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Dec 22, 2015 - photocatalytic conversation of CO2 into renewable fuels by optimized HNb3O8/G hybrids yields 8.0-fold improvements in CO evolution amou...
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Synergistic Effects in Nanoengineered HNb3O8/Graphene Hybrids with Improved Photocatalytic Conversion Ability of CO2 into Renewable Fuels He Liu, Haitao Zhang,* Peng Shen, Feixiong Chen, and Suojiang Zhang* Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Layered HNb3O8/graphene hybrids with numerous heterogeneous interfaces and hierarchical pores were fabricated via the reorganization of exfoliated HNb3O8 nanosheets with graphene nanosheets (GNs). Numerous interfaces and pores were created by the alternative stacking of HNb3O8 nanosheets with limited size and GNs with a buckling and folding feature. The photocatalytic conversation of CO2 into renewable fuels by optimized HNb3O8/G hybrids yields 8.0-fold improvements in CO evolution amounts than that of commercial P25 and 8.6-fold improvements than that of HNb3O8 bulk powders. The investigation on the relationships between microstructures and improved photocatalytic performance demonstrates that the improved photocatalytic performance is attributed to the exotic synergistic effects via the combination of enhanced specific BET surface area, increased strong acid sites and strong acid amounts, narrowed band gap energy, depressed electron−hole recombination rate, and heterogeneous interfaces. and effective atomic coordination number on the surface.24 Superior stability and better electrocatalytic performance of AuNP/MoS2 films for the oxygen reduction reaction (ORR) were attributed combinationally to the positive onset potential of the gold nanoparticles and the four-electron oxygen reduction properties of ultrasmall MoS2 nanoparticles.26 Unexpected surprisingly high performance of ORR and oxygen evolution reaction (OER) existed in Co3O4/graphene oxide hybrid was assigned to the formation of new bond and changes in the chemical bonding environment.30 In addition, improved electrocatalytic activity was realized in graphene/Pt−Ni nanohybrid as poisonous intermediates could be removed by abundant oxygen-containing groups existing on the hybridized graphene surface.32 Although noble metal nanoparticles are identified to be efficient cocatalysts for many reactions, the scale-up of their usage is largely hindered.33,34 Interestingly, nanocarbons (such as fullerene, nanodiamond, carbon nanotube, and graphene) have demonstrated promising functions as promoters or cocatalysts for many kinds of catalytic reactions.35−39 Especially, graphene, a 2-dimensional (2D) sheet of sp2-hybridized carbon, has drawn considerable attentions in functionalizing and tailoring TiO2 catalytic ability.40−47 Enhanced photocatalytic activity in graphene-based composites was assignable to the high conductivity and intense light absorption of gra-

1. INTRODUCTION CO2 reduction and solar energy conversion into fuels and chemicals have simultaneously attracted considerable attention due to the desire of regulating global warming as well as fuel crisis.1 Activation of CO2 has long been an intensive challenge.2 Among different available methods, photocatalytic reduction of CO2 with H2O into hydrocarbons is the most promising route from the view of mild operating conditions and efficient utilization of solar energy.3−6 Tremendous efforts have been made to exploit complex metal oxides (such as niobates, titanates, vanadates, and germanates) as catalysts for the photocatalytic reduction of CO2, and attractive progress in improving their photocatalytic performance has been realized in recent years.7−14 Systemically understanding and precisely controlling the microstructures and surfaces/interfaces of hybrids are prerequisites to realize their full potentials in practical settings.15,16 One dependable solution to enhance the photocatalytic activity is fabricating multicomponent hybrids with programmable microstructures and strongly coupled interactions by utilizing nanocrystals as basic building blocks.17−19 Abundant exotic synergistic effects could be obtained by nanoengineered energy gap, exposed facets, defects, and interfaces, as the composition, functionality, and morphology of complex hybrid systems can be tuned at different dimension scales.20,21 Generally, synergistic effects have been found in hybridized heterometallic nanoparticles, metal/oxide, graphene/oxide, and graphene/metal systems.22−32 Improved catalytic performance in heterometallic nanocatalysts was caused by the local strain © XXXX American Chemical Society

Received: September 5, 2015 Revised: November 12, 2015

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Langmuir phene.40,41,48−53 In addition, the low-dimensional feature of graphene favors the electronic coupling with its hybridized partner, which results in greatly enhanced photoreduction ability.15,51−54 So far, graphene has been hybridized with TiO2,40−42,45−47 ZnO,55−60 and WO361−63 in order to improve their photocatalytic activity. Note that Ti- and Nb-based metal oxides have long been pursued extensively as they possess nontoxicity, chemical inertness, and high stability under light irradiation.64−68 Comparing to that of Ti-based catalysts, hybridization of low-dimensional Nb-based metal oxide catalyst was rarely explored. Remarkably, lamellar niobium-containing solid acids (KNb3O813,69 and HNb3O870,71) exhibit attracting performance because their valence band top and conduction band bottom are located at desirable potential levels. KNb3O8 crystallizes in orthorhombic structure (space group Amam, No. 63), which consists of negatively charged sheets of linked NbO6 octahedral units and K+ ions intercalated between sheets.13 Such a low-symmetric crystal structure facilitates electron transfer, and its protonic acidity favors the adsorption of water and some organic molecules.72 Additionally, HNb3O8 holds more stable photocatalytic activity than traditional TiO2.73 Herein, we report the fabrication of layered HNb3O8/G nanohybrids with numerous heterogeneous interface and pores through an exfoliation−restacking process. Usually, exfoliated single-crystal nanosheets exhibit much higher catalytic activities than that of their bulk counterparts due to their extremely high 2D anisotropy, which can offer enhanced specific surface area, and increased Brønsted and Lewis acidity as well as shortened transfer path of photogenerated carriers.74−77 We then design to introduce nanosheets into photocatalytic reactions, which not only widens their application but may also provide unprecedented opportunities for understanding the interactions between active sites and reactants at a molecular level.74,78 In another aspect, low-dimensional HNb3O8 nanomaterials tend to stack together because of their layered nature and high surface energy during preparation process, resulting in a substantial loss of catalytic active sites and higher resistance for electron transfer and diffusion of reactant molecules. Therefore, one new route should be developed to reserve the advantage of HNb3O8 nanosheets and induce some exotic synergistic effects to enhance its catalytic performance. To address this issue, a newly developed exfoliation−restacking route was applied to reserve the advantage of HNb3O8 nanosheets and induce some exotic synergistic effects to enhance its catalytic performance. The relationships between microstructures and improved photocatalytic performance were studied systemically to unveil the mechanism of synergistic effects induced by heterogeneous surface/interface and interactions.

