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Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction

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Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction catalysts Communication Reducing the Photodegradation of Perovskite Quantum Dots to Enhance Photocatalysis in CO2 Reduction Hanleem Lee 1,*,†, Meeree Kim 2,3,† and Hyoyoung Lee 2,3 1 Advanced Photonics Research Institute (APRI), Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Korea 2 Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Korea; [email protected] (M.K.); [email protected] (H.L.) 3 Department of Chemistry, Sungkyunkwan University (SKKU), 2066 Seoburo, Jangan-gu, Suwon 16419, Korea * Correspondence: [email protected]; Tel.: +82-10-6247-2230 † These authors contributed equally to this work. Abstract: Solution-processed perovskite quantum dots (QDs) have been intensively researched as next-generation photocatalysts owing to their outstanding optical properties. Even though the intrin- sic physical properties of perovskite QDs have been significantly improved, the chemical stability of these materials remains questionable. Their low long-term chemical stability limits their commercial applicability in photocatalysis. In this study, we investigated the photodegradation mechanisms of perovskite QDs and their hybrids via photoluminescence (PL) by varying the excitation power and the ultraviolet (UV) exposure power. Defects in perovskite QDs and the interface between the perovskite QD and the co-catalyst influence the photo-stability of perovskite QDs. Consequently, we designed a stable perovskite QD film via an in-situ cross-linking reaction with amine-based silane materials. The surface ligand comprising 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)) and 5-hexynoic acid improved the interface between the Ni co-catalyst and the perovskite QD. Then, ultrathin SiO2 was fabricated using 3-aminopropyltriethoxy silane (APTES) to harness the strong surface binding energy of the amine functional group of APTES with the perovskite QDs. The Ni co-catalyst content was further increased through Ni doping during purification using a short surface Citation: Lee, H.; Kim, M.; Lee, H. ligand (3-butynoic acid). As a result, stable perovskite QDs with rapid charge separation were Reducing the Photodegradation of successfully fabricated. Time-correlated single photon counting (TCSPC) PL study demonstrated that Perovskite Quantum Dots to Enhance the modified perovskite QD film exhibited slow photodegradation owing to defect passivation and Photocatalysis in CO2 Reduction. the enhanced interface between the Ni co-catalyst and the perovskite QD. This interface impeded Catalysts 2021, 11, 61. https:// the generation of hot carriers, which are a critical factor in photodegradation. Finally, a stable red doi.org/10.3390/catal11010061 perovskite QD was synthesized by applying the same strategy and the mixture between red and green QD/Ni(ppy)/SiO displayed an CO reduction capacity for CO (0.56 µmol/(g·h)). Received: 16 December 2020 2 2 Accepted: 2 January 2021 Published: 5 January 2021 Keywords: perovskite QD; CO2 conversion; photocatalyst; photodegradation Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- 1. Introduction nal affiliations. Rapid industrialization and urbanization result in high energy consumption. The amount of energy required to support the modern lifestyle is increasing as the interactions between humans and devices increase through the Internet of Things. To generate the necessary energy, fossil fuels, nuclear energy, and natural energy sources such as solar, water, and Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. wind power are used. Despite the global trend toward renewable energy, which generates This article is an open access article energy via natural energy sources, the energy conversion efficiency of renewable energy distributed under the terms and con- systems is far behind those of fossil fuels and nuclear energy. Moreover, the by-products of ditions of the Creative Commons At- energy conversion using fossil fuels and nuclear energy cause serious problems around tribution (CC BY) license (https:// the globe. In particular, the release of carbon dioxide (CO2) from fossil fuels gives rise to creativecommons.org/licenses/by/ various global problems such as a rise in the greenhouse effect. To satisfy the demands 4.0/). of our age, namely a high energy consumption and lower environmental pollution, we Catalysts 2021, 11, 61. https://doi.org/10.3390/catal11010061 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 61 2 of 18 need to develop both pollutant removal methods and renewable energy sources. Thus, renewable energy conversion via a carbon-neutral process is critical for next-generation energy systems. Several researchers have