Elsevier

Ceramics International

Volume 48, Issue 1, 1 January 2022, Pages 920-930
Ceramics International

Enhanced photoelectrochemical activity using NiCo2S4 / spaced TiO2 nanorod heterojunction

https://doi.org/10.1016/j.ceramint.2021.09.176Get rights and content

Abstract

The photoelectrochemical (PEC) process is one of the most promising techniques for converting solar energy directly into clean fuels and for environmental remediation applications. One-dimensional (1D) TiO2 nanostructure arrays are extensively studied morphologies for a wide range of catalytic processes, including photocatalysis (PC) and PEC reactions for water splitting. In this study, a well-aligned spaced TiO2 nanorod (TNR) arrays were uniformly grown on a fluorine-doped tin oxide (FTO) substrate, and thermal exfoliation of the TNRs, which appeared as small islands, was investigated. The calcination time, which ranged from 1 to 5 h at 400 °C, had a significant impact on the agglomerated TNRs islands, which tended to be uniformly exfoliated as spaced nanorods and eventually improved the PEC performance. The regularly arranged spaced nanorods allow for the efficient transfer of internal charge carriers within the TNRs. Furthermore, the heterojunction formation with NiCo2S4 via the successive ionic layer adsorption and reaction (SILAR) method, substantially improve PEC performance, owing to reduced charge carrier recombination at the interface of the heterojunction. The Mott–Schottky analysis strongly supports the improved charge carrier density at the heterojunction interface. The improved life time of the charge carriers was investigated by comparing the time-resolved photoluminescence spectra for the TiO2/NiCo2S4 heterojunction with pristine TNRs, which was eventually confirmed by incident photon-to-current efficiency analysis.

Introduction

The ever-increasing demand for energy and environmental protection has prompted research for the discovery of efficient technologies and innovative nanomaterials across the world. Researchers in the field of materials science have developed a wide range of novel nanomaterials to tackle these demands. As a result, one-dimensional (1D) nanomaterials have become the most fascinating choice of nanomaterials because their nanoscale properties function exclusively in photoelectrochemical (PEC) applications [[1], [2], [3]]. More importantly, the PEC process has become a promising technique for solar fuel production through electrochemical water decomposition via oxidation and reduction reactions. Therefore, the development of smart PEC electrode materials (anode & cathode electrodes) for the water splitting process has become an efficient approach [3,4]. Although a wide variety of nanomaterials, including 0D, 1D, two-dimensional (2D), and their corresponding composites, have been tested for PEC processes, 1D nanostructures, such as nanorods and nanotubes, evidently rank first [2,3,[5], [6], [7], [8]]. The nanoscale properties of these nanostructures, such as high surface area and uniaxial flow of charge carriers (electrons) along the length of the axis, make electrochemical research more interesting. Among all 1D nanostructure metal oxides, TiO2 and α-Fe2O3 (hematite) nanorods have become the most promising electrode materials for electrochemical solar fuel production via water splitting owing to their natural availability, chemical stability in alkaline electrolytes, strong support-catalyst interaction, high resistance to photo corrosion, and nontoxicity [3,[9], [10], [11], [12], [13]]. The conduction and valence band potentials of TiO2 are particularly suitable for driving water reduction (H+/H2) and oxidation reactions (H2O/O2)– ECB(TiO2) = ca. −0.2 V vs. reversible hydrogen electrode (RHE) and EVB(TiO2) = ca. 3.0 V vs. RHE.10 However, the only limitation of TiO2 is its wide bandgap (i.e., 3.2 and 3.0 for anatase and rutile polymorphs, respectively), which restricts visible light absorption. Therefore, additional modifications of TiO2 nanorod arrays (TNRs) could improve light absorption properties, as well as effective charge transfer properties by forming a heterojunction with other narrow bandgap materials of appropriate bandgaps and band-edge potentials, as well as fine-tuning of experimental parameters to obtain well-defined spatial nanorod arrays [10,[15], [16], [17]].

Essential and highly influential experimental parameters have always been encouraged to improve the charge transfer separation in TiO2 photoanodes. Zhao et al. [16] investigated the effect of growth time (4–32 h) and annealing temperature of TiO2 grown on fluorine-doped tin oxide (FTO) substrates using the hydrothermal method on the photocurrent conversion efficiency of dye-sensitized solar cells. Compared to unannealed nanorod arrays, the optimized nanorod arrays had a 400% enhanced efficiency, which was attributed to improved adhesion and electric contact between TiO2 and FTO, as well as a reduced number of recombination sites. As a result of these investigations, we carried out in-depth studies on TNRs to further improve the PEC performance. We focused on experimental parameters that have not yet been investigated, such as different calcination times at different time intervals. Interestingly, these investigations yielded promising results, such as improved photocurrent densities and reduced impedance values.

