The synthesis and analysis of uniformly distributed plasmonic platinum nanoparticles on an array of gallium-doped zinc oxide nanopagodas, showcasing improved photocatalytic performance. -

The synthesis and analysis of uniformly distributed plasmonic platinum nanoparticles on an array of gallium-doped zinc oxide nanopagodas, showcasing improved photocatalytic performance.

Introduction

Zinc oxide (ZnO), II–VI n-type semiconductor with a direct energy gap of 3.37 eV and a large exciton binding energy of 60 meV, is a good candidate material to substitute silicon in pursuit of the next generation electronic and photonic devices. The global interest in the photocatalysis of harmful pollutants stems from concerns about environmental purification and the conversion of solar energy. The majority of photocatalysts employed in the degradation of organic pollutants for wastewater treatment exist in nanopowder form. To eliminate these particulates post wastewater treatment, filtration and/or centrifugation processes are essential. The most studied noble metal/ZnO hybrid nanocomposites are Au and Ag, whereas there are few studies on Pd and Pt. It demonstrates superior and more rapid photodegradation capabilities. Furthermore, they offer the added advantages of convenience and easy recyclability compared to conventional nanopowders and wet solution methods. Additionally, our discussion covers the impact of Pt nanoparticles on the photocatalytic activities of Pt/GZO hybrid nanocomposites, exploring the involved photocatalytic mechanisms.

Methodology

The GZO seed layer was applied onto Si (100) substrates through room temperature radio frequency (RF) magnetron sputtering, employing a 1% GZO target. The GZO seed layer exhibits excellent electrical properties, making it well-suited for serving as back electrodes in subsequent measurements and applications of optoelectronic devices. GZO nanopagodas (NPGs) were manufactured on the previously deposited GZO seed layer using the metal–organic chemical vapor deposition (MOCVD) technique. Platinum nanoparticles (Pt NPs) were synthesized and adorned onto the surfaces of GZO nanopagodas (NPGs) using the atomic layer deposition (ALD) method. The molar ratios of the Ga/Zn precursor were maintained at 0.360 for all GZO nanopagodas (NPGs). Through X-ray photoelectron spectroscopy (XPS), the Ga content was identified to be approximately 5.4%. Additionally, pure ZnO nanowires (NWs) devoid of Ga doping were synthesized using the identical procedure.

The nanostructures were analyzed using a field emission scanning electron microscope (FESEM) coupled with an Oxford Instruments INCA energy-dispersive X-ray spectroscopy (EDS) system, specifically the JEOL JSM-6500. The crystal structures of Pt/GZO hybrid nanocomposites were investigated using an analytical transmission electron microscope (AEM) (JEOL, JEM-2010) and a spherical-aberration corrected field emission atomic resolution analytical electron microscope. The ultraviolet (UV) photocatalytic performance of pristine GZO nanopagodas (NPGs) and Pt/GZO hybrid nanocomposites with varying atomic layer deposition (ALD) cycles was assessed using Rhodamine B (RhB) as a representative indicator. At regular 20-minute intervals, each sample was retrieved from the RhB solution to monitor the reaction, and UV–visible absorption spectra were recorded using a UV/vis Spectrometer.

Results: Transmission electron microscopy (TEM) images illustrate the highly successful outcome of the atomic layer deposition (ALD) process in producing Pt nanoparticles (NPs) on GZO nanopagodas (NPGs). This effectiveness surpasses that of particles prepared through the chemical reduction of Pt-containing salts in an aqueous solution. GIXRD confirms the presence of Pt NPs on GZO NPGs. Analysis of photocatalysis indicates that the photocatalytic activities of Pt/GZO hybrid nanocomposites are influenced by several factors as indicated by the listed reactions. These factors encompass (i) optical properties and (ii) surface plasmonic effects. The photodegradation efficiency of Rhodamine B (RhB) in Pt/GZO hybrid nanocomposites with fewer than 30 ALD cycles exhibited a significant enhancement with increased loading content of Pt nanoparticles (NPs).

Initially, the fabrication process involved creating 1D Pt/GZO hybrid nanocomposites by first producing GZO nanopagodas (NPGs) through metal–organic chemical vapor deposition (MOCVD) and subsequently depositing Pt nanoparticles (NPs) onto GZO NPGs using atomic layer deposition (ALD). Both GZO NPGs and Pt NPs demonstrated exceptional crystal quality, as evidenced by high-resolution transmission electron microscopy (HR-TEM) and low-temperature photoluminescence (PL). Notably, Pt/GZO hybrid nanocomposites with 30 ALD cycles exhibited a substantial improvement in photocatalytic activity under 254 nm UV illumination, surpassing that of pristine GZO NPGs by approximately 2.7 times.

The competing effects of decorated Pt NPs on absorbance/transmittance, electron-hole separation/recombination, and the localized surface plasmon resonance (LSPR) effect highlighted the existence of an optimal Pt NPs loading content. This content exhibited the maximal photocatalytic activity for 1D Pt/GZO hybrid nanocomposites, and in this study, it was found to be at 30 ALD cycles. The results of Rhodamine B (RhB) photodegradation reactions of Pt/GZO hybrid nanocomposites with varying ALD cycles under 365 nm UV illumination supported the conclusion that the LSPR effect of decorated Pt NPs was maximal for Pt/GZO hybrid nanocomposites with 30–50 ALD cycles, consistent with the findings under 254 nm UV illumination.

Conclusion: Pt NPs played a dual role by serving as electron sinks, effectively separating photogenerated electrons and holes, and creating additional active adsorption sites for photocatalysis. Furthermore, Pt NPs acted as plasmonic centers, transferring additional photoexcited hot electrons through the direct electron transfer (DET) route, as well as electromagnetic energies through the local electromagnetic field (LEMF) and radiative energy transfer (RET) mechanisms into GZO NPGs. This process resulted in the generation of more photogenerated electrons and holes for catalysis. However, it was observed that Pt NPs reduced the transmittance of light to GZO, leading to a decrease in the photocatalytic activity of GZO.

Reference:

Chiu, H.M., Yang, T.H., Hsueh, Y.C., Perng, T.P. and Wu, J.M., 2015. Fabrication and characterization of well-dispersed plasmonic Pt nanoparticles on Ga-doped ZnO nanopagodas array with enhanced photocatalytic activity. Applied Catalysis B: Environmental, 163, pp.156-166.