Mariam N. Al-Hinai†
ac,
Reda Hassanienb,
Nicholas G. Wrightc,
Alton B. Horsfallc,
Andrew Houltona and
Benjamin R. Horrocks*a
aChemical Nanoscience Laboratory, School of Chemistry, Newcastle University, Bedson Building, NE1 7RU, UK. E-mail: b.r.horrocks@ncl.ac.uk; Tel: +44(0)191 2225619
bDepartment of Science and Mathematics, Faculty of Education, Assiut University, New Valley Branch, El-Kharja 72511, Egypt
cSchool of Electrical and Electronic Engineering, Newcastle University, Merz Court, NE1 7RU, UK
First published on 4th March 2013
Electroless templating on DNA is established as a means to prepare high aspect ratio nanowires via aqueous reactions at room temperature. In this report we show how Pd nanowires with extremely small grain sizes (<2 nm) can be prepared by reduction of PdCl42− in the presence of λ-DNA. In AFM images the wires are smooth and uniform in appearance, but the grain size estimated by the Scherrer treatment of line broadening in X-ray diffraction is less than the diameter of the wires from AFM (of order 10 nm). Electrical characterisation of single nanowires by conductive AFM shows ohmic behaviour, but with high contact resistances and a resistivity (∼10−2 Ω cm) much higher than the bulk value for Pd metal (∼10−5 Ω cm @20 °C). These observations can be accounted for by a model of the nanowire growth mechanism which naturally leads to the formation of a granular metal. Using a simple combing technique with control of the surface hydrophilicity, DNA-templated Pd nanowires have also been prepared as networks on an Si/SiO2 substrate. These networks are highly convenient for the preparation of two-terminal electronic sensors for the detection of hydrogen gas. The response of these hydrogen sensors is presented and a model of the sensor response in terms of the diffusion of hydrogen into the nanowires is described. The granular structure of the nanowires makes them relatively poor conductors, but they retain a useful sensitivity to hydrogen gas.
It is now widely recognised that hydrogen gas sensors are essential for safety reasons in any widespread use of hydrogen as a low-carbon energy source.34,35 Hydrogen is highly flammable and becomes explosive when its concentration exceeds 4% in air.36 The sensors are required to be rapid, sensitive, reliable, cheap, simple to operate and to have high selectivity. The required selectivity can be obtained by using the ability of palladium to sorb hydrogen gas.37 Electronic sensors have advantages over optical and other devices with respect to the simplicity of the sensor structures and compatibility of the sensor fabrication process with miniaturization and the current IC process.38 However, bulk Pd-based hydrogen sensors suffer from fundamental problems including long diffusion-limited response times and irreversible resistance changes resulting from the large internal stress caused by the expansion that occurs upon excess hydrogen absorption.39 Sensors have therefore been designed using various Pd-based nanomaterials, including nanowires, as transducers.40–54 Interest in nanowires for use in sensor technology has grown and therefore Pd/DNA nanowires may be attractive for hydrogen sensing. In particular, DNA-templated nanowires offer a simple and cost effective sensor fabrication process. In this report we describe the chemical and structural characterisation of Pd/DNA nanowires, their electrical behaviour and discuss the response of a Pd/DNA nanowire network as a hydrogen sensor.
Our standard preparation of Pd/DNA nanowires:
10 μL of freshly-prepared K2PdCl4(aq) (3 mM) was added to 20 μL of λ-DNA (500 ng μL−1) in the presence of 5 μL sodium citrate (aq) (0.5 mM). The Pd(II) ions were reduced to Pd(0) metal by addition of 5 μL aqueous dimethylaminoborane (DMAB, 10 mM) or NaBH4 (10 mM). The solution was thoroughly mixed and allowed to react at room temperature for 2–24 h as indicated in the results section. 24 h reaction times were required to form smooth continuous nanowires. Use of NaBH4 or DMAB as reducing agent produced similar nanowires; some differences between the two are noted in the electrical characterisation below.
Slightly different preparation methods were used for UV-Vis spectroscopy and XRD analysis because these techniques require larger samples. CT-DNA was employed for reasons of cost.
