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May 19, 2016 - One-Pot Hydrothermal Route to Synthesize the Bi-doped Anatase. TiO2 Hollow Thin Sheets with Prior Facet Exposed for Enhanced. Visible-L...
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One-Pot Hydrothermal Route to Synthesize the Bi-doped Anatase TiO2 Hollow Thin Sheets with Prior Facet Exposed for Enhanced Visible-Light-Driven Photocatalytic Activity Weiwei Wang, Dan Zhu, Zhi Shen, Jin Peng, Jie Luo, and Xiaoheng Liu* Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China S Supporting Information *

ABSTRACT: A simple one-pot hydrothermal synthesis route has been employed to fabricate the bismuth-doped (Bi-doped) anatase TiO2 hollow thin sheets with {001} facets exposed. Controlling BiVO4 precursor concentration plays a key role in tuning the morphology and the Bi doping concentration of TiO2 hollow thin sheet catalysts. The photocatalytic activity of as-prepared catalysts was evaluated through the photodegradation of different organic dyes under visible light irradiation (>400 nm), including methylene blue (MB), methyl orange (MO), rhodamine-B (RhB), and p-nitroaniline (PNA). Results showed that the optimal dopant of 0.8 atom % Bi in TiO2 achieved the best photocatalytic activity, especially for possessing a much higher photodegradation of PNA, which could be ascribed to the results of photoinduced charge separation and transfer combined with low bulk recombination of charge carriers. This discussion demonstrates that the design of new TiO2 nanostructures for application in solar energy conversion could be easily achieved by coupling Bi cation-doping and active facets with hollow thin sheet morphology.

1. INTRODUCTION

photocatalytic activity of semiconductor photocatalysts, which is dedicated to the high surface photoinduced charge separation and transfer combined with low bulk recombination of charge carriers.3 Among all reported semiconductor photocatalysts, titanium dioxide (TiO2) is one of the most widely used photocatalysts due to its chemical stability, nontoxicity, and relatively low coat.5 However, pure TiO2 only absorbs UV light with a wavelength less than 390 nm, limiting its practical environmental application by using solar light with more than 43% of visible light, due to its large band gap (3.2 eV).6 Therefore, to

Recently, semiconductor photocatalysts have been paid more attention for handling the worldwide energy shortage and counteracting environmental degradation.1,2 From the perspective of photochemistry, the formation of photoexcited electrons and holes could occur on the surface of a semiconductor, resulting from the absorption of ultraviolet (UV) or visible light and consequent formation of photoexcited electron−hole pairs when the energy of the incident photons matches or exceeds the semiconductor bandgap.3 Upon photoactivation, the recombination of photon-generated charge easily occurs between the photogenerated electrons (e−) in the conduction band (CB) and photogenerated holes (h+) in the valence band (VB), which limits the efficiencies of converting photonic energy into chemical energy.4 Therefore, at present, the vast majority of research works mainly focus on how to improve © 2016 American Chemical Society

Received: Revised: Accepted: Published: 6373

February 17, 2016 May 8, 2016 May 19, 2016 May 19, 2016 DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

Article

Industrial & Engineering Chemistry Research

concentration in the precursor of Ti plays a key role in tuning the morphology and the Bi doping concentration of TiO2 hollow thin sheets. The related possible mechanism of crystal growth and enhanced visible-light-driven photocatalytic activities of as-prepared hollow Bi-doped anatase TiO2 were also discussed. And results showed that the optimal dopant of 0.8 atom % (atom % is atomic percentage) Bi in TiO2 achieved the best photocatalytic activity especially for possessing a much higher photodegradation of PNA, which could be ascribed to the synergetic effects of photoinduced charge separation and transfer combined with low bulk recombination of charge carriers. To the best of our knowledge, this discussion demonstrates that the design of new TiO2 nanostructures for application in solar energy conversion could be easily achieved by coupling Bi cation doping and active facets with hollow thin sheet morphology, and it also confirmed that the strategy of coupling of tailored facets with dopants has scientific significance for enhancement photocatalysis of a wide band semiconductor.

