Functionalized Graphite Platelets and Lead Sulfide Quantum Dots

Article pubs.acs.org/JPCC

Functionalized Graphite Platelets and Lead Sulfide Quantum Dots Enhance Solar Conversion Capability of a Titanium Dioxide/ Cadmium Sulfide Assembly P. Naresh Kumar,† Sudip Mandal,† Melepurath Deepa,*,† Avanish Kumar Srivastava,‡ and Amish G. Joshi‡ †

Department of Chemistry, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502205, Andhra Pradesh, India ‡ CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India S Supporting Information *

ABSTRACT: A photoactive electrode comprising lead sulfide (PbS) and cadmium sulfide (CdS) quantum dots (QDs) and functionalized graphite platelets (FGPs) was prepared by assembling them onto titanium dioxide (TiO2), which functioned as the wide band gap semiconducting scaffold. The QDs were cumulatively capable of harvesting portions of visible and infrared regions of solar spectrum, and FGP served as electron conduit. Graphite platelets (GPs) were noncovalently functionalized using 1pyrenecarboxylic acid (PCA) to yield FGP. The insertion of PCA between GP layers to yield few-layer graphene or FGP was confirmed by high-resolution transmission electron microscopy and Raman and X-ray photoelectron spectroscopic analyses. Fluorescence quenching, emission decay analyses, and energetics of the TiO2/FGP/ PbS/CdS electrode demonstrated excited electron deactivation via a cascade mechanism. Photoexcited electrons propagate from PbS to CdS to TiO2 and to the external circuit through FGP, which had a suitably poised Fermi level at 4.52 eV. The role of FGP in working as an efficient electron acceptor and PbS as a red wavelength absorbing layer was evidenced in the form of enhanced external and internal quantum efficiencies achieved for the TiO2/FGP/ PbS/CdS electrode over the entire solar spectrum compared with the TiO2/CdS electrode. This was accomplished using cells with Sn2−/S2− as the redox couple and a multiwalled carbon nanotube-based counter electrode. The best overall power conversion efficiency of the TiO2/FGP/PbS/CdS photoanode-based cell is 3.82%, which is greater by 54% compared with that of the TiO2/CdS cell. Our studies demonstrate the prowess of using a near-infrared absorber like PbS and an electron acceptor like FGP in realizing remarkable improvements in solar-cell performance metrics.

1. INTRODUCTION Photoactive quantum dots (QDs) are promising for the realization of high-efficiency photovoltaic devices at a low cost.1 Adjustable band gap by size tuning, high extinction coefficient, good photostability, and the possibility to use multiple exciton generation (the generation of more than one exciton per impinging photon) are some unique features of QDs that render them to be competitive alternates to dye sensitized solar cells (DSSCs).2−4 Among the various quantum dot solar cells (QDSCs) based on CdSe,5 CdS,6 PbSe,7 ZnSe,8 PbS,9 InP,10 and CuInS2,11 which have been explored in the past in terms of photovoltaic performance and charge-transport behavior, CdS and PbS QDs are attractive because they allow a broader utilization of the solar spectrum. While CdS QDs typically absorb in the visible region with a pronounced absorption in the 400 to 550 nm wavelength range,12 PbS QDs exhibit a distinctive absorption in the near-infrared (NIR) region, >800 nm.13 Therefore, by use of PbS and CdS QDs as cosensitizers, the resulting photoanode is expected to exhibit an optimal coverage of the solar spectrum. Furthermore, both QDs can be easily prepared by successive ionic layer absorption © 2014 American Chemical Society

and reaction (SILAR) method, which is a room-temperature solution-phase process and is highly scalable.2 In a previous report, a nanocomposite CdS/PbS/TiO2 photoanode-based QDSC, with aqueous Sn2−/S2− as the redox couple and Cu2S as the counter electrode, was constructed, and this cell was characterized by an overall power conversion efficiency (PCE) of 2.21% under one sun illumination.14 In another attempt, HF-treated TiO2 nanostructures were used as the semiconducting support for PbS/CdS QDs, and Sn2−/S2− and Cu2S were employed as the electrolyte and counter electrode, respectively. This cell showed a PCE of 2.1 ± 0.2%.15 For a regenerative liquid-type cell containing PbS/CdS attached to mesoporous titania as the photoanode and a cobalt complex as the redox mediator, an incident photon to current conversion efficiency (IPCE) in excess of 50% and a PCE of 2% was achieved under 0.1 sun irradiance.16 Among previous reports on PbS QD solar cells, a notable study was on Received: May 28, 2014 Revised: July 21, 2014 Published: July 28, 2014 18924

dx.doi.org/10.1021/jp5052408 | J. Phys. Chem. C 2014, 118, 18924−18937

The Journal of Physical Chemistry C

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Scheme 1. Schematic Representation for Preparation of TiO2/FGP/PbS/CdS Electrodea

a

Right-hand panel illustrates PCA inserted between GP to yield FGP.

