Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 973−980
pubs.acs.org/JPCL
Role of Structural Distortion in Stabilizing Electrosynthesized BlueEmitting Phosphorene Quantum Dots Manila Ozhukil Valappil,†,‡ Krati Joshi,†,‡ Lisa John,⊥ Sailaja Krishnamurthy,§ Bikash Jana,# Amitava Patra,# Vijayamohanan K. Pillai,†,‡ and Subbiah Alwarappan*,†,‡ †
CSIR-Central Electrochemical Research Institute, Karaikudi 630003, Tamilnadu, India Academy of Scientific & Innovative Research, CSIR- Human Resource Development Centre, (CSIR-HRDC) Campus Postal Staff College Area, Ghaziabad 201 002, Uttar Pradesh, India ⊥ Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, Tamilnadu, India § Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, Maharashtra, India # School of Materials Science, Indian Association for the Cultivation of Sciences, Raja S C Mullick Road, Poddar Nagar, Jadavpur, Kolkata 700032, West Bengal, India
J. Phys. Chem. Lett. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/18/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Luminescent phosphorene quantum dots (PQDs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Herein, we investigate the structural distortion of black phosphorus (BP) under an applied electric field to yield blue luminescent PQDs [average diameter 8 ± 1.5 nm (N = 60)]. The electrosynthesized PQDs exhibit photoluminescence emission independent of excitation wavelength with 84% quantum efficiency. Structural distortion that occurred during the transformation of BP to PQDs is confirmed by results obtained during transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Further, using first-principles-based density functional theory, calculations on oxygenated and nonoxygenated PQDs augment the experimental observations that an optimum oxygen content maintains the structural integrity of PQDs, above which the structural robustness of PQDs is drastically diminished.
P
thesis.11,12 This top-down approach consists of ultrasonication (probe sonication or bath sonication) of BP in organic solvents mostly in N-methyl 2-pyrrolidone (NMP). Aqueous solvents such as deaerated water are also used to synthesize PQDs.13 However, phosphorene nanostructures are prone to degradation by light and the presence of oxygen, and thereby, the stability of PQDs synthesized by these methods is questionable. One of the major challenges during liquid phase exfoliation-based PQD synthesis is the choice of a more stabilizing solvent that can cap the PQDs as well as perform exfoliation.14 Electrochemical synthesis is considered as one of the facile top-down approaches for QDs from 2D materials.15−17 Unlike liquid phase exfoliation techniques, it endows flexibility in choosing the appropriate potential for size tunability, electrolyte, supporting electrolyte, etc. Electrochemical exfoliation for the synthesis of QDs of graphene, WS2, MoS2, MoSe2, and graphene nanoribbons is well-known.16−20 However, till date, there are no reports on the electrochemical synthesis of
hosphorene, with a single or a few layer of black phosphorus (BP), is at the forefront of research due to its monoelemental 2D structure.1 A typical phosphorene layer can be defined as covalently bonded phosphorus atoms in a puckered honeycomb structure.2 Despite the discovery of layered BP in 1914, its renaissance as a unique 2D system came to light when it recently exhibited remarkable performance in field effect transistors.3 Subsequently, several methods for the efficient exfoliation of BP have been explored. Due to its layered van der Waals structure, BP can be readily cleaved into phosphorene by mechanical exfoliation.4 Interestingly, BP has a direct bandgap that varies from 0.3 eV for bulk to ∼2 eV for a single layer.5 Such a direct bandgap dictates the optoelectronic properties of BP for device applications. Moreover, zerodimensional quantum dots (QDs) exhibit comparatively better performance than 2D phosphorene in view of optoelectronics due to quantum confinement and edge effects.6 Tiny fragments of 2D materials exhibit similar size-dependent bandgap features as noticed in graphene, MoS2, and WS2 QDs that are useful in bioimaging and photonic applications.7−9 Synthesizing phosphorene quantum dots (PQDs) from bulk BP by mechanical exfoliation is challenging. Predominantly, in several cases, exfoliation results in the formation of few-layer phosphorene.4,10 Alternatively, a few research groups have reported an organic liquid phase exfoliation technique for PQD syn© XXXX American Chemical Society
Received: November 30, 2018 Accepted: February 13, 2019
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DOI: 10.1021/acs.jpclett.8b03600 J. Phys. Chem. Lett. 2019, 10, 973−980
