Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO2

Jan 19, 2012 - ... measured by variable angle spectroscopic ellipsometry (VASE). ... were obtained with a CCD (Photon MAX 1024, Princeton Instruments)...
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Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO2 Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy Shengye Jin,†,‡ Alex B. F. Martinson,*,‡,§ and Gary P. Wiederrecht*,†,‡ †

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, United States § Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

bS Supporting Information ABSTRACT: The presenting surface of TiO2 is one of the key factors that influence the photoinduced charge injection process from covalently bound chromophores. However, the dependence of electron transfer (ET) on TiO2 surface properties (structure, defects, and facets) remains poorly understood due to the difficulties of deconvoluting the signal from a multitude of surface binding sites in highly heterogeneous ET systems. In an effort to correlate TiO2 surface features with ET, we compare the photoinduced ET dynamics from single quantum dots (QDs) to polycrystalline TiO2 thin films (pc-TiO2) grown by atomic layer deposition (ALD) with that of porous TiO2 nanoparticle films (np-TiO2) by utilizing single-particle fluorescence spectroscopy. Unlike the broad distribution of ET rates (deduced from fluorescence lifetimes) on np-TiO2, QDs on pc-TiO2 exhibit two narrowly distributed ET rates that we attribute to reduced site heterogeneity. Variable temperature pc-TiO2 annealing studies suggest that the double-peaked distribution of ET rates is related to TiO2 surface defects, where QDs undergo more rapid ET. Further modification of pc-TiO2 with a submonolayer of Al2O3 enables the selective exclusion of the more rapid ET pathway. More generally, this study provides insight into the role of surface defects in photoinduced ET into crystalline semiconductor oxides.

’ INTRODUCTION Photoinduced interfacial electron transfer (ET) from covalently bound molecules or particles to a semiconductor oxide plays a critical role in solar cells,1,2 photocatalysis,3,4 and molecular electronics.5 Quantitating, understanding, and ultimately controlling ET dynamics are essential in order to improve performance in devices utilizing electron injection. Interfacial ET dynamics in various combinations of molecule/semiconductor oxide systems have been extensively studied,6 13 with ET rates ranging from femtoseconds to nanoseconds depending on the composition of semiconductor oxide, the tether chemistry, and the structure of the molecule. Recently, quantum dots (QDs) have garnered significant attention due to their exceptionally large extinction coefficients, tailorable electronic properties, and broad absorption spectra compared to molecular chromophores. As such, they are promising materials to enable meaningful improvements in solar energy conversion efficiency.14 16 Studies examining the dependence of QD structure, shape, and size on the ET dynamics have been reported previously.17 21 However, studies examining the dependence of ET on the metal oxide are lacking. Specifically, the dependence of interfacial ET dynamics on the physicochemical properties of the electron donor/acceptor interfaces is still poorly understood and inadequately timeresolved. For example, in a recent review of electron transfer dynamics in dye-sensitized solar cells (DSSCs), the extent to which electron injection can proceed directly into localized states r 2012 American Chemical Society

was identified as a “key and currently unresolved question”.22 This is at least partially due to the ambiguity inherent in investigating ET through traditional ensemble measurements, which report only the average properties of the processes. Single-molecule or single-particle spectroscopy is a powerful technique to resolve the more precise information that is inevitably lost in ensemble averaging. It has proven valuable for the deconvolution of heterogeneous dynamics in photocatalysis23 25 and interfacial ET.13,26 35 For example, Majima and co-workers have recently revealed crystal-face-dependent TiO2 photocatalysis using a single-molecule detection technique.36 Their findings suggest that the photocatalytic activities of different TiO2 single crystal facets are affected by the distinct surface properties exhibited by each crystal face. Photoinduced interfacial ET dynamics from molecules and QDs to wide bandgap semiconductor oxides (i.e., TiO2 and SnO2) have also been investigated on single molecule or particle levels.13,30 35 However, to date, these studies have been restricted to nanoparticle films and all reveal a wide distribution of ET dynamics within each system. This heterogeneity may be at least partially attributed to a multitude of oxide surface binding sites induced by different structures, crystal facets, and defect chemistries. As a result of the complexities Received: December 6, 2011 Revised: December 21, 2011 Published: January 19, 2012 3097

