High‑Efficiency Visible‑Light Photocatalysts: Uniform Cu₂O Nanoparticle Decoration on Silicon Nanowire Arrays with Minimal Reflectivity

Abstract

By applying a two‑step electroless deposition at ambient temperature, we achieved highly uniform Cu₂O nanoparticle coverage along the sidewalls of high‑aspect‑ratio silicon nanowires (SiNWs). Comparative studies between aggregated (A‑Cu₂O/SiNWs) and dispersed (D‑Cu₂O/SiNWs) morphologies revealed that the uniform decoration enhances photodegradation efficiency by more than three‑fold relative to aggregated structures and nine‑fold compared with bare SiNWs. This performance is attributed to reduced carrier recombination and superior light absorption in the dispersed configuration.

Background

Cu₂O, with a direct band gap of 2.0–2.2 eV, is a promising, low‑toxic, and abundant photocatalyst capable of degrading organic pollutants under visible light [1–4]. However, its practical deployment has been hindered by aggregation and morphological instability in aqueous media. SiNWs, known for their hydrophilicity, mechanical robustness, and chemical stability, provide an attractive scaffold for anchoring Cu₂O, offering additional light‑trapping benefits through multiple scattering [8–13]. The key challenge lies in achieving uniform Cu₂O deposition on the high‑aspect‑ratio SiNW sidewalls using inexpensive solution processes. Here we present a facile two‑step electroless route that yields uniform Cu₂O/SiNW heterostructures, enabling efficient charge separation and high‑visibility photocatalysis.

Methods / Experimental

Materials

p‑type Si (100) wafers (Czochralski), AgNO₃ (99.85 %), HF (48 %), HNO₃ (65 %), CuSO₄·5H₂O (98 %), and methylene blue (MB) were sourced from Acros and Fisher Scientific. All chemicals were used as received.

Fabrication of Si Nanowire Arrays

Si wafers were cleaned and immersed in a 0.02 M AgNO₃ / 4.8 M HF mixture under gentle stirring at room temperature. After rinsing with deionized water, the samples were etched in 63 % HNO₃ for 15 min to remove residual Ag, followed by DI water rinsing and vacuum storage.

Synthesis of Cu₂O Nanoparticles

Two electroless strategies were employed:

• Method 1 (Aggregated): SiNWs were directly immersed in 0.015 M CuSO₄ / 4.5 M HF for 3 min, leading to Cu aggregation at the nanowire tips. These samples were denoted A‑Cu₂O/SiNWs.
• Method 2 (Dispersed): SiNWs were first exposed to 0.015 M CuSO₄ for 15 min, then sequentially treated with HF to trigger uniform Cu²⁺ reduction along the wire surfaces. These were labeled D‑Cu₂O/SiNWs.

All samples were rinsed with DI water and thermally oxidized at 90 °C for 30 min.

Characterizations

Morphology and composition were examined by FESEM (Hitachi JSM‑6390) and EDX (Oxford INCA 350). TEM (JEM‑2100F) provided detailed nanoparticle distribution after sonication in ethanol. XRD patterns were recorded on a Rigaku Multiflex diffractometer (Cu‑Kα). Optical reflectance was measured with a Varian Cary 5000 UV‑Vis‑NIR spectrophotometer. Photocatalytic activity was evaluated in a PanChum PR‑2000 reactor under 580 nm illumination, using 0.2 mM MB. Samples were pre‑equilibrated in the dark for 40 min, then irradiated; aliquots were withdrawn every 10 min, diluted, and analyzed by a Shimadzu UV‑2401 PC spectrophotometer.

Results and Discussion

The two deposition routes yielded distinct morphologies (Fig. 1). In Method 1, Cu aggregates formed predominantly at the nanowire tips, as shown in Fig. 1c, limiting light penetration. In contrast, Method 2 produced a dense, uniform coating of Cu₂O along the entire sidewalls (Fig. 1d), as confirmed by SEM and EDX spectra (Fig. 2a). XRD analysis (Fig. 2b) revealed characteristic Cu₂O peaks at (111), (200), and (220) for both morphologies, along with a (200) Cu peak indicating incomplete oxidation.

Optical reflectance measurements (Fig. 2c) showed that D‑Cu₂O/SiNWs exhibit an average reflectivity of only 3.8 %, slightly higher than bare SiNWs (1.4 %) but markedly lower than aggregated samples (7.7 %). Photoluminescence spectra (Fig. 2d) displayed a 522 nm peak for all samples, with D‑Cu₂O/SiNWs showing the lowest intensity, indicating efficient suppression of electron‑hole recombination.

Photocurrent studies (Fig. 2e) further confirmed superior charge separation: at 4 V bias, the photocurrent increments were 0.216 mA (SiNWs), 0.527 mA (A‑Cu₂O/SiNWs), and 0.823 mA (D‑Cu₂O/SiNWs). Particle size analysis (Fig. 2f) revealed average diameters of 41.5 nm (aggregated) and 36.4 nm (dispersed), suggesting similar nucleation kinetics.

Photocatalytic performance (Fig. 4a) demonstrated that D‑Cu₂O/SiNWs reduced MB concentration to 34.7 % after 100 min, outperforming A‑Cu₂O/SiNWs (55.4 %), planar Cu₂O/Si (62.1 %), and bare SiNWs (77.1 %). The enhanced activity arises from reduced recombination, improved light absorption, and efficient electron transfer to Cu₂O conduction band.

Kinetic analysis (Fig. 4c) indicated that the second‑order model best describes the degradation, with the highest rate constant for D‑Cu₂O/SiNWs (Fig. 4d). A schematic band diagram (Fig. 4e) illustrates that photoexcited electrons from SiNWs migrate to Cu₂O, initiating MB oxidation while holes remain in Si, thereby minimizing recombination.

Reusability tests over three cycles showed negligible decline in XRD or SEM features (Additional File 1), confirming the structural stability of the photocatalysts.

Conclusions

We have developed a scalable, low‑cost two‑step electroless process that uniformly decorates SiNWs with Cu₂O nanoparticles, yielding heterostructures that significantly suppress carrier recombination and enhance visible‑light photocatalysis. The D‑Cu₂O/SiNWs achieve up to nine‑fold faster degradation of methylene blue compared to bare SiNWs. These findings provide a robust platform for advanced photocatalytic devices in water treatment, solar fuels, and beyond.

Abbreviations

A

Aggregated

D

Dispersed

DI

Deionized

EDS

Energy‑dispersive X‑ray

FESEM

Field emission scanning electron microscopy

SiNWs

Silicon nanowires

TEM

Transmission electron microscopy

XRD

X‑ray diffraction

PL

Photoluminescent