Hybrid nanostructures for photoelectrochemical water splitting

Mahdi SHAHREZAEI

Doctoral thesis

Abstract:

Titanium dioxide (TiO2) has garnered significant attention in the fields of photocatalysis and photoelectrocatalysis due to its unique properties. However, the rapid recombination rate of photogenerated electron-hole pairs poses a considerable challenge, impeding the full potential of pure TiO2 to generate substantial hydrogen (H2) through photocatalytic processes. As a result, this realization has spurred a comprehensive investigation into modifying TiO2 surfaces, particularly by incorporating noble metals and carbonaceous nanomaterials. These additional components serve as effective electron transfer mediators, playing a crucial role in significantly enhancing H2 production. The primary objective of this doctoral thesis was to uncover specific conditions that could enhance the photocatalytic and photoelectrochemical capabilities of TiO2 nanostructures. To achieve this, our initial focus was on synthesizing both compact and discrete one-dimensional (1D) TiO2 nanotubes. These nanostructures combine the advantages of nanoparticles with the unique property of facilitating accelerated electron movement through a direct pathway. Furthermore, subsequent efforts were directed towards creating discrete TiO2 nanotubes/platinized cyanographene (G-CN and G-CN/Pt) heterojunctions. This step aimed to comprehensively explore their potential in enhancing photoelectrochemical water splitting (PEC-WS) performance. The study began with synthesizing discrete TiO2 nanotubes on a Ti foil through electrochemical anodization. Subsequently, a composite was generated using electrodeposition, incorporating pristine G-CN and G-CN/Pt. These composite materials underwent testing for their photoelectrochemical water splitting activity, which was then compared to the performance of bare TiO2 nanotubes. The results revealed that the introduction of G/CN and G-CN/Pt onto the TiO2 nanotubes led to enhanced PEC-WS activity. Further insights were gained through electrochemical impedance spectroscopy, VOCD analysis, measurements of photogenerated electron transfer time, and electron lifetime assessments. These investigations confirmed that surface functionalization of discrete TiO2 nanotubes with G-CN and G-CN/Pt effectively mitigated overall charge carrier recombination and improved charge transfer time. In another study, we introduced a novel method to create lattice defects on pristine compact TiO2 nanotubes, serving as an active support for securely anchoring highly effective platinum single atom (Pt-SA) co-catalysts. The main innovation in this study lies in the ability to precisely control surface reduction of TiO2 nanotubes through tunable and adjustable surface modification using ultrasonication treatment. Our approach introduces lattice defects under mild conditions through ultrasound, avoiding destructive effects. Additionally, our wet chemical method for forming Pt-SA is simple and accessible in any chemistry lab, without requiring specialized or expensive equipment. This investigative endeavor gains further significance due to the scarcity and costliness of noble metals. Through this innovative approach, we not only effectively harness these precious materials but also enhance overall catalytic efficiency. The physico-chemical properties of the synthesized materials were characterized using XPS, high-resolution TEM, UV-Vis DRS, and photoelectrochemical methods. Photocatalytic H2 evolution efficiencies from methanol photoreforming of pristine, Pt-SA decorated, and Pt-nanoparticle (NP) decorated TiO2 nanotubes were compared. Our research demonstrates that the uniform and well-dispersed decoration of Pt-SA on TiO2 nanotubes leads to a remarkable enhancement in Pt-surface-amount-normalized photocatalytic activity for hydrogen evolution compared to that of the pristine and Pt-NP decorated TiO2 nanotubes, respectively.

Abstract:

Titanium dioxide (TiO2) has garnered significant attention in the fields of photocatalysis and photoelectrocatalysis due to its unique properties. However, the rapid recombination rate of photogenerated electron-hole pairs poses a considerable challenge, impeding the full potential of pure TiO2 to generate substantial hydrogen (H2) through photocatalytic processes. As a result, this realization has spurred a comprehensive investigation into modifying TiO2 surfaces, particularly by incorporating noble metals and carbonaceous nanomaterials. These additional components serve as effective electron transfer mediators, playing a crucial role in significantly enhancing H2 production. The primary objective of this doctoral thesis was to uncover specific conditions that could enhance the photocatalytic and photoelectrochemical capabilities of TiO2 nanostructures. To achieve this, our initial focus was on synthesizing both compact and discrete one-dimensional (1D) TiO2 nanotubes. These nanostructures combine the advantages of nanoparticles with the unique property of facilitating accelerated electron movement through a direct pathway. Furthermore, subsequent efforts were directed towards creating discrete TiO2 nanotubes/platinized cyanographene (G-CN and G-CN/Pt) heterojunctions. This step aimed to comprehensively explore their potential in enhancing photoelectrochemical water splitting (PEC-WS) performance. The study began with synthesizing discrete TiO2 nanotubes on a Ti foil through electrochemical anodization. Subsequently, a composite was generated using electrodeposition, incorporating pristine G-CN and G-CN/Pt. These composite materials underwent testing for their photoelectrochemical water splitting activity, which was then compared to the performance of bare TiO2 nanotubes. The results revealed that the introduction of G/CN and G-CN/Pt onto the TiO2 nanotubes led to enhanced PEC-WS activity. Further insights were gained through electrochemical impedance spectroscopy, VOCD analysis, measurements of photogenerated electron transfer time, and electron lifetime assessments. These investigations confirmed that surface functionalization of discrete TiO2 nanotubes with G-CN and G-CN/Pt effectively mitigated overall charge carrier recombination and improved charge transfer time. In another study, we introduced a novel method to create lattice defects on pristine compact TiO2 nanotubes, serving as an active support for securely anchoring highly effective platinum single atom (Pt-SA) co-catalysts. The main innovation in this study lies in the ability to precisely control surface reduction of TiO2 nanotubes through tunable and adjustable surface modification using ultrasonication treatment. Our approach introduces lattice defects under mild conditions through ultrasound, avoiding destructive effects. Additionally, our wet chemical method for forming Pt-SA is simple and accessible in any chemistry lab, without requiring specialized or expensive equipment. This investigative endeavor gains further significance due to the scarcity and costliness of noble metals. Through this innovative approach, we not only effectively harness these precious materials but also enhance overall catalytic efficiency. The physico-chemical properties of the synthesized materials were characterized using XPS, high-resolution TEM, UV-Vis DRS, and photoelectrochemical methods. Photocatalytic H2 evolution efficiencies from methanol photoreforming of pristine, Pt-SA decorated, and Pt-nanoparticle (NP) decorated TiO2 nanotubes were compared. Our research demonstrates that the uniform and well-dispersed decoration of Pt-SA on TiO2 nanotubes leads to a remarkable enhancement in Pt-surface-amount-normalized photocatalytic activity for hydrogen evolution compared to that of the pristine and Pt-NP decorated TiO2 nanotubes, respectively.

Keywords

TiO2 nanotube arrays, platinized cyanographene, photoelectrochemical properties, single-atom catalysts, hydrogen evolution, photocatalysis

Language used: English

Date on which the thesis was submitted / produced: 16. 8. 2023

Thesis defence

• Supervisor: Ing. Štěpán Kment, Ph.D.

Palacký University Olomouc

Faculty of Science

Doctoral programme / field:
Chemistry / Physical Chemistry