Water-Induced Charge Transport Processes in Titanate Nanowires: An

Article pubs.acs.org/JPCC

Water-Induced Charge Transport Processes in Titanate Nanowires: An Electrodynamic and Calorimetric Investigation Henrik Haspel,† Noémi Laufer,† Valéria Bugris,† Rita Ambrus,‡ Piroska Szabó-Révész,‡ and Á kos Kukovecz*,†,§ †

Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1. H-6720 Szeged, Hungary Department of Pharmaceutical Technology, University of Szeged, Eötvös u. 6. H-6720 Szeged, Hungary § MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla tér 1. H-6720 Szeged, Hungary ‡

ABSTRACT: Elongated oxide nanostructures have gained much attention in the past decade due to their unique mechanical, optical, and electrical properties. Despite the vast amount of theoretical and experimental work on these materials, the mechanism of water-related electrical conduction in these systems has remained unsolved. In this study, the charge transport processes in hydrothermally synthesized trititanate nanowires (TiONW) at varying relative humidity (RH) have been investigated. Parameters characterizing these processes were extracted from dielectric spectroscopy (DRS) and ionic transient current (ITIC) measurements at room temperature. The dc conductivity varies exponentially with increasing RH. It is suggested to stem mainly from the exponentially increasing charge carrier concentration, while carrier mobility seems to have a much weaker influence on the long-range charge transport. The changes in the constituent parameters of dc conductivity are thought to be due to the changes in the amount, surface structure, and thermodynamic state of adsorbed water, which has been confirmed in the case of ionic mobility with moisture sorption and calorimetric (DSC) investigations. However, the microscopic origin of the exponentially increasing carrier concentration with RH still remains an open question.

1. INTRODUCTION Various titanium oxides have attracted much attention in the past decades because of their unique properties and have found applications of environmental interest in dye-sensitized solar cells,1 photocatalysts for the degradation of pollutants,2 and gas-sensing devices.3 One-dimensional (1D) nanostructures such as nanorods, nanotubes, nanowires, and nanobelts constitute a novel class of functional materials that have gained considerable attention due to their size- and morphologydependent physicochemical properties and potential applications. Recently, the water sensing properties of 1D titanate nanocrystals have also been studied.4−6 The charge transport mechanism in humid nanostructures7−10 as well as in cellulose materials11−13 has been extensively studied to elucidate the underlying processes governing the water sensing properties. The principle of humidity sensing is the change in the dielectric properties of the used material due to the varying water content, caused by water swelling, adsorption, surface dissociation, or capillary condensation. These dielectric changes in hygroscopic solids of very different kinds during water uptake bear striking similarities. This suggests a common mechanism; however, up until now the microscopic description of the underlying processes is still missing. It is widely accepted that surface ionic conduction dominates in these humid materials, which could explain why materials of very different chemical origins exhibit similar characteristics in water-related conductivity © 2012 American Chemical Society

variation. Because titanate nanostructures might have semiconducting properties besides the large amount of ions in their structure, they might be expected to exhibit electronic and ionic conduction in parallel (mixed ionic-electronic conductors, MIEC) as is well-known for ceramic materials.14 However, it is quite likely that ionic conduction dominates at near room temperature from low relative humidity on,9,11,12,15−21 and the conduction may be assigned either to protons or to exchangeable cations (Na+, K+) balancing the charge of the lattice.9,11,15,17−19,21,22 Protons could either act as counterions in the structure or come from the dissociation of adsorbed water molecules, mainly at coordinatively unsaturated metal sites, like oxygen vacancies. A typical example is water dissociation on TiO2 surfaces: On perfect rutile TiO2(110), water prefers to adsorb molecularly.23,24 It can dissociate spontaneously over oxygen vacancies; however, it is adsorbed only molecularly on titanium vacancies.25 The dissociative adsorption of water has been widely investigated on different metals26 and metal oxides by scanning probe microscopy,23,24,27−29 desorption techniques,30 photoelectron spectroscopy (XPS),31−34 and theoretical calculations.25,35 Furthermore, protons are also suggested to be generated via surface enhanced autoprotolysis of water molecules.9,19,21 The Received: May 11, 2012 Revised: August 7, 2012 Published: August 8, 2012 18999

dx.doi.org/10.1021/jp304605k | J. Phys. Chem. C 2012, 116, 18999−19009

The Journal of Physical Chemistry C

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liquid-like state is present, where each water molecule has two to three hydrogen bonds on average. This structure has been referred to in the literature as liquid-like water. Infrared (IR) spectroscopy has been extensively used as an indicative tool for investigating the structure of these water types, because the peak position of the OH stretching vibration is very sensitive to the degree of hydrogen bonding.46−51 Analyzing the thermodynamic nature of adsorbed water with calorimetric methods (DSC) allowed the identification of three different types of adsorbed water that is, water in at least three different thermodynamic states.52−55 In this Article, we differentiate nonfreezing bound water, and two types of freezing bound water. Water undetected by DSC is called nonfreezing bound water, and that detected by DSC but with a melting temperature lower than 0 °C is called freezing bound water. The term “bound” is used to indicate that this type of water is significantly influenced by the molecular forces of the sample surface. In this contribution, we will show the connection between the differently identified (IR, DSC) water types by means of the moisture sorption properties of titanate nanowires (TiONW) and reveal the possible role of these waters in the charge transport processes. As analysis tools, electrodynamic methods (dielectric relaxation spectroscopy (DRS) and ionic transient current measurements (ITIC)) were chosen as analytical tools. The variation of charge transport parameters with the humidity of the environment was discussed in terms of the type, amount, structure, and thermodynamic state of water present in the TiONW structure and on its surface.

