Enhanced solar photocatalysis of TiO2 nanoparticles and nanostructured thin films grown on paper

Titanium dioxide nanoparticles and nanostructured thin films were simultaneously synthesized using a microwave-assisted hydrothermal method. The synthesis formed very fine particles, appearing as nanospheres in the 11 nm size range. As for the nanostructured films, they have displayed similar structural characteristics to the nanoparticles, with thickness of 130 nm. These films covered uniformly and homogenously the Whatman paper, while maintaining its flexibility. The materials processed had their photocatalytic activity assessed from rhodamine B degradation under solar radiation (91% degradation after 40 min for the powder material and 68% after 6 h for the nanostructured thin films). Reusability experiments were also carried out, revealing superior performance concerning the Degussa P25, the most common photocatalyst used. The results of the present work can be thought as an option for the existing photocatalysts activated under solar light, namely for water purification, as it simultaneously produces enhanced photocatalytic powders and photocatalytic papers fully disposable and that can be easily recycled.


Introduction
Water is an essential resource for all known forms of life, and it is used in every industrial sector. Despite covering 70% of the Earth's surface, just 2.5% of this water is fresh for consumption and even this small percentage is not accessible for every human in the planet Earth. This scarcity and misuse led to the necessity to develop techniques and products that facilitated the treatment of wastewater and helped decreasing the amount of people without access to it.
Since the beginning of the millennium, companies and governments [1][2][3][4] have been addressing sustainability issues, approaches to decrease the problems caused by human activities and industrial sectors and ways to make responsible choices regarding natural resources' exploitation, which culminated in the Sustainable Development Goals by the UN, such as the Goal 6: Clean Water and Sanitation. To make this possible some metal oxides, like Titanium Dioxide (TiO 2 ), are being used as photocatalytic agents to breakdown different water pollutants.
Photocatalysis is described as the process of altering reactions' rate upon exposure to a light source. In this process, the material absorbs photons that will give sufficient energy to an electron (e -) in the valence band to get excited and jump to the conduction band, leaving behind a positive charge particle (h + ) and generating an electron-hole pair, which will be responsible for reducing and oxidizing the compounds on the photocatalyst surface [26][27][28]. The water splitting induced by the TiO 2 photocatalytic properties is a widely studied phenomenon, in which this material, through the reduction and oxidation of the solution, originates O 2 • and OH • radicals that are effective in the decomposition of organic substances, pollutants and microorganisms. In the field of photocatalysis, TiO 2 appears as a common solution for several applications related with water treatment since this material displays strong oxidizing abilities for the organic pollutants' decomposition, its hydrophilicity, chemical stability and low cost [29][30][31][32]. TiO 2 films can be integrated in flexible and rigid substrates and used as water filters. Despite being an efficient process, TiO 2 photocatalysis is most likely to be activated by UV irradiation, which only comprises a reduced portion of the solar spectrum. Taking advantage of the rest of the solar spectrum is the future of photocatalysis [8,27,33,34]. Thus, approaches for making TiO 2 photoactive under solar radiation are highly sought.
The synthesis method for the TiO 2 as a photocatalyst material is a key factor in its final cost and performance and should also take in account the type of substrate used for deposition. Several methods have been reported in the literature for the production of TiO 2 films for photocatalytic applications, including CVD [35,36], spray pyrolysis [35,37], sol-gel [35,38], hydrothermal synthesis [35,39], and microwave-assisted synthesis [7,35,40]. The latter synthesis route enables the use of flexible substrates, like paper [6,14], and allows reproducibility while being an inexpensive, fast and simple method.
The following study will describe the synthesis and further characterization of TiO 2 nanoparticles and nanostructured thin films grown on paper and produced simultaneously by microwave-assisted hydrothermal technique. The main goal was the production of stable, environmentally friendly, and cost-effective TiO 2 materials with improved photocatalytic behaviour under solar radiation, and also achieving an enhanced disposable photocatalytic paper.
2. Experimental procedure 2.1. Materials Both TiO 2 nanoparticles and nanostructured thin films were synthesized simultaneously using a microwaveassisted hydrothermal method. The materials were produced at least 3 times to demonstrate the technique reproducibility. The TiO 2 synthesis used deionized water, titanium (IV) isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 , TTIP, 97% from Sigma Aldrich), and hydrochloric acid (HCl, 37% from Merck). For the microwave synthesis, it has been mixed 57.5 ml of water on 2.5 ml of HCl under stirring during 5 min 2 ml of TTIP was then added to the solution and stirred for 10 min prior to synthesis. A CEM Focused Microwave Synthesis System (CEM Discover SP) was used in this work. The microwave parameters were set at 75 min, 100 W, 110°C and 200 Psi, for synthesis time, power, temperature, and pressure, respectively. Quartz vessels of 35 ml were used, and a piece of the Whatman substrate was added for the TiO 2 film to grow. Afterwards, 20 ml of the mixed solutions were transferred into it. The microwave parameters selected have been previously employed in [6,14,41]. when using paper as substrate.
The substrate used was the Whatman chromatography paper grade 2 with sizes of 2×2.5 cm 2 . The selection of this paper was due to the absence of impurities or additives associated to its uniform hydrophilic properties [6,42]. There was no need of a seed layer to assist TiO 2 growth [6].

Characterization techniques
The x-ray diffraction (XRD) measurements were achieved using a diffractometer PANalytical's X'Pert PRO MPD equipped with a X'Celerator 1D detector and using CuKα radiation. XRD diffractograms were recorded in the 20-75°2θ range with a step size of 0.033°. The rutile, anatase, brookite simulated powder diffractograms were obtained using PowderCell [43] and crystallographic data from [44].
Raman spectroscopy measurements were carried out using an inVia Qontor confocal Raman microscope from Renishaw. It has been used a 17 mW He-Ne laser operating at 532 nm, with a 10 s exposure time and settings of 3 accumulations. The Raman spectra were recorded in the range of 100-700 cm −1 .
Surface scanning electron microscopy (SEM) images were obtained with a Carl Zeiss AURIGA CrossBeam SEM-FIB microscope equipped for energy dispersive x-ray spectroscopy (EDS) analyses. The nanoparticles and films' dimensions have been estimated using SEM images and ImageJ software [45].
The optical bandgap was estimated through diffuse reflectance measurements carried out at room temperature using a Perkin Elmer lambda 950 UV/VIS/NIR spectrophotometer equipped with a diffuse reflectance module (integrating sphere with 150 mm diameter and internally coated with Spectralon). A standard reflector sample was used calibration (reflectance, R=1.00 from Spectralon disk). The reflectance (R) measurements were carried out from 250 to 800 nm.

Photocatalysis experiments
The TiO 2 nanoparticles and nanostructured thin film had their photocatalytic behaviour assessed by the photodegradation of rhodamine B (RhB) from Sigma-Aldrich at room temperature. The samples were positioned at the bottom of a beaker with 50 ml of the RhB solution (20 mg l −1 ) and left to stir for 30 min in the dark to establish absorption-desorption equilibrium. Absorbance measurements were taken every 10 min up to 70 min in the case of powder materials, and every 2 h up to a total of 6 h in the case of paper-based materials, using Shimadzu UV-3101PC UV-vis-NIR Scanning Spectrophotometer. For powder materials, 4 ml of the rhodamine B solution with the photocatalyst was collected after light exposure and separated by centrifugation for 5 min at 6000 rpm. After absorbance measurements, the 4 ml solution was returned for further measurements. The reusability experiments consisted in recovering the powder and centrifuging it with further discard of the total liquid. The recovered slurry was dried at 50°C for 3 h. The recovered dried powder was then poured into fresh solution and exposed to solar light for 70 min along several weeks [22]. The commercial Degussa P25 TiO 2 was tested in the same proportion as the produced powders (20 mg l −1 ).
The irradiation was achieved by using a LED solar simulator LSH-7320 (AM 1.5 spectrum) with an intensity of 1 Sun, and the photocatalytic experiments considered the International Standard ISO 10678 [46].

