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Size-tunable fabrication of multifunctional Bi2O3 porous nanospheres for photocatalysis, bacteria inactivation and template-synthesis† Fan Qin,a Huiping Zhao,a Guangfang Li,a Hao Yang,a Ju Li,a Runming Wang,a Yunling Liu,b Juncheng Hu,c Hongzhe Sund and Rong Chen*a Multifunctional Bi2O3 porous nanospheres (PNs) with tunable size have been successfully synthesized via a
Received 28th December 2013 Accepted 28th February 2014
facile solvothermal method. The obtained Bi2O3 porous nanospheres demonstrate outstanding
DOI: 10.1039/c3nr06870f
performance in visible-light-driven photocatalysis for Cr(VI) and organic dye removal, inactivation of Gram-negative and Gram-positive bacteria, as well as template-synthesis for fabrication of bismuth-
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related hollow nanostructures.
Introduction Recently, multifunctional nanomaterials have attracted considerable research interest due to their great potential in diverse applications of catalysis, magnetic separation and biomedicine, which could simultaneously exhibit novel magnetic, optical, electronic, plasmonic and semiconducting properties.1–4 Extensive studies focused on fabricating complex multifunctional nanosystems that contain two or more nanoscale components or incorporate nanomaterials with different functional molecules.5–7 However, the construction of a multifunctional architecture by deliberately combining different useful functions into one material is still limited and remains a challenge.6,8 Consequently, many efforts have been made to realize the expectation by mediating the size, morphology and structure of nanomaterials, which strongly affect their properties and applications. Porous/hollow nanostructures with designed chemical components, controlled morphologies and tunable sizes are good candidates owing to their high surface area, low density and high loading capacity, which endow them with multifunctional capabilities.9–13 For example, mesoporous SiO2 could be used as a drug delivery carrier in biomedicine,8,14 an excellent noble metal or metal oxide support in catalysis and chemical sensing,15,16 as well as a hard template in fabricating novel
hollow nanostructures.17 It is highly desirable to develop novel multifunctional nanostructures with advanced performance. Currently, bismuth-containing nanomaterials have been intensively studied because of their remarkable properties and applications.18–24 Particularly, as an important p-type semiconductor, bismuth oxide (Bi2O3) is of great interest and has been extensively applied in photovoltaic cells, gas sensing, fuel cells and photocatalysis.25–29 Over the past decades, a great volume of publications has demonstrated the synthesis and applications of various Bi2O3 nanostructures.26,29–35 Unfortunately, there is no report achieving the incorporation of different functions into single-component Bi2O3 nanostructures. Herein, we report the size-tunable synthesis of Bi2O3 porous nanospheres (PNs) with multiple enhanced properties via a facile solvothermal method for the rst time. To demonstrate the multifunctionality of the obtained Bi2O3 PNs for a variety of applications, Bi2O3 PNs were used as a visible-light-driven (VLD) photocatalyst for Cr(VI) and organic dye removal, an antibacterial agent for growth inhibition of S. aureus and E. coli, as well as a bismuth precursor and template for fabricating other bismuthcontaining hollow nanostructures. This work demonstrates various outstanding performances of Bi2O3 PNs which endow them with great potential for further applications, and provides a solution for integration of different functions into one material.
a
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China. E-mail: [email protected]; Fax: +862787195671
b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
c
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, PR China
d
Department of Chemistry, The University of Hong Kong, PR China
† Electronic supplementary 10.1039/c3nr06870f
information
5402 | Nanoscale, 2014, 6, 5402–5409
(ESI)
available.
See
DOI:
Experimental Materials Bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), potassium bichromate (K2Cr2O7) and thioacetamide (TAA) were purchased from Aladdin. Poly(vinylpyrrolidone) (PVP, Mw ¼ 10 000) and commercial Bi2O3 powder (99.8%) were purchased from Aldrich (USA). Ascorbic acid (AA) and Na2SeO3 powder were purchased from Lancaster. Ethylene glycol (EG), urea, nitric acid (HNO3),
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TeO2 powder, NH4VO3 and Na2WO4 were obtained from Sinopharm Chemical Reagent Co. (China). All the reagents were of analytical grade and used directly without further purication.
Table 2 Experimental conditions for preparation of bismuth-containing hollow nanospheres
Sample
Bi precursor
Reagent
Synthesis of Bi2O3 nanospheres
Bi2S3 Bi2Se3
Bi2O3 PNs (S2) Bi2O3 PNs (S2)
Bi2Te3
Bi2O3 PNs (S2)
BiVO4 Bi2WO6
Bi2O3 PNs (S2) Bi2O3 PNs (S2)
Thioacetamide (TAA, 0.05 g, 0.7 mmol) Na2SeO3 powder (0.035 g, 0.2 mmol) and ascorbic acid (AA, 0.3 g, 1.7 mmol) TeO2 powder (0.032 g, 0.2 mmol) and ascorbic acid (AA, 0.3 g, 1.7 mmol) NH4VO3 (0.012 g, 0.1 mmol) Na2WO4 (0.033 g, 0.1 mmol)
In a typical experimental procedure, Bi(NO3)3$5H2O (0.182 g, 0.375 mmol) was dissolved in HNO3 solution (5 mL, 1 M), then urea (0.08 g, 1.35 mmol) and EG (25 mL) were added into this solution. Then the mixture was transferred into a stainless steel autoclave with Teon liner. The autoclave was sealed and maintained at 150 C for 3 h. The obtained products were centrifuged and washed with deionized water for ve times and nally dried in a desiccator for a few days for further characterizations (S1). Other samples were also prepared under identical reaction conditions by varying the amount of PVP and using different precursors. The detailed procedure is same as described above and all the experimental parameters are listed in Table 1. Synthesis of bismuth chalcogenides and bismuth-based oxysalt nanostructures The synthesis of bismuth chalcogenides (Bi2S3, Bi2Se3, Bi2Te3) and bismuth-based oxysalt (BiVO4 and Bi2WO6) was simply achieved via the hydrothermal process by utilizing the obtained Bi2O3 PNs (S2) as a bismuth precursor and template. In the synthesis, different reaction precursors were dissolved in 5 mL of deionized water, respectively. The details are summarized in Table 2. Then the precursor source solutions were mixed with the 5 mL dispersion of Bi2O3 PNs (S2) at room temperature. The mixture was then hydrothermally treated for 12 h at 150 C. The products were collected by centrifugation and thoroughly washed with deionized water ve times.
were converted from reection to absorbance by the Kubelka– Munk method. The Brunauer–Emmett–Teller (BET) specic surface area of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). Atomic force microscopy (AFM) images were recorded on a Nanoscope IIIa Multimode SPM (Veeco Co., USA). The electron spin resonance (ESR) method for detecting the generation of reactive oxygen species (ROS) in Bi2O3 suspensions was performed by the ESR spectrometer (EMX-8/2.7, Bruker) at room temperature with DMPO as the radical trap. Cr(VI) adsorption measurement Cr(VI) solutions with different concentrations were prepared by using K2Cr2O7 as the Cr(VI) source. In a typical adsorption procedure, 0.02 g of Bi2O3 sample was added to 20 mL Cr(VI) solution (40 mg L1) under stirring. At each given time interval, 2 mL suspension was sampled and centrifuged to remove the solid adsorbents. The concentration of Cr(VI) during the adsorption was measured with a Shimadzu UV2800 spectrophotometer. All the measurements were carried out at room temperature.
Characterizations Powder X-ray diffraction (XRD) was carried out on Bruker axs D8 ˚ Scanning electron microscopy Discover (CuKa ¼ 1.5406 A). (SEM) images and energy dispersive X-ray (EDX) spectra were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were recorded on a Philips Tecnai 20 electron microscope. UV-vis diffuse reectance spectra (DRS) were recorded on a UV-vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and
1 Experimental nanomaterials Table
conditions
for
preparation
of
Bi2O3
Sample
Reactant
PVP (g)
Size (nm)
S1 S2 S3 S4 S5 S6 S7
Urea Urea Urea Urea NaOH NaOH NaOH
— 0.15 0.3 0.45 0.15 0.3 0.45
350 180 140 160 130 100 —
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Photocatalytic activity measurement The photocatalytic activities of Bi2O3 nanospheres for the degradation of Cr(VI) were performed under visible light irradiation (500 W Xe lamp with a 400 nm cut-off lter, Beijing Changtuo Technology Co. Ltd.). In a typical photocatalytic procedure, 0.02 g Bi2O3 sample was added into 20 mL Cr(VI) solution (40 mg L1). Prior to irradiation, the suspension was stirred in the dark for 2 h to reach adsorption–desorption equilibrium. Then, the solution was exposed to visible light irradiation under magnetic stirring. At each given time interval, 2 mL suspension was sampled and centrifuged to remove the solid photocatalyst. The concentration of Cr(VI) during the degradation was monitored by colorimetry using a Shimadzu UV2800 spectrophotometer. The photocatalytic activities of the Bi2O3 porous nanospheres for Congo red (CR) and Rhodamine B (RhB) degradation were also evaluated by the same procedure upon visible light irradiation. In each experiment, 0.02 g Bi2O3 sample (S6) and 30 mL CR solution (50 mg L1) or RhB solution (105 M) were used in the photocatalysis. The dark adsorption– desorption equilibrium time is 1 h. All the measurements were carried out at room temperature.
