A new silver-based precursor as ink for soft printing techniques

Article abstract of DOI:10.1002/ejic.201100930

A new, easily obtainable silver-pyrrolidone complex that is suitable for printing applications was synthesized by the reaction of silver nitrate and 2-pyrrolidone (Pyl) at room temperature. According to single-crystal X-ray crystallographic studies, the product [Ag(Pyl)₂]NO₃ crystallizes in the monoclinic space group C2/c (no. 15) [a = 5.358(1) A, b = 15.217(3) A, c = 14.986(3) A, β = 99.296(6)°]. Highly concentrated solutions of the complex can be obtained in an ethanol/water mixture, thereby allowing for the manufacture of thin films by means of dip coating. Subsequent UV irradiation and moderate-temperature treatment yielded compact films of elemental silver with thicknesses of about 100 nm and sheet resistances down to 6.5. Furthermore, microcontact printing (μcp) and embossing of [Ag(Pyl)₂]NO₃ were performed, also followed by UV treatment. By means of the photoreduction of the complex and subsequent moderate-thermal treatment, defined structures of elemental silver lines were obtained. We present an easily obtainable silver-based precursor suitable for printing applications. The simple reaction of silver nitrate and 2-pyrrolidone (Pyl) yields a stable [Ag(Pyl)₂]NO₃ complex, the crystal structure of which was solved. Highly concentrated solutions can be obtained in ethanol/water, thereby allowing for manufacture of thin films through coating or printing techniques. Copyright

Full text of DOI:10.1002/ejic.201100930

FULL PAPER
DOI: 10.1002/ejic.201100930
A New Silver-Based Precursor as Ink for Soft Printing Techniques
Julia Fritsch,[a] Benjamin Schumm,[a] Ralf Biedermann,[a] Julia Grothe,*[a] and
Stefan Kaskel[a]
Keywords: Silver / Pyrrolidone / Photoreduction / Thin films / Patterning
A new, easily obtainable silver–pyrrolidone complex that is
suitable for printing applications was synthesized by the re-
action of silver nitrate and 2-pyrrolidone (Pyl) at room tem-
perature. According to single-crystal X-ray crystallographic
thin films by means of dip coating. Subsequent UV irradia-
tion and moderate-temperature treatment yielded compact
films of elemental silver with thicknesses of about 100 nm
and sheet resistances down to 6.5 Ω. Furthermore, microcon-
studies, the product [Ag(Pyl)2]NO3 crystallizes in the mono- tact printing (μcp) and embossing of [Ag(Pyl)2]NO3 were per-
clinic space group C2/c (no. 15) [a = 5.358(1) Å, b
=
formed, also followed by UV treatment. By means of the pho-
toreduction of the complex and subsequent moderate-ther-
mal treatment, defined structures of elemental silver lines
were obtained.
15.217(3) Å, c = 14.986(3) Å, β = 99.296(6)°]. Highly concen-
trated solutions of the complex can be obtained in an etha-
nol/water mixture, thereby allowing for the manufacture of
with polymers such as poly(N-vinyl-2-pyrrolidone) (PVP)
or others.[21,22] In addition to their stabilizing effect, PVP
or pyrrolidone derivatives support the photoreduction of
silver ions to elemental silver.[23] Based on these conditions,
we created an easily obtainable complex of silver nitrate and
2-pyrrolidone. Both silver nitrate and 2-pyrrolidone are
highly soluble in water or water/ethanol mixtures. Similar
to PVP, the organic 2-pyrrolidone ligand should have a sta-
bilizing effect and support photoreduction. Due to these
facts, the generated complex can be used as an ink for direct
soft printing methods such as microcontact printing (μcp)
or embossing processes that offer a way to fabricate con-
ductive transparent substrates.
In the following we describe the structure of the new,
easily obtainable silver complex synthesized by mixing sil-
ver nitrate and 2-pyrrolidone (Pyl) in water. In addition, we
demonstrate its suitability as an ink by using dip coating for
compact silver films and soft print methods for structured
coatings.