Results and discussion
TiO 2 nanoparticles and the nanostructured TiO 2 thin film were successfully synthesized under microwave irradiation using a cellulose-based substrate. i.e., Whatman paper. The produced nanostructures and thin film were systematically investigated and correlated to their final photocatalytic behaviour under solar radiation. All samples showed that the process is reproducible and reliable, where deviations observed on the set of the materials' characteristics analysed were below 4% and 10% for the powder and thin films, respectively. The approach presented opens-up to the simultaneous production of photocatalytic powders and disposable photocatalytic paper activated under solar light. Figure 1(a) depicts the SEM images of TiO 2 nanoparticles. The microwave synthesis resulted in very fine particles, appearing as nanospheres. These nanoparticles appear in an agglomerate, since after drying the powder there is aggregation in larger particles in the micrometer range. The average sphere diameter calculated was 11±0.5 nm. The powder produced was also analysed by x-ray diffraction and the XRD diffractograms are presented in figure 1(b). Most peaks of the experimental diffractogram could be assigned to the anatase phase, nevertheless the presence of brookite could also be observed (as a minor second phase or impurity). It has not been detected peaks coming from impurities such as Ti(OH) 4 and the XRD results demonstrate that the materials are well crystallized and highly nanostructured. This mixture of phases was previously reported with small amounts of brookite in TiO 2 anatase nanocrystals when synthesized by hydrothermal synthesis [47]. The mean particle size calculated from Scherrer's equation was 5 nm [48]. In analogous studies, HCl played a central role in the determination of the final TiO 2 crystalline phase. Both studies reported the presence of a mixture of rutile and brookite when using higher amounts of acid [14,22]. When comparing with [14], where 5 ml of HCl has been used, it can be concluded that after a certain amount of acid, the formation of anatase phase is preferred. In the present work, 2.5 ml of HCl resulted in mostly anatase with residual presence of brookite.

Structural characterization
In figures 2(a) and (b) it is presented the SEM images of the TiO 2 nanostructured films grown on the Whatman paper substrate without any seed layer. The paper roughness promoted nucleation and fixation of the TiO 2 nanostructures without any additional process [6,14,41]. After microwave synthesis, the Whatman paper fibres remain unaltered, and when magnifying the observed area, it is clear the presence of nanostructures forming a continuous film. The nanoparticles resultant from the microwave synthesized powder uniformly covered the paper substrate. The inset shows the cross section of the film, and it could be observed a compact film of agglomerated nanoparticles, with thickness around 130 nm. From EDS analysis, it can be observed the homogenous distribution of Ti within the substrate (figure 2(c)), together with the presence of C and O (figures 2(d) and (e)), where C is related to the use of a paper substrate. No impurities were detected.
From previous studies, it has been concluded that paper hinders the TiO 2 signal on XRD measurements [6,14], and for that reason Raman spectroscopy has been employed. The Raman spectra of the TiO 2 nanoparticles and the thin film grown on Whatman paper are presented in figure 3. The Raman spectrum of the pristine Whatman paper is also shown for comparison, and the minor bands marked with the star are associated to the substrate. The Raman spectra attested for the presence of the anatase and brookite phases (more evident in the powder Raman spectrum). The Raman bands associated to anatase were detected with a blue shift and can be assigned to 153 cm −1 (E g ), 204 cm −1 (E g ), 405 cm −1 (B 1g ), 512 cm −1 (B 1g +A 1g ), and 640 cm −1 (E g ) for anatase [47,[49][50][51]. This Raman band shift can be related to the structural defects present in the TiO 2 lattice, oxygen vacancies [47][48][49][50][51][52] or minor deviations from the TiO 2 films' stoichiometry [53]. Some Raman bands associated to brookite have also been observed (246, 322 and 365 cm −1 [54,55]). In fact, the Raman spectra of brookite phase display a characteristic intense band at 153 cm −1 , which can be similar and coincident with E g mode of anatase at 144 cm −1 [47]. However, being brookite a minor phase or impurity, it can be assumed that the 153 cm −1 band comes from anatase phase. XRD measurements on powder confirmed the Raman spectroscopy results ( figure 1(b)).

