MO2 (M=Si, Ti, Sn) Composites: Insight into

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CsPbBr3 Nanocrystal/MO2 (M=Si, Ti, Sn) Composites: Insight into Charge Carrier Dynamics and Photoelectrochemical Applications Jin-Feng Liao, Yang-Fan Xu, Xu-Dong Wang, Hong-Yan Chen, and Dai-Bin Kuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14988 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Applied Materials & Interfaces

CsPbBr3

Nanocrystal/MO2

(M=Si,

Ti,

Sn)

Composites: Insight into Charge Carrier Dynamics and Photoelectrochemical Applications Jin-Feng Liao, Yang-Fan Xu, Xu-Dong Wang, Hong-Yan Chen, and Dai-Bin Kuang* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China KEYWORDS CsPbBr3, nanocrystal, perovskite, transient absorption, photoelectrochemistry

ABSTRACT Though coating CsPbBr3 NC with an outer layer has been regarded as an effective strategy to address its instability issues, deep investigations into the electronic interaction between CsPbBr3 NC and coating layer have yet to be conducted. In this study, the dynamics of hot carrier and charge carrier of CsPbBr3 nanocrystal with various MO2 (M=Si, Ti, Sn) coating layers have been comprehensively studied. Combined transient optical characterizations (time-resolved photoluminescence and ultrafast transient absorption) and photoelectrochemical measurements reveal that coating with insulating SiO2 accelerates the hot carrier relaxation and enhances radiative recombination by passivating surface traps, while efficient charge carrier separation and extraction observed after coating with SnO2 and TiO2. The electron injection from CsPbBr3 NC to SnO2 (1.14·108 s-1) is 2-fold faster than to TiO2 (5.4·107 s-1) owing to the lower conduction band edge and higher electron mobility of SnO2. Particularly, the first time fabricated CsPbBr3 NC/SnO2

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composite exhibits the superior stability against UV light and moisture as well as the best photocurrent response in this study. This work has implied that rational design of the coating layer for perovskite NC can not only improve the stability but also tailor the electronic and optoelectronic properties for various applications.

INTRODUCTION All inorganic CsPbX3 (X=Cl, Br, I) perovskite nanocrystal (NC) has attracted extensive attention in the field of optoelectronic applications such as solar cells,1,2 light emitting diodes (LEDs),3,4 lasers5,6 and photocatalysts7,8 by virtue of the remarkable optical and electronic properties (e.g. large absorption coefficient, high carrier mobility and low recombination rate).9,10 Nevertheless, there still remains a thorny challenge hindering the extended commercial uses, which resides in its extreme sensitivity to moisture, UV light, heat and oxygen, as a result of low formation energy and intrinsic ionic crystal nature.11,12 To address the instability issues, outer shell coating has been recently deemed as the most straightforward and potent approach. Accordingly, a variety of polymers (e.g. PVP, PS)13-15 and inorganic oxides (e.g. SiO2, Al2O3)16-20 have been explored for the encapsulation of perovskite NC. However, most of these coating materials are electric insulating, thus restrict the further applications in optoelectronic devices or photocatalysis since the charge extraction and transportation are significantly restrained. Therefore, developing a charge transfer coating material has been urgently called for to not only improve the stability but also facilitate the charge collection from perovskite NC. In addition, there remains a gap in the basic understanding of the electronic interaction between perovskite NC and their coating shell. It is well perceived that the dynamics of the photogenerated electron-hole pairs play a critical role in determining the operating parameters of the relevant


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optoelectronic devices. In this regard, transient optical measurements such as time-resolved photoluminance (TRPL) and transient absorption (TA) spectroscopy have been widely utilized to probe into the ultrafast photophysical process, including the generation and recombination of the charge carrier,21-23 the hot carrier relaxation24-26 and the interfacial charge transport properties.2729

These fundamental studies have provided a profound guidance in the optimizations on the halide

perovskite based devices such as solar cells and light emitting diodes. As for halide perovskite NC, the ultrafast interfacial charge transfer process at the solid-liquid interface from CsPbBr3 NC to the small organic molecular dissolved in solution has been widely demonstrated.30,31 However, the study focused on the solid-solid charge transfer dynamics at the interfaces of encapsulated CsPbBr3 NC with different coating layers has yet to be carried out. Therefore, it is of significant importance to give insights into the impact of different coating layers on the charge carrier dynamics of halide perovskite NC/coating layer composite to further unlock their potentials in various applications. Herein, a series of CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites have been fabricated and the charge carrier dynamic has been comprehensively studied using the TRPL, TA and photoelectrochemical characterizations for the first time. As a result, coating CsPbBr3 NC with the insulating oxide SiO2 can effectively passivate the surface traps, leading to the significant promotion of radiative recombination. Additionally, coating with SnO2 and TiO2 facilitates the charge carrier separation and transfer in CsPbBr3 NC and the electron injection rate between CsPbBr3-xClx NC/SnO2 (1.14·108 s-1) is more efficient than that between CsPbBr3 NC/TiO2 (5.4·107 s-1). The systematical evaluation of different coating shells on the charge carriers of CsPbBr3 NC opens a new venue to rational surface modifying perovskite NC coating with another outer shell material to improve the stability


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and tune the photoelectric properties for further developing perovskite NC-based applications.

RESULTS AND DISCUSSION

Figure 1. TEM images of (a) CsPbBr3 NC, (b) CsPbBr3 NC/SiO2, (c) CsPbBr3 NC/TiO2 and (d) CsPbBr3-xClx NC/SnO2. CsPbBr3 NC was synthesized by a previously reported hot-injection method.9 The yellow precipitate was collected by centrifugation and then dispersed in hexane. As displayed in the transmission electron microscopy (TEM) images (Figure 1a and Figure S1a), the as-synthesized CsPbBr3 NC crystallized into cubic shape and possessed an average size of 18 nm. The powder Xray diffraction (PXRD) patterns of CsPbBr3 NC (Figure 2) demonstrated that all peaks were assigned to the orthorhombic phase (ICSD 97852). As for encapsulating CsPbBr3 NC with different coating layers (SiO2, TiO2, SnO2), the corresponding precursor (tetraethyl orthosilicate as Si precursor, tetrabutyl titanate as Ti precursor and tin isopropoxide as Sn precursor) was added into the CsPbBr3 NC colloidal solution and stirring at 70 oC in air for 30 mins. With the assistance


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of the moisture in air and hexane, the Si/Ti/Sn precursors adsorbed on the surface of CsPbBr3 NCs in situ hydrolyzed into the corresponding oxide (SiO2/TiO2/SnO2) to fulfill the encapsulation process. A mild temperature was set at 70 oC to accelerate the hydrolysis rate of these oxide precursors for the circumvent of the degradation of CsPbBr3 NCs during the long period exposure in air. Figure 1b-d showed a thin amorphous layer was successfully coated onto the surface of the CsPbBr3 NC with different thickness (SiO2: ~1 nm; TiO2: ~3 nm, SnO2: ~3 nm). As presented in Figure S1 b-d, the low magnification TEM images demonstrated that although the morphology of CsPbBr3 NC has transformed from cube to sphere after coating, the sizes have been commendably preserved. The XRD patterns of all the CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites displayed no apparent changes compared with that of pristine CsPbBr3 NC (Figure 2), indicating the phase integrity of CsPbBr3 phase was unspoiled during the encapsulation process. The characteristic diffraction peaks of SiO2/TiO2/SnO2 were failed to be figure out in the XRD pattern, plausibly due to the amorphous nature of the coating layers. To confirm the existence of MO2 (M=Si, Ti, Sn), the energy-dispersive X-ray (EDX) mappings of CsPbBr3 NC/MO2 composites were conducted, which revealed that the Si, Ti and Sn elements were homogeneously distributed on CsPbBr3 NC, respectively (Figure S2-4). The surface elemental analysis with X-ray photoelectron spectroscopy (XPS, Figure 3) indicated that the main peaks of Cs 3d, Pb 4f, Br 3d possessed the consistent binding energy for CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites samples, suggesting that the chemical state of CsPbBr3 NC remained unspoiled and the Si, Ti, Sn elements were not doped into the CsPbBr3 NC. However, the high-resolution scans for corresponding CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites showed the peak (103 eV) can be assigned to the Si (IV) in SiO2,32 and the two peaks (459.6 eV and 465.4 eV) can be attributed to the Ti (IV) to


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TiO2,33 whereas the peak (487 eV) was originated from the Sn (IV) in SnO2,34 confirming the formation of SiO2, TiO2 and SnO2 onto the CsPbBr3 NC (Figure S5).

