Synthesis of Au or Pt@Perovskite Nanocrystals via Interfacial Photoreduction

: The surface modiﬁcation of perovskite nanocrystals (NCs) (i.e., their decoration with noble metals) holds great promise with respect to the tailoring of their properties but has remained a challenge because perovskite NCs are extremely sensitive to water and alcohols. In this study, Au or Pt@CsPbBr 3 NCs were successfully synthesized by photoreduction at the water/hexane interface. First, Cs 4 PbBr 6 NCs were synthesized through the hot-injection method. Then, Cs 4 PbBr 6 was transformed into CsPbBr 3 and subjected to noble metal modiﬁcation, both at the interface. The synthesized CsPbBr 3 NCs exhibited a cubic perovskite phase and had an average size of approximately 13.5 nm. The deposited Au and Pt nanoparticles were crystalline, with a face-centered cubic lattice and average diameters of approximately 3.9 and 4.4 nm, respectively. The noble metal modiﬁcation process had almost no effect on the steady-state photoluminescence (PL) emission wavelength but affected the charge-recombination kinetics of the CsPbBr 3 NCs. Time-resolved PL decay spectral analysis indicated that the ﬂuorescence lifetimes of the Au and Pt@CsPbBr 3 NCs were shorter than those of the pure CsPbBr 3 NCs, probably owing to the quenching of the free charges because of electron transfer from the perovskite to the noble metal nanoparticles. a rhombohedral phase, and the diffraction peaks at approximately 22.4 ◦ , 27.8 ◦ , 28.7 ◦ , 29.0 ◦ , and 30.4 ◦ correspond to the (101), (610), (230), (121), and (330) planes, respectively [17]. On the other hand, the product formed after the water treatment exhibits a pure cubic CsPbBr 3 perovskite phase, and its diffraction pattern contains peaks related to the (100), (110), and (200) planes [17]. These results conﬁrm that a change in the crystal structure from Cs 4 PbBr 6 to CsPbBr 3 was successfully realized at the water/hexane interface.


Introduction
Organic-inorganic lead halide perovskite (OIHP) materials with a general ABX 3 (A = organic cation, B = metal cation, and X = halide anion) formula have attracted significant attention in the field of optoelectronics because of their unique optical and semiconducting characteristics [1][2][3]. For instance, their use has rapidly increased the power conversion efficiency of solar cells from 3.81% [4] to more than 25% [5] within a few years. However, owing to the instability of OIHP, including its extreme sensitivity to oxygen and moisture, as well as its poor photo-and thermostability [6], a number of perovskite analogues have been developed. Replacing the A-site organic groups with inorganic cations to construct all-inorganic halide perovskites (IHPs) is a potential strategy for improving the stability of OIHP while maintaining its optical and electrical properties [7].
Cesium lead halide perovskite (CsPbX 3 ) nanocrystals (NCs), an IHP material, show great promise for use in a range of fields owing to their high photoluminescence quantum yield, narrow emission width, and tunable band gap, which covers the entire visible range [8]. They were first reported by the Kovalenko group in 2015 [9]. Since then, considerable progress has been made in the synthesis of CsPbX 3 NCs. For instance, the hot-injection method [10,11], the solvothermal method [12], ultrasonication [13], roomtemperature precipitation [14], and chemical vapor deposition [15] have been employed to prepare CsPbX 3 NCs with controllable shapes and compositions.
