Letter pubs.acs.org/NanoLett
Wet-Chemical Approaches to Porous Nanowires with Linear, Spiral, and Meshy Topologies Dachi Yang and Luis F. Fonseca* Department of Physics, University of Puerto Rico at Rio Piedras, San Juan, Puerto Rico 00931 S Supporting Information *
ABSTRACT: We report universal approaches for porous nanowires (NWs), and porous NWs with spiral and meshy topologies that have been developed via anodic aluminum oxide (AAO) confined wet-chemical synthesis. Materials such as CuOx, Pd, and Cu NWs are taken as examples for porous NWs and porous NWs with spiral and meshy topologies. Immediate benefits are demonstrated in hydrogen sensors as examples. We observed that hydrogen concentrations as low as 0.2% (v/v) were detected, that critical temperatures of the reverse sensing behavior as low as 239.9 K were measured and that better baseline-stability was confirmed compared with those fabricated with pure Pd NWs. Our approaches are anticipated to work on the synthesis of the porous NWs of other materials that could be obtained via wet-chemistry with potential as candidates for the next generation nanodevices (e.g., gas sensors) and other applications (e.g., catalysts). KEYWORDS: Porous nanowires, wet chemistry, linear, spiral, meshy, hydrogen sensors
P
Here, universal synthesis approaches to porous NWs and porous NWs with spiral (PS-NWs) and meshy (PM-NWs) topologies have been developed by first coelectrodepositing intermediary and main materials inside the channel of AAO template, and subsequently totally or partially dissolving the intermediaries. For the P-NWs, we take two examples: the first system consists on coelectrodepositing WOy (intermediary) and CuOx (main material) where WOy is totally dissolved to form P-CuOx NWs, while in the second one P-PdCu NWs are achieved by direct electrodeposition of Cu (intermediary) and Pd (main material) inside the AAO-channels. For the PS-, and PM-NWs cases, we take Pd and Cu as an example for the synthesis of PS-PdCu NWs and PM-PdCu NWs by first synthesizing NWs with precontoured shapes and then partially etching away Cu. Theoretically, any two materials that are amenable to coelectrodeposition could be used to synthesize porous NWs, and one can easily extend the approaches to other materials. The synthesis of P-NWs includes the coelectrodeposition of the intermediaries and main materials inside the anodic aluminum oxide (AAO) nanochannels and subsequent etching of the intermediaries, as is schematically illustrated in Figure 1a,b. To understand the process, the AAO template was partially dissolved, and then both CuOx and WOy intermediary were seen distributed uniformly in field emission scanning electron microscope (SEM) image Figure S1a of the Supporting Information. By a close-up magnification, the
orous nanowires (P-NWs) that own high surface-tovolume (s/v) ratio have attracted increasing interest. A number of applications with excellent performance such as in ultralow-reflection solar cell,1 high-sensitivity gas sensors,2,3 high-efficiency catalyst,4−6 high-rate-capability lithium ion battery,7 great-performance fuel cell,8 excellent-ability water treatment,9 surface-enhanced Raman scattering10 and high-rate supercapacitors,11 make porous NWs an important investigation. Accordingly, the synthesis of porous NWs with desired materials is an essential step in building nanodevices, and significant efforts have been devoted on synthesis strategies.1−5,7−12 Additionally, nanostructures with novel morphologies such as meshy13 and spiral14−16 shapes have been explored in future nanodevices and nanotechnology in general. However, until today no general approaches have been reported on the synthesis of porous NWs and porous NWs with spiral and meshy shapes, which may have potential in high-performance nanodevices such as new generation gas sensors with enhanced both stability and sensitivity. As an example of application, palladium (Pd)-based hydrogen sensors show baseline instability caused by the volume expansion due to the α−β phase transition.17,18 Recently, temperature-dependent investigations16,19 on both multiple and individual Pd NWs-built hydrogen sensors within 120−370 K temperature range revealed that at certain critical temperature the predominant sensing mechanism changes from the typical bulk response to the percolation-type behavior, which leads to a temperature-instability. From the perspectives of materials, it is reasonable to design NWs with porous topologies to enhance the stability by limiting the α−β phase transition as well as the sensitivity by elevating the s/v ratio. © 2013 American Chemical Society
Received: September 4, 2013 Revised: October 15, 2013 Published: October 31, 2013 5642
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elsewhere.14,20 Otherwise, Pd nanosprings could be obtained after chemical etching.14 As for the reason why we can obtain P-PdCu NWs via one-step deposition, it might be that NH3 in Pd(NH3)Cl2 assists the remaining Pd2+ to be codeposited with Cu2+; at the same time, the applied higher potential leads to hydrogen bubble generation that may contribute to the porous formation.21 For the PS-PdCu NWs cases with the electrolyte and the electrodeposition parameters being further optimized, arrays of PS-PdCu NWs are achieved by the coelectrodeposition of NWs with precontoured shapes and then proper etching, as is schematically illustrated in Figure 2a−c. A bundle of PS-PdCu
Figure 1. Schematic illustration of the synthesis strategies and the characterization of CuOx porous NWs. (a) Co-electrodeposition of CuOx-WOy NWs. (b) CuOx porous NWs after chemically etching away the WOy. (c) The SEM image of the NWs of CuOx and WOy after the AAO template being partially dissolved. (d) The TEM image of high-yield CuOx NWs arrays after totally removal of the WOy. (e) Close-up magnification.
