Impact of Nanostructuring on the Photoelectrochemical Performance

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Impact of Nanostructuring on the Photoelectrochemical Performance of Si-TaN Nanowire Photoanodes 3

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Ieva Narkeviciute, and Thomas F. Jaramillo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08690 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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The Journal of Physical Chemistry

Impact of Nanostructuring on the Photoelectrochemical Performance of Si/Ta3N5 Nanowire Photoanodes Ieva Narkeviciute† and Thomas F. Jaramillo*† †

Department of Chemical Engineering, Stanford University, Stanford, California 94035, USA

*E-mail: [email protected]

Abstract: The nanostructuring of light absorbing materials in photoelectrochemical applications can potentially improve the performance of charge transport limited semiconductors by increasing incident light absorption as well as the electrochemically active surface area. However, a drawback associated with an increase in electrode surface area is the increased effect of surface recombination on device performance. To understand the interplay of the positive and negative impacts of nanostructuring, we studied these effects by varying the nanowire length and thereby surface area, on the photoelectrochemical performance of tandem core-shell Si/Ta3N5 photoanodes. Si/Ta3N5 nanowires of different lengths, 1.2 to 3.3 µm, were fabricated by changing the reactive ion etch duration by which the Si nanowires are formed and subsequently characterized by optical UV-Vis reflectance measurements, effective charge carrier lifetime measurements, and photoelectrochemical ferrocyanide oxidation. Overall, we show that as the nanowire length is increased, the photovoltage decreases due to decreasing effective carrier lifetimes that arise from higher surface recombination. On the other hand, the device photocurrent increases as the nanowires become longer due to increasing electrochemically active surface area and decreased light reflection, which in turn increases absorption due to light trapping within the nanowires. Balancing these effects is crucial toward developing high performance devices. 1 ACS Paragon Plus Environment


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Introduction: The photoelectrochemical (PEC) production of hydrogen by way of water splitting is a promising means of storing energy from solar radiation in the form of chemical bonds, producing a fuel that can be transported and utilized as necessary.1-2 A tandem system with two light absorbers—a photoanode and a photocathode— which can drive the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), respectively, has been shown to be able to convert water to hydrogen and oxygen more efficiently than a single light absorber.3-4 Recently, tantalum nitride (Ta3N5) has emerged as a promising candidate photoanode material due to its reasonably sized bandgap for a tandem system (2.1 eV)3 and valence and conduction band positions that straddle the HER and OER redox potentials.5 Despite a sufficiently negative conduction band position and an expected photocurrent onset close to 0 V versus RHE, the photovoltage obtained from Ta3N5 is substantially lower than expected, and photocurrent does not typically onset until applied potentials of at least 0.6 to 0.8 V versus RHE.5-10 Hence, achieving earlier photocurrent onset with Ta3N5-based devices is a significant scientific challenge for future implementation of Ta3N5 in water splitting technologies. One strategy that has been used in our previous work to help improve the photovoltage of Ta3N5 devices is to pair Ta3N5 with other semiconductors to form tandem heterostructures, e.g. nanostructured tandem Si/Ta3N5 photoanodes.11 In that work, we found that the incorporation of a semiconducting, n-type Si added an additional 200 mV of photovoltage to the device. Optimization of the synthesis conditions and therefore the morphology and composition of Ta3N5 in these tandem Si/Ta3N5 systems can improve the device photovoltage by an additional 100-200 mV.12 Furthermore, device modeling calculations show that a tandem system with Si as a photocathode and Ta3N5 as