mechanism of conduction in humid systems is not yet clear: hopping transport between surface groups10 and diffusion of protonated species in the condensed water layers7,8 were suggested at low to moderate and high water content, respectively. Furthermore, in the deliquescent layers, Grotthuss proton shuttling36 is also suggested. The easiest dielectric parameter to follow is dc conductivity, defined by

σdc = nqμ

(1)

where σdc is the frequency-independent (DC) conductivity, q is the charge, n is the concentration, and μ is the mobility of the ionic charge carriers. To obtain in-depth information on the charge transport, not only the dc conductivity but also its constituent parameters (μ, n) should be investigated in a wide relative humidity range. The relative humidity (RH), the ratio of actual vapor pressure (p) of water to the saturated vapor pressure (p0) at a particular temperature expressed in percentage (100·p/p0), is commonly used to measure humidity of the atmosphere surrounding the investigated material. Under ambient relative humidity conditions, metal and metal oxide surfaces are partially covered with molecular H2O and/or its dissociated species OH. This thin film can have a profound effect on the chemical and physical properties of the substrate surface; thus it plays a crucial role in physical,37 chemical,38 environmental,39 and biological processes.40 Despite the relevance of the interfacial water on metal oxide surfaces under environmental conditions, the adsorption, bonding structure, and reaction of water on these surfaces is poorly understood. Therefore, studying the structure of thin water films continues to be an active research topic. Recently, numerous review articles were published on the structure of water on the surface of various solids, even on ice.41−44 Theoretical and experimental evidence shows that wetting is controlled by the presence of OH groups on the surface, acting as nucleation sites for water adsorption.26,31 The surface OH groups stabilize water molecules via strong hydrogen bonds: donating hydrogen bonds from H2O to surface OH is ∼0.2 eV stronger than that between two water molecules;32 hence the mixed OH + H2O layers are more stable and energetically favored than the pure water layer. It is therefore expected that in moisture adsorption, water no longer interacts with the metal and O2− ions at the interface, but rather it interacts with this fully saturated hydroxyl layer.34 The hydrophilic/hydrophobic character of a surface seems to be determined by the ability of the surface to form OH groups.32 The thickness and structure of thin film water are determined by the interaction forces between the surface and the adsorbed water molecules.45 As the moisture sorption continues, water layers with varying degrees of hydrogen bonding appear on the surface. When water is completely self-associated, each water molecule forms four hydrogen bonds with its nearest neighbors. This structure is very similar but not equal to bulk water; thus the corresponding term “ice-like” indicates a structure where water molecules exist in a collapsed, strongly surface-associated (hydrogen-bonded) state. In the literature, this structure has also been referred to as quasi-ice, amorphous solid, and structured hydration layer.46 Because surface-induced structuring effects begin to diminish when newly adsorbed molecules can no longer sense the underlying surface,47 the disorder of the H-bonded film increases and H-bonding coordination decreases as the coverage increases beyond the first molecular layers. As the thickness of the layer continues to grow, a more

2. EXPERIMENTAL SECTION 2.1. Titanate Nanowire Synthesis. The nanowires were prepared56 by mixing 2 g of anatase TiO2 into 140 cm3 10 M aqueous NaOH solution until a white suspension was obtained, then aging the suspension in a closed, cylindrical, Teflon-lined autoclave (diameter 4 cm, height 14 cm) at 130 °C for 72 h while rotating the whole autoclave intensively at 10 rpm around its short axis, and finally washing the product with deionized water and neutralizing with 0.1 M HCl acid solution to reach pH 7, at which point the slurry was filtered and the nanowires were dried in air at 70 °C. 2.2. Characterization. The completion of the nanowire synthesis was verified by transmission electron microscopy (TEM) using a FEI Tecnai G2 20 X-Twin instrument operating at 200 kV. Product structure was characterized by powder X-ray diffraction (XRD) using a Rigaku Miniflex 2 instrument working with Cu Kα radiation. Elemental analysis was performed with a Röntec QX2 energy dispersive spectrometer. The thermogravimetric curves were obtained by a MOM DerivatographQ TGA instrument with a heating rate of 10 °C/ min from room temperature to 1000 °C in air. The water adsorption isotherm was measured at 25 °C. The samples were allowed to equilibrate to constant weight in desiccators where the relative humidity was kept constant between 6% and 100% using saturated salt solutions.57,58 The accurate water content was determined from TGA curves. The specific surface area of the sample was calculated from the water adsorption isotherm using the Guggenheim−Anderson−de Boer (GAB) equation:59 m (a) = 19000

mmon ·c·k·a (1 − ka)(1 + (c − 1)ka)

(2)



