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Electron transfer dynamics and yield from gold nanoparticle to different semiconductors induced by plasmon band excitation
Chemical Physics Letters 701 (2018) 126–130
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Electron transfer dynamics and yield from gold nanoparticle to different semiconductors induced by plasmon band excitation L.C. Du a,⇑, W.D. Xi a, J.B. Zhang b, H. Matsuzaki c, A. Furube c,d,⇑ a
Institute of Atomic and Molecular Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China c National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8565, Japan d Department of Optical Science, Tokushima University, 2-1, Minamijosanjima-cho, Tokushima 770-8506, Japan b
a r t i c l e
i n f o
Article history: Received 24 October 2017 In final form 17 April 2018 Available online 18 April 2018
a b s t r a c t Photoinduced electron transfer from gold nanoparticles (NPs) to semiconductor under plasmon excitation is an important phenomenon in photocatalysis and solar cell applications. Femtosecond plasmoninduced electron transfer from gold NPs to the conduction band of different semiconductor like TiO2, SnO2, and ZnO was monitored at 3440 nm upon optical excitation of the surface plasmon band of gold NPs. It was found that electron injection was completed within 240 fs and the electron injection yield reached 10–30% under 570 nm excitation. It means TiO2 is not the only proper semiconductor as electron acceptors in such gold/semiconductor nanoparticle systems. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon resonance (SPR) has been widely studied in the surface science field since R. H. Ritchie had done the initiating work in 1957 [1]. SPR is a wave of collective motion of numerous conductive electrons induced by the electric field of incident light, which are sensitive to the condition of circumstance such as the shape and size of the metal and the surrounding refractive index [2,3]. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light gives rise to enhancement of fluorescence, Raman scattering light, secondharmonic light from absorbed molecules. The plasmon features of the gold nanoparticle have many applications such as photocatalysis and solar cell devices [4–6] due to the stability of gold nanoparticles and charge transfer between gold nanoparticles and contacting semiconductors. It is already well-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 to the Fermi electron distribution through electron– electron scattering (<100 fs), cooling of hot electron through electron–phonon scattering (1–10 ps), and heat dissipation from the gold nanoparticles to the environment through phonon–phonon scattering (100 ps) [7–10]. Considering these dynamics, only ⇑ Corresponding authors at: Institute of Atomic and Molecular Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China (L.C. Du). E-mail addresses: [email protected] (L.C. Du), [email protected] (A. Furube). https://doi.org/10.1016/j.cplett.2018.04.043 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.
electrons with high energy before deactivating through electron– electron scattering (less than 100 fs) seem to have a chance of electron transfer going across the surface of gold nanoparticle. Efficient electron transfer, however, may occur in such a short time if electronic coupling with the electron acceptor is large enough. Contact between gold and semiconductor can form a metal– semiconductor Schottky junction [11,12], which possibly blocks the charge separation because electrons are difficult to go across the interface. The interaction between gold and semiconductor can be influenced by the plasmon-induced strong electronic field upon photo-excitation. More than a decade ago, Y. Tian and T. Tatsuma proposed the mechanism that the charge separation was accomplished by electron transfer from the photo-excited gold particles to TiO2 [13]. In our previous study using femtosecond spectroscopy, electron injection from the 10 nm gold nanoparticle to TiO2 nanoparticle (P25) had been directly observed [14]. We have proved plasmon excitation induced free electron transfer occurred in gold/TiO2 systems by using femtosecond IR probe transient spectroscopy. We found electron injection process was completed within 50 fs and the yield was about 40%. But the electron injection mechanisms are still not fully clear. TiO2 usually works as a very excellent electron acceptor because TiO2 has a larger density of states in the conduction band due to the d-orbital nature, differing from the other typical metal oxides such as ZnO and SnO2, whose conduction band basically is composed of the s or sp orbitals of metal atoms. To clarify the electron injection mechanism, three types of different semiconductors (TiO2, SnO2, and ZnO) are used as electron acceptor, respectively.