CuOx NWs are observed to be enwrapped by disordered and multiangular WOy and AAO template (Figure 1c). After the total removal of the WOy, a bundle of CuOx NWs with uniform diameter and similar porous shapes are shown in Figure S1b of the Supporting Information, which suggests a high-yield fabrication of P-CuOx NWs with similar geometrical parameters. In order to observe the pore morphology, a close-up transmission electron microscope (TEM) characterization in Figure 1e was done from the bundle (Figure 1d), and the pore distribution are seen both in and around its surface and the pore sizes are estimated between 1−5 nm in diameter. Additionally, the diameter of NWs is measured ∼55 nm, smaller than those of the nanochannels of the AAO template (∼65 nm). The difference in diameter is attributed to some space of the AAO-nanochannels taken by WOy. In our experiments, the intermediaries play crucial role in achieving the P-NWs, and thus the selection of intermediaries is thus of great importance for the coelectrodeposition and subsequent etching. In our cases, WOy and Cu are feasible to be coelectrodeposited. Meanwhile, WOy can be completely dissolved with either acid (i.g., H3PO4) or alkali (i.g., NaOH) and thus porous CuOx NWs are formed without WOy dopants. We did the energy dispersive X-ray spectroscopy analysis (EDS; Figure S2, Supporting Information) to further confirm the composition of CuOx NWs before and after removal of the WOy. It should be pointed out that the porous CuOx NWs show cylindrical shapes rather than spiral and meshy ones obtained according to the following description. On the other hand, if the intermediaries were partially incorporated into the main materials, porous NWs made of intermediary and main materials can be achieved. For example, Cu (intermediary) and Pd (main material) were directly codeposited for P-PdCu NWs by optimizing the electrodepositing parameters and the electrolytes without further etching procedure (Figure S3, Supporting Information). Here we employed the electrolytes of PdCl2 and Pd(NH3)Cl2 with pH 7−8, which is different from the work published
Figure 2. The scheme of the synthesis strategies and the characterization of PS-PdCu NWs. (a) Co-electrodeposition of PdCu NWs with precontoured shapes. (b) Removal of the AAO template. (c) Partially etching away the Cu. (d) The SEM image of a bundle of PSPdCu NWs. (e) Close-up magnification from (d). (f) The TEM image of dual NWs.
NWs in Figure 2d and other SEM images in Figure S4a,b of the Supporting Information indicate that our preparation was largescaled. By the enlargement in Figure 2e, the arrays of PS-PdCu NWs spiral shapes are observed from which the outer diameters are seen at ∼60 nm that corresponds to those of the channels of the utilized AAO template. Additionally, the TEM image in Figure 2f and other TEM images (Figure S4c,d, Supporting Information) further confirm that the NWs are porous and spiral in shape. Moreover, the EDS spectrum (Figure S5, Supporting Information) indicates that the synthesized P-NW is formed by Pd and Cu with the atomic fraction Cu (∼23%) and Pd (77%), respectively. The TEM image (Figure S6, Supporting Information) further confirms a porous and spiral structure. The selected area electron diffraction (SAED) pattern and the resolved crystal planes shown in the high resolution TEM (HR-TEM) image reveal that PS-PdCu NWs are polycrystalline. The formation of PS-PdCu NWs was determined by the initial electrodeposition of porous PdCu NWs with precontoured shapes (Figure S7, Supporting Information). Under acidic solution, H+ ions can be adsorbed onto the AAO channel surface resulting in a compact H+ layer, and Pd2+ and Cl1− combine into [PdCl4]2−; under the interaction between H+ adsorbed on the AAO channel surface and the [PdCl4]2− ions, periodic spiral shapes were formed during electrodeposition,14,20 while Cu2+ is less sensitive and is evenly electrodeposited. Meanwhile, the porous morphology was formed with the hydrogen bubble generation produced due to the 5643
