12410
J. Phys. Chem. B 2005, 109, 12410-12416
Photoconductive Properties of Organic-Inorganic Hybrid Films of Layered Perovskite-Type Niobate Kazuko Saruwatari,*,† Hisako Sato,†,‡ Tomochika Idei,† Jun Kameda,† Akihiko Yamagishi,†,‡ Atsushi Takagaki,§ and Kazunari Domen| Graduated School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan, Core Research for EVolutional Science and Technology (CREST), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan, and Graduated School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan ReceiVed: February 1, 2005; In Final Form: April 11, 2005
A hybrid film of layered niobate and an organic amphiphile was prepared by the Langmuir-Blodgett (LB) method. Trimethylammonium-exchanged perovskite-type niobates ((CH3)3NHSr2Nb3O10) were exfoliative to form an aqueous suspension. A monolayer of octadecylamine was produced on such an aqueous dispersion as a template for a hybrid film. A hybrid film was transferred as a Y-type LB film onto a hydrophilic glass plate or an ITO substrate. The structure of a deposited film was investigated with X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and atomic force microscopy (AFM) measurements, indicating a layerby-layer structure with a single or double sheet of niobate as an inorganic composite. From the cyclic voltammogram on an ITO electrode modified with the Y-type 10 layered film, the lower edge of the conduction band of a niobate layer was determined to be - 0.6 V (vs Ag/AgCl). ac impedance and dc measurements were carried out on 1, 5, and 10-layered LB films (2 mm (electrode spacing) × 8 mm (width)) with aluminum electrodes. The freshly deposited samples behaved as an insulator under the illumination of 280 nm light (2.04 × 1016 quanta s-1). Photoconductivities appeared, however, when they were preirradiated with a 150 W Xe lamp (ca. 2 × 1018 quanta s-1) for 0.5-8.5 h. The process was denoted as photomodification. From the FT-IR and XRD results, it was deduced that the photomodification of LB films caused the decomposition of organic templates (octadecylammonium) accompanied by the collapse of layer-by-layer structures. dc analyses on the 5- and 10-layered films after photomodification also showed that they behaved as a photosemiconductor under UV light illumination.
Introduction Layered perovskite-type oxides are known as a treasury of functional materials because of their unique physical properties such as electrical conductivity1,2 and superconductivity3,4 and their interesting chemical reactivity such as ion exchangeability and photocatalytic activity.5,6 When alkali metal ions in the interlayer spaces of layered perovskites are ion-exchanged with organic compounds, the resultant intercalation compounds create a possibility to develop a new type of composite materials with unprecedented properties.7,8 Moreover, the combination of this character with thin film preparations may open a further chance to prepare an organized nanostructured film. Such films may be used as a surface modifier, as sensors, and as electronic devices. As sensor devices, in particular, the films may exhibit high sensitivity due to their huge surface areas. The systematic preparation of thin films has been explored with self-assembly (SA) and Langmuir-Blodgett (LB) methods in the electrochemical studies of nanocrystalline semiconductors.9-12 In the case of the synthesized nanosheets, both the SA and LB methods were also utilized to form highly organized thin films.13-15 Our group has applied the LB method to prepare * To whom correspondence should be addressed. Fax: +81-3-58414682. E-mail:
[email protected]. † Graduated School of Science, The University of Tokyo. ‡ CREST. § Tokyo Institute of Technology. | Graduated School of Engineering, The University of Tokyo.