Figure 2. XRD spectra of CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites.

Furthermore, the Fourier Transform Infrared Spectroscopy (FTIR) has been measured and the result was displayed in Figure S6. Nearly identical signals were observed in CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites by comparing with that of pristine CsPbBr3 NC. Furthermore, no signal that can be indexed to the precursor was found in the FTIR spectra, which indicated the twice rinsing procedure by hexane thoroughly removed the residue un-reacted precursors. Therefore, based on the above results of TEM, XRD, element mapping and XPS, it can be concluded that amorphous SiO2, TiO2 and SnO2 layers have been successfully coated on the surface of CsPbBr3 NC respectively through the mild solution phase process.


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Figure 3. XPS spectra of CsPbBr3 NC, CsPbBr3 NC/SiO2, CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2. Optical properties of CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites were investigated by UV-vis absorption and PL spectra. As shown in Figure 4a, a sharp absorption band edge of 520 nm and a PL emission peak at 525 nm were featured for CsPbBr3 NC, which were maintained the same after coating with SiO2 and TiO2 layers. However, both the absorption band gap and the PL emission peak of CsPbBr3 NC/SnO2 composite were significantly blue-shifted compared with that of pristine CsPbBr3 NC owing to the formation of CsPbBr3-xClx. As depicted in Figure S5, the characteristic peak of Cl was found in CsPbBr3 NC/SnO2, whereas undetected in the CsPbBr3 NC. The PL peak position of CsPbBr3 NC was not changed after adding SnBr4 (Figure S7), which consequently demonstrated the Sn4+ was innocent for such phenomenon.


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Figure 4. (a) UV-vis, (c) PL and (d) time-resolved PL spectra of CsPbBr3 NC, CsPbBr3 NC/SiO2, CsPbBr3 NC/TiO2 and CsPbBr3 NC/SnO2 solutions. (b) Photography of CsPbBr3 NC, CsPbBr3 NC/SiO2, CsPbBr3 NC/TiO2, CsPbBr3-xClx NC/SnO2 solutions under daylight (up) and UV light (bottom, λ=365 nm). Furthermore, the PL peak of CsPbBr3 NC colloidal solution was continuously blue shifted when adjusting the amount of Sn precursor (Figure S8) due to the increasing Cl doping in the CsPbBr3xClx.

The ion chromatography measurements (Figure S9) verified that chlorine existed in the

commercial reagent Tin (IV) isopropoxide, which resulted in the formation of CsPbBr3-xClx and consequently led to the blue shift in optical spectra. Although the PL emission peak position of CsPbBr3 NC/SiO2 and CsPbBr3 NC/TiO2 remained unchanged compared with uncoated CsPbBr3 NC while their emission intensity was greatly altered after coating. As presented in Figure 4b-c,


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the PL intensity was obviously enhanced by a factor of 1.7 for CsPbBr3 NC/SiO2 compared to CsPbBr3 NC. In contrast, the PL emission intensity of CsPbBr3 NC/TiO2 was sharply quenched to 30% of the CsPbBr3 NC and weak emission was also witnessed in CsPbBr3-xClx NC/SnO2. Meanwhile, the photoluminescence quantum yield (PLQY) of CsPbBr3 NC (37%) has been remarkably promoted to 64% after coating with SiO2 layer suggesting an enhanced radiative recombination, while it was reversely decreased dramatically to 7% and 3% when coated by TiO2 and SnO2, respectively (Table S1), indicating the increased consumption of photoinduced charge carriers via a new nonradiative pathway. Quantitative understanding of the carrier dynamics was further provided by the time resolved PL (TRPL) spectra. Figure 4d clearly presented an accelerated PL decay for CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2 composites while a retarded decay for CsPbBr3 NC/SiO2 compared with pristine CsPbBr3 NC. The PL decay curves were fitted with a model of two exponential functions and the resulting parameters were listed in Table S2. The major difference between CsPbBr3 NC and CsPbBr3 NC/SiO2 composite in PL lifetime lay in the slow time component τ2 mainly related to the radiative recombination process,20 which was significantly prolonged and its contribution (A2) to the average lifetime was also vastly increased after coating with SiO 2. Contrariwise, in the case of CsPbBr3 NC/TiO2 and CsPbBr3 NC/SnO2, the fast time component τ1 mainly dominated by the charge transfer process was shortened. Meanwhile its contribution A1 was increased from 17% (CsPbBr3 NC) to 25% (CsPbBr3 NC/TiO2) and further increased to as high as 60% (CsPbBr3-xClx NC/SnO2), suggesting favorable charge transfer from CsPbBr3 NC to SnO2 and TiO2. To investigate the impact of Cl doping on the PL lifetime of CsPbBr3 NC, the nanocrystals with the same emission wavelength as occurred between CsPbBr3 NC with TiO2 and SnO2. It is noteworthy that the average lifetime of CsPbBr3-xClx NC/SnO2 composite was much


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shorter than that of CsPbBr3-xClx indicating the accelerated decay was originated from the SnO2 coating rather than Cl doping. The PL decay was characterized as presented in Figure S10, with a fitted average lifetime of 47 ns, slightly lower than that of CsPbBr3 NC (53 ns), indicating that the sharply reduced PL lifetime for CsPbBr3-xClx NC/SnO2 (13 ns) should be attributed to the charge transfer rather than Cl doping. Accordingly, the radiative and nonradiative recombination rate have been calculated based on the PLQY and PL decay lifetime to illustrate the effects of different MO2 (M=Si, Ti, Sn) on the charge carrier dynamics of CsPbBr3 NC. As listed in Table S2, the radiative recombination rate κrad of CsPbBr3 NC (6.98·106 s-1) has been improved by a factor of 1.4 after modifying with SiO2 coating layer, consisting with the enhanced PL emission intensity and PLQY. However, the nonradiative recombination rate κnon of CsPbBr3 NC was dramatically boosted by coating with SnO2 (7.74·107 s-1), suggesting that the electrons can be efficiently transferred from CsPbBr3 NC to TiO2 and SnO2. Moreover, in the case of CsPbBr3 NC/TiO2, CsPbBr3-xClx NC/SnO2, the nonradiative recombination rate is an order of magnitude higher than the radiative recombination rate indicating the vast majority of the photoinduced electrons are more inclined to be extracted out from CsPbBr3 NC rather than recombine directly with holes via emitting photons. Femtosecond (fs) TA spectroscopy characterizations were further carried out to provide supplementary analysis on the process such as ultrafast charge transfer and hot carrier relaxation which were unable to be captured on the aforementioned TRPL due to the restricted time resolution.35,36 A pump pulse of 400 nm was used to solely excite the CsPbBr3 NC, while a white light continuum probe pulse was employed to record the induced absorption changes (ΔA) as functions of both wavelengths and delay time. The representative TA spectra plots of CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites at different delay time were depicted in Figure 5. During the initial time delay, the TA spectra were dominated with a positive photoinduced


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absorption (PIA) peak at 530 nm, which was vanished rapidly. Simultaneously, the ground state bleaching (GSB) signal located at 510 nm became increasingly prominent owing to the state filling.37 In accordance with the UV-vis spectra, the GSB and PIA signal peak of CsPbBr3 NC/SnO2 were correspondingly blue shifted on account of the Cl doping. The carrier density on the bandgap can be rationally interpreted by the ΔA intensity at the GSB signal. Specifically, comparing with the CsPbBr3 NC, the GSB peak intensity at the same delay time was largely decreased after coated with SnO2 and TiO2, while enhanced after coating with SiO2, which coincided with the aforementioned PL results in the sense that SnO2 and TiO2 can efficaciously lower down the charge density by extracting electrons from the CsPbBr3 NC, while SiO2 can passivate the surface states of the CsPbBr3 NC to eliminate the trap fillings. The decay kinetics of the GSB were fitted with a three-exponential parameters model, and the parameters were summarized in Table S3. Similar to the results derived from the TRPL measurement, the average decay lifetime (τaverage) of CsPbBr3 NC/SiO2 (9240 ps) was much longer than that of CsPbBr3 NC (7930 ps) while the τaverage of CsPbBr3 NC/TiO2 (5545 ps) and CsPbBr3-xClx NC/SnO2 (3740 ps) were much shorter than the uncoated CsPbBr3 NC. To further elucidate the charge transfer kinetics between CsPbBr3 NC with SnO2 and TiO2, the electron transfer rate constant κet was estimated by comparing the bleaching recovery lifetimes in the presence and absence of charge transfer layers.38 According to the equation (1) κet=1/τ(CsPbBr3 NC/MO2) - 1/τ(CsPbBr3 NC)

(1)

the κet was calculated to be 5.4·107 s-1 for CsPbBr3 NC/TiO2 and 1.14·108 s-1 for CsPbBr3-xClx NC/SnO2, indicating the electron injection rate from CsPbBr3 NC to SnO2 was two times faster than to TiO2. It is of particular to note that the electron injection rate is similar to the above calculated nonradiative recombination rate based on PL, indicating that the electron transfer is the


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main nonradiative process for CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2. As illustrated by the Marcus theory,39 the driving force plays a critical role in the electron transfer rate.40,41 The driving force between CsPbBr3-xClx NC/SnO2 is 0.4 eV larger than that of CsPbBr3 NC/TiO2 (Figure S11), which stands a chance for a faster electron injection. Of particular note is that the Cl doping plays a negligible impact on the bottom of conduction band of CsPbBr3 NC,42 which thus would not alter the driving force between CsPbBr3 NC and SnO2. The PIA signal intensity decreased at the lower pump energy in the Figure S12 and the PIA decay time was nearly identical to the buildup of bandedge bleach (Figure S13), which validated the hot carrier cooling dynamics could be investigated by supervising the PIA signal.43 As presented in Figure 5f, the fitted hot carrier cooling time for CsPbBr3 NC was 482 fs, which was gradually decreased to 436 fs (CsPbBr3 NC/SiO2), 389 fs (CsPbBr3 NC/TiO2) and further to 277 fs (CsPbBr3-xClx NC/SnO2). As illustrated in Figure S10, when the hot carrier relaxed from the higher excited state to the conduction bottom band, it can be captured by the intraband surface trap states. For CsPbBr3 NC/SiO2 composite, accelerated hotcarrier relaxation process from the higher exited states to the CB band can effectually eliminate the charge trapping,43 facilitating the subsequent radiative recombination which accounted for the enhanced the PL intensity and PLQY. Accelerated hot carrier cooling time for CsPbBr 3 NC/TiO2 and CsPbBr3-xClx NC/SnO2 indicate the photogenerated hot carriers probably can dissociate quickly and further transfer to the SnO2 and TiO2.


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Figure 5. Femtosecond transient absorption (fs-TA) spectroscopy under 400 nm pump pulse of (a) CsPbBr3 NC, (b) CsPbBr3 NC/SiO2, (c) CsPbBr3 NC/TiO2 and (d) CsPbBr3-xClx NC/SnO2. And the corresponding TA dynamics probed at the position of (e) GSB and (f) PIA features. CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites were further deposited on FTO glass by centrifugation cast as photoelectrodes for photoelectrochemical measurements. As shown in Figure S14, the as-prepared thin films were compact and possessed a thickness of ~2 μm. The


