1 Effect of Grain Boundary Cross-Section on the Performance of

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Effect of Grain Boundary Cross-Section on the Performance of Electrodeposited CuO Photocathodes 2

Chandan Das, Ashish K. Singh, Yooun Heo, Garima Aggarwal, Sandeep K. Maurya, Jan Seidel, and Balasubramaniam Kavaipatti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10103 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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The Journal of Physical Chemistry

Effect of Grain Boundary Cross-Section on the Performance of Electrodeposited Cu2O Photocathodes Chandan Das,a Ashish K. Singh,a Yooun Heo,b Garima Aggarwal,a Sandeep K. Maurya,a Jan Seidel,b and Balasubramaniam Kavaipatti a,* a Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai-400076, INDIA b School of Materials Science and Engineering, UNSW Australia, Sydney NSW 2052, AUSTRALIA

Abstract Large-grained Cu2O photocathodes in a superstrate configuration on F-doped SnO2 (FTO) coated glass substrate are synthesized via two-step electrodeposition. Only sub-micrometer sized grains were obtained during single-step electrodeposition in the potential window (-0.31 to -0.7 V vs Ag/AgCl) of stable Cu2O formation. We observe reductive decomposition of the Cu2O to Cu metal in the potential range of -0.7 to -0.98 V; bulk reduction of Cu2+ in the solution to Cu metal occurs only beyond -0.98 V. In the potential window of stable Cu2O deposition, only the growth of the few nuclei occurs until a certain time. Minimal nucleation on the pristine FTO sites occurs during this period of deposition. The time to secondary nucleation is ~ 6 min at -0.31 V and ~ 15 s at -0.37 V. Interrupting the deposition at -0.31 V after 6 min and increasing the potential to -0.37 V leads to uniform, large grains (~ 3 µm) of Cu2O. Photoinduced conducting atomic force microscopy reveals shunting and the presence of sub-bandgap states at the grain boundaries of Cu2O. Also, the lower carrier concentration (~ 1016 cm-3) in the large-grained Cu2O film obtained from Mott-Schottky analysis suggests a lower rate of Auger recombination. Thus, lowering the grain boundary cross-section in the two-step deposited film leads to a 30% increase in photocurrent at 0.0 V vs RHE.

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1. Introduction Photoelectrochemical (PEC) water splitting for hydrogen generation using earth abundant materials as a photocatalyst is one of the promising ways to store solar energy.1,2 In the materials perspective, p-type Cu2O is suitable as a photocathode due to its good light absorption in the visible range (Eg ≈ 2 eV, direct; α ≈ 104 cm-1), electrical properties and band positions that straddle the water reduction potential (offset: ~ -0.7 V vs RHE).3,4 Under 1 sun illumination, the maximum achievable photocurrent (theoretical) from Cu2O photocathodes is 14.7 mA cm-2 and the solar-to-hydrogen (STH) efficiency of 18.1%.5,6 The electrical properties of Cu2O vary according to the synthesis method and parameters used for deposition. Under certain deposition conditions, the hole mobility of 100 cm2 V-1 s-1 have been demonstrated.7 Among the various methods, electrodeposition is comparatively simpler and cost-effective to grow Cu2O at room temperature and atmospheric condition. The morphology, crystal orientation, and shape of the crystals are often controlled using specific additives, pH, bath temperature and a dramatic effect on device performance was observed.8– 12

The ease of varying the parameters in electrodeposition makes it one of the widely used

methods for the growth of Cu2O films.4,13–15 For this reason, electrodeposited Cu2O thin films, albeit with a protective layer have been investigated as photocathodes for H2 generation.14,16–19 The protective layers are used to arrest the photodegradation of the active Cu2O layer.20,21 However, there are some caveats to such a device architecture, which has motivated us to further investigate the material and device modifications in this low-cost material for enhanced performance. The protective coating along with cocatalyst nanoparticles (to enhance HER kinetics) on top of the electrode as well as the hydrogen bubbles forming at the electrode-electrolyte interface cause light scattering at the surface. In this situation, back illumination of the photocathode will 2 ACS Paragon Plus Environment

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minimize the losses due to the device architecture. This is one of the reasons to undertake the study on F-doped SnO2 (FTO) transparent substrates, enabling back illumination of light during PEC experiments. Although Cu2O electrodeposition has been studied on various substrates, it is mandatory to have an ohmic contact between the semiconductor and the substrate for optimal charge carrier transfer at the interface. Reportedly, Au, FTO, ITO (Sn doped In2O3), and Ti are a few suitable substrates for Cu2O photocathodes.22 Among all these substrates FTO is transparent and relatively inexpensive, hence used for photocathodes fabrication in this study. Another major issue associated with reduction in device performance is caused by the grain boundaries, which is higher in a film comprising smaller grains. Grain boundaries are strong electron trapping sites and a potential barrier to the charge transport process. Minimization of the grain boundaries would result in better hole mobility, thereby enhancing the device performance. Major et al. studied the grain size effect on CdTe based solar cells and observed a significant improvement in the large grained CdTe film.23 Similarly, large grained perovskite solar cells also have shown high performance due to exhibiting excellent carrier transport properties.24,25 These are a few reasons that have led to investigating on the grain size effect of Cu2O on PEC performance. The FTO substrates used in this study are polycrystalline in nature. If the nucleation is sparser, only on some of the ideally oriented planes of FTO grains that have lower interface energy with the Cu2O and follows a progressive growth, a Cu2O film comprising large grains is expected.26 In this article, we demonstrate the growth of Cu2O thin films with grain size (~ 3 µm) larger than the film thickness (~ 1.5 µm). The nucleation and grain growth was investigated by scanning electron microscopy (SEM) analysis and chronoamperometry experiments. Curbing the secondary nucleation on the FTO substrate by a two-step potential deposition led



