Activation of CdSe Quantum Dots after Exposure to Polysulfide - The

Subscriber access provided by Washington University | Libraries

Article

Activation of CdSe Quantum Dots Post Exposure to Polysulfide Laurie Ann King, Weijie Zhao, Lefteris Danos, and D. Jason Riley J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2014 Downloaded from http://pubs.acs.org on June 25, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17

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

The Journal of Physical Chemistry

Activation Of CdSe Quantum Dots Post Exposure To Polysulfide Laurie A. King1, Weijie Zhao2, Lefteris Danos3, D. Jason Riley1. 1

Department of Materials, Imperial College London, SW7 2AZ, United Kingdom

2

Department of Physics, National University of Singapore, Singapore 117542

3

Department of Chemistry, Lancaster University, LA1 4YB, United Kingdom

Email: [email protected]

Abstract: Polysulfide electrolyte is the most commonly utilized electrolyte in quantum dot sensitized solar cells (QDSSCs). Recently, however, a redshift in the relative positions of the absorbance and incident photon conversion efficiencies of CdSe QDs in polysulfide solutions has been reported. Here, we conduct extensive analyses of CdSe QD films comparing those exposed to polysulfide solution with unexposed films; indeed a redshift in the absorbance is observed post exposure. Most strikingly, we observe that a photocurrent response from films immersed in sodium sulfite is only observed post exposure to polysulfide solution. We demonstrate this is due to a combination of both QD ripening and a change in the QD capping layer. The results demonstrate that a polysulfide processing step significantly alters the photoelectrochemical performance of QDs.

KEYWORDS: Photoelectrochemical Cells, Polysulfide Electrolyte, Sensitized Photoanodes, Quantum Dots

ACS Paragon Plus Environment

1


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

Page 2 of 17

Introduction Since early studies began in the 1970s, cadmium chalcogenides (CdX where X = S, Se or Te) have attracted significant attention as photoelectrodes in photoelectrochemical cells (PECs) due to the suitability of their band gaps for solar photon harvesting, (≤2.4 eV).1–5,5–8 More recently, metal chalcogenide quantum dots (QDs) have been utilized as efficient light harvesting media in quantum dot sensitized solar cells (QDSSC)9,10 as well as in Colloidal QD (CQD) solar cell devices.11 Semiconductor QDs such as CdS, CdSe,12,13 PbS14 and InP15 have drawn particular attention as sensitizers in QDSSCs. In such devices, absorption of a photon leads to the creation of an electronhole pair within the QD which can then be separated and extracted from the cell at the anode and cathode. The photoexcited electron in the QD conduction band is extracted via injection into the TiO2 and subsequent diffusion through a porous TiO2 network to the conductive glass substrate. Conversely, the hole resides in the QD valence band and is removed by reducing species in the redox couple. The oxidized redox species diffuses towards the counter electrode where it is regenerated by the electrons completing the cell. The most frequently employed redox couple in QDSSCs is the aqueous sulfide/polysulfide electrolyte.16 Instability in CdSe QDSSCs utilizing polysulfide electrolyte has, however, been observed.17 Where instability has been observed, a spectral mismatch in the onset of the optical absorbance and photocurrent IPCE (incident photon to current efficiency) of the QD and QDSSCs was reported. Specifically, a redshift in the QD sensitized photoelectrode photocurrent spectra by up to 25 nm relative to the onset of the QD absorbance was recorded in polysulfide electrolyte. On the basis of SEM-EDX elemental identification of sulfur in the QD sensitized photoanode post polysulfide exposure, analogous to the 1970’s CdSe PEC literature,18–20 photon driven S/Se substitution mechanisms were suggested as the cause of CdSe QD instability.17 When sodium sulfide is dissolved into an aqueous medium the sulfide ions predominantly exist as HS- ions as dictated by the following equilibrium; + ↔ +

(1)

The fact that the equilibrium of reaction 1 lies to the right is illustrated by the observation that a 0.1 M aqueous Na2S solution has a pH of 13 such that SH- ions dominate the solution. When sulfur is dissolved in the presence of sodium sulfide, various hydrolysis and complexation reactions occur (below) forming polysulfide species denoted Sn2- where typically n = 2 – 5.21 + →

(2)

The polysulfide redox couple is generally written as: ↔ + ( − )

(3)

The aqueous polysulfide redox couple (S2-/Sn2-) is chemically very complex, and remains poorly defined with respect to chemical species. The electrolyte is also limited with regards to stability by the gradual disproportionation of polysulfide species to sulfide and oxosulfur species.22 Despite the limitations, polysulfide electrolyte remains the most popular redox couple for many aqueous semiconductor solar cells due to its ability to support a relatively (compared to other non-corrosive to QDs electrolytes) high open circuit voltage and the acclaimed stability during solar cell operation. The exact redox couple is known to vary with electrolyte composition, conACS Paragon Plus Environment

2

Page 3 of 17

centrations and solution pH.21,23 Indeed modifications of the electrolyte composition (ratio of Na2S:S) alongside the introduction of ionic liquids,22 alternative solvent mixtures24 and selenium17 has led to enhanced efficiencies for CdSe QDSSCs. Here, we report a systematic study whereby the performance of a CdSe photoanode immersed in sodium sulfite electrolyte was monitored both before and after submersion in a polysulfide solution. Photocurrent spectroscopy revealed significant device performance enhancement as a consequence of exposure to the polysulfide solution. Additionally, a redshift in the absorbance first excitonic peak is observed, coincident with the photocurrent spectra. Extensive chemical, optical and structural analyses were conducted to decipher the influence of the polysulfide solution on the CdSe QD photoanode. Results and Discussions Optical and Photoelectrochemical Analysis Photocurrent spectra of the CdSe photoanode were recorded in sodium sulfite electrolyte (pH 9.0) before and after exposure of 9.0) before and after exposure of the electrode to polysulfide solution (pH 12.5). The average absorbed photon conversion efficiencies (APCE) and incident photon conversion efficiencies (IPCE) were calculated and are presented in 1.2x10 -3 1.0x10

0.8 APCE - Treated polysulfide APCE - Treated alkaline LHE - Treated polysulfide LHE - Treated alkaline

-3

b.

2.5 IPC E IPC E IPC E Abs Abs Abs -

3.0x10-4 0.6

-4

2.5x10

8.0x10 -4

- u ntrea ted - p olysulfide trea ted - a lkalin e tre ated untr eated polysulfide tr eated alka line tr eated

4.0x10 -4

IPCE

0.4

2.0

1.5

2.0x10-4

6.0x10 -4

LHE

APCE

3.5x10-4

-4

1.5x10

1.0

1.0x10-4 0.2

2.0x10 -4

0.5

-5

5.0x10 0.0 450

500

550

600

650

0.0

0.0 700

450

Normalized Absorbance

a.

0.0 500

550

600

650

700

Wavelength (nm)

Wavelength (nm)

Figure 1 (a and b respectively). The spectra are overlaid with the light harvesting efficiencies/absorbance. Prior to polysulfide exposure, no photocurrent response was recorded for any of the tested CdSe untreated films in sulfite electrolyte. Intriguingly, post exposure to polysulfide solution, photocurrent was, however, switched on for all tested photoanodes. To distinguish the influence of polysulfide species from that of high alkalinity (pH) a third electrode was exposed to an alkaline solution of pH comparable to the polysulfide (pH 12.5).

1.2x10 -3 1.0x10


-3

b.

3.5x10-4

2.5 IPC E IPC E IPC E Abs Abs Abs -

-4

3.0x10 0.6

-4

2.5x10

8.0x10 -4

- u ntrea ted - p olysulfide trea ted - a lkalin e tre ated untr eated polysulfide tr eated alka line tr eated

1.5

4.0x10 -4

IPCE

0.4

LHE

2.0x10-4

6.0x10 -4

2.0

-4

1.5x10

1.0

1.0x10-4 2.0x10

0.2

-4

0.5

-5

5.0x10 0.0 450

500

550

600

650

0.0 700

0.0 450


a.

APCE

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


0.0 500

550

600

650

700

Wavelength (nm)

Wavelength (nm)


3


Figure 1. (a) The absorbed photon conversion efficiency (APCE) for the untreated (blue), polysulfide treated (red) and alkaline (black) CdSe photoanodes. The APCE spectra is overlaid with the respective light harvesting efficiency of the samples. Incident photon conversion efficiencies (IPCE) of the same samples are plotted in (b). Photocurrent was recorded at 0 V against a Ag/AgCl reference electrode in 0.5 M sodium sulfite. The absorbance of a treated CdSe film is shown for comparison.

Absorbance untreated Absorbance treated polysulfide Absorbance treated alkaline PL untreated PL treated polysulfide

Normalized PL


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

Page 4 of 17

450

500

550

600

650

700

750

800

Wavelength (nm)

Figure 2. Normalized absorbance and photoluminescence of the CdSe untreated (blue) and polysulfide (red) and alkaline (black) treated samples. No photoluminescence was observed for the alkaline treated sample.

