Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor

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Tuning anatase surface reactivity towards carboxylic acid anchor groups Mariana Cecilio de Oliveira Monteiro, Patrik Schmuki, and Manuela Sonja Killian Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03044 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Tuning anatase surface reactivity towards carboxylic acid anchor groups Mariana C. O. Monteiro,1 Patrik Schmuki,1,2 Manuela S. Killian1,* (1) Department of Materials Science, Friedrich-Alexander University Erlangen-Nürnberg, Martensstr. 7, 91058 Erlangen, Germany (2) Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia

Abstract The effect of different post-treatments on TiO2 anatase surface reactivity was investigated, in order to obtain the best techniques for enhancing anatase performance in diverse applications, e.g. in photocatalysis and especially as photoelectrodes for dye-sensitized solar cells (DSSCs). Different posttreatments of compact anodic anatase TiO2 were compared, including O2 plasma, UV irradiation, immersion in H2O2, vapor thermal treatment and post-anodization, evaluating the increase of the amount of OH reactive groups on the surface and removal of surface contamination. In XPS spectra, the increase of OH groups is evident by the O1s peak at higher binding energy. ToF-SIMS principal component analysis demonstrated that treatments performed in aqueous media lead to a cleaner surface, with substantial removal of electrolyte residues. Stearic acid and the organic dye N719 were adsorbed to the differently post-treated anatase and adsorption was evaluated by contact angle and dye desorption measurements. A higher loading with molecules containing carboxylic acid functionalities was confirmed by both techniques on the treated samples. The post-treatments that presented the highest amounts of dye were used to prepare photoelectrodes, and these were tested in DSSCs where the efficiency values doubled in comparison with the non-post-treated electrode.

Introduction TiO2 surfaces have a wide range of applications in the field of dye sensitized solar cells (DSSCs)1, biomaterials2–4 and photocatalysis5. Particularly anatase is largely used due to its high chemical stability and electronic properties. For many applications, the modification of anatase with a functional monolayer is needed or desired and one of the most widely used anchor groups are carboxylic acids (-COOH). A defined mean to produce TiO2 surfaces is by anodization and annealing.6 As formed, the layers are amorphous and after annealing they are converted into crystalline anatase.

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Amorphous TiO2 surfaces surmount anatase TiO2 in terms of reactivity towards –COOH anchoring groups due to the high presence of OH surface termination.7 However, it is well known that for diverse electrochemical or photoelectrochemical applications, the performance is drastically increased if amorphous TiO2 is crystallized to anatase.8 One of the ways of carrying out this transformation is by annealing the amorphous TiO2 in air or O2 environment at temperatures exceeding 400 ºC. The anatase surface is less reactive towards carboxylic acids (–COOH), which are important anchor groups for surface modification.9 Considering TiO2 use in DSSCs, for example, the surface treatment of anatase becomes necessary and has shown to improve surface binding to the –COOH anchoring groups. Different surface treatments have been reported and are claimed to clean the surface, and/or modify the electronic structure, surface chemistry, maximizing available anchor sites for monolayer formation. Among the most common methods are plasma treatments,10 ultra-violet (UV) irradiation,11 immersion in acids and bases,12 and others13. However, even though there are many well established forms of treating anatase TiO2 reported in the literature, there is a lack of information comparing those treatments in order to select the most effective one. The latter is crucial, considering that not every treatment is suitable for every application. In the case of DSSCs, for example, treating the anatase TiO2 surface with peroxide has shown to improve dye coverage, therefore improving cell efficiency.14 Films of TiO2 may contain suboxide forms, as Ti2O3 and TiO in combination with defects such as oxygen vacancies. Plasma treatment of anatase TiO2 surface, for example, is known to improve its affinity with carboxylic groups, by increasing surface hydrophilicity and potentially modifying the Ti3+/Ti4+ ratio. The actual mechanism through which surface reactivity is improved is still not yet clear as different work in the literature present divergent hypotheses. On one hand, authors reported that after treating TiO2 with O2 plasma, oxygen vacancies and defects were introduced to the oxide film and more Ti3+ species were formed.15–24 On the other hand, some authors stated that low temperature O2 plasma treatment actually leads to the reduction of the number of oxygen vacancies and defects, leading to the formation of a homogeneous stoichiometric TiO2 surface.25–29 We believe that the treatment does not lead to a fully stoichiometric surface and its actual effects must still be better understood. Even though the mechanism behind it is still not well defined in the literature, there are several works reporting the improvement of dye coverage on compact anatase treated with O2 plasma for application in DSSCs.8,17,21-24 Different studies reported also on the use of H2O2 as an oxidizing agent for TiO2 surface treatment.33-35 Zhou et al.36 stated that the immersion of TiO2 in aqueous H2O2 may lead to the formation of Ti-OOH bonds, which may cause a red shift to 400-500 nm visible light absorption and generate interstitial oxygen vacancies and Ti3+ on the TiO2 structure. The interaction of water with metal oxide surfaces is also a very fundamental and a largely studied reaction37–40 that influences the degree of


