Manganese/TiO2 Composites Prepared and Used for Photocatalytic Active Textiles

Manganese/TiO2 composites are prepared by a solvothermal process starting from the precursors titanium propoxide and manganese nitrate. The solvothermal processes are driven at temperatures of 140 °C or 180 °C. The formation of anatase as crystalline TiO2-phase was determined by XRD for preparation conditions at 180 °C. The occurance of crystalline phases in nanometer scale is determined further by TEM. The prepared manganese/TiO2-composites are further investigated as coating onto viscose textile. The photocatalytic activity of those composites was determined by degradation of organic dye stuff under illumination with UV-light. The effect of photocatalytic dye degradation is also investigated in presence of H2O2. The prepared textile coatings exhibit a high capability for dye decomposition under the chosen arrangement of investigations. For this reason, the prepared coated textile materials could be of high interest for industrial applications, for example, as filter material for cleaning waste water from dyeing processes. (doi: 10.5562/cca2097)


INTRODUCTION
A photocatalytic process is a process which is accelerated in presence of light and a photocatalyst. A photocatalyst is a material, which has no influence on the process under dark conditions. However, in case of illumination, the photocatalyst is able to accelerate a reaction. Most photocatalysts are semiconductors containing an electronic structure with certain bandgap between a valence band and a conduction band. 1−4 The photocatalytic reaction starts with the transport of electrons from valence to conduction band induced by light absorption. To overcome the bandgap in this process, the absorbed light needs to have a certain energy, which is in relation to the wavelength of light. For this, the photoreaction is started by the absorption of light with a certain maximal wavelength determined by the bandgap of the photocatalyst. If light is used with wavelengths larger the maximal wavelength, the photoreaction cannot start, because the light does not contain the necessary energy to transport electrons from the valence to the conduction band. 4 One frequently reported photocatalytic process is the photooxidation with oxygen. This kind of photooxidation is developed for several applications, e.g. degradation of organic dyes in wastewater, degradation of chlorophenol or oxidation of gaseous benzene. 3,5−7 An interesting topic is also the use of photocatalytic materials as coating on fiber materials to reach self-cleaning fabrics, which eliminate dirt by photooxidative processes. Examples are given for different fiber materials like cotton, keratins or polyester. 8−10 Beside the degradation of dye, toxic compounds or dirt, also bacteria can be degradated and eliminated by photooxi-dation. This degradation of bacteria is also defined as antimicrobial effect caused by a photooxidative process. 11,12 One of the probably best investigated photocatalysts is titanium oxide. TiO 2 can occur in amorphous form or in three different types of crystalline phases as rutile, anatase or brookite. For photooxidation with oxygen the TiO 2 crystalline phase anatase contains the highest photocatalytic activity. 3,13 For photooxidation with other oxidative agents, like hydrogenperoxide and also for the other TiO 2 crystalline phase like rutile, a high photocatalytic activity can be observed. 13 The disadvantage of TiO 2 as photocatalyst is probably that due to the large band gap of 3.2 eV the photocatalytic effect works only with UV light. 1 An application with visible light is with pure TiO 2 not possible, which closes the door for economic daylight applications, e.g. for waste water treatment.
For this, many efforts have been done to modify TiO 2 in order to induce a photoactivity with visible light. Beside the general enhancement of photoactivity of TiO 2 , the generation of photoactive properties using visible light is in the focus of current research. The modification of TiO 2 is done by combination with other materials to prepare so-called doped TiO 2 or TiO 2 composites. For modification of TiO 2 in order to reach activity with visible light a wide range of different materials are investigated and only a short overview can be given in this introduction. Main modifications of TiO 2 are done by doping with non-metals as nitrogen, fluorine, carbon, boron or sulphur. 14−19 Other groups report that the modification with metal are suitable to reach a photoactivity of TiO 2 for visible light. Examples are given for the doping with cobalt, iron, antimony, chromium or vanadium. 20−23 Beside doping also the use of composites is reported, as for example composites of TiO 2 with silver or silver compounds, manganese oxides, cadmium sulfide or tungsten oxide. 24−27 Also many efforts have been made to prepare TiO 2 coatings instead of TiO 2 powders in order to improve their usability in technical processes. TiO 2 coatings are prepared on different types of substrates like metal, glass or textiles. 28,33 Especially, the TiO 2 coatings on textiles are of high interest, because textiles are highly flexible, have a large surface areas and can be easily fit to the demands of a photoreactor. 34 Beside TiO 2 catalysed reactions with O 2 as oxidative agent, also many investigations have been performed with the stronger oxidative agent hydrogenperoxide. 13,35−38 These experiments were done to improve the decomposition rates of toxic or other hazardous compounds. 39−41 With this background also other semiconductor materials like ZnO, ZnS or CdS were investigated with view on their photocatalytic activity in presence of hydrogenperoxide. 42−44 Under this aspect, manganese oxide is another interesting material to catalyse oxidation reactions, which is able to enhance the degradation of hydrogenperoxide, and by this, accelerate the oxidative reaction. 45−47 Manganese compounds are also reported to act as photocatalysts with oxygen as oxidative agent. 48,49 For this reason, many attempts have to be performed to combine both materials, titanium and manganese oxide, to get a composite material with advantageous catalytic activity. 50−56 However, it is also reported that, by addition of Mn to TiO 2 the photoactivity decreases. 57 Because manganese oxide is a catalyst for the splitting of hydrogenperoxide, these manganese/titanium oxide composites should be interesting materials for photooxidative reactions with H 2 O 2 . Nevertheless, in contrast to the strong investigations with oxygen as photoactive agents, the properties of these composites are less investigated in presence of H 2 O 2 .
Therefore, the aim of this study is first the preparation of photoactive TiO 2 modified with manganese and second the testing of these materials in respect to photocatalytic properties with oxidative agents O 2 and H 2 O 2 . The preparation is done using a sol-gel process under solvothermal conditions. The resulting TiO 2 sol can be used as coating agent onto textiles or for the preparation of photoactive powders. The photoactivity is tested as function of degradation of dye under illumination.