The optical absorbance spectra of the QD films are shown in Figure 2, revealing a redshift in the onset of the absorbance post-exposure to polysulfide electrolyte. Specifically, the first QD excitonic absorbance peak position shifts from 560 nm to 576 nm in polysulfide solution. Niitsoo et al. have previously reported CdSe QD ripening in alkaline solutions in general.25 Conversely, we do not observe crystal growth in films exposed to sulfite solution at pH 9.0. However, at higher alkalinity (pH 12.5 and hence comparable to that of polysulfide electrolyte) we observe the switching on of photocurrent as well as a red shift in the both the absorbance onset and peak position. Comparison between the photocurrent data and optical absorbance for both the polysulfide and alkaline exposed films reveals that the photocurrent onset mimics that of the corresponding absorbance spectra. With regards to the photoluminescence (PL) spectra (Figure 2), the peak position is also redshifted from 562 nm (untreated) to 592 nm when polysulfide treated (Δλ max = 30 nm). In addition to the spectral shift, the photoluminescence peak width (FWHM) also broadens after exposure to a polysulfide solution from 33 nm to 43 nm. Peak broadening is also observed in the optical absorbance spectra, inferring a broadening in the particle size distribution. To separate the influence of high alkalinity (pH) and polysulfide species on the CdSe films, a comparison between the absorbance (Figure 2) and photocurrent spectra (Figure 1) of the alkaline and polysulfide treated films must be made. Whilst both are red shifted from the untreated film, the onset of photocurrent and absorbance for the alkaline only treated film is approximately 20 nm further red shifted than the polysulfide treated. With regards to spectral shape, it is also apparent that towards the red region (500 – 700 nm) the photocurrent shape is significantly less “peak-like” in the polysulfide treated relative to the alkaline exposed sample. Towards the blue region (450 – 500 nm), however, the polysulfide exposed sample reveals a stronger signal in the photocurrent spectra (contrary to the alkaline exposed sample). This is discussed in light of further measurements in the further discussions. Time resolved photoluminescence spectroscopy (TR-PL) was used to monitor and compare the photoluminescence decay rate of the QD sol, untreated and treated QD films and hence probe the kinetics of electron injection. The decay spectra for each sample (excited at 560 nm) are shown in Figure S2. Relatively, the QD sol has the longer photoluminescence (PL) lifetime than 4 ACS Paragon Plus Environment

Page 5 of 17

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


the solid film samples. Post treatment, the QD film PL lifetime is vastly reduced. The average lifetime photoluminescence decay rates (τAVG) are shown in Table 1. Electron-transfer (injection) rate constants can be calculated for this system following the mathematical analysis utilized by other groups for QD injection into a TiO2 substrate.26,27 The calculation assumes that all photoexcited electrons in the QD films decay by either electron injection into the conductive glass (FTO) or radiative relaxation (emission of a photon); where τQD+FTO and τQD are the measured average photoluminescence decay time constants for QD-FTO film and QD sol respectively. Calculated electron transfer rates (ket) are also tabulated in Table 1 where ket is calculated by Eq (1). The rate of electron transfer is enhanced by 7 times post the polysulfide treatment. =

−

(1)

-1

Sample

τAVG(ns)

ket(s )

Untreated CdSe film

18.6

0.017

Polysulfide treated CdSe film

6.6

0.114

CdSe QD sol

26.8

Table 1. Average decay rate for the PL decay spectra shown in Figure S2. The rates of electron tranfers were calculated by Eq 1.

In addition to these optical (redshift) and photoelectrochemical (switching on of photocurrent) observations, extensive analysis of the chemical and morphological changes of the photoanodes were undertaken. Chemical Analysis The chemical composition of QD films post exposure to polysulfide was initially analysed by ICPOES and SEM-EDX. Post exposure, sulfur was shown to reside in the films in addition to selenium (Table S1 and Figure S3). Additionally, the elemental ratio of Se:Cd was shown to decrease post exposure inferring dissolution of selenium. Given the importance and dependence of QD attachment and photoelectrochemical properties on the surface capping ligands,28–30 all films were prepared from the same batch of QDs which was subject to the same purification procedure. To probe the influence of polysulfide electrolyte on the presence and structure of the organic capping layer FT-IR was conducted (Figure S4). FTIR spectra pre and post exposure to polysulfide were found to be identical with regards to peak positions, verifying that organic ligands are present in the sulfide exposed QD films and that they remain unchanged with respect to chemical identity and structure. To probe the chemical structure of the sulfur introduced in the CdSe films Raman spectroscopy was conducted (Figure 3). The Raman peak at 208.2 cm-1, dominant in the untreated sample, has elsewhere been shown to correspond to three Lorentzians. The most prominent mode was identified as the first order longitudinal optical (LO) phonon for Cd-Se at 207.6 cm-1.31,32 Post polysulfide treatment, a new Raman band appears at 300.5 cm-1 whilst the original peak at 208.2 cm-1 appears suppressed, broadened and redshifted to 205.8 cm-1. The new peak originates predominantly from the Cd-S LO, and hence evidences the presence of Cd-S post polysulfide exposure.


5


To verify either core/shell or alloyed CdSe/S structure post polysulfide exposure, comparison of our Raman spectra with that of Aubert et al.33 on alloyed QDs and Tschirner et al.32 whose work is on CdSe/S core/shell was made. These studies revealed distinct differences in the Raman spectra for the two systems. Specifically, for alloyed CdSe1-xSx QDs the frequency of the LO phonon resonance was shown to shift as a function of composition (x) between the pure CdS and CdSe frequencies. In our spectra (Figure 4) the two LO phonon resonance peaks (CdSe and CdS) almost exactly align with the bulk frequencies (dashed black lines on Figure 4). Thus we conclude the formation of a core/shell structure rather than an alloyed CdSe1-xSx structure. Untreated Polysulfide treated

Intensity (A.U)

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

Page 6 of 17

200

300

400 -1

Wavenumber (cm )

Figure 3. Raman spectra of the untreated (blue) and treated (red) CdSe QD films. FTO substrate is shown for comparison (black). The normalized Raman spectra were measured with 473 nm excitation. The black dashed lines identify the bulk CdSe -1 -1 (~210 cm ) and CdS (~301 cm ) longitudinal optical phonon.

