Nanocrystalline Boron-Doped Diamond as a Corrosion Resistant

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Energy, Environmental, and Catalysis Applications

Nanocrystalline Boron-Doped Diamond as a Corrosion Resistant Anode for Water Oxidation via Si Photoelectrodes Petr Ashcheulov, Andrew Taylor, Vincent Mortet, Aleš Poruba, Florian Le Formal, Hana Krýsová, Mariana Klementová, Pavel Hubik, Jaromír Kope#ek, Jan Lorincik, Jun-Ho Yum, Irena Kratochvilova, Ladislav Kavan, and Kevin Sivula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08714 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Nanocrystalline Boron-Doped Diamond as a Corrosion Resistant Anode for Water Oxidation via Si Photoelectrodes Petr Ashcheulov1*, Andrew Taylor1, Vincent Mortet1,2, Aleš Poruba3, Florian Le Formal4, Hana Krýsová5, Mariana Klementová1,6, Pavel Hubík1, Jaromír Kopeček1, Jan Lorinčík7, Jun Ho Yum4, Irena Kratochvílová1, Ladislav Kavan5 and Kevin Sivula4 1

Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21, Prague, Czech Republic

2

Faculty of Biomedical Engineering, Czech Technical University in Prague, Sítna sq. 3105, 272 01, Kladno, Czech Republic 3

4

Fill Factory s.r.o., Televizní 2618, 756 61, Rožnov pod Radhoštěm, Czech Republic

Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 6, 1015 Lausanne, Switzerland

5

J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejskova 3, 182 23, Prague 8, Czech Republic 6

University of West Bohemia, New Technologies - Research Centre, 306 14, Pilsen, Czech Republic 7

Research Centre Řež, 250 68, Husinec-Řež, Czech Republic

KEYWORDS Nanocrystalline

diamond,

protective

coating,

transparent

conductive

film,

silicon

photoelectrodes, photoanode, water splitting

ACS Paragon Plus Environment

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ABSTRACT

Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical water splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal towards a suitable protection of otherwise unstable semiconductors used in photoelectrochemical cells. Here we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes

show

no

sign

of

performance

degradation

after

a

continuous

photoelectrochemical treatment in neutral, acidic and basic electrolytes. Deposition of cobalt phosphate (CoPi) oxygen evolution catalyst onto the BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of BDD layers to substitute transparent conductive oxide coatings such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) frequently used as protective layers in Si photoelectrodes.

INTRODUCTION Storing solar energy for later usage during sunlight deficiency periods is a primary economic and scientific interest.1-4 Sunlight-driven splitting of water into hydrogen (H2) and oxygen (O2) is envisaged as the most viable and environmentally-friendly concept for the production of


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molecular hydrogen, denominated as solar fuel.4,5 Hydrogen can be used directly as the energy carrier to power fuel cells or participate as an intermediate component in the production of more complex

hydrocarbons.5,6

Sunlight-driven

water

splitting

can

be

performed

via

photoelectrochemical (PEC) devices, typically comprised of light-absorbing semiconductors.7,8 Depending on the nature of the semiconductor (p-type or n-type), direct contact with a liquid results in a rectifying junction at the semiconductor-liquid interface which facilitates photogenerated carrier separation and triggers either the oxygen evolution reaction (OER) or hydrogen evolution reaction (HER)9,10. A huge research effort has been devoted to the development of a wide range of the photoelectrode materials for solar-driven water splitting, such as oxide semiconductors (e.g. TiO2, Fe2O3, WO3, BiVO4, SrTiO3, KTaO3, BaTiO3, Cu2O, CuFeO2)11-17 and Group II-VI, III-V, IV semiconductors (e.g. Si, a-SiC, InP, GaP, CdS, CdSe, GaAs, CdTe)18-25. Unfortunately, under photoelectrochemical operation conditions these materials face either low solar-to-fuel conversion efficiencies and/or limited electrochemical stability.26 Series connection of a p-type photocathode and n-type photoanode semiconductors in a dual-absorber tandem cell has been pursued to enhance the overall PEC energy conversion efficiency, while methods to circumvent corrosion and/or passivation of unstable semiconductors benefited in improved chemical stability.26,27 Considerable progress has been made in developing photoelectrodes based on silicon (Si) – it absorbs a significant fraction of the solar spectrum and has been proven to be an important material in photovoltaics.28 However, the application of Sibased photoelectrodes in solar-driven water splitting devices faces a critical limitation caused by the intrinsic instability of Si under continuous PEC operation in aqueous media.29 Recently, many materials have been studied for the protection of both Si photoanodes and photocathodes.30-38 As such, Si photoanodes were stabilized for water oxidation by the use of


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ultrathin Ni/NiOx films,30 by atomic-layer deposition (ALD) of a TiO2 tunnel barrier layer31 or by the use of thick “leaky” TiO2 film.32 Thin film TiO2 and Ti/TiO2 layers have also been reported to protect Si photocathodes.33-34 Metal oxides such as indium-doped tin oxide (ITO)35-37 and fluorine-doped tin oxide (FTO),38 commonly labeled as transparent conductive oxides (TCO), have been implemented as protective layers on Si photoelectrodes. However, any decrease in the amount of dopants in tin-based oxides during the electrochemical treatment usually results in significant Ohmic losses and reduced energy conversion efficiency of the water-splitting device.29,36,39-41 From this perspective, in order to make sunlight-driven water splitting technology feasible, the development of new protection strategies for photoelectrodes, which have decades long stable operation, are of strong interest. Diamond in the form of thin conductive films meets the requirements of a coating material for effective protection of Si from its inherent corrosion weakness.42 Initially, being a wide bandgap (5.4 eV) semiconductor, diamond acquires metallic conductivity (p-type, resistivity of ~10-3 Ω cm) with heavy doping with boron atoms, thus enabling bulk charge carrier transport.43,44 Borondoped diamond (BDD) can be produced by a variety of chemical vapor deposition (CVD) techniques.45 Due to its high stability against corrosion, low background current, chemical inertness and extreme overpotential for the OER,46-49 BDD electrodes are applied in electroanalysis,50

electrosynthesis,48,51

spectroelectrochemistry,52,53

and

wastewater

purification.48,54 A low catalytic activity along with a high overpotential for the OER observed for classical BDD electrodes have impeded their use in the water electrolysis applications.54-56 However, electrochemical properties of BDD electrodes usually depend on the type of diamond (single crystal or polycrystalline), boron-doping level and amount of non-diamond (sp2 carbon) impurities.47,57-59 It has been reported that a significant fraction of non-diamond inclusions within


