Alloyed Shell Quantum Dots

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Cation Exchange in Zn-Ag-In-Se Core / Alloyed Shell Quantum Dots and their Applications in Photovoltaics and Water Photolysis Ganga Halder, Anima Ghosh, Sahanaz Parvin, and Sayan Bhattacharyya Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03743 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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Chemistry of Materials

Cation Exchange in Zn-Ag-In-Se Core / Alloyed Shell Quantum Dots and their Applications in Photovoltaics and Water Photolysis Ganga Halder, Anima Ghosh, Sahanaz Parvin and Sayan Bhattacharyya* Department of Chemical Sciences and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur - 741246, India *Email for correspondence: [email protected] Abstract Quantum dot sensitized solar cells (QDSSCs), especially those based on ternary AgInX2 (X = S, Se) QDs, suffer from very low power conversion efficiencies (PCEs) owing to presence of high density of trap states and imperfect surface coverage by surfactants. This has similar adverse effect on water photolysis when the solar cells are integrated with the electrolyzer. Passivation of QD surface with an inorganic shell is vital and temporally regulated cation exchange can come to the rescue whereby an alloyed shell can minimize the interfacial trap states which are mostly abundant in core/shell QDs with sharp interfaces. Herein Zn-Ag-In-Se (ZAISE) core QDs were cation exchanged with Cd2+ ions to create an alloyed shell of Ag-In-ZnCd-Se. When the QDs were used as absorbers, the PCE was enhanced from 3.01% for core ZAISE QDs to 4.71% for core/alloyed shell QDs by optimizing the shell thickness. 4.71% is also one of the superior PCEs in AgIn(S/Se)2 based photovoltaic (PV) systems. The density of states (DOS) investigated via density functional theory (DFT) formalism shows that the feasibility of charge transporation varies with the thickness of alloyed shell. Furthermore, a solar-to-hydrogen efficiency (STH) of 2.66±0.06% was achieved when four QDSSC devices were integrated with a water splitting electrolyzer constructed with earth-abundant catalysts.

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Introduction Semiconductor QDs have become attractive absorber materials for third generation PV technologies because of their unique optoelectronic properties such as size controllable band gap tunability, high absorption coefficient, ability to generate multiple excitation by single photon and large carrier lifetime.1-5 The QDSSCs fabricated with colloidal QDs pave the way for solution processed, facile and low-cost PV technology to meet our future energy demands.6 However, in spite of the theoretically predicted PCE up to 32%,3 QDSSCs suffer from their limiting PCE due to existence of numerous unwanted recombination pathways both within the QDs as well as at the interfaces of QD/metal oxide (MO), QD/MO/electrolyte and counter electrode (CE)/electrolyte.2,7 The presence of very high density of surface trap states in the QDs as well as the MOs and their interfacial defect states lead to the trapping of ‘hot’ electrons resulting in many non-radiative decay channels.8 These surface trap states mainly originate from the unsaturated dangling bonds present on the surface of MO and QD due to imperfect coverage of surfactants during synthesis.9 Hence, passivating the QD surface is crucial to achieve high performance QDSSCs.9-11 The surface modifiers can be molecular linkers such as 3mercaptopropionic acid (MPA), benzenedithiol, 1,2-ethanedithiol as well as atomic linkers like halide ions. The surface passivation strategies not only lower the trap state density but also improve the photoluminescence (PL) quantum yield, carrier mobility and air stability of the QDs.12-15 The growth of an epitaxial inorganic shell around the core QDs (Type-I core/shell QDs) can also reduce surface trap state density besides improving the chemical and photochemical stability of the QDs.16,17 However, optimization of shell thickness is essential to achieve high PCE since inappropriate shell thickness acts as a barrier for tunneling the electrons and holes.18,


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A very thin overlay of large band gap semiconductor can effectively suppress surface trap

states caused by unsaturated dangling bonds although this coating cannot suppress the intrinsic defect states within the QD lattice.17 Especially in case of core/shell QDs, lattice mismatch between the core and shell leads to charge carrier trapping at the interface. Very recent reports have shown that these interfacial defects can be further reduced by creating a gradient interface between the core and shell, owing to a gradual change in chemical composition at the interface.20,21 Recently, copper and silver based ternary and quaternary alloyed nanostructures have received significant attention for their interesting photophysical properties.22-26 However, these absorbers suffer from very high density of surface trap states due to their non-stoichiomertic composition.27 Passivating these intrinsic carrier trapping sites is indeed a challenge and more so when the primary aim is to improve the PCE of QDSSCs fabricated with the alloyed QDs as absorbers. Inorganic passivation along with a suitable band alignment to the energy levels of the core is advantageous over organic passivating ligands which have the tendency to hinder interfacial charge transport between the layers. In this work, ZAISE QD with a gradient alloyed shell of Ag-In-Zn-Cd-Se having a broad light harvesting range from visible to NIR were synthesized by hot injection method followed by cation exchange with Cd2+ ions. Oleylamine (OAm) was used as a surfactant in the reaction medium which plays an important role in selective cation exchange of In3+ and Zn2+ rather than Ag+. The reduction of core diameter occurs during the formation of alloyed shell, the thickness of which was controlled by changing the reaction time. The hydrophobic long chain OAm capped core/shell QDs were made water soluble and immobilized onto mesoporous TiO2 films via capping ligand induced self assembly. Through several optimization steps, the best PCE obtained is 4.71% when the shell was grown 3 ACS Paragon Plus Environment


for 20 min. DFT calculations elucidate the influence of alloyed shell thickness on the propensity of charge transportation which governs the PV performance. Furthermore the practical solution of future energy crisis is possible not only by fabricating relatively efficient QDSSC, but also converting the solar energy into chemical energy through H2 production. In addition to electrochemical, photoelectrochemical and photocatalytic water splitting, artificial photosynthesis devices are also designed by external integration of solar cells with a 2-electrode electrolyzer.28 Moving beyond noble metals, earth-abundant catalysts are the need of the hour,29,30 and transition metal based catalysts like NiFe-LDH (NiFe-layered double hydroxide) and NiMo alloy are well known electrocatalysts for their excellent performance in water splitting.31 The best performing PV device was integrated with an electrolyzer consisting of NiFe-LDH (+) // NiMo-alloy (-) couple for unassisted solar driven water splitting. Although external integration of an electrolyzer with hybrid perovskite solar cells is the usual norm,28,32 this is the first instance where QDSSCs are being used and a STH of 2.66±0.06% is obtained, which is directly proportional to the efficiency of the PV device. Results and Discussion Structural characterization of the QDs A two step synthesis procedure was employed to prepare the core/shell QDs. In the first step, ZAISE QDs were synthesized by hot injection method at 180°C. In the second step, a gradient alloyed shell was grown around the QD core by cation exchange with Cd2+ ions at a moderate temperature of 120°C. The shell thickness was altered by controlling the time allowed for cation exchange and thereby the QD samples are designated as CS0, CS5, CS10, CS20 and CS25 when the cation exchange reaction was continued for 0, 5, 10, 20 and 25 min, respectively. 4 ACS Paragon Plus Environment

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The powder X-ray diffraction (XRD) patterns in Figure 1a show that the QDs crystallize in tetragonal crystal structure with space group I42d according to the JCPDS pattern 75-0118. The crystallite size of the QDs is 3.1-3.8 nm, calculated using the Scherrer equation after subtracting the instrumental broadening. The transmission electron micrographs (TEM) show that the QD diameter is around 5-6 nm with a narrow size distribution (Figure 1b-f and Figure S1a-f). A slight increase in QD diameter is observed with increasing reaction time. The interplanar spacing and the indexed selected area electron diffraction (SAED) patterns (inset of Figure 1b-f) confirm high crystallinity of the as prepared QDs. The monodispersity of the QDs is also evident from the high angle annular dark field – scanning TEM (HAADF-STEM) images (Figure S2). Energy dispersive analysis of X-rays (EDAX) measurements confirm near stoichiometric inclusion of Ag, In, Zn, Cd and Se (Figure S3). The Zn:(Ag+In) ratio of CS0 is maintained at 44 at%, close to the earlier validated best performing QDs.25 On moving from CS0 to CS25, the atomic ratio of Cd2+ gradually increases from 0 to 0.78 with respect to (Ag+ + In3+) whereas the ratio of Zn2+/(Ag+ + In3+) decreases from 0.44 to 0.24, respectively which indicates the partial replacement of Zn2+ with Cd2+ during cation exchange. The exchange reaction kinetics is faster from CS0 (0.44) to CS5 (0.33) and becomes sluggish from CS10 (0.29) to CS25 (0.24) implying that Cd2+ can primarily replace Zn2+ near the QD surface than at the core. Cd2+ also replace In3+ over longer durations of cation exchange especially from CS10 to CS25. However in all cases the atomic % of Ag+ ions remains unaltered. These experimental findings could be explained in terms of hard-soft acid-base interaction principles developed by Parr and Pearson in 1983. The experimental absolute hardness values of Ag+, In3+, Zn2+ and Cd2+ are 6.96 , 13, 10.88 and 10.29, respectively.33 Also, OAm was used as a ligand in our synthesis methodology which is known to behave as a hard 5 ACS Paragon Plus Environment


base.34 Therefore, formation of In-OAm, Zn-OAm and Cd-OAm complexes during cation exchange reaction leads to comparatively more stable acid-base complexes than Ag-OAm. Overall the nearly equal hardness of Zn2+ and Cd2+ is responsible for the partial replacement of Zn2+ by Cd2+ ions as shown in Figure 2a. The cationic radius also plays a decisive role in the cation exchange reaction whereby the nearly identical ionic radii of Cd2+ (95 pm) with Zn2+ (74 pm) help to accelerate the exchange reaction. As the ionic radius of Cd2+ is slightly higher, an increment of interplaner spacing is also observed from 0.339 nm for CS0 to 0.362 nm for CS20 (Figure1b-e). Moreover, the presence of OAm in the reaction medium promotes the extraction of In3+ (ionic radii: 144 pm) by forming a more stable In-OAm complex than Cd-OAm as represented in Figure 2a and Figure S3. The extraction of In3+ ion from the lattice could reduce the interplanar spacing as the ionic radii of In3+ is higher than Cd2+ and Zn2+ as shown in Figure 1f. However, exchange of Ag+ by Cd2+ could not be observed under this reaction condition as interaction of Cd2+ with OAm results in the formation of relatively less stable Ag-OAm than CdOAm. The existence of alloyed shell is confirmed from X-ray photoelectron spectroscopy (XPS) which provides surface environment of the QDs. While the presence of Zn2+ state is verified from Zn 2p3/2 spectra in the binding energy range 1021.14-1021.76 eV, Ag+ and In3+ states are confirmed from Ag 3d5/2 (367.49-368.36 eV) and In 3d5/2 (444.3-445.22 eV) spectra, respectively (Figure S4).35,36 The appearance of Cd 3d5/2 level at ~405.6 eV implies successful cation exchange by Cd2+ ions.37 In cation exchange, analyses of the relative intensity of the cationic states at the QD surface are of relevance over the Se 3d5/2 level (54.26-54.38 eV).36 As shown in Figure 2b and Table S1, Cd2+/Ag+, Cd2+/In3+ and Cd2+/Zn2+ ratios, obtained from the relative area of deconvoluted peaks, increase with increasing cation exchange time and found to be ~2 fold 6 ACS Paragon Plus Environment

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higher than the EDAX results due to screening of core by the shell.38 The maximum exchange of Zn2+ by Cd2+ ions is observed due to their similar hardness as discussed earlier. The presence of all the core ions i.e. Ag+, In3+ and Zn2+ at the QD surface even after 25 min of cation exchange in CS25 confirms the alloyed shell nature of the QDs. Moreover, the In 3d and Cd 3d peak positions are not shifted with increasing shell thickness restating the core/shell architecture (Figure S5).39 Density of states - based electronic structure analysis The cation exchange by Cd2+ ions is likely to alter the charge distribution within the QDs. The DOS calculations of the hybrid systems as investigated via DFT formalism can elucidate the charge distribution between ZAISE and CdSe. The general bonding characteristics of different electronic states can be demonstrated by the partial and total DOS (PDOS and TDOS, respectively) of CS0, CS5, CS20 and CS25 structures (Figure 3). Chalcopyrite AgInSe2 has inherent Coulombic attraction between the charged defects i.e. InAg2+ (metal In-on-Ag anti-sites) and 2VAg1- (Ag vacancies) and DOS is calculated by substitution of Ag+ and In3+ by Zn2+ diffusing inside the lattice and thereafter CdSe is incorporated as the shell. The upper valance state of AgInSe2 mainly consists of strongly hybridized Ag-3d and Se-4p levels whereas the conduction bands near Fermi level are composed of hybridized In-5s, In-5p and Se-4s, Se-4p (Figure S6a). The obtained band gap from optimized AgInSe2 unit cell is 1.47 eV which is in good agreement with the experimental 1.50 eV.25 The intermediate energy sub-bands located at about -5.8 and -4.5 eV mainly consist of Se-4s and In-5s with a nominal admixture of Ag-3d, In5p and Se-4p levels (Figure S6b). From PDOS plots in Figure S7a, it is evident that Zn-3d bands mostly occupy the relatively deep level of valance band at around -6 eV and do not hybridize directly with the band consisting of In3+, Ag+ or Se2- states. In other words, depending on a 7 ACS Paragon Plus Environment


certain Zn2+ occupancy in Ag+ (or In3+) sites (here Zn:(Ag+In) = 0.45), Zn2+ acts as donor (or acceptor) and modifies the Fermi level. Due to Zn2+ occupation at Ag+ sites, Zn 3d10 is strongly hybridized with Se 4p4 and the Fermi level is lifted towards the conduction band due to the donor defect ZnAg1+ (Figure 3a). Similarly, the valence band side will be pushed down primarily due to the formation of ZnIn1- states when Zn2+ occupies the In3+ sites and plays an acceptor role. As the p-d admixture reduces, the calculated band gap increases to 1.58 eV (Figure S8) which can effectively enhance the PV performance. In case of CS5 (Figure 3b), the change of TDOS upon element substitutions indicates that the preoccupied Zn2+ of the outermost lattice sites are first replaced by Cd2+ which then gradually occupies the core positions of Zn2+. Here Cd2+ atom vacancies act as a donor. This observation is consistent with the previously reported results for Mn-substituted CuInSe2,40 and site occupations of Zn in AgInSe2.41 The changes in charge distribution as a function of Cd2+ content have a first-hand correlation to the charge transport propensity which has direct implications on the application of the cation exchanged QDs. In depth analysis of PDOS shows that although there exists a minor contribution of 5s (highest DOS at 3.36 eV) and 4d levels (mostly at intermediate and higher valance states) of Cd2+ in TDOS of the system but charge transportation within the system is dominated by Ag-d, Zn-d, Zn-s, In-s and In-p levels (Figure S7b). This happens due to the low percentage of Cd present in CS5. When CdSe shell thickness increases more Zn2+ sites can be occupied by Cd2+ (Figure 3c) in addition to the tendency of Cd2+ to fill the vacancies primarily created by ZnAg1+ or ZnIn1-. In CS20 (Figure S7c), the Cd-3d bands mostly occupy the highest hybridized bands on the valance band side around -7.8 to -7.4 eV which are deeper than the Zn3d level at -6 eV. When the shell thickness is increased further in CS25, Cd2+ starts occupying the In-sites and results in asymmetry of the system thereby resisting charge mobility (Figure 3d). 8 ACS Paragon Plus Environment

