or Niobium

Article pubs.acs.org/JPCC

Electronic Band Structure of Exfoliated Titanium- and/or Niobium-Based Oxide Nanosheets Probed by Electrochemical and Photoelectrochemical Measurements Kosho Akatsuka,† Genki Takanashi,† Yasuo Ebina,† Masa-aki Haga,‡ and Takayoshi Sasaki*,†,§ †

International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan § CREST, Japan Science and Technology Agency, Kawaguchi, Saitama, 332-0012, Japan S Supporting Information *

ABSTRACT: Exfoliated two-dimensional (2D) unilamellar nanosheets of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− were deposited layerby-layer to produce multilayer films on indium−tin−oxide (ITO)-coated glass electrodes, and their electrochemical and photoelectrochemical properties were explored. The layer-by-layer assembly process via sequential adsorption with counter polycations was monitored by UV−visible absorption spectra and X-ray diffraction measurements, which confirmed the successful growth of films, where nanosheets and polycations are alternately stacked at a separation of 1.6−2.4 nm. Exposure to UV light totally removed polycations, producing inorganic films. Cyclic voltammetry on Ti and/or Nb oxide nanosheet electrodes thus fabricated showed reduction/oxidation (Ti3+/Ti4+ and Nb4+/Nb5+) peaks associated with insertion/extraction of Li+ ions into/from intersheet galleries of the films. The extent of the redox reaction is found to be governed by the cation density in the nanosheet gallery. Anodic photocurrents of the oxide nanosheet electrodes were observed under UV light irradiation. These action spectra showed close resemblance to optical absorption profiles of the colloidal nanosheets, indicating that the photocurrent was generated from the nanosheets. Their analysis indicates that the nanosheets of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− are all indirect transition-type wide-gap semiconductors with bandgap energies of 3.44, 3.68, 3.64, and 3.53 eV, respectively. These values are larger than those for corresponding parent layered oxide compounds before delamination, suggesting confinement effects into 2D nanosheet structure. Furthermore, the value was invariable for the films with a different number of nanosheet layers, indicating that quantized nanosheets were electronically isolated with each other. In addition, photocurrent generation was measured as a function of applied electrode potential, and the flatband potential was estimated from the photocurrent onset values as −1.12, −1.33, −1.30, and −1.29 V vs Ag/Ag+, for Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets, respectively, providing a diagram of electronic band structure for the nanosheets.

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structures. Nanosheets in this system include Ti0.91O20.36−, Ti0.87O20.52−, Ti3O72−, Ti4O92−, and Ti5O112− as Ti end members,4 Nb3O8−, Nb6O174−, and Ca2Nb3O10− (and related nanosheets derived from layered perovskites) as Nb end members,6c−e,7 and TiNbO5−, Ti2NbO7−, and Ti5NbO143− as intermediate members.6a,b In the past decade, considerable attention has been directed at the fabrication of nanofilms and flocculated samples with a designed nanostructure by using these nanosheets as a building block, and it has been demonstrated that a range of useful functionalities can be developed by this approach.16 Takagaki et al. reported that nanosheets such as TiNbO5− and Sr2Nb3O10− work as a strong solid acid catalyst for esterification of acetic acid and

INTRODUCTION The research boom in graphene has intensified interest in other types of two-dimensional (2D) materials.1 Various layered compounds such as clays,2 metal chalcogenides,3 oxides,4−10 and hydroxides11,12 have been delaminated into colloidal unilamellar layers via soft-chemical procedures. The resulting inorganic nanosheets are characterized by their ultimate 2D anisotropic shape and extremely small thickness of around 1 nm. In contrast to graphene, the nanosheets have a rich variety in terms of composition and structure. In particular, oxide nanosheets are attractive because of their high diversity and tunability over composition and structure. A number of transition-metal or rareearth-metal based oxide nanosheets have been synthesized,4−12 and a range of intriguing properties have been found in them.13−15 Among them, Ti and/or Nb oxide nanosheets are very important due to their useful functionalities. This ternary Ti/Nb oxide system provides a series of nanosheets with varied © 2012 American Chemical Society

