorbance. The calculated band gaps of CuBiS2 were 1.33 and 2.29 eV. Our CuBiS2 had two band gaps due to the secondary phases of Bi2S3 and Cu3BiS3 with low band gaps of 1.3 eV43,44 and 1.2 eV,45,46 respectively. Because the weight ratio of CuBiS2 on SiO2 sphere was very low (about 2.92% atomic, based on EDS analysis), CuBiS2 on SiO2 had only a small intensity contribution to the visible light absorbance with the calculated optical band gap near 2.3 eV. Ag nanoparticles also contributed to the visible light absorbance because of the important surface plasmonic resonance,17 as supported by the SiO2/CuBiS2/Ag and SiO2/ TiO2/Ag data. The TiO2 coating gave the sphere absorbance
shape, most of the mapping images look similar; however, the Ti mapping could confirm the TiO2 phase deposited as a slightly dense and continuous layer on the sphere support. Figure 5 shows the UV−vis−NIR absorbance spectra for SiO2, CuBiS2, SiO2/CuBiS2 (S/CuBiS2), SiO2/CuBiS2/Ag (S/ CuBiS2/Ag), SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag). The SiO2 sphere had only a little light absorbance in the visible wavelength range. If the wavelength axis in DRS measurement is extended to the UV range, SiO2 will have a strong light absorbance at 250 nm wavelength. The calculated optical band gap showed SiO2 had a band gap value of 4.63 eV. The CuBiS2 with a little secondary phase of Bi2S3 and Cu3BiS3 had good visible light 13635
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Figure 4. (a) HAADF STEM image and elemental mapping of (b) Si, (c) O, (d) Ti, (e) Cu, (f) Bi, (g) S, and (h) Ag.
composite catalyst has been proposed. However, most of the reported studies have focused on two-component systems because of the difficulty of controlling uniformity in the processes of adding the third nanoparticle onto two-component composite particles. Our SiO2 sphere with a size of ∼500 nm is much larger than the nanoparticles with the size smaller than 10 nm and can provide locations for multilayered nanoparticles to display the compounding advantages. Photocatalytic Degradation Tests. Figure 6 shows the changes in UV−vis spectra of AB 1 dye on UV irradiation in the
Figure 5. Comparison of UV−vis diffuse reflectance spectra for the light absorption of SiO2, CuBiS2, SiO2/CuBiS2 (S/CuBiS2), SiO2/ CuBiS2/Ag (S/CuBiS2/Ag), SiO2/TiO2(S/T), SiO2/TiO2/Ag (S/T/ Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag).
capability in the range of UV light with the calculated optical band gap of about 3.4 eV. However, after Ag nanoparticles were coated on SiO2/TiO2, the composite sphere has visible light optical band gap of 2.8 eV, as supported by the SiO2/TiO2 and SiO2/TiO2/Ag data in Figure 5. The tunable visible light band gap of TiO2 achieved by varying the Ag content in TiO2/Ag composite also has been reported by Hari et al.47 The SiO2/ TiO 2 /CuBiS 2 /Ag composite sphere combined the UV absorbance of TiO2, the visible light absorbance of CuBiS2, and the surface plasmonic resonance absorbance of Ag nanoparticles. The optical band gap of the SiO2/TiO2/ CuBiS2/Ag composite sphere was 2.9 eV as calculated from its DRS data. This composite catalyst has advantages because it utilizes more than two components. The concept of a
Figure 6. Changes in UV−vis spectra of AB 1 Dye in the presence of SiO2/TiO2/CuBiS2/Ag composite catalyst.
presence of SiO2/TiO2/CuBiS2/Ag composite catalyst. The dye degradation under UV irradiation was referred to the intensity drop of the peak located at 615 nm. After the sample was stirred for 30 min in dark conditions, there was a 55% dye adsorption and the catalyst color changed from gray to dark blue before the UV light started to irradiate the dye solution at t = 0 min. After irradiation for 10 min, the dye solution was completely discolored by composite catalyst and the catalyst color returned back to gray. The transition between dark blue and colorless for the AB 1 dye under irradiation provides us a 13636
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The Journal of Physical Chemistry C good indicator to differentiate adsorption and degradation by the photocatalyst. From the data shown in Figure 6, we can conclude that the AB 1 dye can be adsorbed by the photocatalyst in the dark conditions and that both the adsorbed dye on the photocatalyst and the dye in the solution can be degraded under UV irradiation in 10 min. The degradation time of 10 min indicates that the SiO2/TiO2/CuBiS2/Ag composite particle is a very powerful photocatalyst. The accelerated photoreaction can be attributed to the special design and the materials selected for our composite catalyst as well as the dye adsorption by the catalyst. To confirm the dye degradation, the catalyst under UV irradiation for 30 min was washed and dried for FTIR analysis, as shown in Figure 7. To easily analyze the peaks of the FTIR
Figure 8. Photocatalytic activities of SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag) composite catalysts in the presence of ultraviolet irradiation for 10 ppm AB 1 dye degradation.
