Proton Conductivity (2)DOI: 10.1002/anie.201205225

Reply: High Proton Conductivity of Water Channels ina Highly Ordered NanowireNoah Johnson und Hai-Feng Ji*

conductivity · hydrogen bonds · nanowires ·proton transport · whiskers

Our group recently published a Communication in thisjournal, in which we discussed our finding of a trimesic acid/melamine nano/microwire with a proton conductivity of5.5 Scm�1.[1] It has since been questioned how much of thismeasured conductivity can be attributed to the nano/mi-crowire complex. Recently, we sent several old devices toKreuer and Wohlfarth for impedance testing.[2] Since Dr.Wang, who had performed the original experiments, had leftthe group, we picked several devices out of a number of de-vices prepared by him. However, we could not identify theone that was used for the experiments reported in the above-mentioned publication. These old devices, along with devicesprepared from Kreuer and Wohlfahrt, did not show the highconductivity we had demonstrated in our Communication andthe results are shown in Kreuer�s Correspondence in this is-sue.

The difference may be due to the unique difficulty ofmeasuring the conductivity of this system. The nature of oursupramolecular crystal means that the wires are held togetherby p–p interactions and hydrogen bonds; there are no cova-lent bonds keeping the system intact. This makes the wiresvery fragile, and attempts to move them from their originalsubstrate will very likely fracture the crystal, resulting in a nullconductivity measurement. On the other hand, as we havediscovered, measuring on their original substrate can result inundesired parallel current flow, which will exaggerate themeasured conductivity. We undertook conductivity measu-rements to attempt to correct this issue. The goal was tocompare the conductivity change of a substrate against ourmeasurement area (Figure 1). While some parasitic currentwas expected, we were attempting to determine whether itwas significant. Under atmospheric conditions, the conduc-tivity of the clean polyethylene substrate was found to bebelow the noise limit of our instrument, measuring less than1 nA of current between �5 and 5 V. While the wire showedsome small conductivity, it was still very minor. Comparingthe same two areas in 100 % saturated air showed a change inconductivity for both (Figure 2). However, while the currentthrough the substrate rose by two orders of magnitude, thecurrent through the wire increased by more than six orders of

magnitude. From this we can conclude that while there isindeed a parallel current which is inflating the calculatedconductivity, the sample on its own is still proton conductive.

However, the performance of the the single wire devicesstill varies from device to device, depending on the quality ofthe nanowires. Furthermore, the heat of thermal evaporationor sputtering can potentially damage the wires, while singlewire samples tend to fracture in transport, giving complex ornull results.

For this reason, we prepared a network of nanostructuresto introduce redundancy into the system. The two new deviceswere made of a network of nano/microwires on a plasticsurface and were sent to Kreuer and Wohlfarth, who didimpedance measurements. Contrary to the expectation on

Figure 1. Comparison of conductivity of a clean polyethylene substrateto a microwire sample under atmospheric conditions.

Figure 2. Comparison of conductivity of a clean polyethylene substrateto a microwire sample under 100% relative humidity (RH).

[*] N. Johnson, Prof. Dr. H. JiDepartment of Chemistry, Drexel UniversityPhiladelphia, PA 19104 (USA)E-Mail: hj56@drexel.edu

.AngewandteKorrespondenz

2 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 2 – 4� �

These are not the final page numbers!

a consistent conductivity of the nanowire network, the twosamples also showed significantly different results (Figure 3from Kreuer and Wohlfahrt): one showed 10 000 fold con-ductivity enhancement from dry to 100 % RH conditions,another only showed less than 10 fold enhancement. Theirresults on our new samples confirmed that the system isconductive, however, the exact magnitude of the conductivityis difficult to determine. It is noteworthy that the conductivityis not occurring through the surface of the plastic, as theplastic is neither proton nor electron conductive.

In summary, we would like to amend our statement thatthe trimesic acid/melamine nanowires have the highest protonconductivity to date, with a measured conductivity of5.5 Scm�1. Based on all the experiments Kreuer and Wohl-fahrt, and our group have conducted, we conclude that thequality of the nanowire is crucial for its high proton conduc-tivity. While the complexities of the measurement make exactdetermination difficult, the nanowires are capable of trans-

porting protons. The nanowires seem to be prone to defects,with conductivities drastically changing depending on nano-wire quality, sample preparation, and measurement condi-tions. Our future goal is to solve the problems related to thestability/fragility of the system in order to obtain a re-producible result for practical use of this new family of pro-ton-conductive material.

Eingegangen am 3. Juli 2012Online verçffentlicht am &&. && &&&&

[1] H. Wang, X. Xu, N. M. Johnson, N. K. R. Dandala, H.-F. Ji, An-gew. Chem. 2011, 123, 12746 – 12749; Angew. Chem. Int. Ed. 2011,50, 12538 – 12541.

[2] K.-D. Kreuer, A. Wohlfarth, Angew. Chem. 2012, DOI: 10.1002/ange.201203887; Angew. Chem. Int. Ed. DOI: 10.1002/anie.201203887

Figure 3. Impedance measurement of a trimesic acid/melamine nanostructured network. Insets show magnified view of the connection (left) witha macroscale view of the device (right).

AngewandteChemie

3Angew. Chem. 2012, 124, 2 – 4 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.de

These are not the final page numbers! � �

Korrespondenz

Protonenleitf�higkeit (2)

N. Johnson, H. Ji* &&&&—&&&&

Reply: High Proton Conductivity of WaterChannels in a Highly Ordered Nanowire

Nanodr�hte aus Trimesins�ure und Mel-amin sind nachweislich protonenleitend,das Ausmaß der Leitf�higkeit h�ngtjedoch stark von der Qualit�t der Nano-dr�hte, der Probenvorbereitung und denMessbedingungen ab. Diese Problemem�ssen gelçst werden, um reproduzier-bare Ergebnisse zu erhalten und damitdiese neue Familie protonenleitenderMaterialien praktisch nutzen zu kçnnen.

.AngewandteKorrespondenz

4 www.angewandte.de � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 2 – 4� �

These are not the final page numbers!