Introduction
Transparent conductive films play a key role in several
optoelectronic applications such as flat-panel displays, or-
ganic light-emitting diodes (OLEDs), and photovoltaics
(PV).[1–4] With increasing demand for structured trans-
parent electrodes, printings of functional materials like
transparent conductive oxides,[5,6] conductive polymers,[7]
carbon-based materials,[8,9] or metals[10,11] have attracted
much attention in recent years.[12,13]
In particular, silver-based materials have been studied
due to their advantages like very low resistivity, stability,
and flexibility compared to transparent conductive oxides
or polymers and carbon-based materials. However, the
challenge in using silver is the fabrication of transparent
materials. Due to that fact, several methods for the prepara-
tion of ultrathin films such as chemical vapor deposition or
vacuum-thermal evaporation techniques have been investi-
gated.[14–16] Also, different printing techniques of metallic
materials to obtain structured patterns with higher trans-
parency have been reported recently. One way is to use sim-
ple metal solutions such as silver nitrate, which can be con-
Results and Discussion
verted into silver at high temperatures (above 400 °C).[17]
A
Synthesis and Structural Characterization of [Ag(Pyl)2]NO3
second way is to use metallo-organic precursors or stabi-
lized metal solutions, which can be converted into metals at
lower temperatures.[18–20] Another well-known method is
the generation of silver-nanoparticle suspensions stabilized
The reaction between aqueous solutions of silver nitrate
and 2-pyrrolidone at room temperature under exclusion of
light led to the formation of colorless crystals of [Ag(Pyl)2]-
NO3 (Scheme 1).
[a] Department of Inorganic Chemistry, Dresden University of
Technology
Bergstrasse 66, 01069 Dresden, Germany
Fax: +49-351-46337287
E-mail: Julia.Grothe@chemie.tu-dresden.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100930.
Scheme 1. Synthesis of the [Ag(Pyl)2]NO3.
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A New Silver-Based Precursor as Ink for Soft Printing Techniques
The linear 1D polymeric silver complex poly-[Ag(Pyl)2]- the silver complex decomposes in two steps (see the Sup-
NO3 crystallizes in the monoclinic space group C2/c (no. porting Information, Figure a), whereas the major organic
15) with four formula units per unit cell. The formula unit part (Ͼ60%) evaporates up to a temperature of 220 °C.
contains the 2-pyrrolidone molecules, one silver atom, and This mass loss can be attributed to the 2-pyrrolidone ligand
one nitrate anion. The silver atom and the nitrate anion and parts of the nitrate. The second step at about 400–
occupy special positions on twofold axes. The Ag atom ex- 440 °C results in the volatilization of the residual nitrate.
hibits an irregular AgO6 coordination geometry, which con- The final residue could be identified as elemental silver by
sists of two oxygen atoms from monodentate-coordinated using powder X-ray diffraction (PXRD) and supports the
2-pyrrolidone molecules and four oxygen atoms from two results from the elemental analysis (see the Supporting In-
bidentate-coordinated nitrate anions. The Ag–O distances formation, Figures b and d).
are in the range of 2.358(2)–2.683(4) Å. The nitrate anions
have a linker function and connect the structure into 1D
polymeric chains, which are located along the [100] direc-
tion (Figure 1). The silver atoms are connected to a linear
polymeric chain by μ2-O bridging oxygen atoms O4 with a
significantly longer Ag1–O4 distance [2.683(1) Å] than
Ag1–O2 [2.536(4) Å]. In the crystal structure, the 1D poly-
meric chains are connected into the 2D network by inter-
molecular N–H···O hydrogen bonds (Figure 2) [N2–
H3N···O1i: D···A 2.952(4) Å, D–H···A 163°; i: 0.5 – x, 0.5 –
y, –z].
Figure 3. PXRD pattern of the silver complex (black) compared to
the pattern that was calculated from the solved structure (dashed
line).
