Solar-driven photoelectrochemical hydrogen generation based on colloidal core/shell quantum dots.

Wang, Kanghong (2022). Solar-driven photoelectrochemical hydrogen generation based on colloidal core/shell quantum dots. Thèse. Québec, Doctorat en sciences de l'énergie et des matériaux, Université du Québec, Institut national de la recherche scientifique, 147 p.

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Colloidal quantum dots (QDs) are considered as one of the promising candidates for next-generation solar-to-fuel energy conversion strategies due to many unique optical and optoelectronic properties. Comparing to directly transferring to electricity, there is an alternative to convert solar energy into hydrogen (H2) fuels. H2 is one of the green energy resources with high power density and the by-product is only water. Thus, colloidal QDs have been investigated a lot for photoelectrochemical (PEC) H2 generation. However, this strategy still suffers from low solar-to-hydrogen (STH) efficiency as well as poor stability performance which restrict their wider application. In my thesis, I used colloidal QDs as the solar harvester, sensitizing TiO2 for PEC H2 evolution. The focus of my work is to design proper structure of core/shell QDs to enhance the absorption from solar spectrum as well as facilitating the carrier separation/transfer, leading to improved efficiency from solar energy to H2. In my work, I first investigated the core size effect in CdSe/CdS core/shell QDs for PEC H2 generation. Generally, I synthesized four types of CdSe QDs with different size, varying from 2.8 nm to 4.8 nm in diameter by controlling the reaction time (2-10 min) under the same temperature (260 ℃) via hot-injection approach. Afterwards, successive ionic layer adsorption and reaction (SILAR) method was further applied to coat similar thickness of CdS shell by controlling six monolayers, forming four types of CdSe/CdS core/shell QDs. Then transmission electron spectroscopy (TEM) and X-ray diffraction (XRD) pattern reveal that all four CdSe/CdS core/shell QDs were all ascribed to the same crystal structure with different size. In which the CdSe is turning to wurtzite (WZ) crystal structure while CdS is zinc blende (ZB) crystal structure. Also, the TEM images showed that the overall size of QDs is becoming larger with longer reaction time. The ultraviolet-visible (UV-vis) absorption (Abs) and photoluminescence (PL) spectra revealed that the position of the first-excitonic peaks vary from 585 nm to 635 nm while the peaks in PL spectrum vary from 595 nm to 648 nm. Finally, when four types of core/shell QDs were applied for PEC H2 generation, the saturated photocurrent density (Jph) varied from 8.8 mA/cm2 to 17.4 mA/cm2 under one sun illumination (AM 1.5 G, 100 mW/cm2 ). The PEC results indicated that the core size has significate influence on the band alignment as well as the absorption range of core/shell QDs. In detail, the optimized PEC performance is based on the QDs with core size of 3.5 nm which balanced the absorption range of QDs as well as optimized carrier transfer efficiency. In my second work, I investigated the composition effect of the CdS shell based on CdSe/CdS core/shell QDs. Generally. I synthesized three types of core/shell QDs. The first one is CdSe/CdS core/shell QDs with six monolayers of pure CdS shell. Then I engineered the shell composition into CdSexS1-x alloyed shells. The two alloyed QDs are in the form of CdSe/(CdSe0.5S0.5)4/(CdS)2 and CdSe/(CdSexS1-x)5/CdS which denoted as CdSe/Alloyed#1 QDs and CdSe/Alloyed#2 QDs, respectively. The difference between the two alloyed QDs is the ratio of Se/S in each layer. When I applied the three types of core/shell QDs for PEC H2 generation, the Jph can reach 6.2 mA/cm2 for CdSe/CdS QDs. For alloyed QDs, the Jph can achieve as high as 15.1 mA/cm2 and 17.5 mA/cm2 for CdSe/Alloyed#1 QDs and CdSe/Alloyed#2 QDs, respectively. The dramatic improvement of PEC performance is mainly because the alloyed layers can not only enhance the absorption but also increase the carrier transfer efficiency by providing intermediate gradient layers. Thus, the unfavorable recombination can be largely reduced. Though the PEC performance is quite promising, the stability performance is not qualified for long-term application. Based on this issue, in my third work, I concentrated on addressing the issue of poor stability performance of QDs based photoanode. One important reason for poor stability performance is the surface traps which will become the recombination center for carriers. One of the strategies is to coat a wide-bandgap semiconductor to passivate the surface. Based on this, I developed three types of QDs, CdSe/CdS core/shell QDs, CdSe/CdS/ZnS core/multiple shell QDs and CdSe/CdSexS1-x/CdS/CdyZn1-yS/ZnS alloyed QDs. When I applied three types of QDs for PEC H2 generation, the saturated photocurrent density can reach 12.6 mA/cm2 for CdSe/CdS core/shell QDs while 14.2 mA/cm2 for CdSe/CdS/ZnS core/multiple shell QDs. For alloyed QDs, the saturated photocurrent density achieved unprecedent 20.5 mA/cm2 under one sun illumination (AM 1.5 G, 100 mW/cm2 ). The PEC results indicated that the ZnS shell can act as the protection and passivation shell for QDs, reducing the recombination events on the surface of QDs. Moreover, the alloyed intermediate layers can play the role of reducing the energy barrier between the shells, increasing the charge transfer efficiency accordingly. Then, I used three QDs for the stability measurement for two hours under one sun power. For CdSe/CdS core/shell QDs based photoanode, only 55 % of the initial current value was maintained for 2 hours. With the coating of ZnS shell, the CdSe/CdS/ZnS core/multiple shell QDs based photoanode can maintain 84 % of the initial current value after 2 hours while the alloyed QDs based photoanode showed the best stability performance, 93.4 % of the initial current value after 2-hour continuous illumination. This promising stability performance of alloyed QDs based photoanode also can be attributed to the effect of ZnS shell coating as well as the alloyed layers. Afterwards, I also investigated the impact of the shell thickness of the ZnS. Among one to three monolayers of ZnS shell, two monolayers of ZnS shell is proved to be the optimized shell thickness for PEC H2 generation. In other words, one monolayer of ZnS is too thin to protect the QDs as well as passivate the surface of QDs while three monolayers of ZnS is too thick which will reduce the charge transfer efficiency.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Rosei, Federico
Co-directeurs de mémoire/thèse:	Sun, Xuhui
Mots-clés libres:	PEC cell; quantum dots; core size effect; optical properties; alloyed shells; gradient band alignment; carrier transfer efficiency; charge recombination; theoretical calculation; core/multiple shell quantum dots; stability performance
Centre:	Centre Énergie Matériaux Télécommunications
Date de dépôt:	25 janv. 2023 16:07
Dernière modification:	25 janv. 2023 16:07
URI:	https://espace.inrs.ca/id/eprint/13166

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