An Improved Strategy for High-Quality Cesium Bismuth Bromine

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An Improved Strategy for High-Quality Cesium Bismuth Bromine Perovskite Quantum Dots with Remarkable Electrochemiluminescence Activities Yue Cao, Ziyi Zhang, Lingling Li, Jian-rong Zhang, and Jun-Jie Zhu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01918 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Analytical Chemistry

An Improved Strategy for High-Quality Cesium Bismuth Bromine Perovskite Quantum Dots with Remarkable Electrochemiluminescence Activities Yue Cao, Ziyi Zhang, Lingling Li, Jian-Rong Zhang,* and Jun-Jie Zhu* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China Corresponding Authors: Jun-Jie Zhu*; E-mail: [email protected]. Tel: + (86)2589687204, and Jian-Rong Zhang*; E-mail: [email protected] Yue Cao and Ziyi Zhang contributed equally to this work

ABSTRACT Low-toxic trivalent bismuth, with the isoelectronic structure (6s26p0) and similar ionic radius to divalent lead, represents a promising candidate for constructing lead-free perovskites. Herein, cesium bismuth halide perovskite quantum dots (Cs3Bi2Br9 QDs) were synthesized via a comprehensively improved ligand-assisted reprecipitation method with the additions of γ-butyrolactone, trace distilled water, and tetrabutylammonium bromide, as well as the aid of ultrasonic technology. The asprepared QDs displayed remarkable monodispersity, outstanding stability, and highly passivated surfaces with a near single-component PL decay, thus affording superb optical properties with photoluminescence quantum yield up to 37 %, outperforming all the reported bismuth-based perovskites. Furthermore, Cs3Bi2Br9 QDs were first attempted for electrochemiluminescence (ECL) and exhibited a stable and efficient ECL response following either annihilation or coreaction ECL mechanism. Not only did the optical properties and stability of Cs3Bi2Br9 QDs were greatly improved in this work, but their electrochemical behaviors and ECL natures were also investigated

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systematically for the first time, demonstrating the significant potential to extend this environment-friendly bismuth-based perovskite into ECL domain.

INTRODUCTION Lead halide perovskite semiconductors have become a research hotspot recently owing to their impressive optical and electrical attributes, such as high photoluminescence quantum yield (PLQY), narrow emission bandwidth, tunable band gap, strong optical absorption capacity, low exciton binding energy, high charge motility, and long charge carrier lifetime.1-4 Benefiting from these “magic” optical and electrical properties,5,6 lead halide perovskites have realized rich applications in solar cells,7,8 photodetectors,9,10 lasers,11,12 light-emitting diodes,13,14 capacitors,15 and so on.16 Unfortunately, lead is recognized as a highly toxic element,17,18 which is one of the principal contradictions for the mass production and commercialization of lead-based perovskites. Therefore, it is obligatory to explore novel eco-friendly lead-free perovskite materials for the ever-expanding applications. Stability challenge has become another bottleneck hindering the further development of halide perovskites due to their ionic salt nature.17 The inherent vulnerability of halide perovskites to water, oxygen, and light exposure19-21 not only becomes one of the major hurdles to optoelectronic applications but also prevents halide perovskites from being optical probes, in spite of their outstanding optical performance. Accordingly, many efforts have been devoted to addressing this circumstance, mostly via embedding perovskites nanocrystals (NCs) into polymers,19 amorphous alumina (AlOx) matrixes,20 and mesoporous silica particles.21 However, these methods just retard the degradation


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process and the stability issue has not been radically solved yet. Even worse, the optical and electrical properties usually degrade after the wrapping behaviors. Therefore, the next progress should be carried out with more research and investment efforts to construct stable perovskites still with powerful optical performances. The most direct strategy to address the issues of toxicity and stability is to replace lead with other low-toxic, environmentally benign metals,17,18,22 which are capable of forming perovskite crystals. Trivalent bismuth with the isoelectronic structure (6s26p0) and similar ionic radius to divalent lead has been a promising candidate for constructing low-toxic perovskites, which can also solve the problem of poor stability suffered by lead-based analogs due to stronger antioxidant capacity.17 By now only a few reports were published concerning the synthesis of bismuth-based perovskites by hot-injection method or ligand-assisted reprecipitation method (LARM).23-27 All these works reported that bismuth-based perovskite NCs are more stable and ecology-friendly, but their optical properties were still greatly inferior to lead-based counterparts whose PLQYs have been up to 90 %.6 Thus, it is urgently desired to develop new synthetic approaches to obtain highly luminescent and stable bismuth-based perovskites. ECL describes a photoemission phenomenon that occurs during the energy relaxation process of the excited substances triggered by a modulated potential at the surface of a working electrode.28-32 Since the first detailed investigation on Si NCs by means of ECL technology,30 ECL has been acknowledged as a versatile technique to probe the redox nature and the surface state of NCs.30-32 Recently, it has been reported that lead-based perovskites could emit ECL.33,34 Unfortunately, they are easily destroyed owing to the