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Figure 1. Idealized schematic of photoinduced interfacial ET from QDs to nanoparticle TiO2 (np-TiO2) and polycrystalline TiO2 thin film (pc-TiO2).

associated with a highly defective and heterogeneous surface, a direct correlation between surface structure and function (ET) in these systems is precluded. It is therefore highly desirable to study single molecule and QD ET processes on a more welldefined (yet still practical) semiconductor oxide substrate. Specifically, a planar, polycrystalline TiO2 thin film (pc-TiO2) is expected to present a more homogeneous surface due to its 2D nature and larger, oriented grains, relative to a porous TiO2 nanoparticle film (np-TiO2). Therefore, we hypothesize that studying ET on pc-TiO2 may reveal additional information that relates structure to ET dynamics. Among the various methods for preparing pc-TiO2, we have selected atomic layer deposition (ALD) for its versatility37 and literature precedence in fabricating sensitized solar conversion devices.38 43 Herein, we compare photoinduced interfacial ET dynamics in a QD/pc-TiO2 system to those observed in QD/np-TiO2 systems. Figure 1 depicts a highly idealized schematic of the experiment. By utilizing single-particle photoluminescence spectroscopy, we infer a significantly simpler picture for electron injection into pc-TiO2 versus np-TiO2. On pc-TiO2, two narrowly distributed ET rates (originating from nominally identical QDs) are observed within the very broad distribution of ET rates to np-TiO2. Combined with firing temperature dependent studies of pc-TiO2 preparation, this study experimentally reveals the role of the surface defects in the photoinduced ET processes for the first time. Further modification of the TiO2 surface chemistry with selective submonolayer deposition of Al2O3 affords additional insight by nearly complete suppression of one (of the two) ET pathways.

’ EXPERIMENTAL SECTION Materials. Water-soluble CdSe/CdS2ML/ZnCdS1ML/ZnS1ML core/multishell QDs with carboxylic acid functional groups were purchased from Ocean Nanotech, LLC, USA. A single batch was used to ensure valid comparisons. A commercial TiO2 paste (Dyesol, DSL 18NR-T) containing ∼20 nm anatase particles was used to prepare np-TiO2 thick films. The ALD metal precursors titanium isopropoxide (TTIP) and trimethylaluminum (TMA) were used as received from Sigma-Aldrich. Water was obtained from a Millipore water purification system. Sample Preparation. Glass coverslips were carefully solvent cleaned and blown dry with N2. np-TiO2 films were fabricated by doctor-blading the commercial paste over a single Scotch tape spacer on glass coverslips. The Al2O3 and pc-TiO2 samples were fabricated by loading glass coverslips onto the center sample

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stage of an ALD tool (Savannah 200, Cambridge Nanotech) at the desired growth temperature. Metal oxides (Al2O3 or TiO2) were deposited by dosing TMA or TTIP, respectively, alternately with water. The Al2O3 (control) and pc-TiO2 films were grown at 225 °C with (H2O) (N2 purge) (metal precursor) -(N2 purge) using timings (in seconds) of 0.015 7 x 7 with x = 0.015 and 0.15 for TMA and TTIP, respectively. Sufficient cycles were repeated to obtain metal oxide thin films of ∼100 nm thickness as deduced from spectator silicon substrates measured by variable angle spectroscopic ellipsometry (VASE). Samples were subsequently fired in a quartz tube furnace under flowing ultrapure oxygen at a ramp rate of 2 °C/min, held for 30 min, and allowed to cool naturally. For samples modified with a submonolayer of Al2O3, freshly fired TiO2 films were loaded into the ALD at 150 °C and subjected to a (H2O) (N2 purge) (metal precursor) (N2 purge) (H2O) (N2 purge) sequence of 0.015 20 0.015 20 0.015 20. Just prior to single particle measurements, a QD water solution with a concentration of 33 pM was spin-cast onto the metal oxide samples. These samples were dried in air and then washed with water (Millipore, 18.2 MΩ) to remove weakly absorbed QDs. The single molecule measurements were conducted in air under standard ambient conditions. As such, some water adsorption on the metal oxide surface is expected. Instruments. The single particle measurements were carried out with a home-built scanning confocal microscope. The QDs were excited by frequency-doubled pulses from a Ti:sapphire laser at 400 nm wavelength with a repetition rate of 5 MHz (after pulse picker). The samples were placed on a piezo scanner and the excitation beam (∼300 nW) focused through an objective (60, NA 1.35, oil immersion, Olympus). The resulting epifluorescence was collected by the same objective, traveled through a band-pass filter (565 to 625 nm), and was finally detected by a fiber coupled single photon avalanche photodiode (SPAD, Micro Photon Devices). The instrument response function for fluorescence lifetime measurements has a full width at half-maximum of 80 ps. The output of the SPAD was recorded and analyzed by a Time-Correlated Single Photon Counting (TCSPC, PicoQuant) module. Wide-field-illuminated single QD fluorescence images were obtained with a CCD (Photon MAX 1024, Princeton Instruments).

