Members of the PLASAR group mag. phys. Rafaela Radičić as first author and Ph.D. Nikša Krstulović as the research leader in cooperation with colleagues Ph.D. Silvija Vdović, mag. phys. Vedran Brusar and Ph.D. Dino Novko from the Institute of Physics and colleagues from the Jožef Stefan Institute published a paper in the journal Applied Surface Science. In the paper, they present the method of laser synthesis of Ag and ZnO heterojunction nanoparticles and show their advanced photocatalytic properties, especially highlighting the photocatalysis of organic dyes using a sunlight simulator. The mechanism of photocatalysis of Ag-ZnO heterojunctions using visible light was also determined.

UV and solar-driven photocatalysis of organic dyes using ZnO-Ag heterojunction nanoparticles synthesized by one-step laser synthesis in water

Rafaela Radičić, Andrea Jurov, Janez Zavašnik, Janez Kovač, Vedran Brusar, Silvije Vdović, Dino Novko, Nikša Krstulović, Appl. Surf. Sci. 669 160498 (2024).

DOI: 10.1016/j.apsusc.2024.160498

In this research, Ag-ZnO heterojunction nanoparticles (NPs) were synthesized with facile one-step pulsed laser ablation (PLAL) in the water. XRD patterns showed that ZnO crystallized in a hexagonal wurtzite structure, while the Ag peaks correspond to the fcc crystal structure. Through XPS, it was confirmed that the Ag is in a metallic state. From TEM images ZnO-Ag heterostructures are visible. The PL spectra show that ZnO-Ag NPs have a lower recombination rate of the e- – h+ pairs, implying a higher photocatalytic activity. The photocatalytic efficiency of ZnO-Ag NPs was carried out under UV and solar irradiation against Methylene Blue (MB), Rhodamine B (RhB), Crystal Violet (CV), and Methyl Orange (MO) dyes in water solution. It was confirmed that the presence of Ag increases the photocatalytic efficiency against all studied pollutants. Ultrafast transient absorption spectroscopy was employed to understand the photoexcitation of the charge carriers involved in the photocatalytic process of ZnO-Ag NPs under the influence of visible irradiation. This research demonstrates a simple one-step synthesis of Ag-ZnO heterojunctions via the PLAL method for efficient solar-driven photocatalysis against various organic pollutants.

Fig. 1. Schematic view of the experimental PLAL set-up for the ZnO-Ag heterojunction synthesis.

Fig. 2. ln(C/C0) as a function of time for the photocatalytic degradation of (a) MB, (b) RhB, (c) CV, and (d) MO in the presence of the pure ZnO and ZnO-Ag under solar irradiation.

A gradual decrease in the absorbance curves is observed while ZnO-Ag nanocomposite shows enhanced photocatalytic activity against all mentioned dyes with respect to the pure ZnO. The photodegradation percentage of the pure ZnO is 91, 46, 77, and 24 % for the MB, RhB, CV, and MO, respectively. In the case of the ZnO-Ag, the photodegradation percentage is 96, 65, 81, and 34 % for the MB, RhB, CV, and MO, respectively. The best photocatalytic activity was demonstrated in the case of MB, which was 96 % photodegraded in 90 min. Furthermore, RhB showed the most significant difference between the photocatalysts, where ZnO-Ag outperformed the ZnO by 40 % within 120 min.

Fig. 3. (a) Transient absorption map of ZnO-Ag NPs in water, excited with 440 nm pump pulses and probed with white light supercontinuum. Probe delay is shown with a linear time scale up to 10 ps and with a logarithmic time scale from 10 ps to 1.5 ns. Color bar limits are set to visualize the lower value and decaying signals. (b) Time dynamics of bleaching signal in ZnO-Ag NPs and Ag NPs in water, probed at 400 nm, corresponding to the Ag NP LSPR absorption peak. Both time traces show fast initial decay of about 3 ps, while ZnO-Ag NPs show additional decay of approximately 1 ns.

Under UV photoexcitation, the e- will be excited to the semiconductor’s conduction band (CB), migrating to the ZnO-Ag interface and, eventually, transferring to the Ag. Simultaneously, interband excitation in Ag NPs will excite e- to the Ag conduction band. The role of Ag NPs is to suppress the exciton recombination, prolonging e- – h+ pairs lifetime. The formed Schottky barrier can prolong the lifetime of e- – h+ pairs through the junction, accelerating their charge transfer rate [58]. Transient absorption spectroscopy was used to understand the ZnO-Ag NPs photocatalysis process under visible irradiation. Transient absorption spectra of the ZnO-Ag NPs excited with 440 nm are shown in Fig. 3a.

After excitation with visible light, hot free e- are formed near the Ag NP surface that can diffuse to ZnO. Hot electron injection is the main mechanism of the emerged Ag visible light response where e- are transported into the ZnO CB [59, 60]. Later, photoexcited charge carriers lose their energy mainly due to electron-phonon interaction in both ZnO and Ag. In Ag NPs, this process can be evaluated by fitting the exponential decay kinetics of the LSPR bleaching signal at 400 nm using a 3 ps time constant (Fig. 3b.).  

Overall, results confirm slower photoexcited charge carriers’ recombination rate in the ZnO-Ag NPs due to charge separation and slower electron thermalization as ZnO introduces new strong electron-phonon scattering channels that lead to hot-phonon bottleneck effect as observed in halide perovskites [62,63]. According to the literature [64, 65, 59] and our transient absorption measurements, a schematic illustration of the ZnO-Ag photodegradation mechanism under UV and visible irradiation against organic dyes is proposed (Fig. 4.).

Fig. 4. Schematic illustration of ZnO-Ag photodegradation mechanism under UV and visible irradiation.

Summarized photocatalytic mechanism reactions: