Nanostructured Titania: the current and future promise of Titania nanotubes

Abstract

Titania nanotubes (TiNT) combine unique nanotubular morphology with the desirable electronic, optical, and chemical properties of nanostructured titania. This perspective provides an overview of the three major synthesis methods for TiNT and the corresponding physical and chemical characteristics of the material. Promising applications in photocatalysis are explored with special emphasis on recent insights in the photooxidative and photoreductive activity of TiNT materials. The major challenge for future work is to connect observed improvements in catalytic activity with an understanding of the fundamental processes in photocatalysis in order to tailor the structure of TiNT composites to targeted functions.

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Kevin C. Schwartzenberg

Kevin Schwartzenberg completed his B.S. in Chemical Engineering at Purdue University in 2007. He has completed his M.S. and is currently pursuing his PhD in Civil and Environmental Engineering at Northwestern University under the advisement of Professor Kimberly Gray. His primary research focus is photocatalysis for energy and environmental applications.

Kimberly A. Gray

Kimberly Gray is a Professor in the Department of Civil and Environmental Engineering, with a secondary appointment in Chemical and Biological Engineering, at Northwestern University. She is a member of the Center for Catalysis and Surface Science at Northwestern and is one of the group leaders in the Institute of Catalysis for Energy Processes. Her areas of expertise are photocatalysis and environmental chemistry and her current research interests include the synthesis and characterization of photoactive, nanostructured composite materials for energy and environmental applications. She is the author of over 100 papers and 4 invention disclosures, and lectures widely on energy and sustainability issues.

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Introduction

Beginning with carbon nanotubes in the early 1990's, the synthesis of nanotubular materials has since expanded to many other chemistries. Titania nanotubes (TiNT) combine the unique, nanotubular morphology with the desirable electronic, optical and chemical properties of nanostructured titania and have emerged as a material of great interest in the fields of electronics, catalysis, solar energy, coatings and others.^1–3 The past decade has seen the development of three distinct synthesis processes and numerous variations thereof producing an extensive range of materials with several promising applications. This perspective will provide a brief overview of the synthesis, characteristics, and applications of TiNT with a focus on their photocatalytic properties and highlight those issues that present opportunity for further research and development.

Synthesis techniques

The most common procedures for the synthesis of TiNT are anodic oxidation, sol–gel template methods, and hydrothermal methods.

Anodic oxidation of Ti foils

Anodic oxidation is commonly employed to coat the surfaces of metals to decrease corrosion or wear, or improve the adhesion of polymers to the surface.⁴ In anodic oxidation, the metal to be coated acts as the anode of an electrochemical cell, which also includes a cathode (usually another metal such as platinum), an electrolyte mixture, and a power supply. It has been observed that for the anodic oxidation of titanium, the structure of the oxide film can be either compact, or porous depending on the nature of the electrolyte. Zwilling and co-workers showed that the presence of HF_(aq) in an electrolyte solution of chromic acid led to the formation of a columnar, porous oxide layer approximately 75 nm thick.⁵ Gong and colleagues subsequently prepared highly ordered arrays of TiNT up to 250 nm in length using a similar method and HF electrolyte solution.⁶ The mechanism for the formation of TiNT from anodic oxidation relies on three interacting processes:

i. electric field assisted dissolution of Ti metal ions into the electrolyte solution

