Highly Efficient Hydrogen Production via Hierarchical ZnO@TiO₂ Hollow Spheres

Background

Hydrogen (H₂) remains a leading contender for clean, renewable energy. Since the seminal photoelectrochemical water‑splitting work by Fujishima and Honda, TiO₂ has attracted extensive research for solar‑driven H₂ production. However, the high recombination rate of photogenerated electrons and holes limits its practical efficiency. Numerous strategies—such as forming heterojunctions, doping, and metal loading—have been pursued to mitigate these drawbacks. In particular, semiconductor–semiconductor heterojunctions with compatible band alignments are highly effective at suppressing recombination and extending carrier lifetimes.

ZnO shares many desirable attributes with TiO₂, including non‑toxicity, low cost, and chemical robustness. Crucially, its conduction band (CB) and valence band (VB) lie at higher potentials than those of TiO₂, enabling electron transfer from ZnO to TiO₂ once a heterojunction is established. This charge separation promotes the reduction of water to H₂ while holes in TiO₂ are scavenged by sacrificial agents.

Morphology also profoundly influences photocatalytic performance. Hollow spheres, for instance, offer large specific surface areas, reduced charge transport distances, and enhanced light scattering due to internal reflections. Prior work has shown that cage‑like titania hollow spheres outperform solid counterparts in hydrogen evolution. Yet, most studies focus on doped composites for pollutant degradation rather than hydrogen production. Here, we synthesize intact ZnO@TiO₂ hollow spheres and evaluate their photocatalytic hydrogen evolution capabilities.

Methods

Synthesis of Hierarchical ZnO@TiO₂ Hollow Spheres

Using a template‑free hydrothermal approach, 0.015 mol each of Ti(SO₄)₂, Zn(NO₃)₂·6H₂O, NH₄F, and 0.06 mol urea were dissolved in 50 mL deionized water. After 60 min of stirring, the mixture was sealed in a Teflon‑lined autoclave and heated at 180 °C for 12 h. The resulting white precipitate was washed with ethanol, dried at 60 °C, and collected as ZnO@TiO₂ hollow spheres. Bare TiO₂ and ZnO were prepared under identical conditions for comparison.

Pt‑Loaded ZnO@TiO₂ Samples

To enhance catalytic activity, Pt was photodeposited onto the ZnO@TiO₂ spheres. The spheres were dispersed in 10 vol % triethanolamine and an H₂PtCl₆ solution, purged with N₂ for 30 min, and irradiated under broadband (>300 nm) light for 2 h. Pt loadings were adjusted via precursor concentration and reaction time, quantified by ICP (PE5300DV).

Characterization

Morphology and structure were examined by FESEM (Hitachi), TEM (Tecnai F20), STEM, and HRTEM. Elemental distribution was mapped with EDS on a Tecnai G2 F20 S‑TWIN. Phase identification used Cu‑Kα XRD (M21X, MAC Science). Specific surface areas were measured by N₂ adsorption (BET, Belsorp‑mini II). Pore size distributions were derived from BJH analysis.

Photoelectrochemical Measurements

Photocurrent was recorded on a CHI 660D workstation using a three‑electrode cell (FTO working electrode, Pt counter, SCE reference). Electrodes were fabricated by doctor‑blading a slurry of 0.25 g catalyst, 0.06 g PEG (MW 20 000), and 0.5 mL ethanol onto 1 × 4 cm FTO. The electrolyte was a 0.35 M Na₂S–0.25 M Na₂SO₃ aqueous solution. Illumination was provided by a 300 W xenon lamp (Perfectlight) at 100 mW cm⁻².

Photocatalytic Hydrogen Production Tests

Hydrogen evolution was conducted in a sealed quartz reactor at ambient temperature and pressure. A 300 W xenon lamp illuminated the system. Evolved H₂ was quantified online by a thermal conductivity detector GC (5A molecular sieve column, N₂ carrier). Experiments used 100 mg catalyst in 80 mL H₂O and 20 mL alcohol; the solution was purged with N₂ for 15 min before irradiation.

Results and Discussion

SEM images (Fig. 1a) reveal that the ZnO@TiO₂ spheres are uniformly spherical with an average diameter of 1.45 µm. A broken sphere (Fig. 1b) confirms the hollow nature, while TEM (Fig. 2a) shows a dark core and bright shell, further validating the hollow architecture. High‑magnification TEM and STEM images (Fig. 2b‑c) display a rough surface composed of nanoscale subunits, forming a hierarchical heterostructure. EDS mapping (Fig. 2d‑f) confirms homogeneous distribution of Zn, Ti, and O throughout the spheres.

HRTEM (Fig. 3) demonstrates clear lattice fringes corresponding to the wurtzite ZnO (100) plane (0.28 nm) and anatase TiO₂ (101) plane (0.35 nm). The abrupt transition between these phases at the ZnO–TiO₂ interface confirms the heterojunction formation, essential for efficient charge separation.

Nitrogen adsorption–desorption isotherms (Fig. 4) exhibit type IV behavior with hysteresis, indicating mesoporosity. BET analysis shows the ZnO@TiO₂ spheres possess a surface area of 102 m² g⁻¹—over four times higher than pristine ZnO (23 m² g⁻¹) and TiO₂ (35 m² g⁻¹). The increased surface area provides abundant active sites for hydrogen evolution.

Photocurrent measurements (Fig. 5a) reveal that ZnO@TiO₂ spheres generate 3.38 mA cm⁻², 2.61‑ and 2.17‑fold higher than ZnO and TiO₂, respectively, confirming superior charge separation. Correspondingly, hydrogen evolution rates (Fig. 5b) reach 0.152 mmol h⁻¹ g⁻¹ for ZnO@TiO₂, compared to 0.039 and 0.085 mmol h⁻¹ g⁻¹ for ZnO and TiO₂ alone. Pt loading further boosts performance; 1.5 at % Pt yields the highest rate, while maintaining structural integrity over five 30‑h cycles (Fig. 5d), evidencing excellent durability.

Mechanistically, upon illumination, electrons in ZnO and TiO₂ are excited to their CBs. Because ZnO’s CB is more negative, electrons transfer to TiO₂, where they reduce water to H₂. Simultaneously, holes migrate to ZnO and are scavenged by the sacrificial agent. The hollow architecture amplifies light absorption via internal scattering, extending the effective photon path length and generating more charge carriers (Fig. 6).

Conclusions

We have demonstrated a facile hydrothermal route to synthesize hierarchical ZnO@TiO₂ hollow spheres that exhibit superior photocatalytic hydrogen evolution (0.152 mmol h⁻¹ g⁻¹) under simulated sunlight. The synergistic effects of the heterojunction, enhanced surface area, and light‑scattering hollow structure lead to improved charge separation and photon utilization. The catalyst remains stable after repeated use, underscoring its potential for scalable, low‑cost hydrogen production.