Carbon Nanostructures Decorated with Titania: Morphological Control and Applications

: Nanostructured titania (TiO 2 ) is the most widely applied semiconducting oxide for a variety of purposes, and it is found in many commercial products. The vast majority of uses rely on its photo-activity, which, upon light irradiation, results in excited states that can be used for diverse applications. These range from catalysis, especially for energy or environmental remediation, to medicine—in particular, to attain antimicrobial surfaces and coatings for titanium implants. Clearly, the properties of titania are enhanced when working at the nanoscale, thanks to the increasingly active surface area. Nanomorphology plays a key role in the determination of the materials’ ﬁnal properties. In particular, the nucleation and growth of nanosized titania onto carbon nanostructures as a support is a hot topic of investigation, as the nanocarbons not only provide structural stability but also display the ability of electronic communication with the titania, leading to enhanced photoelectronic properties of the ﬁnal materials. In this concise review, we present the latest progress pertinent to the use of nanocarbons as templates to tailor nanostructured titania, and we brieﬂy review the most promising applications and future trends of this ﬁeld.


Titania Properties and Uses
Titanium dioxide or titania is certainly the most studied semiconducting oxide due to its well-established photo-activity. Well-known features that render it so attractive are its low cost, negligible toxicity, high stability, easy handling, and resistance to chemical corrosion. This semiconductor has the known ability to absorb light in the ultraviolet (UV) wavelength range and generate excited charges, electrons and holes, which separate in the conduction and valence band, respectively, and are at the core of its photo-activity. This key property of TiO 2 has been widely investigated firstly in catalysis and later in medicine, especially to attain antimicrobial coatings for medical implants [1]. The wide applications of titania for air and water remediation, cultural heritage preservation, and self-healing, as well as microbial inactivation and the mitigation of SARS-CoV-2 spreading onto surfaces, have been recently reviewed [2] and are thus not discussed in detail in this review.
Despite the many advantages offered by titania, this material also suffers from some drawbacks, such as a rapid recombination of the photo-excited charge carriers and the relatively wide band gap (in the range of 3.2 eV) which implies a poor utilization of solar light due to the small percentage of UV radiation of sunlight reaching the Earth's surface.
Despite the many advantages offered by titania, this material also suffers from some drawbacks, such as a rapid recombination of the photo-excited charge carriers and the relatively wide band gap (in the range of 3.2 eV) which implies a poor utilization of solar light due to the small percentage of UV radiation of sunlight reaching the Earth's surface. Remedies that have been developed to address these issues include the use of suitable supports [3], doping with other elements [4], embedding in composites [5], and the development of suitable nanostructures [6], also following green protocols [7]. These approaches have indeed allowed the practical widespread use of TiO2-for instance, for biomass conversion processes [8]-thanks to the improved hydrothermal stability of the catalyst [9]. In particular, the combination of titania with noble metals and carbon nanostructures has allowed the development of high-performing gas sensors [10] and photocatalysts [11]. Dye-sensitizers combined with titania allow metal-free photocatalysts to be attained for hydrogen production using visible light [12]. Hydrogen-peroxide production is another area in which titania is highly promising [13], as well as solar cells [14].
The morphology and crystallinity of the titania phase are important parameters in the determination of the resulting photo-electronic properties of the final material [15]. However, only a few studies compare the photo-activity of the three phases of titania (rutile, brookite, and anatase; Figure 1) [16], while the majority of works focus on the generally best-performing anatase phase [17]. Titania can be produced by several methods, with sol-gel [18], hydrothermal [19], and solvothermal [20] processes being the most popular, and in some cases also microwave-assisted methods [21]. However, other approaches are also often used, such as anodic oxidation [22] and atomic layer deposition [23], and others are under continuous development; for instance, the use of laser beams [24], molecular layer deposition [25], and air-plasma spraying [26].

Carbon Nanostructures Properties and Uses
Carbon nanostructures represent a large family of materials based on carbon characterized by a diversity of morphologies and structures ( Figure 2) [27]. Typically, they present very interesting electronic and conductive properties that arise from the extended conjugation of sp 2 atoms, although exceptions exist. For instance, nanodiamonds contain mainly sp 3 -hybridized carbon atoms, as the name suggests [28]. Nanocarbons have been engineered to feature sp-hybridized carbon atoms as well, especially for energy conversion and storage applications [29]. In particular, sp-hybridized one-dimensional "synthetic carbon allotropes" are emerging as attractive molecular wires for advanced applications in electronics and opto-electronics [30].

