J. Phys. Chem. B 2001, 105, 4545-4557
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FEATURE ARTICLE Ultrafast Electron Transfer Dynamics from Molecular Adsorbates to Semiconductor Nanocrystalline Thin Films John B. Asbury, Encai Hao, Yongqiang Wang, Hirendra N. Ghosh, and Tianquan Lian* Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: September 26, 2000; In Final Form: January 24, 2001
Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)2(SCN)2 (dcbpy ) 2,2′-bipyridine-4,4′-dicarboxylate) sensitized TiO2 thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO2. The injection process was characterized by a fast component, with a time constant of SnO2 > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO2 sensitized by three adsorbates, Re(Ln)(CO)3Cl [Ln is a modified dcbpy ligand with n ()0, 1, 3) CH2 units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH2 units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)2X2 (X2 ) 2SCN-, 2CN-, and dcbpy) sensitized SnO2 thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.
1. Introduction Electron transfer (ET) between molecular adsorbates and semiconductor nanomaterials has been a subject of intense research interest in recent years.1-5 This process is intimately related to the application of semiconductor nanomaterials in photography,6 solar energy conversion,4,7,8 photocatalytic waste degradation,9 and quantum dot devices.10,11 For example, dyesensitized nanocrystalline semiconductor thin film solar cells12 may emerge as a potential cost-effective alternative to siliconbased cells.4,7,8 The most efficient cells of this type, based on Ru(dcbpy)2(NCS)2 [dcbpy ) (4,4′-dicarboxy-2,2′-bipyridine)] (or Ru N3) sensitized nanocrystalline TiO2 thin films, can achieve a solar to electric power conversion efficiency of about 10%.12,13 The high conversion efficiency can be attributed to efficient solar energy harvesting by the sensitizer and high incident photon to current conversion efficiency (IPCE).4,7,8 A high IPCE requires a fast electron injection rate from the sensitizer to the semiconductor and a much slower back electron* To whom correspondence should be addressed: e-mail
[email protected].
transfer rate to the sensitizer.4 A schematic of the interfacial ET processes in dye-sensitized semiconductor nanomaterials is shown in Figure 1. Also shown in Figure 1 are the energy levels of the adsorbates (shown in Figure 2) to be discussed in this paper. The operation of this solar cell and many devices based on nanocrystalline materials is directly related to the charge transfer and carrier relaxation/combination dynamics. For this reason, in addition to interfacial ET, carrier relaxation and recombination dynamics in metal and semiconductor nanoparticles have also been actively studied in recent years.14-19 Interested readers can refer to recent review articles for more details on that subject.14,16,17,20-22 In this article, we will focus on ultrafast interfacial charge transfer from molecular adsorbates to semiconductor nanocrystalline thin films. Electron transfer dynamics between molecules and bulk solids are still poorly understood1,2,23 compared with ET in homogeneous solution.24-27 The interfacial nature of these processes has hindered their experimental studies in the past. The study of bulk surfaces often demands sophisticated surface science methods for samples in ultrahigh vacuum (UHV) chambers.28
10.1021/jp003485m CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001
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Figure 1. Schematic diagram of electron injection and the energetics of sensitizers and semiconductors. The left half depicts a two-state electron injection model. k1 is the electron injection rate from the upper state (states above the conduction band edge), and k2 is the rate of relaxation from the upper state to the lower state (states below the conduction band edge). The estimated conduction band edges (VCB) for the three semiconductors are indicated by the dotted lines. The positions of the solid lines indicate the redox potential of the ground (S) and relaxed 3MLCT (S*) state of the sensitizers.
Figure 2. Structure of Re(L1)(CO)3Cl (ReC1A) and Ru(dcbpy)2X2.
