Article pubs.acs.org/Langmuir
A Highly Efficient UV−Vis−NIR Active Ln3+-Doped BiPO4/BiVO4 Nanocomposite for Photocatalysis Application Sagar Ganguli,† Chanchal Hazra,† Manjunath Chatti,† Tuhin Samanta,† and Venkataramanan Mahalingam*,† †
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur, West Bengal 741252, India S Supporting Information *
ABSTRACT: In this Article, we report the synthesis of Ln3+ (Yb3+, Tm3+)-doped BiPO4/BiVO4 nanocomposite photocatalyst that shows efficient photocatalytic activity under UV−visible−near-infrared (UV−vis−NIR) illumination. Incorporation of upconverting Ln3+ ion pairs in BiPO4 nanocrystals resulted in strong emission in the visible region upon excitation with a NIR laser (980 nm). A composite of BiPO4 nanocrystals and vanadate was prepared by the addition of vanadate source to BiPO4 nanocrystals. In the nanocomposite, the strong blue emission from Tm3+ ions via upconversion is nonradiatively transferred to BiVO4, resulting in the production of excitons. This in turn generates reactive oxygen species and efficiently degrades methylene blue dye in aqueous medium. The nanocomposite also shows high photocatalytic activity both under the visible region (0.010 min−1) and under the full solar spectrum (0.047 min−1). The results suggest that the photocatalytic activity of the nanocomposite under both NIR as well as full solar irradiation is better compared to other reported nanocomposite photocatalysts. The choice of BiPO4 as the matrix for Ln3+ ions has been discussed in detail, as it plays an important role in the superior NIR photocatalytic activity of the nanocomposite photocatalyst.
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INTRODUCTION The discharge of dye-containing effluents into water bodies by textile industries is a major threat to the aquatic environment, as the toxicity and carcinogenic properties of these effluents are of serious concern for marine life.1 While there are several photounstable dyes that degrade quickly when exposed to light, dyes used by industries are generally quite photostable, i.e., they do not bleach significantly in the presence of light. This demands biological oxidation and chemical treatments to remove such dyes.2 However, the use of heterogeneous semiconductor-based photocatalysts is considered a costeffective alternative.3,4 There are two different mechanisms by which dyes degrade in the presence of a semiconductor photocatalyst. One, upon illumination, dye molecules adsorbed on suitable semiconductors can inject electrons from its excited state into the conduction band of the semiconductor and degrade subsequently.5 One such example is the degradation of Rhodamine B dye adsorbed on TiO2 by such an electron transfer mechanism.6 Alternatively, semiconductor photocatalysts in the presence of light can generate reactive oxygen species, which in turn degrades these pollutants. However, for effective photocatalytic activity by the second mechanism, the energy of the incident light must be equal or greater than the band gap of the photocatalyst. TiO2 and ZnO, the most widely studied photocatalysts so far, possess large bandgap and thus become active only in the presence of UV light.7,8 Bismuth © XXXX American Chemical Society
phosphate (BiPO4) is a new type of inorganic oxy-acid photocatalyst that shows superior photocatalytic activity compared to P25 (commercial titania) under UV irradiation.9,10 Nevertheless, UV light consists only ∼5% of the solar spectrum, thus restricting the practical application of these photocatalysts. Lately, different strategies like introduction of noble metals, cationic and anionic substitutions, etc. into these photocatalysts have been done to extend their absorption window into the visible region, which constitutes about 49% of the solar spectrum.11−13 Bismuth vanadate (BiVO4) has recently attracted considerable attention due to its high photocatalytic activity under visible light. In fact, the composites of BiPO4 and BiVO4 received much research attention as they show superior photocatalytic activity in both UV and visible region.14,15 The efficiency of these composites is higher than that of BiPO 4 and BiVO 4 , when used individually.14,15 In order to further increase the efficiency of photocatalysts, researchers are interested in widening the absorption window of photocatalysts into the near-infrared (NIR) region as it covers about ∼46% of the solar radiation. However, NIR light with low photon energy cannot directly induce the activity of most semiconductors in the process of Received: September 5, 2015 Revised: December 6, 2015
