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SrBi2O4 under visible light irradiation
Applied Catalysis B: Environmental 69 (2006) 17–23 www.elsevier.com/locate/apcatb
Photocatalytic decomposition of acetaldehyde and Escherichia coli using NiO/SrBi2O4 under visible light irradiation Xuexiang Hu, Chun Hu *, Jiuhui Qu State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China Received 12 October 2005; received in revised form 29 April 2006; accepted 17 May 2006 Available online 30 June 2006
Abstract A monoclinic structure SrBi2O4 was prepared by coprecipitation method and characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and diffuse reflection UV–vis spectra (DRS). Photocatalytic activity of the catalysts was evaluated through the degradation of acetaldehyde and Escherichia coli (E. coli) under visible light irradiation (l > 420 nm). The results indicated that monoclinic structure SrBi2O4 shows visible light activity and its photocatalytic activity was greatly enhanced when further loaded with NiO by the impregnation method. This is attributed to NiO promoting the electron–hole separation and interfacial charge transfer. The FT-IR spectra of the used NiO/SrBi2O4 indicated that some intermediates such as acetic acid, H2O, CO2 were formed for the degradation of acetaldehyde. The determination of intracellular K+ leakage with the inactivation of E. coli verified that the outer membrane of the cell is destroyed, causing the cell to die under visible light excitation of NiO/ SrBi2O4. ESR studies revealed that OH, O2 were involved as the active species in the photocatalytic reaction. A possible visible light photocatalytic mechanism was proposed. # 2006 Elsevier B.V. All rights reserved. Keywords: Bacteria; Monoclinic structure; NiO/SrBi2O4; Toxic organic pollutant; Visible light photocatalyst
1. Introduction Semiconductor photocatalysis has been the focus of numerous investigations because of its application for conversion of solar energy to chemical energy stored in gaseous hydrogen, quantitative decontamination and purification of air and wastewater [1–4]. Among various oxide semiconductor photocatalysts, TiO2 was intensively investigated because of its photostable, nontoxic, cheap and active properties. However, TiO2 is effective only under ultraviolet light irradiation (l < 380 nm) due to its large band gap (3.2 eV). The UV light occupies merely ca. 4% of the whole solar energy while the visible light accounts for 43% of the solar energy. Therefore, it is necessary to develop visible light photocatalysts with high activity to utilize efficiently solar light, which involves water splitting and organic contaminants degradation under visible light irradiation. There are two ways to exploit the photocatalysts responsive to visible light irradiation: the first
* Corresponding author. Tel.: +86 10 62849171; fax: +86 10 62923541. E-mail address: [email protected] (C. Hu). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.05.008
involves the modification of some UV active oxides, such as TiO2, InTaO4 and SrTiO3, turned into visible light photocatalysts. The second is the development of a new material to utilize solar energy efficiently. The former has been largely investigated by substitutional doping of metals [5–7] or of nonmetals (C, N) [8– 10] to obtain photocatalytic activities under visible light irradiation. Although dopants such as N, C, Fe, Cr, V could induce the visible light absorption, they also serve as sites for electron–hole recombination that leads to low activities, and there is concern of the stability of substituted anions under reaction conditions. On the other hand, there have only been a few reports on the development of new visible light material for the destruction of contaminants, such as CaBi2O4 [11], BiVO4 [12], PbBi2Nb2O9 [13], Bi2WO6 [14], CaIn2O4 [15]. The variety of visible light photocatalysts is still very limited. Therefore, it is still crucial to develop new visible light photocatalytic materials with high activity to utilize sunlight more efficiently. The photocatalytic decomposition of organic contaminants requires that the valence band (VB) of the photocatalyst must meet the potential level of oxidizing the organic contaminants. The deeper the VB of a semiconductor, the stronger its oxidative activity, and the higher the photocatalytic properties
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of the material are for decomposition of the organics. In addition to the VB position, the mobility of photogenerated carriers also influences the activity of a photocatalyst. The dispersion of the VB can influence the mobility of the photogenerated holes. So the VB of a photocatalyst is a key factor for the effective photocatalytic decomposition of organic contaminants. For oxide semiconductor photocatalysts, it is indispensable to control VB with orbitals of some elements instead of O2p. Bi3+ with 6s2 configuration is a candidate for valence-band-control element [16]. For bismuth (III)-based semiconductors, the VB control by the Bi6s or hybrid Bi6s–O2p orbitals is expected to provide possibilities for the development of visible light photocatalysts [17,18]. The hybridization of the Bi 6s and O 2p levels would push up the position of the valence band, giving even smaller band gap compared with compounds that do not contain Bi in their structures [13]. And this hybridization also makes the valence band largely dispersing, which favors the mobility of holes in the valence band and is beneficial to the oxidation reaction. In the present work, SrBi2O4 powders, bismuth (III)-based semiconductors, were synthesized by co-precipitation method. Acetaldehyde was typical pollutant in indoor air, as model compound to investigate the activities of SrBi2O4 samples under visible light irradiation (l > 420 nm). Effects of NiO loading on the photocatalytic activity of SrBi2O4 catalyst were further examined. Moreover, the photocatalytic activities of NiO/SrBi2O4 for bacteria were also investigated under visible light irradiation (l > 420 nm). 2. Experimental section 2.1. Synthesis of the photocatalysts Stoichiometric amounts of Sr(NO3)2 and Bi(NO3)35H2O were dissolved in deionized water. Then a solution of ethylene diamine tetraacetic acid in ammonia and citric acid was added to the above aqueous solution. The pH value was adjusted to 4.0 by the addition of ammonia. During this process, the mixed solution was stirred continuously until a white precipitate was formed. Subsequently, the precipitation was dried at 160 8C to remove water completely. Then a voluminous and fluffy black precursor material was obtained. The precursor material was calcined at 350 8C for 10 h and were finally calcined at 750 8C in muffle furnace for 4 and 12 h in air, respectively. A NiO co-catalyst was loaded by impregnation method. Assynthesized samples calcined at 750 8C for 12 h were put into in 0.1 mol/L Ni(NO3)2 aqueous solution. The suspension was evaporated at 70 8C by magnetically stirring. Then the dried powder was calcined at 300 8C for 1 h. 2.2. Characterization Powder X-ray diffraction was recorded on a Scintag-XDS˚ ). 2000 diffractometer with a Cu Ka radiation (l = 1.54059 A The size and shape of the particles were observed using a Hitachi S-3000N scanning electron microscopy (SEM). UV–vis absorption spectra of the samples were recorded on a UV–vis
