Synthesis of Nanomaterials Using Self-Assembled

Copyright © 2012 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 12, 1–19, 2012

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates Latika Menon1! ∗ , Christiaan Richter2 , Adam Friedman3 , Zhen Wu4 , and Eugen Panaitescu1 1

2

Department of Physics, Northeastern University, Boston, MA 02115, USA Department of Chemical Engineering, Rochester Institute of Technology, Rochester, NY 14623, USA 3 Code 6361, US Naval Research Laboratory, Washington, DC, 20375, USA 4 Software Engineering Institute of East China, Normal University, Shanghai, 200062, P. R. China

CONTENTS 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Anodization of Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Nanoporous Alumina Templates . . . . . . . . . . . . . . . . . . . . 2.2. Theoretical Understanding of Pore Formation . . . . . . . . . . 2.3. Branched Pores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Synthesis of Periodic Porous Alumina Templates . . . . . . . 3. Anodization of Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Nanoporous/Nanotubular Titania Templates . . . . . . . . . . . . 3.2. Titania Nanotubes Fabricated in Chlorine Media . . . . . . . . 4. Synthesis of Nanomaterials Inside Nanoporous Templates . . . . 4.1. AC Electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. DC Electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 3 5 5 6 7 10 13 13 15

noteworthy milestones are: (1) the work of Keller et al.4 in 1953 describing porous alumina as a duplex structure consisting of porous and barrier layers. Keller also studied the relationship between pore structure (pore diameter and ordering) and applied voltage. (2) The first review paper dealing with anodic oxide films on aluminum by Diggle and Downie in 1968.5 (3) The studies of the Manchester group led by Thompson and Wood in the decades from 1970 to 1990.6 The 1970 article by O’Sullivan and Wood “The Morphology and Mechanism of Formation of Porous Anodic Films on Aluminium”7 is one of the most cited articles on anodization of aluminum to obtain porous alumina structures.

1. INTRODUCTION The electrochemical oxidation of metals dates back to the early 1800’s. The early scientific interest was supplemented by commercial interest in the use of anodic oxide protective and decorative coatings on aluminum and aluminum alloys. The technique was widely used as a means to coat tableware, kettles, car bodies and other commodities.1! 2 However, it was only after the availability of microscale characterization techniques, such as scanning electron microscopy, that it was also discovered that well-ordered nanoscale structures can be obtained by electrochemical anodization.3

2. ANODIZATION OF ALUMINUM The majority of publications on electrochemical anodization phenomena deal either with the anodization of aluminum or of silicon. In the case of aluminum ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2012, Vol. 12, No. xx

2.1. Nanoporous Alumina Templates The basic process of anodization to prepare nanoporous alumina is as follows: anodization is carried out under dc conditions with Al foil as anode and Pt mesh as cathode. The electrolytes used are strong acids with low pH values. Acids typically used are 15% sulfuric acid, 3% oxalic acid and 5% phosphoric acid. During anodization, it has been observed that the variation of the anodization current as a function of time follows the behavior shown in Figure 1. During the first few seconds, the net current, J , decreases rapidly with time until a minimum is reached. This is followed by an increase in the current density which finally reaches a constant value. The net current may be thought of as arising due to two effects: current, Jb , due to growth of barrier layer which decreases rapidly with time and current, Jp , due to pore growth which increases with time and finally reaches a steady state value. The stages of porous structure growth as a function of time are as follows. Immediately after switching on the voltage, a

1533-4880/2012/12/001/019

doi:10.1166/jnn.2012.6637

1

REVIEW

Keywords:

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Menon et al.

Latika Menon

REVIEW

Christiaan Richter

Adam Friedman

Zhen Wu

Eugen Panaitescu

2

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Menon et al.

2.2. Theoretical Understanding of Pore Formation

0.025 0.023

Current (A)

0.021 0.019 0.017 0.015 0.013 0.011 0.009 0.007 0

50

100

150

200

Anodization Time (sec)

Fig. 1. Typical anodization current versus time for a sample anodized at 40 V under dc conditions.

Written in terms of their chemical reactions these two processes are most commonly formulated as: (Formation of aluminum oxide) 2Al"s# + 3H2 O → Al2 O3 "s# + 6H+ "aq# + 6e−

Reaction "1#

(Dissolution of aluminum oxide) Al2 O3 "s# + 6H+ "aq# → 2Al3+ "aq# + 3H2 O Reaction "2#

Fig. 2. (a) (top) Cross-section scanning electron microscopy image showing an array of vertically arranged pores in nanoporous aluminum oxide, (b) (bottom) top view of the same sample anodized at 40 V in 3% oxalic acid.

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Recognizing that these processes involve the counter diffusion of oxide carrying and Al ions7! 16 some authors5! 17 suggested that field-assisted dissolution may also occur by the following pathway (illustrated in Fig. 3): (Net diffusion of Al3+ across barrier layer) Al(s) → Al3+ "aq# + 3e−

Reaction "3a# 3

REVIEW

barrier layer starts to form explaining the sudden decrease in current in the initial stages of anodization. Subsequently, fine featured pores begin to develop which is enhanced with increased anodization time. This corresponds with the increase in anodization current, J , seen in Figure 1. Finally, a steady state is reached when the pore structure is stabilized and correspondingly the anodization current reaches a more or less constant value. The pores are parallel to each other and grow perpendicular to the surface. In addition to the porous layer, a thin U-shaped barrier layer of aluminum oxide forms at the interface of aluminum. Figure 2(a) shows a top-view scanning electron microscopy image of a porous alumina template prepared by anodization in 3% oxalic acid at 40 V. The side view is shown in Figure 2(b).