Although the advances made in the synthesis of CsPbX 3 NCs have been impressive, it is known that heterostructured NCs formed by tailoring the properties of two or more dissimilar materials usually exhibit several interesting functionalities. An example is the surface modification of CsPbX 3 NCs at the individual particle level [16,17]. However, it remains a challenge to use conventional sol-gel methods for surface modification of CsPbX 3 NCs because of their water and alcohol intolerance. Recently, Hu et al. demonstrated an effective sol-gel process for modifying the surfaces of CsPbX 3 NCs at the interface of water and a nonpolar solvent and were able to produce CsPbX 3 /metal oxide Janus NCs with improved stability [17]. Thus, interfacial synthesis is the key to preparing heterostructured NCs and is different from the other reported methods for preparing CsPbX 3 /metal [18,19] and CsPbX 3 /SiO 2 [20] heterostructures. Inspired by their report, herein, we performed photoreduction at the water/hexane interface to produce noble metal@CsPbBr 3 NCs. First, Cs 4 PbBr 6 NCs were synthesized through the hot-injection method. Subsequently, the Cs 4 PbBr 6 was transformed into CsPbBr 3 and subjected to noble metal modification, both at the interface. Owing to the localized surface plasmon resonance effect [21], high catalytic activity [22], and ease of charge separation [23] of the noble metal nanoparticles, the noble metal@CsPbX 3 NCs have the potential to play an important role in photodetectors, light-emitting diodes (LEDs), solar cells, and photocatalysts.

Results and Discussion
We propose the following mechanism to explain the synthesis process ( Figure 1). When the hexane suspension containing the Cs 4 PbBr 6 NCs comes in contact with water, the Cs 4 PbBr 6 is transformed into CsPbBr 3 , which accompanies the stripping of CsBr through the hexane/water interface and removing partial hydrophobic capping ligands (i.e., oleic acid (OA) and oleylamine (OAm)) [17]. Subsequently, the noble metal precursor in contact with the interface reacts with the electrons photogenerated from CsPbBr 3 (methanol was used as a sacrificial agent). According to a previous report [8], the CsPbBr3 NCs obtained through this water-triggered transformation process exhibit enhanced stability against moisture compared with those formed through the hot-injection method. Thus, Cs 4 PbBr 6 NCs were chosen to prepare the Au or Pt@CsPbBr 3 NCs instead of CsPbBr3 NCs presynthesized by the hot-injection method.
Catalysts 2021, 11, x FOR PEER REVIEW 2 of 10 precipitation [14], and chemical vapor deposition [15] have been employed to prepare CsPbX3 NCs with controllable shapes and compositions. Although the advances made in the synthesis of CsPbX3 NCs have been impressive, it is known that heterostructured NCs formed by tailoring the properties of two or more dissimilar materials usually exhibit several interesting functionalities. An example is the surface modification of CsPbX3 NCs at the individual particle level [16,17]. However, it remains a challenge to use conventional sol-gel methods for surface modification of CsPbX3 NCs because of their water and alcohol intolerance. Recently, Hu et al. demonstrated an effective sol-gel process for modifying the surfaces of CsPbX3 NCs at the interface of water and a nonpolar solvent and were able to produce CsPbX3/metal oxide Janus NCs with improved stability [17]. Thus, interfacial synthesis is the key to preparing heterostructured NCs and is different from the other reported methods for preparing CsPbX3/metal [18,19] and CsPbX3/SiO2 [20] heterostructures. Inspired by their report, herein, we performed photoreduction at the water/hexane interface to produce noble metal@CsPbBr3 NCs. First, Cs4PbBr6 NCs were synthesized through the hot-injection method. Subsequently, the Cs4PbBr6 was transformed into CsPbBr3 and subjected to noble metal modification, both at the interface. Owing to the localized surface plasmon resonance effect [21], high catalytic activity [22], and ease of charge separation [23] of the noble metal nanoparticles, the noble metal@CsPbX3 NCs have the potential to play an important role in photodetectors, light-emitting diodes (LEDs), solar cells, and photocatalysts.