Studies of adsorbates on semiconductor or metal surfaces by these techniques have yielded great insights into small molecule/ solid interactions. Efforts to develop in situ, direct, and interfacespecific spectroscopic techniques for studying bulk solid/liquid interfaces have resulted in some elegant yet complicated techniques, such as sum frequency (SFG) and second harmonic (SHG) generation,29-31 and surface restricted grating techniques.32 Direct time-resolved measurement of ultrafast dynamics of molecules in bulk solid/liquid interfaces has proven to be difficult, except for a few cases where the system under study is photostable.28,29,32-38 For strongly luminescent semiconductor electrodes, time-resolved photoluminescence has been used successfully to measure electron-transfer dynamics in the solidliquid interface.39-41 Much of the previous understanding of interfacial charge transfer was obtained from steady-state photocurrent measurements in electrochemical cells.1,2,4,23,42-45 Unfortunately, obtaining ET rates from steady-state photocurrents is often quite complicated because the photocurrent also depends on many other interfacial and bulk properties, such as carrier transport and mass transport in electrolyte, in addition to the interfacial ET rates.1,23 As a result, this technique is often limited to process in which interfacial ET is the rate-limiting step. This is typically on the slower than nanosecond time scale.46
Asbury et al. In recent years, there has been a rapid development in the synthesis and characterization of nanometer-size metal and semiconductor particles.14,16,47 In addition to their fascinating properties resulting from quantum size confinement effects, their large surface areas and small sizes also enable the study of their surface properties by time-resolved absorption techniques. Interfacial processes of these materials can be readily studied without sophisticated surface science techniques. As a result, there have been many studies of electron-transfer dynamics in dye-sensitized semiconductor nanoparticles and thin films by time-resolved laser spectroscopy techniques, such as transient absorption and fluorescence lifetime measurement, in the visible and near-IR region.32,48-72 Although much insight regarding interfacial ET has been gained through these studies, transient absorption studies in the visible and near-IR region are often hindered by spectral overlap of absorptions from various electronic states (e.g., excited, oxidized, and ground states), as well as stimulated emission. Fluorescence quenching studies can be sometimes complicated by non-ET-related quenching pathways, such as energy transfer among sensitizer molecules,56 and the dynamic fluorescence Stokes shift.60 Because of these complexities, a systematic study of the dependence of interfacial ET rates on various adsorbate/nanoparticle properties has proven to be difficult. To systematically study ET dynamics at the solid-liquid interface, new in situ techniques that are capable of assigning the ET process unambiguously in many adsorbate/nanoparticle systems and that complement the existing visible/near-IR transient absorption and fluorescence quenching techniques are needed. Femtosecond mid-IR spectroscopy73 provides such an approach for these interfacial problems because it can directly study the dynamics of electrons in the semiconductor in addition to the dynamics of the adsorbates. As demonstrated in bulk74,75 and quantum well76 and, more recently, nanocrystaline77-90 and quantum dot91-96 semiconductor materials, valence band holes and conduction band electrons in semiconductors have strong absorptions in the infrared region. These absorptions consist of free carrier absorption, which is often broad and increases with wavelength, intraband transitions between different valleys (or subbands) within the conduction or the valence bands, and absorptions of trap states.97 Since the IR absorption of electrons is direct evidence for the arrival of electrons inside the semiconductor, it provides an unambiguous spectroscopic probe for studying interfacial electron transfer between adsorbates and semiconductors. Furthermore, since IR absorption of injected electrons is a common feature of any semiconductor, femtosecond mid-IR spectroscopy can be used for any adsorbate/ nanoparticle system, allowing a systematic study of interfacial ET in different adsorbates, semiconductors, and solvent environments. It should be pointed out that injected electrons in semiconductors can also be monitored in the near-IR region.48,49 However, in this region it may be difficult to avoid spectral overlap with the electronic transitions of large dye molecules.49 Injected electrons can also be probed in the far-IR and microwave region. Direct measurement of microwave absorption of injected electrons in dye-sensitized nanocrystalline thin films has been demonstrated with nanosecond time resolution.67 Ultrafast terahertz spectroscopy98-100 should provide another convenient probe for ultrafast electron injection dynamics. Recently, transient mid-IR spectroscopy was used to study electron transfer in sensitized nanoparticles and thin films by Heimer and Heilweil89,90 and by our group.77-88 We demonstrated that this technique could be used to simultaneously probe the injected electrons and the vibrational spectra of the