Several theoretical models have been proposed in order to explain the self-ordered growth of porous alumina (see Refs. [6–15] and references therein). Thompson and Wood6 attributed the inherent instability of ‘field focusing’ as the mechanism for pore creation in the barrier oxide. In 1970, O’Sullivan et al.7 concluded that the pore initiation in anodic films occurs by the merging of locally thick oxide regions (which is related to the substrate structure) and the subsequent concentration of current into the thinner regions. The pores grow perpendicular to the surface such that their diameter is proportional to the applied voltage. The steady state barrier-layer thickness, pore diameter and inter-pore separation are directly proportional to the applied voltage. The barrier-layer thickness and behavior at the pore bases are determined principally by an equilibrium between oxide growth in the barrier layer (the metal/oxide interface) and field-assisted dissolution at the pore bases (oxide/electrolyte interface). The oxide growth is due to the migration of oxygen containing ions O2− /OH− from the electrolyte through the oxide layer at the pore bottom. The oxide dissolution is due to the migration of Al3+ ions which drift through the oxide layer and are ejected into the electrolyte. The general consensus is that pores form as a result of competing oxidation and dissolution processes. This understanding, as first formulated in the widely referenced work of Thompson, Wood and O’Sullivan et al.,6! 7 suggests that oxidation and dissolution proceed by the following mechanisms: (1) the growth of aluminum oxide, either at the inter-face between aluminum and alumina or within the barrier layer due to the counter migration of Al3+ ions and OH− and O2− ions and (2) the dissolution of aluminum oxide at the interface between the alumina film and solution.

REVIEW

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Fig. 3. A schematic representation of the field induced counter diffusion of aluminum and oxygen atoms (or ions). Also indicated is the dissolution of aluminum ions into the electrolyte according to the mechanism of reaction 3(a) (i.e., there is no net oxygen dissolution). Oxidation of alumina occurs at the oxide layer-aluminum interface.

with the understanding that the electrons are transferred directly to species in the electrolyte,18 for instance by proton reduction:19 (Net diffusion of Al3+ across barrier) 3H+ "aq# + Al"s# → Al3+ "aq# + 23 H2 Reaction "3b# In order to investigate the oxide dissolution mechanism we sputtered an aluminum thin film onto a silicon

Menon et al.

substrate.20 Subsequently these silicon wavers with aluminum film were anodized for various short time intervals as can be seen in Figure 4. At the same time the anodization currents were recorded. From a detailed SEM analysis of the samples anodized for the various time intervals we could determine the exact amount of Al dissolved and the exact amount oxidized (from SEM images). An excellent match between the recorded external current data (number of electrons transferred around the external circuit) was observed and the amount of Al dissolved and the amount oxidized if we make the following assumption. Once an oxygen atom enters the oxide (i.e., oxidizes the aluminum) it does not dissolve. That is, the experiment indicated that only reaction 3 as shown in Figure 3 occurred while reaction 2, if present was of negligible magnitude.20 Several models have been proposed to understand poreformation. Jessensky et al.,12 explained the observed periodicity in the pore structure under special anodization conditions. According to them, a possible origin of forces between neighboring pores is the mechanical stress associated with the volume expansion that occurs during the conversion of aluminum to aluminum oxide. The material can only expand in the vertical direction, since oxidation takes place at the entire pore bottom simultaneously. All of the oxidized aluminum does not contribute to oxide formation since some of the Al3+ ions remain mobile in the oxide under the applied voltage. Therefore, the relative thickness of the porous alumina layer compared to the consumed aluminum was found to vary with voltage and electrolyte

Fig. 4. (a) Cross-section Scanning electron microscopy images of samples anodized for varying times ranging from 1–49 s. (b) Magnified image showing pore structure for a sample anodized for 35 s. (c) Scanning electron microscopy images of samples anodized for 49 s under 40 V showing the U-shape bottom that suggest that the relatively short time anodization is sufficient to form a fully developed pore structure. Reprinted with permission from [20], Z. Wu et al., J. Electrochem. Soc. 154, E8 (2007). © 2007, The Electrochemical Society.

4

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Menon et al.

J. Nanosci. Nanotechnol. 12, 1–19, 2012

porous templates in 1% phosphoric acid. For increasing soaking time, the pore diameter is found to increase.28! 29 For example, for a template prepared by anodization in 15% sulfuric acid at 10 V, the pore diameter is of the order of 9 nm. The pore diameter can be increased to a value of 19 nm by soaking in 1% phosphoric acid for 30 mins. By controlling the soaking time, the diameter of the pores can be controlled down to a few nanometers. This is particularly useful in the investigation of various properties of the embedded nanowire, as a function of tiny changes in diameter. 2.3. Branched Pores As mentioned earlier, thickness of the barrier layer is proportional to the applied voltage. This implies that if the anodization voltage is reduced during anodization, the thickness of the barrier layer also drops. This happens because of the nucleation of pores at the pore bottoms. From each original pore, several new pores nucleate and increase in diameter, leaving the new equilibrium pore structure characteristic of the lower anodizing voltage.7 This implies that by reducing the voltage during anodization, before the state of equilibrium is achieved (corresponding to the lower anodization voltage), a branched pore structure can be obtained in the alumina. This method of synthesizing branched alumina template has attracted attention due to its usefulness in the synthesis of Y -junction carbon nanotubes which have potential applications in nanoelectronics. It may be added that if the anodization voltage is increased during anodization, the opposite effect is observed, namely the pores merge and the barrier layer thickens. Figure 5 shows an SEM images of a typical bifurcated nanotemplates. 2.4. Synthesis of Periodic Porous Alumina Templates One obvious drawback of the porous alumina technique of nanofabrication is the lack of long range ordering. The extent of the ordering can be improved slightly by annealing the Al foil at ∼500 C prior to electropolishing and anodization.12 This helps to enhance the grain size in the metal and to obtain homogenous conditions for pore growth over large areas. Masuda et al.,30 developed a multi-step anodization technique to improve the periodicity of the porous structure. As usual the Al foil is first degreased and electropolished. This is followed by a short 5 min anodization in acid. This leads to the formation of a textured aluminum surface. The thin layer of porous aluminum oxide formed is dissolved in a mixed solution of 0.2 M chromic and 0.4 M phosphoric acid solution at 60 % C for about 5 minutes. The sample is then reanodized for about 12–18 hours to create long-range ordering. Long time anodization causes the cells to rearrange and reduces the number of defects and dislocations. The oxide film 5

