Fabricating CsPbX3-Based Type I and Type II Heterostructures by

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Fabricating CsPbX3-Based Type I and Type II Heterostructures by Tuning the Halide Composition of Janus CsPbX3/ZrO2 Nanocrystals Haiyu Liu, Yeshu Tan, Muhan Cao, Huicheng Hu, Linzhong Wu, Xiaoya Yu, Lu Wang, Baoquan Sun, and Qiao Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00001 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Fabricating CsPbX3-Based Type I and Type II Heterostructures Composition

by of

Tuning Janus

the

Halide

CsPbX3/ZrO2

Nanocrystals Haiyu Liu, Yeshu Tan, Muhan Cao,* Huicheng Hu, Linzhong Wu, Xiaoya Yu, Lu Wang, Baoquan Sun,* Qiao Zhang* Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, People’s Republic of China E-mail: [email protected] (M.C.); [email protected] (B.S.); [email protected] (Q.Z.) KEYWORDS: CsPbX3 nanocrystals, ZrO2, type I composite, type II composite, LED ABSTRACT: Fabricating CsPbX3-based heterostructure has proven to be a feasible way to tune their photophysical properties. Here, we report the successful fabrication of Janus CsPbX3/ZrO2 heterostructure nanocrystals (NCs), in which each CsPbX3 NC is partially covered by ZrO2. According to the band alignment, CsPbBr3/ZrO2 and 1 ACS Paragon Plus Environment

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CsPbI3/ZrO2 can be indexed as type I and type II composites, respectively. The type I composites display great enhancement in photoluminescence quantum yield (PLQY, from 63% to 90%) and PL lifetime (from 12.9 ns to 66.1 ns) because of the charge carrier confinement and passivation effect provided by ZrO2. In contrast, the Type II composites can be used in photocatalytic reduction of CO2 because electrons and holes are effectively separated and accumulated in ZrO2 and CsPbI3, respectively, under irradiation. Janus CsPbBr3/ZrO2 NCs showed much higher stability than pristine CsPbBr3 against polar solvents treatment. A stable and highly efficient light emitting device with luminous efficiency up to 55 lm W-1 is fabricated by using CsPbBr3/ZrO2 NCs as the green light source. This work may not only enrich the family of surface passivated perovskite materials, but also provide a good example for the rational design of specific composite in the metal halide perovskite field.

Over the past several years, we have witnessed an explosive development in the field of all-inorganic cesium lead halide perovskite nanocrystals (CsPbX3 NCs). Since the first report by Kovalenko group in 2015,1 significant advances regarding the preparation methodologies,2,3 morphology and composition control,4-8 property regulation9-11 and potential applications12 of CsPbX3 NCs has been achieved. Until now, CsPbX3

NCs

with

excellent

photophysical

properties,

including

high

photoluminescence quantum yield (PLQY), narrow emission bandwidth and widely tunable emission spectra, have been successfully obtained.13 However, the development of CsPbX3 NCs is still in its infancy stage. Many challenges should be solved before it can be applied in practical fields, such as their unsatisfactory efficiency and poor stability.

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Fabrication of heterostructure has proven to be a feasible method to engineer the surface properties of CsPbX3 NCs. In addition to polymers14-21 and inert shells,9,22-26 semiconductors have also been introduced to improve the stability and photophysical properties. More importantly, by using another semiconductor with different conduction and valence bands, one can easily tune the energy or charge transfer process, resulting in improved photophysical properties. Based on the different band positions, one can fabricate three different types of semiconductor composites, named as type I, in which the conduction band (CB) and the valence band (VB) of semiconductor A are respectively higher and lower than the corresponding bands of semiconductor B; type II, in which both CB and VB levels of semiconductor A are higher than the corresponding levels of semiconductor B; type III is similar to that of the type II heterojunction except that the staggered gaps do not overlap.27 However, most of previous reports are focused on CsPbX3-based type II composite. For instance, composite CsPbBr3/MoS2 have been fabricated,28 in which electrons can transfer from the conduction band of CsPbBr3 to that of MoS2 while holes still localize in the valence band of CsPbX3. Similar type II heterojunctions have also been reported in the CsPbBr3/TiO229 and CsPbBr3/ZnS30 composites, in which the decreased PL lifetime suggested an improved charge diffusion and separation, leading to enhanced photovoltaic performance. In comparison, type I band alignment usually favors the carrier confinement in the semiconductor with narrower bandgap, resulting in enhanced radiative recombination. To date, only two type I structures of CsPbBr3/Cs4PbBr6 and CsPbBr3/Rb4PbBr6 system with enhanced PLQY were reported and were used for LEDs with superior performance.31-34 It should be noted that most of the reported hybrid systems are in the multiple-particle level, which is not ideal for film formation in LEDs.


