Highly Conductive 2D Metal-Organic Framework Thin Film Fabricated by Liquid-Liquid Interfacial Reaction Using One-Pot-Synthesized Benzenehexathiol

Article abstract of DOI:10.1021/acs.langmuir.8b03938

Metal-organic frameworks (MOF) are studied extensively in applications like catalysts, gas storage, and sensors due to their various functional groups and structures. Two-dimensional (2D) MOFs such as triphenylene-based materials show excellent charge transport properties, but thin-film fabrication and organic ligand synthesis are difficult. In this work, we synthesize thiol-based organic ligand, benzenehexathiol (BHT), by a simple one-pot reaction. This facile method is safer and faster than conventional synthesis procedure that requires using liquid ammonia as solvent. Two novel 2D MOF materials, Ag₃BHT₂ and Au₃BHT₂, are fabricated by coordinating BHT with either silver (Ag) or gold (Au) ions through liquid-liquid interfacial reaction. The Ag₃BHT₂ thin film reaches a high electrical conductivity of 363 S cm^-1, which has potential applications in electronic devices and sensors.

Full text of DOI:10.1021/acs.langmuir.8b03938

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Highly conductive 2D metal-organic framework thin film fabricated by liquid-
liquid interfacial reaction using one-pot synthesized benzenehexathiol
Iu-Fan Chen, Chun-Fu Lu, and Wei-Fang Su
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03938 • Publication Date (Web): 05 Dec 2018
Downloaded from http://pubs.acs.org on December 5, 2018
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Highly conductive 2D metal-organic framework thin
film fabricated by liquid-liquid interfacial reaction
using one-pot synthesized benzenehexathiol
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Iu-Fan Chen, Chun-Fu Lu and Wei-Fang Su*
Department of Materials Science and Engineering, Advanced Research Center of Green Materials
Science and Technology, National Taiwan University, Taipei City 10617, Taiwan.
KEYWORDS. metal-organic framework, conductivity, one-pot synthesis, benzenehexathiol, film
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ABSTRACT. Metal-organic frameworks (MOF) are studied extensively in applications like catalysts,
gas storage and sensors due to their various functional groups and structures. Two-dimensional (2D)
MOFs such as triphenylene-based materials show excellent charge transport properties but thin film
fabrication and organic ligand synthesis are difficult. In this work, we synthesize thiol-based organic
ligand, benzenehexathiol (BHT), by a simple one-pot reaction. This facile method is safer and faster
than conventional synthesis procedure that requires using liquid ammonia as solvent. Two novel 2D
MOF materials, Ag3BHT2 and Au3BHT2, are fabricated by coordinating BHT with either silver (Ag) or
gold (Au) ions through liquid-liquid interfacial reaction. The Ag3BHT2 thin film reaches high electrical
conductivity of 363 Scm-1, which has potential applications in electronic devices and sensors.
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INTRODUCTION
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Two-dimensional (2D) materials, like graphene1-2, boron nitride3, transition metal
dichalcogenides4-6 and silicon nanosheets7, are widely utilized in electronic and optoelectronic
devices, such as transistor, sensor or electrode. However, these materials are fabricated under
high demanding chemical conditions and complicated procedures, so large efforts are dedicated
to seek alternating graphene analogous materials. Therefore, 2D metal-organic frameworks
(MOF)8-11 have attracted great research interests recently. Benefiting from their unique chemical,
physical and optoelectronic properties by the planar coordination of metal ions and π-conjugated
aromatic organic compounds, 2D MOFs are regarded as one of the potential candidates to
replace conventional 2D materials. Additionally, most of them can be prepared through normal
chemical procedures with low-cost and high yield, and offer excellent optoelectronic properties.
For example, D. Sheberla et al.12 reported devices fabricated by electrically conductive
Ni3(HITP)2 that function as electrochemical double layer capacitors due to its high conductivity
and high surface area. The device can have critical advancement in renewable energy
technology. S. R. Zhang et al.13 constructed a new 2D MOF material named NENU-503, which
displays highly selective and recyclable ability as a fluorescent sensor of detecting nitroaromatic
explosives.