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a photoanode could reach a solar-to-hydrogen efficiency as high as 15%.3 These results motivate further studies into two semiconductors in a tandem structure. While employing Si in tandem heterostructures can improve photovoltage, nanostructured Si can also serve as a high surface area scaffold for Ta3N5 to improve photocurrent, by allowing for both efficient charge carrier collection and sufficient light absorption. Due to the poor mobility and low lifetimes of charge carriers in Ta3N5,8, 13 nanostructuring and orthogonalizing light absorption and charge carrier collection1, 14 has been a successful strategy to increase photocurrent from Ta3N5 in the form of nanorods, nanowires and nanotubes.7, 11, 15 This nanostructuring strategy enables high surface area and efficient charge extraction because minority carriers do not have to travel distances greater than their diffusion lengths and are collected efficiently at the semiconductor-liquid junction. Therefore, increasing the electrochemically active, or accessible, surface area has the potential to improve PEC performance in Ta3N5 devices.16-17 Another advantage of nanostructuring is optical property improvement by decreasing light reflection and increasing light absorption. Black Si, produced by reactive ion etching, has been shown to effectively suppress light reflection over a wide range of incident wavelengths and angles, and these antireflective properties improve with increasing Si wire length.18 In other work, ordered Si nanowire arrays produced by nanosphere lithography and subsequent reactive ion etching increase light absorption by highly efficient light scattering and thereby increase the path length of solar radiation by a factor of 73.19 Tapered Si microwire arrays have also been shown to have extremely low reflectivity and absorption enhancement due to coupling to waveguide modes.20 Therefore, a nanostructured Si scaffold has the opportunity to increase light absorption for thin Ta3N5 layers. The nanostructured core-shell system can provide both 3 ACS Paragon Plus Environment


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beneficial light absorption and carrier collection properties, however, the advantages associated with increased surface area and optical properties do not come without potential drawbacks— namely the significant increase in surface recombination at the semiconductor interfaces that can decrease short circuit current and open circuit voltage by increasing dark saturation current.21 The optimal device morphology would depend on the interplay of these competing effects which motivates the study herein. In this work, we fabricated dual-absorber Si/Ta3N5 devices of different aspect ratios and studied the effects of nanowire length on the photoelectrochemical performance of tandem core-shell Si/Ta3N5 photoanodes to optimize their performance for applications in PEC water splitting cells. While a similar approach has previously been shown to be successful for single absorber photovoltaic19, 21-22 and photoelectrochemical water splitting applications, 23-27 fewer studies exist for tandem PEC systems.28-30 By varying the deep reactive ion etch time in the fabrication procedure, we obtained Si nanowires ranging in length from 1.2 ± 0.2 µm to 3.3 ± 0.3 µm and thereby roughness factors ranging from 6.4 to 15.4. To characterize and understand optical property improvements in the nanostructure, we performed UV-Vis reflectance measurements. We also measured the effective carrier lifetime (τeff) to study the effect of increased surface area on the quality of the Si cores in the tandem core-shell architecture. The overall trends seen in this work are a decreasing photovoltage and increasing photocurrent with increasing nanowire length due to increased surface area and increased light absorption, respectively. This study highlights the important considerations that need to be taken into account when designing tandem nanostructured devices.


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Experimental Methods: Si/Ta3N5 device synthesis and characterization Core-shell tandem Si/Ta3N5 devices were synthesized as previously reported11 on single crystal 500 μm thick, 100 mm diameter, P doped n-type Si (100) wafers with 1-5 Ω cm resistivity (semiconducting n-type Si) and As doped Si (100) wafers with 0.001-0.005 Ω cm resistivity (degenerately doped, n+ Si) (WRS Materials and University Wafer Inc., respectively). n-type Si was used for the fabrication of Si/Ta3N5 photoanodes, while n+ Si was used for roughness factor measurements, as described below. Briefly, the Si nanowires were fabricated by a top-down pseudo-Bosch process etching of a Si wafer (Surface Technology Systems ICP Deep Reactive Ion Etcher) where the length of the Si nanowires was varied by different etch times (19, 15, 13, and 11 minutes) and the dimensions of the wires were imaged and measured using crosssectional scanning electron microscopy, SEM (FEI Magellan 400 XHR) and imageJ analysis. Subsequently a conformal thin film (~ 12 nm) of Ta2O5 was deposited onto the nanowires by thermal atomic layer deposition (ALD) using pentakis(dimethylamino)-tantalum (PDMAT) (99% Strem Chemicals) and water in a Cambridge Nanotech Savannah ALD reactor. Finally the Ta2O5 was nitrided to Ta3N5 at 800 °C in a 3-zone Mellen fused silica tube furnace (Mellen Company SC12.5R, three zones) for 8 hr in anhydrous ammonia, NH3, (99.995% Praxair). The samples were stored in a desiccator under vacuum. The thickness of the Ta3N5 film after nitridation was ~10 nm as measured by cross-sectional SEM. Optical UV-Vis measurements of Si wafer reflectance were measured in a custom-fitted integrating sphere (8” AdaptaSphere, Labsphere Inc.), the inside of which is coated with a back-