In addition, many other studies have revealed that pre-treatment and post-treatment experiments can also improve the PEC activities. In this regard, hydrogen treatment of TNRs, heterojunction formation with other metal oxides, bimetallic chalcogenides, and doping with nonmetal/metals, to name a few, have improved the PEC water splitting efficiencies [6,10,[17], [18], [19]]. For example, hydrogen-treated TiO2 nanowires (H-TNWs) were prepared by annealing in a hydrogen atmosphere at temperature ranges of 200–550 °C. In comparison with pristine TiO2 nanowires, the H–TNW samples demonstrated substantially enhanced photocurrent density in the entire potential window. Hydrogen treatment increases the TiO2 nanowire donor densities by three orders of magnitudes by creating a high density of oxygen vacancies that serve as electron donors. The improvement in photocurrent densities is further enhanced by bandgap engineering, heterojunction formation, and altering band-edge potentials with TiO2 [20]. Single metal or bi-metal oxides are frequently used to alter or improve the electrochemical and catalytic properties of TiO2. For example, composites based on BiVO4–TiO2 nanowires [21], TiO2–CdS–NiO [22], and TiO2–In2O3 [23] have been extensively studied for PEC reactions. Subsequently, Ni-based bi-metal oxides and chalcogenides have also worked out in this particular research. The electrochemical performance has been improved by NiCo2O4 nanostructures grown on Ni foam with oxygen vacancies [24,25], and AB2X4 (A, B = transition metals and X = Sulfur)-type metal chalcogenides of NiCo2S4 nanostructured thin films have also been studied for PEC overall water splitting [26,27]. Therefore, it is assumed to be a promising material for electrochemical reactions. Fortunately, NiCo2S4 is a visible light active narrow bandgap material with bandgap energy that varies from 1.2 to 2.1 eV and has been widely studied for various energy applications, such as hydrogen production and supercapacitors [[28], [29], [30], [31]]. NiCo2S4 can also function as a hydrogen generation co-catalyst in photocatalytic activities embedded on support catalysts [8,32].

Hence, we aimed to form a TiO2/NiCo2S4 heterojunction on FTO substrates for the first time for PEC water splitting applications. In the case of NiCo2S4, the combination of these materials exhibits beneficial catalytic properties toward overall water splitting owing to their enhanced electronic conductivity, different valence states (redox couples, i.e., Co3+/Co2+ and Ni3+/Ni2+), capacity to interact with water molecules to form M − O bonds, and good water oxidation potential despite the gradual decrease in stability during long-term operation [26,33,34]. In addition, according to the Engel–Brewer theory, the electronic interaction between Ti and NiCo (both having hypo-d-electron character) can improve the electrocatalytic activity, as well as the optical and electrical properties, resulting in an increased surface area and improved stability in TiO2 support catalysts [[35], [36], [37]]. As a result of this research, we constructed a novel heterojunction photoanode to improve PEC performances in water splitting applications. In summary, we succeeded in improving the PEC performance of TiO2 by heterojunction construction with NiCo2S4 nanolayers, as well as narrow optimization of exceptional parameters.

Section snippets

TNR syntheses

The selected anode material was synthesized using the hydrothermal method [38], which involved mixing 15 mL of distilled water with 15 mL of concentrated HCl (35–37%) in a beaker and stirring for 5 min. Subsequently, varying amounts (0.2, 0.4, and 0.8 mL) of titanium butoxide (Sigma Aldrich), were added and stirred for 15 min. After that, the above solution was transferred into a Teflon-lined autoclave with two FTO substrates (1.5 cm × 2 cm) placed at an angle of 45° with the FTO side facing

XRD analysis

Fig. 3 depicts the crystal structure of the photocatalysts measured by XRD. The TNRs annealed in the temperature range of 300–500 °C reveal rutile-type TiO2 nanorods grown in (101) and (002) facet directions (Fig. 3(a)) [[42], [43], [44], [45]]. As the temperature increased, the rutile phase intensity also increased, confirming a well-defined crystalline structure, which is a crucial factor for the fast transfer of photogenerated charge carriers upon light irradiation [45]. The increased rutile

Conclusions

For the first time, efficient photoelectrode nanocomposite materials with heterojunctions between TiO2 nanorods and NiCo2S4 nanosheets have been prepared using the hydrothermal and SILAR methods. The structural and morphological analysis confirmed the nanorod and heterojunction formation and revealed a clear difference in the annealing temperature variation, which had a significant impact on the PEC performance. The PEC studies demonstrate that TNR optimization and heterojunction formation

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

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1l1A3A01041454, NRF-2021R1A2C2008447).

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