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BDD can substantially alter its electrochemical characteristics – narrowing the width of the potential window and reducing the overpotential for both the OER and HER, therefore making BDD electrodes a prospective candidate material for water electrolysis.60 Recently, conductive BDD electrodes have been shown to effectively protect underlying Si electrodes (p-Si|BDD) operating under dark conditions from corrosion in a highly acidic environment for the HER.61 Following our reported fabrication of diamond-based electrodes with superior optical and electrical characteristics, similar to that of TCO electrodes,62 herein, we report on the development of highly conductive thin BDD layers, for the protection of buried junction Si photoelectrodes63 (shown schematically in Figure 1). The objective of the present study was to verify the ability of BDD electrodes to effectively substitute commonly used transparent conductive oxides (ITO, FTO), particularly in the context of photelectrochemical water oxidation via Si photoelectrodes. Under photoelectrochemical treatment in various aqueous electrolytes (acidic, neutral, alkaline), thin BDD layers enable a stable performance of the PV component and protect the underlying Si against chemical instability. Functionalization of the BDD surface with a cobalt phosphate (CoPi) oxygen evolution catalyst greatly reduces the overpotential for the OER and yields water oxidation via the BDD-protected Si photoelectrodes (npp+-Si|BDD|CoPi) comparable to that of ITO and FTO-coated Si photoelectrodes (npp+-Si|ITO|CoPi, npp+Si|FTO|CoBi).35,38


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Figure 1. (a) Schematic illustration of a photoelectrode based on BDD electrode and buried pnjunction Si solar cell for water oxidation and reduction. The PV component (np-Si) is responsible for incident light absorption and charge separation across the pn-junction, while oxygen evolution occurs at the BDD electrode/electrolyte surface and hydrogen evolves at the externally wired metal cathode. The BDD surface can be further interfaced with an oxygen evolution catalyst to facilitate the water oxidation process. (b) Band diagram of the Si|BDD photoelectrode, where due to the position of Fermi level (which is below the valence band maximum for the degenerately doped BDD electrode), hole carriers can transfer towards the BDD/electrolyte interface to participate in water oxidation.

RESULTS AND DISCUSSION Nanocrystalline boron-doped diamond electrodes For the development of BDD electrodes with suitable electrical characteristics we concentrated our research efforts on careful examination of the BDD layer deposition parameters (See Supporting Information Table S1), which are known to affect a number of the layers’ properties: morphology, electrical conductivity, chemical composition, optical transparency and layer


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thickness.62,64-68 Three deposition regimes have been investigated for BDD electrodes fabrication: 250 °C, 500 °C and 730 °C (see Supporting Information S2-S5 for additional discussion on fabrication of BDD electrodes). The fabricated BDD layers were found to be conformal and free of discernible openings (pinholes) or impurities and demonstrate similar morphological characteristics, i.e. a distinct crystalline structure with diamond crystallites consisting of a mixture of orientations (Figure S1a-f). All obtained BDD electrodes demonstrate a high concentration of boron atoms within the diamond layers (i.e. above 1020 at/cm3) as accessed by secondary ion mass spectroscopy (SIMS) analysis (Figure S2). Besides the impact on boron incorporation, the BDD deposition temperature has been shown to considerably affect its electrical characteristics. A high deposition temperature (730 °C) readily promotes boron incorporation into the diamond crystal lattice, such that the concentration of electrically active boron acceptors determined via Hall Effect measurements ~1.89 × 1021 cm3 (Figure 2a) is analogous to the total boron concentration measured by SIMS analysis ~1.8 × 1021 at/cm3 (Figure S2a). Such a result, however, does not hold true for BDD layers prepared using lower temperatures of 550 °C and 250 °C. For these cases boron concentration values display significant variance between Hall concentration values (Figure 2a) and SIMS atomic boron concentration (Figure S2b,c), indicating possible partial compensation of boron atoms by hydrogen or incorporation of boron atoms into interstitial positions in the diamond lattice along with possible clustering of boron atoms forming electrically inactive sites.69 An abrupt increase of electrical resistivity of BDD electrodes obtained at 550 °C and 250 °C is primarily an outcome of the reduced number of active electron-acceptor boron atoms participating in the conduction mechanisms (Figure 2b and Table S2).


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Figure 2. Concentration of active boron atoms and mobility values (a), and corresponding sheet resistance and resistivity values (b) of BDD layers deposited at three temperature regimes on non-conductive glass substrates, as measured by Hall effect measurements. Bright field TEM image of Si|BDD interface (c) and energy filtered TEM map of Si|BDD interface where bright areas represent carbon atoms (d) and bright areas represent silicon atoms (e). Minority carrier lifetime maps for npp+-Si solar cell without BDD layer (f), with BDD layer deposited at 250 °C (g), 550 °C (h) and 730 °C (i). Typical current-voltage (J-V) curve of npp+-Si|BDD photoelectrode under simulated 1 Sun illumination (j).

Buried junction Si photoelectrodes stabilized by nanocrystalline boron-doped diamond After successful coating of buried junction Si wafers by BDD, maximizing the Si|BDD photoelectrode electrical output characteristics (photovoltage, photocurrent) is of utmost importance for appropriate solar-to-fuel energy conversion. In general, the flow of photocurrent generated within the solar cell component of a Si|BDD photoelectrode may be impeded by various resistive losses associated with (i) resistance at the interface between Si and BDD layer, (ii) resistance of the BDD layer, (iii) carrier recombination at the interface and in the Si solar cell and (iv) resistance at metal wire contacts to Si (and BDD layer). Besides, additional electrical losses associated with the instability of protective coating and/or changes of the surface states of the coating may arise during long-term operation.39 Although Si is known to make good Ohmic contact with metals, possible voltage drops at the Si|BDD interface may originate from the defective nature of such an interface. The very fact that electrically non-conductive nano-sized diamond seeds were used as nucleation sites for subsequent BDD layer growth might impose some limitations onto the charge carrier transfer


