One-dimensional TiO2 (1D TiO2) nanomaterials with unique structural and functional properties have been extensively used in various fields including photocatalytic degradation of pollutants, photocatalytic CO2 reduction into energy fuels, water splitting, solar cells, supercapacitors and lithium-ion batteries. In the past few decades, 1D TiO2 nanostructured materials with a well-controlled size and morphology have been designed and synthesized. Compared to 0D and 2D nanostructures, more attention has been paid to 1D TiO2 nanostructures due to their high aspect ratio, large specific surface area, and excellent electronic or ionic charge transport properties. In this review, we present the crystal structure of TiO2 and the latest development on the fabrication of 1D TiO2 nanostructured materials. Besides, we will look into some critical engineering strategies that give rise to the excellent properties of 1D TiO2 nanostructures such as improved enlargement of the surface area, light absorption and efficient separation of electrons/holes that benefit their potential applications. Moreover, their corresponding environmental and energy applications are described and discussed. With the fast development of the current economy and technology, more and more effort will be put into endowing TiO2-based materials with advanced functionalities and other promising applications.

A review of one-dimensional TiO2 nanostructured materials for environmental and energy applications

Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO)

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO2) and iron oxide (hematite, α-Fe2O3) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO2 and α-Fe2O3 in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO2 and α-Fe2O3 photoelectrocatalytic properties. The present review describes various synthetic methods and modifying concepts of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on α-Fe2O3/TiO2 heterostructures, for PEC applications. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Photoanodes based on TiO2 and α-Fe2O3 for solar water splitting – superior role of 1D nanoarchitectures and of combined heterostructures

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

One‐dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Solar driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes represents a promising approach for a sustainable and environmentally friendly production of renewable energy vectors and fuel sources, such as dihydrogen (H2). In this context, titanium dioxide (TiO$_2$) and iron oxide (hematite, $\alpha$-Fe$_2$O$_3$) are among the most investigated candidates as photoanode materials, mainly owing to their resistance to photocorrosion, non-toxicity, natural abundance, and low production cost. Major drawbacks are, however, an inherently low electrical conductivity and a limited hole diffusion length that significantly affect the performance of TiO$_2$ and $\alpha$-Fe$_2$O$_3$ in PEC devices. To this regard, one-dimensional (1D) nanostructuring is typically applied as it provides several superior features such as a significant enlargement of the material surface area, extended contact between the semiconductor and the electrolyte and, most remarkably, preferential electrical transport that overall suppress charge carrier recombination and improve TiO$_2$ and $\alpha$-Fe$_2$O$_3$ photo-electrocatalytic properties. The present review describes various synthetic methods, properties and PEC applications of 1D-photoanodes (nanotubes, nanorods, nanofibers, nanowires) based on titania, hematite, and on $\alpha$-Fe$_2$O$_3$/TiO$_2$ heterostructures. Various routes towards modification and enhancement of PEC activity of 1D photoanodes are also discussed including doping, decoration with co-catalysts and heterojunction engineering. Finally, the challenges related to the optimization of charge transfer kinetics in both oxides are highlighted.

Photoanodes Based on TiO$_2$ and $\alpha$-Fe$_2$O$_3$ for Solar Water Splitting Superior Role of 1D Nanoarchitectures and of Combined Heterostructures

The photocatalytic degradation of the non-biodegradable azo dye Acid Orange II (AO II) was performed via a heterogeneous Fenton-like process using Fe incorporated TiO2 nanotube arrays (Fe–TiO2 NTAs) as a photocatalyst in the presence of H2O2 under UV irradiation. Fe–TiO2 NTAs were prepared by an ultrasound-assisted impregnating-calcination method. The effects of reaction conditions including the amount of Fe incorporated into TiO2 NTAs, initial concentration of H2O2, and initial pH of the AO II solution on the photocatalytic degradation rate of AO II were investigated. The results revealed that the Fe–TiO2 NTAs prepared by ultrasonic deposition for 25 min exhibited the highest photocatalytic activity compared with pure TiO2 NTA and other Fe–TiO2 NTA samples. In the presence of H2O2, the photocatalytic degradation rate of AO II was significantly enhanced. About 80% AO II can be decomposed at pH 2.96 by using Fe–TiO2 NTAs prepared by ultrasonic deposition for 25 min as a photocatalyst with 3 mM H2O2 after 120 min of UV irradiation, which was 1.7 times higher than that without H2O2 (∼30%). Very interestingly, there was an obvious synergistic effect between the photocatalytic process of Fe–TiO2 NTAs and H2O2 oxidation. Furthermore, the Fe–TiO2 NTA photocatalyst exhibited excellent stability in the heterogeneous Fenton-like process. Thus, we demonstrated that the heterogeneous Fenton-like process combining the Fe–TiO2 NTA photocatalyst with H2O2 oxidation has a promising application in practical wastewater treatment.

