Photoelectrochemical Formation of Polysulfide at PbS QDs-Sensitized

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Photoelectrochemical Formation of Polysulfide at PbS QDs-Sensitized Plasmonic Electrode Xiaowei Li, Paul David McNaughter, Paul O'Brien, Hiro Minamimoto, and Kei Murakoshi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02045 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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The Journal of Physical Chemistry Letters

Photoelectrochemical Formation of Polysulfide at PbS QDs-Sensitized Plasmonic Electrode Xiaowei Li*†, Paul D. McNaughter§, Paul O’Brien§‡, Hiro Minamimoto†, and Kei Murakoshi*† †Department §School

of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan

of Chemistry and ‡School of Material, the University of Manchester, Oxford Road,

Manchester, M13 9PL, United Kingdom Abstract Effective electron-hole separation is a key to enhance photoenergy conversion of semiconductor-quantumdots-(QDs) sensitized plasmonic solar cells. However, in contrast to intense studies on electron transfer, hole transfer from QDs and consequent chemical reactions with donors in electrolytes remain unclear. Herein, in-situ electrochemical surface-enhanced Raman scattering (SERS) measurement on a PbS QDssensitized TiO2/Au/TiO2 photoelectrode indicated formation of cycloocta-sulfur (-S8) via tuning the electrochemical potential. A photocurrent density of 100 nA/cm2 was recorded simultaneously even with an extremely low QDs loading. Two-dimensional correlation analysis of the SERS revealed subsequent formation of S8− and S42− at −1.1 ~ −0.1 V (vs. Ag/AgCl), S8 from −0.3 V, and S52− and S62− at ≥0.2 V via complex disproportionation reactions. The sensitive detection is attributed to the enhanced electromagnetic field of localized surface plasmon resonance, which provide a better understanding of charge separation processes in QDs-sensitized solar cells.

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Colloidal semiconductor quantum dots (QDs) have attached intense attention as light harvesters to modify photoenergy devices because of their tunable electrical and optical properties based on sizedependent quantum confinement. Lead sulfide (PbS) has a relatively larger Bohr radius of 18 nm, and thus exhibits a strong quantum confinement effect that enables bandgap tuning over the full solar spectral range. PbS QDs-sensitized titanium dioxides (TiO2) performed efficiently in photoenergy conversion1-2 and in photocatalytic reduction of carbon dioxide,3-4 especially in the presence of localized surface plasmon resonance (LSPR) of metal nanoparticles (NPs). The excitation of size-controlled PbS QDs with bandgap energy Eg = 2.3 eV by plasmonic Au NPs enhanced photoelectric conversion over an expanded spectral range including short-wavelength visible and near-infrared light.5 It also modified the electromotive force and output power density based on multiple exciton generation (MEG)6-8–effective PbS QDs with a narrow bandgap energy Eg = 0.9 eV,9 which is attributed to a highly enhanced density of electrons excited by energy overlapping between MEG and LSPR. Electron transfer processes from photoexcited PbS QDs to TiO2 or molecules have been extensively studied;10-11 whereas, hole transfer between PbS QDs and inorganic electrolytes has received less attention. Sulfide redox couples (S2−/S), instead of triiodide/iodide (I3−/ I−) electrolytes, for hole scavenging plays a crucial role in stabilizing lead chalcogenide QDs-based solar cells (QDSC) against degradation under solar irradiation. The dynamics of interfacial hole transfer process from QDs to polysulfide has been discussed in terms of transient absorption,12 and sulfur reduction in lithium-sulfur batteries has been extensively studied on during discharge.13 However, potential-dependent polysulfide oxidation pathways in QDSC are still unclear. Previous work investigated plasmon-induced photo-polymerization of pyrrole monomers localized at spatially selective sites by hole carries damped from LSPR,14 but the oxidation processes of molecules at active reaction sites requires further investigation. Herein, taking PbS QDs sensitized TiO2/Au NPs/TiO2 as an electrochemical surface-enhanced Raman scattering (EC-SERS) prototype, we investigated distinct spectral characteristics of cycloocta-sulfur (-S8) formation simultaneously with photocurrent collection, by positive tuning of the electrochemical potential. We performed two-dimensional correlation spectroscopy (2DCOS) to determine potential-dependent sulfur production and to propose polysulfide oxidation mechanisms. The present work paves a way to further understand the electron-hole separation and polysulfide oxidation for QDs-based solar cells. Characterization of PbS QDs-sensitized TiO2/Au/TiO2. An electrode of Au nanoparticles (Au NPs) embedded in a TiO2 thin film (TiO2/Au/TiO2) was prepared as reported previously.5, 9 Au NPs with an average diameter of 20.6 ± 6.3 nm (Figure 1a) have a LSPR peak maximum at 580 nm in the range of 1.77–2.58 eV, according to extinction spectra (Fig. S1 in SI). The thickness of the second TiO2 layer was estimated to be around 7～8 nm based on the deposition time and reported film growing rates.13 Oleic acid-capped PbS QDs with a 0.92 eV bandgap energy and an estimated average diameter of 4.6 nm (donated as OP-1344 according to the absorption maximum, Fig. S1 in SI), were drop-casted onto the substrate with a particle density of 2.3×1012 cm−2. Both the Au NPs and the PbS QDs have strong resonances with 514 nm (Eex=2.4 eV) laser excitation that matches the LSPR region and exceeds the MEG threshold of 2.6 times of the PbS QDs bandgap energy.15