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Among various types of MOFs materials, planar metal bis(dithiolene) complexes have drawn
noticeable attention owing to their excellent electronic properties. The five-member ring
coordinating by dithiolene with a metal ion provides strong π-d interaction14 to achieve high
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electron transfer. Moreover, thiol-based aromatic compounds, including benzenehexathiol (BHT)
and triphenylenehexathiol (THT), are highly reactive with metal ions and could easily form
ordered planar coordinated structure at room temperature and normal pressure. X. Huang et al.15
reported their studies of Cu-BHT, which shows the highest electrical conductivity (1580 Scm-1)
in coordination MOF materials reported in the literatures and displays ambipolar charge transport
behavior. R. Dong et al.16 synthesized single-layered conductive THTNi nanosheets that
exhibited good performance of electrocatalytic hydrogen production from water. Thus, thiol-
based aromatic compounds have been intensively studied in the design and fabrication of novel
2D nanosheets. In general, the thiol-based aromatic compounds are synthesized in anhydrous
liquid ammonia as solvent. One must be very careful for the usage of liquid ammonia because
the anhydrous ammonia is caustic, corrosive, and harmful when it contacts with human body,
especially eyes, lungs, and skin. Anhydrous ammonia incidents are usually severe because brief
exposure to 2,500 to 6,500 ppm of anhydrous ammonia can lead to death17. Moreover, the
synthesis procedure includes several complicated and time-consuming steps at low temperature
and in controlled environment. Therefore, a simple and safe method to synthesize thiol-based
aromatic compounds is of great interest for further studies and applications in electronics and
sensors.
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In this work, we successfully developed a novel method to synthesize BHT compound by one-
pot reaction without the usage of liquid ammonia. The reaction can be complete in 6 hours, and
the reaction yield can reach up to 85%. This approach is not only safer but also simpler than
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conventional method to synthesize thiol-based aromatic organic compounds. With an efficient
and safe way to synthesize high-purity BHT, 2D MOF materials can be easily developed. We
synthesized two new 2D MOFs by coordinating BHT with Ag or Au ions to obtain Ag3BHT2 and
Au3BHT2 respectively. The MOF films were prepared by two different methods. The first one is
the direct thin film formation through liquid-liquid interfacial reaction. Two immiscible liquid
systems, which dissolve metal ion salts and BHT respectively, are used as solvents. The thin film
would form at their interface. The second one is the pressed thick film from BHT MOF powder,
which was synthesized through the direct mixing metal ion and BHT solutions. The structures
and electrical properties of two films are carefully evaluated. Four-probe measurement indicates
that the conductivity of Ag3BHT2 can reach 363 Scm-1, which is much higher than most of the
reported coordinated polymers18 and conductive MOF materials19-22. From the XRD analysis,
Ag3BHT2 film has hexagonal lattice structure to obtain planar conjugation and π-π stacking to
transport electrons, leading to high electrical conductivity. On the other hand, Au3BHT2 is almost
insulated due to its disordered structure. Our work provides a more efficient and safer method to
synthesize thiol-based aromatic compounds with high reaction yield and purity. Furthermore, we
report a brand new, high electrically conductive 2D MOF material, Ag3BHT2, which has
potential in applications of electronic devices and sensors.
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RESULTS AND DISCUSSION
Synthesis of BHT, Ag3BHT2 and Au3BHT2. The simple one-pot synthesis method (Scheme 1)
is based on two consecutive SN2 reactions. The overall reaction consists of three stages. First, the
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nucleophilic aromatic substitution occurs on hexahalogenobenzene, replacing the halide anions
with alkylthiol groups. The procedure is assisted by the high polarity solvent of 1,3-dimethy-l-2-
imidazolidinone (DMEU) for a favorable SN2 reaction. Second, another nucleophilic substitution
cleaves the aliphatic groups due to the presence of excess alkylthiolate anions, producing
benzenehexathiolate anions. Sodium particles are added into the solution in order to stabilize the
benzenehexathiolate anions. Finally, sodium hexathiolate is treated with hydrochloric acid to
obtain benzenehexathiol (BHT). This method provides several advantages: (1) it is simpler,
cheaper and safer than the conventional method to synthesize aromatic thiol-based compounds,
which needs the usage of liquid ammonia; (2) the using solvent, excess sodium alkylthiolate and
sodium particles can be easily removed by water, leading to high purity, and (3) high reaction
yield can be easily achieved due to the one-pot reaction.
