Enhanced Photoelectrochemical Water Splitting with TiO₂ Nanosheet Arrays, Layered SnS₂, and CoOx Nanoparticles

Abstract

Converting solar energy into hydrogen fuel via photoelectrochemical (PEC) water splitting offers a sustainable solution to global energy and environmental challenges. Conventional TiO₂ nanomaterials suffer from limited visible‑light absorption and rapid electron–hole recombination. Here, we rationally design a TiO₂ nanosheet array photoanode decorated with layered SnS₂ absorbers and CoOₓ nanoparticles. The hybrid architecture delivers a 3.6‑fold increase in photoconversion efficiency (0.44 %) compared with bare TiO₂ nanosheets and a 2.0‑fold improvement over TiO₂/SnS₂ alone. CoOₓ not only serves as an efficient water‑oxidation catalyst but also protects the absorber from photocorrosion, yielding superior long‑term stability. This study demonstrates a viable route toward high‑efficiency, durable PEC devices.

Background

Solar‑driven hydrogen production via PEC water splitting is a key technology for clean energy. TiO₂ is attractive because of its chemical robustness, favorable band edges, and earth‑abundant composition, yet its wide band gap (~3.2 eV) limits absorption to the UV region, and high recombination rates further reduce performance. Two‑dimensional (2D) TiO₂ nanosheet arrays with exposed {001} facets provide short electron‑transport paths and enhanced charge separation. However, their practical efficiency remains limited by narrow light absorption and surface recombination. Strategies to broaden the spectral response include ion doping, plasmonic coupling, and heterojunction formation with narrow‑bandgap semiconductors. SnS₂ (band gap ~2.4 eV) offers strong visible‑light absorption and a suitable band alignment for type‑II charge transfer with TiO₂. Additionally, cobalt‑based oxides (CoOₓ) act as robust water‑oxidation catalysts, lowering over‑potential and preventing photocorrosion.

Methods

Chemicals and Reagents

Tetrabutyl titanate, ammonium hexafluorotitanate, tin(IV) chloride pentahydrate, thioacetamide, cobalt(II) acetate tetrahydrate, ammonium hydroxide, hydrochloric acid, acetone, and ethanol were used as received.

Preparation of TiO₂ Nanosheet Arrays

TiO₂ nanosheets were grown on FTO glass via a hydrothermal method: 10 mL HCl + 10 mL DI water, 0.4 mL tetrabutyl titanate, 0.2 g ammonium hexafluorotitanate, followed by 170 °C hydrothermal synthesis for 10 h. Post‑annealing at 550 °C for 3 h improved crystallinity.

Fabrication of TiO₂/SnS₂ Hybrid

SnS₂ was deposited by solvothermal growth: 10 mL ethanol + 10 mM SnCl₄ + 30 mM thioacetamide at 80 °C for 1 h, then annealed at 250 °C in Ar for 2 h.

Synthesis of TiO₂/SnS₂/CoOₓ Photoelectrodes

CoOₓ nanoparticles were anchored by solvothermal deposition: 0.25 mL NH₄OH in 18 mL ethanol + 5 mM cobalt acetate, heated at 120 °C for 1 h, followed by thorough rinsing.

Characterization

XRD (Cu Kα), SEM/EDS, TEM/HRTEM, Raman (633 nm), UV–Vis diffuse reflectance, XPS, and PEC measurements (three‑electrode, 0.5 M Na₂SO₄, pH 6.8, 100 mW cm⁻² AM 1.5G). Photocurrent–potential curves were recorded at 10 mV s⁻¹, and stability tests were conducted for 2 h under continuous illumination.

Results and Discussion

The TiO₂/SnS₂/CoOₓ architecture is illustrated in Scheme S1 (supplement). SEM images (Fig. 1a–c) reveal uniform, vertically aligned TiO₂ nanosheets (~280 nm thick, 1 µm high). SnS₂ coating renders the surface rougher (Fig. 1b), while CoOₓ loading leaves the nanosheet morphology largely intact (Fig. 1c). HRTEM confirms single‑crystalline TiO₂ {001} planes (0.23 nm) and SnS₂ {100} planes (0.32 nm). XRD patterns (Fig. 2a) match anatase TiO₂ and hexagonal SnS₂; CoOₓ peaks are absent due to low loading. Raman spectra show TiO₂ Eg, B1g, A1g, and Eg modes; the SnS₂ A1g peak at 314 cm⁻¹ confirms successful SnS₂ deposition. UV–Vis absorption (Fig. 2b) indicates a blue shift to 380 nm for TiO₂, broad visible absorption for TiO₂/SnS₂, and no further shift upon CoOₓ addition. Optical band gaps are 3.2 eV (TiO₂) and 2.4 eV (SnS₂).

XPS confirms Ti⁴⁺, Sn⁴⁺, S²⁻, and mixed Co²⁺/Co³⁺ species; Co at ~4.3 at % (Fig. 3f). PEC performance (Fig. 4) shows negligible dark current for all samples; under illumination, TiO₂/SnS₂ yields 0.24 % photoconversion efficiency, while TiO₂/SnS₂/CoOₓ reaches 0.44 %—a 3.6‑fold gain over bare TiO₂ and 2.0‑fold over TiO₂/SnS₂. Photocurrent density at 1.23 V vs. RHE jumps from 0.31 mA cm⁻² (bare) to 1.05 mA cm⁻² (triple hybrid), a 3.38‑fold increase. Electrochemical impedance spectroscopy (Fig. 4d) shows reduced charge‑transfer resistance (R_ct: 3780 Ω → 1650 Ω) for the hybrid, indicating efficient interfacial charge separation.

Long‑term stability (Fig. 5) demonstrates a 54 % photocurrent drop for TiO₂/SnS₂ and only 18 % for TiO₂/SnS₂/CoOₓ after 2 h, underscoring CoOₓ’s protective role. The proposed band alignment (Fig. 6) illustrates type‑II transfer: photoexcited electrons in SnS₂ CB move to TiO₂ CB, while holes transfer from TiO₂ VB to SnS₂ VB, where CoOₓ catalyzes water oxidation and suppresses recombination.

Conclusions

We have fabricated a 2D TiO₂/SnS₂/CoOₓ heterojunction photoanode that delivers a 3.6‑fold increase in photocurrent and 1.8‑fold higher photoconversion efficiency relative to TiO₂/SnS₂. The synergy of enhanced visible‑light absorption, efficient charge separation, and robust CoOₓ catalysis yields a stable, high‑performance PEC platform for solar‑driven hydrogen production.

Availability of Data and Materials

Datasets are available from the corresponding author upon reasonable request.