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hydroxylation of the surface and therefore its reactivity towards different molecules and functional anchoring groups.41 However it has not been significantly discussed in the literature if water vapor treatment of anatase surface is a way to improve its reactivity towards carboxylic acids. As shown in a number of reports,38,42,43 OH groups are generated on metal oxides surfaces by this technique; therefore it is of potential interest also for application on anatase. Ultra-violet light has also been largely used in order to treat titanium oxide surfaces, even though the mechanism behind it is until nowadays discussed and opinions may differ.44–47 Kiwi et al.48 investigated the effect of UV treatment on TiO2 and observed a hydrophilic conversion of the surface under UV light irradiation, by OH∙ radical formation. Sakai et al.49 proposed a detailed mechanism for the effect of UV light on this hydrophilic conversion, which can be found in the correspondent reference. An important remark is that testing of the samples must be done when they are freshly treated. It has been shown that storage of the samples changes the hydrophilic characteristics of the treated surface.48,49 An increase in contact angle of UV treated samples was observed after storing them for 1-10 days in the dark, most likely caused by adsorption of contamination from air. In the present work, we will compare different post-annealing treatments in terms of binding to carboxylic acid groups, due to their significance with regards to DSSCs and other processes. The aim of this study is to provide the reader with a coherent summary of available methods, to facilitate an appropriate choice of treatment without the necessity to screen all of them for their own purpose. We also propose a treatment not presented before in the literature, involving a second anodization of TiO2 after annealing. We evaluate if this treatment is capable of creating a similar surface like the non-annealed, amorphous TiO2, without compromising the beneficial electronic properties of the anatase TiO2. Surface reactivity was determined by evaluating the adsorption of stearic acid and N719 dye, and as well as with surface analytic and electrochemical methods.

Experimental Titanium foil sheets (Advent Research Materials, 99.6%, 0.125 mm thickness, 1 cm² surface area) were cleaned in an ultrasonic bath first with ethanol and then with water for 5 minutes each, afterwards thoroughly rinsed and dried with N2. Anodization of the cleaned Ti foils was carried out in 1 M H2SO4 solution as electrolyte and using a Pt electrode as cathode. Anodization was performed for 20 minutes at 20 V in order to obtain a 50 nm thick amorphous compact oxide layer. Some of the samples were further annealed for 3 hours in a furnace at 450 °C in air in order to obtain anatase TiO2. Anatase samples were submitted to different post-treatments: A) Immersion in 10 ml H2O2 (Sigma Aldrich, 30% w/w in H2O) for 10 minutes or 18 hours; B) UV-light (HeCd laser, Kimmon, Japan)