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Langmuir solar energy conversion.16 Researchers are in search of promising materials that can absorb NIR photons and convert that energy into visible ones. In this regard, lanthanide ions (Ln3+) are interesting, as they are able to convert NIR photons into visible ones via the upconversion process.17−27 Therefore, the design of composite semiconductor materials with upconverting Ln3+ ions is one of the ways to perform NIR photocatalysis. For example, Zhu et al. have recently reported the photocatalytic activity of BiVO4/CaF2:Er3+,Tm3+,Yb3+ composite under both visible and NIR illumination.28 Similarly, NIR-responsive photocatalytic activity and mechanism of NaYF4:Yb3+,Tm3+@ TiO2 core−shell nanoparticles have been investigated in detail by Qin et al. in the recent past.29 However, to the best of our knowledge, there is only one report of photocatalyst that shows activity covering all three regions of the solar spectrum, i.e., under UV, visible, and NIR. In this work, Liu et al. have reported a broad spectrum photocatalyst by assembling Bi2WO6 nanosheets on TiO2 nanobelts, which can harness UV, visible, and NIR light to decompose organic contaminants in aqueous solution.30 In this Article, we have synthesized a Ln3+-doped nanocomposite photocatalyst that shows efficient photocatalytic activity under UV−vis−NIR illumination. This is achieved by doping upconverting Ln3+ ion pairs (Yb3+, Tm3+) into BiPO4 nanocrystals (NCs), which exhibit strong emission in the visible region upon excitation with a NIR laser (980 nm). A nanocomposite of these NCs with vanadate has been prepared by stirring at elevated temperatures. The intensity of the characteristic emission peak of Tm3+ at 475 nm selectively decreases in the nanocomposite. We believe this is due to the significant transfer of upconverted energy from Tm3+ emission to BiVO4, which in turn produces reactive oxygen species for photodegradation of dyes. Moreover, under UV and visible illumination, the nanocomposite shows superior photocatalytic activity, which surpasses most of the previous reports on similar systems.
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white dispersion (as shown in the digital image Figure 1a). The dispersion was then heated to ∼90 °C. 700 mg NH4VO3 was added to
Figure 1. Digital images: (a) aqueous dispersion of Ln3+-doped BiPO4 NCs, and (b) aqueous dispersion of BiPO 4:Yb 3+,Tm 3+/BiVO4 nanocomposite. the dispersion under continuous stirring. The reaction mixture was further stirred for 6 h, by which time the color of the solution turned yellow (as shown in the digital image Figure 1b). The suspension was centrifuged and washed with deionized water and ethanol several times to remove any unreacted reactants. The final nanocomposite (yellow in color) was dried at 60 °C under vacuum. Characterization. The X-ray diffraction (XRD) patterns were collected using a Rigaku-SmartLab diffractometer attached with a D/ tex ultra detector and Cu Kα source operating at 35 mA and 70 kV. The scan range was set from 10 to 60° 2θ with a step size of 0.02° and a count time of 2 s. The samples were well powdered and spread evenly on a quartz slide. Infrared spectroscopy measurements were carried out on a Perkin Elmar FT-IR spectrometer 1000 with a resolution of 2 cm−1 and averaged over four scans. Raman spectra of all the samples were recorded on a Horiba Jobin Yvon LabRAM HR800 micro-Raman spectrometer, using a 1800 gmm−1 grating. The samples were excited with a 633 nm He−Ne laser line. Field emission scanning electron microscopy (FESEM) images were taken on a SUPRA 55-VP instrument with patented GEMINI column technology. Prior to loading the samples into the chamber, they were coated with a thin film of gold−palladium in order to avoid charging effects. Transmission electron microscopy (TEM) images were taken on a UHR-FEG-TEM, JEOL; JEM 2100F model using a 200 kV electron source. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI 5000 Versa Prob II, FEI Inc. instrument. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed with a Thermo Scientific XSERIES2 ICP-MS instrument. 2 mg of the nanocomposite was dissolved in 5 mL suprapure nitric acid by heating overnight at 110 °C. Then the required amount of that solution was added to ultrapure water according to the required concentration. The PL spectra were measured on a Horiba Jobin Yvon spectrometer equipped with a 450 W Xe lamp. The excitation and emission light was dispersed using a Czerny-Turner monochromator with an optical resolution of 1 nm. The emitted photons were detected using a Hamamatsu R928 detector. The output signal was recorded using a computer. The PL lifetime measurements were performed with a Horiba Jobin Yvon Fluorolog CP machine equipped with a pulsed Xe source operating at 25 W. For upconversion measurements and photocatalytic experiments under NIR irradiation, NCs were excited with a 980 nm diode laser from RgBLase LLC at a power density of 105 W/cm2, which was coupled with a fiber with a core diameter of 100 μm. UV−vis diffuse reflectance spectroscopic measurements were