Results and Discussion
We propose the following mechanism to explain the synthesis process ( Figure 1). When the hexane suspension containing the Cs4PbBr6 NCs comes in contact with water, the Cs4PbBr6 is transformed into CsPbBr3, which accompanies the stripping of CsBr through the hexane/water interface and removing partial hydrophobic capping ligands (i.e., oleic acid (OA) and oleylamine (OAm)) [17]. Subsequently, the noble metal precursor in contact with the interface reacts with the electrons photogenerated from CsPbBr3 (methanol was used as a sacrificial agent). According to a previous report [8], the CsPbBr3 NCs obtained through this water-triggered transformation process exhibit enhanced stability against moisture compared with those formed through the hot-injection method. Thus, Cs4PbBr6 NCs were chosen to prepare the Au or Pt@CsPbBr3 NCs instead of CsPbBr3 NCs presynthesized by the hot-injection method.  To explore the morphologies of the synthesized Cs 4 PbBr 6 and CsPbBr 3 NCs, transmission electron microscopy (TEM) imaging was performed. Figure 2a shows that the original Cs 4 PbBr 6 NCs are quasispherical with an average diameter of approximately 13.9 nm. The high-resolution TEM (HRTEM) image in the inset clearly shows that the lattice spacing is 0.122 nm and is in good agreement with that of the (1 0 1) plane of rhombohedral Cs 4 PbBr 6 . The size distribution of the NCs was relatively narrow (Figure 2b), indicating that the NC growth process was well controlled. After the water-triggered transformation process, the NCs show a cube-like structure with an average edge length of approximately 13.5 nm. Further, the d-spacing of 0.152 nm, as measured using the HRTEM image ( Figure 2c and its inset), could be assigned to the (1 1 1) plane of cubic CsPbBr 3 . The size distribution of these NCs was similar to that of the Cs 4 PbBr 6 NCs ( Figure 2d). To explore the morphologies of the synthesized Cs4PbBr6 and CsPbBr3 NCs, transmission electron microscopy (TEM) imaging was performed. Figure 2a shows that the original Cs4PbBr6 NCs are quasispherical with an average diameter of approximately 13.9 nm. The high-resolution TEM (HRTEM) image in the inset clearly shows that the lattice spacing is 0.122 nm and is in good agreement with that of the (1 0 1) plane of rhombohedral Cs4PbBr6. The size distribution of the NCs was relatively narrow (Figure 2b), indicating that the NC growth process was well controlled. After the water-triggered transformation process, the NCs show a cube-like structure with an average edge length of approximately 13.5 nm. Further, the d-spacing of 0.152 nm, as measured using the HRTEM image (Figure 2c and its inset), could be assigned to the (1 1 1) plane of cubic CsPbBr3. The size distribution of these NCs was similar to that of the Cs4PbBr6 NCs (Figure 2d). The transformation was also monitored through ultraviolet (UV)-visible (vis) absorption and photoluminescence (PL) measurements. As shown in Figure 3a, two sharp peaks were observed at 230 and 314 nm (solid black line). These could be assigned to the pristine Cs4PbBr6 NCs [24]. The spectrum shows that the Cs4PbBr6 NC suspension did not exhibit any distinct absorption peaks in the visible-light region, in accordance with the colorless appearance of the suspension. After the reaction with water, the sharp peak at 314 nm disappeared and a new absorption peak emerged at approximately 510 nm (solid red line), indicating the transformation of Cs4PbBr6 into CsPbBr3. The corresponding PL spectrum The transformation was also monitored through ultraviolet (UV)-visible (vis) absorption and photoluminescence (PL) measurements. As shown in Figure 3a, two sharp peaks were observed at 230 and 314 nm (solid black line). These could be assigned to the pristine Cs 4 PbBr 6 NCs [24]. The spectrum shows that the Cs 4 PbBr 6 NC suspension did not exhibit any distinct absorption peaks in the visible-light region, in accordance with the colorless appearance of the suspension. After the reaction with water, the sharp peak at 314 nm disappeared and a new absorption peak emerged at approximately 510 nm (solid red line), indicating the transformation of Cs 4 PbBr 6 into CsPbBr 3 . The corresponding PL spectrum shows a sharp peak at 532 nm, with a small full-width-at-half-maximum value. Figure 3b shows the X-ray diffraction (XRD) pattern of the original Cs 4 PbBr 6 NCs, as well as that of the product after the water-triggered transformation. The Cs 4 PbBr 6 NCs exhibit  [17]. On the other hand, the product formed after the water treatment exhibits a pure cubic CsPbBr 3 perovskite phase, and its diffraction pattern contains peaks related to the (100), (110), and (200) planes [17]. These results confirm that a change in the crystal structure from Cs 4 PbBr 6 to CsPbBr 3 was successfully realized at the water/hexane interface.
