Sylwia Mozia
email: sylwiam@ps.pl

Ewa Borowiak-Paleń

Jacek Przepiórski

Barbara Grzmil

Tomoki Tsumura

Masahiro Toyoda

Joanna Grzechulska-Damszel

Antoni W Morawski

Physico-chemical properties and possible photocatalytic applications of titanate nanotubes synthesized via hydrothermal method

Keywords: A. nanostructures, B. chemical synthesis, C. Raman spectroscopy, D. thermogravimetric analysis (TGA)

Titanate nanotubes (TNTs) were prepared from TiO 2 P25 via hydrothermal method.

The reaction temperature was 130 or 140ºC and the reaction time was 24 or 48h. The samples were characterized by transmission electron microscopy, X-ray diffraction, thermogravimetry and N 2 adsorption as well as Raman, FTIR-DRS and UV-Vis/DR spectroscopy. The obtained samples exhibited similar properties, regardless of the preparation temperature and time. The most notable difference between properties of TNTs prepared under different conditions was observed in case of BET surface area, which was increasing from 386 m 2 /g to 478 m 2 /g along with increasing the reaction time and temperature. Based on TEM, XRD and TG measurements we have suggested that the structure of TNTs was H 2 Ti 2 O 4 (OH) 2 . The TEM and Raman spectroscopy measurements showed that the obtained products contained also low amount of anatase phase. The TNTs exhibited no photoactivity towards degradation of model A c c e p t e d m a n u s c r i p t azo dye Acid Red 18. However, TNTs were successfully applied for photocatalytic generation of CH 4 and H 2 in a solution of acetic acid. The amount of methane produced with application

of TNTs synthesized at 140°C was about 2.5 times higher than that generated with use of TiO 2 P25. To the best of our knowledge this is a first report on the photocatalytic generation of hydrocarbons using TNTs in the current state of the art.

Introduction

Since Kasuga et al. [START_REF] Kasuga | Formation of Titanium Oxide Nanotube[END_REF] described a novel hydrothermal method for titanate nanotubes (TNTs) formation, the investigations on preparation and properties of these species have attracted attention of numerous researchers. More than a decade later, the structure of TNTs as well as their properties are still a problem under discussion. Although the hydrothermal method of synthesizing of TNTs seems to be quite simple, the products obtained in different laboratories differ significantly from one to another. Due to the fact that different TNTs precursors (amorphous TiO 2 , anatase, rutile, mixture of both phases, such as P25, etc.) and reaction conditions were applied, the obtained nanotubes exhibited different surface areas, pore volumes, sodium content, etc. The main inconsistencies, however, concern crystal structure of nanotubes and their photocatalytic activity. The present debate over the crystal structure of titanate nanotubes, as summarized by Ou et al [START_REF] Ou | Review of titania nanotubes synthesized via the hydrothermal treatment: Fabrication, modification, and application[END_REF], is among the following: anatase TiO 2 , lepidocrocite H x Ti [START_REF] Nian | Hydrothermal synthesis of single-crystalline anatase TiO 2 nanorods with nanotubes as the precursor[END_REF][START_REF] Zhang | The evolution of morphology and crystal form of titanate nanotubes under calcination and its mechanism[END_REF][5][START_REF] Zhang | Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H 2 Ti 2 O 4 (OH) 2[END_REF] and Na x H 2-

x Ti 3 O 7 [START_REF] Morgado | Effects of thermal treatment of nanostructured trititanates on their crystallographic and textural properties[END_REF][START_REF] Morgado | A study on the structure and thermal stability of titanate nanotubes as a function of sodium content[END_REF][START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF] seem to be the most probable. Recently, Kochkar et al. [START_REF] Kochkar | Optimization of the alkaline hydrothermal route to titanate nanotubes by a Doehlert matrix experience design[END_REF] presented the results of the investigations on the effect of different preparation conditions on TNTs structure. They reported that, dependent on the reaction time and temperature, as well as NaOH concentration, different structures, such as amorphous titania, anatase TiO 2 , bititanates (H 2 Ti 2 O 5 •H 2 O) and trititanates (Na 2 Ti 3 O 7 ) could be obtained. Amorphous phase was also observed in case of samples treated with highly concentrated (20M) NaOH [START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF].

The photocatalytic activity of TNTs has been another unanswered question. In the "Introduction" section of numerous papers [START_REF] Gao | Preparation and characterization of titania nanotubes with mesostructured walls[END_REF][START_REF] Miyauchi | Low-reflective and super-hydrophilic properties of titanate or titania nanotube thin films via layer-by-layer assembly[END_REF][START_REF] Kasuga | Formation of titanium oxide nanotubes using chemical treatments and their characteristic properties[END_REF][14][START_REF] Bavykin | Protonated titanates and TiO 2 nanostructured materials: synthesis, properties and applications[END_REF] it can be found that titanate nanotubes have potential applications in photocatalysis. However, the data on this subject which are presented in literature are rather limited and inconsistent. Yu et al [START_REF] Yu | Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes[END_REF] investigated photocatalytic oxidation of acetone in air in the presence of TNTs synthesized hydrothermally at 150°C for 48h. The authors found that prior to calcination, the nanotubes showed no photocatalytic activity. Similar observations were published by Qamar et al. in [START_REF] Qamar | Preparation and photocatalytic activity of nanotubes obtained from titanium dioxide[END_REF]. The researchers reported that the as-prepared nanotubes, with or without sodium contents, showed no photocatalytic activities towards amaranth degradation. Baiju et al. [START_REF] Baiju | Hydrothermal processing of dye-adsorbing one-dimensional hydrogen titanate[END_REF] presented results on the photocatalytic degradation on methylene blue (MB) on a mixed nanobelts-nanotubes titanate obtained via hydrothermal method at 150°C for 30h. The authors found that the sample exhibited very high adsorptivity towards MB. However, the photocatalytic activity was very low. Tachikawa et al. [START_REF] Tachikawa | Photoinduced charge separation in titania nanotubes[END_REF] investigated the photocatalytic one-electron oxidation reaction of 4-(methylthio)phenyl methanol (MTPM) in presence of titanate nanotubes, using time-resolved diffuse reflectance spectroscopy. The efficiency of the reaction was found to be rather low, although the TNTs exhibited some photocatalytic activity. On the other hand, significant activity of H titanate nanotubes towards synthetic dye sulforhodamine degradation under UV light was reported by Zhu et al. [START_REF] Zhu | Hydrogen titanate nanofibers covered with anatase nanocrystals: A delicate structure achieved by the wet chemistry reaction of the titanate nanofibers[END_REF]. After 1h of irradiation about 15% of the dye A c c e p t e d m a n u s c r i p t was photodegraded. Nakahira et al. [START_REF] Nakahira | Synthesis of nanotube from a layered H 2 Ti 4 O 9 • H 2 O in a hydrothermal treatment using various titania sources[END_REF] reported 60 to 70% photodegradation of HCHO in the presence of TNTs. According to Xiao et al. [START_REF] Xiao | Synthesis and characterization of WO 3 /titanate nanotubes nanocomposite with enhanced photocatalytic properties[END_REF], titanate nanotubes synthesized hydrothermally at 130°C for 24h revealed photocatalytic activity toward Rhodamine B (RhB) degradation with efficiency similar to that of P25. Photocatalytic activity of as-prepared TNTs towards methylene blue (MB) degradation and oxidation of NO to NO 2 was reported by Inagaki et al. [START_REF] Inagaki | Structure and photoactivity of titania derived from nanotubes and nanofibers[END_REF]. Similarly, Geng et al. [START_REF] Geng | Nitrogen-doped TiO 2 nanotubes with enhanced photocatalytic activity synthesized by a facile wet chemistry method[END_REF] observed photodegradation of MB in the presence of TNTs. After 7h of irradiation about 25% of MB was degraded (vs. ca. 5% during photolysis), what was accounted for the photosensitized capability of MB molecules. Recently, Costa et al. [START_REF] Costa | TiO 2 nanotubes as recyclable catalyst for efficient photocatalytic degradation of indigo carmine dye[END_REF] reported a considerable photocatalytic activity of titanate nanotubes towards degradation of indigo carmine dye, although it was lower than that of anatase TiO 2 . In the presence of TiO 2 the dye solution was completely decolorized during 60 min of the reaction, whereas the time of decolorization in the presence of the nanotubes was almost twice longer (110 min). The authors found that a great advantage of TNTs in comparison with traditional TiO 2 catalyst was that nanotubes were easily recovered from the solution. As reported by the authors, the nanotubes can be recycled and re-applied in numerous photodegradation cycles, maintaining 90% of their activity after 10 cycles of the reaction. In contrast, the precursor of TiO 2 catalyst lost its activity during the second catalytic cycle [START_REF] Costa | TiO 2 nanotubes as recyclable catalyst for efficient photocatalytic degradation of indigo carmine dye[END_REF].

