Efficient Visible‑Light Hydrogen Production with Eosin Y‑Sensitized g‑C3N4/GO Hybrid Loaded with PtNi Alloy Cocatalyst

Abstract

Combining a platinum–nickel (PtNi) alloy cocatalyst with a g‑C₃N₄/graphene oxide (GO) hybrid, sensitized by the organic dye Eosin Y, delivers an unprecedented hydrogen evolution rate of 5.89 mmol g⁻¹ h⁻¹ under visible‑light irradiation. The PtNi alloy, synthesized by a straightforward chemical‑reduction and sonochemical approach, reduces Pt loading while preserving catalytic activity. Comparative studies show the PtNi system is 1.54 × more active than pure Pt and 1,178 × more active than pure Ni, highlighting the synergistic interaction that promotes electron transfer and increases reactive sites. Photoluminescence quenching and transient photocurrent measurements confirm that the alloy enhances charge‑carrier separation, thereby driving the hydrogen evolution reaction (HER) efficiently.

Background

Solar‑driven hydrogen generation offers a clean alternative to fossil fuels. To make this technology practical, a visible‑light responsive photocatalyst and an efficient, low‑cost cocatalyst are essential. While noble‑metal Pt is highly effective, its scarcity and expense motivate the development of bimetallic alloys that replace part of Pt with cheaper transition metals such as Ni. Previous studies on PtNi or PtCo alloys on semiconductor hosts have demonstrated comparable or superior HER activity. Here, we report the first use of an Eosin Y‑sensitized g‑C₃N₄/GO hybrid decorated with a PtNi alloy, achieving record‑breaking hydrogen evolution from water under simulated sunlight.

Materials and Methods

Preparation of g‑C₃N₄ and GO

Polymeric g‑C₃N₄ was obtained by thermal condensation of urea at 600 °C for 2 h. GO was synthesized via a modified Hummers’ method and freeze‑dried. The g‑C₃N₄/GO composite (mass ratio 2:1) was ultrasonically dispersed in water to promote intimate contact.

PtNi Alloy Deposition

Pt and Ni salts were co‑added to an anhydrous ethanol dispersion of g‑C₃N₄, followed by NaBH₄ reduction. Simultaneous loading of Pt and Ni (1:1 molar ratio) yielded uniformly distributed PtNi nanoparticles (average diameter <5 nm) on the g‑C₃N₄/GO matrix. The resulting composite (PtNi/GO‑0.5%) was collected, washed, and dried at 60 °C.

Photocatalytic Tests

Hydrogen evolution was measured in a Pyrex reactor containing 100 mL of 20 % (v/v) triethanolamine (TEOA) aqueous solution (pH = 7) with 50 mg of photocatalyst and 50 mg of Eosin Y. A 300‑W xenon lamp (λ > 420 nm) illuminated the suspension, and H₂ was quantified by gas chromatography.

Results and Discussion

Structural Characterization

X‑ray diffraction confirmed the graphitic‑like structure of g‑C₃N₄ (2θ = 13.8° and 27.4°). TEM images revealed a thin, layered morphology and confirmed the presence of small, well‑dispersed PtNi nanoparticles. XPS analysis showed Pt in a near‑zero oxidation state, while Ni exhibited a slight shift to lower binding energy, indicative of alloy formation. The Pt:Ni ratio was determined to be 9:11 by XPS, close to the targeted 1:1 molar ratio.

Hydrogen Evolution Performance

The PtNi/GO‑0.5% composite achieved 5.89 mmol g⁻¹ h⁻¹, outperforming Pt/GO (3.82 mmol g⁻¹ h⁻¹) and Ni/GO (0.005 mmol g⁻¹ h⁻¹). Optimizing the Pt/Ni ratio revealed a maximum at 1:1; increasing Ni beyond this ratio reduced HER activity due to a loss of Pt active sites. The optimal PtNi loading was 0.5 wt % relative to the g‑C₃N₄/GO matrix; higher loadings hindered light absorption and slightly decreased performance.

Stability

Four consecutive 4‑hour irradiation cycles showed only a <5 % drop in H₂ production, demonstrating good photostability attributed to the robust g‑C₃N₄ framework and covalently bound Eosin Y.

Mechanistic Insights

Diffuse‑reflectance UV‑Vis spectra showed that the PtNi alloy enhances charge separation rather than light absorption. Photoluminescence quenching and transient photocurrent measurements confirmed the most efficient electron‑hole separation in the PtNi/GO system, correlating with the highest HER rate. A proposed photocatalytic cycle involves electron transfer from excited Eosin Y to g‑C₃N₄/GO and then to the PtNi alloy, where protons are reduced to H₂. Holes oxidize the sacrificial TEOA.

Conclusion

The study demonstrates that a PtNi alloy cocatalyst, synthesized via a simple reduction/sonication route, dramatically boosts visible‑light photocatalytic hydrogen evolution on an Eosin Y‑sensitized g‑C₃N₄/GO hybrid. With a 5.89 mmol g⁻¹ h⁻¹ rate, this system offers a cost‑effective alternative to pure Pt, paving the way for scalable, sustainable hydrogen production.