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μkT = qD

where m(a) is the amount of water sorbed by a gram of TiONW at sorbate activity a, mmon is the amount of water required to form one monolayer coverage over the same unit, and c and k are energy constants that measure the difference of the chemical potentials of the sorbate molecule in the upper sorption layers and in the monolayer, and in the upper sorption layer and in the bulk, respectively. At k = 1, the GAB isotherm reduces to the original BET equation.60 A Mettler-Toledo DSC821e differential scanning calorimeter equipped with a cooling device was used to determine the types of adsorbed water. DSC curves were obtained upon heating at a scan rate 10 °C/min in the temperature range from −50 to 25 °C in dry nitrogen flow. The samples were allowed to stabilize for 5 min at −50 °C. Twenty milligrams of titanate nanowire was used for each run in sealed aluminum capsules. The FT-IR measurements were performed in reflection mode with a Bruker Vertex 70 connected to a Hyperion 2000 microscope. The sample was prepared via drop-casting from an aqueous TiONW dispersion on a polished aluminum plate, and its specular reflection spectrum was measured at 97 RH%. The IR spectrum of bulk water was measured in the same instrument with the use of an ATR objective equipped with a germanium crystal. The NIR measurement was carried out in a Thermo Scientific Antaris II FT-NIR Analyzer. 2.3. Dielectric Measurements. The dielectric properties were measured61 by inserting the sample powder into a concentric cylindrical capacitor. To avoid density-dependent conductivity variation,12 the sample was measured in powder form, without pressing it into a pellet. The complex impedance of the cell was measured using a Novocontrol Alpha-A frequency response analyzer (FRA) equipped with a ZG2 interface and a somewhat modified BDS1200 sample holder. The response analyzer applied a sinusoidal voltage with a frequency between 10 mHz and 1 MHz with a 21/2 logarithmic spacing and an amplitude of 3 V (peak) to the sample cell electrodes. The FRA measures the phase and amplitude relations between the applied generator signal and the detected sample current. The complex conductivity function was then calculated from the raw impedance data. The relative humidity dependence of the dielectric properties was measured in a closed, grounded metal vessel containing saturated salt solutions maintaining the desired RH levels. All measurements discussed here were performed at 25 °C. 2.4. Transient Current Measurements. The transient current measurements were conducted by an ACM Instruments Gill AC multipurpose electrochemical station. A voltage of 3 V was applied across the sample, and the resulting current was measured. The sample was drop-casted onto an interdigitated copper electrode configuration from a TiONW/ethanol−water suspension and dried in air at 70 °C. Because copper electrodes are blocking ionic species, information about the number n and mobility μ of ionic charge carriers can be obtained by fitting the transient current I to62−64 I (t ) =

AσdcU ⎛ μU ⎞ exp⎜ − 2 t ⎟ ⎝ d ⎠ d

(4)

where k is the Boltzmann constant, and T denotes the absolute temperature.

3. RESULTS AND DISCUSSION In Figure 1 we present characterization data of the titanate nanowires used in this study. The starting anatase particles were

Figure 1. Representative TEM images (a) and XRD pattern (b) of the hydrothermally synthesized titanate nanowires (TiONW). The XRD pattern of the starting anatase material is also shown for comparison.

converted into nanowires with an average length of a few micrometers and average diameter ∼60 nm as measured by TEM (Figure 1a). The XRD profile of the product material (Figure 1b) agrees well with previous literature results, and, in particular, the diffraction peaks at 2Θ = 11.1°, 25.1°, and 29.5° prove the formation of the trititanate structure.65−67 The peak at 2Θ ≈ 11° is characteristic of the interlamellar distance (about 7 Å according to the TEM images in Figure 1a), and therefore it can be used for monitoring the water loss of the structure.68 The corresponding EDS measurement (not shown here) verified the presence of sodium ions originating from the alkaline treatment in the structure. Therefore, the

(3)

where t is the time elapsed after the onset of the potential U, σdc is dc conductivity defined earlier by eq 1, A is the electrode surface area, and d is the electrode spacing. The diffusion coefficient D of a charged species is related to the mobility via the Einstein equation: 19001



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system protons are the main charge carriers, because the applied measurement techniques are not suitable for determining the nature of the charge carriers. However, earlier findings in the literature on moisture adsorbing materials show that in these systems at around room temperature ionic conduction is dominant9,11,12,15−22 and charge carriers are suggested to be protons and either hydroxyl or impurity (e.g., Na + ) ions.9,11,12,15,17−19,21,22 The presumed large amount of protons was suggested to form during adsorption by the surface enhanced autoprotolysis of water molecules. Several authors have found that the dissociation of the adsorbate is enhanced by the surface hydroxyls and the formerly adsorbed layers.9,19,21 Furthermore, protons can also be generated by the applied bias. In the dielectric spectra of humid materials at low frequencies, the so-called low-frequency dispersion69,70 (LFD) has been observed. One subcategory of LFD is the quasi-dc process, in which the complex permittivity follows a fractional power law dependence on frequency. Jonscher suggested that chemical reactions at electrode−sample interfaces could play an important role in the development of LFD. This low-frequency dispersion has also been observed in TiONW; however, in DRS measurements with an applied voltage ranging from 50 mV to 3 V, the characteristics of the RH-dependent specific conductivity were the same. Hence, it can be concluded that, although chemical reactions at the electrodes may occur in the investigated system, they do not influence our conclusions about the specific conductivity. We further assume that this is also true for the constituents of the conductivity. Because there is no sound theoretical basis for analyzing long-time transient current processes,63,64 we analyze only the first region of the current decay in details. In Figure 2b the solid line denotes the exponential fit to the first decay of the current response, being caused most likely by protons. The charge transport parameters associated with this region were obtained from the fit via eqs 1, 3, and 4 and are also displayed in the plot. Figure 3a−c shows the charge transport parameters (σdc, μ, D, n) as a function of the relative humidity at 25 °C in the 6− 100 RH% range, and the constituent parameters of the dc conductivity were compared to that of the bulk water. In Figure 3a the logarithm of the dc conductivities obtained from both dielectric and transient current measurements is depicted. Conductivity increases exponentially with RH as found by several authors for, for example, poly(monosubstituted)acetylene,71 conductive polymers,72,73 poly vanadium−molybdenum acid,74 and porous aluminum oxide.75 Albeit many attempts were made to create a theory that could explain the exponential dependence on either humidity75,76 or moisture content77−79 basis in heterogeneous humid systems, to the best of our knowledge, no widely accepted theory is available in the literature yet. We thus performed exponential fits to both data sets; the fitted lines and the corresponding equations are also included in the plot. The results from DRS and ITIC measurements often differ from each other by some tens of percent as in, for example, Carbopol gels.80 This is also the case here; the slopes of the lines, that is, the exponents of the exponential fits, differ from each other by ∼40%, while between the intercepts a 2-fold difference was obtained. Furthermore, we calculated an apparent conductivity from the almost constant region of the current decay (at t = 3600 s), and the current−voltage characteristics (I−V) were also measured at 84 RH% (not shown here for the sake of clarity). The resulted dc conductivities from the four different methods (DRS, ITIC, current, I−V) are in fair agreement. The similar results from