studied CO2 conversion systems over the past decade. Among the various strategies proposed, photocatalytic CO2 conversion has received con- siderable attention, because it utilizes sunlight to convert pollutants into chemical fuels. This process not only converts solar energy into chemical fuels; however, it can reduce CO2 generation [1]. For photocatalytic CO2 conversion, the photocatalyst is the key material. Properties of the photocatalyst should include light absorption, exciton generation, charge separation, surface absorption/desorption, and surface redox reactions. Metal oxides, lay- ered double hydroxides, metal–organic frameworks, and nano-dimensional materials have all been examined as photocatalysts [2]. Metal oxides such as TiO2, ZnO, NiO, and Al2O3 are considered as the standard owing to their high stability and low cost [3]. However, the large band gap of these material limits their performance owing to low energy absorption from sunlight. Recently, perovskite materials have been intensively studied as photocatalysts owing to their strong light absorption, high absorption coefficients, low exciton binding energies, long charge-carrier diffusion lengths, multiexciton generation effects, and facile bandgap modulation [4–6]. However, their low stability in polar media limits their applicability as photocatalysts. Studies have attempted to enhance the stability of perovskites via templates. Kuang et al. improved the stability of perovskites by using graphene oxide as a template [7]. CsPbBr3 nanocrystals were placed on a graphene oxide substrate. Owing to the high mobil- ity of graphene oxide, the charge separation of the CsPbBr3 nanocrystals was dramatically improved. Moreover, the graphene oxide acted as a template for the CsPbBr3 nanocrystals, thus preventing the aggregation of CsPbBr3 nanocrystals during the photo-reaction. To improve stability, the encapsulation of perovskite in a metal–organic framework (PCN- 221(Fex)) was applied [8]. MAPbI3-PCN-221(Fex) demonstrated better stability and activity than pristine perovskite and PCN-221(Fex). Although several studies have demonstrated better stability and enhanced catalytic behavior by using various templates, the mechanism of photodegradation of the hybrid system is unclear. Thus, it is necessary to investigate the photodegradation behavior of the hybrid system. In addition, the development of per- ovskite quantum dots (QDs) with high stability in chemical environments is indispensable to the successful production of high-efficiency photocatalysts. In this study, we investigated the photodegradation of encapsulated perovskite QDs. The design of the encapsulated material included an in-situ cross-linking reaction with 3-aminopropyltriethoxy silane (APTES). The surface ligand, which comprised 2,6-bis(N- pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)) and the silane material, was carefully selected to preserve optoelectrical properties while driving the in-situ cross-linking reac- tion. This short surface ligand facilitated co-catalyst doping during the synthesis reaction and purification. In particular, the multi-π-electron-conjugated structure of Ni(ppy) im- proved the interface between the Ni complex and perovskite QD in addition to enhancing electron transfer and storage capacity during the photocatalytic reaction. Moreover, an ultrathin SiO2 coating formed through in-situ cross-linking of APTES effectively prevented ion migration and the vaporization of organic cations, resulting in better operational sta- bility. A time-resolved photoluminescence (PL) study demonstrated that the modified perovskite QD film exhibited slow photodegradation because of rapid charge separation and defect passivation. As a result, our system achieved an CO2 reduction capacity for CO of 0.56 µmol/(g·h). 2. Results 2.1. The Effect of Surface Lligand on Co-Catalyst Doping The photodegradation of perovskite QDs is one of the main factors limiting their photocatalytic efficiency. To understand the photodegradation behavior, we investigated the photophysical properties of various types of perovskite QDs: Pristine QD, encapsulated Catalysts 2021, 11, 61 3 of 18 perovskite QD (QD/SiO2), co-catalyst-doped perovskite QD (QD/Ni(ppy)), and encapsu- lated co-catalyst-doped perovskite QD (QD/Ni(ppy)/SiO2). Organic–inorganic perovskite QDs were used in this study because of their conduction band alignment with the CO2 reduction reaction [9]. Figure1a is a schematic illustration of the synthesis of QD/Ni(ppy)/SiO 2. The reaction conditions were modified from a previous ligand-assisted precipitation method [10]. The pristine QDs was synthesized using oleic acid and oleylamine as a surface ligand (method 3.2). The transmission electron microscopy (TEM) image of a pristine QD demonstrated a cubic
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