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J. Phys. Chem. C 2009, 113, 6454–6462
Plasmon-Induced Charge Separation and Recombination Dynamics in Gold-TiO2 Nanoparticle Systems: Dependence on TiO2 Particle Size Luchao Du, Akihiro Furube,* Kazuhiro Yamamoto, Kohjiro Hara, Ryuzi Katoh, and M. Tachiya National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: December 2, 2008; ReVised Manuscript ReceiVed: February 19, 2009
To study plasmon-induced charge-transfer mechanisms between excited gold nanoparticles and TiO2, ultrafast visible-pump/infrared-probe femtosecond transient absorption spectroscopy was used and charge separation and recombination dynamics in gold-TiO2 nanoparticle systems were explored. TiO2 nanoparticles of different diameters (9, 20, 30, and 50 nm) were chosen as electron acceptors for gold nanoparticles with a 10 nm diameter. Electron transfer from gold nanoparticles to the conduction band of TiO2 was observed by the transient absorption of electrons in the conduction band of TiO2 at 3440 nm after optical excitation of the surface plasmon band of gold nanoparticles at 550 nm. It was found that electron injection was completed within 50 fs and the electron injection yield reached 20-50%. The charge recombination decay within 1.5 ns was nonexponential and was strongly dependent on the particle diameter of TiO2. Larger TiO2 particles resulted in longer charge recombination times because of the longer diffusion length of electrons in those TiO2 particles. 1. Introduction Solar cells, visible light-sensitive photocatalysts, and biosensors developed on the basis of the surface plasmon resonance properties of noble metal nanoparticles may be promising applications in the future.1-24 Surface plasmon resonance has been actively researched from both scientific and technological viewpoints for decades.25 Spherical gold nanoparticles are easily synthesized1,26,27 and functionalized,28 and nanoparticles of noble metals such as gold and silver exhibit widely varying absorption profiles in the visible light region because of pronounced surface plasmon absorption, which is caused by the collective oscillation of free conductive electrons induced by the electric field of the incident light. For example, the plasmon band of a 10 nm gold nanoparticle solution is located at a wavelength of around 510 nm. Surface plasmons are sensitive to the properties of nanoparticles, such as shape and size.29,30 Shifts in plasmon resonance are caused by changes in the local index of refraction, such as a change induced upon adsorption of molecules on a metal nanoparticle surface. A strong electromagnetic field generated by the free electron oscillation near the metal surface interacts with molecules in the field. This interaction can cause unique enhancement of optical processes, such as fluorescence, Raman scattering,31-33 absorption,34,35 and photocurrent enhancement.36 Recently, the sensitivity of plasmon absorption has been used to investigate the electron dynamics in gold nanoparticles with femtosecond laser pulse excitation.37-42 For example, El-Sayed’s group have used an ultrafast pump-probe technique to study the electron dynamics of gold nanoparticles involved in plasmon oscillations.13,29,38,43-45 Link et al. have reported the influence of a nonequilibrium electron distribution in electron dynamics and the size and shape dependence of the electron-phonon relaxation in gold nanoparticles.45 Ultrafast spectroscopy shows that when gold nanoparticles were excited by laser excitation, the excited gold nanoparticles are relaxed to the ground state * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81-29-861-2953. Fax +81-29-861-5301.
within a subnanosecond domain: excited electrons are first formed above the Fermi level, and then the electronic gas thermalizes internally through electron-electron collisions and electron-phonon coupling before being externally cooled by phonon-phonon coupling. The good electron-storing properties of gold nanoparticles and the particles’ ability to act as electron transfer sites on nanostructures have been proven by electrochemical studies.2,3,7,8 Electron transfer between metals and photoexcited semiconductors, such as that from the conduction band of the semiconductor to a gold nanoparticle, is an important phenomenon in photocatalysis. For example, TiO2 nanoparticles loaded with noble metals have been widely applied in photocatalytic water-splitting reactions.46 Earlier research on semiconductor-metal composites has demonstrated that the deposition of metal on semiconductors can improve the efficiency of photocatalytic redox processes.47-50 Photoinduced electron transfer from gold nanoparticles to semiconductors is also interesting because this reaction offers effective utilization of sunlight owing to the strong plasmon band of gold nanoparticles. When a metal is in contact with a semiconductor, the height of the Schottky barrier is very important. Contact between bulk gold and bulk TiO2 can form a metal-semiconductor Schottky barrier of ∼1.0 eV.51,52 This Schottky barrier can be difficult for electrons to overcome, thus making interfacial charge separation unfavorable.23 The tails of wave functions of the free electrons in metals decay into the semiconductor53 and the possibility of charge transfer from metal to semiconductor, which strongly depends on the barrier height, will be small for tunneling electron transfer. These facts imply that it is difficult to achieve electron transfer from photoexcited gold nanoparticles to semiconductors. Nevertheless, electron transfer in a gold-TiO2 system has been reported by some groups,54,55 although the electron transfer mechanisms have not been fully elucidated. Recently, Tatsuma’s group reported that gold nanoparticles can be used as photosensitizers in place of common dye molecules in dye-sensitized solar cells.55 They have verified that a gold-TiO2 system with an appropriate electron
10.1021/jp810576s CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
Plasmon-Induced Charge Separation
Figure 1. Plasmon-induced charge transfer in the gold-TiO2 nanoparticle system.
