Keywords

1 Introduction

With the advent of industries that produce a large number of pollutants in effluents, including pharmaceuticals, textiles, and chemicals, the amount of pollutants has grown almost exponentially in the last 30 years according to the UN survey in 2018. For this reason, in the context of reducing the number of emerging pollutants in industrial effluents. In this idea, titanium dioxide has become a highly researched material with high potential for commercial and industrial applicability. However, one of the major drawbacks is the fact that the material has a gap within the UV light spectrum and is considered a high bandgap semiconductor, which restricts its applications. Although this characteristic represents a depreciation of the material, since this part of the spectrum is smaller when compared to the whole spectrum available, the fact that the TiO2 is low cost, with high physical-chemical stability and easy to manufacture makes it an attractive material for photocatalytic applications.

To increase the industrial use and applicability of nanostructured TiO2, as a base element in the decomposition of pollutants, processes such as doping and decoration are widely used because of the ease of synthesis in conjunction with titanium dioxide. One of the most successful approaches is the use of transition and noble metals together with TiO2 nanostructures. In addition to allowing access to a more significant portion of the available light spectrum, thereby increasing the photocatalytic activity, the presence of these metals still allows an improvement of yield and applicability in the decomposition of different chemical compounds present in effluents. This chapter presents a condensation of the primary metals used together with TiO2.

This chapter aims to show the latest advances in the application of photocatalytic processes for the decomposition of pollutants and H2 production using metals such as Fe, Cu, Ag, and Au together with TiO2 nanostructures for their simplicity of synthesis and low cost for direct application.

2 Iron-Doped TiO2

Although having wide application potential, titanium dioxide nanotubes (TNTs) face a major drawback regarding their large band gap, an aspect that limits photoresponse almost exclusively to the ultraviolet region [1]. Modifying TiO2 with other narrow bandgap materials is a widely recognized method to minimize such drawback, as it causes a red shift of the absorption edge to the visible light region [2,3,4]. Moreover, selective metal ion doping, especially iron in its Fe3+ form [5, 6], significantly broadens light response range and inhibits charge-carrier recombination as the ions act as charge traps for the photogenerated electron-hole pairs [7,8,9,10,11]. Several studies also pointed out to an enhanced photocatalytic activity for Fe-doped catalysts both under UV [11,12,13] and visible light irradiation [14,15,16,17]. As demonstrated by Yu et al. [18], such high performance of Fe3+/TiO2 is attributed to the accumulation of photogenerated holes in the valence band (VB) of TiO2 and the catalytic reduction of oxygen by photoreduced Fe2+ species on TiO2 surface (Fig. 1).

Fig. 1
figure 1

Schematic diagram illustrating the possible photocatalytic mechanism of Fe3+/TiO2, involving interfacial charge transfer (arrow 1) and multielectron reduction processes. The band gap excitation is indicated by arrow 2. Reprinted with permission [18]

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Regarding Fe/TNTs, Asiltürk et al. [19] found that Fe3+ doping may prevent particle agglomeration due to the formation of well-defined crystalline nanoparticles (NPs) with high surface area, two key aspects to improving photocatalytic performance [19]. Compared to undoped, electrochemically anodized TNTs, Fe3+ doped nanotubes have shown a six-fold increase on photocurrent density and a doubled degradation rate measurement [1]. Using the crystal structure of TiO2 as a matrix for fixating Fe3+ NPs is also presented as an alternative to having them dispersed in an aqueous suspension, thus avoiding the detrimental nanoparticle aggregation where many particles are hidden from light absorption [5].

Fe2O3 has been pointed out as another strong candidate for being a low cost, non-toxic and highly stable form of iron oxide [20, 21]. Concerning morphology, aggregating Fe NPs in adequate amounts to TNTs does not affect the highly ordered nanotube construction, as demonstrated by Zhang et al. [1] in Fig. 2, where SEM images compare undoped two-step anodized TiO2 NTs with Fe3+-doped TiO2 NTs, also fabricated via a two-step anodization process. Xie et al. [22] reinforce this conclusion by successfully electrodepositing ZnFe2O4 nanoparticles within self-organized and highly ordered TiO2 nanotube arrays while minimizing the clogging of the tube entrances (Fig. 3).

