Sensitization of ZnO Single Crystal Electrodes with CdSe Quantum

Oct 2, 2014 - Department of Chemistry and School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071, United States. Langmuir , 2014 ...
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Sensitization of ZnO Single Crystal Electrodes with CdSe Quantum Dots Yongqi Liang, James E. Thorne, Meghan Kern, and Bruce A Parkinson Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5023888 • Publication Date (Web): 02 Oct 2014 Downloaded from http://pubs.acs.org on October 12, 2014

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Sensitization of ZnO Single Crystal Electrodes with CdSe Quantum Dots Yongqi Liang, James E. Thorne, Meghan E. Kern and B. A. Parkinson* Department of Chemistry and School of Energy Resources University of Wyoming

Abstract CdSe quantum dots (QDs) were attached to single crystal ZnO(0001) and ZnO(1100) substrates using capping groups, 4-mercaptobenzoic acid, 2-mercaptoacetic acid, 3-mercaptopropionic acid, 8-mercaptooctanoic acid and 11-mercaptoundecanoic acid as bifunctional linker molecules. The spectral response and photosensitization yields of the adsorbed QDs were studied with photocurrent spectroscopy. Atomic force microscopy (AFM) was used to verify the surface structure of the ZnO crystals and to examine the coverage and arrangement of the QDs on the single crystal surface. The inner-sphere aqueous redox couple Sx2-/S2-, often used as a regenerator for chalcogenide based QDs, as well as outer-sphere redox couples such as ferrocene, were able to regenerate the photoexcited CdSe QDs and suppress their photocorrosion. Differences in the binding of the QDs to different ZnO crystal faces are also reported. *[email protected]

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Introduction Quantum dots (QDs) have attributes that make them attractive for solar energy conversion.1 Besides having size tunable absorption properties, the quantized energy levels may promote effects such as multiple exciton generation an approach to surpassing the Shockley-Queisser efficiency limit for single junction solar cells.2,3 QDs have been increasingly used as sensitizers for sensitized solar cells.4 For instance the injection and collection of photogenerated electrons from CdSe QDs has been accomplished on various substrates: ZnO nanowire arrays5, mesoporous TiO2 films,6,7 TiO2 single crystals2,8,9 and TiO2 nanotube arrays7. To increase the absorption of solar photons, nanostructured films consisting of either nanocrystals or nanowire/nanotube arrays are usually adopted as high surface-area substrates to increase the loading of QDs. These nanostructured films usually have no built-in electric field (space charge layer) but still enable rapid separation of the injected charges to minimize back electron transfer or recombination.

Properly doped single crystal electrodes develop space

charge layers at the electrolyte interface that quickly sweep the photoinjected electrons from the QDs away from the interface. By minimizing (or avoiding) the back-transfer of electrons, single crystal substrates thus provide a platform to study the electron injection process and to evaluate and optimize the redox couples used as regenerators for QD sensitized devices.10 In addition, the nearly atomically flat surfaces of the single crystals allows for the use of scanning probe microscopies to determine both the coverage and state of aggregation of the adsorbed QDs. To optimize the binding and facilitate electron transfer from the photoexcited QDs, engineering the interfaces between the QDs and the substrates is critical. Surfactants with long chain alkyl groups are often used as capping ligands during the synthesis of QDs but can interfere with the electronic interaction of the QDs with the substrate and thus prevent facile electron transfer.11,12 Therefore smaller bifunctional molecules, with a group that selectively binds to the surface of the electrode and one that binds selectively to the QD surface, are often used to covalently attach the QDs to the electrode surface.13 Carboxylic acid groups bind strongly to ZnO substrates14 and the thiol group is known to bind strongly to CdSe.15 Therefore, we investigated the role of various bifunctional (MXA) mercaptocarboxylic acid linker molecules with different hydrocarbon chain

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lengths to better understand and control the electron injection process and photocurrent response in CdSe QD sensitized single crystal ZnO electrodes.