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adsorbate.77,85,86 We have used the transient mid-IR signal of injected electrons to study both the forward and backward electron-transfer dynamics in TiO2 nanoparticle colloids and thin films.77-88 The absorption of injected electrons in TiO2 thin films was shown to be very broad in the 3-7 µm region.78,80,81 In addition, we have observed similar broad absorption features for electrons in other semiconductors, such as ZnO,82 SnO2,101 CdS, CdSe, and PbS.102 We have applied this approach to TiO2 nanoparticles sensitized with Ru(dcbpy)2(SCN)2 and related compounds,80,81,83,84 coumarin 34378 and related organic dyes, and smaller ions such as Fe(II)(CN)64-77 and SCN- species.103 In this paper, we will focus on the interfacial electron injection dynamics from adsorbate molecules to semiconductor nanocrystalline thin films. The paper is organized as follows: In section 2, the theoretical background for interfacial ET is introduced. The experimental method used for this study is briefly described in section 3. Studies of electron injection processes from various adsorbate molecules to semiconductors are discussed in section 4, followed by a summary and future work. 2. Theoretical Background Electron transfer between molecular adsorbates and bulk semiconductors (or metals) involves electron transfer between discrete molecular states and a dense manifold of highly delocalized electronic levels in the solid. The basic theoretical framework for describing electron transfer in bulk solid/liquid interfaces was developed by Marcus,104 Gerischer,105,106 and Levich and Dogonadeze107 in the 1960s. For nanomaterials that are not in the quantum-confined size regime, the electronic structure of the bulk material still applies, but there is a more significant contribution from surface defect states. Both delocalized band states and localized trap states may contribute to the electron transfer process. Unfortunately, so far the extent and nature of the trap states have not been fully characterized for nanomaterials. Therefore, a quantitative comparison of experiment with theory is still difficult. For this reason, we will emphasize major qualitative trends expected for ET in different dye-sensitized semiconductors on the basis of classical Marcus ET theory.24 (a) ET between a Donor and an Acceptor State. Electron transfer between a discrete donor and acceptor level in solution can be described by the conventional transition-state theory:24,25,27
kET ) keVeffe-E*/kBT
(1)
where Veff is the effective frequency of motion along the reaction coordinate and ke is the electronic transmission factor. The activation barrier (E*) is related to the sum of the free energy difference, ∆G0, and the total reorganization energy, λ. The transmission factor is given by the Landau-Zener coefficient for curve crossing:24,25,108
ke )
2P0 (1 + P0)
[
P0 ) 1 - exp -
|Hif|2π3/2 hVeffxkBTλ
]
(2)
where Hif is the electronic coupling matrix element. In the limit of weak coupling, i.e., the nonadiabatic limit, the exponential function in the above expression can be expanded and truncated at the second term, leading to the familiar expression for nonadiabatic electron-transfer rate:24-26,109
kET )
2π p
|H|2
x4πλkBT
[
exp -
]
(λ + ∆G0)2 4λkBT
(3)
This expression can also be obtained by use of Fermi’s golden rule at the high-temperature limit.26,27,109 In the limit of strong coupling, the electronic transmission factor becomes 1 and the electron-transfer rate becomes24,26,110
kET ) Veffe-E*/kBT
(4)
In this limit, the activation barrier may be severely modified by coupling. In the case of a small barrier (E* , kBT), the reaction rate may be directly determined by the effective frequency νeff, which in some cases can be the solvation time.26,110 (b) Interfacial ET. The above expressions are derived for ET between two discrete electronic states. For interfacial ET, one needs to take into account the continuous bands in the solid as well as the distribution of trap states.111-113 For example, the expression for nonadiabatic ET from metal to molecules in solution, a common case for electrochemical study, can be derived by including the population distribution for an electron near the Fermi level and the density of states in the solid.24,45,104,105,111-113 Electron Injection Process. The electron injection process to be discussed in this paper involves electron transfer from the excited state of an adsorbate molecule to the conduction band of a semiconductor nanocrystalline thin film as shown in Figure 1. Under no external bias voltage, we can assume negligible electron population in the conduction band of wide band gap materials prior to electron injection and negligible interaction of injected electrons. The reactant state corresponds to the electron in the excited state of the dye, and the product states correspond to the electron in the semiconductor conduction band. There is only one reactant state, which connects to a continuum of product states, each corresponding to the injected electron at a different electronic level in the semiconductor, as shown in Figure 3. In the nonadiabatic limit, the total ET rate from adsorbate excited state to semiconductor can be expressed as the sum of ET rates to all possible accepting states in the semiconductor. Adopting an approach similar to that of Marcus and co-workers,111-113 we can write down an expression for the total ET rate from adsorbate to semiconductor:
KET ) 2π p
∫0∞dE F(E)|Hh (E)|2
1
x4πλkBT
[
exp -
]
(λ + ∆G0 - E)2 4λkBT
(5) where ∆G0 ) ECB - Eox is the energy difference between conduction band edge and the redox potential of adsorbate excited state; F(E) is the density of state at energy E from the conduction band edge, which can contain both the bulk state and surface states; H(E) is the average electronic coupling between the adsorbate excited state and different k states in the semiconductor at the same energy E; and λ is the total reorganization energy. Furthermore, an inhomogeneous distribution of adsorbate/ semiconductor interactions exists, giving rise to a distribution of electronic coupling matrix elements, H, and hence injection rates. If a distribution function g(H) is assumed, the total change of injected electron population, Ne(t), is given by the integration over the distribution
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Asbury et al.
Figure 4. Calculated electron injection rate from adsorbate excited state to conduction band as a function of EOX(S*) - ECB for different reorganization energies.
Figure 3. Schematic diagram of potential energy as a function of nuclear configuration for electron injection from an adsorbate excited state (S*) to a semiconductor conduction band (CB). The reactant state (S*-CB) connects to a continuum of product states (S+-CB-), corresponding to the electron in different energy levels of the conduction band.