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From PDOS we can observe noticeable reduction in the DOS values of In-5s from CS20 to CS25 at -4.8 eV of valance band position. Although the increasing shell thickness from CS20 to CS25 enhances CdSe percentage within the system and the ZAISE lattice in particular, PDOS of both the systems shows that DOS of Cd-3d bands at relatively deep levels of the valance band decreases to half its value. Conversely with thicker graded shell from CS20 to CS25, the deep level DOS of Zn-3d increases (Figure S7d) indicating hindered charge transportation of valence and conduction band states. Optical properties of QDs The absorption spectra of all the QDs (Figure 4a) show light harvesting ability from visible to NIR which is an essential criterion to design a high performance QDSSC. The band gap of CS0, CS5, CS10, CS20 and CS25 QDs are 1.59, 1.79, 1.75, 1.73 and 1.72 eV; respectively as calculated from the Tauc plots (Figure 4b). The experimental values match well with the DFT calculated band gap (Figure S8). The increase in band gap with cation exchange is due to reduction of ZAISE core size by forming an alloyed shell with Cd2+ on the surface of the core.42 The relative PL intensity increases drastically from CS0 to CS5 upon shell growth and the maximum PL intensity is observed for CS20, beyond which it becomes almost constant (Figure 4c and Figure S9). This increment of PL intensity is due to effective reduction of surface defect states of core QDs by the formation of the alloyed shell.43 The alloyed shell also reduces the interfacial trap states mostly observed in core/shell QDs with sharp interfaces.20,21 A blue shift of ZAISE PL peak position is also observed from CS0 to CS5 upon shell growth which attests to the reduction of core due to construction of alloyed shell (Figure 4c). A red shift of PL peak position is also observed on moving from CS5 to CS25 due to increase in the overall diameter of the QDs (Figure S9). 9 ACS Paragon Plus Environment


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Time resolved PL decay measurements were performed for all the colloidal core and core/shell QDs in chloroform at an excitation wavelength of 402 nm (Figure 4d and Figure S10) to explore the effect of alloyed shell on charge recombination processes and the decay curves were fitted to look into the individual photophysical processes. The PL decay curves can be well fitted with tri-exponential functions by the equation: I(t) = A1exp (―t/1) + A2exp (―t/2) + A3exp (―t/3)

(1)

The average lifetime was calculated by equation (2): = ∑Ann2/∑ Ann

(2)

Here, A1, A2 and A3 denote the contribution of 1, 2 and 3 components, respectively. The average lifetimes of CS0, CS5, CS10, CS20 and CS25 QDs are 176.3, 201.9, 213.7, 217.2 and 218.3 ns, respectively (Table 1). After forming the alloyed shell, the average lifetime of the QDs increases. The τ2 component (58.6−68.6 ns) is related to surface trap state emissions.44 The contribution of this component is highly dependent on the reaction temperature and time as shown in Table S2. When temperature of the reaction is increased, Cd2+ ions have higher chance to diffuse from the QD surface to the lattice sites. As evident from Table 1, A1 component increases from CS0 to CS5 since during cation exchange, Cd2+ ions primarily reside at surface sites before reaching the lattice sites thus increasing the surface trap state contribution. As the reaction progressed from CS5 to CS25, the contribution of this component is lowered which indicates that the passivation of QD surface occurs due to formation of alloyed shell and thereby, the reduction of surface defect sites takes place.45 The slow decay component (τ3 ~ 191.9−234.6 ns) is contributed from donor-acceptor recombination between the Ag+ vacancies as donor sites


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and In3+ and Se2- vacancies as acceptor sites.35, 44-46 The faster decay component of CS0 (τ1 ~2 ns) arises due to the non-radiative recombination process.47 Upon shell growth from CS5 to CS25, the life time of this component increases to ~10 ns which indicates lesser chances of nonradiative recombination. Application of the cation exchanged QDs in photovoltaics For making the photoanode, the OAm capped QDs need to be made water dispersible by ligand exchange of OAm by 3-MPA molecules in order to bind the QDs to the mesoporous TiO2 film. The attachment of capping ligands onto CS20 QD surface both before (CS20-OAm) and after ligand exchange (CS20-MPA) was probed by fourier transform infrared (FTIR) spectroscopy (Figure S11) and 1H nuclear magnetic resonance (NMR) spectroscopy (Figure S12). The absence of S-H at ~2569 cm-1 in the FTIR spectra of CS20-MPA clearly indicates the attachment of 3-MPA through sulfur atoms.48 A significant broadening of 1H NMR signals is observed for CS20-OAm in CDCl3 and CS20-MPA in D2O when compared to free OAm and 3MPA molecules, respectively, due to faster spin relaxation, a well studied phenomenon in case of ligands bound to QD surface (discussion S1).49-51 The photoanode was fabricated by depositing MPA capped water dispersible QDs onto mesoporous TiO2 film electrodes by capping ligand induced self-assembly approach followed by 8 cycles of ZnS by successive ionic layer adsorption and reaction (SILAR) as surface passivation layer. The counter electrode was prepared of a paste of microwave synthesized CuS powder.52 The PV cells were made by sandwiching a droplet of electrolyte between the assembled working and counter electrodes. To understand the effect of shell thickness, the current density (J) – voltage (V) measurements were carried out as shown in Figure 5a and all the extracted solar cell 11 ACS Paragon Plus Environment


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parameters namely short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF) and PCE are enlisted in the Table 2. A systematic increment of VOC is observed from 0.37 V (CS0) to 0.53 V (CS25) with increasing shell thickness which owes to suppression of charge recombination due to formation of alloyed shell.16 This result also shows a good agreement with PL decay measurement where a reduction of surface trap states is observed with increasing shell thickness (Table 1). JSC also increases from CS0 (15.81 mA/cm2) to CS20 (19.06 mA/cm2). However, further thickening of shell can decrease the photocurrent in CS25 (15.43 mA/cm2) and PCE follows the same trend as that of JSC. This behavior is consistent with the charge transportation response to alloyed shell thickness as elucidated by DOS calculations and can be further explained by equation (3):4 (3)