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Antibacterial activity measurement The bacteria used were cultured in the Luria Bertani (LB) liquid medium at 37 C for 24 h. Before the antibacterial test, all the samples and materials in the experiments were sterilized at 121 C for 20 min. The antibacterial activities of Bi2O3 samples towards S. aureus and E. coli were evaluated by colony counting, minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) methods. The colony counting test was performed by mixed 2 mL 105 CFU mL1 diluted bacteria and 2 mL Bi2O3 sample (4 mg mL1) into tubes, then the mixture was incubated for 24 h at 37 C with shaking at 150 rpm. Then, 10 mL of the mixture were plated onto LB agar plates and the number of the colonies was counted aer incubation for 24 h at 37 C. The MIC value was determined by mixing 2 mL 105 CFU mL1 bacteria and 2 mL of serial doubling diluted dispersion of the samples into the tubes and incubated for 24 h at 37 C. The MIC value was recorded as the lowest concentration of the sample inhibiting the visible growth of microorganisms. The MBC value is determined by spreading 10 mL of the mixture of bacteria suspension and samples with different concentrations from the clarifying tube on LB agar plates, which were incubated for 24 h at 37 C. Then the number of the colonies was counted. Photocatalytic inactivation assay The photocatalytic inactivation of E. coli over Bi2O3 samples (S2) was conducted under visible light irradiation by using a 500 W xenon lamp with a 400 nm cutoff lter as the light source. All glass apparatuses and materials used in this experiment were autoclaved at 121 C for 20 min to ensure sterility. The bacterial cells were incubated in LB agar plates at 37 C for 24 h, then dispersed in sterilized saline (0.9% NaCl) and adjusted to the required concentration (105 CFU mL1). In the experiment, 30 mL bacterial cell suspension with 100 mg mL1 catalysts was stirred upon visible light irradiation throughout the experiment. At different time intervals, aliquots of the sample were withdrawn. 10 mL samples were then immediately spread on LB agar plates and incubated at 37 C for 24 h to determine the number of viable cells. For comparison, two control experiments were conducted along with treatment experiments. The light control was carried out in the absence of Bi2O3 nanospheres (S2) under visible light irradiation, and negative control without photocatalyst and visible light irradiation. All the treatment and control experiments were performed in triplicates. The reaction temperature of photocatalysis was maintained at 25 C. Bacteria imaging by AFM The culture of E. coli or S. aureus was grown overnight to 108 CFU mL1 in LB medium at 37 C. Bi2O3 samples (S2) (20 mg mL1 for S. aureus and 120 mg mL1 for E. coli) were added to the suspension, and the mixture was incubated at 37 C for 2 h with continuous shaking. The bacteria cells in 2 mL suspension were isolated by centrifugation and washed twice immediately with PBS buffer solution (200 mM, pH ¼ 7.4). The samples were then
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xed in 2.5% glutaraldehyde at 4 C for 2 h. Then the samples were centrifuged and washed with PBS buffer solution (200 mM, pH ¼ 7.4) to remove residues of glutaraldehyde. And then the samples were washed twice with sterilization of deionized water. Finally, the samples were resuspended in the sterilization of deionized water and 20 mL bacterial suspensions were added dropwise on the cover slip and dried in air. Culture with Bi2O3free served as control. E. coli and S. aureus cells were imaged in ambient air with a tapping mode AFM (Nanoscope IIIa Multimode, Veeco Co., USA) before and aer being treated with Bi2O3 nanospheres (S2). The AFM was equipped with an E-type scanner and a rectangular silicon cantilever with a resonance frequency of 290–320 kHz. Real time scanning was performed with a scan rate of 1 Hz, scan angle of 0 , and resolution of 256 256 pixels. Measurement and processing of the AFM images were performed with Version 5.31r1 soware (Veeco Co., USA). Measurement of leakage of the reducing sugars To detect the leakage of reducing sugars through bacteria membrane, Bi2O3 samples (S2) and bacterial suspension (S. aureus and E. coli) were added into 10 mL LB medium with a nal concentration of 100 mg mL1 Bi2O3 and 6 108 CFU mL1 bacteria. Control experiments were conducted without Bi2O3. The cultures were incubated at 37 C with shaking at 150 rpm. Then the sample was centrifuged at 10 000 rpm, the concentrations of reducing sugar of the supernatant were determined immediately by the 3,5-dinitrosalicylic acid method aer being treated with Bi2O3 for 0 and 12 h. All experiments were performed under sterile conditions and in triplicate.
Results and discussion Fig. 1a shows the typical XRD pattern of the Bi2O3 product synthesized from HNO3 treated Bi(NO3)3 and urea in the absence of PVP via a solvothermal method (S1). All the diffraction peaks are well indexed to Bi2O3 (JCPDS 76-2478), indicative of the high purity of Bi2O3. The SEM image illustrates that the product is composed of large-scale uniform solid Bi2O3 nanospheres with an average diameter of 350 nm (Fig. 1b). TEM images (Fig. 1c and d) further demonstrate the spheric morphology and smooth surface of Bi2O3 nanospheres. Interestingly, Bi2O3 porous nanospheres were fabricated under identical experimental conditions when PVP was introduced into the reaction system. As illustrated in Fig. 2a, large quantities of Bi2O3 porous nanospheres with an average diameter of 180 nm were obtained in the presence of 0.15 g PVP (S2). TEM images clearly reveal that the spheric Bi2O3 has a relatively uniform size (Fig. 2b) and porous structure (Fig. 2c), which consists of numerous interconnected nanoparticles. The HRTEM image (inset of Fig. 2c) illustrates a well-resolved interplanar d-spacing of 0.326 nm, which is corresponding to the (111) lattice plane of Bi2O3. It has a BET area of 9.8 m2 g1 and a total pore volume of 0.078 cm3 g1, both of which are larger than that of solid Bi2O3 nanospheres (S1, 8.2 m2 g1 and 0.018 cm3 g1) (Fig. S1, ESI†). More importantly, the porous structure and size of Bi2O3 nanospheres could be further
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Fig. 1 (a) XRD pattern, (b) SEM image, (c and d) TEM images of solid Bi2O3 nanospheres (S1) prepared by the solvothermal method in the absence of PVP.
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mediated by varying the amount of PVP. Fig. 2e–h show SEM and TEM images and the size distribution diagram of Bi2O3 porous nanospheres prepared in the presence of 0.3 g PVP (S3), which reveals that the average diameter of Bi2O3 porous nanospheres decreased to 140 nm and a more obvious porous structure could be observed. Further increasing the PVP amount to 0.45 g (S4), however, the average diameter of Bi2O3 porous nanospheres increased to 160 nm and the product maintained its porous structure (Fig. 2i–l), which might be ascribed to the increased viscosity of the reaction system with the increase of the PVP amount. It indicates that PVP plays a critical role in the fabrication of Bi2O3 porous nanospheres, which not only controls the size but also mediates the structure of the nal product. We further found that the size of Bi2O3 porous nanospheres would decrease when NaOH was used as a reagent, instead of urea. Under identical conditions, uniform Bi2O3 porous nanospheres with an average diameter of 130 and 100 nm could be obtained in the presence of 0.15 g (S5) and 0.3 g (S6) PVP, respectively, as depicted in Fig. 2m–t. TEM images of individual Bi2O3 nanospheres (S5 and S6) also reveal their distinct porous structures. Bi2O3 porous nanospheres with the smallest size (S6) possess the largest BET surface area of 23.4 m2
Fig. 2 SEM, TEM images and size distribution histograms of Bi2O3 PNs prepared by the solvothermal method: (a–d) S2, (e–h) S3, (i–l) S4, (m–p) S5, (q–t) S6, respectively.
