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Metal-phosphate bilayers for anatase surface modification Mariana Cecilio de Oliveira Monteiro, Gihoon Cha, Patrik Schmuki, and Manuela Sonja Killian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16069 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018
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Metal-phosphate bilayers for anatase surface modification Mariana C. O. Monteiro,1,2 Gihoon Cha,1 Patrik Schmuki,1,3 Manuela S. Killian1,* (1) Department of Materials Science, Friedrich-Alexander University Erlangen-Nürnberg, Martensstr. 7, 91058 Erlangen, Germany (2) Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300RA Leiden, The Netherlands (3) Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia *E-mail
[email protected] Abstract Compared to many other metal oxides, anatase TiO2 shows a relatively lower reactivity towards carboxylic acid anchor groups. The latter is crucial for application, for example, in dye sensitized solar cells, where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. In order to improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, ToF-SIMS and XPS results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes’ depth and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces towards carboxylic acid terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all the modified electrodes with phosphate-Co treatment leading to the highest JSC values and to an efficiency improvement of 48%.
1. Introduction Titanium dioxide anatase is extensively used as electrode for dye sensitized solar cells (DSSCs) as it is cheap, stable and has suitable electronic properties. Even though DSSCs are cheaper than silicon panels, their application is still limited due to, among others, the lower efficiencies obtained in comparison with first generation solar cells. In DSSCs, efficiency can be influenced by different factors such as charge recombination at the semiconductor–dye–electrolyte interface or in some cases, limited
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light harvesting.1 In view of bringing DSSC to a competitive position, many efforts have been made in order to improve the dye coverage of the surface, consequently improving light harvesting. The dye load and binding mode may also affect both injection and recombination dynamics as an influence of the electronic coupling between the dye and TiO2. There are different approaches that can be used to improve dye loading, which may involve increasing the surface area or improving the reactivity of the metal oxide surface towards the dye anchor groups.2 Up to now, ruthenium polypyridyl dyes are the ones which have shown one of the highest lightto-energy conversion efficiencies in DSSCs due to their photophysical, photochemical and electrochemical properties. They bind to the metal oxide surface through carboxylic acid anchor groups and, therefore, the surface reactivity towards these groups is essential for obtaining satisfactory solar cell efficiencies.3 Potential ways of improving the surface reactivity are based on treating the surface with UV irradiation4, plasma5 or acids. These treatments are known to increase the surface hydroxylation degree, generating more reactive sites for dye adsorption through condensation reactions. However, neither all treatments are suitable for nanostructures, nor cause significant improvements. On the other hand, surface modification techniques with different degrees of complexity have also been reported. Tuning properties such as surface area, charge recombination rate and dye loading has been achieved through, for instance,treatment with TiCl46, coating with other oxide layers7 or immobilization of different nanoparticles on the surface.8 Foreign oxide layers on top of anatase have been largely applied to coreshell nanocrystals, however, only a few studies have shown their application in 1D electrodes.9 Metal oxide layers deposited on the surface of anatase not only can have a positive influence in decreasing charge recombination, but may also be used as a way to improve light harvesting. Spampinato et al.10 were able to demonstrate the covalent surface modification of a metal oxide with a different metal oxide layer, using phosphate groups as intermediates. A prerequisite for this modification is a high affinity of both metal oxides to phosphate/phosphonic acid groups, which fortunately is valid for many materials (e.g., TiO2, ZrO2, Co3O4, NiO, Al2O3, Ta2O3, HfO2).11,12 Raman et al.13 carried out a stability study of metal oxide films coated with self-assembled monolayers using different alkyl acids. Phosphonic acid head groups were the only ones that were shown to form stable bonds with all the metal oxides tested, being resistant to rinsing and ultra-sonication. Phosphate groups can be immobilized on metal oxide surfaces through different methods, e.g., immersion in, spray-coating with or anodization in phosphate containing electrolytes,14–16 or immersion in POCl310. Vitale et al.17 recently reported a successful stepwise surface priming strategy based on zirconium phosphate-phosphonate chemistry, which was applied to TiO2 nanoparticles.