Ne(t) ) N0[1 -
∫0∞ g(H)e-K
ET(H)t
dH]
(6)
where N0 is the total number of excited molecules. We assume that injection is the only pathway for deactivation of excitedstate molecules. This equation predicts a non-single-exponential injection process. To obtain some insight into the qualitative behavior of ET rate, we have evaluated eq 5 in some simple cases. If we assume that the accepting states are dominated by contribution from conduction band states and that the states near the conduction band edge consist of only one band, the density of states obeys the following simple relationship:
F(E) dE )
(2m*)3/2 xE dE 2π2p3
(7)
where m* is the effective mass of electrons in the conduction band. If we further assume that the effective electronic coupling is independent of the energy, we can calculate the injection rate, KET, as a function of Eox(S*) - ECB as shown in Figure 4. The injection rate increases with the increase in the energy difference between adsorbate excited state and the conduction band edge, Eox(S*) - ECB. This results from the increase in the density of accepting states in the solid at higher energy above the conduction band edge. A larger total nuclear reorganization energy leads to a smaller ET rate in the region of small driving force, because of a higher activation barrier for ET. This trend can also be understood in terms of Gerisher’s theory of ET.105 The dependence on density of states can be much more complicated than the simple case shown in Figure 4. The ET rate depends not only on the total density of states but also on the density of states coupled to the adsorbate electron-donating state. This point was clearly illustrated in a recent theoretical study of nonadiabatic ET on metal surfaces.112 This issue has been well examined in a previous study of intramolecular vibrational redistribution processes.114 Furthermore, the quantitative details of the trend shown in Figure 4 will change if the
density of states does not follow the simple relationship shown in eq 6 or if the average electronic coupling varies with the energy of the adsorbate. In the adiabatic limit, the dependence shown in Figure 4 is no longer valid. If the coupling is strong enough that barrierless ET is possible, the injection rate is then limited by the fastest nuclear motion in the system or the electronic dephasing time in the solid.32 It no longer depends on the energy of the injecting state. In this limit, injection on a 80% has been reported for solar cells based on Ru N3-sensitized TiO2 thin films,12,13 indicating an efficient electron injection process. However, the rate for electron injection was investigated only in the past few years. We have recently studied the electron injection dynamics in Ru N3-sensitized TiO2 thin films using femtosecond mid-IR spectroscopy.80,81,84 To unambiguously identify the electron injection process, we compared the transient IR absorption signal in the Ru N3sensitized TiO2 films with those of naked TiO2 films and Ru N3-sensitized Al2O3 thin films. The same excitation power at 400 nm was used for all samples. Shown in Figure 5a is one such comparison of transient IR signals measured at 2115 cm-1 with 1.1 µJ of 400 nm excitation. There is negligible transient IR signal for Ru N3-sensitized Al2O3 (dotted line in Figure 5a) and naked TiO2 (not shown).84 The sensitized TiO2 film shows a broad absorption signal in the mid-IR region, ranging from 3 to 7 µm.80 This broad absorption was attributed to injected electrons in TiO2 and has been observed in colloidal TiO2 nanoparticles,77-79 bulk TiO2 single crystals,78 and nanocrystalline thin films.80,81,83-86 The MLCT excited states of Ru N3 molecules lie above the conduction band edge of TiO2. On the other hand, because Al2O3 has a much higher conduction band edge position,84 electron injection from Ru N3 to the Al2O3 conduction band is not likely, consistent with the lack of transient IR signal in this system, as shown in Figure 5a. The unsensitized TiO2 films show negligible signal because of their small absorbance at the excitation wavelength.
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Figure 5. (a) Electron injection kinetics in RuN3-sensitized TiO2 (b) and Al2O3 (···) thin films measured at 2115 cm-1 after 400 nm excitation. The rise time is well fit with a 50 fs exponential function convoluted with an instrument response, as indicated by the solid curve. (b) Longer time kinetics in RuN3-sensitized TiO2 thin film measured at 2150 cm-1 after 400 nm excitation. Shown in the inset is a kinetics trace extending to 1 ns.
Since the observed transient IR absorption in sensitized TiO2 thin films shown in Figure 5a is due to injected electrons, its rise time is the electron injection time from the sensitizer excited state to TiO2. The rise time can be well fit by a singleexponential rise function convoluted with the instrument response function. The best fit to the data at 2115 cm-1 yielded a 50 ( 25 fs rise as shown by the solid line in Figure 5a. The error bar reflects a 50% increase in the χ2 of the fit. A similar rise time has been observed at probe wavelengths from 2000 to 2200 cm-1.80,84 In addition to this fast injection component, there appears to be a minor component with a slower rise time of about 2 ps, as shown in Figure 5b. We found that the amplitude of the second component (
