Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in

Letter pubs.acs.org/JPCL

Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in Gas and Liquid Environments Jessy B. Rivest,†,⊥,∥ Guo Li,†,‡,⊥ Ian D. Sharp,‡ Jeffrey B. Neaton,†,¶ and Delia J. Milliron*,†,§ †

The Molecular Foundry and ‡The Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ¶ Department of Physics, The University of California, Berkeley and Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California 94720, United States § McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Controlled attachment of molecules to the surface of a material can alter the band structure energies with respect to the surrounding environment via a combination of intrinsic and bonding-induced dipoles. Here, we demonstrate that the surface potential of an application-relevant material, anatase TiO2, can be tuned over a broad energy range of ∼1 eV using a family of dipolar phosphonic acid-based adsorbates. Using TiO2 as an example, we show with photoelectron spectroscopy that these adsorbates are stable in a liquid environment (propylene carbonate). More interestingly, the tunability is substantially retained and follows trends in the computed bound dipole. The electrochemical surface potential is shown to vary over 600 meV, the highest range in electrolytes to the best of our knowledge. Using density functional theory calculations, we rationalize the measured trends and show that the effective dipole upon molecular adsorption and not the intrinsic dipole of the isolated molecules correlates with observed changes in surface potential. Control of the effective dipole, through judicious choice of robust surface species, can allow in situ tuning of energy levels and functionality at active surfaces for energy conversion and storage, biosensing, and molecular electronics. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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this effect can enable development of new active materials with optimal optoelectronic or catalytic function. The electric field associated with the adsorbed molecular dipoles results in a potential energy step ΔΦ across the adsorbate layer, biasing the energetics of charge carriers traversing the interface. The magnitude of this step is proportional to the substrate-normal dipole μz and the number density of the molecular adsorbates N, according to the Helmholtz relationship8

unctional interfaces between organic molecules, solvents, and semiconductors are central to many emerging technologies. Surface adsorbates for enhanced functionality are being explored for applications such as optoelectronics, catalysis, sensing, and energy storage. In the last few decades, much progress has been made in using surface molecules to alter energy levels of solid-state materials with respect to the surrounding environment.1−5 However, systematic control, particularly in liquid environments, has been elusive, and there remains a significant need for understanding the mutual changes of the structural and electronic properties, as well as stability, of the semiconductor surface and adsorbate molecules arising from surface bonding. Optoelectronic or catalytic properties of functionalized surfaces are fundamentally related to alignment between the substrate Fermi level and band edge positions and those of the adjacent materials. Modifications of this alignment upon adsorption are well-known to be significant; self-consistent interactions between the molecule and surface rearrange the electron density, inducing a so-called interface dipole,6,7 and modify the valence and conduction band edge energies as a step function relative to a common reference in vacuum, solution, or other environments. A validated framework for understanding © 2014 American Chemical Society

ΔΦ =

Nμz εε0

(1)

where εε0 is the local effective dielectric constant. Work function shifts at gold surfaces with thiol adsorbates have been reported previously, demonstrating shifted surface potentials with molecules possessing different dipoles.9 Tindoped indium oxide (ITO)10 and TiO211 have been functionalized with benzoic acids to shift their work functions. More recently, phosphonic acids have been shown to have high Received: May 25, 2014 Accepted: June 25, 2014 Published: July 1, 2014 2450

