Ultrafast Electron Trapping and Defect-Mediated Recombination in

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Ultrafast Electron Trapping and Defect-Mediated Recombination in NiO Probed by Femtosecond Extreme Ultraviolet Reflection-Absorption Spectroscopy Somnath Biswas, Jakub Husek, Stephen Londo, and L. Robert Baker J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01865 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry Letters

Ultrafast Electron Trapping and Defect-Mediated Recombination in NiO Probed by Femtosecond Extreme Ultraviolet Reflection-Absorption Spectroscopy Somnath Biswas, Jakub Husek, Stephen Londo, and L. Robert Baker∗ Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210 Received August 7, 2018; E-mail: [email protected]

Abstract: Understanding the chemical nature of defect sites as well as the mechanism of defect-mediated recombination is critical for the rational design of energy conversion materials with improved efficiency. Using femtosecond extreme ultraviolet (XUV) spectroscopy in conjunction with x-ray photoelectron spectroscopy (XPS), we present results on the ultrafast electron dynamics in NiO prepared with varying concentrations of defect states. We find that oxygen vacancy defects do not serve as the primary recombination center, but rather the recombination rate scales linearly with the density of Ni metal defects. This suggests that grain boundaries between Ni metal and NiO are responsible for fast carrier recombination in partially reduced NiO. Our kinetic model shows that the photoexcited electrons self-trap via small polaron formation on the subpicosecond time scale. Additionally, we estimate an absolute measurement of small polaron formation rates, direct versus defect mediated recombination rates, and the small polaron diffusion coefficient in NiO. This study provides important parameters for engineering NiO based materials for solar energy harvesting applications.

The electronic structure of NiO has been a subject of interest for several decades owing to its potential to serve in numerous energy conversion applications. This includes the use of NiO as a hole transport layer in photovoltaic and dye sensitized solar cells, and as a protecting layer in technologically important semiconducting photoanodes. 1–5 Additionally, NiO based mixed metal oxides (e.g. Ni-Fe, NiCo, Ni-Cu-based oxides) have been shown to accelerate the kinetically slow oxygen evolution reaction tremendously. 6–8 Surface electronic properties of NiO play a central role in determining the efficiency of these energy conversion processes, and ultrafast charge trapping and defect-mediated recombination are central reasons for reported poor efficiency in NiO based energy conversion materials. 9–13

Surface electronic structure in semiconductor materials has long been described by the presence of surface mid-gap states that are usually associated with various structural defects such as oxygen or metal atom vacancies, adatoms, and grain boundaries. 14–17 These mid-gap states often play an undesirable role in surface photochemistry acting as trap states and recombination centers for otherwise redox active charge carriers. 18–20 Much effort has been made to eliminate or passivate these defect states in metal oxides with varying degrees of success. 9,10,21–23 To facilitate these efforts, a better understanding is required regarding the chemical and structural origin of these recombination centers as well as the exact dynamics of photogenerated charge carriers in these states. A theoretical study suggests that electronic transport in NiO occurs via a small polaron hopping mechanism. 24 Polarons form via the coupling between photoexcited electrons and optical phonons and this results in an ultrafast deformation of the lattice. 25,26 The experimental observation of small polaron formation in NiO has not been explored due to the unavailability of the spectroscopic techniques sensitive to this ultrafast lattice deformation. However, proper understanding of the photoexcited charge carrier trapping via small polaron formation, diffusion of polarons to defect sites, and defect mediated recombination is crucial to design novel NiO based energy conversion materials with improved efficiency. NiO is nominally a p-type semiconductor but can be made either n or p-type depending on preparation conditions. 27,28 In this material oxygen vacancies act as an electron donor and Ni atom vacancies act as a hole donor. 29 It has been speculated that surface defects in NiO thin films are responsible for carrier trapping and account for poor charge mobility in this material, 9,30,31 but little is known about the chemical nature of the defects responsible for recombination or the actual kinetics of photoexcited charge carriers in these states. Exposure of Ni metal to air at room temperature oxidizes the surface in a self-limiting reaction resulting in approximately 1 nm of NiO on Ni metal. 32 At elevated temperature, deeper oxidation occurs by a diffusional process where Ni+ ions migrate through the NiO overlayer and react with surface oxygen. 33,34 There is no thermodynamically stable nickel oxide phase containing Ni+ ; 35 consequently, partially oxidized NiOx will exist as a solid solution of Ni metal and near stoichiometric NiO, with increasing amounts of NiO following oxidation at higher temperature. X-ray diffraction and electron microscopy confirming this bi-phase mechanism for oxide growth has been reported previously. 33 Based on this picture, two types of defects are expected to exist in NiOx samples prepared here. The first are oxy-