Experimental section Chemicals. CdO (99.998%, Alfa-Aesar), oleic acid (OA, 90%, Alfa-Aesar), 1-octadecene (ODE, 90%, Acros), Se (99.999%, Alfa-Aesar), Tri-n-octylphosphine (TOP, 90%, Alfa-Aesar), toluene (HPLC 99.9%, Fischer), hexane (>97.9%, EMD) 1-butanol (ACS bulk, 99%, Alfa-Aesar), ethanol (absolute purity, Pharmco-Aaper), methanol (99.9%, Fisher scientific), tetramethyl .

ammonium hydroxide pentahydrate (TMAOH 5H2O, 98%, Alfa-Aesar), and ethyl acetate (99.98%, EMD), were used without further purification. 2-mercaptoacetic acid (MAA, >98%, Sigma-Aldrich), 3-mercaptopropionic acid (MPA, 99+%, Aldrich), 4-mercaptobenzoic acid (MBzA, 90%,

Aldrich), 8-mercaptooctanoic acid

(MOA, 95%,

Aldrich),

and

11-

mercaptoundecanoic acid (MUA, 95%, Aldrich) were used as the linker molecules between the CdSe QDs and the ZnO substrates. MXA will be used as a generic abbreviation for these linker molecules in the following text. Synthesis and purification of the CdSe QDs. Oleic acid capped QDs were synthesized by following a standard hot-injection procedure with minor modifications. In a typical reaction, 0.256 g CdO (2.0 mmol) was mixed with 1.6 ml oleic acid (5.0 mmol) and 8.0 ml ODE in a 3neck flask. A clear solution was obtained after heating up to 190°C for 2 h under a N2 atmosphere. The Se precursor was made by heating 0.145 g Se (~1.8 mmol) in a mixture of 1.22 g TOP (2.7 mmol) and 4.0 ml ODE at ~100°C for ~10 min. After cooling to room temperature, the Se precursor solution was quickly injected into the Cd precursor solution. After 3 min, the 3-neck flask was removed from the heating bath and the reaction was quenched by adding 20 ml of toluene. CdSe QDs with the exciton absorption peak ranging from 530 nm to 550 nm were obtained for different batches, and the diameters of the QDs are estimated to be from 2.7 nm to 3.0 nm.16 The QDs were then purified by precipitation/redispersion cycles. Typically, 4.0 ml of the toluene quenched dispersion was taken and 20 ml butanol and 20 ml methanol were added to make the dispersion flocculate. After 3 hours, 13 ml of the suspension was centrifuged and the upper

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solution was decanted. After being washed with methanol, the flocculated QDs were then resuspended in 1.5 ml of hexane. 6 ml of butanol and 6 ml of methanol were then added to reflocculate the QDs. The purification was repeated for another cycle with the hexane/butanol/methanol mixture. The purified QDs were finally suspended in hexane and stored in the dark. Ligand exchange of CdSe QDs. Following a reported procedure17 oleic acid molecules on QDs were exchanged for thiol terminated bifunctional carboxylic acid ligands (MXA) before being .

used to sensitize the ZnO (0001) crystals. 1.0 g of TMAOH 5H2O (0.092 M) was dissolved in 60 ml CH3OH with a suitable amount of MXA molecules added (i.e. 0.070g for MBzA, 7.6 mM). 0.8 ml of QDs dissolved in hexane (Optical Density, ODexciton= ~40) was added under continuous stirring. The mixed solution was refluxed for more than 6 hours under a N2 atmosphere. The final clear methanol suspension of QDs was flocculated with excess ethyl acetate (volume ratio 1:10). The dispersion was centrifuged and the solid was collected after decanting the upper solution. The red solid was dried in the air and then re-dissolved in ethanol. The solution was then diluted to a concentration (~2 µM, ODexciton= 0.2) before being used for sensitization. To increase the chemical stability of the QD suspension during adsorption to ZnO, TMAOH solution in ethanol (0.1 M) was added to adjust the pH to 8~11 for the QD suspension. In later tests a modified version of the ligand exchange procedure was used. For this procedure 1mL of 1 M MXA in acetonitrile was added to 1 mL of CdSe suspended in hexanes. After the QDs had completed flocculated, the supernatant was removed and the QDs were dried using nitrogen. Once dried 2 mL of 48 mM NaOH in water was added to resuspend the QDs. Adsorption of MXA-capped CdSe QDs on ZnO. Hydrothermally grown, n-type, 10 mm X 10 mm X 0.5 mm oriented ZnO wafers (both side polished, Princeton Scientific Corporation) were cut into smaller pieces and mounted in epoxy using a standard procedure.8 Back ohmic contacts to ZnO electrodes were obtained using Ga-In alloy. The identities of the Oface (000-1) and the Zn-face (0001) were verified by the etching behavior in 20% HNO3.18 The epoxy-mounted ZnO electrodes were immersed for 1 hour in the QD suspension to allow for QD adsorption and then rinsed with C2H5OH and deionized H2O.