EXPERIMENTAL SECTION
Materials. Bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], Yb2O3, Tm2O3, Y2O3, Er2O3, sodium trifluoroacetate [CF3COONa], and ammonium dihydrogen phosphate [NH4H2PO4] were purchased from Sigma-Aldrich. Ammonium metavanadate [NH4VO3], 1 (M) HNO3 (70% pure), ethylene glycol, and ethanol were purchased from Merck. Methylene Blue (MB) was purchased from Lobachemie. Trifluoroacetic acid [CF3COOH] was purchased from Spectrochem. All chemicals were used without further purification. Synthesis. Yb3+ and Tm3+ codoped BiPO4 NCs were synthesized following the procedure of Zhang et al.31 Briefly, 0.1 mmol of Yb2O3 and 0.025 mmol Tm2O3 were stirred with concentrated HNO3 and heated at 95 °C to produce Yb(NO3)3 and Tm(NO3)3, respectively. 2.5 mL of ethylene glycol and 0.795 mmol of Bi(NO3)3·5H2O were then added and stirred for another 30 min to make a transparent solution. 32.5 mL of an aqueous solution containing 1 mmol NH4H2PO4 was added to the above solution under vigorous stirring. The resulting white suspension was further stirred for 30 min at room temperature to attain homogeneity. The suspension was centrifuged and washed with deionized water and ethanol several times to remove any unreacted reactants. The final product (white in color) was dried at 60 °C under vacuum and calcinated at 900 °C for 6 h at a heating rate of 5 °C min−1. For comparative studies, Ln3+ (Ln = Yb, Tm) undoped BiPO4 NCs were prepared using the same protocol without adding Ln3+ ions into the ethylene glycol solution of Bi(NO3)3 at the initial stage. To prepare BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite, 250 mg of as prepared BiPO4 NCs were sonicated in 100 mL water to prepare a B
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Langmuir carried out on Jasco V670 spectrometer. Room-temperature optical absorption spectra of all the samples were recorded on a Hitachi U4100 spectrophotometer. BET surface area measurements were carried out by N2 adsorption at 77.3 K using a Quantachrome Novawin2 instrument. Zeta potentials of the NCs were measured on a Malvern Zetasizer Nano instrument. Photocatalytic Measurements. The photocatalytic activities of the as-prepared nanocomposite under full solar irradiation and that under only NIR light irradiation were determined by performing degradation of MB dye under a Xe lamp (Newport, Standford) with 140 W power (70 mA) and a 980 nm NIR laser, respectively. To evaluate the photocatalytic activity of the nanocomposite under full solar irradiation, 30 mg of the nanocomposite was added into a beaker containing 30 mL of MB solution (5 × 10−5 mol L−1). Prior to irradiation, the mixture was stirred in the dark for 1 h to reach adsorption−desorption equilibrium. Subsequently, the solution was exposed to radiation using a solar simulator with continuous magnetic stirring. Photocatalysis experiments were further performed with a UV (400 nm) cutoff filter to evaluate the efficiency of the photocatalyst under UV-free condition. To evaluate the photocatalytic activity of the nanocomposite under 980 nm NIR irradiation (power density = 105 W/cm2), 10 mg of the nanocomposite was added to 10 mL of MB solution (1 × 10−5 mol L−1) followed by stirring in dark for about an hour. The mixture was then irradiated with a 980 nm laser. In all cases, aliquots of the suspensions were collected at regular irradiation time interval and the slurry samples, including the photocatalyst and MB solution, were centrifuged (6000 rpm, 5 min) to remove the photocatalyst particles in order to assess the rate of decolourization and degradation photometrically. The solutions were analyzed by a UV−vis-NIR spectrophotometer, and the characteristic absorption of MB at 670 nm was used to monitor the photocatalytic degradation. All measurements were carried out at room temperature.
Figure 2. (A) XRD patterns of (a) hexagonal BiPO4, (b) BiPO4 after annealing at 900 °C (mixture of LTMP and HTMP) (c) BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite, and (d) tetragonal BiVO4. (B) FTIR spectra of (a) BiPO4, (b) BiVO4, and (c) BiPO4:Yb3+,Tm3+/ BiVO4 nanocomposite. The region showing vanadate peaks are indicated in the rectangular box. (C) Raman spectra of (a) BiPO4, (b) BiVO4, and (c) BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite.
