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 neutral 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, layered 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 mobility 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 perovskite 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 reaction. 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) improved 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 stability. 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 encapsulated 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 shape of approximately 5 nm side length (Figure1b). The photoluminescence quantum yield (PLQY) of this material was 71% with a full width at half maximum (FWHM) of approximately 27 nm (Figure1c). For QD/SiO 2 fabrication, the as-synthesized pristine QDs were re-dispersed in an APTES/toluene solution. Further, the 5 µL of ammonia was to ensure the silanization of the APTES (method 3.3) [11,12]. The QD/SiO2 demonstrated a PLQY of 69% and an FWHM of approximately 21 nm (Figure1d). The PL peak position shifted from 511 to 516 nm after SiO2 coating. In addition, the position of the band edge in the ultraviolet–visible (UV–Vis) spectra changed from 520 nm for pristine QD to 529 nm for QD/SiO2. The TEM image in Figure1e indicates that the size of QD/SiO 2 increased to 15–30 nm. We determined the energy-dispersive X-ray spectroscopy (EDS) line profile for the TEM image in Figure1f. The EDS line profiles of several QDs were determined and all displayed similar behaviors. Figure1g indicates that QD/SiO 2 has a core-shell structure consisting of a perovskite QD core and a SiO2 shell. The radius of the core was approximately 10 nm, which was larger than that of the pristine QD, and the SiO2 shell thickness was approximately 5 nm. The larger particle size of perovskite QD/SiO2 resulted in a redshift of the PL peak; however, the PLQY of QD/SiO2 did not significantly change compared with pristine QD. We predict that core-shell structure enhanced the quantum confinement of QD/SiO2, leading to a higher PLQY. Quantum confinement with size had a smaller impact on PLQY of QD/SiO2 [13]. The synthesis process for QD/Ni(ppy) is similar to that of pristine QDs. For QD/Ni(ppy), a trace of Ni(ppy) was mixed with the other precursors. The 5-hexynoic acid was additionally inserted with original surface ligands (method 3.5). Additional Ni(ppy) was added during purification to increase the Ni(ppy) content. During purification, a short alkyl ligand (i.e., 3-butynoic acid) was combined with Ni(ppy) in methanol to facilitate ligand exchange [14] and the co-catalyst doping reaction (method 3.6). Without the Ni addition step during purification, QD/Ni(ppy) demonstrated a PLQY of 71% with a PL peak at 509 nm (FWHM ~26 nm in Figure2a). The absorption band edge displayed blueshift. Ni(ppy) appeared to prevent the lattice growth of perovskite QDs during ligand-assisted precipitation [15]. However, when additional Ni(ppy) was introduced through ligand exchange during purification, the PL peak shifted to 511 nm (Figure2b). However, the PLQY of QD/Ni(ppy) dropped to 52% with a FWHM of 26 nm because of the Ni(ppy)- facilitated charge separation in the perovskite QDs [16]. Finally, QD/Ni(ppy)/SiO2 was fabricated via a modified ligand-assisted precipitation method, described in method 3.8. Once QD/Ni(ppy) was obtained, the QD/Ni(ppy) was re-dispersed in the APTES/toluene solution. Subsequently, 5 µL of ammonia was added to silanize the APTES. Then, Ni(ppy) was additionally inserted through the purification process. The obtained QD/Ni(ppy)/SiO2 demonstrated a PLQY of 59% with a FWHM of 24 nm (Figure2c). Catalysts 2021, 11, 61 4 of 18 Catalysts 2021, 11, x FOR PEER REVIEW 4 of 20

Figure 1. ((aa)) Schematic Schematic illustration illustration of of synthesis synthesis process process of of QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2; (b2);( TEMb) TEM image image of of Catalysts 2021, 11, x FOR PEER REVIEW 5 of 20 pristinepristine QD; UV-vis spectra; spectra; PL PL spectra spectra of of (c ()c )pristine pristine QD QD and and (d ()d QD/SiO) QD/SiO2; (e2,;(f) eTEM,f) TEM image image of of QD/SiO2 and (g) corresponding EDS line profile. QD/SiO2 and (g) corresponding EDS line proﬁle.

The synthesis process for QD/Ni(ppy) is similar to that of pristine QDs. For QD/Ni(ppy), a trace of Ni(ppy) was mixed with the other precursors. The 5-hexynoic acid was additionally inserted with original surface ligands (method 3.5). Additional Ni(ppy) was added during purification to increase the Ni(ppy) content. During purification, a short alkyl ligand (i.e., 3-butynoic acid) was combined with Ni(ppy) in methanol to facilitate ligand exchange [14] and the co-catalyst doping reaction (method 3.6). Without the Ni addition step during purification, QD/Ni(ppy) demonstrated a PLQY of 71% with a PL peak at 509 nm (FWHM ~26 nm in Figure 2a). The absorption band edge displayed blueshift. Ni(ppy) appeared to prevent the lattice growth of perovskite QDs during lig- and-assisted precipitation [15]. However, when additional Ni(ppy) was introduced through ligand exchange during purification, the PL peak shifted to 511 nm (Figure 2b).

However, the PLQY of QD/Ni(ppy) dropped to 52% with a FWHM of 26 nm because of Figure 2.Figurethe UV-vis Ni(ppy)-facilitated 2. UV-vis spectra spectra and PL and spectra PLcharge spectra of separation of(a) ( aQD/Ni(ppy)) QD/Ni(ppy) in withoutthe without perovskite Ni Ni dopingdoping viaQDsvia puriﬁcation purification [16]. Finally, process process (P1-2), (b) QD/Ni(ppy)(P1-2), with (b) Ni QD/Ni(ppy) doping via with purification Ni doping process via puriﬁcation (P2-2), and process (c) QD/Ni(ppy)/SiO (P2-2), and (c2). QD/Ni(ppy)/SiO . QD/Ni(ppy)/SiO2 was fabricated via a modified ligand-assisted precipitation method, de-2 scribed in method 3.8. Once QD/Ni(ppy) was obtained, the QD/Ni(ppy) was re-dispersed When we synthesizedWhen QD/Ni(ppy), we synthesized we addedQD/Ni(ppy), Ni(ppy) we with added a short Ni(ppy) alkyl with chain a (i.e.,short alkyl chain (i.e., in the APTES/toluene solution. Subsequently, 5 µL of ammonia was added to silanize the 5-hexynoic acid) at5-hexynoic the reaction acid) pot. at Whenthe reaction QD/Ni(ppy) pot. When was QD/Ni(ppy) fabricated without was fabricated a short without a short APTES. Then, Ni(ppy) was additionally inserted through the purification process. The alkyl chain ligandalkyl (see process chain ligand P1-1 in (see Figure process3a), Ni(ppy) P1-1 in could Figure not 3a), effectively Ni(ppy) interact could withnot effectively interact obtained QD/Ni(ppy)/SiO2 demonstrated a PLQY of 59% with a FWHM of 24 nm (Figure the perovskite QDswith because the perovskite the densely-packed QDs because oleic the aciddensely-packed and oleylamine oleic prevented acid and oleylamine the prevented 2c). diffusion of Ni(ppy)the to diffusion the perovskite of Ni(ppy) QD surface to the [ 17perovskite,18]. As a QD result, surface QD/Ni(ppy) [17,18]. formedAs a result, QD/Ni(ppy) formed during P1-1 displayed no significant Ni peak in the X-ray photoelectron spectroscopy (XPS) spectrum (Figure 3b). In contrast, when QD/Ni(ppy) was synthesized via a short alkyl chain ligand (P1-2 process in Figure 3a), we could distinguish various Ni com-

ponents via XPS. The deconvoluted XPS Ni 2p spectra of P1-2 in Figure 3b revealed several oxidation states (i.e., Ni3+ at 861.1 eV, Ni(OH)2 at 857.5 and 878.9 eV, NiO at 855.6 and 875.5 eV, Ni(ppy) at 853.7 and 873.0 eV, and metallic Ni at 852 eV) [19]. Even though a Ni2+ precursor was used in the reaction, Ni(ppy) underwent oxidation and reduction. In particular, Ni3+ was formed when the Ni precursor reacted with the vacant positions of methylammonium (MA+) or Pb2+ cations in perovskite structure [20]. Thus, we obtained QD/Ni(ppy) with a high Ni3+ content as well as NiO via P1-2 owing to in-situ doping reaction.