sample was trititanate nanowire (TiONW) described by the formula (Na,H)2Ti3O7. In Figure 2a the real part of the complex conductivity function (σ*(ω) = σ′(ω) + iσ″(ω)) of TiONW in the 10 mHz

Figure 2. The real part of the complex conductivity of TiONW at different relative humidities in double logarithmic representation (a), and current response after application of a potential step from 0 to 3 V (b) at 29 RH%. Two exponentially and a further, nonexponentially decaying regions were identified. Only the first region of the response is displayed, where the solid line denotes an exponential fit to the current response. The charge transport parameters associated with this region are also displayed.

to 1 MHz frequency range is shown at different relative humidities in double logarithmic representation. The spectra of TiONW equilibrated at 6 RH% and 9 RH% run very close to each other in the whole measured frequency window, and, therefore, the 6 RH% spectrum was not depicted in Figure 2a for clarity. However, the corresponding result is included in the discussion. The conductivity is almost independent of the frequency in the low-frequency region and is equal to the dc conductivity σdc. Dispersion in conductivity is observed at higher frequencies, with the transition point from dc to dispersive conductivity shifting toward higher frequencies with increasing relative humidities. The peaks in the dispersive regime are caused by interfacial processes stemming from the heterogeneous nature of the fibrous titanate structure. The dc conductivity values used in this work were determined from the low frequency plateau of the real part of the conductivity function. Figure 2b shows the current response after a potential step from 0 to 3 V in TiONW at 29 RH%, while in Figure 2b inset, the initial current decay important for our ITIC analysis is only depicted. Two exponentially regions and a further, nonexponentially decaying region were identified. Figure 2b depicts only the short-time response. Because the charge carriers in the material are most likely protons and sodium ions as in many similar water adsorbing systems,9,11,22 the first two exponential current response regions are likely to be caused by these ionic species. The third, nonexponential decay could stem from electrochemical reaction in the sample/electrode interface. There is however no direct evidence supporting that in this 19002



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with relative humidity are shown in Figure 3b. Additionally, the inset depicts D and the mobility in linear representation. We discussed our results in terms of diffusion coefficient, but the use of ionic mobility would also be sufficient because mobility equals 38.9217 times the diffusion coefficient at room temperature (μ = 38.9217D). Therefore, the right axis in Figure 3b is just a multiplication of the left axis. We plotted our ITIC results in this way to facilitate comparison with results published by other research teams. The diffusion coefficient increases continuously in the whole RH region with a total variation of about 1 order of magnitude between 4.48 × 10−7 and 6.01 × 10−6 cm2/s. The diffusion coefficient of proton in bulk water is extraordinarily high, about 93.11 × 10−6 cm2/s, which stems from the Grotthuss proton shuttling in the bulk. The structure of the adsorbed water on TiONW differs from that of the bulk, and, in addition, a continuous water layer through the whole sample cell cannot be developed, particularly in the low RH regime. Therefore, perfect proton shuttling in the measured TiONW/air sample cannot take place, and somewhat lower diffusion coefficients have been obtained. In Figure 2b at least three distinct regimes can be distinguished: initially a steep increase at relative humidities less than ∼20 or ∼55 RH%, a plateau region with almost constant diffusion coefficients in the ∼55−90 RH% region, and mobility increasing again above 90 RH%. The accuracy of the measurements does not allow one to decide whether the first increasing part levels off at ∼20 RH% as indicated by the logarithmic plot or at ∼55 RH% as suggested by the linear plot. Once the dc conductivity and the mobility are known, the charge carrier concentration (n) can be extracted via eq 1. Figure 3c shows n as a function of the relative humidity. Although the water adsorption isotherm will be discussed in detail only in Figure 5b, the number of adsorbed water molecules present on TiONW surface at various RH values is also included in panel (c) for comparison purposes. In TiONW at 100 RH%, a conductivity similar to bulk water was obtained (as can be seen later in IR and DSC measurements in Figures 6 and 7), yet the measured diffusion coefficient was lower than that of the bulk. This is only possible when the charge carrier concentration is relatively high. Because of the autoprotolysis of water, the proton concentration in bulk water is about 1013− 1014 1/cm3 from ∼3 × 1022 1/cm3 water molecules, while it continuously increases with increasing RH in TiONW between 4.74 × 1010 and 4.09 × 1015 1/cm3 from 1016−1017 1/cm3 adsorbed water molecules via the previously suggested enhanced autoprotolysis. The strong dependence of the number density of protons on adsorbed water molecules is depicted in Figure 3c. The number of charge carriers is 106−102 orders of magnitude lower than the number of adsorbed water molecules, and the difference decreases with increasing humidity. The variation of n with RH seems to obey an exponential law with an exponent close (−15%) to the exponent of the dc conductivity characteristics determined from ITIC measurements and depicted in Figure 3a. The moderate increase in diffusion coefficient and the very similar exponential behavior of the charge carrier concentration and dc conductivity strongly suggest that in our case the latter exerts the main effect on the moisture-related electrical processes in TiONW. In other watersensitive systems, the large variation in dc conductivity with RH has also been attributed mainly to the increase of the charge carrier concentration in the material.11