RESULTS AND DISCUSSION Structure and Morphology. Powder XRD measurements were performed for all three samples to evaluate their phase. The XRD pattern of BiPO4: Yb3+,Tm3+ NCs prepared at room temperature matches well with that of the standard hexagonal phase BiPO4 (Figure 2A(a)) (JCPDS: 15-0766). However, after calcinating the samples at 900 °C, a mixture of hightemperature monoclinic phase (HTMP) and low-temperature monoclinic phase (LTMP) appears in the XRD pattern (Figure 2A(b)). These results are consistent with the observations by Zhang et al.31 The XRD pattern of the final yellow nanocomposite (Figure 2A(c)) does not clearly show any characteristics of vanadate. We believe that the relatively smaller percentage of vanadate in comparison to BiPO4 limits the detection ability of XRD measurement. To confirm the presence of vanadate in the nanocomposite, vibrational spectroscopic measurements were carried out. The FTIR spectrum of the nanocomposite (Figure 2B) clearly shows characteristic peak of vanadate (VO43−) around 745 cm−1. The other peaks observed at 1067, 1005, 958, 619, and 552 cm−1 correspond to BiPO4. In Raman spectroscopy measurements (shown in Figure 2C) of the nanocomposite, characteristic peaks of both vanadate (137, 350, 822 cm−1) and phosphate (176, 232, 614, 991, 1039 cm−1) are observed. XPS analysis of the nanocomposite was carried out to get information about its elemental composition. The survey spectrum (Figure S1a) shows the presence of all the elements in the nanocomposite. As shown in Figure S1b, characteristic peaks of Bi are observed around 159 and 165 eV, which correspond to its 4f7/2 and 4f5/2 levels, respectively. Characteristic peaks of V 2p level also appear around 517 and 524 eV, which closely match with V peaks of BiVO4.32 The presence of Yb and Tm in the nanocomposite is confirmed from their
characteristic peaks around 180 eV. To further quantify the composition of different ions in the nanocomposite, ICP-MS analysis was carried out. The ICP-MS data (Table S1a) also confirm the presence of Bi3+, V5+, Yb3+ and Tm3+ in the nanocomposite. The calculated concentration for the elements from the ICP-MS results are shown in Table S1b. The results indicate that the molar ratio of Bi3+:V5+:Yb3+:Tm3+ is 136:150:40:1. To investigate the morphologies of calcinated Ln3+ doped BiPO4 NCs and the nanocomposite, microscopic analysis was carried out (shown in Figure 3). SEM images reveal that calcinated BiPO4 NCs are rice shaped with an average length and diameter of 70 and 30 nm, respectively (Figure 3a). However, upon BiVO4 coating over the BiPO4 particles, the SEM study indicates formation of aggregation of the particles. The evaluation of the diameter of the particles suggests the absence of core/shell formation (Figure 3b). This implies the likely formation of the nanocomposite after coating with BiVO4. This is further supported by TEM analysis (Figure S2a) of the final yellow product. The presence of both vanadium (elemental V) and phosphorus (elemental P) in the nanocomposite is also confirmed from EDS analysis (Figure S2b). Photoluminescence Studies. The as-synthesized hexagonal Yb3+,Tm3+-doped BiPO4 NCs did not show upconversion luminescence (UCL) upon 980 nm laser excitation. However, after annealing the sample at 900 °C, a strong emission at 475 nm is observed under 980 nm excitation (Figure 4). Annealing leads to the transformation of BiPO4 from hexagonal to monoclinic phase, which is crucial for UCL due to loss of water molecules in the lattice of hexagonal phase.31 The strong emission at 475 nm produced via UC is assigned to 1G4 → 3H6 transition of Tm3+ ions. In addition to the strong peak, the emission spectrum shows weak emission peaks at 648 and 692
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Figure 3. SEM images of (a) BiPO4:Yb3+,Tm3+ NCs. Inset shows rice shaped particles and (b) BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite.