’ RESULTS AND DISCUSSION The atomic force microscope (AFM) images of representative np-TiO2 and pc-TiO2 films used in this study are shown in Figure 2a and b, respectively. The pc-TiO2 ALD film shows typical polycrystalline grain structure, the average lateral dimensions of which we estimate to be 500 nm. Several absent peaks in the XRD pattern (Figure 2c) of ALD-grown pc-TiO2 suggest significant structure, with only (101) and some (200) anatase planes oriented parallel to the substrate. This orientation, combined with the large platelets visible by AFM, suggests that the exposed surface is largely (101). These results are in agreement with calculated surface formation energies, which also suggest a presenting (101) face of anatase is favored.44 The np-TiO2 film is also composed of anatase grains but is randomly oriented. The XRD peak widths (indicative of minimum grain size as measured perpendicular to the plane of the substrate) correspond to a vertical grain size of 13 nm and at least 34 nm for np-TiO2 and pc-TiO2, respectively. The morphology of pc-TiO2 films shows no apparent dependence on the O2 firing 3098

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Figure 2. AFM images of a representative (a) porous np-TiO2 film and (b) pc-TiO2 film after firing both up to 500 °C under flowing O2. (c) Offset X-ray diffraction (XRD) patterns of the TiO2 films shown in panels a and b. The peaks are referenced to anatase TiO2 pattern 00-021-1272. (d) TEM image of the CdSe/CdS2ML/ZnCdS1ML/ZnS1ML core/multishell QDs used in this study. The average diameter of the QDs is estimated to be 5.66 ( 0.16 nm. (e) Ensemble fluorescence decays (black circles) of the QDs in water. Red solid line is the biexponential fit with fitting parameters (A, amplitude; τ, lifetime; τave, amplitude-weighted average lifetime) listed beside.

Figure 3. (a) UV vis absorption (black solid line) and emission (red solid line) spectra of the core multishell QDs in water. (b) An idealized and approximate energy level diagram for the QDs and TiO2 with arrows illustrating expected electron transfer processes. (c) Wide-field-illuminated fluorescence image of single QDs on a pc-TiO2 film (500 °C). (d) Typical fluorescence intensity (green solid line) and lifetime (red square line) trajectories of a single QD on pc-TiO2. The black dashed line indicates the average lifetime of 16 ns calculated over the lifetime trajectory for this particular QD.

temperature as evidenced by additional AFM (Figure S1, Supporting Information) and XRD (Figure S2, Supporting Information). The UV vis and photoluminescence spectra of the watersoluble CdSe/CdS2ML/ZnCdS1ML/ZnS1ML core/multishell QD used in this study are shown in Figure 3a. The QDs are capped

with octadecylamine and finally an ultrathin carboxylic functionalized polymer,45 which is used to covalently bind the QD to the oxide surface. Shown in Figure 2d is the TEM image of the QDs. Their averaged diameter is estimated to be 5.66 ( 0.16 nm. As shown in Figure 3b, the conduction and valence band levels of the QD can be estimated to be 3.8 and 5.8 eV (vs vacuum) 3099

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Figure 4. Total (left) and corresponding average (right) lifetime histograms (bars) of QDs on Al2O3, np-TiO2, and pc-TiO2 films fired at indicated temperatures. Solid lines are Gaussian fits with fitting parameters P (peak center) and W (width) listed in Table 1.

