Subscriber access provided by UNIV OF DURHAM
Energy Conversion and Storage; Plasmonics and Optoelectronics
On the Optoelectronic Properties of CuI Photoelectrodes Adam Balog, Gergely F. Samu, Prashant V. Kamat, and Csaba Janáky J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03242 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019
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 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 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.
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 15 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 Letters
On the Optoelectronic Properties of CuI Photoelectrodes
Ádám Balog,1 Gergely F. Samu,1,2,3 Prashant V. Kamat,3,4 Csaba Janáky1,2,* 1Department
of Physical Chemistry and Materials Science, Interdisciplinary Excellence Centre, University of Szeged, Rerrich Square 1, Szeged, H-6720, Hungary 2ELI-ALPS
Research Institute, Szeged, Dugonics sq. 13, 6720, Hungary
3Department 4Radiation
of Chemistry and Biochemistry, University of Notre Dame
Laboratory, University of Notre Dame, Notre Dame, Indiana, 46556, United States
Abstract Detailed mechanistic understanding of the optoelectronic features is a key factor in designing efficient and stable photoelectrodes. In this letter, in situ spectroelectrochemical methods were employed to scrutinize the effect of trap states on the optical and electronic properties of CuI photoelectrodes, and to assess their stability against (photo)electrochemical corrosion. The excitonic band in the absorption spectrum and the Raman spectral features were directly influenced by the applied bias potential. These spectral changes exhibit a good correlation with the
alterations
observed
in
the
charge
transfer
resistance.
Interestingly,
the
population/depopulation of the trap states, that are responsible for both the changes in the optical and electronic properties, occur in a different potential/energy regime. Although cathodic photocorrosion of CuI is thermodynamically favored, this process is kinetically hindered, thus providing good stability in the photoelectrochemical operation.
ACS Paragon Plus Environment
1
The Journal of Physical Chemistry Letters 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
Page 2 of 15
Table of Contents (TOC) Bound
Bound electron
-0.1 V
hole
Excitonic Abs. Change
0.5 V
Electrical resistance
Free
Free electron
hole
ACS Paragon Plus Environment
2
Page 3 of 15 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 Letters
Photoelectrochemical (PEC) methods hold the promise to unite the functions of solar cells and electrolyzers, thus directly converting sunlight to fuels.1 At the same time, after almost five decades of research on different semiconductor systems (e.g., metal oxides and chalcogenides), the magic bullet to attack the problem is still to be found.2,3 Both new materials and new methods are needed to develop photoelectrodes with enhanced PEC performance. The recent wave of excitement triggered in the solar energy community by lead halide perovskites has also generated an interest to employ halide-based semiconductors as photoelectrodes.4–7 At the same time, chemical, electrochemical, and photoelectrochemical corrosion remains a major issue that needs to be better understood and circumvented.8,9 In this letter, we show how in situ spectroelectrochemical measurements can contribute to the better understanding of the optoelectronic properties of CuI, a prominent member of the metal halide family. The choice of material was deliberate, as the optoelectronic properties of CuI makes it a suitable material for thermoelectrics,10 flexible transparent p–n diodes,11 thin film transistors,12 hole-transporting layers in solar cells,13,14 or even for photocatalytic CO2 reduction.6 Notably, the oxide counterpart (Cu2O, cuprous oxide) of CuI is a frequently studied and promising photocathode material for solar fuel generation.15,16 At the same time, it is prone to photocorrosion, which severely limits its applicability, unless some protection strategy is employed.17,18 There are examples in the literature where spectroelectrochemistry provided valuable insights into the energetics, defect sites, and charge transfer properties of semiconductors. For example, the redox transformation of conjugated polymers (organic semiconductors) was investigated by applying the combination of two in situ techniques at the same time, to follow both spectroelectrochemical and conductance changes.19,20 As for inorganic semiconductors, the presence of electrons in the accumulation layer of TiO2 was probed through spectroelectrochemistry.21 The flatband potential of semiconductors can be conveniently
ACS Paragon Plus Environment
3
The Journal of Physical Chemistry Letters 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
Page 4 of 15
estimated from the onset potential of the absorbance change.22 The difference in spectral response and onset potential can also be used to distinguish the flatband potential and trapping of electrons at the defect sites.23 Spectroelectrochemical diffuse reflectance spectroscopy is also appropriate to seek information on electron traps and distinguish different chemical species generated during photoexcitation of the semiconductor.24–26 As discussed above, the existing literature is oxide- (and especially TiO2) centric. However, there are precedent spectroelectrochemical studies on other non-oxide materials (e.g., CdSe,27 CsPbBr3,28 CuInS2,29 and Cu:ZnSe/CdSe30) as well. Here, we present how combination of two methods can furnish new insights on the optoelectronic properties of CuI. UV–visible spectra were recorded for the spin-coated CuI layers before and after annealing at 150 °C for 10 minutes (Figure 1A). The annealed layers showed sharp features at 407 nm and 337 nm, which correspond to the excitonic peaks of CuI.14,31 The bandgap, which was calculated from the Tauc plot derived for a direct allowed transition,6 gave a value of 3.01 eV (Figure S1) and it is in close agreement with values reported in the literature.32 SEM images demonstrated that a homogeneous film developed on the surface of the ITO supports (Figure 1B). The coverage, however, was not perfect since small holes formed within the film. The layer thickness was 700–750 nm as determined from side-view SEM images (not shown here). PEC measurements were performed to probe the photoactivity of the prepared layers under simulated sunlight (Figure 1C) in both argon and oxygen saturated media. The photocurrents are cathodic in polarity and the overall shape of the curve bears the hallmarks typical for a p-type semiconductor. The photocurrent onset appeared at ~0.2 V and the spikeshaped current transients indicated significant charge carrier recombination. The photocurrents under solar irradiation are notably smaller compared to those measured for the oxide counterpart, which is predominantly rooted in the larger bandgap energy value (3.01 eV vs. 2.20 eV).15 Furthermore, in argon saturated electrolyte negligible photocurrents were observed
ACS Paragon Plus Environment
4
Page 5 of 15
compared to the oxygen saturated solution. This confirms that CuI is capable of reducing dissolved oxygen in the electrolyte.
0.5
C
B
as-is ITO/CuI annelaed ITO/CuI at 150 °C for 10 min
(a) (b)
Current density / A cm-2
A Absorbance
0.4 0.3 0.2 b
0.1 a
0.0
400
600
800
1 m
1000
Wavelength / nm
0 -10 -20
Ar-saturated solution O2-saturated solution
-30 -40 -0.4 -0.3 -0.2 -0.1 0.0
0.1
1
0.2
0.3
0.4
Potential vs. Ag/AgCl / V
Figure 1. A: UV–vis absorbance spectra of ITO/CuI electrodes before and after annealing at 150 °C for 10 min. B: SEM image of the ITO/CuI annealed electrode. C: Photovoltammograms of the annealed CuI/ITO electrode. The measurement was recorded in argon and oxygen saturated 0.1 mol dm−3 Bu4NPF6/dichloromethane electrolyte, using a solar simulator as the light source (AM1.5), with an additional UV cut-off filter (
