**3. Carbon-based nanostructures**

The development of heterogeneous photocatalysts by combining metal sulfides and different carbon nanomaterials has been explored as an effective strategy to obtain high-performance photocatalysts. Owing to delocalized electrons from the conjugative π-system, graphitic carbon nanostructures are good at accepting and shuttling the photogenerated electrons from semiconductor photocatalysts; hence, effectively separating the electron–hole pairs [89–93]. For instance, Wan *et al*. have shown that the synergistic influence of charge-carrier migration, advanced excited states, and suitable Fermi levels between CdS phases and graphene leads to enhanced photoactivity and stability [94]. Also, Lv *et al*. have shown that graphene attached to semiconductors can efficiently accommodate and transport electrons from the excited semiconductor, which not only hindered charge recombination but also improved charge transfer, giving rise to high photocatalytic efficiency [89]. These works confirmed the relevant role of graphene, among the carbon-based nanomaterials, in aqueous colloidal chemistry processes, such as heterogeneous photocatalysis. Thus, in this chapter, graphene and its derived nanostructures are used as illustrative examples in the fabrication of carbon-supported metal-sulfides photocatalysts.

Graphene is a 2D material formed by a one-atom-thick planar layer of sp2 -hybridized carbon atoms that resemble a chicken-wire-shaped lattice, presenting outstanding electronic, thermal, and mechanical properties [95]. Graphene is the basic structural material of graphite, which result from the overstacking of graphene monolayers *via* van der Waals forces, resulting in interspaced neighboring layers that are 0.34 nm far apart [96, 97]. The carbon atoms in each graphene sheet establish covalent bonds due to the overlapping of trigonal planar sp<sup>2</sup> hybrid orbitals. The overlapping of the perpendicular unhybridized p*<sup>z</sup>* orbitals accounts for the formation of the VB and the CB, respectively composed of filled π orbitals and empty π\* orbitals [98].

The mechanical exfoliation of graphite creates free-standing graphene sheets, as shown by Novoselov and Geim, who used sequential micromechanical cleavage of graphite using the "scotch-tape method." The authors were honored with the Nobel Prize in Physics in 2004, 6 years later to such an important finding [98, 99]. The direct exfoliation of bulk graphite produces layers of graphene with good quality and crystallinity, low defect densities, and high conductivity, but frequently, at a low yield

[100]. As such, graphene layers can be obtained by the chemical exfoliation of a lowcost raw material bulk graphite, which applied together with selected chemicals produce graphene and graphene derivatives, such as GO and reduced graphene oxide (rGO) [100–102]. Although water is a first-choice medium for the production of graphene-based materials, the hydrophobic nature of pristine graphene sheets tends to promote their restacking, which makes exfoliation challenging. The use of surfactants during the exfoliation processes has been considered to overcome this limitation because they allow exfoliated layers to remain suspended and avoid overstacking [101, 103]. The success of the exfoliation processes is overcoming the van der Walls forces by increasing the distance between the layers *via* chemical intercalation. Ideally, to obtain good dispersion of graphene layers, the solvents should have surface tensions of 40 mJ/m<sup>2</sup> [97, 101, 104]. Therefore, graphene can be exfoliated by the sonication of graphite in dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), pyridine, and perfluorinated compounds [98, 101, 104, 105]. For instance, Hernandez *et al*. have used sonication-based exfoliation of graphite in NMP to obtain a final material containing graphene monolayers (28%) and nanosheets less than six atomic layers thick, almost in quantitative yield [106]. Commonly used sonication exfoliation processes involve shear forces and cavitation mechanisms, which involve the growth and collapse of micrometer-sized bubbles, acting on the bulk material precursor and causing their exfoliation [97].

GO is composed of sp<sup>2</sup> graphene layers with a high content of oxygen-containing functional groups, such as hydroxyl, epoxy, carboxylic, and carbonyl groups [107]. The UV–visible absorption spectra of GO suspensions show an absorption peak ascribed to π-π\* electronic transitions of aromatic C-C bonds and n-π\* transitions of the oxygen-containing groups, at around 230 nm and 315 nm, respectively [108]. The aqueous suspensions of GO are normally stable due to the hydrophilic character of oxygen-containing groups present in the sheets'surfaces, namely at the edges. Colloidal stability is favored by the electrostatic repulsion that arises due to anionic groups that form due to extensive proton dissociation in such functional groups, over a certain pH range. On the other hand, the presence of out-of-planar C-O covalent bonds increases the interlayer distance from 0.34 to 0.65 nm, therefore decreasing the energy needed to separate the graphene layers [96, 98, 107]. The hydrophilic nature of oxidized graphite facilitates water to be adsorbed into its lamellar structure, showing a further increase in the interlayer distance to 1.15 nm [109]. For instance, the use of polar solvents (e.g., ethanol, acetonitrile, and dimethyl sulfoxide) allows the preparation of stable colloids but either flocculation or aggregation occur when nonpolar organic solvents are used as the dispersing medium [107].