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Figure1. (a) Schematic of the electrochemical SERS measurements. (b) SEM image of the fabricated TiO2/Au/TiO2 electrode. Potential-dependent SERS spectra. In-situ EC-SERS spectra of a PbS QDs/TiO2/Au/TiO2 electrode were recorded by varying the electrochemical potential over −1.1 V ~ 0.5 V (vs. Ag/AgCl) in an aqueous electrolyte of 0.05 M Na2S and 0.1 M NaOH (Figure 1). As the potential tuned to the positive direction, the enhancement of Raman bands at 474, 222 and 155 cm−1 were investigated (Figure 2a). These bands were assigned to the internal vibrations of the crown-like orthorhombic cycloocta-sulfur (-S8) based on factorgroup symmetry,16-17 with 7 (ν(S–S), E2 (ag) stretching at 474 cm-1), 2 (δ(S–S–S), E2 (ag) bending at 222 cm-1) and 8 (δ(S–S–S) A1 (b3g) bending at 155 cm-1), respectively. Allotropes of monoclinic sulfur, -S8 and -S8, were ruled out because temperatures above 369 K are required for phase transition by melting -S8.18 LSPR-induced local heating of Au NPs embedded into TiO2 was not sufficient for the phase transitions. The increased intensities for these vibrational modes at the positive electrochemical potentials resulted from a chemical reaction where holes generated in the photoexcited PbS QDs oxidized the sulfide S2− donor in the electrolyte (Figure 2c) to polysulfide radical dianions Sn2− and monoanions Sn− intermediates, and finally, S8.5, 12 (Details will be discussed below.) The apparent increase in the 474 cm-1 (purple), 222 cm-1 (red) and 155 cm-1 (blue) signals in Figure 2b were observed when the potential was controlled more positive than −0.4 V. The simultaneously recorded photocurrent increased stably with the potential (Figure S2) because of efficient electron-hole separation in the excited PbS QDs. This occurred because the Fermi level moved downwards as the more positive potential increased the potential gradient in TiO2 (Figure 2c). This boosted the electron injection into TiO2 while restricted the recombination, consequently enhancing the oxidation reaction between holes and the donor. As previously reported, the observed photocurrent was attributed to both the enhanced electromagnetic field5 (dark orange area in Figure 2c), by coupling dielectric PbS QDs with plasmonic Au NPs, and the MEG of OP-1344 excited by LSPR with an energy exceeding 2.6 times of the bandgap.139 An exponential relationship between the simultaneously measured photocurrent density and S8 Raman intensities was established (Figure S3). The photocurrent gradually saturated as the potential became more positive than −0.3 V, while the formation of S8 saturated at 0.1 V. This was ascribed to the different dynamics on the time scales of electron and hole transfer. Electron transfer from PbS QDs into TiO2 NPs has a femtosecond timescale11 which is much more rapid than both the hole-scavenging by the S2−