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Clean ZnO surfaces were recovered by a photooxidation procedure using UV illumination from a 60 W Xe lamp (Oriel). The ZnO electrodes were biased at +0.54 V vs. NHE using a potentiostat (174A, Princeton Applied Research) during the 3 minute photooxidation in 0.1 M NaClO4 aqueous solution. The complete removal of adsorbed CdSe QDs was indicated by the near-zero IPCE measured at the exciton peak wavelength in photocurrent spectra. Assuming a 100% Faraday efficiency for photooxidation, ~75 nm of ZnO was etched off by a charge density of ~0.1 coulomb/cm2 was passed. The ZnO substrates were then polished with colloidal silica slurry (0.05 µm size, MTI) and treated with 3 M NaOH to obtain flat surfaces. 10 mm X 10 mm X 0.5 mm (1100) ZnO crystals (both side polished, MTI) were polished with colloidal silica slurry (20 nm size, MTI). The crystals were sonicated in water for an hour to remove residual polishing compound. They were then annealed at 1000oC for approximately 8 hours to yield reproducible terraces.

The crystals were immersed in MXA-capped QD

suspension for 1 hour to allow for QD adsorption and then rinsed with deionized H2O. Sensitization experiments were performed by mounting the ZnO crystals, with an ohmic back contact made with a Ga-In alloy, in an electrochemical cell with an O-ring (Ф6 mm). AFM characterization. Tapping mode AFM (Digital Instruments Nanoscope IIIA controller and a multimode SPM) and Asylum Cypher were used to characterize the morphology of the ZnO substrates both with and without adsorbed QDs. Photoelectrochemical characterization. The ZnO single crystal electrodes with adsorbed CdSe QDs were immersed into an electrochemical cell with a coiled Pt wire as the counter electrode. The electrochemical characterization was carried out in either 0.1 M NaClO4 aqueous solution or 0.1 M tetrabutylammonium perchlorate (TBAP) solution in CH3CN. 10 mM Na2S or 2 mM K3Fe(CN)6/K4Fe(CN)6 was used as the redox couple in the aqueous electrolyte. 1 mM ferrocene was used as the redox couple in organic electrolyte. Photocurrent measurements to obtain incident photon current efficiencies (IPCE) used monochromatic light obtained by passing the white light from a 50 W tungsten-halogen lamp (NRC 780, Newport) through a monochromator (Jarrel-Ash). Suitable long-pass filters (Edmund

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Optics) were selected to remove the second order component. A light intensity of 10-50 uW/cm2 was typically used for IPCE spectral collection. The ZnO electrodes were biased at 0.0 V via a current preamplifier (Stanford Research 570), and the photocurrent signal (chopped at 13.5 Hz) was fed into a lock-in amplifier (Stanford Research 830). The amplified signal was then collected via a computer interfaced through a data acquisition card (USB1408 FS, Measurement Computing). The photocurrent spectra were then converted to the IPCE spectra with the power spectra of the monochromatic light.

Results and Discussion Two strategies have been used to sensitize oxide semiconductor substrates with QDs. Physical adsorption or chemical bonding of pre-synthesized QDs or formation of the QDs directly on the substrate through either a successive ionic layer adsorption and reaction (SILAR) or a chemical bath deposition (CBD) process.19 Loading of pre-synthesized QDs, as we do in this paper, has the advantage that the size and shape of the QDs is well controlled in the synthetic procedure.20 Three different procedures to chemically bind pre-synthesized CdSe QDs (long chain acid surfactant capped) to substrates (ZnO as an example) via a bi-functional MXA linker have been reported.8

The first is to expose the ZnO to the MXA linker and then add surfactant capped

CdSe QDs, the so-called in situ procedure that is widely reported on nanocrystalline substrates6,21. The second is to mix the ZnO and CdSe QDs and then add MXA linker and thirdly, mix the CdSe QDs and MXA then add ZnO, the so-called ex situ procedure.

Functionalizing the

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substrate with the MXA ligand (thiolated substrates ) and then exposing the surface to the QDs with the long chain surfactants (the in situ procedure) has been less reproducible and in our hands and often results in very low coverages of QDs8,23. Therefore chemical binding using QDs capped with short chain bifunctional ligands, often from ligand exchange reactions, was used in this study21. It is worth noting that physisorption might still be occurring during QD loading onto the substrates even when an excess of linker molecules is present.23 ZnO was selected as the semiconductor oxide substrate9 for several reasons. Firstly, it has a suitable electronic structure with a bandgap of 3.2 eV~3.4 eV and a conduction band edge positioned at 4.1 eV - 4.4 eV below the vacuum level24,25. The conduction band position allows for the injection of photoexcited electrons from lowest excited state of CdSe QDs in various

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electrolytes.26 The large band gap allows for sensitization by small CdSe QDs since there will be no direct photoexcitation of ZnO in the visible region of the spectrum (band-edge excitation of ZnO starts at ~387 nm). The co-existence of electrical conductivity and optical transparency in ZnO allows for the determination of the optical absorbance of the QD layers and subsequent quantification of APCE (Absorbed Photon-to-current Conversion Efficiency) values from IPCE (Incident Photon-to-current Conversion Efficiency) values. In addition atomically flat surfaces of ZnO can be prepared by treating in alkaline solution (3.0 M NaOH) and subsequent 800°C annealing.27 Alkaline treatment of the as-received ZnO wafers produces flat terraced surfaces over large areas (>5 µm x 5 µm as seen in Figure 1A).

