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Free standing membranes to study the optical properties of anodic TiO2 nanotube layers

In the present work we investigate various optical properties (such as light absorption and reflectance) of anodic TiO2 nanotubes layers directly transferred as self-standing membranes onto quartz substrates. This allows investigation in a transmission geometry which provides significantly more reliable data than measurements on the metallic Ti substrate. Light transmission and reflectance measurements were carried out for layers of thicknesses varying from 1.8 to 50 micrometer, and the layers were investigated in their amorphous and crystalline form. A series of wavelength-dependent light attenuation coefficients are extrapolated and found to match the photocurrent vs. irradiation wavelength behavior. However, a feature specific to anodic nanotubes is that their intrinsic carbon content causes a sub-bandgap response that is proportional to the carbon contamination content in the TiO2 nanotubes. Overall the extracted data provide valuable basis and understanding for the design of photo-electrochemical devices based on TiO2 nanotubes.


Introduction
Among the different metal oxide semiconductors, titanium dioxide (TiO 2 ) is by far the most widely investigated material, owing to its low cost, large availability, high (photo-)chemical stability, and, most importantly, to the proper position of valence and conduction bands that make TiO 2 a most promising candidate for several catalytic and photo-electrochemical applications. [1][2][3] TiO 2 , having in its anatase and rutile forms bandgap of ~ 3.2 and 3.0 eV, [4] respectively, is widely explored as a photocatalyst for water splitting, since valence band holes and conduction band electrons generated upon super-bandgap irradiation are able to oxidize and reduce water into O 2 and H 2 , respectively. [5] In addition, owing to its relatively high electron mobility, [6][7][8] TiO 2 scaffolds have found wide application as anodes for photo-electrochemical (PEC) water splitting, [5,9,10] and for dye-sensitized solar cells (DSSCs). [11,12] Owing to the extended use in such light driven applications and devices, large efforts have been given to study TiO 2 optical properties. On the one hand, TiO 2 is explored as a solid state light absorber, and suitable analytical tools for this purpose are measurements of absorbance, transmittance and reflectance. [13][14][15][16][17][18] On the other hand, since as anticipated TiO 2 is often used as an illuminated anode immersed into a suitable electrolyte, photo-electrochemical techniques are employed to determine parameters such as the semiconductor bandgap, the light absorption threshold, and the wavelength dependence of the generated photocurrent.
In this work we investigate by different techniques the optical properties of anodic TiO 2 nanotube layers, the growth of which is based on the simple anodization of a piece of Ti metal under self-organizing electrochemical conditions. [19,20] This fabrication approach has gained large attention in the last decade as it is a straightforward and versatile nanostructuring technique that can lead to several controlled geometries of self-organized structures by adjusting the electrochemical conditions, and it allows for the growth of oxide films that are back-contacted, 3 and therefore ready to be used, e.g., as an electrode in photo-electrochemical applications. In addition, the obtained TiO 2 architectures show a series of functional features that are, namely, the large specific surface area ascribed to the nanostructured morphology, [19] their onedimensionality that leads to preferential percolation pathways for photogenerated electrons, the orthogonal separation of charge carriers and the reduced hole diffusion pathway towards the TiO 2 surface. [21][22][23] Noteworthy, while a number of measurements on some optical properties of TiO 2 nanotube layers have been reported in the literature, the possibly most important geometry, i.e., the measurement of light transmittance is so far unexplored. The reason for this is that TiO 2 nanotube layers are typically grown on Ti metal foils and therefore cannot be used for light transmission measurements.
In this work we illustrate how by an adequate anodic growth sequence followed by a controlled chemical etching, TiO 2 nanotube layers can be detached from the Ti metal substrate and transferred as self-standing membranes onto quartz slides (without the use of an adhesive layer). We show that the lift-off and transfer strategy can be used to form tube membranes as thin as of ~ 1.8 µm. A further key of this approach is that we avoid the use of intermediate layer deposited between the tube arrays and the quartz slide, which would interfere with the optical measurements. Note that in all prior work using membranes, e.g., for solar cells, films of TiO 2 nanoparticles are typically deposited onto the quartz to improve the membrane adhesion and its mechanical stability. [24][25][26] We report for our membranes a complete set of light transmission and reflectance data, in relation to the irradiation wavelength, the TiO 2 nanotube layer thickness and their amorphous or crystalline nature. Particularly, we extract from the transmission results a series of wavelengthdependent light attenuation coefficients, and show that these can be used as a constant specific of 4 anatase-TiO 2 -tube films that describes how monochromatic light is attenuated when passing through the nanotube layer. We explore also different annealing conditions for the tube layers and show that their light absorption and photo-electrochemical properties in the visible range are related to the carbon content in the anodic films.