a heterojunction solar cell with a ZnO/PbS QD/MoO3−V2Ox/ metal configuration, which delivered an NREL-certified PCE of 4.4%.17 Yet another depleted heterojunction cell with PbS QDs with a purposefully engineered interface yielded a PCE of 3.5%.18 Another recent study employed a PbS QD-sensitized TiO2 as photoanode, a CuxSy as the counter electrode, and aqueous polysulfide as the electrolyte; the cell showed a PCE of 1.1% under 1 sun illumination.19 In the past, in-depth analyses of electron transport in PbS/CdS core/shell QDs have also been carried out.20,21 Although charge-transport mechanisms and photovoltaic parameters have been examined at length, in previous reports on PbS/CdS QDs, there continues to be enough scope for further manipulating this photoanode architecture for the realization of high-performance solar cells. To this end, incorporation of an electron acceptor such as a carbon nanostructure (carbon nanotubes or reduced graphene oxide (RGO) or fullerene), which is chemically compatible with the QDs, can boost the current collection capability of the electrode. While RGO as an electron acceptor is attractive due to a large surface area and high electron mobility,22,23 the widely used preparation methods of RGO involve the use of corrosive acids and high temperatures.24 The harsh processing tends to rupture the covalently bonded framework of carbon atoms in graphene, and as a consequence, the benefit of graphene such as a large 2D electronic conductivity cannot be tapped when this material is integrated into a practical device. Noncovalent methods to produce exfoliated graphene sheets by direct cleavage of graphite are preferred to circumvent this issue of poor structural quality in graphene sheets. In previous studies, noncovalently functionalized graphene nanosheets with high structural integrity were prepared from graphite by using 1-pyrenecarboxylic acid (PCA).25,26 Here we used the same cleaving agent, PCA, but modified the processing method by using graphite platelets (GPs) as the precursor to form functionalized graphite platelets (FGPs). We present a heretofore unreported photoanode architecture comprising PbS/CdS QDs as photosensitizers grown on a TiO2 semiconducting scaffold, the latter pre-enmeshed with FGP as electron conduits. We used the Sn2−/S2− redox couple as the hole scavenger and an electrophoretically deposited layer of multiwalled carbon nanotubes (MWCNTs) as the counter electrode to construct QDSCs. In the past, MWCNTs have been successfully employed as counter electrodes to yield highperformance DSSCs.27,28 The successful exfoliation of GP by 1-

PCA to yield few-layer graphene in the form of FGP was confirmed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman studies, and high-resolution transmission electron microscopy (HRTEM) analyses. Chargetransport dynamics in the TiO2/FGP/PbS/CdS photoanode was followed by fluorescence quenching and lifetime measurements. The comparison of solar-cell performance metrics15,29,30 and internal and external quantum efficiencies (IQE and EQE)31,32 of photoanodes with different compositions brought out the omnipotent role played by FGP in improving photovoltaic parameters. Our study shows how by simply tailoring the composition of the photoanode by use of FGP, substantial improvements in solar energy conversion can be realized.

2. EXPERIMENTAL METHODS 2.1. Chemicals. Cadmium acetate (Cd(CH3COO)2), sodium sulfide (Na2S), lead nitrate Pb(NO3)2, acetyl acetone, and solvents (methanol and toluene) were obtained from Merck. GP (98% carbon), titanium tetrachloride (TiCl4), Triton X-100, MWCNTs (purity >98%), and PCA were procured from Aldrich. TiO2 powder (P25) was a free gift from Evonik. Deionized water with a resistivity of ∼18.2 MΩ cm was obtained from a Millipore Direct-Q3 UV system. Inorganic transparent electrodes of SnO2:F coated glass (FTO, sheet resistance: 14 Ω/sq) were obtained from Pilkington and were cleaned in a soap solution, 30% HCl solution, double-distilled water, and acetone, in that sequence prior to use. 2.2. Preparation of Photoanodes. FTO substrates were left submerged in an aqueous TiCl4 (0.04 M) solution at 70 °C for 20 min, and a dense TiO2 layer was applied over this layer using a paste of TiO2 powder by doctor blading. The paste was prepared by dispersing TiO2 powder (0.3 g) homogeneously in a clear solution of acetyl acetone (1.5 mL), ultrapure water (8.5 mL), and Triton X-100 (20 mg). The as-deposited TiO2 plates were heated to 80 °C for 30 min, which was followed by annealing at 500 °C for another 30 min. To increase light scattering, one more layer of TiO2 was also deposited using the same steps as previously mentioned, followed by heating and annealing as performed for the active layer. The TiO2-coated FTO substrate was finally dipped in an aqueous TiCl4 (40 mM) solution for 30 min at 70 °C; the resulting TiO2 film was rinsed in distilled water and annealed at 500 °C for 30 min. 18925