Our ICP, SEM-EDX and Raman investigations verify sulfur is introduced into the CdSe films. During the 1970-80s, there were several studies into the stability of CdSe/polysulfide photoelectrochemical cells. Varying degrees of stability (ranging from the minute to month timescale) were finally accredited to differences in crystal structure and face, electrolyte composition and photocurrent density.20 Through elemental analysis, it was demonstrated that under illumination the cadmium (Cd) concentration of the electrolyte was constant,2 conversely, as verified by XPS, selenium (Se) concentrations at the crystallite surface were shown to decrease. The intensity of sulfur (S) levels was shown to increase simultaneously.18,19 Thus, under illumination, a selenium/sulfur (Se/S) substitution mechanism was proposed in which the top layer of CdSe is gradually replaced by CdS. The thickness of the substituted CdS layer at the surface of the photoelectrode was estimated to be of the order of tens to hundreds of Angstroms. Cell “stabilization” was shown to correspond to quenching of the Se/S substitution. The proposed mechanism for Se/S substitution is shown below. In summary, upon absorption of two photons CdSe forms two electron-hole (e--h+) pairs which can lead to the dissolution of the Se resulting in the formation of Cd2+. Due to the high concentration of S2- in the electrolyte solution Cd2+ will re-precipitate with S2- adsorbed at the crystal surface forming CdS. Thus the overall reaction is the replacement of CdSe with CdS.

+

(2)

+ ( ) → ( ) + (!")

(3)

(!") + (!") → (!")

(4)


6

Page 7 of 17

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

The Journal of Physical Chemistry ( ) + ( )(! ) → #( )

(5)

The majority of the CdSe single crystal photoanode/polysulfide literature studied light driven S/Se substitution reactions. However, in reference 18 it was demonstrate that S/Se substitution also occurs in the dark, albeit at a lesser rate. Here, our samples were only exposed to polysulfide electrolyte in the dark. Evidenced by our Raman and ICP analysis, we do however see the introduction of sulfur into the films. Specifically, we report the formation of Cd-S bonds. Additionally, given the broadening of the peaks observed in the optical absorbance and PL, we propose a similar outcome for CdSe QDs exposed to polysulfide in the dark, whereby the CdSe is dissolved into the polysulfide electrolyte solution and sulfur/polysulfide species bind to cadmium that redeposits as CdS at the surface of the QD. Morphology Alongside the chemical analysis of the QD films, SEM imaging of our films reveal a vast increase in surface cracking post polysulfide treatment (Figure 4 c). The induced cracks are up to 70 μm in length and up to 800 nm in width. The untreated film is relatively more continuous, however, in some regions randomly distributed, non-regularly shaped pores (~0.2 – 1.5 μm in diameter) (Figure 4 a) are observed. Samples were also imaged by cross-sectional profiles revealing the depth of the cracking (Figure 4 b and d). Cracking has previously been observed for ethanedithiol treated PbS QD (solid) films.34 This was attributed to induced stress in the films upon excess thiol treatment. Here we observe that polysulfide treatment results in a widening of the size distribution of the QDs in addition to CdS formation at the QD surfaces. We propose that both of these will induce stress in the films and hence we observe the appearance of cracks in our SEM images.

a.

b.

c.

d.

Figure 4. Typical SEM top-down (a and c) and cross section images (b and d) images of untreated (a and b) and polysulfide treated (c and d) CdSe films.

Further Discussions A shift in the first excitonic peak position infers a redistribution of electronic distribution within the QD. Excitonic delocalization (redshifts) have been observed for both inorganic core/shell QD ACS Paragon Plus Environment