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scattering barium sulfate BaSO4 layer, illuminated by a 1000 W Xe lamp (Newport Corporation).31 Lifetime measurements Carrier lifetime measurements were performed on a Sinton Instruments WCT-100 wafer lifetime measurement tool in quasi-steady state photoconductance decay mode.32-33 The wafer inductance was tuned for each wafer and an optical constant of 0.74 was set to account for the reduced reflectance off of the Si nanostructures as calculated from PC1D simulations from a model provided by Sinton Instruments. Planar Si recombination lifetimes were measured with the standard optical constant of 0.7. Carrier recombination lifetimes are reported at a minority carrier density ∆p, which is equivalent to excess carrier density ∆n of 8 x 1014 cm-3. Lifetimes were measured on > 12 cm2 Si wafer pieces coated with 12 nm of Ta2O5 on the front surface and without any processing on the unpolished back surface. (Photo)electrochemical Testing All (photo)electrochemical testing was performed using a Bio-Logic VSP potentiostat in a three electrode configuration with a reference electrode and a Pt wire counter electrode. Capacitive current measurements were performed to determine the roughness factor (RF) of the nanostructured Si. Cyclic voltammograms (CVs) at sweep rates of 25, 50, 75, 100, 200 and 300 mV/s in a voltage range of -0.82 to -0.72 V versus Hg|HgSO4 (-0.11 to -0.01 V versus RHE) were collected in 0.5 M H2SO4 electrolyte (pH 0.3, Sigma Aldrich 99.999% metals basis) where faradaic processes did not contribute to any redox features (Figure S1a). Photoelectrochemical performance was evaluated in a solution of 0.1 M K4Fe(CN)6 and 1 mM K3Fe(CN)6 (pH 8.5-9.0) (99%+ Acros). The Fe(CN)63-/Fe(CN)64- reversible redox couple 6 ACS Paragon Plus Environment

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requires a single electron transfer to oxidize Fe(CN)64- to Fe(CN)63-. The reversible potential for Fe(CN)63-/Fe(CN)64- at the concentrations mentioned above is 0.98-1.01 V versus RHE. PEC experiments were performed in a quartz cell with a Ag|AgCl reference electrode in Ar-purged electrolyte. To make PEC devices, Ga-In eutectic (99.99% Aldrich) and a wire attached to the eutectic using conductive carbon paste (Electrodag, Ted Pella DAG-502) were used to make electrical contact to the backside of the Si. Epoxy (Loctite 9462 Hysol) was used to mask the back and sides of the electrode and a digital photograph of the electrodes was taken and analyzed with ImageJ to measure active electrode areas between 0.1-0.2 cm2. Electrodes were illuminated by a 1000 W Xe arc lamp (Newport) equipped with a water filter to remove infrared radiation. The light intensity was calibrated by summing above-bandgap photons such that the number of incident photons was equivalent to that of an integrated AM 1.5G spectrum for λ < 590 nm11, 34 using a spectrometer (Ocean Optics Jaz EL 200-XR1). Illuminated and dark linear sweep voltammograms (LSVs) were collected at a scan rate of 10 mV/s. Results and Discussion Nanowire characterization Si nanowires of different lengths were fabricated by varying reactive ion etch time and the architectures were characterized by scanning electron microscopy and electrochemical capacitance measurements. In previous studies, the Si nanowires were approximately 3 µm in length after etching for 19 min. In this study, we systematically fabricated shorter nanowires by decreasing etch time, aiming to explore effects on photovoltage and photocurrent of the tandem devices. Figure 1 shows scanning electron micrographs of Si nanowires reactive ion etched, from left to right, for 19, 15, 13 and 11 minutes and coated with Ta2O5 by ALD. The pseudo-Bosch



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deep reactive ion etching process produces “black” Si nanowires that are tapered at the tips. The wire length and base diameter dimensions after etching and Ta2O5 deposition are summarized in Table 1, where a 0 min etch time refers to an unetched, planar Si wafer. As the etch time is decreased, the wire length as well as the wire base diameter decrease due to a truncation of the conically shaped nanowires at an earlier point in the etch process. Overall, the estimated aspect ratio (wire length to wire base diameter) decreases from 18.3 after a 19 min etch to 10 after an 11 min etch. The packing density of nanowires for all nanowire lengths studied was roughly 26 nanowires per µm2.