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Scheme 1. Synthetic scheme of benzenehexathiol (BHT) by one-pot reaction.
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In traditional organic nucleophilic substitution reaction, bromide anion and chloride anion are
used extensively as leaving groups due to their electronic stability and the weak carbon-bromide
or carbon-chloride bonds. Fluoride anion, on the other hand, is a considerably weaker leaving
group than chloride and bromide because carbon–fluorine bond is a strong polar covalent bond.
However, fluoride anion can serve as good leaving group in special cases. L. Testaferri et al.23
reported the substitution reaction by hexafluorobenzene (C6F6) not only has shorter reaction
time, but also reacts at lower temperature than hexachlorobenzene (C6Cl6) to substitute halide
and form hexaalkylthiolbenzene. Fluorine has the highest electron density among all halogen
elements, which results in the strong electronic repulsion in a small benzene ring. Therefore,
fluoride anion is easier to be substituted than other leaving groups owing to the presence of
strong repulsive force.
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To compare the reaction time and reaction yield of different reagent, hexafluorobenzene
(C6F6) and hexabromobenzene (C6Br6) were chosen as starting materials. To achieve high yield,
aliphatic groups must have high efficiency in both nucleophilic substitution reactions. Various
alkylthiolate salts, including sodium methylthiolate (CH3-SNa), sodium ethylthiolate (C2H5-SNa)
and sodium isopropylthiolate (i-C3H7-SNa) were employed to study the effect of aliphatic group
on the reaction. The results are summarized in Table 1, and there are two significant phenomena
in these reactions.
Firstly, the reactions by employing C6F6 as starting material (Run 1-3) all have higher reaction
yields, and require less reaction time than those using C6Br6 (Run 4-6) when the same type of
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alkylthiolate salt is used. The reactions by C6Br6 (Run 4-6) all need to take over 30 hours to be
complete. The first SN2 substitution on C6F6 can occur at 0°C, while the temperature of the
reactions of C6Br6 have to be above 100°C to replace halide ions. In common organic
substitution reaction, fluoride is the weakest leaving group, and bromide and chloride are better
choices for normal organic synthesis due to their electronic stability. However, our study shows
the totally contrast results. With the narrow space in an aromatic ring, the fluorides produce
strong electronic repulsion with each other, leading to the easy substitution of fluorides in C6F6.
Therefore, Run 1-3 with C6F6 as starting material not only possess higher reaction yield, but also
can be complete at lower temperature and in shorter time than that Run 4-6, which used C6Br6 as
reagent.
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Second, the runs with i-C3H7-SNa have much shorter reaction time than those with C2H5-SNa
and CH3-SNa. The reaction times of runs with C2H5-SNa are shorter than which with CH3-SNa.
The larger aliphatic group both in chain length and spatial arrangement exhibits stronger
inductive effect and steric effect than the smaller one. Thus, it makes alkylthiol to be stronger
nucleophile. The substitution reaction of alkylthiol with the halogenated benzene is easier to take
place. The larger the aliphatic group is, the shorter the reaction time is. The branched chain of
isopropyl group can accommodate more negative charge than the linear chain, leading to high
stability, so its nucleopholicity is the largest with abnormal high substitution yield24. In Table 1,
Run 3 has the shortest reaction time (6 hours) and the highest reaction yield (85%), with starting
materials as C6F6 and i-C3H7-SNa. Run 2 (C6F6 and C2H5-SNa) has similar yield as Run 1 (C6F6
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and CH3-SNa), while the former has shorter reaction time (18 hours) than the latter (31 hours).
The similar trend is also observed in Runs 4-6. Run 6 (C6Br6 and i-C3H7-SNa) has second
highest reaction yield among all runs, confirming the excellent nucleophilic characteristic of
isopropyl groups. Run 4, with C6Br6 and CH3-SNa, has the longest reaction time (72 hours) and
the lowest reaction yield (58%) among all reactions, corresponding to the poor leaving ability of
bromide ion in the first SN2 reaction, and weak inductive effect and steric effect of methyl groups
in both reactions. To sum up, we obtain an efficient, simple and rapid method to synthesize BHT
by using C6F6 and i-C3H7-SNa as starting material, and the reaction has a yield of 85% in 6
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hours.