inherent affinity with moisture resulting in a poor ECL performance, which hindered their potential applications under normal conditions. Considering the great improved stability and low-toxicity of Cs3Bi2Br9 QDs, they might be more suitable for ECL emitters. To the best of our knowledge, however, bismuth-based perovskites have not yet been led into ECL domain, and many open questions about the redox nature and charge transfer in Bi-based perovskites still remain to be studied. In this work, high-quality Cs3Bi2Br9 QDs with significantly enhanced PLQY (up to 37 %) and stability were synthesized via a modified LARM that involves all-round improvements in the whole reaction process including presolvent, antisolvent, ligands, and reaction conditions (Scheme 1). The excellent optical and electrical properties of as-prepared Cs3Bi2Br9 QDs suggest their promising potentials as candidates of ECL emitters. Thus EC behaviors and ECL mechanisms were investigated systematically for the first time with a glassy carbon electrode (GCE) modified with Cs3Bi2Br9 QDs in our work. Such efforts are attempted to open a novel avenue to design environmentally friendly perovskite as efficient emitters for potential ECL applications.

EXPERIMENTAL SECTION Synthesis of Cs3Bi2Br9 QDs Cs3Bi2Br9 QDs were synthesized following a modified LARM.25 Typically, 0.0426 g of CsBr, 0.0601 g of BiBr3, 0.0390 g of tetrabutylammonium bromide (TBABr), and 33.0 μL of oleylamine (OAm) were first dissolved in a mixture solution of dimethyl sulfoxide (DMSO, 1.0 mL) and γ-butyrolactone (GBL, 2.0 mL). They were


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ultrasonicated subsequently until a clear precursor solution was obtained. Then, 1.0 mL precursor solution was slowly dropped into the antisolvent of ethanol (10.0 mL) containing 40.0 μL of distilled water (H2O) and 1.0 mL of oleic acid (OA) at the room temperature under ultrasonic irradiation for 10 min. After centrifugation at 8000 rpm for 10 min, the final colloidal suspension was obtained. Cs3Bi2Br9 precipitates were also collected via gradient centrifugation between 4000 rpm and 9500 rpm both for 10 min. Preparation of Cs3Bi2Br9|GCE The surface of GCE with a diameter of 5 mm was successively polished using 0.3 μm and 0.05 μm alumina, and carefully washed with distilled water and ethanol. After the freshly cleaned GCE was dried under a nitrogen atmosphere, 10.0 μL of Cs3Bi2Br9 QDs ethanol dispersion was dropped onto the GCE. Eventually, solid Cs3Bi2Br9 QDs film was self-assembled on the electrode surface (Cs3Bi2Br9 QDs|GCE) after dried naturally. EC and ECL measurement All the EC and ECL experiments were measured in the specially prepared electrolyte solution, an organic mixture of acetonitrile and toluene (2:1, v/v) containing 0.05 M tetra-n-butylammonium hexafluorophosphate (TBAPF6), with a three-electrode system containing a modified GCE working electrode, a platinum wire auxiliary electrode, as well as a silver wire quasi-reference electrode. The redox nature of QDs was investigated with differential pulse voltammetry (DPV) profiles with fewer disturbances of noise and charging current, and the ECL measurements were triggered



with cyclic voltammetry (CV) profiles. All potentials quoted in this manuscript were against Ag quasi-reference electrode. All the ECL signals were collected with a PMT operated with a working voltage at 800 V and a triple amplification series.

RESULTS AND DISCUSSION Feasibility of all-round modification strategy The proposed synthetic method for highly luminescent Cs3Bi2Br9 QDs derives from a recently reported green LARM,25 namely a process by transferring precursor solution prepared by dissolving precursors (CsBr and BiBr3) in a “good” solvent (DMSO) into a “poor” antisolvent (ethanol) for precipitation with the assistant of ligands. LARM undergoes a reaction process that can finish within a few seconds under mild conditions, which are beneficial to be extended to gram-scale preparation for mass production.35 As convenient as LARM is, the obtained products usually display more defects with inferior optical properties to those synthesized by the classical hot-injection operation.36 During the whole process, the presolvent, antisolvent, ligands and operation conditions are determining factors controlling the crystallization process of perovskites. Therefore, an overall modification strategy with the usage of additives including GBL, trace distilled water, and TBABr, as well as the aid of ultrasonic technology, was conducted for the preparation of high-quality Cs3Bi2Br9 QDs in this work (Scheme 1). To evaluate the feasibilities and advantages of these modifications, the gradual optical evolution was investigated in detail. The effect of presolvent was first taken into consideration. The Lewis basicity, qualified by Gutmann’s donor number (DN), indicates the coordinating capacity of the presolvent