Materials and Preparation
A TiO 2 -sol is prepared using the following procedure. Into 226 ml ethanol an amount of 2 g Pluronic P123 (blockcopolymer supported by BASF) is dissolved. To this solution an amount of 14 g titaniumtetrapropoxide is given under stirring. At last, under strong stirring 1.8 ml 1N-HNO 3 is added. To combine this TiO 2 -sol with manganese an ethanol based solution of Mn(NO 3 ) 2 ×4H 2 O (4 % w/w) is used. To 50ml of the TiO 2 -sol the manganese nitrate solution is added under stirring in amount of 0.05 ml up to 5 ml. 10 minutes after these solutions are prepared, they are treated in a solvothermal process at 140 °C or 180 °C in a steel autoclave with Teflon vessel (Berghof Instruments GmbH, Germany; Hochdruck-Laborreaktor BR-500). The process duration is set to 2 hours. The annealing to reach the final process temperature took 45 minutes, while for the subsequent cooling at least 2 hours were necessary. All TiO 2 -sols were applied onto viscose fabrics by dip-coating. After application the fabrics were dried at room temperature and finally a thermal treatment at 120 °C for 1 hour was done. The viscose fabric was made from multifilament yarn (supplied by KOB-Wolfstein, Germany) with a weight of 240 g/m 2 and was used for coatings as received. For X-ray diffraction (XRD) investigations and for transmission electron microscopy (TEM) the coating solutions were also dried at room temperature and grinded to powders.

