Comparative Study on pH-Dependent Photocatalytic Degradation of 4-Nitrophenol using ZnO-Based Photocatalyst

Nitrophenolic compounds are a prominent class of environmental pollutants. They are widely discharged from the pesticide, pharmaceutical, and dye manufacturing industries.^1,2 Among these, 4-nitrophenol (4-NP) has been classified as a priority pollutant by the U.S. Environmental Protection Agency due to its high toxicity, carcinogenicity, and persistence in aquatic ecosystems. Thus, it is essential to develop effective remediation technologies.

Degradation strategies for 4-NP involve either its reduction to 4-aminophenol or its complete mineralization via advanced oxidation processes. One of the principal advanced oxidation processes is the homogeneous Fenton reaction, which utilizes hydrogen peroxide (H₂O₂) and ferrous ions(Fe²⁺) to generate hydroxyl radicals (·OH). This reaction is summarized by the equation:

The resulting hydroxyl radicals can non-selectively degrade a wide range of persistent organic pollutants, including 4-NP. However, the practical application of this process is hindered by the generation of iron sludge, a narrow operational pH range, and inefficient Fe³⁺/Fe²⁺ cycling.³

To overcome these limitations, heterogeneous Fenton-like catalysts, such as ZnO, have been investigated. In this system, the reaction occurs on the surface of the ZnO catalyst, which mitigates sludge formation and expands the operational pH window.^4,5

The mechanism involves the adsorption of iron ions onto the ZnO surface, followed by the surface-mediated generation of radicals. As a semiconductor, ZnO can also facilitate the continuous regeneration of the active Fe²⁺ species from Fe³⁺ using photogenerated electrons (e^–), thus overcoming a key drawback of the homogeneous process. The key steps on the ZnO surface can be described as follows:

This surface-based catalytic cycle enhances both the efficiency and stability of the degradation process.

The catalytic activity of pristine ZnO is constrained by its wide bandgap (~3.37 eV), which limits its photoactivity to the UV region, and by the recombination of photogenerated charge carriers.⁶ A strategy to overcome these limitations is the decoration of the ZnO surface with noble metal nanoparticles, such as silver (Ag). The integration of Ag nanoparticles extends light absorption into the visible spectrum via localized surface plasmon resonance(LSPR) and promotes charge separation through the formation of a Schottky barrier at the Ag-ZnO interface.^7–9 This enhanced charge separation is hypothesized to accelerate the regeneration of Fe²⁺ (Step 1) and the decomposition of H₂O₂ (Step 2), thereby improving overall catalytic activity. While the photocatalytic performance of ZnO/Ag nanocomposites has been reported, their Fenton-like activity for 4-NP degradation has not been systematically investigated.

Herein, we report the synthesis and characterization of Ag nanoparticles, ZnO nanorods, and Ag-decorated ZnO nanocomposites. The Fenton-like catalytic activities of these materials were evaluated for 4-NP degradation in both acidic and neutral pH environments. The catalytic performance was compared among the three catalysts by analyzing their reaction kinetics. This work contributes to the rational design of next-generation heterogeneous catalysts with improved efficiency and stability for nvironmental remediation.

The surface morphologies of the samples were characterized by high-resolution SEM, as shown in Figure 1. Figure 1a reveals that the sample consists of densely packed spherical Ag nanoparticles. The particle size analysis (inset) indicates an average diameter of approximately 45 nm, showing a tendency toward aggregation due to homogeneous nucleation. The ZnO nanorods sample (Figure 1b) is composed of discrete, uniform, pencil-like nanorods. The Ag/ZnO nanocomposite (Figure 1c) shows that the Ag nanoparticles are well-distributed on the surfaces of the ZnO nanorods. Notably, these decorated Ag nanoparticles were found to be significantly smaller, with an average diameter of 20 nm, compared to the pure Ag nanoparticles. This observation confirms the successful formation of each nanostructure.

The formation of Ag nanoparticles was characterized using UV-Vis spectroscopy, as shown in Figure 2. The spectrum exhibits a strong and distinct absorption band with a maximum (λ_max) at ~445 nm. This characteristic band is attributed to the localized surface plasmon resonance (LSPR) of the Ag nanoparticles, which originates from the collective oscillation of conduction electrons on the nanoparticle surface upon excitation by incident light. The λ_max value is consistent with values reported in the literature for spherical Ag nanoparticles, confirming their successful synthesis.¹⁰

The crystal structure and phase purity of the synthesized materials were investigated using XRD (Figure 3). For the ZnO nanorods sample, all observed diffraction peaks (red line) can be perfectly indexed to the hexagonal wurtzite structure of ZnO (ICSD 67849; black line). The prominent peaks located at 2θ values of 31.7°, 34.4°, 36.2°, 47.5°, and 56.6° correspond to the (100), (002), (101), (102), and (110) crystal planes of wurtzite ZnO, respectively. These sharp and narrow peaks indicate the high crystallinity of the prepared ZnO nanorods. In addition, the diffraction peaks correspond to the wurtzite ZnO phase, confirming the high phase purity of the sample without any other impurities.

