Enhanced Visible‑Light Photocatalysis via Z‑Scheme Ag3PO4/TiO2 Heterojunctions

Abstract

We present a simple, scalable two‑step route to Ag₃PO₄/TiO₂ heterostructures that deliver outstanding visible‑light photocatalytic performance. The composites were comprehensively characterized by X‑ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‑resolution TEM (HR‑TEM), energy‑dispersive X‑ray spectroscopy (EDX), X‑ray photoelectron spectroscopy (XPS), and UV‑vis diffuse reflectance spectroscopy (DRS). The highly crystalline structures exhibit a uniform distribution of TiO₂ nanoparticles on Ag₃PO₄ surfaces, yielding a pronounced Z‑scheme charge‑transfer pathway. In Rhodamine B (RhB) degradation tests, TiO₂400/Ag₃PO₄ achieved ~100 % removal within 25 min under visible illumination, with a reaction rate constant of 0.02286 min^-1—twice that of Ag₃PO₄ alone and 6.6 times higher than pristine TiO₂400. The catalyst maintained >95 % activity after four recycles. Radical‑trapping experiments identified holes (h⁺) and superoxide anions (O₂·^-) as the primary oxidative species, while hydroxyl radicals (·OH) contributed partially. A detailed Z‑scheme mechanism is proposed, explaining the synergistic electron–hole separation and the suppression of Ag₃PO₄ photochemical corrosion.

Background

Semiconductor photocatalysts have become pivotal for environmental remediation and solar‑energy conversion, owing to their low cost and chemical stability. Titanium dioxide (TiO₂), a prototypical photocatalyst, offers excellent photocorrosion resistance and high oxidative potential, yet its wide band gap (3.0–3.2 eV) limits activity to the UV region. Silver orthophosphate (Ag₃PO₄) possesses a narrower band gap (~2.45 eV) and a quantum yield exceeding 90 % under visible light, but suffers from photocorrosion that generates metallic Ag, diminishing long‑term performance. Coupling TiO₂ with Ag₃PO₄ can harness complementary band structures, enhance charge separation, and mitigate Ag₃PO₄ degradation. Z‑scheme heterojunctions, in particular, preserve the strong redox potentials of both components while enabling efficient interfacial electron transfer.

Methods

Hydrothermal Preparation of Nano‑TiO₂

A mixed solution of P123 (0.4 g) in ethanol (7.6 mL) and deionized water (0.5 mL) was stirred until complete dissolution (solution A). Separately, butyl titanate (2.5 mL) and concentrated HCl (12 mol L^-1, 1.4 mL) formed solution B, which was added dropwise to A. After 30 min stirring, ethylene glycol (32 mL) was introduced and the mixture was stirred another 30 min. The suspension was sealed in an autoclave at 140 °C for 24 h, then cooled, centrifuged, washed, and dried at 80 °C for 8 h. Calcination at 300, 400, or 500 °C produced TiO₂300, TiO₂400, and TiO₂500, respectively.

Preparation of TiO₂/Ag₃PO₄ Photocatalyst

TiO₂ (0.1 g) was dispersed in 30 mL AgNO₃ solution (0.612 g) by ultrasound for 30 min. Subsequently, 30 mL Na₂HPO₄.12H₂O (0.43 g) was added, and the mixture stirred for 120 min at room temperature. After centrifugation, washing with deionized water and anhydrous ethanol, the solids were dried at 60 °C. The resulting composites were labeled TiO₂300/Ag₃PO₄, TiO₂400/Ag₃PO₄, and TiO₂500/Ag₃PO₄. Pure Ag₃PO₄ was synthesized under identical conditions without TiO₂.

Characterization

XRD patterns were recorded on a D/MaxRB diffractometer (Cu Kα, 35 kV, 0.02° s^-1, 10–80°). SEM (JEOL JSM‑6510/2100) and TEM (JEOL JSM‑2100) provided morphological insights at 10 kV. EDX coupled with TEM confirmed elemental composition. XPS (ESCALAB 250, 300 W Cu Kα, 3 × 10^-9 mbar) supplied valence‑state information, calibrated to the C 1s line at 284.6 eV. UV‑vis DRS spectra (400–800 nm) were obtained with a Lambda 35 spectrophotometer, and band gaps were derived from Kubelka–Munk plots.

Photocatalytic Activity Measurement

Photodegradation of RhB (10 mg L^-1) was carried out in a 50 mL suspension containing 50 mg of catalyst. After 30 min in the dark (to reach adsorption equilibrium), the mixture was irradiated with a 1000‑W Xe lamp (visible light) while maintaining the temperature at 25 °C using a water bath. Absorbance at 553 nm was recorded every 5 min; the decolorization ratio was calculated as C_t/C₀. The experiment was repeated four times to evaluate recyclability.