in the nanocomposite rules out the possibility of radiationreabsorption between BiPO4:Yb3+,Tm3+ and BiVO4, because the fluorescence lifetime of the donor does not change in case of radiation-reabsorption process.29 To further confirm that the quenching of 475 nm peak is not associated with radiationreabsorption process, photoluminescence studies of the physical mixtures of BiPO4:Yb3+,Tm3+ NCs and BiVO4 NCs were carried out. Only a small decrease in the luminescence intensity at 475 nm was observed (Figure 4), compared to that of pure BiPO4:Yb3+,Tm3+ NCs. This confirms that quenching of the blue emission is likely associated with some kind of energy transfer mechanism from Tm3+ to BiVO4. The upconversion quantum efficiencies of all samples were determined by comparing with bulk NaYF4:Yb3+, Er3+ for which the absolute quantum efficiency is reported.33 The quantum efficiencies of BiPO4:Yb3+,Tm3+ NCs and the nanocomposite were found to be 1.44% and 1.20%, respectively. The UC spectra measured from the samples along with the calculations are provided in the Supporting Information (SI; Figure S5). Photocatalytic Studies. The strong quenching of the blue emission from Tm3+ in the nanocomposite motivated us to evaluate its NIR photocatalytic activity by performing degradation of MB dye under 980 nm NIR irradiation. A plot of C/C0 versus time (t) reveals that the nanocomposite was able to degrade about 40% of MB in 12 h of NIR illumination (Figure 5). However, for the mixture containing BiVO4 NCs and BiPO4:Yb3+,Tm3+, only 10% of MB degradation is noted under similar conditions. This clearly suggests the importance of the nanocomposite in degrading MB dye efficiently. Moreover, when both BiVO4 and BiPO4:Yb3+,Tm3+ were used individually, only 2% and 4% of MB degradation was noted, respectively. Only 4% dye degradation was observed when BiPO4/BiVO4 (without Ln3+) nanocomposite was used as a photocatalyst suggesting the importance of upconverting Ln3+ ion pairs behind NIR photocatalytic activity. Moreover, the nanocomposite adsorbs about 35% of MB dye from a 1 × 10−5 mol L−1 aqueous solution under dark condition in 1 h, while BiPO4:Yb3+,Tm3+ NCs adsorbs only about 25% dye under similar conditions. As a result, the nanocomposite is able to effectively remove about 61% dye in total from an aqueous 1 × 10−5 mol L−1 MB solution via both adsorption and NIR photocatalysis (Figure S6A). To further investigate the photocatalytic performance of the nanocomposite, additional experiments were carried out under
Figure 4. Upconversion luminescence spectra of (black) BiPO4:Yb3+, Tm3+ NCs (blue) mixture of BiPO4: Yb3+, Tm3+ NCs and BiVO4 NCs and (red) BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite upon 980 nm excitation.
nm corresponding to 1G4 → 3F4 and 3H4 → 3H6 transitions in Tm3+, respectively. The intensity ratio of the blue emission (475 nm) to the 648 nm emission is ∼8.5. For the BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite, notable spectral differences were observed (Figure 4). The intensity of the blue emission (475 nm) decreases significantly, while the intensities of the other two peaks are barely affected. The intensity ratio of the blue emission (475 nm) to the 648 nm emission reduces from 8.5 to 1.06. The selective decrease in the intensity of the 475 nm emission compared to the emissions in the red region suggests a likely energy transfer/absorption of the 475 nm emission by BiVO4 in the nanocomposite. To gain more insight, UV−vis diffuse reflectance measurements of both BiPO4 NCs and the nanocomposite were carried out (Figure S3). The absorption spectrum of the nanocomposite shows a peak starting at 550 nm, while for BiPO4 NCs, it starts around 340 nm. Therefore, we can speculate that in the nanocomposite, the photon energy around 475 nm generated via upconversion in BiPO4:Yb3+,Tm3+ NCs has good probability to get transferred to BiVO4. Fluorescence lifetime studies for both BiPO4:Yb3+,Tm3+ NCs and the nanocomposite were performed to further understand the process (Figure S4). The fluorescence lifetime curves of 1G4 (475 nm) levels of Tm3+ ions in BiPO4:Yb3+,Tm3+ NCs and the nanocomposite were recorded at the excitation wavelength of 374 nm. The average decay time of 1G4 level decreases significantly in the nanocomposite (8 μs) compared to BiPO4 NCs alone (10 μs). This shortening of the fluorescence lifetime of Tm3+ ions D
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suggests that the nanocomposite outperforms P25 as a photocatalyst under identical conditions. We propose the following mechanism for the observed photocatalytic effect. Briefly, the valence band edge potential (EVB) of BiVO4 and BiPO4 are 2.77 and 3.96 eV, respectively, and their homologous conduction band edge potential (ECB) values are 0.31 and 0.03 eV.14 Upon excitation using a solar simulator, an exciton generates in both the BiPO4 and BiVO4 parts of the photocatalyst. The electron in the conduction band and the hole in the valence band react with oxygen and water molecules to produce O2•− and HO•, respectively. We believe that these reactive oxygen species (e.g., HO•, O2•−, etc.) can oxidize the organic dye molecules.14,15 A scheme illustrating the photogenerated charge separation in the nanocomposite and subsequent generation of reactive oxygen species, which is responsible for dye degradation, is shown in Figure 7. The
Figure 5. Plot of concentration change of MB dye over time in aqueous solutions under NIR illumination in the presence of (black) BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite, (green) mixture of BiPO4:Yb3+,Tm3+ and BiVO4, (blue) BiPO4:Yb3+,Tm3+, (brown) BiPO4/BiVO4 nanocomposite, and (red) BiVO4. The green line shows the degradation of MB dye solution under NIR illumination in the absence of any photocatalyst.