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to the growth of large-grained Cu2O films exhibiting superior photocathode properties in comparison to the films with smaller grains. We discuss the reasons for this enhancement on the basis of lower grain boundary shunting and carrier concentration in the large-grained Cu2O films. 2. Experimental Section Electrodeposition of Cu2O was carried out using a three-electrode system, equipped with Biologic SP-150 Potentiostat/Galvanostat. F-doped SnO2 (FTO) coated glass substrates (TEC-15, Dyesol, Australia) were the working electrode with a large area Pt foil as the counter electrode and Ag/AgCl in 4 M KCl (E = 0.197 V vs RHE) as the reference electrode. The FTO substrates were ultrasonically cleaned in soap solution, 1% H2SO4, DI water and IPA respectively and then dried via N2 flushing. The precursor electrolyte for Cu2O deposition comprised of a lactate stabilized CuSO4 solution in DI-water. The required amount of CuSO4 was added to lactic acid (chelating agent) in the weight ratio of 1:6 and stirred vigorously for 1 h. 100 mL water was added to the above composition and stirred overnight such that all the Cu2+ ions form a lactate complex. The pH of the bath was maintained at 12 by adding 2 M KOH. The depositions were carried out at different potentials inside the Cu2O phase field under potentiostatic electrodeposition mode at 55 °C. The structural and phase composition of the deposited films were identified by XRD (Rigaku smart lab). The morphology and thickness of the films were obtained from FE-SEM (Zeiss Ultra 55). The phase purity of the bulk material was cross-verified by XPS (ULVAC-PHI) depth profile analysis. Conducting atomic force microscopy (c-AFM) measurements were carried out using an AFM instrument (AISTNT SmartSPM 1000) under monochromatic light of various wavelengths.


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The photoelectrochemical analysis was carried out using a three-electrode system in 1 M Na2SO4 electrolyte. The 1 sun simulated light was obtained from a Xe-lamp (Enlitech) equipped with A.M. 1.5G filter. Pt cocatalyst nanoparticles were electrodeposited from 1 mM H2PtCl6 precursor at -0.1 V (vs Ag/AgCl) for 15 min. The electrolyte for PEC experiment was sufficiently purged with Ar gas to remove the dissolved oxygen. The Ar purging continued for the duration of the experiment, although at a lower flow rate. The MottSchottky analysis was carried out in 0.1 M CH3COONa using a three-electrode system under constant Ar purging.8 3. Results and Discussion 3.1 Linear sweep voltammetry of the lactate stabilized Cu(II) electrolyte

Figure. 1. (a) Linear sweep voltammetry (LSV, cathodic) plot of the lactate stabilized Cu(II) electrolyte at pH 12. The onset potential is -0.2 V for Cu2+ to Cu+ reduction process. Inset is a magnified part of the LSV between -0.6 to -1.1 V. (b) LSV (cathodic) of Cu(II) electrolyte till -0.7 V (I, dotted red line). Later, the sample was further analyzed via LSV in KOH electrolyte (II, red line). The LSV of a continuous 8µm thick film under similar reaction condition (III, blue line). The pH of the electrolyte was maintained at 12 during LSV. Inset is SEM image of the sample after LSV in Cu(II) electrolyte till -0.7 V.

Cathodic linear sweep voltammetry (LSV) in the electrolyte used for Cu2O deposition was recorded in the range of +0.6 V to -1.2 V at different scan rates (Figure S1a). The characteristic features (reduction of Cu2+ ions to Cu+ and Cu at different potential stages) of all the voltammograms are similar, except for larger cathodic currents at higher scan rates. 5 ACS Paragon Plus Environment


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The voltammogram obtained at a scan rate of 20 mV s-1 is shown in Figure 1a, exhibiting characteristic features corresponding to the various reduction processes. Firstly, the onset potential of the reduction of Cu2+ to Cu+ is -0.2 V vs Ag/AgCl electrode; more evident in the inset of Figure S1a. On further cathodic scanning, the current increases and reaches a maximum at -0.7 V. The potential range from -0.2 to -0.7 V can be attributed to the Cu2+ reduction to Cu+, resulting in Cu2O deposition. Later, the current gradually reduces till -0.84 V and again starts increasing till -0.98 V. The latter rapid increase in current after -1.06 V can be ascribed to the bulk reduction of Cu2+ to Cu. The intermediate features in the potential range of -0.7 to -0.98 V were not clearly observed in the earlier studies.9,27 Instead, the LSV for the lactate stabilized Cu(II) electrolyte at pH 9 featured a plateau within this range when a Cu2O pre-layer was used.28 The plateau formation was attributed to a Schottky like rectifying junction at the semiconductor/solution interface. When no Cu2O pre-layer was used, only the two features corresponding to the Cu2+ to Cu+ reduction and Cu2+ to Cu reduction processes were observed.9 Therefore, it is imperative to investigate the intermediate reduction processes in our deposition bath conditions. In the following, the reduction processes are elucidated. We performed LSV at a scan rate of 20 mV s-1 in lactate stabilized Cu(II) electrolyte till the first current maximum (at -0.7 V) was attained as shown in Figure 1b (I, red short dash). With an onset potential of -0.2 V, this implies that the deposition occurred for a total of 25 s. This results in the formation of a film with Cu2O islands as shown in the inset of Figure 1b. Cathodic scans beyond this potential can exhibit various reduction processes either at the FTO/electrolyte interface or the Cu2O/electrolyte interface. The possible reduction processes are (i) further deposition of Cu2O (Cu2+ to Cu+), (ii) metallic Cu deposition (Cu2+ to Cu), and (iii) decomposition of the Cu2O islands to Cu metal (Cu+ to Cu).