Carbon Nanostructures Properties and Uses
Carbon nanostructures represent a large family of materials based on carbon characterized by a diversity of morphologies and structures ( Figure 2) [27]. Typically, they present very interesting electronic and conductive properties that arise from the extended conjugation of sp 2 atoms, although exceptions exist. For instance, nanodiamonds contain mainly sp 3 -hybridized carbon atoms, as the name suggests [28]. Nanocarbons have been engineered to feature sp-hybridized carbon atoms as well, especially for energy conversion and storage applications [29]. In particular, sp-hybridized one-dimensional "synthetic carbon allotropes" are emerging as attractive molecular wires for advanced applications in electronics and opto-electronics [30].  [31]. The nano-onion is reproduced with permission from [32], copyright © 1996, Elsevier.
Among the most studied carbon nano-allotropes are zero-dimensional fullerenes [33], which can be considered as soccer-ball shaped structures, one-dimensional carbon nanotubes (CNTs), which have a tubular morphology [34], and two-dimensional graphene-based materials [35], which generally consist of nanosheets. Other well-known examples include nano-onions (CNOs) [36], which consist of concentric fullerenes, and nanohorns (CNHs), which arise from clusters of nanocones [37]. In recent years, the class of carbon dots has attracted attention thanks to their ultrasmall structure (<10 nm), which provides them with characteristic luminescent properties [38].
Among the most studied carbon nano-allotropes are zero-dimensional fullerenes [33], which can be considered as soccer-ball shaped structures, one-dimensional carbon nanotubes (CNTs), which have a tubular morphology [34], and two-dimensional graphenebased materials [35], which generally consist of nanosheets. Other well-known examples include nano-onions (CNOs) [36], which consist of concentric fullerenes, and nanohorns (CNHs), which arise from clusters of nanocones [37]. In recent years, the class of carbon dots has attracted attention thanks to their ultrasmall structure (<10 nm), which provides them with characteristic luminescent properties [38].

Carbon Nanostructures as Templates for Titania Nanomorphologies
The efficiency of titania photo-activity can be significantly improved through nanostructuring and heterostructuring with carbon nanomaterials, allowing the enhanced use of the solar spectrum and better charge separation [87]. The addition of carbon nanostructures can lead to an increase of the adsorption capacity, of the absorption of visible light, and of the lifetime of photogenerated electron-hole pairs. In particular, CNTs have been widely used as supports for the growth of nanosized titania. The advantages of interfacing TiO 2 with CNTs include not only improved structural stability, but also electronic communication between the two phases, whereby charge carriers (electrons or holes) may be transferred through the phase boundaries through several proposed mechanisms. For instance, a general view is that CNTs can efficiently scavenge the titania photoexcited electrons, thus retarding the electron-hole recombination rates and resulting in enhanced photocatalytic activities [88]. It is worth noting, however, that alternative hypotheses have been advanced; for example, on the basis of transient absorption experiments, where the photogenerated holes are transferred from the TiO 2 to the CNT [89], confirming the complexity of the electron transfer dynamics, which for example could depend on the level of functionalization (in particular, with oxygenated groups) of the CNT surface. Other reports have suggested that CNTs can act as photosensitizers, with the electrons excited and injected from the CNTs into the TiO 2 conduction band, with subsequent electron transfer from the titania valence band into the CNTs, thus yielding a charge separation state (electron-hole) on the metal oxide. However, such sensitization effects assume a semiconducting character of the CNT, which cannot always be taken for granted. In general, the micro-structure of the CNT and its functional group distribution, doping, and method of preparation deeply affect the photochemical response. Furthermore, Ti-C and Ti-O-C bonds can be formed during calcination, thus resulting in a "doping" effect with the formation of intragap states within the titania and a consequent improved absorption of visible light [88].
The use of carbon nanomaterials to template titania with nanomorphological control was reviewed in 2016 [90]; thus, we focus here on the progresses made over the last five years. An overview of selected examples is provided in Table 1. A summary of the physicochemical properties of carbon nanostructures after decoration with titania, including the type of surface functionalization used to anchor the metal oxide, is provided in Table 2.
Carbon nanodots are not present in the table since they are typically not used as templates; rather, they are added onto preformed titania, for instance on anodized nanotube arrays for uses in photocatalysis [91] and sensing [92]. Readers with a particular interest in graphene quantum dots incorporated in titania nanostructures are referred elsewhere [93]. Furthermore, studies that used commercially available titania nanoparticles are not included, since our focus is on the influence of carbon nanomaterials on their formation. 1 ALD = atomic layer deposition. 2 @ denotes core@shell structure. 3 SWCNTs = single-walled carbon nanotubes. 4 C 60 is a fullerene with 60 carbon atoms. 5 MWCNTs = multi-walled carbon nanotubes. 6 PTO = potassium titanium oxide oxalate. 7 TDMAT = tetrakis (dimethylamino)titanium. 8

Fullerenes
Fullerenes have been intensively studied for photocatalytic applications, as well as in combination with other semiconductors as photocatalyst enhancers [175]. The use of fullerenes as nano-templates for titania nucleation and growth was mainly developed in previous years [88], despite a modern renaissance of their use in solar cells [176].
Zinc-functionalized fullerene was recently combined with nanostructured titania for water remediation; however, the two nanomaterials were formed separately and only later combined to make nanostructured composites [177]. The interface between fullerenes and titania has been deeply investigated. It was recently found that defect states in the band gap of titania are quenched by C 70 while an interfacial state appears, showing a barrier-free extraction of charges ( Figure 3) for the next generation of organic solar cells [178]. However, in studies such as this one, fullerenes are added onto preformed titania.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 2 Figure 3. C70/titania band diagrams derived from the photoemission spectra, with energy gap based on optical absorption data. Reproduced with permission from [178], copyright © 2021 American Chemical Society.

