Synthesis and electrochemical stability of ultrahigh aspect-ratio

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Synthesis and electrochemical stability of ultrahigh aspectratio nanoporous gold after calixarene-phosphine ligand removal Yuhui Xie, Christian Schöttle, Ying Li, Carlo Carraro, Xinya Zhang, Alexander Katz, and Roya Maboudian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00754 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Synthesis and electrochemical stability of ultrahigh aspect-ratio nanoporous gold after calixarenephosphine ligand removal Yuhui Xie1,2‡, Christian Schöttle1‡, Ying Li3, Carlo Carraro1, Xinya Zhang2, Alexander Katz1*, Roya Maboudian1*

1Department

of Chemical and Biomolecular Engineering, University of California,

Berkeley, Berkeley, California 94720, United States 2School

of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou, 510640, China 3School

of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096,

China

ACS Paragon Plus Environment

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KEYWORDS: nanoporous gold assemblies, calixarene-phosphine ligand removal, UV/ozone treatment, electrochemical stability, nitrobenzene sensing.

ABSTRACT: Leveraging our previous report on synthesis of calixarene-capped ultrahigh aspect ratio nanoporous gold, we now report a new class of nanoporous gold comprising removed calixarene-phosphine ligands, using UV-ozone treatment. The removal of the calixarene ligands by this treatment is supported by XPS measurements on gold clusters. TEM further shows the extraordinary stability of the ~1 nm building blocks comprising the nanoporous gold wall, after UV-ozone treatment and subsequent strongly reducing electrochemical environments. Sensing of nitrobenzene is used as a method of characterization, to show that the surface chemistry of the nanoporous gold assemblies has radically changed after the UV-ozone treatment.

Gold is widely used in a variety of applications such as catalysis, optics, biotechnology and sensing, with a crucial attribute of gold in these applications being its long-term stability.1-3 An emerging frontier in the area of gold-containing materials is the synthesis


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of nanoporous gold, which is highly interconnected and possesses a large active area compared to other forms of unsupported gold.4 Different synthetic methods relying on templating have been developed to control porosity and structure of nanoporous gold,5-7 and methods have also been developed to control the surface functionalization of nanoprous gold, which is crucial to several applications.8-9

Previously, we reported the synthesis of nanoporous gold of extraordinarily high aspect ratio,10 comprising walls down to 10 nm in thickness and pore diameters of several hundred nm, and demonstrated its stability under strongly reducing electrochemical conditions, which otherwise sinter similarly sized gold clusters lacking the protective bulky calixarene-phosphine ligands. While functionalization of the nanoporous gold surface with calixarene-phosphine ligands can be advantageous for certain applications (e.g., providing binding sites for small molecules via synthetically tailorable calixarene cavities), the removal of the calixarene-phosphine ligands is also of importance in order to provide more access to the underlying metal surface and to enable further adjustment of the surface chemistry of the pores. Originally, we had suggested that removal of calixarene-


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phosphine ligands from its surface should be possible without compromising the integrity of the nanoporous gold scaffold, and supported this hypothesis with data that demonstrated nanosized patches of ligand removal via beam damage under the highenergy electron beam of a scanning transmission electron microscope.10

Now using a mild chemical oxidative route, we demonstrate here that it is indeed possible to remove nearly all of these protecting calixarene-phosphine ligands, while still retaining similar morphology comprising ultra-high-aspect-ratio nanoporous gold (i.e., wall thickness on the order of 10 nm and pore size on the order of hundreds of nm). We also assess the electrochemical stability of the resulting nanoporous gold, which lacks protective organic ligands. Typically coarsening and sintering of gold is observed under reducing electrochemical conditions,10-11 which is problematic for long-term stability in electrochemical applications.12 We have investigated whether the resulting nanoporous gold may exhibit greater robustness under these conditions. If so, this could provide an important clue as to what attributes could help preserve metal colloidal structural stability


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under strongly reducing electrochemical conditions – a grand challenge that encompasses many metals and electrocatalytic systems.11, 13

We thus synthesized hexagonally ordered nanoporous gold assemblies consisting of calixarene-phosphine-capped gold cluster building blocks on glassy carbon electrodes, as previously described.10 In an attempt to remove the protecting calixarene-phosphine ligands, which are large enough to allow access to the nanoporous gold surface during growth but cover enough of the surface to prevent gold aggregation during synthesis under electrochemically reducing conditions (based on a bioinspired concept),14 we subsequently treated the resulting nanoporous gold on the electrode with a variety of oxidizing approaches. These approaches consisted of various oxidizing environments and oxidant concentrations and contact times, comprising aqueous hydrogen peroxide, oxygen plasma, and UV/ozone treatments. The overall synthesis process is as shown in Scheme 1.