HNb3O8-HCl with GNs (10 mL) were also synthesized in the same conditions, and they were referred as HNb3O8-Bulk/G-II and HNb3O8-HCl/G-II, respectively. The molar ratios of Nb to C for HNb3O8/G hybrids were estimated from the TGA data in Figure S1. 2.2. Characterizations of Materials. The morphologies and microstructures were investigated by using a transmission electron microscope (TEM, JEOL-2010) with an accelerating voltage of 200 kV and a scanning electron microscope (SEM, JEOL JSM-7001F). Atomic force microscopy (AFM) images were observed on a Nanoscale IIIA (Digital Instrument) in a taping mode with a silicon tip cantilever (force constant: 20 mN m−1). Crystal structure was analyzed by an Xray diffractometer (XRD, Rigaku, Smartlab) equipped with a Cu Kα1 radiation source (9 kW, λ = 0.154 06 nm) and a 1D silicon strip detector (D/teX Ultra 250). Chemical environment of hybrids was analyzed by an X-ray photoelectric spectroscopy (XPS, ESCALAB 250Xi, Thermal-Fisher). Samples were transferred to the sample chamber and irradiated with a monochromotized Al Kα X-ray source (1486.71 eV photons) at a vacuum of 10−9−10−10 Torr. The spectra were calibrated by C 1s core level (284.6 eV) peak. Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet 380 spectroscopy. Materials’ porosity was characterized by N2 adsorption− desorption curves collected from a Micrometritics ASAP2020 apparatus. The Raman spectra were recorded by a microspectrometer (LabRAM HR800, Horiba Jobin-Yvon) with an excitation laser at 514 nm. Thermogravimetric analysis (TGA) was performed on a thermogravimetry/differential thermal analyzer (STA7200RV, Hitachi High-Tech) at a heating rate of 10 °C min−1. The photoluminescence spectra were detected on an F-4600 fluorescence spectrophotometer (Hitachi High-Tech) using a 320 nm excitation. The evaluation of CO2 uptake capacity of the samples was conducted on an intelligent gravimetric analyzer (IGA-100A, HIDEN). Typically, 20 mg of samples was placed in a quartz basket and pretreated in a vacuum at 70 °C for 6 h to remove moisture and impurities. Then CO2 uptake amounts were recorded at 25 °C at the pressure of 500, 1000, 3000, 5000, 8000, and 10000 mbar, respectively. Every pressure point was kept for 30−300 min to reach the adsorption equilibration. NH3 temperature-programmed desorption (NH3-TPD) measurement was conducted on an Autochem II 2920 apparatus (Micromeritics). Typically, 100 mg of sample was heated up to 120 °C in a helium flow and then maintained for 60 min. Subsequently, the sample was cooled to 50 °C and treated with a 10% NH3−He flow for 120 min; then the sample was purged in a helium flow until the baseline was stable. The desorption profile was measured by the thermal conductivity detector in flowing helium at a heating rate of 10 °C min−1 up to 650 °C. 2.3. Assessment of Photocatalytic Performance. The photocatalytic reduction was conducted in a tubular reactor (200 cm3) with a quartz window vertically facing the light source. Typically, 10 mg of catalyst was uniformly dispersed on a flat glass with an area of 4.8 cm2 and excited by a 300 W xenon arc lamp (light range: 200−1100 nm, simulated sunlight including UV, vis, and NIR/IR spectrum). Compressed CO2 was allowed to pass through a water bubbler to generate a mixture of CO2 and H2O. The mixture then flowed through the reactor for 2 h to ensure the elimination of air. The flow rate was maintained at 1 mL min−1, and the light was turned on to start the reaction. The blank experiment with identical condition was also conducted, except that compressed CO2 was replaced by the high purity argon gas. The outlet gas was recorded on a gas chromatograph (GC-7890A, Aglient) equipped with a MolSieve 13X packed column and a flame ionization detector (FID). The FID used here is equipped with a nickel reforming furnace to detect the amounts of CO in the outlet gas.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Porous HNb3O8/G Hybrids. Exfoliated HNb3O8 nanosheets were obtained via a proton exchange and osmotic swelling process (more synthetic details are provided in the Supporting Information). The freeze-dried nanosheets were named as Nb3O8-TBA. Flocculated HNb3O8 was obtained by treating with HCl solution and collected by centrifugation (referred as HNb3O8-HCl). Exfoliated graphene oxide (GO) nanosheets were synthesized by the modified typical Hummers’ method. HNb3O8/G hybrids were fabricated via coflocculation of HNb3O8 and GO nanosheets with HCl solution as precipitant, followed by a reduction treatment. Five samples (referred to HNb3O8/G-I, II, III, IV, and V) were prepared with different volume of GO solution consumption of 5, 10, 20, 30, and 40 mL, respectively. In contrast, the hybrids of HNb3O8-Bulk and

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. Layered HNb3O8/G nanohybrids with numerous heterogeneous interface and pores here were fabricated through an exfoliation−restacking process. XRD was used to monitor the crystal structural evolution of B

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Langmuir hybrids. All diffraction peaks of HNb3O8 bulk powder (Figure 1a) can be indexed to orthorhombic structure (JCPDS No. 01-

Figure 1. Represetative XRD patterns of (a) HNb3O8 bulk powder, (b) Nb3O8-TBA freeze-dried powders, (c) flocculated HNb3O8-HCl nanosheets, and (d) HNb3O8/G hybrids.

075-2182). HNb3O8 is isostructural with KNb3O8 and is composed of 2D Nb3O8− anion slices built by corner- and edgesharing NbO6 octahedra, and the H+ cations are located between the slices.13 Compared with that of bulk precursor, Nb3O8 nanosheets (Figure 1b,c) exhibited much weaker and broader diffraction peaks while the in-plane diffraction peaks at low angle (2θ < 10°) were retained after exfoliation− aggregation,79 indicating the loss of periodicity along b-axis.80 This could serve as strong evidence for the existence of HNb3O8 nanosheets with single layer.74 The (020) diffraction peak is the characteristic of layered structures for samples. The shift of (020) peak to lower 2θ angle during the exfoliation process (Figure 1a,b) implied the expansion of the gallery height (1.12 nm for the bulk precursor and 2.09 nm for the exfoliated nanosheets). The gallery height along b-axis shrank from 2.09 to 1.26 nm (Figure 1c) after the intercalated TBA+ cations were replaced by protons. No typical diffraction peaks of graphene in the HNb3O8/G hybrids (Figure 1d) were observed. The shift of (020) peak to lower angle implied the intercalation of GNs between HNb3O8 nanosheets. The GNs were stacked in a parallel fashion with the primary lamellar structures, leading to an increase of interlayer spacing to 1.38 nm. These results indicate that GO nanosheets could prevent the agglomeration of HNb3O8 nanosheets in some extent, thus expanding the interlayer height of neighboring nanosheets. Atomic force microscope (AFM) observation revealed that the exfoliated nanosheets had a thickness of ca. 1.79 nm and lateral size of ∼300−600 nm (Figure 2a). Such a thickness value is much larger than the crystallographic thickness of the host layer (Nb3O8− anion slices: 0.75 nm; HNb3O8 layer: 0.814 nm).81 Generally, the larger thickness by AFM can be ascribed to surface adsorption of solvents (water molecule) or guest species on nanosheets.82 Bulk particles (Figure S2) could be exfoliated into single-crystal nanosheets with distinctive 2D morphology. They tended to flocculate after being mixed with acid, forming porous powders with irregular shapes (Figure 2c). Compared with pure HNb3O8 flocculates, the nanocomposites hybridized with GNs (Figure 2d) exhibited crimped lamellar structures and more channels between the slices were reconstructed. The chemical compositions of HNb3O8/G hybrids were measured by energy dispersive X-ray spectroscopy (EDS), which revealed unambiguously the presence of

Figure 2. (a) AFM and (b) TEM images of exfoliated HNb3O8 nanosheets. (c) SEM images of HNb3O8-HCl flocculates. (d) SEM, (e) TEM, and (f) HRTEM images of HNb3O8/G hybrids.