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donor on a nanosecond scale,12 and a 1-μs radiative recombination detected as photoluminescence.19 The slow hole scavenging could limit the electron-hole pair separation in QDs and affect the overall conversion efficiency, even though multiple excitons are generated. TiO2/Au/TiO2 and TiO2 control experiments were also performed under the same conditions; however, significant S8 vibrations were not observed (Figure S5 in SI). In addition, increasing the excitation intensity up to 1.61 mW (see Figure S6 in SI) yielded only TiO2 phonon modes that were assigned to anatase.20-21 Photocurrent generation by bare TiO2 or TiO2/Au/TiO2 substrates is extremely limited compared with PbS QDs sensitized ones.5 The QDs loading was extremely low within a monolayer, and a photocurrent up to 100 nA/cm2 was investigated with a rather weak laser intensity of 10 mW/cm2, possibly due to the MEG of the narrow bandgap QDs used previously,9 and the enhanced electromagnetic field via coupling of Au NPs and large-sized dielectric PbS QDs that increase light scattering efficiency at their gap.22 The constant intensity of the 155 cm−1 band at potentials more negative than −0.4 V suggested that limited hole transfer does occur. Formation of sulfur by electrochemical oxidation at this negative potential range23 is ruled out because of the rectification of TiO2. Although photocurrents were not collected at potentials more negative than −0.65 V because of the restricted potential gradient in TiO2, electron-hole pairs were produced from QDs under laser irradiation and hole transfer competed with recombination processes.12 Moreover, the vibrational modes of the oleic acid (OA), the surface modification of PbS QDs, were not detected because of the rather lower excitation intensity and also because the increased distance between PbS QDs and Au NPs by the second layer of TiO2 precluded direct contact.24 Although the spacing of OA and produced S8 from Au NPs are the same, the enhancement on vibrations are different. Functional group-dependent Raman enhancement was confirmed previously24 that rather than the olefin group or polymethylene chains of OA, the most easily detected vibrations was the carboxyl group (COO-) linking with PbS QDs. This implies that detecting OA faces a problem of a decayed gradient of the LSPR electromagnetic field at a distance ca.～2 nm, the OA chain length, farther away from the TiO2 surface, in comparison with detecting the S8 formed on the TiO2 surface with a gradually increased amount by the excited QDs. Furthermore, Au-S stretching25 at 310 cm−1 was not observed, indicating that the second layer of TiO2 covered the Au NPs and inhibited electron quenching between QDs and Au NPs. All these observations indicated that the formation of S8 occurred on the TiO2 surface in the nanoscaled gap between the QDs and Au NPs, where an enhanced electromagnetic field was generated and excited holes were confined.