REVIEW

composition. Jessensky et al.,12 investigated the influence of mechanical stress on the structural features by varying the anodization voltage and the acid. The volume expansion was measured using two different methods, namely using a mechanical profiler and by measuring the current efficiency for oxide formation. The current efficiency increases with increasing voltage. According to Jessensky et al.,12 optimal conditions for the formation of ordered nanopores occurs at a current efficiency corresponding to a moderate expansion (∼1.2–1.4) of aluminum during oxidation. In the case of sulfuric acid, the most ordered pores were obtained using 20 wt.% sulfuric acid at 1 % C at an anodizing voltage of around 18.7 V. In the case of oxalic acid, the most ordered pores were obtained for a 0.3 M solution at an anodizing voltage of about 40 V. No ordered domains were observed in the cases of contraction or very strong volume expansion. Other authors too, have arrived at similar conclusions regarding the conditions for longrange pore ordering.21–23 The dependence of pore diameter and pore density on the fabrication conditions has been studied quite extensively.4! 7! 24–26 The smallest pore diameters are obtained by anodization in 15% sulfuric acid at low voltages and the largest pore diameters are obtained for anodization in 5% phosphoric acid at higher voltages. As a function of anodization voltage, pore diameter and barrier layer thickness increases with increasing anodization voltage. In contrast, pore density and the inter-pore separation decrease with increasing anodization voltage. O’Sullivan et al.,7 showed that the pore density varies inversely as a function of V 2 while the pore diameter is directly proportional to V (pore cross-section is proportional to V 2 #. This implies that films grown at different voltages under otherwise similar conditions have the same porosity. More specific dependence of film parameter as a function of anodization conditions have been obtained by various groups. All of them are consistent with the general dependence mentioned above. AlMawlawi et al.,27 showed that the dependence of pore density $ (expressed in units of pores/Å2 ), as a function of anodizing voltage is given by $ = %/(d + &V )2 where % is a constant ∼1.15, d is the pore diameter, & is a constant also dependent on the acid and temperature. An exponential dependence of pore density as a function of anodization voltage, has been obtained by Palibroda et al.13 They showed that the pore density, $ and the diameter, d of the pores can be expressed as $' = 1'6 · 1012 exp"−"4'76 V /V ∗ ## cm−2 and d = 3'64 + 18'9 V /V ∗ nm where V ∗ , the critical voltage determined empirically by V ∗ = 4'2 + 20'5c(H+ # where c(H+ # is the proton concentration of the acidic electrolyte. Pore densities are typically of the order of 1010 – 1011 pores/cm2 . Barrier layer thickness is in the range of tens of nanometers. Pore diameters are in the range of 8–200 nm. Further control of the pore diameters (over nanometer length scales) may be obtained by soaking the

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Menon et al.

REVIEW

Fig. 5. SEM image of bifurcated alumina template. Anodization conditions yield 40 nm branch diameters and 20 nm stem diameters. Note the region of bifurcation where the larger pores form Y -shaped junctions.

formed is then removed by soaking in the same mixed solution for about 3–4 hours. The film is reanodized for an appropriate length of time, depending on the required pore length. Using this procedure, it is possible to obtain a very well-ordered array of pores. The regular arrangement of the pores extends over large domains, defect-free in each of these domains. The size of the ordered domains increases with increasing anodization time. A highly periodic structure obtained after two step anodization in 3% oxalic acid is shown in Figure 6. Such well-ordered pore structures with smaller and larger pore diameters may be obtained by anodization in sulfuric acid at lower voltages and phosphoric acid at larger voltages, respectively.31! 32 A second method has been developed by Masuda et al.,21! 33 using a molding procedure. An Al foil is first annealed at 400 C for 1 hr to facilitate deformation in the molding process and is then electropolished. An array of hexagonally arranged convexes is fabricated on a master using conventional electron beam lithography. The spacing between the convexes is chosen to match the cell spacing during subsequent anodization at the corresponding voltage. Masuda et al.,33 used SiC single-crystal wafer as the master material. SiC is mechanically strong and is thus suitable for electron beam lithography. Onto the SiC wafer, a hexagonal array of convexes is arranged using e-beam lithography. This master is then pressed onto the Al surface using an oil press at room temperature and at an approximate pressure of about 5 ton · cm−2 . Under this pressure, the master could be detached from the Al and reused. The pressed Al is then anodized resulting in ordered pores over long ranges ∼mm2 . Thus, a pretextured Al can cause the development of pore arrays ordered over long ranges. However, it may be added that appropriate self-ordering conditions are also essential for the growth of ideally ordered pore configuration. For example, in oxalic acid, pretexturing can lead to well-ordered pore arrays over very long ranges only at voltage ∼40 V. For other voltages, pretexturing cannot maintain ideal ordering over long ranges. In the case of 6

Fig. 6. Atomic force microscopy image showing a nanoporous alumina membrane with pore diameter 50 nm.

phosphoric acid, Masuda et al.,21 observed that the appropriate anodization voltage is 200 V in order to maintain long range order on pretexturing. This is also the condition for naturally occurring long-range order in the absence of pretexturing.31 In the case of sulfuric acid the appropriate voltage is 20–25 V.12! 30 The appropriate voltage also depends on other conditions, for example, concentration of electrolyte, temperature of solution, etc. Masuda et al.,34 investigated the effect of deficiency sites in the initial pressed Al surface, on long range order. They demonstrated that the deficiency sites are compensated automatically during the anodization that is, the anodized pore array does not exhibit the corresponding deficiency. Li et al.8 used a technique similar to Masuda’s nanoindentation method to form a well-ordered array of nanopores on aluminum. A positive poly (methyl methacrylate) (PMMA) resist is first cast on the substrate with a thickness of about 100 nm. Hexagonal pattern squares are then written on the resist using an electronbeam system. The irradiated parts are removed with a developer. The pattern is then transferred to the aluminum substrate by using a wet chemical etch in phosphoric and nitric acid. After removing the resist with a remover, the aluminum is anodized in oxalic acid under constant voltage. The anodization voltage is chosen such that the final inter-pore spacing matches the pitch of the e-beam prepattern. The prepattern thus helps to guide the pore growth, resulting in a well-ordered pore array.

