High‑Performance Flexible Photocatalytic Paper: Cu2O and Ag Nanoparticle‑Decorated ZnO Nanorods for Visible‑Light Degradation of Organic Dyes

Introduction

Metal‑oxide nanomaterials have attracted sustained research interest over the past two decades due to their versatile roles in photonics, electronics, energy storage, sensing, and environmental remediation. ZnO nanostructures, in particular, are prominent because of their facile synthesis, tunable morphology, and benign cost. ZnO nanorods (NRs) supported on solid substrates offer distinct advantages—appropriate band alignment, chemical stability, and a reduced need for post‑reaction separation—making them attractive for photocatalytic dye degradation.

Photocatalytic degradation hinges on the generation of highly reactive hydroxyl radicals (•OH). The fundamental reactions involve hole‑mediated oxidation of hydroxide ions or water (Eq. 1–2) and electron‑mediated reduction pathways leading to reactive oxygen species (Eq. 3–6). ZnO’s wide band gap, however, limits visible‑light absorption; thus, decorating ZnO NRs with narrow‑band‑gap semiconductors or noble‑metal NPs is a common strategy to extend the absorption edge into the visible region and suppress electron–hole recombination.

While single‑NP decoration (e.g., Cu₂O, Ag, CdS, CuO, ZnFe₂O₄, Au) has been extensively studied, simultaneous decoration with both a narrow‑band‑gap semiconductor and a noble metal can synergistically enhance visible‑light harvesting and charge separation. The semiconductor NPs contribute additional absorption pathways, whereas the noble‑metal NPs provide plasmonic effects that facilitate rapid electron transfer across heterojunctions.

Flexibility is a key attribute for practical photocatalytic substrates. Paper offers a lightweight, biodegradable, and inexpensive platform that can be easily handled and scaled. Although ZnO NRs on paper have been explored mainly for sensing and electronic devices, few studies have addressed their photocatalytic performance. Here, we report the solution‑phase growth of Cu₂O/Ag‑decorated ZnO NRs directly on kraft paper and evaluate their efficacy in degrading RhB under visible illumination.

Methods

Growth of ZnO NRs on Paper

ZnO NRs were hydrothermally grown on a 10 × 10 cm² kraft paper. A ZnO seed layer was first deposited by dropping a 10 mM zinc acetate/3 mM NaOH (ethanol) solution onto the paper heated to 90 °C. After drying, the seeded paper was immersed in a 25 mM zinc nitrate/25 mM hexamethylenetetramine (HMTA) aqueous solution and heated to 95 °C for 7 h. The resulting paper was rinsed with deionized water and dried under nitrogen.

Decoration of Cu₂O and Ag NPs on ZnO NRs

Cu₂O NPs were deposited by immersing the ZnO NR paper in a 0.1 mM CuSO₄ solution and irradiating with 254‑nm UV lamps (1 W each) for 1.5 h at 60 °C. Ag NPs were formed by a 50 mM AgNO₃ immersion followed by 1 min UV exposure. Photoreduction proceeds via the generation of electron–hole pairs in ZnO, reduction of Cu²⁺ to Cu, subsequent oxidation to Cu₂O, and reduction of Ag⁺ to Ag. For co‑decorated samples, Cu₂O deposition was performed first, followed by Ag. Four variants were produced: ZnO, Cu₂O/ZnO, Ag/ZnO, and Ag/Cu₂O/ZnO.

Photocatalytic Measurement

Photocatalytic activity was assessed by degrading a 100‑mL, 10‑µM RhB solution under a 300‑W halogen lamp. The paper was pre‑equilibrated in the dye solution for 1 h in the dark, then a fresh solution was used for photodegradation with gentle stirring. Every 10 min, a 50 µL aliquot was withdrawn and its absorbance recorded using a fiber‑coupled Si photodiode array spectrometer.

Results and Discussion

Figure 1a shows the transformation of a brown kraft paper into a gray substrate after ZnO NR growth. The paper remained crack‑free after repeated rolling (≈ 2 cm radius), confirming the robust adhesion of the NRs. SEM images (Fig. 1b–f) reveal hexagonal ZnO NRs with diameters ranging from 50 to 300 nm. Cu₂O and Ag NPs are clearly visible on the NR surfaces in the co‑decorated sample.

a Photographs of a kraft paper substrate (left) and the paper with as‑grown ZnO NRs (right). b SEM images of the ZnO NR paper. c–f SEM images of ZnO, Cu₂O/ZnO, Ag/ZnO, and Ag/Cu₂O/ZnO NRs, respectively.

EDS spectra (Fig. 2a–c) confirm the presence of Cu and Ag peaks in the corresponding samples, verifying successful NP decoration. TEM analysis (Fig. 3) shows well‑defined Cu₂O and Ag NPs on the NR surface. Interplanar spacings measured at 0.179 nm (Cu₂O) and 0.236 nm (Ag) match JCPDS standards, confirming crystalline identities.

a–c EDS spectra of the Cu₂O/ZnO, Ag/ZnO, and Ag/Cu₂O/ZnO NRs, respectively.

a TEM image of an Ag/Cu₂O/ZnO NR. b–c High‑resolution TEM images of a Cu₂O and an Ag NP, respectively. d–e Corresponding Fourier transform patterns.

a–b High‑resolution TEM images of a Cu₂O NP and an Ag NP, respectively.