dx.doi.org/10.1021/jz501050f | J. Phys. Chem. Lett. 2014, 5, 2450−2454

The Journal of Physical Chemistry Letters

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adsorption energies to metal oxides, resulting in robust functionalization.12 In metal oxide systems, the effect of dipolar surface functionalization has been used advantageously in lightemitting diodes,10 photoelectrochemical cells,13−15 and dyesensitized solar cells.3 Reported shifts in the photovoltaic opencircuit voltage vary between 10s and 100s of mV,3,16 and turnon voltages in LEDs have been reported to shift in excess of 1 V.17,18 However, these device characteristics depend on other material properties that may be influenced by surface molecules, such as the recombination rate and interfacial structure; therefore, the realization of surface potential changes (ΔΦ) in these cases remains unclear. To our knowledge, there has not been a direct, systematic comparison of surface potential shifts across different application-relevant environments. Here, we present a systematic study of anatase titanium dioxide functionalized with phosphonic acid adsorbates of varying molecular dipole moment. Using photoelectron spectroscopy (PES) and contact potential differentiometry (CPD), we show that Φ can be tuned by over 1 eV based on molecule selection. We calculate the bound dipole strengths and resulting band energy shifts with density functional theory (DFT), explaining our measurements quantitatively. To demonstrate broad applicability of our approach, we extend our experiments to additional metal oxides. Finally, we measure the flatband potential in electrolyte for comparison with the air and vacuum environment and observe similar trends. Anatase TiO2 films were fabricated using a sol−gel method adapted from ref 19 with the porogenic agent omitted to obtain dense, planar thin films. Surface functionalization was accomplished by vapor-phase deposition, heating the films at 160 °C under vacuum in a quartz tube containing the desired molecular surface species (see the Supporting Information (SI) for further details). Adsorbates used were para-substituted phenyl- and benzylphosphonic acids. Methylphosphonic acid (MPA) was used as a control as atmospheric physisorption is expected to vary considerably between bare and functionalized surfaces, making a nominally bare substrate an unreliable control. DFT calculations were performed with the VASP code (v 5.2.12),20 using projector augmented wave potentials.21 We employed the Perdew−Burke−Ernzerhof generalized gradient approximation to the exchange−correlation functional22 for all studies and used vdW-DF223,24 to test the effects of van der Waals interactions (which turned out to be negligible) on the results. The most stable facet25 of anatase TiO2(101) was used as a model substrate, and we computed ΔΦ as the difference between the band edges of bare and surface-functionalized TiO2 substrates relative to vacuum. All of the metastable adsorption configurations 12,26 of nine phosphonic acid molecules, including the five used in the experiments, at three molecular densities (from 1.29 to 5.16 molecules/nm2), were systematically investigated. The thermally averaged values of the effective dipole (μbound) and the change in band energies were calculated for each molecule at each coverage. Further details of our calculations are in the SI. The family of phenylphosphonic acid adsorbates used here enabled the tuning of the band edge energies over a range of 1 eV (Figure 1A). This shift was consistent whether measured in vacuum (with PES) or in air (with CPD, measuring the sample work function with respect to a Au reference), which is reasonable given the similar dielectric constants of these two

Figure 1. (a) Work function of TiO2 versus DFT-calculated electric dipole strength of bound surface molecules. The work function was measured on the same sample set by CPD (red squares) in air and PES (black circles) in vacuum. The error bars on the experimental measurements are ±50 mV. Theoretical calculations (open black squares) are for number densities of N = 1.29 molecules/nm2. (b) Calculated free versus bound dipoles. Para- substituents are labeled (above points) for each phosphonic acid family (colored boxes).

environments. To reference the differential CPD values and the calculated ΔΦ to absolute work functions, the MPA sample was used as a matching standard with PES data. Interestingly, despite a lack of detailed knowledge of the experimental packing densities, we find that the PES and CPD results agree with the computed ΔΦ at N = 1.29 nm−2.27 The agreement between theory and experiment enables us to quantitatively understand the origin of these shifts in work function. In Figure 1A, the measured and computed work function shifts, ΔΦ, are plotted versus the calculated bound dipoles of the adsorbed molecules. The data show a linear relationship between the computed dipolar strength and the energetic shift, ΔΦ. The slope of this line should be N/ε0 because our computed dipoles include depolarization effects (ε). The fit of the experimental data to a single line suggests that the areal number density of adsorbates, N, is likely to be similar across the series; we obtain a slope of ∼1.3/nm2/ε0 from the line in Figure 1A. Furthermore, we clarify the microscopic origins of the energetic shifts by comparing the computed bound dipole (μbound) with the free (gas-phase) molecular dipole (μfree) (Figure 1B). All computed μbound are significantly more negative than μfree, even causing a reversal of the sign of μ in the case of two data points falling in quadrant IV. This can explain the reports in prior studies that the sign of 2451



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determine the flatband potential by extrapolating the linear portion of 1/C2 versus E to the x-axis intercept, where the capacitance of interest, C, is that of the semiconductor space charge layer. This capacitance varies with applied potential E, which changes the degree of band bending in the semiconductor.30 Remarkably, we measure a range of Φ ≈ 0.6 eV in the electrolyte, as compared to a span of ∼1 eV in vacuum (Figure 3). This suggests that, while the influence of the molecular surface adsorbate is reduced in the presence of electrolyte, it still is possible to tune Φ significantly.