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during P1-1 displayed no signiﬁcant Ni peak in the X-ray photoelectron spectroscopy (XPS) spectrum (Figure3b). In contrast, when QD/Ni(ppy) was synthesized via a short alkyl chain ligand (P1-2 process in Figure3a), we could distinguish various Ni components via XPS. The deconvoluted XPS Ni 2p spectra of P1-2 in Figure3b revealed several oxidation 3+ states (i.e., Ni at 861.1 eV, Ni(OH)2 at 857.5 and 878.9 eV, NiO at 855.6 and 875.5 eV, Ni(ppy) at 853.7 and 873.0 eV, and metallic Ni at 852 eV) [19]. Even though a Ni2+ precursor was used in the reaction, Ni(ppy) underwent oxidation and reduction. In particular, Ni3+ was formed when the Ni precursor reacted with the vacant positions of methylammonium Catalysts 2021, 11, x FOR PEER REVIEW(MA +) or Pb2+ cations in perovskite structure [20]. Thus, we obtained QD/Ni(ppy) with6 of 20 a

high Ni3+ content as well as NiO via P1-2 owing to in-situ doping reaction.

FigureFigure 3.3.( a(a)) Schematic Schematic illustration illustration of of several several different different perovskite perovskite QDs; QDs; QD/Ni(ppy) QD/Ni(ppy) withoutwithout shortshort surfacesurface ligandligand (P1-1),(P1-1), QD/Ni(ppy)QD/Ni(ppy) via in-situ NiNi dopingdoping withoutwithout NiNi dopingdoping duringduring purificationpurification (P1-2),(P1-2), QD/Ni(ppy) QD/Ni(ppy) viavia P1-2P1-2 withwith onetimeonetime NiNi dopingdoping duringduring purification purification (P2-1), (P2-1), QD/Ni(ppy) QD/Ni(ppy) viavia P1-2P1-2 withwith twicetwice NiNi dopingdoping during during purification purification (P2-2), (P2-2), QD/Ni(ppy) QD/Ni(ppy) viavia 2 2 P2-2P2-2 withwith SiO SiO2 coating coatingvia via spin spin coating coating (P3/SiO (P3/SiO2);); ((b)) XPSXPS NiNi 2p2p spectraspectra ofof P1-1,P1-1, P1-2,P1-2, and and P2-1; P2-1; ( c(c)) Ni/Pb Ni/Pb atomicatomic ratioratio andand Si/Pb atomic ratio of QD/SiO2, P1-2/SiO2, P2-1/SiO2, P2-2/SiO2, and P3/SiO2; (d) XPS Si 2p spectra of P2-1/SiO2 and P2-2/SiO2; Si/Pb atomic ratio of QD/SiO2, P1-2/SiO2, P2-1/SiO2, P2-2/SiO2, and P3/SiO2;(d) XPS Si 2p spectra of P2-1/SiO2 and (e) XPS Pb 4f spectra of pristine QD, P1-2/SiO2, and P2-2/SiO2. P2-2/SiO2;(e) XPS Pb 4f spectra of pristine QD, P1-2/SiO2, and P2-2/SiO2.

ToTo increaseincrease catalyticcatalytic efﬁciency,efficiency, the the co-catalyst co-catalyst mobility mobility should should bebe improved.improved. MobilityMobility isis enhancedenhanced when Ni Ni is is in in the the form form of of metallic metallic Ni Ni0 or0 or Ni(ppy). Ni(ppy). We We further further developed developed the theNi(ppy) Ni(ppy) doping doping process process via a via ligand a ligand exchange exchange strategy strategy (P2-1 (P2-1and P2-2 and processes P2-2 processes in Figure in Figure3a). In3 P2-1,a). In ligand P2-1, exchange ligand exchange with Ni(ppy) with Ni(ppy)was conducted was conducted once, while once, in P2-2, while ligand in P2-2, ex- ligandchange exchange with Ni(ppy) with was Ni(ppy) conducted was conducted twice. P2-1 twice. and P2-2 P2-1 were and P2-2executed were using executed perovskite using perovskiteQDs produced QDs via produced P1-2. During via P1-2. ligand During exchange, ligand exchange,a ligand with a ligand a shorter with alkyl a shorter chain alkyl (i.e., chain3-butynoic (i.e., 3-butynoicacid, which acid, is shorter which than is shorter 5-hexynoic than 5-hexynoicacid) was used acid) to was replace used tothe replace bound theoleic bound acid and oleic oleylamine acid and oleylamine as much as as possibl muche. as P2-1 possible. perovskite P2-1 perovskiteQDs contained QDs similar contained ox- similaridation oxidation states to those states in to thoseP1-2 perovskite in P1-2 perovskite QDs (Figure QDs (Figure3b). However,3b). However, the intensity the intensity of the peak at 852 eV corresponding to metallic Ni0 dramatically increased, while the intensity of the peak at 861.1 eV corresponding to Ni3+ decreased after Ni(ppy) doping, indicating a co-catalyst composition conducive to charge separation. In addition, the total amount of Ni increased as the number of ligand exchange cycles increased (Table 1).

Table 1. Ni/Pb atomic ratio and dominant chemical composition of the product obtained via P1-1, P1-2, P2-1, and P2-2.

Process Ni/Pb (%) Dominant Component P1-1 0 - P1-2 0.13 NiO/Ni3+ P2-1 0.62 NiO/Ni0

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of the peak at 852 eV corresponding to metallic Ni0 dramatically increased, while the intensity of the peak at 861.1 eV corresponding to Ni3+ decreased after Ni(ppy) doping, indicating a co-catalyst composition conducive to charge separation. In addition, the total amount of Ni increased as the number of ligand exchange cycles increased (Table1).

Table 1. Ni/Pb atomic ratio and dominant chemical composition of the product obtained via P1-1, P1-2, P2-1, and P2-2.

Process Ni/Pb (%) Dominant Component P1-1 0 - P1-2 0.13 NiO/Ni3+ P2-1 0.62 NiO/Ni0 0 P2-2 4.20 Ni(OH)2/Ni

We next measured the atomic ratio of Si and Ni after chemical SiO2 coating using APTES (Figure3c). QD/Ni(ppy)/SiO 2 was fabricated using QD/Ni(ppy) from three different processes, namely P1-2, P2-1, and P2-2. QD/SiO2 exhibited a Si content of 2.5%. When QD/Ni(ppy) was coated with SiO2, the Ni content reduced slightly. The P1-2 product contained 0.13% Ni without SiO2 coating, which reduced to 0.10% with SiO2 coating. The P2-1 and P2-2 products yielded similar results. The Ni content reduced from 0.62% to 0.54% for P2-1 and from 4.20% to 4.12% for P2-2 after SiO2 coating. Interestingly, QD/Ni(ppy)/SiO2 fabricated via P2-1 displayed a lower Si content than QD/Ni(ppy)/SiO2 fabricated via P2-2. This was attributed to the large solvated radii of the Ni(ppy)-rich QDs. Even though the Si content of QD/Ni(ppy)/SiO2 produced via P2-2 was higher than those of other types of QD/Ni(ppy)/SiO2, the Ni content was higher than the Si content, which is preferred for enhanced catalytic behavior. Finally, we examined the Ni and Si contents resulting from different Si coating methods, namely chemical (in-situ silanization) and physical (P3/SiO2 in Figure3a). For P3/SiO 2, QD/Ni(ppy) was fabricated via P2-2 and subsequently deposited on the desired substrate. Then, a solution of 10 µM of APTES in benzene was spin-coated onto the QD/Ni(ppy) film and annealed at 50 ◦C for 5 min in a vacuum. Unlike chemical silanization, the Ni content rapidly decreased to 1.09%, while the Si content increased to 35%, indicating that an effective shell coating was only achieved via chemical silanization. Moreover, we examined the XPS Si 2p spectra (Figure3d) of QD/Ni(ppy)/SiO2 produced via the different processes (i.e., P2-1/SiO2, P2-2/SiO2) to confirm the chemical composition of the SiO2. A single peak appeared for both P2-1/SiO2 and P2-2/SiO2; however, the peak position shifted from a XPS binding energy (Ebinding) of 1 1 102 eV for P2-1/SiO2 to 104 eV for P2-2/SiO2. In general, Si 2p 2 and /3 peaks appear at 102 eV. The shape of the resultant peak should be asymmetric because of the overlapping 1 1 of the Si 2p 2 and /3 peaks. However, the XPS Si 2p spectra of P2-1/SiO2 displayed a Gaussian shape, which indicated that the peak of the Si 2p spectra originated from the oxide form of Si [21]. Thus, the peak shift of the binding energies was attributed to the doping effect [22,23]. Figure3d shows that the N-type doping became stronger as ligand exchange with Ni(ppy) increased (Ebinding (P2-1) < Ebinding (P2-2)). Moreover, we examined the XPS Pb 4f spectra of pristine QDs, P1-2/SiO2, and P2-2/SiO2 (Figure3e). An n-type shift for P2-2/SiO2 and a p-type shift for P1-2/SiO2 relative to pristine QDs occurred. Both peak shifts were driven by strong chemical interactions between the perovskite QD, Ni(ppy), and the SiO2 shell, which contributed to the redistribution of charge in the perovskite QDs [24]. This confirms the PL results in Figure2 where the wavelength of the peak for P1-2/SiO 2 decreased and that of the peak for P2-2/SiO2 increased compared to pristine QD.