7


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

Page 8 of 17

structures35,36 and for aromatic (relative to non-conjugated) organic capping ligands.37,38 In our untreated films the QDs are surrounded by an organic capping layer, a system where the radial probabilities of finding an electron or hole are predicted to be minimal outside the CdSe QD boundary. Post polysulfide exposure, a redshift in the absorbance/PL and photocurrent spectra is observed in addition to absorbance/PL peak broadening (Figure 2) evidencing an increased crystal size distribution of the QD films. However, chemical analysis (ICP) reveals the introduction of sulfur into the films, and indeed Raman spectroscopy specifies the introduction of CdS bonding. Given that CdS is a higher band bap material relative to CdSe, a blue shift would be predicted for Se/S substitution only. Films were therefore prepared and exposed to a high alkaline solution (comparable pH to that of polysulfide) to distinguish the influence of pH from that of polysulfide species and the proven CdS formation. In both polysulfide and alkaline treated cases the photocurrent was switched on, and, for both, a red shift is observed in the onset of absorbance and photocurrent spectra. Despite these similarities, the red shift is 20nm greater in the case of the alkaline exposed film. We therefore propose, in the case of the polysulfide exposure there exist two competing mechanisms. The introduction of a CdSe/CdS core/shell type structure due to selenium/sulphur substitution reactions in polysulfide species (which leads to a blue shift), however, this shift is overshadowed by crystal growth and hence a red shift is observed overall. Alongside the optical changes observed, the change in QD/film must also rationalize the onset of photocurrent post exposure to polysulfide and alkaline solution. The presence of cracking in the films post polysulfide exposure (Figure 4) indicates that the treatment induces stress in the films. Given that photocurrent is also switched on in the high alkaline exposed film, we propose that post exposure, the spacing between individual QDs has been reduced. We therefore postulate enhanced (or, indeed the onset of ) electrical conductivity through the film, and thus the onset of photocurrent. Conclusions Our investigations show that exposure to polysulfide solution significantly shifts the absorption, photoelectrochemical action spectrum (in sulfite electrolyte), time resolved photoluminescence and chemical structure of TOPO-capped CdSe QD films. Through systematic characterisation we demonstrate that these observations are due to a combination of QD crystal ripening (due to high pH) leading to a red shift and sulfur substitution in the formation of CdS (slightly reducing the red shift). Experimental Materials Precursors were prepared using cadmium acetate (Cd(Ac)2, 99.99 +%, ChemPur), trioctylphosphine (TOP, Fluka, 90%, technical grade), 1-hexadecylamine (HDA, 90%, Aldrich, technical grade), trioctylphosphine oxide (TOPO, 90%, Aldrich, technical grade), 1-tetradecylphosphonic acid (TDPA, 98%, Alfa Aesar), selenium powder (Se, 99.99%, Aldrich), and toluene (99.9%, VWR International); all were used as supplied. TEC8 glass plate fluorinated tin oxide (FTO) coated glass slides (1 x 2.5 cm) (Dyesol) were used as optically transparent electrodes (OTE). Calibration standards and sample solutions for ICP-OES measurements were prepared using S (1000 mg/L S in 2% HNO3, Fluka Analytical), Cd (1000 mg/L Cd in 2 % HNO3, Fluka Analytical), Se (1000 mg/L Se in 2% HNO3, Fluka Analytical) and nitric acid (HNO3, 70%, Fisher) and concentrated hydrochloric acid (HCl, 36 %, VWR). Polysulfide electrolyte was prepared with 0.1M sodium sulfide nonahydrate (Na2S.9H2O, > 99.99 %, Aldrich) and 0.1 M sulfur powder, (S, sublimed, ~100 mesh, 99.5%, Alfa Aesar). The electrolyte 8 ACS Paragon Plus Environment

Page 9 of 17

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


(hole scavenger) used for photocurrent measurements was aqueous 0.5 M sodium sulfite (Na2SO3, ≥ 98 %, Sigma-Aldrich). CdSe QD synthesis CdSe QDs were synthesised by hot injection technique by the route of Mekis et al.39 In brief, TOPSe (0.158 g selenium into 2 ml TOP) and TOPCd (0.13 g cadmium acetate in 3 ml TOP) precursors were prepared prior to QD synthesis. The precursors were left to stir overnight prior to QD synthesis. A one-pot hot injection synthesis was used to prepare the QDs. TOPO (8.0 g), HDA (5.0 g), and TDPA (0.15 g) were degassed under vacuum, and refluxed at 120 ˚C for 1.5 hour. TOPSe was then added to the pot and the reaction temperature increased to 300 ˚C. TOPCd was subsequently injected rapidly under vigorous stirring resulting in the nucleation of CdSe QDs. The reaction temperature was immediately reduced to 280 ˚C for particle growth. Once the desired QD size was reached (as determined by absorbance spectroscopy) the reaction was quenched in cold toluene. Post synthesis, the QD sol was purified following the procedure outlined in reference 29. One batch of QDs was utilized for all of the experimental work presented in this paper. The average diameter of the QDs was 3.3 nm (estimated from the excitonic peak position determined by absorbance spectroscopy of the QD sol and the empirical fit for CdSe QDs in reference 40). Preparation of Cadmium Selenide Quantum Dot Photoanodes A concentrated CdSe QD paste was prepared by centrifugation of the QD sol and re-dispersion in a 1 ml toluene and 1 ml ethanol mix. 15 μL of QD sol was used to prepare films on the clean, 10 x 25 mm conductive fluorine doped tin oxide (FTO) using the doctor-blade technique. Samples were left to dry in the dark for > 24 hours prior to further measurement. Polysulfide Electrolyte Samples subject to “polysulfide exposure” were prepared on FTO and then submersed in a 0.1 M Na2S, 0.1 M S aqueous solution for 30 mins in the dark. The temporal evolution of the polysulfide treatment was monitored with respect to time using UV-Vis absorbance spectroscopy (Figure SI). The optical evolution of UV-Vis absorbance saturated/plateaued after ~800 s (13.3 min). Polysulfide treatment was hence defined as the exposure of a CdSe QD film to polysulfide solution for 30 mins. Post exposure to polysulfide, a treated film was rinsed of excess polysulfide by repeated submersion in distilled water. Polysulfide treated films are referred to as “treated” films hereafter. For each set of characterisation/analysis (except photocurrent measurements) one CdSe film was prepared and cut in half; one half was exposed to polysulfide solution, the other left “untreated”. For the photocurrent spectra, untreated films were first measured in sodium sulfite solution; the sample was then treated with polysulfide solution; finally the photocurrent spectra were re-recorded, again in sodium sulfite. This was repeated for three different samples. Alkaline Exposure Samples subject to “alkaline treatment” were prepared as above on FTO. Samples were then submersed in a sodium sulfite solution adjusted to pH 12.5 by sodium hydroxide. The temporal evolution of the electrode was monitored by UV vis until the absorbance shift subsided (after 1 week). UV-Vis Absorbance ACS Paragon Plus Environment