Fig. 2
figure 2

SEM images of: a TiO2 NTs in the two-step anodization. b Fe3+/TiO2 NTs. Reprinted with permission [1]

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Fig. 3
figure 3

a Top-view and b side-view SEM images of the TiO2 nanotube arrays. The inset shows the images of TNTs at high magnification. Reprinted with permission [22]

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Cong et al. [23] studied the performance of α-Fe2O3/TNTs composites for photoelectro-Fenton degradation using phenol, a compound usually found in wastewater discharged from a variety of industries [24, 25], as a model pollutant with an initial concentration of 10 mg/L. The α-Fe2O3/TNT complex was synthesized over anodized nanotubes by two different techniques, a dipping method in aqueous suspension of α-Fe2O3 NPs (fixed-Fe2O3 system 1) and an electrochemical deposition (fixed-Fe2O3 system 2), for which the electrodes fabricated via the second one presented higher removal efficiency of phenol and stronger photoresponse under visible light irradiation due to the uniform fixation of Fe NPs on TNTs. To further investigate the role of α-Fe2O3 NPs, phenol degradation was also carried in a dispersed system, where α-Fe2O3 NPs were added into the aqueous phenol solution instead of being attached to TNT electrodes. Responses to three different Fenton-related processes, electro-Fenton, visible light photoelectro-Fenton, and UV-visible light photoelectro-Fenton, are depicted in Fig. 4. After a 60 min treatment, electrochemically deposited α-Fe2O3/TNT electrodes presented 100% phenol removal efficiency in the UV-visible light photoelectro-Fenton process, whereas the dispersed system required double the amount of time to reach similar rates. The increased performance of Fe2O3 modified TNTs could be linked to the fact that incorporating α-Fe2O3 NPs effectively suppressed the recombination of electron-hole pairs, thus allowing them to act as heterogeneous catalysts for phenol degradation [23].

Fig. 4
figure 4

Phenol degradation in a dispersed system, b fixed-Fe2O3 system 1, and c fixed-Fe2O3 system 2. Reprinted with permission [23]

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ZnFe2O4 appears as an opportunity to harvest most of the visible portion of the solar spectrum [26], also presenting an alternative to overcome the separation and recycling process of the powdered ZnFe2O4 and improving the energy conversion efficiency when aggregated to TiO2 in a ZnFe2O4/TNT composite [27]. Loading highly ordered TiO2 nanotube arrays with ZnFe2O4 NPs enhances absorption in both UV and visible light regions promotes greater separation of photoinduced electron-hole pairs and presents a more effective photoconversion capability than undoped TNTs [22, 27]. Moreover, Xie et al. [22] demonstrated that the photocurrent density of the composite ZnFe2O4/TNT (Z-TNT) electrode was more than 5.5 times higher than that of the TNT electrode alone (Fig. 5), implying that ZnFe2O4 can be effectively used to sensitize TiO2 nanotube array electrodes. Regarding its applicability, the synthesized electrode was found to possess excellent photoelectro-catalytic activity for degradation of 4-chlorophenol, a toxic and non-biodegradable pollutant present in wastewater of industrial activities [28, 29] under UV light illumination, presenting 100% efficiency with an applied potential bias of 0.8 V, 16% higher than the degradation efficiency measured for the unloaded TNT electrode [27].

Fig. 5
figure 5

a Transient photocurrent responses of the TNTs and the Z-TNTs samples under the bias potential of 0.8 V versus SCE. Three ZnFe2O4/TNT samples were analyzed, each containing a distinct concentration of Fe(NO3)3 and Zn(NO3)2: (1) 0.5 M and 0.25 M; (2) 1.0 M and 0.5 M; (3) 2.0 M and 1.0 M, respectively. Reprinted with permission [22]

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Titanium nanotube composites are also attractive photocatalysts for photo-electrochemical (PEC) solar water splitting applications due to their high-efficiency energy conversion, good chemical stability, and corrosion resistance in aqueous environments [30,31,32,33]. PEC splitting of water is an ideal, low-cost renewable method of hydrogen (H2) production that integrates solar energy collection and water electrolysis in a single photocell [30, 34, 35]. Figure 6 presents the principle of photocatalytic water splitting reactions.