Experimental
Prior to anodization, Ti sheets with a thickness of 0.125 mm (99.6 % purity, Advent Materials, UK) were cleaned in acetone, ethanol, and distilled water (15 min for each treatment) in an ultrasonic bath, and then were dried in a N 2 stream.
The TiO 2 nanotube layers were grown by electrochemical anodization in a glycol ethylenebased electrolyte containing 0.15 M NH 4 F and 3 vol% H 2 O (all reagents were provided by Sigma Aldrich). The electrochemical experiments were carried out always in a two-electrode electrochemical cell, with the Ti foil and a Pt gauze being the working and the counter electrodes, respectively. The applied potential was provided by a LAB/SM 1300 DC power supply (ET System). The thickness of the nanotube layers was adjusted by the anodization time. After the anodization, the nanotube films were rinsed with ethanol and dried in a N 2 stream. The lift-off procedure for the nanotube membranes, their transfer onto optically transparent quartz slides and their annealing are described in the Results and Discussion Section (and are also illustrated in Scheme 1). The membranes were exposed to thermal treatment using a Rapid Thermal Annealer (Jipelec Jetfirst 100 RTA), with a heating and cooling rate of 30°C min -1 . The quartz slides were 2.5 cm x 1.5 cm x 0.1 cm and were provided by GVB GmbH.
For morphological characterization, a field-emission scanning electron microscope (Hitachi FE-SEM S4800) was used. The cross-sectional images were obtained by cracking the TiO 2 nanotube layers. X-ray diffraction analysis (XRD, X'pert Philips MPD with a Panalytical 5 X'celerator detector) using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å) was used for determining the crystallographic composition of the samples. Energy-dispersive Xray spectroscopy (EDAX Genesis, fitted to SEM chamber) was also used for the chemical analysis of the anodic layers.
where ℎ is the measured photocurrent density (mA cm −2 ), λ is the incident light wavelength (nm), and ℎ is the intensity of the light source at a specific wavelength (mW cm −2 ).
The nanotube layers used for the photo-electrochemical measurements were grown from Ti foils (up to a thickness of ~ 10 µm) under the same experimental conditions adopted for fabricating the membranes. These nanotube layers were investigated both as amorphous and 6 crystalline. In the case of crystalline films, the nanotube layers were converted to anatse phase by annealing at 500°C (in the RTA), in air, for 1 or 4 h. The compact anodic films were produced in aqueous electrolytes (1 M H 2 SO 4 ), at 20 V for 20 min, and were annealed under the same conditions used for the nanotube layers.

Results and Discussion
Scheme 1 illustrates the procedure we used to produce the TiO 2 nanotube membranes studied in this work. To fabricate membranes of highly ordered nanotubes, we employed a doubleanodization approach that previously was explored particularly for the growth of anodic alumina nanopores layers. [27] For this we perform a first anodization of the Ti foil at 60 V for 20 min to grow a ca. 10 µm-thick tube layer that after anodization is removed by sonication. The removal of such a tube layer leaves on the surface of the Ti metal substrate an array of dimples, i.e., a replica of the tube bottom morphology.
The Ti surface is then anodized for different times, from 4 min up to 14 h, to grow nanotube layers with a thickness of ca. 1.8-50 µm. During this anodization step the dimples on the Ti surface act as preferential initiation site for the anodic growth, leading to highly-ordered and aligned structures. [28] Subsequently the sample is exposed to an annealing in air at low temperature (250°C) to dehydrate and improve the chemical stability of the tube layer. Then the sample is anodized once more to grow a tube layer underneath the annealed layers. After anodization the samples are treated in a 30% H 2 O 2 aqueous solution at room temperature for ca. 3 h. Owing to the higher chemical stability of the upper anodic layer, this treatment leads to the chemical dissolution of the lower tube layer, so that the upper one is detached from the substrate as entire self-standing TiO 2 nanotube membrane. [12,29,30] 7 After detachment, the tube membranes of different thickness are transferred onto quartz slides in a tube-top-down configuration, covered with a porous ceramic block and left overnight in air at room temperature for drying. When the porous block is removed, the membranes stick well to the quartz glasses and have good mechanical stability even after annealing.
The TiO 2 nanotube membranes were explored in view of their optical properties both in amorphous (as-formed) and crystalline state. The crystallization was generally carried out by exposing the tube membrane deposited on quartz to a thermal treatment in air, at 500°C for 1h. In some cases, the thermal treatment implied also a second annealing under the same conditions for another 3 h. In any case, another quartz slide was placed on the membranes before annealing (i.e., the membrane was sandwiched between two quartz slides) to avoid cracking and curling effects. is chemically etched, resulting in a flow-through tube morphology. [31] Moreover, from the crosssectional view ( Fig. 1 (b,d,f)) one can clearly see that in general the anodic layers grow as arrays of highly aligned and ordered nanotubular structures, and their thickness is precisely adjusted (by the anodization time) over a quite large thickness range.
As reported in the literature, as-formed anodic TiO 2 nanotube layers are amorphous, and this is confirmed by the XRD data in Fig. 2 (a). XRD also showed their crystallization to anatase TiO 2 by a thermal treatment in air at 500°Cthe absence of the Ti foil underneath the tube layers 8 avoid the formation of Ti oxide from the Ti metal. [32][33][34] It is also evident that the 4 h-long annealing, compared to a thermal treatment of 1 h, leads to tube membranes with higher degree of crystallinity as the intensity of the main anatase peak (at ca. 25.3°) increases with the annealing time. The XRD data in Fig. 2 (b) show that the intensity of the anatase signal increases with the thickness of the tube membranes (annealed in air, 500°C), which is ascribed to the larger content of crystalline oxide in thicker tube layers. However, as expected for sub-bandgap illumination of both amorphous and crystalline layers, the light transmission is much higher than that measured in the UV range. In addition, the intensity of transmitted light for thick films (even up to 50 µm) significantly increases upon their conversion into anatase TiO 2 , and the longer the irradiation wavelength the larger the relative increase of I/I 0 .
Overall these results confirm that upon crystallization into anatase TiO 2 , the membranes show a low transmission of UV light compared to visible light, in line with the bandgap of anatase The data compiled in Fig. 3 can also be used to estimate a wavelength dependent light absorption coefficient for the TiO 2 nanotube layers. The light absorption of a solid state system (i.e., the tube layers) can be described, as for a light absorber in a liquid solution, using the Lambert-Beer law that can be expressed as in equation (1): Where A is the light absorption at a specific wavelength (λ), 2 , is the wavelengthdependent light attenuation (absorption) coefficient (typical constant of the light absorbing medium), and t is the thickness of the tube membraneas the light attenuation increases with the optical path length through the light absorber it is reasonable to assume that the thicker the tube layer the larger the light absorption. [35,36] Noteworthy, the light absorption coefficient 2 , in equation (1) not only relates to a certain irradiation wavelength, but also depends on the optical density of the light absorber, i.e., the effective optical density of the TiO 2 nanotube layers, that is assumed to be constant (as ascribed to the uniform tube morphology). The Lambert-Beer law also allows for relating the light absorption (A, i.e., Absorbance) to the light transmittance (T, i.e., Transmittance) as described by equation (2) By rearranging equation (4), equation (5) can be obtained: Therefore, according to equation (5), a plot of = − log 10 0 vs. the thickness of the tube membranes is ideally a linear function (as explained below, for A < 2.5) from the slope of which the wavelength-dependent light absorption coefficient of TiO 2 nanotube layers (i.e., 2 , ) can be extrapolated.
The light transmission data, i.e., − log 10 0 , for anatase TiO 2 nanotube membranes are plotted for the different irradiation wavelengths in Fig. 4 (a) as described in equation (5), that is, vs. the layer thickness (t). Data points that lead to A values larger than 2.5 are not taken into account (for the sake of avoiding instrumental deviation from the Lambert-Beer law). The reason is that for A > 2.5, (%) = 0 100% < ~ 0.3%, that is, for a calculated absorbance of 2.5 the measured intensity of transmitted light is less than 0.3% the full intensity emitted by the laser, which is in the range of instrumental error of the power meter. Nevertheless the as-formed layers show a clearly low light transmission under both UV and visible light irradiation and, even more, one can see from the optical picture in Scheme 1 that these membranes are colored, i.e., the films interfere with visible light, and the thicker the membrane the darker is its color. Hence, a most likely explanation for this is that the relatively high 2 , values along with the color of the amorphous layers are ascribed to the intrinsic carbon content of anodic nanotubes (i.e., carbon species up take of the oxide layers during the anodization in the organic-based electrolyte [20,37] this aspect is described below with more details).
On the contrary, the light transmission data of crystalline tube membranes lead to A values almost always < 2.5 and this allowed for a reliable evaluation of 2 , . According to the model and assumptions described above, the linear fitting of the data in Fig. 4 (a)  13 Interestingly, the wavelength dependence of the calculated 2 , values (i.e., the trend of 2 , vs. the irradiation wavelength) clearly matches the experimental IPCE spectrum of the anatase TiO 2 nanotube layer. This indicates that the model in equation (5) holds in the case of anatase nanotube layers, and the evident match in Fig. 4 (b) between the calculated 2 , values and the experimental photocurrent spectrum confirms the validity of the assumption that the measured photocurrent is directly proportional to the light absorption coefficient of anatase TiO 2 nanotube layers, as expressed in equation (6) ℎ, =∝ 2 , where ℎ, is the measured photocurrent at a specific wavelength. [38][39][40] Also, the trend of 2 , vs. the irradiation wavelength allows for a qualitative evaluation of the optical bandgap of anatase TiO 2 nanotubes, that matches the photo-electrochemically measured bandgap.