spectrophotometer (Hitachi UV-3010) with an integrating sphere attachment for their reflectance in the range of 200–800 nm and BaSO4 was the reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed with an Axis Ultra spectrometer (Kratos, UK) using Mono Al Ka (1486.6 eV) radiation at a power of 225 W (15 mA, 15 kV). Fourier Transform Infrared Spectroscopy (FT-IR) investigations were made on a FT-IR Spectrometer (Nicolet 5700). Zetapotential of catalysts (1.0 g/L) in KNO3 solution (1 mM) were measured with Zetasizer 2000 (United Kingdom, Malvern Co). 2.3. Photocatalysis experiments 2.3.1. Photocatalytic degradation of organic pollutants under visible light The photocatalytic activities of as-prepared samples were evaluated by the degradation of acetaldehyde in gas phase. The light source for photocatalysis was a 350-W spherical Xenon lamp (Shanghai DianGuang Device Ltd.). Light passed through an IR water filter and a UV cutoff filter (Ø 30 mm, l > 420 nm) and then was focused onto a 550 mL cylindrical Pyrex glass batch reactor. The intensity of the incidence illumination through the cutoff filter was 1.48 mW/cm2. The photocatalytic decompositions of acetaldehyde were performed with 0.5 g of the powdered photocatalyst placed at the bottom of reactor at room temperature in a gas-closed system. Humidity and pH were not controlled in this experiment. The reaction gas mixture (1 atm) consisted of 15 ppm CH3CHO, 21% O2 and N2 balance gas. Prior to commencing irradiation, the reaction system was equilibrated for about 120 min until no changes in the concentration of acetaldehyde and CO2 were monitored. The changes in the acetaldehyde and CO2 concentration were obtained by a gas chromatography (GC) equipped with a flame ionization detector (N2 carrier) and the catalytic conversion furnace. 2.3.2. Measurements of bactericidal activity Escherichia coli (E. coli DH 4a), a Gram-negative bacterium, were used as model bacteria in this study, which were purchased from the Institute of Microbiology, Chinese Academy of Sciences. It was incubated in Luria–Bertani (LB) nutrient solution at 37 8C shaking for 18 h, and then washed with 0.9% saline solution by centrifuging at 4000 rpm. The treated cells were then re-suspended and diluted to 5 108 colony forming units per milliliter (CFU/mL) with 0.9% saline. The diluted cell suspension and photocatalyst were added to a 100-mL beaker with a cover. All materials used in the experiments were autoclaved at 121 8C for 25 min to ensure sterility. The final photocatalyst concentration was adjusted to 0.2 g/L, and the final bacterial cell concentration was 5 106 CFU/mL, the reaction volume was 30 mL. The intensity of the incidence illumination was 2.8 mW/cm2. The reaction temperature was maintained at 25 8C. The reaction mixture was stirred with a magnetic stirrer throughout the experiment. A bacterial suspension without photocatalyst was irradiated as a control. Before and during the light irradiation, an aliquot of the reaction solution was immediately diluted with saline and an appropriate dilution of the sample was incubated
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Fig. 1. XRD of samples: (a) SrBi2O4 calcined for 4 h; (b) SrBi2O4 calcined for 12 h; (c) Ni 1.0 wt.%/SrBi2O4.
at 37 8C for 24 h on nutrient agar medium, and then the colonies were counted to determine the number of viable cells. To monitor the change of K+ ions concentration during the inactivation of the E. coli, at every time interval, 1 mL of the illuminated bacterial suspension was withdrawn and filtered through a Milllipore filter (pore size 0.22 mm) for ICP-MS analysis on OPTIMA 2000 (Perkin-Elmer Co.) All experiments were repeated three times. 3. Results and discussion 3.1. Photocatalysts characterization Fig. 1 shows the X-ray diffraction patterns of as-synthesized samples calcined at 750 8C for different times. The samples
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calcined for 4 h mainly consisted of SrBi2O4, Sr2.25Bi6.75O12.38 and Bi2O3 crystallites. With increasing calcination time to 12 h, the intensity of the diffraction peaks of Sr2.25Bi6.75O12.38 obviously decreased, and the peaks of Bi2O3 disappeared, while the intensity of SrBi2O4 gradually increased. XRD results indicated that monoclinic structure SrBi2O4 formed predominant phase and the crystallinity became better. Fig. 2 shows the SEM morphology of the powders calcined at 750 8C for different times. The sample calcined for 4 h exhibited a honeycomb-like shape and the particles aggregated (Fig. 2a), while sample calcined for 12 h was well crystallized and some particles possessed a plate shape with about 0.2–5 mm of particle sizes (Fig. 2b). The results indicated that monoclinic structure SrBi2O4 with plate shape was obtained by prolonging calcination time to 12 h. Furthermore, The atomic ratio of Bi/Sr in the structure of the samples was 16.47:10.89, 21.34:10.66 for 4 and 12 h respectively by the measurement of energy dispersive analysis of X-rays (EDX). The value of Bi/Sr in the sample calcined for 12 h was close to the ideal value of 2:1, which agrees with the result of XRD. XRD patterns and SEM images revealed that calcination time had significant influences on the crystallinity and morphology of final products. With prolonging calcination time, the crystallinity and purity of SrBi2O4 samples were improved and the formation of the impurity phase was inhibited. The addition of Ni 1.0 wt.% (NiO/SrBi2O4) had no influence on the crystal structure of SrBi2O4 (Fig. 1c), and the plate morphology was still retained in the NiO/SrBi2O4 sample (Fig. 2c). No XRD diffraction peaks of oxide nickel species
Fig. 2. SEM images of samples: (a) SrBi2O4 calcined for 4 h; (b) SrBi2O4 calcined for 12 h; (c) Ni 1.0 wt.%/SrBi2O4.