elemental Nb and C components in the hybrids (Figure S3). TEM image of an individual nanosheet (Figure 2b) exhibited a low contrast, revealing its ultrathin thickness. In contrast, HNb3O8/G hybrids (Figure 2e) exhibited a curled characteristic, reconfirming the incorporation of buckling and folding GNs.83 In the high-resolution TEM investigation, HNb3O8 nanosheets contacted with graphene intimately through interfacial interaction (Figure 2f). Structural and contrast difference can be clearly observed between HNb3O8 and graphene nanosheets. The well-defined lattice fringes with a lattice spacing of 0.56 nm corresponded to the (040) planes in lamellar HNb3O8 sample (PDF, file no. 00-044-0672). TEM elemental mapping (Figure S4) showed the hybrids exhibited sandwiched lamellar structures with GNs intercalating into the interlayer of HNb3O8 nanosheets. The curled edge of GNS can be observed from the C elemental mapping (Figure S4d), demonstrating the buckling and folding characteristic of soft GNs. These results imply that hierarchical nanocomposites were obtained through the restacking of exfoliated nanosheets of soft graphene and hard HNb3O8, forming a sandwiched lamellar structure. Therefore, the fabrication procedure of HNb3O8/G hybrids could be proposed and is illustrated in Scheme 1. FT-IR spectra were used to monitor the phase evolution during the exfoliation process. The absorption bands positioned at 997.5, 909.6, and 807.9 cm−1 can be assigned to the octahedral vibrations of NbO6 in layered KNb3O8 (Figure 3a), and the absorption bands around 700 and 600 cm−1 were C

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Langmuir Scheme 1. Schematic Illustration of the Crystal Structure of Typical HNb3O8 Nanosheets and the Fabrication Procedure for HNb3O8/G Hybrids

Figure 4. Raman spectra of (a) bulk KNb3O8 precursor, (b) HNb3O8 bulk powder, (c) Nb3O8-TBA aggregates, (d) HNb3O8 nanosheets flocculates, and (e) HNb3O8/G hybrids.

Nb3O8-TBA and flocculated HNb3O8 nanosheets increased to 962 and 982 cm−1 (Figure 4c,d), respectively, indicating that the interaction between Nb and O ions increased accordingly after the crystallographic symmetry was frustrated as exfoliated slabs are of 2D symmetry. This higher Raman shift implied that the bond distance of NbO terminal double bond was shortened and the electron density in NbO increased accordingly, leading to a slight electron transfer from O ion in Nb−O single bond to Nb ion in NbO6 octahedron. This transfer of electron cloud makes easier loss of H+, resulting into the stronger protonic acidity in exfoliated nanosheets because the bridging OH groups in Nb(OH)Nb functions act as strong Brønsted acid sites. The peaks positioned at 898 and 660 cm−1 (Figure 4a) were assigned to the characteristic modes of NbO6 octahedron, and the bands from 200 to 350 cm−1 were ascribable to the distorted octahedrons and structural deviation from Oh symmetry.85 It is clear that the peaks at 236 and 660 cm−1 (Figure 4c,d) became broadened after exfoliation. Generally, the asymmetric broadening peaks and shift of Raman lines in nanomaterials can be explained by the phonon confinement model.87,88 Phonon confinement in nanoscale materials may lead to a breakup of selection rules, and therefore non-zonecenter phonons will also participate in scattering, resulting into an asymmetric broadening of peaks and shift of the optical phonon Raman line.87,88 The Raman lines of hybrids with GNs (Figure 4e) showed a drastic broadening feature, indicating that bare nanosheets were separated by GNs, reaffirming the results of XRD analysis. Additionally, the two peaks observed at 1352 and 1595 cm−1 in the HNb3O8/G hybrids (Figure S5) were attributed to the D band and G band of graphene, respectively,89 demonstrating the presence of GNs in the hybrids. Of particular note is the intensity ratio of the D band and G band, ID/IG, which is a measure of the relative concentration of local defects or disorders (particularly the sp3hybridized defects) compared to the sp2-hybridized domains.90 It can be seen that the ID/IG ratio was 1.12 for GO and decreased to 1.05 for HNb3O8/G hybrids (Figure S5), indicating more graphitization upon hydrothermal reduction process.90−92 The XPS technique was employed to discern the chemical environment of hybrids. The C 1s XPS spectrum of bare GO (Figure S6) suggested the abundance of various oxygencontaining functional groups on the GO surface. The deconvoluted peak located at the binding energy of 284.6 eV was attributed to the C−C, CC, and C−H bonds.93 The

Figure 3. FT-IR spectra of (a) bulk KNb3O8 precursor, (b) HNb3O8 bulk powder, (c) Nb3O8-TBA, and (d) HNb3O8-HCl nanosheets.

attributed to the v3 mode vibrations in the corner-shared NbO6 octahedron.84,85 The spectrum of HNb3O8 powder (Figure 3b) was identical to that of KNb3O8 powder, indicating the high stability of Nb3O8−1 slabs. The absorptions at 2960, 2874, and 1380 cm−1 of freeze-dried exfoliated nanosheets (Figure 3c) were ascribed to the stretching and bending vibrations of C−H in CH3 of TBA+, and the bands at 1200−1000 cm−1 were assigned to stretching vibrations of C−N and C−C in TBA+. After protonation, the absorption peaks from TBA+ disappeared and only the modes of NbO6 octahedron remained (Figure 3d), indicating the total removal of TBA+ ions. Since Raman frequencies of niobium oxides depend strongly on the niobium−oxygen bond order (e.g., a higher niobium− oxygen bond order corresponding to a shorter bond distance will shift the Raman frequency to higher wavenumbers), Raman spectra of as-prepared and protonated precursor as well as nanosheets bonded with organic species and proton (Figure 4) were investigated in order to understand the exfoliation and restacking process. The sharp and strong band at 954 cm−1 of bulk KNb3O8 sample was characteristic of symmetric stretching mode of the shortest NbO terminal double bond, which stuck out into the interlayer.84 Once protonated, the vibrational frequency decreased from 954 to 940 cm−1 (Figure 4b), indicating the increase distance of NbO bond. Such an increase could be due to the newly formed NbO---H bonds with a terminal oxygen in the adjacent layer, considering that K+ ions had been exchanged by proton and some water molecules were also intercalated between the interlayers.86 Therefore, the interaction between Nb and O tended to decrease on account of the polarization effect. The vibrational frequency of NbO terminal double bond in aggregated D

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Figure 5. XPS curves for (a) C 1s and (b) Nb 3d core level in typical HNb3O8/G hybrids, (c) representative N2 sorption isotherms of HNb3O8 nanosheets and HNb3O8/G hybrids and their corresponding pore size distribution curves, and (d) BET surface area, pore volume, and pore size of hybridized samples (pore size: BJH desorption average pore diameter value) (Bulk: HNb3O8 bulk powder, NSs: HNb3O8 nanosheets, II, III, IV, and V: HNb3O8/G-II, III, IV, and V).

deconvoluted peaks centered at the binding energies of 286.5, 288.1, and 289.5 eV were assigned to the C−OH, CO, and OC−OH oxygen-containing carbonaceous bands, respectively.93 For HNb3O8/G hybrid, a significant loss of the oxygencontaining functional groups was observed based on the C 1s XPS spectrum in Figure 5a, implying considerable reduction of GO via a hydrothermal reduction treatment.94 Figure 5b displays a representative Nb 3d spectrum. The core line spectra of the Nb 3d spin−orbit doublet for as-prepared and reduced samples peaked at 207.1 and 209.8 eV, which agreed well with that of LiNbO3 and Nb2O5,95 and the peak area ratio of doublet corresponded to the ratio of photoelectron cross sections. Systemic analysis revealed that no detected changes took place on the Nb 3d doublet features in different compounds. Effects of hybridization of GNs on the porosity characteristics of HNb3O8/G nanocomposites were evaluated. The isotherms of nanosheets and HNb3O8/G hybrids were of IV type, implying the formation of mesoscale pores.96 The hysteresis loops (Figure 5c) of flocculated nanosheets and hybrids resembled H 3 type according to the BDDT classification,97 which is a representative characteristic of slitshaped pores formed by the stacking of nanosheets. Our analysis revealed that BET surface area increased from 4.0 m2 g−1 of bulk HNb3O8 powder to 28.8 m2 g−1 of flocculated nanosheets. Interestingly, the specific surface area could be promoted further by hybridization due to the intercalation of GNs into the HNb3O8 nanosheets, as intercalated GNs can prevent direct agglomeration of inorganic lamellar nanosheets via electrostatic interaction. As illustrated in Figure 5d, the BET surface area could be tuned from 28.8 to 46.0 m2 g−1. There is no doubt that enhanced BET surface areas will provide more surface active sites, which is helpful for the absorption of CO2

