Bi-Doped Hexaniobate with Visible

ARTICLE pubs.acs.org/JPCC

Pillared Nanocomposite TiO2/Bi-Doped Hexaniobate with Visible-Light Photocatalytic Activity Xue-Tao Pian, Bi-Zhou Lin,* Yi-Lin Chen, Ji-Dong Kuang, Ke-Zhi Zhang, and Li-Mei Fu Key Laboratory for Functional Materials of Fujian Higher Education, College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, P. R. China ABSTRACT: Under the strategies of doping to extend the absorptive response region of a semiconductor and fabricating a heterojunction structure to suppress the recombination of the photogenerated electronhole pairs, a TiO2-pillared Bi-doped hexaniobate nanocomposite was prepared via a solid state reaction and a subsequent exfoliationrestacking route. The as-prepared pillared nanocomposite was characterized by powder X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, UVvis, and N2 adsorptiondesorption measurements. It was revealed that the host 2D sheet structure was retained after doping, exfoliation, and restacking. The pillared nanocomposite is mesoporous with a high specific surface area (more than 110 m2/g) and shows a good photocatalytic activity in the degradation of methylene blue under visible light irradiation. A photoexcitation model in the semiconductorsemiconductor pillared photocatalyst was proposed based on the results of XPS, UVvis, and photoelectrochemical studies.

1. INTRODUCTION Semiconductor-based photocatalytic degradation of organic pollutants has attracted great interest since it provides a potential solution to many environmental pollution problems that mankind is currently facing.1,2 It is well-known that when a semiconductor absorbs a photon with energy greater than or equal to band gap energy an electron would be promoted from the valence band to the conduction band, leaving a hole in the valence band. If this charge separation is valid, these induced electrons and holes can be used for efficient photocatalytic degradation of organic pollutants. Holes can react with the surface-bound H2O or OH to generate a powerful oxidant such as hydroxyl radicals, and the conduction band electron may be picked up by the dissolved oxygen species to form superoxide anion radicals. These active radicals will oxidize the organic pollutants.3,4 The photocatalytic efficiency of photocatalytic procedures strongly depends on the photogenerated electronhole recombination rate and solar energy utilization.1,5,6 To efficiently utilize solar energy, many strategies have been employed to modify oxide semiconductors sensitive to visible light, including metal and nonmetal doping,712 superficial deposition of precious metals,13 dye sensitization,14 and coupling with two different semiconductors.1517 Among them, introduction of transition metal doping states will lead to an improved visible light photocatalytic activity due to the band gap narrowing, resulting from the creation of doping energy levels below the conduction band.10,18 It has demonstrated that hybridizing two different semiconductors, leading to formation a heterojunction structure, can greatly decrease the photogenerated charge carries (electronhole pairs) recombination probability and increase r 2011 American Chemical Society

the lifetime of charge carries, thus promoting photocatalytic efficiency.4,1517 Recently, several efforts have been made to explore inorganic layered semiconductors, through inserting other semiconductor guest particles into the interlayer spacing of the 2-D semiconductor host lattices, which provides one of the most effective ways to fabricate a heterojunction structure. Such pillared nanocomposites exhibit porous textures, high specific surface areas and improved photocatalytic activities, predominantly attributed to the effective spatial separation of photogenerated electronhole pairs between guest and host.1922 Potassium hexaniobate K4Nb6O17 presents a layered structure and has been used as a photocatalyst under UV irradiation (λ < 385 nm),23 its photocatalytic activity can be improved by doping with Fe3þ,24 ion-exchanging with Sn2þ,25 and pillaring with SnO226 and TiO2.27 In recent years, Bi-based oxides has been found to be very active under visible light irradiation, which is attributed to the hybridized valence band by O 2p and Bi 6s so as to narrow the band gap.28 In the present paper, in order to enhance the photocatalytic activity of K4Nb6O17, we synthesized bismuth-doped K4Nb6O17 by solid-state reaction, which has an extended absorption edge up to the visible light region. Subsequently, an exfoliationrestacking route has been employed to prepare pillared nanocomposite TiO2/BixNb6xO17, which exhibits high surface area with mesoporosity. The photocatalytic properties were evaluated by the degradation of methylene blue (MB) under visible light irradiation. It was demonstrated that the