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Among the three possible processes, testing in an electrolyte devoid of Cu2+ ions would provide information regarding the electrochemical decomposition of the Cu2O islands. The solid red curve in Figure 1b shows the LSV of the sparsely grown film (inset) recorded in KOH electrolyte at pH 12. The reduction process has an onset potential of ~ -0.7 V, with the current maximum at -0.98 V. The only possible contribution to the current in this voltammogram would be the Cu+ to Cu reduction. A current maximum is observed in this potential range due to the limited Cu+ available in the Cu2O islands. The LSV of a thick (~ 8

µm), continuous Cu2O film [Figure 1b, blue curve (III)] exhibits the same onset potential as the Cu2O islands. However, the current maximum is not observed in this potential range as there is ample Cu+ in the case of continuous film. Beyond -1.3 V, the bulk material peels off due to the fast reduction of Cu2O. Having established that the Cu+ to Cu reduction occurs beyond -0.7 V, we revisit the features observed between -0.7 V and -0.98 V in Figure 1a. The current at any potential beyond -0.7 V should have contributions from both Cu formation and Cu2O deposition as these potentials are cathodic with respect to both the onset potentials. However, it can be observed from Figure 1a that the current is slightly reduced between -0.7 V and -0.84 V. For instance, the current at -0.77 V is -3.38 mA cm-2, a difference of ~ 0.01 mA cm-2 compared to the current at -0.7 V. From Figure 1b, curve II, the contribution to the total current from Cu+ to Cu reduction at -0.77 V is -0.2 mA cm-2. Thus, the current arising from Cu2+ to Cu+ reduction [in Cu(II) electrolyte] should decrease at potentials lower than -0.7 V. This decrease in the Cu2+ to Cu+ reduction current may arise due to the following two reasons. Firstly, the electrical double layer increases with increasing potential, thereby limiting the charge transfer process for Cu2O formation. Secondly, the Cu2O islands may experience a potential drop at the surface after acquiring a certain size due to the higher resistance of Cu2O. 7 ACS Paragon Plus Environment


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This decrease in the total current continues till a potential is reached (in this case, -0.84 V) wherein the Cu+ reduction current overcompensates for the decrease in current of the Cu2+ reduction process. Therefore, an increase in the total current is observed beyond -0.84 V. The other feature in this potential range is the local maximum at ~ -0.98 V. This is possible if both the reduction processes slow down at potentials beyond -0.98 V. As noted earlier, the current due to the Cu2+ to Cu+ reduction process is continuously decreasing or is saturated at some potential beyond -0.7 V. Also, it is clearly seen from Figure 1b (II, red curve) that the current associated with the Cu+/Cu0 reduction beyond -0.98 V decreases due to the decreasing amount of Cu2O available. If there is sufficient Cu2O, the current would have continuously increased on further cathodic sweeping as in the case of a thick film (Figure 1b, blue curve). This decrease in the total current beyond -0.98 V continues till the overpotential for the bulk reduction of Cu2+ to Cu0 is reached triggering a rapid increase in current after -1.06 V (Figure 1a).

Figure. 2. Reaction pathway of Cu(II)L2 (Cu precursor) during linear sweep voltammetry under cathodic scan in a basic electrolyte (pH 12). Here, L indicates lactate and L- indicates lactate ions. E indicates the specific potential for the various reduction reaction. Here, E1 < E2 < E3.

Based on these observations, Figure 2 shows a schematic of the various reduction processes that the lactate stabilized Cu(II) ions [Cu(C3H5O3)2, Cu(II)L2] undergo at different potentials. Here E indicates potential, wherein E1 < E2 < E3. The E1 range from -0.2 V to -0.7 8 ACS Paragon Plus Environment

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V, attributed to the Cu2O formation. Metallic Cu may form via different pathways. If a potential over -1.06 V is applied, which is termed as E3, the Cu(II)L2 directly reduces to Cu. On the other hand, metallic Cu formation at a lower potential (just after -0.7 V) may occur via consecutive reduction of Cu2+ to Cu+ to Cu. 3.2 Electrodeposition as a function of potential and structural characterization of the samples

Figure. 3. XRD patterns of the electrodeposited films at different potentials. The peaks corresponding to the FTO, Cu2O, and Cu are shown with different symbols.