and H2O vapor and therefore for the gas-phase catalytic reactions. The pore-size distribution of HNb3O8 nanosheets (inset of Figure 5c) mainly ranged from 3.1 to 4.0 nm with the average pore diameter of 3.5 nm, indicating that the pore channels are in the mesoporous region and the as-exfoliated HNb3O8 maintains the free-standing nanosheet form.74 For the HNb3O8/G hybrids, the pore-size distribution extended from 3.0 to 140 nm (inset of Figure 5c), reaching the macroporous regime. These facts demonstrated that the mesoporous structure of the pristine nanosheets was inherited after the incorporation of GNs, and the use of graphene can further expand the pore size. The increasing macroporous distribution can provide more heterogeneous interfaces and readily accessed channels for reactants to diffuse efficiently to the active sites, allowing more CO2 to be absorbed in the interface channel, which can be proved by the higher CO2 uptake capacity of HNb3O8/G hybrids than that of HNb3O8-HCl (Figure S7). In Figure 5d, it can be seen that the BET surface area of the hybrids enhanced with the increasing amounts of GNs, reconfirming the function of GNs in improving the total surface area of lamellar nanocomposites. However, pore size and pore volume did not linearly increase with the increasing amounts of GNs, implying that moderate ratio of GNs intercalating in the hybrids could optimize the pore structure of the obtained hybrids, thus resulting in an optimal synergistic effect in improving the photocatalytic efficiency. 3.2. Acid Properties of Exfoliated Nanosheets and Hybrids. Exfoliation−aggregation of protonated layered compounds does not always result in strong solid acids. Generally, the formation of bridging OH groups, M(OH)M′, which are intrinsic to the crystal structure of the 2D sheets, are essential to endow the nanosheet with strong acidity.79 Typical E

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Figure 6. (a) NH3-TPD curves for HNb3O8 bulk powder, HNb3O8 flocculates, and typical HNb3O8/G hybrids, (b) photoluminescence (PL) emission spectra, (c) UV−vis diffuse reflectance spectra, and (d) band gap energies of HNb3O8 nanosheets and different HNb3O8/G hybrids.

contributes notably to the higher photocatalytic activities observed in the solid acids. Thus, the stronger protonic acidity and increasing acid amount in layered hybrids would favor the separation of electron−hole pairs and boosting the photocatalytic reduction of CO2. 3.3. Optical Properties of Exfoliated Nanosheets and Hybrids. Figure 6b shows emission spectra of bulk HNb3O8 powder and hybrids with different contents of GNs. The intensity of the PL signal is proportional to the recombination rate of photogenerated electron−hole pairs.103 Obviously, HNb3O8 bulk powder showed the strongest PL signal, and the intensity of nanosheet systems decreased with increasing GNs. These results indicate that the electron−hole pair recombination occurred rapidly in the bulk powder. HNb3O8 nanosheets obtained by exfoliation process shortened the transfer path of photogenerated carriers; thus, the PL signal for the nanosheets decreased. Additionally, the PL signal of HNb3O8/G hybrids was strongly attenuated due to hybridization of GNs, indicating that graphene could suppress electron−hole pair recombination in HNb3O8 nanosheets. The depression of electron−hole pair recombination in the hybrid nanosheets indicates a potential candidate for photocatalytic reaction. The optical properties were studied to evaluate the light absorption capacities and estimate band gap energies. As displayed in Figure 6c, the absorption edge of hybrids shifted to higher wavelength and a strong absorption in the visible light region appeared. In addition, the absorption intensity increased with increasing hybridized GNs. It should be noted that the absorption intensity and red-shift of hybrids were positively correlated with the content of GNs in the nanocomposites, as observed in many graphene−semiconductor complexes.41,104−107 The band gap energies decreased with increasing GNs content according to the Kubelka−Munk equation.107 The calculated band gap decreased from 3.15 eV

NH3-TPD (m/e = 16) curves of HNb3O8 bulk powder, flocculated HNb3O8 nanosheets, and HNb3O8/G hybrids are displayed in Figure 6a. The desorption peak of bulk precursor positioned at 517 K with a shoulder peak at about 493 K. In contrast, the NH 3 -TPD profile of flocculated HNb 3 O 8 nanosheets was composed of three distinct peaks at 471, 625, and 763 K. The peak at 471 K was plausibly ascribable to the preserved acid sites in the layered structure. The exfoliation− aggregation process exposed inevitably intrinsic −OH groups on individual 2D sheets, resulting in more strong acid sites on the nanosheets. Therefore, additional peaks at 625 and 763 K were assigned to strong acid sites and Nb(OH)Nb functions as strong Brønsted acid sites.98−100 The HNb3O8/G hybrids exhibited similar profile with peaks at 479, 634, and 775 K, which were about 10 K higher than those corresponding peaks of the bare nanosheets. The strong acid amounts were 0.19 mmol g−1 for the aggregated-nanosheets and 0.23 mmol g−1 for the hybrids, similar to the previous reports.79 The enhanced acid amount in the hybrids was ascribed to the more strong acid sites on the nanosheets without aggregation. Although the enhanced acidity is not favorable for the adsorption of CO2 due to the acid character of CO2 molecule, the enhanced surface area and broader pore-size distribution favor the diffusion of CO2 and its solubility in the interlayer H2O, leading to the high efficiency in the storage of CO2 (Figure S7). The photocatalytic performance of lamellar solid acids depended strongly on the acidities of samples.101 The protonic acidity of the lamellar solid-acid facilitated the intercalation of water molecules. Previous studies on photocatalytic hydrogen evolution from water splitting revealed that the photogenerated electron−hole pairs might migrate to and separate at the interlayer surface of some lamellar solid-acid samples more easily than in other places, as the photogenerated holes at the interlayer surface can be readily trapped by the intercalated water molecules to form the active hydroxyl radicals.101,102 This F

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Figure 7. Yields of CO (a) and CH4 (b) versus irradiation time in the photocatalytic reduction of CO2 in the presence of H2O; (c) accumulated yield of CO and CH4 for 4 h under light irradiation.

for pure flocculate to 2.87 eV for HNb3O8/G-IV (Figure 6d), implying that more light will be absorbed in the presence of GNs. 3.4. Photocatalytic Reduction of CO2. To evaluate the effects of microstructure and intercalated GNs on the catalytic performance, photocatalytic CO2 conversion reactions were conducted. Representative photocatalytic CO and CH 4 evolutions versus irradiation time were studied in Figure 7a,b. The blank experiment with identical condition and in the absence of CO2 showed no appearance of CH4 or CO, implying that the carbon source was completely derived from the photocatalytic reduction of CO2. In our study, bulk HNb3O8 powders possessed the catalytic activity close to that of commercial TiO2 power (Degussa P25), providing CO and CH4 with amounts of 25.3 and 2.0 μmol under 4 h light irradiation (200−1100 nm), respectively. The exfoliated HNb3O8 nanosheets were able to produce 4.2 μmol of CH4, without obvious enhancement in CO yield. Interestingly, the yield of CO was dramatically improved via the hybridization of GNs, while the yield of CH4 enhanced slightly or even decreased. Intercalation of GNs between HNb3O8 nanosheets was crucial to achieve an optimal synergy interaction. The yield rate of CO increased first and then decreased with increasing GNs content (Figure 7c); the best photocatalytic performance for reducing CO2 into CO was found in sample HNb3O8/G-II, whose activity is 8.0 times higher than that of commercial P25 and 6.1 times than that of the pristine HNb3O8 nanosheets. Although the increasing amount of GNs led to higher BET surface area, lower bandgap energy, and depressed electron−hole recombination rate, further increase of intercalated GNs deteriorated the photocatalytic efficiency due to the light shielding effect of GNs.108−111 With the increase in GNs loading, the excess GNs would increase the scattering and absorbance of photons, shielding the light from reaching the surface of HNb3O8