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g1 and a total pore volume of 0.15 cm3 g1 (Fig. S1, ESI†). However, only Bi2O3 nanoparticles and irregular nanoplates were obtained when more PVP was employed (S7, Fig. S2, ESI†). In this fabrication, HNO3 pretreatment of Bi(NO3)3 is essential to form sphere-like Bi2O3 nanostructures. If untreated Bi(NO3)3 was directly used under the same experimental conditions, Bi2O3 nanotubes were fabricated, as reported in our previous work.33 Besides, we have also successfully prepared Bi hollow nanospheres from HNO3 treated Bi(NO3)3 in the absence of urea.36 Simultaneously, PVP and OH were vital factors for fabricating size-tunable Bi2O3 PNs. Based on our experimental results and reported literatures, we reasoned the formation process of Bi2O3 PNs as follows. In the initial stage, Bi–PVP complexes formed through the coordinative bonding between Bi3+ and carbonyl oxygen of PVP, and the excess PVP wrapped the Bi–PVP complexes to generate micelles.37,38 The formed Bi–PVP complexes could slowly release Bi3+ as the reaction proceeded, which would react with OH ions provided by urea or NaOH. This process slowed down the nucleation and subsequent crystal growth of Bi2O3 particles, leading to the formation of tiny nanoparticles. The coated PVP molecules prevented the adjacent Bi2O3 crystal particles from quick aggregation, resulting in the formation of porous structures assembled by numerous interconnected nanoparticles. The adequate PVP also modulates the size of Bi2O3 nanospheres with the increase of the PVP amount to a certain extent, which is probably due to the different viscosity of the reaction system regulated by the amount of PVP. However, our understanding of the mechanism is still limited and further investigation is needed in our further work. Initially, we expected a relatively high specic surface area and adsorptive capacity of Bi2O3 porous nanospheres, considering its porous structure. However, the largest BET surface area of Bi2O3 PNs is only 23.4 m2 g1 (S6). Fig. 3a shows the Cr(VI) adsorption capacities of Bi2O3 nanospheres with different diameters under an initial concentration of 40 mg L1 (S1, S2, S5 and S6). The Cr(VI) adsorption measurement demonstrates a size-dependent Cr(VI) adsorption capacity over Bi2O3 nanospheres. Fig. 3b shows the adsorption isotherm of Cr(VI) removal of sample S6. The Langmuir adsorption model (qe ¼ qmbCe/ (1 + bCe)) is used to calculate the maximal adsorption capacity, which represents the relationship between the amount of heavy metal adsorbed at equilibrium (qe, mg g1) and the equilibrium solute concentration (Ce, mg L1), where qm (mg g1) is the maximal adsorption capacity corresponding to complete monolayer coverage and b is the equilibrium constant (L mg1). According to the adsorption isotherm, the maximum Cr(VI) adsorption capacity of Bi2O3 porous nanospheres with smallest diameter (S6) is 24.5 mg g1. Although it is higher than those of many reported metal-oxide adsorbents,39–42 the remaining Cr(VI) is still a huge potential threat for human health. In order to eliminate Cr(VI) completely and efficiently, photocatalysis over Bi2O3 nanospheres was performed under visible light (VL) irradiation, which is one of the most promising and clean strategies to thoroughly reduce highly toxic and carcinogenic Cr(VI) species to less harmful Cr(III) species.43 Fig. 3c shows the variation in the Cr(VI) concentration (C/C0) with irradiation time over Bi2O3 nanospheres (S1, S2, S5 and S6), commercial Bi2O3 (C–Bi2O3)
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Fig. 3 (a) Cr(VI) removal capacities of Bi2O3 nanospheres with different diameters with an initial Cr(VI) concentration of 40 mg L1 (S1, S2, S5 and S6), (b) adsorption isotherm of Cr(VI) removal of sample S6, (c) variation in Cr(VI) concentration (C/C0) with VL irradiation time over Bi2O3 nanospheres (S1, S2, S5 and S6), P25 and C–Bi2O3, (d) plot of ln(C00 /Ct) versus time for Cr(VI) photodegradation over Bi2O3 nanospheres, C00 and Ct are the initial Cr(VI) concentration after adsorption equilibrium and the Cr(VI) concentration at time t, respectively.
and P25 under visible light irradiation. It is clearly observed that the Cr(VI) concentration hardly changed in the absence of Bi2O3 nanospheres upon 2 h visible light irradiation. However, nearly 100% Cr(VI) could be removed over S6 and S5 within 20 and 120 min upon visible light irradiation, respectively. Sample S2 also could reduce about 70% Cr(VI) aer 2 h visible light irradiation. While Bi2O3 solid nanospheres (S1) and P25 were only able to partially degrade Cr(VI) aer 2 h visible light irradiation (photodegradation efficiency is only about 30% and 10%, respectively). All Bi2O3 porous nanospheres present much higher photocatalytic efficiency than solid nanospheres (S1) and P25, indicating a structure- and size-dependent photocatalytic activity. The highest photoactivity of S6 is probably attributed to its unique porous structure, largest BET surface area and smallest size. The pH effect on the photocatalytic activities of Bi2O3 nanospheres is also investigated. It is found that Cr(VI) could be efficiently removed over a wide pH range from 3 to 11 (Fig. S3, ESI†), suggesting that Bi2O3 nanospheres could be a potential candidate for Cr(VI) photoreduction in practical applications. The kinetics of the photocatalytic reduction of Cr(VI) were also investigated. The obtained kinetic data of the Cr(VI) reduction reaction over Bi2O3 nanospheres (S1, S2, S5 and S6) t to a pseudo-rst-order model as expressed by ln(C00 /Ct) ¼ kCrt, where C00 and Ct represent the Cr(VI) concentration before and aer the irradiation, t is the irradiation time, and kCr is the apparent rate constant. As shown in Fig. 3d, Bi2O3 porous nanospheres (S6) present the fastest reduction rate, compared with other Bi2O3 nanospheres upon visible light irradiation. The kinetics data also demonstrate the size-dependent photoreduction rate of Bi2O3 nanospheres. The results indicate that Bi2O3 porous nanospheres should be a novel ideal visible light responsive photocatalyst for Cr(VI) removal in aqueous solution by utilizing solar energy.