a single layered film of inorganic nanosheets by hybridizing with organic monolayers.15-18 Hybridization occurs at an airwater interface by the sticking of inorganic nanosheets exfoliated in an underlying aqueous dispersion to a floating amphiphilic monolayer. In this article, we have prepared layered niobates intercalating three different kinds of cations (K+, H+, and (CH3)3NH+). In particular, a layered niobate intercalating (CH3)3NH+ was exfoliative in water. Its aqueous dispersion was used as a subphase for the LB method as described above. For the purpose of developing a sensor device based on organic/niobate hybrid films, we have investigated the electrical conductivities by means of alternating current (ac) and direct current (dc) analyses. Since our method inevitably includes an organic layer as a template, the study has been focused on the role of an organic layer in electrical properties. As a result, it has been shown that the elimination of organic templates is possible simply by irradiating a film with a Xe lamp. Due to this procedure (denoted by photomodification treatments), the electric photoconductivity increased remarkably. The reasons for such enhancement of conductivity are discussed on the basis of the structural data obtained by applying various spectroscopic methods. Experimental Section Materials. KSr2Nb3O10 (K-type) was synthesized by reacting appropriate mixtures of K2CO3, SrCO3, and Nb2O5 powders at
10.1021/jp0505476 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/04/2005
Hybrid Films of Layered Perovskite-Type Niobate 1150 °C in air for 24 h.19 An excess of 15 wt % alkali metal carbonate was used to compensate the loss of the alkali component due to evaporation. A K-type powder was refluxed in 6 M HCl at 60 °C for 16 h to be acidified to a HSr2Nb3O10 (H-type) powder.20 The obtained H-type powder was mixed with 30% trimethylamine solution (6 times excess of H+) in an autoclave at 60 °C for 72 h to be transformed into a trimethylammonium (TMA) type niobate ((CH3)3NHSr2Nb3O10). After being washed with distilled water three times, the mixture was centrifuged at 3000 rpm for 10 min. The precipitate (TMAtype sample) was exfoliated in pure water by being stirred as an aqueous dispersion for more than 1 week. An aqueous dispersion of a TMA-type powder thus prepared was stable for severalmonths.Octadecylammoniumchloride(CH3(CH2)17NH4+Cl-; ODAH+Cl-) was prepared by reacting octadecylamine with a stoichiometric amount of hydrochloric acid. The salt was purified by recrystallization from methanol twice. Film Preparation. Hybrid thin films were prepared by a Langmuir trough (10 cm × 13 cm) (USI System, Japan). Surface pressure was measured with a Wilhelmy balance. The temperature of the trough was controlled by circulating water at 20 °C. A mixed solvent of chloroform and methanol (9:1 in volume) containing 4.05 × 10-4 mol dm-3 ODAH+Cl- was spread onto an aqueous dispersion of TMA-type niobate (0.512 g dm-3). After waiting 30 min at zero surface pressure, the surface was compressed at a rate of 10 cm2 min-1. A hybrid monolayer was deposited onto a hydrophilic glass plate (15 mm × 8 mm) or an ITO substrate (10 mm × 8 mm) at a surface pressure of 20 mN m-1 with the transfer ratio of 0.75 ( 0.25 by the perpendicular dipping method. For comparison, we also prepared the cast films, which were prepared on glass plates (15 mm × 8 mm) by dropping an aqueous dispersion of one of the K-, H- and TMA-type powders. Instruments. Samples were characterized with an X-ray diffraction (XRD) apparatus (Rint 2000, Rigaku, Japan) operated with Cu KR radiation and a Ni filter, and IR spectrometer (JASCO FT-IR 460). Thermogravimetric (TG)-differential thermal analysis (DTA) measurements (2000S MAC Science Co., Ltd.) were performed to estimate the weight percentage of trimethylammonium cations in the structure. Scanning electron microscopy (SEM; S-4500 Hitachi, Japan) and atomic force microscopy (AFM; Nanoscope III scanning microscope, Digital Instruments) were used to observe the surface structure of a deposited film in the range of micrometers to nanometers. ac impedance measurements were operated at a constant voltage of 1.0 V for frequencies from 2 MHz to 50 Hz at room temperature with an LC meter (Hioki 3532-50, Japan). dc electrical conductivity measurements were made at room temperature using a dc voltage current source monitor (ADVANTEST TR6143). To eliminate the effects due to atmospheric air, both ac and dc measurements were carried out under vacuum (0.1 Torr) using a glass cell. Both edge regions (∼6 mm × 8 mm) of a glass plate having deposited LB or cast films were coated by aluminum as electrodes through vacuum coating methods, leaving a central region (2-4.5 mm × 8 mm) uncoated as an electrode spacing. For photoconductivity measurements, the samples were illuminated by a 150 W Xe lamp (Hamamatsu Photonics, Japan) through an interference filter (280 nm) and a 10 cm quartz cell containing pure water for 30 min until the stable values were obtained (pretreatment). In this setup, the temperature rise of sample films was found to be ∼5 °C. The intensity of light was 2.04 × 1016 quanta cm-2 s-1 as measured by chemical actinometry using potassium tris(oxalate)iron(III).21,22 In the case of LB films, an intense UV light was
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12411
Figure 1. X-ray powder diffraction patterns of synthesized layered niobates: (a) K-type; (b) H-type; (c) TMA-type.
