Noble‑Metal‑Free Photocatalytic Hydrogen Generation with Cd0.5Zn0.5S Quantum Dots on Ni2P Porous Nanosheets

Abstract

We report a cost‑effective, noble‑metal‑free photocatalyst comprising 7‑nm Cd0.5Zn0.5S quantum dots (QDs) uniformly anchored on 15–30‑nm Ni2P porous nanosheets. The high‑surface‑area Ni2P framework facilitates efficient charge separation and provides abundant active sites for the hydrogen evolution reaction (HER). Optimal performance is obtained at 1.5 wt % Ni2P, delivering a hydrogen production rate of 43.3 µM h–1 mg–1 and a solar‑to‑hydrogen efficiency (STH) of 1.5 %. Systematic studies of optical, photoluminescent, and electrochemical properties, supported by density‑functional theory, reveal that Ni2P enhances electron transfer, suppresses recombination, and supplies catalytic sites, while excessive Ni2P shading reduces light absorption. This strategy offers a scalable route to high‑efficiency, precious‑metal‑free photocatalysts.

Background

Photocatalytic hydrogen production has emerged as a promising route to sustainable energy, with Cd_xZn_{1–x}S materials gaining attention due to their narrow band gap and visible‑light activity. Traditional co‑catalysts such as Pt, Co‑Pt, and Au effectively enhance HER but incur high cost, limiting industrial adoption. Non‑precious alternatives—carbon allotropes, phosphides, and sulfides—have shown promise; Ni2P, in particular, has been successfully integrated with CdS nanowires/rods, yet the use of quantum dots (QDs) offers superior surface area and tunable band structure. We therefore explore a reverse heterostructure where Cd0.5Zn0.5S QDs are loaded onto Ni2P nanosheet arrays to synergistically combine high surface area with efficient charge separation.

Methods/Experimental

Synthesis of Ni2P

Ni(OH)2·xH2O precursors were calcined at 500 °C in the presence of NaH2PO2 to yield black Ni2P powder.

Synthesis of Ni2P‑Cd0.5Zn0.5S Nanocomposites

Ni2P was dispersed in ethanol and mixed with ZnCl2/CdCl2 precursors in ethylene glycol. After 180 °C reflux, Cd0.5Zn0.5S QDs formed on Ni2P. The Ni2P content (0.5–5 wt %) was varied by adjusting the volume of Ni2P suspension. Pure Cd0.5Zn0.5S QDs were prepared without Ni2P for comparison.

Characterization

SEM, TEM, HRTEM, and EDX mapped the morphology and composition. XRD confirmed the hexagonal Ni2P and cubic Cd0.5Zn0.5S phases. XPS verified the elemental states. UV–Vis spectroscopy measured optical absorption; PL assessed charge recombination. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) evaluated charge transfer and HER activity.

Photocatalytic Hydrogen Evolution

Photocatalysts (1–100 mg) were dispersed in a Na2S/Na2SO3 mixture and irradiated with a 300 W Xe lamp (λ>420 nm). Hydrogen evolution was quantified by GC. STH was calculated from the standard equation using the 300 mW cm–2 incident power.

Results and Discussion

Morphology and Composition

SEM images reveal flower‑like Ni2P nanosheets composed of 15–30 nm cross‑linked sheets. After Cd0.5Zn0.5S loading, 7‑nm QDs uniformly decorate the nanosheets. XRD patterns show characteristic peaks of both Ni2P and Cd0.5Zn0.5S; the Ni2P signal diminishes at higher loadings. XPS confirms the presence of Ni, P, Cd, Zn, and S without contaminants.

Photocatalytic Performance

At 1 mg catalyst, the 1.5 wt % Ni2P composite achieves 43.3 µM h–1 mg–1, 3.4× higher than pure Cd0.5Zn0.5S. Increasing Ni2P beyond 3 wt % reduces activity due to light shading. Stability tests over 16 h show negligible decline. Scaling up to 100 mg in a 150 mL reactor yields 700 µM h–1 and an STH of 1.5 %.

Optical and Electrochemical Analysis

UV–Vis spectra exhibit a 506 nm absorption edge for Cd0.5Zn0.5S; Ni2P contributes additional visible absorption, though higher Ni2P reduces <506 nm absorption. PL intensity decreases with Ni2P loading, indicating suppressed recombination. EIS reveals that Rct drops from 17.3 kΩ (Cd0.5Zn0.5S) to 5.5 kΩ (3 wt % Ni2P), evidencing enhanced conductivity. LSV shows Ni2P has low HER overpotentials (84 mV at 10 mA cm–2).

Band‑Structure Insight

DFT calculations confirm metallic Ni2P with a Fermi level at 1.03 V vs NHE, lower than Cd0.5Zn0.5S CBM (–1.04 V vs NHE). Electrons thus transfer from Cd0.5Zn0.5S to Ni2P, driving charge separation and HER at Ni2P sites.

Conclusions

The Cd0.5Zn0.5S/​Ni2P heterostructure delivers noble‑metal‑free, efficient photocatalytic hydrogen generation. Optimized at 1.5 wt % Ni2P, it achieves 1.5 % STH and high stability, illustrating the dual role of Ni2P as both charge‑transfer facilitator and HER co‑catalyst. This design principle is applicable to other photocatalytic systems.