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irradiation for 30 minutes on samples immersed in ultra-pure water or 1 M NaOH; C) Exposure to water vapor for 4 hours at 80 °C; D) O2 plasma treatment for 2 minutes (SmartPlasma, 80 W, 0.1 mbar) followed by immersion in H2O for 6 h; E) Post-anodization for 2 min at 20 V in 1 M H2SO4 electrolyte. In order to compare the reactivity towards carboxylic acids, post-treated samples were immersed in 2.5 mM stearic acid (SA) CH3(CH2)16COOH (Sigma Aldrich, 95%) solutions for 1 hour, prepared in different solvents: ethanol (Merck, 96%) and acetonitrile (Sigma Aldrich, 99.8%). After immersion, samples were rinsed with the respective solvent to remove non adsorbed molecules and dried with N2. Characterization of the stearic acid monolayer was done by contact angle measurement using a macroscope (WILD, Type 246634, Heerbrugg, Switzerland) combined with a camera from Leica and the corresponding software (LAS V3.7). Ultra-pure water 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, Ti 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 C1s 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 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). As ToF-SIMS showed to be a suitable technique for investigating molecules attached to metal oxide surfaces,50 a ToF-SIMS 5 instrument from ION-TOF (Münster) was used for surface analysis. The samples were irradiated with a pulsed Bi3+ liquid metal ion beam (25 keV). The beam was electrodynamically bunched down to below 1 ns. Positive and negative spectra were recorded in high resolution mode (Δm/m > 8000 at

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rastered area was 100 μm × 100 μm in each case. To ensure static conditions, the primary ion dose density (PIDD) was kept at 5 × 1011 ions/cm2. Signals recorded in negative polarity were used for evaluation. They were identified using the accurate mass as well as the isotopic pattern and calibrated to CH2−, C2−, CN− and CNO−. For the integration of the signals Poisson correction was used. At least 5 different spots were measured per sample. For principal component analysis (PCA) a peak list was generated according to signals present on the as anodized anatase sample. Topography images were taken with a field emission scanning electron microscope (SEM) Hitashi S 4800. For dye-sensitization, Ru-based N719 dye (Ruthenizer 535-bisTBA, Solaronix) was used. Samples were immersed for 24 hours in a 300 mM solution of the dye in acetonitrile and tert-butyl alcohol (volume ACS Paragon Plus Environment

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ratio: 1:1). After dye-sensitization, the samples were rinsed with acetonitrile to remove non-chemisorbed dye and dried with N2. Dye loading was evaluated on a UV-vis spectrometer (Lambda XLS UV/VIS spectrophotometer, PerkinElmer) by measuring the absorbance of N719 at λ = 505 nm desorbed from 1 cm² samples by 500 μL of 10 mM KOH solution for 1 hour. The concentration of adsorbed dye was calculated by using the Beer-lambert law, A = εlc where A is the intensity of the UV-vis absorption spectra at 505 nm, ε is the molar extinction coefficient of N719 dye at 520 nm (ε = 10260 M-1 cm-1), l is the path length of the light beam (l = 1 cm) and c the dye molecular concentration. To evaluate the photovoltaic performance, the sensitized samples were combined with a Pt coated fluorine-doped glass counter electrode (TCO22-15, Solaronix) using a polymer adhesive spacer (Surlyn, Dupont). Electrolyte (IoLiLyt SB-163, IoLiTec) was introduced into the space between the electrodes. Using back-side illumination, the current-voltage was measured under simulated AM 1.5 illumination provided by a solar simulator (300 W Xenon Lamp Power Supply - XPS400, Solarlight), applying an external bias to the cell ranging from -1 to 1 V with 0.05 V steps and measuring the generated photocurrent with a Keithley model 2420 digital source meter. The active area measured was 0.79 cm2.