Received: October 11, 2010 Revised: January 27, 2011 Published: March 22, 2011 6531

dx.doi.org/10.1021/jp1097553 | J. Phys. Chem. C 2011, 115, 6531–6539

The Journal of Physical Chemistry C as-prepared pillared nanocomposite exhibited an excellent visible light photocatalytic activity for the degradation of the pollutants.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. All chemicals were of analytical grade and were used as received. Similar to the method employed in ref 26. Bi-doped K4Nb6O17 (denoted as K4Nb6xBixO17) was prepared by a solid state reaction with a stoichiometric mixture of Nb2O5, K2CO3, and Bi2O3 in molar ratios of 1:1.25:1% at 1100 °C for 10 h in a corundum crucible. Energy dispersive X-ray spectroscopy (EDS, Oxford 7021) gave an atomic ratio of Nb:Bi = 1:0.006. The protonated doped hexaniobate (denoted as HNb6xBixO17) was obtained by refluxing a suspension of K4BixNb6xO17 in a 6 M HNO3 solution at 60 °C for 1 week, similar to the protonated hexaniobate.26,29 During the proton exchange reaction, the acid solution was replace with a fresh one every 2 days. The resultant solid product was centrifuged, washed with deionized water, and air-dried at room temperature for 24 h. Subsequently, the protonated sample was exfoliated into nanosheets by reacting with TBAþOH in the molar ratio of TBAþOH/HNb6xBixO17 being 1:1. Typically, 0.5 g of proton niobate was added to 250 mL of TBAOH aqueous solution, and ultrasonicated at ambient temperature for 2 h. The exfoliation of niobate sheets was achieved by penetrating large TBAþ molecules into the interlayer, opening up the interlayer to delaminate the powder into single nanosheets. After the exfoliation reaction, a small fraction of incompletely exfoliated particles (about 5%) was removed from the dispersed nanosheets by means of a centrifugation at 4000 rpm for 10 min. Then the pH value of the colloidal suspension was adjusted to 8 by 0.5 M HNO3 carefully. A monodispersed TiO2 nanosol was prepared by hydrolysis of tetrabutyl titanate, C16H36O4Ti, according to the procedure reported previously.30 A certain amount of C16H36O4Ti was added slowly to a mixed solvent system of acetylacetone and 2-propanol in 2:5 in volume. The resultant mixture was stirred at room temperature for 15 min and then slowly added to 0.1 M HNO3 aqueous solution under vigorous stirring at ambient temperature for 24 h. The as-prepared TiO2 sol was slowly dispersed into the obtained niobate colloidal suspension with a Ti/Nb molar ratio of 1:2 under vigorous stirring. After stirring at room temperature for 12 h, an amount of 0.5 M HNO3 was slowly dropped under vigorous stirring to adjust the pH value of the mixed suspension to around 3, and flocculation was observed. Then, the mixture was continuously kept stirring for 5 h and aging for 2 days to ensure completion of the intercalation reaction. The floccule was separated by centrifugation at 4000 rpm, washed several times with ethanol/water mixed solvent (1:1 in v/v) and deionized water to remove other soluble products. The product was finally dried in a desiccator under vacuum at 100 °C for 24 h and designated as sol/Nb6xBixO17. For comparison, a sample flocculated without TiO2 loading was prepared as well, and designated as eH-Nb6xBixO17. Finally, TiO2-pillared layered niobate (TiO2/Nb6xBixO17) was obtained by calcining sol/ Nb6xBixO17 at 300 °C for 2 h with an increment of 1 °C/min. Energy dispersive X-ray spectroscopy gave an atomic ratio of Nb: Ti = 1:0.31. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were collected at ambient temperature on a Bruker D8 Advance diffractometer using Ni-filtered Cu KR radiation

