Predominated Thermodynamically Controlled Reactions for

Mar 27, 2018 - Department of Chemistry, Indian Institute of Science Education and Research , Kolkata 700064 , India. J. Phys. Chem. ..... Interestingl...
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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Predominated Thermodynamically Controlled Reactions for Suppressing Cross Nucleations in Formation of Multinary Substituted Tetrahedrite Nanocrystals Suman Bera, Anirban Dutta, Sankararao Mutyala, Dibyendu Ghosh, and Narayan Pradhan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00680 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Predominated Thermodynamically Controlled Reactions for Suppressing Cross Nucleations in Formation of Multinary Substituted Tetrahedrite Nanocrystals Suman Bera, Anirban Dutta, Sankararao Mutyala, Dibyendu Ghosh¥ and Narayan Pradhan* 

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India. ¥ Department of Chemistry, Indian Institute of Science Education and Research, Kolkata, India. TOC graphic

ABSTRACT: Group I-II-V-VI semiconducting Cu12-xMxSb4S13 (M = ZnII, CdII, MnII and CuII) substituted tetrahedrites nanostructures remains a new class of multinary materials which have not been widely explored yet. Having different ions, the formation process of these nanostructures has always the possibility of formation of cross nucleations. Minimizing the reaction time, herein, a predominantly thermodynamically control approach is reported which decoupled the quaternary nucleations with their possible cross nucleations. As a consequence, possible cross nucleations were prevented and series of nearly monodisperse intriguing substituted tetrahedrite nanostructures are formed. The possible LaMer plot for this single- and also multi-materials nucleations is also proposed. Further, bandgaps of all these new materials are calculated and preliminarily the applicability of these materials is tested for photoelectrochemical water splitting.

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Multinary nanocrystals having multivalent cations are extremely important among the developed functional nanomaterials for their efficient photo-induced charge carrier mobility.1-11 Series of such nanocrystals in group I-III-VI and I-II-IV-VI semiconductors are extensively studied and explored for designing both light emitting and harvesting devices.1,3,9,12-19 Unlike binary semiconductors, the chemistry of designing these nanocrystals never observed straight forward. In most cases, a secondary diffusion growth process was followed after formation of initial binary seed nanocrystals.20-28 The case goes even more complicated when more number of cations are present in the reaction system opening more possibilities in the formation of cross nucleations. There is no such generic approach or well established mechanism which would be adopted for designing new quaternary nanocrystal systems. Literature reports reveal that even though ternary and quaternary semiconductors are widely studied, but these are very much limited in comparison to widely explored binary nanocrystals.1,29-32 On the other hand, while group III elements are extensively studied,1-2,20 multinary nanocrystals involving group V elements are again limited.32-36 Tetrahedrite is a particular phase of copper antimony sulfide material having composition CuI(12-x)CuIIxSb4S13. This has also very unique crystal structure where Cu is present both in Cu(I) and Cu(II) oxidation states. Interestingly, while Cu(I) ions are present trigonally as well as tetrahedrally coordinated; but Cu(II) are present on facets and only tetrahedrally coordinated (Figure 1a).37-38 The diversity of this system is possible by substitutions of other bivalent ions in place of Cu(II) and monovalent ions Cu(I) and this might led to composition variable different semiconductors having tunable functional properties. While theoretically the importance of these substituted tetrahedrite nanostructures were predicted; but the experimental findings and their new

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functional properties were not explored. Since, these are quaternary systems where group I, II, V and VI elements were present, the cross nucleations leading to mixture of nanostructures remained always as possible alternative products. Hence, synthesis of these nanocrystals requires proper manipulations of the involved physical processes with balanced combination of thermodynamics and kinetic controlling parameters. It is widely known that Sb(III) ions in solution promptly react with sulfide ions and form its sulfide.39-40 Antimony sulfide (Sb2S3) is a one dimensional material having layered structure with two dimensional van der Waals interaction and in standard colloidal protocols, this typically attains tube, sheet and rod shapes even at low temperature.39-40 This brings more challenges for driving reactions leading to exclusive quaternary zero dimensional nanocrystals. In a facile approach, following fast injection of all precursors together and fast cooling, nearly monodisperse phase pure substituted tetrahedrite nanocrystals are reported. This minimized reaction time suppressed the cross nucleations and the reactions were allowed to be governed predominantly by thermodynamically controlled. The protocol was also observed generic where Cu(II) were selectively replaced with Cd(II), Mn(II) and Zn(II) ions leading to solution processed three new semiconducting nanomaterials. Further, the bandgap of all these substituted nanostructures were measured and preliminarily these were explored for photoelectrocatalytic (PEC) water reduction and observed superior catalytic activities than Cu12Sb4S13 nanocrystals. Synthesis of substituted tetrahedrite CuI(12-x)MIIxSb4S13 nanostructures were carried out following thermal decomposition of diethyldithiocarbamate (DDTC) complexes of all cations. Figure 1b shows the thermogravimetric analysis plots of DDTC Complexes of Cu, Sb, Zn, Cd and 3 ACS Paragon Plus Environment

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Mn. From these plots the injection temperature was fixed where all precursors could immediately be decomposed.

The best temperature window for all four nanostructures

remained within 250-260 oC. Figure 1c presents the temperature vs. time plots for the injection and cooling rate in achieving the desired products. Allowing more annealing time or slow cooling of the reactions always led to mixed products and the best case was for sharp cooling which exclusively generated tetrahedrite products.

Figure 1. (a) Crystal structure of tetrahedrite crystal. (b) Thermogravimetric (TG) plots of DDTC complexes of CuII, ZnII, CdII and MnII. CuII changes to CuI in amine medium and hence only CuII complex were taken. (c) Reaction temperature vs. time plot for annealing, sharp cooling and normal cooling. Among different procedures, only sharp cooling without annealing favored quaternary tetrahedrite nanostructures.

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Figure 2. (a-b) Wide area TEM images in different resolutions of Cu-Zn-Sb-S (CZAS), (Size: 15±2 nm).(c-d) wide area TEM images of Cu-Cd-Sb-S (CCAS) (Size: 22±2 nm), (e-f) Wide area TEM images for Cu-Mn-Sb-S (CMAS) (Size: 24±2 nm)nanocrystals and (g-h) Wide area TEM images of Cu-Sb-S (CAS) (Size: 25±4 nm) nanocrystals. (i) Size distribution histograms for the nanocrystals calculated from the average of 100 numbers of particles. Figure 2 (a-f) (and Figure S1-S3) show the transmission electron microscopy (TEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of Zn, Cd and Mn substituted tetrahedrite nanocrystals and also the standard tetrahedrite (CAS) (Figure 2g-2h). Only Zn substitution led to cube shape and in all other turned to spherical particles. Importantly, for all cases, the nanostructures were nearly monodisperse and these were obtained with high precession. Figure 2i shows the size distribution histograms for all these nanocrystals.

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Figure 3. (a) Powder XRD patterns for CuI(12-x)MIIxSb4S13(M=Zn, Cd, Mn, Cu) nanocrystals. (b) Raman spectra of all these nanocrystals,(c) X-ray photoelectron spectra of Cu in CZAS (M =Zn) showing absence of CuII in the nanocrystals. (d) HRTEM of single nanocrystal of CZAS, CCAS and CMAS, and their selected area FFT patterns. Further establishing the exact composition and structure of these nanostructures, different characterizations for detailed analysis were carried out. Figure 3a presents the powder XRD patterns of all four tetrahedrite nanostructures with M = Zn, Cd, Mn and Cu. Peak pattern of all cases were observed identical and this resembled with bulk tetrahedrite crystal phase (JCPDF = 025-0324); but their positions were narrowly shifted. This is possibly because of the incorporation of new cations which influenced the d-spacing. Figure 3b presents the Raman spectra showing peak at ~356 cm-1 which is typical characteristic for the tetrahedrite Cu12Sb4S13.44 Tetrahedrite nanocrystals are a particular class of phase among four phases of CuSb-S chalcogenides. From the literature reports, It was found that every Phase of Cu-Sb-S has

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its own characteristic Raman peak such as chalcostibite shows a strong Raman peak at ∼337 cm−1, tetrahedrite at ∼354 cm−1, and famatinite at ∼333 cm−1 (strong peak), 283 and 369 cm−1 (low-intensity peak).44 The red shift for M = Zn, Cd and Mn indicates the substitution in place of CuII.44Tetrahedrite nanocrystals shows strong Raman peak due to the vibration of M-S bonds and the shift in the frequencies of these peaks is observed only due to different force constants fM−S.44 During formation of substituted tetrahedrite (M= Zn, Cd, Mn) the CuII-S bonds are replaced by MII-S and hence change in force constants are in such extent that Raman peak position of the substituted Tetrahedrite remains nearly at same position (~362 cm-1), but slightly red shifted from the normal Cu-based tetrahedrite at ~356 cm-1. Figure 3c (Figure S4) presents the XPS spectra of CZAS for identifying the MII substitution and in this case no satellite peak for CuII was observed confirming CuII ions in the tetrahedrite were replaced with ZnII ions. Furthermore, high-resolution transmission electron microscopy (HRTEM) of single nanocrystal of all substituted tetrahedrites are presented in Figure 3d and the analysis of FFT pattern again confirmed the tetrahedrite phase in all cases. For obtaining exact composition, ICP-MS of all samples were carried out (see Table S1) and x for all cases was observed ~ 2 confirming almost equivalent amount of CuII were replaced by the substituent bivalent ions. Energy dispersive spectra for all samples were also measured (Figure S5) and the compositions in all cases were also found almost same or within the proximity as obtained from ICP-MS data. Elemental mapping on the HAADF-STEM images showing the presence of Cu, M (Zn and Cd), Sb and S are shown in Figure S6 confirming the presence of other divalent ions. As the parent compound CuI(12-x)CuIIxSb4S13 (CAS) is already

