Precise Control of Photoinduced Electron Transfer in Alternate

Sep 8, 2014 - The alternate layered structure was synthesized by the thiol–ene click reaction between ... Progress in Surface Science 2016 91 (4), 1...
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Precise Control of Photoinduced Electron Transfer in Alternate Layered Nanostructures of Titanium Oxide−Tungsten Oxide Dai Mochizuki,* Kozue Kumagai, Masato M. Maitani, Eiichi Suzuki, and Yuji Wada* Department of Applied Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: The alternate layered structure was synthesized by the thiol−ene click reaction between the alkylthiol-modified tungsten oxide layer and the alkene-modified titanium oxide layer. The interlayer distances between the titanium oxide layer and the tungsten oxide layer were controlled to 0.72, 0.94, 1.01, and 1.14 nm by changing the carbon number of the functional groups. Photoinduced electron transfer from the titanium oxide layer to the tungsten oxide layer depends on the interlayer distance of the titanium oxide−tungsten oxide alternate layers from 0.7 to 1.1 nm. The alternate layers of narrow interlayer distance showed high photocatalytic activity in decomposition of methylene blue. The amount of the photoexcited electron transfer from titanium oxides to tungsten oxides was quantitatively measured by the reduction of Ag ions with the electrons stored in tungsten oxide. Because the rate of photoinduced electron transfer should be proportional to the amount of electron transfer, the tunneling decay constant β was estimated to be 0.63 Å−1 in the alternate layer samples, indicating that electrons transfer from titanium oxide to tungsten oxide by throughspace tunneling.

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INTRODUCTION Electron-transfer reactions have led many researchers to study the fundamental chemistry of these processes.1 A long-term goal of this study is to achieve an understanding of photoinduced electron-transfer reactions that is sufficiently advanced to allow a system to be designed to convert solar energy into chemical potential.2 Synthetic models, such as a supramolecular system, have been prepared, and the dependence of the electron-transfer rate constant on the donor− acceptor distance has been studied using such systems.3 Precise tuning of the donor−acceptor distance at the nanometer scale is essential for controlling electron transfer. Electron transfers between two or more semiconductors have been investigated extensively in the donor−acceptor system.4−12 In a common approach for the donor−acceptor system using two semiconductors, one semiconductor is linked with the other semiconductor through the bifunctional molecular linkers, for which each end of the molecule selectively anchors to the donor and acceptor.6 The electrontransfer rate is expected to decrease exponentially with the distance between the donor and the acceptor because of Marcus theory of outer-sphere heterogeneous electron-transfer reactions.7,8 Charge-transfer systems between acceptor and donor semiconductor particles have been synthesized by linking PbS, PbSe, CdSe, and CdTe quantum dots (QDs) to TiO2, ZnO, and SnO2 nanoparticles.9−12 Wang et al. controlled photoinduced electron-transfer processes from semiconductor QDs molecularly bridged to a metal oxide phase by tuning the carbon number of the bridging molecule.12 However, an accurate measurement of the distance between the interface of the two semiconductor particles is impossible because of the © 2014 American Chemical Society

connection of the spherical surface, the electron-transfer rate was discussed using the nominal lengths of the bridge molecules. Two-dimensional nanosheets are available and can form greater interfacial areas between the metal oxides.13 Nanosheets obtained by the exfoliation of a layered metal oxide maintain the properties of the layered metal oxide and develop the novel functionalities. Alternate layered structures can be synthesized by mixing a liquid dispersion of double-hydroxide nanosheets (with positively charged surfaces) with a liquid dispersion of metal oxide nanosheets with negatively charged surfaces.14 Alternate layers of double-hydroxide nanosheets and titanate nanosheets have exhibited high photocatalytic activities for the evolution of O2.15 Although these methods are reliable for synthesizing alternate layered structures, we have no other choice than to choose layered double hydroxides as a cationic nanosheet, in contrast to various choices of anionic nanosheets. Therefore, a challenge remains in establishing a versatile and chemically “soft” methodology for controlling the interlayer distance (i.e., the distance between the alternate layers), and thus for controlling the functionality of the material. We have proposed a new method for creating an alternate layered structure, in which the interlayer distance is controlled at the nanoscale by click chemistry.16 Click chemistry enables the easy generation of stable connections between specific functional groups.17 Simultaneous with our report, heterojunctions between titanium oxides and tungsten oxides Received: August 6, 2014 Revised: September 7, 2014 Published: September 8, 2014 22968