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obtained on a HRTEM FEI Tecnai G2 F30 STWIN with a FEG source operating at 300 kV. For TEM, a thin layer of the GP or FGP sample was carefully transferred onto a carbon-coated copper grid of 3.05 mm diameter, and the solvent was evaporated. For the electrode sample, a thin layer was extracted from FTO into water, dispersed in water, and then transferred to the grid, and water was evaporated. XRD patterns of electrodes or GP/FGP were recorded on a PANalytical, X’PertPRO instrument with Cu−Kα (λ = 1.5406 Å) radiation. Surface morphology analysis of electrodes or GP/FGP was performed using a field-emission scanning electron microscope (Carl Zeiss Supra 40 FE-SEM). XPS of GP and FGP was carried out on a PerkinElmer 1257 model operating at a base pressure of ∼4.2 × 10−8 Torr (100 W, 15 kV) with a nonmonochromatized Mg Kα line at 1253.6 eV, an analyzer pass energy of 60 eV, and a hemispherical sector analyzer capable of 25 meV resolution. The core-level spectra were deconvoluted using a nonlinear iterative least-squares Gaussian fitting procedure. For all fitting multiplets, the full widths at half-maximum (fwhm) were fixed accordingly. Corrections due to charging effects were taken care of by using C(1s) as an internal reference and the Fermi edge of a gold sample. Jandel Peak FitTM (version 4.01) program was used for the analyses. Raman spectra were recorded for the electrodes on a Bruker Senterra dispersive Raman microscope spectrometer; the laser excitation wavelength was fixed at 785 nm. The optical absorption spectra of the films were measured in the diffuse reflectance mode and converted to absorbance using Kubelka− Munk function, and for solutions of GP or FGP, spectra were measured in absorbance mode in quartz cuvettes on a UV−visNIR spectrophotometer (Shimadzu UV-3600). Photoluminescence (PL) spectra of films were measured on a Horiba Fluoromax-4 fluorescence spectrometer; a suitable filter was utilized during the measurement, and the background correction was also applied. Time-correlated single photon counting (TCSPC) method was used for deducing emission lifetimes with a Horiba Jobin Yvon data station HUB functioning in the TCSPC mode. A nano LED diode emitting pulses at 370 nm with a 1 MHz repetition rate and a pulse duration of 1.3 ns was employed as an excitation source. Lightscattering Ludox solution (colloidal silica) was used to acquire the instrument response function (prompt). A long-pass 500 nm filter was placed in front of the emission monochromator for all measurements. Horiba Jobin Yvon DAS6 fluorescence decay analysis software was used to fit the model function (biexponential decays) to the experimental data, with appropriate correction for the instrument response. Current versus potential (I−V) data of QDSCs were measured using a Newport Oriel 3A solar simulator with a Keithley model 2420 digital source meter. A 450 W xenon arc lamp was the light source that provided a light intensity of 100 mW cm−2 of air mass (AM) 1.5G illumination; the spatial uniformity of irradiance was confirmed by calibrating with a 2 cm × 2 cm Si reference cell traceable to NREL and reaffirmed with a Newport power meter. Before collection of I−V data, each electrode was allowed to reach equilibrium at open-circuit. Photovoltage decay versus time measurements were carried out on QDSCs by using a tungsten-halogen lamp as the light source coupled to an Autolab PGSTAT 302N, which recorded the chronopotentiometric data in dark after the open circuit voltage (VOC) in each cell had acquired a saturated value. IQE and EQE were recorded using a Quantum Efficiency Measurement System, Oriel IQE-200, capable of measurements compliant

For preparing FGP, we adapted the procedure from a previous report,26 PCA (5 mg) was first dissolved in 20 mL of ultrapure water, and a clear solution was obtained by adding ammonium hydroxide to the PCA/water dispersion. The pH of this solution was maintained at 10. 50 mg of GPs was then added to this dispersion, and the mixture was sonicated for a few hours. The resulting dispersion was centrifuged, and the supernatant was collected in ultrapure water. This supernatant was washed repeatedly (to remove the larger aggregates) by subjecting it to two cycles of sonication and centrifugation (at 5000 rpm) using ultrapure water as the solvent; finally, the supernatant liquid containing the dispersed exfoliated GP or FGP was collected and used. The photograph of the homogeneous and stable dispersion of FGP is shown in Scheme 1. FGP was deposited electrophoretically over the TiO2 film from a solution of FGP in ultrapure water. A constant potential of 60 V was applied for 1 h using a Tarsons electrophoresis power supply, and a Pt rod was used as the counter electrode. The color of the solution faded, and the color change of TiO2 from white to dark gray indicated the formation of the TiO2/ FGP film. The film was rinsed in water and dried. PbS QDs were deposited over the TiO2/FGP electrode by using successive ionic layer adsorption and reaction (SILAR) method by using clear solutions of Pb(NO3)2 (0.03 M) and Na2S (0.03 M) in methanol as lead and sulfide precursors, respectively. The two solutions were taken in two separate beakers. The TiO2/ FGP plate was dipped in the Pb(NO3)2 solution for 2 min and rinsed in methanol to remove excess ions, followed by drying. It was then dipped in the Na2S solution for 2 min, again followed by a methanol rinse and drying, and a greyish-black layer of PbS was formed over the TiO2/FGP electrode. TiO2/PbS electrodes were obtained by processing TiO2 plates to two cycles of SILAR in Pb(NO3)2 and Na2S solutions. CdS QDs were deposited over the TiO2/FGP/PbS by using the SILAR method, from clear solutions of Cd(CH3COO)2 (0.1 M) and Na2S (0.1 M) in methanol as cadmium and sulfide precursors, respectively. The two solutions were again taken in two separate beakers. The TiO2/FGP/PbS film was dipped in the Cd(CH3COO)2 solution for 2 min, rinsed in methanol to remove excess ions, followed by drying. It was then dipped in the Na2S solution for 2 min, again followed by a methanol rinse and drying, and this is regarded as one cycle of CdS deposition. In a similar way, six more cycles were performed and a TiO2/ FGP/PbS/CdS electrode was obtained. TiO2/PbS/CdS and TiO2/CdS electrodes were obtained by processing TiO2/PbS and TiO2 plates to seven cycles of SILAR in Cd(CH3COO)2 and Na2S solutions. The sequential deposition process is illustrated in Scheme 1. MWCNTs (200 mg) were added to 60 mL of 1:3 v/v solution of H2SO4/HNO3 (each of 6 M strength) and refluxed for 12 h at 80 °C. After cooling to room temperature, the reaction mixture was diluted with ultrapure water (50 mL) and then washed with ultrapure water until the supernatant showed neutral pH. Using ashless fast filter paper (Ø150 mm), the resulting brown solid was collected, dried at room temperature, and dispersed in water by sonication for 2 h. By using electrophoretic deposition at 60 V, a thin layer of functionalized MWCNTs was deposited over FTO/glass substrates, and the films were washed in water and dried in air and used as counter electrode for photovoltaic measurements. 2.3. Characterization Techniques. HRTEM images of GP or FGP and TiO2/FGP/PbS/CdS electrode samples were 18926