Fig. 6
figure 6

The general principle of photocatalytic water splitting reactions. Reprinted with permission [35]

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Moreover, H2 has been considered an attractive energy carrier, achieving much higher conversion efficiency than the conventional fossil fuels, while offering a more sustainable [33], environmentally benign energy source with lesser emissions [36]. Unfortunately, large-scale application of most photocatalytic systems is restricted by the fact that noble metal-based co-catalysts are still the first option for achieving reasonable activity rates. Therefore, seeking cheap, earth-abundant and high-performance alternative materials is indispensable to achieve cost-effective, highly efficient water splitting process [32]. Momeni and Ghayeb [37] produced Fe/TNTs composites for water splitting applications with varying amounts of the iron source potassium ferricyanide (K3Fe(CN)6), for which the ideal concentration was found to be 9 mM. Compared to undoped TNTs, the samples exhibited a red-shift of absorption edge and a band gap decrease, as well as a dramatic increase in photocurrent at the ideal Fe concentration. Regarding composite efficiency, Fig. 7a illustrates that the amount of H2 evolved on the 9 mM Fe/TNTs sample (named FeNTs-2), after 240 min was more than 6,5 times the one verified on the undoped TiO2 sample (NTs), and the hydrogen production showed no obvious decay after 6 continuous runs of photocatalytic reaction, as seen in Fig. 7b, thus proving the recyclability of the system.

Fig. 7
figure 7

a Photocatalytic H2 production of different samples over irradiation, with applied external voltage [1.50 V (vs. Ag/AgCl)]. b H2 evolution for different samples as a function of running times (reusability test of samples). Reprinted with permission [37]

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3 Copper-Doped TiO2

Exhibiting great potential for applications in the fields of solar energy conversion [38,39,40], photocatalytic hydrogen (H2) production [32, 41, 42] and degradation of hazardous components [2, 43], earth-abundant and low-cost transitional metals such as cobalt (Co), nickel (Ni) and copper (Cu) [44, 45] have been successfully loaded on semiconductors in order to improve photocatalytic processes. According to Ran et al. [32], applying such metals as cocatalysts promote charge separation due to the Schottky barrier formed at the metal/semiconductor interface.

Regarding solar water splitting, hydrogen not only is vital for many industrial processes but also is thought to be an attractive, clean energy vector when combined with efficient fuel cells. The overall photocatalytic water splitting reaction involves three major steps, those being light absorption by a semiconductor to generate electron-hole pairs, charge separation and migration to the surface of the semiconductor and finally surface reactions for H2 or O2 evolution [32] (Fig. 8). However, solar photocatalytic water splitting is still a challenging promise for sustainable H2 production, mostly because the ideally described use of platinum (Pt) as cocatalyst endures a series of limitations linked to its high-cost and relatively low availability [32, 41].

Fig. 8
figure 8

Schematic illustration of photocatalytic water splitting over a semiconductor photocatalyst loaded with \({{\text{H}}_{2}}^{ - }\) and \({{\text{O}}_{2}}^{ - }\) evolution cocatalysts. Reprinted with permission [32]

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In this context arises an increasing interest in the aforementioned transition metals, especially Cu species, for the narrow band-gap characteristic of both CuO and Cu2O extends the photoresponse of TiO2 into the visible light region [2, 38,39,40, 43, 46], making them a sustainable candidate for TiO2 doping to enhance hydrogen production [44, 47]. On that note, Xu et al. [48] reported that CuO-loaded TiO2 exhibited a photocatalytic H2 production rate in methanol aqueous solution about 3 times higher than that of some Pt/TiO2 electrodes (\(18{,}500\, \upmu \text{mol}\,\text{h}^{-1}\,{\text{g}^{-1}}_{\text{catalyst}}\) to 6000 [49] and 6900 [50] \(\upmu \text{mol}\,\text{h}^{-1}\,{\text{g}^{-1}}_{\text{catalyst}}\), respectively). Furthermore, it was found that Cu-doping process does not influence the morphology of TiO2 samples [43, 51], and different chemical states of Cu species, as well as their distribution ratio over TiO2 notably influence H2 production activities, with Cu species aggregated to its surface promoting charge transfer more efficiently than those in TiO2 lattice [42].

The stability of the system is also to be observed, for leaching of non-metals is one of the main reasons for catalyst deactivation in liquid media [41]. Gombac et al. [41] tested copper leaching during and after photocatalytic reactions carried with a Cu/TiO2 electrode both under reducing and oxidizing conditions, those being argon flow and inert atmosphere and exposure to air, respectively. Results indicated that under reducing conditions Cu leaching is marginal and, if operative, can be minimized by UV irradiation. On the other hand, dramatic leaching is observed under oxidized conditions.