Figure 3. Charge transport parameters: the logarithm of the dc conductivities (a), the diffusion coefficient and the mobility of the main carrier ion (proton) in logarithmic and linear scale (b), as well as the charge carrier concentration and the total number of water molecules (c) as a function of relative humidity in the 6−100 RH% range. All three parameters were obtained simultaneously by extracting the initial transient current response, while in panel (a) the dc conductivity from the dielectric spectroscopic (DRS) measurements is also depicted. The amount of water molecules in panel c was calculated from the adsorption isotherm detailed in Figure 5b. The solid lines in panels a and c are exponential fits to the data points; the corresponding equations are also indicated.

these independent techniques validate the applicability of these methods further. The overall variation in dc conductivity is about 4 orders of magnitude according to DRS and about 6 orders of magnitude according to ITIC, which can be considered as typical variations in humidity sensitive systems.9,11,71,74 The humidity dependence varied exponentially in the whole investigated RH regime, as there is no breakpoint at medium RH levels. Contrary to earlier findings for anatase thick films,7 dielectric properties varied significantly with temperature. By definition, the specific conductivity of ultrapure water is between 0.055 and 5 × 10−6 S/cm (ASTM Standard Type I-IV water), but it is strongly influenced by the dissolved ions and even nonpolar gases from the surrounding atmosphere.81,82 The dc conductivity of TiONW at 100 RH% is about 0.1−0.01 × 10−6 S/cm, which is close to the value of that of the purest water. The similarity of the TiONW conductivity to that value is somewhat expected. At 100 RH%, 1.2 g/g water is adsorbed on the nanowires, while bulk-like water layers are present on the surface (as can be seen later in IR and DSC measurements in Figures 6 and 7). Because humid hydrophilic materials were suggested to exhibit surface conduction, a similar-to-bulk conductivity is expected. That is exactly what we measured in our case. The mobility and the diffusion coefficient (D) of the carrier ion (proton) were also extracted by fitting the initial current response. The variations of the logarithm of D 19003



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(d200) in elongated titanate nanostructures. During structural water loss, this interlamellar distance decreases, and the (200) diffraction line shifts toward higher 2Θ degrees68 as the structure shrinks. Therefore, we used the (200) reflection as an indicator of the structural water uptake/loss and investigated its RH dependence. The results can be seen in Figure 5a, where

The investigation of the water content of TiONW in the broadest possible humidity range is of great importance. In general, water gaining processes can be classified as either structural water uptake or sorption onto the surface and into the pores of the sample. In Figure 4 the TGA plots of samples

Figure 4. Thermogravimetric curve of a titanate nanowire sample saturated at 97 RH%. The different weight loss regions are indicated.

saturated at 6 and 97 RH% are shown. Because we investigated nanowires in the 6−100 RH% range, these two TGA responses differ from each other to a great extent. Three distinct regions can be distinguished in each curves. Up to ∼180 °C are the physisorbed and chemisorbed water losses,83 while from 180 to 370 °C the structural water losses take place. Above 250 °C, in parallel with water loss, topotactic transformation of (Na, H)2Ti3O7 to the intermediate phases of (Na, H)2Ti6O13 and (Na, H)2Ti12O25 may occur. At temperatures below 400 °C, the removal of the remaining water results in the formation of monoclinic TiO2-(B) nanowires. A further increase in temperature above 400 °C results in the transformation of TiO2-(B) to anatase. At temperatures as high as 1000 °C rutile microfibres are formed.68 The major difference between the TGA curves in Figure 4 is the amount of physisorbed water. The quantity of chemisorbed + structural water is the same within the experimental error. Both the decay slope and the characteristic temperature of the second steps are very similar to each other. Furthermore, the quantity of the chemisorbed water was also expected to be the same in the whole series. It was demonstrated earlier in heat of immersion measurements84 that mild heating (up to about 75 °C) in αFe2O3 causes only physisorbed water loss, while surface hydroxyls remain intact. Increasing the outgassing temperature gradually removes the underlying surface hydroxyl groups, which was in accordance with the earlier results obtained on γAl2O385 and ThO2.86,87 Similar behavior is expected in the case of titanate nanostructures. Because our samples were measured in different RH atmospheres at room temperature only, the expected constant amount of chemisorbed water content is identifiable. It is important to clarify which type of water is involved in the humidity-induced changes of the dielectric properties. It is expected from the TGA results and from the limited number of active sites for water dissociation that all sites are already occupied from very low RH on27−34 and the chemisorbed water does not play a significant role in the investigated RH range. The possible involvement of the structural water in the charge transport mechanism was further studied by means of XRD patterns. The diffraction line at about 2Θ = 11° in the XRD pattern (Figure 1b) is related to the interlamellar distance

