Nanoscale Investigation of Photoinduced Hydrophilicity Variations in

Oct 23, 2013 - Laboratory for Energy and Nanosciences, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates. § Masdar Institut...
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Nanoscale Investigation of Photoinduced Hydrophilicity Variations in Anatase and Rutile Nanopowders M. V. Diamanti,*,† K. R. Gadelrab,‡ M. P. Pedeferri,† M. Stefancich,‡ S. O. Pehkonen,§ and M. Chiesa‡ †

Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milan, Italy ‡ Laboratory for Energy and Nanosciences, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates § Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates ABSTRACT: The photoactive properties of TiO2 are employed to develop surfaces with self-cleaning capabilities. Clearly, the fine-tuning of such surfaces for different applications relies on a holistic understanding of the different aspects that induce the self-cleaning behavior. Among those, the mechanisms responsible for the photoinduced surface alteration in the TiO2 allotropes are still not completely understood. In this study, TiO2 polymorphs nanopowders are investigated by combining the high spatial resolution observables of recently developed atomic force microscopy (AFM) based force spectroscopy techniques with diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). Phase maps under irradiated and nonirradiated conditions for anatase and rutile suggest the existence of two distinct behaviors that are further discerned by energy analysis of amplitude and phase vs distance curves. Independently, surface analysis of anatase and rutile by means of DRIFTS spectroscopy reveals a readily distinguishable coexistence of dissociated water and molecular water on the two phases, confirming the stronger photoactivity of anatase. The peculiarity of the surface interaction under UV exposure is further investigated by reconstructing the force profiles between the oscillating AFM tip and the TiO2 phases with the attempt of gaining a better understanding of the mechanisms that cause the different hydrophilic properties in the TiO2 allotropes.

1. INTRODUCTION The benefits of titanium dioxide (TiO2) in the field of environmental purification, as well as in energy production devices, have been a subject of scientific investigation for the last few decades, leading to constantly increasing government and industrial funding, and to an exponentially growing number of publications in the field. In fact, this wide band gap semiconductor acts as photocatalyst in the removal of pollutants from water and air,1−4 which places it, within purification technologies,5−7 in the class of advanced oxidation processes.3,8,9 Additionally, TiO2 nanostructured films can be exploited as anodes in water splitting,10 or in dye-sensitized solar cells (DSSCs),11 and although their efficiency is far from that of conventional Si- or GaAs-based solar cells, these devices deserve attention due to the low cost of materials and production techniques, and to the potential efficiency upturns this developing technology can still experience.12 Nonetheless, the majority of industrial applications of photoactivated TiO2 lie in the production of self-cleaning surfaces,13,14 the uses of which range from antifogging rear-view mirrors,15 to selfcleaning windows,16 tiles17 and, more generally, building materials18−20 to textiles.21,22 This effect relies on the coexistence of two mechanisms: photocatalysis and superhydrophilicity, where the former degrades the functional © 2013 American Chemical Society

groupssuch as carboxylic groups and fatty acidsby which pollutants adhere to a surface, while the latter forces water to spread completely on the surface, carrying away particulate matter and degraded contaminants.14,23 TiO2 photocatalytic reactions basically involve the formation of highly reactive radical species by interaction of photogenerated electrons and holes with water and oxygen adsorbed on TiO2 surfaces. The redox power of such species is high enough to induce the degradation of inorganic and organic compounds, in liquid or gas phase, coming in contact with the photoactivated TiO2 surface.1,24,25 On the other hand, mechanisms generating photoinduced superhydrophilicity are less clear. Traditionally, water contact angle measurements provide macroscopic evidence of the superhydrophilicity of TiO2 surfaces under exposure to ultraviolet (UV) and/or visible irradiation, but fail to provide a mechanistic explanation to these observables. From a surface chemistry point of view, this phenomenon is generally attributed to the reduction of surface Ti4+ to Ti3+, which reduces the bond strength between the corresponding titanium ion and the closest oxygen ion; the latter is then Received: September 8, 2013 Revised: October 17, 2013 Published: October 23, 2013 14512