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Figure 2. (a) EC-SERS spectra of OP-1344/TiO2/Au/TiO2 for potentials ranging over −1.1 ~ 0.5 V (vs. Ag/AgCl) in an aqueous electrolyte of 0.05 M Na2S and 0.1 M NaOH. The spectra and corresponding photocurrent signals were recorded simultaneously. The samples were irradiated for 30 s with a 514.8-nm, 0.08-mW laser. Dashed lines indicate the major internal vibrations of -S8. Partial data adapted from ref (26). Copyright 2016 American Chemical Society. (b) Potential-dependent Raman intensities of S-S bending at 155 cm−1 (S5−, S7− at negative potentials and 8 at positive potentials, red), at 222 cm−1 (2, blue), and S-S stretching at 474 cm−1 (7, purple). (c) Schematic of a PbS QDs/TiO2/Au/TiO2 electrode in contact with a sulfide donor. The conduction and valence band edges were calculated with regard to the average size evaluated from the first 1344-nm exciton. The flat band potential of anatase TiO2 in a neutral aqueous solution. The enhanced near-field generated by LSPR is confined between the Au NP and the PbS QD.5, 22 The 474 cm−1 (7, purple) and 222 cm−1 (2, blue) intensities were almost zero at potentials more negative than −0.3 V, while the 155 cm−1 (8, red) intensity was still observed in this region (Figure 2b). 2D correlation analysis was performed using the EC-SERS spectra in Figure 2a after subtracting baselines and spectral averaging. The synchronous map, Φ(1, 2), has three positive correlation peaks centered at (474, 222), (474, 155) and (222,155) cm−1 (see red areas in Figure 3a), confirming that all the Raman intensities of these peaks increased when the potential value became positive. The corresponding correlation peaks on the asynchronous map, (1, 2), however, were rather complex with both positive and negative cross peaks not only for the vibrations noted above, but also for their adjacent Raman shifts (Figure 3b). This indicated that there were at least six other vibrations mixed with the three major S8 vibrations. These

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vibrations were tentatively assigned, according to the high-resolution Raman intensity and full-width at halfmaximum (FWHM) of -S8 at 15 K,16-17 to 1 (A1 (ag) stretching at 480 cm-1), 5 (E1 (ag) stretching at 468 cm1),

2 (A1 (b1g) bending at 216 cm-1), and 8 (E2 bending at 161cm-1 (b2g) and possibly 149 cm-1 (b1g)),

respectively. The band at 228 cm-1 could not be assigned to -S8 because the -S8 crystal does not have Raman-active factor group components around 230 cm-1, even at ultra-low temperatures. Considering the S8 formation processes from the S2− donor, the 228 cm-1 band was assigned to pentasulfide dianion S52−, and the band at 480 cm−1 and 161 cm−1 could be alternatively attributed to S52− or S62− and S5− or S8−, respectively (see more details in Table 1).27 The sequential order of these bands can be determined from the synchronous and asynchronous maps according to Noda’s rules,28 where, in the case of 1>2 andΦ(1, 2)>0, the corresponding (1, 2)>0 ((1, 2) 468 cm−1 > 149 cm−1 > 222 cm−1> 474 cm−1 > 155 cm−1> 228 cm−1 > 161 cm−1 > 480 cm−1 during positively potential tuning (details see Table S1 in SI). This order strongly suggested that the potential dependence of the SERS spectra was closely related to the formation of S8. One hypothetical reason is that the three major S8 bands had potential-dependent blue-shifts during formation. However, there was no robust evidence to support this assumption when considering both the physical state macroscopically and the fine factor group symmetry spectrally. For the former, as the S8 product increased at positive potentials, dissolved S8 could have become saturated due to its rather limited solubility in water, and thus deposited on the surface of TiO2 to obstruct electron transfer from QDs to TiO2, which was confirmed by the reduced photocurrent at 0.3 ~ 0.5 V. The physical state transition implies that the S8 vibrations will exhibit a red-shift because of the increased intermolecular association;29 however, this contradicts the sequential order. Concerning the latter, if the bands at 149, 155 and 161 cm−1 are all attributed to the 8 vibration of -S8 with molecular symmetries of b1g, b3g, and b2g, respectively, the intensity ratios of these three fine vibrations over the range −1.1 ~ −0.3 V conflict with the Raman spectra at 300 K,17 the heavy overlap of these vibrations at room temperature, and the absence of Raman-active -S8 symmetry at 228 cm−1. For these reasons, the vibrations adjacent to 474, 222 and 155 cm−1 of -S8 were attributed to other polysulfide mono-anions or dianions.