2.2. The Photodegradation Phenomina of Perovskite QD Next, we investigated the photophysical properties of the four different perovskite QDs: Pristine QD, QD/SiO2, QD/Ni(ppy), and QD/Ni(ppy)/SiO2. Figure4 demonstrates the photodegradation behavior of the QD solutions in ambient conditions. Each solution was exposed to UV light (1 mW), and a non-polar solvent, benzene, was used. The Catalysts 2021, 11, 61 7 of 18

pristine QD displayed dramatic photodegradation as the duration of the exposure increased (Figure4a ). The PL intensity halved after 20 min of UV exposure and the peak width increased because of defect generation. The PL intensity was almost zero after 30 min. In contrast, QD/Ni(ppy) fabricated via P1-2 displayed slightly higher PL intensity (4%) after 5 min of UV treatment, and an 11% loss of the original PL intensity after 30 min Catalysts 2021, 11, x FOR PEER REVIEWof UV treatment (Figure4b). Photodegradation appeared to have been suppressed8 of 20 by co-catalyst doping. We hypothesize that charge separation induced by the co-catalyst prevented hot-carrier generation and resulted in better photophysical stability [25].

FigureFigure 4. Photodegradation 4. Photodegradation of QD of QD solution. solution. PL PLspectra spectra depending depending on onthe the duration duration of UV of UV exposure exposure time with (a) pristine QD, (b) QD/Ni(ppy) via P1-2, and (c) QD/Ni(ppy)/SiO2 via silanization of P2- time with (a) pristine QD, (b) QD/Ni(ppy) via P1-2, and (c) QD/Ni(ppy)/SiO2 via silanization of 2; (d) Excitation power versus PL intensity plot for pristine QD, QD/SiO2, P1-2, and P2-2. P2-2; (d) Excitation power versus PL intensity plot for pristine QD, QD/SiO2, P1-2, and P2-2.

TheThe SiO SiO2 coating2 coating inhibited inhibited photodegradation photodegradation in QD/Ni(ppy). in QD/Ni(ppy). Figure Figure 4c shows4c shows the re- the lationshiprelationship between between PL intensity PL intensity and andUV exposure UV exposure time time for QD/Ni(ppy)/SiO for QD/Ni(ppy)/SiO2 produced2 produced via P2-2.via A P2-2. 29% A enhancement 29% enhancement in PL in intensity PL intensity was wasobserved observed after after 10 min 10 min of UV of UV treatment, treatment, however,however, the the intensity intensity decreased decreased as asthe the UV UV ex exposureposure time time increased increased further. further. However, However, afterafter 30 30min min of ofUV UV exposure, exposure, QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2 still2 still exhibited exhibited a higher a higher PL PL intensity intensity than than thatthat of the of the fresh fresh sample. sample. Interestingly, Interestingly, the PL the peak PL peak of P1-2 of P1-2was slightly was slightly red-shift red-shift as the asUV the exposureUV exposure time increases, time increases, while whileno significant no signiﬁcant PL shift PL was shift observed was observed for P2-2/SiO for P2-2/SiO2. In gen-2. In eral,general, the perovskite the perovskite QD demonstrates QD demonstrates the red-shifted the red-shifted PL with PLthe withlarger the size larger of QD. size Under of QD. theUnder UV exposure, the UV exposure,perovskite perovskite QD underwent QD underwent crystal reconstruction crystal reconstruction with the Ni with content; the Ni thecontent; crystal thereconstruction crystal reconstruction was facilitated was facilitated without withouthot carrier hot generation, carrier generation, leading leading to the to collisionthe collision with the with nearby the nearby QDs. On QDs. the On other the hand, other the hand, P2-2/SiO the P2-2/SiO2 did not2 didshow not the show PL peak the PL shiftpeak because shift because the SiO the2 coating SiO2 coatingprevented prevented the collision the collision behavior, behavior, but it only but it facilitated only facilitated the crystalthe crystal reconstruction reconstruction within within one QD. one QD. To elucidate photostability enhancement due to the SiO2 coating, we conducted a PL-versus-power experiment. The results are shown in Figure4d. As the excitation power increased from 0.3 to 1 mW, the PL intensity of pristine QDs at 2.43 eV increased at rate of 3.50. The PL intensity at 2.40 eV of QD/SiO2 increased more rapidly at a rate of 7.91. The PL intensity of pristine QDs rapidly plateaued with increasing excitation density. The numerous defects in pristine QDs limited the number of carriers available

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 20

To elucidate photostability enhancement due to the SiO2 coating, we conducted a PL- versus-power experiment. The results are shown in Figure 4d. As the excitation power increased from 0.3 to 1 mW, the PL intensity of pristine QDs at 2.43 eV increased at rate Catalysts 2021, 11, 61 of 3.50. The PL intensity at 2.40 eV of QD/SiO2 increased more rapidly at a rate of 7.91. The8 of 18 PL intensity of pristine QDs rapidly plateaued with increasing excitation density. The numerous defects in pristine QDs limited the number of carriers available to generate excitons, resulting in a lower PL changes for pristine QDs compared with QD/SiO2. We hy- to generate excitons, resulting in a lower PL changes for pristine QDs compared with pothesize that the SiO2 coating plays an important role in passivating the surface defects QD/SiO . We hypothesize that the SiO coating plays an important role in passivating of perovskite2 QDs [26]. In contrast, the PL2 changes of the QD/Ni(ppy) with excitation the surface defects of perovskite QDs [26]. In contrast, the PL changes of the QD/Ni(ppy) power decreased in inverse proportion to the amount of Ni(ppy) doping. The PL changes with excitation power decreased in inverse proportion to the amount of Ni(ppy) doping. of QD/Ni(ppy) with excitation power produced via P1-2 and P2-2 were 2.65 and 1.87, re- The PL changes of QD/Ni(ppy) with excitation power produced via P1-2 and P2-2 were spectively. This phenomenon was attributed to the limited number of minor carriers in 2.65 and 1.87, respectively. This phenomenon was attributed to the limited number of QD/Ni(ppy). After Ni(ppy) doping, charge carriers in the QDs effectively moved to the minor carriers in QD/Ni(ppy). After Ni(ppy) doping, charge carriers in the QDs effectively Ni(ppy), resulting in a loss of PL and a slower PL intensity increase with increasing exci- moved to the Ni(ppy), resulting in a loss of PL and a slower PL intensity increase with tation power [27]. Next, we observed the photodegradation of pristine QDs, QD/SiO2, increasing excitation power [27]. Next, we observed the photodegradation of pristine QDs, QD/Ni(ppy), and QD/Ni(ppy)/SiO2 in the form of films. First, the relationship between QD/SiO2, QD/Ni(ppy), and QD/Ni(ppy)/SiO2 in the form of ﬁlms. First, the relationship photodegradationbetween photodegradation and UV power and UV was power examin wased examined for pristine for pristine QDs (Figure QDs (Figure 5a) and5a) andfor QD/Ni(ppy)/SiO2 (Figure 5b). Pristine QDs only sustained PL up to 0.08 mW UV exposure. for QD/Ni(ppy)/SiO2 (Figure5b). Pristine QDs only sustained PL up to 0.08 mW UV Atexposure. 0.6 mW and At 0.60.3 mWmW andUV exposure, 0.3 mW UV pristine exposure, QDs pristineexhibited QDs PL exhibitedenhancement PL enhancement for the first 20for min; the however, ﬁrst 20 min; PL however, degraded PL with degraded further with UV further exposure. UV exposure.The PL enhancement The PL enhancement during

shortduring low-power short low-power UV exposure UV was exposure because was of becausedefect repair of defect via UV repair light via [28]. UVHowever, light [28 ]. SiO2 coating prevented the photodegradation of perovskite QDs. At 0.6 mW, 0.3 mW, and However, SiO2 coating prevented the photodegradation of perovskite QDs. At 0.6 mW, 0.08 mW UV exposure, QD/Ni(ppy)/SiO2 displayed PL enhancement for 40 min and the 0.3 mW, and 0.08 mW UV exposure, QD/Ni(ppy)/SiO2 displayed PL enhancement for PL40 intensity min and was the PLmaintained. intensity was maintained.