ΔΦ is not always consistent with that of μfree.3,16,28 Interestingly, a linear relationship is computed between μbound and μfree for phenylphosphonic acid adsorbates (blue trace in Figure 1B). A slope of less than unity reflects depolarization associated with coupling to neighboring molecules and changes in molecular geometry upon binding. The intercept suggests a constant induced dipole with adsorption (μind = μbound − μfree = −1.21 D), which accounts for the negative shift from μfree to μbound. The relatively constant induced dipole and depolarization are computed for rigid molecules whose geometries only slightly change upon adsorption. For benzylphosphonic acid adsorbates (orange), greater conformational flexibility obfuscates any linear trend, and no systematic correlation is observed. Generalizing this paradigm, the same molecules were bound to two other metal oxides, ITO and indium-doped zinc oxide (IZO). These samples showed similar ∼1 eV ranges of work function (Figure 2), suggesting that the number density and

Figure 3. Flatband potential (ΔVfb, green) of TiO2 measured in electrolyte (right axis), as compared to the work function (Φ, black) of the same samples measured in vacuum with PES (left axis). ΔVfb values are referenced to the para-3-fluoro sample and are measured at 2 kHz with electrochemical impedance spectroscopy.

Literature reports of metal oxide surface potentials measured in electrolyte are limited to indirect evidence from device metrics, and the majority focus on solid electrolytes. Studies in liquid electrolyte demonstrate quite modest shifts of ∼100 mV (an upper-bound value based on the photovoltaic open-circuit voltage).3 Our work indicates that device performance may be affected more substantially than has yet been reported in liquid electrolyte by using an optimized molecular design and deposition strategy. On the basis of our mechanistic understanding of surface potential shifts, we expect that the 40% reduction of tuning range that we observe is due to a combination of (i) depolarization of molecular species by electrolyte intercalation or surface ordering (double layer), (ii) changed molecular orientation in the presence of electrolyte (likely negligible), and (iii) partial desorption of dipolar molecules. In addition, the presence of surface defect states on TiO2 could lead to Fermi level pinning, thereby reducing the impact of work function changes on the observed built-in potential. As desorption of surface molecules would significantly hamper practical application, we characterize the stability in the electrolyte and under operational conditions. We electrochemically cycled the TiO2 samples versus lithium metal 10 times in a half-cell configuration. We then left the substrates soaking in the electrolyte. After 2 months, samples were thoroughly rinsed and analyzed by PES. The intensity ratio of integrated P 2p to Ti 2p peaks, indicating the presence of surface adsorbates, was of similar magnitude in pristine- and cycled-soaked samples (Figure S7, SI).

Figure 2. Work function of TiO2 (gray), ITO (blue), and IZO (red) versus adsorbed dipolar surface species, measured by PES.

molecular orientations of adsorbates on various metal oxides are similar. Therefore, the trends in adsorbate-induced work function shifts for different metal oxides are consistent with a nearly constant but oxide-specific μind, which nevertheless does not influence the achievable range of ΔΦ. Thus, our understanding of (a) intrinsic molecular and (b) induced/ binding contributions to bound dipoles and the resulting energetic shifts is applicable across a broad range of relevant metal oxides. While our strategy yields a large range of surface potential shifts in vacuum and air, lower ΔΦ may be anticipated in condensed-phase environments. Direct characterization of energetic shifts of the same materials in different environments would be of great value in evaluating the potential for practical use of molecular surface potential tuning. For example, in applications such as catalysis and electrochemical storage, the tuning of Φ may result in improved chemical stability, enabling high-voltage batteries or more efficient catalysis.29 We therefore sought to measure the flatband potential of our samples when immersed in liquid electrolyte, to compare with the same samples whose work function was measured in vacuum. Electrochemical data were taken in an argon glovebox using 1 M tetrabutylammonium perchlorate in propylene carbonate electrolyte. Mott−Schottky analysis at 2 kHz was used to 2452