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the etching temperature and duration under certain NW diameter and concentration of the etching solution. Besides, in order to observe the atomic structure of the PM-PdCu NWs, HR-TEM characterization in Figure S9 of the Supporting Information was done, from which the Cu and Pd atomic planes was seen either mixed or separated. In our experiments, the porosity of PM-PdCu NWs are further widened by etching the presynthesized P-NWs using CuCl2 and HCl solution. However, if excessive etching is applied, a collapsed structure can be obtained in Figure S10 of the Supporting Information. Hydrogen sensors built with these new PdCu-based NWs benefit from their porous topologies as demonstrated in the following examples. A representative hydrogen sensor prototype in Figure S11 of the Supporting Information was built with PS-PdCu NWs in which multiple NWs were first integrated in between interdigitated electrodes (IDE) and then the system was wire-bonded to a chip carrier.16,19 Similarly, sensors with Pand PM-PdCu NWs were built. The current versus voltage (I− V) curves of the sensors built with these three types of NWs (Figure S12, Supporting Information) confirm an ohmic contact between the integrated NWs and the electrodes. To investigate the baseline stability, we submitted the sensors to hydrogen absorption−desorption cycles. Meanwhile, we tested the baseline stability by exposing them to air, and then observed the ability of recovery. Figure 4 shows the raw data in current versus time plots of (a) PM-PdCu (0.5% v/v H2 in Ar), (b) P-PdCu (3% v/v H2 in Ar), and (c) PS-PdCu (2% v/v H2 in Ar) sensors. Although the baseline was not fully saturated at the time of data collection in case (a) after exposure to air, we observed that in 420 s the current recovers to normal cycling conditions. Similarly, for case (b) (baseline before saturation) and case (c) (baseline after saturation) they both recovered after exposure to air at 400 and 850 s, respectively. For comparison, a similar sensor with pure Pd NWs was tested the plots (Figure S13, Supporting Information). Although the sensor’s baseline was already saturated and well-stabilized, either over-recovery or under-recovery were shown, in which current did not return to normal cycling conditions. These observations suggest that P-, PS-, and PM-PdCu NWs show better baseline stability than pure Pd NWs do. To evaluate the hydrogen sensors working at low temperatures, we investigated the temperature stability of the sensors built with these new porous NWs by testing the sensors within the 150−370 K temperature range. A temperature-activated bimodal response was observed changing from the increasing resistance mode RH (+) (bulk response) to a decreasing resistance mode RH (−) (percolation mode) with reducing temperature. Figure 5a−c reveals the ΔR versus time plots of
applied potential when palladium and copper are coelectrodeposited.21 Subsequently, Cu was partially etched away from the precontoured porous PdCu NWs using a solution of CuCl2 and HCl, and thus PS-PdCu NWs were achieved. Simultaneously, the preformed pores in and around the NWs surface become widened by the CuCl2 and HCl solution, and thus PS-PdCu NWs with larger pores (Figure S4c,d and Figure S6a, Supporting Information) were obtained. We should point out that the Cu fraction in the PS-PdCu NWs cannot be controlled yet. For the PM-PdCu NWs cases, as is schematically shown in Figure 3a−c via optimizing the technique of chemical etching,
Figure 3. The scheme of the synthesis strategies and the characterization of PM-PdCu NWs. (a) The electrodeposition of P-PdCu NWs. (b) After the removal of the AAO template. (c) PM-PdCu NWs achieved by properly widening the holes of the as-synthesized P-PdCu NWs. (d) The TEM image of a bundle of PM-PdCu NWs. (e) Closeup magnification from (d). (f) The TEM image of one PM-PdCu NW with higher magnification.
the PM-PdCu NWs were achieved by controlled widening of the pores of the P-PdCu NWs (Figure S3, Supporting Information). The TEM image in Figure 3d shows a bundle of PM-PdCu NWs from which the uniform diameter and porous shapes of NWs are observed. By further magnification in Figure 3e,f, the meshy structure is revealed, where the meshlike pores (∼5 nm in diameter, Figure 3f) uniformly distribute along the NW. More SEM and TEM images in Figure S8 of the Supporting Information suggest that the pore size of PM-PdCu NWs can be tuned as desired by controlling
Figure 4. The baseline stability. At atmospheric pressure and temperature of 310.8 K, the current/time plots were obtained for (a) PM-PdCu NWs, 0.16 V, and alternate Ar 100%/0.5% (v/v) hydrogen; (b) P-PdCu NWs, 0.16 V, and alternate Ar 100%/3% (v/v) hydrogen; (c) PS-PdCu NWs, 0.14 V, and alternate Ar 100%/2% (v/v) hydrogen. 5644
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Figure 5. The ΔR response of hydrogen sensors with P-, PS-, and PM-PdCu NWs at atmospheric pressure. The critical temperature of the reverse sensing behavior is (a) 264.2, (b) 257.2, and (c) 239.9 K. The temperature dependence of the response modes: negative values (below Tc) corresponds to the RH (−) mode and positive ones to the RH (+) mode.