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reported through in different method,37,38, 45-47 the entire study emphasized with the three new substituted tetrahedrite nanostructures. However, while the chemistry of formation of these nanostructures was analyzed, the swift injection of the mixture of precursors at a certain reaction temperature followed by fast cooling was observed as the turning point for obtaining such exclusive quaternary nanostructures. This was optimized carrying out several trial and error reactions. While the optimized temperature was between 250-260 oC; but with lower temperature injection or allowing slow cooling or carrying out annealing even at desired temperature led to mixture of nanocrystals (Figure S7-S9 and all the reaction conditions are summarized in Table S2). The common byproduct in all cases is antimony sulfides and along with this binary sulfides of the bivalent ions are formed, due to the thermal decomposition of the respective precursor in low temperature. It is also established that in presence of alkylamines, these single source precursors would decompose catalytically at lower temperature than their thermal decomposition temperatures and the reaction is controlled kinetically.42 Predominantly kinetically and thermodynamically controlled reactions were already stated in literature.41,48 In contrary, either in octadecene or alkylamine, we observed identical results and exclusive quaternary tetrahedrite nanostructures were the ultimate products. Hence, the predominantly thermal control reaction indeed helped here for obtaining exclusive quaternary tetrahedrite nanostructures. Even though all the precursors were single source; two types of thiols were used in the reaction protocol, where t-DDT was introduced to the reaction system along with precursors and 1-DDT was used during purification. In absence of t-DDT, polydispersed nanocrystals were obtained. A representative TEM image for CZAS nanocrystals is presented in 8 ACS Paragon Plus Environment

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Figure S10 which showed the particles were immature cube structure and also were highly agglomerated. Hence, we assumed here t-DDT helped for instant supply of sulfide ions which triggered fast and matured growth of tetrahedrite nanostructures. Furthermore, 1-DDT was used during purification which helped in removing unreacted precursors and enhanced dispersity (Figure S11). The arresting observation noticed that the compositions also did not tune in either case irrespective of the variations of precursor concentrations in the initial mixture. The amount of CuII ions in all nanocrystals were replaced (as shown in XPS) and the same amount of other MII were incorporated. Cu-DDTC being fast reactive, it was assumed that its concentration remained limiting for the formation of respective nanocrystals and only proportion amount of other precursors decomposed. This became the only possibility in obtaining exclusive tetrahedrite or substituted tetrahedrite nanocrystals. On the other hand, interestingly, the shape of Zn substituted nanocrystals (CZAS) were cube shaped and all others were spherical. At this stage, it indeed became difficult to identify the mechanistic path of different shapes for different metal substitutions as the reactions were very fast. However, for the cube case in CZAS, the collected immediate samples suggested that these were formed from intermediate quasi spherical dots (Figure S12) suggesting all nanostructures were initially spherical. Hence, it could be assumed here for all three substituted tetrahedrites were initially formed spherical particles. But, in case of CZAS the high energy (111) facets of the dots were grown leading to nanocubes which was one of the established growth processes for formation of cube nanostructures.49 However, certainly more theoretical supports are required for establishing the same. 9 ACS Paragon Plus Environment

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Further, tuning the size of these nanostructures was also attempted by varying the reaction parameters. Interestingly, for CZAS while precursors are injected at 280 oC, mixture of two size nanocubes was obtained (Figure S13). The injection at high temperature and subsequent sharp decomposition quickly reduced the reaction temperature and hence secondary nucleations of same material were facilitated leading to two size particles. This suggests that the size may be tuned though needs further optimization. However, this all injection and fast cooling approach is ideal for single size particles as this would not lead to time dependent growth. Moreover, these reactions were intentionally cooled for stopping cross nucleations. For different size, new reaction at different temperature injection is desired for tuning the density of nucleation and subsequent fast growths. Interestingly, these tetrahedrites formation did not also require a fixed ratio of three precursors and changing the ratio +/- 25% molar ratio) different from 1:1:1 also led to similar nanostructures. However the size variation among different substituted tetrahedrites (M= Zn, Cd, Mn and Cu) suggests the variation of nucleation density from one systems to other as Cu-precursors played the determining role.