Cu2O thin films were grown potentiostatically on FTO substrates for 15 min at various potentials indicated as black dots in Figure 1a. The samples deposited at various potentials will be denoted as F1 (-0.31 V), F2 (-0.37 V), F3 (-0.48 V), F4 (-0.7 V), F5 (-0.84 V), and F6 (-0.98 V). Although the onset potential is -0.2 V, the deposition rate is extremely slow below -0.31 V. Figure 3 shows the XRD patterns of F1-F6. A phase pure Cu2O is obtained at any potential within -0.31 to -0.7 V (JCPDS Card No. 05-0667). At -0.84 V, which is already 9 ACS Paragon Plus Environment


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within the Cu+/Cu reduction zone, XRD reflections corresponding to both Cu and Cu2O are observed. Deposition at -0.98 V resulted in mostly Cu along with minor Cu2O. The presence of Cu2O with Cu indicates that the potential above -0.7 V and below -0.98 V associated with a consecutive reduction of L2Cu(II) to Cu2O followed by Cu+ to Cu in multiple steps as shown in Figure 2. The rate of Cu2O reduction to Cu increases with increasing the potential within the potential zone -0.7 to -0.98 V. Further increasing the potential, the direct reduction of L2Cu(II) in solution to Cu metal occurs, and the XRD pattern shows only trace amount of Cu2O in the film. The intensity ratio of the (200) to (111) peak of Cu2O (I200/I111) is lower than that for a completely polycrystalline film (Figure S2, JCPDS card no: 05-0667) and it decreases with increasing the deposition potential. Thus, our films are (111)-textured. As the Cu2O formation requires OH- ions (Figure 2), the reaction rate will also be faster if the concentration of OH- ions is higher. It is reported that increasing the rate of reduction leads to faster growth along the (111) direction.28 However, the accurate mechanism explaining the effect of pH on the orientation of the Cu2O film remains to be explored. The SEM images in Figure 4 show that all the grains are densely packed and perpendicular to the substrate. The grains at low potentials (-0.31 and -0.37 V) are comprised of sharp pyramids with truncation in a few grains (Figure S3). The truncation in the grains above -0.48 V was not observed. The grain size under the lowest possible potential, -0.31 V is within 1-1.5 µm, which is the maximum obtainable size under this deposition condition (Figure 4a). At higher potential within the phase field of Cu2O, the sizes are within 100-300 nm (Figure 4d).


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Figure. 4. SEM cross-sectional view of the deposited films at different potential; (a) -0.31 V, (b) -0.37 V, (c) -0.48 V, (d) -0.7 V and (e) -0.84 V. Reduce in grain size as a function of potential (f). The blue (*) shows the ~3 times larger grains via two-step potentials deposition, which will be discussed in later sections. The insets show the Cu2O deposited for 30 s at different potentials.

Scharifker et al. observed that the number density of active sites for nucleation varies with overpotential.29 The reduction in grain size with increasing the potential is mainly caused by the increasing number density of active sites, resulting in dense nucleation. Also, at higher potentials, the amount of Cu(II)L2 is higher near the substrate. In this scenario, the 2-D (lateral growth) growth of the nuclei is sterically hindered, thereby favoring further nucleation according to the nucleation-growth-collision theory. The density of Cu2O grains within 30 s of deposition is shown as inset at various potentials; the large area images can be found in the supporting information (Figure S4). The grains are isolated from each other due to sparser nucleation at lower potential. On the contrary, the grains become denser at higher potentials. The cross-sectional SEM images of the films deposited for 15 min (Figure 4) reveals that the isolated seed grains acquire a certain 11 ACS Paragon Plus Environment


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size until they are closely packed, and later grow perpendicular to the plane with further deposition. As the seed grains at higher potentials are very dense, the final film is comprised of smaller grains. Figure 4f clearly shows a decreasing order of grain sizes with increasing the deposition potentials. More interestingly, the grain sizes of up to 3 µm (shown as a blue star in Figure 4f) could be obtained using a modified electrodeposition method, which will be discussed in later section. To better understand the influence of potential on grain size under different potential scenarios, a systematic investigation was carried out at two different potentials (-0.31 and -0.37 V) as a function of time. 3.3 Single-Step Electrodeposition: Secondary nucleation as a function of Time and Potential

Figure. 5. The SEM plan view images for the Cu2O crystals grown as a function of time at two different potentials. (a) -0.31 V for 6 min, (b) -0.31 V for 8 min, (c) -0.37 V for 15 s, and (d) -0.37 V for 30 s. The grains inside dotted yellow circles correspond to the second stage nucleation. In case of (c) and (d), the magnified insets are provided for more clear observation of the secondary grains. (e) is corresponding to the j vs t transients at -0.31 and -0.37 V for 60 min deposition.

Figure 5 shows the SEM images of the Cu2O, deposited at -0.31 and -0.37 V at two different time durations. These potentials were chosen as to obtain sparser nucleation at the initial stage followed by their growth as a function of time. The experiments were carried out for 15 s, 30 s, 2 min, 4 min, 6 min and 8 min at both the potentials, which can be found in the 12 ACS Paragon Plus Environment