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across the Si|BDD interface. In addition, the presence of an interfacial SiO2 layer may also impede charge transfer at the interface. However, the presence of a thin layer of SiO2 can also be advantageous for minimizing the interface charge recombination (caused by the geometric mismatch factor) due to smoothening of the transition at the Si|BDD junction.35 Another factor to consider is formation of local inhomogeneities (such as SiC, SiOC, amorphous and graphitic carbon) within the Si|BDD interfacial junction during BDD layer growth. Transmission electron microscopy (TEM) analysis of the Si|BDD junction reveals details about the composition of the interface. One can discern the presence of a 10-50 nm thin defective interface at the junction (Figure 2c and Figure S3). According to electron energy loss spectroscopy (EELS) complemented by energy-filtered transmission electron microscopy (EFTEM) and high resolution transmission electron microscopy (HRTEM) the interface between Si and BDD layer accommodates regions containing amorphous carbon, carbon (Figure 2d) and silicon (Figure 2e) which also suggests formation of SiC (Figure S3 and Figure S4). Although periodicities characteristic of SiC were not observed in electron diffraction (ED) data (Figure S3), a periodicity of about 2.5Å was detected in HRTEM analysis, which likely corresponds to polytypes of SiC (Figure S5).67,68 Furthermore, elemental mapping of the interface reveals the appearance of oxygen atoms present at the junction, indicating possible SiO2 presence (not shown). Enhancement of the oxygen content at the Si|BDD junction is also apparent from SIMS analysis (Figure S2). The degenerate character of BDD electrodes is essential for reducing bulk electrical resistance and contact resistance contributions, therefore allowing effective hole carrier transport from Si into the BDD layer. In the case of np-Si|BDD structures, the photoelectrodes’ electrical characteristics were impeded due to a rather resistive p-Si surface, which is in agreement with


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the previous report.35 Yet, in the case of the highly conductive p+-Si side of npp+-Si, the electrical conductivity of the npp+-Si|BDD structure is greatly enhanced due to the readily charge carrier transfer through the BDD layer, as in the case of BDD electrodes grown on metal supports, e.g. optically transparent composite BDD/Ti-mesh electrodes displaying sheet resistance values below 50 Ω/sq.62 Remarkably, these similar low sheet resistance values are found for commonly used thin film FTO and ITO electrodes.35,38 In the case of BDD layers deposited at 730 °C, the measured sheet resistance values of p+-Si|BDD structures were 45 Ω/sq in comparison to bare BDD electrodes with a value of 1500 Ω/sq (Figure 2b), therefore demonstrating the contribution to electrical transport from the underlying conductive Si substrate. From Hall effect and SIMS analysis it follows that bare BDD electrodes obtained at 730 °C are degenerately doped with boron atoms. Therefore, BDD layers obtained at 730°C might be treated as electrodes having metal-like characteristics, where a space charge region is spanning merely a few nanometers into the BDD, such that hole carriers are able to undergo tunneling transport from the valence band into the electrolyte solution or across the Si|BDD interface. On the contrary, layers obtained at lower temperatures display reduced values of Hall carrier concentration of 5.2 × 1020 cm3 and 3.9 × 1020 cm3, respectively. Such results, however, do not suggest a possible transition from metal-like behavior to a normal p-type semiconductor, generally occurring within this range of carrier concentration (~4 × 1020 cm3).43 In fact, results of Raman and SIMS measurements on samples obtained at 550 °C and 250 °C provide clear indication of a heavy doping regime (“metallic” type), i.e. boron concentration above 9 × 1020 cm3 and the appearance of Raman peaks at ~1200 and ~500 cm-1 (Figure S1g) implying a shift in position of the Fermi level close to or below the valence band maximum.70 Hence, the observed reduction of Hall carrier concentration and thus increase in electrical resistivity of BDD


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electrodes obtained at temperatures lower than 730 °C is linked to the incorporation of nonelectrically active boron defects, which may be due to passivation of boron atoms by hydrogen. To optimize Si|BDD performance, optimized electrical contacts to BDD layers and n-Si were examined. Due to degenerate doping, Ohmic contact to n-Si was reached by means of Ag paste, while Ti/Au has been used to contact the BDD layer (Figure S6).71,72 The utilized Czochralski-type monocrystalline Si wafers are known to contain oxygen impurities which usually act as recombination centers for photogenerated charge carriers after high temperature treatment of Si.73,74 For this reason, the process of BDD layer deposition onto Si and high temperature processing of Si|BDD photoelectrodes were studied in connection with possible changes in the rate of carrier recombination processes in Si wafers. At first, buriedjunction Si wafers not subjected to BDD layer deposition were characterized by mapping of the minority carrier lifetime,74 related to the recombination of electrons and holes in the bulk and on the dangling bonds at the Si surface. To reduce recombination at the Si surface full passivation of n-Si (front side of Si photoelectrode) has been achieved via coating with a silicon nitride layer (SiNx), which later acted as an efficient antireflection layer. Following deposition of BDD layers at the three temperature regimes onto p+-Si (i.e. the rear side of the Si photoelectrode) changes in the minority carrier lifetime distribution of the Si|BDD photoelectrodes are observed (Figure 2fi). For Si|BDD photoelectrodes with BDD layers deposited at 250 °C measured minority carrier lifetime values (12 µs) were comparable to values measured on non-processed Si wafers (15 µs) indicating a good passivation of the Si surface by the BDD layer and no apparent activation of carrier recombination by oxygen impurities (Figure 2g). Increase in the growth temperature to 550 °C resulted in an increase of carrier lifetime values to 20 µs most likely attributed to improved passivation of the Si surface by BDD and improved carrier transport through Si|BDD


