Quantifying Bulk and Surface Recombination Processes in

Letter pubs.acs.org/NanoLett

Quantifying Bulk and Surface Recombination Processes in Nanostructured Water Splitting Photocatalysts via In Situ Ultrafast Spectroscopy Kannatassen Appavoo, Mingzhao Liu, Charles T. Black, and Matthew Y. Sfeir* Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: A quantitative description of recombination processes in nanostructured semiconductor photocatalystsone that distinguishes between bulk (charge transport) and surface (chemical reaction) losses is critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity. KEYWORDS: Water-splitting, transient emission spectroscopy, heterostructure nanowire, recombination pathway, photoelectrochemical cell, light harvesting

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could act concomitantly. These include the fact that the surface layers may (i) act as cocatalysts (e.g., Pt, RuOx),27 (ii) modify both the interfacial recombination and the Helmholtz layer (e.g., FeOOH and NiOOH),22 (iii) passivate surface states at the electrode/electrolyte interface or affect the space-charge layer width to decrease electron−hole recombination pathways (e.g., TiO2, Al2O3, and Ga2O3).28,29 Transient absorption studies (in the microseconds−seconds range) on photocatalytic systems such as WO3, TiO2, and α-Fe2O3 reveal that the photogenerated charge carriers must be long-lived (>microseconds) as a prerequisite for water oxidation but indirectly suggest that a significant fraction of the initially photogenerated carriers are lost via recombination pathways.30−35 All together, these results suggest that both picosecond and microsecond time scales are important, and that the material requirements for efficient transport may be disentangled from those promoting efficient chemical reactivity (Figure 1a,b). However, quantifying each individual process selectively, including charge separation, transport, and chemical reaction steps, while identifying the rate-limiting steps remains challenging.

enerating chemical fuels using photocatalysts has been investigated for several decades now for its potential as a renewable solar energy technology.1−6 However, generating hydrogen-fuel by photoelectrochemical (PEC) water-splitting for example, remains a particularly complex materials science problem7 due to stringent requirements on the electronic8−10 and surface structure11−13 of the semiconductor photocatalyst. Efficient photocatalytic water splitting requires both a high quantum yield for transport of charge carriers to the reactive interface (QYtrans) and for charge transfer (QYchem). Most spectroscopic investigations have focused on the latter process, which occurs on nanosecond time scales or longer, from localized surface sites.14−19 Much less attention has been paid to the ultrafast transport process and how it can be optimized in photocatalyst design. Indeed, physical separation of photoexcited electron−hole pairs must occur sufficiently fast in order to avoid loss of the absorbed photon energy through recombination.20 The critical time scales are set by the intrinsic properties of the semiconductor with typical excited state lifetimes in the range of 1−100 ps.21 Significant performance gains, both in terms of watersplitting efficiency and resistance to harsh environments, have been observed when utilizing distinct ultrathin (∼1 nm) surface reaction layers.22−26 The increase in water-splitting performance has been attributed to several factors, which potentially © 2015 American Chemical Society

Received: October 20, 2014 Revised: December 30, 2014 Published: January 7, 2015 1076

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Figure 1. Schematics and measured performance of heterostructured photocatalysts. (a) Schematic illustrating the concept of separability of the charge separation and transport process from the chemical reaction process. (b) Details of the photophysics following light absorption, where CB is the conduction band, VB is the valence band, Ef is the Fermi level, and τ is the nonradiative and radiative recombination before charge carriers are separated. (c) Experimental data for a model zinc oxide nanowire (ZnO NW, ∼ 50 nm in diameter and length of ∼500 nm) conformally coated by a 1 nm TiO2 surface layer system with IPCE obtained after each critical step of the fabrication process and at different potentials. Here, ag-NW is asgrown ZnO NW, t-NW is treated ZnO NW, and surface is 1 nm thin TiO2 conformal layer.