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Figure 3. (a) Synchronous and (b) asynchronous 2D correlation spectra from Figure 2a. The top and right SERS spectra are the averaged spectrum from over the range −1.1 V ~ 0.5 V. The red and blue areas represent the positive and negative cross peaks, respectively. Table 1 summarizes the potential regions that those vibrations were significantly discernable and their possible assignments based on previous experimental agreement with DFT simulations27 and further corrections,30 giving 216 cm−1 (S8−, −0.4 ~ −0.1 V) > 468 cm−1 (S42−, −0.4 ~ −0.2 V) > 149 cm−1 (S8−, −1.1 ~ −0.1 V) > 222 cm−1 (S8, −0.3 ~ 0.5 V) > 474 cm−1 (S8, −0.3 ~ 0.5 V) > 155 cm−1 (S8 or S5−, S7−, full range) > 228 cm−1 (S52−, −0.1 ~ 0.5 V) > 161 cm−1 (S5−, S8−, full range) > 480 cm−1 (S52−, S62−, 0.2 ~ 0.5 V). Briefly, octasulfide mono-anion S8− and tetrasulfide dianion S42− formed in a relatively negative potential range of −1.1 ~ −0.1 V, then the cycloocta-sulfur ring of S8 was produced from −0.3 V. Large amounts of S52−and S62− were then formed at potentials more positive than 0.2 V, via oxidation by holes from photoexcited PbS QDs and also via complex disproportionation reactions including S8 and other polysulfide species. This would be consistent with thermodynamically feasible reaction pathways for the formation of S8 (Scheme 1) based on published Gibbs energies Go at 298 K31-33 and charge-discharge processes on lithium-sulfur batteries.34 A sulfide dianion S2− from the electrolyte first oxidizes into a sulfide anion S−, which then exothermically forms disulfide dianion S22− (Eq. 1). These processes repeat, increasing the sulfur chain to form tetrasulfide dianion S42− and octasulfide dianion S82−; the latter then scavenges holes to form a thermodynamically stable cycloocta-sulfur ring of S8. The consumption of S42− and S32− via multiple routes with Gibbs free energy of e.g., ca. −1512 and −1498 kJ/mol at ca. 298 K in aqueous solution indicated the feasibility of the holeassisted reactions, which agreed with the spectral characteristics that these two types of polysulfide dianions were merely observed during the SERS measurements (details will be discussed below). Other oxidation reactions and disproportionation reactions evolving polysulfide anions Sn− , dianions Sn2−, and polysulfur Sn occur simultaneously (e.g., Eqs. 2, 3), resulting in complex pathways.30 The 2D correlation analysis provided direct spectral evidence of the sulfur oxidation processes that were neglected during research on lithiumsulfur batteries.35

Table 1 Raman shifts and the internal mode assignments of the main peaks Raman shift / cm-1 480 474 468 228 222 216 161 155 149 444 246

Assignment ν(S–S)

δ(S–S–S)

δ(S–S–S) ν(S–S) δ(S–S–S) a Internal

S52−, S62− -S8 S S52− -S8 S8− − S5 , S8− -S8, S5−, S7− S8− -S8 -S8

a Internal vibrations

Order

Potential range V vs. Ag/AgCl

 7   2   8  10 11

 5 2 7  1  6 3 − −

   −0.3 ~ 0.5 −0.4 ~ −0.2 −0.1 ~ 0.5 −   −0.4 ~ −0.1 full range full range −1.1 ~ −0.1 −0.1 ~ 0.5 −0.3 ~ 0.5

vibrations of -S8. ν stretching mode and δ bending mode.

The reason that the band at 155 cm−1 had fluctuated intensities in the potential region of −1.1 ~ −0.4 V (Figure 1b) could also be attributed to the formation of S5− or S7−. Along with the S8 vibrations of 10 (E3 (ag) stretching at 444 cm-1) and 11 the (E3 (ag) bending at 246 cm-1) (Table 1), the onset potential of the S8 production was c.a. −0.3 V, which is less negative than that of photocurrent generation at c.a. −0.5 V. In

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addition, in the potential range 0.3 ~ 0.5 V, the 222 and 474 cm−1 intensities were reduced when the 155 cm−1 band increased. One aspect was that the solid S8 deposition reduced not only the photocurrent (Figure S2), but also the S8 production, which therefore diminished the 7 and 2 vibrations of S8; another was the consumption of S8 via disproportionation reactions that increased the formation of S5− or S7−.