3. ANODIZATION OF TITANIUM The demonstration of nanostructured surfaces by anodizing titanium was discovered very recently. First, in 1997 Darque-Ceretti and co-workers demonstrated poreformation35 and then in 2001 Grimes and co-workers J. Nanosci. Nanotechnol. 12, 1–19, 2012

Menon et al.

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

demonstrated the formation of nanotubes.36 This may seem surprising in the light that the anodization of titanium has been studied, just like aluminum, for many years.37 The only apparent reason for the delay in discovering anodized nanostructured titania is the fact that whereas in aluminum one can observe pores fairly rapidly after anodizing aluminum foil in a wide range of acids, in titanium on the other hand in addition to the acid one also needs to add fluorine to the electrolyte. Additionally, if the pH is not low enough nanostructure may take several hours to appear. 3.1. Nanoporous/Nanotubular Titania Templates

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Fig. 7. Schematic showing the dissolution rate in nanotube walls and the etch rate in the nanotube floor as a function of time. The anodization time given correspond roughly to anodizing in 0.5 wt% HF (or an electrolyte with pH around 1) where a steady state length (∼500 nm) is reached after approximately 20 min.

buffers, bases and milder acids to adjust the pH and fluorine ion content. Salts such as KF, NH4 F and NaF totally dissociate in aqueous solution and then hydrolyze with water to form HF: F− + H2 O ! OH− + HF

(1)

Besides, HF is a relatively mild acid and in acidic solutions (pH < 3'45) more than 50% of the fluorine exists in the form of HF. As a result pH and fluorine ion concentration is closely related (and solutions with KF, NaF or NH4 F and no additional acid will be basic). Grimes and co-workers found that they could grow nanotubes up to 4.4 (m using a solution of 0.1 M KF as fluorine source, 1 M H2 SO4 as acid, 0.2 M citric acid (trisodium citrate) presumably serving as buffer and NaOH as base to be added until the pH was as desired (4.5). The anodization voltage was 25 V and the anodization time was 20 h.64 Longer nanotubes of ∼6 (m were also demonstrated after anodization at 25 V44! 46! 64 for about 17–20 h using the same electrolyte. The following dissolution mechanism was first proposed:46! 58! 63 TiO2 + 6F− + 4H+ → TiF2− 6 + H2 O

(2)

They suggest that fluorine plays an important role in dissolving titania and that nanotube length is limited in acidic electrolytes because tube-wall dissolution is too high. Yet interestingly, it follows from the fundamental equilibrium: HF ! H+ + F−

(3)

that the fluorine ion content is highest in more basic solutions where the equilibrium in (3) is shifted to the right. 7

REVIEW

There are three important milestones in the history of titania nanopore and tube arrays: the discovery of nanoporous titania, the discovery of anodized titania nanotubes and the innovations that made it possible to fabricate micron length titania. Zwilling and Darque-Ceretti used electrolytes consisting of chromic acid combined with a small amount of hydrofluoric acid.38! 39 These were the first reports of the formation of a nanoporous structure in anodized titania. It was clear that the nanoporous structure formed only when sufficient HF was added to the electrolyte mixture. In 2001 the Grimes group discovered that titania nanotubes could be grown by using an electrolyte consisting primarily of HF acid (0.5 wt%) together with the application of high anodization voltages.36 Their focus was mainly on promising sensor applications,40–42 and photocatalytic applications.43–48 In 2003, Schmuki et al.49 used a mixture of sulfuric acid and a small amount of HF (0.15 wt%) and demonstrated ‘tube-like structures.’ They explored the use of alternative acid combinations,50–61 for example, phosphoric acid59 (as reported also by Zhao et al.62 ) and acetic acid50 (Grimes group also used acetic acid only as a minor ingredient in combination with HF or other sources of fluorine like NH4 F). The Grimes group also demonstrated that unlike anodized alumina, where pore length increases indefinitely with anodization time, both porous titania and titania nanotubes reach a ‘steady state’ length when anodized. That is, after typically 10 to 20 minutes of anodization the rate of etching of the pore (or tube) floor equals the rate of dissolution of the pore (or tube) walls so that the pore (or tube) depth does not show any further increase with additional anodization time (see Fig. 7). For potential applications in sensors and photocatalysis, increased length of the nanotubes is desired. An increase in length of the nanotubes enhances the effective surface area of the nanotubes and also reduces failures in devices such as high temperature sensors, where the electrode material can diffuse and come into contact with the unanodized part of the titanium substrate.63 Recognizing that there appears to be a connection between pH and/or F− and the dissolution rate of titanium dioxide during anodization Grimes and co-workers experimented with the use of other fluorine salts (as fluorine ion source besides HF) and combine

REVIEW

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

For instance, a typical acidic solution like 0.5 wt% HF has a fluorine ion concentration of approximately 0.01 M. On the other hand a pH = 4'5 solution with 0.1 M KF that is typical of formulations used to grow ‘long’ nanotubes64 has a F− ion content of around 0.1 M (see Table I) – approximately ten times higher. (KHF = 7'1 × 10−4 at 24 % C.) Hence, the longest nanotubes, and presumably lowest wall dissolution, is found in more basic electrolytes with relatively higher fluorine content. How is this consistent with the standard theory that it is fluorine ions that is the primary agent responsible for dissolution? We suggest that it is the concentration of H+ ions (or strictly speaking hydronium ions) that is primarily rate determining for nanotube wall dissolution. Hence, the mechanism of Eq. (2) could still explain the observed dissolution but we observe that at high pH it is the low H+ concentration and not the F− concentration that is the limiting reagent. However, hydroxyl ions can also partake in the solvation of dissociated titanium cations, in reactions like for instance: TiO2 + 4OH− + 4H+ → Ti"OH#4 + H2 O

(3)

or: TiO2 + 2OH− + 2F− + 4H+ → TiF2 "OH#2 + H2 O

(4)