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interface (Figure 2h). A further increase in the deposition temperature to 730 °C led to the reduction of minority carrier lifetime values to 7 µs in comparison to 20 µs measured for the 550 °C case, which is supposedly linked to the loss of photogenerated carriers on activated oxygen impurities regardless of good passivation of the Si solar cell by the BDD layer (Figure 2i). As a part of Si|BDD phototelectrode preparation, a high temperature (1 min at 800 °C) sintering process of Ag paste through a SiNx dielectric layer was required for the formation of an electrical contact to the Si front side (n-Si). After high temperature processing Si|BDD photoelectrodes maintained their minority carrier lifetime values (Figure S7), simultaneously BDD layers demonstrated unchanged characteristics as observed by SEM and Raman analysis (Figure S8a,b). Based on the points discussed above, during simulated 1 Sun illumination (AM 1.5G, 100 mW cm-2) of the Si|BDD phototelectrodes, current-voltage (J-V) characteristics are seen to be affected by contributions from various parasitic resistive losses. In the case of a npp+-Si structure with BDD layer deposited at 730 °C, the phototelectrode exhibited an open-circuit voltage (Voc) of 0.54 V, short-circuit photocurrent density (Jsc) of 33.5 mA cm-2, fill factor (FF) of 41%, and a power conversion efficiency of 5.3% (Figure 2j). Similarly, in the work of Pijpers et al.35 the J-V characteristics of Si|ITO photoelectrodes were reported to be of a comparable magnitude, i.e. Voc = 0.57 V, Jsc = 26.7 mA cm-2 and FF of 47%. When considering the albeit apparent modest values of FF and efficiency for the Si|BDD photoelectrodes, one must take into account the fact that the collection of photogenerated carriers from the BDD layer during the solid-state J-V characterization occurred at the electrical contact area of ~ 2 - 3 mm2, whereas the active area of the BDD layer available for electrochemical reactions was generally of 0.8 - 1 cm2 (Figure S9). Hence, the modest J-V performance of Si|BDD photoelectrodes is explained by the increased path length over which charge carriers travel before collection at the contact on top of BDD


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layer. The path length increase results in higher series resistance and possibly recombination of photogenerated charge carriers, which in turn influences J-V characteristics. However, under working conditions where Si|BDD photoelectrode is immersed into the electrolyte and hence the entire active area (1 cm2) of the BDD layer is in direct contact with the solution, photogenerated carriers will require a shorter path length, due to vertical carrier transfer from Si into BDD layers and thereafter to the electrolyte, as a result reducing contribution to the Ohmic losses and effectively improving FF and efficiency of the photoelectrode. Additional characterization of npp+-Si|BDD photoelectrode using pseudocurrent-voltage measurements (Suns-Voc), in which acquired J-V curves are unaffected by the series resistance (no flow of current) and solely reflect generation and recombination processes revealed the increased magnitudes of pseudo-fill factor (78 %), pseudo-efficiency (19.1 %) and Voc (0.614 V) (Figure S10). Such results support an assumption that any losses of photocurrent and a voltage drop within the examined Si|BDD photoelectrodes would originate mainly as a result of series resistance losses in the device, i.e. predominantly due to the bulk resistance of the BDD layer and the contact resistance between Si and BDD layer. This assumption was further supported by J-V measurements of npp+-Si structures with BDD layers deposited at 550 °C and 250 °C. These Si|BDD photoelectrodes displayed a significant variation in J-V characteristics - decreased Jsc, Voc and FF values, which are directly attributed to the higher bulk resistivity of BDD layers obtained at lower temperature regimes (Figure S11). However, further investigations are needed to gain more insights on the charge carrier transport across the Si|BDD interface.

Photoelectrochemical activity of buried junction Si|BDD photoelectrodes


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For a BDD layer to serve as a prospective anode material, in addition to the requirement of good electrical conductivity, long-term stability of its physical and chemical properties under electrochemical reactions is of equal importance for the realization of solar-to-fuel conversion via fabricated buried junction Si|BDD photoelectrodes. Previous articles report that the corrosion resistance and mechanical stability of BDD electrodes in severe electrochemical environments is essentially due to the high atomic density and sp3 hybridized tetrahedral carbon bonding within the diamond grains.47,75,76 Although BDD electrodes are usually acknowledged for their high overpotential for oxygen and hydrogen evolution due to limited adsorption of intermediate species and lack of catalytic sites, the electrocatalytic character and width of the electrochemical potential window of BDD electrodes have been reported to depend on certain factors.57,58,60,77-80 Nanocrystalline and microcrystalline BDD electrodes, in spite of their common origin, possess grains of significantly different sizes – a factor which can account for the distinct variability in their electrochemical behavior. In contrast to commonly used microcrystalline BDD electrodes (1 – 10 µm thick), nanocrystalline (100 – 500 nm) films contain a considerably higher density of grain boundaries, which leads to a greater magnitude of the non-diamond (sp2 carbon) fraction present in the layer.62,81 These sp2 carbon impurities manifest different electrocatalytic activity compared to the surrounding diamond grains (sp3 rich), significantly influencing the overall nanocrystalline BDD electrodes’ electrochemical properties, i.e. narrowing of the working potential window and faster electrontransfer kinetics.60,80 It should be noted that although the presence of non-diamond carbon results in microstructural damage under anodic polarization (oxidation), it is mainly confined to the electrode’s surface granting the high dimensional stability of BDD electrodes.82 Aside from the influence of sp2 impurities, the dual character of BDD electrodes’ electrocatalytic activity is also


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evident upon an increase in boron dopant density above the semiconductor/metallic threshold (in the case of degenerate doping > 1020 atoms cm-3), which results in a significant negative shift in the onset of anodic current towards a lower overpotential for the OER.60,78,79 High levels of boron doping are known to deteriorate the sp3 bonding environment within the diamond lattice, forming various defect sites which alongside non-uniform boron uptake for individual diamond grains (dependent on the crystallographic [111],[110] and [100] growth orientations), results in variations of electrocatalytic activity on the BDD electrode’s surface.57 Additionally, the BDD electrodes’ surface termination (caused by interruption of the bulk structure at the surface) is known to significantly influence electron transfer kinetics.58 Hydrogen-terminated surface (hydrophobic character), which is standard for as-grown CVD BDD layers, is known to have a finite lifetime, eventually becoming hydrophilic as the prevalence of chemical groups (C=O, C– OH, COOH) dominates the surface bonds converting the surface to oxygen-terminated.47,82