direct the flow of photogenerated minority carriers and to correlate the measured carrier dynamics to the electrical properties of the photoelectrochemical device. Importantly, we found by direct means that losses occurring on picosecond time scales represent a major loss channel in nanostructured photocatalytic materials with relatively high carrier densities (>1019 cm−3 for oxides), even when the size of the nanostructure is below its carrier diffusion length. Furthermore, we determine that TiO2 surface layer plays a role in both protecting the light-absorbing core (as shown by Liu et al.24) and in enhancing the overall activity. These results validate strategies for efficient visible-light absorbing photocatalysts that separately optimize the individual transport (bulk) and chemistry (surface) processes using heterostructured materials. A major difficulty in optimizing a material’s photocatalytic activity is that even minute structural modifications can drastically impact the overall efficiency. As a model system, the incident photon conversion efficiency (IPCE) of four ZnO nanowire (ZnO-NW) materials synthesized simultaneously under identical conditions varies between 41 and 83% (at +1 VAgCl/Ag), depending on the type of postgrowth processing (Figure 1c), even though the samples are all highly crystalline with no perceivable differences in X-ray diffraction patterns (for full fabrication details, see Methods and Supporting Information Figure S1). The nanowire materials, diameter of ∼50 nm and length of ∼500 nm, were synthesized by a hydrothermal process and evaluated in their as-grown form (ag-ZnO), after annealing at 500 °C and cleaning by oxygen plasma (t-ZnO), and after conformal coating with an ultrathin (1 nm) surface layer of titania (ag-ZnO/TiO2 and t-ZnO/TiO2). We see that all postprocessed samples showed improved efficiency compared to the ag-ZnO with multicomponent systems (agZnO/TiO2 and t-ZnO/TiO2) outperforming their single component counterparts at all measured biases (Figure 1c).

In this Letter, we develop a novel experimental approach to separately measure losses during charge separation and interfacial charge transfer in order to understand their individual impacts on the overall photocatalyst efficiency. We quantify and understand origins of these differences in photoanodic performance between different materials through concurrent electrical characterization and in situ optical studies carried out in a customized PEC cell, designed for this purpose (Figure 2). Critically, we utilize parasitic emission signatures of the light-absorbing material as in situ markers to selectively track the complex ultrafast carrier dynamics following photon absorption. Because transient emission studies preferentially probe the photogenerated excitons, this allows us to directly monitor the recombination processes that occur on time scales shorter than the interfacial charge transfer ones. In order to observe the effects of femtoseconds−picoseconds time scale charge separation and recombination, we employ a highefficiency transient emission spectrometer based on the optical Kerr effect that resolves weak broadband emission with a temporal resolution of ∼500 fs (see Methods).36 This approach selectively tracks a specific quasiparticle (photoexcited excitons) with well-defined spectral signatures,which is in sharp contrast to transient absorption spectroscopy where deconvoluting the broad absorption features of various excited species is necessary. In addition, the fact that overlap between the electron and hole wavefunctions is necessary for light emission provides additional spatial information within the absorptive core, as shown later. Another advantage compared to absorption-based measurements is that nonemissive surface layers do not contribute to the overall signal, though their effect on the resulting dynamics on the absorbing core can be readily quantified. Performing these measurements in a working cell at various applied potential allows us to tune the depletion region to 1077