Results and Discussion A difference in reactivity of amorphous and anatase TiO2 towards the adsorption of carboxylic acids was observed, see Figure 1. The former shows a contact angle comparison of a self-assembled monolayer of stearic acid adsorbed to the two different surfaces under identical conditions. A higher surface coverage with the molecule leads to a more hydrophobic surface. The lower coverage of anatase can be explained by a lack of hydroxyl termination for anatase after the annealing treatment.51

Figure 1. Contact angle measurement of amorphous and anatase TiO2 showing distinct surface reactivity towards carboxylic acids after immersion in 2.5 mM stearic acid for 1h.

The lack in hydroxyl termination in anatase was confirmed by XPS analysis. The O1s spectra normalized to the total intensity can be seen in Figure 2. A shoulder at higher binding energy is more pronounced in the amorphous than in the anatase sample, due to the presence of more OH groups. Peak


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fitting was done on the amorphous spectrum and OH species and adsorbed water were identified. In various applications these OH groups on the anatase surface act as reactive sites for condensation reactions and binding of molecules through carboxylic acid groups (–COOH). Increasing the hydroxylation of anatase surface may assure a larger surface coverage with –COOH terminated molecules.

Figure 2. O1s XPS spectra comparing the amorphous and anatase TiO2.

In the present work crystalline anatase TiO2 compact oxide samples were obtained by anodization and annealing as described in the experimental section, with an oxide layer thickness of 50 nm. We compared the effect of different post-treatments of anatase (performed after annealing) with regard to the adsorption of molecules with carboxylic acid functional groups. The treatments described in detail are: A) immersion in hydrogen peroxide (H2O2), which acts as an oxidizing agent for the formation of hydroxyl terminations on the surface; B) immersion in water or NaOH and exposure to UV light for 30 minutes. It is expected that under UV light irradiation photo-generated holes will cause a hydrophilic conversion of the surface. 52 This hydrophilic conversion is enhanced by dissociative water adsorption53 and the presence of OH radicals. C) exposure to water vapour at moderate vapour pressure, D) exposure to O2 plasma followed by immersion in water, E) post-anodization of anatase. After the post-treatments A) to E) were performed, SEM images of the anatase samples were acquired in order to evaluate their influence on the TiO2 surface morphology. As annealed anatase TiO2 samples were used as reference and will be referred to as “non-treated”. The images can be found in the supporting information (S-1) and show that no obvious morphological change is observable on the SEM size scale by any of the treatments. The latter is expected, as the treatments may mainly influence the surface chemistry and cleanliness, do not generating significant morphology changes. In order to evaluate changes in the ACS Paragon Plus Environment

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surface chemistry, samples were submitted to XPS analysis to investigate alterations in surface composition of TiO2. The XPS data showed that all the samples consist of mainly Ti and O, with C detected as impurities. Traces of nitrogen can also be observed, as after anodization, annealing and the post-treatments the samples were rinsed with water and blow-dried under a N2 gas stream. XPS results can be seen in Figure 3 and Table 1.

Figure 3. Atomic concentration of O, Ti, C and N species in anatase non-treated and submitted to the different posttreatments.

From the O1s XPS spectra shown in Figure 4 it can be seen that the post-treatments enhance the hydrophilic character of the TiO2 surface. The latter is indicated by an increase in OH, observed as a shoulder at higher binding energy. After deconvolution and integration of the peak areas of the spectra obtained after the different post-treatments, the relation between the amount of O-H and O-Ti was calculated and is shown in Table 1. It can be seen that all of the treatments lead to increased hydroxylation of the surface. The higher the amount of OH termination of an oxide, the more potential anchoring centers for condensation reactions, for example with –COOH, exist.


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Figure 4. O1s XPS spectra of the non-treated anatase TiO2 and post-treated with treatments A)-E).