ii. electric field enhanced oxidation of Ti metal to create TiO₂

iii. dissolution of Ti and TiO₂ due to etching by F⁻ ions.³

The relative rates of these processes control the formation of the TiNT, and thus, the aspect ratio. Beranek and coworkers found that increasing the duration of anodization initially caused a corresponding increase in length, but this response eventually leveled off at about 500 nm.⁷ Early attempts to increase the TiNT aspect ratio focused on controlling the pH near the open end of the growing tube, thereby limiting the chemical dissolution of the oxide. In this manner, the length was increased from 500 nm to 7 μm.^8,9 Subsequent efforts involved the use of organic electrolytes such as formamide/N-methylformamide (FA/NMF), dimethyl sulfoxide (DMSO), ethylene glycol/ammonium fluoride (EG/NH₄F),^10,11 or focused on the choice of fluoride counterion.¹² Due to the incorporation of these organic compounds into the oxide layer, the dielectric breakdown potential is lowered, allowing the use of higher anodization potentials. Under these conditions, highly polar electrolyte systems such as FA/NMF can be used to increase the length of TiNT arrays by accelerating the rate of growth of the pores. In contrast, the length of TiNT arrays was observed to increase in DMSO electrolyte due to decreased etching of the TiO₂ at the mouth of the tube. Arrays prepared using EG/NH₄F electrolytes exhibited growth characteristics similar to those prepared with FA/NMF.¹¹ By optimizing the water content of these electrolyte solutions near 5 wt%, the length of the TiNT arrays was extended to 1000 μm.¹³ A more extensive account of anodic oxidation synthesis techniques can be found in the review of highly ordered TiNT arrays by Mor and colleagues.¹⁴

Major advantages of TiNT synthesized by anodic oxidation include their highly tunable length, their uniform orientation, and the fact that their synthesis leaves them essentially immobilized on the Ti precursor material. Potential limitations include a closed end geometry and the relatively small volume between tubes.² In addition, this technique may be difficult to scale up for production of commercial quantities of TiNT.

Template methods

Template methods have long been used for the fabrication of carbon nanotubes.¹⁵ The principle has also been applied to the synthesis of TiNT. In a typical process, Ti-containing precursor reagents undergo sol–gel hydrolysis. The TiO₂ formed is allowed to polymerize or deposit onto the template. Subsequent removal of the template material and calcination yields crystalline TiNT.² The literature reports a wide variety of templates. The pores of anodically oxidized aluminua have been used as a template for TiO₂ deposition in a sol–gel process using tetrabutyl titanate in ethanol.¹⁶ Self assembled organic surfactant templates, such as laurylamine hydrochloride are also used in the presence of tetraisopropyl orthotitanate.¹⁷ Electrochemical deposition has also been used in conjunction with template molecules.¹⁸

Advantages of this approach include tight control over the size and morphology of the TiNT based on the geometry of well-understood template materials. However, the template material is typically dissolved or discarded, which means more materials are required for this synthesis than for others. The expenses associated with this added material and extra processing step may pose a barrier to commercial scale up.

Hydrothermal synthesis

Another well known technique for the synthesis of TiNT is the hydrothermal process, which employs high temperature and pressure under alkaline conditions to generate single layer nanosheets of titania which self-assemble (roll) into tubes due to the high surface energy of sheets.¹⁹ Kasuga and co-authors reported a simple hydrothermal process using titania nanoparticles, which were synthesiszed in the lab by a sol–gel method.²⁰ Subsequent syntheses have used commercially available titania powders as the starting material.^1,19,21–28 The titania is suspended in an aqueous solution of NaOH (typically 10 M). The typical hydrothermal processing takes place inside a PTFE-lined, stainless steel vessel, which is then sealed and placed inside an autoclave. The vessel is allowed to cook inside the autoclave at autogenic pressure and temperatures from 110–120 °C in excess of 20 h. During this process, some of the Ti–O–Ti bonds are broken and Ti–O–Na and Ti–OH bonds are formed. Subsequent re-arrangement yields lamellar intermediates consisting of TiO₆ octahedra with Na⁺ and OH⁻ intercalated between sheets.²⁶ The tubes are then acid washed with HCl, which facilitates the exchange of intercalated Na⁺ ions with H⁺ ions. The degree of exchange is largely determined by the final pH of the acid wash solution, with a lower pH corresponding to a higher degree of exchange. This process produces a charge variation in the surface layer of the lamellar intermediate which, when strong enough, overcomes the inter-layer binding energy causing a sheet to peel off of the surface and roll up into a tube.^26,29 Subsequent washes with distilled water bring the pH of the wash back to 5–6. The resulting material is then dried in an oven and calcined. The morphological and surface properties of TiNT synthesized with this method can be tuned by varying the calcination temperature over the range of 200–400 °C. The nanotubes begin to collapse to nanorods above 400 °C and to nanopowders above 600 °C.¹ Advantages of the hydrothermal method include its relative simplicity and high yields, making it an excellent candidate for commercial scale up.