L.C. Du et al. / Chemical Physics Letters 701 (2018) 126–130
Here we report that plasmon-induced electron transfer dynamics in gold nanoparticle loaded with different semiconductor systems of TiO2, SnO2, and ZnO. Transient absorption kinetics up to 1 ns in the IR range were measured by exciting plasmon band of gold nanoparticles located at 570 nm. Electron injection yields for different gold/semiconductor systems were also studied. The transient absorption kinetics probed at 3440 nm reflected that the free electrons are injected into the conduction band of these semiconductors, clearly indicating that TiO2 is not the only suitable semiconductor in such metal–semiconductor systems showing lightenergy conversion. 2. Experimental section 2.1. Sample preparation Bare TiO2 (99.8%TiO2, anatase, Aladdin, China) and SnO2 (AlfaAesar, China) NP films with the average diameter of 10 nm were prepared by respectively mixing the powders with two droplets Triton and a certain amount of acetylacetone using the doctor blade method. ZnO (99.9% ZnO, Cw-nano, Shanghai) NP film with the average diameter of 30 nm was produced by electric deposition method. All samples were made by heating at 450 °C for 1 h. Aumodified procedure was done after heating, and the above heated samples were put into a beaker in 100 mL 0.001 M HAuCl4 and 1 mL CH3OH, then irradiated with a UV lamp for 30 min. Dyesensitized TiO2, SnO2 and ZnO films using an efficient sensitizer Ru-complex dye (N719) were prepared as the reference samples with 100% electron injection efficiency to the semiconductor NP, by immersing a bare TiO2, SnO2 or ZnO film in a N719 dye solution for 12 h. The molecular structure of N719 dye is shown in Fig. 1. 2.2. Femtosecond transient absorption spectrometer Transient absorption kinetics were measured by the femtosecond VIS-pump/IR-probe transient absorption spectrometer based on an amplified Ti: sapphire laser system (1 kHz repetition frequency, 800 nm center wavelength, 150 fs pulse width) combined with two optical parametric amplifiers. The pump beam radius on the film surface was about 0.36 mm. The details of our spectrometer had been described previously [15,16]. The gold/semiconductor NP and N719/semiconductor NP films were placed in air and stable enough during the transient absorption measurements. The samples were mechanically scanned to avoid the degradation of N719 dye and gold nanoparticle due to pump light irradiation during the experiments. All the transient absorption measurements were performed at 295 K. 2.3. Steady state spectrometer The prepared samples scatters visible light strongly. A spectrophotometer (Shimadzu, MPC-3100) equipped with an integrat-
Fig. 1. The molecular structure of N719 dye.
127
ing sphere was used to accurately obtain the absorption fraction for each sample. Details of the procedure are described as below. An incident light with intensity I0 enters the integrated sphere through the entrance window. First the samples were placed before the sphere to measure the transmission fraction (FT) for each gold/semiconductor and N719/semiconductor sample. FT can be obtained from the ratio of transmitted light IT to the incident light I0 (Eq. (1)). Then the sample was placed behind the sphere to obtain the reflection fraction (FR) for each gold/semiconductor and N719/semiconductor sample. FR can be obtained from the ratio of reflected light IR to the incident light I0 (Eq. (2)). So the absorption fraction FA can be calculated from FT and FR (Eq. (3)) because the total of FA, FT and FR is 100%.
FT ¼ IT =I0
ð1Þ
FR ¼ IR =I0
ð2Þ
FA ¼ 1 ðFR þ FT Þ ¼ 1 IT =I0 IR =I0
ð3Þ
3. Results and discussion 3.1. Optical characterization Fig. 2(a,b,c) show each FA as a function of wavelength for the TiO2, Au/TiO2, N719/TiO2, SnO2, Au/SnO2, N719/SnO2, ZnO, Au/ ZnO, and N719/ZnO systems. For Au/TiO2 system, the absorption band at 550 nm originating from the plasmon band of gold nanoparticles were observed. The FA value was close to 0.8 due to strong absorption of gold nanoparticles. For Au/SnO2 system, the FA value was close to 0.4 due to weaker absorption of gold nanoparticles at 550 nm. For Au/ZnO system, the FA value was close to 0.35 due to weaker absorption of gold nanoparticles at 515 nm. For N719/TiO2 and N719/SnO2 systems, the FA values for the absorption band peak at around 530 nm were also close to 0.6–0.8. For N719/ZnO system, the FA values for the absorption band peak at around 515 nm were close to 0.4. The plasmon band peak of different semiconductor loaded with gold nanoparticle is slightly different each other due to the particle size effect of gold, different refractive index and reflection characteristics of semiconductors. It is known that large refractive index and large Au diameter make red-shift to the peak wavelength. The band peak of bare TiO2, SnO2, ZnO absorption give the value at around 380 nm, 369 nm, 360 nm. We chose 570 nm light as a pump pulse to excite the plasmon band of gold nanoparticle. 3.2. Electron injection in different semiconductor systems Transient absorption signals of Au/TiO2, N719/TiO2, bare TiO2, Au/SnO2, N719/SnO2, bare SnO2 and Au/ZnO, N719/ZnO, bare ZnO systems observed at 3440 nm upon excitation at 570 nm were shown in Fig. 3(a,b,c). All amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm. At this probe wavelength, absorption due to free electrons injected from gold nanoparticle into the conduction band of TiO2, SnO2 or ZnO can be monitored. Bare TiO2, SnO2 or ZnO show no transient absorption signals due to no absorption at 570 nm (Fig. 2), the excitation intensity at the sample point is 1mw. For Au/semiconductor systems, we found the electron injection process was completed within less than 240 fs (the time resolution of our apparatus) [14] and following was the decay process. From the data, There may be a small rising component after response rise. This might be relaxation of injected electrons as we discussed in our previous work [16,17]. The following decay was assigned to the back electron transfer from the semiconductor to gold nanoparticle. The
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Fig. 2. (a), (b), (c) FA as a function of the wavelength for TiO2, Au/TiO2, N719/TiO2, SnO2, Au/SnO2, N719/SnO2, ZnO, Au/ZnO, and N719/ZnO systems.
black dots in Fig. 3(a) are the electron injection dynamics for N719/ TiO2 film. There was no decay observed up to 1000 ps. A plateau appeared after 300 ps, and it can be assumed that it gives the 100% electron injection efficiency [18]. The black dots in Fig. 3(b) are the electron injection dynamics for N719/SnO2 film. It gives a plateau at 300 ps, and then showed a decay. The black dots in Fig. 3(c) are the electron injection dynamics for N719/ZnO film, giving the 100% electron injection efficiency [19]. We assume that dye/metal oxide systems give 100% electron injection efficiency. This assumption comes from former study in Katoh et al. group
Fig. 3. (a), (b), (c) The transient absorption signals for Au/TiO2, N719/TiO2, bare TiO2, Au/SnO2, N719/SnO2, bare SnO2, Au/ZnO, N719/ZnO and bare ZnO systems observed at 3440 nm under excitation at 570 nm. All amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm.
