Introduction To Nanoscience And Nanotechnology

Published Date: 02 Nov 2017

by Ruslan Garifullin, 20903644

Term Paper

PHYS 520 Introduction to Nanoscience and Nanotechnology

1. Introduction

Titania (TiO2) is certainly one of the most studied transition metal oxides in materials science (over 40000 publications in last decade). This material has attracted enormous interest due to its interesting properties and found applications in dye-sensitized solar cells, photoelectrolytic cells, photocatalysts and biomedicine. Bulk titania is known for its non-toxicity, eco-friendliness, corrosion resistance. The most eminent functional features are exceptional biocompatibility (biomedical applications), unique ionic and electronic properties of TiO2. Titania in all of its crystal forms is a wide band gap semiconductor (ca. 3 eV) with appropriate band-edges that make it utile in solar cells, photoelectrolytic cells and photocatalytic reactions. Photogenerated excitons (electron–hole pairs) can be exploited for splitting water into oxygen (O2) and hydrogen (H2), or can be used for the degradation of hazardous wastes such as organic pollutants in ground waters or toxic air pollutants.1-3 The first milestones laid in titania research area were indisputably the reports by Fujishima and Honda in early 70s on photoelectrolytic water splitting on a TiO2 anode, 4 and the works by Gerischer and Tributsch,5 Dare-Edwards et al.,6 and Gratzel and O’Regan7 that introduced the use of titanium dioxide for solar energy conversion in the 80s and 90s.8 Over the past two decades, the spectrum of potential applications has become only wider towards devices with increasingly sophisticated photovoltaic, electrochromic, self-cleaning or antifogging properties, biomedical coatings, sensors, and smart-surface coatings.9-12

2. TiO2 nanostructures

Titania along with its various crystalline forms (anatase, rutile and brookite) also presents interest based on architecture of its nanostructures, namely nanowires, nanotubes and quantum dots.

2.1 TiO2 Nanotubes

From the moment of discovery of carbon nanotubes,13 the combination of intriguing molecular geometry and exclusive properties inspired the field of nanotechnology and triggered huge efforts in chemistry, physics and materials science. One dimensional (1-D) nanostructures possess exciting electronic properties, such as high electron mobility and quantum confinement effects, high specific surface area, and also demonstrate high mechanical strength.14-16 Despite that carbon still holds the name of the most explored nanotubular material, a wide spectrum of other materials (e.g. transition metal oxides and sulfides) have been synthesized in a 1-D or quasi-1-D geometry (nanowires, nanofibers, nanorods and nanotubes) and have also exhibited novel properties and characteristics.

Fabrication of 1-D titania nanostructures is achieved by various routes comprising sol–gel methods, template-directed methods, solvothermal approaches and by electrochemical anodization.11,17-20 One of the first endeavors to obtain titania nanotubes is most likely the study reported by Hoyer,21 who utilized an electrochemical deposition technique into an ordered Al2O3 template. Later methods primarily included other template-directed methods, sol–gel techniques, and solvothermal methods with or without templates19,22-27 and atomic layer deposition (ALD) into the template.28,29 Many of the processes are acid-catalysed hydrolysis reactions of titanium alkoxide precursors (e.g. titanium isopropoxide) followed by the condensation reactions. For instance, TiO2 sol was filled into the pores of the Al2O3 template, then a thermal processing was applied, and finally the Al2O3 template was selectively washed out by dissolution.25 TiO2 nanotubes are also prepared by using fibrous templates of suitable amphiphilic surfactants above their critical micelle concentration; these amphiphilic surfactant molecules assemble in a solvent to give a worm-like micelles, which are consequently utilized as templates for titania preparation. In described approach, nanotube formation is carried out on the surface of fibrous templates (micelles or reverse micelles) with a cylindrical exterior surface. The titanium precursors can successfully react on the surface of a template, and after removal of the template (calcination), a nanotube structure is achieved.30 Usually TiCl4 or Ti[O-iPr]4 solutions are used as the titanium precursor. Moreover, by varying water to micelle ratios31,32 crystallite size can be controlled.