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Figure 3. Cyclic voltammograms (CV) of BDD layers grown at three deposition temperatures as measured in 0.1M potassium phosphate (KPi) solution at pH 7 (a). CV curves of FTO and BDD (730 °C) electrodes with and without cobalt phosphate (CoPi) water oxidation catalyst measured in dark (b). CV curves of npp+-Si|BDD phototelectrodes in 0.1M KPi solution upon light illumination (c) and current-voltage curves of the npp+-Si|BDD phototelectrodes after performance of consecutive CV scans, where BDD was deposited at 730 °C (d). In the case of CV measurements the scan rate was 25 mV s-1


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Taking into the account the aforementioned factors, the electrochemical response of nanocrystalline BDD layers obtained at three different temperature regimes were evaluated by cyclic voltammetric (CV) measurements. Fabricated Si|BDD photoelectrodes were submerged in a potassium phosphate (KPi, 0.1 M, pH7) aqueous electrolyte solution and an external potential was applied directly through the BDD layers, such that the npp+-Si photovoltaic component was electrically bypassed (Figure 3a). Taking into account the high boron dopant concentration (metallic character) in the BDD layers, hydrogen-termination of the layers was not expected to significantly influence the surface conductivity due to a potential drop at the BDD/electrolyte interface.77 Nevertheless, an anodic pre-treatment of the BDD layers was initially performed to intentionally induce oxygen-termination on the surface and partially reduce possible sp2 impurities from the surface. Under these measurement conditions, when the thermodynamic water oxidation potential is 1.23 V versus the reversible hydrogen electrode (RHE), the onset of oxygen evolution reaction (OER), with a steady increase in anodic current, occurs at ~1.9 - 2 V vs RHE for all examined BDD layers (Figure 3a), whereas the onset for the HER with potentials more negative than -0.6 V was dependent on the BDD layer deposition temperature which is in accord with several other reports (Figure S12). The measured values of OER overpotential (ca. 600 mV for all layers) support previous discussions on the generally low catalytic activity of BDD electrodes. However, due to their nanocrystalline nature and metal-like behavior, all studied BDD layers demonstrate considerably lower overpotential towards oxygen evolution when compared to classical microcrystalline BDD electrodes.83 The electrochemical potential window width of ca. 2.5 V for BDD layers obtained at 730 °C was the narrowest among the examined layers, which is consistent with an assumption of higher electrical conductivity (Figure S12). It should be noted, that due to the presence of grains and


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grain boundaries, the conductivity and electron-transfer rates of nanocrystalline BDD electrodes vary across the surface, and thus the anodic current measured during CV scanning represents the sum of currents through various diamond grains possessing different electrical properties. In addition, high electronic density of states (DOS) near the Fermi level known for BDD electrodes with high boron doping levels (> 1020 – 1021 cm-3)70 might greatly enhance the adsorption of intermediate hydroxyl (OH) radicals onto the BDD surface, which are later consumed in OER and possibly in other competing chemical reactions. Finally, residual sp2 carbon present at the surface could potentially facilitate electrocatalytic processes and thus affect the width of the electrochemical potential window.60 Nevertheless, highly positive applied potentials during the OER treatment are known to contribute to a gradual removal of the sp2 impurities from the BDD surface, thus a prolonged operation in the OER conditions can eventually induce an increase of the BDD potential window width. Differences in the BDD layers’ electrical properties are evident in the variation of the slopes of the CV J-V curves visible in the anodic direction. BDD layers grown at 730 °C (possessing higher conductivity values) exhibit a steady increase in anodic current upon an increase in positive potential, whilst BDD layers fabricated at lower temperatures demonstrate considerably lower anodic currents with applied potential. On the basis of their superior electrical properties and good electrochemical performance, BDD layers produced at 730 °C were selected for the comparison with FTO electrodes, which were recently utilized for the protection of buried junction Si photoelectrodes in aqueous media (Figure 3b).38 Both ITO and FTO electrodes, due to their high optical transparency and electrical conductivity have been reported to protect a large variety of semiconductor photoelectrodes.29,3539

Although, FTO or ITO coatings might resist corrosion to some extent at neutral pH

electrolytes, they usually tend to degrade84-87 and lose their properties under anodic oxidizing


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conditions during long-term operation or at extreme pH, thus inhibiting the efficiency of water splitting devices.29 Comparison of FTO and BDD electrocatalytical characteristics (CV J-V measurements) revealed a comparable behavior of electrodes, i.e. the onset of oxidizing anodic current located at 1.9 V vs RHE (Figure 3b), which is in agreement with previous reports on the catalytic inactivity of TCO electrodes.35,87 Nevertheless, for the FTO electrode the anodic current increases much faster with applied positive potential than for BDD layers. Such an observation might be partially ascribed to the charge carrier transfer through a bulk material of a more resistive nature (nanocrystalline BDD) along with BDD electrodes sluggish kinetics towards oxygen evolution, therefore leading to higher overpotential values to attain a given anodic current when compared to FTO electrode. Sluggish kinetic behavior at potentials close to the thermodynamic O2 evolution is related to the chemical inertness of the BDD surface on which oxygen evolution takes place through the formation of weakly adsorbed OH radicals.54 Although, at high applied potential the competing OER reaction eventually overrides the oxidation of organics and primarily oxygen evolution takes place through mediation of OH radicals. The low catalytic activity and large overpotential for oxygen evolution (or hydrogen evolution) have been commonly addressed via surface functionalization with catalytic materials (catalysts) exhibiting a low activation energy.6,88,89 A wide variety of materials (noble metals, transition metals, metal oxides) have been shown to significantly improve OER of otherwise non-catalytic semiconductors in water splitting devices.90 Recently, Nocera and co-workers pioneered a cobalt phosphate (CoPi)6,91 OER catalyst, which exhibits high activity in natural waters and demonstrates a continuous self-repair mechanism, which allows for re-deposition of the catalyst during operation.