photocatalysts. Furthermore, the excess GNs can block the active sites of the catalyst and favor the formation of recombination centers and accelerated electron−hole recombination. These results reveal that proper addition ratio of GNs is critical to achieve an optimal synergistic interaction between GNs and the semiconductor.112 The photocatalytic reduction of HNb3O8-Bulk/G-II and HNb3O8-HCl/G-II samples was also conducted in contrast, and they exhibited poorer photocatalytic performance than their counterparts HNb3O8-Bulk and HNb3O8-HCl, respectively, regardless of the yields of CO and CH4 (Figure S8). As layered HNb3O8 of the two samples were wrapped in the GNs with large surface areas, the exposed active sites on HNb3O8 were reduced. Additionally, the photocatalytic performance of the two samples was much lower than that of HNb3O8/G-II. This could result from the light shielding effect of GNs wrapped on the surface. Thus, only the structures with GNs intercalated into HNb3O8 nanosheets can boost the photocatalytic efficiency of CO2 reduction, as the intimate interfacial contact of two nanosheets in atomic level in hybrids can provide extraordinary high carrier mobility and outstanding optical properties. Such a greatly improved photocatalytic performance can be attributed to the synergistic effects from increased specific surface area, minimized charge barrier, improved protonic acidity, and narrowed band gap energy. (1) The ultrathin nature of HNb3O8 nanosheets allows charge carriers to transport rapidly onto the surface active sites to participate in the photoreduction reaction.40 (2) Increased specific surface area of hybrids can offer more active adsorption sites and readily accessible channels for reactants to the active sites. (3) The intercalation of GNs between neighboring HNb3O8 nanosheets provides abundant sites serving as energetically favorable sites for fast migrating photogenerated electrons from HNb3O8 nanosheets to GNs, trapping electrons to prevent the G

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recombination of electron−hole pairs. The longer lifetime and mean free path for electrons on GNs enable energetic electrons to cover a larger area of the graphene surface, thereby increasing the likelihood of interaction with adsorbed CO2.48 (4) The introduction of GNs narrows the band gap of nanosheets, resulting in an obvious red-shift of the absorption edge of hybrids compared with the bare nanosheets. Therefore, the absorption range of light can be efficiently extended, and a more efficient utilization of the solar spectrum could be achieved. (5) The porous feature and broad pore width characteristic allow the gas to move quickly in the hybrids, which is also responsible for the enhanced photocatalytic performance. (6) The protonic acidity of exfoliated HNb3O8 nanosheets allows them to be easily hydrated, and the enhancement in the strong Brønsted acidity and acid amount in the hybrids favors separation of the electron−hole pair at the interlayer sites.101 Note that the intercalation of GNs favors high selectivity for CO formation compared to the slight improved yield rate of CH4.113,114 The high selectivity of CO was believed to be due to the compromise between charge transfer and thermodynamics. CO formation is a two-electron transfer process and CH4 formation involves eight-electron transfer through the reaction routes:36,40 CO2 → CO → C· → CH 2 → CH4

(1)

CO2 + 2H+ + 2e− → CO + H 2O

(2)

CO2 + 8H+ + 8e− → CH4 + 2H 2O

(3)

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03359. Experimental details; TGA curves of flocculated graphene, HNb3O8 nanosheets and HNb 3O8/G-V hybrids in different atmosphere; SEM images of HNb3O8 bulk powder and HNb3O8 nanosheets; SEM EDS spectrum and TEM elemental mapping of typical HNb3O8/G hybrids; Raman spectra of GO and reduced HNb3O8/G hybrids; C 1s XPS spectrum of original GO; CO2 uptake capacity of HNb3O8-Bulk, HNb3O8-HCl and HNb3O8/G hybrids; photocatalytic reduction of CO2 for HNb3O8-Bulk/G-II and HNb3O8-HCl/G-II samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86-10-82544875; Fax +86-1062558174 (H.Z.). *E-mail [email protected]; Tel +86-10-82544875; Fax +86-1062558174 (S.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21271175, 21127011), National Basic Research Program of China (973 program, No. 2014CB239701), and Instrument and Equipment Research and Development Project of CAS (No. YZ201221).

The transferred electrons to graphene would diffuse quickly over the large area of carbon framework, benefiting from the increasing electrical mobility of graphene.40 Therefore, the accumulation of photoinduced electrons is restrained and local electron density is decreased, thus favoring two-electron reactions for the formation of CO.

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REFERENCES

(1) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Non legacy Worlds. Chem. Rev. 2010, 110, 6474−6502. (2) Zhai, Q.; Xie, S.; Fan, W.; Zhang, Q.; Wang, Y.; Deng, W.; Wang, Y. Photocatalytic Conversion of Carbon Dioxide with Water into Methane: Platinum and Copper(I) Oxide Co-catalysts with a CoreShell Structure. Angew. Chem., Int. Ed. 2013, 52, 5776−5779. (3) Tu, W.; Zhou, Y.; Zou, Z. Photocatalytic Conversion of CO2 into Renewable Hydrocarbon Fuels: State-of-the-Art Accomplishment, Challenges, and Prospects. Adv. Mater. 2014, 26, 4607−4626. (4) Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons. ACS Nano 2010, 4, 1259−1278. (5) Izumi, Y. Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond. Coord. Chem. Rev. 2013, 257, 171−186. (6) Das, S.; Daud, W. Photocatalytic CO2 transformation into fuel: A review on advances in photocatalyst and photoreactor. Renewable Sustainable Energy Rev. 2014, 39, 765−805. (7) Wang, W.-N.; An, W.-J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276−11281. (8) Chen, D.; Ye, J. Hierarchical WO3 hollow shells: Dendrite, sphere, dumbbell, and their photocatalytic properties. Adv. Funct. Mater. 2008, 18, 1922−1928. (9) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L.

4. CONCLUSION In summary, a general approach to porous HNb3O8/G nanohybrids was developed by interfacially engineering the unique 2D materials of organic graphene and inorganic solid acid nanosheets through a two-step exfoliation−restacking process. The intimate interfacial contact in hybrids was found to be responsible for the extraordinary high carrier mobility and outstanding optical properties, resulting into strong synergistic coupling effects and thereby great enhancement in photocatalytic performance for CO2 reduction. The enhanced activity of nanohybrids was attributed to their unique layered and porous structure, high specific surface area, high efficiency of electron−hole separation, and extended light absorption range realized by graphene. The results provide a better understanding of the photocatalytic behavior in organic−inorganic heterostructures and a novel insight into rational design and controllable synthesis protocol of 2D nanohybrids. The structural integration of HNb3O8 nanosheets with GNs will be a new promising strategy to develop a highly efficient and low-cost non-noble-metal cocatalyst for photocatalytic applications. The hybrids can also be utilized in other fields, such as dye-sensitized solar cells and electrochemical applications. H

DOI: 10.1021/acs.langmuir.5b03359 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

