Enhanced Visible‑Light Photocatalysis of Methylene Blue and Phenol with N‑Doped ZnO/g‑C₃N₄ Nanocomposites

Abstract

We synthesized N‑doped ZnO/g‑C₃N₄ composites by a simple, low‑cost sol‑gel route and confirmed their structure through XRD, FE‑SEM, HRTEM, FT‑IR, XPS, and UV‑vis DRS. Compared with pure N‑doped ZnO, the composite’s absorption edge shifted to lower energies, expanding visible‑light uptake and boosting charge‑carrier separation. As a result, the N‑doped ZnO/g‑C₃N₄ photocatalyst achieved superior degradation of methylene blue (MB) and phenol under visible‑light irradiation, while maintaining high stability over multiple cycles. The enhanced activity is attributed to the synergistic heterojunction between N‑doped ZnO and g‑C₃N₄, which optimizes band alignment and suppresses electron–hole recombination.

Background

Solar‑driven photocatalytic degradation of organic pollutants is a promising strategy for environmental remediation. Zinc oxide (ZnO) is attractive for its low cost, high activity, and nontoxicity, yet its limited visible‑light absorption, rapid charge‑carrier recombination, and photocorrosion hinder practical use. Various strategies—such as nonmetal doping, semiconductor coupling, and surface sensitization—have been employed to address these limitations. Nitrogen doping, in particular, extends ZnO’s light‑absorption into the visible region with minimal lattice distortion.

Graphitic carbon nitride (g‑C₃N₄) is a metal‑free polymeric semiconductor that has shown promise in pollutant degradation, water splitting, and CO₂ reduction. However, its rapid electron–hole recombination limits performance. Forming a heterojunction with ZnO can improve charge separation, as electrons transfer from g‑C₃N₄’s conduction band to ZnO’s, enhancing photocatalytic activity. While N‑doped ZnO/g‑C₃N₄ hybrids have been studied for dye degradation, their effectiveness against volatile organic compounds like phenol remains underexplored.

In this work, we prepared N‑doped ZnO/g‑C₃N₄ composites via a sol‑gel method and demonstrated their superior photocatalytic degradation of MB and phenol under visible light, along with a mechanistic discussion.

Methods

Preparation of N‑Doped ZnO/g‑C₃N₄ Nanocomposites

g‑C₃N₄ was produced by calcining melamine (5 g) at 550 °C for 4 h, followed by milling. The powder was dispersed in water, sonicated, and centrifuged to remove unexfoliated sheets.

For N‑doped ZnO synthesis, equimolar zinc acetate and urea were dissolved in ethanol; g‑C₃N₄ was added and stirred. The mixture was heated at 80 °C for 5 h, dried, then calcined at 400 °C for 1 h to yield a composite containing 50 mol % N‑doped ZnO (denoted N‑ZnO/g‑C₃N₄). A similar protocol without urea produced ZnO/g‑C₃N₄.

Characterization

XRD (Rigaku D/max 2000, Cu Kα) confirmed phase composition. FE‑SEM (Zeiss Ultra 55) and HRTEM (FEI Tecnai G² F20) revealed morphology. FT‑IR (Nicolet Nexus‑870) spanned 400–4000 cm⁻¹. XPS (Thermo Fisher K‑Alpha) calibrated to C 1s 284.6 eV. BET surface area was measured (TriStar‑3000). UV‑vis DRS (Shimadzu UV‑3600) used BaSO₄ as reference.

Photocatalytic Activity

Degradation tests employed 10 mg L⁻¹ MB or 5 mg L⁻¹ phenol in 100 mL solutions. A 300 W Xe lamp with a 420 nm cutoff delivered 120 mW cm⁻² visible light. Catalysts were added at 0.5 g L⁻¹ (MB) or 5 g L⁻¹ (phenol). Solutions were stirred at 250 rpm, equilibrated in the dark for 30 min, then irradiated at 25 °C. Samples were withdrawn every 30 min, centrifuged, and analyzed by UV‑vis at 664 nm (MB) or 270 nm (phenol). Recyclability was tested over four cycles. Total organic carbon (TOC) was monitored with a Shimadzu TOC‑2000. Scavenger experiments used EDTA (h⁺), isopropyl alcohol (•OH), and benzoquinone (O₂⁻).