Another well-established solution-based method to synthesize 1-D titania nanostructures is the solvothermal method, which was first reported by Kasuga et al.19,23 In this method, bulk titania powder is treated with sodium hydroxide solution in a teflon-coated autoclave at 100–150°C for several hours, followed by hydrochloric acid treatment.19,23,33 The general formation of the tubular architecture is based on exfoliation of titania crystal planes in the alkali medium and stabilization in the form of Ti-O-Na+ ionic species. This step is succeeded by a nanolayer sheets rolling into tubes during cooling or HCl treatment processes. The cause for the rolling-up is still under debate, because several experimental factors are vital to achieve a material with tube geometry. These tubular structures are made of multiple shells, with an inner diameter of ca. 5 nm, a shell spacing of less than 1 nm, and an average tube diameter of ca. 10 nm. Amount, length and size distribution of fabricated nanotubes are dependent on the specific reaction conditions, which introduces certain control over mentioned parameters.33

Atomic layer deposition technique makes it possible to conformably coat surfaces of templates (e.g. porous Al2O3) with consequent atomic layers, one after the other, by using alternating cycles of exposure to a titania precursor followed by purge process and hydrolysis.29,34,35 Al2O3-template approaches allow nanotube fabrication with vertical alignment to the substrate, but at the same time in certain cases they have critical template removal step.

All these titanium precursor solution- or template-based processes result in individual tubes or loose agglomerates of the same or bundles that are dispersed in a solution, and frequently a wide distribution of nanotube lengths is obtained. Implementation of the nanotubular structures in electrically contacted devices is usually achieved by compact placement of the nanotubes into layers on an electrode surface. However, this process results in a random orientation of the nanotubes on the surface of an electrode, which seriously impedes many advantages of the 1-D directionality.

Figure 1. The electrochemical anodization and anodic morphologies: a) I) metal electropolishing, II) compact anodic oxide formation, III) self-ordered oxides, IV) fast oxide nanotube formation, V) ordered nanoporous layer formation. Examples of morphologies of obtained structures: b) highly ordered nanoporous Al2O3,36 c) highly ordered TiO2 nanotubes d) disordered TiO2 nanotubes grown in bundles.

In contrast to aforementioned techniques, the electrochemical anodization method leads to TiO2 nanotube arrays aligned perpendicular to the substrate surface (Figure 1) with well-defined and controllable well-defined nanotube length. The nanotubes are attached to the metal surface and, therefore are already electrically connected and easy to work with. The use of an electrochemical anodization method allows almost any shape of titanium surfaces to be coated with a dense and defined nanotube layer and is, thus a very easy to scale up structuring process. Fabrication of titania nanotube in the form of the arrays by electrochemical anodization can be summarized in four synthesis generations. The first generation employs aqueous HF-based electrlolytes such as HF, HNO3/HF, H2SO4/HF, H2Cr2O7/HF, CH3COOH/NH4F, H2SO4/NH4F, H3PO4/HF and H3PO4/NH4F (Figure 2).37 The second generation uses buffered electrolytes which include NaF or KF in buffered environment (Figure 3).38 The third generation utilizes polar organic electrolytes such as formamide, dimethyl formamide, dimethylsulfoxide ethylene glycol, diethylene glycol, glycerol and methanol/H2O solutions of HF or its salts (Figure 4).39 The fourth generation comprises fluoride-free electrolytes such HCl, aqueous H2O2 and aqueous HCl/H2O2 based electrolytes (Figure 5).40

Figure 2. FESEM cross-sectional views of tapered nanotubes obtained: (a) by ramping the anodization voltage from 10 to 23 V over a 30 min period, 0.43 V/min, then holding the voltage at 23 V for 10 min, (b) by initially anodizing the sample at 10 V for 20 min then increasing the voltage at 1.0 V/min to 23 V then held constant at 23 V for 2min, (c) straight nanotubes obtained by applying a constant 23 V for 45min. Here, d and D denote diameters of the conical apex and conical base,respectively.37

Figure 3. Lateral view of the tubes synthesized in solutions at different pHs (pH>1).38

Figure 4. FESEM images of titania nanotubes grown in formamide-based electrolyte at 35 V for 48 h; (a) cross-section at lower magnification, (b) cross-section at high magnification, and (c) top surface image.39

Figure 5. FESEM images of titania nanotubes anodized at 10 mA/cm2 for 30 min in: (a) methanol and water mixture (50 vol% methanol, 1 mass% HF) (b) methanol and water mixture (90 vol% methanol, 1 mass% HF).40