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Figure 4. Schematic presentation of LaMer plot for the nucleation and growth of multinary tetrahedrite nanocrystals (Plot A) and different cross products (Plot B). Since these nanocrystals were synthesized following a fast cooling approach, the formation pathways were further proposed as per LaMer plot of nucleation and growths.43 As Cu precursor remained the limiting factor and at high temperature injection of all precursors triggered the quaternary tetrahedrite nanocrystals, for the ideal case no further nucleations of cross products were interfered. Hence, both nucleations and growths of I-II-V-VI nanocrystals ended within certain time frame and it is shown under the plot A in Figure 4. On the other hand, if more time was allowed, nucleations of cross products were formed along with quaternary nucleations and these were grown as expected for standard protocol (Plot B of Figure 4). This mechanism followed only for injections at 260 oC or above. However, for lower temperature injection, the expected plot should follow similar to plot B. We believe, this is a new kind of mechanism where the reaction is designed to prevent the secondary nucleations and would help for understanding the mechanism of exclusive multinary nanocrystals.

Figure 5. (a) Absorption spectra of different tetrahedrite and substituted nanocrystals with M = Zn, Cd, Mn and Cu. Inset shows the digital image of the nanocrystals solutions of different compositions. (b) Band positions for all nanocrystals calculated from cyclic voltammetry (see 11 ACS Paragon Plus Environment

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substituted

tetrahedrite

photoelectrocatalysts. Dotted lines represent for dark and solid lines under illumination. (d) The time-dependent photocurrent density with illumination switched ON and OFF of modes of different photocathodes at −0.5 V vs. SCE. As these nanocrystals show optical absorption in visible spectrum, these nanocrystals could be ideal for light harvesting and hence, these were further explored as photoelectrocatalysts for hydrogen evolutions. Figure 5a shows the UV-Vis absorption spectra for CAS and three substituted tetrahedrites. All these show absorption in visible spectral window. Further, their band gaps were derived via cyclic voltammetry (Figure S14) and presented in Figure 5b. Since, band position of all these tetrahedrite nanocrystals were suitable for hydrogen evolution (Figure 5b) they were further explored as Photoelectrocatalytic (PEC) hydrogen evolution. Details of the PEC experiment are provided in the supporting information. Figure 5c shows the linear cyclic voltamogram (LSV) for all as synthesized tetrahedrite nanostructures. Interestingly, it was observed that for all the substituted tetrahedrite (M = Zn, Cd and Mn), the activities remained superior to the unsubstituted tetrahedrite. The photocurrent density obtained for these tetrahedrites nanocrystals are found to be comparable to standard CdS systems.50 Similarly, the photoresponse behavior of these nanostructures modified photoelectrodes (Figure 5d) also followed the same trend and the substituted tetrahedrites were observed better than Cu12Sb4S13. The half-cell solar-to-hydrogen (HC-STH) Efficiency for each case has been shown in Figure S15. Since, each nanostructure contains different metal ions, difference in their photo active behaviors were expected. However, the

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observed trend of Zn>Cd>Mn>Cu may be due to the change in surface-to-volume ratio. Higher the surface-to-volume ratio, higher is PEC activity. As the surface-to-volume ratio for M= Zn>Cd>Mn>Cu, the PEC activity is also in the same order. There might be some other factors e.g. extinction coefficient, charge transfer resistance etc. responsible for the activity differences. In conclusion, the physical process for a generic synthetic protocol is reported for fabricating nearly monodisperse substituted tetrahedrite nanocrystals. These were performed by adopting a fast injection and fast cooling of all precursors together which helped in obtaining exclusive substituted tetrahedrite nanocrystals without any cross nucleations. Investigating the formation processes and comparing with different controlled reactions, it was established that these reactions were mostly thermodynamically controlled. Being, the approach is facile and leads to exclusive quaternary nanocrystals, this may be extended for synthesize variety of multinary systems. As these nanostructures absorb visible light, their bandgaps were further calculated and the preliminary study as photocathode on PEC water reductions were reported. As these materials are new in the multinary systems, these might be further implemented for photovoltaic and other light harvesting applications. ASSOCIATED CONTENT Supporting Information Four supporting figures with additional TEM images, XRD, Raman, XPS, cyclic voltamograms are available in the supporting information. This material is available free of charge at xxxxx