Coupling Cu NPs with self-organized, highly ordered TiO2 nanotube arrays (Cu/TNTs) creates an even more efficient opportunity to harvest sunlight when compared to randomly oriented TiO2 nanoparticles [2]. Cu/TNTs heterojunction also favors the separation of photogenerated electron-hole pairs, ultimately improving photoelectrical conversion ability under solar light irradiation [40, 43]. Hua et al. [43] confirmed such aspect by varying deposition time of Cu NPs over TNTs, for which the increase in deposition time also meant an increase in the transient photocurrent response (Fig. 9). Moreover, uniformly dispersed Cu NPs ensured the electron migration by forming a strong interaction with TNTs, thereby improving charge transfer and separation.

Fig. 9
figure 9

Electrochemical impedance spectroscopy (EIS) spectra of Cu/TNTs electrodes, recorded at the open circuit potential under simulated solar light irradiation. The semicircle diameter of EIS, related to charge transfer resistance and separation efficiency at the contact interface between the electrode and electrolyte, was substantially decreased with increasing the deposition time, which could be attributed to Cu2O and CuO improving the harvesting of visible light and reducing the transfer impedance of electrons. Reprinted with permission [43]

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In the same study, Hua et al. [43] evaluated the catalytic efficiency of Cu/TNTs electrodes for degrading of diclofenac (DCF), a non-steroidal anti-inflammatory drug vastly researched for its endocrine disrupting and adverse effects and also one of the most frequently detected pharmaceuticals in municipal wastewater [52]. Among the four processes tested, electrochemical (EC), direct photolysis (DP), photocatalysis (PC) and photoelectrocatalysis (PEC), the PEC process was found to be the most efficient with a DCF degradation efficiency of 71.9%, pointing to the important role played by an applied potential bias of 0.5 V in effectively separating electron-hole pairs and prolonging lifetime of the photogenerated charge carriers (Fig. 10).

Fig. 10
figure 10

Degrading DCF by electrochemical process (curve A), photolysis (curve B), photocatalysis (curve C) and photoelectrocatalysis (curve D) over Cu/TNTs electrode. Reprinted with permission [43]

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Similar research utilizing the previously mentioned catalytic processes (EC, DP, PC, and PEC) was carried by Hou et al. [2] for 4-chlorophenol (4-CP) decomposition under UV light irradiation. As depicted in Fig. 11 the photocurrent density of the Cu2O/TNT electrode was measured to be more than 5 times the value for the non-loaded TNTs. Again, the PEC process utilizing a Cu2O/TNT electrode was the fastest among the four alternatives, achieving nearly 100% of 4-CP degradation in 120 min, a value at least 20% higher than the one measured for the PEC process with a non-loaded TNT electrode (Fig. 12). Similarly, maximum photoconversion efficiency was observed at an applied bias of 0.1 V, corroborating the idea proposed by Hua et al. [43] that a potential bias inhibits the recombination of photogenerated electron-hole pairs.

Fig. 11
figure 11

Variation of photocurrent density versus bias potential (vs. SCE) in 0.01 M Na2SO4 solution for the Cu2O-loaded TiO2 nanotube array electrode and TiO2 nanotube array electrode under Xe lamp (400–600 nm, 33 mW cm−2) irradiation. Reprinted with permission [2]

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Fig. 12
figure 12

Variation of 4-CP concentrations by photoelectrocatalytic (PEC) technology with TiO2 nanotube array electrode and Cu2O-loaded TiO2 nanotube array electrode under UV light illumination (I0 = 1.4 mW cm−2, 0.2 V vs. SCE bias potential applied). Reprinted with permission [2]

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Cu2O appears yet in structures other than NPs. As an example, Yang et al. [46] incorporated both Cu0 NPs on the inner walls (Fig. 13a) and Cu2O nanowires onto the top surface of TNTs (Fig. 13b) for p-nitrophenol (PNP) degradation, creating an intercrossed network with extended absorption in the visible light range without blocking the nanotubes (Fig. 13c).

Fig. 13
figure 13

SEM images of a Cu-loaded TiO2 NTs and b Cu2O ultrafine nanowires modified TiO2 NTs. c UV-vis absorption spectra of TNTs (curve 1) and Cu2O/TNTs (curve 2), demonstrating that the Cu2O-modified TNTs have intense absorption in the visible light range. Reprinted with permission [46]

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Used for drug, fungicide and dye manufacturing, the priority pollutant is generally degraded by strong oxidants only, thus impeding purification of PNP-contaminated wastewater due to its stability to chemical and biological degradation [53, 54]. However, the authors proved that under solar light PNP can be effectively degraded by the Cu2O/TNT p-n junction network without the addition of oxidants, at a rate 2.3 times higher than the unmodified TiO2 NTs (1.97 µg/min cm2 versus 0.85 µg/min cm2, respectively).