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specimens exhibited a greater light absorption throughout the visible wavelengths due to the grayed color of the catalyst. Fig. 4 shows the changes of the zeta-potential with pH of solution. The isoelectric point of SrBi2O4 and NiO/SrBi2O4 was 10.15 and 11.54 pH units, respectively. The result implied that there were alkaline groups on the surface of catalysts. The excess alkaline group might be arisen from the formation of hydrate during the preparation. Therefore, NiO/SrBi2O4 surface was positively charged in general solution. 3.2. Photocatalytic activities of catalysts under visible light
Fig. 3. UV–vis diffuse reflectance spectra of the samples: (a) SrBi2O4 calcined for 4 h, (b) SrBi2O4 calcined for 12 h; (c) Ni 0.5 wt.%/SrBi2O4; (d) Ni 1.0 wt.%/ SrBi2O4; (e) Ni 1.5 wt.%/SrBi2O4.
were observed at NiO/SrBi2O4 samples. This was presumably due to the combination of its low content, small particle size and highly dispersed on the surface of the SrBi2O4 particle. Determination of the oxidation state of nickel was carried out by measuring Ni 2p3/2 binding energy (BE) with XPS. The BE 854.0 was assigned to NiO [19]. The concentration of Ni in the surface of the catalyst was 5.54 at% from the data of XPS. Based on the dosages of Ni(NO3)2 and SrBi2O4 (Ni 1.0 wt.%), the concentration of Ni in SrBi2O4 was 1.40 at% which was smaller the experimental value. It demonstrated that most NiO was supported on the surface of SrBi2O4. The UV–vis absorbance spectra of the as-prepared samples are shown in Fig. 3. The absorption characteristics of sample calcined for 4 h were quite similar to that of sample calcined for 12 h except for slightly shorter onset wavelength. From Fig. 3, the optical absorption spectra of SrBi2O4 calcined for 12 h showed visible light absorption band 400–540 nm. With the loading of NiO, the NiO/SrBi2O4 displayed the same absorption edge as SrBi2O4. However, the NiO/SrBi2O4
Fig. 4. Plot of the zeta-potential as a function of pH for SrBi2O4 and NiO/ SrBi2O4 suspensions (1.0 g/L) of catalyst in the presence of KNO3 (1 mM).
3.2.1. Photocatalytic activities of catalysts prepared under different conditions The photocatalytic activities of the catalysts were evaluated by the photodegradation of acetaldehyde in the gas phase under visible light irradiation. Fig. 5A illustrates the photocatalytic decomposition of acetaldehyde on the catalysts calcined for different times under visible light irradiation (l > 420 nm). Acetaldehyde was not degraded in the absence of photocatalyst. In contrast, it was degraded by different rates with various
Fig. 5. Photocatalytic decomposition of acetaldehyde over different catalysts: (A) the sample calcined for different time and (B) the sample loaded with different amount of NiO under visible light irradiation (l > 420 nm, 1.48 mW/cm2).
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Fig. 6. Selectivity toward CO2 with the visible light irradiation time over SrBi2O4 and Ni 1.0 wt.%/SrBi2O4.
catalysts under visible light irradiation. The photocatalytic activity of the sample calcined for 4 h was lower than that one calcined for 12 h, although the energy structure of the former was also similar to that of the latter. Based on the results of XRD and SEM, monoclinic structure SrBi2O4 favored photocatalytic reaction. However, under illuminated SrBi2O4 system, the concentration of CH3CHO did not decrease any more after 2 h irradiation, even prolonging irradiation time. In this experiment, the conversion of acetaldehyde to CO2 was less than 18% (as shown in Fig. 6). Fig. 5B shows the effect of different loading amount of NiO on the photodegradation of acetaldehyde. Acetaldehyde was greatly photodegraded and completely removed for 2 h irradiation under the same conditions (Ni 1.0 wt.% curve in Fig. 5B). The conversion of acetaldehyde to CO2 was more than 90% (Fig. 6), and no significant inactivation of the catalyst was observed. With the loading amount of NiO increasing, the degradation rate of acetaldehyde increased and reached a certain value at Ni
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1.0 wt.% and Ni 1.5 wt.%. And the degradation rate of acetaldehyde decreased at Ni 2.0 wt.%. The results indicated that the loading amount of NiO existed optimum value. Furthermore, FT-IR spectra of different samples were shown in Fig. 7. The fresh, acetaldehyde-adsorbed and used SrBi2O4 exhibited almost same spectra except a little bit change at around 1451 cm 1, indicating the less intermediates formation, while the fresh, acetaldehyde-adsorbed and used Ni 1.0 wt.%/ SrBi2O4 (NiO/SrBi2O4) displayed different FT-IR spectra. Compared with the fresh NiO/SrBi2O4 spectra, a new absorption peak appeared at 1426 cm 1 for the asymmetric and symmetric CH3 bending vibration of acetaldehyde in acetaldehyde-adsorbed NiO/SrBi2O4 before reaction. For NiO/ SrBi2O4 after photocatalytic degradation of acetaldehyde, several new peaks were observed. A strong new peak emerged at 1383 cm 1 for the CH3 bending vibrations of acetic acid, with the peak at 1426 cm 1 decreasing. A stronger new peak attributable to adsorbed water, a weak new peaks for C O groups of acetic acid appeared at 1626, 1750 cm 1, respectively. The intensity of two peaks at 2425 and 2494 cm 1 for adsorbed CO2 significantly increased. The results also verified that acetic acid, CO2 and H2O were produced with the degradation of acetaldehyde, and the formed acetic acid gradually accumulated on the surface of the catalyst during the course of the reaction. It has been found that the photoactivity of the catalyst decreased when this catalyst was used repeatedly. Nevertheless, after the addition of Pd, this phenomenon was avoided. The Pd/NiO/SrBi2O4 catalyst did not significantly deactivate in the time scale of these experiments. The further details about the Pd/NiO/SrBi2O4 catalyst will be reported in another work. 3.2.2. Bactericidal activity of NiO/SrBi2O4 under visible light irradiation The bactericidal activities of NiO/SrBi2O4, P25 TiO2 and Ni 1.0 wt.% P25 TiO2 (NiO/P25 TiO2) were evaluated by the
Fig. 7. FT-IR spectra of SrBi2O4 and NiO%/SrBi2O4: (a) before reaction; (b) acetaldehyde-absorbed; (c) after reaction.
