Photoactive CoreâShell Nanorods as Bifunctional Electrodes for

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Photoactive Core-shell Nanorods as Bifunctional Electrodes for Boosting the Performance of Quantum Dot Sensitized Solar Cells and Photoelectrochemical Cells Dibyendu Ghosh, Anima Ghosh, Md. Yusuf Ali, and Sayan Bhattacharyya Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02504 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Chemistry of Materials

Photoactive Core-shell Nanorods as Bifunctional Electrodes for Boosting the Performance of Quantum Dot Sensitized Solar Cells and Photoelectrochemical Cells Dibyendu Ghosh, Anima Ghosh, Md. Yusuf Ali 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 The performance of solar energy conversion and storage devices is contingent to the appropriate design of photosensitizers that make the electrodes. In quantum dot sensitized solar cells (QDSSCs) the power conversion efficiency (PCE) is mainly limited by the impediments of electron-hole pair generation in the trap state abundant QDs at the photoanode and below expected catalytic activity of the counter electrode (CE). Employing photoactive CEs, that too with precisely tuned core/shell nanorods (NRs) array is thereby a new approach in addressing these limitations. Herein CdS/Cu2S core/shell p-n junction NRs increase photon harvesting over a wider solar spectrum as well as demonstrate enhanced catalytic activity to reduce the oxidized polysulfide electrolyte. The NRs were also employed as photoanodes in photoelectrochemical (PEC) water splitting. The NRs with varying shell thickness were synthesized by meticulously controlled cation exchange and their vertical caxis orientation not only offers high surface area to react with the electrolyte but also generates low sheet resistance. With a graded interface for better charge flow, electron tunnelling in the CE increases the photocurrent. In QDSSCs, when the Cu2S shell is thicker, Cu-d and S-p hybridization facilitates the tunnelling of holes from Cu2S shell to thinner CdS core increasing the PCE of QDSSC device, well validated by density functional theory (DFT) calculations. In PEC water splitting, the thin shell CdS/Cu2S NR photoanode however provides the highest photon-to-current conversion efficiency since thicker Cu2S shell creates larger depletion region increasing the recombination of charge carriers. 1 ACS Paragon Plus Environment

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1. INTRODUCTION Among the cost-effective strategies to convert and store solar energy in the form of chemical fuels, colloidal QD based photovoltaic (PV) devices, with rapid progress in PCE exceeding 12%, and PEC cells have attracted considerable attention.1,2 The credence in their ability to reach and even cross the Shockley−Queisser limit in QDSSCs,3 is due to the tuneable optoelectronic properties of QDs such as high absorption coefficient and multiple exciton generation.4−8 In order to boost the PCE of QDSSCs, although the photoanodes have received major attention,9−14 effective design of the electrolyte,15,16 and CE also play integral roles.17−20 Different materials have been employed as CE and among them the metal chalcogenides, such as CuxS,17,21−27, PbS,28,29 CoS,30,31 and Cu-Zn-Sn-S,32 have been manifested as premier electrocatalysts to reduce the oxidized electrolyte Sn2− to nS2−. Despite having low charge transfer resistance (RCT), the metal chalcogenides suffer from high sheet resistance, for which the graphene based composites have become viable alternatives.18,33-35 Two of the major bottlenecks to achieve a high PCE are the inefficient QD loading on TiO2 photoanode and the inability of a single type of QD to absorb significant portion of the solar spectrum. While oriented nanostructures in the photoanode can achieve an optimized loading, so far limited attempts have been made to increase the absorption spectrum. The tandem architecture of QDs in a solid state device although can increase the spectrum but unable to achieve a reasonable PCE since the thick QD layer interrupts the charge transfer process.36 On the contrary, employing a photoactive material in the CE that can enhance the range of absorption of the solar spectrum is a promising approach. Teng and co-workers first reported the use of a photoactive CE in QDSSC,37 whereas Song, Wan, Hu and co-workers demonstrated a tunnel junction based on ITO@Cu2S nanowire CE for liquid junction QDSSC.38 In this context, developing bifunctional CEs that are excellent electrocatalysts for

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hole transport as well as solar light absorber covering a significant spectral range is the need of the hour to fabricate tandem QDSSC devices. Not only as bifunctional CEs in QDSSCs, a decent photoactive material might also have potential applications in solar water splitting, which is a promising way of simultaneously harvesting and storing solar energy in the chemical bond of hydrogen molecule.2,39,40 In a PEC cell, Fermi energy of the semiconductor and the redox potential of water at the semiconductor/water interface play vital roles in transferring photogenerated electrons/holes to the water molecules. Here too the recombination of photogenerated carriers reduces the conversion efficiency of a PEC, which augments the necessity of designing efficient photoanodes by low-cost earth-abundant materials. Photoanodes made of heterostructures show higher conversion efficiency than the individual semiconductor components due to better solar light harvesting and effective charge separation.41,42 Among the recent reports, CdS/Cu2S co-sensitized TiO2 branched NRs demonstrated PEC conversion efficiency around 7.74% at -0.467 V versus Ag/AgCl.43 In order to elucidate the mechanistic principles of our bifunctional CE, we have synthesized p-n junction based core/shell NR arrays configured with n-type CdS core and ptype Cu2S shell (referred as CdS/Cu2S) as the CE of a QDSSC with the photoanode made of either CdS or CdSeTe QDs and polysulfide as the electrolyte. The light harvesting property of the core/shell NR arrays is further evaluated in the form of photoanodes in PEC water splitting. By incorporating a CdS core inside the shell of a well established hole scavenger Cu2S, a p-n junction is created that can harvest more photons for the device. Direct growth of c-axis oriented unidirectional CdS NRs on the conductive substrate and precisely controlled hetero-epitaxial growth of Cu2S shell via cation exchange have the advantages of superior charge transport across the graded core/shell interface and low sheet resistance. Additionally the three-dimensional (3D) architecture provides sufficiently active surface area to react with