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Figure 1. AFM images of the exfoliated nanosheets from organically modified-layered titanate (a) and tungstate (d). The insets in the AFM images present the cross-sectional height profiles along the dashed line. The distributions of the lateral size (b and e) and the thickness (c and f) of the nanosheets from the organically modified-layered titanate and tungstate, respectively, were determined by counting 100 sheets.

trons in titanium oxide in the layered material can easily be transferred to tungsten oxide when the material has a small interlayer distance.

nanoparticles via click chemistry have been proposed to conjugate metal oxide nanoparticles.18 Click chemistry between two-dimensional nanosheets has been used to form heterojunctions between two types of nanosheets, with large areas and small interlayer distances; the distance between layers is precisely controlled by the size of the functional groups. Our method is a promising method for producing alternate layers with accurate interlayer distances, which should lead to materials with enhanced functionalities. The nanoconjugation of some metal oxides is expected to lead to enhancements and innovations in their photoinduced charge separation abilities. One of the best candidates is a photocatalyst that consists of titanium and tungsten oxides. Tungsten oxide has a conduction band energy that enables the transfer of excited electrons from titanium oxide to it.19,20 Charge separation efficiency and photocatalytic activity have been enhanced using combinations of titanium and tungsten oxides,21−23 and such photocatalysts have been synthesized by the incipient wetness method24 and the sol−gel method,25 among others. The photocatalytic activities between the two metal oxides were affected by the interfacial area at each grain boundary of the two metal oxides. Therefore, the larger interfacial area in the alternate layered structure is the best candidate for the photoinduced charge separation. In this study, titanium oxide−tungsten oxide alternate layers with different interlayer distances were formed by changing the number of carbons in the functional groups used in the click reaction. We quantitatively demonstrate that the distance between titanium oxide and tungsten oxide affects the photoexcited charge separation efficiency. Photoexcited elec-

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EXPERIMENTAL SECTION Synthesis of Titanium Oxide−Tungsten Oxide Alternate Layers. Titanium oxide−tungsten oxide alternate layers were synthesized according to our previous reports.16 Layered titanates (HxTi2‑x/4□x/4O4; □, vacancy; x ≈ 0.7) were reacted with allyltrimethoxysilanes (denoted as C3ene) or (7-octenyl)trimethoxysilanes (C8ene) to immobilize the alkene groups. Layered tungstates (H2W2O7) were reacted with (3-mercaptopropyl)trimethoxysilanes (C3SH) or (11-mercaptoundecyl)trimethoxysilanes (C11SH) to immobilize thiol groups. Organically modified layered metal oxides were ultrasonicated in N,Ndimethylformamide (DMF) and exfoliated into the single nanosheets. The dispersion liquid of the nanosheets was centrifuged at 3000 rpm to remove layered materials which were not exfoliated. Azobis(isobutyronitrile) (AIBN), as a radical reaction initiator, was added in the mixture of the dispersion liquid of the nanosheets. This solution was reacted under nitrogen for 24 h in 80 °C. After the reaction, a yellow powder was collected by centrifugation at 3000 rpm. Four kinds (C3ene + C3SH, C8ene + C3SH, C3ene + C11SH, C8ene + C11SH) of click-reacted samples having different length of interlayer methylene chains (denoted as C6-, C11-, C14-, and C19-TiO2/WO3, respectively) were obtained by combination of organically modified layered metal oxides. Photocatalytic Activities in Decomposition of Methylene Blue. C6-, C11-, C14-, and C19-TiO2/WO3 were 22969

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dispersed into 2 × 10−5 M methylene blue (MB) ethanol solution. After 24 h, the clicked samples adsorbed MB in their interlayers and the color of the samples was changed to blue. For comparison, click-reacted layered titanate and tungstate were synthesized by thiol−ene reaction of the same kinds of nanosheets (denoted as TiO2 and WO3) using the same method. Physical mixture of TiO2 and WO3 (same Ti/W ratio as alternate layers) was also used as a control sample. The samples adsorbing MB were strewn thinly on one side of the double-sided tape; the other side of the tape was put on the glass slides. The samples absorbing MB on the glass slides were irradiated with UV−visible light (λ > 290 nm) through the cutoff filter from a Hg−Xe lamp. Absorbance intensity of MB was measured by diffuse reflectance spectrum, and percentage of degradation was estimated. Infrared Spectroscopies of Conduction Band Electrons in Tungsten Oxides. C6-, C11-, C14-, C19-TiO2/ WO3, or physical mixture were taped strewn thinly on one side of the double-sided tape on the glass slides. The samples were irradiated by the UV−visible light, and the absorbance at the wavelength of 1000 nm was measured for the storage of the photoexcited electrons in the conduction band of the metal oxides. The selected wavelengths of irradiation were 405 and 300 nm. Only tungsten oxide absorbs the light of 405 nm from a LED lamp, and mainly titanium oxide absorbs the light of 300 nm through a bandpass filter from a Hg−Xe lamp. Each intensity of light was controlled to 0.2 mW/cm2. Reduction of Silver Ion Using Stored Electrons in Tungsten Oxides. C6-, C11-, C14-, C19-TiO2/WO3, or control samples (0.01 g) were dispersed in water (20 mL) under ultrasonication. The dispersion liquid was irradiated by the light at the wavelength of 300 nm with the intensity of 0.4 mW/cm2 from an upper side of a light-shielding vessel for 30 min. After the irradiation, AgNO3 aq. solution (100 ppm, 20 mL) was added to the dispersion liquid. The solution was stirred for 30 min under dark conditions. After the dark reaction, the samples were removed by centrifugation, and the supernatant was analyzed by inductively coupled plasma− optical emission spectroscopy (ICP-OES). The powders also reacted with AgNO3 solution without irradiation. The adsorbed amount of Ag+ in the interlayers was estimated from this result without irradiation.