JSC = ηLHE × ηinj × ηcc

where ηLHE is the light harvesting efficiency of the photoanode, ηinj and ηcc are the charge injection and collection efficiencies, respectively. A thicker alloyed shell than CS20 lowers ηinj by acting as a barrier layer and inducing blocking of TiO2 pores. Best photovoltaic performance is observed for QDSSC based on CS20 QDs with JSC = 19.06 mA/cm2, VOC = 0.490 V, FF = 0.520 and PCE = 4.71%. This value of PCE is 56% higher as compared to the QDSSCs based on CS0. The increment is even more significant when compared to 2.11% for ZnO/ZnS/AgInS2 nanotube arrays,53 and 3.57% for the earlier reported TiO2/ Zn2+-diffused AgInSe2 QD/ amorphous TiO2/ ZnS/ SiO2 configuration.25 Electrochemical impedance spectroscopy (EIS) studies can explain the trend of photovoltaic performance with QD alloyed shell thickness. EIS measurements were carried out in dark by applying forward bias voltage from near VOC to -0.2 V. This is a well accepted


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technique to understand the charge recombination processes in QDSSCs. The charge recombination resistance (Rrec) and chemical potential (C) are extracted by fitting the Nyquist curves and plotted against different bias voltages as shown in Figure 5b,c. A gradual increment of Rrec is observed with increasing shell thickness although C remains the same in all the cases implying that different shell growth in the QD sensitizers have no effect on the TiO2 conduction band position. Highest Rrec is observed for QDSSCs based on CS25 which indicates that thicker shells can more efficiently suppress the charge recombination at the interface of TiO2/QD/electrolyte even though they act as chemical barrier hindering electron transfer from QD to MO.16 Dark current measurement also follows the similar trend (Figure 5d) which further confirms the effective inhibition of charge recombination at the photoanode/electrolyte interfaces. Moreover, this alloyed shell can minimize the unwanted charge recombination of photoexcited electrons with the electrolyte as well as it prevents back electron transfer from TiO2 to QD as depicted in Figure 6, thereby improving the device performance. Water Photolysis After increasing the efficiency of the QDSSC device by shelling ZAISE core QDs with an alloyed shell of Ag-In-Zn-Cd-Se, the next task is to assemble this device with an electrolyzer, NiFe-LDH (+) // NiMo-alloy (-) for water photolysis (Figure 7a). When NiFe-LDH is used as anode for oxygen evolution and NiMo alloy as the cathode for hydrogen evolution in a typical two electrode configuration, this couple conducts overall electrochemical water splitting at a cell voltage of 1.56 V to reach a current density of 10 mA/cm2, which is only 10 mV higher than noble metal based IrO2 (+) // Pt/C (-) electrolyzer (Figure S13). The thermodynamic potential for overall water splitting is 1.23 V whereas for QDSSC driven water photolysis the VOC provided by one QD solar cell is only ~0.5 V. Therefore, to achieve a minimum voltage of 1.23 V, four 13 ACS Paragon Plus Environment


QDSSC devices need to be connected in series. This series connection is further connected through wires with the electrolyzer immersed in 1 M KOH as electrolyte whereby simulated solar irradiation provides sufficient energy to split water. The J-V response of the integrated solar cells is shown in Figure 7b, which yields a VOC of 1.64 V and JSC of 3.18 mA/cm2 with solar to electric efficiency (PV) of 2.93±0.05%. The operating current density (JOP) (2.16 mA/cm2) and operating voltage (1.28 V) is the intersection point between J-V curve of the solar cells in presence of light and J-V curve of the electrolyzer. The STH was calculated by multiplying JOP, thermodynamic potential for water splitting divided by total incident solar irradiance (S) as elaborated in discussion S2. The operating point is very close to maximum power point of the solar cell confirming minimal energy loss and effective conversion of chemical energy to electrical energy. To confirm JOP which corresponds to STH of 2.66±0.06%, a chronoamperometric measurement was carried out under unbiased solar light driven configuration for 500 s (Figure 7c). Conclusions ZAISE QDs with a gradient alloyed shell of Ag-In-Zn-Cd-Se were synthesized by hot injection method followed by cation exchange with Cd2+ ions at a moderate temperature. The thickness of the shell was controlled by changing reaction time at a particular concentration of Cd2+ ions in the reaction medium keeping the temperature constant. OAm plays a crucial role in selective cation exchange of In3+ and Zn2+ with Cd2+. Both the core and core/shell QDs show broad photon harvesting from visible to NIR range. Band gap increases for core/shell QDs due to reduction of core size by the formation of alloyed shell around the surface of the core QDs during cation exchange. PL spectra also show a blue shift and the relative PL intensity increases due to passivation of surface states. PL decay measurement also shows that surface trap state 14 ACS Paragon Plus Environment

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contribution decreases owing to cation exchange. DOS calculations demonstrate the need of optimizing the alloyed shell thickness, which beyond a certain optimum thickness retards the charge transportation in the system. The QDSSCs were fabricated with ligand exchanged QDs ensuring maximum loading onto the photoanode and by applying 8 ZnS SILAR cycles. PCE was enhanced from 3.01% with CS0 QDs to 4.71% for CS20 core/shell QD based QDSSCs. This cation exchange mediated enhancement in PCE is one of the best for AgInX2 (X = S, Se) QD sensitized solar cells. EIS measurement of the QDSSCs based on the core/shell QDs have higher charge recombination resistance than those with the core QDs. Applying the gradient alloyed shell around the core QDs also switch off the charge recombination pathways between the photogenerated electron and the electrolyte. This surface passivation strategy can be applied for other ternary QD systems to enhance the PCE further. When four best performing devices were integrated and connected with NiFe-LDH (+) // NiMo-alloy (-) electrolyzer, a STH of 2.66±0.06% was achieved. The utilization of QDSSCs along with low cost transition metal based catalysts offers many degrees of freedom for re-designing the architecture, and for incorporating new light absorbers and catalysts. Experimental Section Materials Silver Nitrate (AgNO3, Sigma Aldrich, 99.5%), indium chloride (InCl3, Sigma Aldrich, 99.999%), cadmium chloride monohydrate (CdCl2.H2O, Merck India), selenium powder (Sigma Aldrich, 99.5%), OAm (Sigma Aldrich,  98%), 1-octadecene (Sigma Aldrich, technical grade, 90%), 1-dodecanethiol (Sigma Aldrich,  98%), zinc acetate dihydrate (Zn(OAc)2.2H2O, Merck India, 98%), sulfur powder (purified, Merck India), sodium sulfide flakes (Na2S.9H2O, purified, 15 ACS Paragon Plus Environment


Merck India), potassium chloride (purified, Merck India), nickel nitrate hexahydrate (Ni(NO3)2.6H2O, 98%, Merck India), iron nitrate nonahydrate (Fe(NO3)3.9H2O, 98%, Merck India), nickel sulfate hexahydrate (NiSO4.6H2O, 98%, Merck India), ammonium molybdate ((NH4)6Mo7O24, 97%, Merck India), sodium chloride (NaCl, 99%, Merck), tri-sodium citrate dihydrate (Na3C6H5O7.2H2O, 98%, Merck India), aqueous ammonia solution (aq. NH3, 27 wt%, Merck India), potassium hydroxide pellets (KOH, Merck India, ≥85%), Toray Carbon Fiber Paper (CFP, Alfa Aesar), chloroform (Emplura, Merck India), ethanol (Absolute, Changshu Yangyuan, China) and methanol (Sigma Aldrich, ACS spectrophotometric grade, 98%) were used without further purification. Synthesis methodologies Synthesis of ZAISE QDs (CS0) The synthesis of Zn-AISE QDs was carried out by following our previous reported method.25 In brief, 0.5 mmol AgNO3, 0.5 mmol InCl3.4H2O and 0.5 mmol Zn(OAC)2.2H2O were mixed with 6 mL OAm and 12 mL octadecene. After purging out dissolved O2, the reaction mixture was heated to 88oC. 2 mL Se-precursor (1 mmol Se in 1 mL OAm and 1 mL dodecane thiol) was quickly injected and the mixture was additionally heated at 180oC for 5 min. After quenching in water bath, the QDs were precipitated with ethanol followed by centrifugation, before dispersing them in CHCl3 for further use. These QDs were taken as the core to prepare Zn-AISE/CdSe core/shell QDs. Preparation of Cd-OAm complex solution Cd-OAm complex solution was prepared by following our previous reported method with slight modification.54 In a three neck round bottomed flask, 1 mmol CdCl2.H2O and 10 mL OAm were 16 ACS Paragon Plus Environment

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loaded followed by evacuation and refilling with N2 three times at 60˚C. Then the mixture was stirred under N2 and heated to 170oC for 30 min to prepare the Cd-OAm complex solution. This mixture was cooled to 120oC and kept under N2 atmosphere for further use. Synthesis of Zn-AISE/CdSe core/shell QDs In the cation exchange mediated synthesis of core/shell Zn-AISE/CdSe QDs, initially the presynthesized Zn-AISE QDs were redispersed into 12 mL octadecene and 6 mL OAm. The dispersion was loaded into a three neck round bottomed flask and cycled between vacuum and N2 three times at 40oC. Thereafter, the required amount of Cd-OAm complex solution was injected into the QD dispersion and heated to 120oC for different specified durations to grow the shell. The shell thickness was in fact optimized by varying the reaction time and concentration of the Cd-precursor solution. Finally, the reaction mixture was quenched in a water bath to stop further growth of QDs and the product was isolated by precipitation with ethanol followed by centrifugation at 7000 rpm. The QDs were redispersed in chloroform for further use. Preparation of water soluble MPA-capped Zn-AISE/CdSe core/shell QDs The as synthesized OAm capped QDs were made water soluble by replacing the long chain hydrophobic group with a short chain bidentate molecule, MPA. To prepare the water soluble QDs, the pH of a solution of 500 μL MPA, 10mL MeOH and 2 mL distilled water was adjusted to 12 by adding 5 M NaOH. The MPA-MeOH solution was added in a chloroform dispersion of 0.5 mmol of Zn-AISE QDs and stirred for 2 h. The mixture was allowed to stand for few minutes to separate the phases. After discarding the bottom organic layer, the upper aqueous layer containing the QDs was washed with acetone to remove free MPA molecules and QDs were redispersed in water for further use. 17 ACS Paragon Plus Environment


Synthesis of CuxS Nanostructures CuxS nanostructures were synthesized by following our previously reported literature method.52 10 mmol Cu(NO3)2.6H2O, 10 mmol sulfur powder and 200 mL ethylene glycol were loaded into a round bottomed flask and irradiated in a microwave chamber at 125oC for 15 min at 600 W. After completion of the reaction, the product was isolated by centrifugation at 5000 rpm for 10 min and dried at 80oC for 1 h and kept for further use. Preparation of NiFe-LDH/CFP electrode NiFe-LDH was electrodeposited on cleaned CFP by following our previously reported method.31 Briefly, CFP of area 0.5  1 cm2 was immersed in an aqueous electrolyte solution containing 0.3 mM Ni(NO3)2.6H2O and 0.3 mM Fe(NO3)3.9H2O. The electrodeposition was carried out by using CFP as working electrode, Ag/AgCl (3 M KCl) as reference electrode and Pt wire as counter electrode at a constant potential of -1.7 V for 360s. A reddish brown film was uniformly deposited on the working electrode, which was washed with deionized water and dried for 6 h in vacuum for further use. Preparation of NiMo-alloy/CFP Electrode Bimetallic NiMo-alloy was similarly electrodeposited on fresh CFP of area 0.5  1 cm2, from a solution containing Ni(SO4)2 (0.017 M), Na3C6H5O7 (0.016 M), (NH4)6Mo7O24 (0.36 mM) and NaCl (0.28 M). Here the electrodeposition was carried out at a constant cathodic current density of -100 mA/cm2 for 1h. After electrodeposition, a dark grey film formed on the electrode which was washed with deionized water followed by drying in vacuum for 6 h. Fabrication of QDSSCs


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Preparation of TiO2 photoanode Fluorine doped tin oxide glass (FTO) (Solaronix, TCO 22-7, 2.2 mm thickness) was used as a substrate to make the photoanode. Three different layers of TiO2 were deposited on this substrate after cleaning it ultrasonically with soap water followed by washing with distilled water and ethanol. At first, a very thin layer of compact TiO2 layer was grown onto the substrate by dipping into an aqueous solution of 40 mM TiCl4 at 85°C for 50 min followed by washing with distilled water and ethanol. After that, the doctor blading technique was employed to deposit the active and the light scattering layers. ~10 m thick of active layer was deposited on compact TiO2 layer by using a TiO2 paste (Solaronix, Ti-Nanoxide T/SP, particle size ~20 nm) followed by annealing at 550°C in air for 1 h in a furnace. The light scattering layer (Solaronix, Ti-Nanoxide R/SP, particle size ~100 nm) was then prepared on the top of active layer (thickness ~4 m) followed by annealing at above mentioned conditions. Sensitization of TiO2 photoanode MPA-capped QDs were redispersed into deionized water and 50 μL of MPA was added to the dispersion. TiO2 films were immersed into this dispersion for sensitization after maintaining the pH of 12 with 5 M NaOH and kept for 24 h. Finally, the QD sensitized TiO2 films were washed with distilled water followed by ethanol and kept in dark for further use. Preparation of ZnS passivating layer The QD deposited TiO2 films electrode was passivated with a very thin layer of ZnS by SILAR technique. At first, the QD-sensitized TiO2 photoanode was dipped into an aqueous solution of 0.1 M Zn(OAc)2.6H2O solution for 1 min followed by rinsing with distilled water. The electrodes were further dipped into 0.1 M Na2S.9H2O solution for another 1 min. The electrodes 19 ACS Paragon Plus Environment


were further washed with distilled water to remove from unreacted S2- ions. Total 8 cycles of ZnS by SILAR was employed onto the photoanode for effective surface passivation. Preparation of CuxS counter electrode CuxS paste was prepared by stirring a mixture of CuxS nanostructures and polyvinyledene fluoride (10: 1 w/w) into N-methyl 2-pyrrolidone. The paste was deposited onto FTO by doctor blading technique and dried at 80oC for 1 h for further use. Electrolyte 2 M Na2S, 2 M S and 0.2 M KCl solution in methanol/H2O (3:7 v/v) was used as the electrolyte. Preparation of QDSSCs The QD-sensitized photoanode and the counter electrode were assembled using a 50 μm thick parafilm spacer in a sandwich configuration keeping a 50 μL droplet of polysulfide electrolyte in between them. The active area was chosen by covering a mask onto the working electrodes. The active area of the electrode was 0.2 cm2. Characterization The powder XRD measurements were performed with a Rigaku powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation. TEM and HAADF-STEM images were recorded with the DST-FIST facility, IISER Kolkata, JEOL, JEM-2100F. EDAX spectra were obtained with an Oxford Instruments X-Max with the INCA software attached to the Field emission scanning electron microscopy (FESEM). UV-Vis absorbance spectra of the QDs were recorded with Jasco V-670 spectrophotometer. Room temperature PL spectra were obtained with Horiba Jobin