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with the metal ions center.37 GBL with a low DN played a significant role in our synthesis. It could regulate high DN presolvent (typically DMSO), resulting in a weaker interaction with the metal ions center, favoring instead complexation between metal and halide ions to finer control over crystallization of perovskites. However, excessive GBL caused an inferior performance instead because the DN was too low leading to superfluous bismuth-halogen salt and a very fast rate of subsequent crystallization.38 As expected, the PL intensity of the Cs3Bi2Br9 QDs increased obviously with the addition of GBL (Figure 1A) and reached the maximum with an optimized volume ratio of 1:2 (DMSO/GBL) (Figure S1, Supporting Information). The PLQY raised accordingly from 18 % to 25 % with the appropriate addition of GBL (Figure 1C). Also, the element contents of the prepared Cs3Bi2Br9 QDs were more consistent with the stoichiometric ratio than those of the untreated QDs (Table S1, Supporting Information). Simultaneously, a more negative zeta potential was observed for the GBL-treated QDs, ensuring higher stability in ethanol (Figure 1D). To illustrate the solvent effect of GBL, UV-vis absorption spectroscopy was subsequently applied. The typical absorption peak of BiBr3 (0.2 mM) experienced a red shift in either DMSO or mixture of DMSO and GBL (1:2, v/v) with increased concentration of bromide supplied by TBABr (Figure S5A, B; Supporting Information). The final absorption peak centered at around 380 nm, which meant the transformation from BiBr4-(λ=367 nm) to BiBr63-(λ=384 nm).39 Significantly, the absorption peak shifted more apparently in the mixture than in single DMSO after adding the same amount of TBABr (Figure S5C, Supporting Information), which confirmed that GBL could regulate DMSO in favor of the formation of BiBr63-



for subsequent crystallization. As for antisolvent, a minor amount of water was added into ethanol to improve the luminescence of Cs3Bi2Br9 QDs. This idea root in the results that the PLQY of Cs3Bi2Br9 NCs could be enhanced obviously after treated with trace water in stability tests reported by Yang et al.24 and Leng et al.25 They tentatively attributed this phenomenon to the passivation effect of the formed hydrate and BiOBr, respectively. But the direct introduction of water into antisolvent during the reaction process has not been tried yet. Excessive water might result in the decomposition of perovskites, thus an optimization experiment of water volume was conducted (Figure S2, Supporting Information). PL intensity increased with the addition of water (Figure 1A) and PLQY climbed from 25 % to 26 % with an appropriate addition volume of 40.0 μL (Figure 1C). A clear red-shift of PL spectra could be observed after the treatment of trace water providing direct evidence for the formation of the hydration layer on the QDs surface (Figure 1A), and the result was identical to the previous report.24 Water molecules could protonate to form H3O+ with the assistance of excessive OA in solution, and H3O+ then rapidly gathered on the negative surface of QDs resulting in a positive zeta potential (Figure 1D). It is worth noting that OAm partly desorbed from the QDs surface in the process as a result of the competition of H3O+, which could be testified by the EDX detailed results showing a decreased nitrogen content after the treatment of distilled water (Table S1, Supporting Information). As regards ligands, the key to acquiring striking performance of perovskite is to form a fully covered hydrophobic ligands layer, which could encapsulate and passivate the


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QDs surface. Here, TBABr was selected as the most suitable ligand in the present system. On the one hand, TBABr could supply enough bromine ions to facilitate the interaction with trivalent bismuth (Figure S5, Supporting Information). On the other hand, TBABr with quaternary ammonium groups could improve the dissolving ability of precursors, control the formation of perovskites, and passivate surface defects effectively.27,36,40 As shown in Figure 1A, the PL intensity increased significantly with the TBABr additive as ligands, and the use of a suitable amount of TBABr (0.0130 g in per milliliter precursor solution) realized the improvement of PLQY from 26 % to 31 % (Figure 1C; Figure S3, Supporting Information). In addition, the markedly increased nitrogen content in EDX detailed results indicated the successful linkage of TBABr to QDs surface (Table S1, Supporting Information). The bromide in TBABr might intercalate into the octahedral structures in perovskite cells, promoting the attachment of the alkyl chains to the QDs and forming a strong chemical interaction between the TBABr and perovskite interface, which were certified by the apparent blueshifts of both PL and absorption spectra (Figure 1A and 1B), along with a more negative zeta potential of the TBABr-capped QDs (Figure 1D). Besides, the operation condition was optimized in terms of the reaction temperature, and room temperature about 25 °C was the optimal condition for the preparation of Cs3Bi2Br9 QDs, displaying a slight climbed PLQY to 33 % (Figure S4, Supporting Information). Meanwhile, ultrasonic irradiation was introduced in the preparation process to further improve the optical performance of Cs3Bi2Br9 QDs. It was widely exploited as a convenient operation to regulate the nucleation process in chemical