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Figure 1. (A) XPS of Ni 2p3/2 for the NiO thin film annealed at 100◦ C, 200◦ C, 300◦ C and 400◦ C. The presence of additional peak at 852.7 eV (shaded red) in case of NiO thin film annealed at 100◦ C, 200◦ C, 300◦ C shows the presence of Ni metal along with NiO overlayers. (B) XPS of O 1s for NiO thin film annealed at different temperatures. The peak at 531 eV (shaded magenta) shows the presence of oxygen vacancy defects. The peak at 529.4 eV (shaded orange) is associated with the presence of oxide lattice. At higher annealing temperature the amount of lattice oxygen increases with simultaneous reduction of the oxygen vacancy defects.

gen vacancy defects due to diffusion of Ni+ ions in NiO, and the second are grain boundaries between Ni metal and NiO phases. It has been shown that in cases where the lattice mismatch is small, the oxide will adopt the phase of the metal substrate, while if the lattice mismatch is large, a dislocation defect will characterize the phase boundary. 33,34 Ni metal fits in the latter category with a large lattice mismatch of 19 % between NiO and nickel metal. 33,36 Both of these defects can be detected and quantified by XPS measurements described below. Figure 1A compares the Ni 2p3/2 XPS of NiO thin films annealed at different temperatures. In all cases, the broad multiplet structure shaded in blue is consistent with the presence of Ni2+ . The peak at 853.8 eV is specifically associated with a cd9 L electron configuration, where c denotes a core hole and L denotes a hole on the ligand, and this peak confirms the existence of NiO at the surface of each sample. 37 The remaining blue-shaded peaks at energies above 853.8 eV do not represent distinct chemical species; instead, these are satellite features resulting from variations in multi-electron interactions in the core-hole excited state of Ni2+ . 38,39 For the case of NiO thin films annealed at lower temperatures (100◦ C, 200◦ C and 300◦ C), we observe an additional peak at 852.7 eV (shaded red). This peak position is aligned well with the reported binding energy of Ni metal. 27,38 This observation indicates the presence of Ni metal in partially oxidized NiO thin films. For the sample annealed at 100◦ C, Ni metal contributes 24.3% of the total intensity, and this value decreases steadily with increasing annealing temperature. The sample annealed at 400◦ C shows no detectable Ni metal indicating that at this temperature the Ni surface is almost fully oxidized. We observe no direct evidence for Ni+ in the fits to the Ni 2p3/2 XPS spectra; however, we note that Ni+ can be difficult to differentiate from Ni2+ in an XPS measurement

due to the largely overlapping multiplet structure between these two species. 38,40 Consequently, to confirm the presence of Ni+ , we rely on the detection of oxygen vacancies, which are associated with undercoordinated Ni+ metal centers. Fits to the O 1s spectra in Figure 1B show that each sample is composed of three components with binding energies of 529.4 eV, 531 eV, and 532 eV. The peak at 529.4 eV (shaded orange) matches the known binding energy for lattice oxygen in NiO, 41,42 while the peak at 531 eV (shaded magenta) matches recent reports for oxygen vacancies in transition metal oxides. 42–44 This assignment as oxygen vacancy defects is further confirmed by the steady decrease of this feature with increasing oxidation temperature from 100 to 400◦ C. The final peak at 532 eV is assigned as surface hydroxyl groups, which are unavoidably present on these samples that have been exposed to ambient. 42–44 From these assignments we are able to identify both types of defects present in these NiOx samples, namely Ni/NiO grain boundaries indicated by the Ni metal signature in Ni 2p3/2 spectra and oxygen vacancy defects indicated by the spectral signature in the O 1s spectra. The percentage of both defects for samples prepared at each annealing temperature are reported in Table 1, and details of this XPS analysis are provided in the Supporting Information. To probe the ultrafast electron dynamics in these materials, we have employed extreme ultraviolet reflectionabsorption (XUV-RA) spectroscopy. 45 XUV spectroscopy probes core-to-valence transitions, which are element specific and provide detailed electronic structure information, including the transient oxidation state of the Ni metal center. Using femtosecond pulses of XUV light produced by tabletop high harmonic generation enables direct observation of ultrafast electron dynamics in an optical pump, XUV probe experiment. We have recently demonstrated that this technique combines the benefits of x-ray absorption