9


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

Page 10 of 17

UV-Vis spectroscopy was conducted on a Perkin Elmer, LambdaBio10 spectrophotometer, with a spectral resolution of 2 nm using 1 cm path length quartz cuvettes. Photoluminescence Spectroscopy PL measurements were conducted in 1 cm quartz cuvettes. The samples were illuminated by a 75W USHIO Xe short arc lamp bulb in conjunction LPS-220B Photon Technology International (PTi) lamp power supply. Light passed through a PTi monochromator with 3 nm resolution. Emission from the sample was collected at 90 ̊ to the incident light beam by a Dolan Jenner Optical Fibre to a 25 μm slit, passed through a second monochromator (also PTi with 3 nm resolution) and detected by a PTi 814 Photomultiplier detection system. Time Resolved Photoluminescence Time-correlated single photon counting (TCSPC) was used to measure Picosecond time-resolved emission spectra (TRES) and fluorescence decays using a FluoTime200 spectrometer (PicoQuant) equipped with a TimeHarp300 TCSPC board (PicoQuant) and a Hamamatsu photomultiplier (PMA-185). The excitation source was a 440 nm picosecond pulsed diode laser (PicoQuant, LDH440) driven by a PDL800-D driver (PicoQuant) operated at a variable pulse repetition rate (10-40 MHz). The emission from the QD films on FTO was collected perpendicular to the excitation laser beam. The emission arm was fitted with a long pass filter (HQ460LP, Chroma) before the monochromator (Scientech 9030). The full width half maximum (FWHM) of the system’s instrument response function (IRF) was 200 ps. The fluorescence decay curves were analysed using the FLUOFITsoftware (PicoQuant, version 4.2.1) based on multi-exponential models involving an iterative reconvolution process. The appropriateness of the fits was assessed by the reduced chi2 value (less than 1.20 accepted) and a visual inspection of the fit and data match. Inductively Coupled Plasma- Optical Emission Spectroscopy (ICP-OES) Three samples were prepared for ICP analysis; untreated CdSe film, polysulfide treated CdSe film, and an FTO control. Each sample was digested in 4 ml aqua regia solution followed by 30 min of sonication. Each solution was analysed for Cd, Se and S content against 0, 1, 5 and 20 ppm calibration solutions. The FTO sample was used as a baseline for each elemental composition and thus these baseline concentrations were subtracted from sample concentrations. The quoted values are averaged for the two dilutions prepared. Elemental ratios were calculated for data interpretation thus avoiding discrepancies due to sample size, area and thickness. Scanning Electron Microscopy (SEM) A high-resolution field emission gun scanning electron microscope (FEG-SEM) (LEO Gemini 1525) fitted with Oxford Instruments INCA energy dispersive and wavelength dispersive x-ray spectrometer was used for all SEM imaging. SEM-EDX analyses were performed in 7 randomly selected regions across the surface of the untreated and polysulfide treated films. Photocurrent Measurements A computer controlled Autolab potentiostat (μAutolabI) was used to control the potential. A 50 W xeon lamp powered by a LPS-220B power supply (Photon Technology International (PTi)) illuminated the sample through a PTi SID-101 monochromator. Photocurrent signals recorded on QD only films were recorded by photocurrent transients; at each wavelength a time delay was inACS Paragon Plus Environment

10

Page 11 of 17

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


troduced such that the photocurrent was not recorded until the signal plateaued. The difference between the dark and light current was calculated and thus used for photocurrent efficiency calculation. Photocurrent transients were recorded with 0.5 M sodium sulfite hole scavenger (pH 9.0) against a Ag/AgCl reference electrode. AUTHOR INFORMATION Corresponding Author * D. Jason Riley. Email: [email protected]

Present Addresses Laurie A. King. Chemistry Department, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT LAK and DJR acknowledge support from the EPRC (Award number: EP/G031088/1). The authors would also like to acknowledge the referee for extremely useful and insightful comments leading to clearer rational of the data.

