## 3 Results and Discussion
### Structural Characterization  
Figure 2 depicts the SEM images of pristine Au paper together with the TiO\({}_{2}\) nanostructured films. As observed in analogous studies [10; 43], the TiO\({}_{2}\) nanostructured films were grown on the cellulose-based substrate without any seed layer and directly on the Au contact. It has been previously stated that the paper roughness facilitates nucleation and fixation of the TiO\({}_{2}\) nanostructures [10]. The structure of the deposited Au film is presented in Figure 2a, and no similarity to the TiO\({}_{2}\) films could be observed. The Au contacts appear as a uniform and homogeneous layer, without any visible porosity. The cellulose fibers of the Whatman paper could be observed after microwave synthesis, however with the magnified SEM image, it is clear that the substrates were uniformly covered, forming continuous TiO\({}_{2}\) nanostructured films. Moreover, the structural differences observed comparing the acids are evident. The TiO\({}_{2}\) film synthesized with nitric acid is composed by densely and closely packed nanoparticles joined to form spherical-like structures nanoaggregates, with \(\sim\)30 nm of mean diameter (Figure 2b). From the cross-section, it can be seen that the nitric-based film is highly porous, with film thickness around 350 nm. When it turns to the hydrochloric-based TiO\({}_{2}\) film (Figure 2c), it can be observed a rougher overall structure, but formed by fine densely agglomerated nanoparticles (\(\sim\)10 nm). From FIB measurements, it could also be observed nanoporosity, with a film thickness of 200 nm. In the case of the acetic acid, a compact and homogeneous film could be observed after synthesis. The nanostructured film was composed by nanoparticles with sizes around 10 nm, and thickness varying from 300 to 500 nm. Low porosity could be observed from the cross-section, in fact the acetic film revealed to be thicker than the other materials synthesized (Figure 2d). The expressive structure differences observed regarding the acid used are expected to be related to the microwave synthesis path. Figure S1 shows the microwave pressure profile associated to the acid used. It is clear the synthesis using nitric acid reached highest pressure values throughout the synthesis, followed by hydrochloric acid and then acetic acid. The microwave pressure/temperature relation to the outcome of nanoparticles has been previously reported. [PERSON] et al. [49] suggested the direct effect of applying higher microwave synthesis pressure to the morphology and size of the nanomaterials. It has been also demonstrated the influence of the solvent used during syntheses and the final synthesis temperature/pressure with a direct effect on the nanoparticle structure [39; 50]. These effects are related to the microwave interactions to the reagents or precursors, and solution medium, that will influence the nucleation and crystallization rates [51]. EDS analyses were carried out on all films synthesized, confirming the homogeneous distribution of Ti along the substrates (Figure 3). Au comes from the contacts deposited, and no other impurities were detected. The significant presence of C and O is also related to the Whatman paper substrate used [43]. Moreover, it has been noticed that the Ti EDS map is more intense for the acetic-based film, with is expectable since this is the thicker film synthesized (Figure 3l). Raman spectroscopy and X-Ray diffraction measurements were carried out for the TiO\({}_{2}\) nanostructured films together with the pristine Au paper. A considerable contribution from the substrate was observed on XRD diffractograms, hindering the TiO\({}_{2}\) signal (Figure 4a). The XRDcharacteristic peaks of native cellulosic fibers are clearly observed at 22.7\({}^{\circ}\) and 35\({}^{\circ}\), in agreement to the literature [43; 52]. Nevertheless, at \(\sim\)25\({}^{\circ}\), a slightly peak was detected, which can be associated to the presence of anatase.