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I-t curve was recorded under a bias of -0.4 VAg/AgCl and an alternative light irradiation (150 mW cm-2). The incident light was provided by a Xeon lamp, with the UV region being cut off by a 420 nm filter to avoid the stimulation of the glasses and the coating shell. Under visible light irradiation, photo-induced electron-hole pairs were immediately generated in CsPbBr3 NC and subsequently separated due to the external bias while the electrons would migrate to the surface of the photoelectrode. As depicted in Figure 6a, the photocurrent density of the pristine CsPbBr3 NC electrode was 21 μA cm-2, which decreased to 17 μA cm-2 after coating of SiO2 whereas boosted up to 41 μA cm-2 and 52 μA cm-2 for CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2, respectively. The increased photocurrent density for CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2 composite compared to CsPbBr3 NC further elucidated that the dramatically shortened PL and TA lifetime after coating with TiO2 and SnO2 shells were inclined to be originated from the enhanced electron extraction rather than the inducing of trap states. To interpret the different photocurrent densities from I-t curves, the photoelectrochemical impedance spectra (PEIS) were further recorded. As shown in Figure 6b, compared to the pristine CsPbBr3 NC, the arcs of CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2 were significantly decreased, indicating more favorable charge injection from the photoelectrodes into the electrolyte.44 Moreover, the arc of CsPbBr3-xClx NC/SnO2 was smaller than that of CsPbBr3 NC/TiO2 which might be attributed to the high electron mobility and low conduction band level for SnO2.34 Hence, the PEIS accompanied with the TRPL and TA tests reasonably demonstrate the charge transfer ability and photocurrent density variations of different coating layers, which was schematically illustrated in Figure 6c.


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Figure 6. Photoelectrochemical characterizations. (a) amperometric I-t curves recorded at bias of -0.4 VAg/AgCl; (b) photoelectrochemical impedance spectroscopy (PEIS) plots recorded at bias of -0.4 VAg/AgCl under visible light (>420 nm, 150 mW cm-2), (c) schematic illustration of the effect of different coating materials on the charge carrier dynamics of CsPbBr3 NC, (d) photostability of different thin films (CsPbBr3 NC, CsPbBr3 NC/SiO2, CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2) under irradiation of 365 nm UV light. Modifying the surface of the CsPbBr3 NCs with another mechanically robust and airtight material has a positive impact on the stability. To test the protective impact of the coating shells on CsPbBr3 NC, the photostability test were measured by exposing the CsPbBr3 NC thin film and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composite thin films fabricated by centrifugation cast to UV light (365 nm) for 10 hours, during which time the PL spectra were monitored as a function of irradiation time (Figure S15). As presented in Figure 6d, an obvious PL intensity degradation was observed for pristine CsPbBr3 NC films, probably due to the surface decomposition or aggregation


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of CsPbBr3 NCs, which was further corroborated the TEM images presented in Figure S16. As a comparison, the CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites were more insensitive to the illumination. To be more specific, the CsPbBr3 NC/MO2 (SiO2, TiO2, SnO2) maintained 60%, 82%, 94% of the original PL peak intensity after 10 h aging, respectively. Note that such superior resistance for CsPbBr3 NC/TiO2 and CsPbBr3-xClx NC/SnO2 toward UV light exposure probably due to thicker shell than SiO2. To highlight the superior protective advantage of the SnO2 coating, moisture stability was conducted by comparing CsPbBr3-xClx NC/SnO2 and pristine CsPbBr3 NC thin film under high humid air (humidity of 70%) for 6 days. As shown in Figure S17, the PL peak intensity of uncoated CsPbBr3 NC declined to 50% of its original intensity while the PL intensity remained about 70% after coated with SnO2. Moisture stability test was also performed on the control sample CsPbBr3-xClx NC (Figure S18) and a similar resistance to the humid air compared with CsPbBr3 NC, which highly emphasized the admirable protective effect inorganic oxide shell toward harsh conditions like moisture and UV light exposure.