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However, it is quite difficult to achieve single-particle-level fabrication with designed bandgap alignment. Here, we reported the successful fabrication of both type I and type II composite CsPbX3/ZrO2 heterostructure on a single particle level. Based on the band alignment, CsPbBr3/ZrO2 NCs could be assigned as type I composite, while CsPbI3/ZrO2 NCs could be indexed as type II composites. The type I composite demonstrated extraordinarily longer PL lifetime (5-fold longer), as well as higher PLQY than plain CsPbBr3 NCs, which can be attributed to the carrier confinement as well as surface passivation effect provided by ZrO2. In contrast, the type II composite (CsPbI3/ZrO2) showed poorer photoluminescence performance than neat CsPbI3 NCs because the electrons were transferred to ZrO2. But they can be used in photocatalysis because of the efficient charge separation. Furthermore, the partial coverage of ZrO2 dramatically improved the stability of CsPbBr3 NCs. The application of Janus CsPbBr3/ZrO2 NCs in LED has been demonstrated, which showed improved performance and stability. RESULTS AND DISCUSSION Janus CsPbBr3/ZrO2 composite NCs were obtained by using a modified interfacial approach developed recently by our group.35 Monodisperse Cs4PbBr6 NCs were first synthesized through a hot-injection method (TEM images were shown in Figure S1). Then, Cs4PbBr6 NCs were mixed with Zr(OC4H9)4 in hexane, followed by the rapid injection of water under vigorous vortex for 5 min. The system was kept undisturbed for 12 h. According to the amount of Zr precursor used, the samples were denoted as CsPbBr3/ZrO2-N (N = 2.5~20, see details in Methods). As a typical example, CsPbBr3/ZrO2-5 was characterized by transmission electron microscopy (TEM). As shown in Figure 1a, monodisperse Janus structures composed of a condensed particle and a seemingly hollow part was observed. High-resolution TEM (HRTEM) image 4 ACS Paragon Plus Environment

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(inset in Figure 1a) revealed a clear lattice spacing of 0.58 nm, which is in good agreement with the (110) planes of orthorhombic phase of CsPbBr3 (ICSD #01-0727929). No clear lattice spacing was found in the light part, which was probably ascribed to the relatively poor crystallinity of ZrO2 formed at room temperature. To investigate the elemental distribution, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 1b) and energy-dispersive spectroscopy (EDS) mapping (Figure 1c−g) were carried out, which further confirmed the presence of Janus structure.

Figure 1. (a) TEM image of CsPbBr3/ZrO2-5. The inset shows the HRTEM image of a single nanoparticle. (b) HAADF-STEM image and (c-g) elemental mapping images 5 ACS Paragon Plus Environment