the electrolyte. The charge transport strongly depends on the relative core and shell dimensions, the mechanism of which is understood from DFT calculations. 2. RESULTS AND DISCUSSION Vertically aligned CdS NRs were first grown on FTO coated glass substrates by hydrothermal method (See experimental section, for details). Depending on the duration of cation exchange of Cd2+ by Cu2+ ions, progressively thicker Cu2S shell could be obtained until a complete exchange takes place to obtain Cu2S NRs. The schematics in Figure 1a-d show the sequential cation exchange steps starting from CdS NRs to thin shell CdS/Cu2S NRs with 20% Cd2+ ions exchanged by Cu2+, thick shell CdS/Cu2S NRs when 60% Cd2+ is exchanged, and finally Cu2S NRs obtained by total cation exchange. The vertically aligned hexagonal shaped NRs have an average diameter ~200 nm and length 500-600 nm as evidenced from field emission scanning electron microscopy (FESEM; Figure 1e). The dimensions remain visibly unaltered even after complete cation exchange (Figure 1e-l). While the elemental composition of the NRs, proportionate with the extent of cation exchange, is confirmed by energy dispersive X-ray (EDX) analysis from 5-6 locations (Supporting Information, Figure S1), the uniform distribution of constituent elements along the length of the vertically aligned NRs is evidenced from the cross-sectional FESEM image of a representative sample of thin shell CdS/Cu2S NRs (Figure S2). The structural information from X-ray diffraction (XRD; Figure S3) reveals crystalline nature of the vertically oriented NRs. The lattice orientation is more evident from transmission electron microscopy (TEM) where the NR edges show lattice spacing ~0.335 nm corresponding to (002) plane of CdS (Figure 1i,m). As observed from the high resolution TEM (HRTEM) images in Figure 1m-p, the NR growth direction is [002] along the c-axis and this orientation does not show any observable change after cation exchange. Though it is arduous to identify the core/shell interface due to similar lattice spacing, the splitting of diffraction spots in the


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selected area electron diffraction (SAED) pattern of thick shell CdS/Cu2S NRs (inset of Figure 1o) discern the core/shell heteroepitaxial interfacial junction between the CdS core and Cu2S shell with a graded interface. After complete cation exchange, the Cu2S NR growth direction remains unaltered (Figure 1p). In fact the high-angle annular dark-field scanning TEM (HAADF−STEM) images of CdS (Figure 2a), thin shell CdS/Cu2S (Figure 2b), thick shell CdS/Cu2S (Figure 2c) and Cu2S NRs (Figure 2d) along with elemental mapping validate the core/shell architecture of the NRs. The line scan profiles show the Cu2S thin shell width to be 3-5 nm (Figure 2b) and the thick shell of 30-35 nm (Figure 2c). The sharp contrasts in the HAADF-STEM maps confirm successful cation exchange steps in creating core/shell NRs with graded interface. Generally CdS acts as a direct band gap n-type semiconductor and when combined with a p-type semiconductor like Cu2S, p-n junction is obtained. The diode nature could be observed for both the core/shell NRs but absent in CdS and Cu2S NRs (Figure S4). Prior to apprising on the device performance it is pertinent to understand the choice of such a photoactive material. The UV-vis-NIR absorption spectra of the CdS photoanode show a limited absorption of the solar spectrum up to ~490 nm (Figure S5) which is disadvantageous considering that a major part of the spectrum does not contribute to exciton generation. Such a narrow absorption is demonstrated both with 8−10 µm thick compact and mesoporous TiO2 layer deposited FTO coated glass substrate (referred to as meso-TiO2), and when CdS QDs are deposited onto this layer via successive ionic layer adsorption and desorption (SILAR) up to 8 cycles. On the contrary, the CdS/Cu2S core/shell NRs can extend the absorption beyond 750 nm which is close to the band-edge absorption of Cu2S (Figure S6) harvesting more solar energy for the device. This trend is reflected from the current density – voltage (J−V) characteristics in Figure 3a. At least 5 QDSSC devices were tested with each of the above photoactive NR CEs along with CdS QD photoanode and polysulfide electrolyte (see



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experimental section for details of device fabrication). PCE remains low at ~0.18% when CdS NRs are used as the CE, whereas with thin shell CdS/Cu2S NR CE, PCE is improved to ~2.60 %. PCE is enhanced significantly to ~4.48 % when the CE is composed of thick shell CdS/Cu2S NRs where the open-circuit voltage (Voc) is 577 mV and short-circuit current density (Jsc) is 14.01 mA/cm2 (Table 1). This performance is even better than the 100% cation exchanged Cu2S NR CE demonstrating PCE of ~3.81 %, brass/Cu2S, Pt and the previously employed Cu1.18S nanoplates, (Table 1).17 The superior performance of thick shell CdS/Cu2S NRs is also reflected from the incident photon to current conversion efficiency (IPCE) plots in Figure 3b. The IPCE spectrum of the QDSSC devices with thick shell CdS/Cu2S NR CE is additionally extended towards higher wavelength up to ~600 nm because of excess generation of excitons at the core/shell interface. The integrated Jsc calculated from IPCE is ~13.65 mA/cm2 for thick shell CdS/Cu2S NRs, commensurate with the Jsc of 14.01 mA/cm2 obtained from J-V curve (Figure 3b). Besides being a co-photosensitizer, a more desirable electrocatalytic activity of the thicker shell NRs could be well understood from the electrochemical impedance spectral (EIS) analysis performed with a symmetric cell configuration and polysulfide electrolyte. The Nyquist plots demonstrate quite low RCT (6.1 Ω) for the thick shell NRs than with only Cu2S NRs, CuxS nanoplates and the thin shell CdS/Cu2S NRs (Figure S7a and Table 1). A lower RCT signifies favourable reduction kinetics at the CE-electrolyte interface even if the series resistance (Rs) is almost comparable. The Tafel polarization curves (Figure S7b) directly relate the exchange current density (Jo) with RCT through equation (1): Jo = RT/nFRCT

(1)

where R is the gas constant, T is the absolute temperature, n is the number of electrons involved in polysulfide reduction and F is the Faraday constant.17,30,44 With lower RCT the thick shell CdS/Cu2S NRs possess higher Jo as compared to the other CEs. The limiting


