Hui Jin Jinab,
Chaeyong Seongac,
Gyu Wan Choiab,
Ji-Youn Seo*b and
Min-Kyu Son*a
aNano Convergence Materials Center, Emerging Materials R&D Division, Korea Institute of Ceramic Engineering & Technology (KICET), Jinju 52851, Republic of Korea. E-mail: minkyu.son@kicet.re.kr
bDepartment of Nano Fusion Technology, Pusan National University, Busan 46241, Republic of Korea. E-mail: j-y.seo@pusan.ac.kr
cDepartment of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea
First published on 3rd January 2024
In this study, selenium (Se)-rich antimony selenide (Sb2Se3) films were fabricated by applying a solution process with the solvents ethylenediamine and 2-mercaptoethanol to optimize the photoelectrochemical (PEC) performance of the Sb2Se3 photocathode. Various antimony (Sb)–Se precursor solutions with different molar ratios of Sb and Se (Sb:
Se = 1
:
1.5, 1
:
3, 1
:
4.5, 1
:
7.5, and 1
:
9) were prepared to attain Se-rich fabrication conditions. As a result, the Se-rich Sb2Se3 films fabricated using the Sb–Se precursor solution with a molar ratio of Sb
:
Se = 1
:
7.5 exhibited an improved PEC performance, compared to the stoichiometric Sb2Se3 film. The charge transport was improved by the abundant Se element and thin selenium oxide (Se2O3) layer in the Se-rich Sb2Se3 film, resulting in a decrease in Se vacancies and substitutional defects. Moreover, the light utilization in the long wavelength region above 800 nm was enhanced by the light-trapping effect because of the nanowire structure in the Se-rich Sb2Se3 film. Hence, the optimal Se-rich Sb2Se3 photocathodes showed an improved photocurrent density of −0.24 mA cm−2 at the hydrogen evolution reaction potential that was three times higher than that of the stoichiometric Sb2Se3 photocathodes (−0.08 mA cm−2).
The morphological characteristics of the Sb2Se3 film are a key parameter affecting the PEC performance of Sb2Se3 photocathodes. The morphological characteristics can be easily controlled by adjusting the fabrication process. A compact and pinhole-free Sb2Se3 photocathode can be fabricated by controlling the cooling speed in a close space sublimation method.20 The fast-cooling process is effective for suppressing the growth kinetics of Sb2Se3, resulting in a smooth Sb2Se3 film with few pinholes. The different solvents used in the solution process also determine the morphology of the Sb2Se3 photocathodes. A combined solution of ethylenediamine (EDA) and 2-mercaptoethanol (2-MER) is advantageous for fabricating a thin and compact Sb2Se3 film, whereas, a solution with thioglycolic acid (TGA) and ethanolamine (EA) is suitable to form the nanowire/rod-based Sb2Se3 films.23 In the solution process using the EDA/2-MER solvent, the nanostructured Sb2Se3 film can be obtained by controlling the concentration of Se/Sb in the precursor solution.24,25 On the other hand, the length and diameter of nanowire/rod can be controlled by changing the concentration of TGA/EA and the spin coating iteration.26,27 In this method, a planar Sb2Se3 film with few pinholes is beneficial for improving the charge transport in the Sb2Se3 photocathode, whereas a nanowire/rod-based Sb2Se3 film is favorable to enhancing light utilization. Therefore, it is crucial to control the morphology of the Sb2Se3 film by adjusting the fabrication process parameters to optimize the PEC performance of the Sb2Se3 photocathode.
The compositional characteristics of Sb2Se3 films are a potential factor affecting on the PEC performance of Sb2Se3 photocathodes, as well as their morphological characteristics. Recently, it has been reported that Se-rich Sb2Se3 films enhance the efficiency of Sb2Se3-based photovoltaic cells because such films have fewer Se vacancies and substitutional defects acting as charge recombination centers.28,29 However, a reduction in efficiency has been observed in the excessively Se-rich Sb2Se3 photovoltaic cells. Thus, these considerations may be applied in photocathodes for PEC water splitting because their basic operation principle is similar to that of photovoltaic cells. Hence, optimal Se-rich Sb2Se3 films should be prepared to attain Sb2Se3 photocathodes with a high PEC performance.