Table 1. Synthesis of benzenehexathiol by different starting materials and reagents.
Run
Starting Material
Reagent
Temp(°C)
Time(h)
Yield(%)
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C6F6
C6F6
CH3-SNa
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0
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C2H5-SNa
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CH3-SNa
C6F6
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C6Br6
C6Br6
C6Br6
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100
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C2H5-SNa
i-C3H7-SNa
Being highly reactive with metal ions, BHT can easily coordinate with Ag or Au ions. The
MOF can be obtained by direct interaction between BHT and metal ions (Scheme 2). One BHT
molecule can coordinate with six metal ions, composing a six-fold symmetric 2D planar
structure. The films of BHT-based MOF was prepared by two methods: (1) thin film: direct film
formation through liquid-liquid interfacial reaction (Figure 1) and (2) thick film: pressed film
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from BHT MOF powder (Figure 2). The thick films were used to perform the analyses of
Fourier transform infrared spectrometer (FT-IR). Both thin films and thick films were used to
characterize the bonding by X-ray photoelectron spectroscopy (XPS), crystallinity by X-ray
diffraction analysis (XRD) and conductivity of the materials by four-point probe. The detailed
procedure is described in the section of experimental method. The reaction time of Ag3BHT2 is
shorter than that of Au3BHT2, for both thin film and powder. The formation of Ag3BHT2 thin
film can be observed by naked eyes in 30 minutes at the liquid interface, while Au3BHT2 thin
film needs 6.5 hours to reach the thickness to be seen. The concentration of both metal ion and
BHT solution should be carefully controlled. When a concentration of precursor higher than
0.1% is used, the thin film at the liquid interface would be too thick to sustain and collapse. The
MOF thin film could not be seen if the concentration of the solution is lower than 0.015%.
Ag3BHT2 powder is precipitated immediately after mixing the BHT and AgNO3 aqueous
solution; however, the synthesis of Au3BHT2 by BHT and HAuCl4 solution needs to be heated at
50°C for 30 minutes. The shorter reaction time of the Ag3BHT2 due to the higher reactivity of
Ag ions than that of Au ions25.
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Scheme 2. The formation of Ag3BHT2 or Au3BHT2.
Figure 1. The formation of the Ag3BHT2 thin film by liquid-liquid interfacial method. (a) Beginning of the reaction.
(b) The formation of the thin film. (c) Film transfer onto the substrate.
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Figure 2. The formation of the Ag3BHT2 powder pressed thick film. (a) MOF powder formation by adding metal
ion solution into BHT aqueous solution. (b) The Ag3BHT2 powder. (c) The powder pressed thick film.
Materials Characterization. To determine the bonding between metal ions and BHT, FT-IR
was employed to study BHT, using thick films of Ag3BHT2 and Au3BHT2 for high absorption
intensity. In Figure 3, the FT-IR of BHT has a peak at 2500 cm-1, which corresponds to the
sulfur-hydrogen bond, while the Ag3BHT2 and Au3BHT2 show no absorption at the same range.
The difference means that the hydrogen atoms on BHT were removed to coordinate with metal
ions.
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Figure 3. FT-IR spectra of BHT powder (black), the Ag3BHT2 thick film (red) and the Au3BHT2 (blue) thick film.
The chemical states of metal ions and sulfur in MOFs were determined by XPS. In Figure
4(a), the peak of Ag 3d3/2 at 367.5eV corresponds to Ag(I) in the Ag3BHT2 thin film. The
measured binding energy differs from that of unbonded Ag (368.3eV)26, confirming the
coordination bonding of silver ions and sulfur ions. Figure 4(b) shows S 2p3/2 and 2p1/2 peaks at
161.9eV and 163.1eV that indicates the sulfur ions bond with metal ions27. Thus, the
coordination between Ag and BHT is well defined in the Ag3BHT2 thin film. In Figure 4(c), the
binding energy of Au has two peaks at 84.9eV and 88.6eV, which matches to Au(I) 4f7/2 and 4f5/2
respectively27. The result points out that Au(III) ions have been reduced when they coordinated
to BHT. Figure 4(d) also shows the binding energy of S 2p3/2 and 2p1/2 representing the
coordination by BHT and Au ions.