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(λ = 1.5418 Å) under the accelerating voltage of 36 kV at a scanning rate of 2° at 2θ min1 from 2° to 50°. Simultaneous TG/DTA measurements were performed from ambient temperature to 800 °C in flowing N2 at a rate of 10 °C 3 min1 on a TA 2910 thermalanalyzer. Scanning electron microscope (SEM) and field emission scanning electron microscope (FE-SEM) images were observed with Hitachi S-3500N and S-4800 microscopes, respectively. High-resolution transmission electron microscope (HR-TEM) images were taken using a JEOL JEM-2100 with an accelerating voltage of 200 kV. The samples were suspended in ethanol and sonicated over 10 min. Subsequently, a drop of the supernatant dispersion was placed onto a carbon film supported by a copper grid. Diffuse reflectance UVvis spectra were recorded on a Shimadzu UV-2550 spectrophotometer equipped with an integrating sphere 60 mm in diameter using BaSO4 as a reference. Specific surface area and porosity measurements were carried out on a Nova 1200e instrument at liquid-nitrogen temperature using ultrapure nitrogen gas as the adsorbate, in which the samples were degassed at 120 °C for 1 h in flowing N2 prior to the measurements. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Escalab MK II spectrometer (Scientific Ltd., U.K.) with nonmonochromatic Al KR X-ray (1486.6 eV). The pressure in the chamber during the experiments was less than 106 Pa. The analyzer was operated at 20 eV pass energy with an energy step size of 0.1 eV. Photoelectrochemical measurements were carried out in a conventional three-electrode quartz glass cell using a PAR 2273 potentiostat/ galvanostate. Nb6xBixO17 nanosheet electrode ITO/(PEI/ Nb6xBixO17)10 was prepared similar to the reported procedures31 and served as the working electrode. The counter and reference electrodes were Pt-black wire and Ag/AgCl in KCl, respectively. Propylene carbonate solution containing 0.1 mol LiClO4 was used as the supporting electrolyte and a Philips TUV 4W/G4 UV-lamp (λmax = 365 nm) was used as the excitation light source with an intensity of 2.8 mW/cm2. 2.3. Photocatalytic Reaction Test. The photocatalytic activities of the as-prepared photocatalysts were evaluated by the photodegradation of MB solution in a cylindrical quartz vessel in response to visible light at ambient temperature. A total of 50 mg of the samples was dispersed in 100 mL of MB aqueous solution (20 mg/L) in a 150-mL vessel. Before illumination, the mixtures were magnetically stirred for 60 min in the dark to ensure the establishment of adsorptiondesorption equilibrium of MB on the sample surfaces. Subsequently, the photocatalytic reactivity tests were carried out in aerobic conditions. A Changtuo 500 W Xe arc lamp with a 400 nm cutoff filter was employed as the visible light source. At given intervals, 3 mL of the suspension was extracted and subsequently centrifuged at a rate of 5000 rpm for 10 min to remove the particles of catalyst. The concentration change of MB was then determined by measuring the absorbance at 664.5 nm as a function of irradiation time using a Shimadzu UV-2550 spectrophotometer. The photodegradation efficiency (X) of MB was given by X = (C0 Ct)/C0, where C0 is the initial concentration of MB and Ct the concentration at time t.

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction and Microscopic Morphology Analyses. The powder XRD patterns of the layered potassium

hexaniobate K4Nb6O17, Bi-doped niobate K4Nb6xBixO17 (x = 0.036) protonated doped hexaniobate HNb6xBixO17, restacked protonic niobate eH-Nb6xBixO17, sol/Nb6xBixO17 6532

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The Journal of Physical Chemistry C

Figure 1. Powder XRD patterns of K4Nb6O17 (a), K4Nb6xBixO17 (x = 0.036) (b), protonic niobate HNb6xBixO17 (c), restacked protonic niobate eH-Nb6xBixO17 (d), sol/Nb6xBixO17 nanocomposite (e), TiO2/Nb6xBixO17 nanocomposite (f), and TiO2 nanoparticle (g), where ( for TiO2 and b for Bi-doped niobate sheets.

nanocomposite, TiO2/Nb6xBixO17 nanocomposite, and TiO2 nanoparticle are represented in Figure 1. The XRD pattern of the as-prepared Bi-doped K4Nb6xBixO17 is found to be similar to that of K4Nb6O17, which indicates that the doped niobate maintains the same layered structure. Compared with those of K4Nb6O17, the positions of corresponding diffraction peaks of Bi-doped niobate are slightly shifted to low angles, which may be attributed to that the Nb5þ ions in lattice were partially replaced by the dopant Bi3þ where the ion radius of Bi3þ (0.103 nm) is greater than that of Nb5þ (0.069 nm).32 The strong and sharp (040) reflection with d = 0.94 nm of K4Nb6xBixO17 was observed, indicates that the well-ordered layered structure was formed. After acid exchanging, this (040) peak was shifted to a higher angle (from 2θ = 9.4° to 11.7°). The corresponding d value was decreased, which can be attributed to the partial replacement of Kþ for Hþ in the niobate interlayer space. The acid-exchanged product has an average composition of K1.2H2.8Nb5.964Bi0.036O17, determined by energy dispersive X-ray spectroscopy. Such partial ion-exchange is similar to that in K4Nb6O17,26,29 and is different from those in layered Na2Ti3O7, K2Ti4O9, and KNb3O8, whereas the alkali cations in the latters can be completely replaced by protons via ionexchange.33,34 However, after the exfoliated niobate sheets were restacked without TiO2 loading, the strong reflection at 2θ = 11.7° of HNb6xBixO17 was vanished (Figure 1d), which implies that the as-prepared HNb6xBixO17 was almost completely exfoliated into single sheets. The positions of the broad peaks at 2θ = 4.8° and 9.5° are comparable with those of K4Nb6xBixO17 (4.7° and 9.4° in Figure 1b), which can be predominantly attributed to the fact that the Kþ and the proton cations were encapsulated into the interlayer region again in some disorder. Upon reacting with the TiO2 sol particles, the exfoliated niobate sheets were restacked, leading to the formation of