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sensors with P-, PS-, and PM-PdCu NWs exposed to 1, 2, and 3% (v/v) hydrogen, respectively. All cases show similar trends with critical temperatures of 264.2, 257.2, and 239.9 K, respectively. For details, the measurements at various temperatures can be found in Figure S14 (P-PdCu NWs), Figure S15 (PS-PdCu NWs), and Figure S16 (PM-PdCu NWs) of the Supporting Information, respectively. These values are significantly lower than what were observed in Pd NWs-based sensors (287 K),19 which suggests that P-, PS-, and PM-PdCu NWs have better temperature-stability than pure Pd NWs sensors. Additionally, we should mention that our sensors were tested with an applied voltage of 0.1−0.2 V, and hydrogen concentrations as low as 0.2% (v/v) can be detected (Supporting Information Figures S14c, S15c, and S16b), as suggest higher sensitivity and lower power consumption. Our previous research disclosed that the reverse sensing behavior is attributed to a competition between the increasing resistivity of the resulting hydride and the formation of percolation paths due to the volumetric expansion that becomes more important at lower temperatures.19 The lowering of the critical temperatures confirms more stable sensors especially when working below room temperature. It has been reported that silver atoms doping of Pd can reduce or prevent the α−β phase transition of PdHx.22 Similarly, in our cases the Cu atoms doping Pd NWs also contribute to limit the phase transition and thus enhance the baseline and temperature stability. Moreover, the porous morphologies allow the expansion of Pd with reduced damage and cracks generation during the hydrogen absorption−desorption cycles thus bringing better stability to the sensor In summary, porous NWs and porous NWs with spiral and meshy topologies have been synthesized following universal wet-chemical approaches by first coelectrochemically depositing the intermediary and main materials and then etching away the intermediary species. The case of CuOx NWs, here presented in detail, might be used to synthesize porous NWs with materials such as metal oxide semiconductors by properly selecting the intermediaries that can be obtained via coelectrodeposition. From the PdCu NW cases, Pd and other metal with porous and linear, porous and spiral, and porous and meshy topologies might be synthesized. All the hydrogen sensor prototypes built with P-, PS-, and PM-PdCu NWs showed higher stability than those built with pure Pd NWs. It is recommended that further research on P-, PS-, and PM-PdCu NWs be undertaken in catalysis applications working within a wide-temperature range.
ASSOCIATED CONTENT
S Supporting Information *
The experimental details, the SEM and TEM images, and EDS spectrum of porous CuOx NWs, the TEM images of the PPdCu NWs, the TEM images of the PS-PdCu NWs, the SEM image and corresponding EDS spectrum of PS-PdCu NWs, the HR-TEM image of the PS-PdCu NWs, the SEM images of PSPdCu NWs before and after etching, the SEM and TEM images of PM-PdCu NWs before and after etching, the HR-TEM image of PM-PdCu NWs, the TEM image of PM-PdCu NWs after excessive etching, a representative prototype of the hydrogen sensors built with multiple PS-PdCu NWs, the I−V curves of the P-, PS- and PM-PdCu NWs integrated into IDEs, the baseline stability of multiple pure Pd NWs, the detailed ΔR response plots of the hydrogen sensor with P-PdCu NWs, the detailed ΔR response plots of the hydrogen sensor with PSPdCu NWs, and the detailed ΔR response plots of the hydrogen sensor with PM-PdCu NWs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
D.Y. conceived the synthesis experiments, carried out microstructural analysis, and the hydrogen sensing test under diverse temperatures. L.F. contributed to the design of the sensor testing setup. D.Y. and L.F. wrote the paper. Both authors contributed to the analysis of this manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Gary W. Hunter (NASA GRC) for helpful discussions and for providing the IDE electrodes. We thank Miss Jennifer Carpena-Núñez for the help on TEM characterization and wire-bonding, Mr. Luis Valentiń for the use of the sensor test facilities, and Mr. Oscar Resto for the HR-TEM images. This work was supported by NASA URC (Grant NNX08BA48A). The use of the UPR Nanoscopy Facility at UPR (NSF Grant 1002410) is acknowledged.
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NOTE ADDED AFTER ASAP PUBLICATION This article was published on the web on November 4, 2013. Figures 2 and 3 have been modified. The correct version was published on November 5, 2013.
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