However, recent work from our laboratory suggests that field assisted dissolution (as is happening in particular at the nanotube floor) may for the most part not happen by mechanisms as shown in Eqs. (2)–(4) as is commonly assumed.65 We found that once oxygen enters the oxide layer, i.e., forms titanium oxide or dioxide, it rarely leaves. Instead, the more electronegative ions tend to migrate inward towards the titanium substrate under the influence of the applied bias and it is titanium atoms/ions that are in effect ‘left behind’ that is dissolved. Hence, for this mechanism the dissolution reaction should be written as: Ti4+ "m# + TiO2 "o# + 6F− → TiF2− 6 "aq# + TiO2 "o#

(5)

with (m) denoting ‘metal,’ (o) denoting ‘oxide’ and where it is understood that the means salivation remains flexible as suggested above (Eqs. (2)–(4)). A final complication is the fact that protons (H+ # can play a role in titania surface embrittlement and hence in the high rates of nanotube wall dissolution observed. A recent study touching on exactly this topic was recently published by Tanaka et al.66 Several other reports on the anodization of titanium need mention here. These reports contain no radical Table I.

Approximate ion contents of two typical electrolyte solutions.

Solution

aH+ ≈[H+] (M)

aF− ≈[F−] (M)

0'013 0'000012

0'013 0'96

0.5 wt% HF (0.25 M HF) pH = 4'5 with 0.1 M KF

8

Menon et al.

new breakthroughs but rather explore finer points like the time resolved growth characteristics,67 controlling morphological aspects,51! 68 wettability characteristics,61! 69 electrochemical measurements,70 photoelectrochemical measurements,60 annealing71 and different acid combinations68 etc. Raja et al. published a study addressing the theory of titania nanotube formation.72 Two studies, making use of the new longer titania nanotubes in photocatalytic application have also recently appeared, that of Park and Bard73 and that of Quan et al.74 The dependence of pore diameter on voltage in titania nanotubes and pores mirror that of aluminum in the sense that pore or tube diameter increase with anodization voltage.65 In a very interesting study Grimes and coworkers found that lowering anodization bath temperature increase the titania nanotube wall thickness.47 Hence, the anodization parameters of electrolyte pH, bath temperature and voltage allow for considerable leverage in ‘tuning’ the morphological characteristics. But for its effect on pH and possible effect on fluorine content (as when using HF) the nature of the acid used does not appear to have a significant impact on the final form of the titania nanotubes obtained. In our laboratory, we have studied the dependence of pore parameters on fabrication conditions.65 Three sets of samples were prepared. The individual sets of samples were anodized in electrolytes with acid concentrations of 0.5 wt% HF, 0.75 wt% HF and 1.0 wt% HF respectively. Platinum mesh was used as the cathode and the Ti foil (Alfa Aesar 99.99% pure) was placed at the anode. The distance between the electrodes was maintained at 5 cm and the sample size was maintained at around 3 cm2 for all of the samples studied. Anodization voltage was varied in the range of 2–25 V. The anodized samples were viewed under the scanning electron microscope (SEM). Both cross-section and top view SEM images were obtained. From these images, pore and tube parameters were obtained as a function of voltage. First, we discuss our results on TiO2 samples anodized in 0.5 wt% HF under various anodizing voltages ranging from 2–25 V. The total time of anodization was kept fixed at 20 minutes for all the samples studied. It may be added that almost no change in template thickness for anodization times 10 minutes and above was observed. A typical current versus time curve for the anodization process is shown in Figure 8. Initially, the current is found to drop sharply and reaches a minimum at around 10 seconds after which it rises slowly and becomes nearly constant beyond ∼10 minutes. Although the titanium foil is still being etched away after 10 mins the pore or tube structure reaches a steady state that does not change under further anodization. In an electrolyte with relatively low acid concentration (0.5 wt% HF) nanoporous arrays are found to form for anodizing voltages 4–8 V. A scanning electron microscopy (SEM) image of a typical sample is shown in Figure 9. J. Nanosci. Nanotechnol. 12, 1–19, 2012

Menon et al.

Fig. 8. Typical anodization current versus time for a sample anodized in 0.5 wt% HF electrolyte at 18 V.

Fig. 9. A nanoporous titanium dioxide arrayed formed by anodization in 0.5 wt% HF acid at an applied bias of 7.5 V.

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Fig. 10. SEM image of a titania template anodized at 10 V in 0.5% HF yielding an array of nanotubes. The surface of the sample has been cracked removing much of the porous oxide layer and revealing the titanium substrate. As can be seen from this image many tubes have been broken off from the titanium substrate allowing for an accurate measurement of nanotube length/thickness of the oxide layer.

occurs when moving from the titanium conductor to the electrolyte TiO2 semi-conductor interface. In addition, the common assumption that little to no electron tunneling occurs through the barrier layer is critically dependent on the actual thickness of this layer. The degree to which tunneling can occur determines the amount of electrolysis that will occur in addition to the field-enhanced electrochemical dissolution and oxidation, the processes responsible for the etching of nanopores and tubes. The barrier layer thickness for the nanoporous samples were determined from cross-section SEM images. The barrier layer thickness is found to increase linearly as a function of voltage, especially over lower voltages (see Fig. 13). Upon cracking, pores typically split in half allowing the direct measurement of the floor thickness of a large number of pores per sample. The tubes rarely break in half prohibiting the direct measurement of barrier layer thickness at the bottom of tubes.

Fig. 11. Top view SEM image of a sample anodized at 9 V in 0.5% HF. Existence of both pores and tubes are clearly seen. The tubes appear disconnected from each other while the pores are attached to each other.