The atomic concentration percentages (at%) were obtained from the peak areas and are displayed in Table 1. The latter also shows the ratio of OH/OTi as derived from the deconvolution of the O1s peaks and the calculated stoichiometry of the oxide. Table 1. Atomic concentration (at%), hydroxyl termination and oxide stoichiometry due to the post-treatments (Ti at% calculated from the peak area of the O1s XPS spectra).

non -treated H 2O 2 UV H2O UV NaOH H2O(g) O2 plasma post-anodized

O1s 62.19 66.76 64.63 65.41 45.13 66.17 65.80

at% Ti2p C1s 26.26 11.18 27.84 5.02 27.70 7.66 26.02 8.48 18.56 36.11 26.23 6.78 27.53 5.98

N1s 0.37 0,39 0.05 0.09 0.21 0.82 0.70

peak fit O-H/O-Ti (%) 12 14 16 16 17 20 15


calculated at% TiO2+TiO(OH)2 28.60 28.03 28.27 27.96 18.16 28.12 28.12

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Assuming that no oxygen is bound to C or N, the presence of different oxide species was evaluated by calculations based on stoichiometry and the OH/O-Ti ratio. A surface composition of a combination of TiO2 and TiO(OH)2 only slightly exceeds the actually measured Ti content and, thus, is probable. The deviation could be caused by the presence, to a smaller extent, of further oxide species, e.g., Ti2O3 or TiO. The spectra containing the Ti2p peaks were also deconvoluted and can be found in the supporting information (S-2). It has to be taken into account, however, that for the vapor thermally and peroxided treated samples adsorption of H2O/H2O2 can also lead to an increase in the apparent OH signal in XPS. Furthermore, the high amount of carbon found in the water vapor treated sample is attributed to contamination obtained as a consequence of the method. The influence of the post-treatments on the surface composition was also evaluated by ToF-SIMS. Principal component analysis (PCA) has been largely used for the interpretation of ToF-SIMS data, it can help identify the major sources of differences within a sample or between samples, determine where certain compounds exist on a sample, or verify the presence of compounds that have been engineered onto the surface.54 Therefore, PCA was carried out in order to evaluate the ToF-SIMS data and scores and loadings obtained from the negative spectra are shown in Figure 5 a) and b) respectively. The scores show the relationship between the samples (spectra) and are a projection of the original data points onto a given principal component axis. The loadings show which variables (peaks) are responsible for the separation seen in the scores plot. Together the scores and loadings represent a concise summary of the original data.54 PC1 separates the samples according to their largest differences in the sample set and contains 67 % of the total variance. For most samples in the scores plot a very narrow distribution can be observed, demonstrating a high homogeneity of lateral surface composition. Figure 5 also shows that the post-treatments involving aqueous media (H2O2, UV H2O, UV NaOH and O2 plasma) present clear differences compared to the others. This is mainly related to a cleaner surface due to the removal of contamination, especially sulfur compounds (electrolyte residues). In XPS the sulfur content was in the noise level range, however, as ToF-SIMS has ppm sensitivity these residues can be observed.


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Figure 5. a) Scores and b) loadings obtained from principal component analysis (nominal masses displayed).

In the negative loadings, fragments of SO- (m/z = 48.06), SO3- (m/z = 80.06), SO4- (m/z = 96.06), SO4H- (m/z = 97.06) originally from the electrolyte are present in larger amounts at the surface of the nontreated, water vapor treated and in a smaller amount on the post anodized samples. Additionally, carbohydrate related fragments C- (m/z = 12.01), CH- (m/z = 13.01) and C2H- (m/z = 25.01) are more prominent in the negative loadings. The positive loadings which correspond to H2O2, UV and O2 plasma treatments, differ from the other samples mainly due to fragments as O2- (m/z = 15.99), OH-