However the controlled loading of CdSe QDs onto the ZnO substrates is not trivial and three challenges need to be addressed. The first challenge is to control the linking chemistry of the bifuncitonal molecules to ZnO surfaces. Various binding configurations are possible for the carboxylate group on ZnO as carboxylates can either bind to a single zinc atom by an ester type bond (or bidendate coordination) or bridge between two zinc atoms.28 The QD loading process onto ZnO due to reaction between the carboxylic acid group and the hydroxyl group29 is sensitive to conditions such as pH, solvent, and temperature etc. In this study the pH for the QD solutions with all the MXA linkers is kept between 8 and 11 despite the possible repulsion for QD binding within this pH region, since both the caboxylate group and the surface of ZnO are negatively charged. The second challenge is to understand the complex surface chemistry of CdSe QDs. Our synthetic procedure produces CdSe QDs capped mostly by oleic acid on Cd sites with a minor amount of TOP on Se sites.30 After the ex situ ligand exchange step, MXA molecules replace the oleic acid molecules on the Cd sites31 and the chemical formula for the dispersed QDs can be approximated by (CdSe)-(MXA-)n-(TMA+)n. The coverage of MXA ligands not only affects the chemical stability of QDs in electrolytes,32 but it will also influence the electronic properties of the QDs (such as the quantum yield of fluorescence)33. It is possible that the solubility of QDs in polar solvents does not guarantee a full replacement of the oleic acid molecules since a small amount of MXA can solubilize QDs in polar solvents.34 To maximize the binding of surface Cd atoms by MXA, a ~10-20 times excess, over the number of surface Cd atoms, is added during

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the ligand exchange step.

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The excess ligand concentration for MPA and MAA capping

molecules typically needs to be higher than for MBzA, MOA or MUA. While the concentration of TMAOH was kept the same (~0.1 M) to deprotonate all the SH groups of the MXA molecules, a larger ratio needed for MAA and MPA suggests that the driving force for ligand exchange is lower for MPA and MAA than that for MBzA, MOA or MUA. This difference in the driving force is tentatively attributed to the variation of adsorption states of MXA molecules on CdSe QDs.35 It is possible that the MPA and MAA molecules are adsorbed on neighboring sites via both thiol and the carboxylate groups36 while for MUA and MBzA, due to the dispersive forces among the uncharged long alkyl chains or rigid aromatic rings, adsorption on CdSe QDs may form more compact layers. The third challenge arises from side-reactions during adsorption of CdSe QDs on to the ZnO substrate. The ZnO substrates are immersed into the suspension of QDs during the loading process of CdSe QDs, and impurities in the suspension might compete for the binding sites by adsorbing on the ZnO substrates. Small amounts of oleic acid, TOP, and TMA+ cations might still be present even after many purification cycles (flocculation and dissolution) for the CdSe QDs capped with MXA ligands.

Free MXA molecules will always be present in the

suspension37 since free MXA molecules are in equilibrium with bound MXA.38 The free MXA molecules may adsorb on the Zn sites either via the COOH group or via the SH group28 and partially block the adsorption of QDs on ZnO.

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Figure 1. (A): AFM topography of ZnO (0001) substrate and a line profile (indicated as the white line). (B) AFM topography of a layer of MPA capped CdSe QDs on ZnO (0001) and a line profile (indicated as the white line).

Despite the challenges our procedures were often able to produce high coverages of QDs on ZnO surfaces. AFM images of the bare ZnO substrates and the ZnO substrates with adsorbed CdSe QDs are shown in Figure 1. The surface of ZnO shows a step-terrace structure (Figure 1A) with irregular terraces. The terraces are typically 50 nm-120 nm in width and can extend over several microns in length.

The step heights vary from 0.3 nm to 0.7 nm but most are ~0.5 nm,

corresponding to a double-layer of Zn-O in c lattice direction.27 The 0.3 and 0.7 nm step heights indicate that some (000-1) surfaces are also exposed on the primarily (0001) surface. After adsorption of CdSe QDs, uniform sized small features in the image can be attributed to QDs (Figure 1B). There are some QD clusters but most appear to be forming a near monolayer coverage. The line profile shows that the height of the nanoparticles is 3.6 nm, consistent with the diameter of QDs including the chain length of the MPA ligands (3.8 nm).