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To understand the visible-light-response photocatalytic property of Bi2O3 nanospheres, the diffuse reectance spectra (DRS) of Bi2O3 nanospheres (S1, S2, S5 and S6) are measured (Fig. S4, ESI†). It reveals that an intense absorption occurs in the UV-light region for all Bi2O3 nanospheres, and the absorption edge is around 350 nm. Meanwhile, weak photo-absorption in the visible-light region could be also observed, indicating that Bi2O3 nanospheres could also respond to visible light in photocatalysis. The band gap energy could be determined by the equation of ahn ¼ A(hn Eg)n/2, in which a, h, n, Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. Among them, n is determined by the type of optical transition of the semiconductor and also depends on the polymorph (i.e., n ¼ 1 for direct transition and n ¼ 4 for indirect transition). According to the reported literatures,26,29 we use n ¼ 1 for Bi2O3 nanospheres. The band gap energy is estimated to be about 3.47, 3.15, 3.42 and 3.54 eV for the S1, S2, S5 and S6, respectively. In order to conrm this value, a tting model is calculated, as reported in our previous study.44 The plots of (ahn)2 versus energy and (ahn)1/2 versus energy in the absorption edge region are shown in Fig. S5 (ESI†). It is observed that the (ahn)2 versus energy plot is almost the perfect linear relationship, while the (ahn)1/2 versus energy deviates from the tted straight line. This feature suggests that the absorption edge of Bi2O3 nanospheres is caused by direct transitions, that is, n ¼ 1. We further extend the photocatalytic application of Bi2O3 PNs for other pollutants in wastewater. The photocatalytic activity of Bi2O3 PNs (S6) was also evaluated by degrading Rhodamine B (RhB) and Congo red (CR) under visible light irradiation, respectively. As shown in Fig. S6 (ESI†), Bi2O3 PNs also display good visible light driven photodegradation performance for organic dyes, indicating that Bi2O3 PNs are potential photocatalysts for water treatment. However, the photodegradation efficiency of organic dyes hardly improved aer 2 h irradiation. In addition, it was found that Bi2O3 porous nanospheres did not present good repeatability for the photocatalysis, which was probably due to photocorrosion of bismuth oxide under visible light irradiation. Efforts on improving its stability and repeatability will be devoted in our future study. In this work, it is proposed that slight visible light absorption ability make a great contribution to photocatalytic activity of samples. Bi2O3 products could be excited under visible light, although the band gaps are large. More importantly, porous structures of Bi2O3 benet carrier transportation during the photocatalytic process, and inhibit combination of photoexcited electrons and holes, which ultimately lead to enhanced visible-lightdriven photocatalytic activity, similar to the case of enhanced photocatalytic activity over macroporous TiO2.45 Although there are numerous studies regarding the antibacterial effect of inorganic metal oxides, few reports concern bismuth oxide.24 Our previous studies also demonstrated no inhibition of Bi2O3 nanoparticles against Helicobacter pylori,18 however, little is known regarding the interaction of Bi2O3 porous nanospheres with other bacteria. We then investigated the antibacterial activities of Bi2O3 nanospheres and C–Bi2O3
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against Gram-positive S. aureus and Gram-negative E. coli. As shown in Fig. 4a, Bi2O3 nanospheres exhibit excellent sizedependent antibacterial activity towards S. aureus, which is much better than C–Bi2O3. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Bi2O3 nanospheres towards S. aureus were generally observed to be in the range of 1–8 mg mL1 and 4–16 mg mL1, respectively (Table S1, ESI†), which also conrms the highly efficient and size-dependent antibacterial activities of Bi2O3 nanospheres. Noticeably, the growth inhibition of E. coli was not observed in the colloidal suspension containing Bi2O3 PNs (S2, le of Fig. 4b), indicative of no toxic effect of Bi2O3 nanospheres to E. coli. However, Bi2O3 PNs (S2) exhibit outstanding photocatalytic bactericidal performance toward E. coli upon visible light irradiation (right of Fig. 4b). Only 10% E. coli could survive aer 2 h visible light irradiation. The control experiment demonstrates no photolysis of bacterial cells under visible light irradiation alone, suggesting that Bi2O3 PNs are truly highly efficient visible-light-driven photocatalysts for bacterial inactivation. The photoinduced bactericidal performance of Bi2O3 PNs could be ascribed to the reactive photo-generated oxidative species (ROS, such as –OH, –O2 and h+) upon visible light irradiation, which could attack the bacterial cell wall and membrane, cause the leakage of intracellular content, and eventually resulting in bacterial death.46 In this study, ROS are also detected from Bi2O3 porous nanospheres upon visible light irradiation, as conrmed by the electron spin resonance spectra(ESR) with DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin trap in Bi2O3 PN (S2) suspension before and aer visible light irradiation (Fig. S7, ESI†). Hence, the bacteria inactivation is a photoinduced oxidation process.
Fig. 4 (a) Inhibition rate of Bi2O3 nanospheres and C–Bi2O3 against S. aureus, (b) inhibition rate of Bi2O3 PNs (S2) against E. coli upon VL irradiation off (left) and on (right), (c) leakage of reducing sugars from bacteria cells of S. aureus (left) and E. coli (right) treated with Bi2O3 PNs (S2), (d and e) topography images of S. aureus (d) and E. coli (e) cells before and after Bi2O3 PN (S2) treatment.
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To understand the different antibacterial effects of Bi2O3 nanospheres towards S. aureus and E. coli, topography changes of bacterial cell walls of S. aureus and E. coli treated with Bi2O3 porous nanospheres (S2) were investigated by atomic force microscopy (AFM). As shown in Fig. 4d and e, S. aureus cells show obvious bacterial membrane destruction and intracellular content leakage aer 2 h incubation with Bi2O3 porous nanospheres (S2), while E. coli cells maintain the original rod-shaped morphology and a smooth surface. The different damages on S. aureus and E. coli were further conrmed by the membrane leakage of reducing sugars from bacteria. As depicted in Fig. 4c, the leakage amount of reducing sugars from S. aureus cells treated with Bi2O3 PNs (S2) is up to 167.1 mg mL1, but only 39 mg mL1 in the control experiment, suggesting that Bi2O3 porous nanospheres may accelerate the reducing sugar leakage from S. aureus bacterial cytoplasm. However, there is no obvious increase in the membrane leakage of reducing sugars from E. coli treated with Bi2O3 porous nanospheres. It is probably due to the difference in cell structures of Gram-negative and Grampositive bacteria. The research on the specic mechanism of bactericidal performance of Bi2O3 porous nanospheres is ongoing in our studies. Besides, the obtained Bi2O3 porous nanospheres could serve as the bismuth precursor and template to fabricate other bismuth-containing hollow nanospheres via a facile hydrothermal method. As shown in Fig. S8 (ESI†), the diffraction peaks in XRD patterns are well indexed to Bi2S3 (JCPDS 89-8964), Bi2Se3 (JCPDS 12-732), Bi2Te3 (JCPDS 08-27), BiVO4 (JCPDS 14-688) and Bi2WO6 (JCPDS 39-256), respectively, indicating that pure Bi2E3 (E ¼ S, Se and Te), BiVO4 and Bi2WO6 products are obtained by using Bi2O3 PNs as a precursor. Fig. 5 shows SEM and TEM images of Bi2E3 (E ¼ S, Se and Te), BiVO4 and Bi2WO6 products. It clearly reveals that all the products present sphere-like hollow nanostructures and have a relatively uniform size. More importantly, the estimated average diameter of the obtained bismuth-containing hollow nanospheres is around 180 nm, which is consistent with the size of the starting material, Bi2O3 porous nanospheres (S2). It indicates that Bi2O3 porous nanospheres serve as a sacrice template in the fabrication of bismuthcontaining hollow structures. The formation processes of Bicontaining hollow nanostructures from Bi2O3 porous nanospheres might be attributed to the fundamental solid-state phenomenon, which is called the Kirkendall effect that deals with the movement of the interface between the diffusion couple.47 For example, once thioacetamide (TAA) solution is introduced to the reaction system, TAA molecules are uniformly distributed and easily adsorbed on the surface of Bi2O3 porous nanospheres through intermolecular interactions. Subsequently, Bi2O3 porous nanospheres were transformed into Bi2S3 due to its lower solubility during the hydrothermal process, nally resulting in the formation of Bi2S3 hollow nanostructures. This process is analogous to the reported formation of hollow and core–shell structures.48,49 It is believed that the hollow structure with large fraction of void space and functional shells endows the obtained bismuthcontaining nanomaterials with technological signicance and
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SEM and TEM images of bismuth-containing hollow nanospheres prepared by using Bi2O3 PNs (S2) as a precursor and template: (a–c) Bi2S3, (d–f) Bi2Se3, (g–i) Bi2Te3, (j–l) BiVO4 and (m–o) Bi2WO6. Fig. 5
potential applications in a wide range of catalysis, energy storage and conversion, gas sensing and biomedicine.
Conclusions In summary, we have successfully synthesized uniform Bi2O3 porous nanospheres with tunable size by employing PVP via a facile solvothermal method. In particular, the prepared Bi2O3 porous nanospheres demonstrate for the rst time the multifunctional property in photocatalysis, bacteria inactivation and template-synthesis. They display outstanding visible-lightdriven photocatalytic performance for Cr(VI) and organic dye removal in aqueous solution, as well as exhibiting remarkable antibacterial activity toward S. aureus and highly enhanced visible-light-driven photocatalytic inactivation of E. coli. Moreover, Bi2O3 porous nanospheres could be applied as a bismuth precursor and template to fabricate bismuth chalcogenides (Bi2S3, Bi2Se3, Bi2Te3) and bismuth-based oxysalt (BiVO4 and Bi2WO6) hollow nanospheres. It is anticipated that the multifunctional Bi2O3 porous nanospheres will lead to many possibilities for creating more novel bismuth-related nanostructures with multiple functionalities.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21171136 and 21371139), International Cooperation Program of Hubei Province (2011BFA021), Key Program of Wuhan Science and Technology Bureau (201260523183), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2014-09) and the High-tech Industry Technology Innovation Team Training Program of Wuhan Science and Technology Bureau (2014070504020243).