Figure 1. Scanning electron micrographs of core-shell n Si/Ta2O5 nanowires etched for a) 19, b) 15, c) 13 and d) 11 min, showing the decreasing nanowire length and base diameter with deep reactive ion etch time. Table 1. Si/Ta3N5 Nanowire Dimensions and Roughness Factor as a Function of Deep Reactive Ion Etch Time Etch time (min) 19 15 13 11 0

Wire length ± σ (µm) 3.3 ± 0.3 2.5 ± 0.2 1.8 ± 0.2 1.2 ± 0.2 0

Wire base diameter ± σ (nm) 180 ± 30 150 ± 40 140 ± 20 120 ± 40 N/A

Roughness factor 15.3 11.9 8.8 6.4 1

The roughness factor, or surface area, of the Si nanowires was determined by non-faradaic electrochemical capacitive current measurements.35-36 Because electrochemical capacitance is difficult to measure on semiconducting materials due to the space charge region in the 8 ACS Paragon Plus Environment

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semiconductor hindering accurate double-layer charging of the ions in the electrolyte, degenerately doped n+ Si was used in these measurements as it exhibits behavior that is more metallic in nature and achieves more accurate double-layer charging. Capacitive current or charging current, ic is proportional to cyclic voltammetry scan rate, υ and the electrochemical surface area, A as ∝ 36-37 and thus can be used to approximate differences in electrode areas as a function of scan rate. Capacitive current was measured between -0.11 and -0.01 V versus RHE where discernable faradaic processes were not occurring (Figure S1a). The average of the absolute values of the anodic and cathodic currents at -0.07 V versus RHE was determined and plotted as a function of sweep rate (Figure S1b) for planar and nanostructured n+ Si etched for 19, 15, 13, and 11 min. The ratio of capacitive current at a given sweep rate between different nanowire lengths gives the roughness factor of the nanowire electrodes, where the planar Si current is normalized to a roughness factor (RF) of 1. As there can be small variations in etch rate in the tool, the nanowire length of the n+ wires was rigorously measured from cross-sectional SEM and imageJ analysis and a standard curve of nanowire length versus roughness factor (as measured from capacitance) was constructed (Figure S1c). A line of best fit was applied to the data yielding the linear equation RF = 4.362L + 1.087, where L is wire length; and from this standard curve and equation, the RF of n-type Si in Si/Ta3N5 devices was determined and summarized in Table 1. Thus, by nanostructuring, we increased the surface area of the electrodes by as much as 15 times with the longest etch time of 19 min. Optical measurements One of the major advantages of the nanostructured Si involves the antireflective and lighttrapping properties of these electrodes. To quantify the antireflective properties of the nanowire arrays, we measured reflectance inside an integrating sphere on planar n Si and etched n Si wires 9 ACS Paragon Plus Environment


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of 3.3, 2.5, 1.8, 1.2 µm lengths still attached to the Si substrate. To get an accurate measurement of reflectance from the Si nanowire substrate, a baseline of light scattering (second and third light interactions with the substrate) occurring within the integrating sphere was taken with the sample rotated off axis, not in the direct path of the light beam.31 As shown in Figure 2, the planar Si has similar reflectance as reported by other sources,18, 21, 38 but the etched Si nanowires are much less reflective, with reflectance decreasing as wire length increases over all wavelengths reported from 360 to 800 nm. At 590 nm, which is the absorption edge of Ta3N5, the reflectance of planar Si is 33.7%. For the 1.2, 1.8, 2.5, and 3.3 µm long wires, the measured reflectances are 4.5, 2.5, 2.0, and 2.1% respectively. Figure S2 shows the reflectance on a logarithmic scale to facilitate easier differentiation in reflectance values for the different nanowire lengths.