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showing the elemental distribution. All scale bars in (b-g) are 10 nm. (h) XRD patterns of ZrO2 and CsPbBr3/ZrO2 NCs matching with orthorhombic CsPbBr3 (ICSD #01-0727929) and ZrO2 (JCPDS #50-1089). (i) UV-vis absorption spectra of pristine Cs4PbBr6 NCs (black solid line) and CsPbBr3/ZrO2-5 (green solid line), as well as PL spectrum of CsPbBr3/ZrO2 at  = 365 nm (green dash line), inset showing the CsPbBr3/ZrO2 solution under UV light irradiation ( = 365 nm). X-ray diffraction (XRD) characterization was employed to characterize the crystal structure of the samples. Pure ZrO2 powder was prepared through the same method and its XRD pattern was plotted in Figure 1h. Two broad peaks at ca. 30.5° and 50.5° are observed in ZrO2, which is ascribed to the low crystallinity (Figure S2).36-38 Zr precursor played a significant role in the formation of Janus heterostructure (Figure S2). For CsPbBr3/ZrO2-5, majority of peaks are indexed as orthorhombic phase of CsPbBr3 (ICSD No. 01-072-7929), while ZrO2 could be observed as well. The as-prepared CsPbBr3/ZrO2 solution exhibited bright green emission under UV light irradiation, as shown in the inset of Figure 1i. The change of optical properties has also been monitored during the reaction process, as shown in Figure 1i. For the original Cs4PbBr6 solution, two absorption peaks at 230 nm and 314 nm can be clearly observed, while no PL emission has been observed in the range of 200-1000 nm. After the reaction, these two absorption peaks gradually declined, while a new absorption peak at 507 nm appeared, suggesting the complete transformation from Cs4PbBr6 to CsPbBr3. Accompanied with the variation of absorption spectra, a sharp PL peak at 514 nm with full width at half maximum (FWHM) of 14 nm emerged. The narrow FWHM indicated excellent homogeneity of CsPbBr3 component in the as-obtained samples.


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Figure 2. (a) Photographs of Janus CsPbBr3/ZrO2 NCs with different amounts of Zr(OC4H9)4 precursor under UV light ( = 365 nm). From left to right: CsPbBr3, CsPbBr3/ZrO2-2.5, -5, -7.5, -10, -15 and -20. The corresponding (b) PL lifetime curves and (c) PL intensity versus the amount of Zr precursor ( = 365 nm). (d) PL lifetime and quantum yield of CsPbBr3/ZrO2 with different amounts of Zr(OC4H9)4 precursor. Table 1. PL lifetime for Janus CsPbBr3/ZrO2 NCs with different amounts of Zr(OC4H9)4 precursor. The PL lifetime is fitted by a biexponential decay function. Samples

A1

A2

1 (ns)

2 (ns)

χ2

Avg (ns)

CsPbBr3

0.1

0.9

40.6

10.6

1.16

12.9

CsPbBr3/ZrO2-2.5

0.15

0.85

65.7

15.0

1.00

22.6

CsPbBr3/ZrO2-5

0.26

0.74

96.5

25.9

1.09

45.0

CsPbBr3/ZrO2-7.5

0.36

0.64

99.0

26.4

1.04

52.5

CsPbBr3/ZrO2-10

0.46

0.54

104.3

34.3

1.15

66.1

CsPbBr3/ZrO2-15

0.59

0.41

26.4

94.4

1.07

56.6

CsPbBr3/ZrO2-20

0.71

0.29

19.5

75.4

1.14

35.6

To get insight into the reaction process, the time-dependent morphology evolution process of CsPbBr3/ZrO2 was monitored by TEM, as shown in Figure S3. During the 7 ACS Paragon Plus Environment

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process, the spherical Cs4PbBr6 NCs turned into cube-like particles (Figure S3a) along with the enlargement of ZrO2 on one side of the cube (Figure S3b-d). With the growth of ZrO2, the PL lifetime of the corresponding product reached the maximum at 12 h, as shown in Figure S4. As a result, the reaction time was fixed at 12 h for all samples. CsPbBr3/ZrO2 composites with different amount of ZrO2 were prepared by tuning the amount of Zr precursor while the other parameters were kept constant. In the absence of Zr(OC4H9)4, neat CsPbBr3 nanocubes were obtained (Figure S5a). As shown in Figure S5b-d, the size of ZrO2 increased with the increased amount of Zr(OC4H9)4. However, free ZrO2 formed and the CsPbBr3 turned into particles with nonuniform size and shape when the amount of Zr precursor exceeded 10 mol (Figure S5e, f). The incorporation of ZrO2 would significantly affect the optical properties of the products. From Figure 2a and S6, it can be found that the brightness of the green emitting solution varied with the amount of ZrO2 under both daylight and UV light irradiation. PL intensity of various samples was shown in Figure 2c. From the results, the highest PL intensity was obtained for CsPbBr3/ZrO2-10, which exhibited a 3-fold increasement compared to pristine CsPbBr3, indicating the significant improvement in PLQY. As expected, the corresponding maximum PLQY could reach up to 90% for CsPbBr3/ZrO2-10 while that of the naked ones was only 63% (Figure 2d). The PL lifetime measurements were also carried out on these samples, as shown in Table 1 and Figure 2b. Similar to the PL intensity and PLQY, an obvious volcano trend appeared, in which the longest PL lifetime was 66.1 ns for CsPbBr3/ZrO2-10. Further increase in ZrO2 amount resulted in a less uniform size distribution of CsPbBr3 because excess Zr precursor would generate undesirable hydrolytic byproducts that might deteriorate the structure of CsPbBr3. From the above data and discussion, it can be speculated that longer lifetime in comparison with pristine CsPbBr3 was caused by the incorporation 8 ACS Paragon Plus Environment