2.1.1 TiO2 nanotubes applications

Highly-ordered and vertically-aligned titania nanotube arrays synthesized by anodization of Ti presents a material architecture that offers a large internal surface area without a parallel decrease in geometric and structural order. The precisely oriented nanotube arrays offer excellent electron percolation pathways for directional charge transfer between interfaces. Titania nanotube arrays have outstanding charge transfer and carrier life-time properties, which enable a variety of advanced applications such as sensors,41-46 dye sensitized solar cells,47-49 H2 generation by water photoelectrolysis,50,51 photocatalytic reduction of CO2 under outdoor sunlight,52 and supercapacitors.53 Moreover, the nanotube arrays have demonstrated great applicability in biomedical applications such as biosensors, molecular filtration systems, drug delivery systems, and tissue engineering.54-56

Figure 6. Time-variant resistance change before and after self-cleaning from a layer of motor oil with UV light.43

The application of titania nanotube arrays in H2 sensing demonstrates remarkable, and readily measurable properties of such materials. At room temperature, in response to 1000 ppm H2 1-mm long TiO2 nanotube arrays show an unprecedented change in electrical resistance of about 9 orders of magnitude;39,45 this is the greates known sensitivity of any material, to any gas, at any temperature. In their use as H2 sensors, the titania nanotube arrays have excellent photocatalytic properties, because they can clean themselves from extreme contamination with exposure to ambient UV light, and thus effectively recovering their gas sensing ability (Figure 6).42,43 The self-organized nature of the material allows for the precise design and control of the geometrical characteristics. This allows one to obtain a material with specific light absorption and light propagation characteristics.49,57,58 The geometry of the nanotube arrays appears to be close to ideal architecture for water photoelectrolysis, where under ultraviolet spectrum illumination (320–400 nm) 30-mm long nanotube arrays give a photoconversion efficiency of 16.25%. 39,47

Figure 7. Room temperature resistance variation of TiO2 nanotube array sample crystallized by annealing at 480°C.39,45

TiO2 nanotube array films can be easily converted into sensors by sputtering of Pt electrodes onto their top surface, to which electrical contact can be attached by wire-bonding. The typical response of a 1-mm long TiO2 nanotube array sample with 30 nm pore size is shown in Figure 7; ambient atmosphere is switched between air and 1,000 ppm hydrogen in nitrogen at room temperature.44 Upon exposure to H2, a reduction in resistance of about 9 orders of magnitude is observed, without hysteresis and a minimal baseline low-level drift over 12 h. A variation in electrical resistance of 7.2 orders of magnitude is observed as response to 20 ppm hydrogen. With drift compensation,46 the low-level linear sensitivity indicates the potential to detect H2 at 200 parts per trillion.45 Longer TiO2 nanotube arrays exhibit lower sensitivity with substantially longer response to recovery times.59 The successful application of H2 sensors have been demonstrated in clinical trials as an indicator of neonatal necrotizing enterocolitis, measuring transcutaneous H2 concentrations of a few ppm from an unheated skin temperature sensor.45 Under the patronage of a hydrogen economy, the described sensors are perfect devices in monitoring H2 concentrations, including the presence of dangerous H2 leaks.

Figure 8. Operation principles and energy levels of nanocrystalline DSSC.

Grown by anodizing and crystallized titania nanotube arrays are used as the electron transport material in dye sensitized solar cells (DSSC), providing large surface areas with directional charge transport along the axis of the nanotubes. DSSC operating principle is depicted in Figure 8. The cell consists of mesoporous metal oxide layer of anatase titania (Figure 9), the surface of which is stained with a monolayer of photosensitizer, usually a Ru complex dye tethered to the TiO2 surface by a carboxylate functionalized bipyridyl ligand. The absorption of these types of complexes in visible spectrum is based on a metal to ligand charge transfer. The carboxylate groups of a dye are directly coordinated to the surface of TiO2 nanotube arrays. Photoexcitation of dye molecules leads to injection of electrons into the TiO2 CB, and redox species such as I-/I3- couple in the electrolyte reduce the oxidized dye molecules back to their original state. Dye regeneration by the I- happens via the SCN- group and, thereby, intercepts the recapture of the CB electron by the oxidized dye. Consequently, I- is restored by the reduction of I3- at the counter-electrode, and finally when electron travels through external load the circuit achieves its completion.60 In 2001, nanocrystalline DSSCs by use of "black dye" achieved 10.4% (1.5 AM) solar to power conversion efficiency in full sunlight.61 Recently, this record has been extended to over 11% by using the N3 dye in conjunction with guanidinium thiocyanate, a self-assembly facilitating additive which makes possible to increase the open circuit voltage.62

Figure 9. An illustrative drawing of the TiO2 nanotube dye-sensitized solar cell.63

Figure 10. The schematic representations of two water photoelectrolysis approaches.