The total light balance in the optical UV-Vis integrating sphere measurements is: = % + + + +

(1)

where I0 is incident light, A% is absorptance, T is transmittance, Rs is specular reflectance, Rd is diffuse reflectance, and S is scattered light (forward-scattered light off of the back of the sample).31, 39 Because the specular and diffuse reflectance measurements were taken with the nanowires attached to the underlying ~ 500 µm thick Si substrate, light is not being transmitted through nor scattering off of the back of the Si and the T and S terms in the light balance are equivalent to zero. Therefore, the remaining light balance is I0 = A% + Rs + Rd, thus light that is not being reflected by the samples is absorbed. Because light reflectance is nearly completely quenched with the nanostructured Si wires and the Rs + Rd terms approach zero, we can conclude that nearly all incident light is being absorbed within the Si. Previous studies on the absorptance


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of reactive ion etched Si nanowires that were lifted off of the Si substrate showed that as the nanowire length was increased, more light trapping occurred within the nanostructure and effectively all of the light was absorbed by the nanowires.19 This effect has also been shown for tapered Si microwires and an additional effect contributing to efficient light absorption beyond light trapping is the enhanced coupling of incident light into waveguide modes.20 Therefore, the black Si substrates used in our study enable efficient light absorption in the Si/Ta3N5 samples by reducing light reflection and enhancing light trapping.

Figure 2. UV-Vis optical reflectance of the black Si scaffolds with Si nanowires of different lengths. Figure S2 shows the same measurements on a logarithmic reflectance scale. Carrier lifetime measurements To assess the effects of nanostructuring Si on the charge carrier properties and lifetimes, we measured the effective minority carrier lifetimes, τeff of the n-type Si cores using photoconductance decay measurements. τeff encompasses several recombination mechanisms including the bulk processes (radiative, Auger and Shockley-Read-Hall recombination) and surface recombination:40-41

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We performed these measurements in a quasi-steady state configuration where the effective minority carrier lifetime is found as a function of excess carrier density, ∆n under steady state photogeneration, G:32-33, 42 #$%% =

∆' (

.

(3)

Lifetime depends on carrier injection level, hence we report it at a given minority carrier density, ∆p, where ∆p=∆n because electron-hole pairs are photogenerated upon light illumination.33 As τeff encompasses both surface and bulk lifetimes, in theory, a high-quality passivating layer such as a thermal silicon dioxide,43 silicon nitride,44 or alumina45 should passivate the surface states of Si well enough to yield a sufficiently low surface recombination velocity and therefore a quantifiable bulk lifetime.33 In our system, we employed a passivation layer in the form of Ta2O5 by atomic layer deposition. The Ta2O5 is also the precursor to Ta3N5 and while this passivation layer does not perform as well as the more conventional passivation layers noted above, it is relevant to the PEC devices that we are fabricating and serves the purpose of measuring the effects of surface recombination with different length Si nanowires. Figure 3 shows the effective lifetimes as a function of nanowire length on the bottom x-axis and roughness factor on the top x-axis. There is nearly an order of magnitude decrease in effective lifetime as the length, and thereby roughness factor of the nanowires increases. Whereas the planar Si (RF 1) has a lifetime of ~160 µs, the value decreases to less than 20 µs for the 3.3 µm long wires (RF 15.3). Decreases in effective lifetime with nanostructuring due to increased surface recombination has previously been reported in the Si photovoltaic literature.21, 46-47 This decrease in lifetime is due to an increased surface area to volume ratio, resulting in a higher density of defects at which recombination can occur.22 The increased surface recombination 12 ACS Paragon Plus Environment

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could also be exacerbated due to the surface damage endured by the Si surface during reactive ion etching. Normally, after reactive ion etching, the top several nanometers are residue that are removed during the cleaning steps, the subsequent layer is a heavily damaged Si crystal lattice (10-20 nm), then another layer with impurities and particles from the etching gas (50-80 nm), and finally vacancies that can diffuse 1-1.5 µm into the Si.48-49 Naturally, all of these defects can contribute to Shockley-Read-Hall charge carrier recombination and therefore decrease the effective lifetime of the samples after nanowire etching.49 Previous studies have shown that the surface damage sustained by reactive ion etched Si contributes to a higher surface recombination velocity (roughly 6-fold higher) compared to chemically etched Si.50-51 Thus, from an increase in surface area and damage to the Si surface, we found that there is a decrease in effective minority carrier lifetime with an increase in Si nanowire length using quasi-steady state photoconductance measurements.

Figure 3. Effective minority carrier lifetime as a function of Si nanowire length (bottom axis) and roughness factor (top axis).



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Photoelectrochemical testing After characterizing the properties of the Si nanowires in terms of length, roughness factor, and effective minority carrier lifetime, we tested the photoelectrochemical activity for ferrocyanide oxidation in a 0.1 M K4Fe(CN)6 and 1 mM K3Fe(CN)6 electrolyte.

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Photovoltage trends

The first performance metric that we evaluated was the photovoltage of the devices. One method to determine photovoltage is by the potential at which the photocurrent onsets—where an onset at a lower applied potential represents a higher photovoltage. First, we examine the onset potential from linear sweep voltammograms in tandem Si/Ta3N5 photoanodes of different nanowire lengths shown in Figure 4a. For statistical significance, we tested at least three samples at each nanowire length and plotted the photocurrent onset potential as a function of nanowire length in Figure 4b. The LSVs of the other samples are included in Figure S3. We observe that as the nanowire length decreases, the onset potential for photocurrent shifts towards more negative potentials, as evaluated at the potential at which a geometric photocurrent density of 0.1 mA/cm2geo is achieved. This shows that higher photovoltages are achieved with shorter nanowires. This result is corroborated by open-circuit potential, OCP, measurements shown in Figure 4c. In OCP measurements, the difference between the electrode potential in the dark and illuminated is the photovoltage, and a larger difference between dark and illuminated OCP defines a higher photovoltage. For the OCP measurements, the steady state dark OCP value varied by about 40 mV over the course of the experiments, so to facilitate direct comparison of photovoltage, the dark OCP for all samples were shifted to the same value. Like the LSVs in Figure 4a, the OCP 14 ACS Paragon Plus Environment

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measurements in Figure 4c also show that the photovoltage increases as the nanowire length decreases. Thus, from LSV and OCP measurements, we have shown that the Si/Ta3N5 device photovoltage increases as the nanowires become shorter.

Figure 4. a) LSVs of photoelectrochemical ferrocyanide oxidation illuminated and in the dark (dashed lines) b) open-circuit potential in of 0.1 M K4Fe(CN)6 and 1 mM K3Fe(CN)6 and statistics of c) photocurrent onset voltage measured at 0.1 mA cm-2 and d) photocurrent density at 0.8 V vs. RHE for Si/Ta3N5 in a planar configuration and with different Si nanowire lengths. To understand the differences in photovoltage with nanowire length, we consider the differences in surface area and surface recombination of these electrodes. Recall that in the lifetime measurements, shorter nanowires exhibited longer lifetimes due to a lower surface area to volume ratio and thus lower surface recombination. In photovoltaic cells, increased surface area and thereby increased surface recombination leads to an decrease in open-circuit voltage and



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short circuit current22, 52 and we expect a similar impact on a photoelectrochemical cell. Hence, we ascribe the higher photovoltage in shorter nanowires to their lower surface area.53 Again, drawing from the photovoltaics literature, the expected decrease in photovoltage of an ideal system due to increased surface area is 60 mV for every order of magnitude increase in surface area.19, 53-55 Analysis of our data in Table 1 and Figure 4b shows that a two-fold increase in roughness factor results in a 90 mV difference in photocurrent onset, or photovoltage, between the 1.2 µm long wires (RF 6.4) and the 3.3 µm long wires (RF 15.3). This more substantial decrease in photovoltage is associated with the increase in the number of interfaces—Si/Ta3N5, Ta3N5/electrolyte—that likely contain defect sites which can also contribute to recombination. Overall, by varying the nanowire length in Si/Ta3N5 photoanodes, we have shown that decreasing the nanowire length increases the device photovoltage.