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current density (Jlim) measured from the Tafel polarization curve (Table 1) is another relevant parameter which mainly determines the diffusion velocity of ionic carriers (polysulfide) between the electrodes. A higher Jlim for thick shell NRs implies better ionic diffusion.17,44 The reason behind a better performance with thicker Cu2S shell is well understood from the energy band diagram in Figure 3c. Since the Cu2S shell is made by cation exchange, increase in shell thickness effectively reduces the core diameter keeping the total NR diameter fixed. With thin Cu2S shell (left panel Figure 3c) photogenerated holes as well as holes from the electrolyte scarcely pass through the thick CdS core since the valance band edge lies much lower and therefore tunnelling of holes becomes stringent. When the Cu2S shell is thicker, the CdS core is thinner and the probability of hole tunnelling increases. Another possible reason is the formation of a thin intermediate layer of Cu doped CdS at the Cu2S/CdS interface facilitating hole tunnelling due to large Debye length.45 The thick Cu2S shell could also directly transfer the holes to FTO at the CE due to its degenerate nature and thereby increase the QDSSC performance. The device stability with different CEs is tested up to 250 h, shown in terms of normalized (against maximum value) device parameter PCE (η) (Figure 3d), Jsc, Voc and FF (Figure S8). In case of thick shell CdS/Cu2S NR CEs only 13% drop in PCE is observed whereas brass/Cu2S has 28% drop and with Pt CE, η is reduced to zero barely after 100 h of operation. It is worthwhile to mention that both CdS and Cu2S NRs show better stability than brass/Cu2S. To check for any morphological or compositional changes due to cation exchange post 250 h stability tests, HAADF−STEM elemental mapping of the representative thick shell CdS/Cu2S NR CE was performed (Figure S9). The mapping and line scan confirm that the core/shell structure is maintained with its original composition even after such long stability tests. The increased gradience observed in the core/shell structure may arise during scratching of the NRs from FTO for sample preparation.



Besides CdS photoanode, the meticulously designed photoactive core/shell NR CEs are also tested with CdSexTe1-x (x = 1) QD photoanodes, since the latter have shown tremendous prospects in fabricating high performance QDSSCs with PCE > 6%.46 The CdSeTe QDs synthesized by hot injection technique,46,47 have a diameter ~4.8 nm with its lattice spacing complying to (111) reflection of zinc blende structure and the SAED pattern confirming high crystallinity of the QDs (Figure 10). The absorption spectrum shows absorption onset at 803 nm and the corresponding PL spectrum show emission maxima at 747 nm (Figure S11). The photoanode was prepared by depositing water soluble CdSeTe QDs on mesoporous TiO2 along with a thin CdS shell coated by SILAR. The J-V curves (Figure 4a) show that thick shell CdS/Cu2S NR CE has an increment in PCE ~30% (Table S1) as compared to brass/Cu2S. A maximum PCE ~8.51% with Voc of 651 mV and Jsc of 19.7 mA/cm2 is achieved with the thick shell CdS/Cu2S NR CE, thus validating its improved QDSSC performance. The solar cell performance of the other NR along with Brass/Cu2S based CEs is shown in Figure 4b and listed in Table S1. Since CdSeTe QDs absorb light in the same spectral range as CdS/Cu2S NRs, the tandem effect is minimized letting small amount of light to reach the photoactive CEs after being absorbed by the photoanode and thus the degree of increment in PCE is smaller than that in CdS QDSSCs. The universality of our photoactive NRs is established through their utilization as photoanodes in PEC water splitting with a typical three electrode system where Pt acts as the CE and Ag/AgCl as reference (see schematic, Figure 5). An aqueous solution of 0.5 M Na2SO4 (pH 7.2) was used as the electrolyte and 0.2 M Na2SO3 was employed as the hole scavenger. Upon solar light illumination from the back, FTO side, the photo-generated electrons are transferred to FTO and transported via external circuit to the Pt electrode for H2 evolution. The photo-generated holes are scavenged through SO32- oxidation. With thin shell CdS/Cu2S NRs, the photocurrent is enhanced drastically to 8.12 mA/cm2 from 3.05 mA/cm2


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for CdS NRs at 0.5 V versus reversible hydrogen electrode (RHE) (Figure 6a). Unlike in QDSSC, here by employing thick shell CdS/Cu2S NRs the photocurrent remains at a modest 6.78 mA/cm2 and decreases further to 1.32 mA/cm2 when complete cation exchanged Cu2S NRs are used as photoanode. Applied bias photon-to-current conversion efficiency (ABPE) has been calculated by the equation:48 = [

×( )

]

(2)

where Jphoto is the photocurrent density, E0rev is the standard state reversible potential (which is 1.23 V for the water-splitting reaction), P is the intensity of the incident light (100 mW/cm2), and Eapp is the absolute value of the applied potential. Using thin shell CdS/Cu2S NR photoanode, ABPE of 6.79% is achieved but decreases slightly to 6.13% with thick shell NRs (Figure 6b and Table 2). The photocurrent as well as ABPE decreases largely in case of Cu2S NRs. In this particular system the photocurrent does not alter between front and back illumination. To understand the PEC activities EIS analysis is performed with the same PEC setup under 1 sun illumination and 0.5 V vs RHE applied bias (at maximum ABPE). The Nyquist plots (Figure S12a) consist of two semicircles, the first semicircle at higher frequency region can be attributed to the semiconductor NRs in the photoanode and the second semicircle in the lower frequency region is due to the components at the electrical double layer and the semiconductor/electrolyte interface. The equivalent circuit (Figure S12b) is modelled to fit the Nyquist plots according to the interfaces involved in the PEC process. CdS being an ntype semiconductor forms a depletion region (DL1) at the CdS NR/electrolyte interface (indicated by red dotted line) and modelled by three resistances, series resistance RS arising from the interfacial resistance between FTO, electrolyte and the electrical connection, bulk resistance RSC of the CdS layer and recombination resistance RRC at the CdS/electrolyte interface. The two capacitors are depletion layer capacitor CSC and Helmholtz layer capacitor 9 ACS Paragon Plus Environment


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CH. In case of core/shell NRs another parallel combination of junction resistance RC and depletion layer capacitance CDL are introduced due to p-n junction formation. The fitted data in Table 2 indicate a small change in RS (105.7-151.3 Ω) albeit with significant changes in RSC and CSC. RSC increases from 661.3 Ω for CdS NRs to 804.6 Ω for thin shell CdS/Cu2S NRs and abruptly increases to 3389 and 4617 Ω for thick shell CdS/Cu2S and Cu2S NRs, respectively. In the case of core/shell NR photoanodes, the increments in the bulk resistance are commensurate with the junction resistance RC. CSC is associated with the depletion capacitance at semiconductor/electrolyte interface and increases from 10.8 µF for CdS NRs to 15.1 µF for thin shell CdS/Cu2S NRs indicating decrease in the depletion layer. However the increase in Cu2S shell thickness also increases the depletion region at the core/shell interface. This depletion region augments the recombination of carriers passing through and this is precisely the reason behind lower photocurrent for thick shell CdS/Cu2S NRs. Since the parallel combination of RRC and CH in the equivalent circuit (Figure S12b) is associated with the recombination processes at the surface defect sites, an increase in RRC reveals that surface recombination sites are less abundant in core/shell structure, which increases the photocurrent generation. The charge carrier density (NA for p-type or ND for n-type) of the photoanode materials also plays significant role in PEC water splitting. The charge carrier densities of the CdS NRs, CdS/Cu2S core/shell NRs and Cu2S NRs have been derived from using the slopes of Mott−Schottky plots (Figure 6c) and from equation (3):42,49 $