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carried out under pulsed excitation as a function of applied potential (Figure 3a,b) shows that the band-edge emission intensity (Figure 3c,d) is minimized under the most anodic potential (maximum depletion width) relative to flat band conditions. The magnitude of the change is, as expected from calculations of the depleted volume fraction, highly sensitive to the carrier density with the ag-ZnO showing a modulation of only 24% (with a depleted volume of 54%) and the t-ZnO/ TiO2 sample showing a modulation of 80% (depleted volume of 77%). The schematics below Figure 3c,d illustrate this concept with light emission only taking place in regions where the minority (circles) and majority (hashes) carriers significantly overlap. At −1 VAgCl/Ag, minority carriers are confined at the center of the nanowire and the majority carrier density is distributed throughout the NW while under anodic bias, the minority carrier density is accelerated to the surface, where majority carriers have been depleted. The fact that emission from the ag-ZnO sample is less modulated at forward bias (24% decrease) than predicted from calculations of the depleted volume fraction (54% decrease) suggests that bulk defects are a major contributor to the overall recombination. This idea is reinforced by a similar lack of enhancement in the as-grown sample under cathodic potential conditions (only 19% increase) under which minority carriers are driven away from the surface. In stark contrast, t-ZnO/TiO2 exhibits a drastic emission increase of ∼141%, characteristic of a system where bulk defects play a minor role and where surface trapping/recombination processes are no longer major decay channels. Furthermore, a direct signature of defect states for the ag-ZnO is observed in the visible region of the voltagedependent emission data (∼575 nm, Figure 3a), corresponding to radiative recombination characteristic of midgap states distributed throughout the bulk of the nanowire. This emission in the visible spectrum reaches maximum intensity near the flatband potential due to potentiostatic filling of those acceptor states (Supporting Information Figure S3).37 The t-ZnO/TiO2 shows no emission from defect states in the visible range. While the time-integrated data suggest that ultrafast recombination processes play a significant role, quantifying these effects requires measuring the emission lifetime at different potentials (Figure 4). For straightforward comparison with Figure 1c IPCEs (AM 1.5G), detailed ex situ fluencedependent emission intensity measurements to rule out nonlinear effects in this regime and show that adding a surface layer modifies the measured emission lifetime only due to changes to the surface recombination velocity are carried out (see Supporting Information, Figures S4−S6). Hereafter, from a direct measurement of the rates and the calculated depleted (Vdepl/Vtot) and neutral (Vneutral/Vtot) volume fractions, we can determine the upper bound of the IPCE for a material with complete extinction of the incident light

The further improvement in the performance of the multicomponent materials (ag-ZnO/TiO2 and t-ZnO/TiO2) implies that the surface is a nontrivial source of recombination because changes to the bulk of the material, where absorption and charge transport occur, are minimal with the low-temperature atomic layer deposition process used to deposit the titania layer. The exact physical origin of the increased efficiency cannot be determined from device characterization alone because these measurements cannot distinguish between various possible recombination processes.

Figure 2. Schematics of PEC cell for electrical and in situ ultrafast optical studies. (a) Ultrafast pulse (white) excites the nanowire samples found in the PEC cell while simultaneously varying applied voltage. Broadband PL is collected in two modes, time-integrated and time-resolved (TR). For TR measurement, emission is collected and focused using a pair of parabolic mirrors onto a Kerr medium, situated between a set of polarizers, while an additional pulse (black; ∼ 100 fs, λ = 800 nm) time-delayed by a mechanical stage is focused on the same spot and gates the emission. (b) Graphical representation at the focus of the working nanowire photoanode under positive bias. Emission from the ZnO NW photocatalysts is monitored during the O2 evolution, following photon absorption.

In contrast to the electrical measurement, the bias-dependent emission properties provide much information about the recombination mechanisms of various nanostructured photocatalysts. For example, annealing ZnO leads to a reduction in the majority carrier density by a factor of ∼4.6 and a corresponding increase in the relative depleted volume fraction under anodic biasing conditions (calculated using Mott− Schottky analysis of capacitance−voltage data, Supporting Information Figure S2). We note that we are operating in a critical regime, where these modest changes in carrier density drastically modulate the depleted volume fraction of the nanowire. A calculation of the depleted volume as a function of bias is shown in Figure S2, where the limits of low (1018 cm−3) and high carrier density (1020 cm−3) are defined relative to the nanowire physical properties. This increase in the nanowire depleted volume directly correlates to a systematic decrease in the parasitic light emission process, consistent with the physical picture of an increased transport of minority carriers to the reactive interface. In situ time-integrated emission studies

