Nitrogen-doped titanium dioxide: An overview of material design and dimensionality effect over modern applications

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 27 (2016) 1–29

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews journal homepage: www.elsevier.com/locate/jphotochemrev

Nitrogen-doped titanium dioxide: An overview of material design and dimensionality effect over modern applications Shahzad Abu Bakar a,b,∗ , Caue Ribeiro b a b

Department of Chemistry—Federal University of São Carlos, Rod. Washington Luiz, km 235, CEP: 13565-905, São Carlos, SP, Brazil Embrapa CNPDIA, Rua XV de Novembro, 1452, 13560-970, CP 741, São Carlos, SP, Brazil

a r t i c l e

i n f o

Article history: Received 11 March 2016 Accepted 13 May 2016 Available online 19 May 2016 Keywords: TiO2 N-doping Semiconductor Photocatalysis Dimentionality 0-D nanoshperes 1-D nanofibers/nanotubes 2-D nanosheets/thin films 3-D interconnected architecture Dye-sesitized solar cells Lithium-ion batteries

a b s t r a c t TiO2 material has gained attention as the most studied semiconductor material for photocatalytic purposes, including their use in devices for clean energy production, such as solar cells and water splitting systems. However, the wide band gap of this material limits applications to UV light, which also confines the use of solar irradiation as the energy source. Much research in the last years is showing the ability of N doping into TiO2 to promote light absorption in the visible range but, to date, it is still controversy if this doping is beneficial to the photocatalytic process, as well as the synthetic methods are not well stabilized yet. Then, this paper summarizes the recent advancement in the structural design perspective of N-doped TiO2 photocatalyst, in a critical analysis of its application for environmental purposes. We reported the dimensionality effect associated with modified N-doped TiO2 structure for its characteristics properties and photocatalytic performance; counting more specifically its charge transportation, surface area, adhesion, reflection and absorption properties. A concise view of the doping effect over morphology in 0, 1, 2 and 3-dimensional ranges was provided, in order to understand which effects are also occurring on the materials besides the photocatalytic response. Furthermore, selected recent and significant advances in the area of renewable energy applications for modified N-doped TiO2 were assessed with the particular importance given towards the electricity generation by dye-sensitized solar cells and lithium-ion batteries rechargeable for electric energy storage. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Opening of the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Synthesis procedures: general prospective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Synthesis of N-doped TiO2 nanospheres (0-D materials) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Synthesis of N-doped TiO2 nanofibers and nanotubes (1-D) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 N-doped TiO2 nanosheets and thin films (two-dimensional) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 N-doped TiO2 interconnected architecture (three-dimensional) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1. Dye-sensitized solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.2. Li-ion batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

∗ Corresponding author at: Department of Chemistry—Federal University of São Carlos, Rod. Washington Luiz, km 235, CEP: 13565-905, São Carlos, SP, Brazil. E-mail addresses: shazad [email protected] (S.A. Bakar), [email protected], [email protected] (C. Ribeiro). http://dx.doi.org/10.1016/j.jphotochemrev.2016.05.001 1389-5567/© 2016 Elsevier B.V. All rights reserved.

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Shahzad Abu Bakar did his master from The University of Punjab in Analytical chemistry (2005) and M.Phil from Quaid-I-Azam University Islamabad in analytical/inorganic chemistry (2007). Then, he joined National centre for physics as a research associate in 2009. In 2013, he was awarded TWAS-CNPq scholarship for Ph.D. Now, Shahzad Abu Bakar is pursuing his Ph.D. degree with the supervision of Prof. Dr. Caue Ribeiro de Oliveira at Sao Carlos Federal University (UFSCar) and at National Nanotechnology Laboratory for Agribusiness (LNNA-Embrapa), Brazil. His main research interests include the synthesis of anion-doped TiO2 nanoparticles for potential target applications in heterogeneous photocatalysis. Caue Ribeiro de Oliveira is Materials Engineer (Sao Carlos Federal University – UFSCar, Brazil, 1999) and PhD in Physical Chemistry (UFSCar, 2005). Since 2007 he is Senior Researcher of Embrapa – Brazilian Agricultural Research Corporation and Professor in Chemistry Graduation Program of UFSCar. Currently he is coordinator of AgroNano Network, a group of about 150 researchers from different Embrapa Centers and Universities working on applications of nanotechnology for agribusiness. He is also vice-coordinator of National Laboratory of Nanotechnology for Agribusiness (LNNA), a multiuser facility supported by Embrapa. Research interests include nanoparticle synthesis, catalytic and photocatalytic role of nanoparticles and nanostructure preparation for slow/controlled release of agrochemicals.

1. Introduction 1.1. Opening of the review In recent years, the assembly of design principles, synthesis and new applications of N-doped TiO2 material has remained a topic of intense interest due the increased light absorption, which could allow a more efficient photoactivation by solar light [1–4]. However, literature still has many controversies about the application of this material, as well as a large number of synthetic methods, were reported. Then, it is still a challenge state to opt the best options to produce these materials and how these methods influence in other properties, such as morphology. Then this review summarizes the variety of different structure dimensionality obtained by such methods, which is one of the most effective approaches to applying the unique properties of cited materials in practical applications. The easy preparation of these semiconductors materials may endow with various structural properties regarding their dimensional classification by an array of physical and chemical methods, greatly extending the arsenal of TiO2 materials for most common application, such as in mechanical energy harvesting using inertial energy harvesting and kinematic energy for applications such as DSSc and Lithium-ion batteries. The information collected in this review discusses the fundamental and critical aspects of structure design and dimensionality for N-doped TiO2 materials having a high specific surface area, looking for the current applications and challenges associated by emphasizing concepts of its physics and chemistry. This review paper mainly focuses on highlighting and summarizing various structures dimensionality of N-doped TiO2 , in an attempt to organize the information regarding the structure design of N-doped TiO2 with their properties, synthetic methods, and applications. The development of photocatalyst using semiconductor nanoparticles has been the subject of considerable attention in the recent era, because of its obvious applications for a broad range of research areas, including current and potential applications such as electronic devices, sensors, energy-related fields and environmental problems [5–9]. Since the breakthrough of water splitting reported by Fujishima and Honda in 1972 [10], extensive and intensive research has been carried out in the investigation of photocatalytic properties of certain materials

Fig. 1. Schematic representations of photogenerated electron and hole formation upon irradiation of UV light on TiO2 .

to convert abundant, long lasting, and clean solar energy into another form of energy such as chemical energy. In this regard, the under-investigated photocatalysts were subjected to perform oxidation/reduction process for the generation of hydrocarbons [11] and hydrogen [10,12–15] (i.e. beneficial for energy related field) and for the removal of pollutant and bacteria from the air, in water and wall surfaces [16–37]. In broad terms, photocatalytic reactions are divided into two classes; (i) catalyzed photoreaction and (ii) sensitized photoreaction, depending on the approach of initial photoexcitation process. In catalyzed photoreaction, the initial photoexcitation step took place on the catalyst surface as a consequence of electron and/or energy transfer into ground state molecule. In the second case, the adsorbent molecules (e.g. dye molecule) underwent photo-excitation and then interact with ground state photocatalyst [38]. TiO2 , as a first generation material, has the strong oxidizing ability and has been most extensively studied for many applications [28,39–43] such as disposition of contaminants in aqueous and atmospheric ecosystems, super-hydrophilicity [44], and decomposition of organic pollutants [30,31]. These characteristics applications are related due to its appropriate properties namely; chemical stability, long durability, nontoxicity, low cost, and transparency to visible light [28,39–43]. The basic principle of photocatalysis process is started with the formation of photogenerated charge carriers, and namely h+ –e− pairs upon the absorption of ultraviolet (UV) light (in the case of TiO2 due to the high band gap), as shown in Fig. 1 [5,7,24,45–47]. The photogenerated charge carriers (h+ –e− pairs) recombined quickly after irradiation of UV light, which generated heat. Therefore, a few of the photo-generated charge carriers become available for the initiation of photo redox reactions. The efficiency of photocatalytic processes depends upon the lifetime of photo-generated charge carriers and the time scale of interfacial electron transfer. The photo-generated holes diffused on the surface of TiO2 and, interacted with adsorbed water molecules and/or other compounds i.e. hydroxyl groups. As a result, hydroxyl radicals (• OH) [7]) were produced, which alongside photogenerated holes oxidized nearby organic molecules on the surface of TiO2 photocatalyst. In the meantime, electrons from conduction band participated in reduction processes and reacted with molecular oxygen yielding in superoxide radical anions (O2•− ). Two basic principles accounting for the conversion of light energy into a useful generation of charge carriers are electron photogeneration/recombination and electron transfer on the surface of a photocatalyst. Therefore, major issues concerned for photocatalysis includes; how to increase the efficiency of charge carriers separation/transport in semiconductor nanoparticles? Another important factor which accounts for the application of TiO2 is the super hydrophilic nature of the TiO2 surface and originated from the chemical confrontation of changes on the surface. It was found that the contact angle under the UV-light irradiation is reduced to less than 5◦ in the case of TiO2 [44,47]. Therefore, the

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Fig. 2. Graphical representation of materials structural dimensionality with outcome hotline applications.

adsorbed species or water molecules on the surface of TiO2 react directly with the holes and, as a result, • OH radicals are produced with the consumption of bulk holes. The remained small portion of holes interact with the lattice oxygen sites or with the TiO2 itself. This interaction caused flagging of titanium and oxygen lattice bonds, and responsible for the generation of new hydroxyl groups. It was induced that the higher surface super hydrophilic capability of TiO2 material is originated due to high surface energy for the single coordinated hydroxyl group which has very low thermodynamic stability under UV-light irradiation. Considerable attention has been given towards the morphological design of TiO2 nanostructured materials such as nanorods [48–56], spheres [57–78], fibers [79–93], interconnected architectures [94–104], and sheets [103,105–114] as shown in Fig. 2. The widespread use of nanostructured TiO2 materials is not limited for photo-catalysis but also in dye-sensitized solar cells (DSSCs) [115–117], lithium-ion batteries [118,119], and electrochromic displays [120]. A major challenge in photocatalysis is the utilization of semiconductor materials (i.e. TiO2 ) for ongoing environmental and energy-related applications. Whereas, TiO2 shows photocatalytic activity under UV light irradiation due to its wide band gap (3.2 eV), and UV light consists of a small fraction (5%) of the solar spectrum compared to the visible light (45%) [115–117]. Therefore, efforts have been made to reduce this threshold energy barrier for optical response of TiO2 to move from UV to visible spectrum range. For this purpose, various approaches have been employed to reduce the band gap energy of TiO2 such as doping with metal cations [121–124] and with some anions (C [125–129], S [130–134], F [135–138], B [139–141], P [142–144] and N [145–148]) or generating oxygen vacancies [149,150]. However, doping of TiO2 with cations has been limited because of the requirement for expensive ion-implantation facilities, thermal instability of doped TiO2 and electron trapping by the metal center to enhance the rate of recombination [151,152]. N-doped TiO2 material has been synthesized so far for various modified applications. Asahi et al. [153] reported that N-doped TiO2 showed enhanced photocatalytic activity under visible light irradiation for the photodegradation of gaseous acetaldehyde and methylene blue. Since then this material has received a lot of attention. The implantation of N impurity into TiO2 crystal lattice