= (2/ !!" )[#( &)/#']$ %

(3)

where e is the electron charge, ε the dielectric constant of the material, ε0 the permittivity of vacuum, and V, the applied potential. The positive slope of the CdS NRs conforms the n-type nature whereas the negative slope for Cu2S NRs confirms its p-type characteristics (Figure 6c). The co-existence of positive and negative slopes in CdS/Cu2S core/shell NRs is 10 ACS Paragon Plus Environment

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according to their p–n junction feature. With the ε values 8.9 (CdS) and ∼4 (Cu2S),42 ND is 7.5× 1019 for CdS NRs and increases by one order of magnitude to 5.4 × 1020 (calculated from the positive slope) when Cu2S thin shell is formed suggesting smooth separation of charges. When Cu2S shell gets thicker, ND again decreases to 1.1 × 1019 since the electrons and holes are trapped in the larger depletion layer of the p–n junction. To quantify evolved H2 gas at the Pt electrode, the NRs were continuously illuminated under 1 sun at 0.5 V vs RHE and the evolved H2 gas was collected over time for quantification by eudiometric gas collection technique. Figure 6d shows the amount of H2 gas evolved using different NR based photoanodes wherein thin shell CdS/Cu2S shows the best performance producing 21 µmol H2 after 1h constant illumination. Correspondingly with thick shell CdS/Cu2S NRs only 11 µmol is obtained whereas in the absence of light, gas evolution is not observed. The mechanistic details could be understood from the DFT calculation of the CdS and CdS/Cu2S core/shell NRs wherein our assumptions of charge transport are well corroborated (Figure 7). The total density of states (TDOS) and partial density of states (PDOS) of CdS NRs (Figure 7a,b), thin shell CdS/Cu2S NRs (Figure 7c,d), thick shell CdS/Cu2S NRs (Figure 7e,f) and Cu2S NRs (Figure 7g,h) were calculated using the self-consistent full-potential linearized augmented plane wave (FP-LAPW) method, as implemented in WIEN2K package.50 The dotted line represents the Fermi energy (EF) and DOS represents electronic structure of the entire material. In pure CdS, with a hexagonal crystal structure, band gap is essentially the energy difference between highest occupied (bonding) S-3p level and the lowest unoccupied (antibonding) Cd-5s level. In case of Cu2S, most of the Cu-d character is concentrated in the upper valence band level and the contributions from Cu-s, S-s and S-p in side of conduction band play a key role in the formation of band gap. From PDOS, a strong hybridization between Cu-d and S-p states is apparent in between -1.5 eV to -0.05 eV. Cu-d and S-p has higher DOS in the valence band side that crosses the Fermi level. When the Cu2S 11 ACS Paragon Plus Environment


shell is thicker, TDOS shifts noticeably above the Fermi level due to Cu-d and S-p hybridization facilitating the tunnelling of holes from Cu2S shell to CdS core, beneficial for CdS/Cu2S NRs to be used as CE in QDSSCs. In PEC water splitting, the core/shell NR photoanode is kept in forward bias condition for collecting the electrons (Figure 5). For thick shell CdS/Cu2S NRs and a similar up-shift of the valence band level will allow hole tunnelling towards FTO side leading to higher recombination possibilities which decrease the photocurrent. On the contrary, lesser chances of hole tunnelling in thin shell CdS/Cu2S NRs retards the detrimental recombination process. In accordance with the experimental results, the 100% cation exchanged Cu2S NRs show only a p-type distribution of DOS. Absorption coefficient calculated from DFT (Figure S13) also matches with the experimental UV-visNIR spectra for the core/shell NRs (Figure S6). 3. CONCLUSIONS In summary, a precisely tuned bifunctional core/shell nanostructure is demonstrated as photon harvesting catalytically active CE, enabling the enhancement of PV performance. The photoactivity of the NRs also allowed effective chemical conversion through PEC water splitting. In QDSSCs, the CdS/Cu2S core/shell NR arrays with p-n junction architecture can trigger effective reduction of the oxidized polysulfide electrolyte and simultaneously broaden the absorption range in the solar spectrum to enhance the generation of electron-hole pairs. The overall QDSSC and PEC efficiencies are strictly governed by CdS core and Cu2S shell dimensions, meticulously controlled by cation exchange, where the optimized core/shell morphology facilitates electron tunnelling through the CdS core. While a thicker Cu2S shell demonstrates improved PV performance, the thinner shell is more effective as a photoanode in PEC water splitting. The crucial carrier transport mechanism is well understood by DOS calculations suggesting the tunnelling pathway which governs the recombination processes