9

REVIEW

The cross-section images were obtained by cracking the surface of the samples to be imaged. When applied anodization potentials of 9 V or higher is used with the same acid concentration as before the pattern formation at the surface is found to change to an array of tubes (see Fig. 10). It may be noted from the SEM images that the nanotube structure is intact even upon cracking. At the intermediate voltage, 9 V, an interesting transition phenomenon is observed. Figure 11 shows the top-view SEM image of a sample anodized at 9 V. It may be noted that in different regions of the same sample, either pores or tubes form. It appears that here a porous structure is apparently in the process of peeling off to reveal a tubular structure below. Barrier layer (see Fig. 12) thickness is a critical parameter with regard to the modeling and understanding of electrochemical etching phenomena. It determines the magnitude of the resistance, and subsequently the magnitude of the significant drop in electrical potential, that

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

REVIEW

Fig. 12. The TiO2 barrier layer of a sample made at 12 V in a 1 wt% HF solution seen from below. Normally these ‘U-shaped’ domes are attached to the titanium substrate into which the TiO2 layer has been oxidized. But, on this SEM image a strip of nanotubes have been peeled and flipped upside down revealing the bottom of the barrier layer.

Additionally, the thickness of the porous layer as a function of voltage has been measured. In the case of the porous samples, at lower voltages, the thickness of the porous layer (or length of pores) is found to rise gradually (see Fig. 14). However, as the critical voltage (where the transition from pores the tubes occurs) is approached pore length rises more steeply. This feature is also seen in the barrier layer thickness data (Fig. 13). That is, at lower voltages there appears to be a linear increase in barrier layer thickness that accelerates as the critical voltage for pore to tube transition is approached. In the same curve is also shown the thickness of the porous layer of a sample anodized at 9 V, though we have noted that this sample is in the intermediate stage with the existence of both pores and nanotubes. As reported by Gong et al.36 we have also observed that nanotube structures (similar to that observed in 0.5 wt%

Fig. 13. Barrier layer thickness of nanopores as a function of voltage. Reprinted with permission from [65], C. Richter et al., J. Nanosci. Nanotechnol. 7, 704 (2006). © 2006, American Scientific Publishers.

10

Menon et al.

HF electrolyte) can be grown at higher acid concentrations. However, we have made an interesting observation with respect to the threshold voltage corresponding to nanotube formation. The threshold voltage for nanotube formation of ∼9 V reported for the electrolyte with concentration 0.5% HF is found to shift to a higher voltage, namely around 11 V for 0.75 wt% HF and around 12 V for 1 wt% HF (see Fig. 14). Thus, the increase in fluorine concentration and acidity that result from using a higher HF concentration appear to shift the transition from pores to tubes to higher voltages. The accurate determination of the threshold voltage requires care because, unlike in the 0.5 wt% HF electrolyte where tubes already appear after about 5 minutes of anodization and reach a steady state in 10–20 minutes, tubes appear in higher acid concentration electrolytes only after 30 to 60 minutes of anodization from below a peeling disordered oxide layer. The slower formation of ordered tubes close to threshold voltages in higher acid concentrations may be another clue towards understanding the process of tube formation and the pore to tube transition with increasing voltage. Below the threshold voltage for tube formation in the 1 wt% HF electrolyte we observed that pore formation becomes unstable; that is pore formation becomes spotty and the pores are badly ordered. Finally, as was the case with pore length, the lengths of the tubes were also found to increase with increasing voltage. However, in contrast to pore length this rise is less rapid and more or less levels off at higher voltages. 3.2. Titania Nanotubes Fabricated in Chlorine Media A major breakthrough in the field of anodic titania nanotubes was the ability to make micrometer length

Fig. 14. (a, bottom) Pore length as a function of applied dc voltage in 0.5% HF. The data point at 9 V represents the sample where both pores and tubes form (as shown in Fig. 3). (b, top) Tube length as a function of applied dc voltage in 1% HF. Reprinted with permission from [65], C. Richter et al., J. Nanosci. Nanotechnol. 7, 704 (2006). © 2006, American Scientific Publishers.

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Menon et al.

Fig. 15. Scanning electron microscopy image of titania nanotubes (side view) fabricated by anodizing titanium foil at 16 V in an electrolyte consisting of 0.5 M formic acid and 0.9 M NH4 Cl.

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Fig. 18. In some areas it was observed that nanotubes grow in two perpendicular directions forming a fairly regular interwoven ‘weave’ of nanotubes. Shown in the picture is the SEM image of such a weave with tubes growing both in the same plane with the page (around 45% with the horizontal) and perpendicular to the page (at this magnification the perpendicular tubes appear as little nodules surrounding the tubes that are parallel to the page). Anodization conditions for this particular sample were 15 V in 0.02 M HCl and 0.4 M NH4Cl.

more rapid and the tubes are significantly thinner (diameters are ∼20 nm) and longer (tubes range from 5 to 50 (m) using the new chlorine based electrochemistry. It can be seen from Figure 16 that these structures are indeed nanotubes and not nanowires. Nanotube diameters appear to be relatively independent of the anodization voltage or electrolyte used and are typically ∼20 nm with wall thicknesses ∼4 nm. The formation of the new chlorine based tubes proceeds extremely rapidly. After only 80 seconds of anodization tube bundles of up to 6 micrometers have been observed, a length that can be obtained only after 17 h of anodization in fluoride electrolytes. Nanotubes were obtained with applied voltages ranging from 13 V to 25 V. For lower voltages, such as an applied bias of 7 V, no tubes were observed even after anodizing for 45 minutes. Voltages lower than 8 V generate an anodization current that is less than 10 mA, approximately 100 times smaller than the 1 A range currents that abruptly emerge for voltages of 10 V and higher. The rate of tube formation appears to scale with voltage, being faster for a higher applied voltage. The nanotube formation process consumes the titanium foil completely within several minutes – typically ‘corroding’ it from its edges releasing tube bundles into the electrolyte. A SEM image of such a tube bundle is shown in Figure 17. The longest tube bundle observed is approximately 60 (m – close to half the foil thickness. Around the edges nanotubes appear to grow in two perpendicular directions – inward from both the front and the sides of the foil. Consequently the tubes in these regions form a very regular interwoven ‘weave’ structure (Fig. 18). The elemental composition of the reported nanotubes was determined by Energy Dispersive X-Ray (EDX). A quantative analyses of this spectrum of tube bundles 11