according to the previously reported method.35,46,47 Although our experiments are performed dry, the conduction band edge position of TiO2 may be estimated to be similar to TiO2 at pH 2 7 ( 4.4 to 4.1 eV vs vacuum),48 suggesting that the ET from QDs to TiO2 will be energetically allowed with tunneling through the ultrathin shells. Slow ET dynamics due to the presence of shells are expected and preferred in this study so that the fluorescence of single QDs is not significantly quenched. A wide-field-illuminated fluorescence image of the QD sensitized pc-TiO2 film in Figure 3c indicates that well-separated single QDs (bright spots) are achieved by spin-casting the dilute QD solutions. In contrast, no bright spots were observed from bare pc-TiO2 films at the same excitation power. Furthermore, the number of bright spots on pc-TiO2 is similar with that on Al2O3 film with a similar loading level, suggesting that the single QDs are not dramatically quenched (hence not observed in the fluorescence images) by ET on pc-TiO2. The examined single QDs in this study are therefore representative of the whole population. The fluorescence intensity and lifetime trajectories of a single QD in Figure 3d are obtained by focusing the excitation laser on a single bright spot and collecting the emitted photons. The intensity trajectory is constructed by plotting the numbers of photons (within a 50 ms bin time, in unit of kHz) as a function of their arrival times. The lifetime points along the lifetime trajectory are determined by fitting the delay time histograms of the detected photons within a 0.5 s window with a single exponential function. The trajectories exhibit the characteristic blinking activity of a single QD49 58 with a positive correlation between fluorescence intensity and exciton lifetime.27,28,34,56 58 The high intensity level is typically referred to as the on state and the low intensity (background) level as the off state. A detailed discussion on the blinking dynamics of single QDs is provided in the Supporting Information.

Typical fluorescence intensity and lifetime trajectories of single QDs on Al2O3, np-TiO2, and pc-TiO2 films are shown in Figure S3, Supporting Information. Compared with QDs on Al2O3 (a control substrate, where no ET can occur and the intrinsic fluorescence properties of single QDs are measured), QDs on both np-TiO2 and pc-TiO2 are found to spend a much longer time in the low intensity (off) state and have considerably shortened lifetimes. The shortening of lifetimes is not caused by the change of dielectric constant between TiO2 and Al2O3. The intrinsic lifetimes (without ET) for QDs with transition dipole moments parallel and perpendicular to the substrate surface are calculated to be 23.7 and 22.1 ns for Al2O3 and 21.2 and 18.2 ns for TiO2, respectively (see Supporting Information for the detailed calculation). The reduced fluorescence lifetime relative to the control is caused by energy transfer or charge transfer between the QD and TiO2. As the TiO2 film has a finite absorbance at 400 nm and the QD absorbs into the visible, we cannot discount the possibility of energy transfer from TiO2 to the QDs, however unlikely. Regardless, the method by which the QDs are excited is irrelevant in the current study. Energy transfer from QDs to TiO2 is ruled out due to the lack of spectral overlap between the QD emission and TiO2 absorption. We therefore attribute the shortening of lifetimes to the presence of interacial ET from QDs to TiO2, which is energetically allowed (see Figure 3b). ET from CdSe QDs (without shell) to np-TiO2 has previously been studied with transient absorption and timeresolved fluorescence quenching spectroscopy as well as confirmed by photocurrent generation in CdSe-sensitized solar cells.59 Furthermore, the ET activities of the QDs on np-TiO2 and pc-TiO2 observed in this study are consistent with previously reported blinking-induced intermittent ET dynamics in similar QD (core shell)/TiO2 ET system (see Supporting Information for details).34 3100

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Table 1. Gaussian Fitting Parameters P (Peak Center) and W (Width) for the Histograms in Figure 4

total (average)

A12O3

np-TiO2

pc-TiO2 500 °C

pc-TiO2 400 °C

pc-TiO2 300 °C

pc-TiO2 as deposited

P1 (ns)

20(19.7)

11.5(12.5)

15.3(15.0)

15.5(15.4)

14.5(14.5)