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in different visible light illuminated catalyst suspensions. No obvious changes in the concentration of K+ were observed under the following several conditions: visible light irradiation without photocatalyst; visible light illuminated P25 TiO2, respectively. In contrast, in the visible light illuminated NiO/ SrBi2O4 suspensions, K+ immediately leaked out and promptly increased paralleling the inactivation of E. coli with irradiation time. The results demonstrated a notable destruction in the structure of the outer membrane and the cytoplasmic membrane of the E. coli cell, resulting in the cell death under visible light illuminated NiO/SrBi2O4. 3.3. Mechanism of NiO/SrBi2O4 photocatalysis under visible light
Fig. 8. (A) Survival of E. coli (an initial concentration of 5 106 CFU/mL) vs. reaction time and (B) Leakage of K+ ion from E.coli cells vs. reaction time under visible light irradiation (l > 420 nm, 2.8 mW/cm2).
inactivation of bacteria E. coli in water (Fig. 8A) under visible light irradiation. No obvious changes in survival were observed under the following several conditions: visible light irradiation without photocatalyst; NiO/SrBi2O4 in dark; visible light illuminated P25 TiO2. In contrast, a prompt decrease in survival was observed in the illuminated NiO/SrBi2O4 suspensions, fewer E. coli cells survived at irradiation time 60 min. The bactericidal activity of NiO/SrBi2O4 was much higher than that of P25 under visible light. In addition, no obvious changes in survival were observed in visible light-irradiated NiO/P25 TiO2 suspensions. NiO did not play any positive effect in TiO2 photocatalyst as it enhanced the photoactivity of SrBi2O4 under visible light irradiation. The results implied that NiO predominantly promoted the photogenerated electron–hole separation because SrBi2O4 could be excited by visible light to generated electron–hole pair but TiO2 could not. K+ exists universally in bacteria [20,21]. It plays roles in the regulation of polysome content and protein synthesis. Therefore, the destroyed cell membrane would result in leakage of intracellular K+ ions that paralleled cell death. Fig. 8B shows K+ leakage with the inactivation of the E. coli
The photocatalytic oxidation of organic compounds is mainly considered to be controlled by the following processes: (1) the semiconductor absorb photons and generate electron– hole pairs; (2) the photoinduced electron–hole pairs are separated; (3) the oxidation of the organic compounds by the photohole in the VB or formed OH radical; (4) the reduction of oxygen by the photoelectron in the CB [11]. Step (2) is crucial for many semiconductors because photogenerated electrons and holes easily recombine and lead low activity. The principal method of slowing the electron–hole recombination is thought to be through the loading of cocatalysts, such as NiO, on the surface of the semiconductors. Based on the XPS data, NiO was supported on the surface of SrBi2O4 by the form of NiO. When NiO was loaded on the surface of SrBi2O4, the photocatalytic activity was improved efficiently. This was suggested by the photocatalytic decomposition of acetaldehyde and bacteria. The ESR spin-trap technique (with DMPO) was used to obtain information on the active radicals involved in the photocatalytic process (Fig. 9). Since the O2 radicals in water were very unstable and undergo facile disproportionation rather than slow reaction with DMPO [22,23]. Consequently, the involvement of O2 radicals in NiO/ SrBi2O4 and SrBi2O4 system was examined respectively in methanol using a Nd:YAG laser (532 nm) as the irradiation source (Fig. 9A). The six characteristic peaks of the DMPO– O2 adducts were observed only in NiO/SrBi2O4 dispersion under visible light irradiation. No such signals were detected in dark. It meant that irradiation was essential for the generation of O2 on the surface of the catalyst. No O2 signals were detected in SrBi2O4 systems under the otherwise identical conditions. Similarly, the DMPO–OH species were detected successfully in aqueous NiO/SrBi2O4 dispersion under visible light irradiation (Fig. 9B). Four characteristic peaks of DMPO–OH, 1:2:2:1 quartet pattern were obviously observed. No such signals were detected in dark and visible light-irradiated SrBi2O4 system. The evidence that OH and O2 were produced on the surface of visible light-illuminated NiO/SrBi2O4 provided a solid indication that the catalyst can be efficiently excited by visible light to create electron–hole pairs and that the charge separation was maintained long enough to react with adsorbed oxygen/H2O and to produce a series of active oxygen radicals which finally induced the
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4. Conclusions The monoclinic structure SrBi2O4 was prepared through a co-precipitation method. SrBi2O4 exhibited the spectral response to visible region although its photocatalytic activity was limited. To the best of our knowledge, SrBi2O4 as photocatalyst under visible light is reported for the first time. Furthermore, when NiO was loaded on the surface of SrBi2O4, the photocatalytic activity was greatly improved since NiO suppress the recombination between electrons and holes. The NiO/SrBi2O4 photocatalyst shows highly photocatalytic efficiency for the removal of acetaldehyde and bacteria under visible light. ESR studies revealed that OH, O2 were involved as the active species in the photocatalytic reaction. NiO/SrBi2O4 is thus very promising for the purification of environmental pollutants. Acknowledgement This work was supported by the National Science Foundation of China (20377050, 20577062, 20537020, 50538090). References
Fig. 9. DMPO spin-trapping ESR spectra recorded at ambient temperature in methanol dispersion (for DMPO–O2 ), (A) and aqueous dispersion (for DMPO–OH), (B) under irradiation of 532 nm. (a) SrBi2O4 and (b) NiO/ SrBi2O4 before and after irradiation.
decomposition of pollutants [24,25]. Neither OH nor O2 was formed in the visible light-illuminated SrBi2O4 suspension, indicating that the recombination rate of the photogenerated electron–hole pairs from the SrBi2O4 excited by visible light was higher than that one of their separation. The results demonstrated that NiO on the external surface of SrBi2O4 trapped photogenerated electrons, enhanced the electron–hole separation leading to active species formation and improved the photocatalytic activity, which agrees with previous work [26,27]. The stability of a photocatalyst is important to its application. After photocatalytic degradation reaction of organic contaminants, the crystal structure of the NiO/SrBi2O4 photocatalyst was checked by XRD analysis. The results showed that the crystal structure of the photocatalyst was not changed after the photocatalytic reaction.