REVIEW

nanotubes. The key was to abandon pure HF based electrolytes and use fluoride salts (typically KF, NaF or NH4 F) instead as fluorine ion source. The obtainability, and the nature, of titania nanotubes fabricated by anodization depend strongly on the fluorine content and the pH of the electrolyte used but appears to be for the most part independent of the particular acid used. For ‘short’ nanotubes the favored electrolyte is pure HF36! 42 or HF with a small amount of acetic acid added.50 To make longer nanotubes the base acid usually preferred is sulfuric acid or related sulfates.49! 64 Some other acids used are phosphoric acid (or other phosphates)59 and nitric acid or a nitric + boric acid combination.75 In the latter case Grimes and co-workers observed that addition of the boric acid appears to result in a larger spread of nanotube diameters at a given voltage. Thus all reported anodic nanotube arrays have been synthesized in fluorine containing electrolytes. In fact, the current wisdom is that titania nanotubes cannot be obtained without the presence of fluorine ions in the electrolyte.63 This opinion is based on the hypothesis that fluorine ions are an essential and perhaps unique ingredient for the formation of nanoporous or nanotubular TiO2 because of fluorine’s unique ability to ‘dissolve’ TiO2 and form the 64! 76 complex TiF2− 6 in solution. Hypothesizing that chlorine ions could perhaps play a role similar to that of fluorine ions during the anodization of titanium, we systematically anodized titanium foil in several combinations of different chlorine salts and acids. Most combinations yielded no tubes or pores. However, by adding chlorine salts (f.i. 0.4 M NH4 Cl) to various acids like oxalic (0.5 M), formic (0.5 M) or sulfuric acid (f.i. 0.05 M) a unique new type of titania nanotubes have been obtained (see Fig. 15). The morphological and chemical differences between titania nanotubes fabricated in chlorine and fluorine containing media were much greater than anticipated. The process of nanotube formation is much

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Menon et al.

REVIEW

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Fig. 16. Scanning electron microscopy image of titania nanotubes (top view), with an average outer diameter of 25 nm, and wall thickness of 5 nm. Anodization conditions for the samples shown are (a) 16 V in 0.5 M formic acid and 0.5 M NH4Cl; (b) 16 V in 0.02 M HCl and 0.1 M NH4Cl; (c) 18 V in 0.02 M HCl and 0.4 M NH4Cl; (d) 14 V in 0.5 M formic acid and 0.4 M NH4Cl; (pH 1.6–1.8).

Fig. 17. Scanning electron microscopy image of typical nanotube bundles. The longest such bundle observed consisting of a collection of nanotubes is just over 60 (m in length. (a) “Attack area” showing tens of bundles coming out of the sample. (b) Single bundle of tightly packed nanotubes ordered mainly in one direction (around 10% from vertical). Anodization conditions for this particular sample were 16 V in 0.02 M HCl and 0.1 M NH4Cl (both pictures).

12

J. Nanosci. Nanotechnol. 12, 1–19, 2012

Menon et al.

4. SYNTHESIS OF NANOMATERIALS INSIDE NANOPOROUS TEMPLATES Once nanoporous templates are synthesized, they can be used effectively to prepare nanowire arrays. One of the simplest ways to achieve this is through electrodeposition. In general, the electrodeposition process occurs when a positively charged element in an electrolyte is reduced by a flow of electrons, and then the reduced element deposits on a substrate. Deposition of nanowires follows the same methods as electroplating.78–80 In order to reduce a given number of moles of a substance, n, a certain amount of electric charge, q, is required which is given by q = nne F , where ne is the number of electrons and F is Faraday’s constant. From basic electrodynamics the total charge, q, can be expressed as the integral of the current over the time interval. From this one can determine the number of moles, n, in terms of the applied current which in turn provides information on the thickness of the deposited !material, which is given by, thickness = "ma /"A$ne F ## I dt, where ma is the atomic mass of the deposited material, A is the area, $ is the density of the material, and I is the current. As it applies to the electrodeposition of wires, the thickness is synonymous with the length of the wires. Additionally, the higher the current that is applied, the faster the rate of growth of the nanowires.

to a metal mesh. This mesh is made of a metal that is a good conductor and acts as a counter electrode. Some commonly used metals for this mesh are Pt, Au, Cu, Pd, and Ag. However, there have been cases where conductive non-metals such as graphite have been used.81–84 The other lead is attached to the aluminum below the porous template. The mesh and the template are then immersed in an electrolyte solution, which contains the salt of material to be deposited (see Fig. 19). An AC voltage is applied to the circuit when grains of material from the solution randomly deposit themselves into the template, eventually filling the pores, creating nanowires. The size of the wires depends directly on the dimensions of the pores in the template. As time increases, the electrochemical process leads to reduction in concentration of metal salt leading to change in pH. Sometimes, it is necessary to maintain a certain range of pH values in order for the desired deposition to occur. In these cases, the solution must be buffered prior to and during the deposition process. So, an apparatus to monitor the pH may be added to the experimental set-up. Common acids used to buffer the solution are boric acid, sulfuric acid, and nitric acid. Deposition takes place only during one half (the negative half in the case of metals) of the AC voltage cycle for a time of 1/2f seconds, where f is the applied voltage frequency. If the metal mesh is the anode and the porous template is attached to the cathode, then deposition takes place during the negative half cycle. The rate of deposition increases with increasing voltage as is expected from Ohm’s Law, V = IR. However, for metals, the rate of deposition does not always increase with increasing frequency. At low frequencies, 1/f is relatively large and deposition occurs in the form of a thin film rather than as nanowires. At high frequencies, 1/f is relatively small. So, there is not enough time for the deposition to occur before the voltage and current switch polarity. This makes the rate of deposition extremely long. An optimal frequency range based on the material that one wishes to deposit must be determined. In this range, the rate of deposition will increase with increasing frequency. In general, when

4.1. AC Electrodeposition AC electrodeposition takes place in a specially designed cell or in a beaker. One lead of a power supply is attached J. Nanosci. Nanotechnol. 12, 1–19, 2012

Fig. 19.

Schematic of the electrodeposition apparatus.