15.0(15.0)

W1 (ns)

7.9 (3.9)

12.6(7.3)

6.4(2.3)

7.5 (3.8)

7.1 (3.5)

7.0 (4.3)

P2 (ns)

7.4 (7.7)

8.2(8.3)

9.5(10.1)

W2 (ns)

5.8(3.2)

6.0 (2.7)

5.5 (2.3)

In order to present a statistical analysis of the ET dynamics over the entire population, total lifetime histograms are constructed by summing the data points in the lifetime trajectories of each single QD for each system (typically ∼100 QDs per system) in Figure 4. In addition, the corresponding average lifetime (averaged over the duration of each trajectory as shown in Figure 3d) histograms for QDs on each substrate are presented adjacently. The histograms can be well fit by Gaussian (single or double peaks) functions, leading to the determination of peak positions (P) and distribution widths (W). ET of Single QDs on np-TiO2. The total and average lifetime histograms of single QDs on Al2O3 and np-TiO2 (fired at 500 °C) are shown in Figure 4a and b, respectively. These histograms are well fit by single peak Gaussian functions with fitting parameters listed in Table 1. For QDs on the control substrate (Al2O3), the total lifetime histogram is centered at 20 ns with a distribution width of 7.9 ns. This broad intrinsic lifetime distribution of single QDs is consistent with the observed multiexponential kinetics associated with their ensemble fluorescence decay in water (see Figure 2e). However, the total lifetime histogram of QDs on np-TiO2 is shifted to a shorter lifetime position centered at 11.5 ns. This shifting lifetime distribution is due to the presence of an additional competing electron relaxation pathway, interfacial ET from QDs to TiO2. By taking the Gaussian peak values as the average lifetimes, an average ET rate from QDs to np-TiO2 can be estimated to be 3.7  107 s 1. This value is in rough agreement with previously reported ET rates in a similar system.59 The width of the total lifetime distribution on np-TiO2 is much larger than on Al2O3, indicating a heterogeneous distribution of ET rates. As the QDs used in all systems are nominally identical, any difference in the width of the ET rate distribution must be caused by large site and/or dynamic heterogeneities. The site heterogeneity is a measure of how the ET rates of single QDs vary between different sites, while the dynamic heterogeneity reflects how the ET rate of a single QD at one site changes with time. Both types of heterogeneities are reflected in the total lifetime histograms. However, the average lifetime histograms, which are constructed by the averaged lifetime (calculated over the duration time) of each QD, can remove the effects of dynamic heterogeneity. Still, the average lifetime distribution on np-TiO2 (Figure 4b) is also much broader than on Al2O3 (Figure 4a) indicating the presence of large site heterogeneity on np-TiO2. Such a heterogeneous ET rate on np-TiO2 is consistent with previous reports in similar dye and QD/np-TiO2 ET systems30,32 34 and is likely caused by variation of the ET driving force and electronic coupling strength at distinct binding sites on the TiO2. The driving force is determined by the energy of accepting states on the surface of TiO2 (which may vary by binding site) and the excited state oxidation potential of QDs (which may vary between single QDs that vary in shape and size). The electronic coupling strength is strongly dependent on the QD-TiO2 interaction, which can also be significantly affected by the structure and surface chemistry of np-TiO2 at different binding sites.