was believed to substitute for the lattice oxygen sites and modified the electronic structure due to the introduction of localized states at the top of the valence band. This modification in the electronic structure of TiO2 resulted in the reduction of bandgap and, therefore, a most probable factor for the enhancement of photocatalytic activity under visible-light. It was supposed that due to characteristic features such as comparable size, small ionization energy and high stability of nitrogen atoms as compared to oxygen atoms the incorporation of N into the TiO2 lattice structure could be an accessible opportunity. In 1986, Sato discovered that adding ammonium hydroxide in titania solution gave a precipitate which was calcined at high temperature resulting a yellowish appearance material that showed response under visible light [154]. But at that time, nobody had taken an interest in these findings. After that in 2001 Asahi et al. synthesized N-doped TiO2 material by sputter deposition [153]. They sputtered TiO2 under an N2 /Ar atmosphere, followed by annealing under N2 atmosphere and reported that the synthesized N-doped TiO2 showed the visible light response. Since then, there have been many reports cited dealing with various nonmetals doped TiO2 materials for their photocatalytic activity under visible-light irradiation. A significant effort has been devoted to exploring the underlying mechanisms for photocatalytic reactions under visible-light for N-doped TiO2 particles, and to investigate the electronic, optical and structural properties of N-doped TiO2 nanoparticles. It has been demonstrated that N-doped TiO2 photocatalyst exhibits strong visible light photocatalytic activity as compared to pristine TiO2 . So far, investigated model pollutants that have been effectively degraded by N-doped TiO2 photocatalyst includes methyl orange, phenols, methylene blue and rhodamine B, as well as some gaseous pollutants such as volatile organic compounds, and nitrogen oxides. Then, in the next section, the main synthesis methods in the literature are reviewed, focusing on the strategy used to introduce the dopant and advantages front of other methods, as claimed by each author. In sequence, an overview of methods reporting specific dimensional aspects (0, 1, 2 and 3-D structures) is reviewed, as the main topic of this paper. Finally, an overview of applications is presented, correlating these to the dimensional aspects previously discussed and to the advantages/disadvantages shown about each method.

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2. Synthesis procedures: general prospective Numerous approaches have been employed for the preparation of N-doped TiO2 material including wet chemical methods such as, hydrothermal treatment [51,155–159], sol–gel processes [17,50,99,148,160–162], spray pyrolysis [163,138] and supercritical methods [164], and some dry powder method like implantation techniques [163,165–167], high temperature sintering under a nitrogen containing atmosphere [168–171], and sputtering [168,172–174]. These methods are mainly described either as one step approaches and/or two-step approaches for the preparation of N-doped TiO2 materials. Sol–gel process and hydrothermal method are usually described as soft methods for doping N and/or other dopants into TiO2 crystal lattice while calcining the sample at high temperature. In a typical procedure, titanium trichloride (TiCl3 ) [175] or titanium sulfate (Ti(SO4 )2 ) [176] were used as precursor solution and NH3 solution (NH4 OH) was added into a precursor solution, as N source. Generally, this treatment yielded in precipitates which were calcined at high temperature to obtain anatase phase of TiO2 . Other Ti precursors, which are normally used in the sol-gel method for the preparation of N-doped TiO2 materials includes, titanium(IV) isopropoxide(TTIP) [177], titanium hydroxide (H2 TiO3 ) [178] and titanium tetrachloride (TiCl4 ) [179]. Similarly, other reagents used as N source are urea [180,181], guanidine hydrochloride [182], hydrazine [177], ammonium chloride (NH4 Cl) [183], and triethylamine(TEA) [1,184]. Controlled hydrolysis of TTIP at fixed pH resulted in obtaining TiO2 nanocrystal in a colloidal suspension and TEA was added dropwise to the colloidal solution for introduction of N into TiO2 nanocrystal. A deep yellow crystallite of N-doped TiO2 was obtained by vacuum drying of the treated nanoparticle solution [1,2]. Hydrothermal method and solvothermal method were used to obtain mesoporous Ndoped TiO2 nanoparticles [3]. For this purpose, Ti precursor was dissolved in nitric acid and various organic compounds containing N-donating atom including; urea, thiourea, TEA, and hydrazine hydrate, were added to the resulting solution as the N-doping source. The resulting mixture solution was set for hydrothermal treatment for several hours in a range of heating temperature, and resulting in varied N-doped TiO2 nanostructures. In another attempt, plasma treatment was applied to prepare N-doped TiO2 photocatalyst [4]. In this case, the Ti-precursor solution, e.g. TTIP in deionized water, are vaporized and transferred by inert gas (Ar/N2 /O2 ) into the plasma reactor to meet with excited N2 gas, used as the nitrogen source and implanted into the vaporized TiO2 particles [4]. The plasma treatment was applied to obtain N-doped TiO2 photocatalyst by treating pristine TiO2 either with the mixture of N2 /H2 gas or N2 gas to obtain a yellow powder [185,186]. Several examples can be used to address the effect of synthesis conditions on the morphology of N-doped TiO2 photocatalyst. For example, regarding temperature treatment, Morikawa et al. prepared N-doped TiO2 material through oxidation of TiN in a flow of O2 gas at 400 or 550 ◦ C for 90 min [187]. It was found that the annealing of the sample at a higher temperature (550 ◦ C) results in a homogeneous rutile phase while at lower temperature (400 ◦ C) TiN remains among the other rutile phase of TiO2 . Irie et al. treated anatase TiO2 at 600 ◦ C for 3 h in an atmosphere of flowing NH3 (67%)/argon (Ar) gas mixture to obtain anatase N-doped TiO2 material and noted that change in temperature and flow of NH3 gas changed the amount of N doping into TiO2 [188]. In another effort, Shin et al. noted that increase of annealing temperature for the synthesis of N-doped TiO2 results in an increasing number of O vacancies due to thermal decomposition of ammonia gas into nitrogen and hydrogen gasses (the last acting as an active reducing gas) [189]. It was reported that NH3 gas acts as either a source of N doping or for reduction of TiO2 , during the synthesis of N-doped TiO2

material by annealing of TiO2 in an NH3 atmosphere [188]. In the meanwhile, different N-containing molecules were used for the doping process. For example, Yin et al. prepared N-doped TiO2 powder by a mechanochemical method using urea and hexamethylenetetramine as N sources [190,191]. Typically stated, this method started with the mixing of TiO2 with N source (urea or hexamethylenetetramine) and followed by grounding the mixture in a planetary ball mill for 1 h at a rotation speed of 700 rpm. It was found that high energy mechanical treatment accelerates the phase transformation of anatase to rutile phase of TiO2 , and high-temperature calcination process was required to remove the organic substances from the product by combustion [190,191]. Reactive sputtering process (SP), a type of dry process, is mainly used to fabricate the 2-D thin films of N-doped TiO2 [153,166,192–194,174,173,195]. Main classifications of sputtering process depend on the SP source such as ion beam (IB) SP, direct current (DC) SP, electron cyclotron resonance (RCR) SP, and direct current (DC) SP. Sputtering process can be classified into direct current (DC) SP, radio frequency (RF) SP, ion beam (IB) SP, and electron cyclotron resonance (RCR) SP. Magnetron SP is often used for rapid deposition of N-doped TiO2 thin film dues to the application of magnetic field near the surface of target substrate which confines the plasma particles. More commonly, DC SP (employed when the target is electrically conductive) and RF SP (does not rely on whether the target is electrically conductive or not) have been engaged in the fabrication of N-doped TiO2 thin films [194,174]. Later on, a combination of these SP techniques (e.g. RF-magnetron SP or DCmagnetron SP) has been hired for tailoring of N-doped TiO2 thin films. The target surface (TiO2 or metallic Ti) is sputtered by fast moving N2 /Ar gas mixture and then set for annealing at high temperature (550 ◦ C) for a couple of hours in the continuous flow of N2 gas to obtain yellowish, crystalline, and transparent N-doped TiO2 thin films [173,195]. Atomic layer deposition (ALD) and pulsed laser deposition (PLD) are the most advanced techniques for the fabrication of crystalline N-doped TiO2 thin films. In PLD operation, Ti, TiO, TiO2 , and TiN target was retained at the heated substrate (400 ◦ C) and then bombarded with neodymium-doped yttrium−aluminium−garnet (Nd: YAG) pulsed in the presence of the continuous flow of N2 /O2 /Ar gas mixture [196,197]. Similar findings were observed where Okato et al. bombarded the surface of TiO2 and TiN mixed powders with Krypton−fluorine (KrF) eximer laser and then calcined at 300−500 ◦ C to obtain yellowish appearance N-doped TiO2 thin films [198]. Cheng et al. fabricated N-doped TiO2 thin films by ALD using TiCl4 , NH3 , and H2 O as the precursors [199]. Chemical vapor deposition has gained much intention in recent years for the deposition of N-doped TiO2 thin films in a single step without going for post-calcination treatment to obtain a product. Various types of CVD have been reported for fabrication of N-doped TiO2 thin films includes; atmospheric pressure CVD (APCVD) in which TiCl4 and NH3 or N2 O [200,201], or TTIP and hydrazine [202] and TiCl4 , ethyl acetate [203], and tert-butylamine [204], were used as raw materials. Other types of CVD are plasma-activated CVD (PACVD) using TTIP and Ar/N2 as source materials [170], metal−organic CVD (MOCVD) using TTIP and N2 [205], and plasma enhanced CVD (PECVD) using TTIP, NH3 , and Ar mixture [206]. Another interesting approach for the fabrication of 2-D N-doped TiO2 thin films is the surface treatment of bare TiO2 thin films. For this purpose, Miao et al. fabricated N-doped TiO2 anatase phase thin film with optical response in the visible-light region by sputterdeposited TiO2 thin films on glass slide surface treated with mixed gasses plasma of N2 -H2 and additionally annealed for 2 h in N2 gas at 400 ◦ C [207]. Borras et al. bombarded the surface of anatase TiO2 thin film by N2+ ion implantation using PECVD to obtain N incorporated TiO2 thin films [205]. Jiang et al. reported micro arc oxidation process (a combination of electrochemical followed by a plasma

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chemical process of nitrified surface) to fabricate anatase phase Ndoped TiO2 thin films by nitrifying the surface of pure Ti plate in an ammonia atmosphere at 540 ◦ C in a plasma nitriding furnace [169]. Similarly, Wan et al. reported another simple thermal oxidation process by annealing TiN at above 700 ◦ C in air to fabricate rutile phase N-doped TiO2 thin films [208]. Sol-gel and/or hydrolysis process, a type of wet process, has been reported recently for the fabrication of N-doped TiO2 thin films. For this purpose, isopropanol, tetrabutylorthotitanate, and ammonia or triethanolamine solutions have been used as a precursor for dip coated fabrication of N-doped TiO2 thin films [209,210]. Avisar et al. fabricated N-doped TiO2 thin films from tetrabutylorthotitanate, isopropanol, ammonium hydroxide and triethanolamine precursors solution by thin dip coating on the surface of glass substrate and dried in air at 150 ◦ C for 12 h and followed for calcination at 510 ◦ C for 1 h to obtain crystalline anatase phase thin film [209]. Recently, Kang et al. fabricated N-doped TiO2 thin film from mixing cellulose-containing ␣-terpineol with nanopowder to form viscous N-doped TiO2 gel and pasted it on a substrate for annealing at 400 ◦ C [211]. Xu et al. fabricated N-doped TiO2 thin film using aqueous peroxotitanate (PTA) solution. In a typical fabrication process, TiCl4 and NH4 OH were added dropwise to distilled water in an ice water bath under continuous stirring to obtain a white precipitate, which was filtered and dispersed ultrasonically with the addition of hydrogenperoxide (H2 O2 ) to form a PTA solution. After that, the substrate is dipped into the PTA solution for coating and then calcined in air for 2 h at 500 ◦ C to fabricate crystalline yellowish N-doped TiO2 thin film [210]. In sol-gel method, post treatment of pure TiO2 thin films is also performed to obtain anatase N-doped TiO2 thin film, for example by thermal treatment in ammonia flow [212] or by cathodic magnetron plasma [213] and ionized [214] N2 gas to the surface of pure TiO2 thin film. Qin et al. reported an anodic oxidation of Ti sheet using anode and Cu as a cathode electrode in an ammonium sulfate((NH4 )2 SO4 ) electrolyte solution with NH4 OH at externally applied a voltage of 245 V to fabricate N-doped TiO2 thin film [215].