better utilize the visible light of the solar spectrum, designing, fabricating, and tailoring the physicochemical and optical properties of TiO2 is indispensable. With this view, various reasonable strategies have been developed to alter the electronic band structure and favorable surface structure of TiO2, like metal ion/nonmetal ion doping, modified by a narrow band gap semiconductor (such as BiVO4, Cu2O, CdS, etc.), surface sensitization by organic dyes or semiconductor quantum dots, surface fluorination, and noble metal deposition.7−11 Of all these strategies, doping of metal ions into an anatase TiO2 lattice has been proved to be an efficient method for improving visible light photocatalytic activity with low bulk recombination of charge carriers.6 Recently, Bi-based oxides have been found to be very active under visible light irradiation, which is attributed to the hybridized valence band by O 2p and Bi 6s resulting in band gap reduction.12,13 Especially for the emergence of several reports on semiconductor photocatalysts modified with a Bi-doping ion (such as Bi-ZnO, Bi-doped αFe2O3, and Bi−Ag3PO4),14−16 it demonstrates that trivalent heavy metal Bi3+-doped TiO2 would increase the formation of Ti3+ ions, leading to the enhancement in the photocatalytic activity, as more Ti3+ states may cause more oxygen defects which facilitate the efficient adsorption of oxygen on the TiO2 surface.17 Although there have been several studies on TiO2 nanopowders modified with Bi-doping ions, better Bi-doped TiO2 photocatalysts for wide application of degradation have not been achieved.18−20 In addition, both theoretical calculations and experimental studies have revealed that the performance of TiO2-based photocatalysts depends on not only their size, morphology, and crystal phase (anatase, rutile, or brookite) but also crystal facets.21−23 For an anatase TiO2 crystal, the {001} faceted crystal surface possesses a superior photocatalytic activity for the degradation of organic pollutants ascribed to its higher surface energy and the 100% unsaturated Ti5c atoms on the {001} facets.24 In addition, among the most investigated {101}, {100}, and {001} facets, the average surface energy on the {001} facet is higher than the others (γ{001} (0.90 J m−2) > γ{100} (0.53 J m−2) > γ{101} (0.44 J m−2), γ represents the average surface energy).25,26 Dramatically, Liu at al. demonstrated that the selective adsorption and photocatalytic decomposition of azo dye molecules could be achieved by using hollow TiO2 microspheres with {001} facets exposed and designed surface chemistry.27 It is suggested that the hollow structure morphology with active reactive facets would become an alternative to lowering bulk recombination of the charge carrier. Cheng et al.’s research further confirmed this explanation or conclusion that the enhanced photocatalytic activity of hollow anatase TiO2 single crystals and mesocrystals with dominant {101} facets could be ascribed to the synergistic effects of photoinduced charge separation and transfer, low bulk recombination of charge carriers, and increased surface area.28 On the basis of the above considerations, coupling of tailored facets with dopants for enhancement photocatalysis of anatase TiO2 would be a worthy study. In this work, it was first shown that a simple one-pot hydrothermal synthesis route can be employed to fabricate the Bi-doped anatase TiO2 hollow thin sheets with {001} facets exposed by using two element precursors of BiVO 4 and Ti, leading to a significant improvement in photodegradation of different organic dyes (methylene blue (MB), methyl orange (MO), rhodamine-B (RhB), and p-nitroaniline (PNA)) under visible light irradiation (>400 nm). By contrast, controlling BiVO4 precursor

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. All chemicals were of analytical grade and used as received without further purification. Hydrofluoric acid (HF; 38 wt %, Caution! Very corrosive) was used. Bi-doped anatase TiO2 hollow thin sheets were synthesized by a one-pot hydrothermal method. In a typical procedure, 36.5 mmol of titanium tetrachloride (TiCl4) was dissolved into 28 mL of an aqueous solution containing 24 mL of isopropanol (HOiPr), 4 mL of HF, and 100 mg of polyvinylpyrrolidone K30 (PVP) in a cold bath under constant vigorous stirring for several hours, forming a homogeneous light yellow TiO2 sol (called as Ti precursor); 10 mL of 2.7 mol/L Bi(NO3)3 solution (3 mmol Bi(NO3)3·5H2O dissolved in the mixture solvents of deionized water (H2O) and oleic acid (OA), v/v, 50:50) was mixed with 10 mL of 2.7 mol/L NH4VO3 solution (3 mmol NH4VO3 dissolved in the mixture solvents of deionized water and N,N-dimethylformamide (DMF), v/v, 50:50), forming a homogeneous light yellow BiVO4 precursor (called the BiVO4 precursor). Subsequently, the precursor of BiVO4 was dropwise added into the above precursor of Ti and kept stirring constantly for 2 h, and then the mixture was transferred into a 100 mL Teflon-lined autoclave, heated to 180 °C, and kept at that temperature for 24 h. After reaction and cooling down to room temperature naturally, the precipitate was centrifuged, washed thoroughly with water and ethanol, and dried at 60 °C for 12 h. The asprepared sample was denoted as X-Bi/Ti, where X represented the molar percentage of two precursors of BiVO4 and Ti by controlling BiVO4 precursor concentration in the precursor of Ti (X = 0, 2.7, 5.5, 7.4, 8.2, 11.0, 14.8). In addition, a contrast experiment was carried out to prove the role of the BiVO4 precursor using Na3VO4 instead of NH4VO3 using a 5.5 molar percentage of two precursors of BiVO4 and Ti. 2.2. Characterization. The X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker) with Cu Kα radiation (λ = 1.54178 Å) in the range 10−80° (2θ). The size and morphology of samples were observed using a field emission scanning electron microscope (FESEM, HITACHI S-4800) and transmission electron microscope (TEM, JEM-2100, JEOL) operating at 200 kV, which was also used to do the elemental analysis using an energy-dispersive spectroscopy (EDS, EDAX) detector. The Brunauer−Emmett−Teller (BET) surface area was determined 6374

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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Industrial & Engineering Chemistry Research