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Photocatalytic decomposition of acetaldehyde and Escherichia coli using NiO/SrBi2O4 under visible light irradiation Xuexiang Hu, Chun Hu *, Jiuhui Qu State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China Received 12 October 2005; received in revised form 29 April 2006; accepted 17 May 2006 Available online 30 June 2006
Abstract A monoclinic structure SrBi2O4 was prepared by coprecipitation method and characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and diffuse reflection UV–vis spectra (DRS). Photocatalytic activity of the catalysts was evaluated through the degradation of acetaldehyde and Escherichia coli (E. coli) under visible light irradiation (l > 420 nm). The results indicated that monoclinic structure SrBi2O4 shows visible light activity and its photocatalytic activity was greatly enhanced when further loaded with NiO by the impregnation method. This is attributed to NiO promoting the electron–hole separation and interfacial charge transfer. The FT-IR spectra of the used NiO/SrBi2O4 indicated that some intermediates such as acetic acid, H2O, CO2 were formed for the degradation of acetaldehyde. The determination of intracellular K+ leakage with the inactivation of E. coli verified that the outer membrane of the cell is destroyed, causing the cell to die under visible light excitation of NiO/ SrBi2O4. ESR studies revealed that OH, O2 were involved as the active species in the photocatalytic reaction. A possible visible light photocatalytic mechanism was proposed. # 2006 Elsevier B.V. All rights reserved. Keywords: Bacteria; Monoclinic structure; NiO/SrBi2O4; Toxic organic pollutant; Visible light photocatalyst
1. Introduction Semiconductor photocatalysis has been the focus of numerous investigations because of its application for conversion of solar energy to chemical energy stored in gaseous hydrogen, quantitative decontamination and purification of air and wastewater [1–4]. Among various oxide semiconductor photocatalysts, TiO2 was intensively investigated because of its photostable, nontoxic, cheap and active properties. However, TiO2 is effective only under ultraviolet light irradiation (l < 380 nm) due to its large band gap (3.2 eV). The UV light occupies merely ca. 4% of the whole solar energy while the visible light accounts for 43% of the solar energy. Therefore, it is necessary to develop visible light photocatalysts with high activity to utilize efficiently solar light, which involves water splitting and organic contaminants degradation under visible light irradiation. There are two ways to exploit the photocatalysts responsive to visible light irradiation: the first
* Corresponding author. Tel.: +86 10 62849171; fax: +86 10 62923541. E-mail address: [email protected] (C. Hu). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.05.008
involves the modification of some UV active oxides, such as TiO2, InTaO4 and SrTiO3, turned into visible light photocatalysts. The second is the development of a new material to utilize solar energy efficiently. The former has been largely investigated by substitutional doping of metals [5–7] or of nonmetals (C, N) [8– 10] to obtain photocatalytic activities under visible light irradiation. Although dopants such as N, C, Fe, Cr, V could induce the visible light absorption, they also serve as sites for electron–hole recombination that leads to low activities, and there is concern of the stability of substituted anions under reaction conditions. On the other hand, there have only been a few reports on the development of new visible light material for the destruction of contaminants, such as CaBi2O4 [11], BiVO4 [12], PbBi2Nb2O9 [13], Bi2WO6 [14], CaIn2O4 [15]. The variety of visible light photocatalysts is still very limited. Therefore, it is still crucial to develop new visible light photocatalytic materials with high activity to utilize sunlight more efficiently. The photocatalytic decomposition of organic contaminants requires that the valence band (VB) of the photocatalyst must meet the potential level of oxidizing the organic contaminants. The deeper the VB of a semiconductor, the stronger its oxidative activity, and the higher the photocatalytic properties
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of the material are for decomposition of the organics. In addition to the VB position, the mobility of photogenerated carriers also influences the activity of a photocatalyst. The dispersion of the VB can influence the mobility of the photogenerated holes. So the VB of a photocatalyst is a key factor for the effective photocatalytic decomposition of organic contaminants. For oxide semiconductor photocatalysts, it is indispensable to control VB with orbitals of some elements instead of O2p. Bi3+ with 6s2 configuration is a candidate for valence-band-control element [16]. For bismuth (III)-based semiconductors, the VB control by the Bi6s or hybrid Bi6s–O2p orbitals is expected to provide possibilities for the development of visible light photocatalysts [17,18]. The hybridization of the Bi 6s and O 2p levels would push up the position of the valence band, giving even smaller band gap compared with compounds that do not contain Bi in their structures [13]. And this hybridization also makes the valence band largely dispersing, which favors the mobility of holes in the valence band and is beneficial to the oxidation reaction. In the present work, SrBi2O4 powders, bismuth (III)-based semiconductors, were synthesized by co-precipitation method. Acetaldehyde was typical pollutant in indoor air, as model compound to investigate the activities of SrBi2O4 samples under visible light irradiation (l > 420 nm). Effects of NiO loading on the photocatalytic activity of SrBi2O4 catalyst were further examined. Moreover, the photocatalytic activities of NiO/SrBi2O4 for bacteria were also investigated under visible light irradiation (l > 420 nm). 2. Experimental section 2.1. Synthesis of the photocatalysts Stoichiometric amounts of Sr(NO3)2 and Bi(NO3)35H2O were dissolved in deionized water. Then a solution of ethylene diamine tetraacetic acid in ammonia and citric acid was added to the above aqueous solution. The pH value was adjusted to 4.0 by the addition of ammonia. During this process, the mixed solution was stirred continuously until a white precipitate was formed. Subsequently, the precipitation was dried at 160 8C to remove water completely. Then a voluminous and fluffy black precursor material was obtained. The precursor material was calcined at 350 8C for 10 h and were finally calcined at 750 8C in muffle furnace for 4 and 12 h in air, respectively. A NiO co-catalyst was loaded by impregnation method. Assynthesized samples calcined at 750 8C for 12 h were put into in 0.1 mol/L Ni(NO3)2 aqueous solution. The suspension was evaporated at 70 8C by magnetically stirring. Then the dried powder was calcined at 300 8C for 1 h. 2.2. Characterization Powder X-ray diffraction was recorded on a Scintag-XDS˚ ). 2000 diffractometer with a Cu Ka radiation (l = 1.54059 A The size and shape of the particles were observed using a Hitachi S-3000N scanning electron microscopy (SEM). UV–vis absorption spectra of the samples were recorded on a UV–vis