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Figure 1. SEM images of (a) GP and (b) low- and (c) high-magnification images of FGP (scale bars in panels a, b, and c = 400 nm, 4 μm, and 400 nm, respectively). (d) TEM image of the GP (scale bar = 50 nm). (e) HRTEM image of GP stacks (scale bar = 10 nm). The inset shows the corresponding enlarged view of lattice fringes (arrowhead separation = 0.34 nm). (f) TEM image of FGP (scale bar = 100 nm). (g) Formation of few layer graphene in FGP (scale bar = 10 nm). Insets are the corresponding lattice scale (arrowhead separation = 0.38 nm) and FFT images. (h) Overlapping of few layer graphene sheets in FGP (scale bar = 10 nm).

to ASTM E1021-06. The instrument gave the EQE (also known as IPCE) and IQE directly as a function of wavelength. The light source was a 250 W quartz tungsten halogen lamp, the monochromator path length was 1/8 M, and the spot size was 1 mm × 2.5 mm rectangular at focus. Cyclic and linear sweep voltammograms (CV and LSV) of electrodes and FGP or GP, respectively, were recorded on an Autolab PGSTAT 302N equipped with NOVA1.9 software. The LSV of GP and FGP was recorded at a scan rate of 10 mV s−1 in the potential range of −0.9 to −0.2 V between two Pt electrodes. A square cavity of 0.5 cm2 area was constructed using an insulating adhesive foam tape on a Pt sheet (Figure S1, Supporting Information). The cavity was filled with the FGP or GP

dispersion using a syringe, and another Pt sheet was carefully placed over this assembly to complete the cell. This cell was used for LSV; electrical contacts were taken from the Pt electrodes, which were prevented from short-circuiting by the thick tape. The thickness of the tape was the thickness of the sample (FGP or GP). Brunauer−Emmett−Teller (BET) specific surface area measurement on samples was performed on a Micromeritics, ASAP 2020.

3. RESULTS AND DISCUSSION 3.1. Electron Microscopy Analyses. The SEM and TEM images of GP and GP exfoliated by PCA (the latter designated as FGP) are shown in Figure 1. The SEM image of GP shows 18927



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flakes and particles of graphite lumped together (Figure 1a), and the micrographs of FGP show a clear morphological transformation to wrinkled graphitic sheets (Figure 1b,c). The TEM image of GP (Figure 1d) shows them to be in the form of aggregated flattened flakes, with irregular shapes, and they appear to be interlinked. The corresponding HRTEM image (Figure 1e) reveals parallely arranged lattice fringes with an interfringe separation of 3.4 Å arising from the (002) plane of graphite with a d spacing of 3.39 Å (JCPDS: 75-1621). The lattice fringes are crinkly, which is also evident from the enlarged view shown in the inset of Figure 1e, indicating that the platelets have structural defects. Upon noncovalent functionalization of GP with PCA, the successful exfoliation of the GP, by incorporation of PCA as molecular wedges in between the graphitic sheets, is confirmed by HRTEM. The TEM image of the resulting product referred to as FGP (Figure 1f) shows the sheet like structures, extending over a few hundred nanometers, and the high magnification image (Figure 1g) shows the formation of multilayered graphene. Contrary to the GP, wherein an ordered continuous array of lattice fringes oriented along the 002 reflection was visible, in FGP, only at the periphery of the sheets, a few lattice fringes are seen with an interfringe distance of 0.38 nm. The slight increase in the fringe spacing is also suggestive of incorporation of PCA between the graphitic layers, which debundles the layers and opens them up to yield few-layered graphene. The fast Fourier transform (FFT) pattern of the few layer graphene affirmed their hexagonal crystalline structure, as bright spots arranged in a hexagonal geometry were observed (inset of Figure 1g). Further evidence in support of graphene formation is obtained in Figure 1h, wherein sheets of multilayered graphene are distinctly observed to be overlapping. The dashed lines in the image represent the boundaries of the sheets, and these appear to be randomly intersecting in different regions. X-ray diffractograms of a TiO2 film and films of CdS and PbS QDs grown by SILAR separately on glass substrates are shown in Figure 2. The XRD pattern of TiO2 shows peaks at d = 3.51, 2.43, 1.89, 1.66, 1.48, and 1.26 Å, which concur well with the (101), (103), (200), (211), (204), and (215) planes of the body-centered tetragonal crystal structure of TiO2, respectively, as per PDF number 894921. The XRD pattern of CdS QDs shows broad peaks at d = 3.34, 2.91, 2.06, and 1.76 Å, which can be attributed to the (111), (200), (220), and (311) planes of the face-centered cubic or fcc lattice of CdS (PDF: 652887). PbS QDs produced d lines at 3.42, 2.97, 2.1, 1.79, 1.71, and 1.33 Å, which have been assigned to (111), (200), (220), (311), (222), and (420) planes of PbS with a fcc lattice (PDF: 781901). The TEM image of the TiO2/PbS/CdS film (Figure 3a) shows mingling particles, of no particular shape with indistinctive grain boundaries, thus illustrating a thorough mixing of the three components. The corresponding HRTEM image of the same film (Figure 3b) shows lattice fringes from TiO2, PbS, and CdS to be overlapping, implying good connectivity between the QDs and the oxide. The lattice fringes from the individual components were identified and have been enclosed within dashed (PbS), dotted (CdS), or solid (TiO2) ellipses. The crystallites with a fringe separation of 0.34 nm (panel A′) correspond to PbS QDs oriented in the (111) direction. Domains with interfringe spacings of 0.35 (inset of Figure 3b) and 0.288 nm (panel B′) arise from crystallites of TiO2 and CdS QDs, oriented along (101) and (200) planes, respectively. The TEM image of TiO2/FGP/ PbS/CdS film (Figure 3c) shows the presence of elongated