Analytical Methods
Measurements of wide-angle X-ray scattering (WAXS) were performed to identify the crystalline species in the solvothermally prepared materials. These measurements were performed on powders gained from the liquid coating solutions after drying for 48hours at room temperature and an afterwards grinding. For investigations a vertical X-ray diffractometer D8 (Bruker AXS) equipped with a scintillation detector was used. All measurements were performed with Cu-Kα radiation selected by a secondary graphite monochromator. The received diffraction patterns were identified by using the Powder Diffraction File 2 (PDF-2) database. 58 Transmission electron microscopy TEM was used to determine crystalline size and crystalinity of the prepared materials. TEM samples are prepared with 200 mesh copper grids containing a graphite membrane of 20 nm thickness (supported by Plano GmbH). For preparation, the powder materials were dispensed in water and the copper meshs were dipped into this solution. After drying for at least 12 hours at room temperature the TEM measurements were performed. Asprepared samples are investigated using a JEOL JEM 2100F Field Emission Electron Microscope. The photocatalytic activity of the prepared composites is determined for the composites as coatings on textiles. In all cases, the photoactivity is measured by the degradation of the dye Acid Orange 7 in presence of the composites and UV-light irradiation. 59,60 As light source six UVlamps with a maximum of intensity at 360 nm were used (OMNILUX 18Watt -supplier Eurolite). To evaluate the photoactivity of the coated textiles in case of illumination with visible light a similar set-up is used with 18 Watt white lamps (OMNILUX). These white lamps are used as they are, emitting also a small amount of UV-light. However, experiments are also done after filtering the UV-light from the white-lamp by using a cut-off filter semaSORB PET 400 (sema GmbH, Coswig, Germany). The spectra of the used lamps are described in reference. 34

Material Properties
To gain information about the crystallinity of the prepared materials, all samples (see Table 1) are investigated by WAXS. From this, it can be clearly seen that  (Figures 2 to 4). The crystalline structure of materials formed under preparation conditions of 180 °C can be clearly determined by TEM (Figures 2 and 3). The diameters of these crystallites is smaller than 10nm, so it is stated that under the chosen preparation conditions nanocrystalline anatase is formed. An influence on the structure of these anatase type crystallites by addition of manganese cannot be detected. For sample B1, prepared at 140 °C, no crystalline phases were observed by TEM investigations (Figure 4). This result is congruent with the WAXS results.