[19] where N3 (basically same as N719) sensitized TiO2, ZnO, and SnO2 gave efficiencies more than 80%. The electron injection yield of different gold/semiconductor systems were evaluated by comparing transient absorption by using the corresponding N719/ semiconductor system as reference, where all amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm. The ratio of transient absorption intensity directly corresponds to the ratio of the electron injection yields of these
L.C. Du et al. / Chemical Physics Letters 701 (2018) 126–130
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two systems. Table 1 shows the calculated electron injection yield for Au/TiO2, Au/SnO2 and Au/ZnO systems. The electron injection yield for the Au/TiO2 system is similar to that for Au/SnO2 system, and smaller than that for Au/ZnO system. They gave not so different electron injection yield. As we know, ZnO is a metal oxide having similar energy levels of the conduction and valence bands to those of TiO2, but having smaller density of states of conduction band to that of TiO2, so contrary result was expected to the experimental result [15]. Au/TiO2 system and Au/SnO2 system have the similar electron injection yield, although the conduction band energy level of SnO2 is lower than that of TiO2. The similar electron injection yields may be due to the smaller density of states of conduction band of SnO2 than that of TiO2 [15]. We think effect of surface states as electron acceptor may be involved. From the work of C.R. Crowell and S.M. Sze [20], the electron transfer yield from metal to semiconductor can be as a function of the excess energy of barrier. For hot electron transfer (occurring in <100 fs in our case), the yield is not so sensitive to the effective mass of semiconductor. Thus our result indicated that different semiconductors with different DOS due to their different electron effective masses gave not so different electron injection efficiency. Surface state effect is not the only reason that the yield did not change so much. Both the effects (surface states and hot electron transfer) resulted in our results (not so different electron injection efficiency).
3.3. Charge recombination in different semiconductor systems Fig. 4 shows the normalized transient absorption kinetics of gold assembled with different semiconductor TiO2, SnO2, ZnO systems monitored at 3440 nm under excitation at 570 nm. The experiments are done at the same excitation intensity and experimental conditions. From the Fig. 4, we can see there are differences in the charge recombination time for Au/TiO2, Au/SnO2 and Au/ZnO systems. We checked the excitation power dependence for these gold-semiconductor systems separately to see if there was the second-order recombination process. The data are shown in Fig. 5. The range of 570-nm laser intensity was chosen as 0.5mw
Table 1 The electron injection yield of Au/TiO2, Au/ SnO2, Au/ZnO systems. Sample
Electron injection yield(%)
Au/TiO2 Au/SnO2 Au/ZnO
17 14 26 Fig. 5. Transient absorption decay profiles of different gold–semiconductor films with different excitation intensities, excited at 570 nm and probed at 3440 nm.
Fig. 4. The normalized transient absorption signals for Au/TiO2, Au/SnO2, and Au/ ZnO systems observed at 3440 nm under excitation at 570 nm, the green curve shows the fitting smooth curve, the number in brackets means the multiple time to normalize the kinetics.
and 1mw at the sample point, because the largest 570-nm output through the OPA in our apparatus was nearly 1.2mw at the sample point, and a desirable signal to noise ratio transient absorption signal was difficult to obtain when the laser intensity was very low. Within experimental error, we found that the transient absorption decay time for each gold–semiconductor film remained same with changes in 570-nm laser intensity, and the amplitude of the transient absorption signals for every gold–semiconductor system was almost proportional to the excitation light intensity. These experimental results suggest that there was no nonlinear process involved in the excitation and also no second-order recombination process. It means only one gold nanoparticle was assembled with one semiconductor nanoparticles as our previous studies [16], and therefore the injected electrons in semiconductor simply recombined with the holes in the original gold nanoparticle. There are many factors that can affect the charge recombination process,
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L.C. Du et al. / Chemical Physics Letters 701 (2018) 126–130 Table 2 Estimated values of s1/e for Au/TiO2, Au/SnO2 and Au/ZnO systems. Semiconductor (diameter)
s1/e (ps)
TiO2 (10 nm) SnO2 (10 nm) ZnO (30 nm)
6.2 461 144
Acknowledgment This work is supported by the Natural Science Foundation of China (grant 21003155, grant 11674125), Key project of National Natural Science Foundation of China (grant 20933010), and Jilin Provincial Department of Education Program (JJKH20170777KJ). References