a solar simulator. A plot of C/C0 versus time (t) reveals that the nanocomposite was able to degrade about ∼98% of MB in 100 min (Figure 6). However, when the photocatalytic experiment
Figure 7. Scheme depicting photocatalysis mechanism of BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite.
mechanism of photocatalytic activity of BiPO4/BiVO4 nanocomposites under visible light has been under investigation for some time. Zhu et al. have proposed that PO43− possess a large electron cloud overlapping and thus prefers to attract holes and repel electrons.34,35 Moreover, PO43− ions are difficult to form oxygen vacancies, which are considered as the recombination centers of electron and holes in oxide based photocatalysts.15,36 Under NIR irradiation, an exciton generates in the BiVO4 due to energy transfer from BiPO4:Yb3+,Tm3+. This in turn produces reactive species and degrades MB. To gain insight into the role of each of the reactive oxygen species on the photocatalysis process, experiments with radical and hole trapping agents were carried out. As shown in Figure 8, the photodegradation of MB is greatly inhibited by the addition of benzoquinone (O2•− scavenger) and K2Cr2O7 (electron scavenger). However, photodegradation was hardly suppressed when tertiary butanol (TBA, hydroxyl radical scavenger) and oxalate (hole scavenger) were added. These results indicate that O2•− and electron play dominant role behind the photocatalytic activity of the nanocomposite. The surface area of a photocatalyst plays a major role behind its photocatalytic activity. Generally, the photocatalytic efficiency of any material increases with increase in its surface area. To find out the role of surface area behind the enhanced photocatalytic activity of the nanocomposite in comparison to Ln3+-doped BiPO4 NCs, BET surface area measurement were carried out. BET surface area values clearly indicate almost 30% increase in the surface area upon composite formation (4.981 m2g−1) than that of the BiPO4:Yb3+,Tm3+ NCs (3.8 m2g−1). Therefore, we may infer that the enhancement of surface area is also contributing to the enhanced photocatalytic activity of the
Figure 6. Plot of concentration change (C/C0) of MB over time in the presence of BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite photocatalyst under (black) full solar irradiation, (blue) UV cut-off filter (400 nm), and (red) only UV irradiation. The green line represents the photodegradation of MB dye under full solar irradiation in the absence of any photocatalyst.