For UV-Vis experiments:
2 mL aqueous CT-DNA (162.5 ng μL−1) was mixed with 1 mL freshly-prepared K2PdCl4 (aq) (3 mM) in the presence of 0.5 mL sodium citrate (aq) (0.5 mM). 0.5 mL of aqueous dimethylaminoborane (DMAB, 10 mM) was added dropwise to the pale yellow solution.
For XRD experiments:
Half the quantities employed in the UV-Vis experiment were used. A black precipitate of Pd/DNA was formed and was collected by filtration and washed thoroughly with hot ethanol.
Scanning conductance microscopy (SCM) measurements were carried out in air on the same Dimension Nanoscope V system using MESP probes (n-doped Si cantilevers, with a metallic Co/Cr coating, Veeco Instruments Inc., Metrology Group), with a resonant frequency of ca. 70 kHz, a quality factor of 200–260 and a spring constant of 1–5 N m−1. In SCM experiments, a bias voltage is applied between the tip and substrate. No dc current flows because the substrate is coated with a dielectric, but an electrostatic field is produced between the tip and the substrate. The phase angle between the tip motion and the driving force is related to the force gradient at the tip and is sensitive to the conductance of samples resting on the dielectric as well as their polarizability. The reported SCM phase images show the phase of the tip oscillation at a set lift height above the substrate surface (10–100 nm typ.). The substrates comprised p++Si(100) chips with a thermally grown oxide layer (240 nm) as determined by a spectrometric thin film analyzer (Filmetrics F40).
For conductive AFM (c-AFM) measurements, a constant bias was also applied between the tip and the sample (the tip was grounded). We have previously shown that DNA-templated nanowires facilitate a simple means of making contacts to individual nanowires.4 Briefly, a large amount of nanowires are deposited on the substrate by drop-casting; as the droplet dries, individual nanowires protrude from the main mass of nanowires and are aligned by the receding meniscus. Individual nanowires can be easily imaged by AFM at the edge of the dried mass. Electrical contact to the mass of nanowires was made by applying a drop of In/Ga eutectic to one corner of the chip and to the metallic chuck. c-AFM imaging was performed in contact mode, with an applied bias of 0.5 V. The imaged area was about 1 mm away from the In/Ga contact. The closed loop system of the Dimension V instrument makes it possible to reproducibly position the tip at a point of interest identified in the image of a single Pd/DNA nanowire and to record current–voltage (I–V) curves at that point. The conductance was estimated using the slope of the I–V curve at zero bias.
1. Solid sites are allowed a chance kdiss to dissolve into an empty neighbouring site to form a metal ion at that site.
2. Dissolved metal ions were allowed a chance kdep to deposit and become a new solid site if a neighbouring site was solid.
3. Dissolved metal ions simply move to the neighbouring site if it is empty.
The values of kdiss and kdep were calculated according to the number of solid neighbours of a given site (n). If a site has w solid neighbours, then dissolution was assumed to require the breaking of additional bonds and,
kdiss = k0disse−wn | (1) |
On the other hand, deposition was favoured by the additional bonds formed and kdep is given by:
kdep = k0depewn | (2) |
k0diss, k0dep and w are parameters input to the simulation. This feature of the model is important to allow for the possibility of a surface tension which is an important consideration for the growth process.33 If w = 0, then the number of bonds between sites has no effect on the growth and surface tension effects are absent from the model. The initial state of the system was taken to be hemispherical nuclei of radius 20 sites on the x-axis, representing the DNA template. The simulations were run for 106–107 time steps.