From a practical point of view, the extrapolated 2 , values can be used, as well as the extinction coefficient of a light absorber in a liquid phase, as a wavelength dependent light attenuation coefficient, that is, as a constant parameter that in this case is specific for anatase TiO 2 nanotube layers and describe how the monochromatic incident light is attenuated (absorbed) when it passes through the semiconductor.
Since the anatase TiO 2 membranes are arrays of bottom-opened nanotubes having almost ideally a cylindrical shape (with outer and inner diameters of ~110 and 60 nm, respectively), one can estimate the porosity of the membranes as of ~30%, that is, the TiO 2 anatase phase (i.e., the light absorbing element) occupies 70% of the total geometrical volume. Therefore, to take into account the membrane porosity, the 2 , values measured at 265 and 325 nm (being 0.39 and 0.43 µm -1 , respectively) can be corrected (divided) by a factor 0.7, so to obtain attenuation coefficients that virtually represent how light is absorbed by anatase TiO 2 in the form of a 14 compact film (i.e., porosity ~0%). Upon applying the correction factor, the 2 , values become 0.56 and 0.61 µm -1 (at 265 and 325 nm, respectively)note that these values are higher than those reported above for the tube membranes, and this is in line with expectation that a denser TiO 2 anatase film, owing to a higher optical density, absorbs more light than a porous layer of the same thickness.
The optical properties of the nanotube membranes deposited on quartz substrates were further explored in the visible range by means of diffuse reflectance spectroscopy (Fig. 5). The data in Fig. 5 (a,b) show a clear dependence of the reflectance on the membrane thickness, that is, an evident drop of the reflectance can be seen with increasing the thickness of the nanotube layers, this regardless of their amorphous or crystalline nature.
Besides, an even more relevant aspect is that a dramatic increase of reflectance can be seen after annealing the membranes to anatase TiO 2 . Particularly, thin nanotube membranes (< 10 µm) converted to anatase show in the entire visible range a reflectance of ~ 100% (this is also visible to the naked eye as these layers becomes transparent after annealing). Thicker layers show also an increase of reflectance upon annealing, although their reflectance never reaches 100% in the whole visible range, e.g., for 10-28 µm-thick layers, the reflectance at 450 nm is of ~ 40-80% (noteworthy, these membranes do not turn transparent upon annealing). A possible explanation for these results is that thick membranes show more pronounced light scattering phenomena compared to their thin counterparts (i.e., the thicker the membrane the larger the light scattering).
The increase of reflectance upon air annealing is evident from the data in 16 These results overall demonstrate that a relatively short (e.g., 1 h) annealing step in air is on the one hand long enough to convert the amorphous nanotubes into crystalline anatase phase, and on the other hand is too short to establish complete removal of carbon from the tubes.
Nevertheless, carbon clearly acts as a visible light sensitizer, that is, either by inducing TiO 2 doping or by generating electronic surface states in the bandgap of TiO 2 , [20] it leads to an evident visible light photo-activity, which is in line with well-established findings on C-doping in TiO 2 . [41,42] Accordingly, a longer thermal treatment in air leads to a decrease of the carbon content and, as a direct consequence, to a decrease of the visible light photocurrent.
These results, along with reflectance and light transmission results, provide evidence that the high light absorption of anatase tube layers in the visible range can be attributed to the presence of carbon remnants from the electrolyte. Please note that the photocurrent measured for anodic layers grown in non-organic electrolytes (i.e., aqueous H 2 SO 4 ) was found to be independent of the annealing time ( Fig. 7 (d)), and the reason for this is more likely that the carbon in films grown in water-based electrolyte, which is typically related only to carbon adsorption at the oxide surface ascribed to air-contamination, does not lead to visible light sensitization effects (from EDX analysis the carbon content of as-formed compact TiO 2 layer is of ~ 4.5 at%).

Conclusions
In this work we illustrate an optimized anodic growth-chemical etching sequence for fabricating self-standing TiO 2 nanotube membranes, having a thickness of 1.8 up to 50 µm. The transfer of these membranes onto optically transparent substrates is essential as it allowed for their characterization not only by classical reflectance measurements, but also by light transmission measurements, that to our knowledge are unprecedented for the case of anodically grown TiO 2 nanotube layers. For anatase TiO 2 nanotube films we extracted from the results of light transmission measurements the light attenuation coefficient, and showed that its dependence on the irradiation wavelength is in line with anatase TiO 2 bandgap and nanotube photocurrent spectra. We showed also that the photocurrent measured for TiO 2 nanotubes under sub-bandgap irradiation is ascribed to the carbon present in the anodic films, the content of which can be adjusted by the annealing conditions.