Figure 2. (a) XRD patterns of (i) a TiO2 film, (ii) a CdS QDs film, and (iii) a PbS QDs film, all on glass and Raman spectra of (b) GP and (c) FGP; the solid smooth lines in panels b and c represent Gaussian fits.

ribbon-like shapes wedged between irregular shaped particles; these ribbon-like shapes originate from FGP. HRTEM images of this assembly (Figure 3d,e) show crystallites of QDs and TiO2 to be superimposed on the few-layer graphene sheets. The fringe separations of 0.21 and 0.17 nm (insets of Figure 3d) are attributed to (220) and (311) planes of PbS and CdS, respectively. The interfringe distances of 0.35, 0.34, and 0.2 nm are assigned to (101), (111), and (220) planes of TiO2, PbS, and CdS, respectively (Figure 3e). HRTEM images provide unambiguous proofs in support of the QDs and the oxide being in intimate nanoscale contact with FGP, which can allow rapid electron transport between the components upon photoexcitation of QDs and eventually result in high photocurrents. 3.2. Raman Studies. Raman spectra of GP and FGP are displayed in Figure 2b,c, respectively. The D and G bands are typically observed in graphene oxide or RGO due to defects and the first-order scattering of the E2g mode of the sp2hybridized carbon atoms, respectively.33 In GP, surprisingly, an intense but broad D-band at 1308 cm−1 was observed, possibly due to a large number of edge areas and a small degree of surface oxidation; this was also supplemented by XPS data. The D-peak herein is considerably downshifted compared with the reported values in graphene-based samples (in the range of 1340−1350 cm−1).34 It could be due to a high number of domain boundaries, owing to which the span of the defect vibrational mode is wider. The G-band in GP was observed at 18928



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Figure 3. (a) TEM image of TiO2/PbS/CdS photoanode (scale bar = 50 nm). (b) Corresponding HRTEM image (scale bar = 5 nm) showing lattice fringes enclosed in solid line ellipses arising from TiO2 and in dotted line ellipses corresponding to CdS and in dashed line ellipses due to PbS (fringe separation = 0.18 nm). Enlarged views of fringe patterns marked as A, B, and C in panel b are presented in the adjoining panels (A′ (arrowhead separation = 0.34 nm), B′ (arrowhead separation = 0.288 nm) and as inset of panel b (C′ (arrowhead separation = 0.35 nm)). (c) TEM image of TiO2/FGP/PbS/CdS photoanode. The elongated shapes are due to FGP (scale bar = 600 nm), and panels d and e are the corresponding HRTEM images showing lattice fringes from TiO2, PbS, and CdS enmeshed with the sheet-like structure of FGP (scale bars in panels d and e = 5 and 2 nm, respectively). In panel d, top insets: in the extreme right, line separation = 0.21 nm, and for the adjoining one, line separation = 0.17 nm. In panel e, line separations in the solid, dashed, and dotted ellipses = 0.35, 0.34, and 0.2 nm, respectively.

1583 cm−1, which matches with the G-band of graphite.35 After treating GP with PCA, FGP was formed. Both the D and G bands are upshifted to 1351 and 1595 cm−1, respectively, in FGP. Furthermore, the fwhm of D-band decreased in FGP compared with GP, and the intensity of G-band increased. The upshift of G-band is assigned to the isolated carbon−carbon double bonds that resonate at frequencies greater than that of the graphitic G-band.35 The D-band shift and narrowing is due to insertion of PCA between the graphitic layers, which reduces the aggregation of the graphitic sheets and thus decreases the proportion of structural defects. The ID/IG ratio provides a measure of the quality graphitic layers. In GP, this ratio was found to be 1.75 contrasting with a value of 0.99 in FGP. This clearly indicates the formation of better quality graphitic layers with a less disordered hexagonal network of carbon atoms in FGP. 3.3. XPS Analyses. The deconvoluted C 1s and O 1s corelevel XPS spectra and the survey spectra of GP and FGP are shown in Figure 4 and Figure S2 (Supporting Information). The survey spectra of both GP and FGP show two signature peaks, which arise from C 1s and O 1s. The IO1s/IC1s ratio for GP is 0.46, while it is 0.66 for FGP. This is a preliminary indicator for insertion of PCA between the graphitic platelets, as oxygens from PCA contribute to IO1s in FGP. The deconvoluted C 1s core-level spectrum of GP (Figure 4a) shows three peaks. It shows a principal peak at 284.7 eV corresponding to the C−C linkages of the graphitic backbone,

followed by two low-intensity higher energy components at 287.2 and 290.4 eV. The fwhm was fixed at 2.33 eV. The latter two peaks are ascribed to C−O links (due to oxygen adsorbed by the surface of the GP and possibly a small degree of oxidation as well) and π−π* transitions, respectively. The highenergy region of the C 1s spectrum of FGP is broadened compared with GP (Figure 4b). The deconvolution yielded four components. A primary component is due to C−C bonds (from the graphitic backbone and pyrene of PCA) at 284.6 eV, two peaks at 288.2 and 290.7 eV due to CO and C−O (stem from the −COOH groups in PCA), and the π−π* transitions produce a peak at 294.4 eV. The fwhm was fixed at 3.2 eV. The ICO+C−O/ICtotal ratio, deduced from the areas under the Gaussian fits, roughly represents the level of functionalization by PCA in FGP. This ratio is found to be 0.25 in FGP. The IC−O/ICtotal ratio in GP is 0.15, which is lower, thus indicating the incorporation of PCA between the graphitic sheets in FGP. Further evidence of PCA’s successful inclusion in FGP was obtained from the comparison of the O 1s core-level spectra of GP and FGP (Figure 4c,d). GP shows a quasi-symmetric peak at 532.4 eV corresponding to surface adsorbed oxygen and oxidation to a minor extent. FGP shows a broadened, slightly asymmetric peak, which was deconvoluted into two components at 532.8 and 530.8 eV. These are attributed to C−O and CO bonds of −COOH groups of PCA wedged between the GP. The valence band (VB) spectra of GP and FGP are displayed in Figure 4e. In 18929