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The Journal of Physical Chemistry Letters pristine PQDs by cathodic exfoliation, although few-layer phosphorene has been electrochemically synthesized.21−23 To realize the true potential of PQDs toward various applications, a thorough understanding of their stability, electronic changes, and structural distortion during the transformation into QDs is critical. Computational analysis is a versatile tool to understand such structure−property correlations. Several theoretical investigations have been carried out on various aspects of phosphorene and PQDs. For example, physical properties including thermal conductivity and the role of solvents in the liquid phase exfoliation of BP have been determined using molecular dynamics simulations.14,24−26 Similarly, electronic properties,25,26 optical properties,27,28 transition metals,29−31 and small-molecule adsorption32−36 have been deduced using periodic density functional theory. Despite these elaborate studies, no explicit efforts have been put forth to understand the role of structural distortions in stabilizing PQDs. Experimental evidence on the structural distortion observed in PQDs and their comparison with a known simulated model are still lacking, which motivated us to undertake the present study. Toward this direction, we report a single-step electrochemical method to prepare PQDs from BP and corroborate the observed structural distortions using first-principles-based density functional theory calculations. Scheme 1 represents the electrochemical transformation of bulk BP into PQDs. We carried out experiments for various
Figure 1. (a) Transmission electron micrographs of (a) bulk BP and (b) Electrosynthesized PQDs. (c) High-resolution transmission electron micrograph of PQDs. (d) Particle size histogram of PQDs.
details are provided in the Supporting Information), the transmission electron micrograph shows morphological changes into PQDs (Figure 1b). The average size of the QDs is 7.8 ± 1.6 nm (N = 60). The crystallinity of the QDs is evident from the electron diffraction of a selected area in the inset of Figure 1b and the high-resolution transmission electron micrograph in Figure 1c. The QDs represent lattice fringes with a spacing of 0.23 nm, which can be correlated to the (041) plane of orthorhombic BP using the refined powder diffraction data of BP.38 The QDs exhibit a fair distribution, as shown in the particle size distribution histogram (Figure 1d). The height profile measurement from atomic force microscopic analysis of PQDs (Figure S2, Supporting Information) shows that the PQDs consist of approximately 3−4 layers (the van der Waals spacing of BP is 0.53 nm).39 Figure 2 is the comparative X-ray diffraction patterns of bulk BP and PQDs. BP shows diffraction patterns corresponding to (020), (021), (040), (060), and (132) facets (orthorhombic phase, ICDD Card No. 76-1963). However, upon exfoliation, the intensity of many of the diffraction patterns diminishes for the PQDs due to a decrease in layer thickness. Importantly, the lattice spacing corresponding to the (040) facet increased from 0.26 to 0.28 nm, causing the in-plane P−P bond length to increase in PQDs, as evident from the shift of the diffraction from the (040) plane to a lower 2θ value. This structural distortion due to P−P bond lengthening is attributed to atomic reconstructions in PQDs to maintain stability. A similar shift in the d spacing corresponding to the (040) plane (2θ ≈ 30°) has been reported for ionic liquid-mediated exfoliation of BP.40 UV−Visible and photoluminescence spectroscopy were employed to determine the optical properties of PQDs. The UV−Visible spectrum of PQDs (Figure 3a) exhibits absorption at around 370 nm (3.35 eV). Interestingly, this absorption is stable, and no significant decay is noticed even after 20 days (Figure 3b). This indirectly indicates that there is neither agglomeration nor degradation of PQDs in the given solvent. This strong UV absorption is further supported by photoluminescence spectroscopy (Figure 3c). As evident from
Scheme 1. Representation of Electrochemical Preparation of PQDs from BP
applied potentials and electrolyte-supporting electrolyte systems. On the basis of the yield (∼50%) obtained, an optimized applied potential of −2.0 V was determined (experimental and optimization details are provided in Table S1 and Figure S1 in the Supporting Information). A cathodic potential of −2.0 V was applied on the BP-coated working electrode (against a Pt wire quasi-reference electrode) in propylene carbonate containing 0.1 wt % lithium perchlorate for 24 h. An organic carbonate electrolyte such as propylene carbonate is a good choice for lithium-based salts, and they are widely used in battery technologies.37 Despite employing a nonaqueous electrolyte, oxygen dissolution was avoided by argon purging throughout the course of experiment. The formation of PQDs is confirmed by comparing the transmission electron micrographs of the bulk BP and the PQDs shown in Figure 1. The dark contrast in the micrograph shows that BP has more layers, causing less electron transmission (Figure 1a). After subjecting to 24 h of an applied electric field of 5.2 × 108 V m−1 (which is calculated with respect to a debye length of 3.87 × 10−9 m; calculation 974