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of ZrO2, which could act as the surface passivator for CsPbBr3. The other possibility is that wide-bandgap ZrO2 might confined more carriers into CsPbBr3 region, reducing the probability of electron leakage, as the typical type I composite CdSe/ZnS39-41 quantum dots did. Table S1 summarized some previously reported composites based on CsPbX3 NCs. Type I composite (CsPbBr3/Cs4PbBr6) exhibited prolonged PL lifetime while type II composite (e.g., CsPbBr3/TiO2, CsPbBrxI3-x/ZnS and CsPbBr3/MoS2) displayed reduced one. The opposite PL lifetime trend was likely ascribed to different charge transfer processes, suggesting the direct and strong relationship between the optical properties and heterostructures. In addition, the CsPbBr3 NCs embedded in SiO2 and Al2O3, which were inert oxides revealing the existence of simple passivation effect, did not show a significant improvement in PL lifetime as CsPbBr3/ZrO2 did.9,23,42,43 It should be pointed out that the Janus CsPbBr3/SiO2 composites in our previous report can promote the PL lifetime of naked CsPbBr3 from 11.6 ns to 19.8 ns due to the surface passivation provided by SiO2.35 By contrast, this similar Janus CsPbBr3/ZrO2 nanostructure showed a much longer lifetime (66.1 ns), which might be ascribed to the passivation effect as well as the strongly carriers confinement determined by ZrO2.


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Figure 3. (a-b) UPS and (c) absorbance spectra of ZrO2 obtained from the same approach. The inset in (c) shows the Kubelka-Munk equation applied to the absorbance spectrum. (d) Scheme of the energy band alignment of CsPbBr3/ZrO2 composite NCs. In order to confirm our hypothesis, the electronic band structure of ZrO2 and CsPbBr3 was investigated by ultraviolet photoelectron spectroscopy (UPS) and UV-vis absorption spectroscopy. Figure 3a-b show the UPS spectra of the ZrO2 sample, in which the valence band maximum position was determined to be -8.40 eV with respect to the vacuum level. From the UV-vis absorption spectrum (Figure 3c), ZrO2 powder showed an obvious absorbance peak with a sharp onset in the ultraviolet range. The light absorption coefficient (F(α)) of ZrO2 was calculated from the Kubelka-Munk equation (inset of Figure 3c). The relationship between the incident photon energy (hν) and (F(α)hν)1/2 suggested the optical bandgap energy was 5.00 eV. Combining the bandgap value with the valence band maximum position, the conduction band minimum position was determined to be -3.40 eV with respect to the vacuum level, which was highly consistent with previous reports.44 By using the same approach, the 10 ACS Paragon Plus Environment