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Figure 2. SEM images of K4Nb6xBixO17 (x = 0.036) (a) and TiO2/ Nb6xBixO17 nanocomposite (b). FE-SEM (c) and HR-TEM images of TiO2/Nb6xBixO17 nanocomposite. White arrows in d indicate the TiO2 pillars between the niobate nanosheets.

pillared nanocomposite sol/Nb6xBixO17. As shown in Figure 1e, the reflections at 2.6° and 5.3° with an interlayer spacing of 3.40 nm were observed, which suggests that the monodispersed TiO2 nanosol was intercalated into the interlayer space of the host niobate sheets. After calcination at 300 °C for 2 h, the reflections at 2.7° and 5.3° of sol/Nb6xBixO17 were slightly shifted to 2.7° and 5.5°. The interlayer spacing of TiO2/ Nb6xBixO17 nanocomposite can be determined as 3.32 nm. The decrease in interlayer spacing, compared with sol/ Nb6xBixO17, is attributed to the dehydroxylation and dehydration of the TiO2 guest nanoparticles to oxide pillars and to the pyrolysis of the residual TBA cations in pores of the material. As shown in Figure 1df, the presence of (002) and (400) diffraction peaks, related to in-sheet diffractions, indicating that the niobate sheets in pillared nanocomposites and eH-Nb6xBixO17 retained the two-dimensional sheet structure of the original K4Nb6xBixO17 upon the exfoliation, restacking and calcination processes. It is noted that in the high angle range several diffraction peaks at 25.6°, 38.4° and 48.5° can be distinctly observed, which are attributed to the reflections of anatase TiO2 structure (JCPDF 832243). Deducting from the thickness of the host sheet (ca. 1.23 nm),35 the gallery height of TiO2/ Nb6xBixO17 nanocomposite was determined to be 2.09 nm. This observation is an evidence of the formation of a mesoporous pillared material assembled by exfoliated Bi-doped niobate nanosheets and nanometer-sized anatase TiO2 particles, which was further supported by the results of pore size distribution measurements. Figure 2, panels a and b, demonstrates SEM images of K4Nb6xBixO17 and TiO2/Nb6xBixO17 nanocomposite, respectively. K4Nb6xBixO17 exhibits a plate-like morphology with well-ordered layered structure, whereas the as-prepared TiO2/ Nb6xBixO17 nanocomposite is free of the tabular particles with less distinct edges and steps. The alveolate morphology of TiO2/ Nb6xBixO17 may be mainly originated from the evaporation of water, as the byproducts of transferring TiO2 sol into TiO2 nanoparticles during the 300 °C calcination process. As shown in 6533



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Figure 3. Simultaneous DTA/TG scans of sol/Nb6xBixO17 nanocomposite (x = 0.036).

Figure 2c, their sheets are somewhat deteriorated. The HR-TEM image of TiO2/Nb6xBixO17 (Figure 2d) shows that the nanosheets were supported by TiO2 pillars, forming an ordered multilamellar architecture. It is observable that the nanosheets are parallel to each other with a vertical distance of 3.3 nm and that the pillars are irregularly positioned on the ac plane of the niobate structure. 3.2. Thermal Analyses of Sol/Nb6xBixO17. The weight loss of sol/Nb6xBixO17 nanocomposite below 190 °C was found to be 5.3% with two small endorthermic peaks at 78 and 145 °C (Figure 3), which can be attributed to the removal of the absorbed water and trace adsorbed organic molecules on the surfaces and in pores of the material. The weight loss beyond 190 °C was 6.5%. The exothermic peak at 298 °C in the DTA curve might be raised from the carbonized decomposition of the trace intercalated organic molecules and the dehydroxylation and dehydration of TiO2 nanoparticles. Since it seems not distinct weight loss beyond the exothermic at 400 °C, at which the collapse of the mesoporous structure and the slow transformation of the niobate nanosheets into amorphous metal oxides might occur, and a subsequent crystallization from amorphous to niobate occurred at around 575 °C. This fact suggests that the porosity of the pillared material prepared in the employed way could be guaranteed below 400 °C. In order to preserve the mesoporous texture of sol/Nb6xBixO17 and to complete the pillaring process, the calcination temperature of 300 °C with a slow increment of 1 °C/min was employed in the present study to prepare TiO2/Nb6xBixO17 nanocomposite. 3.3. Nitrogen AdsorptionDesorption Isotherms. N2 adsorptiondesorption isotherms of TiO2 nanoparticle, sol/ Nb6xBixO17, and TiO2/Nb6xBixO17 nanocomposites are displayed in Figure 4. The isotherm of TiO2 is assigned as type I according to the BDDT classification,36 and no distinct hysteresis is indicative of typical microporous adsorbents. However, sol/ Nb6xBixO17 and TiO2/Nb6xBixO17 composites show a type IV isotherm as a result of the mesoporosity. Both of them exhibit an IUPAC type H3 hysteresis loop in the p/p0 range of 0.41.0,37 suggesting the presence of open slit-shaped mesopores. As listed in Table 1, the specific surface areas and the total pore volumes of the as-prepared nanocomposites are drastically increased upon pillaring TiO2 nanoparticle into the interlayers. The specific surface area of TiO2/Nb6xBixO17 was determined to be 114 m2/g, whereas that of restacked eH-Nb6xBixO17 is about 28 m2/g. This improvement may be ascribed to the