Results and Discussion

Structural and Morphological Analysis

XRD patterns (Fig. 1a) confirmed hexagonal ZnO (wurtzite) and the characteristic (002) peak of g‑C₃N₄ at 27.5°. N‑doping caused a slight red shift in ZnO peaks, indicating lattice contraction. Crystallite sizes: N‑ZnO (38.6 nm) < ZnO (45.8 nm), suggesting N suppresses growth. BET areas increased from 15.3 m² g⁻¹ (ZnO/g‑C₃N₄) to 18.5 m² g⁻¹ (N‑ZnO/g‑C₃N₄), implying improved surface area for charge separation.

SEM images (Fig. 2) show sheet‑like g‑C₃N₄ and well‑dispersed ZnO or N‑ZnO nanoparticles on its surface. N‑ZnO/g‑C₃N₄ displays a rougher surface due to uniformly assembled N‑ZnO spheres, enhancing reactive sites.

HRTEM (Fig. 3) revealed lattice fringes of 3.25 Å (g‑C₃N₄ (002)) and 2.43 Å (N‑ZnO (101)), confirming intimate contact.

FT‑IR spectra (Fig. 4) preserved g‑C₃N₄’s characteristic C‑N/C=N peaks (1243 cm⁻¹, 1637 cm⁻¹) and Zn‑O vibrations (400–560 cm⁻¹). Slight blue shifts in g‑C₃N₄ peaks in composites indicate enhanced conjugation.

XPS (Fig. 5) confirmed elemental composition. Zn 2p peaks at 1021.8/1044.9 eV, O 1s at 530.4 eV (ZnO) and 532.0 eV (surface O/H). C 1s displayed C‑C (284.6 eV), sp³ (286.5 eV), sp² (287.8 eV). N 1s peaks at 397.5/398.6 eV (O‑Zn‑N, sp²‑N) and 398.5 eV (C‑N‑C) confirmed N incorporation and intact g‑C₃N₄ framework.

Optical Properties

UV‑vis DRS (Fig. 6) showed g‑C₃N₄ absorption edge at ~470 nm (E_g = 2.63 eV). ZnO edge ~390 nm (E_g = 3.21 eV). N‑ZnO exhibited a red shift, lowering E_g to 3.10 eV. Both composites displayed enhanced visible‑light absorption (400–600 nm); N‑ZnO/g‑C₃N₄ (2.73 eV) had a broader edge than ZnO/g‑C₃N₄ (2.85 eV), favoring photocatalysis.

Photocatalytic Performance

Under visible light, N‑ZnO/g‑C₃N₄ achieved 90 % MB removal in 100 min, outperforming ZnO/g‑C₃N₄ (≈50 %) and the pristine materials. Rate constants (k): N‑ZnO/g‑C₃N₄ = 1.794 h⁻¹, N‑ZnO = 0.316 h⁻¹, g‑C₃N₄ = 0.466 h⁻¹, ZnO/g‑C₃N₄ = 0.937 h⁻¹. The composite retained >85 % activity after five cycles.

Phenol degradation followed similar trends: N‑ZnO/g‑C₃N₄ reached 92 % removal in 8 h, with k = 0.034 h⁻¹, surpassing ZnO/g‑C₃N₄ (k = 0.026 h⁻¹) and g‑C₃N₄ (k = 0.013 h⁻¹). TOC analysis revealed 93 % mineralization of MB after 120 min; phenol mineralization reached 18 % due to intermediate formation.

Active Species and Mechanism

Scavenger tests indicated that holes (h⁺) and superoxide anions (O₂⁻) significantly suppressed MB degradation, while hydroxyl radicals (•OH) had the strongest effect, confirming •OH as the primary oxidant. Band‑edge calculations place N‑ZnO’s CB at –0.45 eV (vs. NHE), enabling reduction of O₂ to O₂⁻ and subsequent •OH formation. Electron transfer from g‑C₃N₄ CB to N‑ZnO CB enhances charge separation, while holes in N‑ZnO VB directly oxidize pollutants.

The proposed heterojunction (Fig. 11) shows efficient electron flow from g‑C₃N₄ to N‑ZnO, suppressing recombination and maximizing reactive species generation, thus explaining the superior photocatalytic performance.

Conclusions

We successfully fabricated N‑ZnO/g‑C₃N₄ nanocomposites via a facile sol‑gel route. The integration of g‑C₃N₄ broadened visible‑light absorption and promoted charge‑carrier separation, while N‑doping narrowed the band gap. Consequently, the composite exhibited markedly enhanced degradation of methylene blue and phenol, with excellent recyclability. Mechanistic insights confirm that the synergistic heterojunction facilitates efficient electron–hole separation and •OH generation.