Carbon Nanofiber Electrodes with High Power and Energy Density. Adv. Funct. Mater. 2011, 21, 2366−2375. (30) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780−786. (31) Yan, J.; Fan, Z.; Sun, W.; Ning, G.; Wei, T.; Zhang, Q.; Zhang, R.; Zhi, L.; Wei, F. Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density. Adv. Funct. Mater. 2012, 22, 2632−2641. (32) Hu, Y.; Wu, P.; Yin, Y.; Zhang, H.; Cai, C. Effects of structure, composition, and carbon support properties on the electrocatalytic activity of Pt-Ni-graphene nanocatalysts for the methanol oxidation. Appl. Catal., B 2012, 111, 208−217. (33) Thomas, J. M.; Johnson, B. F. G.; Raja, R.; Sankar, G.; Midgley, P. A. High-performance nanocatalysts for single-step hydrogenations. Acc. Chem. Res. 2003, 36, 20−30. (34) Gelin, P.; Primet, M. Complete oxidation of methane at low temperature over noble metal based catalysts: a review. Appl. Catal., B 2002, 39, 1−37. (35) Balandin, A. A. Thermal properties of graphene and nanostructured carbon materials. Nat. Mater. 2011, 10, 569−581. (36) Sun, H.; Wang, S. Research Advances in the Synthesis of Nanocarbon-Based Photocatalysts and Their Applications for Photocatalytic Conversion of Carbon Dioxide to Hydrocarbon Fuels. Energy Fuels 2014, 28, 22−36. (37) Serp, P.; Corrias, M.; Kalck, P. Carbon nanotubes and nanofibers in catalysis. Appl. Catal., A 2003, 253, 337−358. (38) Lee, K.; Zhang, J. J.; Wang, H. J.; Wilkinson, D. P. Progress in the synthesis of carbon nanotube- and nanofiber-supported Pt electrocatalysts for PEM fuel cell catalysis. J. Appl. Electrochem. 2006, 36, 507−522. (39) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-based composites. Chem. Soc. Rev. 2012, 41, 666−686. (40) Tu, W.; Zhou, Y.; Liu, Q.; Tian, Z.; Gao, J.; Chen, X.; Zhang, H.; Liu, J.; Zou, Z. Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO2 Conversion into Renewable Fuels. Adv. Funct. Mater. 2012, 22, 1215−1221. (41) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. P25-Graphene Composite as a High Performance Photocatalyst. ACS Nano 2010, 4, 380−386. (42) Tu, W.; Zhou, Y.; Liu, Q.; Yan, S.; Bao, S.; Wang, X.; Xiao, M.; Zou, Z. An In Situ Simultaneous Reduction-Hydrolysis Technique for Fabrication of TiO2-Graphene 2D Sandwich-Like Hybrid Nanosheets: Graphene-Promoted Selectivity of Photocatalytic-Driven Hydrogenation and Coupling of CO2 into Methane and Ethane. Adv. Funct. Mater. 2013, 23, 1743−1749. (43) Zhang, Y.; Tang, Z.-R.; Fu, X.; Xu, Y.-J. TiO2-Graphene Nanocomposites for Gas-Phase Photocatalytic Degradation of Volatile Aromatic Pollutant: Is TiO2-Graphene Truly Different from Other TiO2-Carbon Composite Materials? ACS Nano 2010, 4, 7303−7314. (44) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene oxide as a chemically tunable platform for optical applications. Nat. Chem. 2010, 2, 1015−1024. (45) Zhang, X. Y.; Li, H. P.; Cui, X. L.; Lin, Y. H. Graphene/TiO2 nanocomposites: synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting. J. Mater. Chem. 2010, 20, 2801−2806. (46) Williams, G.; Seger, B.; Kamat, P. V. TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008, 2, 1487−1491. (47) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide. Nano Lett. 2010, 10, 577−583. (48) Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C. Minimizing Graphene Defects Enhances Titania NanocompositeBased Photocatalytic Reduction of CO2 for Improved Solar Fuel Production. Nano Lett. 2011, 11, 2865−2870.

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nat. Mater. 2010, 9, 559−564. (10) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano-photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229−251. (11) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels. Nano Lett. 2009, 9, 731−737. (12) Tanabe, K. Catalytic application of niobium compounds. Catal. Today 2003, 78, 65−77. (13) Suzuki, S.; Teshima, K.; Yamaguchi, A.; Yubuta, K.; Shishido, T.; Oishi, S. Fabrication and photocatalytic performance of highly crystalline nanosheets derived from flux-grown KNb3O8 crystals. CrystEngComm 2012, 14, 987−992. (14) Kapilashrami, M.; Zhang, Y. F.; Liu, Y. S.; Hagfeldt, A.; Guo, J. H. Probing the Optical Property and Electronic Structure of TiO2 Nanomaterials for Renewable Energy Applications. Chem. Rev. 2014, 114, 9662−9707. (15) Liang, Y. T.; Vijayan, B. K.; Lyandres, O.; Gray, K. A.; Hersam, M. C. Effect of Dimensionality on the Photocatalytic Behavior of Carbon-Titania Nanosheet Composites: Charge Transfer at Nanomaterial Interfaces. J. Phys. Chem. Lett. 2012, 3, 1760−1765. (16) Ferey, G. Hybrid porous solids: past, present, future. Chem. Soc. Rev. 2008, 37, 191−214. (17) Li, W.; Wang, F.; Feng, S.; Wang, J.; Sun, Z.; Li, B.; Li, Y.; Yang, J.; Elzatahry, A. A.; Xia, Y.; Zhao, D. Sol-Gel Design Strategy for Ultradispersed TiO2 Nanoparticles on Graphene for High-Performance Lithium Ion Batteries. J. Am. Chem. Soc. 2013, 135, 18300− 18303. (18) Perry, J. J.; Perman, J. A.; Zaworotko, M. J. Design and synthesis of metal-organic frameworks using metal-organic polyhedra as supermolecular building blocks. Chem. Soc. Rev. 2009, 38, 1400−1417. (19) Kamat, P. V. Meeting the clean energy demand: Nanostructure architectures for solar energy conversion. J. Phys. Chem. C 2007, 111, 2834−2860. (20) Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market. Chem. Soc. Rev. 2011, 40, 696−753. (21) Zhang, X. W.; Chan, S. H.; Li, G. J.; Ho, H. K.; Li, J.; Feng, Z. P. A review of integration strategies for solid oxide fuel cells. J. Power Sources 2010, 195, 685−702. (22) Rawalekar, S.; Mokari, T. Rational Design of Hybrid Nanostructures for Advanced Photocatalysis. Adv. Energy Mater. 2013, 3, 12−27. (23) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575−6578. (24) Singh, A. K.; Xu, Q. Synergistic Catalysis over Bimetallic Alloy Nanoparticles. ChemCatChem 2013, 5, 652−676. (25) Costi, R.; Saunders, A. E.; Banin, U. Colloidal Hybrid Nanostructures: A New Type of Functional Materials. Angew. Chem., Int. Ed. 2010, 49, 4878−4897. (26) Wang, T.; Zhuo, J.; Chen, Y.; Du, K.; Papakonstantinou, P.; Zhu, Z.; Shao, Y.; Li, M. Synergistic Catalytic Effect of MoS2 Nanoparticles Supported on Gold Nanoparticle Films for a Highly Efficient Oxygen Reduction Reaction. ChemCatChem 2014, 6, 1877− 1881. (27) Wu, Z.-S.; Wang, D.-W.; Ren, W.; Zhao, J.; Zhou, G.; Li, F.; Cheng, H.-M. Anchoring Hydrous RuO2 on Graphene Sheets for High-Performance Electrochemical Capacitors. Adv. Funct. Mater. 2010, 20, 3595−3602. (28) Jiang, J.; Li, Y. Y.; Liu, J. P.; Huang, X. T.; Yuan, C. Z.; Lou, X. W. Recent Advances in Metal Oxide-based Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166−5180. (29) Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F. Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated I

DOI: 10.1021/acs.langmuir.5b03359 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