TiNT Modification and the use of Dopants

Beyond the three major synthesis methods discussed above, many attempts have been made to combine TiNT with other materials or to otherwise modify the surface of TiNT. Common methods for generating these composites include the soaking of hydrothermally synthesized TiNT in solutions containing the dopant of interest and electrodeposition of dopants onto anodized arrays of TiNT. Functionalized TiNT synthesized in these ways address two primary goals. The first is the addition of dopant materials or coatings to improve and tailor the performance of TiNT in a particular application. For example, dimethyl phosphite has been added to improve visible light response,²¹ various metal dopants including Ni, Mn, Ru, Cu, Ag, Co,²⁵ and Ce²⁸ have been added to improve the catalytic properties of TiNT in the oxidation of contaminants, Au and Pt have been added to facilitate electron transfer in electrochemical biosensors,³⁰ and an allylamine plasma polymer has been used to coat the surface of TiNT for improved biocompatibility in medical implants.³¹ The second goal for composite synthesis is the use of TiNT as a novel substrate or support to enhance the catalytic activity of other materials. Examples of this strategy include the use of TiNT as a substrate to improve the catalytic oxidation of carbon monoxide²⁴ by gold nanoparticles, to enhance the antimicrobial properties of silver,²³ and to increase the capacitance of nickel hydroxide in batteries and supercapacitors.³² These are just a few examples of the uses of TiNT in composite materials and their prevalence will only continue to grow given the potential to improve existing technologies.

Characteristics

The TiNT characteristics of primary interest are morphology, size, crystal phase, and surface area.

Morphology

The morphology of TiNT is highly dependent on the synthesis method. Hydrothermally prepared TiNT, once dried into a powder, are randomly oriented. They are open on both ends (Fig. 1A). In contrast, anodically oxidized titanium produces TiNT in a highly ordered array. The tubes are open on one end and closed at the other, which is fixed to the underlying foil or substrate. There are small void spaces between adjacent tubes (Fig. 1B). If anodization is carried out under constant voltage, the resulting TiNT are cylindrical. However, the use of sweeping voltage results in TiNT which are tapered with a smaller diameter at the open end of the tube.¹⁴ The morphology of TiNT prepared using template methods depends on the template. For example, TiNT prepared using anodized alumina as the template are produced in an ordered array¹⁸ whereas processes that use organic surfactant templates yield a disordered powder.³³

Fig. 1 (A) Hydrothermally synthesized TiNT after calcination at 400 °C. (B) Array of TiNT produced by anodic oxidation followed by calcination at 580 °C. Reproduced with permission from work by Schulte and coworkers.³⁶ Copyright Elesevier Science 2010.

Size

The relevant dimensions of TiNT include the internal diameter, length, aspect ratio, and wall thickness and are summarized in Table 1. Fig. 2 compares the relationship between length and diameter for TiNT synthesized by the three techniques discussed above. TiNT prepared by anodic oxidation exhibit a relatively uniform diameter on the order of 100 nm. The length of the tubes prepared in this way spans four orders of magnitude, resulting in aspect ratios from 2.5 to 8334. This enormous range reflects a continuing effort to increase length by altering synthesis conditions such as electrolyte composition and water content. Electrolyte bath temperature has also been found to influence both length and wall thickness with higher temperatures producing shorter tubes with thinner walls.³⁴