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and hence the photocurrent. These core/shell NRs have huge potential in boosting the photoactive electrodes in solar energy conversion and storage devices. 4. EXPERIMENTAL SECTION 4.1. Materials: Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O, 98%), cadmium acetate dihydrate (Cd(OAc)2.2H2O, ≥ 99%), copper (I) chloride (CuCl, 99.99%), copper (II) nitrate trihydrate (Cu(NO3)2.3H2O, 99.99%), L-glutathione reduced (≥ 98%), sulfur powder (99 %), ethylene glycol (≥ 99 %), cadmium acetate dihydrate (≥ 99 %), Tellurium powder (Te, 99.8%), Selenium powder (Se, 99.99% trace metals basis), 3-mercaptopropionic acid (MPA, >99%), potassium chloride (99%), sodium sulphide flakes (≥ 50 %), zinc acetate dihydrate (≥ 99 %), hydrochloric acid (37 %), ethanol (absolute), titanium tetrachloride (99%), sodium hydroxide (NaOH, 97%), Trioctylphosphine (TOP, 97%), 1-octadecene (technical grade, 90%), chloroform were purchased from Sigma-Aldrich. F:SnO2 (FTO)-coated glass (TCO 22-7), TiO2 paste (Ti-nanoxide T/SP, average size ∼20 nm), scattering TiO2 paste (Tinanoxide R/SP, average size >100 nm), were purchased from Solaronix. 4.2. Synthesis of CdS NR array: CdS NRs were synthesized according to a previous report,51 with slight modifications. FTO coated glass was washed in soap solution, distilled water (DI water), and ethanol under sonication for 20 min each. CdS NR arrays were prepared using a precursor solution of 1 mmol Cd(NO3)2.4H2O and 3 mmol thiourea in DI water. Cleaned FTO-coated glass was then transferred into a Teflon lined stainless steel container containing the precursors and heated to 180oC for 12 h. After deposition, the film was rinsed with DI water and dried naturally. 4.3. Synthesis of core/shell NRs: 0.14 g of copper (I) chloride was taken in a previously evacuated and 30 min N2 purged 50 mL 3-necked flask containing 25 mL of 1 M HCl. The pH was adjusted to 7 by drop wise injection of hydrazine into the flask. The solution was further purged with N2 for 30 min before the temperature was raised to 90°C. After the 13 ACS Paragon Plus Environment


temperature stabilized at 90°C, the solution became transparent due to dissolution of copper (I) chloride albeit the formation of some gray precipitate. The CdS NR array was then dipped into the solution for cation exchange followed by rinsing the substrate in DI water to form the core-shell NRs. The dipping time was varied from 3s to obtain a thin shell and up to 3 min for complete cation exchange to obtain Cu2S NRs. 4.4. Synthesis of CuxS nanoplates: CuxS nanoplates was synthesized according to our previous report.17 In a 250 ml round bottomed flask a mixture of 6 mmol Cu(NO3)2.6H2O and 5 mmol sulfur powder in 100 mL ethylene glycol was irradiated in a microwave chamber for 12 min at 600 W and 125oC under constant stirring at 2000 rpm. After cooling down to room temperature the gray product was centrifuged at 6500 rpm for 15 min, washed with ethanol and dried at 80oC for 1h. 4.5 Synthesis of CdSeTe QDs: CdSeTe QDs were synthesized according to a previous report with slight modifications.46,47 Adequate amounts of Se and Te powder were mixed with TOP and 1-octadecene, v/v, 1:3, followed by sonication to fully dissolve the powders. Separately, Cd(OAc)2.2H2O was stirred with oleic acid and 1-octadecene (v/v, 1:3) in a three necked round bottom flask under vacuum for 30 min, the temperature was raised to 120°C and maintained for another 30 min followed by N2 purging till the solution was clear. The solution temperature was further raised to 280°C and adequate quantity of Se and Te stock solution was swiftly added into the solution and kept for 5 min. The solution was left to cool down and the obtained QDs were precipitated with excess ethanol followed by centrifugation. The precipitate was re-dissolved in chloroform for further use. 4.6 Preparation of water soluble MPA-capped CdSeTe QDs: The water soluble QDs were prepared by replacing the initial hydrophobic surfactants with MPA according to our previous report.52 At first, 500 mL MPA was mixed with 4 mL methanol and 1 mL distilled water. The


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pH of this solution was adjusted to 12 with 5 M NaOH, mixed with the chloroform dispersed QD and stirred overnight. The mixture was allowed to stand for few minutes to separate the phases. The upper aqueous layer containing the QDs was washed with acetone and QDs were re-dispersed in 6 mL water for further use. 4.7. Preparation of QD-sensitized photoanodes: A compact TiO2 layer was deposited on FTO glass by dip coating in 40 mM TiCl4 solution at 80°C for 40 min and washed with DI water and ethanol. 8 µm thick mesoporous TiO2 active layer was then doctor-bladed onto the compact TiO2 layer coated FTO and dried at 100°C for 30 min followed by annealing at 550°C for 1 h in a box furnace. A thin (1 µm) scattering TiO2 layer was similarly doctorbladed on top of the active layer and annealed at 500oC for 1 h. 4.8. CdS and CdSeTe Photoanodes: In case of CdS photoanode, the CdS QDs were deposited on the TiO2 coated FTO glass by SILAR process. The TiO2 films were dipped alternatively in 0.1 M methanol solution of Cd(OAc)2.2H2O and 0.1 M methanol : water (1:1) solution of Na2S.9H2O for 1 min. The deposited films were washed with DI water and dried in air between each step to complete one SILAR cycle. The total number of CdS SILAR cycles was 8 followed by two SILAR cycles of a ZnS passivation layer deposited from 0.1 M aqueous solutions of Zn(NO3)2.6H2O and Na2S.9H2O. Finally, the films were washed with excess amounts of DI water and allowed to dry in air at room temperature. To prepare the CdSeTe QD photoanode, the MPA-capped QDs were deposited by chemical bath deposition for 24 h, followed by washing with DI water, air drying and the deposition of 8 SILAR cycles of CdS and 2 SILAR cycles for ZnS deposition. 4.9. Device fabrication: The solar cell device was prepared by assembling the QD-sensitized photoanode and NR-based CE using a parafilm spacer (50 µm) in a sandwich configuration. 80 µL droplet of polysulfide electrolyte was employed between the two electrodes before assembling them. 15 ACS Paragon Plus Environment