thickness observed between 3 and 4 nm indicated the existence of the small percentage (less than 10%) of the exfoliation of the bilayer structure. Therefore, the organically modified layered titanate and tungstate were exfoliated to the monolayer structure above 90%. The X-ray diffraction (XRD) patterns of the click reaction products are shown in Figure S1a of the Supporting Information. The C6 sample exhibited peaks at 2θ = 5.60 and 11.41° (d = 1.58 and 0.78 nm, respectively), which were assigned to first- and second-order diffraction, characteristics of a layered structure, respectively. The d-values increased with the number of carbon atoms (Figure S1b of the Supporting Information), proving that alternate layered structures were formed. Although we have already reported the click-reacted titanate−tungstate samples with different numbers of carbon atoms,16 the samples in the previous report retained the precursor of tungstate as an impurity. The XRD patterns of the present samples show no (C6 and C14) or negligible (C11 and C18) peaks attributed to the precursor because of the careful treatment in the exfoliation process. The interlayer distances were estimated to be 0.72, 0.94, 1.01, and 1.14 nm, indicating that the click-reacted organic groups were arranged obliquely. Photocatalytic Decomposition in the Interlayer of the Alternate Layered Samples. In alternate multilayers, methylene blue was absorbed between the layers by ion exchange with octylamine.16 The ion-exchange reaction between MB and octylammonium ions remaining in the C6TiO2/WO3 was confirmed by the decrease of the d-value (Figure S2 of the Supporting Information). The photocatalytic activity of the alternate layered samples with MB between the layers was determined by the degradation reaction with MB as a model reaction. Light with a wavelength of 290 nm, or longer, was irradiated to the click-reacted sample and to the organic modified layered metal oxides, and the degradation was evaluated by the diffuse reflection of the absorption spectrum from a decrease in the peak at 670 nm derived from MB. Figure 2 shows the degree of photodegradation of MB plotted against

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RESULTS AND DISCUSSION Synthesis of Titanium Oxide−Tungsten Oxide Alternate Layers. Organically modified layered metal oxides were dispersed in DMF to exfoliate the nanosheets. Figure 1 shows the AFM images of the exfoliated nanosheets of organically modified layered titanate and tungstate. The distributions of the thickness and lateral size in the nanosheets were determined by counting 100 sheets in the atomic force microscopy (AFM) images. The lateral size distributions of the nanosheets decreased to several dozens of nanometers compared with before the exfoliation, suggesting that the nanosheets were ripped by the ultrasonication during the exfoliation. The titanate and tungstate nanosheets have narrow thickness distributions centered at 1.56 and 1.84 nm, respectively. Because the thicknesses of the monolayer nanosheets of titanate26,27 and tungstate28,29 were calculated to be 0.71 and 1.01 nm, respectively, from the crystal structure, the increased thicknesses (0.85 and 0.83 nm) from the inorganic structures were caused by modification of the organic groups of organotrimethoxysilanes and remained octylamine. A greater

Figure 2. Photocatalytic activities of C6 (○), C11 (▽), C14 (*), and C19 (+) click-reacted samples and those of click-reacted layered titanium oxide (Δ), tungsten oxide (□), and a physical mixture of the two (◊).

irradiation time. The alternate layered samples exhibited photocatalytic activities higher than those of TiO2, WO3, and a physical mixture of these metal oxides, and the photocatalytic activities were a sequence from shortest to longest of the number of the carbon atoms in the interlayer in spite of the almost same amounts of carbon in the interlayer spaces (Table S1 of the Supporting Information). The smallest interlayer distance of C6-TiO2/WO3 enhanced the photocatalytic 22970