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Figure 4. XPS spectra of (a) Ag 3d3/2 and (b) S 2p1/2, 2p3/2 in the Ag3BHT2 thin film, (c) Au 4f5/2, 4f7/2 and (d) S
2p1/2, 2p3/2 in the Au3BHT2 thin film.
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Figure 5. XPS spectra of (a) Ag 3d3/2 and (b) S 2p1/2, 2p3/2 in the Ag3BHT2 thick film, (c) Au 4f5/2, 4f7/2 and (d) S
2p1/2, 2p3/2 in the Au3BHT2 thick film.
Worthwhile to mention, the XPS results of MOF thick films (Figure 5) show interesting
differences from those of MOF thin films. First, in Figure 5(a), the peak of Ag 3d3/2 of the
Ag3BHT2 thick film is similar to that of the Ag3BHT2 thin film. However, in Figure 5(b),
different from the result of the thin film, there are unreacted thiols (-SH, 162.5eV and 163.6eV)
and unbonded sulfur ions (-S-, 163.9eV and 165.0eV) remaining in the Ag3BHT2 thick film. The
existence of unbonded sulfur ions and unreacted thiols in the Ag3BHT2 thick film indicates the
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incomplete coordination between BHT and Ag ions due to the rapid formation. In Figure 5(c),
the XPS result of Au of the Au3BHT2 thick film states that Au has two oxidation states. The four
peaks correspond to Au(I) 4f7/2 , 4f5/2 and Au(0) 4f7/2, 4f5/2 (85.9eV and 89.4eV), which means
that the gold ions have different chemical states in the Au3BHT2 thick film. In Figure 5(d), the
peaks of sulfur indicate the signals of unreacted thiols can also be observed. The results of
unreacted BHT and the multivalent Au ions in the Au3BHT2 thick film mean that the
coordination of BHT and Au ions is incomplete. Comparing with the Ag3BHT2 thick film, there
are no signals of unbonded sulfur ions in the Au3BHT2 thick film. Due to the long reaction time,
the sulfur ions might be all consumed by metal ions, leaving no unbonded sulfur ions signal. It is
noted that while unreacted thiols are observed in XPS spectra, there are no absorption signal of
thiol groups detected by FT-IR spectroscopy. This is because of the low concentration of
unreacted thiols, which is below the detection limit of FT-IR. Also, the XPS signals come from
the surface of the materials, where the concentration of defects and unreacted ligands are higher
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than in the bulk material.
Table 2. Thickness and Conductivity of BHT-MOFs.
Sample
Thickness
Conductivity (Scm-1)
290 ± 23.2 (363)
Ag3BHT2 thin film
Ag3BHT2 thick film
Au3BHT2 thin film
Au3BHT2 thick film
276.51 ± 5.53 nm
64.23 ± 0.43 μm
324.72 ± 11.23 nm
89.05 ± 0.12 μm
14.2 ± 3.0 (19.8)
9.15 ± 0.21×10-5 (1.12×10-4)
7.46 ± 1.12×10-5 (8.07×10-5)
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The electrical conductivities of all the thin films and thick films of Ag3BHT2 and Au3BHT2
were measured by four-probe method. The thickness of the film was determined from the cross-
sectional SEM image of samples (Figures S5 and S6). The results are summarized in Table 2. A
represented I-V curve of the measurements is included in Figure S1 in the section of supporting
information. For the Ag3BHT2 thin film, the conductivity can reach up to 363 Scm-1. X. Huang et
al.15, 28 reported the Cu-BHT film with a record conductivity of 1580 Scm-1 in 2015, and their
parallel work of Ag-BHT film was published recently with conductivity of 250 Scm-1. T. Kambe
et al.29 reported the Ni3(BHT)2 in 2014, which has a conductivity of 160 Scm-1; In 2016, the
Ni3(HITP)2 material reported by D. Sheberla et al.12 possesses the conductivity of 40 Scm-1. No
other conductive MOF materials reported to date could reach over 10 Scm-1. The Ag3BHT2 thin
film in this work displays an impressive high conductivity among conductive MOF materials.