Carbon nanotubes (CNT) are 1D materials formed by graphene sheets rolled around a common axis, with diameters reaching between 0.5 and 100 nm, and lengths extending several micrometers or even millimeters [110]. CNT can be single- (SWCNT) or multi-walled (MWCNT) according to the number of graphene sheets rolled-up, that is, a single sheet or more than one, respectively. SWCNTs have diameters in the range of 1–2 nm and MWCNT show typical diameters in the range of 10–100 nm range [111]. Pristine CNT has hydrophobic nature, and their high aspect ratio favors interparticle van de Waals forces mediated by the outer walls, which results in a tendency for CNT aggregation [112]. Thus, non-functionalized CNT dispersed in a liquid medium exists as large bundles, which limit handling and, consequently, their use in many applications. Usually, mechanical disentanglement of CNT bundles is achieved by ultrasonication of the respective dispersions in which shear forces promote the separation of CNT but can also cut such nanostructures.

Nevertheless, the debundling process depends on the modification of the CNT surface by using chemical agents that enhance the compatibility of the CNT with the dispersing medium. Hence, surface modifiers, such as surfactants, homopolymers, and block copolymers, have been used to promote the dispersion of CNT in aqueous environments. In addition, surface oxidation treatments that result in the presence of carboxylic, hydroxyl, and carbonyl functional groups at the end of the tubes and on their sidewalls, also allow better dispersions of CNT in water [113].

Powder X-ray diffraction (XRD) has been used to check the crystalline structure of graphitic materials. Bulk graphite shows a strong Bragg diffraction peak at 26.6° corresponding to the reflection of (002) planes and associated with an interlayer distance of 0.34 nm. The oxidation and exfoliation of graphite increase the interlayer distance changing the peak position of the basal (002) reflection from 26.6 to 11.2°, which corresponds to an interplanar distance of 0.79 nm, as observed for GO materials [98].

Raman spectroscopy has been a key instrumental technique to study graphene materials, such as the surface chemistry of GO and the existence of structural defects. The Raman spectra of graphitic materials are typically characterized by three distinct vibrational bands: the G-, D-, and 2D-bands. The G-band is observed around 1580 cm<sup>1</sup> and is ascribed to the in-plane bending mode of the sp<sup>2</sup> hybridized carbon atoms in graphene. In high-quality graphene, this band is very sharp, suggesting its high crystallinity and non-defect structure. The D-band at around 1350 cm<sup>1</sup> has been associated with the amount and type of defects in the carbon lattice, for example, the existence of sp<sup>3</sup> hybridization or due to vacancies [114]. The extension of such defects in the carbon sheet, either at the edges or topological defects, have been monitored by Raman measurements using such diagnosis band, namely by computing the intensity ratio between the G- and D-bands [98, 104, 114]. In the Raman spectrum of highquality pristine graphene, the D-band is not observed or is very weak, but it is observed in GO samples due to the presence of different oxygen functional groups in the carbon sheets. Hence, the D-to-G Raman band intensity ratio provides useful information on the nature and extension of structural defects that characterize the GO samples [94]. The 2D band is an overtone of the D-band, resulting from a two-photon lattice vibrational process. For true single-layer graphene, such a band occurs as a symmetric feature below 2700 cm<sup>1</sup> [104, 114]. Overstacking of successive layers results in structures of less symmetry with a Raman shift to higher wavenumbers [98]. For example, in graphite and graphite oxide materials, it is observed a broad band at about 2800 cm<sup>1</sup> . The features of the G and 2D bands are particularly useful in exfoliation and surface modification laboratorial tasks because are the first indication for distinguishing between monolayer (or few-layer) graphene and graphite-based materials. Furthermore, it has been shown that Raman methods applied to GO modified with metal sulfides are an alternative strategy to probe the surface of nanocomposite photocatalysts [115].