Figure 5. Variation of the interlamellar distance on the relative humidity calculated from the (200) X-ray diffraction line. The solid line denotes a linear fit to the data; the low slope value indicates a constant interlamellar distance (a). Moisture adsorption isotherm of TiONW at 25 °C calculated from TGA measurements (b). The inset plot enlarges the 6−90 RH% humidity range, where the solid line denotes GAB fit to the data. The monolayer capacity according to this theory is also indicated.

the solid line is a linear fit to the data. Its low slope value means that the d200 distance can be considered constant in the whole RH range studied. The observed d200 ∼7.7 Å value is close to that obtained from TEM analysis in Figure 1a. Hence, it can be concluded that the humidity-induced changes in the electrical properties of TiONW are related solely to the physisorbed water. The moisture adsorption isotherm of TiONW at 25 °C was calculated from TGA measurements and presented in Figure 5b. The inset shows the enlarged view of the isotherm in the 6−90 RH% range, whereas the solid line indicates the GAB fit59 to the data according to eq 2. The steep increase in the adsorbed moisture amount above 90 RH% is the so-called third adsorption stage. This feature always appears in water adsorption measurements because it is caused by the continuous condensation or deliquescence of the adsorbate onto the formerly adsorbed water molecules on the surface. It cannot be described adequately by either the GAB or the BET equation.59 The obtained GAB parameters agree well with previously published data for TiO2/water88−90 or cellulose/ water systems.52 The monolayer capacity is ∼94 mg/g (at about 22 RH%), while the specific surface area calculated from the water molecule surface area of 0.111 Å2/molecule91 is about 350 m2/g, which is twice as much as the value calculated from 19004



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N2 adsorption isotherms (not shown here) at −196 °C using the BET method60 (∼180 m2/g). In different water adsorbing systems, considerable differences in monolayer coverages have also been witnessed by several authors.9,59,92 In the case of ground glass sample, it was suggested that very fine pores are accessible to water molecules but not to noble gas atoms,9 while this phenomenon in different celluloses was explained with a water-specific swelling process.92 Furthermore, the various adsorption isotherms provide different monolayer capacities,62 and hydration of the material93 could also influence the results. Although either of these mechanisms could account for our results, we propose here that the surface structure of adsorbed water is responsible for the observations. The specific surface area is calculated from multilayer sorption isotherms (BET, GAB, etc.) using the monolayer capacity and the adsorbate molecular areas but without considering surface structure. Theoretical studies94,95 predicted a solid-like bilayer of water on silicon oxide surfaces, while combined theoretical and experimental investigations suggested96,97 that ice (0001) grown on Pt(111) surface is terminated as a full bilayer. Furthermore, in TiO2(110) the kink in the adsorption isotherm is thought to indicate an ice-like bilayer formation.31 Additional uncertainty could arise from the ambiguous definition of the adsorbate monolayer. In SPM and XPS studies, a monolayer is defined by one water molecule per unit cell,26 whereas in IR spectroscopy the number of adsorbed water layers is calculated from the overall thickness of the thin water film41,46−50 via the literature value of the ice bilayer thickness (365 pm).98 Therefore, one should compare the thicknesses obtained from different techniques only with particular caution to the used monolayer definition. Information on the structure of the adsorbed water layers can be gained from IR spectroscopic measurements by analyzing the region of the OH stretching vibrations, which is sensitive to the degree of hydrogen bonding.46−50 In general, the OH stretching absorption band between 3000 and 3650 cm−1 is composed of three components.46 The peaks at ∼3220 and ∼3380 cm−1 are attributed to the solid-like and liquid-like water structures, respectively. The small peak at ∼3700 cm−1 is due to the dangling or free OH groups present at the surface. Both solid-like and liquid-like water can be observed on various surfaces;41,43−46 furthermore, sum frequency generation spectroscopic (SFG) studies show99,100 that the dangling OH groups are dominant at the free surface of the adsorbed water. In the NIR regime, two major bands are observed in the regions of 6000−7500 and 5500−4500 cm−1, the former being assigned to the overtone band (νOH) of the stretching vibration of OH group and the latter to the combination band (νOH + δH2O) of the OH stretching vibration and bending vibration of the adsorbed water molecule, respectively.93 Typical FT-MIR and NIR spectra of TiONW equilibrated at 97 RH% are depicted in Figure 6. The IR spectrum of bulk water is also shown for comparison. All three stretching bands are clearly present in TiONW MIR spectrum, while in the NIR spectrum (Figure 6, inset) the appearance of the combination band is indicative of the existence of molecular water on the surface. Because the latter band appears at around 5000 cm−1 close to the wavenumber for the bulk liquid, it implies that the molecularly adsorbed water strongly interacts with the surface OH groups and with each other by forming a strongly hydrogen-bonding network.93 Consequently, in the discussion of the state of adsorbed water, one has to take this structural information into consideration. The spectra of the water

Figure 6. FT-IR spectrum of adsorbed water at 25 °C on TiONW equilibrated at 97 RH%. The peak positions of the strongly associated ice-like and the liquid-like water are represented by dashed and dashed−dotted lines, respectively. The peak related to the free or dangling OH-groups is denoted by a dotted line. In the inset, the NIR spectrum of TiONW is depicted, where the combination and overtone bands are also indicated.