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The Journal of Physical Chemistry Letters

Figure 2. Static XUV-RA spectra of NiO thin film annealed at different temperatures. At higher annealing temperature, the fully oxidized form shows two allowed transitions at 65 eV and 68.5 eV. The shoulder at 58.8 eV in case of NiO thin film annealed at 100◦ C, 200◦ C, 300◦ C confirms the presence of Ni metal in addition to NiO.

spectroscopy, with femtosecond time resolution and surface specificity, where measuring reflection of XUV light from a surface at a near grazing incidence angle has a probe depth of only a few nm. 46 This probe depth is closely comparable to XPS measurements discussed above and enables the study of surface electron dynamics on the femtosecond time scale. Figure 2 first compares the static XUV-RA spectra of NiO thin films annealed at different temperatures. We observe a major peak centered around 64.8 eV in the spectrum of each sample. This peak is associated with the M2,3 -edge (3p → 3d) transition of Ni2+ , indicating the presence of NiO at the surface of each sample. The static M2,3 -edge spectrum in NiO is actually a doublet resulting from two allowed transitions from the 3p6 3d8 ground state (3 F) to the 3p5 3d9 core-hole excited state (3 F and 3 D) as described in the Supporting Information. 46 This doublet gives rise to a second transition also present at 68.5 eV. As annealing temperature is decreased, we observe a red shift of the peak at 64.8 eV and a decreasing intensity of the peak at 68.5 eV. This is an indication of partially reduced NiO in films annealed at lower temperatures and is consistent with XPS measurements showing an increased density of oxygen-vacancy defect states in these same samples. A weak shoulder at 58.8 eV is also observed for the case of NiO thin film annealed at lower temperatures (100, 200, 300◦ C). Because this absorption is lower in energy than Ni+ , we assign this feature to Ni metal. To support this assignment, we have also collected a ground state spectrum of Ni metal which shows a strong absorption at 58 eV as shown in Figure S2. Although XPS measurements show that these samples contain a significant concentration of Ni metal, the intensity of this feature in the XUV-RA spectrum is weak. This is because the M2,3 -edge transition is formally forbidden in Ni0 , which has a 3p6 3d10 electron configuration. NiO is a wide band gap material with an indirect band gap of 4 eV, which requires a 267 nm pump pulse for pho-