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Photocurrent trends

Photocurrent is another important metric to consider when evaluating PEC performance, as it correlates to the solar-to-hydrogen efficiency of a PEC cell. From device physics in photovoltaics, a decrease in photocurrent would be expected with an increase in nanowire length and surface area, assuming similar absorption characteristics.56 However we observe the opposite result where increasing nanowire length leads to increasing photocurrent (Figure 4a,d) as measured at 0.8 V versus RHE, indicating that other factors need to be considered. At this applied potential, only Ta3N5 is the contributing photoanode as n-type Si does not onset until roughly 1 V versus RHE.57-59 Thus, at applied potentials lower than 1 V, e.g. at the 0.8 V used in these studies, the longer nanowires actually improve photocurrent. This could be due to improved light absorption occurring in Ta3N5. Based on the absorption coefficient of Ta3N5 at a wavelength of 540 nm,35 90% of the light should be absorbed within ~ 400 nm of material as 16 ACS Paragon Plus Environment

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calculated using the Beer-Lambert Law. Considering that the Ta3N5 shell thickness is only 10 nm and the aspect ratio of the 3.3 µm long nanowires, light that is at normal incidence on the tandem structure that is not scattered is fully absorbed upon passing through roughly 400 nm of Ta3N5. For a photoelectrochemical device working in the field, incident light may not be exactly normal to the surface and would likely pass through an effectively thinner or thicker layer of Ta3N5, or might be scattered or transmitted through the nanostructure. Therefore, for the light to be absorbed by the Ta3N5 and/or Si, it likely requires multiple passes through the absorber materials during transmission and scattering events.53 When the Si/Ta3N5 nanostructure is composed of longer nanowires, there is more light trapping occurring within the nanostructure19 and these events increase the probability of light absorption in Ta3N5 and therefore increase the photocurrent. The increase in current due to effective light trapping has previously been shown for pure Si nanowire architectures for photovoltaic applications.19 Another advantage beyond light trapping that could increase current in a photoelectrochemical cell is the increased number of active sites present for electrochemical reaction with higher surface area.28 As the nanowire length increases, surface area increases, and thus there are more electrochemically active sites for ferrocyanide oxidation to occur. Thus, we find that the increase in light absorption as well as electrochemically active surface area can overcome the increase in charge carrier recombination and therefore can lead to increased photocurrent with increasing nanowire length. Overall, we have determined that as nanowires become longer, the photovoltage decreases while the photocurrent increases, hence there is a trade-off in performance. This performance trade-off may be avoidable through the exploration of different nanostructures. Because the Si nanowires in this study are very narrow, they likely contain a large number of defects throughout their 17 ACS Paragon Plus Environment


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length from the reactive ion etching process. Thus, increasing the dimensions of the nanowires may improve performance. Furthermore, studies on nanowire packing density and shape may also lead to better performance, similar to the studies performed on photovoltaic InP nanowires and Si microwires.20, 60-62 We note that the studies presented herein are focused on the fundamental photoelectrochemical performance of the core-shell Si/Ta3N5 nanowires using ferrocyanide, rather than on photoelectrochemical water splitting. However, we have shown previously that similarly fabricated tandem Si/Ta3N5 devices11 (with ~ 3 µm long Si nanowires) are capable of water oxidation, demonstrating photocurrent onset comparable to some of the highest performance Ta3N5 photoanodes shown to date, around 0.6 V versus RHE.7, 9, 15, 63 Photocurrents for these devices reach roughly 3 mA/cm2 at the water oxidation potential, which are approximately 50 % of state-of-the-art Ta3N5 photoanodes.7, 9, 64 Conclusion In this study, core-shell Si/Ta3N5 nanowire photoanodes were fabricated with varying nanowire lengths to study the effects of nanostructuring and device surface area on the photoelectrochemical performance. By changing deep reactive ion etch time for the Si nanowire fabrication, we were able to control the nanowire length and diameter and thereby the device roughness factor/surface area. As the nanowire length was increased the reflectance of the Si scaffold was significantly decreased to values as low as 2%. However, the decrease in reflectance associated with long nanowire lengths also caused a significant decrease in the effective lifetime of minority carriers as measured with quasi-steady state photoconductance decay. These concerted effects had interesting implications for the photoelectrochemical