Wavenumber (cm−1) in λ-DNA | Wavenumber (cm−1) in Pd/DNA | Assignment |
---|---|---|
1705 | 1662 | guanine ring |
1409 | 1395 | N–H deformation |
1060 (broad) | 1074 | P–O or C–O backbone stretch |
1112 | PO−2 symmetric stretch | |
1235 | 1239 | asymmetric PO−2 stretch |
Fig. 1 shows the fingerprint region of the λ-DNA infrared spectrum. The most intense feature is a broad unresolved band due to PO2− symmetric stretch modes and P–O or C–O stretches of the phosphate backbone and sugar centered near 1060 cm−1. After Pd-templating, this feature is reduced in intensity, shifted to higher energy and two components are resolved; one at 1112 cm−1 which we assign to the symmetric phosphate mode and another at 1074 cm−1 due to the C–O stretching modes. The other significant changes are in the asymmetric PO2− stretch near 1235 cm−1 (decreased intensity, shifted to 1239 cm−1), the N–H deformation near 1400 cm−1 (shifted to 1395 cm−1) and the modes associated with the nucleobases in the range 1500–1700 cm−1. In particular, the guanine mode at 1705 cm−1 increases in intensity and shifts to 1662 cm−1 after metallisation. Taken together, these changes indicate that there is a strong interaction of the templated material with both the nucleobases and the phosphate backbone.
Fig. 1 FTIR spectrum of Pd/DNA nanowires (blue) and FTIR spectrum of bare λ-DNA (black). 128 scans co-added and averaged, 4 cm−1 resolution. |
UV-Vis absorption spectra were employed to monitor the reduction of Pd(II) ions and the formation of metallic Pd(0) upon addition of the reducing agent. Electronic spectra of metal nanowires show absorption bands due to the excitation of the plasmon resonance. Noble metals that give a distinct plasmon peak, such as Au,56 Ag57 and Cu58 are relatively easy to examine by optical spectroscopy. For Pd, the overlap of the DNA and bands related to the reducing agent and Pd(II) complicates the spectra. Fig. 2 shows the UV-Vis absorption spectra of the Pd/DNA solution at different stages of the template synthesis process. The aqueous calf-thymus (CT-DNA) solution exhibits the characteristic nucleic acid absorption band at 260 nm (curve 1); upon complexation with Pd(II) ions, the dominant absorption maximum shifted to about 300 nm (curve 3). The formation of a Pd(II)-DNA complex was supported by the red-shift of the Pd(II) band to 400 nm from the value observed in aqueous PdCl42− of 390 nm (curve 2). After DMAB reduction to produce Pd metal, the 300 nm peak for the Pd(II)-DNA complex was narrowed, blue-shifted to 290 nm and the 400 nm Pd(II) peak disappeared. A long absorption tail, decreasing with wavelength in the range 300–600 nm, remains (curve 4). The shape of this absorption is consistent with metallic Pd nanostructures.59,60
Fig. 2 UV-Vis absorption spectra at different stages of the synthesis process: absorption spectra of CT-DNA (curve 1); aqueous K2PdCl4 solution (curve 2); the Pd(II)-DNA complex before DMAB treatment (curve 3) and the Pd/DNA solution after DMAB reduction (curve 4) showing the loss of the Pd(II) band near 400 nm. |
A high-resolution transmission electron microscopy (HRTEM) image and the EDS spectrum of a network of DNA/Pd nanowires is shown in Fig. 3. The EDS spectrum consisted mainly of Pd, C, Cu and P peaks. Cu peaks came from the carbon-coated Cu TEM grid, the Pd peak from nanowires, the C peak came from the TEM grid as well as DNA and the small P peak came from the DNA. The EDS analysis confirms directly the presence of Pd on the DNA template.
Fig. 3 (a) HRTEM image of Pd/DNA nanowire network on a carbon-coated Cu grid and (b) the EDS spectrum at the point “x” in the image corresponding to a nanowire. |
Fig. 4 shows the X-ray powder diffraction pattern for a sample of Pd/DNA nanowires. Peaks due to reflections from the (111), (200), (220) and (311) planes of metallic Pd at 2θ values of 39°, 46°, 68° and 82° were observed; these are consistent with a previous report.59 The peaks are very broad and, upon fitting a pseudo-Voigt function to the most intense diffraction peak at 2θ = 39°, we estimate a crystallite diameter of 1.6 nm from the Scherrer equation. No peaks attributable to Pd oxides were observed, which rules out the presence of crystalline oxides.