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Figure 4. Deconvoluted core-level XPS spectra of C 1s of (a) GP and (b) FGP and O 1s of (c) GP and (d) FGP. (e) Valence band spectra showing the DOS near the Fermi level of GP (□) and FGP (○). (f) XRD patterns of (i) GP and (ii) FGP.

both FGP and GP, the position of the VB, which can be equated to the Fermi level, is taken as the distance from 0 eV to the point at which the density of states (DOS) commences to populate. The VB positions are 4.54 and 4.52 eV in GP and FGP; the difference is not very significant. These Fermi level positions agree very well with the reported value of EF = 4.5 eV for RGO.36 X-ray diffractograms of GP and FGP are shown in Figure 4f. GP shows a strong sharp intense peak at 3.35 Å and weak peaks at 2.056, 1.676, and 1.23 Å, corresponding to (002), (101), (004), and (110) reflections of graphite (PDF: 751621). The structural change ongoing from GP to FGP is reflected in the XRD pattern of FGP, as the peak seen at d = 3.35 Å in GP is broader and is observed at a higher d value of 3.8 Å in FGP. This is suggestive of the widening of the graphitic planes to include the PCA molecules in between. Furthermore, the lower intensity peaks that were observed in GP are no longer perceptible in FGP. This indicates the conversion of graphite to few-layer graphene in FGP, thus inducing some degree of amorphicity in the structure compared with the highly crystalline structure of pristine GP. 3.4. Electrical Conduction Behavior. The linear sweep voltammograms of the GP and FGP recorded at a scan rate of 10 mV s−1 are shown in Figure 5. The reciprocals of slopes of the linear fits (V/I) of the two plots in this potential domain yielded the resistances (V/I = R) offered by GP and FGP to

Figure 5. Linear sweep voltammograms of GP (□) and FGP (○) recorded between two conducting Pt electrodes at a constant scan rate of 10 mV s−1. The dashed lines represent the linear fits.

electron propagation. From the resistance values, the roomtemperature electrical conductivities of GP and FGP were determined by using σ (S cm−1) = 1/R(d/a). In this equation, d is the thickness of the sample sandwiched between the two Pt electrodes and a is the area of cross-section of the Pt electrode in contact with the sample. The conductivities of GP and FGP were calculated to be 0.66 and 2.55 S cm−1. The conductivity of FGP is almost four times greater than that of GP, again indicating that the 2D graphitic layered structure analogous to graphene is dominant in FGP, which renders electron movement facile. In comparison, GP is characterized by a 18930



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2.39 and 1.41 eV, respectively. The absorption of TiO2/PbS/ CdS and the TiO2/FGP/PbS/CdS films are reflective of the cumulative effects of both QDs. Both curves display a broad absorption in the 400−500 nm range due to CdS QDs and the weak peak at 800 nm due to the PbS QDs. The fluorescence spectra of FGP dispersion and a thin film of FGP deposited on FTO -coated glass substrate are shown in Figure 7a, obtained at an excitation wavelength of 370 nm. The

stacked-up 3D graphitic structure; therefore, its electronic conduction behavior is expected to be only slightly better than that of bulk graphite. The carbon atoms in a graphene nanosheet (as in the few-layer graphene achieved in FGP) are sp2-hybridized and form a planar hexagonal lattice. The carbon atoms are σ-bonded within the plane but are π-bonded above and below the plane. These p orbitals are perpendicular to the plane and are indistinguishable, ongoing from one carbon to the other. Therefore, the electrons in these orbitals are delocalized. The conjugated π-orbital system allows unhindered electron movement above and below the carbon planes with least scattering. The high level of electronic conductivity in fewlayer graphene has been explained in the past, on the basis of the occurrence of quasiparticles; electrons that act as if they have no mass, like photons, can travel relatively long distances without scattering. These electrons are known as massless Dirac Fermions.37 However, in the 3D graphite structure, like in GP, wherein the graphitic layers are not exfoliated and therefore the single sheets are unavailable, the layers interfere with the behavior of single sheets, and as a consequence conductivity is lowered. Previously, for RGO samples, conductivities of 5.73 × 103 S m−1 have been obtained.38 Our value is comparable to reported values, and this high conductivity of FGP also contributes to improving photoexcited electron transport, when FGP is integrated with CdS and PbS QDs in a solar-cell configuration. 3.5. Absorption and Fluorescence Spectral Analyses. The absorbance spectra of GP-, FGP-, and QD-based films are shown in Figure 6. Both GP and FGP show a cluster of peaks in

Figure 7. Fluorescence spectra of (a) FGP colloid (○) and FGP film on FTO coated glass (□) and (b) CdS QDs on a glass substrate (□), TiO2/CdS (○), TiO2/PbS/CdS (△), and TiO2/FGP/PbS/CdS (◇) on FTO/glass electrodes. (c) Time-resolved fluorescence decay traces of CdS QDs/glass (□), TiO2/CdS (○), TiO2/PbS/CdS (△), and TiO2/FGP/PbS/CdS (◇) on FTO/glass electrodes. Inset in panel b is the deconvoluted fluorescence spectrum of CdS/glass. The excitation wavelength was fixed at λex = 370 nm in panels a−c, and λem was fixed at 530 nm in panel c.