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In the interest of improving the binding affinity of carboxylic acids to anatase, the modification of TiO2 must be performed with materials that bind strongly to both carboxylic acids and phosphate groups. In the present work, we propose the synthesis of metal bilayers in order to improve anatase surface reactivity without significantly altering the material´s bulk properties. In this context, nickel and cobalt were selected for anatase surface modification. The choice was made based firstly on the material´s point of zero charge, which has a strong influence on the binding of molecules to metal oxide surfaces. According to Tewari et al.18 the point of zero charge (pHpzc) of NiO, Ni(OH)2, Co3O4, Co(OH)2 is 11.3, 11.1, 11.4 and 11.5, respectively. On the other hand, the anatase TiO2 average pHpzc reported value is 6.19 As pH values below the pHpzc of a metal oxide will lead to attractive interactions between dissociated carboxylic acid groups with the positively charged surface, the larger the pHpzc of a metal oxide surface, the stronger is the interaction with the dissociated –COO- groups. Nickel oxide, which is a p-type semiconductor with a bandgap energy of 3.6 eV20, is a promising candidate to be applied in bilayer formation. According to the work of Qin21 on NiO dye sensitized solar cells, among different binding groups, carboxylic acids were reported to bind very well to NiO through three different modes. Quinones et al.22 showed that also stable SAMs of alkylphosphates on NiO can be achieved, demonstrating the high capacity of NiO to bind also with phosphate groups. Raman et al.13 demonstrated that both nickel and titanium dioxide mainly bind to phosphate groups in bidentade form, which confirm NiO as a suitable candidate for bilayer formation on anatase TiO2. Stable bonds can also be formed between cobalt and phosphate groups as it is shown in diverse research work regarding CoPi catalyst systems.23,24 Besides, crystalline Co3O4 can offer high conductivity, electrochemical stability and pseudocapacitive behavior. As a supercapacitor, the Co3O4 electrode has been found to have good efficiency and long-term performance.25 Cobalt was also shown to have a high potential in binding to carboxylic acids. Pan et al.26 modified TiO2 nanoparticles by highly doping them with Co2+ (up to 23%). Results showed that it provided better binding of the particles to oleic acid, a molecule that binds to the surface through carboxylic acid terminations. In the present work we propose for the first time the formation of nickel and cobalt oxide phosphate bilayers on anatase 1D nanostructures in order to improve the surface reactivity towards carboxylic acid anchor groups. Optimization of the layer formation method was carried out using a compact anatase oxide in combination with XPS and ToF-SIMS analysis. The developed method was applied to TiO2 nanostructures and these subsequently were used as electrodes to assemble DSSCs so that the effect of the bilayers on photovoltaic performance could be assessed.
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2. Experimental Titanium (Advent Research Materials, 99.6%, 0.125 mm thickness), cobalt (Chempur, 99.9%, 0.125 mm thickness) and nickel (Chempur, 99.9%, 0.125 mm thickness) foil sheets were cleaned in ultrasonic bath first with ethanol and then with water for 5 minutes each, afterwards they were thoroughly rinsed and dried with N2. Anodization of the foils was carried out in different electrolytes and conditions using a Pt electrode as cathode, in order to obtain either compact oxide or nanostructures of comparable dimensions (except for NiO). Samples were also annealed in air in order to obtain crystalline structures. The experimental conditions used for each material can be found in Table S 1 of the supporting information. Anatase TiO2 both compact oxide and nanotube samples were subjected to different surface modification procedures. Priming of the surface with phosphate groups was done by immersing the samples in closed glass bottles containing H3PO4 (Sigma Aldrich, 85%) for 30 min, 1, 4 and 24 h at RT. Afterwards, samples were rinsed with deionized water and dried with N2. Surface modification with Ni and Co bilayers was performed by immersing the primed samples in 5 ml of either NiCl2.6H2O (Sigma Aldrich, 98% purity) or CoCl2.6H2O (Sigma Aldrich, 98% purity) solutions of 0.1 M concentration for 150 min. After being removed from the solutions, samples were thoroughly rinsed with deionized water and dried with N2. In addition, after surface modification with Ni or Co, some samples were heat treated in air at 300 ºC for 1, 2, and 3 h. Reactivity towards carboxylic acid groups was assessed via formation of stearic acid selfassembled monolayers. Samples were immersed in 2.5 mM stearic acid (SA, CH3(CH2)16COOH, Sigma Aldrich, 95%) solutions for 1 h, prepared in acetonitrile (Sigma Aldrich, 99.8%). After immersion, samples were rinsed with the solvent to remove non-adsorbed molecules and were dried with N2. Characterization of the stearic acid monolayer was done by contact angle measurements using a microscope (WILD, Type 246634, Heerbrugg, Switzerland) combined with a camera from Leica and the corresponding software (LAS V3.7). Ultra-pure water (milli-Q-Millipore) was used as liquid, with a droplet volume of 15 μL. The chemical composition of the samples was determined by X-Ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). XPS spectra were recorded on a Perkin-Elmer Physical Electronics 5600 spectrometer using monochromatic Al Kα radiation (1486.6 eV, 300 W) as excitation source. The takeoff angle of the emitted photoelectrons was 45º. The binding energy of the target elements (O 1s, C 1s, P 2p, Cl 2p, Ti 2p, Co 2p, Ni 2p, Cl 2p, N 1s) was determined at a pass energy of 23.5 eV, with a resolution of 0.2 eV, using the binding energy of the Ti 2p3/2 signal as reference. The measured spot had a diameter of 800 μm, and 100 cycles were recorded for each spectrum. The background was subtracted using the Shirley method in
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all spectra. To obtain the molar fractions of each species, the peak areas of the measured XPS spectra were corrected with the photoionization cross-sections of Scofield (σ) and the asymmetry parameter β (orbital geometry), which are contained in the sensitivity factors of the acquisition software (MultiPak V6.1A, Copyright Physical Electronics Inc., 1994-1999). ToF-SIMS was performed on a ToF-SIMS 5 spectrometer (ION-TOF; Münster, Germany) using a 25 keV Bi+ ion beam bunched down to