FigureFigure 5. 5.PhotodegradationPhotodegradation of offilm. ﬁlm. ΔPL∆PL /PL /PL0 intensity0 intensity depending depending on the on theUV UVexposure exposure time time of (a of) (a) pristine QD and (b) QD/Ni(ppy)/SiO2; PL intensity change depending on the 1mW UV exposure pristine QD and (b) QD/Ni(ppy)/SiO2; PL intensity change depending on the 1mW UV exposure time of (c) pristine QD, (d) QD/Ni(ppy) via P1-2, (e) QD/Ni(ppy) via P2-2, and (f) time of (c) pristine QD, (d) QD/Ni(ppy) via P1-2, (e) QD/Ni(ppy) via P2-2, and (f) QD/Ni(ppy)/SiO2 QD/Ni(ppy)/SiO2 via P2-2 with silanization. via P2-2 with silanization.

PhotodegradationPhotodegradation due due to to high-power high-power UV exposureexposure waswas significantlysignificantly different different even even for forshort short exposure exposure times. times. Pristine Pristine QDs QDs exhibited exhibited a 19%a 19% PL PL loss loss after after only only 6 min6 min at at 1 mW1 mW UV UVlight light exposure exposure (Figure (Figure5c). 5c QD/Ni(ppy)). QD/Ni(ppy) produced produced via via P1-2 P1-2 displayed displayed the oppositethe opposite tendency tendency(Figure (Figure5d), namely 5d), namely an increase an increase in PL at 1in mW PL UVat 1 light mW exposure UV light up exposure to 3 min. up Compared to 3 min. to Comparedthe PL spectra to the of PL the spectra solution, of the the PL solution, spectra ofthe the PL film spectra became of broaderthe film because became the broader stacked becauseQD/Ni(ppy) the stacked in the filmQD/Ni(ppy) facilitated in intermolecular the film facilitated charge intermolecular transport between charge the QDtransport and the betweenNi co-catalyst the QD [ 29and]. Inthe contrast Ni co-catalyst to PL enhancement [29]. In contrast during to PL short enhancement low-power during UV exposure, short only the P1-2 sample displayed PL enhancement with high-power UV exposure, which was attributed to defect repair via UV light. We hypothesize that the effective interaction between the perovskite QD and Ni(ppy) in the P1-2 sample inhibited hot-carrier generation under high-power UV light and enhanced PL intensity. In addition, the two discernible PL peaks appeared for P1-2 sample due to the crystal reconstruction and collision behavior under UV exposure, as we mentioned. Hence, QD/Ni(ppy) produced via P2-2 did not Catalysts 2021, 11, x FOR PEER REVIEW 10 of 20

low-power UV exposure, only the P1-2 sample displayed PL enhancement with high- power UV exposure, which was attributed to defect repair via UV light. We hypothesize that the effective interaction between the perovskite QD and Ni(ppy) in the P1-2 sample inhibited hot-carrier generation under high-power UV light and enhanced PL intensity. Catalysts 2021, 11, 61 In addition, the two discernible PL peaks appeared for P1-2 sample due to the crystal9 ofre- 18 construction and collision behavior under UV exposure, as we mentioned. Hence, QD/Ni(ppy) produced via P2-2 did not show PL enhancement with 1 mW UV light exposure (Figure 5e), but it demonstrated a smaller PL loss (10%) than that of pristine QDs. show PL enhancement with 1 mW UV light exposure (Figure5e), but it demonstrated a Finally, QD/Ni(ppy)/SiO2 produced via P2-2 displayed no significant change during 1 smaller PL loss (10%) than that of pristine QDs. Finally, QD/Ni(ppy)/SiO produced via mW. 2 P2-2 displayed no signiﬁcant change during 1 mW. To demonstrate the importance of the interface between the QD and the Ni complex, To demonstrate the importance of the interface between the QD and the Ni complex, we used NiBr2 as the Ni doping precursor. In contrast to Ni(ppy), the PL intensity did not we used NiBr2 as the Ni doping precursor. In contrast to Ni(ppy), the PL intensity did not decrease as the amount of Ni doping increased (Figure 6a). The PL intensity of QD/NiBr2 decrease as the amount of Ni doping increased (Figure6a). The PL intensity of QD/NiBr increased by 20% after a single ligand exchange and by 28% after a double ligand ex-2 increased by 20% after a single ligand exchange and by 28% after a double ligand exchange. change. However, QD/Ni(ppy) displayed excellent PL quenching behavior because of the However, QD/Ni(ppy) displayed excellent PL quenching behavior because of the strong strong interaction between the QDs and Ni(ppy) and the multi-electron-conjugated struc- interaction between the QDs and Ni(ppy) and the multi-electron-conjugated structure of tureNi(ppy), of Ni(ppy), which enhancedwhich enhanced the electron the electron transfer andtransfer storage and ability storage of theability Nico-catalyst of the Ni [co-30]. catalystInterestingly, [30]. Interestingly, when we conducted when we the conducted P2-1 process the without P2-1 pr P1-2ocess pre-treatment, without P1-2 the pre-treat- resultant ment,QD/Ni(ppy) the resultant displayed QD/Ni(ppy) signiﬁcantly displayed lower significantly PL quenching lower behavior PL quenching because behavior of the small be- causeamount of the of immobilizedsmall amount Ni(ppy) of immobilized (Figure6 Ni(ppy)b), but still (Figure exhibited 6b), but a 6% still PL exhibited loss with a 6% 20 µPLM μ lossNi(ppy) with 20 addition. M Ni(ppy) With addition. P1-2 pre-treatment, With P1-2 pre-treatment, the PL intensity the dramaticallyPL intensity dramatically decreased as decreasedthe amount as the of Ni amount doping of increasedNi doping because increased of effectivebecause of charge effective separation charge (Figureseparation6c), (Figureindicating 6c), thatindicating P1-2 promoted that P1-2 interactions promoted betweeninteractions Ni(ppy) between and Ni(ppy) QD during and Ni QD doping during via Nithe doping ligand via exchange the ligand processes exchange (i.e., processes P2-1 and (i.e., P2-2). P2-1 and P2-2).

Figure 6. (a) PL Spectrum of pristine QD, QD/NiBr2 via P2-1, and QD/NiBr2 via P2-2; (b) PL Figure 6. (a) PL Spectrum of pristine QD, QD/NiBr2 via P2-1, and QD/NiBr2 via P2-2; (b) PL Spec- Spectrum of QD/Ni(ppy) with different Ni(ppy) concentrations. The QD/Ni(ppy) is synthesized trum of QD/Ni(ppy) with different Ni(ppy) concentrations. The QD/Ni(ppy) is synthesized via P2- 1via without P2-1 withoutP1-2 process; P1-2 process;(c) PL Spectrum (c) PL Spectrum of QD/Ni(ppy) of QD/Ni(ppy) via P1-2, via QD/Ni(ppy) P1-2, QD/Ni(ppy) via P2-1, viaand P2-1, and QD/Ni(ppy)QD/Ni(ppy) via via P2-2; P2-2; (d (d) )TCSPC TCSPC of of pristine pristine QD, QD, QD/SiO QD/SiO2,2, QD/Ni(ppy)QD/Ni(ppy) via via P2-2, P2-2, QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2 2 viavia P2-2. P2-2.

TheThe TCSPC TCSPC PL PL spectra spectra supported supported this this result result (Figure (Figure 6d).6d). The The PL PL lifetimes lifetimes of of pristine χ2 QD,QD, QD/Ni(ppy) QD/Ni(ppy) via via P2-2, P2-2, QD/SiO QD/SiO2, and2, and QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2 were2 were 6.80 ns 6.80 (χ2 ns = (1.031),= 1.031), 4.87 4.87 ns (χ2 = 1.007), 13.60 ns (χ2 = 1.010), and 6.15 ns (χ2 = 0.984), respectively; χ2 is the reduced chi-squared value. After Ni(ppy) doping, the PL lifetime of QD/Ni(ppy) decreased to less than that of pristine QD because of charge separation. In contrast, the

PL lifetime of QD/SiO2 dramatically increased because of defect passivation. As a result, QD/Ni(ppy)/SiO2 produced via P2-2 demonstrated a longer lifetime than QD/Ni(ppy) produced via P2-2 but demonstrated a signiﬁcantly shorter lifetime than QD/SiO2, indicating that PL quenching via Ni(ppy) still occurred after SiO2 coating. Catalysts 2021, 11, 61 10 of 18