3.1 Silver/TiO2 Nanostructures

The addition of nanostructured silver to titanium dioxide (Ag/TiO2) has been thoroughly studied, mainly in terms of its photocatalytic properties [55,56,57]. Several studies [55, 58,59,60] demonstrate that silver is one of the most suitable materials for industrial application, due to the easy preparation and consequently low cost [58]. Because it is a noble metal, Ag presents characteristics that improve the photocatalytic activity of TiO2 [61] as already mentioned (Fig. 14).

Fig. 14
figure 14

Electron transfer mechanism in silver loaded TiO2. Reprinted with permission [61]

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Amongst the main properties directly linked to the addition of silver to the surface is the facility to dispose of photogenerated electrons on the TiO2 surface, directly attached to the silver fermi energy being just below the conduction band of TiO2, thus causing silver in this way function as a storage point of photogenerated load. Besides, the nanostructure of silver facilitates the photoabsorption process, due to the effects of SPR, which depend directly on the geometry and size of the nanostructures. Plasmon resonance effects can amplify the discharge effect for photoactive surfaces.

The Ag loaded TiO2 (Ag/TiO2), recently have been used in applications that aim pollutants and dyes degradation of wastewater. Seery [62] evaluated the rate of degradation of a model dye, rhodamine (R6G), and samples produced by different methods (irradiation and calcination) and the amount of silver doping. The author [62] relates that the degradation rate of the calcinated sample presented the most efficient catalytic properties (6–50% improvement in catalytic efficiency), that can be attributed to the silver is homogeneously dispersed throughout the material. The degradation rate of (R6G) oscillates among 0.06 min−1 for TiO2 to 0.34 min−1 for 5 mol% Ag–TiO2, which is attributed to the increasingly visible absorption capacity by Ag presence [62].

Concerning the pollutant materials, Li [63] explored the degradation of toluene with TiO2 nanotube powder doped with Ag nanoparticles and compared the photocatalytic efficiency with commercial TiO2 (P25, Degussa). The TiO2 nanotubes were produced from Titanium foils with potenciostatic anodization method. The anodization was performed in a two-electrode configuration, where the Ti foil was used as a working electrode, and platinum foil as the counter electrode. The Ag-doped TiO2 were prepared employing an incipient wetness impregnation method. The Ag/TiO2 powder photocatalytic activity was measured through photo-oxidation of gaseous toluene. The results indicated that achieved efficiency for toluene degradation was 98% after 4 h reaction, under UV-light, these values were better than with pure TiO2, Ag-doping P25 or P25 [63].

Another application that has been extensively studied using Ag/TiO2 is water splitting, to generate gaseous H2 as an alternative energy source. To enhance the process efficiency, Fan [64] produced highly ordered Fe3+-doped TiO2 nanotube arrays (Fe/TiO2 NTs), then Ag nanoparticles were assembled in Ag-Fe/TiO2 NTs. The material assembly with iron was prepared by electrochemical anodic oxidation and the Ag was loaded by microwave-assisted chemical reduction. The author related that Ag-Fe/TiO2 NTs showed higher UV-Vis light absorption and lower electron-hole recombination rate than pure TiO2 (Fig. 15). The photocatalytic activity indicated a higher efficiency by 0.2 mM Ag-0.3 mM Fe/TiO2 NTs samples. if compared with the pure TiO2 catalyst. These results show the potential of photocatalytic material for energy and environment applications [64].

Fig. 15
figure 15

a SEM images of pure TiO2 NTs and b Fe doped and Ag NPs loaded on TiO2 NT. c Mechanism of H2 production by water splitting over Ag–Fe/TiO2 NTs applying UV and visible light irradiation. d Photoluminescence spectra of TiO2 NTs under the excitation of 250 nm Fe/TiO2 NTs, e Ag/TiO2 NTs and f Ag–Fe/TiO2 NTs. g H2 production by water splitting by TiO2 catalysts. (*Ag/TiO2–Ag-doped TiO2). Reprinted with permission [64]

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3.2 Gold

Gold is another noble metal that possesses singular electronic properties. Several researchers [65,66,67] relates that the presence of gold (Au) in nanoparticle form, or combined with other noble metals in TiO2 supports, will improve the photocatalytic degradation of pollutants. In this regard, reports [68, 69], relate researches about the photocatalytic oxidation of pesticides and phenolic compounds applying TiO2, furthermore, reviewed concerning the organic dyes degradation in effluents.