In the XRD pattern of the Ag/ZnO composite (green line), all diffraction peaks corresponding to the wurtzite ZnO phase are retained, suggesting that the crystal structure of ZnO was not altered during the composite formation. In addition to the ZnO peaks, several new diffraction peaks are clearly observed at 2θ values of 38.1°, 44.3°, 64.4°, and 77.5°. These peaks are in excellent agreement with the (111), (200), (220), and (311) planes of the face-centered cubic (fcc) structure of metallic Ag (ICSD 53761; blue line). The presence of distinct peaks for both ZnO and Ag, without any detectable impurity phases, confirms the successful synthesis of a two-phase Ag/ZnO nanocomposite.

The photocatalytic performance of catalysts was evaluated by monitoring the degradation of 4-NP under UV irradiation using time-resolved UV–Vis spectroscopy. Prior to illumination, all solutions containing the catalyst were stirred in the dark for 90 min to establish adsorption–desorption equilibrium, and the concentration after this dark period was defined as C₀. The time-dependent absorption spectra (Figure 4a–f) reveal the progressive degradation of 4-NP over the UV irradiation. The initial spectra show clear pH-dependent differences arising from the acid-base equilibrium of 4-NP. At pH 4, 4-NP exists mainly in its neutral protonated form and displays an absorption maximum at 317 nm. At pH 7, it is largely deprotonated to the 4-nitrophenolate anion, which exhibits a strong band at 400 nm. These two bands correspond to protonated 4-NP and the deprotonated 4-nitrophenolate ion, respectively.¹² Upon interaction with the catalysts, particularly ZnO and Ag/ZnO (at pH 4 and 7), and Ag nanoparticles (at pH 7), the primary absorption band of 4-NP shifts from approximately 317 nm to around 400 nm. However, this shift was negligible for Ag nanoparticles at pH 4. This shift provides spectroscopic evidence for the formation of the deprotonated 4-nitrophenolate species and its subsequent adsorption onto the catalyst surface. As the reaction proceeds, the intensity of the relevant absorption bands decreases for all samples. This decrease is consistent with the visual fading of the solution's yellow color (inset images).

To elucidate the degradation kinetics, the C/C₀ versus time plots and corresponding pseudo-first-order kinetic fits (ln(C/C₀) versus time) are presented in Figure 4g and 4h, respectively. The linear correlation for all catalysts confirms that the degradation follows pseudo-first-order kinetics. Catalytic activity was pH-dependent. Under pH 4, pristine ZnO exhibited superior activity (k = 0.00792 min^-1) over the Ag/ZnO composite (k = 0.00433 min^-1). Conversely, at pH 7, the Ag/ZnO composite showed the highest rate constant (k = 0.00579 min^-1). This pH-dependent reversal in catalytic performance is attributed to the electrostatic interplay between the catalyst surface charge and the ionization state of the reactant. Physicochemical data supports this mechanism: ZnO possesses a point of zero charge (PZC) in the range of approximately 8.7–10.3,¹³ meaning that its surface remains a net positive charge (ZnO–H⁺) under both pH 4 and 7. Simultaneously, given its pKa of 7.16, 4-NP transitions from its neutral form at pH 4 to the negatively charged 4-nitrophenolate anion near pH 7. Consequently, at pH 7, a strong electrostatic attraction forms between the positively charged ZnO surface and the anionic phenolate species, enhancing their local concentration near the catalyst surface. For the Ag/ZnO composite, the presence of Ag nanoparticles further facilitates electron trapping and charge separation across the ZnO/Ag heterojunction, thereby accelerating the degradation process at this optimized pH. At pH 4, however, electrostatic interactions are minimal because the 4-NP molecules remain neutral.

This study reveals a pH-dependent reversal in photocatalytic performance: pristine ZnO showed superior 4-NP degradation at pH 4, while Ag/ZnO excelled at pH 7. This demonstrates that noble metal decoration does not universally enhance photocatalytic activity but depends critically on surface electrostatics, substrate speciation, and reaction conditions. Rational catalyst design must therefore adopt a holistic approach that optimizes the entire catalytic system rather than focusing solely on material composition. These findings provide valuable guidance for developing pH-responsive photocatalysts for environmental remediation.