adsorbed on TiONW at 97 RH% and that of bulk water are similar to each other, but, as it was pointed out by several authors,46−50 they would never match each other perfectly as in the case of surface premelting in bulk water.41,44 This stems first from the fact that as water is adsorbed on a hydrophilic surface, the molecules in the first layers form a strongly hydrogenbonded network. This phase resembles the structure of ice, but will, in fact, never become the same. These layers are present on the surface from very low relative humidities on; thus their IR response will be measured together with the response of the layers above. On the other hand, with ongoing moisture sorption, more bulk-like layers are formed. The thermodynamic state of water molecules in these layers differs from the state of liquid water as it will be discussed later in connection with the DSC measurements (Figure 7). In the bulk-like layers, however, the water molecules are hydrogen bonded to a greater extent than in bulk water; moreover, the ordering effect of the oxide surface is not completely shielded. The extensive hydrogenbonded environment, the ordering effect of the adsorbent surface, and the strongly hydrogen-bonded ice-like structure result in different peak positions and broader line shapes as is indeed seen in Figure 6. The thermodynamic state of the adsorbed water on TiONW was characterized by calorimetric (DSC) measurements; results are depicted in Figure 7. At high humidity when all types of water are present on the surface, two peaks commensurate to each other are visible during freezing, while in the melting curves the peak at lower temperature appears only as a shoulder (Figure 7a). Merging melting peaks with increasing water content is commonly observed in water swelling systems.53,54 In our mesoporous TiONW presumably pore freezing−melting also occurs, but at high (5−10 °C/min) scan rates, it could not be resolved due to peak broadening effect.101 At the different humidity levels, only the melting curves after quenching were further investigated. The observed melting peaks in humid TiONW indicate that crystallization occurred only above 53 RH% at higher-than-monolayer coverages. The first adsorbed water molecules are affected so that their mobility is restricted and no first-order phase transition is observed. These molecules form the first tightly bound nonfreezing water layer, which is most likely equal to the highly hydrogen-bonded structure adsorbed on surface OH-groups (referred as ice-like water earlier). There are interpretations in the literature suggesting 19005



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the other hand, it is possible that not enough water molecules are present in this upper layer for a macroscopic phenomenon like freezing-melting.55 Therefore, we assume that these water molecules are in a nonbulk-like thermodynamic state, which is thought to be equivalent to the intermediate state in Zografi’s argument made for humid cellulose and starch materials52 and with the transitional water layers found by IR,48,49 XPS,31 and NEXAFS45 measurements on metals and metal oxides. This region also corresponds to the slowly changing part of the moisture adsorption isotherm. Considerable differences were also found in the dynamic behavior of water molecules below and above monolayer coverage in, for example, graphite oxide,103 calcium silicate hydrate (C−S−H) gel,104 and rutile.105 The transition of the ice-like to liquid-like water (interfacial melting, surface premelting) is thought to be extremely sensitive to impurities.106 Our nanowires contain a considerable amount of sodium ions from the hydrothermal synthesis, which is suggested to influence the structure of these water layers on the surface. At higher relative humidities, as the adsorption continues, water can also bind to other water molecules not associated with primary OH sites, and thus form a weakly bound liquidlike phase. This was referred to in the adsorption study above as the third sorption stage. The melting of this phase is observed near 0 °C as can be seen in Figure 7a,b and is often called in calorimetric studies as free water. The nomenclature is somewhat misleading because the state of these molecules only approaches that of bulk water, but it is not equivalent with that even at 100 RH%.52,55 This assumption is confirmed in Figure 7b, where the melting peak in the DSC curves above 90 RH% approaches the reference peak of bulk water both in position and in shape as RH increases, but does not become identical to it even at the highest RH. Similar nonbulk behavior was found in different water adsorbent systems, for example, in ligno-cellulosic materials.55 On the basis of these results, we propose the following explanation for TiONW wetting: (1) Well below 6 RH% dissociative water adsorption takes place, and the surface is covered with OH groups acting as nucleation sites for further water adsorption. (2) In the 6−22 RH% region, there is a steep increase in both the adsorbed amount of water and the diffusion coefficient of the main charge carrier (most likely proton). As a highly hydrogen-bonded, so-called, ice-like structure is formed, no phase transition is observed. (3) From 22 to ∼50 RH% water adsorption slows, as new water layers are formed on top of the formerly adsorbed ice-like structure. These water molecules seem to be under strong influence of the substrate surface; thus no crystallization phenomenon occurs. The accuracy of the measurements, however, does not allow one to decide whether the increasing part of the diffusion coefficient levels off at ∼20 RH% or at ∼60 RH%. (4) In the ∼60−90 RH% region, the adsorbed water amount increases slowly. Although water molecules are assumed to be in a transitional thermodynamic state in the ∼20−60 RH% regime, a first-order phase transition is only observed above ∼50 RH% at a temperature of ∼ −25 °C. It suggests that the surface-induced structuring effect diminishes to a great extent only above this RH, which infers the slight increase in mobility and hence in diffusion coefficient; (5) in the very high humidity region between 90 and 100 RH%, a steep increase in adsorbed water amount is observable. With ongoing deliquescence, the properties of these water layers approach that of the bulk, but stay in a nonbulk thermodynamic

Figure 7. Calorimetric (DSC) curves measured at relative humidities indicated on the lines. The melting curve of water is also shown for comparison. The melting peak of the liquid-like water is clearly visible (a,b); it approaches the melting peak of water both in position and in shape as RH increases. The melting peak corresponding to the transitional bound water is seen in panel c in the enlarged section of the original plot. Phase transition could not be detected below ∼53 RH%.