Nano-Onions
Carbon nano-onions consist of concentric fullerenes and have been used usually t template nanostructured titania by means of sol-gel methods. X-ray diffraction analyse confirmed that the nanocarbons were anchored onto anatase, whose microscopy image suggested a certain level of control against the particle agglomeration exerted by th nano-onions, relative to a reference without the template, despite the limited morpho logical control over the titania particles. The nano-onions enhanced the specific surface area average pore size, and pore volume of the composite leading to the adsorption of pollutants as well as its visible-light absorption, overall leading to a more efficient photocatalytic dy degradation as demonstrated on rhodamine B. Finally, the paramagnetic nature of th In an interesting study, fullerenol was used as a template for titania to exploit the buckyball's hydroxyl groups as nucleation sites for the generation of titania nanoparticles by atomic layer deposition using titanium tetraisopropoxide as a precursor [94]. Given the low temperatures used in the process, the resulting titania was amorphous, yet it surprisingly demonstrated photocatalytic activity that was ascribed to the presence of fullerene in the composite material [94].

Nano-Onions
Carbon nano-onions consist of concentric fullerenes and have been used usually to template nanostructured titania by means of sol-gel methods. X-ray diffraction analyses confirmed that the nanocarbons were anchored onto anatase, whose microscopy images suggested a certain level of control against the particle agglomeration exerted by the nanoonions, relative to a reference without the template, despite the limited morphological control over the titania particles. The nano-onions enhanced the specific surface area, average pore size, and pore volume of the composite leading to the adsorption of pollutants, as well as its visible-light absorption, overall leading to a more efficient photocatalytic dye degradation as demonstrated on rhodamine B. Finally, the paramagnetic nature of the nano-onions with a magnetic core allowed for the easy recovery of the composite through magnetic separation [95].
Carbon nano-onion/anatase hybrids have been proposed as innovative replacements for graphite anodes in lithium-ion batteries. Titania was formed with a sol-gel method onto the nanocarbon template, exerting a certain level of both morphological and size control over the nanocrystals, which appeared to be smaller than 10 nanometers in diameter. Furthermore, the presence of the conductive carbon nanostructure overall significantly enhanced the electrochemical properties of the final material relative to the reference without the nano-onions [96].
Carbon nano-onions/anatase materials were also obtained by a hydrothermal route, with various mass compositions, ranging from an excess of the nano-onion to an excess of the titania, as confirmed by thermogravimetric analyses. The best results within the series were obtained with compositions featuring approximately 80-90 wt% nano-onions. In particular, high-resolution transmission electron microscopy (HR-TEM) images ( Figure 4) revealed an intimate contact between the inorganic and the carbon components, with both the graphitic walls of the nano-onions and the lattice of the anatase nanocrystals being clearly visible. These materials demonstrated the highest capacitance and an enhanced electrochemical performance overall, which was rationalized through a cooperative effect of both the electrochemical double layer capacitance from the nano-onions and non-capacitive Faradaic storage regarded as pseudocapacitance from titania [97]. Carbon nano-onions have been proposed for use in dye-sensitized solar cells, thanks to a number of advantageous features, including their limited cost, ease of dispersibility that does not require the use of binders-in contrast with other carbon nanomaterials-and the high level of optical transparency of the final device [179]. Appl. Sci. 2021, 11, x FOR PEER REVIEW Figure 4. HR-TEM images of carbon nano-onion/anatase nanocrystals with different rat two components, consisting of either (a) 89 wt%/11 wt% or (b) 83 wt%/17 wt%, resp White-dotted circles highlight the nano-onion cores; yellow-dotted circles indicate the between the two components. The nano-onion graphitic walls area visible with the typ nm-distance, as is the lattice spacing of 0.35 nm, corresponding to the (101) plane of ana printed from [97], Copyright © 2017, with permission from Elsevier.

Nanocones
The cone-like morphology of carbon nanocones leads to very interesting

Nanocones
The cone-like morphology of carbon nanocones leads to very interesting features, such as an uncommon graphene-sheet stacking depending on the apex (cone) angle, which in turn implies the presence of a number of pentagons within the six-membered ring patterns, creating the conical geometry. In the absence of pentagons, the angle can reach 0 • , thus resulting in a flat disk. Nanocones are normally produced as a mixture of these varied structures with differing geometrical parameters and containing a large excess (approximately 70%) of disks. The nanocones were oxidized to display hydroxyl groups that could act as anchor sites for the inorganic phase, which was grown as a uniform coating reproducing the nanocone morphology with high fidelity through a sol-gel process. The coating included embedded palladium nanoparticles, and the final material displayed very high photocatalytic activity under UV irradiation thanks to an increased surface area and the ability of the carbon framework to scavenge the photoexcited electrons to retard the charge recombination rates. Despite the lower conductivity relative to carbon nanotubes or graphene, nanocones provide other advantages, such as the ease of dispersibility in liquid media. The intimate contact between the organic and the inorganic phases, leading to an increased number of heterojunctions, was responsible for the higher activity in photocatalytic hydrogen production, using ethanol as an environmentally friendly sacrificial donor, which is conveniently a product of fermentation from feedstock (corn, sugarcane, etc.) [98].