Figure 2. FT-IR spectra of synthesized layered niobates: (a) K-type; (b) H-type; (c) TMA-type.
irradiated directly from a Xe lamp before the electrical measurements. This procedure was denoted as photomodification. The intensity of light in photomodification was approximately 2.12 × 1018 quanta cm-2 s-1, which was 100 times higher than that of the light for the photoconductivity measurements. Cyclic voltammograms were recorded with a potentiostat 2020A (Toho Technical Co., Japan), using a Ag/AgCl electrode as a reference electrode. A Pt mesh was used as an auxiliary electrode. The electrodes were immersed in a 0.1 M NaClO4 aqueous solution under nitrogen atmosphere. Results and Discussion Characterizations of Layered Perovskite-Type Niobates. The XRD patterns of synthesized niobates are shown in Figure 1. K-type and H-type niobates gave nearly the same patterns in good agreement with previous works.1,19,20,23 TMA-type niobate gave patterns with weaker peaks than K- and H-types, implying that perovskite layers were partially delaminated into exfoliated single layers.24 The basal reflection peak (001) of TMA-type shifted toward the smaller angle than that of K-types, corresponding to the expansion of layers from 1.51 nm (K-type) to 1.78 nm (TMA-type) due to the increase of the size of an intercalated cation.8 The IR patterns of the synthesized niobates are shown in Figure 2. On acidifying K-type niobate to H-type one, the peak at 920 cm-1, which was assigned to the terminal Nb-O vibration, underwent a 20 cm-1 blue shift. When the H-type niobate was transformed to the TMA-type niobate, the peak shifted back to the lower wavenumber (or a red shift). The
12412 J. Phys. Chem. B, Vol. 109, No. 25, 2005
Figure 3. Simultaneous TG-DTA curves for TMA-type niobates.
Saruwatari et al.
Figure 5. (a) AFM image (5 µm × 5 µm) of exfoliated TMA-type niobates as a scattered cast film. The vertical distance between the two arrowed positions is estimated to be 2.94 nm in (c). (b) AFM image of TMA/ODAH+-type 1-layered LB film after photomodification for 4.5 h. The vertical distance between the two arrowed positions is estimated to be 3.88 nm in (d).
Figure 4. SEM photos of sections of synthesized layered niobates cast films: (a) K-type; (b) TMA-type.