Table 1 Size parameters for TiNT using various synthesis methods

Synthesis method	Inside diameter (nm)	Wall thickness (nm)	Length	Aspect ratio	Reference
Anodic oxidation	100	N/A	250 nm	2.5	6
	60	40	2.3 μm	38	58
	40	12	7.0 μm	175	8
	115	N/A	4.4 μm	38	9
	160	25	134 μm	838	10
	180	24	93 μm	517	11
	150	50	93 μm	620
	135	25	220 μm	1630
	120	N/A	1000 μm	8334	13
Hydrothermal	8	N/A	100 nm	13	20
	3–5	1–2	25–250 nm	3–83	19
	4–6	2–8	50–300 nm	13–75	1
Template	70–100	30–50	8.0 μm	80–115	18
	4–7	3–16	∼102 nm	15–26	38

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Fig. 2 Diameter and length for TiNT synthesized using anodic oxidation (●), template methods (), and hydrothermal method (). See Table 1 for references.

TiNT prepared using template methods have a wide range of sizes as illustrated in Fig. 2. The diameter of the TiNT reflects the geometry of the template and can be controlled over at least an order of magnitude. However, the maximum length of the nanotubes produced using templates methods is low compared to those produced by anodic oxidation, ultimately limiting the aspect ratio of these materials.

Hydrothermally synthesized TiNT have a fairly uniform size with internal diameters on the order of 10 nm and length on the order of several hundred nanometers. This uniformity reflects the consistent synthesis conditions reported in the literature. There is also some model-based evidence that the consistent diameter corresponds to a minimum in the total energy of a rolled layer of titanate at a radius of curvature of 4.3 nm.²⁹ Minor variations may be due to differences in NaOH concentration, processing temperature and time.

Crystal phase

As initially prepared by anodic oxidation, TiNT are found to be amorphous. One report indicated that crystalline TiNT were formed by anodic oxidation without subsequent calcination by increasing the temperature of the electrolyte bath.³⁵ However, calcination or annealing at high temperature is usually required to crystallize the tubes. X-ray diffraction (XRD) reveals that TiNT prepared by anodic oxidation begin to undergo crystalization to anatase at annealing temperatures above 280 °C.³⁶ As annealing temperature is increased to about 430 °C, peaks associated with rutile begin to appear in the XRD spectra. This is a result of the transformation of the crystallites of the underlying oxide layer.^9,37 Growth of rutile within the nanotubes themselves does not occur due to geometrical constraints imposed by the structure of the tube walls which prevent nucleation.³⁶ At even higher temperatures above 680 °C, the XRD spectra show almost exclusively rutile, however at this point the porous architecture of the array collapses.^36,37

XRD and selected area electron diffraction (SAED) have been used to demonstrate that hydrothermally prepared TiNT show anatase structure after calcination.²² However, because of the mechanism of formation of these nanotubes, titanates are sometimes produced. During NaOH treatment, some Ti–O bonds are broken resulting in lamella of sodium titanate (Na₂Ti₂O₅·H₂O). Subsequent acid washes result in the exchange of Na⁺ for H⁺. Below pH 8.0, the lamella roll up into tube structures of Na_2−xH_xTi₂O₅·H₂O. Further lowering of the pH results in a transition to anatase.²⁶

TiNT prepared by various template methods have been shown to be either amorphous¹⁸ or anatase with a very low crystalinity³³ prior to calcination. XRD spectra and SAED patterns of calcined samples reveal anatase crystals.^{16–18,33,38}

Surface area

Because of their high aspect ratio and hollow architecture, TiNT have significantly higher specific surface area than other titania nanomaterials. Table 2 shows the specific surface area for TiNT prepared using several different methods. Tubes produced using hydrothermal and template methods demonstrate an increase in specific surface area by a factor of ∼30 over bulk titania powder and by a factor of ∼6 over Degussa P25, a commercially available mixed phase titania nanocomposite. Reported values for the surface area of TiNT arrays produced by anodic oxidation are relatively low in comparison. This may be due to the presence of a compact titania layer at the base of the tubes which does not have the same porosity as the tubes themselves.

