Photoelectrochemical Properties of GaN Photoanodes with Cobalt

May 30, 2017 - Photoelectrochemical Properties of GaN Photoanodes with Cobalt Phosphate Catalyst for Solar Water Splitting in Neutral Electrolyte. Jum...
0 downloads 15 Views 1MB Size
Subscriber access provided by Binghamton University | Libraries

Article

Photoelectrochemical Properties of GaN Photoanodes with Cobalt Phosphate Catalyst for Solar Water Splitting in Neutral Electrolyte Jumpei Kamimura, Peter Bogdanoff, Fatwa Abdi, Jonas Lähnemann, Roel van de Krol, Henning Riechert, and Lutz Geelhaar J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photoelectrochemical Properties of GaN Photoanodes with Cobalt Phosphate Catalyst for Solar Water Splitting in Neutral Electrolyte Jumpei Kamimura,*,† Peter Bogdanoff,‡ Fatwa F. Abdi,‡ Jonas Lähnemann,† Roel van de Krol,‡ Henning Riechert,† and Lutz Geelhaar† †

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany



Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-

Meitner-Platz 1, 14109 Berlin, Germany

Abstract Cyclic voltammetry measurements are carried out in neutral phosphate buffered electrolyte using n-type Ga-polar GaN thin film photoelectrodes with and without cobalt phosphate (Co-Pi) modification. Without Co-Pi, the variation of the photocurrent with the bias potential exhibits a two-step behavior, and under chopped illumination spikes occur at low bias potential. Thus, in this regime surface recombination is dominant. Co-Pi modification suppresses surface recombination and significantly increases the photocurrent, especially for low bias potentials. At the same time, stability tests reveal that Co-Pi does not protect GaN against photocorrosion. Experiments using H2O2 imply that this photocorrosion is a reductive process, and probably related to the presence of charged surface defects.

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Photoelectrochemical (PEC) water splitting is one of the most interesting methods to produce hydrogen and oxygen using sunlight without emitting CO2. Research in this field was triggered by the first demonstration using TiO2 photoelectrodes.1 Ever since, many researchers have tried to explore stable materials that enable highly efficient solar water splitting. GaN, which has been developed for blue light emitting diodes and lasers,2,3 is one of the promising candidates. The bandgap can be tuned from the ultraviolet to the near infrared region by alloying with In, which allows to cover the entire solar spectrum. In addition, the conduction and valence band edges have been calculated to straddle the H+/H2 and O2/H2O redox potentials for In content up to 50%.4 The PEC properties of GaN have been investigated in several electrolytes.5-13 Among them, mild pH conditions are interesting from an ecological point of view,9-11 but as far as we know all the corresponding studies employed NaCl as an electrolyte. However, this choice of electrolyte may be problematic since the oxidation of Cl- can compete with the oxidation of water.11-13 This competition may limit the water splitting efficiency and hinder a detailed analysis of the oxygen evolution reaction (OER) on GaN photoanodes. In this work, we study the charge carrier kinetics of the OER on GaN films in phosphate buffered electrolyte with pH≈7. Furthermore, we systematically investigate how the OER is affected by the deposition of a catalyst on GaN. In particular, we employ the earth-abundant material cobalt phosphate (Co-Pi) as catalyst,14 which has widely been applied to enhance the performance of oxide photoanodes such as Fe2O3,15 WO3,16 BiVO4,17 TiO2,18 under neutral pH conditions. Our study is of general relevance since GaN is single crystalline and can be used as a model system for water-splitting photoelectrodes.19

2 ACS Paragon Plus Environment

Page 2 of 17

Page 3 of 17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

EXPERIMENTAL SECTION We used nominally undoped n-type Ga-polar GaN templates grown by hydride vapour phase epitaxy (≈5 µm GaN on AlN/sapphire) purchased from Kyma Technologies Inc. The net ionized donor density of the GaN templates was determined to be 5×1017 cm-3 by Mott-Schottky measurements.20 Cyclic voltammetry measurements were performed with a scan rate of 30 mV/s using a standard three-electrode PEC cell with a platinum wire as the counter electrode, a Ag/AgCl electrode as the reference electrode, a neutral solution of 0.1 M KPi (pH≈7) as an electrolyte, and a Xe lamp (intensity ≈100 mW/cm2) as a light source. In other experiments, H2O2 was added in the electrolyte. In both cases, a circular area with a diameter of 6 mm on the GaN surface was exposed to the electrolyte and illuminated. Potentials vs. the Ag/AgCl electrode were converted to the reversible hydrogen electrode (RHE) scale by the equation VRHE = VAg/AgCl + 197 mV + (59 mV×pH), based on the fact that GaN follows the Nernstian pH response.8,21 For the electrical contact, a Ti/Al/Ti layer was deposited by electron beam evaporation on part of the GaN surface, and this contact was not exposed to the electrolyte. For the modification of GaN with Co-Pi, we followed the photoelectrodeposition procedure developed by Nocera et al.14 We used a 0.1 M KPi + 0.5 mM Co(NO3)2 electrolyte solution and fabricated a series of samples in which the amount of charge passed between the electrodes at 0.2 VRHE was varied to 6, 18, 54, and 162 mC/cm2. Scanning electron microscopy (SEM) revealed for the sample with the largest deposited amount of Co-Pi that the GaN film was covered by an almost continuous layer of Co-Pi with a thickness of around 18 nm (see Figure 1). We note that the existence of cobalt and phosphorus in the sample was confirmed by energy dispersive X-ray (EDX) spectroscopy, consistent with previous reports on Co-Pi catalysts (data shown in Supporting Information, Figure S1).14,18 All other Co-Pi film thicknesses were deduced

3 ACS Paragon Plus Environment

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by linear interpolation; the charge densities of 6, 18, and 54 mC/cm2 correspond thus to the thicknesses of ≈0.7, 2, and 6 nm, respectively.

Figure 1. Cross-sectional (a) and top-view (b) SEM images of Co-Pi modified GaN. The amount of deposited Co-Pi corresponds to a passed charge of 162 mC/cm2. RESULTS AND DISCUSSION First, we analyzed the GaN film as a reference without any catalyst. Figure 2(a) shows the corresponding current-voltage (J-V) curves acquired in the dark, under illumination, and under chopped illumination. The dark current is negligible. Under illumination, the J-V curve shows a pronounced two-plateau behavior with the transition occurring at ~0.7 VRHE. This behavior has not been reported before. Under chopped light conditions, transient photocurrent spikes are observed at low bias potential (