Efficient One‑Step Photo‑Ultrasonic Synthesis of rGO/Ag₃PO₄ Quantum‑Dot Composites for Enhanced Visible‑Light Photocatalysis

Abstract

We report, for the first time, a streamlined one‑step photo‑ultrasonic‑assisted reduction that yields reduced graphene oxide (rGO) decorated with 1–4 nm Ag₃PO₄ quantum dots (QDs). X‑ray diffraction, TEM, XPS, FT‑IR, Raman, and UV‑vis spectroscopy confirm uniform dispersion of the QDs on rGO nanosheets. Photocatalytic tests using methylene blue (MB) reveal that a composite with 2.3 wt % rGO degrades 97.5 % of MB in 5 min, outperforming both pure Ag₃PO₄ QDs and rGO/Ag₃PO₄ composites prepared by conventional stirring. The superior performance is attributed to synergistic charge‑carrier separation at the rGO/Ag₃PO₄ interface and the high specific surface area of rGO, which facilitate rapid electron‑hole transfer. These findings demonstrate the potential of the rGO/Ag₃PO₄ QD system for wastewater treatment and solar‑driven hydrogen evolution.

Background

High‑efficiency photocatalysts are pivotal for removing organic pollutants and generating hydrogen. Ag₃PO₄, with its narrow band gap and efficient photo‑excited charge separation, has attracted considerable attention; however, irregular morphology, low solubility, instability, and high cost limit its application. Enhancing Ag₃PO₄ activity hinges on efficient separation of photogenerated electron–hole pairs. Quantum‑dot (QD) size reduction shortens carrier diffusion paths, thus suppressing recombination and boosting activity. Yet, surfactant‑covered QDs often aggregate, reducing their accessibility to pollutants. Loading QDs onto high‑surface‑area supports such as reduced graphene oxide (rGO) mitigates aggregation while providing excellent electronic conductivity and charge‑carrier pathways. rGO, derived from graphene oxide via chemical, hydrothermal, or chemical vapor deposition, offers a two‑dimensional structure with outstanding electronic, mechanical, and thermal properties. Conventional reduction methods are complex and may generate secondary waste. Photo‑ and ultrasonic‑assisted reductions are greener alternatives, generating localized high temperatures and pressures that reduce GO efficiently while simultaneously enabling QD formation. To date, no reports describe a photo‑ultrasonic‑assisted synthesis of rGO/Ag₃PO₄ QD composites.

Experimental Section

Synthesis of rGO/Ag₃PO₄ QDs

Graphene oxide was prepared from natural graphite via the Hummers method. In a typical synthesis, 20 mg GO was dispersed in 50 mL water and sonicated for 30 min. Sodium oleate (2.2 mmol) was added and sonicated for an additional 60 min. After stirring 4 h for ion exchange, 10 mL 0.6 M AgNO₃ was introduced, followed by dropwise addition of 10 mL 0.2 M Na₂HPO₄ under ultrasonic irradiation. After 60 min, the precipitate was collected, washed with hexyl alcohol and ethanol, and dried to yield GO/Ag₃PO₄ QD composites. A 0.3 g aliquot was dissolved in 100 mL ethanol and subjected to simultaneous visible‑light (300 W Xe lamp, 420 nm cutoff) and ultrasonic (20 kHz, 10 mm probe) irradiation for 60 min. The resulting product, after centrifugation and drying at 60 °C, is rGO/Ag₃PO₄ QDs. Samples with rGO weight ratios of 1.5, 2.0, 2.3, 2.5, and 3.0 wt % were prepared and labeled R‑1.5, R‑2, R‑2.3, R‑2.5, and R‑3.

Materials Characterization

Phase identification employed XRD (Cu‑Kα, 2θ = 10°–80°). Morphology was examined by TEM (JEOL JEM‑2010). Chemical states were analyzed via XPS (PHI Quantera SXM). Vibrational modes were probed by FT‑IR and Raman spectroscopy (Horiba JY‑T64000). Optical absorption was recorded with a UV‑vis spectrophotometer (Hitachi U‑3010). Photoluminescence spectra were measured with a Hitachi FL‑4500 spectrophotometer.

Photocatalytic Activity Measurement

Photocatalytic tests used 10 mg of catalyst in 100 mL of 10 ppm MB. After 30 min dark equilibration, the suspension was irradiated with a 300 W Xe lamp (λ ≥ 420 nm). Aliquots were taken every minute for the first 6 min, then every 2 min thereafter. The absorbance at 664 nm was measured by UV‑vis; catalysts were removed by centrifugation before measurement.