Figure 4. Nitrogen adsorption (closed symbols)desorption (opened symbols) isotherms of TiO2 nanoparticle (a), sol/Nb6xBixO17 (x = 0.036) (b), and TiO2/Nb6xBixO17 (c). The inset indicates the pore size distribution curves.

presence of mesoporosity originated from pillaring the exfoliated niobate nanosheets by nanometer-sized TiO2 particles. The pore size distribution curves of sol/Nb6xBixO17 and TiO2/Nb6xBixO17 nanocomposites, calculated by the BarrettJoynerHalenda method,36 are displayed in the inset of Figure 4. Each of them exhibits two pore distribution peaks, 2.0 and 3.9 nm for sol/Nb6xBixO17 and 2.1 and 3.7 nm for TiO2/ Nb6xBixO17, respectively. Judging from the XRD results, the ∼2 nm peak is mainly attributed to the gallery height, and the mesopores of the materials are derived mainly from the platelet stacking of niobate sheets pillared by the intercalated TiO2 particles. The ∼3.8 nm peak with a wide pore distribution may be originated from the house-of-cards type random stacking between the restacked nanohybrid with the metal oxide nanoparticles, similar to that in TiO2-, ZnO-, and CrOx-pillared titanates20,21,38 and SnO2-pillared tantalotungstate.22 In general, the dehydroxylation and dehydration of the encapsulated guest oxide sol particles into oxide nanoparticles during the calcination process will make a pillared composite have a bigger pore position and an increased specific area, like SnO2/HTaWO6,22 R-Fe2O3/titanate,39 and CrOx/titanate nanocomposites.20 However, the peak positions and the specific area of TiO2/ Nb6xBixO17 are slightly lower than the corresponding those of sol/Nb6xBixO17. It may be come from that the heat-treatment led to a part of niobate sheets transfer into an amorphous oxide and some mesoporous texture collapse, resulting in the reduction of the specific surface area and the pore volume. It is noted that after calcination the mesopore with the pore diameter of ∼2 nm was basically maintained. The pillared structure, fabricated from the alternating arrangement of hostguesthost, was basically preserved during calcination process. Such a semiconductorsemiconductor pillared system is favorable for coupling two semiconductors with each other and forming a heterojunction structure, which is expected to be much efficient for the photocatalysis of organic pollutants by suppressing the recombination of photogenerated electrons and holes. 3.4. UVVis Diffuse Reflectance Spectra. Figure 5 compares the UVvis diffuse reflectance spectra of HNb6O17, HNb6xBixO17, sol/Nb6xBixO17, TiO2/Nb6xBixO17, and TiO2 nanoparticle. The KubelkaMunk function is defined as F(R) = (1 R)2/2R, and the optical band gaps Eg are determined 6534



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Table 1. Parameters Obtained from N2 Desorption Measurements samples K4Nb6O17

c

pore volumeb (cm3/g)

average pore sizec (nm)

1.7

∼2

K4Nb6xBixO17 (x = 0.036)

∼2

TiO2 nanoparticle

161

0.12

28

0.04

restacked eH-Nb6xBixO17

a

surface areaa (m2/g)

sol/Nb6xBixO17 nanocomposite

168

0.32

3.8

TiO2/Nb6xBixO17 nanocomposite

114

0.22

3.6

BET specific surface area calculated from the linear part of BET plot. b Total pore volume taken from the volume of N2 adsorbed at p/p0 = 0.98. Average pore Radius was estimated from the BarrettJoynerHalenda formula.

