**2.1. Light paper**

bility, flexibility, mechanical strength, and hydrophobicity, is essential to assure the proper

Paper substrates are generally classified based on their weight or *grammage* and can be divided in three categories, following the suggestion of Tobjörk and Österbacka [43]: Light paper, whose *grammage* is in the range of 12–30 g m−2, or for thickness below 75 µm; standard paper, such as office paper, which is around 80 g m−2 and 100 µm thick; and cardboard (or paperboard), when the *grammage* exceeds 200–800 g m−2, or when the thickness is above 300 µm:

• **Light paper**. In this category, one can place two cellulose substrates that are attracting much attention in the recent years: bacterial cellulose (BC) and nanocrystalline cellulose (NCC).

Bacterial cellulose is a bacteria-produced biopolymer composed of ultrafine nanofibers (<100 nm wide); the major perks of this material, comparing to the nanocellulose obtained from wood, is its purity and crystallinity since it is free of lignin, hemicellulose, and other components present in the vegetable cellulose [64–66]. This material can be obtained from several cellulose-producing bacteria, such as *Gluconacetobacter genus, Agrobacterium tumefaciens*, bacteria of the genera *Pseudomonas*, among others; the cellulose is produced extracellularly since the bacteria excretes the cellulose into an aqueous culture medium, of low molecular weight sugars, directly as nanofibers, which form a porous three-dimensional nanocellulose mesh structure [67–69]. The grown layer is then harvested from the medium, cut and dried. Nanocrystalline cellulose can be obtained primarily from cotton. NCC membranes are prepared through the acid hydrolysis of cellulose process [70]. Cotton is one of the purest cellulose sources, given its higher amount of cellulose (90 wt%), when compared with other vegetal sources that usually contain a mass fraction between 50 wt% (wood) and 80 wt% (flax or hemp) [40, 67, 71]. These membranes are highly transparent, lightweight, and have a smooth surface [40]. The flexibility of some NCC papers and the unique optical properties of nanocrystalline cellulose open a wide range of cost-efficient applications, for instance, smart

• **Standard paper**. There are countless types of paper in this category. The varieties here discussed are the most relevant for this chapter: commercial office paper (COP), raw paper (RP), the *Whatman* filter papers (WFP1 and 4), and the vegetable papers – parchment paper (PP) and tracing paper (TP), with thickness of 60, 135, 180, 205, 52, and 80 µm, respectively. The commercial office paper presented here is an 80 g m−2 *grammage* paper made from the Portuguese paper manufacturer the *Navigator* company. Raw paper (63 g m−2) is fabricated by *Felix Schoeller Group*. The W*hatman* filter papers are used worldwide for chromatography and are a registered trademark of *GE Healthcare*. WFP1 and WFP4 *grammage* is 88 and 96 g m−2, respectively. Parchment paper (41 g m-2) is commonly used in baking as a disposable nonstick surface and the tracing paper studied here, with a *grammage* of 90 g m−2, was provided by *Canson*, a manufacturer of fine art paper and is specially adapted to technical

• **Cardboard**. For this category, the paper studied is called liquid packaging cardboard (LPC) and it is produced by the Finnish company *Stora Enso* [72]. LPC is an aseptic and biodegradable

labels, RFID, smart packaging, or even as support for bio-applications.

device operation.

40 Nanostructured Solar Cells

drawing.

The X-ray diffraction (XRD) diffractogram (**Figure 2a**) provides information regarding the crystallinity index and crystallite size. Bacterial cellulose (BC) and cotton-based nanocrystalline cellulose (NCC) show the presence of characteristic cellulose type I, also defined as native cellulose, given the characteristic peaks at 2*θ* = 14.7°, 16.9° and 22.9° for BC and 14.7°, 16.8° and 22.7° for NCC. These cellulose type I peaks correspond to the 1¯ 10, 110, and 002 crystallographic planes, respectively. The XRD patterns were collected from 10° to 90° (2*θ*), with a scanning step size of 0.016°, with a monochromatic CuKα radiation source (wavelength 1.540598 Å). The crystallinity index, *I* C, was determined using the method proposed by Segal et al. [77]:

$$I\_c = \frac{(I\_{\text{(002)}} - I\_{\text{(am)}})}{I\_{\text{(on2)}}} \times 100\tag{1}$$

where *I* (002) is the maximum intensity of the diffraction of the (002) lattice peak, taken at a 2*θ* angle between 21° and 23°, and *I* (am) is the intensity of the diffraction of the amorphous regions, taken at the minimum on a 2*θ* angle range between 18° and 20° [40, 41].

The inexistence of lignin and hemicellulose in BC leads to a high crystallinity index (~92%) and a crystallite average size of 5.7 nm, higher than NCC which lies around 80% and has a crystallite average size of 7 nm.

Attenuated total reflectance-Fourier-transformed infrared (FTIR-ATR) spectroscopy provides information regarding the chemical species, chemical bonding state of cellulose, and to infer on the water content. Measurements were recorded using an attenuated total reflectance (ATR) sampling accessory (Smart iTR) with the following conditions: incident angle of 45°; 4000–650 cm−1 range; 4 cm−1 resolution; 32 scans; 20°C. The FTIR-ATR spectrum (**Figure 2b**) of BC and NCC resembles that typically obtained for plant-based cellulose and depicts the characteristic cellulose absorption peaks, i.e., hydrogen-bonded OH stretching at 3400 cm−1, C−H and C−O stretching vibrations, 2900 and 1060 cm−1, respectively. The O–H bending vibration assigned to the absorbed water is also observed with the presence of a peak at 1640 cm-1 [40].

**Figure 2.** (a) XRD diffractogram of bacterial cellulose (BC) and cotton-based nanocrystalline cellulose (NCC) dried membranes; (b) FTIR spectrum of BC and NCC dried membranes; (c) total transmittance.

The BC membranes have a smooth structure of long entangled filaments of extruded cellulose fibrils with an average width of 20–100 nm (**Figure 3a**) and a surface *RMS* roughness in the order of 60 nm [68, 69, 78]. As for NCC, one can observe a smooth and compact morphology (**Figure 3b**) and also a needle-like structure in the cross-section inset.

**Figure 3.** SEM micrograph of the surface morphology of (a) bacterial cellulose and (b) nanocrystalline cellulose membranes (NCC) produced by evaporation and the corresponding cross section (inset). Adapted from Gaspar et al. [40], with permission from IOP Publishing.

The low porosity, roughness in the order of nanometers, and high compactness of the cellulose fibers give these paper substrates a high contact angle (~95°C, accessed through 1 µL of deionized water droplet), which translates in a significant hydrophobicity that is beneficial to microfluidic devices [79], self-cleaning and to prevent the negative effects that the absorption of water can have on electronics devices.