Sn 2  + h   Sn  2Sn   S2 n 2 Sn  + h   Sn

Equation 1

1 Sn 2 + S8  Sn 12 8  1 Sn + S8  Sn 1 8 Equation 2 S x 2 +S y   Sm 2  Sn  (x  y; x  y  m  n )

Equation 3

Scheme 1. Proposed reaction pathways for the formation of S8 from polysulfide mono-anions (Sn−) and dianions (Sn2−) by holes generated from photoexcited QDs. Dashed lines present the disproportionation reaction of S8− into S82− and S8. Numbers in parentheses represent the calculated reaction Gibbs energies (kJ /mol) at 285 K in solution from ref. 30, 31 and a32. Furthermore, the presence of Sn− and Sn2− were additionally clarified by analyzing the vibrations from Figure 2a that Raman intensities surmount the noise widths (shadows in Figure 2b), e.g., 408 cm−1 at −0.8 V. Observed vibrational modes were assigned via peak intensities27,

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and the most likely species are

summarized in Table S1, providing evidence for the formation of polysulfide mono-anions Sn− with n=3~8 and dianions Sn2− with n=4~8 during potential tuning (Figure 4a). Figure 4b shows the numbers of occurrences of Sn− and Sn2− at certain potentials in seventeen recordings during potential tuning. The more frequent occurrence of Sn− relative to Sn2− was clarified, which was consistent with previous DFT calculations that polysulfide dianions were relatively unstable.31 S3− and S4− created on lithium-sulfur batteries were not predominantly observed here, partially because the 514-nm laser reached the resonance condition for these polysulfide anions18 and accelerated their oxidation reactions. The successful detection of rarely observed Sn− and Sn2− was attributed to the enhanced electromagnetic field of LSPR and the resonance condition.

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This work confirmed that the charge separation and transfer processes by PbS QDs enhance the photoelectric conversion efficiency of solar cells. Further investigations of the potential-dependent relationship between photocurrent and photoluminescence of size-controlled QDs could provide additional information on charge transfer and radiative recombination in electrochemical environments and would considerably advance development of semiconductor QDs-based solar cells.

Figure 4. (a) Summary of assigned polysulfide monoanions (Sn−) and dianions (Sn2−) at different electrochemical potentials. The Raman peaks had intensities in the range 50 ~ 125 cps. The -S8 formed in the region −0.4 ~ 0.5 V (light red). Details of the assignments are in Table S2 in the SI. (b) Occurrences at certain potentials for Sn− and Sn2− in seventeen recordings during potential tuning. Hollow circles in (a) and light color bars in (b): Sn− and Sn2− from bands at 149, 161, 216, 228, 468 and 480 cm−1. In conclusion, EC-SERS was acquired for a PbS QDs-sensitized TiO2/Au/TiO2 electrode. Vibrations assigned to the cycloocta-sulfur ring of -S8 were investigated and compared with the bare TiO2/Au/TiO2 or TiO2 electrodes. Enhanced Raman intensities of S8 were observed by tuning the electrochemical potential positively, because of efficient electron-hole separation in photoexcited PbS QDs by enlarging the potential gradient in TiO2. 2D correlation analysis of the SERS spectra provided additional information on the potential-dependent formation of S8, including S8− and S42− products in the relatively negative potential range of −1.1 ~ −0.1 V, the creation of S8 from −0.3 V, and the subsequent formation of S52− and S62− at potentials more positive than 0.2 V via complex disproportionation reactions by S8 and other polysulfide species. Thermodynamically feasible reaction mechanisms were promoted. Other probable polysulfide anions and dianions produced during the sulfide oxidation by holes from QDs were investigated by analyzing relatively weak SERS signals that surmounted the noise width, yielding evidence of intermediates. The detection of these spectral characteristics in the present system with extremely low QDs loading was attributed to the enhanced electromagnetic field of LSPR and the resonance condition. Such investigations will provide a better understanding of charge separation processes and should stimulate improved designs of QDs-sensitized solar cells.