CONCLUSIONS In summary, the charge carrier dynamics at the interface of CsPbBr3 NC with different coating layers (SiO2, TiO2, SnO2) have been studied by the ultrafast transient absorption spectroscopy. Coating with insulating SiO2 accelerated the hot carrier cooling process and efficiently enhanced the PL intensity of CsPbBr3 NC by a factor of 1.7 owing to passivating the trap states, while the semiconductor coatings (TiO2 or SnO2) facilitated the charge separation and transfer from CsPbBr3 NC. It has been demonstrated that the electron injection rate from CsPbBr3 NC to SnO2 is two times larger than to TiO2, which contributed to 2.5-fold and 1.3-fold enhancement in photocurrent responses for CsPbBr3-xClx NC/SnO2 (52 μA cm-2) than that of CsPbBr3 NC (21 μA cm-2) and CsPbBr3 NC/TiO2 (41 μA cm-2), respectively. These results offer valuable basic understandings


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and new insights for the appropriate choice of coating layers to stabilize perovskite NC and tune the photoelectric properties for the further applications in the cutting-edge optoelectronic field.

EXPERIMENTAL SECTION Chemicals and Materials: Lead bromide (PbBr2, 99.999%), lead chloride (PbCl2, 99.999%), oleic acid (OA), 1-octadecene (ODE, 90%), tetraethyl orthosilicate (TEOS, 99.999%) were purchased from Sigma-Aldrich. Oleylamine (OAm, 80-90%) and Tin (IV) isopropoxide (99% metal basis, 10% w/v in isopropanol) were purchased from Alfa Aesar. Cesium carbonate (Cs2CO3, 99.99%), tetrabutyl titanate (TBOT, 99%) were purchased from Aladdin. Hexane was purchased from Guangzhou Chemical Reagent Factory. Preparation of CsPbBr3 Nanocrystal (CsPbBr3 NC): CsPbBr3 NC was synthesized through a typical hot-injection method. A Cs-oleate precursor was prepared firstly by mixing 0.814 g of Cs2CO3, 2.5 mL of OA and 40 mL of ODE into a 100 mL 3-neck flask, following by dried at 120 o

C for 1h, and then the temperature was set to 150 oC until Cs2CO3 was totally dissolved. The

Cs(oleate) solution was stored at room temperature and was preheated to 150 oC just before use in the synthesis of CsPbBr3 NC. For the preparation of CsPbBr3 NC, 138 mg of PbBr2 and 10 mL of ODE were loaded into a 50 mL 3-neck flask and dried at 120 oC for 1 h before 1 mL OA and 1 mL OAm injected. The temperature was raised to 180 oC under N2 flow and 1 mL Cs-oleate precursor solution was swiftly injected. The reaction crude was quickly cooled down by an ice water bath. As-synthesized CsPbBr3 NC was obtained by centrifugation at 12000 rpm and then dispersed in 10 mL of hexane for further use without additional washing process. The CsPbBr3xClx NC

was synthesized by anion exchange reaction. The PbCl2 anion stock solution was prepared

by loading PbCl2, OA, OAm and ODE into a three-neck flask and heated to 120 oC with


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trioctylphosphine added, which was further introduced into the CsPbBr3 NC hexane solution under vigorous stirring. Preparation of CsPbBr3 NC/MO2 (M=Si, Ti, Sn) Composites: TBOT and TEOS were diluted into hexane as precursors with a concentration of 0.28 mol/L. To fabricate the CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composites, 300 μL of TBOT, TEOS precursor solution and Tin (IV) isopropoxide precursor were added into CsPbBr3 NC hexane colloidal solution (3 mL) in a sealed vial respectively, and then continuously stirred at 70 oC for 30 min. Subsequently, the precipitates were collected by centrifugation at 10000 rpm and washed with hexane twice before dispersing into hexane. For a fair comparison, the control sample pristine CsPbBr3 NC colloidal solution was also treated at 70 oC stirring. Fabrication of CsPbBr3 Thin Film: The CsPbBr3 NC thin film and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) thin films were fabricated by centrifugally-cast process. The FTO substrates were sonicated sequentially in deionized water, alcohol and acetone for 10 min subsequently. The FTO glasses were sliced into the size of approximately 1 cm x 3 cm and placed inside a 10 mL centrifuge tube with the conduction side facing down before the centrifugally-cast process. CsPbBr3 NC or CsPbBr3 NC/MO2 (M=Si, Ti, Sn) hexane suspension was added into the centrifuge tube and then centrifuged at 6000 rpm for 5 min to deposit the CsPbBr3 NC or CsPbBr3 NC/MO2 (M=Si, Ti, Sn) onto the FTO glasses. The as-prepared thin films were dried at 70 oC for 10 min before the PEIS measurements and the stability test. The thickness of the films applied to the photoelectrochemical characterization and stability tests is about 2 μm. Characterizations: Powder X-ray diffraction (XRD) was characterized on a Rigaku MiniFlex 600 using Cu Kα radiation (λ=1.5418 Å). The absorption spectra were tested by Shimazu UV-3600. Nanostructure was investigated on TEM and HRTEM (HAADF-STEM, Tecnai G2 F30). X-ray