The conductivity could be partly contributed to the strong π-d conjugation between sulfur ions
and silver ions14. Furthermore, the ordered crystal structure of Ag3BHT2 can introduce excellent
π-π stacking effect in the interlayer direction for high charge transport. From the XRD patterns
(Figure 6), the Ag3BHT2 thin film has ordered hexagonal planar structure, leading to the superb
intralayer and interlayer charge transport ability. For the Ag3BHT2 thick film, the conductivity is
19.8 Scm-1, lower than that of the thin film. The difference of conductivity between thin film and
thick film might be caused by crystallinity30 and the presence of unreacted BHT. The thick film
of Ag3BHT2 has weaker XRD intensity than thin film, showing the less ordered structure of thick
film than thin film. Since the electrical conductivity results from the delocalized electron transfer
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through the conjugated planar structure and the π-π stacking from layer to layer, the thick film
pressed from random orientation powder is expected to have lower conductivity than the thin
film. From the XPS analysis, there are some BHT molecules not fully coordinated with silver
ions in Ag3BHT2 thick film as discussed before. Therefore, the packing of MOF thick film is less
ordered and looser than that of MOF thin film. More ordered thin film is expected to have higher
conductivity than that of the thick film.
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Figure 6. XRD patterns of Ag3BHT2 thin film (red), Au3BHT2 thin film (blue) and stimulated result (gray).
On the other hand, both the Au3BHT2 thin film and the Au3BHT2 thick film exhibit low
conductivity despite the high conductive MOFs are demonstrated by using BHT to coordinate
with copper ions in the reference13 and silver ions in this work. The large discrepancy in
Au3BHT2 might due to the disrupted structure presenting in Au3BHT2. The lattice structures of
Ag3BHT2 and Au3BHT2 are speculated to be 2D planar hexagonal coordinated polymer by one
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BHT molecule connecting with six metal ions and each metal ion is coordinated with four
sulfurs. The XRD analysis has shown the structural difference between the thin film of Ag3BHT2
and Au3BHT2. To characterize the MOF thin film, out-of-plane diffraction was employed. For
Ag3BHT2, the out-of-plane diffraction patterns shows the peak of (001) at 29.19° which indicates
the interlayer distance of 3.06 Å. Moreover, the peaks at 9.48°, 16.45° and 19.02° correspond to
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ꢀ
the (100), (210) and (200) of hexagonal lattice structure with lattice constant ꢁ = ꢂ =10.77 Å.
The large full width at half maximum (FWHM) of the diffraction peaks indicates the Ag3BHT2
thin film consisted of small crystal grains. Also, these crystals are polycrystalline because the in-
ꢀ
plane peaks, (100), (210) and (200), are observed in out-of-plane XRD patterns. (Simulation of
XRD patterns also confirmed the hexagonal structure of Ag3BHT2.) The XRD patterns of
Au3BHT2 thin film also display (100) and (001) at the same angle as Ag3BHT2; however, the
weak intensity of peaks and the lack of order diffraction signals indicate that the Au3BHT2 thin
film has lower crystallinity than Ag3BHT2. The Au3BHT2 thin film might be distorted and
deviated from planer structure; that is, the two BHT molecules bonded with an Au ion are not
placed on a same plane, which would cause random orientation and loose packing of the
structure. Without ordered planar structure, charge transport in Au3BHT2 is interrupted in both
planar and interlayer direction, leading to insulate property. For the Ag3BHT2 and Au3BHT2
thick films, the XRD patterns are extremely weak, showing the both samples in low crystallinity
(Figure S2).
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Morphological differences between the thick films and the thin films of Ag3BHT2 and
Au3BHT2 are shown by scanning electron microscopy (SEM) images (Figure 7). The thin film
of Ag3BHT2 is flat (Figure 7(a)) but has cracks and small particles on the surface, while the
Au3BHT2 thin film (Figure 7(c)) is extremely uniform in large area without any micrometer-
scaled pores in the film. The morphology of the Ag3BHT2 powder pressed thick film has larger
roughness than thin film, but still is a large-scale continuous structure (Figure 7(b)). Figure 7(d)
shows the Au3BHT2 thick film is cracked and discontinuous, and the sizes of domain are varied
largely from 10 to 100μm. The differences of morphology between the thin films of Ag3BHT2
and Au3BHT2 might result from the different reaction rate. The slower reaction rate of the
Au3BHT2 thin film formation resulted in the smooth film. MOF thin films have less cracks and
coarseness than powder pressed thick film, resulting in a long-range continuous morphology.