synthesis, finally obtaining produces with effectively improved monodispersity and uniformity.41,42 As expected, the PLQY maintained sustained growth from 33 % to 37 % under ultrasound (Figure 1C; Figure S4, Supporting Information) and the zeta potential was up to -39.9 mV (Figure 1D). Such a high zeta potential along with the teeny size of QDs was responsible for the excellent stability of the Cs3Bi2Br9 QDs ethanol dispersion. On the basis of the above improvements, it is completely feasible for this all-round modification strategy to regulate the whole synthesis process in four aspects including presolvent, antisolvent, ligands, and operation conditions with GBL, distilled water, TBABr, and ultrasonic irradiation, respectively. Under various optimal conditions, the prepared Cs3Bi2Br9 QDs showed the step-up PLQYs with a final value up to 37 % (Figure 1C), which was superior to all the reported bismuth-based perovskites with the best of our knowledge (Table S2, Supporting Information). Characterization of the Prepared Cs3Bi2Br9 QDs Overall inspections of the structure and optical properties of the as-synthesized Cs3Bi2Br9 QDs were performed to better understand their underlying natures. Highresolution transmission electron microscopy (HRTEM) was first employed to observe the morphology of the QDs. As shown in Figure 2A, the Cs3Bi2Br9 QDs exhibited quasi-spherical shapes with a good monodispersity. Moreover, the lattice distance of 3.28 Å (Figure 2B), ascribed to that of the (003) plane in Cs3Bi2Br9 trigonal structure, could be apparently observed from a typical QDs.24,25 The size distribution counted with hundreds of QDs showed an average diameter of 4.80 ± 1.24 nm (Figure 2C),


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suggesting their strong quantum confinement effect. The crystal structure was further confirmed by means of X-ray diffraction (XRD) measurement. As shown in Figure 2D, the Cs3Bi2Br9 QDs displayed a hexagonal structure (P-3m1, a = b = 7.9551 Å, c = 9.8443 Å,  =  = 90º,  = 120º), which was identical to a standard PDF card (JCPDS card no. 44-0714) revealing the successful synthesis of highly crystalline Cs3Bi2Br9 QDs. Unit cell graphs of Cs3Bi2Br9 QDs viewed from various orientations were depicted in Figure 2E, which consists of the metal halide octahedral (BiBr63-) layers and cesium ions (Cs+) filled in the bedding void. In addition, X-ray photoelectron spectroscopy (XPS) was utilized to identify the valence elements (Cs, Bi, and Br) of Cs3Bi2Br9 QDs, which could match well with their corresponding typical peaks reported previously (Figure S6, Supporting Information).25,43 Besides XPS element content analysis and detailed EDX results of the prepared QDs were nearly consistent with the stoichiometric ratio. In sum, all these results demonstrated that QDs were successfully synthesized with high crystallinity. The surface of the Cs3Bi2Br9 QDs solid film captured by scanning electron microscopy (SEM) displayed a smooth and dense morphology with uniform distribution of QDs (Figure 2F), which was beneficial to protect the film from oxygen and water intrusion. The steady-state optical properties of the final products were investigated. As depicted in Figure 3A, the UV-vis absorption spectrum showed an absorption onset at about 390 nm and reached the peak absorption at around 332 nm, which displayed a blue shift about 127 nm compared with the bulk NCs24,25 due to the quantum confinement effect of QDs. The PL excitation spectrum showed a peak ranging from 254 to 352 nm with