2.3. Catalytic Behavior of Perovskite QD

To increase the catalytic selectivity for CO2 conversion, the conduction band of the photocatalyst should be close to the CO2 reduction potential. Red perovskite QDs are known to have excellent band alignment with the CO2 reduction potential. However, red perovskite QDs are impractical to use as photocatalysts for CO2 conversion because of their rapid photodegradation. We improved the photo-stability of perovskite QDs via SiO2 encapsulation to produce a red/green perovskite QD/Ni(ppy)/SiO2 for use as a CO2 conversion photocatalyst. Figure7a shows the schematic band alignment of red/green perovskite QD/Ni(ppy)/SiO2. The solar simulator has a continuous light spectrum from 400 to 1100 nm, with 400–700 nm light contributing approximately 40% of the total irra- diance. The red/green perovskite QDs in this study can absorb a wider range of light compared with the green perovskite QDs. Moreover, red/green perovskite QDs facilitated photocarrier generation efficiency. Higher energies above 500 nm generated multiexcitons in red and green QDs. In particular, the excitons from green QDs were delivered to red QDs through energy transfer and charge transfer, dramatically increasing photocarrier generation in the red QDs. The photo-induced electrons transferred to the Ni complex for use in CO2 reduction. Figure7b shows the PL intensities of various materials. The pristine green and red perovskite QDs that were used to synthesize red/green perovskite QD/Ni(ppy) demonstrated a 78% PLQY and 41% PLQY, respectively. The red/green perovskite QD/Ni(ppy) was fabricated using different amounts of Ni(ppy) doping. When the red perovskite QD/Ni(ppy) and green perovskite QD/Ni(ppy) were mixed, the PL intensities of both reduced significantly even at a low Ni(ppy) content. This PL quenching is evidence of energy transfer and charge transfer between the green and red perovskite QDs. Next, we measured the PL intensities at different Ni(ppy) contents. The PL intensities of both the green QDs and red QDs decreased in inverse proportion to the Ni(ppy) content. Finally, red/green perovskite QD/Ni(ppy)/SiO2 was synthesized. The TEM image of red/green perovskite QD/Ni(ppy)/SiO2 indicated a strong interaction between the red and green perovskite QDs (Figure7c,d). Small green perovskite QDs attached to the surface of large red perovskite QDs. Interestingly, we observed many small perovskite QDs (~10 nm in diameter) similar to the original green perovskite QDs. However, we identified mid-sized perovskite QDs (20–30 nm in diameter) on the surface of the red perovskite QDs. Pristine green QDs normally exhibit diameters less than 10 nm. In the red/green perovskite QD/Ni(ppy)/SiO2, mid-sized perovskite QDs were fabricated via mixing of red and green perovskite QDs during the silanization reaction [31]. Rapid silanization prevents a large number of mid-sized QD formation because the red and green perovskite QD experience less nanocrystal confusion through rapid silanization. There were significantly fewer mid-sized perovskite QDs than small perovskite QDs. In addition, we observed an ultrathin SiO2 coating at the edge of the red/green perovskite QD/Ni(ppy)/SiO2. Finally, the catalytic behavior of different types of perovskite QDs is presented in Figure8: pristine QD, perovskite QD/SiO 2, and red/green perovskite QD/Ni(ppy)/SiO2. We firstly investigated the photocatalytic activity in CO2-saturated water (4%). The pristine QDs demonstrated a low CO2 reduction catalytic activity of approximately 3 µmol/g in Figure8a. The CO 2 reduction rate of pristine QD was higher over a 6 h period than over a 24 h period because QD catalytic activity reduced and finally stopped during the CO2 reduction. Interestingly, the amount of generated H2 gas was also reduced after 18 h. It indicated the fast degradation behavior of pristine QD. The perovskite QD/SiO2 demonstrated significantly lower catalytic activity because the SiO2 coating impaired efficient charge separation for CO2 reduction (Figure8b). The catalytic behavior of red/green perovskite QD/Ni(ppy)/SiO2 was improved. The photocatalytic hydrogen reduction activity of QD/Ni(ppy)/SiO2 improved to 72 µmol/g, compared to that of pristine QD (16 µmol/g) in Figure8c. The perovskite QD/Ni(ppy)/SiO 2 exhibited more consistent activity, and its catalytic behavior still sustained until 27 h, resulting in enhanced catalytic stability after SiO2 coating. However, the CO conversion rate of this material was inferior (Figure8d). To improve the CO 2 reduction rate, we optimized the hole scavenger Catalysts 2021, 11, 61 11 of 18

and the ratio between red and green QDs. The CO2-saturated ethyl acetate in the presence Catalysts 2021, 11, x FOR PEER REVIEWofwater (1%) showed a 15-fold CO2 reduction rate compared to ~4% H2O hole scavenger12 of 20

(Figure8e). Finally, the 1:2 ratio of red/green perovskite QD/Ni(ppy)/SiO 2 demonstrated catalytic activity of approximately 0.56 µmol/g·h for CO (Figure8f).

Figure 7. (a) Energy diagram of red/green perovskite QD/Ni(ppy)/SiO2.; (b) PL Spectrum of green Figure 7. (a) Energy diagram of red/green perovskite QD/Ni(ppy)/SiO ;(b) PL Spectrum of pristine QD, red pristine QD, and red/green perovskite QD/Ni(ppy) with different2. Ni(ppy) con- green pristine QD, red pristine QD, and red/green perovskite QD/Ni(ppy) with different Ni(ppy) centrations. (c,d) TEM image of red/green perovskite QD/Ni(ppy)/SiO2. concentrations. (c,d) TEM image of red/green perovskite QD/Ni(ppy)/SiO2. Finally, the catalytic behavior of different types of perovskite QDs is presented in The photodegradation of perovskite QD/Ni(ppy) and perovskite QD/Ni(ppy)/SiO2 wasFigure investigated 8: pristine via QD, a TCSPC perovskite PL study. QD/SiO The2, PLand lifetime red/green of pristine perovskite red/green QD/Ni(ppy)/SiO QD mixtures2. We firstly investigated the photocatalytic activity in CO2-saturated water (4%). The pris- without SiO2 coating is difficult to observe because of nanocrystal confusion and defect generationtine QDs demonstrated over time. Thus, a low we CO measured2 reduction the catalytic TCSPCPL activity of green of approximately perovskite QD/Ni(ppy) 3 µmol/g (Figurein Figure9a) 8a. and The red CO perovskite2 reductionQD/ rate of Ni(ppy) pristine (Figure QD was9b) higher separately. over a 6 The h period TCSPC than PL over of greena 24 h perovskiteperiod because QD/Ni(ppy) QD catalytic at different activity excitationreduced and power finally levels stopped revealed during significant the CO2 photonreduction. losses Interestingly, as the excitation the amount power of increased. generatedThe H2 gas average was also PL lifetime reduced reduced after 18 from h. It 4.80indicated to 4.09 the ns fast when degradation the TCSPC behavior tests were of conducted pristine QD. at theThe same perovskite spot in QD/SiO the film.2 Thedemon- red perovskitestrated significantly QD/Ni(ppy) lower demonstrated catalytic activity a reduction because in averagethe SiO2 PL coating lifetime impaired as the excitation efficient powercharge increasedseparation but for to CO a lesser2 reduction degree (Figure than the 8b). green The perovskite catalytic behavior QD/Ni(ppy), of red/green even though per- theovskite stability QD/Ni(ppy)/SiO of green perovskite2 was improved. QDs was The better photocatalytic than that ofhydrogen red perovskite reduction QDs. activity We hypothesizeof QD/Ni(ppy)/SiO that good2 improved alignment to of72 theµmol/g, red perovskite compared QD to that and of Ni(ppy) pristine facilitated QD (16 µmol/g) charge transportin Figure 8c. between The perovskite them and QD/Ni(ppy)/SiO prevented a photoinduced2 exhibited redoxmore consistent reaction in activity, the perovskite and its QDs.catalytic In contrast,behavior red/green still sustained perovskite until 27 QD/Ni(ppy)/SiO h, resulting in 2enhanceddisplayed catalytic no significant stability change after atSiO different2 coating. excitation However, power the CO levels, conversion indicating rate high of this stability. material The was PL inferior lifetimes (Figure at 511 8d). nm (FigureTo improve9c) and the 612 CO nm2 reduction (Figure 9rate,d) were we optimized measured atthe the hole same scavenger spot in and the filmthe ratio of the be- red/greentween red and perovskite green QDs. QD/Ni(ppy)/SiO The CO2-saturated2 sample. ethyl Both acetate red in and the green presence PL were of water detected (1%) andshowed their a lifetimes15-fold CO were2 reduction constant rate within compared the same to ~4% excitation H2O hole power scavenger range used (Figure for 8e). the pristineFinally, the QDs. 1:2 The ratio SiO of2 red/coatinggreen appeared perovskite to QD/Ni(ppy)/SiO effectively minimize2 demonstrated photodegradation catalytic viaac- defecttivity of passivation approximately behavior. 0.56 µmol/g·h for CO (Figure 8f).