Therefore, in this study, Sb2Se3 photocathodes were fabricated using a solution process with EDA and 2-MER as solvents as they allow for easy control of the film composition. The concentration of Se in the precursor solution with a fixed concentration of Sb was gradually increased to obtain the Se-rich Sb2Se3 photocathode. Morphological, optical, compositional, and electrochemical analyses were carried out to investigate the effect of the Se-rich Sb2Se3 photocathode on the PEC performance depending on the concentration of Se. As a result, the Se-rich Sb2Se3 photocathode fabricated using the precursor solution with the optimal concentration of Se showed an improved PEC performance because of the enhanced charge transport, as well as the improved light utilization as a result of the nanostructured morphology.
ERHE = EAg/AgCl + 0.059 × pH + 0.197 |
Chronoamperometry (CA) measurements were carried out in the same electrolyte (0.1 M H2SO4) biased at 0 V versus RHE under chopped one sun illumination to determine the stability of the Sb2Se3 photocathodes. The CA measurement is a typical method to evaluate the stability of the photoelectrode in the PEC water splitting system because it is an intuitive technique to recognize the activity of the photoelectrode on the water reduction reaction.30 A decrease in the photocurrent density during the CA measurement often indicates the corrosion of the photoelectrode in the aqueous solution.
To examine the charge transport capability of the Sb2Se3 photocathodes, the electrochemical impedance spectroscopy (EIS) measurement was conducted in a 0.1 M H2SO4 aqueous solution under one sun illumination biased at the hydrogen evolution reaction potential (0 V versus RHE). It was carried out at frequencies from 1 MHz to 100 mHz by applying a sinusoidal potential perturbation of 10 mV. All data of PEC and EIS measurements were acquired by a potentiostat (SP-200, BioLogic Science Instruments).
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Fig. 1 XRD patterns of solution processed Sb2Se3 film with different molar ratios of Sb and Se; 1![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Fig. 2 presents the FE-SEM images of the solution processed Sb2Se3 film with different molar ratios of Sb and Se. The stoichiometric Sb2Se3 film (Sb:
Se = 1
:
1.5) was almost planar as it was formed by the agglomeration of short column-like Sb2Se3 grains. However, the morphology of the Sb2Se3 film was slightly changed into a nanowire structure as the amount of Se powder in the Sb–Se precursor solution was increased. Moreover, the length and diameter of the Sb2Se3 nanowire were continuously increased along with the increment in the amount of Se. In the precursor solution with the excessive Se amount, the longer 1D [Sb4Se7]2− chain bonding along the (001) axis is predominantly formed by the complete reaction with the sufficient Se chains and Sb ions.24 This accelerates the formation of longer Sb2Se3 nanowires with a larger diameter. This finding agrees with the XRD results (Fig. 1). Finally, using the precursor solution with excessive Se (Sb
:
Se = 1
:
9), an irregular nanowire structured Sb2Se3 film was obtained including a large nanowire with a diameter of approximately 400 nm and many voids. The numerous voids in the Sb2Se3 film are likely due to the high viscosity of the precursor solution under the high Se condition. The morphological changes of the Sb2Se3 film, particularly the formation of the Sb2Se3 nanowire, would affect the PEC performance of the Sb2Se3 photocathodes because they provide a sufficiently large surface area for the water reduction reaction, direct routes for the efficient charge transport, as well as advantageous structures for the enhanced light utilization.
Fig. 3a shows the PEC performance of the solution processed Sb2Se3 photocathodes with different molar ratios of Sb:
Se in a strongly acidic aqueous solution (0.1 M H2SO4) under chopped one sun illumination. The PEC performance, particularly the photocurrent density, gradually improved as the Se concentration was increased. The highest photocurrent density (−0.24 mA cm−2 at 0 V versus RHE) corresponded to the Sb2Se3 photocathodes with a molar ratio of Sb
:
Se = 1
:
7.5. By contrast, the photocurrent density slightly decreased in the Sb2Se3 photocathodes with a molar ratio of Sb
:
Se = 1
:
9, although the concentration of Se was further increased. Thus, the PEC performance was related to the variation of the charge transport capability and light utilization in the Sb2Se3 photocathode, resulting from the morphological changes of the Sb2Se3 film.