donor, in the presence of Fe2+/3+, exhibits sufficiently large photocurrent under excitation of the plasmon band, with a maximum incident-photon-to-current conversion efficiency (IPCE) of 26%; this IPCE is not caused by thermal effects. The group clarified that charge separation can occur by electron transfer from the excited gold particles to TiO2.55 More direct evidence of plasmon-induced electron injection from gold nanoparticles to TiO2 is provided by our previous study, which involved an ultrafast pump-probe technique.56 We used femtosecond time-resolved infrared (IR) probe transient spectroscopy to observe electron injection from 10 nm gold nanoparticle to TiO2 P25 nanoparticles; we verified that free electron transfer occurred in gold-TiO2 systems under the photoexcitation of the plasmon band of gold nanoparticles. We have observed the generation dynamics of free electrons in TiO2, which are well-known to give strong intraband absorption in the IR region.57 We have also found that electron injection from gold nanoparticle to TiO2 nanoparticles occurs within 240 fs with an electron injection yield of about 40%. The charge recombination kinetics within 1 ns indicate that most of the injected electrons finally recombine to gold nanoparticle. We proposed that the decay is due to electron diffusion within TiO2 particles but the detailed mechanisms were not clear.56 To clarify the plasmon-induced charge transfer mechanisms, we carried out the studies described in this report. TiO2 particle diameter is an important physical parameter that influences electron transfer mechanisms. To further clarify plasmon-induced electron transfer mechanisms in gold-TiO2 systems (Figure 1), our present work investigates TiO2 particle size effects. Here, we report the TiO2 particle size effect on electron injection yield and back electron transfer kinetics, as studied by femtosecond time-resolved IR probe transient spectroscopy. Our systems were 10 nm gold nanoparticle loaded with different diameter TiO2 nanoparticles (9, 20, 30, and 50 nm). Transient absorption kinetics probed at 3440 nm were measured following excitation of the plasmon band of gold nanoparticle (550 nm). Within our experimental conditions, there was no excitation intensity effect for the back electron transfer kinetics. Interestingly, we found that the electron injection yields were similar for TiO2 particle diameters from 9 to 50 nm; the efficiency was ∼30-50% and within experimental error. The measured back electron transfer kinetics within 1.5 ns depended strongly on the particle size of TiO2, and nonexponential kinetics were attributed to the diffusion of electrons within TiO2 particles. We explain different charge recombination kinetics by using different diffusion distances of electrons within the TiO2 particles. The plasmon-induced charge separation and recombination mechanisms are also discussed in detail. 2. Experimental Section 2.1. Sample Preparation. To make TiO2 films uniformly loaded with gold spherical nanoparticles (gold-TiO2), a col-
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6455 loidal solution of gold spherical nanoparticles (Sumitomo Osaka Cement) with an average diameter of 10 nm was mixed with TiO2 powders of the following particle diameters: 9 nm, 20 nm (95% TiO2, Ishihara Sangyo, Japan), 30 nm (Catalysis Society of Japan, JRC-TIO-13), and 50 nm (Showa Titanium ST series). The technique we used to prepare the films was doctor blade.58 The detailed procedure of doctor-blade technique is, a drop of the paste we make like above mentioned was first put on one side of the substrate, the substrate we used is the glass, then the paste was spread uniformly on the conducting surface of the substrate by using the smooth glass stick with the draw rate around 2 cm/s. The thickness of the gold-TiO2 film was controlled by adhesive tape (thickness ∼15 µm). The resulting gold-TiO2 films was heated at 450 °C for 1 h to decompose organic molecules between the gold and TiO2. To confirm that there was no aggregation of gold nanoparticle, we mixed the paste homogenously and made that less than one gold nanoparticle was supported on one TiO2 nanoparticle by calculating the number ratio between gold and TiO2. To calculate the electron injection yield, dye-sensitized TiO2 films using a ruthenium complex dye (cis-bis-(4,4′-dicarboxy-2,2′-bipyridine) dithiocyanato ruthenium(II), generally referred to as N3) were chosen as reference samples because these systems have a high electron injection yield (∼100%).59 The N3-TiO2 samples were prepared by immersing bare TiO2 films (9, 20, 30, and 50 nm) in the N3 dye solution and were stored in the dark for 12 h at 25 °C to