shows a sharp peak at 532 nm, with a small full-width-at-half-maxim shows the X-ray diffraction (XRD) pattern of the original Cs4PbBr6 N the product after the water-triggered transformation. The Cs4PbBr6 bohedral phase, and the diffraction peaks at approximately 22.4°, 27 30.4° correspond to the (101), (610), (230), (121), and (330) planes, resp other hand, the product formed after the water treatment exhibits a perovskite phase, and its diffraction pattern contains peaks related to (200) planes [17]. These results confirm that a change in the crystal stru to CsPbBr3 was successfully realized at the water/hexane interface.  Figure 4 shows the TEM images of the Au or Pt@CsPbBr3 hybr reacting Cs4PbBr6 NCs with the corresponding noble metal precurso tion at the water/hexane interface. These hybrid NCs were similar in pure CsPbBr3 NCs shown in Figure 1c. Moreover, the Au and Pt na  irradiation at the water/hexane interface. These hybrid NCs were similar in shape and size to the pure CsPbBr 3 NCs shown in Figure 1c. Moreover, the Au and Pt nanoparticles were deposited randomly on the surfaces of the CsPbBr 3 NCs, and their sizes were relatively small, at 3.9 and 4.4 nm, respectively. The HRTEM images show that the formed Au and Pt nanoparticles are crystalline with d-spacings of 0.106 and 0.090 nm, respectively, which correspond to the face-centered cubic (fcc) Au (400) (JCPDS Card No. 04-0784) and Pt (331) planes (JCPDS Card No. 70-2057), respectively. These results confirmed that the photoreduction reaction had been carried out successfully at the water/hexane interface. The X-ray photoelectron spectroscopy (XPS) survey scans of Au@CsPbBr 3 and Pt@CsPbBr 3 NCs are presented in the left of Figure 5, in which the presence of Cs, Pb, Br, and Au (or Pt) elements is confirmed. In the high-resolution XPS spectra (the right part of Figure 5), doublet peaks containing a low energy band (Au4f 7/2 ) and a high energy band (Au4f 5/2 ) are observed at 84.0 and 87.7 eV for Au@CsPbBr 3 , respectively, and 71.0 (Pt4f 7/2 ) and 74.3 eV (Pt4f 5/2 ) for Pt@CsPbBr 3 , which are assigned to metallic Au(0) or Pt(0) [25,26]. The results demonstrate that the noble metal precursors can be effectively reduced through the interfacial photoreduction. Pt) elements is confirmed. In the high-resolution XPS spectra (the right part of Figure 5), doublet peaks containing a low energy band (Au4f7/2) and a high energy band (Au4f5/2) are observed at 84.0 and 87.7 eV for Au@CsPbBr3, respectively, and 71.0 (Pt4f7/2) and 74.3 eV (Pt4f5/2) for Pt@CsPbBr3, which are assigned to metallic Au(0) or Pt(0) [25,26]. The results demonstrate that the noble metal precursors can be effectively reduced through the interfacial photoreduction.   Successful deposition of the Au or Pt nanoparticles was also verified through UV-vis absorption measurements. Figure 6a shows the absorption spectra of suspensions containing the different NCs. Compared with the peaks of the pure CsPbBr3 NCs, the absorption edges of the Au@CsPbBr3 and Pt@CsPbBr3 NCs exhibited a red shift. Furthermore, the Pt@CsPbBr3 NCs showed enhanced absorption for wavelengths greater than 520 nm, while the Au@CsPbBr3 NCs showed additional weak absorption peaks at 420-450 nm. These differences in the absorption spectra may be attributed to the surface plasmonic resonance effect of the deposited noble metal nanoparticles [16]. All the NCs exhibited PL emission peaks at 532 nm, and their full-width-at-half-maximum values were also similar ( Figure 6b). A bright-green PL emission was observed under UV-light irradiation ( Figure  6c). Time-resolved PL (TRPL) measurements were performed to study charge transfer from the perovskite NCs to the noble metal nanoparticles. The TRPL decay data ( Figure  6d) were fitted using a biexponential function of time, t, as shown in Equation 1, and the fitted data are listed in Table 1.