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(m/z = 16.99), O2H- (m/z = 32.99), indicating a higher hydroxylation of the surface, in accordance with the XPS results. Spectra containing fragments with high loadings can be found in the supporting information (S-3). A reduction of the S-containing signals after the UV, H2O2 and plasma treatments as well as an increase in the oxygen and OH signals is clearly visible. UV irradiation also leads to the removal of larger organic contaminants through OH· radicals, as commonly observed in photocatalytic decomposition.55 It is important to notice that O2 plasma treatment was investigated with and without subsequent immersion in water. A detailed discussion of the effect of immersing the sample in water on contact angle, dye loading and surface cleanliness can be found in the supporting information (S-4). Plasma, when not followed by immersion in water, does not lead to a cleaner surface; however it leads to the oxidation of contaminating compounds present on the surface, therefore also causing the formation of groups like SO3-, SO4-, SO4H-. Therefore, immersion in water is extremely recommended after plasma treatment, especially when samples are supposed to be functionalized afterwards. In order to better evaluate the effect of the post-treatments on the anatase surface reactivity towards carboxylic acid groups, samples were immersed in stearic acid, a molecule containing an 18-carbon hydrophobic chain and a carboxylic acid head group. In contact with the metal oxide surface, these molecules tend to form a hydrophobic self-assembled monolayer (SAM), which can be evaluated by contact angle measurements. It is known that solvent choice has a strong influence in the formation of SAMs.9 Therefore, we first compared the stearic acid monolayer formation on as prepared amorphous and anatase TiO2 samples using solvents with different polar strength: ethanol and acetonitrile. Results can be seen in Figure 6 where - as it was also shown in Figure 1 - amorphous TiO2 presents higher reactivity towards carboxylic groups, presumably due to a larger amount of OH groups present on the surface (Figure 2). It can be seen that the solvent polar strength clearly influences the kinetics of the monolayer formation. Both solvents were used in order to compare the anatase post-treatments A) to E).


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Figure 6. Contact angle measurement performed after immersing non post-treated anatase and amorphous TiO2 samples in 2.5 mM stearic acid solution prepared using different solvents.

Anatase samples immersed in stearic acid/ethanol solution were submitted to contact angle measurements and results are shown in Figure 7. It can be seen that all the post-treatments significantly improve surface reactivity towards stearic acid. Treatment with UV light in NaOH solution is especially promising, seen by the more than three times higher contact angle measured. This is a significant improvement, as some of the post-treatments bring anatase surface reactivity to a higher level than the as prepared amorphous sample. The same evaluation was done for anatase samples immersed in stearic acid/acetonitrile solution in order to further compare the post-treatments. As can be seen in Figure 8 using acetonitrile leads to more efficient monolayer formation, due to the lower polarity of acetonitrile in comparison to ethanol. Differences are smaller but still clear, with UV treatment in NaOH and H2O as well as O2 plasma treatment presenting contact angle values around 120 ° while the other treatments range from 100 to 110 °. Compared to the non-treated sample this means an improvement of more than two times on the surface reactivity towards carboxylic acids.


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Figure 7. Contact angle values and comparative images obtained after immersing the post-treated samples in 2.5 mM stearic acid in ethanol solution for 1 hour at room temperature.

Figure 8. Contact angle values obtained after immersing the post-treated samples in 2.5 mM stearic acid in acetonitrile solution for 1 hour at room temperature and comparative contact angle image.


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XPS, ToF-SIMS and the contact results give valuable information about the effects of the studied post-treatments on the anatase surface chemistry, composition and cleanliness. Different mechanisms have been proposed, explaining the source of these effects and based on our results a few conclusions can be drawn regarding each technique. In the case of UV treatment, for example, under UV light irradiation photo-generated electron-hole pairs will cause a hydrophilic conversion of the surface, which is enhanced by dissociative water adsorption53 and the presence of OH∙ radicals.The low stability of NaOH in solution is believed to lead to the enhanced surface reactivity found for the UV NaOH treated sample. In the case of the hydrogen peroxide (H2O2) treatment, the weak acid acts as an oxidizing agent for the formation of hydroxyl terminations on the surface, also in the presence of dissociated water adsorbed to the surface. The presence of water leads also to the dissolution of electrolyte residues, which was observed for other treatments carried out in the aqueous media. According to the results shown, O2 plasma treatment by itself basically leads only to the oxidation of surface species, including electrolyte residues, other inorganic contaminants and adsorbed water. Small organic contaminants, however, can be removed from the surface by coupling with the plasma highly reactive O∙ species. The immersion in water after the plasma treatment probably leads to the removal of the oxidized electrolyte residues by dissolution in the aqueous media, as well as to the dissociative adsorption of H2O on the oxidized surface oxygen, leading to the formation of OH groups. The latter is confirmed by the prominent presence of OH species in the O2 plasma + H2O treated samples, in comparison to the O2 plasma without subsequent immersion in water treated ones (see Figure S-4-5 in the supporting information). The new method proposed, which comprises a short time anodization of the anatase surface, showed to also increase the O-H/O-Ti ratio (to a smaller extent than other treatments) and also improve the surface reactivity towards COOH groups. No morphological changes were observed with SEM, and we believe the bias applied in the presence of the aqueous electrolyte recovers the species that are usually encountered in a freshly anodized amorphous TiO2. These species include OH terminations and also S-containing electrolyte residues. In the case of less complex techniques as the water vapor treatment, water molecules are believed to dissociatively adsorb to the metal oxide surface, generating more OH reactive sites, however, this was shown to be less efficient than the plasma treatment, indicating that the activation energy for dissociative H2O adsorption is decreased on the plasma modified surface. Up to now, ruthenium polypyridyl dyes are the ones which have shown one of the highest light-toenergy conversion efficiencies in DSSCs due to their photophysical, photochemical and electrochemical properties. They are immobilized on the metal oxide surface through carboxylic acid groups and therefore surface reactivity towards these groups is crucial for solar cell efficiency.56 Dye N719 was used in order to compare the effect of the different surface post-treatments on DSSC efficiency. The absorbance spectra obtained as well as the calculated values from the dye desorption experiments are shown in Figure 9 and ACS Paragon Plus Environment

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Table 2. All the post-treatments lead to an enhancement in the amount of dye adsorbed, in agreement with the previously shown contact angle results. Especially for samples treated with UV light, a significant improvement is observed, the amount of dye adsorbed was up to 2.8 times higher than for the non-treated sample. These results are consistent with the trend observed in the stearic acid monolayer formation results. Also, the increase in hydroxyl termination as derived from XPS data indicates that the larger amount of OH groups on the surface was the major reason for the improvement in dye adsorption.

Figure 9. UV-Vis absorption spectra of the solutions containing dye desorbed from each post-treated sample.

Larger differences can be observed among the samples in the dye desorption measurements compared to stearic acid/acetonitrile contact angles. A potential explanation is that with stearic acid at 120° we may already have achieved a saturation in contact angle, i.e., a denser packing does not lead to an increase in contact angle anymore.57 This scenario is plausible, as the 18-C chain will cause a hydrophobic surface even though not in a completely erected geometry. Figure 7 supports the latter hypothesis, as the trend in contact angles obtained from ethanolic solution is in agreement with the results from N719 dye desorption experiments. Table 2. Amount of N719 dye adsorbed on the non-treated and post-treated anatase surfaces.

non treated H2O2 UV NaOH UV H2O H2O(g) O2 plasma post-anodized

nmol cm-² 27.27 ± 1.17 38.31 ± 1.65 72.16 ± 2.40 58.55 ± 2.51 33.57 ± 3.31 45.55 ± 1.11 38.89 ± 0.39

Enhancement (%) 40.48 164.61 114.70 23.11 57.32 42.62


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To verify the influence of the post-treatments on the actual performance in DSSCs, the ones which showed to be the most promising were used in the construction of the photoelectrodes. TiO2 anatase compact oxide samples were synthesized to be used as anode according to the experimental section and post-treated by exposure to UV light for 30 minutes in NaOH and H2O. Cell performance was recorded in a sun simulator under AM 1.5 irradiation and the results obtained are shown below. As it can be seen in Figure 10 and Table 3 the results confirm the positive effect of the post-treatments on the device performance, observed by an increase in Jsc due to the larger amount of dye on the surface. An increase in the Voc can also be observed and is attributed to less charge recombination processes taking place.

Figure 10. DSSC test with non-treated and UV treated samples. Table 3. Values obtained from the DSSC test with non-treated and post-treated samples.

non-treated UV H2O UV NaOH

η (%) 0.035±0.001 0.076±0.002 0.077±0.001

Jsc (mA cm-²) 0. 14 0. 21 0. 23

Voc (V) 0.53 0.58 0.58

FF 0.55 0.58 0.58

Device efficiency more than doubled going from 0.035±0.001 for the non-treated electrode to 0.076±0.002 and 0.077±0.001 for electrodes subjected to UV treatment in H2O and NaOH, respectively. These results are in accordance with what was observed in the contact angle measurements after stearic acid adsorption as well as in the UV-vis dye desorption experiment, where in both cases UV NaOH treatment presented slightly larger coverage than UV H2O. ACS Paragon Plus Environment

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The results obtained in this study show that the post-treatment of anodic anatase TiO2 is highly recommended after annealing, especially for applications where the presence of hydroxyl terminations on the surface is needed. All the post-treatments studied lead to an improvement of anatase surface reactivity towards carboxylic acids and those performed in aqueous media also to a cleaner surface. Among the treatments investigated, UV irradiation demonstrated to assure a better binding of carboxylic acids to the surface and to be highly effective in device application.

Conclusions We have studied different post-treatments after conversion of amorphous TiO2 to anatase by annealing. We compared the main techniques available for improving TiO2 anatase surface reactivity towards carboxylic acids and also assessed newly proposed methods. O2 plasma, UV irradiation, immersion in H2O2, vapor thermal treatment and post anodization were evaluated with regard to their efficiency in increasing the amount of reactive OH groups on the surface and improving surface cleanliness. Through XPS it could be seen that all the post-treatments lead to an increase in the surface hydroxylation degree which is directly related to the number of active sites for adsorption through condensation reactions. ToF-SIMS principal component analysis demonstrated that treatments performed in aqueous media lead to a cleaner surface, with a substantial removal of electrolyte residues, and to larger increase in OH related fragments. Contact angle measurements were carried out after the formation of stearic acid self-assembled monolayers in order to confirm surface reactivity towards carboxylic acid groups. Results demonstrated that all the treatments provide an enhancement in reactivity, especially UV and O2 plasma treatment. Furthermore it was concluded that immersion of the sample in H2O after plasma treatment is highly recommended. In order to evaluate these differences in the context of DSSCs, dye adsorption/desorption experiments were performed and higher dye loading due to the post-treatments was confirmed. The post-treatments that presented the highest amounts of dye were used to prepare photoelectrodes, and these were tested in DSSCs where the efficiency values doubled in comparison with the non-treated electrode. UV treatment performed in NaOH media assures the largest number of –COOH terminated molecules bound to the surface, especially in the case of N719 dye. The latter combined with a reduction in charge recombination was confirmed by a significant efficiency improvement in DSSC.

Acknowledgments The authors would like to thank the DFG research unit FOR1718 FunCOS "Functional Molecular Structures on Complex Oxide Surfaces" for funding and Mr. Gihoon Cha for assistance. We acknowledge also Dan Graham, Ph.D., for developing the NESAC/BIO Toolbox used in this study and NIH grant EB002027 for supporting the toolbox development.


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Supporting information. S1. SEM images of post-treated samples, S2. Additional XPS spectra containing the Ti2p peaks, S3. ToF-SIMS spectra correspondent to the principal component analysis, S4. Comparison of the O2 plasma treatment with and without subsequent immersion in H2O.


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Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor

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