was performed under a solar simulator in the presence of a UV cutoff filter (400 nm), the dye degradation was found to be ∼55% in 100 min (Figure 6). Moreover, the nanocomposite adsorbs about 23% of MB dye from a 5 × 10−5 mol L−1 aqueous solution under dark conditions in 1 h. Therefore, the nanocomposite photocatalyst is able to remove about 99% of dye effectively from a 5 × 10−5 mol L−1 aqueous MB solution by both adsorption and photocatalysis processes under full solar radiation (Figure S6B). Furthermore, to evaluate the overall efficiency of the photocatalytic process of the nanocomposite in comparison with that of existing state of the art photocatalysts, photocatalytic degradation of MB dye was carried out in the presence of a benchmark photocatalyst (P25) under similar conditions. It is evident from Figure S7 that the nanocomposite and P25 degrade about 98% and 32% of MB dye under full solar irradiation, respectively. This result clearly E
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other previous reports on nanocomposites, the intensity of the Ln3+ emission, which quenches to produce excitons, is much weaker, thus decreasing the probability of exciton creation.28,29
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CONCLUSION In conclusion, we have successfully synthesized a BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite photocatalyst that shows high photocatalytic activity under all three regions of the solar spectrum, i.e., UV, visible and NIR. To the best of our knowledge, under full solar irradiation, the observed photocatalytic efficiency of the photocatalyst is either comparable to or better than most of the previous reports. The photocatalyst also shows high dye degradation rate under NIR illumination. This enhanced NIR dye degradation can be attributed to the selective decrease of the highly intense blue emission from Tm3+ ions due to energy transfer to BiVO4 in the nanocomposite. This suggests that for designing UCL based NIR photocatalysts, Ln3+ ions should be doped in hosts, where they are able to show strong emission in the absorption window of the photocatalyst. The present work provides a new guideline for further design of upconversion photocatalysts.
Figure 8. Bar diagram showing the concentration change of MB after 100 min in the presence of BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite photocatalyst and different scavengers.
nanocomposite. The isoelectric point of the nanocomposite photocatalyst was determined by measuring zeta potentials of aqueous nanocomposite dispersions at different pH. As shown in Figure S8, the isoelectric point of the nanocomposite in water is at pH 8.95, as the zeta potential value of the nanocomposite dispersion becomes zero at this pH. Moreover, we would like to point out that the zeta potential of the nanocomposite dispersion does not change significantly upon change in pH. This indicates that the dispersiblility and surface charge of the nanocomposite may not play a major role behind the photocatalytic activity of the material. The MB dye degradation follows apparent first-order kinetics, which is in agreement with a general Langmuir− Hinshelwood mechanism:
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03289. XPS analysis, ICP-MS analysis, TEM image, EDS analysis, UV−vis DRS, lifetime measurement data, quantum efficiency measurement, adsorption and total dye removal studies, photocatalysis with benchmark photocatalyst (P25), isoelectric point determination, rate constant determination, ans table for comparison of photocatalytic ability of different materials (PDF)
r = −dC /dt = k1k 2C /(1 + k 2C)
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where, r is the degradation rate of the dye (mg min−1), C is the concentration of the dye (mg l−1), t is the illumination time, k2 is the adsorption coefficient of the dye (mg−1), and k1 is the reaction rate constant (mg min−1). If C is very small, then the above equation could be simplified to
AUTHOR INFORMATION
Corresponding Author
*E- mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ln(C0/C) = k1k 2t = kt
ACKNOWLEDGMENTS V.M. thanks the Department of Science and Technology (DST) India, Council of Scientific and Industrial Research (CSIR), and IISER-Kolkata for the funding. S.G. and T.S. thank CSIR and UGC, respectively, for their fellowships. C.H. and M.C. thank IISER-Kolkata and KVPY for their scholarships. Authors thank Dr. Sri Sivakumar (IIT Kanpur) for his help in the XPS analysis. The authors would also like to thank A. Sahasrabudhe, A. Pramanik, and S. Bhattacharya for their help in DRS, BET, and zeta potential measurements, respectively.
where, C0 is the initial concentration of the dye, C is the concentration of the dye after a certain time t, k is the rate constant, and t is the time. The slope of a plot of ln(C0/C) versus t for the dye degradation by the nanocomposite shown in Figure S9 provides the rate constant (k) of the photocatalytic process. The rate constants of MB dye removal by the photocatalyst under full solar irradiation and UV-free irradiation were found to be 0.047 min−1 and 0.010 min−1, respectively. Similar studies of MB dye degradation under NIR illumination revealed that the average removal rate constant is 0.04 h−1. A comparison of NIR photocatalytic activity of the nanocomposite with respect to previous reports (Table S2 in the SI) suggests that the BiPO4:Yb3+,Tm3+/BiVO4 nanocomposite shows better photocatalytic activity compared to similar composites reported in the literature. This enhanced NIR photocatalytic activity of the nanocomposite can be ascribed to the intense blue emission at 475 nm from Tm3+ in BiPO4 NCs as well as to the efficient energy transfer of the emission to BiVO4 in the nanocomposite resulting in the creation of an exciton. To the best of our knowledge, in all
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DOI: 10.1021/acs.langmuir.5b03289 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.5b03289 Langmuir XXXX, XXX, XXX−XXX