Active Species Detection

Scavenger studies employed isopropanol (OH·), EDTA (h⁺), and p‑benzoquinone (O₂·⁻), each at ~1 mM, to identify dominant reactive species during MB degradation.

Results and Discussion

Materials Characterization

XRD patterns (Fig. 1) show the characteristic GO peak at 10.7° and rGO at 25°. Ag₃PO₄ QDs and R‑2.3 exhibit peaks of the body‑centered cubic phase (JCPDS 06‑0505). Peak broadening in R‑2.3 indicates an average Ag₃PO₄ QD size of ~3.7 nm, smaller than the 5.1 nm size of Ag₃PO₄ QDs prepared without GO, confirming GO’s role in size control.

TEM images (Fig. 2) reveal uniformly dispersed Ag₃PO₄ QDs (2.81 ± 1.2 nm) on rGO sheets. Compared with samples synthesized by conventional stirring, ultrasonic treatment prevents aggregation and yields finer particles, underscoring the importance of acoustic cavitation.

XPS spectra (Fig. 3) confirm reduction of GO to rGO and the presence of Ag₃PO₄. The O 1s peak shifts to lower binding energy in R‑2.3, indicative of C=O interaction with Ag₃PO₄. C 1s spectra show reduced oxygen functionalities after photo‑ultrasonic reduction.

FT‑IR and Raman data (Fig. 4) further validate the chemical bonding between rGO and Ag₃PO₄ QDs and the partial reduction of GO.

Preparation Mechanism

The synthesis proceeds via (1) Ag⁺–oleate complex formation, (2) adsorption onto GO, (3) precipitation of Ag₃PO₄ QDs, and (4) simultaneous photo‑ and ultrasonic‑driven reduction of GO to rGO, with electron transfer from Ag₃PO₄ to rGO and hydrogen radical generation. This route yields a stable rGO/Ag₃PO₄ QD heterostructure.

Optical Properties

Diffuse reflectance spectra (Fig. 6a) show a red‑shift for rGO/Ag₃PO₄ composites, peaking at 2.3 wt % rGO. Band‑gap analysis (Fig. 6b) yields a reduced gap of 1.62 eV for R‑2.3, compared with 2.23 eV for pure Ag₃PO₄, enhancing visible‑light absorption.

Photocatalytic Activity and Stability

Surfactant dosage studies (Fig. S3) indicate optimal performance at 0.5 g oleate; excess surfactant hinders QD dispersion. Photocatalytic tests (Fig. 7a) show that R‑2.3 degrades 97.46 % of MB in 5 min, outperforming all other samples. The rate constant (k) peaks at 0.057 min⁻¹ for R‑2.3. Recycling experiments (Fig. 7c) demonstrate >90 % activity after five cycles, with XRD confirming structural integrity (Fig. 7d).

Mechanism of Enhanced Photocatalysis

Photoluminescence (Fig. 8a) indicates reduced electron–hole recombination in R‑2.3. Scavenger experiments (Fig. 8b) reveal that holes and O₂·⁻ radicals dominate the degradation process, while hydroxyl radicals play a minor role. The proposed mechanism (Fig. 9) involves visible‑light excitation of Ag₃PO₄, rapid electron transfer to rGO, and subsequent oxidation of MB by holes and superoxide radicals generated on the composite surface.

Conclusions

We have developed a facile, green photo‑ultrasonic‑assisted route to produce rGO/Ag₃PO₄ QD composites with superior visible‑light photocatalytic activity. The rGO support enhances charge‑carrier separation and facilitates rapid electron transfer, leading to high degradation efficiencies and excellent recyclability. This strategy offers a promising platform for environmental remediation and solar‑hydrogen technologies.

Abbreviations

2D

Two‑dimensional

BQ

p‑Benzoquinone

CB

Conduction band

CVD

Chemical vapor deposition

EDTA

Disodium ethylenediaminetetraacetate

GO

Graphene oxide

IPA

Isopropanol

MB

Methylene blue

MO

Methyl orange

QDs

Quantum dots

R‑1.5, R‑2, R‑2.3, R‑2.5, R‑3

rGO content 1.5, 2.0, 2.3, 2.5, 3.0 wt %

rGO

Reduced graphene oxide

RhB

Rhodamine B

W_composite

Weight of composite

W_rGO

Weight of graphene