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Scheme 1. Schematic illustration of the synthetic procedure for ligand-free nanoporous gold comprising (i) reduction of calixarene-phosphine-capped gold clusters to


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synthesize nanoporous gold via H2 bubble templating; (ii) organic-ligand removal via oxidation; and (iii) stability testing under strongly reducing electrochemical conditions.

After the synthesis of nanoporous gold assemblies, we applied treatments consisting of varying durations of aqueous hydrogen peroxide as well as an oxygen plasma. The former was motivated by previous reports of bound phosphine oxidation to phosphineoxide functionality on gold,15 whereas the latter is a well-established method for the removal of organics from metal surfaces.16 All of these methods failed to retain the structure of the nanoporous gold, and caused subsequent pore collapse (see Figures S4, S5 in Supporting Information).

Oxidative treatment via UV/ozone treatment was also applied directly on the electrodes containing the nanoporous gold assemblies for 30 min in an attempt to remove the protective calixarene ligands from the nanoporous gold (labeled Au-np in Scheme 1). Interestingly, after the UV/ozone treatment, the nanoporous structure was preserved, and even the primary-particle size of the gold-cluster building block is still maintained at


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around 1 nm, with no evidence of aggregation (see in Figure 2 and Figure S1). To understand the effect of this treatment on bound calixarene-phosphine ligands, we performed X-ray photoelectron spectroscopy (XPS) of the primary-particle gold-cluster building blocks (labeled Au-c in Scheme 1) before and after UV/ozone treatment (Figure 1). Before the treatment, the dominant peak in the spectrum is the C 1s peak at 284.6 eV, due to the calixarene-phosphine ligands, which cover the surface of the gold cluster, with a carbon to gold ratio of about 23.5 (see Table S1 in supporting Information). The P 2p region of the spectrum shows two peaks at 131 and 134 eV, which can be attributed to phosphine and phosphine-oxide (due to oxidation of the ligand), as reported previously for a closely related gold cluster.17 After the UV/ozone treatment, the C 1s peak is significantly decreased, with a carbon to gold ratio of about 1.4, corresponding to a 16fold decrease in carbon content (see Table S1 in Supporting Information). This slight amount of residual carbon may arise from a remnant of the calixarene or from a minor amount of contamination following UV-ozone treatment, due to the ex-situ nature of the XPS measurements. In contrast to the markedly decreased carbon levels, the P 2p peak shifts to higher energies (134 eV), combined with an increase in peak intensity. This


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indicates phosphorous oxidation of the bound phosphine ligand after the UV/ozone treatment. This result is conceptually similar with previous reports by Böhmer et. al., where P of initial phosphine ligands remained on an iridium surface even after harsh high temperature treatment for 5 hours, which removed the organics.18 The Au 4f peak has the same energy before and after UV/ozone treatment, indicating the oxidative treatment with UV/ozone is sufficiently mild so as to not change the gold oxidation state. This is consistent with the results of prior investigations that used UV/ozone treatment to remove organics while preserving the electronic state of the gold surfaces.19 The XPS results demonstrate that the UV/ozone treatment is effective in removing the calixarene ligands without changing the gold valence state and electronic structure.


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Figure 1. X-ray photoelectron spectra of the nano-gold assemblies before and after UV/O3 treatment, (a) survey scan, (b) P 2p, and (c) Au 4f.