like the size of gold nanoparticle, the size of semiconductor, the diffusion coefficient of electron in semiconductors [16,17], and so on. Lian’s group [21] also reported that the electron recombination dynamics depend on the sample preparation method, which was due to effect of the trap state density. Nelson et al. [22] suggested charge recombination kinetics depend on both electrons trapping/ detrapping dynamics and the rate of electron transfer on the trap site on the semiconductor. In our previous work, we reported that the charge recombination dynamics depend on the size of semiconductor nanoparticle and can be correlated with the diffusion of electrons [16,17]. We found larger particle size and smaller diffusion constant make the charge recombination time longer. Tachiya proposed the diffusion models of reactions on micelles and vesicles in 1987 [23]. This model is for diffusion-controlled reactions of micelle systems and actually is similar to our case, where it is assumed that one molecule is confined within the micelle interior and another one is fixed on the micelle systems. When the molecule in the micelle interior reaches the surface and contacts with another molecule fixed on the surface, the reaction occurs. The mean reaction time of the two molecules holds a relation: s = 1.37 r3/(Dd), where s is the reaction time, r is the radius of the micelles, D is the diffusion coefficient of the molecule in the micelle interior and d is twice the reaction radius (or the reaction diameter). We regard the semiconductor particle surface as the micelle; the injected electron will diffuse within the semiconductor interior and the recombination process will occur at the semiconductor surface. Then we can estimate the charge recombination time by considering the size of semiconductor and diffusion constant of electrons in semiconductor. We analyzed the transient absorption kinetics of Au/TiO2, Au/ SnO2, and Au/ZnO systems. Here, we define the charge recombination time as the time at which the transient signal is equal to 1/e of the maximum value of the signal (s1/e). Estimated values of s1/e are shown in Table 2. As can be seen from Table 2, the charge recombination times (s1/e) of Au/ZnO and Au/SnO2 systems are longer than that of Au/TiO2 system. The charge recombination process can be correlated with the electron diffusion as mentioned above. Therefore, when charge recombination time is longer, the electron diffusion time is longer. The different diffusion coefficient of electron in semiconductors may affect the diffusion time. The different diffusion coefficient for different semiconductor is DTiO2 = 3 105 m2 s1, DSnO2 = 109 m2 s1 [24], and DZnO = 107 m2 s1 [25]. Smaller reported diffusion constants for SnO2 and ZnO as well as the larger diameter of ZnO may be the reason of the observed much slower recombination than TiO2.
4. Conclusion Plasmon induced electron injection from Au NP to different metal oxides (TiO2, SnO2, and ZnO) was observed directly by using IR probe femtosecond transient absorption spectroscopy. The injection efficiencies and the lifetimes (s1/e) were evaluated. SnO2 and ZnO are proved to be efficient electron acceptor from Au NP besides TiO2, even with their small conduction band DOS.
[1] R.H. Ritchie, Plasma losses by fast electrons in thin films, Phys. Rev. 106 (1957) 874–881. [2] S. Link, M.A. El-Sayed, Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods, J. Phys. Chem. B. 103 (1999) 8410–8426. [3] P.K. Jain, K.S. Lee, I.H. El-Sayed, M.A. El-Sayed, Calculated absorption and scattering properties of gold nanoparticles of different size, shape, and composition: applications in biological imaging and biomedicine, J. Phys. Chem. B. 110 (2006) 7238–7248. [4] E.W. McFarland, J. Tang, A photovoltaic device structure based on internal electron emission, Nature 421 (2003) 616–618. [5] L.B. Mark, J.H. Naomi, N. Peter, Plasmon-induced hot carrier science and technology, Nat. Nanotech. 10 (2015) 25–34. [6] C. César, Plasmon-induced hot-electron generation at nanoparticle/metaloxide interfaces for photovoltaic and photocatalytic devices, Nat. Photon. 8 (2014) 95–103. [7] A. Wood, M. Giersig, P. Mulvaney, Fermi level equilibration in quantum dotmetal nanojunctions, J. Phys. Chem. B 105 (2001) 8810–8815. [8] J. Zhou, J. Ralston, R. Sedev, D.A. Beattie, Functionalized gold nanoparticles: synthesis, structure and colloid stability, J. Colloid Interface Sci. 331 (2009) 51– 262. [9] A.J. Bard, M.A. Fox, Artificial photosynthesis: solar splitting of water to hydrogen and oxygen, Acc. Chem. Res. 28 (1995) 141–145. [10] S. Link, A. Furube, M.B. Mohamed, T. Asahi, H. Masuhara, M.A. El-Sayed, Hot electron relaxation dynamics of gold nanoparticles embedded in MgSO4 powder compared to solution: the effect of the surrounding medium, J. Phys. Chem. B 106 (2002) 945–955. [11] M. Jakob, H. Levanon, P.V. Kamat, Charge distribution between UV-irradiated TiO2 and gold nanoparticles: determination of shift in the fermi level, Nano Lett. 3 (2003) 353–358. [12] D.M. Schaadt, B. Feng, E.T. Yu, Enhanced semiconductor optical absorption via surface plasmon excitation in metalnanoparticles, Appl. Phys. Lett. 86 (2005) 0631061–0631063. [13] Y. Tian, T. Tatsuma, Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles, J. Am. Chem. Soc 127 (2005) 7632–7637. [14] A. Furube, L.C. Du, K. Hara, R. Katoh, M. Tachiya, Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles, J. Am. Chem. Soc 129 (2007) 14852–14853. [15] A. Furube, R. Katoh, T. Yoshihara, K. Hara, S. Murata, H. Arakawa, M. Tachiya, Ultrafast direct and indirect electron-injection processes in a photoexcited dye-sensitized nanocrystalline zinc oxide film: the importance of exciplex intermediates at the surface, J. Phys. Chem. B. 108 (2004) 12583–12592. [16] L.C. Du, A. Furube, K. Yamamoto, K. Hara, R. Katoh, M. Tachiya, Plasmoninduced charge separation and recombination dynamics in gold-TiO2 nanoparticle systems: dependence on TiO2 particle size, J. Phys. Chem. C 113 (2009) 6454–6462. [17] L.C. Du, A. Furube, K. Hara, R. Katoh, M. Tachiya, Ultrafast plasmon induced electron injection mechanism in gold–TiO2 nanoparticle system, J. Photochem. Photobio. C: Photochem. Rev. 15 (2013) 21–30. [18] J.B. Asbury, E. Hao, Y.Q. Wang, H.N. Ghosh, T.Q. Lian, Ultrafast electron transfer dynamics from molecular adsorbates to semiconductor nanocrystalline thin films, J. Phys. Chem. B. 105 (2001) 4545–4557. [19] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H. Arakawa, M. Tachiya, Efficiencies of electron injection from excited N3 dye into nanocrystalline semiconductor (ZrO2, TiO2, ZnO, Nb2O5, SnO2, In2O3) films, J. Phys. Chem. B 108 (2004) 4818–4822. [20] C.R. Crowell, S.M. Sze, Quantum-mechanical reflection of electrons at metalsemiconductor barriers: electron transport in semiconductor-metalsemiconductor structures, J. Appl Phys 37 (1966) 2683–2689. [21] E. Hao, N.A. Anderson, J.B. Asbury, T. Lian, Effect of trap states on interfacial electron transfer between molecular absorbates and semiconductor nanoparticles, J. Phys. Chem. B 106 (2002) 10191–10198. [22] J. Nelson, S.A. Haque, D.R. Klug, J.R. Durrant, Trap-limited recombination in dye-sensitized nanocrystalline metal oxide electrodes, Phys. Rev. B 63 (2001) 205321–205330. [23] M. Tachiya, in: Kinetics of Nonhomogeneous Processes, John Wiley & Sons, Chichester, 1987, p. 575. [24] Y. Fukai, Y. Kondo, S. Mori, Highly efficient dye-sensitized SnO solar cells having sufficient electron diffusion length, Electrochem. Commun. 9 (2007) 1439–1443. [25] E.A. Meulenkamp, Electron transport in nanoparticulate ZnO films, J. Phys. Chem. B 103 (1999) 7831–7838.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Electron transfer dynamics and yield from gold nanoparticle to different semiconductors induced by plasmon band excitation L.C. Du a,⇑, W.D. Xi a, J.B. Zhang b, H. Matsuzaki c, A. Furube c,d,⇑ a
Institute of Atomic and Molecular Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China c National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8565, Japan d Department of Optical Science, Tokushima University, 2-1, Minamijosanjima-cho, Tokushima 770-8506, Japan b
a r t i c l e
i n f o
Article history: Received 24 October 2017 In final form 17 April 2018 Available online 18 April 2018
a b s t r a c t Photoinduced electron transfer from gold nanoparticles (NPs) to semiconductor under plasmon excitation is an important phenomenon in photocatalysis and solar cell applications. Femtosecond plasmoninduced electron transfer from gold NPs to the conduction band of different semiconductor like TiO2, SnO2, and ZnO was monitored at 3440 nm upon optical excitation of the surface plasmon band of gold NPs. It was found that electron injection was completed within 240 fs and the electron injection yield reached 10–30% under 570 nm excitation. It means TiO2 is not the only proper semiconductor as electron acceptors in such gold/semiconductor nanoparticle systems. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Surface plasmon resonance (SPR) has been widely studied in the surface science field since R. H. Ritchie had done the initiating work in 1957 [1]. SPR is a wave of collective motion of numerous conductive electrons induced by the electric field of incident light, which are sensitive to the condition of circumstance such as the shape and size of the metal and the surrounding refractive index [2,3]. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light gives rise to enhancement of fluorescence, Raman scattering light, secondharmonic light from absorbed molecules. The plasmon features of the gold nanoparticle have many applications such as photocatalysis and solar cell devices [4–6] due to the stability of gold nanoparticles and charge transfer between gold nanoparticles and contacting semiconductors. It is already well-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 to the Fermi electron distribution through electron– electron scattering (<100 fs), cooling of hot electron through electron–phonon scattering (1–10 ps), and heat dissipation from the gold nanoparticles to the environment through phonon–phonon scattering (100 ps) [7–10]. Considering these dynamics, only ⇑ Corresponding authors at: Institute of Atomic and Molecular Physics, Jilin University, 2699 Qianjin Street, Changchun 130012, China (L.C. Du). E-mail addresses: [email protected] (L.C. Du), [email protected] (A. Furube). https://doi.org/10.1016/j.cplett.2018.04.043 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.