observed blue or red spectral shift might be attributed to the decrease or increase of the interactions of intercalated cations with the terminal Nb-O bonds, respectively. An interlayer cation is known to occupy several kinds of coordination sites between the NbO6 octahedral layers, depending on its size.25 A larger cation such as K+ and TMA cations is located at a larger polyhedral site, interacting with more Nb-O bonds than hydronium ions. This resulted in the red shift of the IR peak. TMA-type niobate showed peaks at 1398 and 1481 cm-1 which were assigned to the symmetric and asymmetric bending vibrations of -CH2- and/or -CH3, respectively. The peaks at 1630 and 3389 cm-1 were assigned to the N-H bending and stretching vibration bands, respectively. These confirmed the intercalation of TMA cations. TG-DTA curves are presented in Figure 3. The first weight loss with 3.6% was observed below 160 °C with weak endothermic peaks in the DTA curve. This was probably originated from the evaporation of surface adsorbed and solvating water molecules. The larger weight loss with 4.9% took place with exothermic peaks between 160 and 450 °C. This was attributed to the combustion of TMA cations. The elemental analysis of the sample was obtained to be C (2.19%), H (1.01%), and N (0.70%). Combining the TG-DTA and elemental analysis results, the chemical formula of the TMAtype sample was proposed to be (H3O+)0.57(TMA)0.43Sr2Nb3O10‚ nH2O (n ) 1) (calculated C (2.12%), H (1.09%), and N (0.82%)). On the basis of this formula, the calculated weight losses due to the elimination of H2O molecules and the combustion of TMA were 3.8% and 3.6%, respectively. Parts a and b of Figure 4 are SEM images showing the cross sections of K- and TMA-type samples before the dispersion, respectively. The K-type sample consisted of plates with a size of ca. 5 µm2 and a thickness of 1-3 µm, while the TMA-type sample consisted of thin wavy layers. Thus the ion exchanging of K+ cations with TMA cations resulted in the exfoliation of stacked layers into single layers, further inducing the flexibility of the exfoliated layers. The flexibility of the niobates was also observed in the AFM image of the cast sample of an aqueous
Figure 6. π-A isotherm curves for monolayers of ODAH+ on (a) pure water and (b) TMA-type niobate suspensions.
dispersion of TMA-type niobates (Figure 5a). There were flat nanosheets with a size of ca. 1-2 µm2 and a thickness of ∼3 nm. Some edges of the nanosheets were overlapped without any breakage. The thickness of a single niobate layer was ∼1.23 nm.8 In the case of the double layers, the thickness was estimated to be ∼2.8 nm as the sum of the thickness of two niobate layers (2 × ∼1.23 nm) and the diameter of an intercalated TMA ion (∼0.4 nm) and/or hydronium ion (∼0.3 nm).20 Based on this, the layers observed in the AFM image corresponded a single or double niobate nanosheets. Preparation and Characterizations of Organic/Layered Niobate LB Films. Solid and dotted curves in Figure 6 show the π-A isotherms when a chloroform solution of ODAH+Clwas spread onto pure water and on a TMA-type niobate suspension (0.518 g dm-3), respectively. On both pure water and a TMA-type niobate suspension, the surface pressure rose from zero at approximately 0.3 nm2 molecule-1 until the film collapsed at 45 mN m-1 and 0.1 nm2 molecule-1. The surface pressure increased more steeply on a TMA-type niobate suspension than on pure water in the molecular area region of 0.1-0.3 nm2 molecule-1. This indicated that the floating monolayer became stiffer due to the attachment of niobate layers. The XRD patterns of 5- and 10-layered LB films are shown in Figure 7. For the 10-layered LB film, three peaks were recognized at 2θ ) 1.31, 3.82, and 5.4°, while two peaks were found at 2θ ) 3.83 and 5.33° for the 5-layered LB film.
Hybrid Films of Layered Perovskite-Type Niobate
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12413
Figure 9. Cyclic voltammograms of 1-, 5-, and 10-layered LB films of niobate electrodes using NaClO4 (0.1 M) as an electrolyte solution.
Figure 7. XRD patterns for 10- and 5-layered LB films of TMA/ ODAH+-type niobates. The time indicates the duration of the photomodification treatment.
Figure 10. Schematic energy band diagram of TMA-type niobate compared with those of Al and ITO.
Figure 8. FT-IR spectra for (a) monolayered, (b) 5-layered, and (c) 10-layered LB films of TMA/ODAH+-type niobates.
Supposing that the peak at 2θ ) 1.31° corresponded to the basal reflection (001) in the LB films, the thickness of LB unit layer was calculated to be 6.73 nm. The other peaks at 2θ ) 3.82 and 5.4° (or 2.31 and 1.64 nm, respectively) were possibly corresponding to (003) and (004) reflections, although the (002) reflection was not detected. Since the LB films were Y-type, they were assumed to have a unit of an ODAH+ double molecular layer sandwiched by two niobate layers. However, the sum of a double niobate layer (∼1.23 nm × 2) and a double ODAH+ layer (∼2.4 nm × 2) was larger than 6.73 nm, suggesting the obliquity of ODAH+ molecules. If there was no intercalation ion between two niobate layers, ODAH+ molecules were estimated to incline by 27° from the normal direction. If hydronium ion and/or a TMA ion was present between double niobate layers, ODAH+ molecules were estimated to incline by 36°. Figure 8 shows the IR spectra of 1-, 5-, and 10-layered LB films in the wavenumber range of 3000-2800 cm-1. The peaks at 2923 and 2853 cm-1, which were assigned to the symmetric and asymmetric -CH2- stretching vibration bands, respectively,26 increased their intensity in proportion to the accumulated number of LB layers. These peaks were considered to arise from ODAH+ and/or TMA ions. The peak due to the C-H stretching vibration in -CH3 (2960 cm-1) was not detected. Thus most of the ions in the hybrid films might be ODAH+ with a very small amount of TMA ions, if any. Figure 9 shows cyclic voltammograms on ITO electrodes (10 mm × 8 mm) modified with 1-, 5-, and 10-layered LB films.
The reduction current was observed at the potential range below -0.6 V (vs Ag/AgCl). The oxidation current was observed at the potential range of -1.0 to -0.6 V. Both currents increased with the increase of layer number or 1 < 5 < 10 layers. These were thought to correspond to the n-doping and n-dedoping processes of niobate layers, respectively. Taking the absolute value of the free energy of the reference electrode (Ag/AgCl) to be 4.66 eV, the location of the conduction band was estimated to be ca. 4.06 eV from the vaccum level. From the extrapolated absorption edge of the electronic absorption spectrum of an aqueous dispersion of TMA-type niobate (280 nm), the band gap was estimated to be 3.76 eV. The energy diagram of a niobate layer is shown schematically in Figure 10. Electrical Properties of LB and Cast Films of Layered Niobates. ac impedance measurements were performed on the TMA/ODAH+-type LB films in the dark and under 280 nm UV irradiation. The results are plotted as the Cole-Cole plots in which the negative sign of the imaginary part (-Χ′′) of the impedance (Z) was plotted against the real part (Χ′) of Z. Parts A and B of Figure 11 show the results for 10- and 5-layered LB films, respectively. As freshly prepared LB hybrid films, they acted as an insulator under both conditions (Figure 11AC). When the films were photomodified by a Xe lamp for 0.54.5 h, the impedance plots under the UV irradiation became smaller with increasing irradiation time. The impedance plots in the dark did not change after photomodification. No further change of impedance was observed by photomodification longer than 4.5 h. The obtained semicircles in the Cole-Cole plots were interpreted in terms of a circuit of one resistance (R) and one capacitance (C) in parallel. In the case of the 10-layered LB film, the R component, which was determined at the intercept
12414 J. Phys. Chem. B, Vol. 109, No. 25, 2005
Figure 11. Cole-Cole plots by ac impedance measurements on 10-, 5-, and 1-layered LB and cast films in the dark and under irradiation of 280 nm UV light. (a) Fresh film with no photomodification; (b)(g) impedance results after photomodification treatments of a Xe lamp for 0.5, 1.5, 2.5, 3.5, 4.5, and 8.5 h, respectively.