Nanohorns
Nanohorns can be considered as clusters of nanocones, and they have also been used as templates for titania. The nanocarbon entanglement features a high surface area that templates a large distribution of "hard-soft" bimetallic sites, where 1.5 nm palladium nanoparticles were embedded within the titania phase while being electrically wired to an electrode by the nanohorn support. This hybrid electrocatalyst activated carbon dioxide reduction to formic acid nearly at zero overpotential in the aqueous phase, while being able to produce hydrogen thanks to a sequential formic acid reduction [99].
In another study, nanohorns templated the formation of anatase with nanoflower morphology through a solvothermal method. The photocatalytic degradation of two model dyes (i.e., methylene blue and methyl orange) was combined with the generation of hydrogen fuel upon solar-light irradiation [100].
Besides the prospective uses in solar cells [180,181], nanohorns are potentially suitable for biological uses thanks to their additional advantageous features relative to other carbon nano-allotropes, such as the ease of oxidation and the high dispersibility in buffer solutions [182]. To this end, they have been used as templates for nanostructured titania with the nanohorn morphology and applied to phosphoproteomics on cancer cell lysates for diagnostics purposes thanks to the well-known binding affinity for titania with phosphorylated peptides (Figure 5) [101].

Appl. Sci. 2021, 11, x FOR PEER REVIEW
Besides the prospective uses in solar cells [180,181], nanohorns are p able for biological uses thanks to their additional advantageous features r carbon nano-allotropes, such as the ease of oxidation and the high dispers solutions [182]. To this end, they have been used as templates for nanostr with the nanohorn morphology and applied to phosphoproteomics on can for diagnostics purposes thanks to the well-known binding affinity fo phosphorylated peptides ( Figure 5) [101].

Nanodiamonds
Nanodiamonds' high thermal conductivity, hardness and friction-resistance, high surface area, chemical inertness, non-toxicity, tunable structure, and excellent opto-mechanical properties have rendered them attractive building blocks in nanotechnology [183]. Nanodiamonds are typically produced by explosive detonation under oxygen-deficient conditions and consist of nanoparticles as small as 4-5 nm with a large portion of sp 3 -hybrized carbon atoms, in addition to sp 2 -hybridized atoms. A hydrothermal method at 70 • C was used as an easy and low-cost production method for the nucleation and growth of titania nanoparticles. Despite the limited control over the particle morphology and the presence of agglomerates, the presence of the nanodiamonds allowed for a reduced recombination rate of photogenerated charge carriers, with an improved photodegradation of bisphenol A as a model organic pollutant [102]. Similar protocols produced nanodiamonds-titania composites that were successfully tested for the photodegradation of other pollutants, thus confirming their versatility towards environmental remediation [103,104]. Further, this kind of nanomaterial has been envisaged as an anodic component for lithium batteries, thanks to the features of nanocarbon, such as its high lithium adsorption capacity, wide surface area, and chemical inertness, as well as is overall high capacitance and the long-term cycle stability of the composites [105].

Single-Walled Carbon Nanotubes (SWCNTs)
The use of SWCNTs is less common than MWCNTs due to the latter being easier to handle and disperse and presenting lower costs of production. In one study, SWCNTs successfully templated the formation of titania through a sol-gel method, and subsequent calcination at 600 • C allowed for the crystallization into anatase/rutile phases onto the tubes and the formation of an aerogel, which was tested for photocatalysis. The addition of platinum nanoparticles as co-catalysts onto the tubes, prior to titania nucleation and growth, was also studied to enhance the activity thanks to a more efficient charge carrier separation [106].
In another work, SWCNTs were first coated with a dendrofullerene ( Figure 6) to provide suitable anchoring points for the nucleation and growth of titania while leading to good performance with regard to photo-catalytic H 2 evolution under visible-light irradiation. Co-axial inorganic-organic nanowires were thus obtained with a high photo-activity that was rationalized in terms of the electron-extracting TiO 2 layer accelerating the electron forward-transfer and the concomitant deceleration of the undesired back-transfer [107].

Multi-Walled Carbon Nanotubes (MWCNTs)
MWCNTs have been widely studied as templates for titania nanostructures, as can be seen from the many entries in Table 1; thus, only a few studies are discussed here. With a sol-gel process in ethanol using a tetrabutyl titanate precursor, it was possible to decorate CNTs with titania nanocrystals, and the materials were used to obtain a film for potential applications as a battery anode [119]. When the same precursor was used in alkaline aqueous solutions with wider, hydroxylated CNTs (average diameter of 50 nm), nanowires were obtained instead, which entangled with CNTs in a network and were envisaged for the same application [126]. The CNT/titanium relative ratio is one of the critical parameters that determines nanomorphology. For instance, when an excess of titania precursor was used in a solvothermal method, nanoflowers were obtained, while increasing the relative amount of CNTs yielded nanowires through which the titania anatase coated the CNTs [130].