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observations on the photoluminescence profile of electrochemically synthesized graphene, MoS2, and WS2 QDs, the PQDs exhibit excitation wavelength-independent photoluminescence emission.16−18 Upon electrochemical exfoliation, the phosphorus atoms at the edges of PQDs are likely to end up with dangling bonds, which will be instantaneously passivated by the oxygen functionalities to balance the structural integrity. Broad absorption bands in the UV−visible spectrum are a characteristic indication of luminophores, and the three emission peaks in the photoluminescence spectrum are attributed to the lowest unoccupied molecular orbital (LUMO)−highest occupied molecular orbital (HOMO) electronic transitions among the energy levels of these luminophores. The energy difference between the peaks at 406 nm (3.05 eV) and 429 nm (2.89 eV) in the photoluminescence emission spectrum is 0.16 eV. Interestingly, a similar energy difference is noticed between the peaks appearing at 429 nm (3.05 eV) and 457 nm (2.71 eV). This energy difference underscores that electronic transition from LUMO−HOMO is responsible for the photoluminescence emission, and the schematic illustration in Figure 4 shows various transitions occurring between the LUMO and HOMO41,42 These luminophores in PQDs (PO, P−OH etc.) have been identified by FT-IR spectroscopy (Figure S3, Supporting Information) and X-ray photoelectron spectroscopy. These functional groups exert a +I effect (electron donating effect) and increase the electron density on the PQD skeleton, causing the HOMO levels to elevate. This results in narrowing the bandgap until the passivation is saturated. Therefore, PQDs exhibit excitation wavelength-independent emission. Such excitation wavelength-independent photoluminescence
Figure 2. X-ray diffraction patterns of BP and electrochemically prepared PQDs.
Figure 3c, PQDs exhibit three peaks at 406, 429, and 457 nm in the photoluminescence emission spectrum. The strongest photoluminescence emission is observed at 429 nm and is responsible for the blue luminescence. The highest photoluminescence emission is observed at an excitation wavelength of 375 nm. This corroborates the broad absorption peak noticed at 370 nm in UV−vis absorption. A Stokes shift of 54 nm is calculated for this transition. Unlike our earlier
Figure 3. (a) UV−visible spectrum of electrosynthesized PQDs (inset: PQDs under 365 nm UV light along with blank electrolyte). (b) UV−visible spectra of PQDs recorded after a few days. (c) Photoluminescence spectra of PQDs exhibiting excitation wavelength independence. (d) Timeresolved fluorescence decay pattern for PQDs recorded at an excitation wavelength of 370 nm and emission wavelength of 420 nm. 975
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The effect of electrochemical exfoliation is further probed by the changes in the cyclic voltammetric response of BP before and after electrochemical exfoliation. Figure S4 shows the superimposed cyclic voltammetric responses of BP before and after exfoliation at −2.0 V for 24 h in a potential window of −2.0 to +2.0 V at 100 mV/s in deaerated propylene carbonate containing 0.1 wt % LiClO4 as a supporting electrolyte. BP exhibits an oxidation peak at +1.5 V and a reduction peak at −1.1 V. The latter is attributed to reduction of trace levels of dissolved oxygen. Upon maintaining the potential at −2.0 V for 24 h, the capacitive current is increased, as can be seen by the change in the nonfaradaic region of the voltammogram. Importantly, the open-circuit voltage changes drastically from −0.002 to −0.67 V versus a Pt wire quasi-reference electrode. A similar shift in open-circuit voltage was observed in our earlier study of the electrochemical exfoliation of WS2.16 This large OCV shift is associated with the thermodynamic feasibility (the free energy change is calculated to be −63 kJ mol−1) of the cathodic exfoliation. Figure 5 shows the comparative high-resolution P 2p and O 1s X-ray photoelectron spectra of BP and PQDs. Fitted curves
Figure 4. Illustration of the excitation-independent photoluminescence emission mechanism of electrosynthesized PQDs.