ET of Single QDs on pc-TiO2. The total and average lifetime histograms of QDs on pc-TiO2 fired at 500 °C are shown in Figure 4c. Unlike QDs on np-TiO2 (whose total lifetime histogram shows a broad and single-peaked distribution), QDs on pc-TiO2 strikingly exhibit two clearly resolved distribution peaks in the total lifetime histogram. The double-peaked lifetime distribution does not result from variable lifetimes exhibited by any single QD (see Figure S3, Supporting Information) but by distinct QDs found on the same substrate that exhibit different (but narrow) lifetimes. The average lifetime histogram, which excludes dynamic heterogeneity, also shows two distribution peaks with peak centers nearly identical to the total lifetime distribution but with an even narrower width. These double-peaked lifetime distributions indicate the presence of two distinct groups of QDs on the pc-TiO2 film with average lifetimes around 15 and 7.5 ns. As the lifetime reduction is a measure of the ET rate (the group with longer lifetime is associated with a slower ET rate, while the group with shorter lifetime is associated with a faster ET rate), this double-peaked lifetime distribution must result from differences in the ET rate between these two groups. To quantitatively separate these two groups and estimate their ET rates, the lifetime distributions are fit by a two-peak Gaussian function, shown as the solid lines in Figure 4c, whose fitting parameters are in Table 1. Here, we take the cross-point of two Gaussians as the threshold to separate the QDs and treat the peak position of each Gaussian as the average lifetime of each group. The first group with slow ET (long lifetime) includes ∼37% of the single QDs, and its averaged ET rate is estimated to be 1.5  107 s 1. The second group with fast ET (short lifetime) covers the remaining 73% of single QDs and exhibits an average ET rate of ∼8.5  107 s 1. It is striking not only that the overall distribution width on pc-TiO2 is smaller than on np-TiO2 but that each of the two subpopulations exhibit very narrow total and average lifetime distributions (6 and 3 ns). These widths are several times smaller than on np-TiO2 and similar to the widths measured on control Al2O3 films that lack ET and, therefore, any ET heterogeneity. We attribute the double-peaked lifetime distribution and decrease in distribution width to the reduced site heterogeneity of pc-TiO2. The two groups of QDs on pc-TiO2 may be due to the presence of two general types of binding sites, each of which has a narrowly distributed driving force and electronic coupling with QDs. It is not surprising to observe less heterogeneous ET on pc-TiO2 than on np-TiO2 when one considers the differences between these two substrates. As shown in the AFM images in Figure 2a and b, the pc-TiO2 has a more uniform surface than porous np-TiO2, which may significantly reduce the heterogeneity of surface binding sites for QDs. The variable np-TiO2 surface, in contrast, may include many different binding environments and therefore widely distributed lifetimes that prohibit the deconvolution of ET dynamics on distinct binding sites. ET Dependence on TiO2 Firing Temperature. To further investigate the origin of the double-peaked lifetime distribution on pc-TiO2, we examine the dependence of ET on the TiO2 3101

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Figure 5. Total (left) and corresponding average (right) lifetime distribution curves of single QDs on 1 and 3 ALD cycles of Al2O3 coated pc-TiO2 films fired at 500 and 400 °C. For comparison, the lifetime distributions of QD on pc-TiO2 without Al2O3 coating (gray bars) are also shown.

firing temperature. The total and average lifetime histograms of single QDs on pc-TiO2 films, which were fired at various temperatures (in addition to 500 °C), are shown in Figure 4d f. Their Gaussian fitting parameters are also listed in Table 1. Experiments on pc-TiO2 films fired at a temperature higher than 500 °C were not conducted due to the potential formation of rutile, the thermodynamically stable phase of TiO2. The QDs on pc-TiO2 fired at 400 °C also display clear double-peaked total and average lifetime distributions with peak positions (populations in percentage) of 8.2 ns (39%) and 15.5 ns (71%). When the firing temperature is limited to 300 °C, the lifetime distributions can still be fit by two peak Gaussians, but the peaks are less well resolved, with peak positions (populations) of 9.5 ns (32%) and 14.5 ns (68%). The lifetime histogram for an unfired pc-TiO2 film is a single-peaked distribution centered at 15.0 ns, although with a small tail at short lifetime position not fit well by the Gaussian. The trend suggests that the population of QDs with short lifetimes (fast ET rates) increases as the firing temperature increases. Additionally, the distribution peaks with long lifetimes stay centered around 15 ns irrespective of firing temperature and are qualitatively similar to the single-peaked distribution on unfired film. It is also important to note that the widths of the distribution peaks do not change significantly with firing temperature. A small firing temperature dependent distribution is also observed on np-TiO2 (see Figure S4, Supporting Information). As the annealing temperature is reduced from 500 to 400 °C, the total and average lifetime distributions are still broad, but the peak positions shift slightly (3 ns) toward longer lifetime positions. Correlating TiO2 Surface Properties with ET Dynamics. The exposure of TiO2 to higher temperature can improve crystallization, alter the trap state distribution, and result in better performance in sensitized devices using TiO2.60 62 As we can now modulate the ET rate distribution with TiO2 firing temperature, this affords the opportunity to correlate ET with a physical and/or chemical change in the surface. The XRD patterns of pc-TiO2 films with similar thickness fired at different temperatures (unfired, 300, 400, and 500 °C) are shown in Figure S2, Supporting Information. All films exhibit peaks characteristic of anatase TiO2 with similar intensities, indicating that firing has little effect on the crystallinity. Different grain orientations could also lead to a different ET rate distribution. However, for all