Sanabrina [66] has investigated the performance of Au and platinum (Pt) nanoparticles, and Au–Pt alloy on anodic TiO2 nanotubes (TiO2 NTs) for photocatalytic degradation. The materials were produced with a different method, that is, metal decoration intrinsically and extrinsically. The intrinsical decoration was obtained using a noble metal-containing titanium alloy for anodic tube growth, as the extrinsic decoration was realized by physical vapor deposition (PVD) method of the Ag and Pt on pure titania tubes (Fig. 16).

Fig. 16
figure 16

a Representation of TiO2 NTs intrinsically decorated by direct anodization of alloy (left) and extrinsically decorated by sputtering and dewetting a noble metal on the top of the NTs anodically growth on Ti foil (right); b top section of the NTs on TiAuPt alloy; c top section of the TiO2 NTs with 1 nm of Au dewetted and d cross-section of the NTs on TiAuPt alloy with magnifications in the top, middle and bottom part identified by different colors black, blue and red. Reprinted with permission [66]

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The results showed enhancement for decomposition of the model pollutant acid orange 7 (AO7) when the Au–Pt intrinsic decoration was evaluated, which can be attributed to the synergistic effect of both noble metals. This effect of Au- Pt intrinsic decoration has revealed to be a better option than the use of pure elements loaded on TiO2 NTs. Sanabrina [66] relates the overall effect is due to the facilitated oxygen reduction reaction, which leads to higher production of reactive oxygen species on the conduction band, which provide an enhanced pollutant degradation rate [66].

Beyond the photocatalysis application, it is possible to cite the H2 production using TiO2 nanotubes (TiO2 NTs) doped with Au. Choi [67] produced Au NP-decorated TiO2 nanotube arrays (TNTs) to apply as photoelectrochemical (PEC) water splitting electrodes for H2 production. To synthesize the TNTs, the author made use of a simple and low-cost method with two-step anodization process, and finally the deposition of a thin film of Au nanoparticles (Fig. 17) [67].

Fig. 17
figure 17

SEM images of Au NP-decorated 2 nd anodized TNT arrays a top-view and b cross sectional view. c EDX analysis of Au NP-decorated 2 and anodized TNT arrays (TNT–Au NP-decorated TiO2 nanotube). Reprinted with permission [67]

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The TiO2 NTs prepared using the two-step anodization process showed better photocurrent stability and efficiency. Furthermore, the Au presence on TNT array increases the photocurrent value in 67.2% to 1.02 mA/cm2. The PEC process water splitting was enhanced and stabilized for charge separation and transport due to reduced cracking after second anodization and the annealing process [67].

Based on the context presented in this chapter, it is important to cite Paramasivam [70], that compared the photocatalytic activity of Au and Ag nanoparticles loaded TiO2 nanotubes and the activity of the unloaded TiO2 substrates. Paramasivam relates an enhancement of photocatalytic activity with TiO2 nanotubular structures compared with a compact TiO2 surface, which possesses higher performance due to SPR enhancement properties due to the presence of Au and Ag.

4 Conclusions

In summary, modifying TiO2 with semiconductor materials, distinctly Fe and Cu ions, creates additional energy levels near the valence and conduction bands of TiO2, thus minimizing the limitations associated with its large band gap via trapping of both electrons and holes. Consequently, it is highly recommended to dope TiO2 with either Fe or Cu ions to obtain superior photocatalytic activity [6, 71]. Furthermore, the nanotubes loaded with Ag provide a doubled degradation when compared with pure nanotubes. The improvement of photocatalytic activity using Ag-TiO2 can be explained considering the deposition method, which a thermal treatment step is recommended as an activation step, otherwise the addition of this metal may lead to decreased activity compared to pure TiO2 NTs. Finally, the presence of the noble metal establish the formation of locally Schottky junctions with a high potential gradient, compared with TiO2/electrolyte interface, established by Schottky barrier, which enables better charge tranfer between the materials. In view of the aforementioned results, it is possible to state that the addition of metallic structures, like Fe, Cu, Ag and Au, to TiO2 NTs have a great potential to be used in photocatalytic reactions and water-splitting applications.