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Cite this: Nanoscale, 2014, 6, 5402
Size-tunable fabrication of multifunctional Bi2O3 porous nanospheres for photocatalysis, bacteria inactivation and template-synthesis† Fan Qin,a Huiping Zhao,a Guangfang Li,a Hao Yang,a Ju Li,a Runming Wang,a Yunling Liu,b Juncheng Hu,c Hongzhe Sund and Rong Chen*a Multifunctional Bi2O3 porous nanospheres (PNs) with tunable size have been successfully synthesized via a
Received 28th December 2013 Accepted 28th February 2014
facile solvothermal method. The obtained Bi2O3 porous nanospheres demonstrate outstanding
DOI: 10.1039/c3nr06870f
performance in visible-light-driven photocatalysis for Cr(VI) and organic dye removal, inactivation of Gram-negative and Gram-positive bacteria, as well as template-synthesis for fabrication of bismuth-
www.rsc.org/nanoscale
related hollow nanostructures.
Introduction Recently, multifunctional nanomaterials have attracted considerable research interest due to their great potential in diverse applications of catalysis, magnetic separation and biomedicine, which could simultaneously exhibit novel magnetic, optical, electronic, plasmonic and semiconducting properties.1–4 Extensive studies focused on fabricating complex multifunctional nanosystems that contain two or more nanoscale components or incorporate nanomaterials with different functional molecules.5–7 However, the construction of a multifunctional architecture by deliberately combining different useful functions into one material is still limited and remains a challenge.6,8 Consequently, many efforts have been made to realize the expectation by mediating the size, morphology and structure of nanomaterials, which strongly affect their properties and applications. Porous/hollow nanostructures with designed chemical components, controlled morphologies and tunable sizes are good candidates owing to their high surface area, low density and high loading capacity, which endow them with multifunctional capabilities.9–13 For example, mesoporous SiO2 could be used as a drug delivery carrier in biomedicine,8,14 an excellent noble metal or metal oxide support in catalysis and chemical sensing,15,16 as well as a hard template in fabricating novel
hollow nanostructures.17 It is highly desirable to develop novel multifunctional nanostructures with advanced performance. Currently, bismuth-containing nanomaterials have been intensively studied because of their remarkable properties and applications.18–24 Particularly, as an important p-type semiconductor, bismuth oxide (Bi2O3) is of great interest and has been extensively applied in photovoltaic cells, gas sensing, fuel cells and photocatalysis.25–29 Over the past decades, a great volume of publications has demonstrated the synthesis and applications of various Bi2O3 nanostructures.26,29–35 Unfortunately, there is no report achieving the incorporation of different functions into single-component Bi2O3 nanostructures. Herein, we report the size-tunable synthesis of Bi2O3 porous nanospheres (PNs) with multiple enhanced properties via a facile solvothermal method for the rst time. To demonstrate the multifunctionality of the obtained Bi2O3 PNs for a variety of applications, Bi2O3 PNs were used as a visible-light-driven (VLD) photocatalyst for Cr(VI) and organic dye removal, an antibacterial agent for growth inhibition of S. aureus and E. coli, as well as a bismuth precursor and template for fabricating other bismuthcontaining hollow nanostructures. This work demonstrates various outstanding performances of Bi2O3 PNs which endow them with great potential for further applications, and provides a solution for integration of different functions into one material.
a
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan 430073, PR China. E-mail: [email protected]; Fax: +862787195671
b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
c
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan, PR China
d
Department of Chemistry, The University of Hong Kong, PR China
† Electronic supplementary 10.1039/c3nr06870f
information
5402 | Nanoscale, 2014, 6, 5402–5409
(ESI)
available.
See
DOI:
Experimental Materials Bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), potassium bichromate (K2Cr2O7) and thioacetamide (TAA) were purchased from Aladdin. Poly(vinylpyrrolidone) (PVP, Mw ¼ 10 000) and commercial Bi2O3 powder (99.8%) were purchased from Aldrich (USA). Ascorbic acid (AA) and Na2SeO3 powder were purchased from Lancaster. Ethylene glycol (EG), urea, nitric acid (HNO3),
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TeO2 powder, NH4VO3 and Na2WO4 were obtained from Sinopharm Chemical Reagent Co. (China). All the reagents were of analytical grade and used directly without further purication.
Table 2 Experimental conditions for preparation of bismuth-containing hollow nanospheres
Sample
Bi precursor
Reagent
Synthesis of Bi2O3 nanospheres
Bi2S3 Bi2Se3
Bi2O3 PNs (S2) Bi2O3 PNs (S2)
Bi2Te3
Bi2O3 PNs (S2)
BiVO4 Bi2WO6
Bi2O3 PNs (S2) Bi2O3 PNs (S2)
Thioacetamide (TAA, 0.05 g, 0.7 mmol) Na2SeO3 powder (0.035 g, 0.2 mmol) and ascorbic acid (AA, 0.3 g, 1.7 mmol) TeO2 powder (0.032 g, 0.2 mmol) and ascorbic acid (AA, 0.3 g, 1.7 mmol) NH4VO3 (0.012 g, 0.1 mmol) Na2WO4 (0.033 g, 0.1 mmol)
In a typical experimental procedure, Bi(NO3)3$5H2O (0.182 g, 0.375 mmol) was dissolved in HNO3 solution (5 mL, 1 M), then urea (0.08 g, 1.35 mmol) and EG (25 mL) were added into this solution. Then the mixture was transferred into a stainless steel autoclave with Teon liner. The autoclave was sealed and maintained at 150 C for 3 h. The obtained products were centrifuged and washed with deionized water for ve times and nally dried in a desiccator for a few days for further characterizations (S1). Other samples were also prepared under identical reaction conditions by varying the amount of PVP and using different precursors. The detailed procedure is same as described above and all the experimental parameters are listed in Table 1. Synthesis of bismuth chalcogenides and bismuth-based oxysalt nanostructures The synthesis of bismuth chalcogenides (Bi2S3, Bi2Se3, Bi2Te3) and bismuth-based oxysalt (BiVO4 and Bi2WO6) was simply achieved via the hydrothermal process by utilizing the obtained Bi2O3 PNs (S2) as a bismuth precursor and template. In the synthesis, different reaction precursors were dissolved in 5 mL of deionized water, respectively. The details are summarized in Table 2. Then the precursor source solutions were mixed with the 5 mL dispersion of Bi2O3 PNs (S2) at room temperature. The mixture was then hydrothermally treated for 12 h at 150 C. The products were collected by centrifugation and thoroughly washed with deionized water ve times.
were converted from reection to absorbance by the Kubelka– Munk method. The Brunauer–Emmett–Teller (BET) specic surface area of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). Atomic force microscopy (AFM) images were recorded on a Nanoscope IIIa Multimode SPM (Veeco Co., USA). The electron spin resonance (ESR) method for detecting the generation of reactive oxygen species (ROS) in Bi2O3 suspensions was performed by the ESR spectrometer (EMX-8/2.7, Bruker) at room temperature with DMPO as the radical trap. Cr(VI) adsorption measurement Cr(VI) solutions with different concentrations were prepared by using K2Cr2O7 as the Cr(VI) source. In a typical adsorption procedure, 0.02 g of Bi2O3 sample was added to 20 mL Cr(VI) solution (40 mg L1) under stirring. At each given time interval, 2 mL suspension was sampled and centrifuged to remove the solid adsorbents. The concentration of Cr(VI) during the adsorption was measured with a Shimadzu UV2800 spectrophotometer. All the measurements were carried out at room temperature.