Preparation of Compact, Thin Silver Films
The suitability of [Ag(Pyl)2]NO3 as an ink for the fabri-
Figure 1. Linker function of the nitrate ion connecting the structure
cation of thin, conductive silver films through soft printing
into 1D polymeric chains.
techniques was tested by the preparation of thin, compact
silver films. For this purpose, glass substrates were coated
with an aqueous/ethanolic solution of the silver complex by
means of dip coating (Scheme 2). Subsequently, the sub-
strates were irradiated with UV light to obtain compact sil-
ver films. To achieve an optimal adhesion of the complex
solution, the microscope slides were pretreated with a pira-
nha solution. With this pretreatment, it was possible to ob-
tain thin, homogeneously coated substrates.
Figure 2. Two-dimensional network of –[Ag(Pyl)2]NO3, which is
connected through intermolecular N–H···O hydrogen bonds in the
1D polymeric chains.
Scheme 2. Schematic description of the dip-coating process to fab-
As shown in Figure 3, the powder X-ray pattern of the
ricate thin, compact silver coatings.
silver complex conforms to the calculated one from the
solved structure. The differences in intensity between sev-
eral peaks are due to the preferred orientation of the acicu-
The photoreduction of the silver complex was examined
lar crystallites in the powdered sample. According to ther- by recording XRD patterns and SEM pictures after dif-
mogravimetric analysis (see the Supporting Information), ferent periods of UV irradiation. As shown in the XRD of
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J. Fritsch, B. Schumm, R. Biedermann, J. Grothe, S. Kaskel
FULL PAPER
Figure 4 (light gray line) and the respective photo, there is sample treated at 220 °C (black line). Furthermore, without
no complete conversion into elemental silver without UV the temperature treatment, the dip-coated layers did not
irradiation. Additionally, the films are not homogeneous show good values in sheet resistance due to the presence of
(Figure 5a). After 10 min of UV irradiation, elemental sil- the organic ligand. To avoid an abrupt vaporization of any
ver resulted (Figure 4, gray line), but as shown in the SEM volatile components after UV irradiation, the samples were
image (Figure 5b), the layer is not homogeneous. An opti- heated slowly to 220 °C at a rate of 5 Kmin–1.
mal silver coating was obtained after 20 min of irradiation
(black line in the XRD of Figure 4, SEM image in Fig-
ure 5c). The broad signal in the thin-film XRDs from 2θ ≈
20 to 30° results from the glass substrate. Additionally, a
thermogravimetric analysis of the UV-reduced sample was
carried out (see the Supporting Information, Figure c). In
contrast to the complex that was not irradiated, the decom-
position took place in only one step at 220 °C, and no fur-
ther decomposition was observed at about 400–440 °C, thus
indicating the decomposition of the nitrate during the UV
reduction. The residual mass fits the content of silver in a
“nitrate-free” complex. Since the thermogravimetric analy-
sis of the UV-reduced sample shows complete decomposi-
tion at 220 °C, the coated substrates were additionally
treated at a temperature of 220 °C after UV irradiation. The
necessity of this additional temperature treatment is dem-
onstrated, for example, in a thicker layer with an UV-irradi-
ation period of 20 min. As shown in Figure 6, the untreated
layer (gray line) shows a slight crystallinity compared to the
Figure 6. XRD pattern of the reduced complex without thermal
treatment (gray) compared to that with thermal treatment (black);
dashed lines: reference of silver ICSD [4-783].
To examine the thickness and surface morphology of the
compact silver films, atomic force microscopy (AFM) and
SEM measurements were used. In Figure 7a, an AFM im-
age of a produced silver coating with optimized reduction
conditions is shown. As seen in the picture, the surface of
the film shows roughness. Surface roughness values were
calculated with Ra = 12.6 nm/Rms = 17.3 nm. Due to that,
the values of sheet resistance were measured by means of a
four-point method spread between 6.5 and 20 Ω in one
sample. However, for the calculation of the specific electri-
cal resistance, we determined the thickness of the silver lay-
ers by means of AFM. In Figure 7b and c, the measurement
of a scratch made with a scalpel with a resulting film thick-
Figure 4. XRD patterns of the reduced complex with the following
irradiation times: 0 min (light gray), 10 min (gray), 20 min (black);
dashed lines: reference of silver [4-783]; photos of the substrates
without UV irradiation (bottom) and after 20 min irradiation (top).