Figure 6. Absorbance spectra of (a) (i) functionalized graphite platelets (FGP) and (ii) graphite platelets (GP) and (b) pristine CdS QDs (○), pristine PbS QDs (△), TiO2/PbS/CdS (□), and TiO2/ FGP/PbS/CdS (◇) electrodes.

FGP colloid shows two peaks at 409 and 430 nm, but the intensities of the emission peaks are weak, indicating FGP to be weakly luminescent. This is also corroborated by the emission spectrum of the FGP film, which is flat and devoid of any peak in the visible region. Because FGP is used in the electrode for solar-cell application by applying it to the FTO substrate, it is the fluorescence response of the FGP film that has direct relevance for analyzing the charge-transport mechanism in the photoanode. The PL spectrum of a film of pristine CdS QDs on glass obtained at λex = 370 nm shows a broad highly

the 200 to 400 nm wavelength range, albeit a slight decrease in their intensities observed in GP. The absorbance profiles are featureless in the visible region (400−800 nm) for both GP and FGP. While pristine CdS QDs show a broad absorption in the 400−500 nm wavelength range, PbS QDs show a peak in the NIR region at 800 nm. From the absorption edges of the QDs, the band gaps of the former and the latter were deduced to be 18931



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Table 1. Kinetic Parameters of Emission Decay Analysis of Photosensitizer Films Deduced from Double Exponential Fitsa

a

sample

B1

τ1 (ns)

B2

τ2 (ns)

⟨τ⟩ (ns)

χ2

CdS TiO2/CdS PbS/CdS TiO2/PbS/CdS TiO2/FGP/PbS/CdS

40.87 30.59 60.13 51.47 58.81

0.472 5.53 9.53 0.28 0.158

59.13 69.41 39.87 48.53 41.19

7.74 0.346 0.361 5.16 2.29

7.446 4.8855 9.3053 4.8944 2.0988

0.9274 1.0422 1.0551 0.8929 0.9333

B is the relative amplitude of each lifetime, τ1 and τ2 are the components of fluorescence lifetime, and χ2 denotes the fit quality.

data were fitted to biexponential curves on the basis of χ2 values, and the plots of the residuals and fitted parameters are summarized in Table 1. In eqs 1 and 2, τi and Bi are decay time constants and amplitudes, respectively, of the individual decay components, and ⟨τ⟩ is average electron lifetime.

asymmetric peak spanning from 475 to 600 nm, indicating it to be composite of multiple components. Deconvolution yielded four peaks: one at 530 nm due to band edge emission and the remaining three components at 509, 568, and 483 nm due to intra gap states. The intensity of the broad peak decreases to 21% of its original value for the TiO2/CdS film suggestive of excited electron transfer from the CB of CdS to the CB of TiO2. However, the intensity of this broad peak experiences a slight increase, by 25% when PbS is incorporated in the TiO2/ CdS assembly. The fluorescence of PbS does not interfere with that of CdS because PbS does not exhibit any luminescence in the visible region. The increase in the emission of CdS QDs is due to photoexcited electron transfer from the CB of PbS to the CB of CdS, as the former lies at 3.52 eV and the latter is poised at 3.61 eV. This renders the electron transfer to be thermodynamically favorable. Lifetime studies further ratify the emission increment. The effect of FGP on the fluorescence of the TiO2/PbS/CdS assembly was also studied. For comparing the fluorescence of TiO 2 /PbS/CdS with that of the TiO 2 /FGP/PbS/CdS electrode, it was necessary to establish that amount of PbS/ CdS that could be loaded onto the TiO2/FGP assembly should be approximately the same as the amount of PbS/CdS loaded onto neat TiO2. For this, the BET specific surface areas (by N2 adsorption) were determined for TiO2/FGP and TiO2, after scrapping the films from FTO surface. These were found to be 27 and 24 m2 g−1, respectively. This suggests that the specific surface area available for anchoring of CdS and PbS QDs in both the electrodes is almost the same. The reason for nearly the same surface area, despite the presence of FGP, is the fact that the high surface area of FGP compensates for the loss in surface area of TiO2 caused by FGP insertion into the pores of TiO2, when FGP is deposited by electrophoresis. To substantiate this claim, the thicknesses of the electrodes was determined from cross-sectional FE-SEM images (Figure S3, Supporting Information). The average thicknesses of TiO2 and TiO2/FGP layers were 4.3 and 4.26 μm. Upon applying 2-PbS and 7-CdS layers by SILAR on each of these electrodes, the average thicknesses of TiO2/PbS/CdS and TiO2/FGP/PbS/ CdS were found to be 6.48 and 6.43 μm, which are comparable. Therefore, the fluorescence spectrum of the TiO2/PbS/CdS assembly can be compared with the emission of the same electrode with FGP. The intensity of CdS emission decreases substantially in TiO2/FGP/PbS/CdS, thus indicating that FGP works as an effective electron acceptor (Figure 7a). Because the Fermi level of FGP lies at 4.52 eV, electron transfer from CdS and PbS to FGP are energetically permissible. This causes the significant quenching of the emission of CdS. 3.6. Emission Decay Studies. To affirm the electron deactivation pathways in the CdS-based films, we measured emission decay by fixing the excitation at 370 nm and monitoring the emission of the different photoanode assemblies at a fixed wavelength of 530 nm (Figure 7c). The fluorescence

f (t ) = B1e−t / τ1 + B2 e−t / τ 2

(1)

⟨τ ⟩ = Σ iBi τi 2/Σ iBi τi

(2)