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valence band maximum and conduction band minimum position of CsPbBr3 were determined to be -6.02 and -3.67 eV, respectively, with respect to the vacuum level, as shown in Figure S7 and S8. Based on the energy level of CsPbBr3 and ZrO2, an energy band structure diagram was proposed, as shown in Figure 3d. From the band alignment, it is found that a type I composite was formed. Once irradiated with light ( = 365 nm), electrons from the valence band of CsPbBr3 were excited to its conduction band. The higher-lying conduction band minimum and lower valence band maximum values of ZrO2 offered a desirable energy level matching with CsPbBr3, which could efficiently confine the carriers in the type I Janus islands. The collection of more excitons could offer further advantage by concentrating carrier density into recombination wells. As a result, more radiative recombination of electrons and holes occurred on the CsPbBr3, which was proposed to be the main reason for the enhancement in optical properties. In addition, similar to our previous work,35 the introduction of ZrO2 could also passivate CsPbBr3 to suppress the trap states on the surface. Consequently, accompanied with the formation of type I composite structure, passivation effect provided by ZrO2 to obtain stronger PL intensity, higher PLQY, as well as longer PL lifetime. Another possible function of ZrO2 is that it might act as the donor of excitons under specific condition. In a control experiment, exciton wavelength of 250 nm was employed. The result (Figure S9) showed that the PL intensity enhancement factor was ca. 6 at  = 250 nm, which was higher than ca. 4 at  = 365 nm. The difference of PL enhancement might be ascribed to the excited state in ZrO2. Under the irradiation of  = 250 nm, ZrO2 in our sample could be excited and carriers and energy might transfer into CsPbBr3, leading to a further increase in radiative recombination. However, CsPbBr3 solution and its based LED film looked undesirably dim under  = 250 nm; the PLQY was as 11 ACS Paragon Plus Environment

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low as 4% (0.6% for pristine CsPbBr3), because 250 nm was far from the optimal excitation wavelength (Figure S10).

Figure 4. TEM images of CsPbX3/ZrO2 composite with different ratios of halide ions. From left to right: (a) CsPbBr2Cl1/ZrO2, (b) CsPbBr2.5I0.5/ZrO2, (c) CsPbBr1.5I1.5/ZrO2, (d) CsPbBr0.5I2.5/ZrO2. (e) UV-Vis absorption and (f) PL spectra of heterostructure CsPbX3/ZrO2 NCs (X = Cl/Br, Br/I). Corresponding photographs under (g) daylight and (h) UV light ( = 365 nm). From left to right: CsPbBr2Cl1/ZrO2, CsPbBr2.5I0.5/ZrO2, CsPbBr1.5I1.5/ZrO2 and CsPbBr0.5I2.5/ZrO2 NCs. One of the most attractive properties of CsPbX3 is their tunable bandgap with different halide components, which has been confirmed by previous reports and our density functional theory (DFT) calculations, as shown in Figure S11 and Table S3. Here we show that CsPbX3/ZrO2 NCs with different halide compositions can be obtained by exposing Cs4PbX6 (X = Cl/Br, Br/I) NCs to water in the presence of Zr precursor. Their TEM images are shown in Figure 4a-d. The absence of CsPbCl3/ZrO2 were ascribed to in-adequation of Cs4PbCl6 for the growth of oxide.35 By controlling the ratio of halide elements, the optical properties including absorbance (Figure 4e), PL emission (Figure 4f) and lifetime (Table S2) of composite particles could be tuned 12 ACS Paragon Plus Environment

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across a wide range of color gamut (480~676 nm), as shown in Figure 4g-h. As expected, these CsPbX3/ZrO2 (X = Cl/Br, Br, Br/I) composites displayed enhanced PLQY compared with the original CsPbX3 NCs (Figure S12), suggesting the possible formation of type I composites fabricated from ZrO2 and CsPbX3 (X = Cl/Br, Br/I) NCs. As shown in Figure S7 and S8, UPS and UV-vis absorption spectra were measured for these NCs, and the energy levels were illustrated in Figure 5a. From the band structures of CsPbX3 (X = Cl/Br, Br/I) NCs, all of them could build up type I composite combined with ZrO2, except that CsPbI3 possessed higher-lying conduction band minimum as well as valence band maximum values compared with ZrO2, which turned out to be a type II rather than type I composite. Electrons efficiently accumulated to ZrO2, leading to separation of photoinduced carrier pairs (Figure 5b). Because of the type II structure, CsPbI3/ZrO2 composite (See Figure S13 for TEM and optical properties) showed only 40% of the PL intensity of original pristine CsPbI3 NCs (Figure 5c-d). In addition, the PLQY decreased from 30% to 16%, indicating the successful separation of charge carriers. As a result, the hybrid materials can be used as efficient photocatalyts for CO2 reduction under visible light irradiation.


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Figure 5. (a) Bandgap distribution of CsPbX3 NCs as well as ZrO2 in this system; (b) scheme of the bandgap distribution of CsPbI3/ZrO2 composite. Solution of pristine CsPbI3 (left) and Janus CsPbI3/ZrO2 (right) NCs dispersed in hexane under (c) daylight and (d) UV light ( = 365 nm). To investigate the colloidal stability of the as-prepared Janus perovskite NCs, CsPbBr3/ZrO2-10 was selected as the sample for water resistance test. Pristine CsPbBr3 NCs prepared from the same method were used as the reference. Prior to comparison, CsPbBr3/ZrO2-10 solution was diluted to an equal PL intensity to CsPbBr3. It should be pointed out that the concentration of CsPbBr3/ZrO2-10 was lower than that of pristine CsPbBr3. In a typical experiment, 2 mL hexane solution was put onto the top layer of 2 mL water in a 5 mL vial, which was then kept undisturbed. As shown in Figure 6a-b, the upper layer of CsPbBr3 showed a gradual blue-shift and green emission quenching in 8 days, which revealed the rapid destruction and poor stability of naked CsPbBr3. On the contrary, CsPbBr3/ZrO2-10 composite kept strong green emission throughout 8 days, indicating an obviously enhanced stability with the protection of ZrO2 (Figure 6c-d). As shown in Figure 6e, the PL intensity of CsPbBr3 solution declined to only 5%, while that of CsPbBr3/ZrO2 was maintained around 80% in 8 days.


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Figure 6. Photographs showing the stability of (a, b) CsPbBr3 and (c, d) CsPbBr3/ZrO210 solution against water under (a, c) daylight and (b, d) UV light irradiation ( = 365 nm). Upper layer: CsPbBr3 in hexane, bottom layer: water. (e) Corresponding normalized PL intensity change of CsPbBr3 and CsPbBr3/ZrO2-10 in 8 days. To take advantage of the enhanced optical properties as well as improved stability, white light emitting device (WLED) was fabricated by combining three emissive layers. On a blue-emitting GaN chip, a red-emitting composite of CdSe-polymethyl methacrylate (PMMA) and the green-emitting composite of CsPbBr3/ZrO2-PMMA were stacked up layer by layer, as shown in Figure 7a. The PL spectrum of the device was shown in Figure 7b. The corresponding initial luminous efficiency of CsPbBr3based WLED was 30 lm W-1 at the current of 1 mA. In CIE diagram in Figure 7c, the color coordinate changed from (0.28, 0.33) to (0.23, 0.21), while the corresponding variation of PL intensity of green emission can be observed attenuating evidently during 120 min (Figure 7d). Meanwhile, the blue emission got higher due to the destroy of blue light absorber CsPbBr3. In comparison, CsPbBr3/ZrO2-10 based device showed a 15 ACS Paragon Plus Environment

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higher luminous efficiency of 55 lm W-1 activated by the current of 1 mA and much higher optical stability. After the decay process, CIE color coordinate slightly changed to (0.27, 0.30), which was still in the white emission area (Figure 7e). The corresponding PL intensity of green emission kept above 90% during the whole process (Figure 7f).

Figure 7. (a) Device architecture of the WLEDs. (b) PL spectra of WLED based on CsPbBr3/ZrO2 NCs operated at current level of 1 mA. Inset shows a photograph of a device in operation. CIE graphs exhibiting the stability of WLEDs constituted of (c) CsPbBr3 NCs and (e) CsPbBr3/ZrO2-10 NCs. (d) Time-dependent PL spectra of WLED based on (d) CsPbBr3 and (f) CsPbBr3/ZrO2-10 NCs under continuous operation current of 1 mA. Besides the WLED device, a series of different colored LEDs have also been fabricated by using CsPbX3/ZrO2 with different halide ions. Distinct from the WLED, these colored LED was fabricated by simply painting CsPbX3 or CsPbX3/ZrO2-10 on a blue-emitting GaN chip. The performance was investigated at ambient condition. As 16 ACS Paragon Plus Environment

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shown in Figure S14a, the initial intensity of green LEDs using CsPbBr3/ZrO2-10 as active layer was 2.8 folds compared to that of CsPbBr3. After 180 min irradiation, the intensity of CsPbBr3 device declined to 70% (Figure S14a, inset I) while the intensity of CsPbBr3/ZrO2-10 kept 90% (Figure S14a, inset II), indicating the higher stability provided by ZrO2 against blue-emission, oxygen and moisture. Similarly, yellow and red LEDs were fabricated based on CsPbBr2I1/ZrO2 and CsPbBr1I2/ZrO2 NCs, respectively. As shown in Figure S14b-c and S15, both yellow and red devices showed 4-time stronger emitting intensity and higher stability against irradiation, oxygen and moisture than that of pure CsPbX3 due to the introduction of ZrO2.

CONCLUSION In summary, monodisperse Janus type I and type II CsPbX3/ZrO2 composite NCs were successfully obtained. The systematic study showed that the type I composites possess dramatically enhanced PL intensity, PLQY and longer lifetime as well as improved stability. The PL lifetimes and PLQY of CsPbBr3/ZrO2 could be tuned by different amounts of ZrO2 in the composite, which could reach up to 66.1 ns and 90% for CsPbBr3/ZrO2, respectively. The enhanced optical properties of CsPbBr3/ZrO2 can be ascribed to the successful fabrication of the type I composite arose from the appropriate energy structures of CsPbBr3 and ZrO2. The well-defined type I composite brought more possibilities to optical applications, in which CsPbBr3/ZrO2 based WLEDs showed excellent efficiency and stability. Meanwhile, the type II composite (CsPbI3/ZrO2) may be very useful in photocatalysis because of the efficient charge separation. This work may push forward the development of rational-designed highly efficient and stable metal halide perovskite NCs.


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METHODS Materials: Lead halide (PbX2, ultradry, 99.999%), 1-octadecene (ODE, tech. 90%), oleic acid (OA, tech. 90%), oleylamine (OAm, 80-90%) were purchased from Alfa Aesar. Cesium carbonate (Cs2CO3, >99.0%), zirconium (IV) tert-butoxide (Zr(OC4H9)4), hexane (Anhydrous, 95%) were obtained from Sigma-Aldrich. All chemicals were used as received without any further purification. Preparation of Cs4PbBr6 NCs: Cs4PbBr6 NCs was synthesized by using a modified method developed by our group.45 PbBr2 (0.0734g), ODE (10 mL), OA and OAm (1 mL each) were kept stirring in a 25 mL round bottom flask and degassed under vacuum at 120 oC for 10 min, then filled with N2. The system was then heated to 140 oC. The Cs-oleate (~0.125 M, 0.8 mL) precursor was swiftly injected into the flask. After 7 s, the reaction was quenched by an ice bath. The product was centrifuged at 8000 rpm for 5 min and then dispersed in 10 mL hexane. For the synthesis of Cs4PbX6 NCs with different halide ions, PbBr2 was simply replaced by other precursors. Preparation of Janus NCs: CsPbBr3/ZrO2 was synthesized following our previous work35 with Zr(OC4H9)4 as precursor. Zr(OC4H9)4 was dissolved in hexane (0.01 M) in a N2-filled glovebox. Different amounts of Zr(OC4H9)4 ( 0~2 mL) were added to 4 mL Cs4PbBr6 solution (0.01 M in hexane) at ambient condition. When precursor was thoroughly mixed with Cs4PbBr6 solution, the water was quickly added into the vigorously vortex reaction mixture and kept stirring for 5 min. Then the mixture was left undisturbed for 12 h and then was centrifuged at 8000 rpm for 5 min to discard the precipitates. The samples were denoted as CsPbX3/ZrO2-N, in which N was the amount of Zr precursor and equaled to 2.5, 5, 7.5, 10, 15 and 20 mol, respectively.