where t is the time after optical excitation, I (t) is the luminescence intensity at t, τ1 is the fast transient lifetime related to the charge-trapping process (nonradiative recombination), τ2 is the slow lifetime attributed to the intrinsic band-to-band recombination (radiative recombination) [27], and A1 and A2 are coefficients corresponding to the contributions of the fast and slow lifetimes, respectively [28]. The τ2 value of the Au or Pt@CsPbBr3 NCs was much smaller than that of the pure CsPbBr3 NCs (Table 1). This can be ascribed to the quenching of the free charges because of electron transfer from the perovskite to the noble metal nanoparticles. Successful deposition of the Au or Pt nanoparticles was also verified through UV-vis absorption measurements. Figure 6a shows the absorption spectra of suspensions containing the different NCs. Compared with the peaks of the pure CsPbBr 3 NCs, the absorption edges of the Au@CsPbBr 3 and Pt@CsPbBr 3 NCs exhibited a red shift. Furthermore, the Pt@CsPbBr 3 NCs showed enhanced absorption for wavelengths greater than 520 nm, while the Au@CsPbBr 3 NCs showed additional weak absorption peaks at 420-450 nm. These differences in the absorption spectra may be attributed to the surface plasmonic resonance effect of the deposited noble metal nanoparticles [16]. All the NCs exhibited PL emission peaks at 532 nm, and their full-width-at-half-maximum values were also similar (Figure 6b). A bright-green PL emission was observed under UV-light irradiation (Figure 6c). Timeresolved PL (TRPL) measurements were performed to study charge transfer from the perovskite NCs to the noble metal nanoparticles. The TRPL decay data (Figure 6d) were fitted using a biexponential function of time, t, as shown in Equation (1), and the fitted data are listed in Table 1.
where t is the time after optical excitation, I (t) is the luminescence intensity at t, τ 1 is the fast transient lifetime related to the charge-trapping process (nonradiative recombination), τ 2 is the slow lifetime attributed to the intrinsic band-to-band recombination (radiative recombination) [27], and A 1 and A 2 are coefficients corresponding to the contributions of the fast and slow lifetimes, respectively [28]. The τ 2 value of the Au or Pt@CsPbBr 3 NCs was much smaller than that of the pure CsPbBr 3 NCs (Table 1). This can be ascribed to the quenching of the free charges because of electron transfer from the perovskite to the noble metal nanoparticles.    Synthesis of Cs 4 PbBr 6 NCs: The Cs 4 PbBr 6 NCs were prepared using the hot-injection method [17]. A cesium oleate solution was prepared by mixing 0.32 g of Cs 2 CO 3 (0.98 mmol), 1 mL of OA, and 16 mL of ODE in a 100-mL three-neck flask. The solution was dried for 1 h at 120 • C under vacuum and then heated in an Ar atmosphere to 150 • C until all the Cs 2 CO 3 had reacted with OA. During a typical run to synthesize the Cs 4 PbBr 6 NCs, OAm (2 mL), OA (2 mL), ODE (20 mL), and PbBr 2 (0.4 mmol) were added to a 100-mL three-neck flask and dried under vacuum for 1 h. The reaction mixture was then heated to 140 • C. Next, 8.8 mL of a hot cesium oleate solution was rapidly injected into the PbBr 2 solution. After 6 s, the reaction mixture was immersed in an ice-water bath for immediate cooling. The product was centrifuged at 12,000 rpm for 5 min to remove the ODE and unreacted OA as well as the OAm. The precipitate was collected and redispersed in hexane and then centrifuged at 3000 rpm for 5 min to remove the oversized Cs 4 PbBr 6 NCs. The concentration of the Cs 4 PbBr 6 NC suspension was adjusted to 1 mg/mL for further synthesis.