Taking into consideration that in a majority of reports already published the structure of TNTs was studied on a basis of TEM and XRD measurements only, we have tried to investigate the physico-chemical properties and structure of the nanotubes in more details.

The comprehensive analysis led to the estimation of the composition of TNTs, which was deduced to be H Interestingly, we have found on a basis of the experiments performed with equal catalyst loadings that the evolution of hydrocarbons was significantly higher in the presence of TNTs than in the process in which commercial P25 was applied.

Experimental

Synthesis of TNTs

Commercial TiO 2 P25 (Evonik, former Degussa, Germany) was used as a starting material for TNTs preparation. The P25 consists of 75% of anatase and 25% of rutile. The BET surface area of P25 is ca. 50 m 2 /g and the crystallite size of anatase is 25 nm (all data according to the manufacturer). TNT were hydrothermally synthesized as follows. 1.5 g of P25 was introduced to a Teflon-lined autoclave (70 cm 3 ) containing 60 cm 3 10M NaOH. The suspension was ultrasonically treated for 1h in order to allow good dispersion of TiO 2 particles in the reaction mixture. After that the autoclave was heated statically at 130 or 140°C

for 24 or 48h (the samples are therefore denoted later as TNT130,24; TNT130,48; TNT140,24 and TNT140,48). After the heat treatment the NaOH solution was decanted and the obtained precipitate was dispersed in 0.5 dm 3 of 0.1M HCl. After 1h the precipitate was separated from the HCl solution and dispersed again in 0.5 dm 3 of fresh 0.1M HCl. After the next 2h the precipitate was washed thoroughly with deionized water (Simplicity, Millipore) until the conductivity of washing solution reached the value of 1 μS/cm or less. Eventually, the precipitate was dried in air at 80 °C for 24h.

A c c e p t e d m a n u s c r i p t 2.2. Characterization of TNTs

Transmission electron microscopy (TEM) images were recorded using FEI Tecnai F20 microscope. Samples were prepared by dispersing the powder in acetone by ultrasonic treatment, and then dropping the suspension onto a copper grid (400 mesh). X-ray diffraction (XRD) analyses were performed on a powder diffractometer Philips X'Pert PRO (CuK α , 35kV, 30 mA). Raman spectra were measured using 785 nm laser line (E laser = 1.58 eV) with a Renishaw InVia Raman microscope spectrometer. FTIR/DRS measurements were performed using Jasco FT-IR 430 (Japan) spectrometer equipped with a diffuse reflectance accessory (Harrick, USA). UV-VIS/DR spectra were recorded using Jasco V 530 spectrometer (Japan) equipped with the integrating sphere accessory for diffuse reflectance spectra. BaSO 4 was used as a reference. Thermogravimetric measurements were carried out with use of STA 449C Jupiter® thermobalance (Netzsch Company, Germany). Thermal analysis was conducted under 25 cm 3 /min air flow in 30-1000 °C temperature range and heating rate was 10 °C /min. BET surface area was determined by N 2 adsorption at 77 K (Autosorb 3, Quantachrome, USA).

Photocatalytic decolorization of Acid Red 18

The photocatalytic reaction was carried out in an open-air glass reactor containing 400 cm 3 of a model solution (30 mg/dm place. Therefore, it can be stated that the system was working in a pure photocatalytic regime.

Photocatalytic generation of methane and hydrogen

The photocatalytic reaction was conducted in a cylindrical glass reactor (Heraeus, type UV-RS-2) equipped with medium pressure mercury vapour lamp (TQ-150, λ max =365 nm).

The volume of the reactor was 765 cm 3 . In the upper part of the reactor a gas sampling port was located. At the beginning of the experiment 0.35 dm 3 of CH 3 COOH (1mol/dm 3 ) and 0.35 g of a catalyst were introduced into the reactor. After that N 2 was bubbled through the reactor for at least 3h to ensure that the dissolved oxygen was eliminated. Then, the N 2 flow was stopped and UV lamp, positioned in the center of the reactor, was turned on to start the photoreaction. The mixture in the reactor was continuously stirred during the experiment.

Gaseous products were analyzed using GC SRI 8610C equipped with TCD and HID detectors, and Shincarbon (carbon molecular sieve; 2m, 1mm, 100-120 mesh) and molecular sieve 5A (3m, 2mm, 80-100 mesh) columns. Helium was used as a carrier gas.

A c c e p t e d m a n u s c r i p t 3. Results and discussion

Physico-chemical properties of titanate nanotubes

Fig. 1 shows TEM images of TNTs prepared under different conditions. The nanotubes were scrolls with unequal number of walls on the two tube sides. The walls usually consisted of 3-4 layers. The distance between the layers, estimated from TEM measurements, was about 0.8 nm, what is in an agreement with the literature data [START_REF] Kochkar | Optimization of the alkaline hydrothermal route to titanate nanotubes by a Doehlert matrix experience design[END_REF][START_REF] Ma | Directly rolling nanosheets into nanotubes[END_REF][START_REF] Toledo-Antonio | Low-temperature FTIR study of CO adsorption on titania nanotubes[END_REF]. The outer diameter of the nanotubes was in the range of 7.5 -11.0 nm, whereas the inner diameter ranged from 4.5 to 6.5 nm. No correlation between the time and temperature of the hydrothermal treatment and the diameters of TNTs could be found. Furthermore, the very weak peak at 2θ ~ 9.5º observed in the patterns corresponds to the distance between the layers in TNTs walls.

According

A c c e p t e d m a n u s c r i p t

spacing of H 2 Ti 3 O 7 is 0.78 nm, which is the same as that they observed in the nanotubes using SAED and HRTEM. Trititanate was also proposed as a structure of TNTs by Morgado et al. [START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF]. They found that the interlayer distance varied between 0.80 and 0.90 nm, i.e. was close to the d values of 0.78 and 0.84 nm in bulk trititanates H 2 Ti 3 O 7 and Na 2 Ti 3 O 7 , respectively.

Lepidocrocite-type titanate nanotubes with general formula H x Ti 2-x/4x/4 O 4 (orthorombic, a=3.783, b=18.735, c=2.978 Å) was obtained by Ma et al. [START_REF] Ma | Nanotubes of lepidocrocite titanates[END_REF]. According to the authors the interlayer distance measured by ED was about 0.8 nm, which was lower than the d 020 (0.93 nm) in lepidocrocite-type titanates. The difference has been attributed to the decrease of interlayer distance as a direct consequence of the loss of hydrated water during microscopic observations carried out under high vacuum. Taking into account this phenomenon, Ma et al. [START_REF] Ma | Nanotubes of lepidocrocite titanates[END_REF] concluded that the lepidocrocite-type titanate is more probable structure than trititanate.

They have explained that the reported interlayer distance in H 2 Ti 3 O 7 of 0.78 nm, which was calculated on a basis of TEM and electron diffraction (ED) measurements cannot be accurate and is most likely underestimated due to the loss of hydrated water under vacuum. Similar conclusions were reported by others [START_REF] Kochkar | Optimization of the alkaline hydrothermal route to titanate nanotubes by a Doehlert matrix experience design[END_REF][START_REF] Ma | Directly rolling nanosheets into nanotubes[END_REF].

Taking into account that the interlayer spacing obtained from TEM measurements (0.8 nm, Fig. 1) is most probably underestimated, what was explained above, we suppose that the structure of the synthesized TNTs is probably H 2 Ti 2 O 4 (OH) 2 . As was mentioned earlier, the peak at 2θ ~ 9.5º (Fig. 2) representing the d spacing of the (200) planes (d 200 ) in H 2 Ti 2 O 4 (OH) 2 corresponds to the distance between adjacent layers in a multilayered nanotube (compare Fig. 1) [START_REF] Zhang | Study on ESR and interrelated properties of vacuum-dehydrated nanotubed titanic acid[END_REF]. Zhang et al. [START_REF] Zhang | Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H 2 Ti 2 O 4 (OH) 2[END_REF] reported that the intensity and position of this peak depends on the amount of inter-and intralayered -OH groups. The authors observed increased intensity of the (200) peak as a result of increasing heat treatment temperature from 100 to 500 °C. Moreover, at temperatures exceeding 300°C the position of this peak was shifted to higher 2θ angles. Considering that the obtained TNTs contained very high amount of water

A c c e p t e d m a n u s c r i p t

(17-20 wt%, see discussion on TG data), the very low intensity of the (200) peak (Fig. 2)

seems to be reasonable.

The relative ratio of 110 to 600 peaks intensities can be used as a measure of sodium content. As shown in Fig. 2 the peak 600 is much less intensive than the 110 one. This suggests replacement of most of Na + ions by H + during washing with 0.1M HCl [START_REF] Ch.-Ch | Structural features of nanotubes synthesized from NaOH treatment on TiO 2 with different post-treatments[END_REF].

As reported by Tsai et al. [START_REF] Ch.-Ch | Structural features of nanotubes synthesized from NaOH treatment on TiO 2 with different post-treatments[END_REF][START_REF] Ch.-Ch | Mesoporous nanotube aggregates obtained from hydrothermally treating TiO 2 with NaOH[END_REF], samples obtained by washing at strongly acidic pH (1.6) were composed of anatase (major one) and protonic titanate. Washing with HCl at pH 0.38 led to formation of anatase phase only. This suggests that the transformation of protonic titanate to anatase under acidic conditions took place. The authors concluded that there was a coexistence of neighboring phases in some pH regimes [START_REF] Ch.-Ch | Structural features of nanotubes synthesized from NaOH treatment on TiO 2 with different post-treatments[END_REF]. Similarly, Lee et al. [START_REF] Ch | Application of titanate nanotubes for dyes adsorptive removal from aqueous solution[END_REF] found that during treatment of TNTs with 0.1M HCl the anatase phase was formed instead of layered titanate structure. This was confirmed by XRD measurements.

In view of the above, using 0.1M HCl in our procedure should result in the presence of anatase phase in the prepared samples (Fig. 2). However, as was mentioned earlier, neither anatase nor rutile peaks were identified in the XRD patterns of TNTs. On the other hand, careful observations of the TEM images (Fig. 1), confirm, in addition the nanotubes, presence some ball-shaped structures in the material. In view of fact that no anatase or rutile were formed, it can be deduced that these structures were amorphous TiO 2 . However, according to the literature [START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF][START_REF] Kochkar | Optimization of the alkaline hydrothermal route to titanate nanotubes by a Doehlert matrix experience design[END_REF] presence of amorphous titania was observed in the case of treatment with highly concentrated NaOH and was not observed in case of 10M NaOH. The XRD analysis can not detect phases at very low concentrations. Therefore, Raman spectroscopy was used to investigate the structure the ball-shaped species. Raman spectra of P25 and titanate nanotubes prepared under different conditions are presented in Fig. 3.

The spectrum of P25 exhibits modes at 143 cm -1 (E g ), 195 cm -1 (E g ), 398 cm -1 (B 1g ),

A c c e p t e d m a n u s c r i p t

active mode at 447 cm -1 (E g ) was also detected. Compared to P25, the TNTs exhibit relative weak Raman peaks what might be attributed to a poor crystallinity. In the spectra measured for nanotubes the peak corresponding to E g mode of anatase is shifted, compared to that in the P25 spectrum, by 5 -10 cm -1 (148 -158 vs. 143 cm -1 ) for TNT130,24 -TNT140,48,

respectively. An occurrence of this peak demonstrates formation of the tetrahedron structure in the nanotubes [START_REF] Qian | Raman study of titania nanotube by soft chemical process[END_REF]. According to Qian et al. [START_REF] Qian | Raman study of titania nanotube by soft chemical process[END_REF] the blueshift might be caused by oxygen deficiency. Particularly intensive peak at 148 cm -1 observed in TNT130,24 spectra may suggest higher, compared to other samples, content of anatase. Moreover, it could be supposed that in case of TNT130,24 some amount of anatase from P25 remained unchanged and did not form the tubular structure. Thus, this peak might represent not only the newly formed anatase, but also the remains of anatase from P25. In case of the other samples the intensity of the discussed peak was significantly lower than that for TNT130,24. This suggests that presence of anatase is not associated with its residues from P25 only, but also, as was reported by Tsai et al. [START_REF] Ch.-Ch | Structural features of nanotubes synthesized from NaOH treatment on TiO 2 with different post-treatments[END_REF], that the transformation of protonic titanate to anatase under acidic conditions took place. Other peaks which might be related to anatase structure were found at 189 cm -1 and 390 cm -1 and were red shifted compared to the peaks measured in P25 sample.

A broad and very weak peak can be observed in spectra of the TNTs samples at about 650 cm - 1 . It is very difficult to assign this peak to a defined mode as one might find even two maxima in this region -one about 636 cm -1 and the second one at about 668 cm -1 . The former maximum might be attributed to E g mode of anatase, whereas the latter one has been reported to be due to Ti-O-Ti [START_REF] Bavykin | Stability of aqueous suspensions of titanate natotubes[END_REF] or Ti-O-Na [START_REF] Cortés-Jácome | In situ thermo-Raman study of titanium oxide nanotubes[END_REF] vibrations in nanotubes. Another peak which might be assigned to Ti-O-Na vibrations in TNT structure was identified by Bavykin et al. [START_REF] Bavykin | Stability of aqueous suspensions of titanate natotubes[END_REF] at 917 cm -1 . However, in the obtained Raman spectra (Fig. 3) this peak was not clearly detected.

This could be explained by the exchange of Na + to H + during the washing step [START_REF] Bavykin | Stability of aqueous suspensions of titanate natotubes[END_REF][START_REF] Cortés-Jácome | In situ thermo-Raman study of titanium oxide nanotubes[END_REF]. The peak at 270 cm -1 with a shoulder at 286 cm -1 can be attributed to TNT only, since it was not

A c c e p t e d m a n u s c r i p t

present in the P25 spectrum. This peak corresponds to Ti-O vibrating modes in titanates [START_REF] Qian | Raman study of titania nanotube by soft chemical process[END_REF][START_REF] Godbole | Synthesis of titanate nanotubes and its processing by different methods[END_REF]. In Fig. 3 a well defined peak at 450 cm -1 can be also observed. This peak could be assigned to E g mode of rutile, although it is blueshifted compared to the rutile peak present in the Raman spectrum of P25 (447 cm -1 ). Moreover, the intensity of this peak in TNTs samples is higher than in case of P25. This might lead to a conclusion that the presence of rutile was not associated with the residues of unreacted P25, but rutile was formed under the conditions applied during preparation. The presence of rutile in the TNTs prepared through sonicationhydrothermal combined method was reported by Ma et al. on a basis of the XRD patterns [START_REF] Ma | Sonication-hydrothermal combination technique for the synthesis of titanate nanotubes from commercially available precursors[END_REF].

The authors supposed that the stable rutile particles did not react quickly enough with NaOH under conditions applied. Some authors [START_REF] Toledo-Antonio | Low-temperature FTIR study of CO adsorption on titania nanotubes[END_REF] attribute the bands at 450 cm -1 to Ti-O-Ti vibrations in titanate nanotubes themselves, not in rutile. Taking into consideration that no diffraction peaks of rutile were observed in XRD patterns (Fig. 2), on the contrary to the results reported by Ma et al. [START_REF] Ma | Sonication-hydrothermal combination technique for the synthesis of titanate nanotubes from commercially available precursors[END_REF], it could be supposed that the discussed bands are associated with TNTs. However, in order to state this unequivocally, further investigations are necessary.

Summing up, study of Raman spectra of the TNTs indicate that the obtained samples were a mixture of different phases, anatase, and nanotubular titanates. The presence of rutile TiO 2 is questionable, although possible. can be assigned to molecular water bending mode [START_REF] Maira | Fourier Transform Infrared study of the performance of nanostructured TiO 2 particles for the photocatalytic oxidation of gaseous toluene[END_REF]. In the range of 3300 -3500 cm -1 the bands due to adsorbed water and hydroxyl groups can be observed in all of the spectra.

Notably, the content of water adsorbed in the titanate nanotubes was significantly higher than

A c c e p t e d m a n u s c r i p t

in P25, owing to the increase of the specific surface areas and pore volumes (compare Table 1).

High water content in the prepared samples was confirmed on a basis of TG measurements. Fig. 5a shows TG curves measured for TNTs prepared under different conditions. It can be clearly seen that more significant weight loss (ΔW) took place in case of the TNTs prepared at 130°C than in case of those prepared at 140°C. The total (i.e. in the range of 24 -1000°C) weight loss amounted to ca. 20wt% and ca. 17wt% for TNTs prepared at 130 and 140°C, respectively. It was found that the highest difference in the ΔW for TNTs prepared at different temperatures was observed below 100°C. The ΔW between room temperature and 100°C was about 8wt% and 5.5wt% for samples prepared at 130 and 140°C, respectively. Taking into consideration that the samples were synthesized during different periods of time, the observed differences might have been due to the adsorption of water on prepared materials during their storage and handling. Moreover, since this weight loss was associated with the release of water physically adsorbed on the samples and cannot be used as a measure of TNTs structure, there is no need to look more deeply into these results. In case of higher temperatures the results obtained for TNTs prepared at 130°C and 140°C were similar one to another. The ΔW in the range of 100-200 °C was ca. 8.0-8.5wt% and in the range of 200-500°C ca. 3.5wt%, regardless of the sample preparation conditions. The weight loss at the temperatures above 100°C is attributed to the dehydration of interlayered and intralayered -OH groups [START_REF] Wang | Surface-modification and characterization of H-titanate nanotube[END_REF]. No weight loss was observed at temperatures above 500°C. associated with high BET surface area (Table 1). From Fig. 5b it can be also found that on the right edge of the discussed peak a weak shoulder at about 140°C is present. This shoulder could be related to the desorption of moisture or it might suggest that at this temperature desorption of chemisorbed water started.

Some authors [START_REF] Morgado | A study on the structure and thermal stability of titanate nanotubes as a function of sodium content[END_REF][START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF][START_REF] Thorne | Formation, structure, and stability of titanate nanotubes and their proton conductivity[END_REF] have tried to calculate the formula of the TNTs from the TG measurements. Thorne et al. [START_REF] Thorne | Formation, structure, and stability of titanate nanotubes and their proton conductivity[END_REF] argued on the basis of the weight loss between 100 and 500°C that the stoichiometry of the TNTs was H 0.67 TiO In case of our samples such a calculation is rather difficult to be performed because the obtained materials were not pure TNTs. As was mentioned earlier, we have supposed that the samples contained some amount of anatase. Since exact content of this "impurity" can not be estimated, calculation of the chemical formula for the TNTs seems to be not possible.

However, because no diffraction peaks related to this phase of TiO 2 were observed in the XRD patterns, an assumption that anatase content in the prepared materials is low, seems to be reasonable. Therefore we tried to estimate a proximate formula of TNTs from TG results.

As we have discussed earlier, on a basis of the XRD measurement of TNTs (Fig. 2

A c c e p t e d m a n u s c r i p t

The reaction (1) might be attributed to the weight loss below 100°C, whereas the reaction (2) might be associated with ΔW in the range of 100-500 °C. The calculated theoretical weight loss due to the reaction (2) equals to ca. 10wt%. This value is lower than the one measured thermogravimetrically (i.e. 11.5-12wt%). The 11.5-12wt% weight loss is, however, closer to the structural water loss of 10wt% due to the thermal decomposition of H 2 Ti 2 O 5 , than to the 7wt% due to the decomposition of H 2 Ti 3 O 7 . Therefore, the structure of H 2 Ti 2 O 4 (OH) 2 seems to be more probable than the H 2 Ti 3 O 7 .

On the other hand, if we go back to the DTG curve and take into consideration the weak shoulder at about 140°C, we might assume that the weight loss due to the release of physisorbed water did not complete below 100°C, but it proceeded up to 140°C. Thus, the weight loss which is attributed to the dehydration of interlayered and intralayered OH groups might be calculated for the temperatures range of 140-500°C. The ΔW calculated in this temperature range was about The obtained data might lead to a conclusion that estimation of TNTs structure on a basis of thermogravimetric measurements cannot give reliable results because the TG and DTG curves obtained for TNTs could be interpreted in several ways. However, it must be stressed here once again that all of the presented calculations were approximates only because the samples contained some amount of anatase, what was found from the Raman spectra. The

A c c e p t e d m a n u s c r i p t

presence of anatase should affect the value of ΔW. It could be supposed that in case of the sample without this "impurity" the weight loss should be even higher than the observed one.

Fig. 6 presents the N 2 adsorption -desorption isotherms of titanate nanotubes prepared under different conditions. All samples showed the type IV isotherms with type H3 hysteresis loops according to IUPAC [START_REF]International Union of Pure and Applied Chemistry, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF] classification. The initial part of the Type IV isotherm is attributed to monolayer-multilayer adsorption. Hysteresis appearing in the multilayer range of physisorption isotherms is usually associated with capillary condensation in mesopore structures. The Type H3 loop is observed with aggregates of plate-like particles giving rise to slit-shaped pores [START_REF]International Union of Pure and Applied Chemistry, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984)[END_REF]. Taking into consideration that titanate nanotubes are multiwalled scrolls which are believed to be formed through sheet folding or wrapping mechanism [START_REF] Morgado | Characterization of nanostructured titanates obtained by alkali treatment of TiO 2anatases with distinct crystal sizes[END_REF][START_REF] Bavykin | Protonated titanates and TiO 2 nanostructured materials: synthesis, properties and applications[END_REF] it seems to be reasonable that they exhibit the H3 loop. The surface properties of TNTs determined on a basis of N 2 adsorption -desorption measurements are presented in Table 1. It can be observed that the BET specific surface area increased along with hydrothermal reaction time and temperature. The lowest BET surface area was observed for sample prepared at 130°C for 24h (386 m 2 /g), whereas the highest surface area was measured in case of TNT140,48 (478 m 2 /g). Similarly, the total pore volume was increasing from 1.31 to 1.69 cm 3 /g for TNT130,24 -TNT140,48, respectively. The obtained values are consistent with the literature data [START_REF] Ch.-Ch | Regulation of the physical characteristics of titania nanotube aggregates synthesized from hydrothermal treatment[END_REF][START_REF] Wang | Effect of annealing temperature on phase transition and optical property of titanate nanotubes prepared by ion exchange approach[END_REF]. For example, Tsai et al. [START_REF] Ch.-Ch | Regulation of the physical characteristics of titania nanotube aggregates synthesized from hydrothermal treatment[END_REF] reported that the BET surface area of TNTs prepared via hydrothermal method at 130°C for 24h was equal to 399 m 2 /g and the pore volume was 1.47 cm 3 /g. Wang et al. [START_REF] Wang | Effect of annealing temperature on phase transition and optical property of titanate nanotubes prepared by ion exchange approach[END_REF] prepared titanate nanotubes at 130°C for 20h. The BET surface area of such prepared samples was equal to 375 m 2 /g. On a basis of N 2 adsorption data the average pore diameter was also calculated (Table 1). The values of this parameter were similar for all the samples and ranged from 13.6-14.2 nm. Taking into consideration that the average inner diameter of TNTs determined from TEM measurements was in the range of 4.5-6.5 nm it is clear that the method of nitrogen adsorption

A c c e p t e d m a n u s c r i p t

overestimates the average diameter of nanotubes. According to Bavykin et al. [START_REF] Bavykin | Protonated titanates and TiO 2 nanostructured materials: synthesis, properties and applications[END_REF] the method accounts for the pores not only inside the tubes but also for the pores between the tubes.

These external pores are usually bigger than the internal pores and the size of these pores depends on the way the nanotubes have been agglomerated into the bundles. Fig. 7 presents UV-Vis diffuse reflectance spectra of titanate nanotubes prepared at different temperatures and for different heating times. For comparison, the spectrum of P25 TiO 2 is also shown. The TNTs spectra, regardless of the samples preparation conditions, exhibit a similar course and are blue shifted for about 20 nm relative to P25. This indicates the increase of the band gap energy. Wang et al. [START_REF] Wang | Effect of annealing temperature on phase transition and optical property of titanate nanotubes prepared by ion exchange approach[END_REF] attributed the shift to hydration and nanosize effect of TNTs. To determine the band gap energy (E g ) of the samples, the Kubelka-Munk method was used. The E g values were calculated from the (F(R)hν) 1/2 versus hν plots, where F(R) = (1-R)/2R [START_REF] Todorova | Preparation of fluorine-doped TiO 2 photocatalysts with controlled crystalline structure[END_REF]. The absorption edges were found to be 378 nm, which value corresponds to the band gap energy of E g = 3.28 eV. The absorption edge and E g determined for P25 were 399 nm and 3.10 eV, respectively. This value is consistent with the already published results [START_REF] Todorova | Preparation of fluorine-doped TiO 2 photocatalysts with controlled crystalline structure[END_REF]. The literature data [START_REF] Wang | Effect of annealing temperature on phase transition and optical property of titanate nanotubes prepared by ion exchange approach[END_REF]51,[START_REF] Khan | Enhanced photoresponse towards visible light in ru doped titania nanotube[END_REF] concerning band gap of titanate nanotubes are inconsistent. Some researchers found that E g of TNT was higher than that of anatase, whereas others reported inverse relation. For example, Yu et al. [51] reported that E g of TNTs prepared from rutile ranged from 3.03 to 3.15 eV, depending on the hydrothermal treatment time. Such low values of E g were associated with a high rutile content in the prepared samples. Khan et al. [START_REF] Khan | Enhanced photoresponse towards visible light in ru doped titania nanotube[END_REF] found the band gap of 3.1 eV, whereas Wang et al. [START_REF] Wang | Effect of annealing temperature on phase transition and optical property of titanate nanotubes prepared by ion exchange approach[END_REF] determined the E g = 3.6 eV. Bavykin et al. [START_REF] Bavykin | Protonated titanates and TiO 2 nanostructured materials: synthesis, properties and applications[END_REF][START_REF] Bavykin | Apparent two-dimensional behavior of TiO 2 nanotubes revealed by light absorption and luminescence[END_REF] reported E g value as high as 3.87 eV. The results obtained during our investigations (Fig. 7) show that E g of TNTs was higher than that of pure anatase (3.23 eV, data not shown), although the difference was negligible.

Summing up, it may be stated that the preparation conditions in the range of the investigations (i.e. temperature of 130-140ºC and time of 24-48h) have no significant effect

A c c e p t e d m a n u s c r i p t

on the the physico-chemical properties of TNTs. Nevertheless, notable increase in BET surface area could be found for samples prepared at higher temperatures and during longer periods of time. Minor differences in TNTs structure were confirmed by the Raman spectra.

The XRD, TG, FT-IR and UV-Vis/DR measurements suggest that all the samples exhibited similar properties.

Photocatalytic behaviour of titanate nanotubes. Decolorization of Acid Red 18

Photocatalytic behaviour of titanate nanotubes. Generation of hydrocarbons and hydrogen

To our best knowledge, no investigations on methane formation in the presence of TNTs and sacrificial agents have been reported up till now. The literature data concerning photocatalytic generation of methane from organic compounds in liquid phase are very limited. All of them concern production of CH 4 from aliphatic acids [START_REF] Kraeutler | Photoelectrosynthesis of ethane from acetate ion at an n-type TiO 2 electrode. The photo-Kolbe reaction[END_REF][START_REF] Kraeutler | Heterogenous photocatalytic synthesis of methane from acetic acid -new Kolbe reaction pathway[END_REF][START_REF] Kraeutler | Direct observation of radical intermediates in the photo-Kolbe reaction -heterogenous photocatalytic radical formation by Electron Spin Resonance[END_REF][START_REF] Sakata | Heterogenous photocatalytic reactions of organic acids in water. New reaction paths besides the photo-Kolbe reaction[END_REF] and alcohols [START_REF] Dey | Methane generated during photocatalytic redox reaction of alcohols on TiO 2 suspension in aqueous solutions[END_REF].

In 1970's Kraeutler and Bard [START_REF] Kraeutler | Photoelectrosynthesis of ethane from acetate ion at an n-type TiO 2 electrode. The photo-Kolbe reaction[END_REF][START_REF] Kraeutler | Heterogenous photocatalytic synthesis of methane from acetic acid -new Kolbe reaction pathway[END_REF][START_REF] Kraeutler | Direct observation of radical intermediates in the photo-Kolbe reaction -heterogenous photocatalytic radical formation by Electron Spin Resonance[END_REF] published a series of papers concerning photocatalytic decarboxylation of acetic acid in the presence of Pt/TiO 2 catalyst. Similarly, Sakata et al. [START_REF] Sakata | Heterogenous photocatalytic reactions of organic acids in water. New reaction paths besides the photo-Kolbe reaction[END_REF] observed formation of methane and ethane during photodecomposition of acetic and propionic acids in the presence of TiO 2 and Pt/TiO 2 . We expect that development of photocatalytic generation of methane might be an interesting alternative for the conventional technology of biogas production based on anaerobic digestion. Aliphatic acids, which are the hole scavengers, are by-products of photocatalytic degradation of most organic compounds in water. This suggests that different wastewaters might be potential sources of methane.

Therefore, we have tried to investigate the possibility of methane generation with application of TNTs. Acetic acid was used as a model compound. The obtained data show that TNTs might be successfully applied for photocatalytic generation of methane and hydrogen in the presence of sacrificial agents, such as acetic acid.

Most of the literature data [START_REF] Kraeutler | Photoelectrosynthesis of ethane from acetate ion at an n-type TiO 2 electrode. The photo-Kolbe reaction[END_REF][START_REF] Kraeutler | Heterogenous photocatalytic synthesis of methane from acetic acid -new Kolbe reaction pathway[END_REF][START_REF] Kraeutler | Direct observation of radical intermediates in the photo-Kolbe reaction -heterogenous photocatalytic radical formation by Electron Spin Resonance[END_REF][START_REF] Sakata | Heterogenous photocatalytic reactions of organic acids in water. New reaction paths besides the photo-Kolbe reaction[END_REF] concerning photocatalytic methane generation, report application of photocatalysts containing Pt, which is an expensive material. Therefore, further investigations on application of TNTs in this process seem to be reasonable.

Conclusions

The effect of the temperature and time of hydrothermal treatment on physico-chemical properties of TNTs was investigated. It was found that the obtained nanotubes exhibited similar properties, regardless of the reaction temperature (130 or 140 °C) and time (24 or 48h).

The most notable difference between properties of TNTs prepared under different conditions was observed in case of BET surface area, which was increasing with increasing the reaction time and temperature. Based on TEM, XRD and TG measurements we suggest that the crystal structure of TNTs was H 2 Ti 2 O 4 (OH) 2 . The TEM and Raman spectroscopy measurements

A c c e p t e d m a n u s c r i p t

showed that the obtained product was not pure TNTs, but contained small amounts of anatase phase. The TNTs exhibited no photoactivity towards degradation of model azo dye AR18.

Thus, it can be concluded that that unmodified, as-received TNTs have no potential application in photocatalytic removal of organic compounds, such as azo dyes, from water.

However, TNTs might be successfully applied for photocatalytic generation of methane and hydrogen in the presence of sacrificial agents, such as acetic acid. The amount of methane produced in the presence of TNTs 140,48 was about 2.5 times higher than in case of TiO 2 P25.

Further investigations on the generation of methane and hydrogen in the presence of TNTs are in progress.

2 A c c e p t e d m a n u s c r i p t

 2 photocatalytic generation of hydrocarbons using TNTs in the current state of the art.

Fig. 2 .

 2 Fig. 2. presents XRD patterns of the starting material (P25) and TNTs prepared under

Fig. 4

 4 Fig. 4 presents the FTIR/DRS spectra of P25 and TNTs obtained under different

Fig. 5bA c c e p t e d m a n u s c r i p t

  Fig. 5b shows, as an example, DTG curve obtained for TNTs synthesized

Fig. 8

 8 Fig. 8 illustrates changes of AR18 concentration during photocatalytic reaction

A c c e p t e d m a n u s c r i p t

Fig. 9 A c c e p t e d m a n u s c r i p t

 9 Fig. 9 presents a comparison of the amounts of different gaseous products evolved

Fig. 1 .TNT140, 48 .Fig. 2 .

 1482 FIGURE CAPTIONS

Fig. 3 .Fig. 4 .Fig. 5 . 24 Fig. 6 .Fig. 7 .Fig. 8 .

 34524678 Fig. 3. Raman spectra of TNTs prepared under different conditions and starting TiO 2 P25.

Fig. 9 .

 9 Fig. 9. A comparison of the amount of products obtained after 5h of photocatalytic reaction

Fig. 1 .Fig. 2 .

 12 Fig.1. TEM images of (a-c) TNT130,24, (d-e) TNT130,48, (f-g) TNT140,24 and (h-j)

Fig. 3 .Fig. 4 .

 34 Fig. 3. Raman spectra of TNTs prepared under different conditions and starting TiO 2 P25.

Fig. 4 .

 4 Fig. 4. FTIR/DRS spectra of TNTs prepared under different conditions and starting TiO 2 P25.

Fig. 5 .

 5 Fig. 5. Thermogravimetric analysis of titanate nanotubes prepared under different conditions;

Fig. 6 .A c c e p t e d m a n u s c r i p tFig. 7 .

 67 Fig. 6. Nitrogen adsorption -desorption isotherms of TNTs prepared under different

Fig. 8 .

 8 Fig. 8. Changes of dye concentration in time during photocatalytic degradation of AR18 on

2 Fig. 9 .

 29 Fig. 9. A comparison of the amount of products obtained after 5h of photocatalytic reaction

  2-x/4x/4 O 4 (x∼0.7, : vacancy), trititanate H 2 Ti 3 O 7 / Na 2 Ti 3 O 7 / Na x H 2-x Ti 3 O 7 , bititanate H 2 Ti 2 O 4 (OH) 2 / Na 2 Ti 2 O 4 (OH) 2 / Na x H 2-x Ti 2 O 5 •H 2 O or tetratitanate H 2 Ti 4 O 9 •H 2 O. From all the above mentioned structures, Na x H 2-x Ti 2 O 4 (OH) 2

	A c c e p t e d	m a n u s c r i p t

A c c e p t e d m a n u s c r i p t the

  experiment. After 15 min of adsorption in the dark, the reaction mixture was illuminated with a mercury lamp emitting UV-A light (Philips Cleo, 6 x 20W, λ max =365 nm).

	The UV
	lamp was positioned above the reactor. After a defined time of irradiation the samples of
	reaction mixture were filtered through a 0.45 μm membrane filter and analyzed. The
	decolorization rate of AR18 was estimated on the basis of changes in UV/VIS spectra (Jasco
	V530 spectrometer, Japan). In order to determine the photostability of the model azo dye
	under the conditions applied a photolysis experiment (i.e. without addition of a photocatalyts)
	was performed. As found, after 5 h of illumination the concentration of the dye in the reaction
	solution remained unchanged. It means that no decomposition of the model compound took
	3 ) of azo dye Acid Red 18 (AR18, C 20 H 11 N 2 Na 3 O 10 S 3 ,
	Chemical Factory Boruta -Kolor Sp. z o.o., Poland) and 0.3 g/dm 3 of P25 or TNTs. The
	photocatalyst loading was selected on a basis of our previous investigations on degradation of
	AR18 in the presence of P25 [26]. The mixture in the reactor was continuously stirred during

  to Zhang et al. [5,6,30,31] the most probable structure of TNTs is Na 2 Ti 2 O 4 (OH) 2 (or H 2 Ti 2 O 4 (OH) 2 in case of the replacement of Na + ions by H + ). These structures can be also presented as Na 2 Ti 2 O 5 • H 2 O and H 2 Ti 2 O 5 • H 2 O, respectively [29]. The H 2 Ti 2 O 4 (OH) 2 exhibits an orthorombic structure, having lattice parameters of a=19.26,

b=3.78 and c=2.99 Å

[5,[START_REF] Zhang | Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H 2 Ti 2 O 4 (OH) 2[END_REF]

. Chen et al.

[START_REF] Chen | The structure of trititanate nanotubes[END_REF]

suggested that the structure of TNTs is H 2 Ti 3 O 7 (monoclinic, a=16.03, b=3.75, c=9.19 Å and β=101.45°). They explained that the interlayer

  2.33 •0.28H 2 O abs or H 2 Ti 3 O 7 •0.8H 2 O abs .

	The same phase composition of TNTs was proposed by Morgado et al. [8,9]. The 7-9%
	weight loss at temperatures from 100 to 500°C observed by the authors was attributed to the
	thermal decomposition of H 2 Ti 3 O 7 according to the following reaction: H 2 Ti 3 O 7 → 3TiO 2 +
	H 2 O.

  7.1-7.3%, being the highest for TNT130,24. This value might suggest that the structure of TNTs was not H 2 Ti 2 O 4 (OH) 2 , but H 2 Ti 3 O to 500°C, what was discussed earlier. In view of this, again, the H 2 Ti 2 O 4 (OH) 2 seems to be more probable structure than the H 2 Ti 3 O 7 .

[START_REF] Morgado | Effects of thermal treatment of nanostructured trititanates on their crystallographic and textural properties[END_REF]

. This way of calculations, however, seems to be rather doubtful. An additional peak in DTG curve at about 135 °C, can be attributed to the release of interlayer water, as was reported by Morgado et al.

[START_REF] Morgado | A study on the structure and thermal stability of titanate nanotubes as a function of sodium content[END_REF]

. If the shoulder observed at about 140°C corresponds to the interlayer water, the calculation mentioned above should not be performed within the 140-500°C range, but for temperatures from 100

TABLE CAPTION Table 1 .

 CAPTION1 Selected properties of TNTs prepared under different conditions.

	A c c e p t e d	m a n u s c r i p t

Table 1 .

 1 Selected properties of TNTs prepared under different conditions. ) Pore volume was determined by nitrogen adsorption at at P/P o =0.995

	Sample	Synthesis	Synthesis	BET surface	Pore volume	Average pore
		temperature [°C]	time [h]	area [m 2 /g]	[cm 3 /g] a)	diameter [nm]*
	TNT130,24 130	24	386	1.31	13.6
	TNT130,48 130	48	406	1.40	13.8
	TNT140,24 140	24	450	1.54	13.7
	TNT140,48 140	48	478	1.69	14.2

a

Acknowledgements

The presented research is co-financed by the Foundation for Polish Science, which has been supported by a grant from Iceland, Liechtenstein and Norway through the EEA Financial Mechanism.

The research has been performed within a frame-work of Polish-Japanese scientific and technological international cooperation joint project for the years 2009-2010.

A c c e p t e d m a n u s c r i p t


Gabriel Bernardo

Ana Charas

Jorge Morgado
email: jorge.morgado@lx.it.pt

Luminescence properties of poly(9,9dioctylfluorene)/polyvinylcarbazole blends: Role of composition on the emission colour stability and electroluminescence efficiency

Keywords: polymers, luminescence, optical properties, transport properties

Luminescence properties of poly(9,9-dioctylfluorene)/polyvinylcarbazole blends: Role of composition on the emission colour stability and electroluminescence efficiency

INTRODUCTION

Conjugated electroluminescent (EL) polymers have been explored for various optoelectronics applications, such as light-emitting diodes (LEDs) [1], photovoltaics [2] and light-emitting field-effect transistors [3]. The molecular nature of these materials makes their properties particularly sensitive to modifications of their chemical structure, either by design or as a result of degradation processes. For instance, significant modifications of their emission spectra, but also of their charge transport ability, are observed upon modifications of their molecular structure. In solid state, interchain excited states (e.g. excimers or charge transfer excited states) can be formed, competing with the intrinsic intrachain excited states for the capture of the excitons energy. Such interchain states can, in working LEDs, be directly populated through on site direct charge recombination, or act as energy traps for the initially formed intramolecular excited states. The combination of energy transfer plus on site recombination originates, in general, a significant amplification of such interchain states contribution to the total electroluminescence, when comparing with their relative importance for light-emission upon photoexcitation (photoluminescence). Aggregates and excimers or exciplexes are the most commonly invoked/observed interchain states in EL polymer films. While the first type of interchain states, possessing an attractive ground state, can be directly Interchain states have been explored to obtain, for instance, white LEDs. However, they usually represent a "degraded state" with respect to the intended emission from the pure system. Besides the formation of interchain states, the emission pattern can also change as a result of degradation processes induced by photooxidation or by secondary reactions following charge injection.

Polyfluorenes, PFs, are among the most studied EL conjugated polymers [4], the homopolymers being blue emitters. Emission colour can be tuned by copolymerisation with lower energy gap comonomers [5] or through blending with low energy gap polymers [START_REF] Morgado | [END_REF]7]. Stable pure blue emission is not easily obtained from LEDs based on neat PFs. Poly(9,9-dioctylfluorene), PFO, and other PF-based LEDs, exhibit a green emission component which grows during device driving, leading to a whitish emission.

The origin of this green emission has been the subject of various reports and remains a matter of debate. First studies attributed this emission to excimer states [8]. Later, the presence of fluorenone defects, formed by photooxidation or electrically induced at the C9 position of PFs, was held responsible for such emission [9]. More recent studies proposed that excimer [10] or charge transfer states involving fluorenone units [11] are the true species responsible for such emission. Chemical strategies, such as modifications of the substituents on C9 of PFs [12], have been proposed to suppress or even eliminate such emission. Other approaches involve PFO chain isolation. In particular, blending of PFO with a photo-and electrically inert polymer (polystyrene) or with an electron-transporting polymer [13] or the PFO intercalation into nanocomposites [14,15], were shown to improve the blue emission stability. PVK was chosen due to its good hole-transporting properties, being widely used in multilayer LEDs. We note that previous studies on blends of PFO with an electrontransporting material and even with polystyrene were shown to improve colour stability and, for low amounts of the additional polymers, to increase device efficiency [16]. In this study we find that blending with PVK improves colour stability and, in some cases, leads to improved efficiency.

EXPERIMENTAL

Absorption spectra were recorded with a CECIL CE7200 spectrophotometer and emission photoluminescence (PL) spectra were obtained with a SPEX Fluorog fluorimeter. LEDs preparation closely followed a previously reported procedure [17].

Oxygen plasma treated indium-tin oxide (ITO)/glass substrates were spun coated with poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid (PEDOT:PSS or PEDOT) (Baytron P, AI4083) and dried in air at 150 o C, for 2 minutes, yielding a film thickness of about 45 nm, as measured with a Dektak 6M profilometer. Films of PFO or PFO/PVK blends, with thicknesses of 70 nm and 100 nm, respectively, were spun coated on top of PEDOT from their chloroform solutions. Finally, aluminum or magnesium cathodes were thermally deposited at a base pressure of 2x10 -5 mbar, defining pixel areas of 4 mm 2 . The devices were tested under vacuum (ca. 10 -2 mbar).

Light output was measured with a calibrated silicon photodiode. The system was calibrated with a luminance meter. External quantum efficiencies (EQE) were estimated emission profile [18]. Electroluminescence, EL, spectra were recorded with an Oriel Instaspec IV spectrograph. PFO/PVK blends are identified according to the weight proportion of the two components (1:1, 1:2 and 1:3) as mixed in chloroform.

RESULTS

Fig. 1 shows absorption and emission spectra of PFO and PVK films deposited by spin coating on spectrosil discs. The emission of PVK, of excimeric nature, shows a significant overlap with PFO's absorption spectrum, which favours energy transfer from PVK to PFO. Fig. 2 compares the effect of the annealing at 150 ºC for 2 hours in air, under indoor daylight, on the emission spectra of films of neat PFO and films of PFO/PVK blends. Upon annealing, the PL spectrum of neat PFO shows the growth of the typical broad green emission centered at about 520 nm. However, no noticeable changes are found in the absorption spectra upon annealing of neat PFO films. As Fig. 2 clearly shows, the blending with PVK improves the colour stability: the intensity of the broad green band in the blends is much smaller compared to neat PFO and it decreases with increasing PVK content. The use of magnesium instead of aluminum, as cathode material, leads to similar effects of blending on the device characteristics, as shown in Fig. 3b). It is worth noting, the Devices based on neat-PFO (Fig. 4a) show quite extensive degradation of the blue emission. The relative intensity of the green band at ca. 520 nm is significant, increasing with the driving voltage. When PFO:PVK is used (Fig. 4b), the green component is still present but, up to the maximum driving voltage, its relative intensity (with respect to the blue peak at about 420 nm) never gets as high as observed in the neat PFO devices.

Further increase of the PVK content (1:2 blend) leads to a significant stabilization of the blue emission. Up to the maximum driving voltage (maximum luminance), the green emission remains with very small intensity (Fig. 4c). Finally, the use of the PFO:3PVK blend does not significantly modify the colour stability of the Al-based devices. The use of Mg cathodes leads to a significant amplification of the green component.

Electroluminescence spectra were also recorded as a function of the driving time.

Contrary to what is observed for LEDs based on neat PFO, with the green component growing in time, we find that the devices based on PFO/PVK blends show much higher spectral stability. For instance, Fig. 5 shows the EL spectra of devices based on the PFO:3PVK blend with Al cathodes as a function of time under a bias voltage of 12V.

The spectral variation is very small. It is worth noting, however, the appearance of an "extra" low intensity band at ca. 620 nm. This is not observed in neat PFO-based devices but is observed in devices based on neat PVK. Its appearance is therefore attributed to a degradation of PVK.

When Mg is used as cathode we find a higher contribution of the green band. The time dependence of the EL spectra of a ITO/PEDOT/PFO:3PVK/Mg device is shown in Fig 5b). It can be observed that, compared with results for the Al-based LEDs, there is a higher contribution of "impurity" emissions at ca. 520 nm and 620 nm. The dilution of PFO with PVK increases the thermal stability with respect to the appearance of the contaminant green emission, as shown in Figure 2. This effect is in agreement with previous studies that show that isolation of PFO chains decreases the importance of the green emission. Two factors can contribute to this observation: i) the mechanism leading to the green-emitting sites may require interchain interactions, as mentioned above, and the blending reduces the probability for such interchain interactions to occur; and/or, ii) the green-emitting sites are of intrachain nature, and isolation simply reduces the probability of the energy transfer from pure PFO sites to these lower energy sites. Therefore, these results, while evidencing the better colour stability against annealing, do not allow us to draw a conclusion on the mechanism behind the green colour. This remains, therefore, a matter of debate and further studies are currently in progress.

PFO and PVK are both good hole-transporting polymers. Hole mobility in PFO has been reported to be around 10 -4 cm 2 /V.s for electric fields around 10 6 V/cm, while electron mobility is found to be around 3 orders of magnitude smaller [19]. Similar values, in the range 4x10 -5 -5x10 -4 cm 2 /V.s, for the hole mobility of PFO were obtained from time of flight studies at a field of 5x10 5 V/cm [20]. Using this same technique, the hole mobility of PVK at 10 6 V/cm was reported to be much lower, around 10 -7 cm 2 /V.s [21]. Therefore, besides the increased disorder associated to the use of blends, the presence of PVK should lead to a decrease of hole mobility.

A c c e p t e d m a n u s c r i p t 9

Using cyclic voltammetry we estimated [START_REF] Morgado | [END_REF] the ionization potential (Ip) and electron affinity (EA) of PFO to be 5.8 and 2.1 eV, respectively, in agreement with previous reports [23]. For PVK the reported values are 5.8 and 2.3 eV [24]. Based on these values, we do not expect significant variation of the hole-injection barrier from PEDOT (workfunction of ca. In terms of the external quantum efficiency, we find that the addition of the PVK significantly reduces the values. In particular, the much lower value found for the PFO:2PVK blend-based LED, when comparing with the devices based on the two other blends, is unexpected and still not explained!

In general we also found that the use of Mg as cathode material, although having a lower workfunction than Al, does not lead to an increase of the current flow through the devices. This is an unexpected result, which we cannot yet explain, but interfacial contamination may be involved. Surprisingly, the EL efficiency of the neat PFO based device is similar for both cathode materials, but increases for the blends-based devices using Mg cathodes as expected for an improved electron injection ability of this material. The observed increase of the light-onset voltage when Mg is used instead of Al is also unexpected, on the same grounds, but is likely related to the decrease of the current when comparing with Al-based devices.

As these are mainly hole-transporting devices, it appears as if the improvement of the cathode electron injection ability is not enough to improve efficiency, which contradicts a priori expectations and usually reported results. A possible explanation is that the improved exciton formation efficiency resulting from a better charge carrier balance leads to an improvement of exciton formation mainly close to the cathode, where metal plasmons are efficient at quenching luminescence. However, we need further studies to confirm if this explanation is correct.

A c c e p t e d m a n u s c r i p t 11

As observed in the PL studies, the dilution of PFO with PVK has a profound effect on the colour stability of the LEDs. In fact, the results of Fig. 4 and 5 show that devices with Al cathodes and using PFO:2PVK or PFO:3PVK exhibit negligible contaminant green emission. The drawback of the use of these blends is the reduction of the EL quantum efficiency. Among the blends used in the Al-based LEDs, the one with composition 1:2 is the one that combines a low light-onset voltage, and a high colour stability. However, this is the one giving the lowest EL efficiency: a maximum EQE of about 0.01% compared to about 0.1% for the neat PFO. The use of a 1:3 composition leads to a significant recovery of the efficiency, while maintaining colour stability (see Fig. 4d). However, the voltage required to attain similar current and luminance values almost doubles.

When the blends are used as emissive layer in the Mg-based LEDs, we now find a much higher efficiency then for the similar devices with Al-cathodes, as mentioned above. In fact, LEDs with the 1:3 blend show a quantum efficiency that is twice that of the LEDs of neat PFO. We find, however, that the purity of the blue emission is lower than that of the Al-based devices (see Fig. Therefore, the use of Mg may, in this case, facilitate the formation of those species.

It is worth noting that, in the EL spectra, the contaminant green emission occurs at the same wavelength range irrespective of the cathode material used (Al or Mg). In addition, the EL green emission peaks at 522 nm with shoulders at 490 and at 580 nm.

In the PL spectrum of annealed PFO (Fig. 2) we find a peak and shoulders at similar wavelengths, although with lower resolution. This observation contrasts with previous reports on additional peaks observed in the EL spectra of polyfluorene LEDs and which were attributed to "interface defects" originating from chemical reactions between the polymer and the cathode metals [26,27].

CONCLUSIONS

The blending of PFO with PVK suppresses the contaminant green emission, centered around 520 nm, observed in the PL spectrum of neat PFO samples annealed in air. This suppression effect increases with the amount of PVK present. This is in agreement with

A c c e p t e d m a n u s c r i p t 3 probed

 3 /identified by UV/Vis absorption spectroscopy, excimers possess, instead, a repulsive ground state and are not detectable by UV/Vis absorption.

A c c e p t e d m a n u s c r i p t 4 Here

 4 , we report on the luminescence properties of PFO/PVK blends and show that the use of the hole-transporting polymer PVK is successful at suppressing the green emission. The molecular structures of these polymers are shown in scheme 1.

A c c e p t e d m a n u s c r i p t 5 taking

 5 into account the spectral response of the photodiode and assuming a Lambertian

Fig. 3 A c c e p t e d m a n u s c r i p t 6 voltages.

 36 Fig.3shows the characteristics of the LEDs based on neat PFO and on their blends with

A c c e p t e d m a n u s c r i p t 7 Fig. 4

 74 Fig. 4 compares the EL spectra of the various devices, at various driving voltages.

A c c e p t e d m a n u s c r i p t 8

 8

A c c e p t e d m a n u s c r i p t 10 the

 10 5.2 eV[25]) into the electroluminescent layer, being this either neat PFO or the PFO/PVK blends ('h §0.6 eV). Furthermore, the presence of PVK should only slightly decrease the electron injection barrier. These values of ionization potentials and electron affinities for PFO and PVK, combined with the spectral overlap between PVK emission and PFO absorption, support the existence of energy transfer of the excited states from PVK to PFO. This is also consistent with the absence of PVK contribution to the emission of the blends (neat PVK emission is centered at ca. 400 nm).As mentioned above, PFO and PVK have the same Ip values. In these devices the electron injection barriers ('e) are much higher than the hole-injection ones (in case of PFO, 'e §2.1 eV, for Al, and 'e §1.6 eV, for Mg, considering a workfunction of 4.2 eV for Al and 3.7 eV for Mg). For PVK the expected electron injection barrier should be slightly decreased, by 0.2 eV. The reduction of the light-onset voltage and the higher currents observed above light-onset voltage for the devices where PFO:PVK and PFO:2PVK blends are used instead of neat PFO, is somewhat unexpected, in view also of the lower hole-mobility of PVK. The small difference in the electron-injection barrier, which controls light-onset and should become more important in the bipolar regime, appears to have a rather relevant effect.For the blend with the highest PVK content, PFO:3PVK, a quite extensive detrimental effect on the current flowing through the devices occurs, which seems to indicate that benefits from improved electron injection are overcompensated by a reduction of charge mobility. We note that both hole and electron mobilities are expected to be reduced in the blends with respect to neat PFO (due to disorder).

A c c e p t e d m a n u s c r i p t 12 fluorenone

 12 4 and 5). The EL spectra of Fig 4 and 5 were recorded at higher voltages for the Mg devices, but the current at such voltages is mostly smaller than that of the corresponding Al-based devices. Therefore, we consider that there is no direct correlation between the increased weight of the green component and the current value, when comparing Al and Mg cathodes. In fact, at similar currents, we find that the Al-based devices show much lower contaminant green emission. It appears as if Mg induces itself a faster degradation. Furthermore, the second contaminant emission at about 620 nm (ascribed to PVK) is more important. A possible explanation may be that the higher electron injection from Mg may favour the formation of the species (such as in the case of PFO) responsible for that emission. It appears that driving the devices at higher voltages, leading to increasing green emission, mainly leads to higher electron currents into the bulk of the device and contaminant species formation.

A c c e p t e d m a n u s c r i p t 13 ofA c c e p t e d m a n u s c r i p t 16 FIGURE CAPTIONS Scheme 1 .

 13161 FIGURE CAPTIONS

Fig. 1 .

 1 Fig. 1. Normalized absorption and emission (PL) spectra of neat films of PFO and

Fig. 2 .

 2 Fig. 2. Effect of the annealing in air, under indoor light at 150 ºC for 2 hours, on the PL

Fig. 3 .

 3 Fig. 3. Current density (J, filled symbols) and luminance (L, open symbols) of the LEDs

Fig. 4 .

 4 Fig. 4. Normalised EL spectra at various applied voltages for devices with Al and Mg

Fig. 5 .

 5 Fig. 5. EL spectra of devices based on PFO:3PVK as a function of time, for devices

Figure 3 A c c e p t e d m a n u s c r i p tFigure

 3 Figure 3

ACKNOWLEDGMENTS

G.B. thanks Fundação para a Ciência e Tecnologia (FCT) for a postdoctoral grant.

Financial support by FCT under the contract POCI/QUI/58291/2004 is acknowledged.