Characterizations Powder X-ray diffraction (XRD) was carried out on Bruker axs D8 ˚ Scanning electron microscopy Discover (CuKa ¼ 1.5406 A). (SEM) images and energy dispersive X-ray (EDX) spectra were taken on a Hitachi S4800 scanning electron microscope operating at 5.0 kV. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were recorded on a Philips Tecnai 20 electron microscope. UV-vis diffuse reectance spectra (DRS) were recorded on a UV-vis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and
1 Experimental nanomaterials Table
conditions
for
preparation
of
Bi2O3
Sample
Reactant
PVP (g)
Size (nm)
S1 S2 S3 S4 S5 S6 S7
Urea Urea Urea Urea NaOH NaOH NaOH
— 0.15 0.3 0.45 0.15 0.3 0.45
350 180 140 160 130 100 —
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Photocatalytic activity measurement The photocatalytic activities of Bi2O3 nanospheres for the degradation of Cr(VI) were performed under visible light irradiation (500 W Xe lamp with a 400 nm cut-off lter, Beijing Changtuo Technology Co. Ltd.). In a typical photocatalytic procedure, 0.02 g Bi2O3 sample was added into 20 mL Cr(VI) solution (40 mg L1). Prior to irradiation, the suspension was stirred in the dark for 2 h to reach adsorption–desorption equilibrium. Then, the solution was exposed to visible light irradiation under magnetic stirring. At each given time interval, 2 mL suspension was sampled and centrifuged to remove the solid photocatalyst. The concentration of Cr(VI) during the degradation was monitored by colorimetry using a Shimadzu UV2800 spectrophotometer. The photocatalytic activities of the Bi2O3 porous nanospheres for Congo red (CR) and Rhodamine B (RhB) degradation were also evaluated by the same procedure upon visible light irradiation. In each experiment, 0.02 g Bi2O3 sample (S6) and 30 mL CR solution (50 mg L1) or RhB solution (105 M) were used in the photocatalysis. The dark adsorption– desorption equilibrium time is 1 h. All the measurements were carried out at room temperature.
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Antibacterial activity measurement The bacteria used were cultured in the Luria Bertani (LB) liquid medium at 37 C for 24 h. Before the antibacterial test, all the samples and materials in the experiments were sterilized at 121 C for 20 min. The antibacterial activities of Bi2O3 samples towards S. aureus and E. coli were evaluated by colony counting, minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) methods. The colony counting test was performed by mixed 2 mL 105 CFU mL1 diluted bacteria and 2 mL Bi2O3 sample (4 mg mL1) into tubes, then the mixture was incubated for 24 h at 37 C with shaking at 150 rpm. Then, 10 mL of the mixture were plated onto LB agar plates and the number of the colonies was counted aer incubation for 24 h at 37 C. The MIC value was determined by mixing 2 mL 105 CFU mL1 bacteria and 2 mL of serial doubling diluted dispersion of the samples into the tubes and incubated for 24 h at 37 C. The MIC value was recorded as the lowest concentration of the sample inhibiting the visible growth of microorganisms. The MBC value is determined by spreading 10 mL of the mixture of bacteria suspension and samples with different concentrations from the clarifying tube on LB agar plates, which were incubated for 24 h at 37 C. Then the number of the colonies was counted. Photocatalytic inactivation assay The photocatalytic inactivation of E. coli over Bi2O3 samples (S2) was conducted under visible light irradiation by using a 500 W xenon lamp with a 400 nm cutoff lter as the light source. All glass apparatuses and materials used in this experiment were autoclaved at 121 C for 20 min to ensure sterility. The bacterial cells were incubated in LB agar plates at 37 C for 24 h, then dispersed in sterilized saline (0.9% NaCl) and adjusted to the required concentration (105 CFU mL1). In the experiment, 30 mL bacterial cell suspension with 100 mg mL1 catalysts was stirred upon visible light irradiation throughout the experiment. At different time intervals, aliquots of the sample were withdrawn. 10 mL samples were then immediately spread on LB agar plates and incubated at 37 C for 24 h to determine the number of viable cells. For comparison, two control experiments were conducted along with treatment experiments. The light control was carried out in the absence of Bi2O3 nanospheres (S2) under visible light irradiation, and negative control without photocatalyst and visible light irradiation. All the treatment and control experiments were performed in triplicates. The reaction temperature of photocatalysis was maintained at 25 C. Bacteria imaging by AFM The culture of E. coli or S. aureus was grown overnight to 108 CFU mL1 in LB medium at 37 C. Bi2O3 samples (S2) (20 mg mL1 for S. aureus and 120 mg mL1 for E. coli) were added to the suspension, and the mixture was incubated at 37 C for 2 h with continuous shaking. The bacteria cells in 2 mL suspension were isolated by centrifugation and washed twice immediately with PBS buffer solution (200 mM, pH ¼ 7.4). The samples were then
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xed in 2.5% glutaraldehyde at 4 C for 2 h. Then the samples were centrifuged and washed with PBS buffer solution (200 mM, pH ¼ 7.4) to remove residues of glutaraldehyde. And then the samples were washed twice with sterilization of deionized water. Finally, the samples were resuspended in the sterilization of deionized water and 20 mL bacterial suspensions were added dropwise on the cover slip and dried in air. Culture with Bi2O3free served as control. E. coli and S. aureus cells were imaged in ambient air with a tapping mode AFM (Nanoscope IIIa Multimode, Veeco Co., USA) before and aer being treated with Bi2O3 nanospheres (S2). The AFM was equipped with an E-type scanner and a rectangular silicon cantilever with a resonance frequency of 290–320 kHz. Real time scanning was performed with a scan rate of 1 Hz, scan angle of 0 , and resolution of 256 256 pixels. Measurement and processing of the AFM images were performed with Version 5.31r1 soware (Veeco Co., USA). Measurement of leakage of the reducing sugars To detect the leakage of reducing sugars through bacteria membrane, Bi2O3 samples (S2) and bacterial suspension (S. aureus and E. coli) were added into 10 mL LB medium with a nal concentration of 100 mg mL1 Bi2O3 and 6 108 CFU mL1 bacteria. Control experiments were conducted without Bi2O3. The cultures were incubated at 37 C with shaking at 150 rpm. Then the sample was centrifuged at 10 000 rpm, the concentrations of reducing sugar of the supernatant were determined immediately by the 3,5-dinitrosalicylic acid method aer being treated with Bi2O3 for 0 and 12 h. All experiments were performed under sterile conditions and in triplicate.
Results and discussion Fig. 1a shows the typical XRD pattern of the Bi2O3 product synthesized from HNO3 treated Bi(NO3)3 and urea in the absence of PVP via a solvothermal method (S1). All the diffraction peaks are well indexed to Bi2O3 (JCPDS 76-2478), indicative of the high purity of Bi2O3. The SEM image illustrates that the product is composed of large-scale uniform solid Bi2O3 nanospheres with an average diameter of 350 nm (Fig. 1b). TEM images (Fig. 1c and d) further demonstrate the spheric morphology and smooth surface of Bi2O3 nanospheres. Interestingly, Bi2O3 porous nanospheres were fabricated under identical experimental conditions when PVP was introduced into the reaction system. As illustrated in Fig. 2a, large quantities of Bi2O3 porous nanospheres with an average diameter of 180 nm were obtained in the presence of 0.15 g PVP (S2). TEM images clearly reveal that the spheric Bi2O3 has a relatively uniform size (Fig. 2b) and porous structure (Fig. 2c), which consists of numerous interconnected nanoparticles. The HRTEM image (inset of Fig. 2c) illustrates a well-resolved interplanar d-spacing of 0.326 nm, which is corresponding to the (111) lattice plane of Bi2O3. It has a BET area of 9.8 m2 g1 and a total pore volume of 0.078 cm3 g1, both of which are larger than that of solid Bi2O3 nanospheres (S1, 8.2 m2 g1 and 0.018 cm3 g1) (Fig. S1, ESI†). More importantly, the porous structure and size of Bi2O3 nanospheres could be further
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Fig. 1 (a) XRD pattern, (b) SEM image, (c and d) TEM images of solid Bi2O3 nanospheres (S1) prepared by the solvothermal method in the absence of PVP.
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mediated by varying the amount of PVP. Fig. 2e–h show SEM and TEM images and the size distribution diagram of Bi2O3 porous nanospheres prepared in the presence of 0.3 g PVP (S3), which reveals that the average diameter of Bi2O3 porous nanospheres decreased to 140 nm and a more obvious porous structure could be observed. Further increasing the PVP amount to 0.45 g (S4), however, the average diameter of Bi2O3 porous nanospheres increased to 160 nm and the product maintained its porous structure (Fig. 2i–l), which might be ascribed to the increased viscosity of the reaction system with the increase of the PVP amount. It indicates that PVP plays a critical role in the fabrication of Bi2O3 porous nanospheres, which not only controls the size but also mediates the structure of the nal product. We further found that the size of Bi2O3 porous nanospheres would decrease when NaOH was used as a reagent, instead of urea. Under identical conditions, uniform Bi2O3 porous nanospheres with an average diameter of 130 and 100 nm could be obtained in the presence of 0.15 g (S5) and 0.3 g (S6) PVP, respectively, as depicted in Fig. 2m–t. TEM images of individual Bi2O3 nanospheres (S5 and S6) also reveal their distinct porous structures. Bi2O3 porous nanospheres with the smallest size (S6) possess the largest BET surface area of 23.4 m2
Fig. 2 SEM, TEM images and size distribution histograms of Bi2O3 PNs prepared by the solvothermal method: (a–d) S2, (e–h) S3, (i–l) S4, (m–p) S5, (q–t) S6, respectively.