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2. EXPERIMENTAL SECTION

removed when a water molecule arrives in contact with the surface and adsorbs onto it. 26 This creates a highly hydroxylated surface layer, which is responsible for hydrogen bonding interactions with water and the consequent increased hydrophilicity of the surface.23,27 Fujishima et al. proposed a further level of complexity in TiO2 surfaces, claiming the observation of hydrophobic and hydrophilic domains of nanometer size alternating across the surfaces, which indicates an amphiphilic character.28 To add a further level of complexity to the picture of TiO2 wettability, Ganesh et al. paved the way for higher performance self-cleaning TiO2 surfaces by achieving superamphiphobicity, which was obtained by generating a siloxane layer over a superhydrophilic TiO2 surface.29 The onset of a superhydrophilic state has also been ascribed to the photodegradation of a thin hydrocarbon film that contaminates the TiO2 surface and decreases its wettability.30 Still, recent XPS investigations of photoactivated TiO2 do not reveal any additional formation of OH groups on its surface upon irradiation; concurrently, several theoretical and experimental studies address the question of the molecular or dissociative character of water adsorption onto TiO2.1,31 Another uncertainty in fully understanding the photoactivated surface variations is related to the different allotropes that titanium dioxide may exhibit. It is commonly accepted that TiO2 has three main allotropic forms, rutile, anatase, and brookite. Rutile is the thermodynamically stable tetragonal structure, anatase only presents lattice distortion with respect to rutile and it is metastable up to temperatures in the order of 500 °C, while brookite, with an orthorhombic lattice, generally attracts less attention, since it is less stable and more difficult to produce.32,33 Between the two main crystal phases, anatase holds a prominent position in photocatalysis and self-cleaning due to the higher photoactivated responses observed, in spite of the lower band gap of rutile that should favor light absorption over a wider wavelength range;34 whether a combination of the two phases can improve photoefficiency or decrease it is still under debate.35 Considering self-cleaning in particular, this is ascribed to a higher hydroxylation of anatase surface compared with rutile, on account of a more defective surface of the metastable phase and the consequent availability of oxygen vacancies and defective sites for water dissociation; again, wellgrounded reasons for differing activities among TiO 2 polymorphs have not been elucidated in detail.13 In this work, we aim at studying the photoinduced surface variations of two TiO2 phases under a different perspective, shedding light on the possible mechanisms responsible for it. This is achieved by combining the high spatial resolution observations obtained by recently developed AFM-based force spectroscopy techniques with diffuse reflectance Fourier transformation infrared (DRIFTS) spectroscopy. To start with, the energy transfer between an AFM tip and the TiO2 samples is investigated by recording phase maps under irradiated and nonirradiated conditions for anatase and rutile. Independently, surface analysis of anatase and rutile by means of DRIFTS spectroscopy reveal a readily distinguishable footprint of water on the two phases. Furthermore, relying on the ability to accurately reconstruct force profiles between the oscillating AFM tip and the two TiO2 phases we can locally quantify the peculiarity of the surface interaction under UV exposure and distinguish the different surface conditions in the TiO2 allotropes.