ensure that the dye was sufficiently adsorbed onto the surfaces of the TiO2 nanoparticles. The N3 dye (Solaronix SA) was used without purification; it was dissolved at a concentration of 0.3 mM in a 50:50 (v/v) mixture of tert-butyl alcohol (Kanto, G grade) and acetonitrile (Kanto, dehydrated). These solvents were used without further purification. The N3 dye is adsorbed strongly onto the TiO2 nanoparticle surface through its carboxy groups.60 The optical transition of the N3-TiO2 films in the visible region is assigned to metal-to-ligand charge transfer, in which an electron is transferred from Ru to one of the bipyridine ligands. The electron injection yield reached 100% for all of the films, as confirmed by very weak phosphorescence compared with that of N3 dye-adsorbed ZrO2 films, for which no electron injection is expected. All of the N3-TiO2 and gold-TiO2 films were 1 cm2 (1 cm × 1 cm) in area and about 10 µm thick. All of the samples were newly prepared before the transient absorption experiments. 2.2. Sample Characterization. To investigate the adsorption of gold nanoparticle on TiO2 after heating at 450 °C for 1 h, transmission electron microscopy (TEM) observation of a gold-TiO2 sample (9 nm TiO2 particle size) was carried out with an OMEGA LEO 922 microscope (CARL ZEISS) operating at 200 kV. The gold-TiO2 sample was scraped off the glass and then ground in an agate mortar to obtain homogeneous powders. The powders were dispersed in ethanol by sonication and dropped on a copper grid coated with a carbon film. TEM images were obtained after the samples had been dried under vacuum for 24 h. 2.3. Femtosecond Transient Absorption Spectrometer. We used ultrafast visible-pump/IR-probe femtosecond transient absorption spectroscopy to measure charge transfer kinetics. Some details of our femtosecond transient absorption spectrometer have been described previously.61 The light source for transient absorption spectroscopy was a femtosecond titanium sapphire laser with a regenerative amplifier (Hurricane, Spectra Physics, 800 nm, 150 fs, 1 mJ/pulse, 1 kHz). The fundamental output of the laser was divided into two beams. One of the beams was used for pump pulse at 550 nm, and another was
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Figure 2. Schematic top view of the integrating sphere used.
used for probe pulse at 3440 nm. To obtain 550 nm light, an 800 nm beam was introduced into an optical parametric amplifier (OPA; TOPAS, Quantronix) with a mixing crystal. The IR (3440 nm) probe beam was obtained by introducing the 800 nm beam into another OPA (TOPAS, Quantronix) with a differential frequency generation crystal. The pump light entered the decay stage to change the pulse distance between pump and probe beams, thus achieving a time delay. We used a chopper to control pump-on and -off. The probe beam was divided into two beams: one beam passed through the sample, and another beam acted as a reference for compensating the intensity fluctuation of each pulse. The pump beam radius on the film surfacewasabout0.36mm.Signalsfrommercury-cadmium-telluride (MCT, Hamamatsu, P3257-10) photodetectors were gated and acquired. In this case, an IR bandpass filter was placed before the MCT detector to efficiently remove IR emission from the excited gold nanoparticle. Transient absorption intensity was calculated from the pulse intensity of the probe with and without excitation, typically using thousands of pulses. The gold-TiO2 and N3-TiO2 films were placed in air during the transient absorption measurements. The samples were scanned to avoid the effect of slight degradation of N3 dye and gold nanoparticle during the experiments. All of the transient absorption measurements were performed at 295 K. 2.4. Steady-State Spectrometer. The samples we prepared have strong scattering properties under visible light irradiation. A spectrophotometer (Shimadzu, MPC-3100) equipped with an integrating sphere was used to accurately obtain the absorption fraction for each sample. For analysis, incident light with intensity I0 entered the integrated sphere through the entrance window. First, the samples were placed in front of the sphere to measure the transmission fraction (FT) for each gold-TiO2 and N3-TiO2 sample. FT was obtained from the ratio of transmitted light, IT, to I0 (eq 1). Then, the samples were placed behind the sphere to obtain the reflection fraction (FR) for each gold-TiO2 and N3-TiO2 sample. FR was obtained from the ratio of reflected light, IR, to I0 (eq 2). The absorption fraction FA then was calculated from FT and FR (eq 3), because the sum of FA, FT, and FR is 100%. Figure 2 presents a schematic top view of the integrating sphere.