3. Synthesis of N-doped TiO2 nanospheres (0-D materials) The importance of dimensional control on nanomaterial synthesis leads to several papers focusing on the morphological aspects of synthesized materials. Zero-dimensional materials (0-D) are the most common product, but aspects such as size distribution and agglomeration control are challenging, especially in doped structures. Spheres of micro/nano-structured N-doped TiO2 materials are now widely studied and used in cutting edge innovations for storage technologies and energy conversion applications such as lithium ion batteries [171,216–218], solar cells [219–221], and fuel cells [222–224]. Therefore, lots of research works have been reported for interesting and useful stuff derived from their unique structures. This results in an enhanced surface to volume ratio of 0-D nanospheres because of their unique structure and reduced the mass and charge transport length. These Ndoped TiO2 nanospheres usually possess outstanding structural and textural features such as high specific surface area, large pore volume, ordered pore networks with uniform and well-defined pore architecture [171]. Furthermore, these structural features are responsible for enhancing the light-harvesting capabilities of these N-doped TiO2 materials and facilitate the bombarded light to access the interior surface [224,225]. Overall, these structure features increased the size of an accessible surface area and, therefore, the rate of mass transfer of organic pollutant adsorption is increased. As a result, enhanced photocatalytic performance is achieved due to more feasible chemical reactions on the surface of the photocatalyst [224,225].

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Monodispersed N-doped TiO2 hollow nanospheres of the above type were successfully synthesized by using sol–gel precursor solution with uniform coating and followed by calcination of PS templates at 500 ◦ C to simultaneously crystallize titania and remove polystyrene (PS) spheres leaving behind a hollow core [226]. In a typical synthesis process, titania solution was prepared by hydrolyzing titanium isopropoxide in absolute ethanol under vigorous stirring and further set for static aging with cationic PS sphere [226]. During the synthetic process, cationic polystyrene spheres were initially used as a template to coat the titanium species by static aging. Afterward, the mixture was further treated with an appropriate ratio of ammonium hydroxide/nitric acid, and then polystyrene spheres were removed by calcination at 500 ◦ C to obtain 0-D N-doped TiO2 hollow nanospheres. The synthesized N-doped TiO2 hollow nanospheres possessed high pore volume and large surface area, and thus showed enhanced photocatalytic decomposition activity. Pan et al. have reported the synthesis of hierarchical N-doped TiO2 hollow microsphere as shown in Fig. 3 by green solvothermal method to have spherein-sphere structure and comprised of nanothrone with exposed anatase {101} facets, having diameter and length in the range of ca.15–25 nm and ca.30–60 nm, respectively [227]. In a solvothermal method, N-doped TiO2 hollow microspheres were prepared from the dissolution of titanium isopropoxide into a 2-propanol solution containing diethylenetriamine to obtain a clear solution and was transferred to a Teflon-lined autoclave operated at 200 ◦ C for 24 h. As a result, yellowish N-doped TiO2 hollow microspheres were obtained after several periods of washing with dry ethanol followed by calcination at 450 ◦ C in air for 2 h. The rough surface of obtained N-doped TiO2 hollow microspheres was comprised of numerous vertically oriented nanothorns and a few fragments of spherical shells (∼5%) co-existed in the final product, which may be due to the structural collapse during calcination or disassembly of solvothermal reactions. The synthesized N-doped TiO2 hollow superstructure possess large accessible surface area, excellent light harvesting properties and exhibited superior photocatalytic activity for the decomposition of organic molecules because of their high surface area, dispersion ability, and highly crystalline form. N-doped TiO2 hollow microsphere shows enhanced photocatalytic performance among the fabricated TiO2 spheres mainly ascribed because of the hollow structure has large specific surface area and also favor for light harvesting (enable light diffraction and reflection of light), and the fully crystallized anatase {101} facets upstanding on the surface [227]. In the meanwhile, Chen et al. reported the preparation of N-doped titanium dioxide microspheres with porous structure by using nitrogen-assisted glow discharge plasma technique at room temperature [228]. The synthesis of N-doped TiO2 microspheres was approached by treating already prepared TiO2 into a discharging tube fitted with a nitrogen-assisted glow discharge plasma system. The resulting Ndoped TiO2 spheres possessed large specific surface area and high potential for multiple reflections and diffraction of light. These properties enhanced the photocatalytic activity of N-doped TiO2 microspheres as demonstrated for methylene blue decomposition as compared to pristine TiO2 . The prepared N-doped TiO2 hollow microspheres were tested for enhancing antibacterial activity, as investigated against E. coli bacteria under visible and dark light conditions, respectively [229,230]. The antibacterial activity reported for N-doped TiO2 microsphere was better than pure TiO2 sphere, expected to arise from their special characteristic structure features which inhibit the growth of bacteria and also high specific surface area promote the efficiency of the antimicrobial performance [229–231]. Recently, a synthesis procedure has been reported for the preparation of N-doped TiO2 hollow nanosphere by using nanostructured TiO2 material loaded into a silica tube reactor for operating CVD

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Fig. 3. Sphere-in-sphere and upstanding nanothorns SEM (a, b) and TEM (c, d) images for N-TiO2 -HMS showing exposed anatase {101} facets on the spherical surface with a 5% collapse N-TiO2 -HMS during the calcination further helped to study the inner nanothorns inner shells having shorter and smoothers morphology than outer. Reprinted from Ref. [232]. with permission.

Fig. 4. Rose flower like TEM images for N-TiO2 (a, b) and quantum dots sensitized-N-TiO2 (c, d), respectively. Reprinted from Ref. [226] with permission.

instrumentation [232]. The temperature of the reactor was raised to a required degree (700 ◦ C) and NH3 gas was introduced into the heated reactor at a rate of 30 cm3 /min for 30 min, to prepare N-doped TiO2 hollow nanospheres with particle size approximately 20 nm. Shu et al. reported the synthesis of N-doped TiO2 hollow spheres with rough spherical morphology using a template-

free process and the resulting spheres were composed of a lot of small crystallites with particle size about 20 nm as shown in Fig. 4 [221]. General procedures adopted for the synthesis of Ndoped TiO2 hollow nanospheres with dimensions in the range of nanometer includes solvothermal synthesis [227] direct reactive plasma route in the air [233], sol–gel method [226,227], templating

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Fig. 5. Well pattern and regularly arranged SEM images of pure TiO2 (a, c) and N-doped TiO2 (b–d, f) first time prepared by nitrogen assisted glow discharge plasma technique for the study of their photocatalytic degradation for methyl blue under solar light. Reprinted from Ref. [233] with permission.

method [220,234] and thermal decomposition of titanium complex [228,235]. Chen et al. prepared porous N-doped TiO2 by nitrogenassisted glow discharge plasma technique as shown in Fig. 5 [228]. In the meantime, Ao et al. prepared N-doped TiO2 hollow spheres by newly developed chemically induced self-transformation mechanism [236]. The resulting N-doped TiO2 hollow spheres showed a strong contrast between dark edges and bright centers with rough outer surface and are loosely packed to each other. N-doped TiO2 hollow spheres with loosely packed arrangement showed enhanced photocatalytic activity due to improved absorption edge and mass transportation of contaminations [236].

4. Synthesis of N-doped TiO2 nanofibers and nanotubes (1-D) One of the major problem associated with powder photocatalyst is the recycling from the solution following the chemical reaction, which is very difficult and agglomeration of the particle into larger size also reduce the performance of photocatalyst during the cycle [237]. One-dimensional (1-D) materials (such as nanofibers and nanotubes) have advantages for photocatalytic reactions due to their high sedimentation rate and high specific surface area. Furthermore, nanofibers and nanotubes shapes materials have a higher surface to volume ratio, high interfacial charge carrier transfer rate, and better reduction in the electron-hole pair recombination rate, which favors for enhanced photocatalytic efficiency [46]. The

beauty of fibers assembly facilitates to obtain self-standing nonwoven mats and is only available to one-dimensional materials as shown in Fig. 6. In the modern industry, nanofibers based photocatalysts are very versatility for post-processing to form large area coatings, films, and porous membranes as well as composites with polymers having elongated nanoparticles [159]. The 1-D N-doped TiO2 materials have been widely used in an extensive range of applications, including photocatalysis [157,238], gas sensor [239,240], DSSCs [211,241,242] and batteries [162,243–245]. The 1-D nanostructures gas sensors of N-doped TiO2 nanofibers showed enhanced sensing performance due to their higher activity, large specific surface area to volume ratio and small size as shown in Fig. 7 [239]. Various methods have been reported for the preparation of N-doped TiO2 nanofibers, and electrospinning method has been acknowledged very useful tool for the fabrication of N-doped TiO2 nanofibers (NFs), due to versatile characteristic features such as simple to operate and production of nanofibers by using high-voltage electric current [183,246]. Fig. 8 shows the setup of operates used for electrospinning technique. Electrospinning method is regarded as an evolution in the synthesis of nanofibers by different materials along the uniaxial alignment and promotes the exploration of different interesting applications and properties which are linked with 1D nanostructures [247]. It has been reported that electrospinning process parameters played an important role for tailoring the morphology of nanofibers, includes molecular weight of the polymer,

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Fig. 6. High resolution TEM images of (a) pristine TiO2 NF, (b) N-TiO2 (A) NF, and (c) N-TiO2 (B) nanofibers after calcinations showed the formation of highly crystalline anatase structure with increased d spacing observed for one step annealed (b) nanofibers as compared to pristine (a) and two step annealed (c) N-TiO2 (B) nanofibers. Reprinted from Ref. [164] with permission.

Fig. 7. (a) and (b) SEM images of well aligned N-doped TiO2 nanofibers in horizontal line at low and high magnifications after annealing at 400 ◦ C with interlining distance of 40 ␮m, (c) TEM image single nanofibere with insert showing crystallite grain size distribution, (d) feroze colored dashed lines marked the interface of fabricated chip in CMOS technology for gas sensing setup. Reprinted from Ref. [244] with permission.

solution concentration, deposition distance and strength of applied electric field. Nanofibers assembled nonwoven mats possess properties such as large volumetric surface area and low resistance to mass transport due to their open structure with the pores between the nanofibers. N-doped TiO2 nanofibers were typically prepared from the electrospinning of a mixture containing titanium tetra butoxide, ethylenediamine, and polymer (e.g. polyethylene oxide) followed by the successive step of calcinations to get rid of unwanted byproducts (polymer) and to crystallize the N-doped TiO2 NFs [183]. Furthermore, processing parameters such as the ratio of reacting species concentration, the strength of the electric field, and importantly the feeding rate of the precursor in the electrospinning process are very important for tailoring the average diameter of the N-doped TiO2 nanofibers. N-doped TiO2 electro-

spun nanofibers have a very high surface area to volume ratio and, therefore, are very good candidate for providing efficient photocatalytic performances for the degradation of organic pollutants such as methylene blue [183]. It is noteworthy to mention that nanofibrous structure photocatalysts are very attractive candidates for practical environmental self-clean-up applications as shown in Fig. 9 [248] and can be used as a mat to eliminate the trouble associated with recycling [248]. Numerous modifications have been adopted to improve the physics and chemistry of electro-spinning process including the alignment of nanofibers by an electric field, the effects of polymer solution concentration, applied voltage, and electrode-to-collector distance [249]. Near-field electrospinning (NFES) has been introduced with several imported parameters like the option for an

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Fig. 8. Schematic representations of Near Field electrospinning apparatuses for the fabrication of N-doped TiO2 nanofibers.