Received: March 13, 2012 Revised: May 15, 2012 Published: May 15, 2012 12426

dx.doi.org/10.1021/jp302417a | J. Phys. Chem. C 2012, 116, 12426−12433

The Journal of Physical Chemistry C

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other reactions.6a,b,17 Hwang et al. showed that the nanosheet flocculates of Ti0.91O20.36− decorated with oxide nanoparticles, for example, CoO or CuO, undergo photocatalytic reactions, water splitting and decomposition of organic compounds, under visible light.18 We also reported related results that a hybrid multilayer film of Ti0.91O20.36− nanosheet and Zn porphyrin can generate photocurrents via dye-sensitization.19 In addition, we found that the films of these nanosheets, particularly Ti0.91O20.36− and Nb3O8−, show high activity for photoinduced wettability conversion, which is useful in applications as a self-cleaning coating of windows.20 Apart from these photochemical functions, we recently found that ultrathin films of some of these nanosheets such as Ti0.87O20.52−, Ca2Nb3O10−, Ti2NbO7−, and Ti5NbO143− exhibit high dielectric properties, the performance of which is far superior to that of films of widely studied high-k materials such as (Ba, Sr)TiO3.21 These attractive functionalities should be deeply related to chemical and physical nature of the nanosheets, particularly, their electronic band structure. The oxide nanosheets above are intrinsically in d0 electronic system and should be categorized as a wide-gap semiconductor or genuine insulator, where valence band states are constituted mainly by 2p orbitals from O and conduction band states are by 3d from Ti/Nb. Quantitative parameters such as potentials of valence-band and conduction-band edges as well as bandgap energy are valuable and important in obtaining deep understandings on the physicochemical properties mentioned above. For example, we can know with such fundamental material parameters how the nanosheets are photoexcited and how strong the reducing and oxidizing power the generated carriers have. However, such information is available only for limited members, Ti0.91O20.36− and Nb3O8− nanosheets.22 Thus, it is of great importance to clarify fundamental aspects related to the electronic structure of other important oxide nanosheets. In this study, we focus on Ti/Nb oxide nanosheets such as TiNbO 5 − , Ti 2 NbO 7 − , and Ti 5 NbO 14 3− in addition to Ca2Nb3O10− as an important Nb end member. We fabricated their multilayer films on ITO-coated glass electrodes and explored their electrochemical and photoelectrochemical properties to deduce their bandgap energy and flatband potential. A diagram of electronic band structure is constructed.

The acid solutions were replaced every 24 h by decantation to ensure complete substitution of alkali metal ions for protons. The resulting protonic oxide compounds, HCa2Nb3O10·1.5H2O, HTiNbO5·0.3H2O, HTi2NbO7·1.0H2O, and H3Ti5NbO14·1.0H2O, were filtered, washed with copious water, and air-dried. The samples were then shaken with an aqueous solution of tetrabutylammonium (TBA) hydroxide, (C4H9)4NOH, to promote exfoliation into unilamellar nanosheets. The solution-to-solid ratio was 250 cm3 g−1, and the TBA concentration was adjusted so as that the TBA dose was equivalent to the molar proton content in the solids. Shaking for 10 days produced colloidal suspensions. Nondelaminated sample was removed by centrifugation at 1500 rpm for 10 min, and the resulting suspensions were used in film fabrication. Fabrication of Multilayer Films. ITO-coated glass electrodes (surface resistance : 10 Ω cm) were cleaned by ultrasonication in acetone, ethanol, and ultrapure water for 30 min each, while quartz glass substrates were cleaned by the normal procedure involving treatments with cH2SO4 and then with (1:1)HCl/CH3OH. The multilayer films of oxide nanosheets were fabricated via sequential adsorption with polycations (PDDA and PEI) according to procedures reported previously.23 The concentrations of PEI and PDDA aqueous solutions were adjusted as 2.5 and 20 g dm−3, respectively, and their pH was controlled to be ∼9 with diluted HCl solution. The nanosheet suspensions were diluted by 20 fold for Ca2Nb3O10− and 50 fold for the others, and their pH was adjusted as ∼9. The cleaned substrate was dipped into the PEI or PDDA solution for 20 min and washed with ultrapure water, followed by flushing by N2 gas flow. The substrate was subsequently immersed in the nanosheet suspension for 20 min, rinsed with ultrapure water, and then dried by N2 flow. These operations were repeated 10 times to fabricate a multilayer film composed of (polycation/nanosheet)10. Then the polycations were decomposed through exposure to UV light (18 MΩ cm) obtained with a Milli-Q filtration system was used throughout the experiments. Synthesis of Titanium and/or Niobium Oxide Nanosheets. Colloidal suspensions of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets were synthesized by delaminating precursory layered compounds of KCa2Nb3O10, KTiNbO5, CsTi2NbO7, and K3Ti5NbO14 according to the reported procedures.6a,b,7 The layered compounds were prepared by solid-state calcination of a stoichiometric mixture of suitable reagents at 1273−1473 K. The obtained polycrystalline samples were converted into their protonic oxides by acid-exchange for 3 days. KCa2Nb3O10 (2.5 g) was treated with 100 cm3 of 5 M HNO3 aqueous solution, while KTiNbO 5 , CsTi 2 NbO 7 , and K3Ti5NbO14 (1 g) were reacted with 100 cm3 of 1 M HCl solution.