4.10. Characterization: UV-vis absorbance spectra were recorded using a Jasco Model V670 spectrophotometer equipped with an integrating sphere. The FESEM images were recorded by Carl Zeiss SUPRA 55VP FESEM. EDX spectra were recorded with the Oxford Instruments X-Max with INCA software coupled to the FESEM. TEM and HAADF-STEM images with elemental mapping were recorded with the DST-FIST facility, IISER Kolkata, JEOL, JEM-2100F equipped with the energy dispersive analysis of X-rays (EDAX) setup and 200 kV electron source. The XRD measurements were carried out with a Rigaku (Mini Flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation. Photovoltaic performance (J−V curves) of QDSSCs were measured using an electrochemical workstation Bio-Logic Science Instruments, Model SP-300), and the illumination source was a 150 W AM 1.5G solar simulator (Peccell technology, Japan, Model PEC-L01). The intensity of the simulated solar light was calibrated to 100 mW/cm2 using a standard silicon solar cell (NREL). EIS measurements were performed by making a symmetric dummy cell on a workstation (Bio-Logic Science Instruments, Model SP-300) in dark at zero bias, with frequency ranging from 1 MHz to 0.1 Hz. Tafel polarization characteristics were also measured on the dummy cells with a scan rate of 50 mV/s. IPCE spectra were recorded using a Newport Apex monochromator illuminator. PEC studies were conducted in a three electrode system where the core/shell NRs on FTO act as working photoanode, Pt as CE and Ag/AgCl as reference electrode. The measurements were performed using Bio-Logic (Model SP-300) electrochemical workstation and 1 sun solar light illumination. The electrolyte was prepared by maintaining pH 7.2 of an aqueous solution of 0.5 M Na2SO4 and 0.2 M Na2SO3 as hole scavenger. 4.11. DFT Calculations: DFT calculations were performed using the self-consistent fullpotential linearized augmented plane wave (FP-LAPW) method, as implemented in WIEN2K package,50 to evaluate the density of states of CdS and CdS/Cu2S core-shell structures. The


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exchange and correlation effects were treated within the Perdew-Bruke-Ernzerhoff (PBE) potential of generalized gradient approximation (GGA),53 and Tran-Blaha modified BeckeJohnson (TB-mBJ) potential.54,55 For studying CdS/Cu2S core-shell structures, a supercell of 40 atoms was generated and optimized from principal hexagonal CdS structure. We modelled the CdS/Cu2S structure with thin Cu2S shell containing approximately 80% CdS and 20% Cu2S and the thick Cu2S shell was assumed to contain approximately 40% CdS and 60% Cu2S. A 5×5×1 division k-point sampling was used for Brillouin zone integration. The separation between core and valance states in the form of cut-off energy was set at -6 Ry. The self-consistent calculations for all structures were iterated until total energy convergences approximate 10-5 Ry with respect to Brillouin zone integration. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: 10.1021/. EDX spectra; elemental mapping and line scan; XRD patterns, p-n junction measurements, UV-vis-NIR spectra, EIS analysis, Stability plots, Tafel polarization curve, HAADF−STEM mapping, CdSeTe TEM image, QDSSC parameters, stability of QDSSCs, DFT calculated absorption coefficient. ACKNOWLEDGMENTS DG and AG acknowledge Science and Engineering Research Board (SERB), Department of Science and Technology (DST) for their fellowship under NPDF sanctions PDF/2016/000069 and PDF/2016/001650, respectively. MYA thanks IISER Kolkata for his fellowship. The financial support from DST-SERB under sanction No. EMR/2016/001703 is duly acknowledged. REFERENCES 17 ACS Paragon Plus Environment


(1) Yu, J.; Wang, W.; Pan, Z.; Du, J.; Ren, Z.; Xuea W.; Zhong, X. Quantum Dot Sensitized Solar Cells with Efficiency Over 12% Based on Tetraethyl Orthosilicate Additive in Polysulfide Electrolyte. J. Mater. Chem. A 2017, 5, 14124–14133. (2) Jiang, C. R.; Moniz, S. J. A.; Wang, A. Q.; Zhang, T.; Tang, J. W. Photoelectrochemical Devices for Solar Water Splitting-materials and Challenges. Chem. Soc. Rev. 2017, 46, 4645– 4660. (3) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510–519. (4) Beard, M. C. Multiple Exciton Generation in Semiconductor Quantum Dots. J. Phys. Chem. Lett. 2011, 2, 1282–1288. (5) Hines, D. A.; Kamat, P. V. Recent Advances in Quantum Dot Surface Chemistry. ACS Appl. Mater. Interfaces 2014, 6, 3041–3057. (6) Halder, G.; Ghosh, D.; Ali, Md. Y.; Sahasrabudhe, A.; Bhattacharyya, S. Interface Engineering in Quantum Dot Sensitized Solar Cells. Langmuir 2018, DOI: 10.1021/acs.langmuir.8b00293. (7) Sambur, J. B.; Novet T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63–66. (8) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; 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. (9) Sahasrabudhe, A.; Bhattacharyya, S. Dual Sensitization Strategy for High-Performance Core/Shell/Quasi-shell Quantum Dot Solar Cells. Chem. Mater. 2015, 27, 4848–4859. (10) 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. (11) 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. (12) Lee, J. –W.; Son, D. –Y.; Ahn, T. K.; Shin, H. –W.; Kim, I. Y.; Hwang, S. –J.; Ko, M. J.; Sul, S.; Han, H.; Park, N. –G. Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent. Sci. Rep. 2013, 3, 1050(1–8). (13) 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. (14) Zhao, Y.; Chen, K.; Zhong, Q.; Yang, S.; Liu, Y. Single Sub-microwire Solar Cells Based on the CdS-Cu2S and CdS-ZnS Core-shell Heterostructures. Prof. Nat. Sci.: Mater. Int. 2017, 27, 182-185. 18 ACS Paragon Plus Environment