Table 2 Specific surface area for commercial titania materials and TiNT prepared by various methods

Material	Specific surface area (m^2 g^−1)	Reference
Bulk TiO2 powder	9.8	1
Degussa P25	50	21
TiNT (Hydrothermal)	261.7	27
	240	1
	274	21
TiNT (Anodic oxidation)	38	13
	19	37
TiNT (Template)	390	33

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Applications

The unique properties of TiNT have led to their use in a number of important applications. For example, it has been demonstrated that the electrical resistance of an array of TiNT produced by anodic oxidation varies by 8.7 orders of magnitude when exposed to alternating atmospheres of 1000 ppm of hydrogen gas and air.³⁹ This discovery has led to the use of TiNT in hydrogen gas sensors. Titania nanomaterials have previously been used in dye sensitized solar cells (DSSC) as the wide band gap semiconductor that routes photogenerated electrons to the anode.⁴⁰ Arrays of titania nanotubes produced by anodic oxidation are a promising alternative to these titania nanoparticles because of their excellent electron percolation properties due to their highly ordered nature with minimal defect and charge trapping sites.³

Perhaps one of the most promising applications for TiNT is in the area of photocatalysis. In this process, semiconductors including titania are used to convert radiant energy to chemical energy. Titania catalysts are of particular interest because of the fact that they are inexpensive, highly active, non-toxic, and relatively stable under ambient environmental conditions.⁴¹ It is well established that titania photocatalysts are robust and general catalysts for a wide variety of oxidative and reductive reactions under both gas and aqueous phase conditions. Recent work has detailed material improvements achieved by nanostructured titania composite materials.⁴² Recently, there has been a great deal of interest in investigating the extent to which TiNT can achieve enhanced reactivity due to the combined effects of such properties as high surface area to volume ratio, high aspect ratio, and a reduced number of interfacial boundaries.

Water splitting

The splitting of water into hydrogen and oxygen gases facilitated by photocatalysts has been touted as a possible alternative to replace fossil fuel use, both as the source of hydrogen and ultimately, as source of primary energy. By coupling the energy of the sun to a photoelectrochemical cell, energy can be stored in the chemical bonds of hydrogen. Fig. 3 shows a schematic of photoelectrolysis. A semiconductor, such as titania, serves as the photoanode of the photoelectrochemical cell, absorbing incident light and producing e⁻/h⁺ pairs. The holes oxidize water at the anode to produce oxygen gas while the electrons migrate through the bulk semiconductor and the external circuit to reduce water to hydrogen gas at the cathode.

Fig. 3 Schematic of water splitting. Incident light excites an electron at the titania anode. The resulting electron hole is filled as water is oxidized to oxygen gas. The electron flows through the external circuit and is used to reduce water to hydrogen gas.

In order to successfully split water, a semiconductor material must be able to efficiently absorb incident light, generate e⁻/h⁺ pairs and then prevent those pairs from recombining before they can react with water. When used in a photoelectrochemical cell, TiNT prepared by anodic oxidation exhibit much higher rates of hydrogen production per watt of incident light than other materials.³⁴ They also exhibit relatively high photoconversion efficiencies in the UV range of 320–400 nm (up to 16.25%).³ This performance is explained to a large extent by the morphology and electronic properties of TiNT. First, the nanotubular architecture creates a large amount of surface area relative to other materials. This increases scattering and effective path length of incident photons resulting in more efficient absorption, which increases with nanotube length.¹¹ The nanotube morphology is also associated with a decrease in bulk recombination. Because of the thin walls and small diameters of TiNT, mobile charge carriers are generated in close proximity to the surface states where holes can be captured within surface bonds.⁴³ However, charge recombination also increases with length.³ Thus, there is a trade off between increased absorption and recombination with respect to length and some optimization is required. Recombination is also sensitive to the amount of band bending, as the e⁻ must overcome this potential to reach the surface where recombination occurs.⁴³ The greater the band bending of the semiconductor at the interface, the lower the rate of recombination. Mor et al. point out that as the size of the grain or nanoparticle decreases, its ability to sustain band bending decreases as well.³⁴ Thus, arrays of TiNT with thicker walls and higher crystalinity display a decreased recombination rate.