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RESULTS AND DISCUSSION Synthesis of Nanosheets. The reaction of layered protonic oxides of HCa 2 Nb 3 O 1 0 ·1.5H 2 O, HTiNbO 5 ·0.3H 2 O,

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HTi2NbO7·1.0H2O, and H3Ti5NbO14·1.0H2O with (C4H9)4NOH solution yielded colloidal suspensions with an opalescent appearance. Figure 1 shows AFM images for specimens obtained

by depositing the nanosheets onto an Si wafer. 2D objects with an average lateral size of ∼500 nm were observed in each sample. Height profile analysis gave values of 2.20, 0.98, 1.05, and 1.35 nm, respectively. These values are similar to those for Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets reported previously, confirming successful delamination into unilamellar nanosheets. In-plane XRD data (Supporting Information, S1) support that the original 2D lattice remained substantially unchanged after delamination. The nanosheets of TiNbO5−, Ti2NbO7−, and Ti5NbO143− are built up from Ti/NbO6 octahedra, which are joined via edge- and corner-sharing into the 2D structures as illustrated in Figure 2. On the other hand, the Ca2Nb3O10− nanosheet has the 2D perovskite structure, in which NbO6 octahedra are linked via corner-sharing. Film Fabrication. The layer-by-layer assembly process of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets on the ITO-coated glass electrode was monitored by UV−visible absorption spectra measured immediately after each deposition cycle (Figure 3, Supporting Information, S2). The absorption bands observed in UV region (λ < 300 nm) are attributable to the oxide nanosheets of semiconducting nature. On the other hand, the small hump observed at around 370 nm is due to interference effects of multilayer nanosheet films on ITOcoated glass electrodes. No such features were observed for the films on quartz glass substrate as control samples, while the strong absorption in the UV region similarly appeared. In all of the films on the ITO-coated glass electrode, absorbance due to the nanosheets increased in a nearly linear fashion with a repetition of deposition

Figure 1. AFM images of Ti and/or Nb oxide nanosheets. (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, (d) Ti5NbO143−.

Figure 2. Structures of Ti and/or Nb oxide nanosheets. (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, (d) Ti5NbO143−. The program VESTA was used.28 12428



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Figure 3. UV−visible absorption spectra in the multilayer buildup process of (PEI/Ca2Nb3O10−)n on ITO-coated glass electrodes. (Inset) Observed peak-top absorbance at 270 nm is plotted against the number of bilayers.

cycles, indicating that the substantially equal amount of nanosheets was deposited on the electrodes at each cycle. Figure 4 depicts XRD patterns for 10-layer films. The as-fabricated films showed a Bragg peak having d-spacing of 1.6−2.4 nm, which

Figure 5. Cyclic voltammograms of 10-layer films of (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, and (d) Ti5NbO143− nanosheet electrodes at different sweep rates (5, 10, 20, 30, 40, and 50 mV s−1).

of 5, 10, 20, 30, 40, and 50 mV s−1. The curves exhibited reduction/oxidation peaks at around −1.5 V (vs Ag/Ag+), which should be associated with Li+ ion intercalation and deintercalation into or from the intersheet spaces during the negative or positive potential scan. The process may be formulated as eq 1 as exemplified for Ca2Nb3O10− nanosheet film.

Figure 4. XRD patterns for 10-layer films of (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, (d) Ti5NbO143− nanosheets. Red lines: asdeposited; blue lines: after prolonged exposure to UV light (λ < 300 nm).

(NH4 +, H+)Ca 2NbV 3O10 + x(Li+ + e−) ⇄ Lix +(NH4 +, H+)Ca 2NbIV x NbV 3 − x O10

(1)

The behavior is similar to that known for n-type semiconductor nanosheets such as Ti0.91O20.36− and Nb3O8−.22 The anodic peak current densities were linear with the square root of the scan rate (Supporting Information, S3), indicating the diffusion-controlled processes of the Li+ ion. Whether Ti or Nb undergoes reduction/oxidation in Ti/Nb oxide nanosheets is not clear because the standard redox potential for Ti4+/Ti3+ and Nb5+/Nb4+ couples is very similar (−0.73 and −0.70 V vs Ag/Ag+, respectively). The electroactive Ti and/or Nb atom number (Γobs) from the integrating area under the anodic peaks of cyclic voltammetry is calculated according to eq 2:

can be explained in terms of a lamellar nanostructure composed of alternately stacked polycation and oxide nanosheets. This result along with UV−visible absorption data clearly indicates successful layer-by-layer growth of multilayer films of (polycation/nanosheet)n. Subtracting the crystallographic thickness of the nanosheets from the d-spacing observed gives 0.5−1.0 nm as the space for the polycation between the nanosheets. Exposure of the films to UV light induced interlayer shrinkage of 0.5−0.6 nm corresponding to such volume (Figure 4). This suggests the decomposition of polycations due to photocatalytic activity of the oxide nanosheets.20 We reported a similar effect for a multilayer film of the Ti0.91O20.36− nanosheet and polycation, where the polycation was photocatalytically cracked into small cations such as NH4+ and/or H3O+ ions to act as charge-balancing species.24 We used the polymer-free films thus-derived in the following electrochemical and photoelectrochemical experiments. Electrochemical Properties. Figure 5 depicts the cyclic voltammograms for the 10-layer film electrodes of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets at sweep rates

Γ=

Q nFS

(2)

where Q is the charge transferred for the anodic peak current, n is the number of electrons transferred (1 in these cases), S is the geometric area of the electrode (1 cm2 in these cases), and F is Faraday's constant = 96 500 C mol−1. Let us take the 10-layer film of Ca2Nb3O10− nanosheets as an example, where the number of 12429



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electroactive Nb ions (Γobs) was estimated to be 1.69 × 10−9 mol cm−2, or 1.02 × 1014 atom cm−2, by integrating the area under the anodic peak (Q = 5.21 × 10−4 C). Since the 2D square unit cell of Ca2Nb3O10− nanosheet (a = 0.390 nm) contains one formula unit (see Figure 2), an ideal monolayer film of the nanosheets has 2.01 × 1016 Nb atom cm−2. Based on these values, the electroactive ratio of Nb4+/Nb5+ is estimated to be 5.1%. Similar calculations for TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheet films gave 4.3%, 7.4%, and 1.1% (Ti3+/4+ and Nb4+/5+), respectively (Supporting Information, Table 1). In Figure 6,

Figure 6. Plot of electroactive ratio of Ti3+/Ti4+ and/or Nb4+/Nb5+ with cation density in the nanosheet gallery. The dotted line is a guide.

these values are plotted against the cation density in the nanosheet gallery (the data for Ti0.91O20.36− and Nb3O8− in previous studies are also included). The reduction process involves Li+ intercalation into the gallery where charge-balancing cations such as NH4+ and H+ (H3O+) already exist. Thus it is reasonably expected that the nanosheet film with a more densely populated gallery is more difficult to undergo electrochemical reduction. The plot in Figure 6 is compatible with this expectation. The electrochemical process should be governed by many factors, but in the 2D systems of the Ti and/or Nb oxide nanosheets, their charge density connecting to the population of chargecompensating cations seems to play a dominating role. Photoelectrochemical Properties. Figure 7 (inset) displays a typical photocurrent response on the 10-layer films of the nanosheets on ITO-coated glass electrodes. All of the electrodes exhibited anodic photocurrents under UV light irradiation at +0.1 V vs Ag/Ag+. The photocurrent was measured by changing the excitation wavelength from 400 to 250 nm at a step of 5 nm. The action spectra thus obtained, or incident photon-to-electron conversion efficiency (IPCE) as a function of the excitation wavelength, are in good agreement with the optical absorption spectra of the colloidal suspensions of corresponding nanosheets, indicating that the photocurrents are generated through the photoexcitation of the semiconducting nanosheets. The incident photon-to-electron conversion efficiency (IPCE) at an excitation wavelength of 270 nm was 2.3, 0.2, 0.3, and 0.1% for Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets, respectively. Comparable data for Ti0.91O20.36− and Nb3O8− nanosheets obtained under similar conditions in the previous studies were 3.3 and 1.8%. The oxide nanosheets composed solely of Ti or Nb show high efficiency in comparison with nanosheets comprising of both Ti and Nb. This trend can be understood by the general concept that a heterometal oxide system works as a recombination center of photogenerated carriers. It should be pointed out that this tendency reflects the photocatalytic

Figure 7. Photocurrent action spectra for 10-layer films of (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, and (d) Ti5NbO143− nanosheet electrodes. Solid curves are the UV−visible absorption spectra of colloidal suspensions of corresponding nanosheets. The inset is the photocurrent response under monochromatic UV light (λ = 275 nm) irradiation at +0.1 V vs Ag/Ag+.