emission is also reported for PQDs synthesized by laser ablation techniques.41,42 For any luminescent material, the photoluminescence quantum yield (PLQY) is an important parameter as it is a direct measure of the fraction of excited molecules returning to the ground state by radiative emission (fluorescence).21 The relative PLQY measurement is executed using typical standard absorption and emission spectroscopy. The integrated emission intensity is compared with the same quantity of a standard reference with known QY, and the relative quantum efficiency of the material is calculated using the following expression43,44 Øsf =
I sA r ns 2 I rA snr 2
Ørf
Øsf and Ørf are the photoluminescence QY of the sample and that of the reference, and Is and Ir are the integrated intensities of sample and reference spectra, respectively. As and Ar correspond to absorbance of the sample and the reference, and ns and nr are the refractive indices of the sample and reference solvents, respectively. In the present study, quinine sulfate monohydrate in 0.5 M H2SO4 (PLQY = 0.54) is taken as the reference (refer to Figure S5 in the Supporting Information). The calculated PLQY is found to be 84%, which is higher than that of the other pristine blue fluorescent PQDs prepared by liquid phase exfoliation in toluene (9%), chloroform (5%), and laser ablation techniques (20.7%).11,41,45 A possible reason for this could be the higher radiative relaxation rate of PQDs. To account for this, the radiative and nonradiative decay constants have been calculated from timecorrelated single-photon counting measurements at an excitation wavelength of 370 nm. Accordingly, Figure 3d shows the fluorescence decay curves, which have been fitted to a biexponential decay curve (Table S2, Supporting Information). The average lifetime (τav) of blue-emitting PQDs is calculated to be 956 ps with decay components of 856 ps (99%) and 10.9 ns (1%), which indicates the presence of species with two distinct lifetimes. To gain more insight into the photoluminescence emission, radiative and nonradiative decay constants are calculated for PQDs. The calculated radiative relaxation rate is found to be 8.6 × 108 s−1, and the nonradiative relaxation rate is found to be 1.77 × 108 s−1. The radiative relaxation rate is 4-fold higher than the nonradiative pathway, which accounts for the higher PLQY. The PLQYs of PQDs synthesized via other methods are relatively low, ranging from 7.2 to 20.7%.41,42,45−47 The higher blue fluorescence of the QDs is beneficial for QD-based blue-light-emitting devices and bioimaging.
Figure 5. (a) X-ray photoelectron P 2p spectrum of bulk BP, (b) O 1s spectrum of bulk BP, (c) P 2p spectrum of PQDs, and (d) O 1s spectrum of PQDs.
for the P 2p spectrum of BP (Figure 5a) show that the P 2p peaks of bulk BP can be deconvoluted into three peaks. The peaks at 129.7 and 130.7 eV correspond to the spin−orbit splitting doublet of the 2p orbital of BP (P 2p3/2 and P 2p1/2, respectively, P0 oxidation state). The peak at higher binding energy (134.3 eV) originates from the surface oxidation of BP to phosphorus oxides (P5+ oxidation state).48,49,51,52 Upon electrochemical exfoliation, a significant decrease is observed in the intensity ratio of P 2p3/2 to P 2p1/2 in PQDs compared to that of BP (Figure 5c). This comes from the presence of defects in PQDs due to edge reconstruction on the P atoms during exfoliation. The P 2p spectrum of PQDs can be deconvoluted into five peaks at 128.0, 130.1, 132.3, and 134.3 eV, which can be correspondingly assigned to the spin−orbit splitting doublet of the 2p orbital into P 2p3/2 and P 2p1/2, O− PO (dangling bond), and P2O5.49,51,52 An O−PO 976
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Figure 6. Models of PQDs geometry-optimized by density functional theory calculations with increasing oxygen content with O/P ratios varying from 6 to 30%