Bi(NO3)3 and NH4VO3 solutions were simply obtained by dissolving Bi(NO3)3·5H2O and NH4VO3 into 1:1 H2O/OA and 1:1 H2O/DMF mixture solvents, respectively. Subsequently, the precursors of BiVO4 and Ti were mixed and kept stirring constantly for 2 h, and then the mixture was transferred into a 100 mL Teflon-lined autoclave, heated to 180 °C, and kept at that temperature for 24 h. Finally, Bi-doped anatase TiO2 hollow thin sheets (7.4-Bi/Ti) were well achieved. Similarly, Bi-doped anatase TiO2 hollow thin sheet samples with different molar percentages of Bi/Ti could be prepared by adjusting the BiVO4 precursor concentration in the precursor of Ti. Notably, this synthesis procedure is facile and simple, and it may be well used to extend the spectral response of TiO2 into the visible region and to improve its photocatalytic activity by doping other cations such as rare earth metals, noble metals, poor metals, and transition metals.6 3.2. XRD Analysis. Figure 1a and b show the XRD patterns of the prepared Bi-doped anatase TiO2 hollow thin sheets with different molar percentages of Bi/Ti. Viewed from Figure 1a, the XRD patterns of all Bi-doped TiO2 samples almost coincided with that of an undoped TiO2 (0-Bi/Ti) sample. All the XRD patterns exhibited that the profiles can be welldefined indexed to the anatase TiO2 (JCPDS No. 21-1272) without any other TiO2 phases or impurity phases induced by the dopants.29 The peaks at 25.28, 36.95, 48.04, 53.91, 55.06, and 62.69° can be assigned to (101), (004), (200), (105), (211), and (204) crystal planes of anatase TiO2, respectively. Obviously, the molar percentage of Bi/Ti increasing up to 5.5 does not present any significant changes in the diffractograms. Nevertheless, the increased dosage up to 7.4 molar percentage of Bi/Ti shows a small peak corresponding to Bi2O3 at 31.8°.19,30 In addition, strong XRD diffraction peaks indicate that as-prepared Bi-doped TiO2 crystals were highly crystallized. Additionally, the XRD patterns of as-prepared Bi-doped anatase TiO2 samples using NH4VO3 and Na3VO4 as different vanadium sources were compared in Figure 2b, controlling a same molar percentage of Bi/Ti (X = 5.5). The same results were observed according to XRD patterns where the peaks match those of anatase TiO2, but a significant difference is also compared in that the Bi2O3 is easily obtained by using Na3VO4 instead of NH4VO3 to prepare the precursor of BiVO4. 3.3. Morphological and BET Analysis. Figure 3a, b, and c show FESEM images of a Bi-doped anatase TiO2 hollow thin sheet sample with 7.4 molar percentage Bi/Ti (7.4-Bi/Ti). The low-magnification FESEM graph exhibited in Figure 3a shows that the prepared sample consists of hollow TiO2 thin sheets partly agglomerated to another and broken. The highmagnification FESEM images were further used to determine the size and structure of single thin sheets of an as-prepared Bidoped anatase TiO2 hollow thin sheet sample, as shown in Figure 3b and c. Results showed that a single hollow thin sheet of a Bi-doped anatase TiO2 sample possesses a width of 800 nm and a thickness of 300 nm with a hollow hole with a diameter of 550 nm. It is clearly observed that the individual Bi-doped anatase TiO2 hollow thin sheet was potentially generated by truncating anatase bipyramids based on the single crystal nucleus of anatase TiO2 (Figure 3d).5 Recently, the preparation of pure anatase TiO2 hollow nanosheets with {001} facet exposed has been studied and its formation mechanism could be ascribed to the selective etching result of HF on {001} faceted anatase TiO2 single crystal surfaces.24 In addition, Shen et al. reported that the preparation of anatase TiO2 hollow nanosheets resulted from the dual roles of the fluorine ion

by nitrogen adsorption−desorption isotherm measurements at 77 K (ASAP 2010). Fourier transform infrared (FT-IR) spectra were collected using a Bomen MB154S (FTIR) spectroscope. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PHI Quantera II SXM X-ray photoelectron spectrometer with an Al Kα excitation source (λ = 8.4 Å). The binding energies were calibrated to the C 1s peak by 284.6 eV. The optical absorbance spectra of the samples were recorded in a UV−visible spectrophotometer (Thermo Fisher, 220) using BaSO4 as a reflectance standard. 2.3. Photocatalytic Activity Measurement. The photocatalytic performance of as-prepared samples were evaluated by using four different organic dyes (methylene blue (MB), methyl orange (MO), rhodamine-B (RhB), and p-nitroaniline (PNA)) as model pollutants in aqueous suspension. The light source for photocatalytic reaction was a 300 W xenon lamp (PLS-SXE300, Beijing Perfectlight Technology Co., Ltd.). All the radiation with wavelengths shorter than 400 nm was removed with a Pyrex-glass filter. The temperature of the reaction system was controlled with a portable air conditioner. The initial concentration of all organic dyes in a 250 mL self-designed quartz photochemical reactor was fixed at 10 mg/L. Before light irradiation, 200 mg catalyst samples were dispersed into 100 mL of organic dye solution and magnetically stirred in the dark for 60 min to establish the adsorption/desorption equilibrium between the pollutants and the surface of the photocatalysts. At given irradiation time intervals, 3 mL of suspension was collected and subsequently centrifuged to remove the catalyst particles. The concentration of organic dyes was monitored by colorimetry with a UV−visible spectrophotometer (UV-1801, Beijing Beifen-Ruili Analytical Instrument (Group) Co., Ltd.).

3. RESULTS AND DISCUSSION 3.1. Fabrication of Bi-doped Anatase TiO2 Hollow Thin Sheets. The preparation of Bi-doped anatase TiO2 hollow thin sheets processed in one step by a simple one-pot hydrothermal synthesis route as shown in Figure 1. Typically, a homogeneous sol of Ti precursor was first formed by adding 100 mg of PVP into TiCl4−HOiPr solution (36.5 mmol TiCl4 dissolved in 24 mL of HOiPr and 4 mL HF solution). Uniform BiVO4 precursor was then prepared by mixing 2.7 mmol of Bi(NO3)3 solution and 2.7 mmol of NH4VO3 solution.