Catalysts 2021, 11, x FOR PEER REVIEW 13 of 20 Catalysts 2021, 11, 61 12 of 18

FigureFigure 8. 8.Photocatalytic Photocatalytic behavior behavior of of (a )(a green) green pristine pristine QD, QD, (b )(b green) green perovskite perovskite QD/SiO QD/SiO2, and2, and (c ,(dc),d red/green) red/green perovskite perovskite CatalystsQD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2021, 11, x FOR PEER22 with REVIEW 4% H 2OO hole hole scavengers. scavengers. Photoc Photocatalyticatalytic behavior behavior of of (e (e) )red/green red/green perovskite perovskite QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO2 14with2 of 20 2 withdifferent different hole hole scavengers scavengers and and (f) red/green (f) red/green perovskite perovskite QD/Ni(ppy)/SiO QD/Ni(ppy)/SiO with2 withdifferent different red/green red/green QD ratio. QD ratio.

The photodegradation of perovskite QD/Ni(ppy) and perovskite QD/Ni(ppy)/SiO2 was investigated via a TCSPC PL study. The PL lifetime of pristine red/green QD mixtures without SiO2 coating is difficult to observe because of nanocrystal confusion and defect generation over time. Thus, we measured the TCSPC PL of green perovskite QD/Ni(ppy) (Figure 9a) and red perovskite QD/ Ni(ppy) (Figure 9b) separately. The TCSPC PL of green perovskite QD/Ni(ppy) at different excitation power levels revealed significant photon losses as the excitation power increased. The average PL lifetime reduced from 4.80 to 4.09 ns when the TCSPC tests were conducted at the same spot in the film. The red perovskite QD/Ni(ppy) demonstrated a reduction in average PL lifetime as the excitation power increased but to a lesser degree than the green perovskite QD/Ni(ppy), even though the stability of green perovskite QDs was better than that of red perovskite QDs. We hypothesize that good alignment of the red perovskite QD and Ni(ppy) facilitated charge transport between them and prevented a photoinduced redox reaction in the perovskite QDs. In contrast, red/green perovskite QD/Ni(ppy)/SiO2 displayed no significant change at different excitation power levels, indicating high stability. The PL lifetimes at 511 nm (Figure 9c) and 612 nm (Figure 9d) were measured at the same spot in the film of the red/green perovskite QD/Ni(ppy)/SiO2 sample. Both red and green PL were detected and their lifetimes were constant within the same excitation power range used for the pristine QDs. The SiO2 coating appeared to effectively minimize photodegradation via defect passivation behavior.

Figure 9. Photodegradation behavior of QD/Ni(ppy) and red/green perovskite QD/Ni(ppy)/SiO2. FigureTime-resolved 9. Photodegradation PL depending behavior on excitation of QD/Ni(ppy) power for (anda) green red/green QD/Ni(ppy) perovskite and QD/Ni(ppy)/SiO (b) red QD/Ni(ppy).2. Time-resolved PL depending on excitation power for (a) green QD/Ni(ppy) and (b) red Time-resolved PL depending on excitation power of red/green perovskite QD/Ni(ppy)/SiO at (c) QD/Ni(ppy). Time-resolved PL depending on excitation power of red/green perovskite 2 511 nm and (d) 612 nm emission wavelength. QD/Ni(ppy)/SiO2 at (c) 511 nm and (d) 612 nm emission wavelength.

3. Materials and Methods 3.1. Materials

Methylammoniumbromide (CH3NH3Br), leadbromide (PbBr2), leadiodine (PbI2), oleylamine (98%), oleic acid (90%), 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide

(Ni(ppy)), Nickel(II) bromide (NiBr2), 3-aminopropyltriethoxy silane (APTES), butynoic acid, hexynoic acid, and N,N-dimethylformamide (DMF) were purchased from Sigma- Aldrich, Seoul, Korea. Benzene, hexane, toluene, acetonitrile, methanol, and methyl acetate (HPCL grade) were purchased from TCI, Seoul, Korea. All chemicals were used without further purification.

3.2. Preparation of Pristine Perovskite QD The green perovskite QDs were synthesized by a well-known precipitation method [32]. All solvents are used in an anhydrate state. The 33.56 mg of CH3NH3Br and 146.8 mg of PbBr2 are dissolved in 10 mL of DMF. Then, 0.1 mL of oleylamine and 1 mL of oleic acid are added into the above solution (Precursor A). The 0.5 mL of ‘Precursor A’ solution is added into 5 mL benzene at 45°C under vigorous stirring. To gather the green emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min. The precipitated material is re-dispersed in toluene (4 mg/mL) and the solution is centrifuged again at 10 krpm 5 min after 0.3 mL of ACN adding. The final product is re-dispersed in hexane.

3.3. Preparation of Perovskite QD/SiO2

The perovskite QD/SiO2 is synthesized by a modified encapsulation method [33]. Per- ovskite QD is firstly fabricated with the same process with pristine QD (method 3.2). The 8 mg of as-synthesized QD is re-dispersed in 10 mL of toluene with 10 μL of APTES. The

Catalysts 2021, 11, 61 13 of 18

3. Materials and Methods 3.1. Materials

Methylammoniumbromide (CH3NH3Br), leadbromide (PbBr2), leadiodine (PbI2), oleylamine (98%), oleic acid (90%), 2,6-bis(N-pyrazolyl)pyridine nickel(II) bromide (Ni(ppy)), Nickel(II) bromide (NiBr2), 3-aminopropyltriethoxy silane (APTES), butynoic acid, hexynoic acid, and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich, Seoul, Korea. Benzene, hexane, toluene, acetonitrile, methanol, and methyl acetate (HPCL grade) were purchased from TCI, Seoul, Korea. All chemicals were used without further purification.

3.2. Preparation of Pristine Perovskite QD The green perovskite QDs were synthesized by a well-known precipitation method [32]. All solvents are used in an anhydrate state. The 33.56 mg of CH3NH3Br and 146.8 mg of PbBr2 are dissolved in 10 mL of DMF. Then, 0.1 mL of oleylamine and 1 mL of oleic acid are added into the above solution (Precursor A). The 0.5 mL of ‘Precursor A’ solution is added into 5 mL benzene at 45◦C under vigorous stirring. To gather the green emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min. The precipitated material is re-dispersed in toluene (4 mg/mL) and the solution is centrifuged again at 10 krpm 5 min after 0.3 mL of ACN adding. The ﬁnal product is re-dispersed in hexane.

3.3. Preparation of Perovskite QD/SiO2

The perovskite QD/SiO2 is synthesized by a modified encapsulation method [33]. Perovskite QD is firstly fabricated with the same process with pristine QD (method 3.2). The 8 mg of as-synthesized QD is re-dispersed in 10 mL of toluene with 10 µL of APTES. The 5 µL of ammonia is added and vigorously stirred for an hour to conduct the silanization of APTES. The solution is centrifuged at 10 krpm for 5 min after 0.3 mL of ACN being added. The final product is re-dispersed in hexane.

3.4. Preparation of Perovskite QD/Ni(ppy) via P1-1 (No Short Ligand) Perovskite QD is fabricated with the modiﬁed synthesis method of pristine QD (method 3.2). All solvents are used in an anhydrate state. The 33.56 mg of CH3NH3Br, 146.8 mg of PbBr2, and 1 mg of of Ni(ppy) are dissolved in 10 mL of DMF. Then, 0.1 mL of oleylamine and 1 mL of oleic acid are added into the above solution (Precursor A). The 0.5 mL of ‘Precursor A’ solution is added into 5 mL benzene at 45 ◦C under vigorous stirring. To gather the green emissive perovskite QDs solution, the solution is centrifuged at 8 krpm for 5 min with 0.3 mL of ACN anti-solvent. The precipitated perovskite QD is re-dispersed in toluene (4 mg/mL) and 60 µL of 1 mM of Ni(ppy) in methanol is added. The solution is stirred for 10 min. Then, the solution is centrifuged again at 10 krpm for 5 min after 0.3 mL of ACN being added. The ﬁnal product is re-dispersed in hexane.