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the C6-TiO2/WO3. Because the bottom of the conduction band and the top of the valence band for WO3 were lower than those for TiO2,30,31 the photoinduced electrons and holes should transfer from the conduction band of TiO2 to that of WO3 and from the valence band of WO3 to that of TiO2, respectively, under the combined light irradiation. The energy level arrangements and the charge-transfer processes between the WO3 and TiO2 are shown in Figure 4. On the other hand,

activities, which was probably due to the effective charge separation in the alternate layers. The enhancement of photodecomposition in the C6-clicked sample was clarified by changing of the irradiation wavelength (Figure 3). The UV−vis spectrum of the click-reacted sample

Figure 4. Energy diagram and charge-transfer process between the TiO2 and WO3.

the effect of the self-decomposition of MB should be taken into account. The light irradiation experiments of solo 620 nm showed slow photodecompositions compared to those of 300 and 405 nm (Figure 3d). The photodecomposition experiments under the irradiation of solo 620 nm were similar among the titanate, tungstate, and C6-TiO2/WO3. The amounts of photodecomposition under combined lights of 620 and 300 nm (Figure 3e) were almost similar to the summation between the amounts of the photodecompositions under the irradiation of 620 and 300 nm. The results of the photodecomposition under 620 and 405 nm exhibited the same behavior as those of 620 and 300 nm (Figure 3f). These indicated that the enhancement of photodecomposition in the alternate layers was not caused by the effect of the self-decomposition of MB. Charge Separation from the CB of the Titanate to the CB of the Tungstate. We used near-infrared (NIR) spectroscopy to investigate charge separation in the materials produced. Broad NIR spectra were observed for photoexcited metal oxides,22,32,33 and excited electrons stored in the tungsten oxide conduction band (CB) absorbed at wavelengths greater than 600 nm (Figure 5a). On the other hand, excited electrons in the titanium oxide conduction band (−0.5 eV vs NHE) reduced O2 to superoxide radicals (−0.1 eV vs NHE) because this conduction band was more negative than that of tungsten oxide (+0.2 eV vs NHE). Holes were simultaneously consumed by the decomposition of organic chains in the interlayers. In fact, XRD peaks arising from the layered structure were weakened or disappeared after photoirradiation. These results indicate that the organic species in the interlayers decomposed. Figure 5b shows the IR absorbance at 1000 nm plotted against irradiation time. The samples were irradiated at 405 nm for 5 min to excite only tungsten oxide, and absorption by excited electrons increased at the same gradient in all of the samples, indicating that the amount of tungsten oxide in each sample was the same. The samples were then simultaneously irradiated at 405 and 300 nm for another 5 min to excite both titanium oxide and tungsten oxide. The absorption increased at

Figure 3. Photocatalytic activities of C6 (○) click-reacted sample, click-reacted titanium oxide (△), and tungsten oxide (□) with different wavelength of irradiation: (a) 300 nm, (b) 405 nm (c), 300 and 405 nm, (d) 620 nm, (e) 300 and 620 nm, (f) 405 and 620 nm.

was different from that reconstructed on the basis of the weighted average of each solo component according to the compositions of titanate and tungstate (Figure S3 of the Supporting Information), suggesting the electronic structure of the click-reacted sample was different from that of the starting materials. Therefore, this clicked sample was expected to exhibit different photocatalytic activities according to the irradiation wavelength. We chose three irradiation wavelengths of 300, 405, and 620 nm. The titanate and the tungstate absorbed the light below 350 and 450 nm, respectively. From the weighted average, the light at 300 nm was absorbed above 70% by the titanate, and the light at 405 nm was absorbed only by the tungstate. Then, the MB adsorbed the light at only around 620 nm. The combined light irradiation with 300 and 405 nm to C6-TiO2/WO3 greatly enhanced the photodecomposion compared to solo titanates and solo tungstates in Figure 3c, whereas the irradiation of solo 300 and solo 405 nm light showed a slight enhancement (Figure 3a) and no change (Figure 3b), respectively. We considered that this great enhancement was caused by an effective charge separation in 22971

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The alternate layered samples were reduced to a greater extent by Ag, indicating that highly efficient photoinduced electron transfer occurred in them. The electron-transfer efficiency was affected by the interlayer distance; smaller interlayer distances led to higher electron-transfer efficiencies. These results are consistent with the photocatalytic decomposition of MB. The rate constant for photoinduced electron transfer, kPET, is defined in eq 1.34−36 kPET−1 = A exp(βd)