Results and Discussion

XRD Analysis

The XRD patterns (Fig. 1) confirm that TiO₂400 crystallizes in the anatase phase (JCPDS 71‑1166). Ag₃PO₄ peaks match JCPDS 70‑0702, while the composites display superimposed reflections of both constituents, indicating successful heterojunction formation. The anatase (101) peak shifts to higher angles with increased calcination temperature, reflecting enhanced crystallinity.

Enhanced Visible‑Light Photocatalysis via Z‑Scheme Ag3PO4/TiO2 Heterojunctions — Figure 1. XRD patterns of TiO₂400, Ag₃PO₄, and TiO₂/Ag₃PO₄ composites.

Morphology and Composition

SEM images (Fig. 2) reveal TiO₂400 as spherical nanoparticles (100–300 nm), Ag₃PO₄ as regular hexagonal crystals (0.1–1.5 µm), and TiO₂400/Ag₃PO₄ as TiO₂ nanoparticles uniformly anchored to Ag₃PO₄ surfaces. TEM and HRTEM (Fig. 2d–e) confirm lattice spacings of 0.3516 nm (TiO₂ (101)) and 0.245 nm (Ag₃PO₄ (211)), indicative of intimate contact. EDX mapping (Fig. 2f) confirms the presence of Ti, O, Ag, and P, validating the composite structure.

XPS Analysis

Survey spectra (Fig. 3a) show Ti, O, Ag, P, and C. Ag 3d peaks at 366.26 and 372.29 eV confirm Ag⁺ oxidation state. P 2p at 131.62 eV indicates PO₄^3-. Ti 2p peaks at 457.43 and 464.58 eV correspond to Ti⁴⁺. O 1s deconvolution reveals lattice oxygen (528.9 eV), TiO₂ oxygen (530.2 eV), and surface hydroxyls (532.1 eV). These results corroborate the successful coupling of TiO₂ and Ag₃PO₄.

Optical Properties

DRS spectra (Fig. 4a) show TiO₂400 absorbing at ~400 nm and Ag₃PO₄ at ~500 nm. The composite extends absorption up to 700 nm, reflecting strong interfacial interaction. Kubelka–Munk plots (Fig. 4b) yield band gaps of 3.10 eV (TiO₂400), 2.45 eV (Ag₃PO₄), and 2.75 eV (TiO₂400/Ag₃PO₄), confirming the suitability for visible‑light activation.

Photocatalytic Performance

Under visible light, TiO₂400 alone degrades 30 % of RhB in 25 min, whereas Ag₃PO₄ achieves 69 %. The TiO₂400/Ag₃PO₄ composite reaches complete decolorization (≈100 %) within the same period (Fig. 5a). Kinetic analysis follows pseudo‑first‑order behavior; the rate constants are 0.00345, 0.01148, 0.00525, 0.02286, and 0.01513 min^-1 for TiO₂400, Ag₃PO₄, TiO₂300/Ag₃PO₄, TiO₂400/Ag₃PO₄, and TiO₂500/Ag₃PO₄, respectively (Table 1). The TiO₂400/Ag₃PO₄ catalyst shows excellent recyclability, retaining >95 % activity after four cycles (Fig. 5c). Trapping experiments with IPA, BQ, and TEOA reveal that holes and superoxide anions dominate the oxidative process; hydroxyl radicals contribute modestly (Fig. 5d).

Z‑Scheme Mechanism

Based on band‑edge alignment (E_CB ≈ 0.45 eV, E_VB ≈ 2.90 eV for Ag₃PO₄; E_CB ≈ –0.24 eV, E_VB ≈ 2.86 eV for anatase TiO₂), a Z‑scheme charge transfer is favored: photogenerated electrons in TiO₂ CB recombine with holes in Ag₃PO₄ VB, leaving highly oxidative holes in TiO₂ VB and strongly reducing electrons in Ag₃PO₄ CB. This configuration preserves the strong oxidizing power of TiO₂ holes and the reducing ability of Ag₃PO₄ electrons, enabling efficient RhB oxidation and O₂ reduction to superoxide. The simultaneous formation of small Ag nanoparticles under illumination further facilitates electron migration and protects Ag₃PO₄ from photocorrosion. A schematic of this pathway is shown in Scheme 1.

Conclusions

The TiO₂400/Ag₃PO₄ heterojunction, fabricated via a facile two‑step method, delivers superior visible‑light photocatalytic activity, achieving complete RhB degradation in 25 min with a rate constant of 0.02286 min^-1. The composite maintains high stability over repeated cycles, owing to efficient charge separation and suppression of Ag₃PO₄ corrosion. Hole and superoxide anion species dominate the degradation process, while a Z‑scheme mechanism rationalizes the observed synergy. This study demonstrates a practical route to high‑performance, durable photocatalysts for environmental remediation.

Availability of Data and Materials

The authors confirm that all materials and data are freely available to readers without restrictive transfer agreements. Full datasets are provided within the article.