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supporting information (Figure S5). Figure 5 presents the critical time when the secondary nucleation occurs under both the potentials. From the Figure S5a and S6a, it can be observed that the density of seed grains within 15 s is sparser at -0.31 V compared to -0.37 V. Potentiostatic deposition at -0.31 V till 6th min leads to deposition mainly on the existing Cu2O islands. According to Markov et al. a growing nucleus develops an exclusion zone due to a local deformation of the electric field around it.30 However, on further deposition, a second stage nucleation was observed after 6th min on the bare FTO substrate shown in yellow circles in the Figure 5b. The size of the secondary grains is almost similar to the size of the grains deposited for 2 min at -0.31 V (Figure S5). It implies that the secondary grains were just nucleated after 6th min of deposition. The formation of secondary grains after 6 min 30 s can be clearly observed from the Figure S7 (supporting information). Once the seed grains are formed (primary or secondary), they start occupying the space by a gradual growth until a continuous film is achieved. A similar type of growth mechanism was also observed at -0.37 V, however secondary nucleation occurs at an early stage, within 30 s of deposition (Figure 5d). Further depositing at -0.37 V follows a progressive route, leading to a film comprising slightly smaller grains. Figure 5e shows the j vs t transient obtained during deposition at -0.31 V and -0.37 V. Although the deposition time is 15 min at each potential for film growth, the j vs t transients are shown for 60 min deposition in order to monitor the deposition characteristics for a longer time. At lower potential, -0.31 V, the current gradually increases till 6th min followed by a slow rise till 9th min and later, reduces continuously till 60 min. On the other hand, a sharp increase in current is observed at -0.37 V till 15th s followed by a slow rise till 30th s. Later, the current slowly reduces till 60 min of deposition. The current density during



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chronoamperometry corresponds to the associated total charge leading to the reduction of Cu2+ to Cu+ per unit time. Thus, the gradual increase in current till 6th min at -0.31 V is associated with a gradual increase in total number of reduced Cu2+ ions to Cu+. As the growth occurs only on the Cu2O rather than fresh nucleation on FTO substrate, it can be inferred that the increase in current is because of the gradual increase in the size of Cu2O with time, thereby increasing the surface area. However, after 6th min of deposition, the sufficiently large Cu2O grains (~ 1 µm) undergo a potential drop due to their higher resistivity.27,31 In this situation the FTO sites that did not serve as nucleation sites become more favorable for fresh nucleation. Similar characteristics were also observed at a slightly higher potential, -0.37 V. However, the secondary nucleation at -0.37 V occurs within 30 s of deposition. 3.4 Two-step electrodeposition: Pathway leading to large grained Cu2O film Figure 6a, b, and c show the SEM images of Cu2O films at different stages of deposition. Figure 6a corresponds to the Cu2O deposited at -0.31 V for 6 min, where the grains are uniform and isolated from each other. This sample was used as a seed layer for further deposition by switching the potential to -0.37 V. The time lag between these two processes is 1-2 s. After 2 min of deposition at -0.37 V (Figure 6b) on the seed grains, no secondary nucleation was observed as against the single-step potential deposition at -0.31 V. Depositing at -0.37 V for 15 min leads to a continuous growth of the seed grains only, resulting in to a film comprising ~ 3 µm sized Cu2O grains. The j vs t plot for the two-step deposition process is shown in Figure 6d. The first step corresponds to the deposition at -0.31 V for 6 min, such that the maximum Cu2O size can be obtained before the secondary nucleation. The deposition was further carried out by switching the potential to -0.37 V as to mitigate the potential drop on the Cu2O surface. Application of -0.37 V in series, the current instantly increases up to 25% (from 0.6 to 0.75 14 ACS Paragon Plus Environment

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mA). The j vs t transient in the Figure 6d is shown for 60 min as to observe the transient pattern for a longer time. A gradual decay in current was observed with time, similar to the single-step potential depositions.

Figure. 6. SEM images depicting the morphology and a continuous growth of primary seed grains with time under two-step potentials deposition. No secondary grains formation is observed under this deposition condition. (a) Deposition at -0.31 V for 6 min, (b) deposition at -0.37 V for 2 min on sample (a), and (c) deposition at -0.37 V for 15 min on sample (a). (d) The corresponding j vs t transient for the two-step potential deposition. The blue part corresponds to the 6 min deposition at 0.31 V and the red part corresponds to the consecutive deposition at -0.37 V.

It is observed that the seed grains grow up to ~ 1 µm size at -0.31 V before the secondary nucleation occurs. On the other hand, the secondary nucleation occurs within a short time at higher potentials, above -0.37 V. Therefore, -0.31 V was used for obtaining a seed layer comprising larger and uniform Cu2O grains. Consecutive use of a slightly higher potential mitigates the potential drop on the semiconducting Cu2O islands, thereby resulting in continuous growth of the primary seed grains. The 25% increase in current at -0.37 V during the second step, as can be seen in the Figure 6d, clearly indicates an increase in potential on the substrate. This method is a promising way to grow 2-3 times larger grains of Cu2O on 15 ACS Paragon Plus Environment


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FTO substrates (Figure 4f, shown as a blue star). It is to note that the seed grains deposited at -0.31 V for 2, 4 or 8 min did not result in large grained Cu2O film [Figure S8, supporting information]. Under this circumstance, it is mandatory to recognize the potential and switching-time precisely to obtain larger grains via a two-step potential electrodeposition method. 3.5 XPS analysis of the two-step deposited film

Figure. 7. XPS analysis of the Cu2O film, deposited via two-step potentials. (a) Atomic Concentration of Cu/O with etching time, (b) high-resolution XPS spectra of Cu and (c) high-resolution XPS spectra of O.