Page 18 of 31

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Chakrapani, V.; Baker, D.; Kamat, P. V. Understanding the Role of the Sulfide Redox Couple (S2-/Sn2-) in Quantum Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 9607– 9615. (16) Kim, H.; Hwang, I.; Yong, K. Highly Durable and Efficient Quantum Dot-Sensitized Solar Cells Based on Oligomer Gel Electrolytes. ACS Appl. Mater. Interfaces 2014, 6, 11245 –11253. (17) 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. (18) Zhang, H.; Yang, C.; Du, Z.; Pan, D.; Zhong, X. Graphene Hydrogel-based Counter Electrode for High Efficiency Quantum Dot-sensitized Solar Cells. J. Mater. Chem. A 2017, 5, 1614–1622. (19) 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. (20) Du, Z.; Pan, Z.; Fabregat-Santiago, F.; Zhao, K.; Long, D.; Zhang, H.; Zhao, Y.; Zhong, X.; Yu, J. –S.; Bisquert, J. Carbon Counter-Electrode-Based Quantum-Dot-Sensitized Solar Cells with Certified Efficiency Exceeding 11%. J. Phys. Chem. Lett. 2016, 7, 3103–3111. (21) Selopal, G. S.; Concina, I.; Milan, R.; Natile, M. M.; Sberveglieri G.; Vomiero, A. Hierarchical Self-assembled Cu2S Nanostructures : Fast and Reproducible Spray Deposition of Effective Counter Electrodes for High Efficiency Quantum Dot Solar Cells. Nano Energy 2014, 6, 200–210. (22) Shen, C.; Sun, L.; Koh, Z. Y.; Wang, Q. Cuprous Sulfide Counter Electrodes Prepared by Ion Exchange for High-efficiency Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 2807–2813. (23) Xu, J.; Yang, X.; Wong T. L.; Lee, C. S.; Large-scale Synthesis of Cu2SnS3 and Cu1.8S Hierarchical Microspheres as Efficient Counter Electrode Materials for Quantum Dot Sensitized Solar Cells. Nanoscale 2012, 4, 6537–6542. (24) Wang, Y.; Zhang, Q.; Li, Y.; Wang, H. CuxS Counter Electrodes in-situ Prepared via the Sulfidation of Magnetron Sputtering Cu Film for Quantum Dot Sensitized Solar Cells. J. Power Sources 2016, 318, 128–135. (25) Hessein, A.; Wang, F.; Masai, H.; Matsuda, K.; El-Moneim, A. A. Improving the Stability of CdS Quantum Dot Sensitized Solar Cell using Highly Efficient and Porous CuS Counter Electrode. J. Renew. Sustain. Energy 2017, 9, 023504. (26) Jiang, Y.; Zhang, X.; Ge, Q.-Q.; Yu, B.-B.; Zou, Y.-G.; Jiang, W.-J.; Hu, J.-S.; Song, W.-G.; Wan, L. J. Engineering the Interfaces of ITO@Cu2S Nanowire Arrays toward



Efficient and Stable Counter Electrodes for Quantum-Dot-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 15448−15455 (27) Jiang, Y.; Yu, B. –B. Liu, J. Li, Z. –H.; Sun, J. –K. Zhong, X.; Hu, J.; Song, W.; Wan. L. Boosting the Open Circuit Voltage and Fill Factor of QDSSCs Using Hierarchically Assembled ITO@Cu2S Nanowire Array Counter Electrodes. Nano Lett. 2015, 15, 3088− 3095. (28) Tachan, Z.; Shalom, M.; Hod, I.; Rühle, S.; Tirosh S.; Zaban, A. PbS as a Highly Catalytic Counter Electrode for Polysulfide-Based Quantum Dot Solar Cells. J. Phys. Chem. C 2011, 115, 6162–6166. (29) Song, X.; Wang, M.; Deng, J.; Ju, Y.; Xing, T.; Ding, J.; Yang, Z.; Shao, J. ZnO/PbS Core/Shell Nanorod Arrays as Efficient Counter Electrode for Quantum Dot-sensitized Solar Cells. J. Power Sources 2014, 269, 661–670. (30) Que, M. L.; Guo, W. X.; Zhang, X. J.; Li, X. Y.; Hua, Q. L.; Dong L.; Pan, C. F. Flexible Quantum Dot-sensitized Solar Cells Employing CoS Nanorod Arrays/Graphite Paper as Effective Counter Electrodes. J. Mater. Chem. A 2014, 2, 13661–13666. (31) Faber, M. S.; Park, K.; Cabán-Acevedo, M.; Santra P. K.; Jin, S. Earth-Abundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1843–1849. (32) Fan, M. -S.; Chen, J. -H.; Li, C. -T.; Cheng K. -W.; Ho, K. -C. Copper Zinc Tin Sulfide as a Catalytic Material for Counter Electrodes in Dye-sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 562–569. (33) Radich, J. G.; Dwyer, R.; Kamat, P. V. Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2-/Sn2- at the Counter Electrode. J. Phys. Chem. Lett. 2011, 2, 2453–2460. (34) Seol, M.; Youn, D. H.; Kim, J. Y.; Jang, J. –W.; Choi, M.; Lee, J. S.; Yong, K. MoCompound/CNT-Graphene Composites as Efficient Catalytic Electrodes for Quantum-DotSensitized Solar Cells. Adv. Energy Mater. 2014, 4, 1300775(1–7). (35) Ghosh, D.; Kapri, S.; Bhattacharyya, S. Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene Dots Immobilized on Zinc Oxide Nanorods. ACS Appl. Mater. Interfaces 2016, 8, 35496-35504. (36) Santra, P. K.; Kamat, P. V. Tandem Layered Quantum Dot Solar Cells : Tuning the Photovoltaic Response with Luminescent Ternary Cadmium Chalcogenides. J. Am. Chem. Soc. 2013, 135, 877–885. (37) Lin, C. Y.; Teng, C. Y.; Li, T. L.; Lee Y. L.; Teng, H. S. Photoactive p-type PbS as a Counter Electrode for Quantum Dot-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 1155 –1162.


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Jiang, Y.; Zhang, X.; Ge, Q. Q.; Yu, B. B.; Zou, Y. G.; Jiang, W. J.; Song, W. G.; Wan, L. J.; Hu, J. S. ITO@Cu2S Tunnel Junction Nanowire Arrays as Efficient Counter Electrode for Quantum-Dot-Sensitized Solar Cells. Nano Lett. 2014, 14, 365–372. (39) Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1, 15010. (40) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (41) Bai, Z.; Yan, X.; Li, Y.; Kang, Z.; Cao, S.; Zhang, Y. 3D-Branched ZnO/CdS Nanowire Arrays for Solar Water Splitting and the Service Safety Research. Adv. Energy Mater. 2016, 6, 1501459. (42) Yu, Y.; Pan, L.; Son, M.; Mayer, M. T.; Zhang, W. –D.; Hagfeldt, A.; Luo, J.; Grätzel, M. Solution-Processed Cu2S Photocathodes for Photoelectrochemical Water Splitting. ACS Energy Lett. 2018, 3, 760–766. (43) Tang, L.; Deng, Y.; Zeng, G.; Hu, W.; Wang, J.; Zhou, Y.; Wang, J.; Tang, J.; Fang, W. CdS/Cu2S Co-sensitized TiO2 Branched Nanorod Arrays of Enhanced Photoelectrochemical Properties by Forming Nanoscale Heterostructure. J Alloys Compd. 2016, 662, 516–527. (44) Ye, M.; Wen, X.; Zhang, N.; Guo, W.; Liua X.; Lin, C. In Situ Growth of CuS and Cu1.8S Nanosheet Arrays as Efficient Counter Electrodes for Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 9595–9600. (45) Böer. K. W. High-field Domains in CdS Adjacent to a Junction of p-type Solar Cells. J. Appl. Phys. 2016, 119, 085703. (46) Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSexTe1-x Alloyed Quantum Dot Sensitized Solar Cells with More than 6% Efficiency and High Stability. ACS Nano 2013, 7, 5215−5222. (47) Yang, J.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. CdSeTe/CdS Type ‑I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9%. J. Phys. Chem. C 2015, 119, 28800−28808. (48) Khan, S. U. M.; Al-Shahry, M.; Ingler Jr. W. B. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science 2002, 297, 2243–2245. (49) Wang, G.; Ling, Y.; Wheeler, D. A.; George, K. E. N.; Horsley, K.; Heske, C.; Zhang, J. Z.; Li. Y. Facile Synthesis of Highly Photoactive α-Fe2O3-Based Films for Water Oxidation. Nano Lett. 2011, 11, 3503–3509. (50) Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2K: An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Propertie;. Schwarz, K. Techn. University at Wien, Austria, 2001.