Fig. 9. Typical FESEM images for N-doped TiO2 nanotubes grown on pyrex substrate of (a) top and (b) cross-sectional views. Digital images of pure and N-doped TiO2 nanotubes coated windows for attestation of self-cleaning ability by quick and dirty test and gas-phase oxidation reaction of CO-oxidation and methane. Reprinted from Ref. [253] with permission.

adjustable distance between electrode-to-collector, introduction of solid tungsten spinner, reduced applied electrostatic voltage and immersion of polymer solution into discrete droplets manners. These improvements of electrospinning method are reported very useful for increasing the photocatalytic activity of N-doped TiO2 nanoparticles [239]. Ruggieri et al. reported the fabrication of controlled parallel aligned and crack free N-doped TiO2 nanofibers by NFES technology [239]. The synthesized nanofibers were in the range of 3–4 mm in length with an internal diameter of 300 nm

and have 1-D pore free structures, and showed a high sense of NO2 gas detection at a very low concentration (1 ppm) at operating temperature of 150 ◦ C. The hydrothermal method has been reported as another conventional route for the synthesis of N-doped TiO2 nanorods [157,250]. Wang et al. reported the formation of V-shaped N-doped TiO2 nanorods as shown in Fig. 10 by using TiN as a precursor by hydrothermal method [250]. The edges of the nanorods have a zigzag shape which is due to inhomogeneous redeposition rate

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Fig. 10. FESEM images of N-doped TiO2 nanorods at low (a) and high (b) magnification. (c) TEM and (d) HRTEM images of N-doped TiO2 straight nanorods with an arrow in (c) showed the zigzag edge of the nanorods, and the inset in (d) is the representing SAED pattern. HRTEM images of V-shaped N-doped TiO2 nanorods are represented in (e) and (f) with corresponding SAED patterns. (g) Represents schematic illustration of the fabrication mechanism of straight and V-shaped N-doped TiO2 nanorods, the four-sided top is clearly indicated. (h) And (I) TEM images of self N-doped TiO2 nanorods and hydrothermally synthesized N-doped TiO2 nanorods, respectively. Reprinted from Ref. [255] with permission.

of the dissolved species during the crystal coarsening process on the surface of a large crystal. The growth direction of nanorod was reported along {001}; perpendicular to the crystal planes and confirmed by matching of inter-plane spacing (0.33 nm) with the reported value. Detailed analysis of the V-shaped N-doped TiO2 nanorods revealed that there are only two types of inner angles; at 114◦ and 135◦ and these values were counter checks from the calculation of boundary planes. In both types of V-shaped nanorods, the grain boundaries of the two branch crystals are all along (101) lattice planes which are symmetrical, indicating that grain boundary has coherent twin shape. It has been proved that the formation of V-shaped N-doped TiO2 nanorod is due to oriented attachment and Ostwald ripening. Anodization has counted another tool for the construction of N-doped TiO2 nanotubes. Anodization is an electrolytic passivation process and is performed in aqueous hydrogen fluoride-based solution (0.5%) which acts as an electrolyte and platinum (Pt) as a counter electrode. During the synthesis of nanotubes, both titanium foil and counter Pt electrodes, which are connected to a high power supply, are soaked in the electrolyte solution [46,238,251,252]. This step resulted in the formation of TiO2 nanotubes [253,254], which were further treated and whether annealed in the presence of NH3 gas [251] or immersed in 1 M NH3 ·H2 O solution for 10 h followed by annealing in a muffle furnace for 2 h to obtain crystalline phase N-doped TiO2 nanotube array electrode [46]. It was found that N-doped TiO2 nanotubes obtained by annealing in ammonia are piled together with almost homogenously ordered and tubelike cylindrical structure with diameters and wall thickness in the range of 100 nm and 20 nm, respectively. This structural characteristic enables the facile diffusion of pollutant molecules at the surface of N-doped TiO2 nanotubes and enhanced the photocatalytic decomposition of organic pollutants. While N-doped TiO2 nanotubes obtained by immersion followed calcination step had

an average diameter of 80 nm and were grown up perpendicular to the substrate with a wall thickness of ∼15 nm [46]. More interestingly, N-doped TiO2 nanotubes with large absorption area and extended visible-light absorption by N-doping showed enhanced the potential for photocatalytic degradation of organic pollutants [46]. Wall-thickness, another important structural characteristic of N-doped TiO2 nanotubes, reduced the chance of photo-induced holes and electrons recombination rate and considerably lowered the carrier diffusion length. Enhanced photocatalytic activity of N-doped TiO2 for the decomposition of hazardous organic pollutants has been encountered due to such structural features and also due to extended visible-light absorption length because of N doping. Other factors, responsible for the enhanced photocatalytic performance of N-doped TiO2 nanotubes array include the morphology and length of the nanotubes. It was found that increase of N-doped TiO2 nanotubes length prominently enhanced the photocatalytic performance due to the high surface area [255,256]. Antony and Mathews fabricated N-doped TiO2 nanotubes by varying the applied voltage using electrolyte mixture containing ethylene glycol, ammonium fluoride and water [255]. The formation of vertically aligned N-doped TiO2 nanotubes was confirmed from the cross-sectional FE-SEM images. The increase of applied voltage resulted in the increase of the diameter, wall thickness, height, and inter-tubular distance, whereas the surface density decreased [255]. Jiang et al. reported very simple, clean and green environmentally friendly solvothermal method for the fabrication of large quantity N-doped TiO2 nanotubes [256]. In this method, nanocrystalline N-doped TiO2 nanotubes were prepared via treatment of protonated titania from ammonium chloride/ethanol/water mixture solution. The as-prepared material inherits the tubular structure with an open end nanotubes having an aspect ratio of several micrometers in length and diam-

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Fig. 11. Schematics illustration of microwave plasma torch apparatus used for the fabrication of N-doped TiO2 nanorods. (b), (c), (d) and (e) are FETEM images of N-doped TiO2 nanorods of high magnification and chemical composition for Titanium (c), oxygen (d) and nitrogen (e) have been selected using EELS in (b), respectively. Reprinted from Ref. [262] with permission.

eter of 10–13 nm, whereas the value of inner and outer diameter was ∼5–8 nm and ∼10–13 nm, respectively. They reported that Ndoped TiO2 nanotubes were composed of interwoven nanotube bundles and having a large surface area with an improved photocatalytic performance for the degradation of organic pollutants (methylene blue was tested for photocatalytic degradation experiment) [256]. It has been evaluated that N-doped TiO2 nanotubes with longer elongated length showed greater photocatalytic activity than pristine TiO2 nanotubes having the same wall thickness as shown in Fig. 11. Highly ordered N-doped TiO2 nanotubes had channels with vertically aligned open mouth and allowed the substrate for an access point of target molecules and in the case of pristine TiO2 nanotubes the structure is disordered which suppressed the diffusion of organic molecules [257]. 5. N-doped TiO2 nanosheets and thin films (two-dimensional) Nanosheet (NS) is a layered material with the diverse and highly untapped source of two-dimensional systems with nanosize flakeshaped and has been studied extensively due to unusual physical phenomena of charge transport confinement to a plane. The inherently high surface area due to an extremely small wall thickness (1–10 nm) and lateral size varies from submicrometer level to several tens of micrometers provide many reaction sites to make them important material for sensing, catalysis, and energy storage applications [258]. Furthermore, the shape of nanosheet brings excellent adhesion to the substrate surface and low turbidity with a highly smooth surface. Due to these phenomenal advantages of nanosheet material enhanced their photocatalytic activity and superhydrophilicity [258,259]. Self-cleaning coating, one of the potential application of nanosheet films, has been raised due to the combination of highly surface smoothness and photocatalytic properties [259]. Semiconducting oxide nanosheet of TiO2 possesses many promising applications. The bandgaps of TiO2 nanosheets is reported ∼3.7 eV, and the p-orbital of O2− and d-orbital of Ti4+ lied in the valence and conduction bands, respectively [260]. The reported large bandgap as compared to bulk TiO2 crystal is cited due to quantum size effect and limits its use for photocatalytic

decomposition of organic pollutants under visible-light irradiation. Enyashin and Seifert [261] and Sato et al. [262] theoretically explained that the electronic structure in the individual nanosheet of lepidocrocite-type titanate layered is nearly independent of morphology and re-stacking process. Therefore, new strategies for modification in the electronic structure of titania NSs are highly desired. For this purpose, tailoring the bandgap energy of TiO2 nanosheet by N-doping for the modification of an electronic structure to extend absorption edge in the visible region has been proven to be an easier and more effective route. It was found that the filled band of N dopant p-orbital exists near the hybridized O2− p-valance band in its original bandgap as investigated by Liu et al. and they adopted two-step solid state synthesis route for the preparation of N-doped TiO2 nanosheets as shown in Fig. 12 [260]. Briefly, the synthesis started from the solid-state reaction between gaseous ammonia (source of N dopant) to the interlayer galleries of the layered Cs0.68 Ti1.83 O4 precursor and resulted in the uniform filling of N dopant over the whole layered titanate particles. The prepared N-doped titanate particles were further treated for ion-exchange in 1 mol/dm3 HCl solution and then set for exfoliation process in tetra-butyl-ammonium hydroxide solution to obtain N-doped TiO2 nanosheets. AFM technique was used to study the primary layer of N-doped TiO2 nanosheet deposited on a silicon wafer [263]. The reported wall thickness of NSs was ∼1 nm and the lateral dimension of the sub-micrometre range having unilamellar nature of individual nanosheets as confirmed by TEM analysis. The peaktop absorbance increase to ca.262 nm after each cycle of synthesis and showed a multilayer growth of N-doped TiO2 nanosheets as compared to the undoped TiO2 nanosheets where the increase in absorbance peak-top was not prominent. This comparative study helped in visualizing that N-doped TiO2 nanosheets have prominent absorption pattern in the visible-light region as compared to pure TiO2 NSs, which is an importance factor for inducing visiblelight photocatalytic activity. Photoanodes made of undoped and N-doped TiO2 nanosheets were exposed to visible light and marked enhancement in photocurrent response was observed for N-doped TiO2 NSs. It was attributed due to the derivation of Fermi level in the N-doped TiO2 NS which elevated the valence band top to the visible-light absorption region and near to small proportion of photoexcitation of oxygen vacancy-related states. The synthe-

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Fig. 12. Schematic representation of expected mechanism for the preparation of N-doped Ti0.91 O2 nanosheets. TBA+ : tetrabutylammonium ion. (a)–(d) show FESEM and TEM images of N-doped TiO2 nanosheets at high and low magnification. Reprinted from Ref. [265] with permission.