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g1 and a total pore volume of 0.15 cm3 g1 (Fig. S1, ESI†). However, only Bi2O3 nanoparticles and irregular nanoplates were obtained when more PVP was employed (S7, Fig. S2, ESI†). In this fabrication, HNO3 pretreatment of Bi(NO3)3 is essential to form sphere-like Bi2O3 nanostructures. If untreated Bi(NO3)3 was directly used under the same experimental conditions, Bi2O3 nanotubes were fabricated, as reported in our previous work.33 Besides, we have also successfully prepared Bi hollow nanospheres from HNO3 treated Bi(NO3)3 in the absence of urea.36 Simultaneously, PVP and OH were vital factors for fabricating size-tunable Bi2O3 PNs. Based on our experimental results and reported literatures, we reasoned the formation process of Bi2O3 PNs as follows. In the initial stage, Bi–PVP complexes formed through the coordinative bonding between Bi3+ and carbonyl oxygen of PVP, and the excess PVP wrapped the Bi–PVP complexes to generate micelles.37,38 The formed Bi–PVP complexes could slowly release Bi3+ as the reaction proceeded, which would react with OH ions provided by urea or NaOH. This process slowed down the nucleation and subsequent crystal growth of Bi2O3 particles, leading to the formation of tiny nanoparticles. The coated PVP molecules prevented the adjacent Bi2O3 crystal particles from quick aggregation, resulting in the formation of porous structures assembled by numerous interconnected nanoparticles. The adequate PVP also modulates the size of Bi2O3 nanospheres with the increase of the PVP amount to a certain extent, which is probably due to the different viscosity of the reaction system regulated by the amount of PVP. However, our understanding of the mechanism is still limited and further investigation is needed in our further work. Initially, we expected a relatively high specic surface area and adsorptive capacity of Bi2O3 porous nanospheres, considering its porous structure. However, the largest BET surface area of Bi2O3 PNs is only 23.4 m2 g1 (S6). Fig. 3a shows the Cr(VI) adsorption capacities of Bi2O3 nanospheres with different diameters under an initial concentration of 40 mg L1 (S1, S2, S5 and S6). The Cr(VI) adsorption measurement demonstrates a size-dependent Cr(VI) adsorption capacity over Bi2O3 nanospheres. Fig. 3b shows the adsorption isotherm of Cr(VI) removal of sample S6. The Langmuir adsorption model (qe ¼ qmbCe/ (1 + bCe)) is used to calculate the maximal adsorption capacity, which represents the relationship between the amount of heavy metal adsorbed at equilibrium (qe, mg g1) and the equilibrium solute concentration (Ce, mg L1), where qm (mg g1) is the maximal adsorption capacity corresponding to complete monolayer coverage and b is the equilibrium constant (L mg1). According to the adsorption isotherm, the maximum Cr(VI) adsorption capacity of Bi2O3 porous nanospheres with smallest diameter (S6) is 24.5 mg g1. Although it is higher than those of many reported metal-oxide adsorbents,39–42 the remaining Cr(VI) is still a huge potential threat for human health. In order to eliminate Cr(VI) completely and efficiently, photocatalysis over Bi2O3 nanospheres was performed under visible light (VL) irradiation, which is one of the most promising and clean strategies to thoroughly reduce highly toxic and carcinogenic Cr(VI) species to less harmful Cr(III) species.43 Fig. 3c shows the variation in the Cr(VI) concentration (C/C0) with irradiation time over Bi2O3 nanospheres (S1, S2, S5 and S6), commercial Bi2O3 (C–Bi2O3)
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Fig. 3 (a) Cr(VI) removal capacities of Bi2O3 nanospheres with different diameters with an initial Cr(VI) concentration of 40 mg L1 (S1, S2, S5 and S6), (b) adsorption isotherm of Cr(VI) removal of sample S6, (c) variation in Cr(VI) concentration (C/C0) with VL irradiation time over Bi2O3 nanospheres (S1, S2, S5 and S6), P25 and C–Bi2O3, (d) plot of ln(C00 /Ct) versus time for Cr(VI) photodegradation over Bi2O3 nanospheres, C00 and Ct are the initial Cr(VI) concentration after adsorption equilibrium and the Cr(VI) concentration at time t, respectively.
and P25 under visible light irradiation. It is clearly observed that the Cr(VI) concentration hardly changed in the absence of Bi2O3 nanospheres upon 2 h visible light irradiation. However, nearly 100% Cr(VI) could be removed over S6 and S5 within 20 and 120 min upon visible light irradiation, respectively. Sample S2 also could reduce about 70% Cr(VI) aer 2 h visible light irradiation. While Bi2O3 solid nanospheres (S1) and P25 were only able to partially degrade Cr(VI) aer 2 h visible light irradiation (photodegradation efficiency is only about 30% and 10%, respectively). All Bi2O3 porous nanospheres present much higher photocatalytic efficiency than solid nanospheres (S1) and P25, indicating a structure- and size-dependent photocatalytic activity. The highest photoactivity of S6 is probably attributed to its unique porous structure, largest BET surface area and smallest size. The pH effect on the photocatalytic activities of Bi2O3 nanospheres is also investigated. It is found that Cr(VI) could be efficiently removed over a wide pH range from 3 to 11 (Fig. S3, ESI†), suggesting that Bi2O3 nanospheres could be a potential candidate for Cr(VI) photoreduction in practical applications. The kinetics of the photocatalytic reduction of Cr(VI) were also investigated. The obtained kinetic data of the Cr(VI) reduction reaction over Bi2O3 nanospheres (S1, S2, S5 and S6) t to a pseudo-rst-order model as expressed by ln(C00 /Ct) ¼ kCrt, where C00 and Ct represent the Cr(VI) concentration before and aer the irradiation, t is the irradiation time, and kCr is the apparent rate constant. As shown in Fig. 3d, Bi2O3 porous nanospheres (S6) present the fastest reduction rate, compared with other Bi2O3 nanospheres upon visible light irradiation. The kinetics data also demonstrate the size-dependent photoreduction rate of Bi2O3 nanospheres. The results indicate that Bi2O3 porous nanospheres should be a novel ideal visible light responsive photocatalyst for Cr(VI) removal in aqueous solution by utilizing solar energy.