Figure 1. Flowchart of the preparation of Bi-doped anatase TiO2 hollow thin sheets (7.4-Bi/Ti). 6375

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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That is to say, the top and the bottom square surface of the asprepared Bi-doped anatase TiO2 hollow thin sheets may be {001} facets, and the eight isosceles trapezoidal surfaces are {101} facets as shown in Figure 3d. The TEM and HRTEM images in Figure 4 further confirm that as-prepared samples consisted of hollow TiO2 thin sheets

Figure 4. Low-magnification (a) and high-magnification (b) TEM images of an as-prepared Bi-doped anatase TiO2 hollow thin sheets sample (7.4-Bi/Ti) and HRTEM images (c and d) of a 7.4-Bi/Ti sample viewed from the recording in the red marked areas A and B, respectively. Inset: the corresponding fast Fourier transform (FFT) patterns (inset of c and d).

partly agglomerated to another, which is consistent with the FESEM analysis. Figure 4a shows a low-magnification TEM image of an as-prepared sample consisting of many anatase TiO2 hollow thin sheets with partly agglomerated structure. And the size of the sample is also consistent with the FESEM analysis. The high-magnification TEM image of the sample further shows that the internal hollow structure has been well formed in the crystal and retained a basic square frame morphology, while some anomalous nanosheets were also observed in the red marked square area (A and B) in the right region of Figure 4b. Based on the theoretical results of the crystallographic construction of anatase TiO2,31 it is easily distinguished that those anomalous nanosheets could be ascribed to the result of selective erosion on the surface of solid TiO2 thin sheets which could be obtained by fluoridemediated stabilization of anatase {001} facets.29 To further confirm our viewpoints, the analysis of high-resolution TEM (HRTEM) images and the related fast Fourier transform (FFT) patterns (inserted in corresponding HRTEM images) could be employed as shown in Figure 4c and d. Figure 4c shows a HRTEM image viewed from the single nanosheets recorded in the A area in Figure 4b. It is clearly identified that the as-prepared sample was indeed comprised of an anatase phase with a lattice fringe spacing of 0.235 and 0.351 nm corresponding to the {001} and {101} crystallographic plane of anatase TiO2, respectively, which is also consistent with the XRD analysis. Dramatically, powerful information could be achieved from HRTEM images that a well-defined lattice fringe over a large area was measured to be about 0.272 nm, which is assigned to the lattice spacing of the (020) atom plane of Bi2O3 and is also consistent with the XRD analysis.30 Furthermore,

Figure 2. XRD patterns of (a) as-prepared Bi-doped anatase TiO2 samples with different molar percentages of Bi/Ti and (b) the 5.5-Bi/ Ti samples obtained by using NH4VO3 and Na3VO4 as different vanadium sources.

Figure 3. Low-magnification (a) and high-magnification (b) and (c) FESEM images of as-prepared Bi-doped anatase TiO2 hollow thin sheets (7.4-Bi/Ti). (d) Crystal evolution of anatase TiO2 nucleus into an as-prepared geometric shape model of a single hollow thin sheet based on FESEM images.

(F−).29 Therefore, in our work, the reason for the formation mechanism for our as-prepared Bi-doped anatase TiO2 hollow thin sheet samples could be found from the above conclusions. 6376

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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Industrial & Engineering Chemistry Research

up to 7.4. However, the increasing content of Bi doping could be observed when the molar percentage of Bi/Ti exceeded 7.4, which is attributed to the existence of Bi2O3 species. On the basis of the above contrast analysis, results revealed that controlling the BiVO4 precursor concentration plays a key role in tuning the morphology and the Bi doping concentration of TiO2 hollow thin sheets. Generally, the formation mechanisms of the hollow structure in a crystal mainly include three categories such as Ostwald ripening,34 gas bubbling,35 and etching.36 We noticed that some similar works on hollow structure TiO2 have been reported, and a consistent conclusion could be ascribed to the role of HF as a stabilizer and etchant.28,29 In this case of Bi-doped anatase TiO2 hollow thin sheets, the internal hollow structure formation as observed in hollow {001} dominant TiO2 could be attributed to the subsequent etching process after full crystal growth in the presence of HF acting as a morphology controlling agent before acting as an etching agent. Simultaneously, Bi cations may easily be introduced in the lattice of TiO2 due to the relatively stable Bi−O bond during the formation of anatase TiO2.15 Therefore, under the current synthetic conditions, using HF and the BiVO4 precursor, the coexistence of nanosheets and hollow thin sheets of Bi-doped anatase TiO2 is reasonable and unavoidable. Since heterogeneous photocatalysis is generally influenced by the surface area and pore structure of catalysts, which relates directly to the ability of surface charge carrier separation and then transfer and recombination of charge carriers, BET surface areas of as-prepared samples were investigated on the basis of nitrogen adsorption and desorption measurements. Values of the BET surface area of all samples are also exhibited in Table 1. It can be seen from Table 1 that the BET surface area of asprepared Bi-doped anatase hollow thin sheets increased first and then decreased with the increase of Bi contents, but only about 10−60 m2/g. Notably, when the amount of Bi exceeds 7.4, a huge change in surface area appeared, which could be resulting from the agglomeration of Bi-doped hollow anatase TiO2 thin sheets according to the analysis of FESEM and TEM. 3.4. FT-IR and XPS Analysis. FT-IR spectra of all asprepared Bi-doped anatase TiO2 samples are shown in Figure 5. FT-IR spectra of all the samples exhibited similar peaks at 3605 and 1623 cm−1, which are assigned to the stretching and

another HRTEM image viewed from the edge on the front side recorded in the B area (Figure 4b) could also be analyzed so that the results of a lattice spacing of 0.189 nm combined with an interfacial angle of 90° could be ascribed to the exposure of the {200} and {020} facets of anatase TiO2.24 On the basis of the above analyses, an as-prepared Bi-doped anatase TiO2 hollow thin sheet was enclosed by two {001} facets on the top and the bottom square surfaces and the Bi2O3 crystal could be formed when the molar percentage of Bi/Ti increased up to 7.4, which also becomes direct evidence for crystal evolution of the anatase TiO2 nucleus into an as-prepared geometric shape model of a single hollow thin sheet. Similarly, the morphology and structure of as-prepared Bidoped anatase TiO2 hollow thin sheets with different molar percentages of Bi/Ti (X = 0, 2.7, 5.5, 8.2, 11.0, 14.8) were also characterized by FESEM and TEM images as shown in Figures S1−S6 (see Supporting Information). Analysis results revealed that all as-prepared samples were composed of hollow thin sheets with a relatively consistent size of about 300 nm except for the undoped sample and a trace doped sample (0-Bi/Ti and 2.7-Bi/Ti). Furthermore, an internal hollow structure was also formed in the crystal and retained a basic square frame morphology with the increasing of BiVO4 precursor concentration. Notably, a similar conclusion could be obtained that the morphology and composition of as-prepared samples are similar to that of the sample of 7.4-Bi/Ti, characterized by clear lattice fringes and FFT patterns. In addition, it was interesting that the flower-like pure anatase TiO2 microspheres could be formed by the same method in our work, and it was composed with a large area of nanosheets with {100} facets exposed. It is probably concluded that the cause of this result is the concentration of the fluorine mediator according to the reported research.25,32,33 In order to prove the role of the BiVO4 precursor, a contrast experiment was carried out by replacing NH4VO3 with Na3VO4 as shown in Figure S7. It is clearly observed that Bi-doped anatase TiO2 hollow thin sheets were achieved under the current synthetic conditions. However, a large difference could also be observed in that the Bi2O3 crystals were formed, which is consistent with above analysis of XRD patterns. In addition, to further investigate the concentration of Bi species doped in TiO2, the energy-dispersive spectroscopy (EDS) of the elements was carried out for all as-prepared samples as shown in Table 1 and Figure S8. Results demonstrated that the introduced Bi species existed in the anatase TiO2 with a small Bi dopant amount. Particularly, the content of Bi doping in TiO2 was just about 0.3 atom % (atom for the abbreviation of atomic) when the molar percentage of Bi/Ti was increased Table 1. Physical Properties of Bi-doped Hollow Anatase TiO2 Thin Sheet Photocatalysts

sample undoped TiO2 (0-Bi/Ti) 2.7-Bi/Ti 5.5-Bi/Ti 7.4-Bi/Ti 8.2-Bi/Ti 11.0-Bi/Ti 14.8-Bi/Ti

BET surface area (m2/g) 1.3599 60.2916 10.8563 14.6304 35.6633 26.9690 20.3451

Bi/TiO2 by EDAX (atom %)

Bi/TiO2 by XPS (atom %)

band gap (eV)

0

0

3.28

0.1 0.2 0.3 1.2 1.3 2.2

0.6 0.6 0.8 1.0 1.2 1.7

3.16 3.16 3.14 3.08 3.02 2.97

Figure 5. FT-IR spectra of as-prepared Bi-doped anatase TiO2 hollow thin sheet samples with different molar percentages of Bi/Ti: (a) 0-Bi/ Ti, (b) 2.7-Bi/Ti, (c) 5.5-Bi/Ti, (d) 7.4-Bi/Ti, (e) 8.2-Bi/Ti, (f) 11.0Bi/Ti, and (g) 14.8-Bi/Ti, respectively. 6377

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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Figure 6. Survey (a) XPS spectrum of as-prepared Bi-doped anatase TiO2 hollow thin sheets with a 7.4 molar percentage of Bi/Ti (7.4-Bi/Ti). Ti 2p (b) XPS spectrum, Bi 4f (c) XPS spectrum, and O 1s (d) XPS spectrum.