IPCE = QY total transQYchem neutral ⎡V ⎛ ⎞⎤ V Γ CS depl ⎟⎥QYchem =⎢ + neutral ⎜⎜ bulk neutral ⎟ Vtot ⎝ ΓR + ΓNR ⎢⎣ Vtot + Γ CS ⎠⎥⎦ (1)

We determine Vtot from scanning electron microscopy of the nanowires, Vdepl (Vneutral = Vtot − Vdepl) using impedance spectroscopy (see Supporting Information), and the ZnO radiative rate (ΓR = 1/(340 ps)) from the literature.38 Because light absorption takes place in both the neutral and depleted 1078

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Figure 3. Bias-dependent time-integrated in situ emission. UV−visible emission spectra versus applied potential for (a) ZnO and (b) ZnO/TiO2 nanowires with a UV excitation pulse fluence of 32 μJ/cm2. Note that in both plots, the color scheme is the same and plotted on the same logarithmic scale; BE is band-edge and DS is defect states. Emission at band-edge for (c) ag-ZnO and (d) t-ZnO/TiO2 nanowires as a function of applied potential. The emission has been normalized with respect to the emission obtained at flat-band conditions. Schematics at key applied potentials are derived from Mott−Schottky analysis of capacitance versus voltage data (Supporting Information Figure S2).

materials with unknown radiative rate, the quantity (ΓR + Γbulk NR ) can be determined under cathodic potential and will be sufficient to determine the IPCE from eq 1. Once the bulk recombination rates are known, the effective rate of minority neutral carrier transport out of the neutral region (ΓCS ) is determined under forward bias. The efficiency of this process is inversely proportional to the bulk recombination rate, because diffusion out of the neutral region must successfully compete with decay of the carriers. We approximate this process by a single rate constant to capture its ensemble behavior. The critical rate constants are derived in more detail in the Supporting Information and are summarized in Table 1. For the ag-ZnO sample, bulk recombination dominates under all biases (Γbulk NR = 1/(24 ps)) and the relatively small depleted volume fraction means that the effective charge separation rate for carriers created in the neutral region is low (Γneutral = 1/(91 CS ps)). A lack of modulation in the ultrafast absorption kinetics as a function of bias was also reported in α-Fe2O3 photoanodes, where it was concluded that 98% of the photogenerated holes decayed within the first few microseconds, before redox reactions occurred.33 However, for the case of the treated ZnO samples, the much smaller bulk recombination rate (Γbulk NR = 1/(110 ps)) means that carriers can efficiently diffuse to the depletion region (Γneutral = 1/(28 ps)) with only minor losses. CS Interestingly, the ∼4.5× change in the bulk recombination rate constant between the ag-ZnO and t-ZnO/TiO2 samples

regions, we model the dynamics of these two initial conditions separately. For carriers generated within the depletion region, we assume that charge separation and transport dominate over recombination processes, such that carriers have unity probability of reaching the surface. Carriers generated within the neutral region must diffuse to the depletion region before recombination in order to be collected. As such, any light emission we observe originates from this region. It is important to note that while the radiative (ΓR) and bulk nonradiative recombination (Γbulk NR ) processes lead to ground state recovery (parasitic for device operation), the process that leads to charge separation and transport from the neutral region to the surface (Γneutral CS ) yields carriers that are available to participate in the chemical reaction. This differs from surface recombination under ex situ conditions in which majority carriers relax via trapping and scattering from interfacial states. neutral The key unknowns in eq 1, Γbulk NR and ΓCS , ultimately determine the upper IPCE limit and can be quantified by measuring the emission lifetimes under bias. Temporal slices at the band-edge of all collected time-resolved 2D emission spectra are plotted at key potentials (Figure 4a) and summarized for the ag-ZnO (Figure 4b) and t-ZnO/TiO2 (Figure 4c) cases. The bulk recombination rate (Γbulk NR ) can be isolated under cathodic potential because, as was discussed above for time-integrated emission measurements, minority carriers are confined to the interior of the wire (Figure 4a) and the surface term can be safely ignored. We note that for 1079