Fig. 4 (a) X-ray diffraction pattern of Pd–DNA powder; the diffraction peaks originate from the (111), (200), (220) and (311) planes of metallic Pd at 2θ values of 39°, 46°, 68° and 82°. (b) pseudo-Voigt functions fitted to the peaks at 39° and 46°. The crystallite size was estimated using the Scherrer equation as 1.6 nm. |
XPS survey spectra indicated the presence of C, N, O, P (from the DNA template), Na (from the buffer or NaBH4 reductant) and Pd and Cl (from PdCl42−). Fig. 5 shows the Pd 3d spectrum. Peaks for both Pd(0) and Pd(II) oxidation states were observed. The doublet at 335.1 eV and 340.4 eV is assigned to Pd(0) and that at 336.4 eV and 341.7 eV to Pd(II) following previous reports.59,61 The feature at 346.3 eV is most likely a plasmon loss band associated with the peak at 335.1 eV. The plasmon energy of 11.2 eV is larger than the 7 eV expected for a surface plasmon and smaller than the bulk plasmon energy of 14 eV,62,63 but may be an effect of the small particle size. The Pd(II) state observed by XPS, but not XRD is interpreted as an amorphous oxyhydroxide coating the metal crystallite surfaces. We suggest this results from aquation of the PdCl42− followed by hydrolysis. PdCl42− is known to undergo exchange of Cl− ligands for H2O in non-complexing acidic solutions, the pH of λ-DNA is buffered at about 8, but some aquation of PdCl42− is still possible.64 Pd(OH2)42+ is unstable to hydrolysis at pH values above the pKa = 2.3.65
Fig. 5 The Pd 3d XPS spectrum of Pd/DNA nanowires. The experimental data are the black symbols and the components making up the theoretical fit are shown as solid lines. |
Fig. 6 AFM images of Pd/DNA nanowires. (a) AFM image of a single Pd/DNA nanowire (scale bar = 1.5 μm); (b) Section of the nanowire in (a) at the indicated points showing the height difference between the Pd/DNA nanowire (≃23 nm) and the bare DNA strand (2 nm). (c) Height distribution of 90 Pd/DNA nanowires, the heights were determined from AFM images. (d) Discontinuous nanowires aligned after a reaction time of 2 h. The scale bar is 1 μm and the height (colour) scale is 30 nm. |
Fig. 7 Schematic representation of the structure of the prepared DNA/Pd nanowires. The Pd crystallite size (≃1.6 nm) is substantially smaller than the nanowire heights observed by AFM (5–45 nm). |
In order to rationalise the AFM and XRD results, we carried out lattice gas simulations of the growth process. The initial configuration was assumed to be a series of spherical nuclei on the DNA template (Fig. 8a). The solid sites are shown in yellow (colour scale values > 1) and the solution sites are shown as purple-black (colour scale values < 1). The solution starts supersaturated with respect to the solid, i.e., where c is input to the simulation as a parameter, along with rate constants for dissolution and deposition of the solid, and c∞ is the concentration of dissolved metal ions at equilibrium. The parameter w determines the additional barrier to dissolution because a site has bonds to its nearest neighbours via eqn (1) and for deposition via eqn (2). This leads to the solid having a bulk surface tension which acts to minimize the area of its surface. The supersaturation can be related to the rate constants, after taking appropriate macroscopic averages, by . We therefore ran simulations with different values of k0diss, as defined in eqn (1), in order to study the effect of the driving force on the templating reaction. Unlike other materials we have studied, Pd deposition is expected to be much less reversible and therefore is modelled by choosing a larger driving force for deposition, i.e., a lower value for k0diss. Fig. 8b,d shows after 104 steps of the simulation the nuclei have grown; the growth is dominated by deposition of atoms at the step edges because of the large value of w chosen, and the crystallites are no longer spherical. When k0diss = 0.01 the crystallites are still separated from each other, but for k0diss = 0.1 there is already substantial overlap of neighbouring crystallites. As the growth continues, the overlap of neighbouring crystallites by dissolution and reprecipitation results in the appearance of a continuous nanowire (Fig. 8e) whose surface roughness has previously been modelled by a linear thermodynamic treatment.33 However, when k0diss = 0.01, events leading to the coalescence of neighbouring crystallites are rare and, though the external surface of the nanowire may appear very similar, there remain internal voids even at long simulation times. Such a simulation cannot model all the details of the actual Pd deposition, but it does indicate that when the deposition process is irreversible, a complex internal structure is a natural consequence. In the actual chemical system, additional features related to the presence of grain boundaries, deposition of oxide and the possibility of further nucleation events will naturally produce a granular morphology as in Fig. 7.