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performance of Si/Ta3N5 photoanodes—as the nanowires became longer, the photocurrent increased but the photovoltage decreased. The increased photocurrent is ascribed to the superior optical properties and electrochemically active surface area of the nanostructured electrodes, while the photovoltage decreases due to the increased surface recombination occurring at the solid-solid and solid-liquid junctions with a higher surface area to volume ratio. Therefore, there is a trade-off between photovoltage and photocurrent with these nanostructured tandem Si/Ta3N5 photoelectrochemical devices. Identifying such trade-offs is a key element to designing tailored architectures for the specific needs of a photoelectrochemical device. To improve performance in the future, different nanostructured device architectures or dimensions should be explored. Furthermore, although the study herein uses ferrocyanide oxidation as a proxy to evaluate performance, water oxidation with similarly fabricated tandem Si/Ta3N5 photoanodes has been reported to have competitive photocurrent onset for Ta3N5 devices, though the photocurrent still needs improvement.11 From the architectures explored in this work, to achieve both high photovoltage and photocurrent, intermediate wire lengths of 1.8-2.5 µm should be used. Understanding the relationship between nanostructuring and device performance is important for light-absorbing materials in both the photovoltaic and photoelectrochemical device fields, where in photoelectrochemistry, the electrochemically active surface area can have an impactful role in performance. The knowledge gained in this study can be translated to other tandem absorber systems, photovoltaic or photoelectrochemical, where multiple junctions and/or surface area are critically important. Acknowledgment This work was supported by the NSF under NSF Center for Chemical Innovation CHE-1305124



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for Solar Fuels. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and Stanford Nanofabrication Facility (SNF) at Stanford University. Supporting Information. Electrochemical capacitance and roughness factor determination, UVVis reflectance on a logarithmic scale, and linear sweep voltammograms that contribute to the statistics of Figure 4 b,d.

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(11) Narkeviciute, I.; Chakthranont, P.; Mackus, A. J. M.; Hahn, C.; Pinaud, B. A.; Bent, S. F.; Jaramillo, T. F., Tandem Core–Shell Si–Ta3N5 Photoanodes for Photoelectrochemical Water Splitting. Nano Lett. 2016, 16, 7565-7572. (12) Narkeviciute, I.; Jaramillo, T. F., Effects of Ta3N5 Morphology and Composition on the Performance of Si-Ta3N5 Photoanodes. Solar RRL 2017, 1700121. (13) Morbec, J. M.; Narkeviciute, I.; Jaramillo, T. F.; Galli, G., Optoelectronic Properties of Ta3N5: A Joint Theoretical and Experimental Study. Phys. Rev. B 2014, 90, 155204. (14) Spurgeon, J. M.; Atwater, H. A.; Lewis, N. S., A Comparison Between the Behavior of Nanorod Array and Planar Cd (Se, Te) Photoelectrodes. J. Phys. Chem. C 2008, 112, 6186-6193. (15) Wang, L.; Zhou, X.; Nguyen, N. T.; Hwang, I.; Schmuki, P., Strongly Enhanced Water Splitting Performance of Ta3N5 Nanotube Photoanodes with Subnitrides. Adv. Mater. 2016, 28, 2432-2438. (16) Li, Y.; Takata, T.; Cha, D.; Takanabe, K.; Minegishi, T.; Kubota, J.; Domen, K., Vertically Aligned Ta3N5 Nanorod Arrays for Solar-Driven Photoelectrochemical Water Splitting. Adv. Mater. 2013, 25, 125-131. (17) Khan, S.; Zapata, M. J. M.; Baptista, D. L.; Gonçalves, R. V.; Fernandes, J. A.; Dupont, J.; Santos, M. J. L.; Teixeira, S. R., Effect of Oxygen Content on the Photoelectrochemical Activity of Crystallographically Preferred Oriented Porous Ta3N5 Nanotubes. J. Phys. Chem. C 2015, 119, 19906-19914. (18) Huang, Y.-F.; Chattopadhyay, S.; Jen, Y.-J.; Peng, C.-Y.; Liu, T.-A.; Hsu, Y.-K.; Pan, C.L.; Lo, H.-C.; Hsu, C.-H.; Chang, Y.-H., Improved Broadband and Quasi-Omnidirectional AntiReflection Properties with Biomimetic Silicon Nanostructures. Nat. Nanotechnol. 2007, 2, 770774. (19) Garnett, E.; Yang, P., Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 2010, 10, 1082-1087. (20) Yalamanchili, S.; Emmer, H. S.; Fountaine, K. T.; Chen, C. T.; Lewis, N. S.; Atwater, H. A., Enhanced Absorption and

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