the CuBiS2/Ag and SiO2/CuBiS2/Ag data, CuBiS2 is apparently little help under UV irradiation. Furthermore, the coupling effect of CuBiS2 with TiO2 apparently plays a major role in the fast photocatalysis. One major issue that needs to be clarified is the hydrophobic nature of pure CuBiS2. With the composite structure on the SiO2 sphere, the SiO2/TiO2/CuBiS2/Ag catalyst can be well dispersed in the dye solution. After the successful dye degradation under UV irradiation, the composite catalysts are also tested for visible light photocatalysis under the Halogen lamp. The difference between the UV and visible light tests was the catalyst and dye contents. As mentioned in the discussion of the experimental procedure, a greater amount of catalyst particles were added for the visible photocatalysis. The same five kinds of catalyst systems are investigated, as shown in Figure 9. Some interesting photocatalytic performances were observed. Because of the hydrophobic nature of CuBiS2, CuBiS2/Ag exhibited a total absence of any responses under visible light testing. The TiO2containing SiO2/TiO2 and SiO2/TiO2/Ag catalysts with good
Figure 7. FTIR absorbance spectra of a reused SiO2/TiO2/CuBiS2/Ag composite catalyst.
results, the SiO2/TiO2/CuBiS2/Ag powder before being used for photodegradation and pure AB 1 dye were also included in the analysis. The rinse of used composite catalyst by alcohol was used to confirm that there was no unreacted blue dye to be removed. The small peak located at 966 cm−1 confirms the Si− O−Ti bonding in the SiO2/TiO2 composite. The strong peak at 1090 cm−1 is contributed from the Si−O−Si bonding in the SiO2 major phase.17 When the SiO2/TiO2/CuBiS2/Ag catalyst before and after testing for photocatalytic experiment are compared, the peak at 1460 cm−1 is related to the peak from AB 1 dye after degradation. This peak is related to the C−C C asymmetric stretch in the aromatic ring, generated from the photodegraded intermediate products.48 These intermediate products are chemically adsorbed on catalyst particle and cannot be removed after the alcohol cleaning procedure. Although there is coverage of the intermediate products on the photocatalyst, they do not affect the photodegradation because of the nature of its monolayer adsorption and its small molecular size. To demonstrate the advantage of our specially designed SiO2/TiO2/CuBiS2/Ag composite particle, five kinds of catalyst systems including CuBiS2/Ag, SiO2/CuBiS2/Ag, SiO2/TiO2, SiO2/TiO2/Ag, and SiO2/TiO2/CuBiS2/Ag are investigated, as shown in Figure 8. As the dye solution was irradiated under UV light, the visible light-excitable CuBiS2-containing catalysts had inferior performance as compared with the TiO2-containing catalysts. Except for SiO2/TiO2/CuBiS2/Ag, the best performance was from SiO2/TiO2/Ag. However, SiO2/TiO2/Ag catalyst was greatly improved for photodegradation under UV irradiation after visible light-activated CuBiS2 was added. From
Figure 9. Photocatalytic activities of SiO2/TiO2 (S/T), SiO2/TiO2/Ag (S/T/Ag), and SiO2/TiO2/CuBiS2/Ag (S/T/CuBiS2/Ag) composite catalysts in the presence of visible light irradiation for 5 ppm AB 1 dye degradation. 13637
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The Journal of Physical Chemistry C performance shown in Figure 8 became ineffective as visible photocatalysts. SiO2/CuBiS2/Ag showed strong adsorption at t = 0 but with very limited photocatalytic capability. SiO2/TiO2/ CuBiS2/Ag catalyst showed an 80% absorbance in the dark conditions with a dark blue color on catalyst particles, but the dye solution became colorless and the catalyst became its own color of gray after 30 min under the visible light irradiation. The color change informs us that the photocatalysis is executed at two steps. The first step is the dye adsorption on the catalyst. The second step is the photoreaction to degrade the dye. With the contact between dye and catalyst after strong adsorption, the photodegradation between dye and catalyst can be accelerated. Reusability of the composite photocatalyst is to repeatedly test the photocatalytic capability of the used composite catalyst in degrading the fresh AB 1 dye. The reusability testing is conducted for four times in the same experimental conditions. After degradation was completed in each run, the catalyst was taken out and washed with deionized water several times followed by methanol cleaning. The catalyst was dried in a vacuum oven at 80 °C for 12 h and used for the next run. An additional three runs were carried out with the used SiO2/ TiO2/CuBiS2/Ag catalyst for ultraviolet and visible-light photocatalytic experiments, as shown in Figures 10 and 11,
Figure 11. Four cycles (a, b, c, d) of SiO2/TiO2/CuBiS2/Ag composite catalyst reusability for the photodegradation experiments under visible light irradiation.