Fig. 8 Lattice gas simulations of the growth of Pd/DNA nanowires on a square lattice of 2000 × 200 sites. The template lies along the x-axis in each figure. The common parameters are w = 0.5, concentration of dissolved metal ions, c = 0.5 (per site) and k0dep = 0.01. (a) t = 0; (b) k0diss = 0.01 and t = 104 time-steps; (c) k0diss = 0.01 and t = 107 time-steps; (d) k0diss = 0.1 and t = 104 time-steps and (e) k0diss = 0.1 and t = 107 time-steps. |
Fig. 9 Electrical characterisation of Pd/DNA nanowires by scanned conductance microscopy. (a) AFM height image of Pd/DNA nanowires (the data scale is 25 nm). (b) and (c) the corresponding SCM phase images of the same nanowires with applied bias voltages of +6 V and −6 V respectively (the data scale is 3°). (d) Phase shift versus applied voltage for a single Pd/DNA nanowire (20 nm diameter) at a lift height of 60 nm aligned on 240 nm thick SiO2 on highly doped Si. (e) A quadratic was fitted to the plots of phase shift against bias voltage. The coefficients of V2 are plotted against the nanowire diameter at a constant lift height of 60 nm. |
It should be noted that, although the majority of the tested nanowires are conductive, the measurements also revealed the existence of non-conductive nanowires in the sample. These findings are consistent with the results of the XPS experiments which indicate the presence of oxide in the sample.
Quantitative measurements of Pd/DNA nanowire conductance were obtained by depositing nanowires on chemically oxidised Si(100) substrates to measure I–V curves using c-AFM following a previously developed method.4 The conductive AFM tip serves as one contact to a nanowire which protrudes from a dense network of nanowires to which an In/Ga eutectic is used to complete the circuit. Fig. 10 shows an SEM image of individual Pd/DNA nanowires stretched out from a dense mass of nanowires deposited from a dried droplet and the I–V curve of a single nanowire selected at the edge of such a dense mass. The dense mass of nanowires serves as the second contact to the nanowire. This technique allows the resistance of a single nanowire and the contacts to be separately determined; a series of I–V curves at different points on the nanowire are recorded and the resistance is then plotted as a function of distance along the nanowire. The conductivity is obtained from the slope of this plot and the measured height; the contact resistance is estimated by extrapolating the plot to zero distance.
Fig. 10 c-AFM measurements of Pd/DNA nanowires. (a) Electron micrograph of the dense mass of Pd/DNA nanowires (lower third of image); single Pd/DNA nanowires are seen protruding from the mass (scale bar = 20 μm). (b) Current–voltage curve with the tip contacting a single nanowire (inset = cAFM current image, data scale = 500 nA, tip/sample bias = 5 V, scale bar = 1 μm). |
Fig. 11 shows the plot of resistance against distance. The slope of the regression lines gives the nanowire resistance per unit length as (5.1 ± 0.2 × 108 Ω cm−1). Using the diameter of the nanowire observed in the contact mode image (≃20 nm), the conductivity was determined as 1.6 × 104 S m−1. This is the largest value of conductivity we observed for single Pd/DNA nanowires. This value is lower than both the bulk conductivity of Pd (9.5 × 106 S m−1)68 and a previous report of a 50 nm diameter Pd/DNA nanowire of 2 × 106 S m−1.14 A conductivity of 3.33 × 105 S m−1 was also reported for a nanowire prepared by reduction of PdO (height 30 nm and length 450 nm61). The conductance of thin nanowires fabricated on DNA templates via Pd(II) reduction have also been reported; the average resistance of a single wire was 800 GΩ at 10 V.68 Our c-AFM measurements on various nanowires revealed resistance values in the range of 0.4–0.8 GΩ for the DNA/Pd nanowires prepared by the reduction of Pd(II) ions with DMAB and in the range of 2–8 MΩ for the nanowires prepared using NaBH4 as a reducing agent.