K2Nb4O11 and KNb3O8 crystals with needle- or plate-like morphology. CrystEngComm 2011, 13, 1304−1313. (70) Nakato, T.; Kuroda, K.; Kato, C. SYNTHESES OF INTERCALATION COMPOUNDS OF LAYERED NIOBATES WITH METHYLVIOLOGEN AND THEIR PHOTOCHEMICAL BEHAVIOR. Chem. Mater. 1992, 4, 128−132. (71) Nakato, T.; Miyamoto, N.; Harada, A. Stable liquid crystalline phases of colloidally dispersed exfoliated layered niobates. Chem. Commun. 2004, 78−79. (72) Li, X.; Kikugawa, N.; Ye, J. Nitrogen-doped Lamellar Niobic Acid with Visible Light-responsive Photocatalytic Activity. Adv. Mater. 2008, 20, 3816−3819. (73) Xiong, J.; Wen, L.; Jiang, F.; Liu, Y.; Liang, S.; Wu, L. Ultrathin HNb3O8nanosheet: an efficient photocatalyst for the hydrogen production. J. Mater. Chem. A 2015, 3, 20627−20632. (74) Liang, S.; Wen, L.; Lin, S.; Bi, J.; Feng, P.; Fu, X.; Wu, L. Monolayer HNb3O8 for Selective Photocatalytic Oxidation of Benzylic Alcohols with Visible Light Response. Angew. Chem., Int. Ed. 2014, 53, 2951−2955. (75) Yanagida, S.; Nakajima, A.; Sasaki, T.; Kameshima, Y.; Okada, K. Processing and photocatalytic properties of transparent 12 tungsto(VI) phosphoric acid-TiO2 hybrid films. Chem. Mater. 2008, 20, 3757− 3764. (76) Umemura, Y.; Shinohara, E.; Koura, A.; Nishioka, T.; Sasaki, T. Photocatalytic decomposition of an alkylammonium cation in a Langmuir-Blodgett film of a titania nanosheet. Langmuir 2006, 22, 3870−3877. (77) Ma, R.; Sasaki, T. Nanosheets of Oxides and Hydroxides: Ultimate 2D Charge-Bearing Functional Crystallites. Adv. Mater. 2010, 22, 5082−5104. (78) Osada, M.; Sasaki, T. Two-Dimensional Dielectric Nanosheets: Novel Nanoelectronics From Nanocrystal Building Blocks. Adv. Mater. 2012, 24, 210−228. (79) Takagaki, A.; Tagusagawa, C.; Hayashi, S.; Hara, M.; Domen, K. Nanosheets as highly active solid acid catalysts for green chemical syntheses. Energy Environ. Sci. 2010, 3, 82−93. (80) Zhai, Z.; Huang, Y.; Xu, L.; Yang, X.; Hu, C.; Zhang, L.; Fan, Y.; Hou, W. Thermostable nitrogen-doped HTiNbO5 nanosheets with a high visible-light photocatalytic activity. Nano Res. 2011, 4, 635−647. (81) Yang, G.; Hou, W.; Feng, X.; Xu, L.; Liu, Y.; Wang, G.; Ding, W. Nanocomposites of polyaniline and a layered inorganic acid host: Polymerization of aniline in the layers, conformation, and electrochemical studies. Adv. Funct. Mater. 2007, 17, 401−412. (82) Shen, P.; Zhang, H. T.; Liu, H.; Xin, J. Y.; Fei, L. F.; Luo, X. G.; Ma, R. Z.; Zhang, S. J. Core-shell Fe 3 O 4 @SiO 2 @HNbMoO 6 nanocomposites: new magnetically recyclable solid acid for heterogeneous catalysis. J. Mater. Chem. A 2015, 3, 3456−3464. (83) Pandey, D. K.; Chung, T. F.; Prakash, G.; Piner, R.; Chen, Y. P.; Reifenberger, R. Folding and cracking of graphene oxide sheets upon deposition. Surf. Sci. 2011, 605, 1669−1675. (84) Yang, G.; Kong, Y.; Hou, W. H.; Yan, Q. J. Heating behavior and crystal growth mechanism in microwave field. J. Phys. Chem. B 2005, 109, 1371−1379. (85) Liu, J. F.; Li, X. L.; Li, Y. D. Synthesis and characterization of nanocrystalline niobates. J. Cryst. Growth 2003, 247, 419−424. (86) Jehng, J. M.; Wachs, I. E. STRUCTURAL CHEMISTRY AND RAMAN-SPECTRA OF NIOBIUM OXIDES. Chem. Mater. 1991, 3, 100−107. (87) Gouadec, G.; Colomban, P. Raman Spectroscopy of nanomaterials: How spectra relate to disorder, particle size and mechanical properties. Prog. Cryst. Growth Charact. Mater. 2007, 53, 1−56. (88) Arora, A. K.; Rajalakshmi, M.; Ravindran, T. R.; Sivasubramanian, V. Raman spectroscopy of optical phonon confinement in nanostructured materials. J. Raman Spectrosc. 2007, 38, 604− 617. (89) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Lett. 2007, 7, 3499−3503.

(49) Yuan, L.; Xu, Y.-J. Photocatalytic conversion of CO2 into valueadded and renewable fuels. Appl. Surf. Sci. 2015, 342, 154−167. (50) Yuan, L.; Yang, M.-Q.; Xu, Y.-J. Tuning the surface charge of graphene for self-assembly synthesis of a SnNb2O6 nanosheetgraphene (2D-2D) nanocomposite with enhanced visible light photoactivity. Nanoscale 2014, 6, 6335−6345. (51) Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J. Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115, 10307−10377. (52) Yang, M.-Q.; Zhang, N.; Pagliaro, M.; Xu, Y.-J. Artificial photosynthesis over graphene-semiconductor composites. Are we getting better? Chem. Soc. Rev. 2014, 43, 8240−8254. (53) Zhang, N.; Zhang, Y.; Xu, Y.-J. Recent progress on graphenebased photocatalysts: current status and future perspectives. Nanoscale 2012, 4, 5792−5813. (54) Zhang, Y.; Tang, Z.-R.; Fu, X.; Xu, Y.-J. Engineering the Unique 2D Mat of Graphene to Achieve Graphene-TiO2 Nanocomposite for Photocatalytic Selective Transformation: What Advantage does Graphene Have over Its Forebear Carbon Nanotube? ACS Nano 2011, 5, 7426−7435. (55) Yang, Y.; Ren, L.; Zhang, C.; Huang, S.; Liu, T. Facile Fabrication of Functionalized Graphene Sheets (FGS)/ZnO Nanocomposites with Photocatalytic Property. ACS Appl. Mater. Interfaces 2011, 3, 2779−2785. (56) Xu, T.; Zhang, L.; Cheng, H.; Zhu, Y. Significantly enhanced photocatalytic performance of ZnO via graphene hybridization and the mechanism study. Appl. Catal., B 2011, 101, 382−387. (57) Chen, Z.; Zhang, N.; Xu, Y.-J. Synthesis of graphene-ZnO nanorod nanocomposites with improved photoactivity and antiphotocorrosion. CrystEngComm 2013, 15, 3022−3030. (58) Williams, G.; Kamat, P. V. Graphene-Semiconductor Nanocomposites: Excited-State Interactions between ZnO Nanoparticles and Graphene Oxide. Langmuir 2009, 25, 13869−13873. (59) Kamat, P. V. Graphene-Based Nanoarchitectures. Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Carbon Support. J. Phys. Chem. Lett. 2010, 1, 520−527. (60) Akhavan, O. Graphene Nanomesh by ZnO Nanorod Photocatalysts. ACS Nano 2010, 4, 4174−4180. (61) Wang, P.-Q.; Bai, Y.; Luo, P.-Y.; Liu, J.-Y. Graphene-WO3 nanobelt composite: Elevated conduction band toward photocatalytic reduction of CO2 into hydrocarbon fuels. Catal. Commun. 2013, 38, 82−85. (62) Guo, J. J.; Li, Y.; Zhu, S. M.; Chen, Z. X.; Liu, Q. L.; Zhang, D.; Moon, W. J.; Song, D. M. Synthesis of WO3@Graphene composite for enhanced photocatalytic oxygen evolution from water. RSC Adv. 2012, 2, 1356−1363. (63) An, X. Q.; Yu, J. C.; Wang, Y.; Hu, Y. M.; Yu, X. L.; Zhang, G. J. WO3 nanorods/graphene nanocomposites for high-efficiency visiblelight-driven photocatalysis and NO2 gas sensing. J. Mater. Chem. 2012, 22, 8525−8531. (64) Shibata, T.; Takanashi, G.; Nakamura, T.; Fukuda, K.; Ebina, Y.; Sasaki, T. Titanoniobate and niobate nanosheet photocatalysts: superior photoinduced hydrophilicity and enhanced thermal stability of unilamellar Nb3O8 nanosheet. Energy Environ. Sci. 2011, 4, 535− 542. (65) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. Protonated titanates and TiO2 nanostructured materials: Synthesis, properties, and applications. Adv. Mater. 2006, 18, 2807−2824. (66) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (67) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891−2959. (68) Wang, L.; Sasaki, T. Titanium Oxide Nanosheets: Graphene Analogues with Versatile Functionalities. Chem. Rev. 2014, 114, 9455− 9486. (69) Madaro, F.; Saeterli, R.; Tolchard, J. R.; Einarsrud, M. A.; Holmestad, R.; Grande, T. Molten salt synthesis of K4Nb6O17, J

