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Au exchange or Au deposition: Dual reaction pathways in Au-CsPbBr heterostructure nanoparticles 3

Benjamin J Roman, Joseph Otto, Christopher Galik, Rachel Downing, and Matthew Sheldon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02355 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Au exchange or Au deposition: Dual reaction pathways in Au-CsPbBr3 heterostructure nanoparticles Benjamin J. Roman,† Joseph Otto, † Christopher Galik, † Rachel Downing, † Matthew Sheldon*,†,‡ †

Department of Chemistry and ‡Department of Material Science and Engineering, Texas A&M University, College Station, Texas 77843, United States

TABLE OF CONTENTS GRAPHIC:

ABSTRACT: We have designed a facile synthetic strategy for the selective deposition of Au metal on all-inorganic CsPbBr3 perovskite nanocrystals that includes the addition of PbBr2 salt along with AuBr3 salt. PbBr2 is necessary because the addition of Au3+ to solutions of CsPbBr3 nanocrystals otherwise results in the exchange of Au3+ ions from solution with Pb2+ cations within the nanocrystal lattice to produce Cs2AuIAuIIIBr6 nanocrystals with a tetragonal crystal structure and a band gap of about 1.6 eV, in addition to Au metal deposition. Including excess

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Pb2+ ions in solution prevents the exchange reaction. Au metal deposits on the surface of the nanocrystals to produce Au-CsPbBr3 heterostructure nanoparticles with an Au particle diameter determined by the Au3+ ion concentration. Fluorescence quenching caused by Au deposition monotonically increases with deposition size, but the fluorescence quantum yield (QY) is significantly greater than if any cation exchange has occurred. Optimized synthesis can produce Au-CsPbBr3 nanoparticles with 70% QY and no evidence of cation exchange.

KEYWORDS: cesium lead halide perovskites; nanocrystal heterostructures; cation exchange; Au deposition; Au-CsPbBr3; Cs2Au2Br6

MAIN BODY: All-inorganic cesium lead trihalide perovskite nanocrystals have shown great promise for optoelectronic applications such as light emitting diodes,1,2 lasers,3,4 and solutionprocessed solar cells, with significantly greater environmental stability than their mixed organicinorganic counterparts, such as methylammonium lead iodide (MAPbI3).5 These all-inorganic materials show narrow emission linewidths with QY exceeding 80% that can be spectrally tuned by controlling the ratio of lead halide salts during synthesis, as first demonstrated by Protesescu et al.6 Additionally, spectral tunability can be achieved post-synthetically with anion exchange reactions at temperatures as low as 60oC. Remarkably, this exchange occurs even when nanocrystals with different halide compositions are in solution together at room temperature, whereby each nanoparticle acts as a halide source and labile oleylamine from the ligand shell transports halides through solution.7–9 Similarly, van der Stam et al.10 reported that divalent metals may undergo cation exchange reactions with cesium lead trihalide perovskites at room temperature in toluene, replacing up to 10% of the Pb cations present in the original all-inorganic perovskite with an isovalent metal. Their work suggested that complete cation exchange on all-


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inorganic perovskite nanocrystals was not possible at room temperature using divalent metals due to the limited metal cation mobility, and thus this technique was limited to doping the perovskite nanocrystals. In contrast to this, however, full cation exchange was recently demonstrated on hybrid organic-inorganic perovskite nanocrystals utilizing a combination of trioctylphosophine and heating.11 A similar level of cation exchange, however, has yet to be demonstrated in all-inorganic perovskite nanocrystals. It was recently demonstrated by Balakrishnan and Kamat that gold can be deposited onto the surface of CsPbBr3 nanocrystals via the reduction of Au salts by the nanocrystal surfactant shell.12 As with similar Au-chalcogenide hybrid nanostructures,13–15 the metal domains can act as electrical contact points for nanoscale optoelectronic devices16 or provide improved catalytic activity.17 Additionally, plasmon-exciton coupling can modify the optoelectronic behavior and excited state dynamics of the heterostructure.18,19 Using the nanocrystal surface ligands as reducing agents, however, compromises colloidal stability. Thus, the size of deposition was limited in this initial report and could not be increased beyond a diameter of 2 nm by the simple increase of Au ion concentration in solution. Additionally, as we show below, under these unstable synthetic conditions, a major side reaction is the uncontrolled exchange of Au3+ and Au1+ with Pb2+ ions in the perovskite nanocrystal lattice. This exchange is evidenced by characteristic diffraction peaks and nearly fully quenched photoluminescence (PL). The exchange reaction should be mitigated if high quality optical materials, or precise control over the morphology of the nanocrystal heterostructure is desired. Here, we report that the concentration ratio of Au and Pb ions in solution will kinetically determine which ion is preferentially driven into the nanocrystal lattice. When both AuBr3 and PbBr2 are added to the nanocrystal solution, excess Pb2+ competitively prevents cation exchange