sized N-doped TiO2 nanosheets were suggested as building blocks for construction of nano-architecture for a wide range of practical applications including photocatalysis and optical-electronic devices [264,265]. Experimental and theoretical studies revealed that use of anatase TiO2 nanosheets with high energy (001) facets have gained much attention due to better and prompt reactivity towards photocatalytic degradation of organic pollutants as compared to the traditional TiO2 nanoparticles with (101) facets and reduced the recombination rate of photogenerated h+ –e− pairs [266]. It has been reported that less-reactive (101) facet is the most dominated facet for TiO2 NS due to their lower surface energy. Therefore, the large bandgap energy and need of very critical conditions for the crystal growth of (001) facets TiO2 nanosheets limit the modification of the TiO2 nanosheets electronic structure to a certain degree. To address these problems, it was required to synthesize visiblelight responsive N-doped anatase TiO2 nanosheets with dominant (001) facets for solar cell applications. Liu et al. and Xiang et al. reported the synthesis of N self-doped intersectional TiO2 NSs from TiN with the dominant (001) facets and examined their photocatalytic activity under visible-light irradiation to about 550 nm absorption edge [266,267]. Fig. 13 shows N self-doped intersectional TiO2 NSs from TiN with the dominant (001) facets. The prepared N-doped TiO2 NSs possess large surface area with much higher visible-light response and were tested for the generation of hydrogen from water splitting under visible-light irradiation. The prepared N-doped TiO2 NSs showed enhanced the ability for the generation of photocatalysis active • OH radical as compared to reported for N-doped TiO2 microcrystallites with exposed (001) facets. Recently, Liu et al. reported the fabrication of N-doped TiO2 NS with a dominant (001) facets by annealing of as-prepared TiO2 nanosheet under NH3 gas atmosphere at 400 ◦ C and examined their photocatalytic activity for the degradation of aqueous pollutant (rhodamine B) under visible light (>400 nm) irradiation [267]. The photocatalytic activity of N-doped TiO2 NSs with dominant (001) facets was compared with commercially available P25 TiO2 nanoparticles (reference) and the photocatalytic phenomena. The N-doped TiO2 NS with high energy (001) facets enabled the decomposition of rhodamine B (RhB) at a higher rate than that for P25 TiO2 particles. The photocatalytic decomposition of higher molecular weight organic compounds has been investigated over N-doped TiO2 nanostructured thin films. Clouser et al. reported the fabrication of N-doped TiO2 nanostructured thin films on the glass substrate

from N-doped TiO2 nanoparticles by sol-gel method and tested for photocatalytic decomposition of stearic acid and polyethelyne glycol (PEG) [268]. Quantitatively N-doped TiO2 nanostructured thin films showed 100 times higher rate for the photocatalytic degradation of organic compounds under UV–vis light irradiation than with only visible-light. It was ascribed that higher energy attributed to UV light and greater light absorption are the main factors for this difference. Dunnill et al. reported that the destruction rate of stearic acid under visible-light irradiation for N-doped TiO2 thin film was significantly higher as compared to undoped TiO2 thin films. It was found that complete removal of stearic acid was achieve–h+ pairs are generated in photocatalytic reaction when initial photon energy corresponds to the band gap of the bombarded semiconductor and they subsequently diffuse from inside bulk to the surface. Therefore, incorporation of N impurity into pure TiO2 crystalline promotes the availability of e− on the surface of thin films, and which made them more feasible to drive the photocatalytic decomposition reaction under visible light [268,269]. Another important application reported for 2-D N-doped TiO2 nano thin films is the photoinduced hydrophilicity under the irradiation of both visible and UV light. N-doped TiO2 NSs possess many important characteristic physical properties such as low turbidity, high smoothness, and good adhesion to the substrate. Modern applications of N-doped TiO2 thin film include the use of selfcleaning and corrosion resistance surfaces [269,270]. He-feng and Bin reported the fabrication of wear resistance and antifriction coating of N-doped TiO2 on stainless steel [270]. The fabrication of N-doped TiO2 coating was successfully achieved by oxidative annealing of sputtered TiN on the stainless steel substrate. It was noted that enhanced wears resistance with very low and stable coefficient of friction was observed for N-doped TiO2 thin film tested by a ball on disc sliding wear. It was predicted that N-doped TiO2 layer can be employed for environment-cleaning, self-cleaning, sterilization and solar cell, and specifically for wear protection applications [270]. Wu and Long et al. investigated the visible light induced self-cleaning property of N-doped TiO2 layer on cotton support surface [271]. The photocatalytic degradation of methyl orange (MB) under visible-light irradiation over pristine TiO2 cotton layer was very weak as compared to N-doped TiO2 coating layer. The photocatalytic degradation of MB was increased to 43% after two hours under visible-light irradiation. This was attributed due to narrowing of banding energy and improvement of optical absorption performance [271].

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Fig. 13. (A) shows the difference in UV–vis absorption spectra for both ((a) P25 titania and (b) N-doped TiO2 ) with the inset showed the appearance of both samples. (B) SEM image shows randomly arranged nanosheets of N-doped TiO2 with inside length and thickness of 80 and 20 nm, respectively. (C) and (D) show TEM of NTNs at low and high magnification with well-defined sheets having a rectangular shape and lattice fringes which are parallel to one of the nanosheet edges. Reprinted from Ref. [271] with permission.

6. N-doped TiO2 interconnected architecture (three-dimensional) Of the various morphological structures, the synthesis and self-assembly of three-dimensional (3-D) interconnected structures from nanoscale building blocks having a unique morphology, orientation and dimension have attracted considerable attention for producing a new class of materials with novel applications. The 3-D structure materials with these distinction characteristics have particular interest due to their novel or enhanced functional performances and potential applications [224]. These 3-D structure materials have a potentially large surface to volume ratios and interwoven porous network which enhanced the photoabsorption and mass-transfer of materials. As a result, enhanced photo-absorption efficiency and efficient diffusion pathways for the guest organic pollutants have been reported into the framework and ultimately supports efficient storage, separation and purification processes. More likely, the 3-D interconnected structure has superior carrier mobility which is very important for the practical point of view. At present, almost all photocatalytic purifier use porous surface ceramic coated with the TiO2 particle. These photocatalytic purifiers work only in UV-light because of a large band gap of TiO2 and also affected by the presence of small dust particles which caused the stripping of TiO2 from the surface tends to the degrade of photocatalytic properties. Therefore, efforts have been made by applying 3-D N-doped TiO2 interconnected structure to drag the photocatalytic activity into visible-light region. Recently, Parida et al. reported the preparation of fibrous and hierarchical meso/macroporous 3-D N-doped TiO2 photocatalyst with enhanced catalytic activity for H2 production as shown in Fig. 14 [224]. For this purpose, titanium isopropoxide droplets were added to the ammonia solution at room temperature to prepare hierarchical and fibrous meso/macroporous N-doped TiO2 nanoparticles without using templates. Highly porous structure resulted due to droplet interaction with ammonia solution and formed a thin dense semipermeable membrane due to subse-

quent hydrolysis and condensation reaction, and followed for calcinations. The hierarchical and fibrous structure of N-doped TiO2 nanoparticles allowed easy channelization of e− for effective surface charge transfer, and as well macro/mesoporosity and nitrogen incorporation played an important role for enhanced photocatalytic production of H2 [224]. Another interesting work was reported by Sreethawong et al. for the comparative study of Ndoped mesoporous-assembled TiO2 and N-doped non-mesoporous assembled commercial TiO2 (Degussa P-25) nanocrystals for photocatalytic H2 production by water splitting under visible-light irradiation [272]. The synthesis process started with the addition of structure directing surfactant and urea (N source) to the Ti precursor solution under sol-gel route to obtain mesoporous assembled N-doped TiO2 nanoparticles with monomodal pore size distribution. The photocatalytic activity of mesoporous-assembled N-doped TiO2 nanoparticle for the production of H2 was superior as compared to N-doped commercial TiO2 [272]. The presence of macropores in the meso-macroporous TiO2 structure material enhanced the photocatalytic activity due to improved photoabsorption and efficient diffusion of molecules with the guest species [273]. Furthermore, these macroporous channels strived as light transfer paths onto the large surface of inner photoactive mesoporous frameworks for the distribution of photon energy as shown in Fig. 15 [273]. Doping of meso/macroporous TiO2 for enhanced photocatalytic activity in the visible-light region has not been carried out so widely. In this case, Shao et al. reported the fabrication of hierarchically meso-macroporous N-doped TiO2 with high surface area and large porosity by the thermal treatment of as-prepared hierarchical meso-macroporous TiO2 with urea solution and tested for the decomposition of water contaminant (methyl orange and rhodamine B) under both UV and visible-light irradiation [273]. The synthesized meso-macroporous TiO2 and N-doped TiO2 contained macrochannels with the macroporous framework composed of wormhole-like arrays. N-doped TiO2 showed good photocatalytic activity with better efficiency for the photocatalytic degradation of

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Fig. 14. Schematic illustration of the fabrication method for hierarchical meso-macroporous N-doped TiO2 . Reprinted from Ref. [229] with permission.

Fig. 15. (a and b) SEM images of the hierarchically mesoporous-macroporous titanium dioxides (MMTD) material prepared at 608 ◦ C and its nitrogen-doping products: N-T-2/350 (c) and N-T-2/450 (d). Reprinted from Ref. [278] with permission.

methyl orange under UV irradiation, as well as the photodegradation of rhodamine B under visible-light irradiation as compared to the pure TiO2 . The photonic crystal structure is another strategy for enhancing optical absorption edge of the material to optimize the photogenerated electron/hole separation rate. The photonic crystal structure is inverse opals and has shown enhanced light harvesting efficiency for photocatalytic applications due to multiple scattering, slow photon effects, high photonic band gaps, and surface areas. These characteristics allowed control of spontaneous emission and localization of photons which are relating to their unique ordered submicron structures [274,275]. It was proposed that improvement of photocatalytic efficiency for the TiO2 could be accomplished by simultaneous physical and chemical modification of crystal structure such as the introduction of photonic crystal

structure and nonmetals doping, respectively. Various methods have been modified to obtain N-doped TiO2 inverse opal structure material and to demonstrate their enhanced visible-light photocatalytic activity [274,275]. Hu et al. reported a novel method for the fabrication of N-doped TiO2 inverse opal films by one step co-assembly of titania precursor and polymer colloidal spheres [276]. This approach is unique is the sense that it eliminates the demerits of other synthesis procedures such as lots of cracks and poor adhesion to the substrates for the fabrication of well-ordered N-doped TiO2 inverse opal structure films as shown in Fig. 16 [276]. The synthesis procedure concentrated on the appropriate amount of TiBALDH (titanium(IV)-bis-lactato-bis-ammonium dihydroxide), which acted as Ti and N source, to lead the fabrication of N-doped inverse opal films at optimized calcination temperature. The prepared films showed high visible-light photocatalytic

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Fig. 16. SEM images of the N-doped TiO2 inverse opal structure after calcinations at the different temperature from (A) 300 ◦ C; (B) 400 ◦ C; and (C) 500 ◦ C. (D) TEM image of the N-doped TiO2 inverse opal structure film with an insert of HRTEM showing macropore wall and calcined at 400 ◦ C. Reprinted from Ref. [281] with permission.