electrons with high energy before deactivating through electron– electron scattering (less than 100 fs) seem to have a chance of electron transfer going across the surface of gold nanoparticle. Efficient electron transfer, however, may occur in such a short time if electronic coupling with the electron acceptor is large enough. Contact between gold and semiconductor can form a metal– semiconductor Schottky junction [11,12], which possibly blocks the charge separation because electrons are difficult to go across the interface. The interaction between gold and semiconductor can be influenced by the plasmon-induced strong electronic field upon photo-excitation. More than a decade ago, Y. Tian and T. Tatsuma proposed the mechanism that the charge separation was accomplished by electron transfer from the photo-excited gold particles to TiO2 [13]. In our previous study using femtosecond spectroscopy, electron injection from the 10 nm gold nanoparticle to TiO2 nanoparticle (P25) had been directly observed [14]. We have proved plasmon excitation induced free electron transfer occurred in gold/TiO2 systems by using femtosecond IR probe transient spectroscopy. We found electron injection process was completed within 50 fs and the yield was about 40%. But the electron injection mechanisms are still not fully clear. TiO2 usually works as a very excellent electron acceptor because TiO2 has a larger density of states in the conduction band due to the d-orbital nature, differing from the other typical metal oxides such as ZnO and SnO2, whose conduction band basically is composed of the s or sp orbitals of metal atoms. To clarify the electron injection mechanism, three types of different semiconductors (TiO2, SnO2, and ZnO) are used as electron acceptor, respectively.
L.C. Du et al. / Chemical Physics Letters 701 (2018) 126–130
Here we report that plasmon-induced electron transfer dynamics in gold nanoparticle loaded with different semiconductor systems of TiO2, SnO2, and ZnO. Transient absorption kinetics up to 1 ns in the IR range were measured by exciting plasmon band of gold nanoparticles located at 570 nm. Electron injection yields for different gold/semiconductor systems were also studied. The transient absorption kinetics probed at 3440 nm reflected that the free electrons are injected into the conduction band of these semiconductors, clearly indicating that TiO2 is not the only suitable semiconductor in such metal–semiconductor systems showing lightenergy conversion. 2. Experimental section 2.1. Sample preparation Bare TiO2 (99.8%TiO2, anatase, Aladdin, China) and SnO2 (AlfaAesar, China) NP films with the average diameter of 10 nm were prepared by respectively mixing the powders with two droplets Triton and a certain amount of acetylacetone using the doctor blade method. ZnO (99.9% ZnO, Cw-nano, Shanghai) NP film with the average diameter of 30 nm was produced by electric deposition method. All samples were made by heating at 450 °C for 1 h. Aumodified procedure was done after heating, and the above heated samples were put into a beaker in 100 mL 0.001 M HAuCl4 and 1 mL CH3OH, then irradiated with a UV lamp for 30 min. Dyesensitized TiO2, SnO2 and ZnO films using an efficient sensitizer Ru-complex dye (N719) were prepared as the reference samples with 100% electron injection efficiency to the semiconductor NP, by immersing a bare TiO2, SnO2 or ZnO film in a N719 dye solution for 12 h. The molecular structure of N719 dye is shown in Fig. 1. 2.2. Femtosecond transient absorption spectrometer Transient absorption kinetics were measured by the femtosecond VIS-pump/IR-probe transient absorption spectrometer based on an amplified Ti: sapphire laser system (1 kHz repetition frequency, 800 nm center wavelength, 150 fs pulse width) combined with two optical parametric amplifiers. The pump beam radius on the film surface was about 0.36 mm. The details of our spectrometer had been described previously [15,16]. The gold/semiconductor NP and N719/semiconductor NP films were placed in air and stable enough during the transient absorption measurements. The samples were mechanically scanned to avoid the degradation of N719 dye and gold nanoparticle due to pump light irradiation during the experiments. All the transient absorption measurements were performed at 295 K. 2.3. Steady state spectrometer The prepared samples scatters visible light strongly. A spectrophotometer (Shimadzu, MPC-3100) equipped with an integrat-
Fig. 1. The molecular structure of N719 dye.
127
ing sphere was used to accurately obtain the absorption fraction for each sample. Details of the procedure are described as below. An incident light with intensity I0 enters the integrated sphere through the entrance window. First the samples were placed before the sphere to measure the transmission fraction (FT) for each gold/semiconductor and N719/semiconductor sample. FT can be obtained from the ratio of transmitted light IT to the incident light I0 (Eq. (1)). Then the sample was placed behind the sphere to obtain the reflection fraction (FR) for each gold/semiconductor and N719/semiconductor sample. FR can be obtained from the ratio of reflected light IR to the incident light I0 (Eq. (2)). So the absorption fraction FA can be calculated from FT and FR (Eq. (3)) because the total of FA, FT and FR is 100%.