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To understand the visible-light-response photocatalytic property of Bi2O3 nanospheres, the diffuse reectance spectra (DRS) of Bi2O3 nanospheres (S1, S2, S5 and S6) are measured (Fig. S4, ESI†). It reveals that an intense absorption occurs in the UV-light region for all Bi2O3 nanospheres, and the absorption edge is around 350 nm. Meanwhile, weak photo-absorption in the visible-light region could be also observed, indicating that Bi2O3 nanospheres could also respond to visible light in photocatalysis. The band gap energy could be determined by the equation of ahn ¼ A(hn Eg)n/2, in which a, h, n, Eg, and A are the absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. Among them, n is determined by the type of optical transition of the semiconductor and also depends on the polymorph (i.e., n ¼ 1 for direct transition and n ¼ 4 for indirect transition). According to the reported literatures,26,29 we use n ¼ 1 for Bi2O3 nanospheres. The band gap energy is estimated to be about 3.47, 3.15, 3.42 and 3.54 eV for the S1, S2, S5 and S6, respectively. In order to conrm this value, a tting model is calculated, as reported in our previous study.44 The plots of (ahn)2 versus energy and (ahn)1/2 versus energy in the absorption edge region are shown in Fig. S5 (ESI†). It is observed that the (ahn)2 versus energy plot is almost the perfect linear relationship, while the (ahn)1/2 versus energy deviates from the tted straight line. This feature suggests that the absorption edge of Bi2O3 nanospheres is caused by direct transitions, that is, n ¼ 1. We further extend the photocatalytic application of Bi2O3 PNs for other pollutants in wastewater. The photocatalytic activity of Bi2O3 PNs (S6) was also evaluated by degrading Rhodamine B (RhB) and Congo red (CR) under visible light irradiation, respectively. As shown in Fig. S6 (ESI†), Bi2O3 PNs also display good visible light driven photodegradation performance for organic dyes, indicating that Bi2O3 PNs are potential photocatalysts for water treatment. However, the photodegradation efficiency of organic dyes hardly improved aer 2 h irradiation. In addition, it was found that Bi2O3 porous nanospheres did not present good repeatability for the photocatalysis, which was probably due to photocorrosion of bismuth oxide under visible light irradiation. Efforts on improving its stability and repeatability will be devoted in our future study. In this work, it is proposed that slight visible light absorption ability make a great contribution to photocatalytic activity of samples. Bi2O3 products could be excited under visible light, although the band gaps are large. More importantly, porous structures of Bi2O3 benet carrier transportation during the photocatalytic process, and inhibit combination of photoexcited electrons and holes, which ultimately lead to enhanced visible-lightdriven photocatalytic activity, similar to the case of enhanced photocatalytic activity over macroporous TiO2.45 Although there are numerous studies regarding the antibacterial effect of inorganic metal oxides, few reports concern bismuth oxide.24 Our previous studies also demonstrated no inhibition of Bi2O3 nanoparticles against Helicobacter pylori,18 however, little is known regarding the interaction of Bi2O3 porous nanospheres with other bacteria. We then investigated the antibacterial activities of Bi2O3 nanospheres and C–Bi2O3
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against Gram-positive S. aureus and Gram-negative E. coli. As shown in Fig. 4a, Bi2O3 nanospheres exhibit excellent sizedependent antibacterial activity towards S. aureus, which is much better than C–Bi2O3. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Bi2O3 nanospheres towards S. aureus were generally observed to be in the range of 1–8 mg mL1 and 4–16 mg mL1, respectively (Table S1, ESI†), which also conrms the highly efficient and size-dependent antibacterial activities of Bi2O3 nanospheres. Noticeably, the growth inhibition of E. coli was not observed in the colloidal suspension containing Bi2O3 PNs (S2, le of Fig. 4b), indicative of no toxic effect of Bi2O3 nanospheres to E. coli. However, Bi2O3 PNs (S2) exhibit outstanding photocatalytic bactericidal performance toward E. coli upon visible light irradiation (right of Fig. 4b). Only 10% E. coli could survive aer 2 h visible light irradiation. The control experiment demonstrates no photolysis of bacterial cells under visible light irradiation alone, suggesting that Bi2O3 PNs are truly highly efficient visible-light-driven photocatalysts for bacterial inactivation. The photoinduced bactericidal performance of Bi2O3 PNs could be ascribed to the reactive photo-generated oxidative species (ROS, such as –OH, –O2 and h+) upon visible light irradiation, which could attack the bacterial cell wall and membrane, cause the leakage of intracellular content, and eventually resulting in bacterial death.46 In this study, ROS are also detected from Bi2O3 porous nanospheres upon visible light irradiation, as conrmed by the electron spin resonance spectra(ESR) with DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin trap in Bi2O3 PN (S2) suspension before and aer visible light irradiation (Fig. S7, ESI†). Hence, the bacteria inactivation is a photoinduced oxidation process.
Fig. 4 (a) Inhibition rate of Bi2O3 nanospheres and C–Bi2O3 against S. aureus, (b) inhibition rate of Bi2O3 PNs (S2) against E. coli upon VL irradiation off (left) and on (right), (c) leakage of reducing sugars from bacteria cells of S. aureus (left) and E. coli (right) treated with Bi2O3 PNs (S2), (d and e) topography images of S. aureus (d) and E. coli (e) cells before and after Bi2O3 PN (S2) treatment.
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To understand the different antibacterial effects of Bi2O3 nanospheres towards S. aureus and E. coli, topography changes of bacterial cell walls of S. aureus and E. coli treated with Bi2O3 porous nanospheres (S2) were investigated by atomic force microscopy (AFM). As shown in Fig. 4d and e, S. aureus cells show obvious bacterial membrane destruction and intracellular content leakage aer 2 h incubation with Bi2O3 porous nanospheres (S2), while E. coli cells maintain the original rod-shaped morphology and a smooth surface. The different damages on S. aureus and E. coli were further conrmed by the membrane leakage of reducing sugars from bacteria. As depicted in Fig. 4c, the leakage amount of reducing sugars from S. aureus cells treated with Bi2O3 PNs (S2) is up to 167.1 mg mL1, but only 39 mg mL1 in the control experiment, suggesting that Bi2O3 porous nanospheres may accelerate the reducing sugar leakage from S. aureus bacterial cytoplasm. However, there is no obvious increase in the membrane leakage of reducing sugars from E. coli treated with Bi2O3 porous nanospheres. It is probably due to the difference in cell structures of Gram-negative and Grampositive bacteria. The research on the specic mechanism of bactericidal performance of Bi2O3 porous nanospheres is ongoing in our studies. Besides, the obtained Bi2O3 porous nanospheres could serve as the bismuth precursor and template to fabricate other bismuth-containing hollow nanospheres via a facile hydrothermal method. As shown in Fig. S8 (ESI†), the diffraction peaks in XRD patterns are well indexed to Bi2S3 (JCPDS 89-8964), Bi2Se3 (JCPDS 12-732), Bi2Te3 (JCPDS 08-27), BiVO4 (JCPDS 14-688) and Bi2WO6 (JCPDS 39-256), respectively, indicating that pure Bi2E3 (E ¼ S, Se and Te), BiVO4 and Bi2WO6 products are obtained by using Bi2O3 PNs as a precursor. Fig. 5 shows SEM and TEM images of Bi2E3 (E ¼ S, Se and Te), BiVO4 and Bi2WO6 products. It clearly reveals that all the products present sphere-like hollow nanostructures and have a relatively uniform size. More importantly, the estimated average diameter of the obtained bismuth-containing hollow nanospheres is around 180 nm, which is consistent with the size of the starting material, Bi2O3 porous nanospheres (S2). It indicates that Bi2O3 porous nanospheres serve as a sacrice template in the fabrication of bismuthcontaining hollow structures. The formation processes of Bicontaining hollow nanostructures from Bi2O3 porous nanospheres might be attributed to the fundamental solid-state phenomenon, which is called the Kirkendall effect that deals with the movement of the interface between the diffusion couple.47 For example, once thioacetamide (TAA) solution is introduced to the reaction system, TAA molecules are uniformly distributed and easily adsorbed on the surface of Bi2O3 porous nanospheres through intermolecular interactions. Subsequently, Bi2O3 porous nanospheres were transformed into Bi2S3 due to its lower solubility during the hydrothermal process, nally resulting in the formation of Bi2S3 hollow nanostructures. This process is analogous to the reported formation of hollow and core–shell structures.48,49 It is believed that the hollow structure with large fraction of void space and functional shells endows the obtained bismuthcontaining nanomaterials with technological signicance and
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SEM and TEM images of bismuth-containing hollow nanospheres prepared by using Bi2O3 PNs (S2) as a precursor and template: (a–c) Bi2S3, (d–f) Bi2Se3, (g–i) Bi2Te3, (j–l) BiVO4 and (m–o) Bi2WO6. Fig. 5
potential applications in a wide range of catalysis, energy storage and conversion, gas sensing and biomedicine.
Conclusions In summary, we have successfully synthesized uniform Bi2O3 porous nanospheres with tunable size by employing PVP via a facile solvothermal method. In particular, the prepared Bi2O3 porous nanospheres demonstrate for the rst time the multifunctional property in photocatalysis, bacteria inactivation and template-synthesis. They display outstanding visible-lightdriven photocatalytic performance for Cr(VI) and organic dye removal in aqueous solution, as well as exhibiting remarkable antibacterial activity toward S. aureus and highly enhanced visible-light-driven photocatalytic inactivation of E. coli. Moreover, Bi2O3 porous nanospheres could be applied as a bismuth precursor and template to fabricate bismuth chalcogenides (Bi2S3, Bi2Se3, Bi2Te3) and bismuth-based oxysalt (BiVO4 and Bi2WO6) hollow nanospheres. It is anticipated that the multifunctional Bi2O3 porous nanospheres will lead to many possibilities for creating more novel bismuth-related nanostructures with multiple functionalities.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21171136 and 21371139), International Cooperation Program of Hubei Province (2011BFA021), Key Program of Wuhan Science and Technology Bureau (201260523183), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2014-09) and the High-tech Industry Technology Innovation Team Training Program of Wuhan Science and Technology Bureau (2014070504020243).
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