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ASSOCIATED CONTENT Supporting Information Materials and methods, synthesis of PbS QDs, construction of electrodes, EC-SERS spectra of TiO2/Au/TiO2 and TiO2 as a function of potential and excitation intensity, 2D correlation analysis results and assignment of polysulfide anions. This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author Xiaowei Li; [email protected] Kei Murakoshi; [email protected]

Author Contributions X.L. conceived the project, fabricated the electrodes and performed the EC-SERS and photocurrent experiments under the supervision of K.M. P.D. M. and P.OB. synthesized the PbS QDs and performed the absorption characterization. X.L. and P.D. M. performed the SEM characterization. X.L. wrote the paper with contributions from discussions with K.M. and H.M.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. JP18K14309, JP16H06506, JP18H05205,) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, especially the aid for Early-Career Scientists is acknowledged. P. OB. and P. D. M. acknowledge funding from EPSRC Grant K010298/1. Supported by the Photo-excitonix Project in Hokkaido University is also acknowledged.

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Figure1. (a) Schematic of the electrochemical SERS measurements. (b) SEM image of the fabricated TiO2/Au/TiO2 electrode. 79x65mm (300 x 300 DPI)


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Figure 2. (a) EC-SERS spectra of OP-1344/TiO2/Au/TiO2 for potentials ranging over −1.1 ~ 0.5 V (vs. Ag/AgCl) in an aqueous electrolyte of 0.05 M Na2S and 0.1 M NaOH. The spectra and corresponding photocurrent signals were recorded simultaneously. The samples were irradiated for 30 s with a 514.8-nm, 0.08-mW laser. Dashed lines indicate the major internal vibrations of α-S8. Partial data adapted from ref (26). Copyright 2016 American Chemical Society. (b) Potential-dependent Raman intensities of S-S bending at 155 cm−1 (S5−, S7− at negative potentials and ω8 at positive potentials, red), at 222 cm−1 (ω2, blue), and S-S stretching at 474 cm−1 (ω7, purple). (c) Schematic of a PbS QDs/TiO2/Au/TiO2 electrode in contact with a sulfide donor. The conduction and valence band edges were calculated with regard to the average size evaluated from the first 1344-nm exciton. The flat band potential of anatase TiO2 in a neutral aqueous solution. The enhanced near-field generated by LSPR is confined between the Au NP and the PbS QD.5, 22 169x135mm (300 x 300 DPI)



Figure 3. (a) Synchronous and (b) asynchronous 2D correlation spectra from Figure 2a. The top and right SERS spectra are the averaged spectrum from over the range −1.1 V ~ 0.5 V. The red and blue areas represent the positive and negative cross peaks, respectively. 170x81mm (600 x 600 DPI)


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Figure 4. (a) Summary of assigned polysulfide monoanions (Sn−) and dianions (Snn2−) at different electrochemical potentials. The Raman peaks had intensities in the range 50 ~ 125 cps. The α-S8 formed in the region −0.4 ~ 0.5 V (light red). Details of the assignments are in Table S2 in the SI. (b) Occurrences at certain potentials for Sn− and Sn2− in seventeen recordings during potential tuning. Hollow circles in (a) and light color bars in (b): Sn− and Sn2− from bands at 149, 161, 216, 228, 468 and 480 cm−1. 169x61mm (300 x 300 DPI)



Scheme 1. Proposed reaction pathways for the formation of S8 from polysulfide mono-anions (Sn−) and

dianions (Sn2−) by holes generated from photoexcited QDs. Dashed lines present the disproportionation

reaction of S8− into S82− and S8. Numbers in parentheses represent the calculated reaction Gibbs energies (kJ /mol) at 285 K in solution from ref. 30, 31 and 32. 169x94mm (300 x 300 DPI)


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TOC 169x169mm (300 x 300 DPI)


Photoelectrochemical Formation of Polysulfide at PbS QDs-Sensitized

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