that this tightly bound layer consisting of hydroxyl groups arises almost exclusively from the dissociation of water molecules on the oxide surface.7,8,10 However, this hypothesis can be questioned as water dissociates mainly at coordinatively unsaturated metal sites,23−35 and these are exactly the vacancies that are already occupied from very low humidities on.31−34 Different kinds of freezing bound water are adsorbed above the nonfreezing water layer with increasing relative humidity. Water may attach to water molecules already adsorbed on the surface by dipole attraction or by weak hydrogen bonds.13 Phase transitions can be first recorded between 53 and 62 RH% at a temperature of about −26 °C. From the very low melting temperature and humidity, one can conclude that these water molecules cannot form a normal ice structure. Although the second layer started to build above 22 RH%, melting could not be detected below ∼50 RH%. There are two possible explanations of this phenomenon. First, because the already adsorbed water molecules have a shielding effect on the interactive forces,102 with ongoing adsorption the effect of the underlying TiONW surface on the structure and thermodynamic state of water in the upper layers41,43,47,55 decreases. On Kelvin probe microscopic measurements in SiO2 films45 revealed that after the first molecular layer, the effect of the substrate on water molecules starts to decrease significantly. The steepest change in the surface potential occurs at ∼50 RH %. This implies that above this low humidity region, the effect of the substrate is so weak that phase transition could occur. On 19006



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water is involved in the humidity-related dielectric processes of TiONW, the variation of the interlamellar distances with RH, that is, the variation of the structural water content, was determined from XRD analysis. It has been pointed out that only adsorbed water contributes to the RH-induced variation of dielectric properties. From the moisture adsorption isotherm and calorimetric measurements, both the amount and the thermodynamic state of the adsorbed water were determined, and changes in ionic mobility in the investigated humidity range were successfully assigned to changes in the structure and thermodynamic state of adsorbed water molecules. Nevertheless, the origin of the exponentially increasing carrier concentration sill remains an open question.

state even at 100 RH%. The diffusion coefficient of the charge carrier increases steeply as these highly mobile layers are developed. Thus, we established connection between the charge carrier mobility variation on RH and the amount and state of water on the surface in titanate nanowires. A proper microscopic mechanism should also account for the phenomenon that the carrier mobility increases significantly during the build-up of the highly hydrogen-bonded ice-like layers at low RH, and also during the development of the liquid-like layers at very high RH. One possible explanation is a mechanism similar to that studied theoretically via DFT calculations in cellulose.22 It showed that when the sodium ion is hindered in its approach to the fixed surface OH by the adsorbed water molecules, it is energetically preferred instead to interact with these more mobile water molecules. This results in an enhancement of the carrier mobility of sodium ion, as its mobility is determined from that point on by the less tightly bound water cluster. In TiONW, sodium ions are thought to be the secondary carriers, but similar behavior could be suggested with protons/ hydronium ions as well. The drop in mobility after the initial increase in MCC tablets11 was explained in this framework,22 on the one hand, by the bonding of Na+ to the oxygen atoms of the surface OH-groups and a less mobile adsorbed water molecules and, on the other hand, by the occupation of the available hopping sites. These effects were suggested to account for the leveling off of the ionic mobility also in TiONW (Figure 3b). A further possibility is that there might be different competing processes, from which one is dominant at low and another one(s) at high coverages, respectively. Our model is not yet suitable to describe the exponential increase of the charge carrier concentration and connect it to the amount, structure, and state of the adsorbed water. Nonetheless, our results strongly suggest that the carrier concentration has the main effect on the moisture-related electrical processes in TiONW (presumably in other humid systems as well); hence the next step in setting up a microscopic model for dc conductivity variation is to find the mechanism of the exponential increase of the charge carrier concentration. Further adsorption and electrodynamic investigations in the much lower RH regime could be that next step in our understanding of these long-range transport processes. Those measurements would allow us to examine the plausibility of the suggested power-law model11,107 mainly at low water coverages, and would help us to reveal the supposed transition from the power-law increase to a linear effective medium theory increase108 in detail.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 36 62 544 620. Fax: 36 62 544 619. E-mail: kakos@ chem.u-szeged.hu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Péter Pusztai for the TEM measurement. Financial support by the TÁ MOP-4.2.1/B-09/1/KONV-2010-0005, “Creating the Center of Excellence at the University of Szeged” program supported by the European Union and cofinanced by the European Regional Development Fund, is acknowledged. Additional support was provided by the OTKA 73676 and NNF2 85899 projects. Á .K. acknowledges support by a HAS János Bolyai research fellowship.

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4. CONCLUSIONS We have analyzed the charge transport mechanism in humid titanate nanowires (TiONW) at room temperature by extracting the variation of dc conductivity, mobility, diffusion coefficient, and charge carrier concentration with the relative humidity of the environment in the 6−100 RH% range. Transient current measurements (ITIC) in combination with dielectric spectroscopy (DRS) are excellent tools to obtain both qualitative and quantitative information about charge transport processes. We have found good correlation in dc conductivity extracted from both techniques, while its constituent parameters provided deeper insight into the underlying mechanism. It seems that changes in charge carrier concentration govern mostly the dc conductivity, while ionic mobility has a less pronounced effect. To unravel which type of 19007



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Water-Induced Charge Transport Processes in Titanate Nanowires: An

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