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Figure 4. In situ broadband ultrafast emission of photocatalysts. Representative time-resolved broadband emission spectra for t-ZnO/TiO2 sample operated at (a) −1 V and (b) +1 V applied potentials. Schematics show where the holes are located in the nanowire with applied potentials. (c) Kinetic traces for ag-ZnO and t-ZnO/TiO2 nanowires obtained by averaging a 10 nm bandwidth near the maximum of the band-edge emission at three critical potentials, −1 V (red), open circuit (black), and +1 V (blue). The lines are two-exponential fits to the data. We note that similar lifetimes to the open-circuit case were obtained when the samples were measured in air and with no applied bias in the photoelectrochemical cell (see Supporting Information, Figure S7).

Indeed, good agreement is obtained between the calculated depleted volume from Mott−Schottky analysis and the measured emission quenching when comparing flat band to forward biased conditions (77% versus 80%). While a greater discrepancy can be seen in ag-ZnO (54% compared to 24%), this can be readily explained using our time-resolved studies, which indicate that significant recombination occurred for carriers within the light-absorbing core region due to the high number of defects. Though not a direct measure of the depletion width in this limit, a higher emission modulation as a function of applied voltage directly correlates with an increased performance. In summary, concurrent in situ optical and electrical studies provide a wealth of quantitative information about the spatial and temporal details of the recombination processes in multilayered semiconductor photocatalytic materials. Critically, achieving a wider space-charge region allows greater charge separation on ultrafast time scales and suppresses recombination on slower time scales (microseconds−milliseconds). While this was achieved here with better material crystallinity and the addition of a surface layer,22 recent studies of doping-controlled modification of the electron/hole diffusion lengths results in similar effect of enhanced water-splitting activity.39,40 This study provides a means to evaluate the efficacy of multifunctional heterostructure designs where each step critical for the water-splitting process with its inherent challenges can potentially be individually addressed with material having specific, desired properties. Such in situ optical studies also allow for high throughput materials characterization and evaluation of fabrication/treatment protocols, even in nonideal structures with varying dimensions.

matches closely to that of the measured carrier density difference (∼4.6×), suggesting a kinetic process that is first order in the number of defects. After all rate constants are determined at anodic bias, the total quantum yield for charge separation and transport (theoretical maximum IPCE) can be determined using eq 1, and knowing the overall IPCE (Figure 1c) we can additionally deduce the surface quantum yield transfer (Table 1). For the high-efficiency material (t-ZnO/TiO2), we find that 94% of photoexcited carriers reach the surface. Together with the measured IPCE of 83%, we infer that 88% of the carriers are utilized in the chemical reaction. By comparing the IPCE of this sample to the identically treated but uncoated sample (t-ZnO) we conclude that in addition to preventing corrosion, the TiO2 surface increases surface activity by ∼14%. We note that direct absorption within the 1 nm TiO2 surface layer is not a measurable contributor to the overall enhancement in activity (see Supporting Information Figure S8). A similar analysis is made for the as-grown ZnO NWs, where the maximum IPCE is calculated to be 63%. This decrease is directly linked to the increased bulk recombination rate and the smaller depleted volume. This means that ∼37% of the IPCE is lost through ultrafast recombination processes, making it a primary bottleneck for photocatalytic efficiency. Using a similar analysis for ag-ZnO and ag-ZnO/TiO2 we find that the TiO2 layer reduces again surface recombination by ∼14%, from 35% to 21% respectively. These kinetic results support the use of in situ timeintegrated emission (steady-state PL) methods to characterize the efficiency of nanostructured material systems with varying length scales, when direct determination of the depletion width is not readily possible (e.g., via impedance spectroscopy). 1080

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of this high-efficiency ultrafast emission spectrometer have been reported in ref 36.