a center at 314 nm, a hallmark of good light absorption capacity. The PL emission spectrum showed a single PL emission peaked at around 389 nm with a narrow full width at half-maximum (FHWM) of 62 nm, suggesting high quality and narrow size distribution of the as-synthesized Cs3Bi2Br9 QDs. It is worth noting that a larger Stokes shift about 57 nm was observed for the QDs compared to that of lead-based counterparts about 15 nm,44 suggesting a negligible self-absorption in favor of lighting applications and optical detections. The inset photographs displayed bright blue PL of the prepared Cs3Bi2Br9 QDs ethanol dispersion under UV light (302 nm) in ambient conditions. The time-resolved PL (TR-PL) spectrum was further measured to reveal the exciton recombination dynamics (Figure 3B). The TR-PL curve could be well fitted to a biexponential function composed of a short-lived constituent (1 = 2.89 ns) and a longlived constituent (2 = 10.47 ns) with the respective percentage of 88.3% and 11.7% (Inserted in Figure 3B). The result was similar to that of previously reported Cs3Bi2Br9 QDs,25 but a significantly improved proportion of the short-lived lifetime was obtained in our work. According to previous reports, 1 was attributed to the direct exciton recombination, while 2 represented the recombination correlated to surface defects.24,25 Therefore, the high proportion of 1 suggested that the PL radiation was mostly triggered by the direct exciton recombination and almost unrestricted by the destructive surface defects, further demonstrating that the QDs were highly passivated, which was responsible for the improved PLQY. In addition, the effective surface passivation was also crucial to subsequent ECL measurements due to the fact that ECL is more sensitive to the surface state of QDs than PL.30,32,45


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Stability of the Prepared Cs3Bi2Br9 QDs The stability has been regarded as a major issue restricting the development of halide perovskites. The stability of the as-synthesized Cs3Bi2Br9 QDs was evaluated in various harmful environments. Figure 4A showed remnant PL intensity of Cs3Bi2Br9 QDs with or without modifications as a function of the storage days under ambient conditions. After exposure to open air, the PLQYs of both QDs got a little improvement in the initial twenty days due to the formation of the hydration layer on QDs surface with atmospheric water (relative humidity, 30-50 %). The final PL intensity of the asobtained QDs remained unabated at 144 % of the initial intensity, and the PL spectrum showed a slight red-shift with obviously increased PL intensity after thirty-day storage (Figure S7, Supporting Information), probably due to passivation effect of the formed hydration layer through the absorption of protonated water molecules (H3O+) on the negative QDs surface (Figure 1D), which indicated the excellent air stability of the prepared QDs. By contrast, the untreated QDs displayed a rapidly decaying PL after twenty days. To figure out this anomalous yet beneficial optical trend, 2.0 mL of distilled water was injected into 10.0 mL of Cs3Bi2Br9 QD ethanol solution. More remarkably, PL intensity continued to rise over the next eight hours (Figure 4B), implying their wonderful stability against water far better than lead-based counterparts which even decomposed with atmospheric water.19-21 It is noteworthy that the PL sensitizing effect of the modified QDs was less apparent than that of the untreated QDs probably because trace water in presolvent had passivated partial surface trap-states in advance. Thermal stability of Cs3Bi2Br9 QDs was tested by annealing the samples at



100 °C and 200 °C for 40 min in air. The samples were placed in the dark to exclude the interference of light exposure, and XRD spectra were utilized to trace the experiment. As shown in Figure 4C, the annealed QDs displayed consistent XRD patterns to the untreated one and still maintained a perfect crystal structure, indicating their high heat-endurance. Electrochemistry of the Prepared Cs3Bi2Br9 QDs EC behaviors of Cs3Bi2Br9 QDs were explored with Cs3Bi2Br9 QDs|GCE as the working electrode by DPV profile in a 2:1 (v/v) acetonitrile/toluene mixture containing TBAPF6 as a supporting electrolyte. TBAPF6 was very difficult to dissolve in single toluene and the acetonitrile alone led to an inefficient ECL. Thus, it was essential to mix the two solvents as the electrolyte solution and the ratio was beneficial for the highest ECL expression, which was optimized in Figure S8 (Supporting Information). In the positive scanning from 0 to +2.0 V (Figure 5A), Cs3Bi2Br9 QDs|GCE displayed three apparent anodic processes (A0, A1, and A2). A0 was a small peak with the onset and crest value at +0.63 V and +0.75 V, and those of A1 were at +0.96 V and +1.23 V, respectively. A2 showed a significantly increased current after +1.75 V. The current baseline increased about 13 μA after the modification of Cs3Bi2Br9 QDs on GCE, suggesting their comparable ionic conductivity. The background current without Cs3Bi2Br9 QDs exhibited a similar anodic process to A0 in air-saturated electrolyte solution with onset and peak value at +0.50 and +0.75 V, respectively, which disappeared in air-free electrolyte solution (Figure S9A), possibly related to the dimeric 1Δ

g

state of dissolved oxygen.46 It has to be mentioned that A0 displayed a more obvious