Optical characterization
Both powder and thin film materials had their optical band gap assessed through reflectance data using the Tauc plot [56,57]. The direct band gap values estimated were 3.07 eV and 3.44 eV for the TiO 2 nanoparticles and nanostructured thin film, respectively (figure 4). The estimated band gaps are within the reported values for the different TiO 2 phases [21,58,59]. Regarding the band gap determination, Reddy et al [60] reported both the direct and indirect band gap measurements, being the direct transition more favourable for anatase TiO 2 nanoparticles with sizes of 5-10 nm. Moreover, the disparity observed between the powder and the thin film materials is expected since the band gap depends on several factors including defects [61], residual strain [62], degree of compactness and densification [63], among others.

Photocatalytic behaviour
Both TiO 2 nanoparticles (powder) and the nanostructured thin film grown on paper had their photocatalytic activity evaluated through the rhodamine B photodegradation under solar radiation. It can be seen the gradual RhB degradation for the powder material ( figure 5(a)), reaching degradation values of 91% after 40 min. From 50 min, the reaction has reached a stable degradation pattern, reaching 93% at 70 min (last measurement) for the powder TiO 2 . The degradation ratio (C/C 0 ) versus exposure time is presented in figure 5(b) for the powder materials, where C is the concentration of the pollutant at each exposure time and C 0 is the initial solution concentration. When compared to the TiO 2 Degussa P25 catalyst, the synthetized powder revealed an enhanced photocatalytic activity under solar radiation (see figure 5(b)). Pure TiO 2 is photo-active under UV radiation, however in the present work, it can be observed an enhanced photocatalytic activity under solar radiation. This behaviour has been reported before and is associated to the presence of nano-sized TiO 2 [64]. Other factors also influence the photocatalytic activity and will be discussed in detail.
The photocatalytic activity relies on numerous factors including the band gap, crystallite size, degree of crystallinity, specific surface area, active facets, among others [7,65,66]. The band gap estimated for the TiO 2 nanoparticles are within what is expected for the different TiO 2 phases. Regarding the crystallite size, and as observed in figure 1(a) and calculated by XRD results, the microwave synthesis resulted in very fine particles  (∼11 nm). It is known that due to quantum size effect, the nanosized particles display enhanced redox ability. The migration of electrons and holes of smaller particles to their surface is facilitated, and the electron-hole recombination is reduced which increases the photocatalytic performance [67]. Thus, it is expected a substantial size contribution to the enhanced photocatalytic activity observed. Moreover, in general the specific surface area increases with the decrease of particle size [66], also contributing to the behaviour observed.
In terms of crystalline phases, XRD, Raman, and SEM (figures 1 and 3) showed that the microwave synthesis produced TiO 2 nanoparticles with mostly anatase and residual amounts of brookite (or impurities). Anatase is largely explored for photocatalytic applications [67], however it has also been reported that a combination of distinct TiO 2 phases will result in higher photocatalytic activity and efficiency than single phases [67-69], reducing the recombination of photogenerated holes and electrons [70]. Moreover, the mixture of anatasebrookite nanomaterials has been reported to have higher photocatalytic activity than just anatase or P25 [69,71,72]. Thus, some contribution to the photocatalytic performance observed can be expected coming from the mixture of anatase and brookite. Reusability tests were carried out for the TiO 2 powder, demonstrating reusability characteristics over time ( figure 5(c)). Nevertheless, as can be observed, there is a clear deterioration with the number of photocatalytic experiments [73,74], which can be related to the powder saturation with rhodamine B [74] or weight loss of the powder during experiments [75]. The blank rhodamine B solution was not significantly influenced under solar radiation, so all the degradation observed is due to the photocatalytic effect of the catalyst in both figures 5 and 6. Nevertheless, it is possible to observe a slight increase in the RhB concentration during exposure to a max of 4% of the initial concentration. This phenomenon can be explained by the evaporation of some of the solvent, which cannot be visible when the photocatalyst is present.
The photocatalytic activity of the TiO 2 nanostructured thin film was also studied and is presented in figure 6. The RhB degradation observed in figure 6(a) is accompanied by a slight hypsochromic shift of absorption bands [76], however in figure 6(b), it is clear its gradual degradation under solar radiation up to 6 h.
When comparing the thin film with the powder material (figures 5 and 6), it has been observed that the nanoparticles had a more effective RhB degradation than the nanostructured thin film. This is associated to the better adsorption activity of powders and photocatalytic efficiency when compared to films as a result of the larger surface area and higher amount of material available for reaction [77]. Nevertheless, the nanostructured thin film has reached a degradation value of 68% after 360 min (6 h). As explained for the powder materials, a size effect contribution to the photocatalytic activity can be thought for the film as well. As observed in figure 2, the film is composed by fine nanoparticles agglomerated to form a compact thin film. Several studies described the higher photocatalytic activity of films composed by nanoparticles [78][79][80], thus as in powder material, it is expected a significant contribution to the observed photocatalytic efficiency coming from the nanosized particles present. Moreover, as can be seen in figure 6(a), the 3D structure of paper contribute to the photocatalytic activity observed [41], associated to partial solution absorption by the Whatman paper which can be observed by the pinkish coloration on it after degradation (see inset in figure 6(a)). The contribution coming from Whatman paper absorption is expected to be ∼27% of the total RhB degradation, thus the RhB photodegradation value of 68% is the joint contribution of Whatman paper and the TiO 2 nanostructured thin film.
Regarding the phase's contribution, both nanoparticles and nanostructured thin film must have the same crystalline phases, even if it could not be seen clearly in figure 3 due to the cellulose signal contribution, hindering some TiO 2 bands. Thus, mixture of TiO 2 phases must have contributed to the enhanced photocatalytic performance just as observed for the powder material. The photocatalytic performance obtained by the nanostructured thin film is largely superior when compared to an analogous studies [6], mostly due to the structural differences observed. Thus, it can be stated that the microwave synthesis parameters selected and acid amount demonstrated to be of great importance to the final photocatalytic activity of the materials produced.
The materials were designed to be a reliable alternative for the commercial photocatalysts used nowadays, and to produce fully disposable materials, that can effectively contribute to the environmental protection, while diminishing production costs. The photocatalytic paper developed in the present study can easily be employed as filters or several other applications, as it easily adapts to different surfaces due to its highly flexibility, despite its green character.