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In that regard, to decrease the observed OER overpotential and to enhance water oxidation, a CoPi oxygen evolution catalyst was deposited by means of electrodeposition onto BDD (npp+Si|BDD) and FTO electrodes (Figure 4a,b). Upon applying a positive potential to the FTO|CoPi electrode and through the BDD|CoPi layer of the npp+-Si|BDD photoelectrode (measurement in dark, PV component is shorted) a significant reduction in the onset potential for water oxidation to 1.6 V vs. RHE was observed (Figure 3b). This result implies an overpotential of ~0.4 V for both examined electrodes (FTO, BDD) and is in agreement with the overpotential reported for CoPi electrodeposited onto metal oxide ITO electrodes.35,91 To demonstrate the constructed npp+-Si|BDD photoelectrodes’ ability to effectively perform water oxidation, anodic currents were measured as a function of applied potential with and without light illumination. In the absence of illumination and an applied positive potential through the n-side of the npp+Si PV component, the n-p junction is under reverse-bias conditions, which should impede anodic current flow through the junction. The observed anodic current at high positive potentials (Figure 3c– brown curve) is insignificant and likely related to leakage shunt-currents across the npp+-Si junction. Under simulated solar illumination, the npp+-Si photoelectrode’s PV component generates photovoltages of ~520 mV, therefore rectifying photogenerated holes in the valence band and electrons in conductive band towards the p+-Si and n-Si sides, respectively. Upon injection into the BDD layer and consequent transport towards the BDD/solution interface, photogenerated hole carriers participate in water oxidation. When additional potential is supplied through the nside, the observed onset of associated oxidation current of illuminated npp+-Si|BDD photoelectrode is reduced to a potential of 1.4 V (Figure 3c – magenta curve). This finding is in


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agreement with the photovoltage available from the PV component (~0.5 V) and hence the observed reduction in OER potential from 1.9 V (measured on BDD layer) to 1.4 V is dictated by the generated photopotential in the photoelectrode. A significant reduction in the OER potential is seen for the npp+-Si|BDD photoelectrode functionalized with a CoPi catalyst (Figure 3c – npp+-Si|BDD|CoPi light). During illumination and supply of an external potential to the n-Si, the onset of water oxidation occurs at ~1.1 V vs. RHE. In this configuration the npp+-Si|BDD|CoPi photoelectrode produces an anodic current of ~1 mA cm-2 at the thermodynamic potential for water oxidation (1.23 V vs. RHE). Photogenerated holes from the Si PV component participate in the water oxidation reaction at the CoPi catalyst. Incorporation of the CoPi catalyst results in a much lower onset potential for water oxidation (both under light and in dark). Notably, such results are comparable to recent results on water oxidation using Si photoelectrodes coated by FTO and ITO layers.35,38 Water splitting requires a minimum thermodynamic potential of 1.23 V. Here, a single npp+-Si photoelectrode generates 0.5 V, while the additional potential was supplied externally. Therefore, to enable unassisted water splitting using photoelectrodes based on buried junction Si, a series connection of three or four Si solar cells is required for sufficient potential to be produced.


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Figure 4. SEM images of CoPi catalyst (a,b) and NiFeO catalyst (c,d) deposited onto npp+Si|BDD phototelectrodes. Chronoamperometric (J-t) curves of npp+-Si|BDD photoelectrode measured under 1 Sun illumination at the current saturation conditions with the potential (3.5V) applied through the n-side of the cell in various electrolytes: 0.1M KPi (e), 1M H2SO4 (g) and 1M NaOH (h). J-t curve (applied potential of 3V) of npp+-Si|BDD photoelectrode functionalized with CoPi catalyst (f).

To test the stability of the protective BDD layer and to track possible changes in electrical characteristics of the underlying npp+-Si PV component, J-V curves of the npp+-Si|BDD


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photoelectrodes were measured after consecutive CV measurements. Solid-state photovoltaic J-V analysis revealed no degradation in the PV component operation characteristics (Figure 3d), which suggest no formation of a SiO2 layer, which can be detrimental to electrical properties, and thus confirms the protective nature of the BDD layer. Additional photocurrent stability measurements were performed under conditions of npp+Si|BDD photoelectrodes’ light-limiting current of ~30 mA cm-2 (Figure S13a,b) and assessed via chronoamperometric (J-t) (Figure 4e-h) and chronopotentiometric (V-t) measurements. No significant changes to J-t characteristics have been observed after photoelectrochemical treatment for 1 hour in neutral (KPi, pH 7) and 20 min in acidic (H2SO4, pH 1) or basic (NaOH, pH 13.6) conditions, again indicating BDD ability to effectively protect the underlying Si and PV component from oxidation in a wide pH range (see supplementary video). Several reports indicate instability of the CoPi OER catalyst during long-term experiments.39,90,92 Therefore, stability tests (J-t) of npp+-Si|BDD|CoPi photoelectrode under light illumination and current saturation conditions (~30 mA cm-2) have been performed in a 0.1 M KPi solution (Figure 4f). During the course of the experiment (20 min) the dissolution of the CoPi catalyst from the surface of photoelectrode has been observed. Studies involving variable thicknesses of a CoPi catalyst could shed more light on this observation, which is however out of the scope of the current work. Selection of additional OER catalysts (NiFeO and Co(NO3)2) revealed inhibited catalytic activity when deposited onto Si|BDD photoelectrodes (Figure S14). SEM analysis shows, that the deposited NiFeO catalyst layer was not homogeneously distributed on the BDD surface (Figure 4c,d) and after consecutive CV measurements the NiFeO catalyst presence has not been observed. Such behavior might be assigned to a weak adsorption of a catalyst layer to a BDD surface causing catalyst detachment. Tests involving various deposition


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methods and utilization of well-known OER catalysts (metals, metal oxides) for water oxidation via Si|BDD photoelectrodes are ongoing within our research team. In addition to stability under OER conditions, where anodic passivation of Si by silicon oxide could mask certain corrosion mechanisms, it is important to demonstrate long-term stability of fabricated Si|BDD photoelectrodes under unbiased conditions. Therefore, npp+-Si|BDD photoelectrodes were soaked in 1M H2SO4 for 24h, which revealed unchanged solid-state J-V characteristics of the underlying PV component and no changes to the BDD morphology (Figure S8c). Furthermore, measurements of the electrical parameters of BDD layers (on Si photoelectrodes and reference glass substrates) accessed before and after the performance of electrochemical treatment and testing in acidic conditions revealed nearly unchanged characteristics (Table S3). These results highlight the ability of BDD-protected buried-junction Si photoelectrodes to operate under harsh acidic conditions (pH1).