spectrophotometer (Hitachi UV-3010) with an integrating sphere attachment for their reflectance in the range of 200–800 nm and BaSO4 was the reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed with an Axis Ultra spectrometer (Kratos, UK) using Mono Al Ka (1486.6 eV) radiation at a power of 225 W (15 mA, 15 kV). Fourier Transform Infrared Spectroscopy (FT-IR) investigations were made on a FT-IR Spectrometer (Nicolet 5700). Zetapotential of catalysts (1.0 g/L) in KNO3 solution (1 mM) were measured with Zetasizer 2000 (United Kingdom, Malvern Co). 2.3. Photocatalysis experiments 2.3.1. Photocatalytic degradation of organic pollutants under visible light The photocatalytic activities of as-prepared samples were evaluated by the degradation of acetaldehyde in gas phase. The light source for photocatalysis was a 350-W spherical Xenon lamp (Shanghai DianGuang Device Ltd.). Light passed through an IR water filter and a UV cutoff filter (Ø 30 mm, l > 420 nm) and then was focused onto a 550 mL cylindrical Pyrex glass batch reactor. The intensity of the incidence illumination through the cutoff filter was 1.48 mW/cm2. The photocatalytic decompositions of acetaldehyde were performed with 0.5 g of the powdered photocatalyst placed at the bottom of reactor at room temperature in a gas-closed system. Humidity and pH were not controlled in this experiment. The reaction gas mixture (1 atm) consisted of 15 ppm CH3CHO, 21% O2 and N2 balance gas. Prior to commencing irradiation, the reaction system was equilibrated for about 120 min until no changes in the concentration of acetaldehyde and CO2 were monitored. The changes in the acetaldehyde and CO2 concentration were obtained by a gas chromatography (GC) equipped with a flame ionization detector (N2 carrier) and the catalytic conversion furnace. 2.3.2. Measurements of bactericidal activity Escherichia coli (E. coli DH 4a), a Gram-negative bacterium, were used as model bacteria in this study, which were purchased from the Institute of Microbiology, Chinese Academy of Sciences. It was incubated in Luria–Bertani (LB) nutrient solution at 37 8C shaking for 18 h, and then washed with 0.9% saline solution by centrifuging at 4000 rpm. The treated cells were then re-suspended and diluted to 5 108 colony forming units per milliliter (CFU/mL) with 0.9% saline. The diluted cell suspension and photocatalyst were added to a 100-mL beaker with a cover. All materials used in the experiments were autoclaved at 121 8C for 25 min to ensure sterility. The final photocatalyst concentration was adjusted to 0.2 g/L, and the final bacterial cell concentration was 5 106 CFU/mL, the reaction volume was 30 mL. The intensity of the incidence illumination was 2.8 mW/cm2. The reaction temperature was maintained at 25 8C. The reaction mixture was stirred with a magnetic stirrer throughout the experiment. A bacterial suspension without photocatalyst was irradiated as a control. Before and during the light irradiation, an aliquot of the reaction solution was immediately diluted with saline and an appropriate dilution of the sample was incubated
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Fig. 1. XRD of samples: (a) SrBi2O4 calcined for 4 h; (b) SrBi2O4 calcined for 12 h; (c) Ni 1.0 wt.%/SrBi2O4.
at 37 8C for 24 h on nutrient agar medium, and then the colonies were counted to determine the number of viable cells. To monitor the change of K+ ions concentration during the inactivation of the E. coli, at every time interval, 1 mL of the illuminated bacterial suspension was withdrawn and filtered through a Milllipore filter (pore size 0.22 mm) for ICP-MS analysis on OPTIMA 2000 (Perkin-Elmer Co.) All experiments were repeated three times. 3. Results and discussion 3.1. Photocatalysts characterization Fig. 1 shows the X-ray diffraction patterns of as-synthesized samples calcined at 750 8C for different times. The samples
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calcined for 4 h mainly consisted of SrBi2O4, Sr2.25Bi6.75O12.38 and Bi2O3 crystallites. With increasing calcination time to 12 h, the intensity of the diffraction peaks of Sr2.25Bi6.75O12.38 obviously decreased, and the peaks of Bi2O3 disappeared, while the intensity of SrBi2O4 gradually increased. XRD results indicated that monoclinic structure SrBi2O4 formed predominant phase and the crystallinity became better. Fig. 2 shows the SEM morphology of the powders calcined at 750 8C for different times. The sample calcined for 4 h exhibited a honeycomb-like shape and the particles aggregated (Fig. 2a), while sample calcined for 12 h was well crystallized and some particles possessed a plate shape with about 0.2–5 mm of particle sizes (Fig. 2b). The results indicated that monoclinic structure SrBi2O4 with plate shape was obtained by prolonging calcination time to 12 h. Furthermore, The atomic ratio of Bi/Sr in the structure of the samples was 16.47:10.89, 21.34:10.66 for 4 and 12 h respectively by the measurement of energy dispersive analysis of X-rays (EDX). The value of Bi/Sr in the sample calcined for 12 h was close to the ideal value of 2:1, which agrees with the result of XRD. XRD patterns and SEM images revealed that calcination time had significant influences on the crystallinity and morphology of final products. With prolonging calcination time, the crystallinity and purity of SrBi2O4 samples were improved and the formation of the impurity phase was inhibited. The addition of Ni 1.0 wt.% (NiO/SrBi2O4) had no influence on the crystal structure of SrBi2O4 (Fig. 1c), and the plate morphology was still retained in the NiO/SrBi2O4 sample (Fig. 2c). No XRD diffraction peaks of oxide nickel species
Fig. 2. SEM images of samples: (a) SrBi2O4 calcined for 4 h; (b) SrBi2O4 calcined for 12 h; (c) Ni 1.0 wt.%/SrBi2O4.