Fig. 11 cAFM measurements of resistance at different points along a single Pd/DNA nanowire. The distance is that between the point at which the tip contacts the nanowire and the dense mass shown in Fig. 10. The nanowire resistance per unit length is the slope of the plot and the intercept at zero distance is the contact resistance. Measurements obtained at different setpoints (tip/sample force) are shown in different colours. |
Fig. 11 also shows the effect of increasing the tip/nanowire contact force by increasing the set point voltage from 0 to 3 V. A clear decrease in the intercept on the resistance axis is observed at higher forces, which is expected if the tip/nanowire junction is the dominant contact resistance. This is expected based on considerations of the area and previous estimates of the resistance at the second contact of 50–200 Ω.24 It is also clear that the contact resistance is very substantial and dominates the raw I–V data.
cAFM is inconvenient for temperature-dependent I–V measurement, therefore two-terminal Au microelectrode devices were fabricated in order to study this aspect. The DNA/Pd nanowires were transferred by the method of molecular combing onto clean Si/SiO2 substrates in the gap between microfabricated Au electrodes. In contrast to the case of polymer nanowires,4 the large contact resistance observed with Pd/DNA samples makes the I–V measurements of single Pd/DNA nanowires by two-terminal measurements difficult and non-linear characteristics were often observed (diode-like characteristics with a turn-on voltage of ≃0.6 V). Treatment of the single nanowires with reducing agents (gaseous hydrogen) did not reliably improve the behaviour. Instead we found that omitting the silanization of the SiO2 resulted in networks of Pd/DNA nanowires with consistent linear I–V characteristics (Fig. 12). Such nanowire networks have the advantage of having multiple routes for current flow within the nanowires and between the nanowire network and the contact pads. The networks exhibited linear I–V curves with measured currents in the nA range compared to the leakage current of the electrodes alone (≃100 fA). The conductance at zero bias increased with temperature over the range 233–373 K, which is opposite to the expected behaviour of a metal. However, similar observations68 have been made and attributed to a conduction mechanism based on the concept of a granular metal. Such behaviour seems natural given the mechanism of growth discussed above and the small grain size observed.
Fig. 12 (a) Two-terminal microelectrode current–voltage curves for a network of Pd/DNA nanowires at temperatures from 233–373 K. (Inset: An AFM height image of the network, the data scale is 20 nm, scale bar = 500 nm). (b) The zero bias conductances, G, from (a) were divided by mean cross-sectional area, A, and ln(G/A) was plotted against T−1/2 to test the Efros–Shklovskii law. |
The single Pd/DNA nanowires also showed increased currents at higher temperatures; the linear portion of the data above the turn-on voltage could be used to make a plot of lnG versus 1/T from which an activation energy of 0.35 eV was derived, but the interpretation of this value is not straightforward. The zero bias conductance of the network was simpler to analyse and the behaviour for a granular metal under conditions where the grains are separated by a tunneling barrier was obtained (Fig. 12). The regression line shows the fit of the Efros–Shklovskii law,69,70 , to the data and T0 = 1850 ± 120 K or in terms of an energy, 0.16 ± 0.01 eV.