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Yovon Fluorolog using a Xe lamp as the excitation source with an excitation wavelength of 450 nm. PL decay profiles were recorded using time correlated single-photon counting spectrofluorimeter from HORIBA Jobin Yvon IBH and the curves were fitted with an iterative fitting program provided by IBH to calculate the fluorescence lifetime. FTIR spectra of the OAm and MPA capped QDs were recorded with a Perkin Elmer spectrum RX1 with KBr pellets. 1H NMR spectra of the QDs before and after ligand exchange were carried out with Bruker 500 MHz NMR spectrometer. The data was analyzed with MestReNova software. XPS spectra were recorded using M/s Thermo Fisher Scientific Instruments and Al Kα source was used with 6 mA beam current and 12KV. Binding energies were calibrated to C 1s at 284.8 eV. Current-voltage measurements were performed by using a solar simulator provided by Newport using a 300W xenon lamp as a light source. The intensity of the light was 100 mW/cm2 adjusted by using a reference silicon solar cell in presence of 1.5 G filter. The electrochemical impedance measurements were carried out by CHI Electrochemical work station. The plots were fitted with inbuilt software in the electrochemical workstation. Electrochemical Measurements All electrochemical measurements were carried out in a conventional two-electrode electrochemical cell in 1M KOH using an electrochemical workstation (Biologic Instruments, VSP-300). The CFP electrodes with electrodeposited NiFe-LDH and NiMo alloy were used as working and counter electrodes, respectively. Before recording the electrochemical activity of the catalyst, all the electrodes were saturated using 20 cyclic voltammetry (CV) scans at a scan rate of 100 mV/s to enhance the hydrophilicity and wettability of the surface. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 10 mV/s in order to minimize the capacitive current and this scan rate is slow enough to ensure steady-state behavior at the 21 ACS Paragon Plus Environment


electrode surface. When the current density was calculated, the working surface area was calculated on a single side.55 DFT Calculations The density functional theory (DFT) calculations were performed by using the self-consistent full-potential linearized augmented plane wave (FP-LAPW) method, as implemented in WIEN2K package.56 The Perdew-Bruke-Ernzerhoff (PBE) functional of generalized gradient approximation (GGA),57 and Tran-Blaha modified Becke-Johnson (TB-mBJ) potential,58,59 were adopted as exchange and correlation effects. We have used k mesh of 4×4×4 sampling in the first Brillouin zone integrations, where the total energy converges to approximately 10-5 Ry. The valance states of Zn, Ag, In, Cd and Se were treated as 3d10 4s2, 4d105s1, 5s25p1, 4d105s2 and 4s24p4, respectively. The other criteria of the optimization for parameterization were adopted following the same procedures as mentioned in the earlier calculation.60 For studying the whole core-shell system and correlating our experimental results, four structures were modeled, namely, CS0 (where we assume the aspect ratio of Zn/(Ag+In) to be 0.45), CS5 (Zn/(Ag+In) is 0.35), CS20 (Zn/(Ag+In) is 0.3) and CS25 (Zn/(Ag+In) is 0.25). The percentage of Cd2+ gradually increases from 0 to 0.75 with respect to (Ag1+ + In3+) upon moving from CS0 to CS25. We optimized all the structures with starting structure of body-centered tetragonal distorted chalcopyrite having four formula unit cell of space group I42d. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: 10.1021/. 22 ACS Paragon Plus Environment

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TEM and HAADF-STEM images; EDX spectra; XPS spectra; TDOS and PDOS plots; Tauc plots; Reaction time dependent PL and decay spectra; FTIR spectra; 1H NMR spectra; Electrochemical water splitting characteristics; STH calculation. ACKNOWLEDGMENTS GH and SP thank University Grants Commission (UGC), New Delhi for their fellowship. AG acknowledges Science and Engineering Research Board (SERB), Department of Science and Technology (DST) for her fellowship under NPDF sanctions PDF/2016/001650. The authors thank Mr. Arjun Halder for XPS measurement and Mr. Surajit Haldar for helping in NMR data analysis. The financial support from Department of Science & Technology – Science & Engineering Research Board (DST-SERB) under Sanction No EMR/2016/001703 is duly acknowledged. SB thanks IISER Kolkata for the academic and research fund. References (1) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid Junction Solar Cells. Chem. Rev. 2010, 110, 6664–6688. (2) Halder, G.; Ghosh, D.; Ali, Md. Y.; Sahasrabudhe, A.; Bhattacharyya, S. Interface Engineering in Quantum Dot Sensitized Solar Cells. Langmuir 2018, 34, 10197–10216. (3) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908–918. (4) Sahasrabudhe, A.; Bhattacharyya, S. Dual Sensitization Strategy for High Performance Core/Shell/Quasi-shell Quantum Dot Solar Cells. Chem. Mater. 2015, 27, 4848–4859. (5) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.; Gao, J.; Nozik, A. J.; Beard, M.C. Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell. Science 2011, 334, 1530−1533. (6) Tian, J.; Cao, G. Semiconductor quantum dot-sensitized solar cells. Nano Rev. 2013, 4, 22578.



(7) Sahasrabudhe, A.; Kapri, S.; Bhattacharyya, S. Graphitic Porous Carbon Derived from Human Hair as 'Green' Counter Electrode in Quantum Dot Sensitized Solar Cells. Carbon 2016, 107, 395–404. (8) Tian, J.; Cao, G. Control of Nanostructures and Interfaces of Metal Oxide Semiconductors for Quantum-Dots-Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1859−1869. (9) Thon, S. M.; Ip, A. H.; Voznyy, O.; Levina, L.; Kemp, K. W.; Carey, G. H. Masala, S.; Sargent, E. H. Role of Bond Adaptability in the Passivation of Colloidal Quantum Dot Solids. ACS Nano 2013, 7, 7680−7688. (10) Abate, M. A.; Chang, J.-Y. Boosting the Efficiency of AgInSe2 Quantum Dot Sensitized Solar Cells via Core/shell/shell Architecture. Sol. Energy Mater. Sol. Cells 2018, 182, 37–44. (11) Zhao, K.; Pan, Z. X.; Mora-Seró, I.; Cánovas, E.; Wang, H.; Song, Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert, J.; Zhong, X. Boosting Power Conversion Efficiencies of Quantum-DotSensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602−5609. (12) McDaniel, H.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I. An Integrated Approach to Realizing High-Performance Liquid-Junction Quantum Dot Sensitized Solar Cells. Nat. Commun. 2013, 4, 2887. (13) de la Fuente, M. S.; Sánchez, R. S.; González-Pedro, V.; Boix, P. P.; Mhaisalkar, S. G.; Rincón, M. E.; Bisquert, J.; Mora-Seró, I. Effect of Organic and Inorganic Passivation in Quantum-Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1519−1525. (14) Chuang, C. M.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796−801. (15) Page, R. C.; Espinobarro-Velazquez, D.; Leontiadou, M. A.; Smith, C.; Lewis, E. A.; Haigh, S. J.; Li, C.; Radtke, H.; Pengpad, A.; Bondino, F.; et al. Near-Unity Quantum Yields from Chloride Treated CdTe Colloidal Quantum Dots. small 2015, 11, 1548−1554. (16) Jiao, S.; Wang, J.; Shen, Q.; Li, Y.; Zhong, X. Surface engineering of PbS quantum dot sensitized solar cells with a conversion efficiency exceeding 7%. J. Mater. Chem. A, 2016, 4, 7214–7221. (17) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Xinhua Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203−9210. (18) Neo, D. C. J.; Cheng, C.; Stranks, S. D.; Fairclough, S. M.; Kim, J. S.; Kirkland, A. I.; Smith, J. M.; Snaith, H. J.; Assender, H. E.; Watt, A. A. R. The Influence of Shell Thickness and Surface Passivation on PbS/CdS Core/Shell Colloidal Quantum Dot Solar Cells. Chem. Mater. 2014, 26, 4004–4013. 24 ACS Paragon Plus Environment