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(3) Rühle, S.; Greenshtein, M.; Chen, S.-G.; Merson, A.; Pizem, H.; Sukenik, C. S.; Cahen, D.; Zaban, A. Molecular Adjustment of the Electronic Properties of Nanoporous Electrodes in Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 18907−18913. (4) Vilan, A.; Shanzer, A.; Cahen, D. Molecular Control over Au/ GaAs Diodes. Nature 2000, 404, 166−168. (5) Cahen, D.; Kahn, A. Electron Energetics at Surfaces and Interfaces: Concepts and Experiments. Adv. Mater. 2003, 15, 271−277. (6) Mönch, W. Semiconductor Surfaces and Interfaces; Springer: Berlin, Heidelberg, Germany, 2001. (7) Tung, R. T. Recent Advances in Schottky Barrier Concepts. Mater. Sci. Eng., R 2001, 35, 1−138. (8) Helmholtz, H. v. Studien über Electrische Grenzschichten. Ann. Phys. (Berlin) 1879, 243, 337−382. (9) Evans, S. D.; Ulman, A. Surface Potential Studies of Alkyl-Thiol Monolayers Adsorbed on Gold. Chem. Phys. Lett. 1990, 170, 462−466. (10) Nüesch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L. Chemical Potential Shifts at Organic Device Electrodes Induced by Grafted Monolayers. Chem. Phys. Lett. 1998, 288, 861−867. (11) Krü ger, J.; Bach, U.; Grätzel, M. Modification of TiO2 Heterojunctions with Benzoic Acid Derivatives in Hybrid Molecular Solid-State Devices. Adv. Mater. 2000, 12, 447−451. (12) Luschtinetz, R.; Frenzel, J.; Milek, T.; Seifert, G. Adsorption of Phosphonic Acid at the TiO2 Anatase (101) and Rutile (110) Surfaces. J. Phys. Chem. C 2009, 113, 5730−5740. (13) Hilal, H. S.; Turner, J. A. Controlling Charge-Transfer Processes at Semiconductor/Liquid Junctions. Electrochim. Acta 2006, 51, 6487− 6497. (14) Hamann, T. W.; Lewis, N. S. Control of the Stability, ElectronTransfer Kinetics, and pH-Dependent Energetics of Si/H2O Interfaces through Methyl Termination of Si(111) Surfaces. J. Phys. Chem. B 2006, 110, 22291−22294. (15) Camacho-Alanis, F.; Castaneda, H.; Zangari, G.; Swami, N. S. Electrochemical Impedance Study of GaAs Surface Charge Modulation through the Deprotonation of Carboxylic Acid Monolayers. Langmuir 2011, 27, 11273−11277. (16) Cho, C.-P.; Chu, C.-C.; Chen, W.-T.; Huang, T.-C.; Tao, Y.-T. Molecular Modification on Dye-Sensitized Solar Cells by Phosphonate Self-Assembled Monolayers. J. Mater. Chem. 2012, 22, 2915−2921. (17) Hotchkiss, P. J.; Jones, S. C.; Paniagua, S. A.; Sharma, A.; Kippelen, B.; Armstrong, N. R.; Marder, S. R. The Modification of Indium Tin Oxide with Phosphonic Acids: Mechanism of Binding, Tuning of Surface Properties, and Potential for Use in Organic Electronic Applications. Acc. Chem. Res. 2012, 45, 337−346. (18) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. Phosphonic Acid Modification of Indium−Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies. J. Phys. Chem. C 2008, 112, 7809−7817. (19) Goh, C.; Scully, S. R.; McGehee, M. D. Effects of Molecular Interface Modification in Hybrid Organic−Inorganic Photovoltaic Cells. J. Appl. Phys. 2007, 101, 114503. (20) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (21) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (23) Lee, K.; Murray, É. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higher-Accuracy van der Waals Density Functional. Phys. Rev. B 2010, 82, 081101. (24) Klimeš, J.; Bowler, D. R.; Michaelides, A. Van der Waals Density Functionals Applied to Solids. Phys. Rev. B 2011, 83, 195131. (25) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (26) Nilsing, M.; Lunell, S.; Persson, P.; Ojamäe, L. Phosphonic Acid Adsorption at the TiO2 Anatase (101) Surface Investigated by Periodic Hybrid HF-DFT Computations. Surf. Sci. 2005, 582, 49−60.