Synthesis of Pt@CsPbBr 3 NCs: During a typical synthesis process, 0.1 mL of methanol was mixed with 0.4 mL of an aqueous H 2 PtCl 6 ·6H 2 O solution (1.448 mM), and the mixture was injected into 10 mL of the Cs 4 PbBr 6 NC suspension. The system was irradiated with a halogen lamp (~20 mW/cm 2 ) for 1 h under vigorous stirring and then kept undisturbed under the ambient conditions for 12 h. The product was centrifuged at 6000 rpm for 5 min. The precipitates were discarded, and the supernatant was collected for characterization. Synthesis of Au@CsPbBr 3 NCs: The synthesis procedure was similar to that for Pt@CsPbBr 3 NCs, except that an aqueous HAuCl 4 ·3H 2 O solution (1.904 mM) was used.
Synthesis of CsPbBr 3 NCs. The synthesis procedure was similar to that of the Pt@CsPbBr 3 NCs, except that pure water without a metal precursor or methanol was used.
Characterization: The UV-vis absorption spectra were recorded using a Shimadzu UV-vis spectrophotometer (UV3600) (Shimadzu, Kyoto, Japan). The morphologies of the NCs were characterized by means of TEM (JEOL JEM-2100HR) (JEOL Ltd., Tokyo, Japan). For the XRD measurements, the NC suspensions were cast on cleaned glass substrates and dried. The measurements were performed using a Rigaku diffractometer (D/max 2500 PC) (Rigaku Corporation, Akishima, Tokyo, Japan) and Cu Kα radiation. XPS analysis was carried out using PHI Quantera II (Ulvac-PHI, Kanagawa, Japan). The PL and TRPL decay measurements were performed using a Horiba DeltaFlex PL system (Horiba, Tokyo, Japan) with a 520-nm pulsed laser for excitation at approximately 25 • C.

Conclusions
In this study, Au and Pt@CsPbBr 3 NCs were successfully synthesized via a reaction at the water/hexane interface, which involved the transformation of Cs 4 PbBr 6 into CsPbBr 3 and its subsequent modification with a noble metal. The prepared CsPbBr 3 NCs exhibited a cubic perovskite phase and an average size of approximately 13.5 nm. The deposited Au and Pt nanoparticles were crystalline, with an fcc lattice and average diameters of approximately 3.9 and 4.4 nm, respectively. The noble metal modification process affected the charge recombination kinetics of the CsPbBr 3 NCs, probably owing to the quenching of the free charges because of electron transfer from the perovskite to the noble metal nanoparticles. This simple strategy for designing and synthesizing perovskite/noble metal nanostructures offers new opportunities to increase their applicability in photodetectors, LEDs, solar cells, photocatalysts, and other such devices. With respect to solar cells, CsPbBr 3 NCs modified with a noble metal may be used to construct perovskite quantum dot-based solar cells. Compared with bare perovskite NCs, those modified with a noble metal can enhance the light-harvesting properties of devices owing to their localized surface plasmon resonance effect. In the case of photocatalysts, a composite of Au or Pt@CsPbBr 3 NCs and reduced graphene oxide or TiO 2 would be suitable for use as a visible-light-responsive photocatalyst for H 2 evolution in aqueous HI solutions. Modification with a noble metal has the potential to enhance the photocatalytic activity of CsPbBr 3 NCs. Further research in these directions is underway in our laboratory.