Indeed, it would seem that TiNT properties make them ideal materials for the photoelectrolysis of water.³ However, they face one major obstacle to their potential use for the generation solar hydrogen fuel: their band gap. TiNT prepared by anodic oxidation and other methods have band gap energies that are very similar to bulk anatase (385 nm) and rutile (410 nm), both of which are in the near UV region.^19,44 This region represents only a small fraction, less than 5%, of the solar radiation spectrum. For this reason, improving the visible light response of TiNT is a high priority. There has been some preliminary success, discussed further below, in the synthesis of TiNT composites with red-shifted band gap energy, but this is likely to be a major focus of continuing research.

Photocatalyzed redox reactions

Titania materials have a long history of use in the catalysis of gas and aqueous phase redox reactions such as the oxidation of organic dyes and VOC’s and the reduction of carbon dioxide to fuels. Fig. 4 shows the basic process of photocatalysis. Incident light excites electrons from the valence band to the conduction band, creating an electron/hole pair (e⁻/h⁺) which then travels through the semiconductor, eventually either recombining or reacting with adsorbed species at the semiconductor interface.⁴⁵

Fig. 4 Photocatalysis. Incident light excites an electron from the valence band to the conduction band, leaving behind an electron hole. These charge carriers either recombine or are transported to adsorbed species for reaction.

Photooxidation

Photooxidation of organic molecules including acetone,⁴⁶ chlorophenol,⁴⁷ and polypropylene⁴⁸ using TiNT catalyst has been reported. Our group's research on photooxidation has focused on acetaldehyde as a model organic indoor air pollutant. It is commonly found in building materials and residential waste and is known to cause “sick building syndrome”.¹ The oxidation of acetaldehyde on titania catalyst is believed to be mediated by activated oxygen species, principally hydroxyl radical, formed by the trapping of photogenerated charge vacancies or holes at surface hydroxyl sites or bound water molecules. A carbonyl radical mediated chain reaction eventually oxidizes acetaldehyde to carbon dioxide and water. The reaction proceeds either directly, as shown in eqn (1), or indirectly with acetic acid as an intermediate, as shown in eqn (2) and (3).¹

2CH₃CHO + 2˙OH + O₂ → 2CH₃˙ + 2CO₂ + 2 H₂O (1)

CH₃CHO + ˙OH + O₂ → CH₃COOH + HOO˙ (2)

CH₃COOH + h⁺ → CO₂ + CH₃˙ + H⁺ (3)

The rates of photooxidation of acetaldehyde by TiNT were influenced by the calcination conditions used in synthesis and hence, in part, by the degree of cystallinity. Schulte, Gray and others explored the catalytic activity of TiNT produced by anodic oxidation and calcined over a range of temperatures (380–680 °C).³⁶ In reactions carried out under 365 nm UV light, a maximum first order rate constant of 0.0458 min⁻¹ was observed for the array calcined at 550 °C, a condition which optimized the fraction of rutile in the base layer of the array. This maximum was attributed to the enhanced activity of the mixed phase base layer and its contribution to the overall decay, which is consistent with previous studies on the effects of mixed phase titania films.⁴² Similar results were observed by Vijayan et al. using hydrothermally synthesized TiNT calcined at 600 °C.¹ Under 365 nm UV light and in comparison to a commercially available titania nano-catalyst (Degussa P25), the TiNT improved the acetaldehyde decay rate by more than a factor of 3. At this elevated calcination temperature, however, the nanotubular structure collapsed to form a nanorod morphology and traces of rutile were detected in the material. In general, increasing calcination temperature presents a tradeoff between increasing crystalinity and decreasing surface area due to loss of the nanotubular morphology, which explains the observed maximum. Electron paramagnetic resonance spectroscopy (EPR) performed on the materials indicated that TiNT calcined at higher temperature exhibited better charge separation behavior due to their greater crystallinity. Calcination temperature provided a simple way to tune the morphological and surface features of the titania nanostructures for particular photocatalytic reactions.¹