Figure 5. Diffuse UVvis spectra of HNb6O17 (a), HNb6xBixO17 (x = 0.036) (b), sol/Nb6xBixO17 (c), TiO2/Nb6xBixO17 (d), and TiO2 nanoparticle (e). The inset indicates the relationships between (F(R)hν)0.5 and photon energy (hν).

by extrapolation of the linear portion of the (F(R)hν)0.5 curve versus the photon energy hν to (F(R)hν)0.5 = 0. The onset wavelengths of K4Nb6O17 and TiO2 are 369 and 382 nm with band gaps of 3.36 and 3.24 eV, respectively. K4Nb6xBixO17 has an extended absorption to the visible light region with a band gap of 3.03 eV. After hybrid and calcination, nanocomposite TiO2/ Nb6xBixO17 exhibits an intense absorbance between the visible light region from 400 to 510 nm. Apart from the possible quantum size effect,39 this might be predominately attributed to the synergistic effect between the host niobate nanosheets and the guest TiO2 nanoparticles, as a result of the electronic coupling between the host and the guest within the contacted interface. Upon calcination, the contact between the host and the guest was improved and the synergistic effect should be enhanced. 3.5. XPS Spectra. The XPS spectra of Bi 4f, Nb 3d, Ti 2p, and O 1s in materials are documented in Figure 6. As listed in Table 2, Bi-dopant makes eH-Nb6xBixO17 have bigger binding energies of Nb 3d and O 1s than eH-Nb6O17, which can be preponderantly attributed to the hybridization of O 2p and Bi 6s.28 The XPS data of Ti 2p in TiO2, sol/Nb6xBixO17, and TiO2/Nb6xBixO17 nanocomposites, ∼458.6 eV for 2p3/2 and ∼464.5 eV for 2p1/2, are consistent with the standard values in anatase TiO2,40 in agreement with the results of XRD analyses. The binding energies of Nb 3d5/2 and 3d3/2 in eH-Nb6xBixO17, sol/ Nb6xBixO17, and TiO2/Nb6xBixO17 nanocomposites are 207.7 and 210.5, 207.4 and 210.2, and 207.5 and 210.3 eV, respectively. These data are consistent with the standard values of Nb 3d in a pentavalent state with a spinorbit splittings of 2.8 eV.41 The data in TiO2/Nb6xBixO17 are slightly greater than

those sol/Nb6xBixO17, implying that there exists electronic coupling between the host and the guest and that the electronic coupling was strengthened after calcination process. On the other hand, the binding energies of O 1s in TiO2 and eHNb6xBixO17 are 530.0 and 531.3 eV, respectively. After TiO2 sol particles intercalated into the interlayer space of the host niobate sheets, the interactions between host sheets and guest nanoparticles resulted in the binding energy of O 1s of the resultant nanocomposite sol/Nb6xBixO17 is shifted to 530.6 eV, greater than that in the guest and lower than that in the host. Upon calcination, the contact between the host and the guest in such a hostguest heterostructure was improved, the electronic coupling between them was strengthened, and the binding energy of O 1s was elevated to 530.7 eV. Such electronic coupling between the host and the guest also resulted in the UVvis absorbance of a nanocomposite in the visible light region elevated, as mentioned above, and eventually result in its enhanced photoactivity. 3.6. Photoelectrochemical Properties of Bi-doped Niobate Nanosheets. Figure 7 shows the dependence of steady state photocurrents on the applied potentials examined in a propylene carbonate solution containing 0.1 mol LiClO4. The transient photocurrent responses of light on and light off recorded at 0.1 V were also illustrated in the figure, where an anodic photocurrent spike produced at the light on was observed. The electrode potential values are given with respect to NHE, in which the value for the Ag/AgCl counter electrode is þ0.1988 V. The onset potential of the anodic photocurrent generation from the 10-layer films of Nb6xBixO17 nanosheet/ITO electrode was determined to be 0.43 V, and no cathodic photocurrents were observed under potentials more negative than this flat-band potential. This characteristic is typical for n-type semiconductor, similar to that of the titania nanosheet electrode.31 The topmost surface of the electrode is the two-dimensional architecture of a Bi-doped niobate nanosheet. The flat-band potential of a semiconductor electrode is generally determined by its topmost surface structure.42 During photocurrent generation, the excited electrons may reduce water and/or molecular oxygen dissolved in the electrolyte, while the produced holes may oxidize water and/or propylene carbonate used as the electrolyte. A full understanding of the redox reactions derived from the excited carriers is not available from the measurements. The photocurrent generation from the anatase TiO2 electrode was also measured under the same conditions. As shown in Figure 7b, its flat-band potential was observed at 0.61 V, consistent with the reported value.31 3.7. Photocatalytic Activities of Catalysts. Figure 8 displays the degradation efficiencies of MB in the presence of TiO2/ Nb6xBixO17 nanocomposites with different Bi dopant amounts after 120 min visible-light irradiation. Similar to Figure 5, doping 6535



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Figure 6. XPS spectra of Bi 4f (A), Nb 3d (B), Ti 2p (C), and O 1s (D) in eH-Nb6xBixO17 (x = 0.036) (a), TiO2 (b), sol/Nb6xBixO17 (c), TiO2/ Nb6xBixO17 (d), and eH-Nb6O17 (e).