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peak current than background at about +0.75 V probably as a result of the extra oxidization of ligands such as OAm and TBABr on the QDs surface, similar to tripropyiamine (TPrA) with anodic oxidation peak at the same position (Figure 5A). In addition, A3 displayed a significantly increased current suppressing the background, which mainly derived from the oxidization of QDs. Thus, the Cs3Bi2Br9 QDs could be continuously oxidized to positively charged states QDs+ and QDs2+ through injecting holes to the HOMO. In the negative scanning from 0 to -1.5 V, GCE displayed two cathodic peaks in air-saturated electrolyte solution around -0.58 V and -1.22 V, respectively (Figure S9B). After removing oxygen by bubbling pure nitrogen, the peak at -0.58 V disappeared and the peak at -1.22 V kept unchanged, demonstrating that the former peak originated from the reduction of dissolved oxygen and the latter one was possibly due to other impurities in the system.33 Cs3Bi2Br9 QDs|GCE showed two cathodic processes including a weak peak C0 with the onset and peak potential at -0.50 V and -0.64 V and a much stronger one C1 with those at -0.75 V and -1.00 V, respectively (Figure 5B). Obviously, C0 arose from the same reduction process with that of the first cathodic peak of bare GCE, indicating that C0 was irrelevant to Cs3Bi2Br9 QDs. Thus Cs3Bi2Br9 QDs could be electrochemically injected electron to the LUMO only once to be a negatively charged state QDs-. The whole EC behaviors of Cs3Bi2Br9 QDs manifested as the injection of electrons and holes were summarized in Table 1 and depicted as below (Reactions 1-3): QDs + e ⟶ QDs-

(1)

QDs – e ⟶ QDs+

(2)



QDs+ – e ⟶ QDs2+

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(3)

Annihilation ECL of Cs3Bi2Br9 QDs|GCE Annihilation ECL mechanism describes a luminescence process of the direct transferring of the exergonic electron between the electrochemically oxidized and reduced QDs,30-32,47 which was investigated with the Cs3Bi2Br9 QDs|GCE as a working electrode triggered by a CV profile between -1.5 V and 2.0 V in the same electrolyte solution as the EC test. As shown in Figure 6, the annihilation ECL of Cs3Bi2Br9 QDs displayed two anodic luminescence including a moderate ECL peak with the onset and peak potential at +1.03 V and +1.50 V and a strong ECL one increasing continuously after +1.75 V, as well as a cathodic luminescence with the onset and peak potential at 0.76 V and -1.11 V, which were almost consistent with the redox processes in DPV profiles (Table 1). We noted that two weak ECL responses at around +0.50 V and -0.21 V were observed with both bare GCE and Cs3Bi2Br9 QDs|GCE, probably attributed to the ECL of dissolved substances or impurities in organic reagents (Figure 6; Figure S10, Supporting Information). More importantly, both the cathodic and anodic ECL signals were stable under ten cycles of continuous CV scanning (Figure S11, Supporting Information) and no ECL signal was found under single positive or negative scanning (Figure S12, Supporting Information), further demonstrating the annihilation ECL mechanism between the redox states of QDs described as follows (Reactions 46): QDs- + QDs+ ⟶ QDs* + QDs0

(4)

QDs- + QDs2+ ⟶ QDs* + QDs+

(5)


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QDs* ⟶ QDs + hν

(6)

ECL transient technology was also utilized to verify this process. We recorded both the reduction and oxidation initiated transient ECL. As Figure S13 (Supporting Information) displays, no ECL signals could be detected in the first initial step of thirty seconds because only electrons or holes were transferred to the LUMO or HOMO. In other words, the QDs were singly positively or negatively charged without charge recombination. When the potential stepped from +1.9 V to -1.3 V or -0.9 V and from 1.3 V to +1.9 V or +1.0 V corresponding to the oxidation and reduction initiated process (a, b, d, and e), the transient ECL signals showed up immediately, indicating that radiative charge transfer took place by electrochemically reducing (or oxidizing) the Cs3Bi2Br9 QDs of oxidative (or reductive) state. Moreover, the respective former displayed significantly increased transient ECL responses due to the complete reduction or oxidation of QDs (a and d). Meanwhile, the ECL signal was almost undetectable with any potential stepping cycles from +1.9 V to -0.8 V or from -1.3 V to +0.9 V because of the unreached lowest reductive or oxidative potentials of QDs (c and f). These performances matched well with the above descriptions of DPV and annihilation ECL process (Table 1). Significantly, transient ECL signals could be detected whether the potential stepped from positive to negative or conversely, which differed from the CH3NH3PbBr3 perovskites whose transient ECL could only be found through reducing the positive states of NCs.34 In addition, the anodic transient ECL intensity was always higher than the cathodic one, suggesting anion radicals of QDs were more stable than cation ones in this ECL system. All the results demonstrated the redox nature of