CONCLUSIONS Inexpensive, electrically conductive and optically transparent boron-doped nanocrystalline diamond layers were integrated into Si photoelectrodes. The BDD layer acts as an anode and allows for photogenerated charge carrier transfer from the underlying Si towards the electrolyte solution to participate in water oxidation. The underlying Si is effectively protected from corrosion by a BDD layer over a wide range of pH conditions (1-14). Developed BDD layers contain sp2 carbon impurities, which reduce the overpotential for water oxidation. Upon functionalization with CoPi catalyst and under simulated light conditions, fabricated Si|BDD photoelectrodes effectively oxidize water, with current densities at the thermodynamic potential of water splitting comparable to that of TCO-coated Si photoelectrodes. Thus, photoelectrodes


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based on the Si and protective BDD layers represent a platform for the development of integrated devices operating over a wide range of pH conditions. Additionally, implementation of nanostructured BDD layers (i.e. porous)93,94 for protection of Si photoelectrodes might further improve the electrocatalytic properties of diamond layers by lowering the overpotential for OER via an increased fraction of sp2 impurities. Efforts in this direction along with long-term stability and water-splitting efficiency testing of Si|BDD photoelectrodes are currently ongoing within our research group.

EXPERIMENTAL Detailed experimental procedures are provided in the Supporting Information. Fabrication of Si|BDD photoelectrodes: Czochralski crystalline p-type Si wafers were selected as a base material. After alkaline texturing, phosphorous diffusion was used to create the np-Si junctions. Aluminum diffusion into Si was employed to create an additional p+-layer on the rear side of the wafers to produce a npp+-Si structure. A SiNx layer was employed on the n-Si surface as a passivation and anti-refection layer. Additionally, SiNx provided a protection of the Si surface from direct contact with the MW-LA-PECVD apparatus substrate holder during BDD layer deposition. The obtained Si wafers were then laser-cut into 2 × 2 cm2 and 1.3 × 1.3 cm2 pieces for the subsequent BDD layer deposition. All BDD layers were deposited using a MW-LA-PECVD system, in a hydrogen rich environment with a fixed ratio of methane (CH4), carbon dioxide (CO2) and diborane (B2H6) at a constant pressure and microwave power, while deposition temperature was varied between 250 and 730 °C. Details on examined growth conditions and MW-LA-PECVD apparatus design are provided in the Supporting Information and Table S1.


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The Si|BDD photoelectrodes were then fabricated by physical attachment of a Cu wire to the BDD layer (through Au/Ti contact) and Ag-grid (n-Si side) by means of Ag paste. Reference samples of FTO coated glass (Solaronix) were fabricated via attachment of a Cu wire to the sample surface by Ag paste. Fabricated np-Si and npp+-Si photoelectrodes were then placed on a glass slide support (adjacent to a front n-Si side) and embedded in corrosion resistant nontransparent epoxy (Araldite 2000, 2014-1), such that the edges of the photoelectrodes and electrical contacts were isolated from the direct contact with the electrolyte. The remaining surface area of BDD layer available for the exposure to the electrolyte was varied between ~0.8 and 1 cm2 (Figure S9). Correspondingly, the light-exposed area was adjusted to be identical to that of catalytic area by masking of the photoelectrode’s front side with non-transparent epoxy. Samples embodied in the epoxy were then dried at a room temperature for >16 hours before proceeding to electrochemical measurements. Photoelectrochemical characterization: Photoelectrochemical experiments were conducted using a three-electrode cell configuration at ambient temperature. A silver/silver chloride (Ag/AgCl in sat. KCl) electrode was employed as the reference electrode while platinum (Pt) wire was used as the counter-electrode, whereas np-Si, npp+-Si and FTO electrodes were used as working electrodes. Aqueous solutions of 0.1 M potassium phosphate (pH 7.0, KPi), 1M NaOH (pH 13.6) and 1M H2SO4 (pH 1) in deionized water were used as supporting electrolytes. Light illumination of the Si|BDD photoelectrodes (from the n-Si side) was provided by a 250-W Tungsten lamp (Oriel 6129), calibrated to 1 Sun intensity (100 mW cm-2) to provide a photon flux equal to the AM 1.5G spectrum (see Supporting Information for the calibration procedure). Cyclic voltammograms (CV) were recorded at a scan rate of either 25 or 100 mV s-1. The measured potentials were converted to the reversible hydrogen electrode (RHE). Prior to CV data


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collection, Si|BDD electrodes were anodically pre-treated by fixing the applied bias at 2.1 V vs RHE to remove possible sp2 carbon impurities from the BDD surface. Catalyst deposition: For electrodeposition of cobalt phosphate (CoPi) oxygen evolution catalyst, the working compartment was charged with a ~10 mL solution consisting of 0.1 M phosphate buffer (KPi, pH 7) and 0.5 mM mM cobalt nitride (Co(NO3)2). CoPi was electrodeposited at 1.1 V vs Ag/AgCl, with deposition times typically of 10 - 20 min, without stirring and without iR compensation. The BDD layer of the npp+-Si|BDD photoelectrode served as working electrode. The reference electrode (Pt wire) was positioned 3 mm from the BDD surface. The same procedure was employed for the deposition of a CoPi on the 1 cm2 FTO electrode. To obtain a cobalt catalyst, drop-casting of 10 µL cm-2 of 10 mM Co(NO3)2 was performed on the BDD surface of the npp+-Si|BDD photoelectrode. To obtain a layer of nickel iron oxide (NiFeO) catalyst, a solution from iron (III) 2-ethylhexanoate (50% w/w in mineral spirits, Alfa Aesar) and nickel (II) 2-ethylhexanoate (78% w/w in 2-ethylhexanoic acid) was prepared and drop-casted onto the BDD surface. UV light treatment (Atlantic Ultraviolet G18T5VH/U lamp – 5.8 W 185/254 nm) was then performed, followed by an annealing step at 100 °C for one hour. Physical characterization: Raman spectroscopy was carried out at room temperature using a Renishaw InVia Raman Microscope at a wavelength of 488 nm and a laser power of 6 mW at the sample to evaluate the quality and BDD layer composition. The surface morphology of deposited BDD layers was examined by scanning electron microscopy (SEM) using a Tescan FERA3 tool. Thickness of BDD layers was evaluated from cross section SEM observations.