TiO2 nanotube arrays have found an application as photoanodes for H2O photoelectrolysis. The TiO2 nanotube array architecture has excellent charge transport properties39,64,65 and also has high surface to volume ratios, which allows a large internal surface area to be in close contact with the electrolyte. Photoelectrolysis is the general term that describes semiconductor-based water splitting by the use of a photoelectrochemical cell (Figure 10).64 One approach is to couple a photovoltaic system and an electrolyzer as a single system. Photovoltaic cells can be combined in series to provide the potential required to split water and then connected to hydrogen and oxygen generating electrodes. In semiconductor–liquid junctions, the water splitting potential is generated directly at the semiconductor–liquid interface. The ability of a semiconductor photoelectrode to drive either the oxidation of water to oxygen, or the reduction of water to hydrogen, or the entire water splitting reaction is determined by its bandgap and the position of the valence and conduction band edges relative to the water redox reactions.64 Besides band edge positions, the semiconductor has to be active over a broad spectral range, and, upon light absorption, the material should efficiently separate the generated charges. Moreover, the immersed semiconductor has to be stable in the electrolyte.

As an example of the remarkable photocatalytic properties of the nanotube arrays, Varghese and co-workers52 recently reported efficient solar conversion of carbon dioxide and water vapor to methane and other hydrocarbons using titania nanotube arrays, with a wall thickness low enough to facilitate effective carrier transfer to the adsorbing species, surface-loaded with nano-dimensional islands of co-catalysts platinum and/or copper (subsequently oxidized to copper oxide). All experiments were conducted in outdoor sunlight. Intermediate reaction products, hydrogen and carbon monoxide, are also detected with their relative concentrations underlying hydrocarbon production rates and dependent upon the nature of the co-catalysts on the nanotube array surface. Using outdoor global AM 1.5 sunlight, 100 mW/cm2, a hydrocarbon production rate of 111 ppm cm-2h-1, or ca. 160 µl/g h, is obtained when the surface of titania nanotube array samples are decorated with both Pt and Cu nanoparticles. This rate of carbon dioxide to hydrocarbon conversion obtained under outdoor sunlight is at least 20 times higher than previously published reports, which were conducted under laboratory conditions using UV light illumination52. Taking into account both H2 and hydrocarbons, the production rate was 273 ppm cm-2h-1. Figure 11 schematically demonstrates an interesting prospect offered by such exciting photocatalytic properties, a high-rate flow via photocatalytic membrane for carbon dioxide reduction.

Figure 11. Scheme of Pt and Cu catalysts loaded flow-through nanotube array membrane for high-rate photocatalytic conversion of carbon dioxide and water vapor into hydrocarbon fuels.52

2.2 TiO2 nanowires and their applications

TiO2 nanowires are also can be prepared by using micellar templates of appropriate amphiphilic surfactants above their critical micelle concentration.17 Alumina template approach can be utilized in nanowire structures as well.29,34,35 The synthesis of anatase and rutile-phase titania nanowires has been achieved via an efficient molten salt-assisted and novel pyrolysis route, respectively (Figure 12).66

Figure 12. TEM image of the as-prepared anatase titania nanowires. (a) General morphology of anatase titania nanowires at 820°C. (b) Typical morphology of the as-prepared anatase titania nanowires at 820°C.66

Similar to titania nanotubes, titania nowires are used in dye sensitized solar cells67, photoelectrolytic cells, photocatalysis etc., nevertheless performance of titania nanowires is inferior to nanotubes.

2.3 TiO2 quantum dots and their potential applications

Recently Hubalek et al. repoted low-cost and rapid template-based synthesis of TiO2 quantum dots (QDs)(Figure 13).68 Hubalek et al. suggests that the nanostructured surfaces with QDs are very promising in the application as a sensor array – fluorescence array detector. In particular, this novel sensing approach can be used for the detection of various biological molecules (DNA, proteins, etc) in vitro (in clinical diagnostics) as well as for in vivo imaging.

Figure 13. SEMimages. SEM characterization of the (a) alumina template, (b) TiO2 QDs on Ti grains.