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specimens exhibited a greater light absorption throughout the visible wavelengths due to the grayed color of the catalyst. Fig. 4 shows the changes of the zeta-potential with pH of solution. The isoelectric point of SrBi2O4 and NiO/SrBi2O4 was 10.15 and 11.54 pH units, respectively. The result implied that there were alkaline groups on the surface of catalysts. The excess alkaline group might be arisen from the formation of hydrate during the preparation. Therefore, NiO/SrBi2O4 surface was positively charged in general solution. 3.2. Photocatalytic activities of catalysts under visible light
Fig. 3. UV–vis diffuse reflectance spectra of the samples: (a) SrBi2O4 calcined for 4 h, (b) SrBi2O4 calcined for 12 h; (c) Ni 0.5 wt.%/SrBi2O4; (d) Ni 1.0 wt.%/ SrBi2O4; (e) Ni 1.5 wt.%/SrBi2O4.
were observed at NiO/SrBi2O4 samples. This was presumably due to the combination of its low content, small particle size and highly dispersed on the surface of the SrBi2O4 particle. Determination of the oxidation state of nickel was carried out by measuring Ni 2p3/2 binding energy (BE) with XPS. The BE 854.0 was assigned to NiO [19]. The concentration of Ni in the surface of the catalyst was 5.54 at% from the data of XPS. Based on the dosages of Ni(NO3)2 and SrBi2O4 (Ni 1.0 wt.%), the concentration of Ni in SrBi2O4 was 1.40 at% which was smaller the experimental value. It demonstrated that most NiO was supported on the surface of SrBi2O4. The UV–vis absorbance spectra of the as-prepared samples are shown in Fig. 3. The absorption characteristics of sample calcined for 4 h were quite similar to that of sample calcined for 12 h except for slightly shorter onset wavelength. From Fig. 3, the optical absorption spectra of SrBi2O4 calcined for 12 h showed visible light absorption band 400–540 nm. With the loading of NiO, the NiO/SrBi2O4 displayed the same absorption edge as SrBi2O4. However, the NiO/SrBi2O4
Fig. 4. Plot of the zeta-potential as a function of pH for SrBi2O4 and NiO/ SrBi2O4 suspensions (1.0 g/L) of catalyst in the presence of KNO3 (1 mM).
3.2.1. Photocatalytic activities of catalysts prepared under different conditions The photocatalytic activities of the catalysts were evaluated by the photodegradation of acetaldehyde in the gas phase under visible light irradiation. Fig. 5A illustrates the photocatalytic decomposition of acetaldehyde on the catalysts calcined for different times under visible light irradiation (l > 420 nm). Acetaldehyde was not degraded in the absence of photocatalyst. In contrast, it was degraded by different rates with various
Fig. 5. Photocatalytic decomposition of acetaldehyde over different catalysts: (A) the sample calcined for different time and (B) the sample loaded with different amount of NiO under visible light irradiation (l > 420 nm, 1.48 mW/cm2).
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Fig. 6. Selectivity toward CO2 with the visible light irradiation time over SrBi2O4 and Ni 1.0 wt.%/SrBi2O4.
catalysts under visible light irradiation. The photocatalytic activity of the sample calcined for 4 h was lower than that one calcined for 12 h, although the energy structure of the former was also similar to that of the latter. Based on the results of XRD and SEM, monoclinic structure SrBi2O4 favored photocatalytic reaction. However, under illuminated SrBi2O4 system, the concentration of CH3CHO did not decrease any more after 2 h irradiation, even prolonging irradiation time. In this experiment, the conversion of acetaldehyde to CO2 was less than 18% (as shown in Fig. 6). Fig. 5B shows the effect of different loading amount of NiO on the photodegradation of acetaldehyde. Acetaldehyde was greatly photodegraded and completely removed for 2 h irradiation under the same conditions (Ni 1.0 wt.% curve in Fig. 5B). The conversion of acetaldehyde to CO2 was more than 90% (Fig. 6), and no significant inactivation of the catalyst was observed. With the loading amount of NiO increasing, the degradation rate of acetaldehyde increased and reached a certain value at Ni
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1.0 wt.% and Ni 1.5 wt.%. And the degradation rate of acetaldehyde decreased at Ni 2.0 wt.%. The results indicated that the loading amount of NiO existed optimum value. Furthermore, FT-IR spectra of different samples were shown in Fig. 7. The fresh, acetaldehyde-adsorbed and used SrBi2O4 exhibited almost same spectra except a little bit change at around 1451 cm 1, indicating the less intermediates formation, while the fresh, acetaldehyde-adsorbed and used Ni 1.0 wt.%/ SrBi2O4 (NiO/SrBi2O4) displayed different FT-IR spectra. Compared with the fresh NiO/SrBi2O4 spectra, a new absorption peak appeared at 1426 cm 1 for the asymmetric and symmetric CH3 bending vibration of acetaldehyde in acetaldehyde-adsorbed NiO/SrBi2O4 before reaction. For NiO/ SrBi2O4 after photocatalytic degradation of acetaldehyde, several new peaks were observed. A strong new peak emerged at 1383 cm 1 for the CH3 bending vibrations of acetic acid, with the peak at 1426 cm 1 decreasing. A stronger new peak attributable to adsorbed water, a weak new peaks for C O groups of acetic acid appeared at 1626, 1750 cm 1, respectively. The intensity of two peaks at 2425 and 2494 cm 1 for adsorbed CO2 significantly increased. The results also verified that acetic acid, CO2 and H2O were produced with the degradation of acetaldehyde, and the formed acetic acid gradually accumulated on the surface of the catalyst during the course of the reaction. It has been found that the photoactivity of the catalyst decreased when this catalyst was used repeatedly. Nevertheless, after the addition of Pd, this phenomenon was avoided. The Pd/NiO/SrBi2O4 catalyst did not significantly deactivate in the time scale of these experiments. The further details about the Pd/NiO/SrBi2O4 catalyst will be reported in another work. 3.2.2. Bactericidal activity of NiO/SrBi2O4 under visible light irradiation The bactericidal activities of NiO/SrBi2O4, P25 TiO2 and Ni 1.0 wt.% P25 TiO2 (NiO/P25 TiO2) were evaluated by the
Fig. 7. FT-IR spectra of SrBi2O4 and NiO%/SrBi2O4: (a) before reaction; (b) acetaldehyde-absorbed; (c) after reaction.