(51) Wong, A. B.; Brittman, S.; Yu, Y.; Dasgupta, N. P.; Yang, P. Core−Shell CdS−Cu2S Nanorod Array Solar Cells. Nano Lett. 2015, 15, 4096–4101. (52) 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. (53) Ivanov, B. A.; Tartakovskaya, V. E. Stabilization of Long Range Magnetic Order in 2D Easy-plane Antiferromagnets. Phys Rev Lett 1996, 77, 386–389. (54) Tran, F.; Blaha, P. Accurate Band Gaps of Semiconductors and Insulators with a Semilocal Exchange-Correlation Potential. Phys Rev Lett 2009, 102, 226401–4. (55) Koller, D.; Tran, F.; Blaha, P. Improving the Modified Becke– Johnson Exchange Potential. Phys Rev B 2012, 85, 155109–8.


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Table 1: QDSSC parameters derived from J-V, EIS and Tafel plot measurements for different CEs. FF

η (%) a

Rs (Ω)

RCT (Ω)

500

Jsc (mA/ cm2) 8.19

0.12

0.33 ± 0.05

19.9

22.1

0.009

0.16

CdS NRs

385

5.85

0.07

0.18 ± 0.02

28.3

687.8

0.002

0.05

Thin shell

535

12.04

0.41

2.60 ± 0.08

19.4

19.7

0.016

0.27

577

14.01

0.55

4.48 ± 0.03

15.8

6.1

0.46

2.58

Cu2S NRs

569

11.48

0.58

3.81 ± 0.05

15.9

8.7

0.17

0.85

CuxS

562

11.52

0.49

3.22 ± 0.04

15.9

10.4

0.08

0.51

514

9.59

0.47

2.35 ± 0.02

10.8

1572

0.012

0.09

CEs

Voc (mV)

Pt

Jo Jlim (mA/cm2) (mA/cm2)

CdS/Cu2S Thick shell CdS/Cu2S

Nanoplates Brass/Cu2S a

Average values obtained from 5 devices.

Table 2: ABPE obtained from PEC water splitting, EIS parameters derived from the fitting of Nyquist and Mott-Schottky plots. Photoanode

ABPEb (%)

RS (Ω)

RSC (Ω)

CSC (µF)

RC (Ω)

CDL (µF)

RRC (Ω)

CH (µF)

NA (ND)

CdS NRs

2.92 ±0.02

114.8

661.3

10.8

--

--

224

1.11

7.5 × 1019

Thin shell

6.79 ±0.04

105.7

804.6

15.1

254.9

9.58

601.4

2.04

5.4 × 1020

6.13 ±0.03

141.5

3389

7.34

1013

0.47

846.5

0.75

1.1 × 1019

1.31 ±0.05

151.3

4617

16.35

--

--

871.8

9.22

2.3 × 1018

CdS/Cu2S Thick shell CdS/Cu2S Cu2S NRs b

Average values obtained from 5 devices



Figure 1: Schematic representation of (a) CdS NRs, (b) thin shell CdS/Cu2S NRs, (c) thick shell CdS/Cu2S NRs, and (d) Cu2S NRs. (e-h) FESEM images of vertically aligned CdS, thin shell CdS/Cu2S, thick shell CdS/Cu2S and Cu2S NRs grown on FTO coated glass substrate, respectively. Corresponding insets show the digital image of the NR-deposited films. (i-l) TEM, and (m-p) HRTEM images of the NRs, respectively. Corresponding SAED patterns are shown in the insets of (m-p). The dotted lines in (n, o) show the region of graded core/shell interface.


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Figure 2: HAADF−STEM images with elemental line scan and mapping of (a) CdS NRs, (b) thin shell CdS/Cu2S NRs, (c) thick shell CdS/ Cu2S NRs and (d) Cu2S NRs. Cd, S and Cu are represented by green, yellow and red colour. The extreme right panels of (b) and (c) represent the merged maps of Cu and Cd, respectively, showing the existence of graded interface.



Figure 3: (a) Current density−voltage characteristic and (b) IPCE curves and representative integrated current densities of CdS QDs sensitized photoanode with different CE. (c) Band diagram and charge transfer mechanism in thin and thick shell CdS/Cu2S p−n junction based CEs. (d) Stability tests up to 250 h with different CEs and CdS QDs sensitized photoanode.


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Figure 4: (a) J-V characteristics and (b) comparative PCE of CdSeTe QDs sensitized photoanode with different CEs.



Figure 5: Schematic of core/shell NRs based photoelectrochemical setup and mechanism involve in water splitting.


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Figure 6: (a) J−V curves of NR based photoanode under simulated chopped AM 1.5G illumination in an electrolyte solution of 0.5 M Na2SO4 at pH 7 and hole scavenger a 0.2 M Na2SO3. (b) ABPE as a function of applied potential and (c) Mott−Schottky plots of NRs obtained from EIS measurements in the dark at a frequency of 1 kHz. (d) The quantity of H2 gas evolved with time at 0.5 V vs RHE applied bias and 1 sun constant light illumination measured by eudiometric gas collection techniques.



Figure 7: TDOS (left panel) and PDOS (right panel) of (a,b) CdS NRs, (c,d) thin shell CdS/Cu2S NRs, (e,f) thick shell CdS/ Cu2S NRs, and (g,h) Cu2S NRs.


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