Silicon/TiO₂ core-shell nanopillar photoanodes for enhanced photoelectrochemical water oxidation

Highlights

• Si/TiO₂ nanopillars core-shell array is prepared with a MACE and ALD methods.

• TiO₂/Si NP photoanodes exhibit enhanced photoactivity for water splitting.

• A maximum photocurrent density of TiO₂/Si NP photoanodes achieves to 1.5 mA/cm⁻² vs. RHE.

• High performance due to enhanced light absorption efficiency and reduced charge recombination.

• TiO₂/Si NP photoanodes shows better photostability in different electrolytes.

Abstract

Nanostructured Si/TiO₂ core-shell nanopillar (NP) photoanodes were synthesized to overcome photodegradation stability of Si and to enhance the efficiency for photoelectrochemical water splitting. The core-shell structures were fabricated by atomic layer deposition of TiO₂ onto Si nanopillars synthesized by metal-assisted chemical etching and nanosphere lithography. Scanning electron microscopy, transmission electron microscopy, Raman and reflectance spectroscopies were utilized to characterize fabricated photoanodes. The obtained Si/TiO₂ core-shell NP arrays exhibit less than 6% of optical reflectance in UV and visible region, providing good optical absorption. Photoelectrochemical (PEC) water oxidation of fabricated photoanodes was studied. We showed that n-Si/n-TiO₂ NP exhibited a larger photocurrent than p-Si/n-TiO₂ due to a barrier at the heterojunction. Optimal morphological parameter of Si/TiO₂ NP for enhanced PEC water splitting were found. We demonstrated its enhanced PEC performance with a photocurrent density of 1.5 mA/cm² under simulated solar radiation with intensity of 100 mW/cm². The relationship between the PEC performance and the electrolyte pH was also discussed. The design of the geometry of Si/TiO₂ core-shell NP arrays offers a new approach for preparing stable and highly efficient photoanodes for PEC water splitting.

Introduction

One of the best approaches in development of renewable energy resources is a hydrogen production technology based on utilization of solar energy in photocatalytic process. Photoelectrochemical (PEC) water splitting is a direct route to capture solar irradiation and to produce hydrogen. A semiconductor immersed in an aqueous electrolyte absorbs photons and generates electron-hole pairs that are separated by a built-in electric field and transported to the semiconductor-electrolyte interphase where they dissociate water molecules into hydrogen and oxygen. One of the main challenges in PEC water splitting is the development of photoanodes with high stability and efficiency [1].

In recent years many publications are focused on application of multidimensional nanostructures for PEC process. Among multidimensional nanostructures, 1D nanostructures (nanowires, nanopillars, nanotubes etc.) is considered to be more efficient among them [2], [3], [4]. Nanopillars (NP) provide a unique approach combining desirable characteristics of excellent photoelectrodes. It is well known that efficient photoelectrodes requires good light absorption, fast photogenerated charge separation and mass transport of reactants. The elongated geometry of NP enables sufficient light absorption. NP arrays poses highly developed surface area, efficient mass transfer, and rapid electron–hole separation along radial axis.

Development of efficient nanostructured photoanodes for water-splitting requires photocatalysts providing a high photocurrent density and good stability. Silicon (Si) is a feasible commercial PEC material, one of the dominant earth-abundant and enhanced solar light absorption semiconductor with a small band gap (E_g = 1.1 eV). Nanostructured Si is commonly used for PEC water splitting due to its enhanced absorption properties and high surface area [5], [6], [7]. For example, porous silicon (PSi) [6] or silicon nanowires (Si-NW) [8] can trap the incident light owing to the surface peculiarities and effectively increase the interfacial area. Oh et al. have shown the photoanode built on the array of silicon nanowires which demonstrates excellent antireflectivity over a wide spectral range (300–1000 nm) and superior hydrogen production in comparison to flat-surface [8].

However, Si photoelectrodes undergoes irreversible changes induced by corrosion and surface passivation during the electrochemical reactions. Moreover, it is thermodynamically impossible to use Si photoelectrode for direct water oxidation because its valence band maximum energy is higher than oxidation potential. Therefore, a composite semiconductor electrode based on the heterojunction should be proposed to overcome these shortcomings. Encapsulation of the Si NP into the photocatalyst shell leads to formation of a complex nanocomposite structure, so-called core-shell, and sustains an effective light absorption and charge separation at the interface [3], [9], [10], [11], [12]. The use of TiO₂ as a shell to protect Si from corrosion is straightforward because it is considered as one of the most stable photocatalytic materials [13], [14], [15]. Because of its high E_g (∼3 eV), TiO₂ shell transmits visible light which can be further absorbed by Si core. Thus, TiO₂-Si interfaces behave as dual absorber tandem photoelectrodes [16], [17], [18], [19].

Among various deposition techniques, the atomic layer deposition (ALD) is a method that enables to conformally and uniformly deposit broad class of materials. ALD provides an occurrence of self-limited chemical reactions which lead to formation of ultrathin film even for complex porous [20], or 1D nanostructures [21], [22]. Application of ultrathin TiO₂ film about 1–2 nm as a protective layer for n-type plane Si surface was firstly introduced by Chen et al. [23]. The process of water oxidation during the research was sustained steadily under various pH conditions, including extreme factors, which led to quick degradation of pure TiO₂ photoanode. An amorphous TiO₂ ALD coverage up to 12 nm applied to p⁺-type of silicon was also introduced by Scheuermann et al. [14] Yu et al. [24] fabricated an efficient photoanode onto black Si wafer using 8 nm TiO₂ ALD layer. In combination with thin film of the oxygen evolution catalysts this heterostructured photoanode is accomplished the photocurrent density of 32.3 mA × cm² at an external potential of 1.48 V. These types of photoanodes also demonstrate a good chemical stability in every pH electrolyte solutions. Another research groups [25], [26], also made a successful attempts of application a TiO₂ film as a protection layer for various nanocomposite photoanodes and showed that the application of TiO₂ thin film effectively prevents passivation and subsequent degradation of Si in harsh electrolytes. Additionally, as it was shown in Ref. [27] the TiO₂/Si n/n heterojunction possess of a sufficient energy barrier which reflects minority holes back to the TiO₂.

In this article, we report a fabrication of Si/TiO₂ core-shell NP photoanodes by a combination of nanosphere lithography, metal-assisted chemical etching (MACE), and ALD. To the best of our knowledge, the comprehensive analysis of Si NP arrays with ALD TiO₂ coverage depending on pH conditions and morphological peculiarities of the nanocomposite structure is not represented in full extent in the literature. The PEC behavior of the pillar-based nanocomposite structure should be correlated with characteristics of TiO₂ layer. Consequently, we performed a comprehensive investigation on the influence of pH conditions, pillar length and TiO₂ thickness onto the PEC efficiency. In this study we prepared arrays of aligned p- and n-type silicon nanopillars covered by ALD TiO₂ layer with various thicknesses and then studied their PEC efficiency depending on pH conditions and morphological peculiarities.