FT ¼ IT =I0
ð1Þ
FR ¼ IR =I0
ð2Þ
FA ¼ 1 ðFR þ FT Þ ¼ 1 IT =I0 IR =I0
ð3Þ
3. Results and discussion 3.1. Optical characterization Fig. 2(a,b,c) show each FA as a function of wavelength for the TiO2, Au/TiO2, N719/TiO2, SnO2, Au/SnO2, N719/SnO2, ZnO, Au/ ZnO, and N719/ZnO systems. For Au/TiO2 system, the absorption band at 550 nm originating from the plasmon band of gold nanoparticles were observed. The FA value was close to 0.8 due to strong absorption of gold nanoparticles. For Au/SnO2 system, the FA value was close to 0.4 due to weaker absorption of gold nanoparticles at 550 nm. For Au/ZnO system, the FA value was close to 0.35 due to weaker absorption of gold nanoparticles at 515 nm. For N719/TiO2 and N719/SnO2 systems, the FA values for the absorption band peak at around 530 nm were also close to 0.6–0.8. For N719/ZnO system, the FA values for the absorption band peak at around 515 nm were close to 0.4. The plasmon band peak of different semiconductor loaded with gold nanoparticle is slightly different each other due to the particle size effect of gold, different refractive index and reflection characteristics of semiconductors. It is known that large refractive index and large Au diameter make red-shift to the peak wavelength. The band peak of bare TiO2, SnO2, ZnO absorption give the value at around 380 nm, 369 nm, 360 nm. We chose 570 nm light as a pump pulse to excite the plasmon band of gold nanoparticle. 3.2. Electron injection in different semiconductor systems Transient absorption signals of Au/TiO2, N719/TiO2, bare TiO2, Au/SnO2, N719/SnO2, bare SnO2 and Au/ZnO, N719/ZnO, bare ZnO systems observed at 3440 nm upon excitation at 570 nm were shown in Fig. 3(a,b,c). All amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm. At this probe wavelength, absorption due to free electrons injected from gold nanoparticle into the conduction band of TiO2, SnO2 or ZnO can be monitored. Bare TiO2, SnO2 or ZnO show no transient absorption signals due to no absorption at 570 nm (Fig. 2), the excitation intensity at the sample point is 1mw. For Au/semiconductor systems, we found the electron injection process was completed within less than 240 fs (the time resolution of our apparatus) [14] and following was the decay process. From the data, There may be a small rising component after response rise. This might be relaxation of injected electrons as we discussed in our previous work [16,17]. The following decay was assigned to the back electron transfer from the semiconductor to gold nanoparticle. The
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Fig. 2. (a), (b), (c) FA as a function of the wavelength for TiO2, Au/TiO2, N719/TiO2, SnO2, Au/SnO2, N719/SnO2, ZnO, Au/ZnO, and N719/ZnO systems.
black dots in Fig. 3(a) are the electron injection dynamics for N719/ TiO2 film. There was no decay observed up to 1000 ps. A plateau appeared after 300 ps, and it can be assumed that it gives the 100% electron injection efficiency [18]. The black dots in Fig. 3(b) are the electron injection dynamics for N719/SnO2 film. It gives a plateau at 300 ps, and then showed a decay. The black dots in Fig. 3(c) are the electron injection dynamics for N719/ZnO film, giving the 100% electron injection efficiency [19]. We assume that dye/metal oxide systems give 100% electron injection efficiency. This assumption comes from former study in Katoh et al. group
Fig. 3. (a), (b), (c) The transient absorption signals for Au/TiO2, N719/TiO2, bare TiO2, Au/SnO2, N719/SnO2, bare SnO2, Au/ZnO, N719/ZnO and bare ZnO systems observed at 3440 nm under excitation at 570 nm. All amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm.
[19] where N3 (basically same as N719) sensitized TiO2, ZnO, and SnO2 gave efficiencies more than 80%. The electron injection yield of different gold/semiconductor systems were evaluated by comparing transient absorption by using the corresponding N719/ semiconductor system as reference, where all amplitudes had been corrected by the absorption fraction at pump light wavelength of 570 nm. The ratio of transient absorption intensity directly corresponds to the ratio of the electron injection yields of these
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two systems. Table 1 shows the calculated electron injection yield for Au/TiO2, Au/SnO2 and Au/ZnO systems. The electron injection yield for the Au/TiO2 system is similar to that for Au/SnO2 system, and smaller than that for Au/ZnO system. They gave not so different electron injection yield. As we know, ZnO is a metal oxide having similar energy levels of the conduction and valence bands to those of TiO2, but having smaller density of states of conduction band to that of TiO2, so contrary result was expected to the experimental result [15]. Au/TiO2 system and Au/SnO2 system have the similar electron injection yield, although the conduction band energy level of SnO2 is lower than that of TiO2. The similar electron injection yields may be due to the smaller density of states of conduction band of SnO2 than that of TiO2 [15]. We think effect of surface states as electron acceptor may be involved. From the work of C.R. Crowell and S.M. Sze [20], the electron transfer yield from metal to semiconductor can be as a function of the excess energy of barrier. For hot electron transfer (occurring in <100 fs in our case), the yield is not so sensitive to the effective mass of semiconductor. Thus our result indicated that different semiconductors with different DOS due to their different electron effective masses gave not so different electron injection efficiency. Surface state effect is not the only reason that the yield did not change so much. Both the effects (surface states and hot electron transfer) resulted in our results (not so different electron injection efficiency).