3.5. Preparation of Perovskite QD/Ni(ppy) via P1-2

The 33.56 mg of CH3NH3Br and 146.8 mg of PbBr2 are mixed with 1 mg of Ni(ppy), 0.6 mL of oleic acid, 0.4 mL of 5-hexynoic acid, and 0.1 mL of oleylamine altogether in 10 mL of DMF (Precursor A). The 0.4 mL of ‘Precursor A’ solution is added into 5 mL benzene at 50 ◦C under vigorous stirring. To gather the green emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min. The precipitated material is re-dispersed in toluene (4 mg/mL), and the solution is centrifuged again at 10 krpm for 5 min after 0.3 mL of ACN is added. The ﬁnal product is re-dispersed in hexane.

3.6. Preparation of Perovskite QD/Ni(ppy) via P2-1

The 33.56 mg of CH3NH3Br and 146.8 mg of PbBr2 are mixed with 1 mg of Ni(ppy), 0.6 mL of oleic acid, 0.4 mL of 5-hexynoic acid, and 0.1 mL of oleylamine altogether in 10 mL of DMF (Precursor A). The 0.4 mL of ‘Precursor A’ solution is added into 5 mL benzene at 50 ◦C under vigorous stirring. To gather the green emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min with 0.3 mL of ACN anti-solvent. The Catalysts 2021, 11, 61 14 of 18

precipitated perovskite QD is re-dispersed in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol is added. The solution is stirred for 10 min and the 60 µL of 1 mM of Ni(ppy) in methanol is additionally injected. The solution is stirred for another 10 min. Then, the solution is centrifuged again at 10 krpm 5 min after 0.3 mL of ACN adding. The ﬁnal product is re-dispersed in hexane.

3.7. Preparation of Perovskite QD/Ni(ppy) via P2-2 The procedure is exactly the same as the P2-1. For P2-2, the product is centrifuged twice. For the ﬁrst centrifugation, the precipitated perovskite QD is re-dispersed in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol and the 60 µL of 1 mM of Ni(ppy) in methanol is added. The solution is stirred for 5 min and the solution is centrifuged again at 10 krpm 5 min after 0.3 mL of ACN adding. The precipitated perovskite QD is re-dispersed again in toluene (4 mg/mL) and 30 µL of 10 mM of 3- Butynoic acid in methanol is added again. The solution is stirred for 10 min and the 60 µL of 1 mM of Ni(ppy) in methanol is additionally injected. The solution is stirred for another 10 min. Then, the solution is centrifuged at 10 krpm 5 min with 0.3 mL ACN adding. The ﬁnal product is re-dispersed in hexane.

3.8. Preparation of Perovskite QD/Ni(ppy)/SiO2 via P2-2 Perovskite QD is fabricated with the same process with QD/Ni(ppy) via P1-2 (method 3.7). The 40 mg of as-synthesized QD/Ni(ppy) is re-dispersed in APTES/Toluene solution (10 µL/10 mL). The 5 µL of ammonia is added and vigorously stirred for 40 min to conduct the silanization of APTES (Solution B). The 30 µL of 10 mM of 3-Butynoic acid in methanol is added in to ‘Solution B’. The solution is stirred for 10 min and the 60 µL of 1 mM of Ni(ppy) in methanol is added. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm 5 min after 0.3 mL of ACN. The precipitated perovskite QD is re-dispersed in hexane (4 mg/mL). The 30 µL of 10 mM of 3-Butynoic acid in methanol is added, and the solution is stirred for 10 min. The 60 µL of 1 mM of Ni(ppy) in methanol is added again. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm 5 min after 0.3 mL of ACN is added. The ﬁnal product is re-dispersed in hexane.

3.9. Preparation of Perovskite QD/NiBr2 via P2-1 and P2-2

The 33.56 mg of CH3NH3Br and 146.8 mg of PbBr2 are mixed with 1 mg of NiBr2, 0.6 mL of oleic acid, 0.4 mL of 5-hexynoic acid, and 0.1 mL of oleylamine in 10 mL of DMF solution (Precursor A). The 0.4 mL of ‘Precursor A‘solution is added into 5 mL benzene at 50 ◦C under vigorous stirring. To gather the green emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min. The precipitated QD is re-dispersed in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol is added. The solution is stirred for 10 min and then centrifuged again at 10 krpm for 5 min after 0.3 mL of ACN is added. For P2-1, the precipitated perovskite QD is re-dispersed again in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol is added again. The solution is stirred for 10 min and 60 µL of 1 mM of NiBr2 in methanol is additionally injected. The solution is stirred for another 10 min. Then, the solution is centrifuged at 10 krpm for 5 min with 0.3 mL ACN being added. For P2-2, the precipitated perovskite QD is re-dispersed in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol is added. The solution is stirred for 10 min and the solution is centrifuged again at 10 krpm for 5 min after 0.3 mL of ACN is added. The precipitated perovskite QD is re-dispersed again in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol and 60 µL of 1 mM of NiBr2 in methanol are added again. The solution is stirred for 5 min and the solution is centrifuged again at 10 krpm for 5 min after 0.3 mL of ACN is added. The precipitated perovskite QD is re-dispersed again in toluene (4 mg/mL) and 30 µL of 10 mM of 3-Butynoic acid in methanol is added again. The solution is stirred for 10 min and 60 µL of 1 mM of NiBr2 in methanol is additionally Catalysts 2021, 11, 61 15 of 18

injected. The solution is stirred for another 10 min. Then, the solution is centrifuged at 10 krpm 5 min with 0.3 mL ACN being added. The ﬁnal product is re-dispersed in hexane.

3.10. Preparation of Red Perovskite QD The red perovskite QD is synthesized by a modiﬁed precipitation method [32]. All solvents are used in an anhydrate state. The 5 mg of CH3NH3Br, 45 mg of CH3NH3I, and 100 mg of PbI2 are dissolved in 2mL of DMF. Then, 0.15 mL of oleylamine and 1 mL of oleic acid are added into the above solution (Precursor A). The 0.1 mL of ‘precursor A’ solution is added into 5 mL benzene at 40 ◦C under vigorous stirring. To gather the red emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min with 1 mL of methyl acetate anti-solvent. The precipitated perovskite QD is re-dispersed in toluene (4 mg/mL). The ﬁnal product is re-dispersed in hexane.

3.11. Preparation of Red Perovskite QD/Ni(ppy)/SiO2

The 5 mg of CH3NH3Br, 45 mg of CH3NH3I, and 100 mg of PbI2 are dissolved in 2 mL of DMF. Then, a trace of Ni(ppy) (1 mg), 0.6mL of oleic acid, 0.4 mL of 5-hexynoic acid, and 0.15 mL of oleylamine is added into above solution (Precursor A). The 0.1 mL of ‘precursor A’ solution is added into 5 mL benzene at 45 ◦C under vigorous stirring. To gather the red emissive perovskite QDs, the solution is centrifuged at 8 krpm for 5 min with 1 mL of methyl acetate anti-solvent. The precipitated perovskite QD is re-dispersed in toluene (4 mg/mL). Then, 10 µL APTES is added into 10 mL of QD solution and vigorously stirred for an hour to conduct the silanization of APTES (Solution B). The 30 µL of 10 mM of 3-Butynoic acid in methanol is added in to ‘Solution B’. The solution is stirred for 10 min and 60 µL of 1 mM of Ni(ppy) in methanol is added. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm 5 min after 1 mL of methyl acetate adding. The precipitated perovskite QD is re-dispersed in hexane (4 mg/mL). The 30 µL of 10 mM of 3-Butynoic acid in methanol is added, and the solution is stirred for 10 min. The 60 µL of 1 mM of Ni(ppy) in methanol is added again. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm for 5 min after 1 mL of methyl acetate is added. The ﬁnal product is re-dispersed in hexane.

3.12. Preparation of Red & Green Perovskite QD/SiO2

The red perovskite QD/Ni(ppy)/SiO2 (method 3.11) and green perovskite QD/Ni(ppy)/ SiO2 (method 3.8) are separately synthesized. The solution with a speciﬁc ratio is mixed in hexane and is stirred for 30 min at RT in ambient condition for hydrolysis of unchained SiO2.