of the plot with the horizontal axis at the low-frequency limit, decreased from 600 to 65 MΩ on photomodification, while the C component, which was determined from the frequency at the top of the semicircle, stayed at the nearly constant value of 2.0 ( 0.2 pF. In the case of the 5-layered LB film, photoconductivity was achieved after the photomodification for 30 min. The R components were stable at 175 MΩ with the C components at 2.27 pF. Conductivity and photoconductivity were also measured under the dc current after the complete photomodification of the LB films (8.5 h). The current in the dark was not measurable in the range of 0-10 V for all LB films. On the other hand, under the UV irradiation, the current increased linearly with voltage in the range of 0-10 V (or Ohmic current) for 10- and 5-layered
Saruwatari et al. LB films, giving resistivity of 69 and 125 MΩ, respectively (Figure 12a,b). Each R component from dc measurements was nearly consistent with the R component obtained from ac impedance measurements (Table 1). Comparing 5- and 10layered LB films, the resistivity of 5-layered LB films was 2 times larger than the one of 10-layered LB films. This was reasonable since the thickness of the film increased 2 times from the 5-layer to 10-layer films. ac impedance measurements on the cast films of K-, H-, and TMA-type niobates were performed in the dark and under 280 nm illumination. K- and H-type niobates acted as insulators with a nearly vertical plot under both conditions, while the TMAtype niobate showed photoconductivity under the illumination of 280 nm with a semicircle for a Cole-Cole plot (Figure 11D, E). The R and C components of the TMA-type niobate were determined to be 125 kΩ and 2.12 pF, respectively. dc measurements showed the Ohmic current in the range of 0-10 V under 280 nm irradiation with the equivalent resistivity of 112 kΩ (Figure 12c). The value was nearly coincident with the R component from the ac measurements (Table 1). Structural Changes during Photomodification Treatments. IR absorption spectra were measured just after the photomodification in order to clarify the structural change of the LB films. The absorption peaks at 2923 and 2853 nm gradually declined with increasing irradiation time (Figure 13), corresponding to the decrease of a radius of the semicircle in the impedance plots as already shown in Figure 11. Since the two peaks in Figure 13 were assigned to the symmetric and asymmetric -CH2stretching vibrations, the declinations of these peaks implied the decomposition of the ODAH+ ions within the LB films, probably due to the photocatalysis by niobate layers. However, these peaks did not disappear completely, so a part of ODAH+ layers was considered to remain. The XRD measurements on the 5- and 10-layered films also showed the reduction of the peak intensity accompanied by the shift toward larger 2θ of all the peaks (Figure 7). All the peaks at 1.31, 3.82, and 5.4° were obviously declined with a slight shift toward the larger angles, 1.43, 4.34, and 5.68°, respectively. This indicated that the layer-by-layer structure of the LB film collapsed due to the decomposition of ODAH+ molecules. However, not all peaks disappeared completely, suggesting that some layer structures still remained. Surface structures as studied by AFM after the photomodification treatment on a single-layered film are shown in Figure 5b. Platelike sheets were observed with good connectivity between sheets. The thickness of each sheet was estimated to be 1-3 nm, which was consistent with the thickness of TMAtype niobates before the formation of LB hybrid films (Figure 5a). Notably, some sheets were characterized by the dark holes surrounded by the bright domains (e.g., the left-side region of the sheet indicated by the arrows in the figure), although no such nanostructure was observed on the TMA-type niobate nanosheet (Figure 5a). The depth of a hole was estimated to be 2-3 nm, which was consistent with the length of ODAH+. Thus, this structure is considered to be evidence of the partial decomposition of ODAH+ cations during photomodification treatment. Interpretation of Photomodification Effects on Photoconductivity. Combining the above data obtained by the FT-IR, XRD, and AFM measurements, it was deduced that the partial elimination of organic parts by photomodification treatments converted the electrical properties of the present hybrid LB films from an insulator to a photosemiconductor (Figure 14). As illustrated in Figure 14, the ODAH+ molecules in the upper
Hybrid Films of Layered Perovskite-Type Niobate
J. Phys. Chem. B, Vol. 109, No. 25, 2005 12415
Figure 12. Steady-state electric current (I) as a function of applied voltage (V) using dc measurements for hybrid niobate thin films: (a) 10-layered LB film; (b) 5-layered LB film; (c) cast film.