Figure 2. (A) Schematic energy diagram for a single-crystalline ZnO electrode sensitized with CdSe QDs employing Sx2-/S2-, Fe(CN)63-/Fe(CN)64- and Fc+/Fc redox couples. The photocurrent is a result of competition between several processes: charge injection, back electron transfer to QDs, electron transfer to electrolyte and hole injection into the electrolyte (regeneration). (B) Simplified schematic structures for CdSe QDs adsorbed on ZnO substrate via bifunctional MXA molecular linkers. On the left, each QD is connected to ZnO via one or more MXA molecules. On the right, the ligands on the QD might get compressed and the QD is adsorbed onto ZnO via physisorption.

1. Different redox couples influence the photovoltage The photovoltage for a sensitized photovoltaic device is determined by the difference between the Fermi level of the oxide substrate and the redox potential of the regenerator. Redox couples with more positive potentials can increase the solar energy conversion efficiency.39 However, the optimization of the redox couples to increase photovoltage for QD-sensitized mesoporous

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film electrodes has not been extensively studied.40,41

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The back electron transfer from the

mesoporous electrodes to many redox couples can be significant resulting in lower photocurrents. The iodide/triiodide (I3-/I- +0.35 V vs. NHE in CH3CN) is widely used as the redox couple for regenerating excited dye molecules, and has been one of the keys to the success of dye sensitized solar cell (DSSC).42 However, I3-/I- is known to be reactive and cause photocorrosion of CdSe QDs.9 The band bending near the surface of single crystal electrodes allows the study of the regeneration process for various redox couples since it can effectively prevent the back transfer of electrons either at short circuit or by applying an additional potential. Transient absorption and fluorescence quenching, are widely used to study electron injection from QDs.43

Compared with these optical spectroscopic techniques, charge injection and

separation, measured as photocurrent, is a more direct method to quantify the electron injection yields from photoexcited QDs (Figure 2A) and is most relevant for the solar energy applications of QDs. Relatively high IPCE values of 60-70% were reported for CdSe QDs sensitization in the literature.5,7,19,21 Figure 3A shows a typical IPCE spectrum for a CdSe QDs sensitized ZnO electrode with MBzA as the molecular linker. The close resemblance of the photocurrent spectrum with the absorption spectrum of the CdSe QDs indicates that the photocurrent is a result of sensitization of ZnO by the CdSe QDs.

Consistent with the literature5, the

polysulfide/sulfide redox couple (Sx2-/S2-) is able to regenerate the photoexcited CdSe QDs. A layer of ZnS might also form on the ZnO surface after being exposed to sulfide anions in aqueous electrolyte44, however it does not appear to have a significant effect on the sensitization of ZnO by the CdSe QDs. The fast hole injection from a photooxidized CdSe QDs to the S2regenerator produces high sensitized photocurrents but the photovoltage is low due to the very negative potential of the Sx2-/S2- redox couple (~ -0.5 V vs. NHE in water, Figure 2A). Outersphere redox couples, like ferrocene and ferrocyanide (not always outer sphere), can have much faster regeneration rates for organic dyes than inner-sphere redox couples like I3-/I- and Sx2-/S2.45,46 A recent report used a ferrocene based regenerator for a DSSC47 suggesting the use of outer-sphere redox couples for CdSe QD sensitized ZnO electrodes.

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A

Abs

0.2

0.1

0.0

ZnO-MUA-CdSe

1

ZnO (0001) -blank 0

450 500 550 600 650 700 wavelength (nm)

IPCE (%)

ZnO-MBzA-CdSe

IPCE (%)

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B

0.02

0.01

0.00 450 500 550 600 650 700 Wavelength (nm)

Figure 3. (A) An IPCE spectrum of a ZnO electrode sensitized with MBzA capped CdSe QDs with a Sx2-/S2- redox couple (solid line). For comparison, the IPCE spectrum of a bare ZnO electrode (dash-dotted line) and the absorbance spectrum of CdSe QDs dissolved in C2H5OH (dashed line) are also shown. (B) IPCE spectra of ZnO electrodes sensitized with MUA capped CdSe QDs in the presence of redox couples Sx2-/S2-(red solid line), Fc+/Fc (blue dashed line) and Fe(CN)63-/Fe(CN)64- (purple dotted line). The sharp decrease in the blue region (