Figure 7 shows the XPS depth profiling data of the two-step deposited film in order to provide a detailed information on the phase purity of the material throughout from surface to the core. During depth profiling (Figure 7a), the Cu and O ratio was obtained to be close to 50/50 without surface cleaning. This is because of the surface oxidation in Cu2O, a spontaneous reaction when exposed to air. After 30 s of etching, the Cu/O ratio was obtained to be 78/22, whereas, ideally, the ratio should be 66/33. The deviation of Cu/O ratio is caused by the O-rich sputtering under Ar plasma.21 The ratio was found to be almost constant throughout the depth, thereby indicating the phase purity of the bulk material. The highresolution (HR) XPS patterns of the Cu2O film (after surface cleaning) corresponding to the Cu and O are shown in Figure 7b and 7c. The distinct peaks of Cu at 933 eV and 948 eV along with O 1s peak at 531 eV confirms the phase purity of the Cu2O.21


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Phase pure Cu2O films were thus obtained in our deposited conditions comprising various grain sizes at different potentials. As the minority carrier diffusion length of Cu2O is close to 0.5 µm, a higher thickness will lead to more recombination of the photogenerated charge carriers. However, the absorption coefficient of Cu2O is 104 cm-1, which requires a thickness around 1.5 – 2.0 µm for efficient absorption of light. Considering these two factors, the film thickness was maintained at 1.5 µm in all the cases. Although the two-step potentials deposition resulted in 3 µm sized Cu2O, the thickness could be maintained to 1.5 µm because of the dominant lateral growth. The film grows perpendicular to the plane only after the film becomes continuous, i.e., when there is no space left for lateral growth. The thickness of ~1.5 µm was found to be effective for electrodeposited Cu2O in the earlier report also.17,20,21 As the rate of deposition varies according to the applied potential, a time-dependent deposition will not result in similar thickness at the same durations.

Therefore, the

depositions were carried out by fixing the charge to 0.8 C cm-2, independent of time, which resulted in ~ 1.5 µm thick film at any potentials. The SEM images presented in Figure S10 (supporting information) are resulted from the 0.8 C cm-2 at -0.31, -0.37 and at two-step potentials. During two-step deposition, the seed layer was initially grown for 6 min at -0.31 V, calculated the total charge, and the remaining charge was deposited at -0.37 V. Thus, Cu2O films comprising different grain sizes (0.2 – 3.0 µm) with 1.5 µm thickness, were obtained at any potentials within the phase field of Cu2O. 3.6 Photoelectrochemical (PEC) analysis Figure 8 shows the illuminated LSV plots of the two-step deposited Cu2O films at various scan rates viz. 1, 5, 10, 20, 40 and 60 mV s-1; the LSV scan of the same film at 20 mV s-1 under dark conditions is also shown in Figure 8 (black circles). At scan rates lower than 20 mV s-1, a diffusion-limited region (plateau) in the j vs E characteristic is observed after 0.4 V 17 ACS Paragon Plus Environment


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(vs RHE). This plateau extends over a larger potential range at scan rates less than 10 mV s-1. As the Cu2O is prone to reductive degradation under PEC experiments in an aqueous electrolyte, the current obtained is inclusive of material reduction along with the water reduction process, as shown by the reactions (R1) and (R2). The parasitic reaction at the material’s surface leads to the formation of metallic Cu followed by the reaction, R1.19,32,33 Cu O 2e 2H → Cu H O

(R1)

2H 2e → H

(R2)

Figure. 8. Linear sweep voltammetry of the two-step deposited Cu2O films at various scan rates such as 1, 5, 10, 20, 40 and 60 mV s-1. The current plateau that appears at 1, 5 and 10 mV s-1 scan rates, is not observable at 20 mV s-1 or higher scan rates.

From the LSV at various scan rates, it is observed that the voltammogram features and the corresponding photocurrent are almost the similar at 20 mV s-1 or higher scan rates. Therefore, further LSV was performed at 20 mV s-1 scan rate using the different photocathodes, F1-F5 along with the two-step deposited Cu2O (FTS). The light insolation was set in chopping mode (on/off) such that the current response can be simultaneously monitored under light and dark conditions. The j vs E plots obtained from light chopping LSV of the Cu2O electrodes, F1, F2, and FTS are presented in Figure 9 (the j vs E plots for F3-F5 are shown in Figure S12-S14). The onset potential was observed close to 0.57 V (vs RHE) for all the electrodes. The 18 ACS Paragon Plus Environment

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voltammogram (sweeping in reverse direction) under light chopping condition, a spike appears in the j vs E plot due to the limiting charge transfer at the electrode-electrolyte interface, stabilizes and gradually the current increases with increasing the potential under cathodic sweeping. A few noisy features also can be observed under sweeping mode, which can be attributed to the simultaneous effects of voltage sweep and repeated light chopping.

Figure. 9. Linear sweep voltammetry (j vs E) under light chopping mode of the three different type of Cu2O films, deposited at -0.31 V, -0.37 V and two-step potentials deposition.