This evidence of robust chemical stability suggests that depolarization is the most likely cause of tuning-range reduction in the electrolyte. Assuming that N is unchanged by electrolyte immersion, the effective dielectric constant obtained from the slope N/ε of the flatband potential versus the dipole (Figure 3) is 1.67, surprisingly low given the relative permittivity of propylene carbonate (64), which will be reduced only somewhat by the addition of salt.31 This finding suggests very low intercalation of electrolyte into the adsorbed layer,32 allowing the electrolyte to only weakly screen the dipolar field of the adsorbed layer, resulting in the large 600 meV range of Φ observable in this liquid environment. We have shown that adsorption of aryl phosphonic acids can strongly tune surface potentials in diverse, application-relevant environments and on a variety of metal oxides. Our DFT framework differentiates molecular and binding contributions to potential shifts and rationalizes the impact of changing the molecular structure by recognizing orientation effects. These results offer a strategy for future use in improving the performance of optoelectronic devices and catalytic systems.

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ASSOCIATED CONTENT

S Supporting Information *

Materials used, TiO2 film preparation and functionalization, and details of PES, CPD, DFT, and electrochemistry. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

J.B.R.: PARC, 3333 Coyote Hill Rd, Palo Alto, CA 94304.

Author Contributions ⊥

J.B.R. and G.L. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed primarily at the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the U.S. Department of Energy (DOE). Theoretical work and PES were supported in part by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. DOE under Award Number DESC0004993, and J.B.R. was supported in part by JCESR, supported through the DOE Office of Science, while D.J.M. was supported by a DOE Early Career Research Program grant. Part of the calculations were carried out at NERSC. We gratefully acknowledge A. Zayak, D. Prendergast, B. Helms, and G. LeBlanc for helpful discussions. The 3-fluoro molecule was provided by W. McClain and J. Schwartz (Princeton), and some TiO2 films were prepared by A. Phillips.

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REFERENCES

(1) MacDonald, J. R.; Barlow, C. A., Jr Work Function Change on Monolayer Adsorption. J. Chem. Phys. 1963, 39, 412−422. (2) Zuppiroli, L.; Si-Ahmed, L.; Kamaras, K.; Nüesch, F.; Bussac, M.; Ades, D.; Siove, A.; Moons, E.; Grätzel, M. Self-Assembled Monolayers as Interfaces for Organic Opto-Electronic Devices. Eur. Phys. J. B 1999, 11, 505−512. 2453



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(27) Experimental determination of surface coverage from XPS requires estimates of surface roughness, electron attenuation lengths (which themselves depend on molecular species and packing), degree of adventitious carbon contamination, and perfect anisotropic data for radiation and photoelectron absorption cross sections. While such measurements are frequently reported and our data would enable a coverage number to be calculated, the compounded uncertainty from these assumptions results in error bars that render the data insufficiently accurate for interpretation in the present case. As a consequence of these limitations, we have intentionally chosen not to present the results of such calculations here. (28) Ray, S.; Cohen, H.; Naaman, R.; Liu, H.; Waldeck, D. Organization-Induced Charge Redistribution in Self-Assembled Organic Monolayers on Gold. J. Phys. Chem. B 2005, 109, 14064− 14073. (29) Yang, S.; Prendergast, D.; Neaton, J. B. Tuning Semiconductor Band Edge Energies for Solar Photocatalysis via Surface Ligand Passivation. Nano Lett. 2011, 12, 383−388. (30) Gomes, W.; Cardon, F. Electron Energy Levels in Semiconductor Electrochemistry. Prog. Surf. Sci. 1982, 12, 155−215. (31) Gavish, N.; Promislow, K. Dependence of the Dielectric Constant of Electrolyte Solutions on Ionic Concentration. arXiv preprint arXiv:1208.5169v1 2012, 1−5. (32) Schwanitz, K.; Mankel, E.; Hunger, R.; Mayer, T.; Jaegermann, W. Photoelectron Spectroscopy at the Solid−Liquid Interface of DyeSensitized Solar Cells: Unique Experiments with the Solid-Liquid Interface Analysis System SoLiAS at BESSY. CHIMIA 2007, 61, 796− 800.

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Phosphonic Acid Adsorbates Tune the Surface Potential of TiO2 in

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