Conclusions
TiO 2 nanoparticles and nanostructured thin films were simultaneously produced using a hydrothermal method assisted by microwave irradiation. Whatman paper has been selected as substrate for the thin film due to its lack of impurities. All materials were composed by a mixture of anatase and brookite polymorphs, with brookite appearing as a minor phase or impurity. It has been observed very fined particles appearing as spheres, that were also observed in the thin film. The nanostructured thin film revealed to be a compact film of agglomerated nanoparticles. Photocatalytic activity was evaluated with the rhodamine B degradation, with the nanoparticles showing the highest photocatalytic activity under solar radiation (91% after 40 min) and revealing reusability characteristics over time. The nanoparticles revealed to have around 12 times higher photocatalytic activity when compared to the commercial Degussa P25 photocatalyst. The nanostructured thin film demonstrated also enhanced photocatalytic performance reaching 68% of RhB degradation after 6 h of solar radiation exposure. The present study showed that this approach is a real and appealing option for the commercial photocatalysts and capable of producing disposable and inexpensive photocatalytic materials in flexible and green substrates.

Acknowledgments
This work is funded by National Funds through the FCT -Fundação para a Ciência e a Tecnologia, I.P., under the scope of the project UIDB/50025/2020-2023. The authors acknowledge Fundação para a Ciência e a Tecnologia for funding the Project ICARUS under the reference PTDC/EAM-AMB/30989/2017. The work was also partially funded by the Nanomark collaborative project between INCM (Imprensa Nacional -Casa da Moeda) and CENIMAT/i3N. A. C. Marques acknowledges funding to the National Foundation for Science and Technology, I P, through the PhD Grants SFRH/BD/115173/2016. Thanks are also due to EC project SYNERGY H2020-WIDESPREAD-2020-5, CSA, proposal n°952169. The authors also acknowledge funding from the European Research Council through the Starting Grant TREND, grant 716510 and from the European Community's H2020 program under grant agreement No. 787410 (ERC-2018-AdG DIGISMART).

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Additional information Competing financial interests
The authors declare no competing financial interests.