not adsorb the dye after being stirred for 30 min in the dark conditions, and it still could perform the photodegradation in 20 min. This example indicates that the photocatalytic capability mainly originates from the composite particle itself with only a small part contributed from the adsorption. In the reusability testing under visible light irradiation (Figure 11), the catalyst adsorbed 78% (Figure 11a), 56% (Figure 11b), 10% (Figure 11c), and 8% (Figure 11d) dye after the sequential tests. The required durations to reach the 99%, 97%, 94%, and 96% degradation were 30, 40, 45, and 45 min, respectively. Results similar to those of the UV tests were obtained. After the third run (Figure 11c), the performance of the catalyst was the same as that of fourth run and were stable. From the reusability tests, the composite catalyst can have a long-term usage for photodegradation with performance for the tests under UV irradiation slightly superior to that under visible light. The surface diffusion-controlled reaction kinetics influenced much more in the UV reusability experiments in Figure 10. The high energy of the UV irradiation resulted in a faster photoreaction; therefore, the reaction rate was limited by diffusion of dye molecules on the surface of the catalyst. The dye molecules will be degraded more quickly as they interact with active sites of the catalyst. Therefore, the total photoreaction rate under UV illumination depends on the slower step of surface diffusion (rate-determining step). The characteristic diffusion length (diffusion distance, x) is related to the characteristic diffusion time (t) through x = (2Dt)1/2, where D is the diffusion constant of the diffusive species. Figure 10 shows the characteristic diffusion time in which the reaction rate is controlled by dye diffusion during the photodegradation. The second, third, and fourth reusability experiments under UV light illumination also experienced a reaction rate controlled by diffusion. Although the adsorption capability of the composite catalyst became lower in the next reusability experiment, the photoreaction on the composite surface was still strong enough to degrade the dye. The higher photon energy interaction that induced many more electron and hole pairs caused the reaction step did not limit the photodegradation rate in Figure 10. Therefore, the UV photodegradation is limited by mass transport of dye molecules via a diffusion mechanism. The photoreaction-controlled kinetics in the visible light reusability experiments in Figure 11 was dominant, which was
Figure 10. Four cycles (a, b, c, d) of SiO2/TiO2/CuBiS2/Ag composite catalyst reusability for photodegradation experiments under ultraviolet irradiation.
respectively. The consecutive tests of reusability under UV irradiation revealed the dye adsorption capability of catalyst decreased from the first run to the fourth run as shown in Figure 10a−d. The catalyst adsorbed 55% (Figure 10a), 15% (Figure 10b), 3% (Figure 10c), and 8% (Figure 10d) dye at t = 0 min after the sequential tests. this result indicates that after the chemical adsorption at the first run, the catalyst surface was covered by the intermediate compounds and was not available for further adsorption at the following reusability tests. The required durations to reach the 99%, 97%, 94%, and 97% degradation of AB 1 dye were 5, 15, 20, and 20 min for the subsequent reusability tests in panels a, b, c, and d of Figure 10, respectively. After the continuous four runs of reusability, the catalyst remained active and could degrade the AB 1 dye in 20 min under the UV irradiation. As a specific example for the third run shown in Figure 10c, we found that the catalyst did 13638
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Figure 12. Photodegradation mechanism of the SiO2/TiO2/CuBiS2/Ag composite catalyst.