Photoreduction

The photoreduction of carbon dioxide has attracted interest as another means of storing the sun's energy in the chemical bonds of fuel molecules. The discovery of methane and methanol production using titania photocatalysts dates back over thirty years⁴⁹ but the catalytic efficiency remains very low and comprehensive understanding of the fundamental mechanism of reaction is elusive. Dey,⁵⁰ and Roy and collaborators⁵¹ have published good reviews of the reduction of CO₂ using titania and other photocatalysts. The overall reaction for the photocatalytic reduction of carbon dioxide to methane, which is an 8 electron process, is shown below in eqn (4).

CO₂ + 2H₂O → CH₄ + 2O₂ (4)

TiNT catalysts with their high surface areas and favourable electronic properties have been employed in an effort to improve efficiencies. Using the same arrays of TiNT discussed above, Schulte and colleagues measured the production of methane under 365 nm UV light.³⁶ Activity was highest for arrays calcined at 480 °C with a methane production rate of ∼3.5 μmol m⁻² h⁻¹. The higher activity of this array was attributed to higher UV absorbance of the material, which contained the largest fraction of anatase (90%). As was the case for oxidation, reduction experiments performed under visible light showed much lower activity, with methane production nearly an order of magnitude lower than under UV light. Vijayan et al. also explored photoreduction using their hydrothermally synthesized TiNT calcined at various temperatures.¹ Methane production was found to increase for samples calcined from 200–400 °C and decrease thereafter, with a maximum production rate of 1.484 μmol g^¬1 after 3 h for materials calcined at 400 °C. This behavior was explained in terms of the interacting effects of surface area, crystallinity, and morphology. The surface area of the TiNT remained relatively constant up to a calcination temperature of 400 °C after which it greatly decreased as the nanotubular structure collapsed to form nanorods. In this same range, the crystallite size as calculated from XRD data using the Scherrer equation increased with increasing calcination temperature. Finally, as reported by Xu and colleagues, photoreduction is more active on the inside surface of TiNT than on the outside due to increased acidic properties caused by surface charge polarity.⁵² TiNT calcined above 400 °C underwent transition to nanorods, thereby eliminating the inner surface and drastically reducing surface area. These phenomena explain the reduced activity for materials calcined at temperatures greater than 400 °C.

Visible light response

A fundamental challenge associated with the use of TiNT, or more generally most titania-based photocatalysts, is to shift the photoresponse of the material to visible light in order to better match the solar spectrum. Among the many strategies to red-shift light absorption, the synthesis of TiNT composites has been explored as a strategy to narrow the band gap of titania but retain the necessary band edge energy for targeted oxidation or reduction reactions. Asapu et al. found that TiNT doped with dimethyl phosphite have a band gap of 2.95 eV as compared with 3.14 eV in undoped materials. The rate constant for the degradation of rhodamine B dye under fluorescent light in TiNT doped with 0.75 wt% P was approximately double that of undoped TiNT.²¹ Vargheses and colleagues reported that nitrogen doping of anodically oxidized TiNT decorated with Cu and Pt nanoparticles shifts the band edge to nearly 540 nm, greatly increasing the visible light absorbance. In photoreduction experiments performed using actual sunlight, they observed a high rate of hydrocarbon generation and claimed significant improvements in CO₂ reduction and increased efficiencies. This work attracted a lot of attention, but it also raised concerns that the higher molecular weight hydrocarbons including hexane, various olefins and branched paraffins did not evolve from the reduction of CO₂; rather the use of opaque adhesive tape in the reactor is thought to be the likely source of C2 and higher hydrocarbons.⁵³ Experimental artifacts such as the presence of contaminants often explain exciting and surprising results. Careful assessment of unexpected findings is imperative. For example, Danon et al. al recently reported that the high temperature treatment of hydrothermally synthesized TiNT with hydrogen produced materials with enhanced visible light activity for acetaldehdye oxidation.⁵⁴ However, it was discovered that the black color was the result of the incorporation of chromium from the stainless steel reactor vessel, not from the hydrogen treatment as was originally believed.⁵⁵ With this knowledge, then, research can be directed to determine the mechanism of the chromium effect on surface structure and how to optimize the substitution of chromium into the TiNT structure in synthesis in order to maximize visible light absorbance and reactivity.