3.13. Preparation of Photocatalytic CO2 Reduction Reaction

Photocatalytic CO2 reduction experiments are set up as shown in Figure 10. The catalyst film is fabricated on a glass membrane substrate (Ø 25 mm). The catalyst-covered substrate is then placed in a stainless steel home-made photoreactor (Volume = 50 cm3) equipped with a quartz window. H2O or EA/H2O is added into the chamber and CO2 gas is continuously flowed over 30 min under dark conditions to remove unknown gases inside the chamber. Then the chamber is filled with CO2 gas and the whole system is sealed up. The chamber is irradiated with a Xe lamp with an AM 1.5G filter to simulate the solar light spectrum (100 mW/cm2); A Si reference cell (BS-520BK, S/N 568, Bunkoukeiki Co., Ltd., Tokyo, Japan) is used for calibration. The 250 µL of gases are collected using a gas-tight syringe and analyzed by gas chromatography (YL 6500GC system, YL Instrument Co, Ltd., Anyang-si, Korea). Catalysts 2021, 11, x FOR PEER REVIEW 17 of 20

mL of methyl acetate anti-solvent. The precipitated perovskite QD is re-dispersed in toluene (4 mg/mL). Then, 10 μL APTES is added into 10 mL of QD solution and vigorously stirred for an hour to conduct the silanization of APTES (Solution B). The 30 μL of 10 mM of 3-Butynoic acid in methanol is added in to ‘Solution B’. The solution is stirred for 10 min and 60 μL of 1 mM of Ni(ppy) in methanol is added. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm 5 min after 1 mL of methyl acetate adding. The precipitated perovskite QD is re-dispersed in hexane (4 mg/mL). The 30 μL of 10 mM of 3-Butynoic acid in methanol is added, and the solution is stirred for 10 min. The 60 μL of 1 mM of Ni(ppy) in methanol is added again. The solution is stirred for another 30 min. Then, the solution is centrifuged at 10 krpm for 5 min after 1 mL of methyl acetate is added. The final product is re-dispersed in hexane.

3.12. Preparation of Red & Green Perovskite QD/SiO2

The red perovskite QD/Ni(ppy)/SiO2 (method 3.11) and green perovskite QD/Ni(ppy)/SiO2 (method 3.8) are separately synthesized. The solution with a specific ratio is mixed in hexane and is stirred for 30 min at RT in ambient condition for hydrolysis of unchained SiO2.

3.13. Preparation of Photocatalytic CO2 Reduction Reaction

Photocatalytic CO2 reduction experiments are set up as shown in Figure 10. The catalyst film is fabricated on a glass membrane substrate (Ø 25 mm). The catalyst-covered substrate is then placed in a stainless steel home-made photoreactor (Volume = 50 cm3) equipped with a quartz window. H2O or EA/H2O is added into the chamber and CO2 gas is continuously flowed over 30 min under dark conditions to remove unknown gases inside the chamber. Then the chamber is filled with CO2 gas and the whole system is sealed up. The chamber is irradiated with a Xe lamp with an AM 1.5G filter to simulate the solar light spectrum (100 mW/cm2); A Si reference cell (BS-520BK, S/N 568, Bunkoukeiki Co., Ltd., Tokyo, Japan) is used for calibration. The 250 µL of gases are collected using a gas- Catalysts 2021, 11, 61 16 of 18 tight syringe and analyzed by gas chromatography (YL 6500GC system, YL Instrument Co, Ltd., Anyang-si, Korea).

FigureFigure 10. 10.Experimental Experimental setup setup used used for for photocatalytic photocatalytic CO CO2 2reduction. reduction.

3.14. TCSPC Measurement 3.14. TCSPC Measurement Optical properties of perovskite QDs: The first excitonic peak position of perovskite Optical properties of perovskite QDs: The first excitonic peak position of perovskite QDs ink is measured using the UV-Vis spectrometer (JASCO V-650 spectrophotometer, QDs ink is measured using the UV-Vis spectrometer (JASCO V-650 spectrophotome- Seoul, Korea). Steady-state photoluminescence (PL) spectra are obtained using two mon- ter, Seoul, Korea). Steady-state photoluminescence (PL) spectra are obtained using two ochromators (SP-2150i and SP-2300i, Acton, MA, USA) systems equipped with a photo- monochromators (SP-2150i and SP-2300i, Acton, MA, USA) systems equipped with a multiplier tube (PMT, Acton PD471, MA, USA) and a Xenon lamp as an excitation light photomultiplier tube (PMT, Acton PD471, MA, USA) and a Xenon lamp as an excitation light source. The PLQY of perovskite QDs solution is obtained relative to a Coumarin 500 (PLQY =∼ 47% in ethanol). The PL decay of perovskite QDs in solution and film is investigated using a time-correlated single-photon counting (TCSPC) measurement. A pulsed diode-laser head (LDH-P-C-378 nm, PicoQuant, Berlin, Germany) coupled with a laser-diode driver (PDL 800-B, PicoQuant, Berlin, Germany) is used as the excitation source with a repetition rate of 10MHz. The excitation wavelength is 405 nm. The PL emission is spectrally resolved using collection optics and a monochromator (SP-2150i, Acton, MA, USA). A TCSPC module (PicoQuant, PicoHarp 300, Berlin, Germany) with a MCP-PMT (Hamamatsu, R3809U-59, Shizuoka, Japan) is used for ultrafast detection. The total instrument response function (IRF) is less than ~140 ps, and the temporal resolution is ~8 ps. The deconvolution of PL decay curve, which separates the IRF and actual PL decay signal, is performed using fitting software (FluoFit, PicoQuant, Berlin, Germany) to deduce the time constant associated with each exponential decay curve.

3.15. TEM and EDS Measurement The transmission electron microscopy (Tecnai G2 F30 S-Twin 300 kV, GA, USA) equipped with an energy-dispersive X-ray spectrometer is used to deﬁne size. The samples are prepared on the carbon-coated Cu grid (product# 01840-F, TED PELLA, INC., Redding, CA, USA). The chemical composition of perovskite QD is detected by the octane Elite EDS System. In addition, TEM is also used to observe the morphology of perovskite QD photocatalysts.

4. Conclusions

We designed a red/green perovskite QD/Ni(ppy)/SiO2 to increase the catalytic behavior of the perovskite QD photocatalyst. A co-catalyst, Ni(ppy), was incorporated into the perovskite QDs. The Ni precursor was mixed with a ligand selected for strong interaction with the surface of the perovskite QDs. The Ni complex was successfully doped via a ligand exchange process. The Ni content was detected by XPS and increased as the concentration of the Ni precursor increased. When the number of ligand exchange cycles increased, the Ni atomic ratio increased simultaneously. The red/green perovskite QD/Ni(ppy) underwent in-situ silanization to produce an ultrathin SiO2 coating. The photodegradation of Catalysts 2021, 11, 61 17 of 18

red/green perovskite QD/Ni(ppy)/SiO2 dramatically improved after SiO2 coating. We studied the photodegradation of various types of perovskite QDs during time-resolved PL tests. Pristine QDs/Ni(ppy) demonstrated signiﬁcant PL degradation as the power of the light source increased. Time-resolved PL spectra indicated that trap-assisted recombination increased after light exposure, leading to photodegradation. With the addition of the SiO2 shell, the photodegradation of perovskite signiﬁcantly reduced. SiO2 passivated the defects in the perovskite QDs, preventing ion migration triggered by the high-power light source. As a result, the SiO2-coated perovskite QDs demonstrated slower photodegradation than pristine QDs at the same power level. Moreover, the SiO2 coating enhanced catalytic stability. In general, the catalytic behavior gradually decreased as the degree of contamination in the catalyst increased. During the absorption/desorption reaction, by-products and/or remaining molecules act as contaminants, sometimes changing the chemical composition of the catalyst, which is an irreversible reaction. Pristine QDs have exposed defects and easily undergo unwanted reactions with neighboring molecules. Thus, the photocatalytic activity of pristine QDs varied with time, while the red/green perovskite QD/Ni(ppy)/SiO2 effectively prevented ion migration and defect contamination, resulting in better operational stability. Our system achieved a CO2 reduction capacity for CO of 0.56 µmol/(g·h). We hypothesize that developing an effective hole scavenger for red/green perovskite QD/Ni(ppy)/SiO2 can further improve the catalytic behavior in the future.

Author Contributions: Conceptualization, H.L. (Hanleem Lee); methodology, H.L. (Hanleem Lee); formal analysis, H.L (Hanleem Lee) and M.K.; investigation, H.L. (Hanleem Lee) and M.K.; resources, H.L. (Hanleem Lee); data curation, H.L. (Hanleem Lee); writing—original draft preparation, H.L. (Hanleem Lee) and M.K.; writing—review and editing, H.L. (Hyoyoung Lee) and H.L. (Hanleem Lee); visualization, H.L.; supervision, H.L. (Hyoyoung Lee); project administration, H.L. (Hyoyoung Lee); funding acquisition, H.L. (Hanleem Lee).These authors contributed equally. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF- 2020R1A6A3A01099729) and the Institute for Basic Science (No. IBS-R011-D1), Korea evaluation institute of industrial technology (20004627). We also thank for APRI (Advanced Photonics Research Institute) in GIST for using TCSPC. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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