Multi-Walled Carbon Nanotubes (MWCNTs)
MWCNTs have been widely studied as templates for titania nanostructures, as can be seen from the many entries in Table 1; thus, only a few studies are discussed here. With a sol-gel process in ethanol using a tetrabutyl titanate precursor, it was possible to decorate CNTs with titania nanocrystals, and the materials were used to obtain a film for potential applications as a battery anode [119]. When the same precursor was used in alkaline aqueous solutions with wider, hydroxylated CNTs (average diameter of 50 nm), nanowires were obtained instead, which entangled with CNTs in a network and were envisaged for the same application [126]. The CNT/titanium relative ratio is one of the critical parameters that determines nanomorphology. For instance, when an excess of titania precursor was used in a solvothermal method, nanoflowers were obtained, while increasing the relative amount of CNTs yielded nanowires through which the titania anatase coated the CNTs [130].
The hollow interior of MWCNTs can also be exploited for various functions. For instance, iron-filled nanotubes could harness a magnetic separation after use for the easy recovery of the photocatalytic system. The tubes were functionalized on their surface to display carboxylic acid groups as anchors for titania nucleation and growth, and a thermal treatment allowed for the formation of anatase nanocrystals on the surface of CNTs (Figure 7) [184].  The hollow interior of MWCNTs can also be exploited for various functions. For instance, iron-filled nanotubes could harness a magnetic separation after use for the easy recovery of the photocatalytic system. The tubes were functionalized on their surface to display carboxylic acid groups as anchors for titania nucleation and growth, and a thermal treatment allowed for the formation of anatase nanocrystals on the surface of CNTs (Figure 7) [184].

Multi-Walled Carbon Nanotubes (MWCNTs)
MWCNTs have been widely studied as templates for titania nanostructures, as can be seen from the many entries in Table 1; thus, only a few studies are discussed here. With a sol-gel process in ethanol using a tetrabutyl titanate precursor, it was possible to decorate CNTs with titania nanocrystals, and the materials were used to obtain a film for potential applications as a battery anode [119]. When the same precursor was used in alkaline aqueous solutions with wider, hydroxylated CNTs (average diameter of 50 nm), nanowires were obtained instead, which entangled with CNTs in a network and were envisaged for the same application [126]. The CNT/titanium relative ratio is one of the critical parameters that determines nanomorphology. For instance, when an excess of titania precursor was used in a solvothermal method, nanoflowers were obtained, while increasing the relative amount of CNTs yielded nanowires through which the titania anatase coated the CNTs [130].
The hollow interior of MWCNTs can also be exploited for various functions. For instance, iron-filled nanotubes could harness a magnetic separation after use for the easy recovery of the photocatalytic system. The tubes were functionalized on their surface to display carboxylic acid groups as anchors for titania nucleation and growth, and a thermal treatment allowed for the formation of anatase nanocrystals on the surface of CNTs (Figure 7) [184].