13

REVIEW

revealed that in addition to titanium and oxygen, the tubes fabricated in organic acids also contained a significant amount of carbon. A quantitative analysis of these spectra indicated that the approximate ratio of these elements (Ti:C:O) as 1:1:2.5. The elemental chlorine content of all samples was 5% or lower. X-ray diffraction spectra were taken to identify the identity of any crystalline phases that may be present. Samples that were annealed for 4 hours at 400 % C in an argon atmosphere showed a distinct anatase pattern while unannealed samples were amorphous. These results exactly mirror similar results obtained for fluoride based tubes. The current consensus regarding fluoride based nanotubes is that they consist of amorphous titania that crystallize to anatase upon annealing at 280 % C or higher.71! 77 Our results strongly suggest that the chlorine based nanotubes also consist of amorphous titania. However, unlike the tubes formed in fluoride media the present tubes have a significant amount of carbon present (around 20 at%) when organic acids are used as electrolyte. The more vigorous chlorine chemistry appears to facilitate the incorporation of significant amounts of carbon derived from the organic anions in the acid into titanium oxycarbide surface layers.

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

depositing non-metallic materials, the frequencies of deposition can be much lower. AC electrodeposition is an ideal process to use when fabricating nanowires. Many different types of wires can and have been manufactured through the use of AC electrodeposition into porous alumina templates including semimetals, magnetic metals, non-magnetic metals, semiconducting metals, superconducting metals (for instance, Pb and Sn), and conducting polymers. Table II gives a brief summary on the conditions: electrolyte used, Table II.

Bi nanowires

Electrolyte solution 85–88

Ni nanowires28! 89

REVIEW

optimal temperature, pH of the electrolyte, of the solutions that have been used for the electrodeposition of various materials. For magnetic nanowires, the easy axis is located along the length of the wires. This enables one to determine the deposition status of the wires using a magnetometer. If wires have formed, one can expect higher coercivities than thin films deposited on the surface. The anisotropic properties of nickel magnetic have led to studies of their applications in magnetic storage devices. Polymer

Deposition parameters for the preparation of nanowires in porous alumina templates under AC conditions.

Material

NiPb nanowires83

Fe nanowires89–91 Co nanowires90–92 CoP nanowires93

CoPt nanowires94–96

FeS2 nanowires97 FePt nanowires94 Prussian blue nanowires (catera-[MFeII FeIII (CN)6 ] with M = Li+ , Na+ , K+ , NH+4 #98! 99 Co1−x Fex nanowires81 Vanadium iron cyanide nanowires100 Fe1−x Nix nanowires82 CdS nanowires89 CdSe nanowires89 CdSx Se1−x nanowires89 Cdx Zn1−x S nanowires89 GaAs nanowires89 Sb/Sb2 O3 nanowires101 Au nanowires91 Ag nanowires91 Pt nanowires102 Pd nanowires102 Cu nanowires91 Polyaniline nanotubes102! 103

14

Menon et al.

75 g/l bismuth nitrate pentahydrate, 65 g/l KOH, 125 g/l glycerol, 50 g/l tartaric acid 0.1 M NiSO4 ∗6H2 O, 45 g/L boric acid 30 g/l Ni(CH3COO)2∗4H2O, 6 g/l Pb(CH3COO)2∗3H2O 6 g/l, 30 g/l H3BO3 50 g/l FeSO4, 25 g/l boric acid 1 M CoSO4 ;92 or 30 g/l CoSO4, 25 g/l boric acid90 50 g/l CoSO4 ∗7H2 O, 30 g/l H3 BO3 10 to 1 mixture of CoCl2 and PtCl2 ;94 or 0.5 g/l PtCl4 , 96 g/l CoSO4 , 42 g/l boric acid95 120 g/l FeSO4 , 45 g/l boric acid 10 to 1 mixture of FeCl2 and PtCl2 0.02 M FeCl3 , 0.02 M K3 Fe(CN)6 , 0.6 M boric acid, 0.5 M KCl 45 g/l boric acid, CoSO4 , CoCl2 , FeSO4 0.02 M NaV04 , 0.02 M K3 Fe(CN)6 , 3.6 M H2 SO4 1 mol FeSO4 to 7 mol water, 1 mol NiSO4 to 6 mol water, boric acid 0.055 M CdCl2 , 0.19 M S in dimethylsulfoxide 0.055 M CdCl2 , 0.19 M Se in dimethylsulfoxide 0.055 M CdCl2 , 0.19 M (Se + S) in dimethylsulfoxide 0.055 M (CdCl2 +ZnCl2 #, 0.19 M S in dimethylsulfoxide Ga(III), As(III) 0.03 M potassium antimonyl tartrate, 0.435 M sodium tartrate dehydrate 1 g/l HAuCl2 , 20 g/l MgSO4 ∗7H2 O 1 g/l AgNO3 , 41 g/l MgSO4 ∗7H2 O, H2 SO4 1 g/l H2 PtCl6 , 176.4 g/l H2 SO4 10 g/l Pd(NH2 #(NO2 #2 , 100 g/l NH4 NH2 SO3 35 g/l CuSO4 , 20 g/l MgSO4 ∗7H2 O, H2 SO4 0.3 M aniline, 1 M HCl, 10 ml 0.5 M toluene-p-sulfonic acid sodium salt, 0.12 M ammonium metavanadate

pH, Temperature pH: 0.9

Characterization Magnetic semimetal/semiconductor

Temperature: 55 % C pH: 5.5

Magnetic

pH: 4.0

Magnetic Magnetic

Magnetic

pH: 3.0–4.0 Temperature: 30 pH: 2.8–395

Magnetic

Temperature: 10

Magnetic Magnetic Magnetic conducting polymer

Temperature: 37

pH: 3.0–3.5

Magnetic

Magnetic Magnetic, Carbon based Magnetic Semiconducting Semiconducting Semiconducting Semiconducting

pH: >2.5 pH: 9

Semiconducting Semiconducting

pH: 1.7 pH: 2

Metallic, non-magnetic Metallic, non-magnetic

pH: