Enhanced Photocatalytic Degradation of Oxytetracycline by WO3/Graphene Nanocomposites and Device Analysis of Photo‑Induced Doping Mechanisms

Abstract

WO₃ integrated with graphene (GR) has emerged as a highly effective photocatalyst for diverse applications. Yet, the exact pathways of charge carrier transfer during photocatalysis remain elusive due to the complex interfacial interactions. In this study, we fabricated WO₃/GR layered films and characterized them using Raman, UV–vis, and SEM techniques. The results demonstrate that p‑doped graphene significantly enhances the structural and optical properties of the WO₃/GR composite. Photocatalytic activity was evaluated through the UV‑driven degradation of the antibiotic oxytetracycline. Cyclic voltammetry revealed a higher photocurrent and a larger impedance resistance for the as‑grown WO₃/GR films synthesized directly on copper foils, contrasting with conventional WO₃ catalysts. To probe the underlying mechanism, we fabricated large‑area WO₃/GR devices on silicon substrates via a modified CVD process and compared them with WO₃ reference devices. Photo‑induced doping was evident from the current–voltage measurements, where the photocurrent was lower than the dark current and the photo‑resistance exceeded the dark resistance—behaviors that differ markedly from pure WO₃. This work establishes a new framework for analyzing charge transfer in WO₃/GR photocatalysts and provides insights into their superior performance.

Introduction

Harnessing solar energy for clean electricity production, notably through photocatalytic water splitting, has become a cornerstone of sustainable technology. Efficient, low‑cost photocatalysts such as WO₃ and TiO₂ have attracted significant attention. The formation of semiconductor composites—especially those incorporating graphene—offers a promising route to improve charge carrier separation and thus enhance photocatalytic efficiency. Graphene’s unique two‑dimensional honeycomb lattice confers exceptional mechanical strength, electrical conductivity, and chemical stability, making it an ideal partner for oxide semiconductors.

Recent reports have highlighted WO₃/GR hybrids as leading photocatalysts for water splitting and pollutant degradation, attributed to their resistance to photocorrosion and efficient electron transport. Extensive research has explored the mechanisms by which graphene improves WO₃ performance, including electron‑acceptor behavior that suppresses electron–hole recombination and the formation of Z‑scheme heterojunctions that facilitate visible‑light activity.

While these studies provide valuable insights, the precise dynamics of photo‑generated charge transfer in WO₃/GR remain incompletely understood. Advanced spectroscopic techniques and device-level measurements are needed to unravel the role of photo‑induced doping and interfacial charge redistribution.

Experimental Section

Film Fabrication
Large‑area graphene films were grown on Cu foils via chemical vapor deposition using methane. The graphene was transferred to SiO₂/Si substrates by etching the Cu with an aqueous Fe(NO₃)₃ solution. A 50‑nm WO₃ layer was deposited from WO₃ powder onto the cleaned Si wafer (275‑nm SiO₂) under argon atmosphere. Cr/Au (5/50 nm) electrodes were patterned by photolithography, electron‑beam deposition, and lift‑off. A control WO₃ film without graphene was prepared identically.

Characterization
Band‑gaps were determined from UV–vis absorption spectra (Shimadzu UV‑2600). Morphology and microstructure were examined by JEOL JSM‑7600F FE‑SEM. Raman spectra were recorded on a Witec system (532 nm laser) at low power to avoid sample damage.

Photocatalytic Testing
Photocatalytic degradation of oxytetracycline (15 mg L⁻¹) was carried out in 20 mL suspensions under 250‑W Hg lamp irradiation (365 nm). The suspension was equilibrated in the dark for 1 h before UV exposure for 160 min. UV–vis absorption was monitored over time to assess degradation.

Electrochemical Measurements
All electrochemical tests used a three‑electrode setup (CHI 604E). WO₃/GR/Cu and WO₃/Cu served as working electrodes, with Pt foil as counter and Ag/AgCl (saturated KCl) as reference, calibrated to RHE. Linear sweep voltammetry (0.1 V s⁻¹, +0.20 to −0.20 V vs. RHE) and impedance spectroscopy (100 kHz–0.1 Hz, 40 mV overpotential) were performed in 0.5 M H₂SO₄. Data were fitted to a simplified Randles circuit.

Optoelectronic Characterization
Photocurrent measurements were performed in a vacuum probe station using an Agilent 1500 A analyzer, with a 253 nm lamp for UV excitation.

Results and Discussion

The Characteristic of the WO₃/GR Film

Figure 1 illustrates the CVD synthesis route and the resulting film morphology. SEM images (Figures 1b–1c) reveal a uniform, smooth WO₃/GR surface with ~100 nm crack gaps. Elemental mapping (Figures 1d–1f) confirms homogeneous distribution of W and O, while C is confined to the crack regions, indicating graphene resides beneath the WO₃ layer.

Schematic of the synthesis and the SEM morphologies of the WO₃/GR heterostructures. a The 50 nm WO₃ powder is positioned in the same ceramic boat at the inlet side of the tube furnace. b × 60,000 and c × 5,000 SEM images. d C e O f WEDS elemental mapping of WO₃/GR

Raman spectra (Figure 2a) show characteristic D (≈1370 cm⁻¹) and G (≈1599 cm⁻¹) bands for the composite, with an I_G/I_D ratio of 1.2—lower than the 2.0 ratio for pristine graphene—indicating increased defect density due to high‑temperature processing. The up‑shift of the G band and the 2D band (≈2700 cm⁻¹) confirms p‑type doping of graphene by WO₃. UV–vis spectra (Figure 2c) reveal enhanced visible‑light absorption and a narrowed band gap from 3.88 eV (WO₃) to 3.68 eV (WO₃/GR), suggesting improved electron–hole separation.

a Raman spectra of as‑prepared samples. b Raman G‑peak mapping of WO₃/GR. c UV–vis absorption spectra. d Band‑gap determination.

The Degradation of Antibiotics Oxytetracycline

Photocatalytic tests under 365 nm UV irradiation show that WO₃/GR degrades oxytetracycline faster than pure WO₃ (Figure 3). The concentration follows pseudo‑first‑order kinetics: ln(C/C₀) = k t, with rate constants k_WO₃ = −0.0045 min⁻¹ and k_WO₃/GR = −0.0054 min⁻¹, indicating a 20 % enhancement.

a UV–vis spectra of oxytetracycline degradation with WO₃. b Degradation with WO₃/GR. c Kinetic plots.

Electrochemical Behavior of the Layered Materials

Cyclic voltammetry (Figures 4a–4b) demonstrates that WO₃/GR/Cu exhibits a lower overpotential (−0.08 V) and higher photocurrent (8.5 mA) compared to WO₃/Cu (−0.06 V, 4 mA). Electrochemical impedance spectroscopy (Figures 4c–4d) shows reduced charge‑transfer resistance (R_et) for WO₃/GR, confirming more efficient electron transport under UV illumination.

Electrocatalytic application of CVD‑synthesized layered materials WO₃/GR and WO₃. a, b CV curves. c, d EIS spectra.

The Charge Transfer Behaviors from WO₃/GR Composite Device

Device measurements (Figure 5) reveal that the WO₃/GR photocurrent (~106× higher) is still lower than its dark current, while the photo‑resistance exceeds the dark resistance—a hallmark of photo‑induced doping. In contrast, the pure WO₃ device behaves as a conventional semiconductor.

Photocurrent and photo‑resistance characteristics of WO₃/GR versus WO₃ devices.

Band‑structure analysis (Figure 6) illustrates that UV photons generate electron–hole pairs in WO₃; the electrons transfer to graphene, leaving positively charged defects in WO₃ that modulate graphene’s Fermi level. This photo‑doping enhances charge separation and lowers recombination, thereby boosting photocatalytic performance.

Charge distribution and equivalent circuit model of the WO₃/GR device under UV illumination.

Conclusion

WO₃/GR layered films exhibit superior photocatalytic degradation of oxytetracycline under UV light, owing to p‑type graphene doping that promotes charge separation. Electrochemical and device studies confirm photo‑induced doping and enhanced electron transport. These findings provide a clear mechanistic framework for designing graphene‑based photocatalysts with improved efficiency.