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Resistivity of BDD layers deposited on insulating (glass) substrates was measured by a fourpoint probe method with tungsten probes arranged into a square of sides 0.63 mm. A Keithley 6221 model was used as a current source and two electrometers, Keithley 6514, for potential probing while a Keithley 2182A nanovoltmeter recorded the potential difference between the electrometers. Current pulses of alternating polarity were applied during the measurement in order to compensate for parasitic thermoelectric signals. The measurement was performed at a temperature of (297 ± 1) K. For Hall carrier concentration and mobility measurements, differential van der Pauw (vdP) technique was employed, in which Ti/Au contacts (Ti adjacent to BDD) were evaporated at the corners of square samples (1 cm2) of BDD layers deposited on glass substrates. A magnetic field of ±0.25 T was applied. Secondary-ion mass spectrometry (SIMS) was used to analyze the composition of BDD layers and to determine boron-dopant concentration. Measurements were performed on the SIMS spectrometer IMS 7f (CAMECA) in depth profiling and negatively charged secondary ions detection modes. Primary ions of Cs+ were used for the analysis with the following parameters: energy of primary ions of 15 keV, impact angle of 23°, current of 5 nA. Secondary ion optics parameters were the following: extraction voltage of 5 kV, analyzed area of 33 µm in the middle of the crater, mass resolution of 4000. The pressure in the analytical chamber during the measurement was maintained at 2 × 109 mbar. Secondary ions 11B, 12C, 16O, 28Si were chosen for signal detection. The atomic boron concentration has been determined using an epitaxial boron doped diamond layer of known boron concentration (2 × 1021 cm3). Transmission electron microscopy (TEM) and scanning TEM (STEM) was carried out on a JEOL JEM 2200FS microscope operating at 200 kV (autoemission Schottky-type electron gun, point resolution 0.19 nm) with an in-column energy filter, a high-angular annular dark-field


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(HAADF) detector and an energy dispersive X-ray (EDX) silicon drift detector Oxford Instruments X-Max. Optical transmittance spectra were measured on BDD-coated glass substrates using a PerkinElmer Lambda 1050 UV-VIS-NIR spectrometer equipped with a 60 mm spectralon integration sphere. Spectra were taken in the 350 - 800 nm range. Microwave photo conductance decay (MW PCD) measurements were employed for minority carrier lifetime mapping using a WT-2000PVN equipment (SEMILAB), which utilizes an IR laser source of 904 nm wavelength with a penetration depth of approximately 30 µm. Electrical characteristics of Si|BDD photoelectrodes were controlled using light illumination from a 300-W Xe arc lamp source. Current–voltage (J-V) curves were obtained under dark conditions and under 1 Sun irradiation (simulated AM1.5G, 100 mW cm-2) calibrated via Si photodiode using a Keithley 2400 source unit.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional discussions to fabrication of boron-doped diamond electrodes, additional experimental details, summary of BDD growth parameters, summary of BDD electrical properties before and after the electrochemical treatment, SEM images of BDD layers, Raman spectra, optical transmittance spectra, SIMS analysis of boron concentration in BDD layers, electron diffraction, EELS spectra of the Si/BDD interface, high resolution TEM images of the Si/BDD interface, Current-voltage results for Ti/Au contacts to BDD, minority carrier lifetime


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maps and SEM images after high temperature processing, photographs of fabricated Si|BDD photoelectrodes, pseudo J-V curves of photoelectrode, Current-voltage curves of Si|BDD photoelectrodes fabricated at various temperatures, CV curves of BDD layers deposited at various temperatures, CV curves demonstrating saturation current density of Si|BDD and Si|BDD|CoPi photoelectrodes, CV curves of photoelectrodes functionalized with various oxygen evolution catalysts, measured open circuit potential of Si|BDD photoelectrode, XPS spectra, SEM images of bare Si photoelectrodes, SEM images illustrating nanodiamond particle seeding on Si photoelectrodes (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions P.A., K.S., I.K. and L.K. conceived the work. P.A., A.T., V.M. prepared and characterized BDD layers. A.P., P.A. prepared and characterized Si wafers. P.A., F.L.F., H.K. obtained and analyzed photoelectrochemical data. M.K. measured and analyzed BDD layers by TEM, HRTEM, EELS and ED. J.K. performed SEM. J.L. performed SIMS measurements. P.A., P.H., J-H.Y. performed photo-electrical characterization. P.A., F.L.F., L.K. and K.S. analyzed the data. P.A. wrote the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This research was supported by the Czech Academy of Sciences via PPLZ Program for postdoctoral researchers no. L100101652, project no. MSM100101602 and Czech Science Foundation (GACR) Grant ID: GAČR 13-31783S. The J.E. Purkyne fellowship awarded to Vincent Mortet by Academy of Sciences of the Czech Republic (ASCR) is gratefully acknowledged. SEM analysis was supported in part by the Ministry of Education, Youth and Sports (MEYS) CR FUNBIO CZ.2.16/3.1.00/21568 (SEM purchase), LO1409 and LM2015088 projects. The TEM characterization was carried out within the CENTEM project, Reg. No. CZ.1.05/2.1.00/03.0088, co-funded by the ERDF as part of the MEYS OP RDI programme and, in the follow-up sustainability stage, supported through MEYS CENTEM PLUS (LO1402) under the National Sustainability Programme. The SIMS characterization was supported by the SUSEN project CZ.1.05/2.1.00/03.0108 (SIMS purchase) and by the project LQ1603 (Research for SUSEN). Support from the MEYS CR grant no. LTC17083 in frame of the EU Action COST CA15107 MultiComp is acknowledged. The authors would like to thank Ladislav Klimša (IoP, Prague), Dr. Nestor Guijarro Carratala (EPFL), Dr. Florent Boudoire (EPFL) and Dr. Mathieu Prévot for the help in obtaining SEM images, Xavier Pereira Da Costa (EPFL) for the electrolyte solutions and Dr. Liang Yao (EPFL) for the assistance with Ti/Au deposition. The authors acknowledge Dr. František Fendrych, Prof. Miloš Nesládek and the MATCON project members for helpful discussions.

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Nanocrystalline Boron-Doped Diamond as a Corrosion Resistant Anode for Water Oxidation via Si Photoelectrodes Table of Contents graphic


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Nanocrystalline Boron-Doped Diamond as a Corrosion Resistant

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