toexcitation. Figure 3A and 3B compares the spectral evolution following photoexcitation of NiO thin films annealed at 400◦ C and 200◦ C, respectively. Immediately upon excitation, the NiO thin film annealed at 400◦ C (Figure 3A) shows two negative features at 65 eV and 69 eV, and one positive feature at 62.5 eV. It has been shown that photoexcitation of NiO promotes an electron from O 2p valence band states to Ni 3d conduction band states, resulting in a one electron reduction of Ni2+ to Ni+ . Consequently, the negative features in the transient spectrum are due to depletion of the Ni2+ ground state while the positive feature is due to absorption by the Ni+ photoexcited state. The photoexcited spectra of NiO annealed at 200◦ C (Figure 3B) show a similar positive feature at 62.5 eV and a negative feature at 65 eV. We note that there is only one ground state bleach feature in the transient state of this material consistent with the presence of a single ground state peak around 65 eV as shown in Figure 2. Additionally, the photoexcited transient states of NiO thin films annealed at 100◦ C and 300◦ C (see Figure S5A and S5B) show similar spectral signatures indicating the formation of charge-transfer excited states in each of these samples as well. In each case the charge transfer state forms within the 100 fs response function of the instrument indicating nearly instantaneous charge transfer on the time scale of these measurements. Visual inspection of the transient spectra of each sample shows that the Ni+ excited state absorption feature blue shifts by ∼0.5 eV following formation of the initial charge transfer state. We assign this blue shift to the self-trapping of the charge carrier via the formation of a small polaron as has been theoretically predicted for NiO. 24 To explain the origin of the blue shift, small polaron formation occurs via the coupling between a photoexcited electron and an optical phonon, and is associated with lattice deformation following charge transfer excitation. This lattice deformation in NiO occurs via the expansion of the oxygen lattice around the localized Ni+ center. 24 Expansion of the negatively-charged oxygen anions in the polaron state serves to decrease the electron density on the photoexcited Ni+ metal center. In core-hole spectroscopy, this results in decreased screening of the core-hole excited state, which increases the M2,3 edge transition energy, and a similar spectral blue shift has been recently observed during small polaron formation in Fe2 O3 25,26 and FeOOH. 47 To extract the transient spectra associated with both the charge transfer and polaron states, we performed singular value decomposition of the transient data for each sample. 48 The resulting spectra are depicted in Figures 3C and 3D for samples annealed at 400◦ C and 200◦ C, respectively, and analogous plots for the 100◦ C and 300◦ C samples are provided in the Supporting Information. In each case, the spectrum of the polaron state blue shifts relative to the initial charge transfer state by ∼0.5 eV for each sample, consistent with the effects of polaron formation on XUV absorption as described above. Using these spectral vectors, we can calculate the amplitude coefficients of each state from spectra obtained as a function of time delay using multivariate regression analysis, and these amplitude coefficients are proportional to the time-dependent population of the charge transfer and polaron states for each sample. Plots of the amplitude coefficients versus time for NiO thin films annealed at 400◦ C and 200◦ C are shown in Figure 3C and 3D, respectively. Results for NiO thin films annealed at 100◦ C and 300◦ C are shown in Figure S5C and S5D, respectively.



Figure 3. (A, B) Spectral evolution following photoexcitation by 267 nm pump of NiO thin film annealed at 400◦ C (A) and 200◦ C (B). (C, D) The inset shows the fitted charge-transfer and polaron states that are used to extract the corresponding spectral amplitude coefficients for the experimental data. The amplitude coefficients are plotted as a function of time with the solid line representing the fit to the experimental data using the kinetic model as shown in Figure 4A. (C) shows the data in case of NiO thin film annealed at 400◦ C while (D) refers to the data of NiO thin film annealed at 200◦ C

The time evolution of both the charge-transfer state and the polaron state differs significantly depending on the annealing temperature of the NiO thin film, which are fit using a global kinetic analysis described below. The difference in small polaron formation kinetics can be seen most clearly in the early time points as shown in Figure S6 of the Supporting Information. Figure 4A schematically depicts the kinetic model used to understand the charge carrier dynamics in photoexcited NiO thin films annealed at different temperatures. Photoexcitation of NiO by a 267 nm pump pulse leads to the generation of a charge-transfer (O 2p → Ni 3d) excited state (Ne ) within the instrument response time (100 fs). This charge transfer state (Ne ) decays to the polaron state (Np ) with a rate constant of kp . This polaron state can then decay to the ground state via two different pathways. The first is spontaneous decay of the polaron, which occurs with a rate constant of krp . The second decay pathway of the polaron consists of diffusion to a defect site (Nd ) with a bimolecular rate constant of kd followed by defect-mediated recombination. Because polaron formation is much faster than recombination as discussed below, this model ignores direct recombination from the charge transfer state, which has a negligible effect on the observed kinetics. We first discuss the effect of annealing temperature on the measured rates of small polaron formation, and then we discuss the role of defects on the subsequent recombination kinetics. Fitting results show that the polaron formation rates decrease with increasing annealing temperature. Comparing the associated time constants in Table 1, it is seen that τp increases moderately from 0.322 ± 0.034 ps to 0.635 ± 0.094