Graphene, Graphene Oxide (GO) and Reduced Graphene Oxide (rGO)
The combination of graphene-based materials and titania is perhaps the most investigated nanocarbon-titania combination, and the topic was reviewed in 2017 [185] and 2019 [186]. Therefore, we focus here on the progresses made in the last two years and, given the vast amount of relevant literature, only selected examples are discussed in this section, while more can be found in Table 1.
Despite the sheet morphology of graphene-based materials, they can be used also to successfully attain elongated nanofibers, as demonstrated by the use of electrospinning techniques. This technique allows continuous nanofibers to be drawn from a liquid, thanks to electrostatic forces in the liquid jet that accelerates through an electric field.
The inclusion of both rGO and a titania precursor in the dispersion of reagents allowed electrospun nanofibers to be attained that after calcination revealed a shell anatase titania round a core with rGO. The nanofibrous exhibited the efficient photocatalytic degradation of a drug and a dye as model pollutants [171].
The enhanced photo-activity arises from synergistic effects, and it critically hinges on the covalent bonds between the two phases, which can be confirmed through the spectroscopic characterization of the Ti-O-C signature. To this end, it can be convenient to use titanium tetrachloride as a titania precursor, as the chloride is an excellent leaving group that can be replaced by the oxygen of the hydroxyl groups present on graphene [153].
Other important parameters that determine the performance of the final materials are clearly the crystallinity and particle size or film thickness of titania [187]. Solvothermal methods have been proven to be appropriate for attaining the complete coverage of graphene oxide flakes by highly homogeneous anatase nanocrystals (approximately 10-20 nm in size) (Figure 8a). In silico investigations confirmed that the presence of oxygenbearing functional groups on graphene oxide acted as anchoring sites for the nucleation of anatase (Figure 8b-d), which was favored over the growth of larger crystals. Comparison with analogous composites obtained using different nnocarbons (i.e., MWCNTs, nanohorns, or graphitized carbon black), revealed that graphene-based material displayed the highest selectivity for phosphopeptides relative to non-phosphorylated peptides. The former could thus be enriched from cancer cell lysates for phosphoproteomics profiling, in a proof-of-concept study for oncological research and potentially for the early detection of cancer [101].
Despite the sheet morphology of graphene-based materials, they can be used also t successfully attain elongated nanofibers, as demonstrated by the use of electrospinning tech niques. This technique allows continuous nanofibers to be drawn from a liquid, thanks t electrostatic forces in the liquid jet that accelerates through an electric field. The inclusion o both rGO and a titania precursor in the dispersion of reagents allowed electrospun nano fibers to be attained that after calcination revealed a shell anatase titania round a core wit rGO. The nanofibrous exhibited the efficient photocatalytic degradation of a drug and a dy as model pollutants [171].
The enhanced photo-activity arises from synergistic effects, and it critically hinge on the covalent bonds between the two phases, which can be confirmed through th spectroscopic characterization of the Ti-O-C signature. To this end, it can be convenien to use titanium tetrachloride as a titania precursor, as the chloride is an excellent leavin group that can be replaced by the oxygen of the hydroxyl groups present on graphen [153].
Other important parameters that determine the performance of the final material are clearly the crystallinity and particle size or film thickness of titania [187]. Solvother mal methods have been proven to be appropriate for attaining the complete coverage o graphene oxide flakes by highly homogeneous anatase nanocrystals (approximately 10 20 nm in size) (Figure 8a). In silico investigations confirmed that the presence of oxy gen-bearing functional groups on graphene oxide acted as anchoring sites for the nuclea tion of anatase (Figure 8b-d), which was favored over the growth of larger crystals. Com parison with analogous composites obtained using different nnocarbons (i.e., MWCNTs nanohorns, or graphitized carbon black), revealed that graphene-based material displaye the highest selectivity for phosphopeptides relative to non-phosphorylated peptides. Th former could thus be enriched from cancer cell lysates for phosphoproteomics profiling, i a proof-of-concept study for oncological research and potentially for the early detection o cancer [101].

Applications
As mentioned in the introduction, nanostructured titania finds a wide variety of applications in areas spanning from energy to medicine, as described below more in detail.

Photocatalysis for Energy and Synthesis
Titania is a promising candidate for photocatalytic applications related to sustainable schemes for energy and environment. For instance, titania is well-studied for the conversion of carbon dioxide into a range of useful fuels or commodities, such as methane, methanol, ethylene, formaldehyde, or formic acid [188]. For hydrogenation reactions, such as those of carbon dioxide and monoxide, titania and titanates have also been used as active supports for Rh catalysts [189]. Additionally, iridium-based catalysts have benefited from research that developed titania-based active supports for the photocatalytic synthesis of benzimidazoles, whereby rutile surpassed anatase in terms of performance, thanks to an increase in the adsorption of the reagents and in the charge transfer from the support, both of which led to higher product yields [190].
The photoelectrochemical generation of hydrogen from water has also enjoyed great prominence, where one-dimensional titania nanostructures demonstrated promising performance [191]. In silico tools are highly valuable to understand the nature of the rate-limiting steps and anticipate the better approaches to improve the efficiency of the process [192]. The further improvement of the photocatalytic performance of titania can be achieved through combination with other metal oxides (such as SnO 2 [193], ZnO [194], CuO [195]), gold [196,197], as well as alkali metals and conducting polymers [198], carbon-doping [199], iron oxides to attain magnetic systems [200], gadolinium [201], various rare earths [202], and bismuth vanadate [203], amongst others. In particular, the use of 4D transition metals was shown to be effective through the production of impurity-originating intra-gap energy states, but also through the semiconductor-metal phase transition [204].
A recent promising approach consists of single-atom doping. In the case of copper, light irradiation leads to electron transfer and the protonation of Cu/titania, and a local distortion around the copper atom stabilizes the deep-trap state on the copper d-orbital and its decoupling from free charges, thus yielding high photocatalytic hydrogen generation activity [205]. Further, the photocatalytic performance of Cu/titania can be improved by spin selection, which is achieved via optical intersite spin transfer or chiral semiconductor coating [205]. Both hydrogen adsorption and spin selection processes increase charge carrier lifetimes by an order of magnitude [205].