Table 2. Binding Energies Obtained from XPS Measurements samples

Nb 3d5/2

Nb 3d3/2

restacked eH-Nb6O17

206.9

209.7

restacked eH-Nb6xBixO17 (x = 0.036) TiO2 nanoparticle

207.7

210.5

sol/Nb6xBixO17 nanocomposite

207.4

TiO2/Nb6xBixO17 nanocomposite

207.5

and pillaring make TiO2/Nb6xBixO17 nanocomposites have an extended absorption to the visible light region, which thus makes them have visible-light driven catalytic activity. After 120 min exposure, 25.8% MB molecules were photodegraded over TiO2/ Nb6O17 nanocomposite. The activities of TiO2/Nb6xBixO17 nanocomposites became higher with the increase of the dopant content of Bi. When x reaching at 0.036, the degradation efficiency of MB was raised up to an extremum. After then, the activity was reduced. The time-dependent degradation of MB over TiO2/Nb6xBixO17 (x = 0.036) and its pillared derivatives are compared in Figure 9. Because MB itself is a dry which is active to visible light, its photodegradation may occur.43 About 4% of MB molecules were degraded after 120 min irradiation without any catalyst in our experimental. The absorption of TiO2 nanoparticle and K4Nb6O17 is not discernible in visible light region (Figure 5), which makes their visible-light driven catalytic activities were lower. However, after 120 min of visible-light exposure, 18.3%, 34.5%, and 42.0% of MB molecules were photodegraded over eH-Nb6xBixO17, sol/Nb6xBixO17, and TiO2/Nb6xBixO17, respectively. The photodegradation curves of MB on different catalysts can be described as the apparent zero-order kinetics in the first

Ti 2p3/2

Ti 2p1/2

O 1s 530.1

458.6

464.5

531.3 530.0

210.2

458.6

464.5

530.6

210.3

458.7

464.5

530.7

step,22,44 dX/dt = k or X = kt, where k is the reaction rate constant. The rate constants k can be obtained by fitting the experimental data in the first 90 min as 0.048, 0.158, 0.319, and 0.425 min1 for TiO2, eH-Nb6xBixO17, sol/Nb6xBixO17, and TiO2/Nb6xBixO17, respectively. The photocatalytic activities of sol/Nb6xBixO17 and TiO2/Nb6xBixO17 nanocomposites are greatly higher than that of eH-Nb6xBixO17. The improved photocatalytic activity of the as-prepared pillared nanocomposite should be predominantly originated from the synergistic effect of its two components. The heterojunction structure, constructed by the host semiconductor nanosheets and the guest semiconductor nanoparticles, and hence the electronic coupling will eventually lead to a markedly improved photocatalytic activity of a pillared nanocomposite. As listed in Table 2, the binding energy of O 1s in TiO2 (530.0 eV) is lower than that in eHNb6xBixO17 (531.3 eV). This observation indicates that the covalency of MO bonds in eH-Nb6xBixO17 is expected to be greater than those in TiO2, leading to lower energy for the valence band, dominantly contributed by O 2p orbitals.1,2 The band gaps of TiO2 and eH-Nb6xBixO17 are 3.24 and 3.03 eV, as the results derived from UVvis diffuse reflectance measurements. On the other hand, the flat-band potentials of TiO2 and 6536



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Figure 7. Dependence of the photocurrents on the applied potentials for 10-layer film of Bi-doped niobate nanosheet electrode (a) and anatase TiO2 electrode (b). The inset indicates the photocurrent generation from the 10-layer film of doped niobate nanosheet electrode at 0.1 V. Figure 9. Photodegradation of methylene blue under visible-light irradiation by blank (a), and over TiO2 nanoparticle (b), eH-Nb6O17 (c), eH-Nb6xBixO17 (x = 0.036) (d), sol/Nb6xBixO17 (e), and TiO2/ Nb6xBixO17 (f).

Figure 8. Photodegradation efficiencies of methylene blue (MB) after 120 min under visible-light irradiation over TiO2/Nb6xBixO17 nanocomposites with different dopant content of Bi.