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with Au ions. Instead, Au3+ is reduced by oleic acid and oleylamine and deposits as Au metal on the surface of the CsPbBr3 nanocrystals. The domain size of the metal deposition increases monotonically with the concentration of AuBr3 in solution. Under these conditions, there is no evidence of cation exchange, and the QY of the resulting Au-CsPbBr3 heterostructures can reach 70%, a high fluorescence yield considering the complete quenching of PL in similar Auchalcogenide heterostructure nanoparticles.20 Alternately, adding a high concentration of only AuBr3 promotes conditions that favor complete cation exchange. The resulting product is Cs2AuIAuIIIBr6, a tetragonal mixed valence perovskite previously studied as a bulk crystal.21 As has been identified for macroscopic crystals,22 Cs2Au2Br6 nanoparticles have a band gap absorption onset at 1.6 eV with a photoluminescence maximum at 812 nm. Additionally, we have determined that the molar ratio of PbBr2 compared to AuBr3 determines the kinetics of deposition versus exchange, rather than the total amount of lead present in the reaction solution. Understanding these reaction dynamics is the first step towards producing lead-free all-inorganic perovskites utilizing this strategy of post-synthesis cation exchange. Experimental. For this study CsPbBr3 perovskite nanocrystals were synthesized as reported by Protesescu et al.6 Once synthesized, they were precipitated, resuspended in 2 mL of hexane, and then centrifuged at 3000 g-forces for 90 minutes. The precipitate was discarded, and the supernatant was diluted with hexane for use in both the exchange and deposition reactions. A concentration of 0.30 μM and 0.60 μM was used for the exchange and deposition reactions, respectively, as calculated using the molar extinction coefficient.23 An AuBr3 solution was made by dissolving the desired amount of AuBr3 in 9 µL ethanol per milligram of AuBr3, and then diluting with 0.5 mL toluene. A PbBr2 stock solution was made by adding 69 mg PbBr2 to a


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solution containing 5 mL 1-octadecene (ODE), 0.3 mL oleic acid, and 0.3 mL oleylamine. This solution was stirred and heated to between 120 and 150oC until it turned clear. It was then cooled and stored for later use. To study the concentration dependence of Au cation exchange, AuBr3 solutions of various concentrations were added to 0.5 mL of the 0.3 μM CsPbBr3 solution and allowed to stir for 16 hours. The final solution was centrifuged at 3000 g-forces for 10 minutes and the supernatant was saved for analysis. To study the deposition of Au metal without cation exchange, we added PbBr2 to the AuBr3 precursor solution to act as a Pb2+ ion source that competitively replaces any Au ions in the nanocrystal lattice. 36.5 µL of the PbBr2 solution and 19 µL of oleic acid were added to the AuBr3 solution per 2.3 mmol of AuBr3. This solution was stirred for 30 seconds, and then swiftly injected into 0.5 mL of the 0.60 μM CsPbBr3 hexane solution. Upon injection, the solution turned orange. To clean, the solvents were evaporated under nitrogen flow until only the oily oleylamine and oleic acid remained, and 0.5 mL ODE was added. The solution was centrifuged at 1300 g-forces for 5 minutes. The supernatant was discarded, and the yellow precipitate was suspended in 0.5 mL hexane.