The structural changes after the UV-ozone treatment were characterized via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging,


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as shown in Figure 2 (see also Figure S1 in supporting information). The porous structure on the nanoscale is maintained without collapse, even though the wall thickness is slightly increased to 10 – 20 nm, from its initial value of 5 – 10 nm.10 As shown in the higher resolution images (Figures 2c, d), there is a minor increase in the granularity of the material but it still consists of small-cluster building blocks in the 1 nm range. No large aggregates are observed. Beam-damage as a result of decomposition of organic ligands (e.g., calixarene phosphines) is not observed on the material after UV-ozone treatment, even after long exposure with a focused electron beam during HAADF-STEM imaging (Figures 2c, d; S2c, d). This is consistent with the XPS results, which demonstrate organic-ligand removal after UV-ozone treatment, and is in stark contrast to our previous reports of nanoporous gold with calixarene-phosphinie ligands, which exhibited extensive beam damage under similar conditions.10

The maintenance of the nanoscale porous structure is further supported by electrochemical measurements of the relative electrochemical active surface area (EASA), shown in Figure 3. After UV/ozone treatment, the EASA is increased by about


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30 ± 15 %, which we surmise to be the result of calixarene removal, which leads to a higher exposed gold surface area.

Figure 2. HAADF-STEM images of the nanoporous gold assemblies after the UV/ozone treatment: (a), (b) overview images and (c), (d) detail images.


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Figure 3. EASA measurements, CV curves of electrode before (a) and after (b) UV/ozone treatment with the scan rate varied from 1 to 5 mV/s in N2-saturated 0.1 M PBS (pH=7.0) solution. (c) Current vs. scan rate behavior for the two materials with the slope of line reflecting the relative surface area of the electrode.


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In order to investigate the stability of the nanoporous structure after the removal of calixarene ligands to strongly reducing electrochemical conditions, the material after UVozone treatment was subjected to a second reductive electrochemical process at a constant potential of -2.1 V vs. Ag/AgCl for 1h, as shown in Scheme 1. The HAADF-STEM images of the samples after the second electrochemical reductive treatment are shown in Figure 4. These results demonstrate no significant change in morphology or any coarsening of the initial nanometer-sized gold-cluster building blocks. The highly stable nanoporous structure is in stark contrast to other reports on unprotected gold nanoparticles, which readily aggregate under such conditions forming large, unstructured, sintered dendrites.10-11

The observed stability of the nanoporous gold after calixarene-phosphine organicligand removal under electrochemically reducing conditions raises the following question: why is our nanoporous gold electrochemically so stable under conditions that are proven to aggregate other gold nanoparticulate assemblies, rendering them unstable? We infer that the answer lies in the remaining P, which was characterized by XPS to remain behind


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after the UV-ozone treatment (Figure 1). We speculate that this remaining P, which is in the form of oxidized P based on XPS data (vide supra), acts as a ligand to protect the gold surface, while allowing access to it in a manner that could not be accomplished with the organic calixarene-phosphine ligands. We also cannot rule out a possible stabilizing role of a slight amount of remaining carbon, which was also shown to persist following UV-ozone treatment (Figure 1).


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Figure 4. HAADF-STEM images of the UV-ozone treated nanoporous gold assemblies after H2 bubble treatment via the electrochemical reduction method: (a), (b) overview images and (c), (d) detail images.

Based on the XPS, EASA and HAADF-STEM results, we conclude that the UV/ozone treatment has successfully removed the calixarene ligands from nanoporous gold assembly while maintaining its nanoporous structure. To further investigate the changes in surface chemistry accompanying calixarene-ligand removal, electrochemical detection of nitrobenzene (NB) was performed. Since nitrobenzene is a common chemical and presents detrimental effects on human health and environment due to its toxicity and carcinogenicity20, the effective detection of nitrobenzene is of great interest.21 Furthermore, a strong interaction between NB and calixarene is expected based on previous reports.22 The cyclic voltammetry (CV) responses to NB of the nanoporous gold assemblies before and after calixarene-phosphine removal via UV/ozone treatment are shown in Figure 5a. Based on a prior report,23 the reduction peak around -0.7 V is attributed to NB reduction to phenylhydroxylamine, while the redox peaks at more positive


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potentials are related to the phenylhydroxylamine to nitrosobenzene redox reaction. For the material treated with UV/ozone, a larger current response is observed, since the removal of calixarene not only increases the EASA of the electrode but also enhances the conductivity of the material. Another observation is that no clear evidence of redox peaks at more positive potentials is observed on the calixarene-capped nanoporous gold (potential window is greater than 0.4 V and is inconsistent with pure redox behavior). In comparison, when using the nanoporous gold after UV/ozone treatment, clear redox peaks at 0 V for oxidation and -0.15 V for reduction are observed. We surmise that the formed oxidized P on the gold changes the surface chemistry of gold and may play a role in the electrocatalysis and adsorption or redox behaviors of the nonoporous gold electrode.24-25