Nb6xBixO17 nanosheets are positioned at 0.61 and 0.43 V, respectively. Therefore, the lower edge of the conduction band for Nb6xBixO17 nanosheets is 0.18 V lower than that for anatase TiO2. The coupling system in the present pillared composites can be depicted in Figure 10, similar to the proposed CdSe/TiO2 model.45 Under visible light excitation, while the photogenerated electrons are reserved in the conduction band of the host Bidoped niobate sheets and subsequently transfer to the surface of the guest particles to be captured by the surface adsorbed O2 molecules, the photogenerated holes in the host sheets will migrate to the valence band of the guest TiO2 nanoparticles by traversing the interface between the host and the guest. The holes will subsequently transfer to the surface of the guest nanoparticles to be trapped by the hydroxyl and water molecules as active radicals to oxidize the organic compounds. Such spatial separation will effectively suppress the rapid combination of the photogenerated holes with the photogenerated electrons, obviously enhance the lifetime of transient electrons and holes, and eventually lead to an enhanced photocatalytic activity. The activity of TiO2/Nb6xBixO17 is higher than that of sol/ Nb6xBixO17, although there are no distinct differences between their mesoporous textures (Table 1). It is convincible that the calcination could improve the electronic coupling and electrical connectivity between the host sheets and the guest nanoparticles in a pillared nanohybrid, which makes the photocatalyst more active. It is noted that the photocatalytic activities of TiO2/

Figure 10. Photoexcitation in TiO2/Nb6xBixO17 semiconductorsemiconductor pillared photocatalyst.

Nb6xBixO17 nanocomposites are not higher with increasing x. This may be relative with the recombination probability of the photogenerated electrons and holes in the heterojunction structure, due to the transfer between different defects derived from introducing Bi. When the dopant content of Bi is too great, a part of Bi centers might be aggregated. The uneven aggregated centers can capture the photogenerated electrons excited from the valence band of the host or immigrated from the conduction band of the guest TiO2. On the other hand, a high specific surface area of a catalyst may promote the reaction rate due to providing the accessible possibility of a large amount of organic molecules on the catalytic surface. For a photocatalyst, the high specific surface area can also provide more active sites to adsorb water molecules and hydroxyl and form different active entities by trapping the photogenerated charges, among which the OH• and HO2• radicals will drive photodegradation reactions.1,2,5 Moreover, for the dye-sensitized path, the more active sites are propitious to adsorb O2 molecules to yield O2• and HOO• radicals by capturing the photoinduced electrons from MB molecules. Under visible-light irradiation, the adsorbed MB molecules will be excited and the electrons will then inject into the conduction band of the semiconductor components.46 The injected electrons will be captured by the surface adsorbed O2 molecules to yield O2• and HOO• radicals in competition with the back excitation to the dye molecules, 6537


The Journal of Physical Chemistry C which is favorable for the adsorption of MB and thus enhances the activity of a mesoporous catalyst. It is noted that the mesoporosity provides less sterical hindrance than that of the microporosity, which is favorable for the adsorption of organic molecules, the diffusion rates of the substrate, product and intermediate molecules and thus enhances the activity of a mesoporous catalyst.22,47 In the cases of eH-Nb6xBixO17, MB molecules hardly penetrate into the interlayers due to the narrow interlayer space. As a consequence, MB photocatalytic degradation only occurs on the external surface of eH-Nb6xBixO17, resulting in a low photocatalytic activity. In contrast, the asprepared nanocomposites sol/Nb6xBixO17 and TiO2/ Nb6xBixO17 possess mesoporous textures and expanded interlayer spaces, which are convincingly helpful to improve their photocatalytic activities.

4. CONCLUSIONS In present study, Bi was doped into layered hexaniobate by solide-state reaction to extend its absorptive response to visible light region. Employing anatase TiO2 nanoparticles as the guest, pillared nanocomposite TiO2/Bi-doped hexaniobate was successfully prepared via an exfoliation-restacking route. The asprepared material has a mesoporous texture with a high specific surface area. It exhibited a high photocatalytic activity in the degradation of MB under visible light irradiation, predominantly attributed to the effective spatial separation of photogenerated electronhole pairs between guest and host. The results demonstrated that it is feasible to fabricate pillared nanocomposites with high visible-light-driven photocatalytic activity to improve sunlight utilization by modifying the components and compositions of the layered semiconductor host lattices and the semiconductor guest nanoparticles. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ86-592-6162221.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (50872037), the Natural Science Foundation of Fujian Province (2010J01040), and the Program for New Century Excellent Talents in Fujian Province University (06FJR01). ’ REFERENCES (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. (2) Thompson, T. L.; Yates, J. T., Jr. Chem. Rev. 2006, 106, 4428–4453. (3) Martin, S. T.; Herrmann, H.; Choi, W.; Hoffmann, M. R. J. Chem. Soc. Faraday Trans. 1994, 90, 3315–3323. (4) Liu, G.; Wang, L. Z.; Yang, H. G.; Cheng, H. M.; Lu, G. Q. J. Mater. Chem. 2010, 20, 831–843. (5) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891–2959. (6) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. Nat. Mater. 2006, 5, 782–786. (7) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625–627. (8) Choi, W. Y.; Termin, A.; Hoffmann, M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1091–1092.

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Bi-Doped Hexaniobate with Visible

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