pc-TiO2 films, the ratio between XRD peak intensities was the same. In conjunction with surface morphology studies by AFM, these results suggest that the firing does not structurally alter the polycrystalline film. Therefore, the double-peaked lifetime distributions on pc-TiO2 (which does change as a function of firing temperature) are most likely not correlated to structural changes observable on the length scale of these experiments (∼10 nm). Additionally, the double-peaked lifetime distributions are not caused by variations in the structure (such as shell thickness and polymer coating) of single QDs. Bimodal ET rates induced by these properties would be observed uniformly on all studied TiO2 substrates and thus would not show the observed dependence on pc-TiO2 firing temperature. Previous reports have established evidence for increasing defect site densities on a TiO2 single crystal surface by thermal annealing.63 66 The defect sites are primarily oxygen vacancies created by eliminating surface oxygen atoms at high temperature that result in shallow subconduction band states. Therefore, one reasonable interpretation of the changing double-peaked lifetime distribution on pc-TiO2 ALD film is the presence of surface defect sites (oxygen vacancies) generated by firing. Theoretical work also suggests that surface defects in TiO2 enhance charge injection in dye-sensitized solar cells.67,68 Intuitively, such enhanced ET may be rationalized by a greater driving force to lower energy (subconduction band) states or stronger electronic coupling between the sensitizers and the defective TiO2 surface. According to previous investigations on the sensitization of QDs on TiO2 single crystals,69,70 the QDs in this study likely bind to TiO2 through their carboxylic acid functional group. In the double-peaked lifetime distributions on pc-TiO2, we propose that the QDs with long lifetime are bound to a less defective TiO2 surface, while the QDs with short lifetime may be bound to more defective sites. This proposed mechanism is consistent with the observed changes in lifetime histograms on pc-TiO2 films fired at the different temperatures, Figure 4c f. On the surface of an unfired pc-TiO2 film, the number of highly defective sites is limited and hence the double-peaked distribution is not observed. As the firing temperature increases, more surface defect sites are produced, leading to a larger population of QDs with short lifetime (fast ET rate). Controlling ET through TiO2 Surface Modification. To further probe and begin to control the two ET pathways, we 3102

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The Journal of Physical Chemistry C also studied the single QD ET dynamics on pc-TiO2 films modified with a submonolayer of Al2O3 using a single exposure to water and then trimethylaluminum (TMA). Recent theoretical work on the subject of Al2O3 ALD on the TiO2 anatase (101) surface suggests that under the typical ALD conditions employed here, no water absorption is expected on a flat (101) terrace after water exposure and subsequent purging.71 In contrast, in proximity to common defects (step edges, O vacancies, and Ti interstitials) water is retained. As a result, the authors find that after one ALD cycle an atomic layer of Al2O3 is favored only where water is retained at defects. This calculation is especially relevant to this work as anatase (101) is the primary surface presented by the pc-TiO2 films. The ultrawide band gap semiconductor (Al2O3) is expected to passivate subconduction band states by filling oxygen vacancies and/or forcing higher energy defect states. Hupp and co-workers report a similar modification of TiO2 and SnO2 electrodes for use in a DSSC by depositing Al2O3 via ALD.72,73 They speculate that a single ALD cycle of Al2O3 selectively passivates surface defects that lead to reduced recombination with redox species in solution, ultimately resulting in better photovoltaic performance. If one ALD submonolayer of Al2O3 reduces the number of available surface defect sites, in this study, we would expect a decrease of the population of QDs with short lifetime (fast ET) in the total and average lifetime distributions. The total and average lifetime histograms constructed from more than 90 single QDs on a pc-TiO2 film modified with one or more ALD cycles of Al2O3 are shown in Figure 5. For the pc-TiO2 films fired at 500 and 400 °C, one ALD cycle of Al2O3 produces lifetime histograms with single-peaked distributions centered close to 15 ns, indicating that the number of QDs with short lifetimes decreases dramatically after less than one atomic layer of Al2O3. The results, which corroborate the theoretical and experimental evidence discussed above, further support the proposed ET mechanism. We hypothesize that the Al2O3 submonolayer shuts down the fast ET pathway by selectively occupying the defect sites such that QDs either do not bind or bound QDs undergo slower ET. The latter is most likely based on the increased occurrence of 15 ns-centered QDs. We also observe that the lifetime distributions of QDs on pc-TiO2 modified with 3 cycles of Al2O3 are further shifted toward the longer lifetime position (see Figure 5), suggesting that the slow ET rate can also be slowed with multiple cycles. This is not surprising, considering that with increasing cycles, a complete Al2O3 layer will form and may act as a tunneling barrier to suppress ET.74,75 We also test this hypothesis on np-TiO2 films. The lifetime distributions are shifted slightly toward longer lifetimes with a smaller population of single QDs with short lifetimes after one cycle of Al2O3 is deposited on a np-TiO2 film (see Figure S5, Supporting Information).