During LSV, the current obtained under dark mode is almost zero. The minor current (-20 µA) obtained at close to 0.0 V (vs RHE) under the dark condition is due to the Pt cocatalyst nanoparticles loaded on the Cu2O photocathodes. During light mode, the maximum current at 0.0 V (vs RHE) was obtained to be -1.6 mA cm-2 with the F1 and F2 electrodes. The electrode, FTS comprising large grained Cu2O resulted in -2.2 mA cm-2 photocurrent, which is ~ 30% higher current compared to the photocathodes F1 or F2. The film comprising smaller grains for the electrodes F3 (Figure S12, supporting information) and F4 (Figure S13, supporting information) generated close to -1 mA cm-2 photocurrent. The electrode F5 was comprised of impurity metallic Cu in them, which in turn resulted in poor photocurrent (Figure S14, supporting information). The inferior performance by the Cu2O photocathode along with impurity metallic Cu may arise due to the following reasons. First, the lowering in the volume of light absorbing material (Cu2O) in the thin film causes low electron-hole pair generation leading to low photocurrent. Secondly, Cu forms a Schottky junction with Cu2O and acts as a recombination center, thereby degrading the photocurrent. Thirdly, an erroneous 19 ACS Paragon Plus Environment


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data may generate if Cu create a shunt path (electrolyte-Cu-FTO). It is to be noted that such shunt path leads to the inferior performance of the devices. Depositing a film at a very low potential (less (-ve) than -0.31 V vs Ag/AgCl in this deposition condition) also may create such shunt paths due to the noncontinuous film. The PEC response of the Cu2O films as a function of deposition potential has shown significant variations in their results. Although the orientation of crystals is predominant in [111] direction at every deposition potential, the (200) faces are more exposed at lower potentials. Also, a few of the grains at lower potentials (-0.31 and -0.37 V) exhibit truncation at the triangular (111) faces parallel to the substrate [Figure S3]. The truncation was almost negligible at higher deposition potentials (above -0.48 V). Kelly et al. demonstrated that the (111) faces of Cu2O are either Cu+ or O2- terminated.19 However, the truncated faces are always O2- terminated, which are prone to photocorrosion driven by the H+ ions during PEC experiments. Thus, the overall current is inclusive of a significant amount of the material reduction and not solely the H+ ions reduction current. It should be noted that the stability of Cu2O is poor under PEC water splitting condition, which necessitates a protective coating for longer stability.20,21 This is not within the scope of the current investigation. From the LSV shown in Figure S11 (supporting information), we observe that the back illumination does not affect the photoresponse of the device, capacitating the use of a non-transparent protective coating in this superstrate device structure. 3.7 Conducting Atomic Force Microscopy (c-AFM) analysis The lower grain boundary cross-section in the large grained Cu2O film would have low defect density leading to the less probability of charge carrier scattering. The role of grain boundaries as shunt paths was investigated via conducting atomic force microscopy (c-AFM) and Mott-Schottky analysis. 20 ACS Paragon Plus Environment

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The photoconductivity of the Cu2O films was measured by adjusting the geometry of the laser light spot such that the contact area underneath the AFM-tip is sufficiently covered. Figure 10 shows the c-AFM images of the two-step deposited Cu2O film (FTS) without application of a bias voltage. Figure 10a shows the topographic image and reveals the surface topographic structure similar to that seen in the SEM image. The corresponding c-AFM images from the same area of the film are shown in Figure 10b-e. The c-AFM image in Figure 10b was taken with no light illumination.

Figure. 10. Atomic force microscopy (AFM) Conducting AFM (c-AFM) of the two-step deposited Cu2O film under light illumination of various wavelengths. The scale bar indicating light-induced short circuit current is given in the top right of the figure.

It is clearly seen that; the entire image appears black under dark condition. This implies that there is no appreciable current flowing through the device (scale bar 0 to +892 pA is given on the top right); the current measured through the entire area in Figure 10b is less than 1 pA, which is the level of the background noise of the instrument. The c-AFM images of Figure 10c – e was taken at successively increasing wavelengths of monochromatic 21 ACS Paragon Plus Environment


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illumination; the monochromatic radiation was chosen such that the energies straddled the bandgap of Cu2O. Illuminating the device with light of wavelength 500 nm (~ 2.48 eV) and 600 nm (~ 2.06 eV) results in a sizeable short circuit current that is seen as the light green areas in Figure 10c and Figure 10d. It should be noted that the energy of the monochromatic radiation used in obtaining these images is ~ 2 eV, the bandgap of Cu2O. As Cu2O is a very good light absorber, this results in some photocurrent generation throughout the sample. The regions in the image representing the bulk regions of the sample, that appear blacker, also represent regions through which a current of ~ 25 pA is flowing, much larger than the background noise of the instrument. The more important point to note is that the current is not uniform over the area of the image, which has both bulk regions and boundary regions. In fact, the regions that represent the boundaries between the particles show an enhanced current in both Figure 10c and d. This enhanced current at the grain boundaries can be attributed to the fact that these grain boundaries act as shunt paths. This effect is detrimental to the photoelectrode performance owing to a reduced shunt resistance leading to lower open circuit voltage. Another contribution to the increased current at the grain boundaries could be the presence of sub-bandgap states in the material. These states make possible the absorption of light wavelengths in addition to those involved in band-to-band transitions, hence we observe an increased short circuit current. Figure 10e is the c-AFM image of the same area as in Figure 10d, albeit with sub-bandgap illumination (E ~ 1.55 eV). Indeed, we observe that the regions of the image that correspond to the bulk of the material do not show any photoactivity; the current in these areas was of the order of the background noise of the instrument. However, a current is observed at the boundary regions even for the sub-bandgap illumination indicating that the boundary regions contain defects with energy levels in the bandgap that are contributing to the current under short circuit. 22 ACS Paragon Plus Environment

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3.8 Mott – Schottky analysis Mott – Schottky analysis of F1, F2 and FTS films were carried out to obtain the carrier density (NA), flat band potential and the type of material. We observed erroneous and inconsistent data (1/C2 vs V) while doing the Mott-Schottky experiments in 1 M Na2SO4, pH ~ 7. Sodium acetate (CH3COONa) has been used elsewhere for impedance spectroscopy of electrodeposited Cu2O.8 It was found suitable for the Mot-Schottky analysis of our samples.

