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Facet-Dependent Photoelectrochemical Performance of TiO2 Nanostructures: An Experimental and Computational Study Chuanhao Li, Christopher Koenigsmann, Wendu Ding, Benjamin Rudshteyn, Ke R. Yang, Kevin P. Regan, Steven J. Konezny, Victor S Batista, Gary W. Brudvig, Charles A. Schmuttenmaer, and Jae-Hong Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5111078 • Publication Date (Web): 06 Jan 2015 Downloaded from http://pubs.acs.org on January 8, 2015
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Journal of the American Chemical Society
Facet-Dependent Photoelectrochemical Performance of TiO2 Nanostructures: An Experimental and Computational Study Chuanhao Li,†a Christopher Koenigsmann,†b Wendu Ding,b Benjamin Rudshteyn,b Ke R. Yang,b Kevin P. Regan,b Steven J. Konezny,b Victor S. Batista,b Gary W. Brudvig,b Charles A. Schmuttenmaer*b and Jae-Hong Kim*a a
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA.
b
Yale Energy Sciences Institute and Department of Chemistry, Yale University, New Haven, Connecticut, 065208107, USA. KEYWORDS: Photoelectrochemical Cells, Time-resolved Spectroscopy, Surface Functionalization, MK-2 dye ABSTRACT: The behavior of crystalline nanoparticles depends strongly on which facets are exposed. Some facets are more active than others, but it is difficult to selectively isolate particular facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO2 nanoparticles with predominantly exposed {101}, {010}, or {001} facets, where 86% to 99% of the surface area is the desired facet. Photodegradation of methyl orange reveals that {001}-TiO2 has 1.79 and 3.22 times higher photocatalytic activity than {010} and {101}-TiO2, respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated atoms. In contrast, the photoelectrochemical performance of the faceted TiO2 nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}TiO2 which yielded an overall cell efficiency of 6.1%, compared to 3.2% for {101}-TiO2 and 2.6% for {001}-TiO2 prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggest a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection and recombination dynamics when MK-2 is bound to {010} compared to other facets, consistent with extensive computational simulations which indicate that the {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as opposed to surface energy alone.
and, thus, catalytic activity is largely dependent upon crystalline structure and exposed facets.9-11 In the case of TiO2, the anatase phase has received the most attention because of its relatively high catalytic activity and electron mobility compared to the rutile and brookite phases.12 In terms of surface structure, computational and experimental results indicate that the {101}, {010} and {001} surface facets of TiO2 exhibit increasing average surface energies of 0.44 J/m2, 0.53 J/m2, and 0.90 J/m2, respectively.13, 14 Studies of the facet-dependent reactivity on singlecrystals and NPs have demonstrated that photocatalytic performance is measurably increased on the higher energy {001} and {010} facets.13-16 This has been attributed to the high surface energy and the predominance of surface Ti4+ sites with five-fold coordination spheres, which are expected to strongly interact with adsorbates on the surface during catalytic reactions.8,16
1. Introduction Solar energy is a promising pathway toward addressing the global challenges of alternative energy needs and environmental sustainability.1 Semiconductor-based materials have been instrumental in converting solar irradiation to energy by facilitating the absorption of photons and the subsequent generation of photoelectrons and holes.2 These can be harnessed to either generate electricity,3 initiate photo-driven reactions, produce chemical fuels (e.g. hydrogen or methanol),4 or form free radical species.5 Amongst many promising semiconductor materials, nanoscale titanium dioxide (TiO2) has been broadly employed in photocatalysis and photoelectrochemical cells due to its advantageous electronic properties, relatively high electron mobility, photochemical stability, and abundance.6 The photocatalytic performance of TiO2 nanoparticles (NPs) depends largely upon the physicochemical properties of the material including the surface area-to-volume ratio, particle size, morphology, crystallinity and surface structure.7,8 Catalytic reactions occur at the surface of NPs
Several researchers have explored routes to synthesize anatase TiO2 via hydrothermal methods with reliable and predictable control over the relative exposure of the {001}, {010}, and {101} facets.13,14,17-22 For example, the growth of
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DSSCs reported in the literature.2 From a computational perspective, a recent study comparing the calculated recombination rates of Ru-based C101 dye (Dyesol Ltd.) bound to the {101} and {001} surfaces suggested that the adsorption of the dye on the {001} facet may slow recombination when compared to the adsorption on the {101} facet leading to higher efficiencies.42 However, neither spectroscopic measurements of the relative electron injection and recombination rates nor simulations of electron injection dynamics have yet been performed as a function of surface facet.