identified from the X-ray photoelectron spectra is not yet clear from these experiments. The oxygen functionalities noticed on the phosphorene moieties are expected to deteriorate the stability of PQDs. In order to gain more insight into the stability of PQDs bearing oxygen functionalities, oxygen passivation on PQDs is studied using first-principles-based density functional theory calculations. The calculations are performed on oxygenated PQDs to evaluate their stability upon sequential oxygen passivation. The average diameter of the PQDs synthesized in the present work is ∼8 nm, which is quite small, and therefore, a representative model of PQD with 54 atoms is opted for the study (more details on the calculations are provided in the Supporting Information). Importantly, we have restricted our simulation studies to only the monolayer, though experimentally we have observed 3−4 layers in electrosynthesized PQDs. PQD without any oxygen is modeled first, followed by its sequential oxygenation with 3, 6, 9, 12, and 16 oxygen atoms, viz., PQD-3O, PQD-6O, PQD9O, and PQD-16O (fully oxygenated model) to construct oxygenated PQDs. As the exfoliation causes the edge planes of PQDs to be more vulnerable for oxygen passivation, our DFT studies are limited to only edge passivation (not basal planes). The oxygen to phosphorus ratio (O/P) is varied from 6 to 30%. The structures of these PQDs are shown in the Figure 6. These geometries exhibit both armchair and ziz-zag edges, as noted in earlier studies.49 To correlate with the experimental results, IR stretching frequencies are calculated for all of the models, and the same are shown in Figure 6b. Interestingly, the PO stretching frequency calculated for all of the models is noted to be around 1100 cm−1, which is in agreement with the experimental IR wavenumber53 of PO stretching in the present work. This value is also in agreement with the range of PO stretching frequencies (1450−1080 cm−1) observed in other phosphorus oxide compounds,53 thereby reaffirming the presence of oxygen functionalities in the PQDs synthesized experimentally. To explain the experimental observations and have better insight into the structure−property correlation within the
dangling bond is considered as a dangling oxygen on the phosphorene surface forming a bond with only one P atom. This causes the P−O bond to be more polar due to a higher electronegativity difference between the dangling oxygen and the phosphorus atom (the lone pair on phosphorus is more attracted to oxygen, thereby increasing the electron density on oxygen). The binding energy of O−PO (dangling) will therefore appear in the higher binding energy region of the P 2p spectrum.50 The shifting of P 2p3/2 and P 2p1/2 peaks to lower binding energy is attributed to air exposure, which is in agreement with the transport measurements performed on BP as a function of air exposure reported in the literature.49,52 The O 1s spectrum of BP (Figure 5b) shows three peaks corresponding to PO (531.1 eV), P−O−P bonding due to bridging oxygen (532.6 eV) and the hydroxyl groups, and P− OH (533.3 eV). However, an additional peak assigned to O− PO due to dangling oxygen is noticed in the O 1s spectrum of PQDs (Figure 5d), which also appears in the P 2p spectrum of PQDs at 132.3 eV. The X-ray photoelectron binding energies of P 2p and O 1s of PQDs are in excellent agreement with those of phosphorene prepared by mechanochemical exfoliation of BP, where the P 2p spectrum exhibited five peaks at 129.7, 130.5, 131.6, 132.8, and 134.3 eV correspondingly assigned to P 2p3/2 and P 2p1/2 of P−P bonds, bridging P−O− P bonding, dangling oxygen bonding O−PO, and P2O5 and the O 1s spectrum exhibited three peaks at 530.6, 532.0, and 533.0 eV that can be correspondingly assigned to PO, P− O−P, and P−OH bondings.51 The oxygenated groups detected on the bulk BP are due to the unavoidable moisture and oxygen adsorption on the surface, and it becomes more prominent upon exfoliation into PQDs to balance the structural distortion. Though X-ray photoelectron spectroscopy is a highly useful analytical technique in identifying the chemical composition, it is important to note that this is a surface characterization technique with limitation of depth analysis up to only 10 nm and capable of data acquisition for a collection of particles. The structural stability of the electrochemically synthesized PQDs with the slightly passivated oxygen functionalities as 977
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Figure 7. (a) Variation of P−P interatomic bond distances as a function of oxygen content and (b) calculated and experimental IR spectra of PQDs.