Table 1. Summary Comparing Performance of All Nanowire Photocatalysts with and without Treatment or Addition of a Surface Layera ag-ZnO IPCE Vdep./Vtotal ΓR Γbulk NR Γneutral CS QYtotal trans QYchem

ag-ZnO/TiO2

41%

50%

t-ZnO 70%

54%

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t-ZnO/TiO2

* Supporting Information

83%

Detailed description of all the fabrication steps, accompanied by X-ray diffraction and fluence-dependence photoluminescence studies that track how each fabrication step changes the efficiency of the heterostructures. Mott−Schottky analysis is used to determine the carrier densities of the samples, together with modeling the changes in the space-charge region as a function of applied bias. Ex situ time-resolved emission measurements are also presented for all samples, while additional in situ measurements are shown. Finally, quantum yield for transport calculations are presented and compared to incident-photon-to-current-efficiencies for the heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

77% 1/(340 ps)

65%

1/(24 ps) 1/(91 ps) 63% 79%

ASSOCIATED CONTENT

S

1/(110 ps) 1/(28 ps] 94% 74% 88%

a

All parameters were obtained under an anodic potential of +1 V, where maximum current was achieved (I−V studies) except for the radiative recombination (ΓRad) parameter that is intrinsic to the material and the non-radiative bulk rate (Γbulk NR ) obtained under cathodic potential, that is, when the minority charge carriers (holes) were shielded from surface effects (Figure 3, schematics).

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Methods. Fabrication of ZnO Nanowires. Single crystalline ZnO-NW arrays grown by a seed-mediated hydrothermal method was chosen in order to minimize grain boundary effects41,42 and maintain efficient majority carrier transport channels43−45 while the vertical nature of the growth on a conductive substrate allowed individual control of each nanowire when electrically biased with efficient turnover frequency (TOF).28,46 In a typical growth, the ZnO seed was deposited on a conductive indium tin oxide (ITO) glass by the atomic layer deposition of a 2 nm ZnO layer, using diethylzinc and water as precursors. The seeded substrate is then placed in the growth solution, an aqueous solution of zinc nitrate and hexamethylenetetramine that is heated to 75 °C, for 120 min. After the growth, the nanowire array is thoroughly washed with deionized water and dried in air. Following this, select samples have undergone various postprocessing techniques, including thermal annealing at 500 °C for 15 min and oxygen plasma cleaning. Atomic Layer Deposition of TiO2. The TiO2 shell is coated over the ZnO nanowire array by atomic layer deposition, using titanium isopropoxide and water as precursors. The deposition is carried out at 250 °C for 100 cycles (nominal deposition rate 0.01 nm/cycle) under 300 mTorr of nitrogen carrier gas flowing at 20 sccm. Optical Characterization. Incident-photon-to-current efficiency was performed in a custom-built three-electrode quartzwindowed photoelectrochemical cell with a 150 W solar simulator with AM 1.5G filter (Newport). The incident was calibrated. Time-integrated emission measurement was carried out with the same laser used for ultrafast emission studies for consistency. The experiments were conducted using a commercial Ti:sapphire laser (800 nm, 100 fs, 3.5 mJ, 1 kHz) with the output of an output of an optical parametric amplifier as the excitation source tuned to 280 nm. A portion of the fundamental beam was focused onto the Kerr material (here benzene with a temporal resolution of ∼500 fs) for gating the ZnO emission. The now gated emission signal is hereafter collimated and refocused onto an imaging spectrometer (iHR320), equipped with a nitrogen-cooled, charge-coupled camera (Symphony II, HORIBA Jobin Yvon). In order to reduce acquisition time and aid analysis of the kinetics by equally weighting the dynamics on every time scale, a progressive time scan was also implemented. Extensive details

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

M.L. fabricated the samples. K.A. and M.L. characterized the electrical and optical properties of the samples. K.A. and M.Y.S. analyzed and interpreted the ultrafast data. M.Y.S. supervised the project. All authors helped write the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research is carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The authors thank M. Hybertsen for his insightful discussions.

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DOI: 10.1021/nl504035j Nano Lett. 2015, 15, 1076−1082