bending vibrations of the surface-bound hydroxyl groups bonding to titanium atoms and surface adsorbed water molecules.37 Besides, the vibration mode at 3150 cm−1 can be ascribed to −OH groups from the oleic acid (or adsorbed water). The broad peak at 2986−2880 cm−1 is assigned to an N−H stretching vibration from adsorbed DMF. The vibration characteristics of TiO2 mainly appear at about 656 and 472 cm−1 with intensive broad bands, which could be assigned to the stretching vibrations of Ti−O bonds and Ti−F bonds from the surface of titanium atoms, respectively.38 Besides the abovementioned main bands, for Bi-doped anatase TiO2, two new peaks at 1490 and 906 cm−1 appeared, and these may be associated with the vibration of the Bi−O bond only observed from the samples with a high molar ratio of Bi/Ti (7.4−14.8).39 This result could further prove the above analysis of the XRD patterns and HRTEM images. Therefore, it is favorable that the species of water or hydroxyl groups adsorbed on the surface of crystal can react with photogenerated holes to produce hydroxyl radicals, which are devoted to lowering the recombination of charge carrier and enhancing the photocatalytic performance of modified TiO2. XPS analysis was performed to investigate the surface chemical states of as-prepared samples. Figure 6 shows the XPS analysis results of as-prepared Bi-doped anatase TiO2 hollow thin sheets sample (7.4-Bi/Ti). After calibrating by the C 1s peak at 284.6 eV, the XPS survey spectrum of the 7.4-Bi/ Ti sample shows that the catalyst mainly contains Ti, O, and Bi elements and a trace amount of F and C as shown in Figure 6a and Figure S9. Two peaks for the Ti 2p at 462.5 and 456.8 eV were assigned to Ti 2p1/2 and Ti 2p3/2 of Ti4+ in Bi-doped anatase TiO2 hollow thin sheets, respectively (see Figure 6b).40 It is clearly observed that the binding energy of Ti 2p is increased compared with that of undoped anatase TiO2 (see Figure S10), resulting from the decreasing electron density around the Ti4+ ions attributed to the doping with some Bi3+

ions. Simultaneously, the Bi 4f XPS profiles and the contents of various Bi species are shown in Figure 6c, Figure S11, and Table 1. Obviously, the two peaks at 158.4 and 163.7 eV could be assigned to Bi 4f7/2 and Bi 4f5/2 states of Bi-doped anatase TiO2 hollow thin sheet samples, respectively.41 However, it is worthy to note that the binding energies of the Bi 4f peaks shift obviously toward higher values with the increase of BiVO4 content (from 2.7 to 7.4). When the content of BiVO4 changed from 8.2 to 14.8, the binding energies of the Bi 4f peaks shift gradually toward lower values, which could be attributed to the increasing formation of Bi2O3 as shown in Figure S11. These results reveal that titanium atoms were partially substituted by the doped bismuth atoms in the hollow TiO2. The analysis of the above XRD and TEM was also further confirmed. Accordingly, the partial substitution of Ti4+ with Bi(3−x) would result in a lower oxidation number, and then the oxygen vacancies attached with holes could be created due to the effect of charge neutrality in the hollow TiO2.15 It is inferred that the Bi−O−Ti bonds in the framework of TiO2 may have been formed in Bi-doped anatase TiO2 hollow thin sheets and lead to the change of oxidation state of Ti and Bi energy. The content of bismuth atoms for 7.4-Bi/Ti sample is ca. 0.8 atom % according to XPS. Although it is discrepant with the result measured by EDS as shown in Table 1, the amount of element atoms of as-prepared samples using XPS could be better for reflecting the proper value because of the effect of partial substitution of Ti4+ with Bi3+ on the outside and the shallow surface of hollow anatase TiO2 thin sheets rather than in the bulk crystal lattice. Figure 6d shows the asymmetric O 1s signals, indicating the existence of different oxygen species on the surface of as-prepared Bi-doped hollow anatase TiO2 samples (Figure S12). After Gaussian−Lorentzian fitting of the peak shape of the O 1s signal, the bands at 528.5, 529.5, 530.5, and 531.9 eV could be attributed to the lattice oxygen (Ti−O), the surface hydroxyl groups (O−H), the oxygen 6378

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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for a direct and indirect band gap semiconductor, respectively).44 When the value of the y axis ((αhv)2 = 0) is zero, an approximation of the band gap energy of the samples can be obtained by the intercept of the tangent to the x axis. Results suggested that the band gaps of as-prepared samples were estimated to be in the range of 3.16 to 2.97 eV as shown in Figure 7b and Table 1. Compared with the undoped sample (0Bi/Ti, 3.28 eV), the doped samples showed only weak visible enhancement from the view of band gap, a reasonable explanation should be considered that the lower content of Bi species (Table 1) doping on the outside and the shallow surface of anatase TiO2 hollow thin sheets rather than in the bulk crystal lattice. In addition, combined with analysis results of BET, although a trace Bi-doped hollow anatase TiO2 sample (2.7-Bi/Ti) exhibits a higher surface area, a higher band gap (3.16 eV) limits its further application as a photocatalyst for degrading the organic pollutants. As a result, Bi doping contributed to absorbing visible light and thus enhance the photocatalytic activity under visible light irradiation. 3.6. Photocatalytic Activity of the As-prepared Samples. The photocatalytic performances of the as-prepared samples were evaluated in terms of photodegradation of different kinds of organic dyes, such as anionic (Methyl Orange, MO), cationic (Methyl Blue, MB; Rhodamine-B, RhB; and pnitroaniline, PNA) dyes. Typically, 200 mg of each sample (0Bi/Ti, 2.7-Bi/Ti, 5.5-Bi/Ti, 7.4-Bi/Ti, 8.2-Bi/Ti, 11.0-Bi/Ti, and 14.8-Bi/Ti) was used as the photocatalyst in this work under irradiation with visible light (>400 nm). The photocatalytic degradation efficiencies of the different dyes for all asprepared samples in a quartz reactor over 6 h are shown in Figure 8. Typical for photodegradation of MB cationic dye as shown in Figure 8a, results showed that the enhancement of photocatalytic activity of the samples with different Bi doping concentrations could be well observed by comparison with the undoped TiO2 sample (0-Bi/Ti). And the 7.4-Bi/Ti sample eliminated 73.8% MB from the solution within 6 h and presented the highest photocatalytic activity among all Bidoped anatase TiO2 hollow thin sheet samples. Compared with the three samples of 2.7-Bi/Ti, 5.5-Bi/Ti, and 7.4-Bi/Ti, we noticed that the photocatalytic activity of the Bi-doped anatase TiO2 hollow thin sheet sample was increased with increasing Bi content over TiO2, but a decreased photoactivity of as-prepared samples could be observed when the Bi content exceeded 1.0 atom % corresponding to the samples of 8.2-Bi/Ti, 11.0-Bi/Ti, and 14.8-Bi/Ti. The reason for the changes of photocatalytic activity of as-prepared Bi-doped hollow anatase TiO2 could be contributed to the Bi3+ content over TiO2 instead of Bi2O3 species and the surface area of samples. In other words, the increasing of Bi2O3 species reduced the photocatalytic activity of catalysts. Furthermore, to determine this Bi doping having the same influence on decolorization of any anionic organic dyes as well as cationic dye (MB), the test of photodegradation efficiencies for the anionic dye (MO) was performed under the same conditions, and the results are shown in Figure 8b.37 Obviously, a similar tendency was also found in degradation of MO anionic dye, and the 7.4-Bi/Ti sample also exhibited the highest photocatalytic activity among all as-prepared samples. Whereas the overall photocatalytic efficiency of degradation of MO for all as-prepared samples was lower than that of MB cationic dye. To better understand our samples, another two cationic dyes for RhB and PNA were selected to further investigate the photocatalytic performance of as-prepared Bi-doped anatase