To examine the charge transport capability of the Sb2Se3 photocathodes, the EIS measurement was carried out under one sun illumination. Fig. 3b illustrates the Nyquist plots of the solution processed Sb2Se3 photocathodes with different molar ratios of Sb:
Se, acquired by the EIS measurement. The plots were fitted based on the equivalent circuit connecting the sheet resistance and the impedance component in series, as shown in the inset of Fig. 3b. In general, the sheet resistance (Rs) is related to the substrate of the electrode.36 Hence, it was almost similar in all Sb2Se3 photocathodes because the same FTO substrate was used. On the other hand, the impedance component was composed of the interface resistance between the electrolyte and the Sb2Se3 photocathode (Rct) and the constant phase element. Table 1 shows the Rct values of the Sb2Se3 photocathodes with different molar ratios of Sb
:
Se, extracted from the semi-circle in the Nyquist plot. It was gradually decreased along with the increment in the Se concentration. Finally, the smallest Rct value (4255 Ω), which was approximately one-fifth of the Rct (19
624 Ω) in the stoichiometric Sb2Se3 photocathode with a molar ratio of Sb
:
Se = 1
:
1.5, corresponded to the Sb2Se3 photocathode with a molar ratio of Sb
:
Se = 1
:
7.5. This result indicates that the charge transport in the photocathode/electrolyte interface was enhanced in the Se-rich Sb2Se3 photocathode and was mainly attributed to the intrinsic characteristics of the Se-rich Sb2Se3 film. The longer nanowire structure in the Se-rich Sb2Se3 film (Fig. 1) facilitated the improved charge transport in the Sb2Se3 photocathode by providing a larger surface area for the water reduction reaction as well as effective charge transport routes. In addition, the fewer Se vacancies in the Se-rich Sb2Se3 substantially decreased the charge recombination centers, resulting in the improved charge transport of the Sb2Se3 photocathodes.28,29 However, the excessively Se-rich Sb2Se3 photocathode with a molar ratio of Sb
:
Se = 1.9 showed a decreased PEC performance, which is likely due to the many voids in the irregular nanowire structure (Fig. 2e) that act as charge recombination sites. The trend of the Rct values agrees with the PEC performance of the solution processed Sb2Se3 photocathodes with different molar ratios of Sb
:
Se (Fig. 3a).
Sb![]() ![]() |
1![]() ![]() |
1![]() ![]() |
1![]() ![]() |
1![]() ![]() |
1![]() ![]() |
---|---|---|---|---|---|
Rs | 62.19 | 46.12 | 44.55 | 50.47 | 44.37 |
Rct | 19![]() |
9411 | 5032 | 4255 | 5507 |
The improved PEC performance of the Se-rich Sb2Se3 photocathode was also confirmed by the optical properties of solution processed Sb2Se3 photocathodes. Fig. 3c displays the transmittance of the Sb2Se3 photocathodes with different molar ratios of Sb:
Se. The transmittance in the wavelength region above 800 nm decreased as the concentration of Se increased. This indicates that the light absorption in the long wavelength region is enhanced along with the increment in the Se concentration. It is a good agreement with the absorbance spectra of Sb2Se3 photocathodes (Fig. S2†). The thickness of deposited Sb2Se3 films was almost similar (approximately 0.7–0.8 μm, Fig. S3†), except for the Sb2Se3 film with a molar ratio of Sb
:
Se = 1
:
9 (1.5 μm) because of the high viscosity of the precursor solution. Thus, it is possible to exclude the thickness of Sb2Se3 as a major parameter affecting the light absorption. Therefore, the obtained result is mainly attributed to the morphological change from a planar Sb2Se3 film to a nanowire structured film, which improves the light utilization in the long wavelength region because of the light trapping effect.24,37 This is one of the main reasons why the Se-rich Sb2Se3 photocathode exhibited an improved PEC performance compared to the stoichiometric Sb2Se3 photocathode, together with an enhanced charge transport. Based on the optical properties, the estimated band gap of the Se-rich Sb2Se3 photocathode fabricated using the Sb–Se precursor solution with excessive Se powders was 1.14–1.15 eV (Table S2†), which is a traditional band gap region of Sb2Se3 films.38
XPS measurements were carried out to further analyze the elemental composition of solution processed Sb2Se3 photocathodes with different molar ratios of Sb:
Se. Fig. 4 displays the XRD spectra of the Sb2Se3 photocathode fabricated by using the Sb–Se precursor solution with a molar ratio of Sb
:
Se = 1
:
7.5, which showed the best PEC performance. In the spectrum of Sb 3d core level (Fig. 4a), two peaks appeared at 537.9 eV and 528.5 eV, which correspond to the bonding of Sb and Se from the Sb2Se3 film.39–41 Two additional peaks appeared at 539.5 eV and 530.2 eV. These peaks are ascribed to the Sb–O bond from the Sb2O3 film on the surface of the Sb2Se3 film.39,40 These peaks were not observed in the XRD spectrum (Fig. 1) because they were the result of the slight oxidation of Sb at the surface of the crystallites. In the spectrum of Se 3d core level (Fig. 4b), three peaks appeared at 52.9, 54.6, and 55.4 eV. The two peaks located at 52.9 eV and 54.6 eV are ascribed to Se2− in the Sb2Se3 film, whereas the peak located at 55.4 eV is ascribed to the elemental Se.39,40 The intensity of the peak located at 55.4 eV began to appear when the Sb
:
Se molar ratio was above 1
:
7.5 (Fig. S4†). This means that the Se-concentrated precursor solution is essential to fabricate the Se-rich Sb2Se3 photocathode, using the solution process. It is thought that the Sb2O3 thin film on the surface and the abundance of Se in the Se-rich Sb2Se3 photocathode facilitate the passivation of Se vacancies and substitutional defects in the Sb2Se3 film, resulting in the reduced charge recombination centers.29 Hence, the PEC performance of the Se-rich Sb2Se3 photocathode was considerably improved, compared to that of the stoichiometric Sb2Se3 photocathode.
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Fig. 4 XPS spectra of the Sb2Se3 photocathode with a molar ratio of Sb![]() ![]() ![]() ![]() |
Photocathode stability is also a critical issue for the practical PEC water splitting, as well as the PEC performance. Hence, a stability test was carried out in a 0.1 M H2SO4 aqueous solution under the continuous PEC operation, using the Se-rich Sb2Se3 photocathode with a molar ratio of Sb:
Se = 1
:
7.5, which showed the best PEC performance. As shown Fig. 5a, the photocurrent density gradually decreased during the PEC operation for 30 min. This outcome was mainly attributed to the morphological and compositional changes of the Se-rich Sb2Se3 photocathodes. Fig. 5b shows a top-view FE-SEM image of the Se-rich Sb2Se3 photocathode with a molar ratio of Sb
:
Se = 1
:
7.5 after the stability test was conducted for 30 min. The diameter of the Sb2Se3 nanowire was slightly enlarged compared to that before the stability test (Fig. 2d). Hence, many voids between the Sb2Se3 nanowires were observed after the stability test. These voids play a role as charge recombination sites, resulting in a decreased PEC performance. In addition, a compositional change was revealed by the XPS spectra of the Se-rich Sb2Se3 photocathode after the stability test (Fig. 5c and d). The two peaks related to the Sb–O bond from the Sb2O3 disappeared in the spectrum of Sb 3d core level (Fig. 5c), whereas the peak ascribed to the elemental Se remained in the spectrum of Se 3d core level (Fig. 5d). This result indicates that the Sb2O3 thin film on the surface of the Se-rich Sb2Se3 photocathode vanishes because of the dissolution of the Se-rich Sb2Se3 film during the PEC operation, which likely induces the exposure of Se vacancies or substitutional defects in the Sb2Se3 film to the electrolyte, thereby diminishing the PEC performance.
Compared to the previously reported PEC performance of Sb2Se3 photocathodes (Table S3†), the photocurrent density is extremely low because our Se-rich Sb2Se3 photocathode consists of only the Sb2Se3 film, without any modifications such as a back contact layer, n-type overlayer, or hydrogen evolution reaction catalyst. However, it is quite competitive, compared to bare or Au underlayered Sb2Se3 photocathodes in the reported literatures. In addition, it is possible to enhance the stability of Se-rich Sb2Se3 photocathodes by introducing a TiO2 protection layer. Therefore, works on the additional modifications using novel and low-cost materials are underway to further enhance the PEC performance and stability of Se-rich Sb2Se3 photocathodes, based on the optimal solution process conditions reported in this work.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra07023a |
This journal is © The Royal Society of Chemistry 2024 |