ps for samples annealed between 100◦ C and 300◦ C, while the sample annealed at 400◦ C shows a more significant increase to 1.66 ± 0.26 ps. Correlation of these lifetimes with sample morphology (Figure S9 and Figure S10) measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM) suggests that these time constants reflect the role of sample crystallinity on electron-phonon scattering rates. Because polaron formation represents a bimolecular reaction between phonons and electrons, the kinetics of this process will depend strongly on the rate of electron-phonon scattering, which determines the time-dependent evolution of phonon population following hot carrier excitation. 26 In general, electron-phonon scattering increases with decreasing crystallinity due to strong scattering at lattice imperfections. 49 AFM and SEM images provided in Figure S9 and S10, respectively, show that significant recrystallization of the NiO thin films does not occur until 400◦ C as shown by the significant increase in surface roughness for this sample indicating the formation of large crystalline domains, consistent with previous reports for other metal oxide materials. 50–54 This increase in crystallinity is consistent with the corresponding decrease in polaron formation rate, indicating that small polaron formation rates in NiO are determined largely by sample crystallinity. Previous XUV measurements in α-Fe2 O3 by Carneiro et. al show that it is possible to deconvolute the rate of polaron formation into the elementary rate constants describing electron-phonon scattering and subsequent bimolecular reaction between free carriers and optical phonons. 26 However, the error associated with a similar analysis performed on this data is too large to provide statistically significant insight. This is primarily


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The Journal of Physical Chemistry Letters a result of the close spectral similarity between the charge transfer and polaron states in NiO, which introduces a larger error in the extracted amplitude coefficients by comparison to α-Fe2 O3 .

existence of a third state associated with polaron trapping at defects, and the fit did not improve when using a threecomponent, instead of two-component, sequential kinetic model. The absence of a measurable third state in the transient data suggests that the defect-meditated recombination is fast compared to polaron diffusion to defects, such that no population of defect-trapped charges accumulates during experiments. Therefore, the effective recombination rate appears to be governed by the diffusion rate of the polaron to defect sites followed by nearly instantaneous recombination. Because defect-mediated recombination is a bimolecular process, the effective rate constant can be written as kd Nd . To illustrate this the following system of differential (Equation 1 and 2) and integrated (Equation 3 and 4) rate equations describes the time evolution for the population of chargetransfer state (Ne ) and polaron state (Np ) in the context of the proposed kinetic model. dNe = −kp [Ne ] dt dNp = kp [Ne ] − krp [Np ] − kd [Np ][Nd ] dt Ne = Ne0 e−kp t kp Ne0 Np = e−kp t − e−(krp +kd Nd )t krp + kd Nd − kp

Figure 4. (A) The schematic of the kinetic model used to fit the experimental data. Photoexcitation by 267 nm pump pulse produces a charge-transfer (Ne ) state. This charge-transfer state decays to a polaron state (Np ) with a rate constant of kp . The polaron state spontaneously decays with a time constant of krp in the absence of defects. In the presence of defects (Nd ), polaron decays via an additional pathway with an effective rate constant of Nd kd . (B) The plot of effective rate constant (Nd kd ) for recombination obtained from the kinetic model versus the percentage of nickel metal defects (Ngd ) measured by XPS. Magenta squares represent an upper limit and green circles represent a lower limit for the grain boundary contribution to defect-mediated recombination.

We now consider the role of surface defects, which are found to strongly influence the recombination kinetics of the polaron state. In the kinetic model described above, Nd represents a general defect site that mediates the recombination. The percentage of grain boundary defects (Ngd ) and oxygen vacancy defects (Nvd ) is shown in Table 1. We note that NiO thin films annealed at 400◦ C have a negligible density of nickel metal defects (Ngd ≈ 0) and the lowest concentration of oxygen vacancy defects (Nvd ) among all the samples studied here as indicated by XPS and static XUV-RA measurements. Therefore, we assume that the polaron state in this sample decays spontaneously to the ground state with a rate constant of krp . It is also possible that the non-zero concentration of oxygen vacancy defects present in this sample also influence the observed recombination rate; however, below we show that oxygen vacancies have an almost negligible effect on the kinetics of defect-mediated recombination by comparison to grain boundary defects. In addition to direct recombination described by krp , the recombination of the polaron in NiO films annealed at 100◦ C, 200◦ C, 300◦ C has an additional defect-mediated decay pathway. This process can be described as the diffusion of polarons to defect sites with a rate constant of kd followed by defect-mediated recombination. The global fitting analysis of the transient data, which shows the existence of the charge-transfer state and polaron state for each sample, finds no evidence for the

(2) (3) (4)