species of lattice oxygen of Bi−O bands, and the (CO), respectively.9,42,43 Therefore, the result of the XPS analysis further confirmed that the Bi species existed mainly in the form of Bi2O3 in all samples. 3.5. UV−vis Absorption Spectra. UV−vis absorption spectra were used to measure the absorbing capacities of the asprepared samples to visible light, and the experimental results are shown in Figure 7 and Table 1. As indicated in Figure 7a, in

Figure 7. UV−vis absorption spectra (a) and the relationships between band gap energy and (αhν)2 (b) of as-prepared Bi-doped anatase TiO2 hollow thin sheet samples with different molar percentages of Bi/Ti.

comparison with UV−vis absorption spectrum of the undoped TiO2 sample (0-Bi/Ti) and Bi-doped anatase TiO2 hollow thin sheets samples, it is obvious that the Bi doping could lead to an absorption increase in the visible region from 412 to 466 nm viewed from a tail or shoulder absorbance in the visible light region, and the absorbance became stronger with the increase of BiVO4 content from 2.7 to 14.8. But, the increase of the Bi content just creates a little influence on the light absorbance, which may be attributed to the Bi species existing on the outside and shallow surface rather than in the bulk crystal lattice. Furthermore, the band energy gaps of as-prepared samples could be well estimated from the Tauc’s plots (Figure 7b), which are calculated by using αhv = A(hv − Eg)n/2 (where α is the absorption coefficient near the absorption edge, h is Planck’s constant, A is a constant, v is the light frequency, and Eg is the absorption band gap energy; among them, n is 1 and 4 6379

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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Figure 8. Photocatalytic degradation of (a) MB, (b) MO, (c) RhB, and (d) PNA with different Bi doping anatase TiO2 thin sheet samples under visible light irradiation.

Figure 9. (a) Plot of −ln(C/C0) vs irradiation time for the 7.2-Bi/Ti sample to determine rate constants for various dyes including MB, MO, RhB, and PNA. (b) Cycling runs of PNA decolorization using the 7.2-Bi/Ti sample.

be calculated as 0.0052 min−1, 0.0025 min−1, 0.0065 min−1, and 0.0068 min−1, respectively. It is obvious that the photodegradation process of PNA is the fastest. Most importantly, the stability of a photocatalyst is always regarded as another important feature for its application, besides activity. Therefore, to evaluate the reusability of the asprepared 7.2-Bi/Ti sample, a cycling test was performed repetitively for five cycles for the degradation of PNA as shown in Figure 9b. It is clearly observed that it possessed a relatively stable photocatalysis with no significant decrease in activity (Figure 9b). Notably, it can be concluded that these Bi-doped anatase TiO2 hollow thin sheets are a good photocatalyst in the application of organic pollutant degradation. To gain fascinating insight into the reaction activity of this Bi-doping TiO2, a contrast experiment was performed to discuss the photodegradation efficiency of PNA over two different Bi-

TiO2 hollow thin sheets samples, and the results are shown in Figure 8c and d. Interestingly, it is found that all samples exhibited a higher photocatalytic activity in RhB and PNA than that in MB. Besides, the 7.4-Bi/Ti sample also exhibited the highest photocatalytic activity among all as-prepared samples and eliminated 99.2% RhB and 99.5% PNA from the solution within 6 h. On the basis of these results, one could conclude that the 0.8 atom % Bi content doping in the anatase TiO2 hollow thin sheet sample (7.4-Bi/Ti) exhibited the optimal photocatalytic performance toward the degradation of both anionic and cationic dyes under visible light irradiation, and it is superior to eliminating cationic dyes. The kinetics of decolorization of both cationic and anionic dyes are indicated in Figure 9a, and the results showed that the degradation process can be simulated by the pseudo-first-order kinetic equation. The decomposition rates of MB, MO, RhB, and PNA over the 7.2-Bi/Ti sample can 6380