Despite the small discontinuous cracks on the surface of the Ag3BHT2 thin film and thick film,
both of them reach the threshold of the percolation network, exhibiting high electrical
conductivity. The morphologies of the Au3BHT2 thin film and thick film have similar trend as
Ag3BHT2. The morphological difference between the MOF thin films and powder pressed thick
film in SEM images are agreed with the results of XRD study.
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Figure 7. SEM images of the Ag3BHT2 (a) thin film and (b) thick film, the Au3BHT2 (c) thin film and (d) thick film.
Ag3BHT2 has higher electrical conductivity than Au3BHT2 due to strong π-d interaction and π-
π stacking via the hexagonal planar conjugated structure, which is determined by XRD analysis.
In the aspect of morphology, the MOF thin films exhibit high ordered and long-range continuous
orientation, leading to higher conductivity than MOF powder pressed thick films, which have
incomplete coordination and disordered morphology. Hence, the conductivity of Ag3BHT2 can
be increased by improving the microstructure through liquid-liquid interfacial method to reach
up to 363Scm-1.
CONCLUSIONS
In this work, we demonstrated a brand new approach to synthesize BHT by a simple one-pot
reaction without using liquid ammonia. The reaction of starting materials as hexafluorobenzene
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and sodium isopropylthiolate can be complete in only six hours, and has 85% of reaction yield.
This method provides a safer and easier way to synthesize thiol-based aromatic compounds.
With four-probe measurement, the electrical conductivity of the Ag3BHT2 thin film can reach
363 Scm-1, which is much higher than most of conductive coordination polymers. The
differences between Ag3BHT2 and Au3BHT2 were investigated; the Ag3BHT2 thin film has
hexagonal lattice planar structure, but the structure of the Au3BHT2 thin film is disordered.
Au3BHT2 is almost insulated due to its random orientation that lacks planar conjugation and π-π
stacking to transport electrons. With large-scaled continuous crystalline morphology and
complete coordination by BHT and Ag ions, the Ag3BHT2 thin film fabricated by liquid-liquid
interfacial method exhibits higher electrical conductivity than powder pressed thick film. With
high conductivity and simple synthesis procedure, Ag3BHT2 is a potential material to be utilized
in the applications of electronic devices and sensors.
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EXPERIMENTAL METHODS
Synthesis of Benzenehexathiol (BHT). The solution of hexahalogenobenzene (14mmol) and
sodium alkylthiolate (0.21mol) with 1,3-dimethyl-2-imidazolidinone (DMEU, 30ml) was stirred
under nitrogen at either 0°C or 100°C. The progress of the reaction was monitored by thin layer
chromatography (T.L.C.). Hexahalogenobenzene was converted into hexaalkylthiolbenzene after
fully reacted. Sodium particles (0.644g, 28 mmol) were added, and the solution was heated to
100°C. The reaction was monitored by T.L.C. Methanol (30 ml) and water (30 ml) were injected
into the flask respectively, and the solution was poured into hydrochloric acid (1.0 M, 150 ml)
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slowly. The yellow solid (3.215 g, 85%) was then precipitated, filtered and washed by water (20
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ml*3) and acetone (20 ml*3). H Nuclear magnetic resonance (NMR, Figure S3) (400 MHz,
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CD3CN): δ (ppm) 2.09 (s, 6H). Electrospray ionization mass spectroscopy (ESI-MS, Figure S4):
[M+2H]2+: 137.07. Calculated for C6S6H6: 270.47 (mg/mol). Found: 272.13 (mg/mol).
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Formation of Ag3BHT2 and Au3BHT2. The MOF thin film was prepared by liquid-liquid
interfacial reaction. BHT (5 mg, 0.0185 mmol) was first dissolved in sodium hydroxide aqueous
solution (5 mM, 10 ml), and the solution was added into a round dish (with 30 mm diameter).