ASSOCIATED CONTENT Supporting Information. Temporal evolution of absorbance spectra (TR-PL), time resolved photoluminescence spectra, inductively coupled plasma-optical emission spectroscopy, SEM-EDX, FT-IR. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. Visible Light to Electrial Energy-Conversion - Stable Cadmium-Sulfide and Cadmium Selenide Photoelectrodes in Aqueous-Electrolytes. J. Am. Chem. Soc. 1976, 98, 1635–1637. Hodes, G.; Manassen, J.; Cahen, D. Photoelectrochemical Energy-Conversion and Storage Using Polycrystalline Chalcogenide Electrodes. Nature 1976, 261, 403–404. Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. Optical to Electrical Energy-Conversion - Cadmium Telluride-Based Photoelectrochemical Cells Employing Telluride-Ditelluride Electrolytes. J. Am. Chem. Soc. 1976, 98. Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. Optical to Electrical Energy-Conversion - Characterization of Cadmium-Sulfide and Cadmium Selenide Based Photoelectrochemical Cells. J. Am. Chem. Soc. 1976, 98, 6855–6866. Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. Study of N-Type Semiconducting Cadmium Chalcogenide-Based Photoelectrochemical Cells Employing Polychalcogenide Electrolytes. J. Am. Chem. Soc. 1977, 99, 2839–2848. Manassen, J.; Hodes, G.; Cahen, D. Photoelectrochemical Energy-Conversion And Storage - Polycrystalline CdSe Cell with Different Storage Modes. J. Electrochem. Soc. 1977, 124, 532–534. Miller, B.; Heller, A.; Robbins, M.; Menezes, S.; Chang, K. C.; Thomson, J. Solar Conversion Efficiency of Pressure Sintered Cadmium Selenide Liquid Junction Cells. J. Electrochem. Soc. 1977, 124, 1019–1021. Miller, B.; Heller, A. Semiconductor Liquid Junction Solar-Cells Based on Anodic Sulfide Films. Nature 1976, 262. Bang, J. H.; Kamat, P. V. Quantum Dot Sensitized Solar Cells. A Tale of Two Semiconductor Nanocrystals: CdSe and CdTe. Acs Nano 2009, 3, 1467–1476. Ruhle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. Chemphyschem 2010, 11, 2290–2304. Kramer, I. J.; Sargent, E. H. The Architecture of Colloidal Quantum Dot Solar Cells: Materials to Devices. Chem. Rev. 2013. Liu, D.; Kamat, P. V. Photoelectrochemical Behavior of Thin CdSe and Coupled TiO2 CdSe Semiconductor-Films. J. Phys. Chem. 1993, 97, 10769–10773. Lee, H. J.; Yum, J. H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Grazel, M.; Nazeeruddin, M. K. CdSe Quantum Dot-Sensitized Solar Cells Exceeding Efficiency 1% at Full-Sun Intensity. J. Phys. Chem. C 2008, 112, 11600–11608. Lee, H.; Leventis, H. C.; Moon, S. J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nuesch, F.; Geiger, T.; Zakeeruddin, S. M.; et al. PbS and US Quantum Dot-Sensitized Solid-State Solar Cells: “Old Concepts, New Results.”Adv. Funct. Mater. 2009, 19, 2735–2742. Zaban, A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots. Langmuir 1998, 14, 3153–3156. Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778–17787. Chakrapani, V.; Baker, D.; Kamat, P. V. Understanding the Role of the Sulfide Redox Couple (S2-/S-n(2-)) in Quantum DotSensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 9607–9615. Cahen, D.; Hodes, G.; Manassen, J. S-Se Substitution In Polycrystalline CdSe Photoelectrodes - Photoelectrochemical Energy Conversion J. Electrochem. Soc. 1978, 125. Noufi, R. N.; Kohl, P. A.; Rogers, J. W.; White, J. M.; Bard, A. J. Semiconductor Electrodes. 19. Investigation of S-Se Substitution In Single-Crystal CdSe And CdS Photoelectrodes By Electron-Spectroscopy. J. Electrochem. Soc. 1979, 126.