FT)IT/I0
(1)
FR)IR/I0
(2)
FA)1 - (FR+FT) ) 1 - (IT/I0) - (IR/I0)
(3)
Then
3. Results and Discussion 3.1. Optical Characterization. Figure 3 shows FA as function of wavelength for the gold-TiO2 and N3-TiO2 systems,
in which the diameter of TiO2 was 9, 20, 30, or 50 nm. For gold-TiO2 systems, the plasmon band peaks of the gold nanoparticles occurred at around 550 nm and were somewhat red-shifted compared with the plasmon band of the gold nanoparticle solution (511 nm); this red-shift was attributed to the high refractive index of TiO2.62 The slight differences in the plasmon absorption peaks of different samples might have been caused by the different effective refractive indices of the TiO2 films prepared from particles of different diameters. For N3-TiO2 systems, the absorption band peaks at around 540 nm were attributed to metal-to-ligand charge transfer absorption of N3 dye. For the N3-TiO2 and gold-TiO2 films, FA values decreased when the TiO2 particle diameter increased because a smaller amount of gold solution was used when preparing the larger TiO2 particle (i.e., smaller specific surface area) films, to obtain the same particle number ratio of TiO2 to gold in every film. For N3-TiO2 films, the larger diameter TiO2 particles absorbed smaller numbers of N3 dye molecules because of the smaller specific surface area of the TiO2 particles. On the basis of these results, we chose an excitation wavelength of 550 nm for transient absorption measurements (Figure 3). For accurate analysis of transient absorption data, the number of absorbed photons in the films is an important parameter for evaluating the electron injection yield. The scattering and reflective properties of the films can decrease the absorption of photons. The transient absorption intensities for all the N3-TiO2 and gold-TiO2 samples were corrected by using experimental transient intensity divided by the corresponding FA, as described in detail herein. 3.2. TEM Characterization. Figure 4 is a TEM micrograph of a gold-TiO2 (9 nm) sample (Figure S1 of the Supporting Information for other images), demonstrating that there was one gold nanoparticle attached on per single TiO2 nanoparticle. The gold nanoparticles were spherical, with a diameter of almost 10 nm. Because the proportion of aggregated gold particles was minor, we assumed that aggregation did not influence our transient absorption measurements. The TiO2 nanoparticles were also spherical with diameters of nearly 9 nm and there was no interconnection. 3.3. Electron Injection in 9 nm TiO2 Loaded with Gold Nanoparticle. In our previous report, by using femtosecond IR probe transient absorption spectroscopy, optical excitation of the 550 nm plasmon band of the gold nanoparticles was found to induce electron injection into the conduction band of Degussa P25 TiO2 nanoparticles, and the electron injection process occurred within 240 fs (the time resolution of our apparatus).56 Here, a more precise experiment was performed to resolve the rise profile of transient absorption caused by injected electrons. A silicon plate was used to obtain the response of our apparatus because when silicon was excited at 550 nm electrons and holes are formed instantaneously. The dynamics of silicon gave a rapid rise, which was used to obtain the time resolution of the apparatus (Figure 5). Figure 5 also shows the risetime of gold-TiO2 (9 nm) kinetics. By comparing this risetime with the risetime of the silicon plate, we determined that the absorption rise of gold-TiO2 (9 nm) was slower than that of the silicon plate. We used convolution analysis to determine the exact electron injection time for the gold-TiO2 (9 nm) system; in this analysis, the reaction was assumed to be single exponential. Among the 10, 50, and 100 fs convolution rise profiles, the risetime of gold-TiO2 (9 nm) fit best with the 50 fs convolution rise profile. These fitting results indicated that the electron injection time from a gold nanoparticle to 9 nm TiO2 was 50 fs or faster. The electron injection time might be
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Figure 3. FA as a function of wavelength in the gold-TiO2 (9, 20, 30, and 50 nm) and N3-TiO2 (9, 20, 30, and 50 nm) systems.
Figure 6. Electron injection dynamics of gold-TiO2 (30 nm) and N3-TiO2 (30 nm) systems.
Figure 4. TEM micrograph of a gold-TiO2 (9 nm) sample.
Figure 5. Rise time of dynamics after convolution analysis of the gold-TiO2 (9 nm) system and for Si, both with excitation at 550 nm and probing at 3440 nm. The right axis corresponds to the gold-TiO2 (9 nm) system.
faster than 50 fs because the 50 fs risetime might be due to the relaxation of injected electrons in the TiO2 conduction band. It is known that the dynamics of photoexcited gold nanoparticles give three representative time constants, which are respectively assigned to relaxation of electrons with a non-Fermi distribution through electron-electron scattering to the Fermi electron distribution (