PQDs, we compare the nonoxygenated (Figure 6a) and oxygenated PQDs (see Figure 6b−f). The figures clearly demonstrate that increasing oxygen content results in structural distortion of PQDs. The magnitude of structural distortion is quantified through calculation of the average P−P interatomic distance before and after the insertion of oxygen atoms in the PQD matrix. Variation in the average interatomic distance as a function of oxygen content is shown in Figure 7a. PQDs without any oxygen functionalities have an average P−P bond distance of ∼2.26 Å. A negligible change is observed in this value upon inclusion of 3−6 oxygen functionalities. In the case of PQD-9O, the average interatomic distance between phosphorus atoms increases to 2.28 Å. Upon further insertion of oxygen functionalities (above 12 atoms for the present model), P−P bonds stretch to nearly 2.31 from 2.26 Å noted for the case of nonoxygenated PQDs. Therefore, the phosphorene skeleton undergoes an important structural distortion. In order to account for this, all of the P−P bond distances in nonoxygenated (PQD) and completely oxygenated (PQD-O) QDs are calculated and compared (inset in Figure 7a). The plot clearly indicates that the values of P−P bond lengths in PQDs vary between a narrow range of 2.25 and 2.26 Å. On the other hand, the P−P bonds in PQD-O vary largely between 2.2 and 2.5 Å. These results imply that the structural integrity of PQDs is still maintained in the presence of optimum oxygen content (PQD-3O, PQD-6O). Nevertheless, higher oxygen content results in P−P bond weakening, and therefore, the structural strength of PQDs weakens drastically. Evidence of this structural distortion is also provided by Hirshfeld atomic charge analysis of oxygenated and nonoxygenated PQDs. An average nominal charge of −0.001 e is noted on the P atoms in the PQDs in the absence of oxygen atoms. However, upon increasing the oxygen content in the PQDs from 3 to 16 oxygen atoms, the average charge transferred to the phosphorus skeleton becomes 0.03 e (PQD3O) and 0.13 e (PQD-16O), respectively. These findings indicate that an increase in the oxygen content leads to an increasingly polarized phosphorus skeleton that can be directly correlated with the stretching of P−P bonds. This increase in the in-plane P−P bond length and increment in electron density has been experimentally corroborated with the X-ray diffraction pattern, X-ray photoelectron spectroscopy, and excitation-independent photoluminescence emission mecha-
nism of PQDs. The observations imply that the phosphorene skeleton balances the structural distortion upon functionalization with minimal or an optimum oxygen content. Concrete evidence for this is the diminished structural robustness in highly oxygenated PQDs. Though the structural distortions caused upon exfoliation of BP into PQDs is balanced by optimum oxygen passivation to a certain extent, the structural collapse posed at higher oxygen content presents experimental challenges in stabilizing PQDs. Such challenges hamper PQDs from devising electronic devices that are exposed to air because the structural degradation caused will be reflected in their electronic properties. Despite explicit efforts to develop novel protection strategies to ensure minimal oxygen termination on PQDs or efficient passivation, the possibility of controlled formation of native oxide imparting stability to pristine BP QDs is intriguing and demands more in-depth studies. In conclusion, we have investigated the structural distortion of BP upon electrochemical transformation to blue luminescent PQDs (average diameter ≈ 8 nm). X-ray photoelectron spectroscopic analysis and the excitation-independent photoluminescence emission mechanism of PQDs confirm the structural distortion. The structural stability of PQDs is maintained by oxygen passivation at the dangling bonds with an optimum oxygen content as revealed from the density functional theory calculations on oxygenated and nonoxygenated PQDs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b03600. Experimental section, details of density functional theory studies and materials characterization, atomic force micrographs, summary of fitted results of the lifetime decay profile, cyclic voltammograms, and FTIR spectrum (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 978
DOI: 10.1021/acs.jpclett.8b03600 J. Phys. Chem. Lett. 2019, 10, 973−980
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The Journal of Physical Chemistry Letters ORCID
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Krati Joshi: 0000-0002-8692-9835 Bikash Jana: 0000-0001-5776-4851 Amitava Patra: 0000-0002-8996-9015 Subbiah Alwarappan: 0000-0001-9559-8989 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.O.V. and K.J. acknowledge CSIR for Senior Research Fellowships, and S.A. acknowledges CSIR for the Indo−Italy bilateral fund. V.K.P. acknowledges the Indo−US Science & Technology Forum (IUSSTF/JC-071/2017) for financial support.
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