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Preparation of WLED: Chloroform and poly-(methyl methacrylate) (PMMA) were mixed to dissolve CsPbBr3 (or CsPbBr3/ZrO2) and CdSe NCs. The CsPbBr3-PMMA film and the CdSe−PMMA film were then casted on the blue GaN chip layer by layer. Preparation of Multi-Color LEDs: CsPbX3 were uniformly painted on GaN chip. Devices based on CsPbX3/ZrO2 were fabricated in the same way. Characterization Methods: Powder X-ray diffraction (XRD) data were collected by using an Empyrean diffractometer (from PANalytical, Netherlands) equipped with monochromatic Cu Ka radiation ( = 1.54056 Å). Transmission electron microscopy (TEM) images were collected by a TECNAI G2 F20 transmission electron microscope with an accelerating voltage 200 kV and a Gatan SC200 CCD camera. Ultraviolet photoelectron spectroscopy (UPS) was carried out on a VG Scienta R4000 analyzer with a monochromatic He I light source (21.2 eV). A sample bias of -5 V was applied to observe the secondary electron cutoff (SEC). The work function (ϕ) can be determined by the difference between the photon energy and the binding energy of the secondary cutoff edge. UV−vis absorption spectra were recorded in a range of 200~800 nm by using an Evolution 220 spectrophotometer in transmission mode. The PL spectra and PLQY were obtained by a FLUOROMAX-4 spectrofluorometer equipped with a xenon lamp. The PL lifetime measurements were recorded on a HORTB-FM-2015 spectrofluorometer and fitted with a triexponential decay. The electro-luminance spectra of WLEDs and multi-color LEDs were collected by a Keithley 2400 Source Meter and a Photo Research PR670 spectrometer. The power efficiency of commercial ultraviolet LED was measured by a Si photodiode, PDB-C203. For the evaluation of photocatalytic performance in CO2 reduction, it was carried out in a 100 mL sealed Pyrex reactor, filled with 20 mL ethyl acetate. Approximately 4 mg of the photocatalyst was added into the reactor. Before the test, the system was degassed to remove air and 19 ACS Paragon Plus Environment

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refilled with CO2. A 100 W Xe lamp with a 420 nm filter was equipped to simulate the solar light illumination, and the light intensity was adjusted to 370 mW cm-2. The gaseous product is detected by gas chromatograph (GC). Theoretical simulation: The density functional theory (DFT) calculations were performed using the projector-augmented wave (PAW) method as implemented within the Vienna Ab initio Simulation Package (VASP) code. The Perdew−Burke−Ernzerhof (PBE) functional of the generalized gradient approximation (GGA) was used for the structural optimization and the band structure calculations. The orthorhombic structures of CsPbI3 and CsPbBr3 were adopted. The Br atoms in the CsPbBr3 structure were partly replaced by I atoms in random to form the CsPbBr2.5I0.5 structure, while the I atoms in the CsPbI3 structure were partly replaced by Br atoms in random to form the CsPbBr0.5I2.5 structure. ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Additional data about the PL lifetime of products at different stages; TEM images, photographs, UV-vis absorption and PL spectra of product obtained by varying reaction parameters; UPS and UV data listed for CsPbX3 with different halide ratios; Performance of devices fabricated with CsPbX3 and CsPbX3/ZrO2 (PDF).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (M.C.); 20 ACS Paragon Plus Environment

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* E-mail: [email protected] (B.S.); * E-mail: [email protected] (Q.Z.) ORCID Qiao Zhang: 0000-0001-9682-3295 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT H.L and Y.T. contribute equally to this work. This work is supported by the National Natural Science Foundation of China (21673150, 21703146), Natural Science Foundation of Jiangsu Province (BK20180097). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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SYNOPSIS Type I and type II Janus CsPbX3/ZrO2 composite NCs are successfully fabricated. The type I CsPbBr3/ZrO2 composite NCs display dramatically enhanced optical properties and improved stability compared to pristine CsPbBr3 NCs and are applied in high-performance light emitting diode devices. ToC figure (For Table of Contents Only)


Fabricating CsPbX3-Based Type I and Type II Heterostructures by

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