Equation 1 represents the differential rate equation for the decay of the charge-transfer state (Ne ) with a rate constant of kp . Equation 2 represents the differential rate equation for population (Np ) of the polaron state. In this differential rate equation the first term represents polaron formation with a rate constant of kp . The second term represents the spontaneous recombination of the polaron in absence of the defects with a rate constant of krp . The third term represents a bimolecular reaction between a polaron and defect site with a rate constant of kd . We note that kd is closely related to the diffusion coefficient of the polaron. Because defectmediated recombination is fast compared to polaron diffusion, this term represents the rate determining step for defect mediated recombination. Therefore, we can treat the product, kd Nd , as the effective rate constant for defect-mediated recombination. It is important to note that both Ne and Np are time dependent, while Nd represents the concentration of defects, which is independent of time. Equations 3 and 4 represent the corresponding integrated rate equations for the population of charge-transfer (Ne ) and polaron states (Np ). In these integrated rate equations, Ne0 represents the initial population of the charge-transfer state. Equations 3 and 4 are convoluted with a gaussian instrument response of 100 fs to fit the experimental amplitude coefficients of the charge-transfer and polaron state. Detailed derivation of the kinetic rate equations and their convoluted form are provided in the Supporting Information. We fit the described kinetic model to the amplitude coefficients of the charge-transfer state and polaron state using a nonlinear regression analysis. In case of the NiO thin film annealed at 400◦ C, the presence of nickel metal defects are below our limit of detection. Therefore, the rate constants for this sample are obtained by considering Nd =0 in Equation 4. Under this approximation, the spontaneous recombination (krp ) of the polaron is the only decay pathway for this sample. From this we determine a value for krp , which then remains fixed while fitting the recombination kinetics for NiO thin films annealed at 100◦ C, 200◦ C, 300◦ C.


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Table 1: Summary of the rate constants and time constant of polaron formation, direct recombination, defect-mediated recombination and defect density in case of NiO thin film annealed at different temperatures. Error bars represent standard errors based on a nonlinear least squares regression of the kinetic model to the measured amplitude coefficients.

Annealing temperature 100◦ C 200◦ C 300◦ C 400◦ C

kp (ps−1 ) 3.11 ± 0.33 1.73 ± 0.19 1.57 ± 0.23 0.603 ± 0.09

τp (ps) 0.322 ± 0.034 0.578 ± 0.06 0.635 ± 0.094 1.66 ± 0.26

Using this approximation, we obtain the rate constant for defect-mediated recombination (kd Nd ) as a function of annealing temperature. A fit to the experimental data using this kinetic model is shown as the solid lines in Figures 3C and 3D for NiO thin films annealed at 400◦ C and 200◦ C, respectively. We have also applied this model to the experimental data of NiO thin films annealed at 100◦ C and 300◦ C as shown in Figure S2C and S2D. All of the rate constants obtained from this analysis are tabulated in Table 1. Assuming no contribution from defect states for the NiO thin film annealed at 400◦ C, the measured rate constant (krp ) for direct polaron recombination is 0.006 ± 0.005 ps−1 (τrp ≈ 163 ps) . This represents an upper limit for the rate of purely spontaneous recombination of the polaron. Because the concentration of oxygen vacancy defects is actually nonzero in this sample, it is possible that this rate includes a contribution from defect mediated recombination at reduced Ni+ metal centers associated with these oxygen vacancy defects. If this contribution to the observed recombination rate is dominant, we would expect the recombination rate to increase linearly with the increased density of oxygen vacancy defects for more defective samples annealed at lower temperature as predicted by the Nd kd scaling parameter in the kinetic model. However, scaling this observed rate of 0.006 ps−1 by the increase in oxygen vacancy concentration measured by XPS, indicates that the most reduced sample annealed at 100◦ C would have a maximum recombination rate of 0.013 ps−1 , which is nearly an order of magnitude lower than the measured total rate (krp + kd Nd =0.117 ps−1 ) for this sample. From this we conclude that oxygen vacancy defects play at most a minor role in the measured recombination dynamics observed here. In contrast, to illustrate the significant role of grain boundary defects on recombination kinetics, we fix the rate constant krp =0.006 ps−1 during the kinetic analysis of all the other samples. As evident from Table 1, the resulting effective rate constant for defect-mediated recombination (kd Nd ) shows a trend with increasing annealing temperature. Given the bimolecular recombination mechanism of a polaron with a defect site, the effective rate constant for recombination (kd Nd ) should follow a linear relationship with the concentration of responsible defects. The magenta squares in Figure 4B shows the effective rate constant for recombination (kd Nd ) obtained from the kinetic model plotted as a function of grain boundary defects (Ngd ) obtained from XPS measurements. We note that the effective rate constant, kd Nd , follows a linear trend with the percentage of grain boundary defects (Ngd ) as expected. This observation confirms that the recombination is mediated through the grain boundary defect sites (Ngd ). We have converted the percentage of the grain boundary defects to molar concentration (M) using the density of the NiO lattice. The slope of the plot is 0.005 M−1 ps−1 and defines the rate constant of polaron hopping (kd ). This value for kd is based on the assumption of an