11


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

Page 12 of 17

(20) Hodes, G.; Manassen, J.; Cahen, D. Effect Of Surface Etching And Morphology On The Stability Of CdSe-Sx Photo-electrochemical Cells. J. Electrochem. Soc. 1981, 128, 2325–2330. (21) Lessner, P. M.; McLarnon, F. R.; Winnick, J.; Cairns, E. J. The Dependence Of Aqueous Sulfur-Polysulfide Redox Potential On Electrolyte-Composition And Temperature. J. Electrochem. Soc. 1993, 140, 1847–1849. (22) Jovanovski, V.; Gonzalez-Pedro, V.; Gimenez, S.; Azaceta, E.; Cabanero, G.; Grande, H.; Tena-Zaera, R.; Mora-Sero, I.; Bisquert, J. A Sulfide/Polysulfide-Based Ionic Liquid Electrolyte for Quantum Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 20156– 20159. (23) Ardoin, N.; Winnick, J. Polysulfide Solution Chemistry at a CdSe Photoanode. J. Electrochem. Soc. 1988, 135, 1719–1722. (24) Li, L.; Yang, X. C.; Gao, J. J.; Tian, H. N.; Zhao, J. Z.; Hagfeldt, A.; Sun, L. C. Highly Efficient CdS Quantum Dot-Sensitized Solar Cells Based on a Modified Polysulfide Electrolyte. J. Am. Chem. Soc. 2011, 133, 8458–8460. (25) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Ruhle, S.; Cahen, D.; Hodes, G. Chemical Bath Deposited CdS/CdSe-Sensitized Porous TiO2 Solar Cells. J. Photochem. Photobiol. -Chem. 2006, 181, 306–313. (26) Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63–66. (27) Hyun, B. R.; Zhong, Y. W.; Bartnik, A. C.; Sun, L. F.; Abruna, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles. Acs Nano 2008, 2, 2206– 2212. (28) Sambur, J. B.; Riha, S. C.; Choi, D.; Parkinson, B. A. Influence of Surface Chemistry on the Binding and Electronic Coupling of CdSe Quantum Dots to Single Crystal TiO2 Surfaces. Langmuir 2010, 26, 4839–4847. (29) King, L. A.; Riley, D. J. Importance of QD Purification Procedure on Surface Adsorbance of QDs and Performance of QD Sensitized Photoanodes. J. Phys. Chem. C 2012, 116, 3349–3355. (30) Kern, M. E.; Watson, D. F. Influence of Solvation and the Persistence of Adsorbed Linkers on the Attachment of CdSe Quantum Dots to TiO2 via Linker-Assisted Assembly. Langmuir ACS J. Surf. Colloids 2012, 28, 15598–15605. (31) Liu Lu, X.-L. X. Raman Analysis of CdSe/CdS Core-Shell Quantum Dots with Different CdS Shell Thickness. J. Phys. Condens. Matter Inst. Phys. J. 2007, 19, 406221-406231. (32) Tschirner, N.; Lange, H.; Schliwa, A.; Biermann, A.; Thomsen, C.; Lambert, K.; Gomes, R.; Hens, Z. Interfacial Alloying in CdSe/CdS Heteronanocrystals: A Raman Spectroscopy Analysis. Chem. Mater. 2012, 24, 311–318. (33) Aubert, T.; Cirillo, M.; Flamee, S.; Van Deun, R.; Lange, H.; Thomsen, C.; Hens, Z. Homogeneously Alloyed CdSe1–xSx Quantum Dots (0 ≤ X ≤ 1): An Efficient Synthesis for Full Optical Tunability. Chem. Mater. 2013, 25, 2388–2390. (34) Barkhouse, D. A. R.; Pattantyus-Abraham, A. G.; Levina, L.; Sargent, E. H. Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency. Acs Nano 2008, 2, 2356–2362. (35) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019–7029. (36) Tan, R.; Blom, D. A.; Ma, S.; Greytak, A. B. Probing Surface Saturation Conditions in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots. Chem. Mater. 2013, 25, 3724–3736. (37) Liang, Y. Q.; Thorne, J. E.; Parkinson, B. A. Controlling the Electronic Coupling between CdSe Quantum Dots and Thiol Capping Ligands via pH and Ligand Selection. Langmuir 2012, 28, 11072–11077. (38) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. The Band Edge Luminescence of Surface Modified CdSe Nanocrystallites: Probing the Luminescing State. J. Chem. Phys. 1997, 106, 9869–9882. (39) Mekis, I.; Talapin, D. V.; Kornowski, A.; Haase, M.; Weller, H. One-Pot Synthesis of Highly Luminescent CdSe/CdS Core-Shell Nanocrystals via Organometallic and “Greener” Chemical Approaches. J. Phys. Chem. B 2003, 107, 7454–7462. (40) Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854–2860.


12

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

Laurie A. King, Weijie Zhao, Lefteris Danos, D. Jason Riley* Journal of Physical Chemistry C


We show that the polysulfide solution structurally, optically and chemically alters the CdSe QDs leading to significantly enhanced photocurrent from a QD modified photoanode immersed in sulfite electrolyte.

Activation of CdSe Quantum Dots Post Exposure to Polysulfide


Untreated

Photocurrent

Page 13 of 17

Treated

λ

13


1.0x10-3

b.

0.6

3.5x10-4

2.5 IPCE - untreated IPCE - polysulfide treated IPCE - alkaline treated Abs - untreated Abs - polysulfide treated Abs - alkaline treated

-4

3.0x10

2.5x10-4

8.0x10-4

1.5

2.0x10-4

6.0x10-4

0.4

4.0x10-4

2.0

1.5x10-4

1.0

1.0x10-4

0.2

2.0x10-4

0.5

5.0x10-5

0.0 450

0.0

500

550

600

Wavelength (nm)

650

0.0 700

450

0.0 500

550

600

ACS Paragon Plus Environment Wavelength (nm)

650

700



IPCE

APCE

2 3 4 5 6 7 8 9 10 11 12 13

1.2x10-3

LHE

a. 1

Page 14 of 17

Page 15 The of Journal 17 of Physical Chemistry Absorbance untreated Absorbance treated polysulfide Absorbance treated alkaline PL untreated PL treated polysulfide

Normalized PL


1 2 3 4 5 6 7 8 9 10 11 12 13


450

500

550

600

650

700

Wavelength (nm)

750

800

b. The Journal of Physical Chemistry Page 16 of 17 Untreated Polysulfide treated

Intensity (A.U)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

200 ACS

Paragon Plus 300 Environment -1

Wavenumber (cm )

400

Page 17 of 17

a.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

c.


b.

d.


Activation of CdSe Quantum Dots after Exposure to Polysulfide - The

Recommend Documents