activity due to exclusive inverse opal structure and nitrogen doping for both color (Rhodamine B (RhB)) and colorless contaminants (tetracycline hydrochloride (TC)). Li and Shang reported the synthesis of N-doped TiO2 inverse opal structure by combining both chemical and physical modifications on n-TiO2 through sol–gel route [277]. The synthesized N-doped TiO2 inverse opal structure material showed enhanced visible-light photocatalytic efficiency for the degradation of organic pollutants. It was proposed due that chemical modification by nitrogen doping extended light absorption edge into the visible region and physical modification by inverse opal structure effect increased the surface area and provides multiple scattering with the slow harvesting of photons near the edges of its photonic gap. The photocatalytic performance of the N-doped TiO2 inverse opal structure was higher for the decomposition of methylene blue (MB) under visible-light irradiation as compared to pure TiO2 inverse opal structure. 7. Applications 7.1. Dye-sensitized solar cells With the increasing demand for utilization of energy resources, alternatives to the utilization of available fossil fuels have become one of the most highlighted tasks for the development of clean, green, cheap and sustainable energy technology for the modern science. In this regards, utilization of solar energy is considered as a promising candidate for renewable energy source due to the utilization of clean, renewable and inexhaustible resources. Recycling of utilized resources for the production of solar energy has been an emerging and most discussed issue for the sustainable social development and environmental protection especially in the most rapid economic growth areas around the world. Furthermore, the development of certain routes and features for the improvement of solar energy-to-electricity conversion efficiency has continued to be an important research area in solar cells. Since the discovery of first dye-sensitized solar cell (DSSCs) based on the single crystal zinc oxide, innovative progress has been made towards the utilization of renewable energy sources. But the performance of DSSCs was very poor and only 3% efficiency for solar-to-electric conversion due to the pitiable exposure of the surface area of

dye molecule adsorption [278]. Therefore, many efforts have been carried out to improve the performance and efficiency of the dyesensitized solar cell includes modifying the design of the DSSCs. More importantly, DSSCs has attracted considerable interest due to the low-cost fabrication process and high solar energy conversion efficiency as compared to the alternative conventional p–n junction silicon solar cells [279]. In 1991, Yun et al. developed a new type of solar cell based on mesoporous TiO2 nanoparticle film electrode by replacing the single crystal ZnO film to enhanced the dye loading and absorption capability of DSSCs [279]. This modification significantly improved the performance of DSSCs to ∼11% and reported an important step toward successful commercialization to beat the market of traditional solid state silicon solar cells as shown in Fig. 17 [221,241,280–282]. Other advantages of the dyesensitized mesoscopic TiO2 solar cell as compared to traditional silicon solar cells include ease of production, lower cost materials, and efficiencies to put-back the amorphous silicon solar cells. In realizing the significance of DSSCs, many academic and industrial R&D research work around the world have been focused on the development of DSSCs devices [221,241,280]. Constituents of DSSCs have been modified for the smooth operation of converting solar-to-electric energy including the use of transparent semiconductor oxide substrate, redox electrolyte light absorber, photoanode and modification of counter electrode [211,241,283]. Special emphasis has been given towards the design of dyes combination as it was revealed that three dyes combination showed different absorption wavelengths to complement the low absorption yield in the near IR region and more interestingly for scavenging the whole solar spectrum for enhanced solar-to-electric conversion efficiency of DSSCs. This modification improved results but limits for DSSCs commercialization due to certain factors such as an inconvenient technique for fabricating the three dyes laminate structure and difficulty for mass production. To address these difficulties, another approach using the light scattering effect on the photoanode has been studied to increase the absorption of longer wavelengths light [211,241,283]. Furthermore, light absorption ability of TiO2 in the visible wavelength region has been increased by fabrication of inverse opal structure TiO2 using photonic crystal materials and they showed increase response of photocurrent above 650 nm in the longer visible wave-

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Fig. 17. (a) illustrate the operating principles and energy level diagram of DSSC and (b) shows diagram show the schematic view of a Dye Sensitized Solar Cell Reprinted from [286] with permission. (c) Represents a schematic presentation of proposed charge transfer processes in a DSSC fabricated on optically-active N-doped TiO2 porous electrode. The process starts with the absorption of the photons by dye and then by N-doped TiO2 electrode for the generation of electron-hole pairs. Reprinted from Ref. [285] with permission. (d) Represents a practical application of DSSC fabricated on Ni substrate. Reprinted from Ref. [287] with permission.

Fig. 18. (a) Cross-sectional SEM images of N-doped TiO2 nanowires/nanoparticles with inset of enlarged part (b) STEM image of N-doped NW with inset HRTEM image of N-doped NW. (b) and (c) elemental mapping of N and Ti in N-doped TiO2 NW, respectively. I–V curves of different samples shown in (e) with inset of dark currents and (f) represents Nyquist diagram for impedance spectra with inset for the equivalent model used to represent interface of cells. Reprinted from Ref. [291] with permission.

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length region [211]. However, the presence of oxygen deficiency in TiO2 nanoparticles has frequently created electron-hole pairs and reacted with the dye or scavenger iodide ions to deteriorate the interfacial charge transfer and caused of the DSSCs short lifetime. To overcome these problems related to TiO2 based DSSCs various metals and non-metals doping into TiO2 crystal lattice have been actively studied [280]. Rather than doping of TiO2 , several other semiconductor oxide materials e.g. Nb2 O5 , ZnO, SnO2 , WO3 , SrTiO3 , CeO2 , Zn2 SnO4 , FeS, and NiO have been applied in developing DSSCs but their conversion efficiencies of DSSCs did not prove to be good enough for commercialization [221]. It was found that N-doped TiO2 composed photoanode in DSSCs enhanced the solar to current conversion efficiency due to more visible-light harvesting and extended electron lifetime from the excited sensitizing dyes on the surface of TiO2 [284,215,285]. Xie et al. reported the fabrication of N-doped TiO2 photoanode by using a hydrothermal method to analyze the effect of N doping level on the photovoltaic performance of DSSCs for dye uptake and the electron transport [284]. The dye absorption was increased with N-doping and consequently enhanced the photoelectron concentration which improved electron lifetime and resistance (R3 ) of the electron transport within photoelectrode. The improvement of photocurrent was prominent for 3% N doping level photoanode to increase the 6.25% performance of DSSCs which is 23.03% higher than that of pure TiO2 based photoelectrode. Moreover, reduced rate of photoelectron recombination and decreased electron transport resistance (R3 ) for 3% N-doped TiO2 photoelectrode favored the rate of charge collection for photogenerated electrons and increased the dye absorption electron transport of photoelectrodes and short-circuit current density (Jsc ) of 11.56 mAcm−2 to show better performance of DSSCs [284]. Qin studied the photoelectric performance of the pure and N-doped TiO2 photoelectrode for DSSCs application with the help of electrochemical impedance spectroscopy and I–V curves, and reported that N-doped TiO2 photoelectrode exhibits higher photoelectric performance as compared to pure TiO2 photoelectrode. It was attributed due to the red shift in absorption edge, synergetic effects of absorption in the visible-light region and lower resistance of the N-doped TiO2 photoelectrode [215]. N-doping in TiO2 crystal lattice extends the absorbance spectra into visible region due to expansion in crystal lattice dimensions and decrease in surface resistance of TiO2 electrode which improve the performance of N-doped TiO2 films based solar cell. Guo et al. fabricated nanocrystalline N-doped TiO2 photoelectrodes using ammonia, urea, and triethylamine as N source by wet chemical methods. The N-doped TiO2 photoelectrodes with different crystallite size and surface area showed 8.32% conversion efficiency for dye-sensitized solar cells [285]. Whereas, conversion efficiencies of 44% and 17% for pure anatase TiO2 photoelectrode (7.14%) and commercial P25 (5.76%) photoelectrode were observed for DSSCs. The dye-sensitized N-doped TiO2 photoelectrode showed high electron transports, longer electron lifetimes, and charge recombination for solar cells as compared to pure and P25 TiO2 based dye-sensitized solar cells (DSSCs). Moreover, enhanced photocurrent (ca.36%) due to high dye uptake and the efficient electron transport has been recorded [285]. A series of needle-like N-doped TiO2 nanocrystals and nanoparticles photoelectrodes with excellent thermostability were fabricated for dye-sensitized solar cells and showed good conversion efficiency of 4.8% and 10.1% in ionic and organic liquid electrolyte, respectively [241]. Fig. 18 shows the complete pattern of cross-sectional SEM/STEM and HRTEM images for Ndoped TiO2 nanowires/nanoparticles and I–V curves of different samples, whereas inset showed the dark currents of cells interface [286]. Furthermore, N-doped sensitized TiO2 films showed faster electron transport, the synergetic effect of higher dye uptake, higher photovoltage and higher stability for solar cell conditions. These characteristics accelerated the dye degradation compared

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to pure TiO2 and proved that they are ideal semiconductor materials for higher conversion efficiency of DSSCs [241]. Kang et al. reported the fabrication of N-doped TiO2 photoelectrodes with varying the N doping level using a simple sol-gel process for DSSCs [211]. It was found that photovoltaic performance of N-doped TiO2 photoelectrodes was approximate 23% higher as compared to pure TiO2 photoelectrodes. This is attributed due to increased light absorption in the near-vis absorbance region and partially to the morphological characteristics [211]. A detailed study of photovoltaic performance and retarded charge recombination in dye-sensitized N-doped TiO2 solar cells showed that formation of O Ti N bonds shift the flat band potential to the negative value which resulted in the improvement of the open circuit voltage for DSSCs and exhibited better stability under high-temperature conditions and soaking conditions under sunlight for more than 1000 h [287]. Mesoporous materials with tunable pore architecture and hierarchical structure have received much interest because of their disordered wormhole mesoporous framework decreases the diffusional barriers [288,289], aiding rapid transport of photogenerated electrons to the photoanode surface and enhanced the conversion efficiency for DSSCs. The key parameters which monitor the performance of DSSCs include porosity, pore size distribution, light scattering, electron percolation and conduction band edge of the mesoscopic TiO2 should be controlled to maintain coordination between dye and electrolyte medium. This could be achieved through control of certain parameters such as hydrothermal growth temperature, binder addition, doping materials, sintering conditions and precursor chemistry [290]. Chen et al. fabricated mesoporous anatase TiO2 photoanodes having high surface area and controlled pore size for DSSCs [291]. The fabricated photoanodes showed longer length for electron diffusion and extended retardation time as compared to commercial TiO2 photoanodes [291]. The conversion efficiency of mesoporous TiO2 photoanodes for DSSCs was reported in the range of 1.5–10%. In 2005, Ma et al. fabricated N-doped TiO2 photoelectrodes for DSSCs and showed enhanced incident photon-to-current conversion efficiency and improved short-circuit photocurrent density in the visible spectrum range of 380–520 nm and 550–750 nm [292]. A systemic investigation has been performed on the mesoporous N-doped TiO2 photoanodes to study the effect of N dopant amount on the performance of DSSCs [283]. N-doped TiO2 photoanodes were fabricated by a simple template-free wet chemical method using parallel macro-channels for enhanced trapping of incident light rays [283]. It was observed that overall conversion efficiency and photocurrent density was increased to 5.01–7.27% and 46% for Ndoped DSSCs as compared to pure TiO2 DSSCs. It was attributed due to reducing charge transfer resistances, high dye uptakes and N dopant amount in N-doped TiO2 photoelectrodes. Recently, Liu et al. reported a comprehensive study on DSSCs performance based on nanocrystalline TiO2 (Non-Meso TiO2 ), mesoporous TiO2 and N-doped mesoporous TiO2 photoanodes which were fabricated by a simple evaporation-induced self-assembly (EISA) method [242]. Mesoporous N-doped TiO2 photoanode showed improved photovoltaic performance with the maximum conversion efficiency for dye-sensitized solar cells as compared to the non-meso TiO2 and meso TiO2 DSSCs. It was ascribed due to the formation of O Ti N linkage which retarded the charge recombination between TiO2 photoelectrode and electrolyte interface. As a result, electron lifetime increased due to an incorporation of nitrogen into TiO2 by replacing the oxygen deficiency [242]. The characteristics of pseudo 3-D mesoporous N-doped TiO2 photoanodes which could improve the efficiency of DSSCs includes the disordered mesoporous structural framework with high surface area to give better dye-loading and a minimum length of diffusion charge carriers for rapid appearance to the surface, small charge storage capacity in the