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photoelectron spectra (XPS) were investigated on a photoelectron spectrometer (ESCALAB 250, Thermo Fisher Scientific). Edinburgh Instruments LTD (FLSP980) was used to test steady-state photoluminescence (PL, excitation at 400 nm) and time-resolved photoluminescence (TRPL) spectra were recorded on the same instrument under an excitation laser at 406 nm with the timecorrelated single-photoncounting (TCSPC) mode. Transient Absorption Spectroscopy (TAS) Characterizations: Transient absorption measurements were detected by equipping a regeneratively amplified Ti: sapphire laser source (Coherent Legend, 800 nm, 150 fs, 5 mJ/pulse, and 1 kHz repetition rate) and Helios (Ultrafast Systems LLC) spectrometers. Portion of the 800 nm output (75%) pulse was frequency-doubled in a BaB2O4 (BBO) crystal, which could generate 400 nm pump light, meanwhile the remaining portion of the output was concentrated into a sapphire window to produce white light continuum (420 nm-780 nm) probe light. The 400 nm pump beam was formed from part of the 800 nm output pulse from the amplifier and the power of it was adjusted by a range of neutral-density filters. The pump beam was focused at the sample with a beam waist of about ~360 µm and the power intensity of was fixed at 14 µJ/cm2 in this experiment. With the aid of the mechanical chopper, the pump repetition frequency was synchronized to 500 Hz. The probe and reference beams could be split from the white light continuum and sent into a fiber optics-coupled multichannel spectrometer by complementary metal-oxide-semiconductor sensors with a frequency of 1 kHz. Photoelectrochemical Characterization: A Zennium electrochemical workstation was equipped to perform the photoelectrochemical studies. The 3-electron configuration was adopted, where the prepared electrode (CsPbBr3 NC or CsPbBr3 NC/SiO2, CsPbBr3 NC/TiO2, CsPbBr3 NC/SnO2) served as the working electrode, the Pt mesh as the counter electrode, and the reference electrode was the Ag/AgCl. The electrolyte solution was prepared by dissolving tetrabutylammonium


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hexafluorophosphate (TBAPF6) in dichloromethane (0.1 M). The incident light irradiation (150 mW cm-2) was provided by a Xeon lamp, with the UV region being cut off by a 420 nm filter to avoid stimulation of the glass substrate and the coating shells. The I-t and PEIS curves were recorded under a bias of -0.4 VAg/AgCl. Stability Test: As for the UV light stability test, the as-prepared CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn) composite thin films assembled by centrifugally-cast process were exposed to the 365 nm light in air and the PL intensity of these thin films were monitored as a function of exposure time. In order to evaluate the humidity stability, the as-prepared CsPbBr3 NC and CsPbBr3 NC/SnO2 composite thin films were placed in a closed box with a humidity about 70% controlled by saturated Mg(NO3)2 solution and the PL intensity was monitored as a function of exposure time.

ASSOCIATED CONTENT Supporting Information: The EDX-mapping, XPS high-resolution plots, cross-sectional SEM images of the centrifugally-casted films of the CsPbBr3 NC and CsPbBr3 NC/MO2 (M=Si, Ti, Sn). The time-resolved photoluminescence decay parameters, and TA decay parameters of CsPbBr3 NC and CsPbBr3 NC/MO2 composite. The PLQY data of CsPbBr3 NC and CsPbBr3 NC/MO2 composite.

AUTHOR INFORMATION Corresponding Author: *Email: [email protected] ACKNOWLEDGMENT


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MO2 (M=Si, Ti, Sn) Composites: Insight into

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