Cs3Bi2Br9 QDs was crucial to their ECL emission. Similar to all-inorganic lead-based perovskites,33 the cesium ion (Cs+) located at the bedding void of metal halide octahedral (BiBr63-) layers might play a significant role in the recombination between the oppositely charged QDs for luminescence radiation. The voltage-adjustable ECL property might make the Cs3Bi2Br9 perovskites as a promising candidate for photovoltaic and optoelectronic applications. Coreaction ECL of Cs3Bi2Br9 QDs|GCE The coreaction route is most widely utilized in ECL domain, which describes a luminescence emission process triggered by a modulated potential in the presence of coreactants. Compared with the annihilation ECL, the coreaction one is easier to implement and usually shows a higher efficiency.46,48 Thus searching for promising coreactants is beneficial to expand potential applications of the Cs3Bi2Br9 QDs. Herein, we selected the typical TPrA and benzoyl peroxide (BPO) as coreactants to test and verify the feasibility of anodic and cathodic coreaction ECL mechanisms of Cs3Bi2Br9 QDs. TPrA was directly added into the electrolyte solution with a final concentration of 10 mM. As shown in Figure 5A, during the positive scanning, TPrA was easily oxidized at a relatively low potential around +0.93 V in this system to generate a short-lived radical cation (TPrA+•), which then deprotonated to form strongly reducing TPrA• radical (Reactions 7 and 8).34,48 The TPrA• radical could inject electrons into the LUMO of Cs3Bi2Br9 QDs producing negatively charged QDs (Reaction 9), which further recombined with electrochemically oxidized QDs to emit ECL signals as in Reactions


Page 18 of 42

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2-6. As shown in Figure 7A, Cs3Bi2Br9 QDs with TPrA as a coreactant exhibited a weak anodic ECL from +0.91 V to +1.72 V and a significantly and continuously increased anodic ECL after +1.72 V. The maximum ECL emission was obtained during the sweep-back process at +1.94 V. The ECL onset potentials were almost identical to the two oxidization processes of Cs3Bi2Br9 QDs in DPV profiles (Table 1). TPrA – e ⟶ TPrA+•

(7)

TPrA+• – H+ ⟶ TPrA•

(8)

QDs + TPrA• ⟶ QDs-

(9)

The cathodic coreaction ECL mechanism was verified by adopting BPO as the coreactant with a final concentration of 10 mM. Contrary to the anodic coreaction route, BPO was first reduced to generate BPO-• in this system (Reaction 10), which could be confirmed by the strong reduction peak from -0.12 V to -1.23 V in DPV profile (Figure 5B). Then BPO-• decomposed into C6H5CO2• with a strong oxidation capacity to inject holes into HOMO of QDs (Reactions 11-13).33 As a result, the BPO-oxidized QDs recombined with electrochemically reduced QDs to emit ECL (Reactions 1 and 4-6). The reduction of BPO occurred ahead of that of QDs in this process; that is to say, the holes injecting was earlier than electrons injecting. As shown in Figure 7B, Cs3Bi2Br9 QDs assisted with BPO as a coreactant displayed a powerful cathodic ECL signal with a start and peak potential at -0.85 V and -1.23 V, respectively. The results could also agree well with the corresponding DPV profiles (Table 1). It is also worth noting that the ECL decreased beyond -1.23 V probably because the high current density was not conducive to charge transfer radiation and electric field-mediated light emission.34,49



Page 20 of 42

BPO + e ⟶ BPO-•

(10)

BPO-• ⟶ C6H5CO2• + C6H5CO2-

(11)

QDs + C6H5CO2• ⟶ QDs+ + C6H5CO2-

(12)

QDs+ + C6H5CO2• ⟶ QDs2+ + C6H5CO2-

(13)

Taking a closer look at the ECL performance, the as-synthesized Cs3Bi2Br9 QDs could generate efficient ECL signals in both the anodic and cathodic coreaction ECL route, demonstrating their great potential in the ECL field. Numerous ECL-related analytical applications with the prepared Cs3Bi2Br9 QDs as ECL emitter could also be envisaged. Searching for more available coreactants for Cs3Bi2Br9 QDs, and trying to realize the ECL phenomenon in aqueous solution will be pushed forward in the future work. Conclusions In this work, Cs3Bi2Br9 QDs were synthesized with an over-all improvement strategy based on the classical LARM with the introduction of GBL, trace distilled water, and TBABr, as well as ultrasonic technology. The as-prepared QDs displayed attractive optical properties superior to available reports regarding bismuth-based perovskites, and excellent stability against air, water, and heating far better than lead-based analogs. A near single-component PL decay kinetics (1 percentage up to 88.3 %) suggested the highly passivated QDs surface which is crucial for the improved PLQY and ECL response. The EC behaviors and ECL mechanisms of Cs3Bi2Br9 QDs were analyzed systematically for the first time and Cs3Bi2Br9 QDs could emit stable luminescence under either annihilation or coreaction ECL mechanism, which might open a new avenue to design highly crystalline and luminescent perovskite QDs as novel emitters


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for ECL-related analysis applications.