3.3. Charge recombination in different semiconductor systems Fig. 4 shows the normalized transient absorption kinetics of gold assembled with different semiconductor TiO2, SnO2, ZnO systems monitored at 3440 nm under excitation at 570 nm. The experiments are done at the same excitation intensity and experimental conditions. From the Fig. 4, we can see there are differences in the charge recombination time for Au/TiO2, Au/SnO2 and Au/ZnO systems. We checked the excitation power dependence for these gold-semiconductor systems separately to see if there was the second-order recombination process. The data are shown in Fig. 5. The range of 570-nm laser intensity was chosen as 0.5mw
Table 1 The electron injection yield of Au/TiO2, Au/ SnO2, Au/ZnO systems. Sample
Electron injection yield(%)
Au/TiO2 Au/SnO2 Au/ZnO
17 14 26 Fig. 5. Transient absorption decay profiles of different gold–semiconductor films with different excitation intensities, excited at 570 nm and probed at 3440 nm.
Fig. 4. The normalized transient absorption signals for Au/TiO2, Au/SnO2, and Au/ ZnO systems observed at 3440 nm under excitation at 570 nm, the green curve shows the fitting smooth curve, the number in brackets means the multiple time to normalize the kinetics.
and 1mw at the sample point, because the largest 570-nm output through the OPA in our apparatus was nearly 1.2mw at the sample point, and a desirable signal to noise ratio transient absorption signal was difficult to obtain when the laser intensity was very low. Within experimental error, we found that the transient absorption decay time for each gold–semiconductor film remained same with changes in 570-nm laser intensity, and the amplitude of the transient absorption signals for every gold–semiconductor system was almost proportional to the excitation light intensity. These experimental results suggest that there was no nonlinear process involved in the excitation and also no second-order recombination process. It means only one gold nanoparticle was assembled with one semiconductor nanoparticles as our previous studies [16], and therefore the injected electrons in semiconductor simply recombined with the holes in the original gold nanoparticle. There are many factors that can affect the charge recombination process,
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L.C. Du et al. / Chemical Physics Letters 701 (2018) 126–130 Table 2 Estimated values of s1/e for Au/TiO2, Au/SnO2 and Au/ZnO systems. Semiconductor (diameter)
s1/e (ps)
TiO2 (10 nm) SnO2 (10 nm) ZnO (30 nm)
6.2 461 144
Acknowledgment This work is supported by the Natural Science Foundation of China (grant 21003155, grant 11674125), Key project of National Natural Science Foundation of China (grant 20933010), and Jilin Provincial Department of Education Program (JJKH20170777KJ). References
like the size of gold nanoparticle, the size of semiconductor, the diffusion coefficient of electron in semiconductors [16,17], and so on. Lian’s group [21] also reported that the electron recombination dynamics depend on the sample preparation method, which was due to effect of the trap state density. Nelson et al. [22] suggested charge recombination kinetics depend on both electrons trapping/ detrapping dynamics and the rate of electron transfer on the trap site on the semiconductor. In our previous work, we reported that the charge recombination dynamics depend on the size of semiconductor nanoparticle and can be correlated with the diffusion of electrons [16,17]. We found larger particle size and smaller diffusion constant make the charge recombination time longer. Tachiya proposed the diffusion models of reactions on micelles and vesicles in 1987 [23]. This model is for diffusion-controlled reactions of micelle systems and actually is similar to our case, where it is assumed that one molecule is confined within the micelle interior and another one is fixed on the micelle systems. When the molecule in the micelle interior reaches the surface and contacts with another molecule fixed on the surface, the reaction occurs. The mean reaction time of the two molecules holds a relation: s = 1.37 r3/(Dd), where s is the reaction time, r is the radius of the micelles, D is the diffusion coefficient of the molecule in the micelle interior and d is twice the reaction radius (or the reaction diameter). We regard the semiconductor particle surface as the micelle; the injected electron will diffuse within the semiconductor interior and the recombination process will occur at the semiconductor surface. Then we can estimate the charge recombination time by considering the size of semiconductor and diffusion constant of electrons in semiconductor. We analyzed the transient absorption kinetics of Au/TiO2, Au/ SnO2, and Au/ZnO systems. Here, we define the charge recombination time as the time at which the transient signal is equal to 1/e of the maximum value of the signal (s1/e). Estimated values of s1/e are shown in Table 2. As can be seen from Table 2, the charge recombination times (s1/e) of Au/ZnO and Au/SnO2 systems are longer than that of Au/TiO2 system. The charge recombination process can be correlated with the electron diffusion as mentioned above. Therefore, when charge recombination time is longer, the electron diffusion time is longer. The different diffusion coefficient of electron in semiconductors may affect the diffusion time. The different diffusion coefficient for different semiconductor is DTiO2 = 3 105 m2 s1, DSnO2 = 109 m2 s1 [24], and DZnO = 107 m2 s1 [25]. Smaller reported diffusion constants for SnO2 and ZnO as well as the larger diameter of ZnO may be the reason of the observed much slower recombination than TiO2.
4. Conclusion Plasmon induced electron injection from Au NP to different metal oxides (TiO2, SnO2, and ZnO) was observed directly by using IR probe femtosecond transient absorption spectroscopy. The injection efficiencies and the lifetimes (s1/e) were evaluated. SnO2 and ZnO are proved to be efficient electron acceptor from Au NP besides TiO2, even with their small conduction band DOS.
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