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AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] ACKNOWLEDGEMENTS DST of India (SR/NM/NS-1383/2014(G), PDF/2016/ 003700 and EMR/2016/001795) acknowledged for funding. S.B. and A.D. acknowledge CSIR for fellowship, S.M. acknowledges SERB NPDF, and N.P. acknowledges DST Swarnajayanti for fellowship. REFERENCES (1) Coughlan, C.; Ibanez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M. Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117, 5865-6109. (2) Aldakov, D.; Lefrancois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mat. Chem. C 2013, 1, 3756-3776. (3) Manna, G.; Jana, S.; Bose, R.; Pradhan, N. Mn-Doped Multinary CIZS and AIZS Nanocrystals. J. Phys. Chem. Lett. 2012, 3, 2528-2534. (4) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I–III–VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167-3175. (5) Leach, A. D. P.; Macdonald, J. E. Optoelectronic Properties of CuInS2 Nanocrystals and Their Origin. J. Phys. Chem. Lett. 2016, 7, 572-583. (6) Debnath, T.; Maiti, S.; Maity, P.; Ghosh, H. N. Subpicosecond Exciton Dynamics and Biexcitonic Feature in Colloidal CuInS2 Nanocrystals: Role of In–Cu Antisite Defects. J. Phys. Chem. Lett. 2015, 6, 3458-3465. (7) Tao, X.; Mafi, E.; Gu, Y. Synthesis and Ultrafast Carrier Dynamics of Single-Crystal TwoDimensional CuInSe2 Nanosheets. J. Phys. Chem. Lett. 2014, 5, 2857-2862. 14 ACS Paragon Plus Environment

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(8) Pundsack, T. J.; Chernomordik, B. D.; Béland, A. E.; Aydil, E. S.; Blank, D. A. Excited-State Dynamics in CZTS Nanocrystals. J. Phys. Chem. Lett. 2013, 4, 2711-2714. (9) Guijarro, N.; Prévot, M. S.; Sivula, K. Enhancing the Charge Separation in Nanocrystalline Cu2ZnSnS4 Photocathodes for Photoelectrochemical Application: The Role of Surface Modifications. J. Phys. Chem. Lett. 2014, 5, 3902-3908. (10) Jara, D. H.; Stamplecoskie, K. G.; Kamat, P. V. Two Distinct Transitions in CuxInS2 Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures. J. Phys. Chem. Lett. 2016, 7, 1452-1459. (11) Singh, S.; Ryan, K. M. Occurrence of Polytypism in Compound Colloidal Metal Chalcogenide Nanocrystals, Opportunities, and Challenges. J. Phys. Chem. Lett. 2015, 6, 31413148. (12) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. Synthesis of CuInS2, CuInSe2, and Cu(InxGa1-x)Se2 (CIGS) Nanocrystal "Inks" for Printable Photovoltaics. J. Am. Chem. Soc. 2008, 130, 16770-16777. (13) Qin, L.; Li, D.; Zhang, Z.; Wang, K.; Ding, H.; Xie, R.; Yang, W. The determination of extinction coefficient of CuInS2, and ZnCuInS3 multinary nanocrystals. Nanoscale 2012, 4, 63606364. (14) Knowles, K. E.; Hartstein, K. H.; Kilburn, T. B.; Marchioro, A.; Nelson, H. D.; Whitham, P. J.; Gamelin, D. R. Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications. Chem. Rev. 2016, 116, 10820–10851. (15) Wu, X.-J.; Huang, X.; Qi, X.; Li, H.; Li, B.; Zhang, H. Copper-Based Ternary and Quaternary Semiconductor Nanoplates: Templated Synthesis, Characterization, and Photoelectrochemical Properties. Angew. Chem., Int. Ed. 2014, 53, 8929-8933. (16) Steinhagen, C.; Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Koo, B.; Korgel, B. A. Synthesis of Cu2ZnSnS4 Nanocrystals for Use in Low-Cost Photovoltaics. J. Am. Chem. Soc. 2009, 131, 12554-12555. (17) Chen, B.; Pradhan, N.; Zhong, H. From Large-Scale Synthesis to Lighting Device Applications of Ternary I–III–VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters. J. Phys. Chem. Lett. 2018, 9, 435-445.

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Wang, X.; Liow, C.; Qi, D.; Zhu, B.; Leow, W. R.; Wang, H.; Xue, C.; Chen, X.; Li, S.

Programmable Photo-Electrochemical Hydrogen Evolution Based on Multi-Segmented CdS-Au Nanorod Arrays. Adv. Mater. 2014, 26, 3506-3512.

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