(1)

β is the tunneling decay constant, d the length of the tunneling barrier, and A a pre-exponential factor that is independent of β. In this study, d is the same as the interlayer distance because electron transfer should be a through-space tunneling phenomenon.36 The rate of photoinduced electron transfer (kPET) should be proportional to the amount of electron transfer (et) because electron transfer linearly increases in Figure 5. Considering the discussion above, eq 2 was developed.

ln(et) = −βd + ln A′

(2)

Figure 5. (a) Near-IR absorption by photoirradiated tungsten oxide and (b) absorbance time courses, at 1000 nm, of C6 (○), C11 (▼), C14 (*), and C19 (+) click-reacted samples and a physical mixture of titanium oxide and tungsten oxide (◊). Each sample was irradiated at 405 nm from 0 to 5 min then irradiated at 300 and 405 nm (at the same time) from 5 to 10 min.

Figure 6 shows that there was a good linear relationship between ln et and d, strongly indicating that the rate of electron

gradients in the click-reacted samples higher than those in the physical mixture of TiO2 and WO3, and the sample with the lowest number of carbons (C6-TiO2/WO3) had the highest gradient. This suggests that a narrow interlayer distance allows electrons to be easily transferred from the TiO2 conduction band to the WO3 conduction band. The electrons stored in tungsten oxide were reacted with Ag+ ions so that electron transfer could be quantitatively assessed. Both titanium oxide and tungsten oxide were excited when irradiated at 300 nm, and excited electrons in the titanium oxide conduction band move to the tungsten oxide conduction band. When the interlayer distance is large, the excited electrons in titanium oxide are difficult to transfer to tungsten oxide; therefore, the remaining electrons in titanium oxide that possess the negative potential (−0.5 eV vs NHE) are consumed in reducing O2 dissolved in water. After irradiation, a Ag solution was added to dispersions of the click-reacted samples, and the results of the reduction of Ag ion are summarized in Table 1. A small amount of reduction of Ag ion was detected in

Figure 6. Interlayer distance versus ln et. The solid line is the approximate best-fit line.

transfer was dependent on the interlayer distance. From the gradient, β was estimated to be 0.63 Å−1 in the alternate layer samples. Through-space electron transfer was confirmed by changing the size of the ligands in semiconductors and organometallics,37 with β values of approximately 0.8 Å−1. These results indicate that electrons transfer from titanium oxide to tungsten oxide by through-space tunneling.

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CONCLUSION We have described a new methodology for constructing alternate nanostructure based on the modification of layered metal oxides with alkenyltrimethoxysilanes or alkylthioltrimethoxysilanes and the subsequent click-reaction between alkene and thiol groups on the exfoliate layers. Titanium oxide−tungsten oxide alternate layers which have different interlayer distance were synthesized by changing the carbon number of the functional groups. The alternate layers of narrow interlayer distance showed high photocatalytic activity in decomposition of methylene blue because of the charge separation between titanium oxides and tungsten oxides. Near-infrared spectroscopies and the reduction of Ag ion were investigated to discuss the charge separation. We demonstrated that the efficiency of photoinduced electron transfer depends on interlayer distance. Such behavior is promising for the design of charge separation systems that

Table 1. Results of Reduction of Ag Ion in the Materials Produced and the Amount of Electrons Transferred material

interlayer distance (nm)

C6-TiO2/WO3 C11-TiO2/WO3 C14-TiO2/WO3 C19-TiO2/WO3 TiO2+WO3

0.72 0.94 1.01 1.14 −

amount of electrons transferred, et (electrons/g) 6.81 2.83 1.55 0.44 0.03

× × × × ×

1020 1020 1020 1020 1020

the physical mixture (which was probably attributable to the electrons transferred from TiO2 to WO3 through a physical contact between the TiO2 and WO3 particles in the solution because the stored electron in CB of TiO2 should be quenched by oxygen dissolved in water), indicating that there was marginal electron transfer between the TiO2 and WO3 particles. 22972

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allow for photocatalytic reactions. The present approach can be extended to the use of other layered metal oxides.

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ASSOCIATED CONTENT

* Supporting Information S

The XRD patterns and UV−vis spectra of the samples, and the relationship between absorbance at 1000 nm and the amounts of electrons. 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]. Tel: +81-3-5734-3735. Fax: +81-3-5734-2879. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grant-in-Aid for Scientific Research (A), Grant-in-Aid for Scientific Research (C), and Grant-in-Aid for Young Scientists (B) from MEXT, Japan

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp507941a | J. Phys. Chem. C 2014, 118, 22968−22974