As shown in Figure 5b, the nanoporus gold assemblies before UV/ozone treatment exhibit good electrochemical response toward NB. Upon incremental addition of NB, the reduction current increases, demonstrating a large linear range from 17 M to 2.6 mM. A further increase in NB concentration increases the current without loss of linearity. This


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wide linear range is rather unique in NB detection.26-27 However, after the calixarene ligands are oxidatively removed via UV/ozone treatment, the material shows a decreased linear range in NB detection even when compared with other porous gold assemblies.28 Considering the surface chemistry changes accompanying the UV/ozone treatment, we surmise the calixarene-phosphine ligands in the nanoporous gold assemblies play a crucial role in the good linearity observed in NB detection. This observation is supported by strong NB absorption from aqueous solution into calixarene cavities, as reported previously.22 After the UV/ozone treatment, we speculate that the oxidized P remaining on the gold surface affects the electrochemical interfaces and interferes with the transport of phosphate ions, which are introduced from the supporting electrolyte, resulting in a more complex detection ability.29-30


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Figure 5. (a) CV curves of the nanoporous gold electrode before and after UV/ozone treatment in presence of 100 M NB in N2 saturated 0.1 M PBS solution (pH=7.0) at scan rate of 50 mV/s. (b) CV curves obtained before UV/ozone treatment electrode with incremental addition of NB from 17 M to 2.6 mM; the insert shows the linear relationship of peaks currents vs. NB concentration at a potential of about -0.7 V. The CV plots for the examined concentration range can be found in Figure S3.


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In conclusion, although the calixarene ligands can play an important role in the first step of constructing high aspect ratio nanoporous gold assemblies as reported in our previous research,

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a new method via UV/ozone treatment is demonstrated to remove the

calixarene ligands from the nanoporous gold assembly, while retaining the porous structure. The HAADF-STEM imaging results demonstrate that the nanoporous structure remains intact after the removal of calixarene ligands, except a slight increase in wall thickness and gold granularity. Moreover, the nanoporous gold and its colloidal building blocks are stable under strongly reducing electrochemical conditions, something not possible for conventional unprotected gold particles. This evidence of electrochemical stability may offer crucial clues in stabilizing colloidal metal nanostructures under strongly reducing electrochemical conditions more generally. Electrochemical sensing studies on nanoporous gold before and after UV/ozone treatment demonstrate that the removal of the calixarene-phosphine ligand leads to a drastically different electrochemical detection behavior toward NB in aqueous solution. The calixarene capped nanoporous gold shows extremely high detection ability toward NB in aqueous solution, in particular an extraordinarily large linear detection range, which is attributed to the strong affinity of NB


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to the calixarene cavity. For the UV/ozone treated nanoporous gold, however, the detection ability toward NB is decreased, but with higher current response. The removal of organic ligands can increase the EASA and conductivity of nanoporous gold, which may be important for some applications. Altogether, our data demonstrates a new facile strategy to synthesize nanoporous materials as well as to enable easy adjustment of the surface chemistry of nanoporous materials that remain stable under strongly reducing electrochemical conditions, comprising nanoporous gold assemblies.

ASSOCIATED CONTENT

Supporting Information.

Additional experimental and methodical details, as well as additional XPS, HAADFSTEM and CV data.

AUTHOR INFORMATION

Corresponding Author


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*Email: [email protected] *Email: [email protected]

Author Contributions ‡These authors contributed equally.

Funding Sources A. K. and C.S. gratefully acknowledge funding from the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Grant DE-FG0205ER15696.

ACKNOWLEDGMENT

The authors acknowledge the support of the Industrial Members of the Berkeley Sensor & Actuator Center. Y.H.-X. thanks the China Scholarship Council for a graduate fellowship (201706150064), and C. S. acknowledges the Deutsche Forschungsgemeinschaft (DFG) for a research fellowship. The work at the Molecular Foundry was supported by the DOE BES, under Contract No. DE-AC02-05CH11231.


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Synthesis and electrochemical stability of ultrahigh aspect-ratio

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