Photocatalysis for Environmental Remediation
The photoactivity of titania has been widely applied for environmental remediation; for instance, through the generation of graphene-bearing membranes for water purification [186,206]. The mechanisms of titania-photocatalyzed dye degradation proceed through the generation of reactive oxygen species (ROS) that trigger the photooxidation of organic molecules [207]. This phenomenon can be applied to a variety of different pollutants. They include dyes, for which titania co-doping with iron and praseodymium was recently reported to significantly narrow the band gap. Moreover, it promoted the generation of oxygen vacancies, which can trap electrons and thus reduce the recombination of charge carriers [208]. Nitrogen and sulfur co-doping represents a very popular strategy to enhance the photodegradation of cationic dyes [209]. Another type of target pollutants consists of volatile organics, whose photo-degradation is heavily influenced by levels of humidity [210]. Another area that requires urgent action is the environmental pollution by drugs, especially antibiotics, whose mechanisms of photodegradation are still the subject of intense investigations [211]. Clearly, the removal of pesticides is also highly sought after, although a number of variables have to be taken into consideration as they may affect the efficiency of the process [212]. Ammonia is a critical pollutant of agricultural concern, both in gaseous and aqueous matrices, for which heterogeneous nanostructures find application as photocatalysts [213]. In fact, ammonia splitting into nitrogen and hydrogen gases is particularly attractive as it would concomitantly generate clean fuel [214]. The fundamental reaction pathways involved in the process are shown in Figure 9 [214].
nanostructures find application as photocatalysts [213]. In fact, ammonia splitting into nitrogen and hydrogen gases is particularly attractive as it would concomitantly generate clean fuel [214]. The fundamental reaction pathways involved in the process are shown in Figure 9 [214].
Sometimes, different photoactive catalysts are combined together to enhance the performance of the final materials [215]. Alternatively, doping can be an effective strategy; for instance, with bismuth [216], tungsten [217], nickel [218], and so on. Figure 9. Scheme of the fundamental reaction pathways of ammonia degradation photocatalyzed by titania. Reprinted with permission from [214], Copyright © 2020, American Chemical Society.

Biomedical Applications and Public Health
The photoactivity of titania nanoparticles has also been recently investigated for applications in medicine. The most popular uses exploit the ability of nanostructured titania to generate ROS upon light irradiation, as these species can be employed for photodynamic therapy, but also for the inactivation of bacteria that are resistant to antibiotics [219]. Another area that is gaining momentum is the development of active coatings and films for food packaging [220]. To this end, the antimicrobial activity phototriggered by nanostructured titania can be boosted upon the inclusion of silver or copper species, for applications that go beyond food packaging, including also active textiles and self-cleaning fabrics [221].
Titania nanotubes are being investigated as vehicles for local drug delivery, especially if functionalized to promote osteogenesis at the bone-implant interface, as recently reviewed [222]. Given the widespread use of titanium implants, the possibility to use nanotopography to modify their surface to render it bioactive is particularly attractive. To this end, the usefulness of a variety of potential designs, including titania nanotube arrays loaded with antibiotics ( Figure 10), has been recently discussed as a potential strategy to address localized infections [223]. Sometimes, different photoactive catalysts are combined together to enhance the performance of the final materials [215]. Alternatively, doping can be an effective strategy; for instance, with bismuth [216], tungsten [217], nickel [218], and so on.

Biomedical Applications and Public Health
The photoactivity of titania nanoparticles has also been recently investigated for applications in medicine. The most popular uses exploit the ability of nanostructured titania to generate ROS upon light irradiation, as these species can be employed for photodynamic therapy, but also for the inactivation of bacteria that are resistant to antibiotics [219]. Another area that is gaining momentum is the development of active coatings and films for food packaging [220]. To this end, the antimicrobial activity phototriggered by nanostructured titania can be boosted upon the inclusion of silver or copper species, for applications that go beyond food packaging, including also active textiles and self-cleaning fabrics [221].
Titania nanotubes are being investigated as vehicles for local drug delivery, especially if functionalized to promote osteogenesis at the bone-implant interface, as recently reviewed [222]. Given the widespread use of titanium implants, the possibility to use nanotopography to modify their surface to render it bioactive is particularly attractive. To this end, the usefulness of a variety of potential designs, including titania nanotube arrays loaded with antibiotics ( Figure 10), has been recently discussed as a potential strategy to address localized infections [223].

Sensing
Nanostructured titania has been investigated to develop a variety of electrochemical sensors, especially as applied to biomarkers for early disease diagnosis. Volatile organic compounds (VOCs) can be useful biomarkers for the early detection of a variety of pathological states, and therefore chemoresistive breath sensors for their detection are investigated, often using semiconducting metal oxides. To this end, various forms of nanostructured titania can be used, such as nanoparticles, nanotube arrays, and nanosheets, as recently reviewed [224]. Zinc-doped titania nanoparticles have been also proposed to develop glutamate sensors, as glutamate is a common food additive but also a useful biomarker, since abnormal levels were linked to pathologies such as epilepsy, Alzheimer's and Parkinson's diseases, ischemia, and amyotrophic lateral sclerosis [225].
Titania nanotubes are being investigated as vehicles for local drug delivery, especially if functionalized to promote osteogenesis at the bone-implant interface, as recently reviewed [222]. Given the widespread use of titanium implants, the possibility to use nanotopography to modify their surface to render it bioactive is particularly attractive. To this end, the usefulness of a variety of potential designs, including titania nanotube arrays loaded with antibiotics ( Figure 10), has been recently discussed as a potential strategy to address localized infections [223]. For all these sensing applications, the inclusion of carbon nanostructures can be beneficial to enhance sensitivity and the performance of the devices generally, as recently reviewed [10]. This is true for a variety of carbon nanomorphologies. For instance, the inclusion of CNTs allowed a lower detection limit and an increased linear range of operativity for an epinephrine sensor [226]. In another recent example, an rGO/titania nanohybrid deposited on a glassy carbon electrode displayed a better performance for the electrochemical detection of a red dye relative to the sensor made with either component, thanks to a larger electroactive surface area and lower charge-transferred resistance [227]. Finally, graphene quantum dots also allowed for an enhanced photoresponse, with higher photocurrents and improved charge carrier separation efficiency, for an NO gas sensor based on titania that operated under UV-light irradiation [228].

