Elsevier

Thin Solid Films

Volume 724, 30 April 2021, 138626
Thin Solid Films

Water oxidation at photoanodes based on hematite films and nanowire arrays

https://doi.org/10.1016/j.tsf.2021.138626Get rights and content

Highlights

  • Photoanodes based on hematite nanowire arrays and hematite films were prepared.

  • The content of oxygen vacancies is higher for hematite nanowires.

  • TiO2 underlayer induced a significant decrease in oxygen vacancy content.

  • Hematite nanowire arrays grown on TiO2 substrate show the highest photoactivity.

Abstract

In this paper we report the results of a photoelectrochemical study performed on photoanodes based on hematite nanowire arrays and films prepared on fluoride-doped tin oxide coated glass (FTO) and FTO/TiO2 substrates, respectively by hydrothermal and spray pyrolysis methods. The hematite nanowires grown on FTO/TiO2 substrate are more stable mechanically, longer (1 µm) and their density on substrate is higher. Hematite film obtained on FTO substrate has a thickness of 92 nm covering uniformly the substrate. X-ray photoelectron spectroscopy measurements showed that hematite samples synthesized on FTO/TiO2 substrate have lower content of oxygen vacancies. The photoelectrochemical performances of the prepared photoanodes are in close connection with the presence or absence of the TiO2 underlayer, with oxygen vacancies content and with their morphological characteristics. Electrochemical impedace spectroscopy was used to investigate the charge transfer kinetics at electrode/electrolyte interface and Mott-Schottky analysis was performed to estimate the flatband potential and the carrier density. TiO2 underlayer led to the formation of defects on the samples surface which induced a positive shift of the flatband potentials compared to that of the bare hematite film. The results showed that the best density photocurrent was obtained with a photoanode of hematite nanowires grown on FTO/TiO2 substrate.

Introduction

Photoelectrochemical (PEC) water splitting has grown into an attractive method for green hydrogen fuel production using sunlight energy, as it could solve the problems connected with the intermittency and storage of solar energy [1], [2], [3], [4], [5], [6], [7]. N-type semiconductor oxides with high chemical stability, as hematite (α-Fe2O3), titanium dioxide (TiO2), bismuth vanadate and tungsten trioxide have been used in this process as photoanodes. The water oxidation is the most important aspect for water splitting process because of the high activation barriers of oxygen generation. To achieve an efficient PEC cell, it is necessary to use a photoanode with properties that include a fast hole migration, a narrow band gap, a large surface area and a valence band maximum energy level more positive than the oxidation potential of water [8], [9], [10], [11], [12]. Hematite has a narrow bandgap (2.1-2.2 eV), suitable for visible light absorption but it has a short lifetime of minor carriers [13] and the kinetics for the transfer of holes at photoanode/electrolyte interface is slow [14]. In addition, its conduction band energy level is 0.4-0.5 eV lower relative to the reduction potential of water [15]. The performance of state-of-art hematite photoanodes has been improved both by shifting of the onset potential towards more cathodic potentials and enhancing of photocurrent density [16]. The onset potential was shifted cathodically by two different paths: modification of photoanode surface with a co-catalyst, mainly cobalt-phosphate [17], [18], [19], [20] to accelerate oxygen evolution reaction kinetics and depositing under or overlayers [21], [22], [23], [24], [25] of other semiconductors (like TiO2) which lead to the creation of homo or heterojonctions. The increasing of photocurrent density was based on chemical [26], [27], [28], [29], [30] and/or structural [14,[31], [32], [33], [34], [35], [36]] optimization. Arrays of one-dimensional (1D) semiconductor nanostructures that include nanowires (NWs), nanotubes, nanorodes have been extensively investigated due to the optoelectronic properties like efficient optical absorption, large specific surface areas, short and continuous path for photogenerated carriers transport and so on [1,[37], [38], [39], [40]]. Indeed, it has been found that, among hematite photoanodes, the most efficient in photoelectrochemical processes are those obtained as nanowire/nanorod arrays [38]. On the other hand, hematite thin films are desirable in PEC processes because can be minimized the charge carriers losses by recombination in thin films. Recently, have been reported a high value of photocurrent density at the potential of reversible oxygen for bare hematite film prepared by spray pyrolysis [41].

In this study, the hematite NWs and hematite films have been prepared on fluoride-doped tin oxide coated glass (FTO) and FTO/TiO2 substrates by hydrothermal and spray pyrolysis methods and the PEC properties of photoanodes have been investigated under water splitting conditions.

Section snippets

Samples preparation

In order to prepare a TiO2 thin film (sample P), a TiOx layer was deposited by spin-coating (3000 rpm for 45 s) on FTO substrate (Solaronix, 7 ohm/square) from a solution of 0.055 M TiCl4 in 1-butanol. After its drying for 1 h at 150°C [23], the sample was transfer to a muffle furnace and maintained at 525 °C for 2h.

Hematite NWs photoanodes were prepared using previously reported methods [42,43]. FTO substrates were cleaned with detergent, water and acetone and sonicated with 2-propanole in an

Morphology and characteristics

The morphology of the hematite NWs grown on FTO substrate (sample P1, Table 1) is shown in the SEM images from Fig. 1. The sample P1 heated at 525 °C shows nanowires with length of 650 nm and an average thickness of 80 nm. The nanowires have different growth directions and show some nanoholes on their surface. After the heat treatment at 750 °C, average diameter of the nanowires increased at 150 nm, their length has decreased and some of them broke.

The hematite NWs annealed at 750 °C exhibited

Conclusion

Photoanodes based on hematite nanowires with nanoholes on their surface and hematite thin films were prepared using as substrates both FTO and FTO/TiO2.

TiO2 underlayer significantly changed the morphology of the hematite nanowires and also oriented growth of hematite films along crystalline direction [110]. Growth of hematite as nanowires led to the formation of a higher content of oxygen vacancies during manufacturing process. TiO2 underlayer induced a significant decrease in oxygen vacancy

CRediT authorship contribution statement

M. Sima: Methodology, Investigation, Writing – original draft. E. Matei: Investigation. E. Vasile: Investigation, Validation. A. Sima: Resources. N. Preda: Investigation. C. Logofatu: Investigation, Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support of Romanian Ministry of Education and Research (Core Program PN19-03, contract no. 21 N/08.02.2019) is gratefully acknowledged.

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