related to the lower photon energy in the visible light. However, the first run (Figure 11a) in the visible light reusability experiment showed the surface diffusion-controlled kinetics. In the first run, the fresh composite catalyst surface was uncovered by an intermediate dye layer; therefore, the photoreaction rate was faster than the dye diffusion rate. After the first run, the composite catalyst was covered by a thin layer of intermediate dye that made the adsorption unavailable. The covered composite surface caused the photocarriers to take more time to react with the dye molecules because of the limiting photon energy of visible light. Consequently, the curve of degradation percentage was linearly proportional to the degradation time, as shown in Figure 11b−d. Under these conditions, photoreaction is the determining step for overall reaction rate. As we explained with the data in Figures 8 and 9, the coupling effect of CuBiS2 with TiO2 apparently plays a major role in photodegradation. The coupling effect between pCuBiS2 and n-TiO2 and its response to the photoenergy can be interpreted as the p−n solar cell behavior. Each contact between p-CuBiS2 and n-TiO2 behaves as a p−n heterojunction diode with a built-in electric field. The schematic presentation of the photocatalytic mechanism proposed in this work is shown in Figure 12. After charge separation, the photogenerated electron will induce water in solution to form a hydroxyl radical and the dissolved oxygen to form an oxygen radical followed by the dye mineralization reaction.48 Each composite catalyst can be viewed as many nano p−n diodes. Without the light illumination, the built-in electric field (electrostatic force) can adsorb the molecules with permanent or induced dipoles and adsorb the ionic species.49,50 Once the nano p−n diodes are illuminated, the photogenerated electron−hole pairs will be separated by the built-in electric field with the photoinduced hole drifting to the p-type end for the oxidation reaction and the photoinduced electron drifting to the n-type end for the reduction reaction. The difficulty in the separation of photoinduced charges can be diminished. However, the built-in electric field at the p−n hetero junction needs to have a good p−n interface to establish the alignment of Fermi levels for both p-CuBiS2 and n-TiO2. As our design with the outer p-CuBiS2 nanoparticles covering on n-TiO2, the oxidation reaction with the dye solution can be dominate over the reduction reaction.
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CONCLUSIONS
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AUTHOR INFORMATION
20% CuBiS2 and 2% Ag nanoparticles-coated SiO2 sphere@ nano TiO2 layer (SiO2@TiO2) composite catalyst, abbreviated as SiO2/TiO2/CuBiS2/Ag, was fabricated by a facile method and characterized. The diffraction peaks identified by XRD were contributed from TiO2, CuBiS2, and Ag with a small intensity of Bi2S3 and Cu3BiS3 as secondary phases. TEM and STEM images showed uniform deposition of TiO2, CuBiS2, and Ag on SiO2 spheres. The DRS UV−vis spectroscopy showed that the composite catalyst has an absorbance in the wavelength range of UV and visible light. As-prepared composites of SiO2/TiO2/CuBiS2/Ag showed excellent photocatalytic activities under UV and visible light irradiation. The Ag nanoparticle was used to accelerate and facilitate the charge separation of the photoinduced electron and hole pairs. The design concept of introducing CuBiS2 onto our composite catalyst is novel and leads to greatly improved visible light photocatalysis. Without coupling with the TiO2, CuBiS2 nanoparticle itself was hydrophobic in nature and totally could not work in dye degradation, but it showed good performance after coupling with TiO2 nanoparticles to form a heterojunction diode with a built-in electric field. As a result, millions of nano p−n diodes on composite catalysts exist in a solution state to interact with dye through the electric field and to enhance photodegradation. Owing to our efficient composite photocatalyst under visible and UV light irradiation, these results show promise for applications in environmental cleaning and other photoinduced reactions.
Corresponding Author
*Fax: +011-886-2-27303291. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by Ministry of Science and Technology of the Republic of China under Grant MOST 103-2218-E-011-015 and by the National Taiwan University of Science and Technology through Grant 103H451201. 13639
DOI: 10.1021/acs.jpcc.5b01970 J. Phys. Chem. C 2015, 119, 13632−13641
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