Pt doping of hydrothermally synthesized TiNT was shown to increase the rate constant of acetaldehyde oxidation under visible light by a factor of 7 relative to undoped TiNT and Degussa P25.²⁷ TiNT with 0.5 mol% Pt was found to be most reactive and had a band gap energy of 3.05 eV compared to 3.16 eV in the undoped TiNT. Although higher concentrations of Pt further narrowed the band gap to 2.64 eV at 4 mol% Pt, these materials were less active due to the fact that the nanotubular morphology collapsed at higher Pt-doping (>1 mol% Pt) and as a result, the surface area and total pore volume also decreased.

Recent work also by Vijayan and colleagues explored the properties of composites of TiNT and single walled carbon nanotubes (SWCNT).⁵⁶ Composites with TiNT : SWCNT ratios ranging from 20 : 1 to 100 : 1 were studied for their activity in acetaldehyde oxidation under visible and UV light. Absorption at wavelengths beyond 400 nm was observed for all composites with the absorption increasing in proportion to the mass of SWCNT. Photoluminescence (PL) was significantly quenched in the composites in comparison to TiNT alone. The rate constants reported for the composites were higher than for TiNT under all illumination conditions and higher than Degussa P25 under most. Increased activity in the visible light region is attributed to the formation of Ti–C and Ti–O–C defect sites which narrow the band gap and act as trapping sites for photogenerated charges.

These encouraging early results point the way to improve the catalytic activity of TiNT materials under visible light. While the composites discussed above exhibited enhanced catalytic properties over their bare TiNT counterparts, efficiencies, on the order of less than 1%, are still very low, especially for the photoreduction of CO₂, and must be increased in order to be commercially competitive.

Current and future implications

The first decade of research on TiNT has been heavily focused on material synthesis and characterization. As a result, three classes of synthestic techniques have emerged. Variation in synthesis conditions and the development of new templates have led to increased control over the critical features of aspect ratio and morphology. Further refinement of post-synthesis treatment has resulted in the ability to manipulate crystalline phase and photoresponse. Additionally, the incorporation of numerous dopants into the TiNT framework produces a suite of diverse TiNT materials adapted to a number of existing technologies significantly improving material performance relative to more conventional forms of nano-titania.

We believe that the next decade of research must go beyond synthesis and characterization to focus on fundamental understanding of the chemical, optical and electronic processes at play in these various applications. It is no longer sufficient to describe that one material serves as a better photocatalyst than another. We must understand the basis of the improvement. This is particularly well exemplified if we consider the promise of TiNT for solar fuel production. Since little is understood about the mechanism of CO₂ reduction, it is difficult to tailor TiNT composite structure to target increased efficiency or to control reaction product yields. Rational photocatalyst design requires knowledge of each of the individual steps in the reaction, from light harvesting, and CO₂ surface adsorption to charge transfer and the role of water. Specifically for TiNT, this means that the differing properties of the inner and outer surfaces must be better understood. The nature of adsorption and recombination centers must be explored. For example, a recent FT-IR study of adsorbed carbon dioxide species on platinized TiNT indicates that the prevalence of acidic and basic surface sites, and therefore the form of adsorbed species, is affected by pre-treatment conditions.⁵⁷ Platinized TiNT treated under H₂ atmosphere appear to activate hydrogen on the surface, allowing the formation of adsorbed bicarbonates.⁵⁷ Only by expanding the knowledge of fundamental processes involved in photocatalysis can the efficiency of TiNT and other photocatalysts be improved.

Acknowledgements

The authors would like to acknowledge the funding provided by the U.S. Department of Energy, under Contract DE-FG02-03ER 15457/A003 (ICEP).

Notes and references

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