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(19) Ghosh, D.; Ghosh, A.; Ali, Md. Y.; Bhattacharyya, S. Photoactive Core-shell Nanorods as Bifunctional Electrodes for Boosting the Performance of Quantum Dot Sensitized Solar Cells and Photoelectrochemical Cells. Chem. Mater. 2018, 30, 6071–6081. (20) Ki Bae, W. K.; Kwak, J.; Park, J. W.; Char, K.; Lee, C.; Lee, S. Highly Efficient GreenLight-Emitting Diodes Based on CdSe@ZnS Quantum Dots with a Chemical-Composition Gradient. Adv. Mater. 2009, 21, 1690–1694. (21) Abdellah, M.; Žídek, K.; Zheng, K.; Chábera, P.; Messing, M. E.; Pullerits, T.; Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core−Shell Quantum Dots Attached to ZnO. J. Phys. Chem. Lett. 2013, 4, 1760−1765. (22) Wu, P. –J.; Yu, J. –W.; Chao, H. –J.; Chang, J. –W. Silver-Based Metal Sulfide Heterostructures: Synthetic Approaches, Characterization, and Application Prospects Chem. Mater. 2014, 26, 3485−3494. (23) Kim, S.; Kang, M.; Kim, S.; Heo, J. –H.; Noh, J. H.; Im, S. –H.; Seok, S. I.; Kim, S. –W. Fabrication of CuInTe2 and CuInTe2-xSex Ternary Gradient Quantum Dots and Their Application to Solar Cells. ACS Nano 2013, 7, 4756–4763. (24) Du, J.; Du, Z.; Hu, J. –S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; Wan, L. –J. Zn-Cu-In-Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201–4209. (25) Halder, G.; Bhattacharyya, S. Zinc-Diffused Silver Indium Selenide Quantum Dot Sensitized Solar Cells with Enhanced Photoconversion Efficiency. J. Mater. Chem. A 2017, 5, 11746–11755. (26) Peng, W.; Du, J.; Pan, Z.; Nakazawa, N.; Sun, J.; Du, Z.; Shen, G.; Yu, J.; Hu, J. –S.; Shen, Q.; Zhong, X. Alloying Strategy in Cu−In−Ga−Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 5328−5336. (27) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and quaternary metal chalcogenide nanocrystals: synthesis, properties and applications. J. Mater. Chem. C, 2013, 1, 3756−3776. (28) Luo, J.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N. G.; Tilley, S. D.; Fan, H. J. F.; Grätzel, M. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014, 345, 1593. (29) Majee, R.; Chakraborty, S.; Salunke, H. G.; Bhattacharyya, S. Maneuvering the Physical Properties and Spin States To Enhance the Activity of La–Sr–Co–Fe–O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation. ACS Appl. Energy Mater. 2018, 1, 3342– 3350. (30) Datta, A.; Kapri, S.; Bhattacharyya, S. Carbon Dots with Tunable Concentrations of Trapped Anti-oxidant as an Efficient Metal-free Catalyst for Electrochemical Water Oxidation. J. Mater. Chem. A 2016, 4, 14614–14624. 25 ACS Paragon Plus Environment


(31) Sahasrabudhe, A.; Dixit, H.; Majee, R.; Bhattacharyya, S. Value Added Transformation of Ubiquitous Substrates into Highly Efficient and Flexible Electrodes for Water Splitting. Nature Commun. 2018, 9, 2014. (32) Kumar, A.; Chaudhary, D. K.; Bhattacharyya, S. High Performance Duckweed-derived Carbon Support to Anchor NiFe Electrocatalysts for Efficient Solar Energy Driven Water Splitting. J. Mater. Chem. A 2018, 6, 18948−18959. (33) Pearson, R. G. Absolute electronegativity and hardness: application to inorganic chemistry. Inorg. Chem. 1988, 27, 734−740. (34) Trizio, L. D.; Manna, L. Forging Colloidal Nanostructures via Cation Exchange Reactions. Chem. Rev. 2016, 116, 10852−10887. (35) Kobosko, S. M.; Kamat, P. V. Indium-Rich AgInS2–ZnS Quantum Dots—Ag-/ZnDependent Photophysics and Photovoltaics J. Phys. Chem. C 2018, 122, 14336–14344. (36) Kshirsagar, A. S.; Khanna, P. K. Titanium dioxide (TiO2)-decorated silver indium diselenide (AgInSe2): novel nano-photocatalyst for oxidative dye degradation. Inorg. Chem. Front. 2018, 5, 2242–2256. (37) Feng, L. –N.; Peng, J.; Zhu, Y. –D.; Jiang, L. –P.; Zhu, J. –J.; Synthesis of Cd2+functionalized titanium phosphate nanoparticles and application as labels for electrochemical immunoassays Chem. Commun. 2012, 48, 4474–4476. (38) Nam, D. –E.; Song, W. –S.; Yang, H. Facile, air-insensitive solvothermal synthesis of emission-tunable CuInS2/ZnS quantum dots with high quantum yields J. Mater. Chem. 2011, 21, 18220–18226. (39) Trizio, L. D.; Prato, M.; Genovese, A.; Casu, A.; Povia, M.; Simonutti, R.; Alcocer, M. J. P.; D’Andrea, C.; Tassone, F.; Manna, L. Strongly Fluorescent Quaternary Cu−In−Zn−S Nanocrystals Prepared from Cu1‑xInS2 Nanocrystals by Partial Cation Exchange Chem. Mater. 2012, 24, 2400−2406. (40) Yao, J.; Wang, Z.; Tol, J. V.; Dalal, N. S.; Aitken, J. A. Site Preference of Manganese on the Copper Site in Mn-Substituted CuInSe2 Chalcopyrites Revealed by a Combined Neutron and X-ray Powder Diffraction Study, Chem. Mater. 2010, 22, 1647−1655. (41) Wang, L.; Ying, P.; Deng, Y.; Zhou, H.; Du, Z.; Cui, J. Site Occupations of Zn in AgInSe2Based Chalcopyrites Responsible for Modified Structures and Significantly Improved Thermoelectric Performance, RSC Adv. 2014, 4, 33897–33904. (42) Huang, B.; Dai, Q.; Zhuo, N.; Jiang, Q.; Shi, F.; Wang, H.; Zhang, H.; Liao, C.; Cui, Y.; Zhang, J. Bicolor Mn-doped CuInS2/ZnS core/shell nanocrystals for white light-emitting diode with high color rendering index. J. Appl. Phys. 2014, 116, 094303.