Commercial TiO2 nanosized powders with two different structures are studied: pure anatase (99.8% pure titanium(IV) oxide from Aldrich Chemical Co.) and pure rutile (99.995% pure titanium(IV) oxide from Aldrich Chemical Co.). BET area and particle size of approximately 10.9 m2/g and 48 nm, respectively, for anatase, and 2.5 m2/g and 240 nm, respectively, for rutile, were reported in previous studies.36,37 Powders are dispersed in distilled water by sonication at 40 °C for 1 h (10−2 mol/L powder concentration); then, a drop of suspension is deposited on a glass slide previously cleansed with ethanol, and dried on a hot plate at 40 °C. AFM experiments are conducted using the Cypher (Asylum Research), operated in amplitude modulation AM-AFM mode, with AC160TS commercial tips (k ≈ 40 N/m and f ≈ 300 kHz) in ambient conditions (25 °C, ≈40% RH). Furthermore, the AFM setup is such that the sample can be UV irradiated in situ. The irradiation source has a spectrum that extends to the visible domain of wavelength (UV−vis). The light source was an Oriel model 66894 Xe and Hg(Xe) 450 W Research Source, with a UV fused silica, 1.3 in. collimated, F/1, 1.5 in., with a fiber bundle focusing assembly, fused silica aspheric, F/2.2, 800 μm spot size, and with a high grade fused silica 36 in. fiber optic bundle. The intensity of this source is ca. 0.6 suns. Surface scans for every material are conducted where phase angle maps are recorded. The cantilever resonance frequency is determined using the thermal method.38,39 For every phase angle scan, this frequency is measured 40 nm above surface and the phase is forced to be at 90° ± 0.2°.40 The stability of resonance frequency is a direct indicator of system stability especially when it is UV−vis irradiated, due to the thermal component of the radiation spectrum. System stability and phase map accuracy, and the effect of UV−vis irradiation on the tip are verified on freshly cleaved mica samples (data not shown). Phase angle maps are recorded in attractive regime at two amplitude set points (A/A0): 60% and 70%, where A0 is the amplitude far from the surface. The phase angle record is considered acceptable only if, after scanning, the resonance frequency changes by less than 5 Hz and the phase changes by less than 0.5°. This guarantees that the system is stable during scanning and the values of recorded phase angle can be compared with and without UV irradiation. Amplitude vs distance curves are also recorded for energy dissipation analysis41,42 and force profile reconstruction43,44 for every material with and without irradiation. The repeatability of experimental results is also investigated by performing multiple experimental sessions on each sample. This is particularly important when AFM tests are performed on nanopowders, where loose adhesion to the substrate and to the rest of the particles might cause ambiguity or spurious signals mainly due to single eventsfor instance, nanoparticles adhering to, or being displaced by the tip. Before every experiment, a series of amplitude vs distance curves are recorded on freshly cleaved mica with increasing free amplitude. Such data are utilized to monitor the tip radius45 and calibrate the system InvOLS (nm/V).46,47 Finally, DRIFTS is performed on both anatase and rutile in the absence and presence of UV−vis light: this analysis is particularly suitable to investigate rough surfaces and powders, as it collects information from diffuse radiation scattered from irregular surfaces. DRIFTS spectra (in Kubelka−Munk units vs a KBr standard) are collected in a DRIFTS HVC-DRP reaction chamber with CaF2 windows, mounted in the front sample compartment of a Bruker 80v FTIR spectrometer. Samples are leveled in the central turret of the HVC-DRP reaction cell, and subjected to humid nitrogen flow (temperature 45 °C, flow rate 40 mL/min, relative humidity 68%); a Netzsch A-drop humidity generator is used to generate and control the moisture level, which is fed through a Netzsch STA449 F3 Jupiter TGA (operating at 45 °C in an isothermal pass through mode). A first series of nonirradiated spectra is collected, allowing powders to equilibrate for at least 30−45 min, then UV−vis irradiation is 14513

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performed in situ (duration of 3 h) and simultaneously a new set of spectra of 300 scans is recorded at time intervals of about 15 min.

UV−vis irradiation. Such scanning mode provides gentle tip− sample interaction and minimizes the possibility of tip contamination. The scans clearly demonstrate different topographies and particle shapes between the two studied nanopowders. The anatase nanopowder is more granular and spherical in shape, while the rutile one shows a more packed and smoother structure. To analyze the scans, a histogram of the phase angle map is constructed to capture variations when samples are irradiated by UV−vis light. The phase histogram of the anatase sample demonstrates a bimodal distribution with a clear influence of topography on the recorded phase angle. The histogram shows a first indication of surface alteration under UV−vis exposure where a peak at a lower phase angle (116°) is created. When compared with the unexposed sample, the closest peak to it is at (121°), meaning that exposure results in lowering of the phase angle of the scan by about 5°, i.e., increasing the energy dissipated by the tip into the sample. As indicated in the previous section, such a reduction in the phase angle is much higher than the uncertainty in the phase angle measurement and cannot be explained by a shift in the tip resonance frequency. When the same experiment is conducted on rutile, a clean single peaked distribution is constructed, see Figure 2. Interestingly, a shift in the opposite direction takes place (4°) when the sample is exposed to UV−vis radiation, meaning that the tip transfers less energy to the sample when rutile is irradiated. Results from the phase angle scans indicate the overall surface property homogeneity. To confirm the behavior obtained from pixel counts in the histogram, amplitude vs distance curves are recorded on different locations on the sample surface. Such curves add a new insight to the study where the variation of properties as a function of tip−sample separation can be captured. The energy transfer between the tip and the sample is again calculated from the recorded amplitude and phase angle41 for irradiated and nonirradiated samples for a range of small amplitude oscillations (3−8 nm). With such oscillation amplitudes, the tip has a higher sensitivity to capture surface properties with minimal interactions with the surface. Figure 3 shows representative energy dissipation plots for both rutile and anatase as a function of the amplitude ratio (A/A0). Every plot is a collection of at least five curves. It is observed that the energy plots confirm the behavior initially suggested by the histograms. When irradiated, rutile exhibits a significant reduction in energy dissipation; such a reduction is more pronounced when the amplitude ratio is small and the tip is

3. RESULTS AND DISCUSSION Surface characteristics of TiO2 have been proven to be sensitive to UV and visible irradiation, giving rise to its unique properties of photocatalysis and superhydrophilicity.28,48 Hence, under UV exposure, complex conservative, as well as dissipative, interactions are expected to prevail when the surface is investigated with a hydrophilic nanosized AFM tip. Phase angle surface imaging would be ideal as an investigation mode to capture such variations. This is related to the fact that phase angle contrast is connected to the magnitude of the energy dissipated per cycle in the tip−sample interaction as the cantilever taps over the surface, implying that phase imaging has the potential to provide physiochemical information about samples with a high spatial resolution. Furthermore, quantifiable mean energy dissipated per cycle can be easily calculated giving an insight into the amount of energy transferred from the tip to the sample. Dissipation has been recently associated, in hydrophilic sample−tip interactions, to the presence of water nanolayers and neck formation49 thus hinting to a potential connecting mechanism between sample hydrophilicity and energy dissipation change. Figures 1 and 2 show sample phase angle scans of anatase and rutile nanopowders in attractive mode, with and without

Figure 1. Phase angle scans and related histograms recorded on anatase nanopowders under no irradiation and under UV−vis irradiation.

Figure 2. Phase angle scans and related histograms recorded on rutile nanopowders under no irradiation and under UV−vis irradiation.

Figure 3. Energy dissipations as a function of amplitude ratio A/A0 recorded on rutile (left) and anatase (right) nanopowders in nonirradiated conditions and under UV−vis irradiation. 14514

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present on the TiO2 surface prior to irradiation, its photoinduced removal cannot be solely responsible for the onset of superhydrophilicity. Alterations in TiO2 anatase leading to an increase in energy dissipation are therefore ascribed to changes in surface chemistry, and more specifically to hydroxylation,28 which increases the strength of the interaction with the hydrophilic silicon tip.54 On the other hand, rutile is known to offer less affinity to water, due to a more stable crystal structure and consequent higher energies in creating oxygen vacancies, which are the most common precursors for surface hydroxylation.28 Studying the problem from a different perspective can be achieved using FTIR incorporating a DRIFTS accessory. Figures 5 and 6 show the absorption

oscillating close to the surface. On the other hand, anatase exhibits the opposite behavior, wherein the energy transfer between the tip and the sample increases when the surface is irradiated, and again the difference is more pronounced when the tip is closer to the surface. Finally, Figure 4 summarizes energy dissipation values integrated in the interval between 60% and 100% of A/A0, as

Figure 4. Integrated values of energy dissipation as a function of driving amplitude, calculated in the case of rutile (left) and anatase (right) nanopowders in nonirradiated conditions and under UV−vis irradiation.

a function of the driving amplitude, in the presence or absence of UV−vis irradiation. A further agreement is observed with previous findings showing that at different free amplitudes, anatase dissipates more energy when exposed to UV−vis radiation, while rutile dissipates less. The distinctive behavior of the energy transfer during UV− vis irradiation in the cases of rutile and anatase should be correlated with variations in surface properties taking place at the tip sample junction. It is worth noting that the access to energy dissipation is calculated on a cycle average basis, with limited information on the nature and the mechanism of energy transfer. Nevertheless, it is expected that possible mechanisms can be related to the activation of kinetic processes that show a dependence on the tip velocity and hence momentum, such as local rearrangement and displacement of atoms, atomic reorientation, and relative motion between atoms.50,51 On the other hand, energy transfer can also be related to permanent modification of atomic configuration by intermolecular forces and dipolar interactions.52 Energy can be even transferred by the creation and destruction of chemical bonds;53 a variety of these interactions can be expected to take place on the surface of TiO2. It is worth mentioning that the variation in the dissipated energy should correspond to an alteration of either the tip or the sample; since the tip was proven to be inert to the presence of UV−vis light (an exact set of experiments are conducted on freshly cleaved mica samples where no phase angle shift or variations in the energy dissipation are observed before and after irradiation), TiO2 accounts for the entire change in the surface response. As reported in the Introduction, photoactivation may increase surface hydrophilicity either by increasing the number of surface hydroxyl groups, or by degrading possible hydrophobic organic contaminants. The latter hypothesis is discarded when tests performed are repeated on freshly irradiated surfaces, after switching UV light off and allowing only a few minutes for the system to equilibrate: the material shows the same behavior observed in the first tests performed on the nonirradiated sample, although no contamination could have occurred in the closed AFM chamber during the short time between irradiated and nonirradiated tests. Thus, if a contamination layer was actually

Figure 5. DRIFTS FTIR spectra of rutile nanopowders in the presence of 68% RH, recorded in nonirradiated conditions and after 1 and 3 h of UV−vis irradiation.

Figure 6. DRIFTS FTIR spectra of anatase nanopowders in the presence of 68% RH, recorded in nonirradiated conditions and after 1 and 3 h of UV−vis irradiation.

peaks of different species present on the TiO2 nanopowders surfaces (rutile and anatase, respectively). It can be seen that the two allotropes surfaces are very different in terms of adsorbed species, with rutile showing a very weak signal of IR absorption at about 3300−3400 cm−1representing the stretching modes of OH groups in molecular water and dissociated waterand they do not change with irradiation time (Figure 5). On the other hand, the signal obtained from anatase is much higher in intensity, with several absorption peaks due to molecular water (at 1650 cm−1 and at 3300−3400 cm−1) and dissociated water hydroxyl groups (at 3300−3400 cm−1 and at 3700 cm−1), see Figure 6. More importantly, 14515

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Figure 7. Conservative force and phase difference profiles for anatase (left) and rutile (right) nanopowders in nonirradiated conditions (line) and under UV−vis irradiation (circles). Light gray small dots represent experimental data.

signals to indicate the presence of distant dependent dissipative phenomena. Nevertheless, in this work we focus on the conservative part of the force profile as a method of capturing and understanding surface alterations taking place when anatase and rutile are exposed to UV−vis radiation. Amplitude vs distance curves are recorded in the repulsive regime (tip experiencing mechanical contact with the sample) at different domains on the sample. The repulsive amplitude curves must demonstrate smooth transition to the repulsive regime to avoid issues with tip stability and discontinuities in amplitude curves. This is normally achieved by using free oscillation amplitudes significantly higher than the critical amplitude.44,62 On the other hand, in order to preserve tip geometry in such a hard tapping condition, the amplitude curve is stopped when the drop in amplitude is about 0.5 nm. It is interesting to realize that such minute reduction (on the order of 1%) in the amplitude is sufficient to reproduce the attractive and repulsive sections in the force curve.62 In addition to the force profile, the phase angle difference between conservative and measured values is utilized as an indication of energy transfer from tip to sample.44 Figure 7 shows the conservative force profile for anatase and rutile with and without UV−vis exposure. Concerning anatase, it is interesting to find that the force of adhesion increases to almost double its value with exposure to UV−vis (2.02 to 3.75 nN). On the other hand, phase angle difference monotonically increases when approaching the surface. It is seen that the phase angle difference increases when exposing the surface to UV−vis indicating larger deviation from the conservative phase angle (higher energy dissipation). Likewise, rutile demonstrates similar behavior in terms of the magnitude of force of adhesion where the force increases from 1.45 to 2.33 nN when the sample is exposed to UV−vis radiation. However, the phase difference signal demonstrates a distinct behavior with UV−vis exposure. The phase angle difference exhibits a clear step-like jump that is an indication of a distance dependent force field: in such force fields, a sudden phenomenon takes place accompanied by a sharp variation in force magnitude that could be related to the presence of capillary neck or bond formation.44 However, in light of the FTIR results, it is believed that bond-like formation is responsible for this behavior for the following reasons: for a capillary neck to form, a relatively thick water (2−3 nm) layer is expected to be present on the sample surface, and possibly on the AFM tip; furthermore, a typical signature of the presence of a water layer is a force independent region that is commonly expected to prevail after the sudden drop in force magnitude.44 In the case of rutile, FTIR data show

anatase shows a small absorption band height decrease at 3300−3400 cm−1 during UV−vis irradiation. It is observed that with longer UV−vis irradiation times, the absorption band value decreases, thus indicating slightly less molecularly bound water and dissociated water species on the anatase surface with time. The absorption bands at ca. 1650 and 3700 cm−1 show practically no variation for anatase within an experimental error, although the baseline does shift upward upon irradiation for anatase at practically all wavenumbers from 1300 to 2800 cm−1, the reasons for these phenomena are beyond the scope of this paper. Such a behavior is almost missing in rutile, again pointing out its low affinity to water resulting from its more thermodynamically stable crystal lattice as compared to anatase. Results obtained from DRIFTS prove the significantly different surface chemistry for both the studied phases of TiO2 and give an idea of the behavior of each phase when irradiated with UV−vis light. This also has direct implications on surface forces sensed by a nanoscopic particle (in this case a nanosized AFM tip) interacting with the surface. In fact, such force fields can be constructed by monitoring evolution of tip dynamics when it approaches and retracts from the sample surface. Forces at the tip−sample junction can be reconstructed in amplitude modulation AM-AFM employing different algorithms that vary in complexity and accuracy,43,55−58 however, the formalism of Katan et al.43 stands out as a robust and relatively easy approach for force reconstruction. The formalism adapts the force reconstruction in frequency modulation FM-AFM by Sader and Jarvis,59,60 by properly accounting for the frequency shift in AM-AFM. Here, the formalism is referred to as Katan−Sader−Jarvis method. The formalism is founded on several assumptions, namely the AFM cantilever is approximated as a simple harmonic oscillator, where the cantilever motion is described by a single frequency, i.e., deflection and higher harmonic contents are assumed to be negligible. Such approximations are acceptable in our experiments as we are using a stiff cantilever (k ≈ 40 N/m) with a high quality factor (Q ≈ 500). The force profile obtained resembles the conservative forces at the tip−sample junction. On the other hand, the formalism accounts for the dissipative nature of the force profile with an effective viscous damping parameter (velocity dependent energy dissipation).59,60 Hence, other forms of energy dissipation (short- and long-range hysteretic damping) pose challenges on their accurate description using the Katan−Sader−Jarvis method.61 Santos et al.44,51,61 discussed the limitations of such approach and further proposed possible solutions to mitigate its limitations; for example, employing the phase angle and energy dissipation 14516

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practically no evidence of the presence of water on the surface. Moreover, the repulsive component of the force field follows directly after the drop in force magnitude, with no force independent domain. On the other hand, bond-like formation is a plausible hypothesis especially when the surface is activated with UV−vis radiation. In fact, the presence of such sudden increase in the adhesive force associated with bond formation would imply the presence of long-range hysteretic behavior where the bond is expected to rupture at a larger separation from the surface. Unfortunately, such distance dependent longrange hysteretic behavior cannot be observed with the Katan− Sader−Jarvis method due to the inherent assumptions of the formalism.43,44,61 Having this new mechanism of energy transfer between tip and sample might be the reason for the drop in energy dissipation noticed in the beginning of the Results and Disussion section: with the presence of a new hysteretic mechanism, the contribution of other dissipation mechanisms is altered resulting in a net reduction in energy transfer. Photoactivation of TiO2 with UV−vis radiation is an important phenomenon that deserves great attention and should be studied at different levels of length scales and complexity. The analysis of AFM for the two different phases of TiO2, anatase and rutile, captures the characteristics of each surface when UV−vis irradiation takes place. This can be observed in the phase angle scans, energy dissipation analysis, and force fields reconstruction. It is interesting to see that every phase of this material has its own footprint and distinct behavior even at the nanoscale. Considering the tip a nanosized particle interacting with the surface, the results of this study can prove valuable in predicting the performance of these surfaces in real environments where dust and particulates are expected to accumulate. Furthermore, the local analysis and measurement of properties should give an insight to design better functioning surfaces of TiO2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39-0223993137. Fax: +39-02-23993180. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Sergio Santos for his valuable comments and advice. REFERENCES

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4. CONCLUSIONS In this work we presented a multitechnique approach to point out variations of surface properties of titanium dioxide allotropes exposed to UV−vis light, involving advanced force spectroscopy analyses performed by AFM and independent DRIFTS measurements. In spite of the challenges related to nanopowder analysis, AFM characterization proved a valuable tool to detect surface alterations on two different spatial resolutions, providing both a localized response and a means to evaluate surface homogeneity. Additionally, DRIFTS analyses offered a reliable support to AFM findings, confirming the different hydrophilic behavior of anatase and rutile allotropes. Their distinct response to UV−vis irradiation further underlined the higher affinity to water exhibited by anatase, resulting from a less thermodynamically stable crystal lattice, which is responsible for its higher hydroxylation degree and consequent better photoinduced performances. Finally, in the debate on the source of photoinduced superhydrophilicitybe it an effect of altered water−TiO2 surface interactions or the photodegradation of surface contaminantsour findings disagree with the latter mechanism proposed, on account of the loss of photoinduced behavior after the interruption of UV−vis irradiation and in the absence of further contamination. 14517

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dx.doi.org/10.1021/la4034723 | Langmuir 2013, 29, 14512−14518