Conclusions and Future Perspectives
The synergy between carbon nanostructures and titania has been extensively investigated over the years and allowed great progress in a variety of fields; in particular, for tailored applications to address urgent societal needs such as clean energy, medicine, and environmental remediation, which is now being extended to the issue of microplastics [229]. It has been undoubtedly demonstrated that the inclusion of nanocarbon into titania-based functional materials leads to the significant enhancement of the materials' properties. In photocatalysis, for example, depending on the efficiency of the titania/carbon contact, a clear reduction in the electron-hole recombination rate has been observed, as the nanocarbon scaffold is able to scavenge the photoexcited electrons. On the other hand, in electrocatalysis, the carbon conductivity has been exploited to facilitate the electron transfer processes occurring at the TiO 2 -based catalytic sites, resulting in enhanced performances. Regardless of the application, it appears that the suitable interface between the carbon and the TiO 2 phases is in most cases an essential parameter to gain definite advantages in performance. For this reason, we have seen that efforts have focused on the development of synthetic protocols that could guarantee adequate phase contact, giving rise to the desired synergistic effects. As an example, the production of co-axial CNTtitania nanostructures, with an intimate contact between the two phases, could be attained through the careful selection of CNT functionalization, so that the support displays suitable oxygen-bearing functional groups to effectively anchor titania, and the optimization of the thermal annealing step to seal the CNT-titania interface [230].
The main limitation of titania for use in photochemical applications, namely its relatively wide bandgap that makes it unresponsive to visible light, has been linked in recent times to a predicted slow decline in the popularity of titania-based materials for photochemical sustainable processes, paving the way for the exploration of new semiconductor materials with a more profitable exploitation of sunlight. However, we foresee that TiO 2 will still have a long life as the leading semiconductor as many authors have shown avenues for expanding its use to incorporate the exploitation of visible light. For example, defect engineering in TiO 2 polymorphs has reached a deep level of understanding, and the controlled alteration of the TiO 2 structure at atomic level can lead to considerable advantages. Black titania, obtained in 2011 by an intensive H 2 treatment that causes reduced Ti states by means of oxygen atom removal, is a symbolic case of a visible-light active TiO 2 material [231]. As the activity under visible light is caused by intragap states, it is frequently hypothesized that combined nanocarbon-TiO 2 could cause a C-doping of the titania at the phase interface, creating new midgap states.
TiO 2 is being revisited in modern times as a component in more complex composites, and the interfacing with carbon nanostructures certainly brings many advantages (some of them illustrated above), in concomitance with a confinement at the nanoscale level of the material, with the expected benefits of nanomaterials. However, TiO 2 junctions with other semiconductors bearing suitable electronic features could make further amends for the intrinsic shortcoming of self-standing TiO 2 . Z-schemes or p-n heterojunctions are the two best known examples to improve photochemical properties. Therefore, opportune multi-phase nanohybrids or nanocomposites featuring nanocarbon and TiO 2 in conjunction with other components bear great potential for unlocking new multifunctionalities and directing these properties to specific purpose.
As we advance our detailed understanding of the mechanisms of interactions and reactions of nanostructured titania, further progress is set to be realized in this fascinating field. New opportunities are arising also from studies on surfaces [232]. The use of photoluminescence spectroscopy is emerging as a key tool to unravel mechanistic details of photocatalytic processes [233]. Finally, chiral nanostructures that can be attained through the templating of self-assembling molecules are also opening the door to new opportunities in non-linear optics, sensing, and photonics [234]. Examples of potential applications include circular polarizers, since titania chiral superstructures showed strong optical activity due to the difference of absorbing left and right-handed circularly polarized light [235]. Optically active films were also obtained from anatase nanocrystals that were spatially organized with long-range chiral nematic ordering, allowing for the selective reflection of circularly polarized light and iridescence [236].
Another emerging area of application lies in electronics and memory devices [114,237,238]. A recent report noted that the doping of titania with hydrogen, deuterium, and lithium led to bipolar conduction and a giant positive magnetoresistance, thus significantly expanding the properties of the materials [239]. Doping with cerium enhanced the magnetic properties of semiconductors from UV-light irradiation, thanks to the ferromagnetic orientation of spin densities near oxygen vacancies in Ce-doped titania, as opposed to the anti-ferromagnetic orientations of those found in undoped titania [240].
In conclusion, it appears that there is still a bright future ahead for the application of titania's great ability to interact with light, especially if maximized through the synergy with other nanomaterials and advanced techniques of characterization and fabrication.