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Fig. 19. (a) Schematic diagram of a DSSC and influence of N-doped TiO2 for enhancement of its efficiency. (b) and (c) HRTEM at low and high magnification show the interconnected nanoparticles with preferred (101) facets having crystallite alignment with SAED pattern. (d) J–V characteristics of DSSC for comparison of photonanodes prepared from P-25 and N-doped TiO2 nanoparticles with the efficiency value given in parentheses. (e) Electrochemical impedance spectra for comparison of DSSC. Reprinted from Ref. [298] with permission.

mesoporous framework with surface unsaturation and nanocrystalline N-doped TiO2 particles are electrically interconnected with good necking and predominant (101) anatase facets to minimize electron-hole recombination as shown in Fig. 19 [293]. Sivaranjani et al. reported the fabrication of mesoporous N-doped TiO2 photoanodes having a large number of bigger pores and electrically interconnected nanosized particles for DSSCs. It was noted that textural features and porosity allowed faster diffusion of charge carriers to surface for their utilization to generate power in about 10 min. A significant enhancement in conversion efficiency (6.9%) for mesoporous N-doped TiO2 photoanodes DSSCs was reported without using any light harvesting protocols and is noted ∼47% higher as compared to wormhole meso TiO2 (<5%) photoanodes [293]. Another rapidly evolving modification of DSSCs is the development of quantum dot sensitized solar cell [294], as a promising low-cost alternative for third generation photovoltaic, and to result in a significant improvement of about 6% for light to electric conversion efficiencies [295,296]. QDSSCs are based on a sandwich dye-sensitized nanocrystalline working electrode and a counterelectrode immersed in electrolyte similar to that of dye-sensitized solar cells. But the unique and intrinsic properties of semiconductor quantum dots photoanodes for QDSCs includes tunable band gap, high extinction coefficient, high photostability and multiple charge carrier generation made them very interesting components for light sensitized solar cells applications [294,297]. QDSSCs promise a high theoretical efficiency up to 44% for its special multi-electron generation character. Despite their pronounce potential, the conversion efficiencies of QDSSCs are still low as compared to conventional DSSCs [297]. Many investigations have been made to improve the low energy conversion efficiency of QDSCs. This is credited to improve the low photovoltage in the QDSCs than DSSCs and lower the fast electron recombination rate by the electronic properties of

the interfaces. For this purpose, different operation parameters of QDSCs are controlled including the development of novel photoanode architectures, different sensitizers, electrolytes and counter electrodes [298,299]. The addition of additive in an electrolyte is another strategy applied to lower down the fast electron recombination rate by adding additive such as tert-butyl pyridine and guanidium thiocyanateare [300]. Other strategies include the formation of a surface passivation layer using metal oxide [301,302] and by introducing co-adsorbents such as a chenodeoxycholic acid (CDCA) [303] and oligomeric poly(ethyleneglycol) carboxylic acid [304]. More fundamental way to prevent charge recombination is to remove the oxygen-deficient sites where the majority of electron recombination occurred. Mora-Seró and Bisquert reported that low energy conversion efficiency in an anatase TiO2 framework is due to existing oxygen-deficient centers. Therefore, either healing or eliminating of oxygen-deficient sites may improve the efficiency of QDSCs by suppressing the electron recombination rate [305]. Ion-doping has been reviewed an attractive approach for healing oxygen deficient TiO2 lattice crystal. For this purpose, particularly N-ion carriers have been cited more favorable for efficient charge separation at interfacial electron transfer sites [153,306]. It was found that introduction of mesoporous structure and N doping into TiO2 crystal lattice enhanced light to energy conversion efficiency of DSSCs. Thus, much of the research work has been devoted to investigating the characterization of N-doped mesoscopic TiO2 sphere for the photovoltaic performance of QDSSCs. It was believed that large surface area of mesoscopic spheres and N-doping would be beneficial approaches due to quantum dot absorption by charge collection facility broaden the absorption capacity. Shu et al. investigated the photovoltaic performance of mesoscopic N-doped TiO2 based quantum dot-sensitized solar cell. Furthermore, a green solvothermal method was used to prepare mesoscopic N-doped TiO2 spheres photoanodes and showed higher

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energy conversion efficiency as compared to pure TiO2 spheres [221]. It was expected that this improvement of photocurrent efficiency for QDSSCs could be due to synergistic effect of efficient electron transport, retarding charge recombination, higher quantum dots loading and enhanced light scattering. These characteristics were reported higher than previous reports and suggested that N-doped TiO2 spheres are good semiconductor material for QDSSCs. Sudhagar et al. fabricated hierarchical nanostructured Ndoped TiO2 photoanodes quantum dot-sensitized solar cells and noted higher energy conversion efficiency for the photovoltaic performance of nearly 145%. It was attributed due to the enrichment of electron transport by inter-particle necking through the TiO2 layer which increased the recombination resistance between the QD Ndoped TiO2 /electrolyte interfaces and cured the oxygen vacancies into TiO2 crystal lattice by N doping [163]. 7.2. Li-ion batteries Recent development in the modern technology of lithiumion batteries has been made for use as energy storage source in portable electric devices such as mobile, computer, cell phones and hybrid electric vehicles, due to their high energy density, low self-discharge rates, improved safety hazards, and good processability. Furthermore, lithium-ion batteries have gained much interest as a talented energy storage technology not only for portable devices but also for blinking energy sources such as solar and wind [307–309]. However, there is rising interest in improving the performance of lithium-ion batteries to obtain better performance and higher capacity for long lasting development of modern electronic portable devices. The most extensively used anode materials for lithium-ion batteries are based on carbon compounds and lithium-containing alloys. Unfortunately, the developed materials limited the possible improvements in the performance of lithium ion batteries due the reduced lithium activity in comparison to pure lithium metal, which decreased the reactivity with the electrolyte and reduces the cell voltage. Therefore, there is always a need to develop new materials for lithium-ion battery electrode to meet the demand of higher energy density and performance [310–312]. In view of electrochemical applications, TiO2 has been vigorously studied as one of the promising applicants in lithium ion batteries for alternative anode materials because of its obvious advantages over conventional graphite electrode such as excellent physicochemical properties, low toxicity and production cost [311,312]. More importantly, its operating voltage is higher than that of >1 V vs. Li/Li+ and the lower tendency of surface reactivity towards electrolyte limits the formation of either harmful Li dendrites or cathodic decomposition of electrolytes and enhance the overcharge protection and safety than conventional graphite electrode. A schematic representation of basic principle for the function of lithium ion batteries is shown in Fig. 20. Anatase TiO2 has been discussed for Li-insertion from the perspective of feasibility. Whereas, instead of high calculated theoretical capacity for TiO2 of 336 mAhg−1 corresponding to 1.0 Li-insertion per 25 unit cell of TiO2 , the highest reported practical capacity in the previous work was 168 mAhg−1 [313]. The low capacity value is supposed to be due to drastic structural transition from tetragonal to orthorhombic crystal system. Furthermore, a low intrinsic electric and ionic conductivity (∼1 × 10−12 S/m) decrease the capability rate of TiO2 electrode which hindered its practical application for anode material in lithium ion batteries [314,315]. Many efforts have been made to overcome these drawbacks including the introduction of nanostructure morphology (having high surface area) of TiO2 material for providing more chance of Li-insertion and reduce the length of ionic and electronic conducting pathways for metal or nonmetal doping [316,60,317,318]. In this regard, nanostructured materials featured a larger surface-to-volume ratio for fast

Fig. 20. Schematic illustration of the basic principle in Li+ batteries.

lithium insertion/extraction kinetics by providing shorter diffusion length and enhance the flux of lithium-ions between the interface of active material and electrolyte to give an excellent power density. Another common strategy applied for the improvement of Li storage capacity is the doping of TiO2 crystal lattice structure with different elements include metal and nonmetal [228]. Whereas, the availability and energy density of the N doping into TiO2 crystal lattice makes it promising candidates for better performance in lithium ion batteries [228,318–320]. Chen et al. reported that incorporation of N atoms into anatase TiO2 structure enhanced the electrochemical activities as compared to pure TiO2 particles having the same pore size and particle size [228]. The N-doped titanium dioxide microspheres were prepared via nitrogen assisted glow discharge plasma technique, operated at room temperature having a porous structure. The N-doped TiO2 microspheres showed better reversibility for lithium insertion/extraction as compared to TiO2 microspheres and P25 due to the smallest potential difference among the anodic and cathodic peaks. The discharge capacity for N-doped TiO2 microsphere was reported at 262 mAhg−1 , much higher than other samples. This indicated that N doping enhanced the electrochemical activity of TiO2 [319–321]. From the electrochemical data interpretation, it was revealed that N doping not only enhanced the reversibility of Li insertion/extraction but also the behavior rate is also increased during the charge-discharge cycle of TiO2 [321]. Han et al. fabricated highly ordered TiO2x Nx nanotubes by an anodization process. The prepared TiO2x Nx nanotubes were further treated with N plasma (consists of mainly atomic N states). The TiO2x Nx nanotubes showed better energy-storage performance for lithium ion batteries as compared to pure TiO2 nanotubes [322]. Higher cycling properties of N-doped TiO2 nanotubes and enhanced charge transfer and higher incorporation of Li+ during cycling as compared to pure TiO2 nanotubes is attributed due to the disparity of charge transfer. The reported initial coulomb efficiency for both pure and N-doped TiO2 nanotubes was 84 and 65% respectively. This shows the significance of electron charge transfer importance in reversibility of Li+ insertion/extraction process. Thus, it was found that 1-D nanostructure N-doped TiO2 nanotubes showed higher energy storage performance for lithium ions batteries as compared to conventional materials. Kim et al. studied the influence of N-doped TiO2 concentration on the porous polymeric electrolyte membrane containing lithium perchlorate (LiClO4 ) on lithium ion conductivity [243]. They observed that N–Tifilled PECs showed a better ability to absorb and retain Li+ ion with highest ion conductivity of 6.7 × 10−4 S/cm, which is of four orders higher in magnitude as compared to pure LiClO4 /PVDF-HFP.

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Fig. 21. Pattern of SEM images for TiO2 (a), N-doped TiO2 nanofibers (b) after calcinations. The presence of bright field in TEM images for TiO2 (c) and N-doped TiO2 nanofibers (d) with their SAED patterns (inset), match to anatase phase. Annular dark-field STEM and HR-TEM images for TiO2 ((e), (g)) and N-doped TiO2 nanofibers ((f), (h)) with the corresponding FFT patterns. Charge−discharge for the first cycle of TiO2 nanoparticles, TiO2 nanofibers, and nitrogen-doped TiO2 nanofiber electrodes with cyclic retention curves are shown in (i) and (j), respectively. (k) Represents the rate performance for TiO2 nanofibers and nitrogen-doped TiO2 nanofiber electrodes. Reprinted from Ref. [223] with permission.

