Optimizing ZnO‑Based Nanohybrids: How Materials, Heterojunctions, and Crystal Orientation Enhance Methyl Orange Degradation

Background

Hybrid nanomaterials that merge exceptional optical, electronic, and magnetic properties have emerged as pivotal agents in environmental remediation and solar‑energy conversion. Recent breakthroughs include graphene oxide composites, TiO₂/BiVO₄ hybrids, 3D‑printed hydrogel composites, and Ru/Li₂O assemblies, each pushing the boundaries of photocatalytic and photovoltaic performance. In particular, semiconductor heterojunctions—especially those pairing n‑type ZnO with p‑type partners—have shown remarkable promise for dye degradation and light‑harvesting applications. Despite progress, the influence of nanostructure orientation and the detailed morphology of the constituent materials on photocatalytic efficiency remain underexplored. Our research fills this gap by presenting a low‑cost, room‑temperature electrodeposition strategy that yields ZnO nanorods coupled with Cu₂O, CuSCN, and NiO nanostructures. We systematically investigate how each variable—material identity, heterojunction architecture, and crystal orientation—shapes the photodegradation kinetics of methyl orange.

Methods

Experimental Materials

All reagents—including indium tin oxide (ITO) glass, zinc nitrate, hexamethylenetetramine (HMT), copper(II) sulfate, sodium hydroxide, lactic acid, potassium thiocyanate, EDTA, triethanolamine, and nickel nitrate hexahydrate—were analytical grade (Sinopharm Chemical Reagent Co., Ltd.) and used without further purification.

Preparation of Nanostructures

Electrodeposition was chosen for its scalability, low processing temperature, and precise size control. In a three‑electrode setup, the ITO substrate served as the working electrode, platinum as the counter, and Ag/AgCl (saturated KCl) as the reference. Reaction parameters for each material are listed in Table 1. Cu₂O deposition required pH 10–12; CuSCN was deposited at pH ≈ 1.5. All samples were rinsed with deionized water and air‑dried; no post‑annealing was performed.

Characterizations

X‑ray diffraction (Rigaku D/Max‑2500, Cu Kα, 1.54 Å) confirmed phase purity. Scanning electron microscopy (Philips‑FEI XL 30‑SFEG) revealed morphology and cross‑sectional structure without coating. UV‑vis diffuse reflectance (Shimadzu UV‑3101PC) assessed optical absorption.

Photocatalytic Decomposition Experiments

Photocatalytic activity was measured via methyl orange degradation under 500‑W Xe lamp irradiation. A quartz reactor contained 3 mL of 20 mg L^–1 MO; the solution was pre‑adsorbed in the dark for 60 min before illumination. Absorbance at 465 nm quantified remaining MO. Experiments were performed at ambient temperature in triplicate.

Results and Discussion

Preparation of Nanostructures and Composition Analysis

All ZnO, Cu₂O, CuSCN, and NiO nanostructures were synthesized by electrodeposition at room temperature. XRD patterns (Fig. 1) confirmed single‑phase wurtzite ZnO, cubic Cu₂O (preferred (111) orientation), rhombohedral β‑CuSCN, and cubic NiO. No secondary phases were detected. UV‑vis DRS (Fig. 1e) showed ZnO absorption below 370 nm; Cu₂O exhibited a band edge at 600 nm (Eg ≈ 2.1 eV); CuSCN and NiO displayed absorption between 350–500 nm, ensuring full overlap with the Xe lamp spectrum.

X‑ray photoelectron spectra of ZnO (a), Cu₂O (b), CuSCN (c), and NiO (d) prepared by electrodeposition, and the absorbance spectra (e) of the four semiconductors.

Design and Morphology of the Heterostructures

Heterojunctions between ZnO nanorods and each p‑type partner were fabricated in two orientations: ZnO/Cu₂O and Cu₂O/ZnO, with similar setups for CuSCN and NiO. SEM images (Figs. 2–5) reveal that ZnO nanorods are hexagonal prisms (200–300 nm diameter, 800–1200 nm length). Cu₂O crystals evolved from cubes to octahedra as the deposition pH increased from 10 to 12, and grew larger with longer times, sometimes aggregating. CuSCN formed dense 3D prisms or nanowires (≈ 80–100 nm diameter). NiO emerged as a meshwork of interconnected particles, producing a high‑surface‑area network. Orientation effects were evident: ZnO nanorods grown on Cu₂O or NiO displayed altered density and spacing compared to those grown on ITO alone, influencing the specific surface area and interfacial contact.

Photocatalytic Activity

Photocatalytic tests (Fig. 6) confirmed that heterojunctions consistently outperform their pristine counterparts. NiO alone exhibited the highest activity among the four semiconductors, attributable to its mesh‑like morphology and superior surface area. The ZnO/NiO (1 min) heterostructure achieved complete MO decolorization in 20 min, outperforming all other configurations, including ZnO/Cu₂O and ZnO/CuSCN. In the ZnO/Cu₂O system, the Cu₂O/ZnO orientation yielded the best performance, suggesting that placing ZnO on top maximizes surface exposure. For ZnO/CuSCN, the 3D CuSCN morphology (ZnO/CuSCN (3D)) delivered superior activity over the nanowire variant, likely due to increased interfacial area and better charge separation. Across all systems, the improved photocatalytic efficiency can be traced to the combined effects of enhanced charge transfer across the heterojunction, reduced electron–hole recombination, and increased active surface sites.

a Relative MO concentration (C_t/C₀) versus time for various catalysts. b UV‑vis spectra of MO after irradiation with each photocatalyst.

Conclusions

We have demonstrated that the photocatalytic degradation of methyl orange can be dramatically enhanced by engineering ZnO‑based heterojunctions with p‑type Cu₂O, CuSCN, and NiO. Key findings include: (1) nanostructure morphology is the dominant factor influencing activity; (2) heterojunctions outperform pristine semiconductors, except when morphology is suboptimal; (3) crystal orientation has a minor role compared to morphology; (4) NiO delivers the best standalone activity among the four semiconductors; and (5) the ZnO/NiO (1 min) heterostructure exhibits the fastest and most complete MO removal. These insights provide a clear pathway for designing next‑generation, low‑cost photocatalysts that address both environmental and energy challenges.

Abbreviations

1D:

One‑dimensional

EDTA:

Ethylenediaminetetraacetic acid

HMT:

Hexamethylenetetramine

MO:

Methyl orange

NW:

Nanowire

SCE:

Saturated calomel electrode

SEM:

Scanning electron microscopy

UV-vis DRS:

UV‑vis diffuse reflectance spectrometry

XRD:

X‑ray diffraction