X‑ray diffraction patterns (Fig. 5a–b) confirm the wurtzite structure of ZnO and preserve crystallinity after NP decoration. XPS spectra (Fig. 5c) reveal Cu⁺ peaks, substantiating Cu₂O formation, with minor Cu²⁺ signals attributable to residual CuSO₄ or trace CuO.

a–b X‑ray diffraction patterns of as‑grown ZnO and Cu₂O/ZnO NRs. c XPS spectrum of Cu₂O/ZnO NRs.

Photoluminescence (PL) spectra (Fig. 6) display a dominant band‑gap emission near 400 nm and a weaker defect emission at 470 nm. NP decoration reduces PL intensity, indicating enhanced charge separation and light absorption by the NPs. The Ag/Cu₂O/ZnO paper shows the lowest PL intensity, reflecting superior charge carrier dynamics.

PL spectra of the four photocatalytic papers.

Figure 7a–b show the time‑dependent absorption spectra of RhB under halogen lamp illumination for ZnO and Ag/Cu₂O/ZnO papers. At 80 min, residual RhB concentrations drop to ~35 % and ~16 % for ZnO and Ag/Cu₂O/ZnO, respectively. First‑order kinetic fitting yields constants of 0.013, 0.016, 0.019, and 0.022 min⁻¹ for ZnO, Cu₂O/ZnO, Ag/ZnO, and Ag/Cu₂O/ZnO, respectively. After correcting for dye adsorption by the paper (0.005 min⁻¹), the adjusted constants are 0.008, 0.011, 0.014, and 0.017 min⁻¹. The co‑decorated sample demonstrates the highest efficiency, surpassing the others by 113 %, 55 %, and 21 %, respectively.

a–b Absorption spectra of the RhB solution as a function of time (10‑min intervals) for ZnO and Ag/Cu₂O/ZnO papers. c ln(C_t/C₀) plots for ZnO in the dark and all four papers under illumination. d ln(C_t/C₀) for Ag/Cu₂O/ZnO under halogen lamp and direct sunlight; fitted kinetic constant under sunlight is 0.041 min⁻¹.

Figure 8 illustrates the influence of UV exposure time on Ag NP loading. At 1 min, only sparse Ag NPs appear; increasing to 1.5 min rapidly amplifies coverage, and 2 min fully coats the NRs. Thus, photoreduction time offers precise control over NP density.

a–d SEM images of Ag/ZnO NRs with photoreduction times of 1, 1.5, 2, and 2.5 min, respectively.

The superior performance of Ag/ZnO over Cu₂O/ZnO aligns with band‑edge considerations. While Cu₂O holes lack sufficient energy (2.1 eV band gap) to oxidize OH⁻ or H₂O, ZnO holes (3.2 eV) can generate •OH efficiently. Ag NPs enhance visible‑light absorption and provide a Schottky junction that promotes rapid electron extraction, thereby facilitating hole‑mediated •OH formation.

The energy band diagrams of ZnO and Cu₂O and the standard reduction potentials for hydroxyl‑radical generation (relative to SHE). Cu₂O holes are insufficient for •OH formation, whereas ZnO holes are adequate.

Table 1 (not shown) compares the Ag/Cu₂O/ZnO paper’s performance with recent substrate‑supported ZnO nanocomposites. Despite varied experimental conditions, the photocatalytic paper demonstrates competitive efficiency while offering flexibility, low cost, and environmental friendliness.

Conclusions

We have fabricated a flexible, low‑cost photocatalytic paper by hydrothermally growing ZnO NRs on kraft paper and decorating them with Cu₂O, Ag, or both NPs via photoreduction. Characterization confirms crystalline NRs, successful NP deposition, and improved charge separation. Photocatalytic tests using a 10‑µM RhB solution reveal first‑order kinetic constants of 0.008 min⁻¹ for ZnO and 0.017 min⁻¹ for Ag/Cu₂O/ZnO under halogen lamp illumination; under direct sunlight, the co‑decorated paper achieves 0.041 min⁻¹. Compared to other ZnO‑based nanocomposites, this photocatalytic paper offers comparable or superior activity while retaining advantages of flexibility, lightweight, nontoxicity, and ease of fabrication. It holds promise for visible‑light degradation of organic dyes and other environmental remediation applications.

Availability of Data and Materials

The datasets used in this study are available from the corresponding author upon reasonable request.

Abbreviations

EDS:

Energy dispersive spectroscopy

JCPDS:

Joint Committee on Powder Diffraction Standards

NP:

Nanoparticle

NR:

Nanorod

PL:

Photoluminescence

RhB:

Rhodamine B

SEM:

Scanning electron microscopy

SHE:

Standard hydrogen electrode

TEM:

Transmission electron microscopy

UV:

Ultraviolet

XPS:

X‑ray photoemission spectroscopy