AgNO3/HAuCl4 (0.025 mmol) was dissolved in the co-solvent of acetonitrile and ethyl acetate
(v/v=1:4, 10 ml). Then, clean acetonitrile/ethyl acetate co-solvent (5 ml) was added to covered
the aqueous layer, forming an organic-aqueous interface. After that, the metal ion solution was
gently injected into the upper layer by syringe. The brown MOF thin film was formed at the
liquid-liquid interface and can be observed by eyes. The Ag3BHT2 film was formed in 30
minutes, while the film formation of Au3BHT2 took 6.5 hours to be fabricated. The silicon wafer
covered with 300nm silicon oxide was placed in the dish in advance, and the liquid was removed
by syringe slowly to make the MOF thin film deposited on the substrate. The MOF powder was
obtained by mixing BHT (10 mg, 0.037 mmol) and AgNO3/HAuCl4 (0.1 mmol) in methanol (30
ml) directly, and small MOF particles were precipitated. Ag3BHT2 particles can form
immediately, and the precipitation of Au3BHT2 needs to heat the solution to 50°C for 30
minutes. The particles were filtered, washed by water and acetone in sequence, dried by vacuum
and pressed into a disk in a mold under a pressure of 50 psi (with diameter of 7 mm). Elemental
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analysis was done by energy dispersive X-ray spectroscopy (EDS) that identifies the presenting
elements and atomic percentage (Table S1 and Table S2).
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Materials Characterization. The MOF thin films were deposited on 20*20 mm2 silicon
substrates with 300 nm thermally grown SiO2. The powder was pressed into a die to form thick
film with diameter of 7 mm. The sheet resistance was measured by four-point probe and Keithley
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2400 equipped with Labview software using current from 10 to 20 mA. H NMR spectrum of
BHT was performed by A525-Bruker AVIII HD 400MHz NMR with d-acetonitrile (CD3CN) as
d-solvent, and ESI-MS was made by FINNIGAN LCQ Mass Spectrometry, with BHT dissolved
in 20 ml dichloromethane (0.56 mM). FT-IR spectra of BHT, thick film of Ag3BHT2 and
Au3BHT2 were acquired by Perkin Elmer Spectrum 100 FT-IR Spectrometer. The out-of-plane
XRD patterns of the MOF films of Ag3BHT2 and Au3BHT2 were measured by Rigaku TTRAX3
X-Ray Diffraction Analyzer (15kW, λ=1.541Å) from 2θ=5° to 35°. The simulations of
diffraction patterns were calculated by Materials Studio 8.0 software. The thickness of thin film
and thick film was determined from the SEM cross-sectional images of samples (Figure S5 and
S6). The SEM surface images and EDS of films and the cross-sectional images of the thick films
were taken by JEOL JSM6510 Scanning Electron Microscopy with 15 and 10kV, respectively.
The SEM cross-section images of the thin films were taken by NOVA NANO SEM 450. The
samples for SEM and EDS were first covered by platinum with the thickness of 5nm. PHI 5000
Versa Probe system (ULVAC-PHI, Chigasaki) with a micro-focused (100μm) Al X-ray beam
was used to obtain XPS spectra of the Ag3BHT2 and Au3BHT2 films.
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ASSOCIATED CONTENT
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Supporting Information. Conductivity measurement of the Ag3BHT2 thin film; 1H NMR and
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ESI-MS analyses of BHT; EDS of the Ag3BHT2 and Au3BHT2 thin films (PDF)
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AUTHOR INFORMATION
Corresponding Author. *suwf@ntu.edu.tw
Author Contributions. The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
Funding Sources. This work was financially supported by the “Advanced Research Center of
Green Materials Science and Technology” from The Featured Area Research Center Program
within the framework of the Higher Education Sprout Project by the Ministry of Education
(107L9006) and the Ministry of Science and Technology in Taiwan (MOST 106-2218-E-002-
021-MY2 & 107-3017-F-002-001). The authors also thank the partial financial support from the
grant of MOST 106-2923-M-002 -004 -MY3.
ACKNOWLEDGMENT. Authors also thanks the technical supports provided by Chien-An
Chen (synthesis), Tzu-Yi Yu (NMR), Yu-Tao Sun (thickness measurement) and Instrumental
Center of National Taiwan University (ESI-MS, FINNIGAN LCQ Mass Spectrometry).
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