NPs with the highest energy {001} facets exposed has been achieved by adding F− ions in the growth media to cap the {001} facet thereby leading to selective growth along the {101} and {010} facets. Facet purity of over 80% has been achieved with nanosheets and nanodisks as predominant morphologies by combining F− with organic capping agents.15,23 It has been reported in all of these cases that increasing the fraction of {001} facets enhances photocatalytic activity. Although the facet-dependent performance of TiO2 has been well studied in the realm of photocatalytic reactions, there are few reports regarding the facet-dependent photoelectrochemical performance particularly in the case of dye-sensitized systems. In these systems, wide-band gap semiconductors such as TiO2 are sensitized by covalently attached organic or inorganic chromophores, which absorb light in the visible spectrum.2,24-26 Photoexcitation of the chromophore results in an ultrafast interfacial electron transfer (IET) into the conduction band of the semiconductor leading to charge separation.27 The resulting hole localized on the chromophore can either be utilized for catalytic reactions such as water splitting,28 or transferred via an electrolyte system to produce electrical current in the case of dye-sensitized solar cells (DSSCs).29 The charge-separation efficiency and photoelectrochemical performance are, therefore, largely dependent on the relative rates of electron injection and recombination, which are affected not only affected by the properties of the semiconductor itself but also its interaction with the chromophore.27,29,30 Specifically, the IET dynamics depend strongly upon the interplay among the driving force for electron injection,31 the strength of the covalent interaction,32,33 the nature of the chromophore’s anchoring group,34,35 anchoring geometry,36,37 and the surface density of chromophores,38 as well as the surface structure, morphology, and crystallinity of the TiO2.39,40 Therefore, studying the facet-dependent photoelectrochemical performance in a rational manner is challenging since all of these factors must be considered.
In this report, we performed a comprehensive study of the facet-dependent performance including i. photocatalysis measurements, ii. characterization of photoelectrochemical performance in DSSC devices, iii. time-resolved spectroscopy, and iv. first-principles calculations of {101}, {001}, and {010} terminated NPs with greater than 86% exposure of the desired facet. To the best of our knowledge, this is the first systematic study of the facet-dependent performance highlighting contrasting trends in photochemical and photoelectrochemical performance in anatase TiO2 nanoparticles. The UVphotodegradation of methyl orange established that the {001} terminated NPs achieved the highest photocatalytic performance of the three facets. Interestingly, the photoelectrochemical performance of the facetted TiO2 NPs sensitized with the MK-2 dye in DSSC devices varied in a different manner: The devices utilizing the {010} terminated NPs exhibited the highest photoelectrochemical performance of 6.1%, which was two-fold higher than that of the {101} and {001} NPs (3.2% and 2.6%, respectively). This is in stark contrast with the photocatalytic results and with prior photoelectrochemical results.43-45 Timeresolved THz and transient absorption spectroscopies measured more efficient electron injection and faster recombination dynamics with {010}-facetted TiO2. Computational results further confirmed that the photoelectrochemical performance and injection efficiency can be attributed to the structure of the different facets, which promotes unique pathways for interfacial electron transfer.
A recent and promising approach has been to study photoelectrochemical performance using DSSC devices because they are well-characterized, inexpensive, and easy to assemble.41-43 Given the high photocatalytic activity of the {001} facet, prior studies have focused on examining the facet-dependent DSSC performance of TiO2 NPs with different percentages of the {001} facet exposed relative to the {101} facet.43-45 For example, a recent report found that increasing the exposure of the {001} facet from 10% in one shape of NP to 80% in another resulted in small but measureable increase of ~8% in DSSC efficiency.44 In another report, TiO2 nanosheets with 75% exposure of the {001} facet showed an enhanced efficiency of 4.6% relative to 4.2% measured for {101} terminated NPs.43 In both cases, accurate interpretation of the facetdependency was difficult since the NPs in these studies consisted of mixtures of the {101} and {001} facets with the desired facet representing 80% or less of the total surface area. Also, variations in performance were not significantly larger than typical device-to-device variations for
2. Experimental Methods 2.1 Synthesis and Characterization The synthesis of the {101}, {010}, and {001}-TiO2 nanoparticles was accomplished by hydrothermal methods. Characterization of the materials including their size and morphology, specific surface area, and band gap was accomplished by scanning electron microscopy (SEM), nitrogen adsorption isotherms, and UV-visible spectroscopy. The details of the synthesis and characterization, timeresolved spectroscopy, and computational methods can be found in Sections 1, 2, and 3 of the Supporting Information, respectively. 2.2 Photodegradation Experiments Methyl orange (MO) was used as a model compound for organic environmental contaminants to test the facetdependent photocatalytic activity. In a typical measure-
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Journal of the American Chemical Society removed by centrifugation. The dye loading was quantified by measuring the changes in the concentration of the loading solution before and after adsorption utilizing UVvisible spectrophotometry. The desorption studies were performed with MK-2 sensitized TiO2 films utilizing a method that is described in detail in Reference 34. The films were prepared identically to those employed in the DSSC devices, however the thickness was reduced from 7 µm to 3 µm to avoid effects of diffusion of the dye through the films. Subsequently, desorption experiments were performed by immersing the sensitized films into a solution containing 0.5 vol% water dissolved in anhydrous dimethylformamide (DMF, Sigma Aldrich,