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Figure 1. (a) Picture of CsPbBr3 (left), Cs2Au2Br6 (middle) and Au-CsPbBr3 (right) in white light (top) and UV excitation (bottom). (b) TEM of CsPbBr3. (c) TEM of CsPbBr3 after gold cation exchange. (d) TEM of CsPbBr3 with gold deposition. (e) TEM of Au-CsPbBr3 heterostructures showing lattice spacings corresponding to the (202) plane of orthorhombic CsPbBr3 (5.8 Å) and the (100) plane of cubic Au (3.7 Å). (f) Powder x-ray diffraction (XRD) diffractogram of CsPbBr3 (blue trace), CsPbBr3 with 3.0 nm deposition (orange trace), and the Cs2Au2Br6 exchange product (yellow trace). A characteristic section of the diffractogram is shown (full diffractograms available in SI Figures S1 – S4). The visible peaks correspond to the (040) and (202) planes of both orthorhombic CsPbBr3 and tetragonal Cs2Au2Br6. The asterisk denotes the peak characteristic of the (111) plane of cubic gold. Results and Discussion. Figure 1 summarizes the major differences between the reaction products of Au metal deposition versus Au cation exchange. Figure 1a shows suspensions of CsPbBr3 nanocrystals as synthesized (left) and after reacting with AuBr3, without and with excess PbBr2 (middle and right, respectively). The difference is stark, both under ambient lighting (top), and under UV lamps (bottom). When viewed using TEM, however, the two products are not as obviously distinct. Figure 1b shows an unreacted sample of CsPbBr3 nanocrystals appearing as cuboids with edge lengths of about 8 to 10 nm (Figure S6). Figures 1c and 1d show the same batch of CsPbBr3 after reaction with a 0.5 mL of a 13.74 mM AuBr3 solution, without and with the inclusion of PbBr2 in the reaction solution, respectively. Both samples appear as cuboid nanoparticles with points of contrast that have a lattice spacing resolvable by HRTEM corresponding unequivocally to cubic gold (Figures 1e, S6 - S14). As discussed by Dang et al.,24 it has been previously observed that electron beam damage may drive the growth of Pb nanoparticles on the surface of CsPbBr3. However, if Pb metal center growth on


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this scale was to be expected from this CsPbBr3 sample, we would expect to see similar points of contrast on the unmodified CsPbBr3 under equivalent TEM imaging conditions, which we do not observe. The points of contrast are steady under TEM, and the CsPbBr3 nanoparticles do not show signs of decomposition within the timescale that they are being imaged. Figure 1f highlights the structural differences in the XRD diffractograms for CsPbBr3 (blue trace), Au-CsPbBr3 (orange trace), and Cs2Au2Br6 (yellow trace). The visible peaks correspond to the (040) and (202) planes of both orthorhombic CsPbBr3 and tetragonal Cs2Au2Br6, while the peak with the asterisk corresponds to the (111) lattice plane of cubic gold, indicating that gold nanoparticles are present in both solutions. Due to the difficulty in cleaning these samples, a large amount of gold is in solution that is not on the surface of a perovskite. As such, the sharp (111) cubic Au peak include contributions both from gold on and off the surface of a nanoparticle. In the sample with PbBr2 added, both CsPbBr3 and cubic Au appear on the diffractogram. Additionally, CsPbBr3 peaks are not shifted from their original positions, indicating a lack of Au(III) or Au(I) incorporation into the lattice. The cation exchange sample, however, has major peaks characteristic of a tetragonal Cs2Au2Br6 perovskite crystal structure, as discussed below. Table 1. Perovskite nanocrystal optical properties due to Au metal deposition or Au cation exchange AuBr3

QY at 510 nm

Amplitude Averaged Lifetime

Deposition Diameter

before

60.01%

5.25 ns

--

0 mM

74.15%

10.05 ns

--

4.58 mM

70.18%

10.11 ns

2.5 ± 0.5 nm

13.74 mM

51.36%

6.27 ns

2.7 ± 0.6 nm


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22.90 mM

11.26%

3.82 ns

3.0 ± 0.7 nm

4.58 mM, no PbBr2

1.64%

heterostructure nanoparticles

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