Fig. 22. A representative view of FE-SEM, TEM and HR-TEM images of nanofibers for TiO2 with the sequence as follow; ((a)–(c) TiO2 nanofibers, (d)–(f) TiO2 hollow nanofibers and (g)–(i)) nitridated TiO2 hollow nanofibers, respectively. Coaxial electrospinning spinneret using a dual nozzle (j) represented for showing the drawing of TiO2 nanofibers (k), TiO2 hollow nanofibers (l), and nitridated TiO2 (m). The specific capacity cycle is shown in (n) for TiO2 nanofibers, TiO2 hollow nanofibers and nitridated TiO2 hollow nanofibers at different current densities. Reprinted from Ref. [331] with permission.

The addition of N-doped TiO2 material not only facilitates the ionmobility and ionic transportation of the PECs but also enhanced the ionic conductivity due to the generation of strong interaction

among the polar groups likewise O and N of N–Ti and Li+ ions. It was observed that an optimum amount of N–Ti could provide more feasible environment for achieving higher ionic transporta-

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tion and ion mobility to improve the performance of Lithium-ion batteries. Thus, synthesis and design of a favourable architecture material for electron or Li+ transport could enhance the performance for lithium storage. For this purpose, 1-D nanostructure material has been attracting attention due to its unique structural characteristics and tunable intrinsic properties such as conductivity and Li+ intercalation [162,244,245,323] Kim et al. fabricated 1-D TiO2 Nx nanofibers by electrospinning process and studied their electronic or electrochemical properties for lithium rechargeable batteries as shown in Fig. 21 [218]. A comparative study was made to evaluate the performance of 0-D nanoparticle with 1-D TiO2 nanofibers for use as anode material in lithium-ion batteries. It was found that 1-D TiO2 nanofibers showed surprisingly higher cyclic retention than 0-D TiO2 nanopowder. More interestingly, N doping into 1-D nanofibers further improved the performance of lithium rechargeable batteries. The presence of 1-D nanostructures such as nanorods or urchin-like structure provides the optimized environment for charge/discharge process in lithium ion batteries due to their dual intrinsic worth such as high surface area, more aspect ratio for fast electron transport without considering large irreversible capacity. The fabricated 1-D N-doped TiO2 nanofibers showed high charge capacity (Li+ insertion) of around 224 mAhg−1 and further cycling for 40 rounds lead the Li+ insertion value to 185 mAhg−1 retentions. This indicates morphological advantages for superior cycling performance including Li+ storage and diffusion process. The charge/discharge process for Li+ was superior for 1-D N-doped TiO2 nanofibers than to both 0-D nanopowder and 1-D TiO2 nanofibres. Recently, Hasegawa et al. studied the performance of hierarchically porous N-doped TiO2 material for Li-ion batteries applications [324]. The N-doped TiO2 material was prepared from macroporous inorganic-organic hybrid gels consisted of titanium complex surrounded by ethylenediamine. High discharge capacity and good cycle performance as anode material for Li-ion batteries were noted for N-doped TiO2 . It was attributed due to enhanced accessibility of the hierarchically porous structure and doping of N atoms into TiO2 crystal structure by a hybrid network. Different factors suggested that high capacity of N-doped TiO2 hierarchically porous structure is ascribed due to improvement of electric conductivity by N-doping, better Li+ diffusion due to improved ionic conductivity, occurrence of side reaction near the crystal surface preferentially where nitrogen atoms located, and distortion of Ti O lattice by nitrogen doping facilitate the electrochemical reaction and Li+ diffusion between the interfaces of active materials and electrolytes. The synthesized hierarchically porous N-doped anatase TiO2 photoanode showed high discharge capacity than the previously reported value (168mAhg−1 ) and better performance for 80 cycles demonstrated superior material for an anode of Li-ion batteries [324]. Various efforts have been made to resolve the problem associated with poor rate capability of TiO2 due to the intrinsic physicochemical properties. For this purpose, thermal nitridation was designed to fabricate nanoscale TiOx Ny . Particular interest has been given to lithium ion diffusivity particularly solid state diffusion of lithium ion and electronic conductivity for use as an alternative anode material in high power lithium rechargeable batteries. Park et al. reported in 2008 that thermal nitridation of Li4 Ti5 O12 under the atmosphere of NH3 increase the rate capabilities of lithium ion batteries due to the formation of conductive nanoscale TiOx Ny layer on the surface and showed a similar character as for oxide based SAFs [325]. It was found that capacity is 6 times higher. After nitridation for 10 min, a mixed-valent intermediate phase plus the core-shell robust structure of nitridated material showed improved electrochemical performance. Han et al. fabricated hollow TiO2 nanofibers via electrospinning method. The prepared TiO2 nanofibers were further set for thermal treatment under NH3 atmosphere to form TiN/TiOx Ny layers as shown in Fig. 22 [326]. The hollow geometry has beneficial for

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large lithium ion flux because of shorter diffusion distance and a large interfacial area between the electrolyte and active material which improved the kinetics of lithium ions. Hollow nitridated TiO2 nanofibers enhanced the electron transport property and significantly improved the capability rate due to synergistic effects of nanosized 1-D hollow geometry and conducting layer, respectively. The discharge capacity and performance rate for nitridated TiO2 hollow nanofibers was almost two times as compared to TiO2 nanofibers at 2C rate. It was attributed due to high electronic conductivity due to TiN which improved the capability rate of TiO2 . But we know that Li is electronically inactive with the TiN material. Therefore, Li transport is blocked due to the formation of thick, discrete and disorder TiN-based surface and optimized the surface nitridation process. To address these issues, another interesting study was conducted by Samiee et al. and showed that capacities rate is more than double for controlled nitrided anatase TiO2 nanoparticle as compared to air annealed anatase TiO2 nanoparticles [327]. This enhancement in capacity rate is attributed due to moderate surface nitridation with the less-disordered nitridated region which increased the surface electronic conductivity and reduced the blockage of lithium transport by discouraging the formation of nanoscale, discrete, and surface amorphous films. It was suggested that controlled doping or thermal treatment atmosphere could improve the capabilities rate of battery materials by surface modification or/and thermodynamics.

8. Conclusions This review summarizes the design principles, synthesis and new applications of N-doped TiO2 photocatalysts and also highlighted emerging opportunities for the application of N-doped TiO2 photocatalyst in various new developing fields. This review has also discussed that doping of N into TiO2 may endow with various structural properties regarding their dimensional classification to greatly extends the arsenal of TiO2 materials and their potential for a spectrum of applications. The information collected regarding zero-dimensional structure N-doped TiO2 nanospheres have a high specific surface area and are tried to figure out the current applications and challenges associated with 0-D N-doped TiO2 photocatalyst, to share the whole image and information to more audience in front of the worldwide research. The 0-D N-doped TiO2 photocatalyst has been viewed as a new emerging class of materials, due to hollow and high energy facet structure and has been rapidly developed with unprecedented speed for diverse photocatalytic applications. Different approaches have been developed for the synthesis of one-dimensional structure of nanofiber and nanotubes shape. Most prominent approaches include electrospinning and/or anodization methods. The design synthesis and unique properties of 1-D N-doped TiO2 have been classified on energy and environmental applications such as photocatalysis, solar cells, sensors, lithium ion batteries, catalysis, and supercapacitors. The discovery of 2-D nanosheet has started a new class of materials science due to their exclusive variety of electronic properties. The 2-D nanosheets are available in the atomically thin layer with mechanically, thermally and electronically stable properties for ultrathin flexible devices. The unique electronic properties of 2-D N-doped TiO2 nanosheets such as band gap tuning and effective e− –h+ masses allow the estimation of charge and spin mobility enhanced the photocatalytic performance. In addition, massless Dirac electrons of 2-D materials exhibit properties which are unknown from the bulk material. The 3-D N-doped TiO2 material has been a promising candidate for modern research interest due to their different structure and intrinsic properties. The porous 3-D macroscopic structure architecture in the N-doped TiO2 monolithic materials has an interconnected structure with high electrical con-

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ductivity, large surface area, and high mechanical strength. These provide potential advantages for separation, storage and purification. Overall, the dimensionality plays a key role to the properties of N-doped TiO2 materials and can be designed to fit the property requirements for different applications in the modern industry. In this review, we have discussed modern applications of N-dope TiO2 photocatalyst including recent progress in Dyesensitized solar cell and Lithium-ion batteries. It was found that selective doping of atomic N states or molecular N2 configurations into TiO2 crystal lattice through a novel and cheap techniques may be very prized in the development of anodic materials for lithiumion batteries. Experimentally, it has been evidenced that successful doping of atomic N states into highly ordered 1-D nanostructure TiO2 nanotube arrays enhanced energy-storage performance for lithium ions batteries. The fabrication of N-mediated nanometer or atomic-scale quantum dots could be very challenging due to their enhanced thermal stability for temperature-sensitive dye materials. An excellent rate of performance and discharge capacity was achieved for the nitridated TiO2 hollow layer for lithium-ion batteries as compared to TiO2 nanofibers by balanced electrode design in both morphology and composition. It was attributed due to current collector, large electrode/electrolyte contact areas, and effective alleviation of mechanical strain from lithium ion insertion/extraction. The presence of hollow geometry and accompany of conducting shell layer in the 2-D nanostructure could provide shorter diffusion length and high electric conductivity for lithium ion, and to improve the rate of capability. The existence of hierarchically porous structure for N-doped TiO2 monoliths could provide faster discharge capacity and enhance cycle performance for anode materials of lithium ion batteries. Microscopic and electronic origins of 3-D morphology due to interstitially located N atoms near the N-doped TiO2 surface higher the conductivity and faster Li+ diffusion for having excellent electrochemical performance, enhancement of cyclic retention, ion mobility, conductivity and changing work function, as confirmed by electrochemical charge/discharge curve and impedance spectra. The influence of nitrogen doping is found to be beneficial on device performance for the development of DSSC, with a significant improvement of photocurrent due to their high surface area. This provides a large surface for dye chemisorption while reducing the amount of electrode material, and reduced charge migration length to allows effectively charge transport from the dye molecules to the electrode. This review has also discussed the effect of parameters influence such as particle size, excellent anatase phase, and suitable concentrations of nitrogen doped TiO2 on the overall conversion efficiency of the DSSCs. The 1-D N-doped TiO2 spheres showed higher energy conversion efficiency for QDSSCs. It was attributed due to their ability for retarding interfacial recombination which causes the negative shift of the flat-band potential and producing a synergistic effect between efficient electron transport to enhanced light scattering and higher quantum dots loading. The 2-D N-doped TiO2 anode films are ideal semiconductor materials for DSSC and achieved high energy conversion efficiency and more dye uptake increase the amount of photocurrent to open circuit photovoltage for a faster electron transport in the N-doped DSSCs. The pseudo-3D nature of mesoporous N-doped TiO2 exhibited a surpassing photovoltaic performance. The 3D mesoporous N-doped TiO2 could successfully retard the charge recombination between photoelectrode/electrolyte interface by healing the surface states or managing the oxygen vacancies upon N-ion doping and enhanced electron lifetime/transport due to inter-particle necking and merger of the oxygen deficiency into TiO2 crystal lattice to form O–Ti–N bonds. Thus, the development of these techniques may offer a renewable approach for potential resource saving. It is of great importance to explore new materials and possible cost-effective techniques involving TiO2 to solving the

challenges in our lives faced by the community for better applications such as renewable energy production and environmental protection.

Acknowledgments The authors greatly acknowledge this work to late Prof. Dr. Tajammul Hussain, who inspired, advised and helped so much to achieve this milestone. The authors also like to thanks the financial support of the Brazilian Agencies CNPq-TWAS, FAPESP and FINEP to this project. The authors like to thank Prof. Dr. Vagner Romito de Mendonc¸a (IFSP, Brazil) for productive discussions.

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