TABLE 1: Resistivity (R) and Capacitance (C) of LB and Cast Films as Measured by ac and dc Analysesa
1 LB 5 LB 10 LB cast
ac R [MΩ] C [pF]
dc R [MΩ]
σ [S m-1]
thickness [nm]
b 175 65 0.125
b 125 69 0.112
b 0.18 0.15 1.34
1.25 11.25 23.75 3743
b 2.27 2 2.12
a Electrical conductivities were calculated from the thickness of the films and the resistivity measured by dc analyses. b No precise measurement was possible due to extremely high resistivity.
Figure 13. FT-IR spectra of 1-, 5-, and 10-layered LB films after Xe lamp irradiation. The spectra correspond to the same samples in Figure 12.
layers near the external surface would be decomposed more easily than those in the inner layers in contact with a glass plate substrate. This was because the upper layers were definitely closer to the UV light than the lower layers. The reasons for such photoconductivity changes are most probably that the organic part behaved as a barrier for electric conduction between niobate nanosheets, while the elimination of such insulating parts resulted in their direct junction to allow the carrier to flow through a film. Furthermore, this may explain differences in photoconductive behaviors among the three types of niobates. For the cast film composed of K- or H-type niobate nanosheets, those cations might interrupt the contact of nanosheets in the same way as ODAH+, leading to low photoconductivity. From the resistance (R) and the thickness of the films, the electrical conductivities of cast and hybrid LB films were estimated as given in Table 1. As for the LB films, the thickness of niobate double layers was estimated to be 2.5 nm from XRD and AFM measurements. Thus the thickness of the 5- and 10-
Figure 14. Schematic diagram showing the collapse of Y-type LB layer structures and changes related to photoconductivities.
layered LB films was calculated to be 4 1/2 times and 9 1/2 times that of two niobate single layers, that is, 11.25 and 23.75 nm as an ideal case, respectively. Here it was taken into account that the first deposition was made as a Z-type film solely in the upward direction. On the basis of this, the conductivity of 5and 10-layered films was obtained to be 0.18 and 0.15 Ω-1 m-1, respectively. On the other hand, the average thickness of the cast film was estimated to be 3700 nm from the SEM measurements. On the basis of this, the electrical conductivity of the cast film was obtained to be 1.34 Ω-1 m-1. Comparing these values, the conductivity of the cast film was about 10 times higher than that of the LB films. The different photocon-
12416 J. Phys. Chem. B, Vol. 109, No. 25, 2005 ductivities are probably induced by the difference in packing densities and the number of contact points of nanosheets between the cast films and LB films. Significance of the Present Findings for the Development of Nanosheet Devices. The photoconductive LB films of layered niobates as obtained in the present work are characterized by the following properties: (1) the magnitude of film conductivity is controlled artificially by changing the deposited layer numbers in the range of nanometer thickness; (2) the conductivity is possibly affected by surface properties such as the adsorption of a gas, especially when the photocatalytic action by a niobate layer is operative on adsorbed molecules, and (3) the conductivity may be changed systematically by changing the elemental compositions of a niobate layer such as the doping of La(III) ions in a layer. It seems promising to combine these properties for developing an inorganic film with several nanometer thickness. In such a film, photoconductivity is monitored for sensing both kinds and amounts of molecular gases with high selectivity. Conclusion K- and H-type layered niobates were found to act as an insulator even under UV irradiation, while niobates intercalating trimethylammonium (TMA) cations behaved as a photoconductor. In the case of the hybrid film with octadecylammonium cations (ODAH+) that was prepared by LB methods, the organic monolayers played an obstacle to electrical conductivity. After illuminating a film with a Xe lamp directly (denoted as photomodification), the hybrid TMA/ODAH+-type LB films increased the photoconductivity, possibly due to the decomposition of organic parts, evoking better connectivity of the layered niobate sheets. It is now desired to determine the carrier species for further understanding the photoconductivity of such a film. Acknowledgment. This work has been financially supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese Government. We are grateful to Mr. Tachikawa for his technical assistance and Prof. Naito at Kanagawa University. Electron microscopy was carried out
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