Acid Orange II degradation through a heterogeneous Fenton-like reaction using Fe–TiO2 nanotube arrays as a photocatalyst

An ultrasound-assisted deposition technique was adopted to construct NiO nanoparticles on TiO2 nanotube arrays (NiO–TiO2 NTAs) which were prepared by three-step electrochemical anodization in an ethylene glycol system. The NiO nanoparticles were found to deposit on the surface of the highly oriented TiO2 nanotubes. NiO–TiO2 NTAs exhibited improved photochemical capability under UV irradiation. Compared with TiO2 NTAs, a more than 2.56-fold enhancement in photocurrent response was achieved using the NiO–TiO2 NTAs. The maximum incident photon to charge carrier generation efficiency (IPCE) values of NiO–TiO2 NTAs and TiO2 NTAs were 83.5% and 22.2%, respectively. The photocatalytic activities of NiO–TiO2 NTAs were evaluated by the photodegradation of methylene blue (MB) aqueous solution. The kinetic constant of photocatalytic degradation of MB using NiO–TiO2 NTAs prepared by ultrasonic deposition for 1.0 h was 1.9 times higher than that using TiO2 NTAs. The enhanced photocurrent response and photocatalytic activity with NiO–TiO2 NTAs were attributed to the formation of the NiO–TiO2 p–n junctions.

An ultrasound-assisted deposition of NiO nanoparticles on TiO2 nanotube arrays for enhanced photocatalytic activity

A novel CdS/TiO2 nanorods/TiO2 nanotube array (CdS/TNRs/TNT) photocatalyst was prepared. The self-organized highly oriented TiO2 nanotube arrays (TNTs) were first synthesized by anodizing Ti sheets. The “flower-like” rutile TiO2 nanorods (TNRs) were then grafted on the TNTs by a hydrothermal method. Subsequently, the CdS quantum dots (CdS QDs) were deposited on the surface of the resulting TNRs/TNTs using a sequential-chemical bath deposition (S-CBD) method. UV-vis diffuse reflectance spectra indicated that the CdS/TNRs/TNTs sample showed significantly enhanced absorption in the range from 350 to 700 nm. The photoelectrocatalytic hydrogen production activities of all samples were evaluated by using Na2S and Na2SO3 as sacrificial reagents in water under a 300 W Xe lamp with a UV-light filter (λ > 420 nm). The results showed that CdS/TNRs/TNTs prepared by hydrothermal reactions for 4 h and S-CBD 15 cycles showed a hydrogen production rate approximately 14 times that of the TNTs. When compared to CdS/TNTs, CdS/TNRs/TNTs showed a 2.3 fold increase in hydrogen production, which can be attributed to the enlarged effective deposition area for CdS QDs by depositing “flower-like” rutile TiO2 nanorods on the TNTs. In addition, CdS/TNRs/TNTs exhibited excellent hydrogen production stability using Na2S and Na2SO3 as sacrificial reagents.

Efficient visible light-induced photoelectrocatalytic hydrogen production using CdS sensitized TiO2 nanorods on TiO2 nanotube arrays

Enhanced photoelectrocatalytic hydrogen production activity of SrTiO3–TiO2 hetero-nanoparticle modified TiO2 nanotube arrays

Photoelectrocatalytic efficiency is mainly dependent on the light absorption of a photocatalyst electrode, charge separation and transport, and surface chemical reactions. To enhance photoelectrocatalytic efficiency, an ordered one-dimensional heterojunction photocatalyst consisting of CuxZn1−xIn2S4 ultrathin nanosheets on electrochemically anodized TiO2 nanotube arrays (CZIS@TNTAs) was successfully synthesized by a solvothermal reaction. It was found that the CuxZn1−xIn2S4 ultrathin nanosheets on the TNTAs significantly enhanced visible light absorption and photoelectrochemical responses, and a near 8.0-fold increase in the photoelectrocatalytic hydrogen production rate was achieved compared to the blank TNTAs. Moreover, the CZIS@TNTAs exhibited excellent photoelectrocatalytic stability. Measurements of the flat-band and electrochemical impedance revealed the feasibility of charge transfer in the CZIS@TNTA hetero-systems, which was supported by PL and photoelectrochemical measurements. The superior photoelectrocatalytic activity of the CZIS@TNTA composite electrodes is mainly attributed to their enhanced light absorption and the separation of photo-generated charges, facilitating electron transport along the 1D TiO2 structure.

Zhi Wu

Papers

Acid Orange II degradation through a heterogeneous Fenton-like reaction using Fe–TiO2 nanotube arrays as a photocatalyst

An ultrasound-assisted deposition of NiO nanoparticles on TiO2 nanotube arrays for enhanced photocatalytic activity

Efficient visible light-induced photoelectrocatalytic hydrogen production using CdS sensitized TiO2 nanorods on TiO2 nanotube arrays

Enhanced photoelectrocatalytic hydrogen production activity of SrTiO3–TiO2 hetero-nanoparticle modified TiO2 nanotube arrays

Enhanced visible light photoelectrocatalytic activity over CuxZn1−xIn2S4@TiO2 nanotube array hetero-structures