Fig. 13 (a) Fractional change in network resistance to pulses of 1700 ppm H2 and pure N2 at temperatures from 330 K to 400 K. (b) Fractional change in resistance and response time against hydrogen concentration at 330 K. Response time is defined as the time to achieve 90% of the steady-state resistance change. |
The device signal, defined as , is clearly reversible and the time for 90% of the signal to be achieved was about 85 s at 2300 ppm hydrogen. Recovery was slightly faster with a 90% recovery time of ≃60 s. The sensitivity is comparable to previous devices,47,50 although the response times are rather slow by comparison with electrodeposited Pd nanowires47 which have larger crystallite sizes. The maximum sensitivity of the sensor is increased from 0.09 to 0.26 with increasing temperature from 330 K to 400 K with good repeatability. Interestingly, this is in contrast to previous reports in which sensor response was observed to decrease with increasing temperature from 323 to 373 K,46 although it should be noted that it has recently been shown that even the sign of the response can change with temperature and that there may be competing effects such as hydride formation and swelling of the material in a single device.75
The response of the network to hydrogen was modelled as that of an ensemble of identical nanowires. The sorption of hydrogen in the nanowires was treated as a standard diffusion problem with cylindrical symmetry:
(3) |
The boundary and initial conditions are C(1,T) = 1; ; and C(R,0) = 0 for 0 ≤ R < 1. The dimensionless groups are defined as C = c(r)/c*; R = r/a and T = Dt/a2. D is the diffusion coefficient of hydrogen in the Pd/DNA nanowire, a is the radius of the nanowire and c* is the concentration of hydrogen at the nanowire surface. r is the radial coordinate and t is the time. The term vanishes for a nanowire which is uniform along its length. We also assume that the process is limited by diffusion in the cylindrical nanowire and not by any barrier at the surface. The radial variation of the dimensionless concentration is given by
(4) |
σ(r) = ΔσHC(R) + σ0 | (5) |
The fractional change in conductance (G) of the nanowire can then be expressed in terms of the integral of 2πrσ in eqn (5) from R = 0 to R = 1:
(6) |
Substituting eqn (4) into eqn (6) gives the final result which can be used to model the data for conductance or resistance changes upon exposure to hydrogen:
(7) |
The system is linear, so the corresponding equation for the diffusion of hydrogen out of the nanowires upon exposure to nitrogen is simply:
(8) |
Eqn (7) and (8) were used as the regression models for the experimental data at each measured temperature (Fig. 14). The sums were truncated after λ20 which was found to give sufficient precision. We estimated the hydrogen atom diffusion coefficient in Pd/DNA at 330 K as 1.4 ± 0.1 × 10−14 cm2 s−1. This is much smaller than the reported diffusion coefficient in bulk Pd of 3.8 × 10−7 cm2 s−1 at room temperature37 and that obtained in a recent electrochemical study of 3.2 × 10−7 cm2 s−1.77 Previous reports also revealed that the hydrogen diffusion coefficient values in Pd thin films (50 nm to 1.34 μm thick) are 2–3 orders of magnitude smaller than of the bulk Pd at 298 K73 and in Pd thin films with thickness of 22.5 nm at 330 K which were reported as 6.9 × 10−11 cm2 s−1.74 Other workers have studied H/D exchange in Pd using molecular beams; the exchange takes place on a timescale of order 10 s for 7 nm diameter Pd nanoparticles at 280 K.78 Our results are therefore not unprecedented for Pd nanostructures. The Pd/DNA nanowires here consist of small crystallites and therefore the low diffusion coefficient may be caused by grain boundary traps.74 We did observe a small difference between response and recovery, but with the recovery process slightly faster (Fig. 15). This suggests that diffusion is not the sole process limiting the response rate.
Fig. 14 Change in nanowire conductance as a fraction of the steady-state value (ΔGmax). The temperature was 330 K, the mean nanowire radius was 20 nm and the hydrogen diffusion coefficient determined using eqn (7) as a regression model was 1.4 ± 0.1 × 10−14 cm2 s−1. |
Fig. 15 The change in diffusion coefficients obtained from the sensor response with increasing [H2] at 330 K. |
The diffusion coefficient is plotted as a function of hydrogen concentration at 330 K (Fig. 15). The results show that DH is roughly constant (∼1.4 × 10−14 cm2 s−1) over the range 600–1800 ppm but decreases to 0.85 × 10−14 cm2 s−1 when [H2] increased to 2300 ppm. This may indicate the onset of the α to β phase transition in Pd which has been reported to occur at concentrations of order 104 ppm.50
Footnote |
† Current address: Department of Engineering, Sohar College of Applied Science, Oman |
This journal is © The Royal Society of Chemistry 2013 |