Photoactivity of Textile Coatings
The photoactivity is determined for the prepared materials after coating on viscose textiles as the decomposition rate of the organic dye stuff Acid Orange 7. First investigations are done with the coated viscose under illumination with UV light and without further addition of H 2 O 2 (Table 1). Also, a testing with a white-lamp was performed. If this determination is performed with uncoated viscose fabric, only an amount of smaller 1 % Acid Orange 7 is decomposed under the chosen set-up.   The samples A2 and C2 prepared solvothermally at 180 °C, which contain crystalline anatase, exhibit a significant photoactivity. All samples prepared at 140 °C, containing only amorphous material, do not lead to a significant photoacive dye decomposition. These results are in good agreement with the literature, which stated that the anatase modification of TiO 2 exhibit higher photocatalytic activity for oxidative reactions with oxygen, compared to other crystalline modifications of TiO 2 or even the amorphous TiO 2 . 3,13 It is also clearly indicated that, the addition of manganese to the anatase lead to a decrease of photoactivity and for sample B2 with highest investigated manganese content the photoactivity is nearly diminished (Table 1). Even small amounts of manganese compared to the TiO 2 content in the prepared materials can lead to a significant decrease of photocatalytic dye degradation. Such a decrease of photoactivity is reported in literature by several groups and is for example a useful property for preparation of TiO 2 materials for sunprotective applications. 61−64 This decreased catalytic efficiency is explained by adsorption of Mn 2+ ions on the surface of TiO 2 . By this, the separation of the photogenerated electron and hole is less effective leading to a higher rate of recombination. 63,64 For all investigated samples no significant photoactivity was determined in case of illumination with the white-lamp. It can be stated that, by combination of titania with managanese no sensibilisation of the photocatalyst for visible light was reached.
Because manganese is supposed to catalyse oxidation reactions with H 2 O 2 , investigations concerning the photoactivity of the prepared materials are also per-formed in presence of H 2 O 2 . For this, the decomposition of Acid Orange 7 is determined under analogous conditions but with the addition of 2.2 % H 2 O 2 during illumination with UV-light, visible light or a white-lamp containing both visible and UV light, respectively. Reference measurements are performed without any kind of illumination.
Before investigating the coated viscose textile, a dye solution without any textile and with the analogous uncoated viscose fabric is investigated as reference ( Figure 5). It can be clearly seen that even if the reactive oxidative H 2 O 2 is added to the dye solution, nearly no additional dye decomposition can be observed in case of no illumination or illumination with visible light. A significant dye decomposition is only be observed in case of illumination with UV light. This decomposition is reduced for the reference with the uncoated viscose fabric, probably due to a kind of shadowing of the dye solution by the presence of the textile.
In contrast, a totally different behaviour of dye decomposition is observed, if the TiO 2 coated viscose fabrics are added to the H 2 O 2 containing dye solution ( Figure 6). In this case the TiO 2 materials solvothermally prepared at lower process temperature of 140 °C lead to a strong decomposition of the dye. This decomposition is even strong, if no illumination is performed. For this, it is stated, that the coated TiO 2 materials do not only work as a photocatalyst for dye oxidation with H 2 O 2 but can also catalyse this reaction in the dark. This strong dye decomposition occurs for the sample A1, containing only TiO 2 , and for the sample B1 and C1 with the manganese addition to the TiO 2 material. Small amounts of added manganese seem to have a small decreasing effect. It is worth to remark that the addition of TiO 2 materials solvothermally prepared at 180 °C do not lead such a dye decomposition without light expo-  sure. With these materials also by illumination with visible light no significant additional dye decomposition is observed. Only in case of UV-light a significant dye decomposition is observed and could be explained by a photocatalysed oxidation of the dye. This behaviour of these anatase containing samples prepared at 180 °C is in good agreement with the photocatalytic behaviour as reported in literature. 3,13 In contrast, there is no simple explanation, why the amorphous TiO 2 coatings from materials solvothermally prepared at 140 °C exhibit a higher catalytic activity. Such a behaviour may be explained in accordance to the different behaviour of different TiO 2 modifications in presence of H 2 O 2 instead of O 2 , which is earlier described for a comparison between both TiO 2 modifications anatase and rutile. 13 It was reported that rutile does not cause a photoactive reaction with O 2 which has been explained by a higher stability of O 2 − species adsorbed on the rutile surface. When H 2 O 2 is present next to rutile a reactive OH radical is formed leading to further photodecomposition reactions. Such a process may also be an explanation for the strong dye decomposition in presence of solvothermally prepared amorphous TiO 2 and H 2 O 2 .

CONCLUSIONS
Manganese/TiO 2 composites are prepared by a sol-gel process under solvothermal conditions. A solvothermal process driven at 180 °C lead to the formation of TiO 2 in the crystalline modification of anatase with crystallite sizes of the crystalline areas of smaller than 10nm. Is the process driven at lower temperatures of 140 °C mostly amorphous material is formed. The prepared materials can be applied as photocatalytic active coating onto textile fabrics. If this photoactivity is determined as oxidative degradation of organic dyestuffs with oxygen, especially the anatase containing samples exhibit a strong photoactivity in presence of UV light. In contrast, under reaction conditions with H 2 O 2 , a stronger oxidative dye decomposition is observed with the amorphous TiO 2 coatings on textiles. This decomposition reaction runs especially under illumination with visible light but also without illumination. Therefore, this materials could be of high interest for the development of cleaning systems for waste water under the use of sun-light without the necessity of an artificial UV source. For applications in waste water treatment it is of especial interest, that the oxidative decomposition with H 2 O 2 is also catalysed in the dark, so it should be possible to develop a waste water cleaning plant working for 24 hours a day without the necessity of an artificial light source.