DOI: 10.1021/acs.langmuir.5b03359 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (90) Chen, Z.; Liu, S.; Yang, M. Q.; Xu, Y. J. Synthesis of uniform CdS nanospheres/graphene hybrid nanocomposites and their application as visible light photocatalyst for selective reduction of nitro organics in water. ACS Appl. Mater. Interfaces 2013, 5, 4309−19. (91) Weng, B.; Yang, M.-Q.; Zhang, N.; Xu, Y.-J. Toward the enhanced photoactivity and photostability of ZnO nanospheres via intimate surface coating with reduced graphene oxide. J. Mater. Chem. A 2014, 2, 9380. (92) Wang, Y.; Wang, W.; Mao, H.; Lu, Y.; Lu, J.; Huang, J.; Ye, Z.; Lu, B. Electrostatic self-assembly of BiVO4-reduced graphene oxide nanocomposites for highly efficient visible light photocatalytic activities. ACS Appl. Mater. Interfaces 2014, 6, 12698−706. (93) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; Ventrice, C. A.; Ruoff, R. S. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy. Carbon 2009, 47, 145−152. (94) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 2006, 16, 155−158. (95) Turcicova, H.; Zemek, J.; Vacik, J.; Cervena, J.; Perina, V.; Polcarova, M.; Bradler, J. Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma. Surf. Interface Anal. 2000, 29, 260−264. (96) Li, X.; Zhuang, Z.; Li, W.; Pan, H. Photocatalytic reduction of CO2 over noble metal-loaded and nitrogen-doped mesoporous TiO2. Appl. Catal., A 2012, 429, 31−38. (97) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas solid systems with special reference to the determination of surface-area and porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603−619. (98) Takagaki, A.; Lu, D. L.; Kondo, J. N.; Hara, M.; Hayashi, S.; Domen, K. Exfoliated HNb3O8 nanosheets as a strong protonic solid acid. Chem. Mater. 2005, 17, 2487−2489. (99) Dias, A. S.; Lima, S.; Carriazo, D.; Rives, V.; Pillinger, M.; Valente, A. A. Exfoliated titanate, niobate and titanoniobate nanosheets as solid acid catalysts for the liquid-phase dehydration of Dxylose into furfural. J. Catal. 2006, 244, 230−237. (100) Tagusagawa, C.; Takagaki, A.; Hayashi, S.; Domen, K. Characterization of HNbWO6 and HTaWO6 Metal Oxide Nanosheet Aggregates As Solid Acid Catalysts. J. Phys. Chem. C 2009, 113, 7831− 7837. (101) Li, X.; Kikugawa, N.; Ye, J. A Comparison Study of Rhodamine B Photodegradation over Nitrogen-Doped Lamellar Niobic Acid and Titanic Acid under Visible-Light Irradiation. Chem. - Eur. J. 2009, 15, 3538−3545. (102) Zhang, L.; Zhang, W.; Lu, L.; Yang, X.; Wang, X. Synthesis, structure and photocatalytic reactivity of layered CdS/H2La2Ti3O10 nanocomposites. J. Mater. Sci. 2006, 41, 3917−3921. (103) Jovic, V.; Chen, W.-T.; Sun-Waterhouse, D.; Blackford, M. G.; Idriss, H.; Waterhouse, G. I. N. Effect of gold loading and TiO2 support composition on the activity of Au/TiO2 photocatalysts for H2 production from ethanol-water mixtures. J. Catal. 2013, 305, 307−317. (104) Wang, W.-S.; Wang, D.-H.; Qu, W.-G.; Lu, L.-Q.; Xu, A.-W. Large Ultrathin Anatase TiO2 Nanosheets with Exposed {001} Facets on Graphene for Enhanced Visible Light Photocatalytic Activity. J. Phys. Chem. C 2012, 116, 19893−19901. (105) Ong, W.-J.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T.; Mohamed, A. R. Self-assembly of nitrogen-doped TiO2 with exposed {001} facets on a graphene scaffold as photo-active hybrid nanostructures for reduction of carbon dioxide to methane. Nano Res. 2014, 7, 1528− 1547. (106) Lee, J. S.; You, K. H.; Park, C. B. Highly Photoactive, Low Bandgap TiO2 Nanoparticles Wrapped by Graphene. Adv. Mater. 2012, 24, 1084−1088.

(107) Zhang, Y.; Pan, C. TiO2/graphene composite from thermal reaction of graphene oxide and its photocatalytic activity in visible light. J. Mater. Sci. 2011, 46, 2622−2626. (108) Jia, L.; Wang, D.-H.; Huang, Y.-X.; Xu, A.-W.; Yu, H.-Q. Highly Durable N-Doped Graphene/CdS Nanocomposites with Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light Irradiation. J. Phys. Chem. C 2011, 115, 11466−11473. (109) Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Graphene Transforms Wide Band Gap ZnS to a Visible Light Photocatalyst. The New Role of Graphene as a Macromolecular Photosensitizer. ACS Nano 2012, 6, 9777−9789. (110) Li, Q.; Guo, B.; Yu, J.; Ran, J.; Zhang, B.; Yan, H.; Gong, J. R. Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets. J. Am. Chem. Soc. 2011, 133, 10878−10884. (111) Xiang, Q.; Yu, J. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2013, 4, 753−9. (112) Zhang, N.; Yang, M.-Q.; Tang, Z.-R.; Xu, Y.-J. Toward Improving the Graphene-Semiconductor Composite Photoactivity via the Addition of Metal Ions as Generic Interfacial Mediator. ACS Nano 2014, 8, 623−633. (113) Li, X.; Pan, H.; Li, W.; Zhuang, Z. Photocatalytic reduction of CO2 to methane over HNb3O8 nanobelts. Appl. Catal., A 2012, 413, 103−108. (114) Li, X.; Li, W.; Zhuang, Z.; Zhong, Y.; Li, Q.; Wang, L. Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2Pillared HNb3O8. J. Phys. Chem. C 2012, 116, 16047−16053.

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DOI: 10.1021/acs.langmuir.5b03359 Langmuir XXXX, XXX, XXX−XXX