’ CONCLUSIONS The highly heterogeneous ET dynamics observed on np-TiO2 mask the structural factors that control the ET process. pc-TiO2, which presents a less varied surface, enables us to begin to deconvolute the detailed ET dynamics on TiO2 as a function of surface chemistry. In this article, we have contrasted the interfacial ET dynamics of single QDs on np-TiO2 and pc-TiO2 films. While single QDs on both substrates follow the previously reported blinking-induced intermittent ET dynamics, QDs on np-TiO2 exhibit a much broader total and average lifetime

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distributions than on Al2O3, suggesting heterogeneous ET dynamics. However, for QDs bound to fired pc-TiO2 thin films, the total and average lifetime distributions show a clear double-peak, revealing the presence of two distinct QD binding sites that exhibit dissimilar ET rates. Each lifetime distribution is considerably narrower than on np-TiO2 and similar to control samples suggesting minimal heterogeneity at each site. The percentage of QDs with short lifetimes becomes larger at higher firing temperatures. We speculate that the QDs with long lifetime (slow ET) originate from QDs that bind to a less defective crystal surface, while the QDs with short lifetime (fast ET) bind on the more defective TiO2 sites. These QDs undergo a faster ET process because of stronger electronic coupling strength and/or larger driving force. This speculation is supported by the study of single QDs on pc-TiO2 that has been modified with a single ALD cycle of Al2O3. The TMA exposure apparently passivates the surface defects on pc-TiO2, resulting in a reduced population of QDs with short lifetimes (fast ET). These studies provide insight into the role of metal oxide surface chemistry in photoinduced interfacial ET process. Additional studies are underway in our laboratories to investigate the generality of these conclusions to other sensitizers including molecular dyes.

’ ASSOCIATED CONTENT

bS

Supporting Information. AFM images and XRD patterns of pc-TiO2 films unfired and fired at each temperature; typical fluorescence intensity and lifetime trajectories of single QDs on different substrates; detailed discussion of single QD blinking and intermittent ET dynamics; total and average lifetime distributions of QDs on np-TiO2 fired at different temperatures and modified with Al2O3. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (A.B.F.M.); [email protected] (G.P.W.).

’ ACKNOWLEDGMENT This work was supported as part of the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001059. Use of the Center for Nanoscale Materials was supported by the Department of Energy, Office of Basic Energy Sciences, through Contract No. DE-AC02-06CH11357. We thank Dr. Matthew Pelton for useful discussions and assistance on the time-correlated single photon counting apparatus. ’ REFERENCES (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (2) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (3) Kamat, P. V.; Meisel, D. Semiconductor Nanoclusters: Physical, Chemical, and Catalytic Aspects; Elsevier: Amsterdam, The Netherlands, 1997; Vol. 103. (4) Serpone, N.; Pelizzetti, E. Photocatalysis, Fundamentals and Applications; John Wiley & Sons: New York, 1989. 3103

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