krp (ps−1 ) 0.006 ± 0.005 0.006 ± 0.005 0.006 ± 0.005 0.006 ± 0.005

τrp (ps) 163 163 163 163

Ngd (%) 24.3 8.1 3.6 0

Nvd (%) 49.4 38.9 30.6 22.8

upper limit value of krp =0.006 ps−1 . However, if we instead assume that the measured rate of 0.006 ps−1 represents the contribution of oxygen vacancy mediated recombination kinetics, we can scale the slope of kd Nd to account for the increasing contribution of oxygen vacancy defects in the more reduced samples (see Table S1), and these results are shown as the green circles and dashed line in Figure4B. This results in a barely reduced slope showing that this assumption can introduce at most an ∼10% error in the reported value of kd . From this narrow range of possible values for kd , we estimate the diffusion coefficient, D = 2.64 × 10−5 cm2 /sec, for polaron hopping in NiO. The details for conversion from the rate constant, kd , to the diffusion coefficient, D, is described in the Supporting Information. This value for D agrees well with other reported values for trapped carrier diffusion. 55,56 A number of recent studies indicate that oxygen vacancies may actually enhance catalytic performance for a variety of photochemical surface reactions, 42–44 while other studies suggest that oxygen vacancies inhibit catalytic activity by facilitating fast charge carrier trapping and recombination. 57,58 These reports represent conflicting design parameters for materials engineering and highlight the need to directly measure defect-mediated charge carrier dynamics in real time with chemical state resolution. The results presented here show that oxygen vacancy defects do not serve as the primary recombination center in NiO. Rather we find that recombination rates scale linearly with the density of Ni metal defects suggesting that grain boundaries between Ni metal and NiO in partially reduced NiO are responsible for fast carrier recombination. These results indicate that an ideal material could maximize the density of catalytically active oxygen vacancy defects while specifically eliminating grain boundaries. Additionally, we show that sub-ps electron trapping does not occur at defect sites but proceeds via small polaron formation in a self-trapping mechanism. These findings resolve important questions related to the mechanisms of carrier trapping and subsequent recombination in NiO and provide parameters for the design of efficient materials based on direct observation of ultrafast surface electron dynamics with chemical state resolution. Associated Content Author Information *Corresponding Author: [email protected] The authors declare no competing financial interest. Acknowledgement This work was supported by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under DOE Grant No. DE-SC0014051. XPS was performed at the OSU Surface Analysis Laboratory, SEM imaging was performed at the OSU Nanotech West Laboratory, and AFM imaging


Nd kd (ps−1 ) 0.111 ± 0.044 0.032 ± 0.013 0.015 ± 0.017 0

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The Journal of Physical Chemistry Letters was performed at the OSU NanoSystems Laboratory. Supporting Information Experimental, M2,3 -Edge Core-Hole Excited States in NiO, Ground State XUV-RA Spectrum of NiO, Contribution of Debye-Waller Factor in the Ground State XUV-RA Spectrum of NiO, Kinetic Model, Time Resolved XUV-RA of NiO thin film annealed at 100◦ C, 300◦ C, Early Time Kinetics of Charge-Transfer State and Polaron State, Rate of Grain Boundary vs Oxygen Vacancy Defect-Mediated Recombination, XPS of Ni 2p3/2 and O 1s, Atomic Force Microscopy, Scanning Electron Microscopy, Calculation of Diffusion Coefficient (D) References

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