activities. As can be seen in Supporting Information (S4), the oxide nanosheets of single metal element underwent faster decomposition of interlayer organic polycation, and the others both of Ti and Nb showed a rather slow reaction. The data is converted into a Tauc plot, the square root of IPCE times the photon energy ((ηhν)0.5) as a function of photon energy (hν) (Figure 8a and Supporting Information, S5).25 A good linear relationship was obtained in all of the systems, indicating that the oxide nanosheets are indirect-type semiconductors. The bandgap energy estimated from the intercept of the tangents to the abscissa axis is 3.44, 3.68, 3.64, and 3.53 eV for Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets, respectively. The bandgap energy values for all the nanosheets are larger than that for the bulk materials before delamination, such as KCa2Nb3O10 (3.33 eV),7d KTiNbO5 (3.51 eV),26 and CsTi2NbO7 (3.56 eV),26 which should be due to the quantum size effect in 2D structures.27 We carried out the measurement and analysis on the nanosheet films with the varied number of layers. The photocurrent enhanced with increasing the nanosheet film thickness. It is important to note that the bandgap energy was found to be independent of the number of stacked nanosheet layers (Figure 8b). This result indicates that the nanosheets stacked are not electronically interacted with each other. We estimated flatband potential edges using the photocurrent onset value for the 10-layer film electrodes (Figure 9a−d). Upon exposure to monochromatic UV light centered at 275 nm, the 12430



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photocurrent was measured by changing the electrode potential from −0.5 to +1.0 V versus Ag/Ag+ at a step of 0.1 V. The photocurrent onset potentials under UV light irradiation were −1.12, −1.33, −1.30, and −1.29 V vs Ag/Ag+, respectively, which can be taken as the flatband potential edges of the nanosheets. Figure 10 depicts a schematic energy diagram of the electric band structure of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets obtained on the basis of these analyses. Now it is clear that the edge positions of valence and conduction bands as well as bandgap energy are dependent on the nanosheets. The information will be useful in obtaining insight into photochemical reactivities reported so far and rational design of nanodevices such as optical devices and energy harvesting/storage systems in the future. The expanded bandgap should be associated with very high dielectric and insulating behaviors of these nanosheet films.

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CONCLUSION

Multilayer nanofilms composed of a series of Ti and/or Nb oxide nanosheets, Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143−, and polycations were fabricated via electrostatic layer-by-layer assembly on ITO-coated glass electrodes. These composite nanofilms were converted into inorganic films by decomposing polycations (PEI or PDDA) in the nanosheet galleries upon exposure to UV light. The obtained films underwent electrochemical redox processes involving reversible Li+ intercalation/deintercalation. The films generated a photocurrent upon excitation with UV light, and analysis of the action spectra revealed that the nanosheets are widebandgap semiconductors with indirect bandgap energy of 3.4−3.7 eV, which are larger than the corresponding layered compounds before delamination due to size quantization effect. The conduction band-edge potential was estimated to be −1.3 to −1.1 V vs Ag/Ag+ from the applied potential dependence. The obtained diagram for electronic band structure should be important to utilize these oxide nanosheets as functional building blocks for various energy devices.

Figure 8. (a) Plot of the square root of IPCE (η) times hν with photon energy for the 10-layer film of Ca2Nb3O10− nanosheet electrodes at +0.1 V vs Ag/Ag+. (b) The bandgap energy estimated for the films with various number of nanosheet layers.

Figure 9. Applied potential dependence of IPCE of Ca2Nb3O10− nanosheet film upon illumination at 275 nm. Inset shows detailed curves around onset potential.

Figure 10. Energy diagram depicting the conduction-band edges, valence-band edges, and bandgap energy for (a) Ca2Nb3O10−, (b) TiNbO5−, (c) Ti2NbO7−, (d) Ti5NbO143−, (e) Ti0.91O20.36−, (f) Nb3O8− nanosheets. 12431



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ASSOCIATED CONTENT

S Supporting Information *

In-plane XRD patterns of Ca2Nb3O10−, TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheets, UV−visible absorption spectra in the multilayer buildup process of (PDDA/TiNbO5−)n, (PDDA/ Ti2NbO7−)n, and (PDDA/Ti5NbO143−)n on ITO-coated glass, dependence of the anodic peak current on the square root of the sweep rate of Ca2Nb3O10− naonsheet film on ITO-coated glass electrodes, changes in multilayer repeating distance depending on UV irradiation time, plot of the square root of IPCE (η) times hν with photon energy for the 10-layer films of TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheet electrodes at +0.1 V vs Ag/Ag+, and applied potential dependence of IPCE of TiNbO5−, Ti2NbO7−, and Ti5NbO143− nanosheet films upon illumination at 275 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by CREST of the Japan Science and Technology Agency (JST) and the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan.

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REFERENCES

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