The average excited electron lifetime in pristine CdS QDs/ glass is 7.44 ns. The short-lived component of 7.74 ns corresponds to deactivation vis-à-vis the intra gap states. The long-lived component of 0.47 ns is assigned to the band edge recombination. The average lifetime decreases to 4.88 ns when CdS QDs are anchored onto TiO2, as in the TiO2/CdS assembly. Here the short-lived component of 0.35 ns arises from CdS QDs, which are in direct contact with TiO2. The long-lived component of 5.53 ns is attributed to the CdS QDs, which are not in direct contact with the oxide particles. The average electron lifetime of CdS QDs increases to 4.9 ns (compared with TiO2/CdS) when PbS is introduced in the TiO2/CdS film. PbS, upon photoexcitation, injects electrons to CdS QDs. The deactivation of electron population in the CB of CdS in the TiO2/PbS/CdS film occurs over a longer a time scale due to the additional electron populace introduced in CdS by PbS compared with the electron deactivation time in TiO2/ CdS. The slight lifetime increase in CdS was also confirmed by measuring the same for a system of PbS/CdS devoid of TiO2 at λem of 530 nm. The average lifetime was 9.3 ns, longer than that of pristine CdS. In line with the emission quenching data, the average lifetime of photoexcited electron in CdS decreases upon integrating FGP with QDs and oxide. The average lifetime is the least (∼2.1 ns) in the TiO2/FGP/PbS/CdS film, owing to the ability of FGP to accept electrons and channelize them to FTO and then to the external circuit. Previously, for a TiO2/CdSe electrode in the presence of stacked carbon nano cups, electron lifetime was found to be 39 to 62 ps.39 The energetics of the assembly is shown in Figure 8. Upon illumination, electrons from the CB of CdS QDs (inherent and those transferred by PbS) are relayed to the CB of TiO2. Electrons then flow from TiO2 to FGP to FTO or from TiO2 to FTO (if FGP is not present in the system). The average emission lifetime was used to estimate the electron-transfer rate so as to include both the short- and longlived components. The rate of electron transfer was obtained from the following expression. k = 1/⟨τ ⟩(TiO2 /FGP/PbS/CdS) − 1/⟨τ ⟩(TiO2 /PbS/CdS) (3)

The apparent electron-transfer rate (k) for electron transfer from the photoabsorbers CdS and PbS to the current collector was deduced to be 0.272 × 109 s−1 in the presence of FGP. FGP promotes electron transfer and propagation; therefore, the 18932



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over a wavelength range of 350 to 1100 nm with the previously described photoanodes are shown in Figure 9a,b. EQE is

Figure 8. Energy band diagram of the TiO2/PbS/CdS/FGP assembly depicting the energy levels of all components and all plausible chargetransfer modes under illumination.

kinetics of electron injection is fast. The electron-transfer modes in the TiO2/FGP/PbS/CdS assembly are shown in Figure 8. The VB and conduction band (CB) positions of CdS and TiO2 were deduced by combining our data from optical absorption and cyclic voltammetry done in a previous report.40 The VB of PbS was fixed from the corresponding oxidation peak observed in the cyclic voltammogram of PbS (Figure S4, Supporting Information). The position of CB in PbS was determined by subtracting the magnitude of optical band gap of 1.41 eV from the VB position. The Fermi level of FGP in the diagram was fixed from the VB spectrum (XPS analyses). Upon illumination, the dominant excited electron transfer and transport pathway in the TiO2/PbS/CdS/FGP electrode is as follows. Excited electrons in PbS are transferred to the CB of CdS and from the CB of CdS; both types of electrons (inherently photogenerated ones and the electrons injected from PbS) are transported to TiO2, and via FGP they reach the current collector. It must be noted that the TiO2, FGP, PbS, and CdS layers are porous because these have been prepared by solution-phase methods. Therefore, despite the PbS layer being sandwiched in between the CdS and TiO2 layers, as per the sequence of deposition, and due to the PbS layer being a single SILAR layer, it is thin enough to permit mingling with the overlying CdS QDs. This was also observed in HRTEM images, wherein lattice fringes are overlapping. Such interpenetrating structures permit unencumbered electron transport from CdS to TiO2, which explains the least electron lifetime obtained for this quaternary system and a superior photocurrent response as well, which will be seen in the later section. 3.7. Photoelectrochemistry of QDSCs. QDSCs were constructed using different photoanodes (TiO2/CdS, TiO2/ PbS, TiO2/PbS/CdS and TiO2/FGP/PbS/CdS), a S2− solution as the electrolyte and MWCNTs deposited on FTO as the counter electrode. The EQE and IQE of the cells measured

Figure 9. (a) EQE, (b) IQE, and (c) J−V characteristics (measured under 1 sun illumination, AM 1.5G) of QDSCs with the following photoanodes: TiO2/CdS (□), TiO2/PbS (○), TiO2/PbS/CdS (△), and TiO2/FGP/PbS/CdS (◇). All experiments in panels a−c were performed using a 0.1 M Na2S in ultrapure water/methanol 3:7 v/v solution employed as the electrolyte and a MWCNT/FTO assembly used as a counter electrode. In panels a and b, a 250 W tungstenhalogen lamp was used as for illumination and the insets are magnified views.

defined as the ratio of number of charge carriers generated to the number of incident photons of a particular energy. IQE is the number of charge carriers generated to the number of photons of a given energy absorbed by the photosensitizer. IQE is usually greater than EQE, as the former is a measure of the effective utilization of impinging photons. The maximum EQE was found to be 30% (λ = 462 nm) for the TiO2/CdS assembly. IQE was found to be 31% at 374 nm and 30% at 462 nm for the same assembly. Both IQE and EQE values declined rapidly at wavelengths above 500 nm, and they were

Functionalized Graphite Platelets and Lead Sulfide Quantum Dots

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