Figure. 11. Mott-Schottky (1/C2 vs V) plots of the different Cu2O films deposited at different deposition conditions. The negative slope indicates p-type nature of the material. The flat band potential was obtained to be close to 0.75 V (vs RHE).

The space charge region under reverse bias in an electrolyte of 0.1 M CH3COONa, pH ~ 7.8 was determined to be +0.4 to +0.75 V (vs RHE). The experiment was carried out inside 23 ACS Paragon Plus Environment


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the space charge potential region with DC bias in the frequency range of 100 Hz to 100 kHz under the dark condition and at room temperature. The carrier concentrations were estimated at moderate frequencies, 1, 3, and 5 kHz in order to avoid parasitic contribution from the surface/interface states’ at higher frequencies. The charge carrier density and flat band potential can be quantified using the following equation.34,35

=

ε ε

E − E −

…… (1)

Here, NA is the carrier concentration, eo is charge of an electron, εo the dielectric constant of the semiconductor, εr the permittivity of free space, C is capacitance, E is the electrode potential, EFB is the flat band potential, k is Boltzmann's constant and T is the temperature. The corresponding Mott-Schottky plots (1/C2 vs V) for the various samples are shown in Figure 11. All the plots exhibit a negative slope, indicating p-type behavior of the material under various deposition conditions. Extrapolating the plots intercepts the voltage axis at ~ 0.75 V (vs RHE), representing the flat band potential (EFB). The carrier density (NA) of the F1, F2 and FTS films was obtained from the slope of the Mott-Schottky plots. Although the flat band potential in all three cases is almost similar, the carrier concentration was found to be one order lower in two-step deposited Cu2O film. Lower carrier concentration can be attributed to fewer defects that are a consequence of the lower grain boundary cross-section in the large grained Cu2O films. The large grained (10 to 100 µm) thermally oxidised Cu2O shows even lower carrier concentration, to the order of 1013 cm-3.36,37 Effect of grain boundaries on mobility is reported for many well-known materials, such as TiO2, Si and InN.38,39 As the grain boundaries are a potential barrier to carrier transport, the mobility gets affected, thereby lowering the device performance.38–40


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4. Conclusion In summary, thin films of Cu2O comprising 0.2 to 3 µm grains at different potentials have been synthesized and the effect of grain size on photocurrent generation is demonstrated. It is observed that the nucleation sites of FTO increase with increasing potential, which governs the grain size in a continuous film. During single-step deposition, the maximum obtainable size is ~ 1 µm (-0.31 V vs Ag/AgCl) and gradually reduces with increasing the potential up to ~ 0.2 µm at -0.7 V. The secondary nucleation that occurs after different time of deposition at different potentials (6 min at -0.31 V and 15 s at -0.37 V) is also a major reason for the reduction of grain sizes during single-step deposition. A modified two-step electrodeposition method; depositing seed grains at -0.31 V for 6 min followed by a slight increase in potential (-0.37 V) in sequence could prevent secondary nucleation, thereby obtaining ~ 3 µm sized Cu2O film. The effective reduction in grain boundary cross-section in the large grained Cu2O film lowers the photoinduced shunt paths, enhances the carrier mobility due to lower scattering, thereby enhancing the photocurrent up to 30%. The use of a transparent FTO substrate will help in using catalytically active, non-transparent robust materials as a protective coating for enhancing the stability of Cu2O photocathodes in this superstrate configuration.



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ASSOCIATED CONTENT Supporting Information: Linear sweep voltammetry (cathodic) of lactate stabilized Cu(II) at different scan rates, Relative intensity of I200/I111 (from XRD) at various deposition potentials, SEM images of the films after 15 min of deposition at different potentials, SEM images of the seed grains after 30 s of deposition, Growth of Cu2O as a function of time at 0.31 V, Growth of Cu2O as a function of time at -0.37 V, Secondary seed grains formation after 6 min 30 s deposition at -0.31 V, Two-Step potentials deposition: Seed layer was deposited at -0.31 V for 2, 4, 6 and 8 min, Tauc plot for band gap calculation, Morphology and thickness of Cu2O films after depositing 0.8 C cm-2, Photoelectrochemical (PEC) performance of the Cu2O photocathodes. AUTHOR INFORMATION Corresponding Author *Tel: + 91-22 2576-7808. E-mail: [email protected] Notes: The authors declare no competing financial interest ACKNOWLEDGEMENTS The authors acknowledge “National Centre for Photovoltaic Research and Education (NCPRE)” and “Centre for Excellence in Nanoelectronics (CEN), IIT Bombay” for providing many instrumental facilities. BK sincerely thanks for the financial support from IRCC, IIT Bombay through Grant No: 12IRCCSG014. CD acknowledges Ministry of Human Resource and Development, India for providing the Ph.D. fellowship. JS acknowledges support by the Australian Research Council through an ARC Future Fellowship and ARC Discovery grants.


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1 Effect of Grain Boundary Cross-Section on the Performance of

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