DOI: 10.1021/acs.iecr.6b00618 Ind. Eng. Chem. Res. 2016, 55, 6373−6383

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photocatalytic degradation mechanism of as-prepared Bi-doped TiO2 hollow thin sheets samples is shown in Figure 11b. On the basis of the above analysis, the Bi-doped anatase TiO2 hollow thin sheet crystals have exhibited improved valence band edges resulting from the coupling between Bi 6s and O 2p electrons that produced antibonding Bi 6s states toward the top of the valence band,45 which contributed to the enhancement of visible light absorption. That is, the photoexcited and separated electrons from the valence band of the as-prepared Bi-doped TiO2 catalyst will further transfer into the internal band of the catalyst, which was formed by the Bi3+ species under visible light irradiation. Furthermore, the active species of hydroxyl radicals (•OH) may be first yielded on the photocatalyst surface, formed by the surface hydroxyl groups (OH−) trapping generated holes, and then the electrons will be transferred to the conduction band and trapped by the dissolved oxygen molecules producing superoxide anions (•O2−).20 After that, the formed superoxide radical anion (•O2−) may either attack the organic molecules directly or generate a hydroxyl radical (•OH) by reacting with hydrion (H+) and photogenerated electrons. Together, the resulting • O2− and •OH radicals can play a vital role in degrading the organic contaminant, acting as two very strong oxidizing agents. What’s more, the effective separation of charge carriers may be another reasonable explanation for the enhancement of photocatalytic activity, because such carriers can transfer between major {001} and minor {101} facets of the hollow thin sheets.28

doped anatase TiO2 hollow thin sheet samples, which are prepared by using NH4VO3 and Na3VO4 as different vanadium sources. As shown in Figure 10, results revealed that the sample

Figure 10. Photocatalytic decomposition of the PNA of two different Bi-doped anatase TiO2 hollow thin sheet samples obtained by using NH4VO3 and Na3VO4 as different vanadium sources with a 5.5 molar percentage of Bi/Ti.

prepared by using NH4VO3 exhibited a higher photocatalytic activity for degradation of PNA than the another sample prepared by using Na3VO4 as a substitute forming the vital BiVO4 precursor to prepare Bi-doped anatase TiO2 hollow thin sheets. It is commonly believed that more oxygen vacancies may be achieved when NH4VO3 was used to form a BiVO4 precursor to prepare Bi-doped anatase TiO2 hollow thin sheets. Although the reason for this phenomenon is uncertain, the increasing of Bi2O3 species could be better for explaining this result, corresponding to the XRD analysis. 3.7. Visible Photocatalysis Mechanism of Bi-doped Anatase TiO2 Hollow Thin Sheets. Considering the synergistic effect of Bi-doped ions into TiO2 samples, optimized surface charge carrier separation/transfer by hollow structure and slightly increased surface area, the XPS valence band of asprepared Bi-doped anatase TiO2 hollow thin sheets sample (7.4-Bi/Ti) was conducted as shown in Figure 11a. The maximum valence band edge position of our hollow Bi-doped anatase TiO2 sample (7.4-Bi/Ti) was determined as 2.38 eV, which was attributed to the effect of hollow morphology according to Cheng’s study.28 In addition, Bi-doped TiO2 favors the photon absorption and then the separation of photoinduced electron−hole pairs. Therefore, the possible

4. CONCLUSIONS In conclusion, Bi-doped anatase TiO2 hollow thin sheets with {001} facets exposed have been successfully synthesized via a simple one-pot hydrothermal synthesis route for water treatment (photodegradation of organic dyes including both cationic and anionic dyes). Controlling the BiVO4 precursor concentration plays an important role in altering the morphology and the Bi doping concentration of TiO2 hollow thin sheets. Under visible light irradiation, Bi doping can significantly increase the photocatalytic performance of TiO2 for decolorization of any anionic organic dyes as well as cationic dye. And the 0.8 atom % Bi content doping in the anatase TiO2 hollow thin sheets sample (7.4-Bi/Ti) exhibited a highest activity toward the photodegradation of both cationic and anionic dyes under visible light irradiation, and it is superior to

Figure 11. (a) XPS valence band spectrum of as-prepared Bi-doped anatase TiO2 hollow thin sheets sample (7.4-Bi/Ti). (b) Schematic illustrations of the possible mechanism of photogenerated carrier transfer of as-prepared Bi-doped anatase TiO2 hollow thin sheet sample. 6381

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eliminating cationic dyes. As a result of the synergistic effect of Bi-doped ions into TiO2 samples, optimized surface charge carrier separation, and then transfer by the highly active {001} facets and hollow structure, the superior photocatalytic performance of as-prepared Bi-doped anatase TiO2 hollow thin sheets samples could be obtained over the degradation of organic pollutant under visible light irradiation, and there was an optimal Bi dopant in TiO2 (0.8 atom %) for degrading both cationic and anionic dyes. Therefore, coupling of tailored facets with dopants for enhancement visible-light-driven photocatalysis of anatase TiO2 has been proved and favors improving the application of wide-band TiO2 semiconductors in more fields.

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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.iecr.6b00618. FESEM, TEM, and HRTEM images; XPS; and EDS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-25-84315943. Fax: +8625-84315054. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No.51272107 and No.51572126), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Fundamental Research Funds for the Central Universities (No. 30920140132038).

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