ASSOCIATED CONTENT Supporting Information Chemicals and materials, apparatus, PLQY measurements, detailed optimization results (GBL, distilled water, TBABr, and reaction conditions), UV-vis absorption spectra for certification of GBL solvent effect, XPS characterization, PL spectrum after thirty-day storage, selection of electrolyte solution, EC and ECL performances of bare GCE, ECL stability test, transient ECL study, EDX detailed results, and recently reported articles regarding Cs3Bi2Br9 QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21834004, 21427807, 21375059, and 21775067).



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FIGURE CAPTIONS Scheme 1. Schematic exhibition of the reaction process for bright Cs3Bi2Br9 QDs with an all-round improvement strategy. Figure 1. (A) The PL emission spectra, (B) the normalized UV-vis absorption spectra, (C) PLQYs, and (D) zeta potentials of the untreated Cs3Bi2Br9 QDs (a) and each stage of the over-all improvements with the optimal usage of additives including GBL (b), trace water (c), and TBABr (d), as well as optimized reaction conditions (e). Figure 2. Structure characterizations of the as-obtained Cs3Bi2Br9 QDs including (A) HRTEM image, (B) HRTEM image of a typical QD, (C) Size distribution histogram, (D) XRD pattern, (E) Unit cell, and (F) SEM image of the solid film constructed with QDs. Figure 3. Optical properties of the prepared Cs3Bi2Br9 QDs in ethanol dispersion. (A) PL excitation spectrum (red), PL emission spectrum (black), UV-vis absorption spectrum (blue), and typical photographs captured under visible and 302 nm UV light in ambient condition (inset); (B) Time-resolved PL decay curve and the detailed fitting data (inset). Figure 4. Stability tests of the as-synthesized Cs3Bi2Br9 QDs. (A) Air stability by recording remnant PL over thirty days of storage under ambient conditions. (B) Water stability by testing remnant PL over eight hours by adding 2.0 mL distilled water into 10.0 mL Cs3Bi2Br9 QDs ethanol dispersion. (C) Thermal stability by tracing the XRD spectra after annealing in air at 100 and 200 °C. The remnant PL represents the ratio of the current PL intensity to the pristine one.



Figure 5. EC behaviors of (A) anodic and (B) cathodic DPV profiles in a binary organic solution of acetonitrile and toluene containing 0.05 M TBAPF6. Figure 6. ECL intensity of bare GCE (black) and Cs3Bi2Br9 QDs|GCE (red) trigged by CV scanning between -1.5 V and 2.0 V in a binary organic solution of acetonitrile and toluene containing 0.05 M TBAPF6. Inset: local magnification around +1.2 V. Figure 7. (A) Anodic ECL and (B) Cathodic ECL of bare GCE (black) and Cs3Bi2Br9 QDs|GCE (red) in a binary organic solution of acetonitrile and toluene containing 0.05 M TBAPF6 with coreactants of TPrA (10 mM) and BPO (10 mM), respectively. Table 1. Detailed EC and ECL behaviors of Cs3Bi2Br9 QDs|GCE in a binary organic solution of acetonitrile and toluene containing 0.05 M TBAPF6.


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Table 1. Detailed EC and ECL behaviors of Cs3Bi2Br9 QDs|GCE in a binary organic solution of acetonitrile and toluene containing 0.05 M TBAPF6. Anodic process (0 ~ +2.0 V) A1 ( Moderate)

Cathodic process (0 ~ -1.5V)

A2 (Strong)

C1 (Strong)

Onset/V

Peak/V

Onset/V

Peak/V

Onset/V

Peak/V

EC (DPV)

+0.96

+1.23

+1.75

CI

-0.75

-1.00

Annihilation ECL

+1.03

+1.50

+1.75

CI

-0.76

-1.11

Coreaction ECL

+0.91

CI

+1.72

CI

-0.85

-1.23

CI represents the current or ECL intensity continuously increasing without peak within the applied potential; Coreaction ECL with TPrA as anodic coreactant and with BPO as cathodic coreactant.



Scheme 1


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Figure 7



For TOC only


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An Improved Strategy for High-Quality Cesium Bismuth Bromine

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