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in different visible light illuminated catalyst suspensions. No obvious changes in the concentration of K+ were observed under the following several conditions: visible light irradiation without photocatalyst; visible light illuminated P25 TiO2, respectively. In contrast, in the visible light illuminated NiO/ SrBi2O4 suspensions, K+ immediately leaked out and promptly increased paralleling the inactivation of E. coli with irradiation time. The results demonstrated a notable destruction in the structure of the outer membrane and the cytoplasmic membrane of the E. coli cell, resulting in the cell death under visible light illuminated NiO/SrBi2O4. 3.3. Mechanism of NiO/SrBi2O4 photocatalysis under visible light
Fig. 8. (A) Survival of E. coli (an initial concentration of 5 106 CFU/mL) vs. reaction time and (B) Leakage of K+ ion from E.coli cells vs. reaction time under visible light irradiation (l > 420 nm, 2.8 mW/cm2).
inactivation of bacteria E. coli in water (Fig. 8A) under visible light irradiation. No obvious changes in survival were observed under the following several conditions: visible light irradiation without photocatalyst; NiO/SrBi2O4 in dark; visible light illuminated P25 TiO2. In contrast, a prompt decrease in survival was observed in the illuminated NiO/SrBi2O4 suspensions, fewer E. coli cells survived at irradiation time 60 min. The bactericidal activity of NiO/SrBi2O4 was much higher than that of P25 under visible light. In addition, no obvious changes in survival were observed in visible light-irradiated NiO/P25 TiO2 suspensions. NiO did not play any positive effect in TiO2 photocatalyst as it enhanced the photoactivity of SrBi2O4 under visible light irradiation. The results implied that NiO predominantly promoted the photogenerated electron–hole separation because SrBi2O4 could be excited by visible light to generated electron–hole pair but TiO2 could not. K+ exists universally in bacteria [20,21]. It plays roles in the regulation of polysome content and protein synthesis. Therefore, the destroyed cell membrane would result in leakage of intracellular K+ ions that paralleled cell death. Fig. 8B shows K+ leakage with the inactivation of the E. coli
The photocatalytic oxidation of organic compounds is mainly considered to be controlled by the following processes: (1) the semiconductor absorb photons and generate electron– hole pairs; (2) the photoinduced electron–hole pairs are separated; (3) the oxidation of the organic compounds by the photohole in the VB or formed OH radical; (4) the reduction of oxygen by the photoelectron in the CB [11]. Step (2) is crucial for many semiconductors because photogenerated electrons and holes easily recombine and lead low activity. The principal method of slowing the electron–hole recombination is thought to be through the loading of cocatalysts, such as NiO, on the surface of the semiconductors. Based on the XPS data, NiO was supported on the surface of SrBi2O4 by the form of NiO. When NiO was loaded on the surface of SrBi2O4, the photocatalytic activity was improved efficiently. This was suggested by the photocatalytic decomposition of acetaldehyde and bacteria. The ESR spin-trap technique (with DMPO) was used to obtain information on the active radicals involved in the photocatalytic process (Fig. 9). Since the O2 radicals in water were very unstable and undergo facile disproportionation rather than slow reaction with DMPO [22,23]. Consequently, the involvement of O2 radicals in NiO/ SrBi2O4 and SrBi2O4 system was examined respectively in methanol using a Nd:YAG laser (532 nm) as the irradiation source (Fig. 9A). The six characteristic peaks of the DMPO– O2 adducts were observed only in NiO/SrBi2O4 dispersion under visible light irradiation. No such signals were detected in dark. It meant that irradiation was essential for the generation of O2 on the surface of the catalyst. No O2 signals were detected in SrBi2O4 systems under the otherwise identical conditions. Similarly, the DMPO–OH species were detected successfully in aqueous NiO/SrBi2O4 dispersion under visible light irradiation (Fig. 9B). Four characteristic peaks of DMPO–OH, 1:2:2:1 quartet pattern were obviously observed. No such signals were detected in dark and visible light-irradiated SrBi2O4 system. The evidence that OH and O2 were produced on the surface of visible light-illuminated NiO/SrBi2O4 provided a solid indication that the catalyst can be efficiently excited by visible light to create electron–hole pairs and that the charge separation was maintained long enough to react with adsorbed oxygen/H2O and to produce a series of active oxygen radicals which finally induced the
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4. Conclusions The monoclinic structure SrBi2O4 was prepared through a co-precipitation method. SrBi2O4 exhibited the spectral response to visible region although its photocatalytic activity was limited. To the best of our knowledge, SrBi2O4 as photocatalyst under visible light is reported for the first time. Furthermore, when NiO was loaded on the surface of SrBi2O4, the photocatalytic activity was greatly improved since NiO suppress the recombination between electrons and holes. The NiO/SrBi2O4 photocatalyst shows highly photocatalytic efficiency for the removal of acetaldehyde and bacteria under visible light. ESR studies revealed that OH, O2 were involved as the active species in the photocatalytic reaction. NiO/SrBi2O4 is thus very promising for the purification of environmental pollutants. Acknowledgement This work was supported by the National Science Foundation of China (20377050, 20577062, 20537020, 50538090). References
Fig. 9. DMPO spin-trapping ESR spectra recorded at ambient temperature in methanol dispersion (for DMPO–O2 ), (A) and aqueous dispersion (for DMPO–OH), (B) under irradiation of 532 nm. (a) SrBi2O4 and (b) NiO/ SrBi2O4 before and after irradiation.
decomposition of pollutants [24,25]. Neither OH nor O2 was formed in the visible light-illuminated SrBi2O4 suspension, indicating that the recombination rate of the photogenerated electron–hole pairs from the SrBi2O4 excited by visible light was higher than that one of their separation. The results demonstrated that NiO on the external surface of SrBi2O4 trapped photogenerated electrons, enhanced the electron–hole separation leading to active species formation and improved the photocatalytic activity, which agrees with previous work [26,27]. The stability of a photocatalyst is important to its application. After photocatalytic degradation reaction of organic contaminants, the crystal structure of the NiO/SrBi2O4 photocatalyst was checked by XRD analysis. The results showed that the crystal structure of the photocatalyst was not changed after the photocatalytic reaction.
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