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(43) Li, W.; Pan, Z.; Zhong, X. CuInSe2 and CuInSe2–ZnS based high efficiency “green” quantum dot sensitized solar cells. J. Mater. Chem. A 2015, 3, 1649–1655. (44) Mao, B.; Chuang, C. –H.; Wang, J.; Burda, C. Synthesis and Photophysical Properties of Ternary I-III-VI AgInS2 Nanocrystals: Intrinsic versus Surface States. J. Phys. Chem. C 2011, 115, 8945–8954. (45) Chevallier, T.; Blevennec, G. L.; Chandezon, F. Photoluminescence properties of AgInS2– ZnS nanocrystals: the critical role of the surface. Nanoscale 2016, 8, 7612–7620. (46) Rao, M. J.; Shibata, T.; Chattopadhyay, S.; Nag, A. Origin of Photoluminescence and XAFS Study of (ZnS)1−x(AgInS2)x Nanocrystals. J. Phys. Chem. Lett. 2014, 5, 167−173. (47) Zhang, B.; Wang, Y.; Yang, C.; Hu, S.; Gao, Y.; Zhang, Y.; Wang, Y.; Demir, H. V.; Liu, L.; Yong, K. –T. The composition effect on the optical properties of aqueous synthesized Cu–In– S and Zn–Cu–In–S quantum dot nanocrystals. Phys.Chem.Chem.Phys. 2015, 17, 25133–25141. (48) Santos, C. I. L.; Carvalho, M. S.; Raphael, E.; Dantas, C.; Ferrari, J. L.; Schiavon, M. A. Synthesis, Optical Characterization, and Size Distribution Determination by Curve Resolution Methods of Water-Soluble CdSe Quantum Dots. Mater. Res. 2016, 19, 1407−1416. (49) Fong, C.; Wells, D.; Krodkiewska, I.; Hartley, P. G.; Drummond, C. J. New Role for Urea as a Surfactant Headgroup Promoting Self-Assembly in Water. Chem. Mater. 2006, 18, 594−597. (50) Grandhi, G. K.; Arunkumar M.; Viswanatha, R. Understanding the Role of Surface Capping Ligands in Passivating the Quantum Dots Using Copper Dopants as Internal Sensor. J. Phys. Chem. C 2016, 120, 19785−19795. (51) Reinhart, C. C.; Johansson, E. Colloidally Prepared 3‑Mercaptopropionic Acid Capped Lead Sulfide Quantum Dots. Chem. Mater. 2015, 27, 7313−7320. (52) Ghosh, D.; Halder, G.; Sahasrabudhe, A.; Bhattacharyya, S. A microwave synthesized CuxS and graphene oxide nanoribbon composite as a highly efficient counter electrode for quantum dot sensitized solar cells. Nanoscale 2016, 8, 10632–10641. (53) Han, J.; Liu, Z.; Guo, K.; Ya, J.; Zhao, Y.; Zhang, X.; Hong, T.; Liu, J. High-Efficiency AgInS2-Modified ZnO Nanotube Array Photoelectrodes for All-Solid-State Hybrid Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 17119−17125. (54) Halder, G.; Bhattacharyya, S. Plight of Mn Doping in Colloidal CdS Quantum Dots To Boost the Efficiency of Solar Cells. J. Phys. Chem. C 2015, 119, 13404−13412. (55) Kumar, A; Bhattacharyya, S. Porous NiFe-oxide Nanocubes as Bifunctional Electrocatalyst for Efficient Water Splitting. ACS Appl. Mater. Interfaces, 2017, 9, 41906–41915.



(56) Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2K, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties (Karlheinz Schwarz, Techn. University at Wien, Austria, 2001). (57) Perdew, J. P.; Burke, K.; Ernzerhof, M. Stabilization of Long Range Magnetic Order in 2D Easy-Plane Antiferromagnets. Phys. Rev. Lett. 1996, 77, 386−389. (58) Tran, F.; Blaha, P. Accurate band gaps of semiconductors and insulators with a semilocal exchange-correlation potential. Phys. Rev. Lett. 2009, 102, 226401. (59) David, K.; Tran, F.; Blaha, P. Improving the modified Becke– Johnson exchange potential. Phys Rev B 2012, 85, 155109. (60) Ghosh, A.; Thangavel, R.; Rajagopalan, M. Electronic and optical modeling of solar cell compound CuXY2 (X = In, Ga, Al; Y = S, Se, Te): first-principles study via Tran–Blahamodified Becke–Johnson exchange potential approach, J Mater Sci 2015, 50, 1710–1717.


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Table 1: PL lifetime decay parameters. QDs CS0 CS5 CS10 CS20 CS25

Reaction Cd/(Ag+In) A1 Time (min) 0 0 1.36 5 0.34 3.30 10 0.48 2.94 20 0.61 2.72 25 0.78 2.94

1 (ns) 1.93 10.6 10.7 9.52 10.3

A2 29.07 30.76 27.81 25.94 26.15

2 (ns) 63.2 66.6 66.9 65.8 66.8

A3 69.57 65.94 69.25 71.34 70.91

3 (ns) 191.9 221.3 231.2 233.1 234.6

2 1.06 1.11 1.06 1.09 1.12

τ (ns) 176.3 201.9 213.7 217.2 218.3

Table 2: Photovoltaic parameters of QDSSCs with different QD shell thickness. QDSSCs CS0 CS5 CS10 CS20 CS25

Composition AgIn0.97Zn0.87Se3.17 AgIn0.99Zn0.58Cd0.67Se3.03 AgIn0.71Zn0.48Cd0.83Se3.15 AgIn0.70Zn0.49Cd1.03Se3.14 AgIn0.60Zn0.39Cd1.24Se3.04

JSC (mA/cm )

VOC (V)

FF

 (%)

15.81 16.83 17.71 19.06 15.34

0.37 0.46 0.47 0.49 0.53

0.53 0.45 0.46 0.52 0.44

3.01±0.011 3.34±0.15 3.70±0.13 4.71±0.15 3.53±0.09

2



Figure 1: (a) Powder XRD pattern of QDs. TEM images and SAED patterns as inset of (b) CS0, (c) CS5, (d) C10, (e) C20 and (f) C25 QDs.


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Figure 2: (a) Schematic illustration of cation exchange reaction. Red, dark yellow, green, blue and yellow spheres indicate silver, indium, zinc, cadmium and selenium atoms, respectively. (b) Variation of surface composition of the QDs, determined from XPS analyses, as a function of cation exchange time.



Figure 3: DOS plots of (a) CS0, (b) CS5, (c) CS20 and (d) CS25. The dotted lines represent the Fermi energy (EF).


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Figure 4: (a) Absorbance spectra of colloidal QDs. (b) Band gap determination of colloidal QDs from Tauc plot. (c) Relative PL spectra and (d) PL lifetime decay of colloidal QDs. Excitation wavelength (ex) = 450 nm. All spectra were recorded by dispersing colloidal QDs in chloroform.



Figure 5: (a) J−V characteristics of the QDSSCs under 1 sun illumination. Plots of (b) recombination resistance and (c) chemical capacitance of the QDSSCs with different applied potentials. (d) Variation of dark current with applied potential in the log scale and (Inset) linear scale.


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Figure 6: Schematic illustration of charge transfer and charge recombination pathways of ZAISE/CdSe core/shell based QDSSCs after applying with ZnS passivating layers by SILAR. Solid arrow, solid curve arrows, dashed curve arrows and dashed arrows represent excitation, electron transfer, redox process and recombination pathways, respectively.



Figure 7: (a) Schematic representation of water splitting device. (b) J-V curves of the integrated solar cell under dark and 1 sun illumination, and NiFe-LDH (+) // NiMo-alloy (-) electrolyzer in a two electrode configuration. (c) current density-time curve of the integrated water splitting device under chopped 1 sun illumination without any external bias.


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Table of Contents (TOC)


Alloyed Shell Quantum Dots

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