Additionally, all coated substrates were treated at 220 °C for
60 min.
Figure 7. (a) AFM image of a compact dip-coated silver film that
was UV-irradiated for 20 min and treated at 220 °C; (b) and (c)
height profile of an AFM micrograph of the compact film
scratched with a scalpel. The AFM cross section (c) shows the
scratched zero level on the substrate and the silver film. To calcu-
late the height, average values of both levels were estimated (dashed
lines). (d) An SEM image of the edge of the silver film on the glass
substrate with a determined film thickness of 114.1 nm.
Figure 5. SEM images of the reduced complex with the following
irradiation times: (a) 0, (b) 10, and (c) 20 min. All samples were
treated after reduction at 220 °C.
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A New Silver-Based Precursor as Ink for Soft Printing Techniques
ness of about 100 nm is shown. For the evaluation of the the lines, which was printed by using the embossing tech-
thickness, the values for both areas (the silver layer and the nique. SEM measurements (Figure 8b) provide evidence of
scratched area) were averaged (dashed lines) and the differ- the compactness of elemental silver in the line that is com-
ence in height was estimated. By considering an average parable to the compact silver films (Figure 5c). The struc-
film thickness of about 100 nm, specific electrical resist- turing can also be followed in an energy-dispersive X-ray
ances of 6.5ϫ10–7 Ωm (6.5ϫ10–5 Ωcm) to 2ϫ10–6 Ωm (EDX) mapping (Figure 8c). For comparison, the mapping
(2ϫ10–4 Ωcm, relative to the sheet resistances between 6.5 was made for silver and oxygen. As seen in the picture, sil-
and 20 Ω) could be estimated. Compared to the resistance ver is present only at the lines, and oxygen (from the glass
of the bulk material (1.587ϫ10–8 Ωm),[24] the films show a substrate) is only visible between the lines. There are only
reduced performance that could be due to the roughness of minor amounts of silver between the lines due to an incom-
the film surface but also grain boundaries in the nanopart- plete takeup of the ink in the printing process.
iculate film with a mean particle size of about 40 nm [calcu-
lated from the single peak widths in the powder pattern of
Figure 4 (black) and the SEM picture of Figure 5c]. To ver-
ify these results, SEM images at a cut edge of the silver film
were recorded. As shown in the picture of Figure 7d, a film
thickness of about 114 nm was examined, which is similar
to the results of the AFM measurements. Slight differences
are due to the surface roughness or the error limit of both
methods.
Printed Silver Films
With the new silver complex, an ink for printing applica-
tions was synthesized. To prove this suitability, we tested
Figure 8. (a) Photo of the silver lines made by using an optical
microscope. (b) SEM image measured in the line to show the com-
pactness of silver in the lines. (c) EDX mapping of silver and oxy-
gen on the structured substrate to show silver only in the printed
area.
the ink in soft printing techniques. We obtained patterned
silver films with two different processes. In Scheme 3i, the
microcontact printing (μcp) process is shown. The precur-
sor solution was adjusted (A) with a polydimethylsiloxane
(PDMS) stamp. Afterwards, the inked stamp was pressed
onto the substrate (B) and the ink had to be transferred to
the substrate (C). With a subsequent precursor–metal trans-
formation, the silver-coated substrate could be obtained. By
By using the μcp technique, it was also possible to obtain
structured silver lines (Figure 9). On the left, an optical
microscope picture of the obtained silver lines is shown. As
seen in the picture, the line width is slightly enlarged relative
to the distances between the lines, which could be due to
smearing of the ink. That was previously observed in the
embossing process but to a lesser extent. The structuring of
silver was again verified by means of EDX mapping.
using
a procedure similar to an embossing process
(Scheme 3ii),[13] the substrate could be coated with a com-
pact precursor solution (A). Subsequently, the PDMS
stamp was pressed into the compact film (B), and after re-
moving a structured coating, the silver complex was ob-
tained (C). Finally, a precursor–metal transformation re-
sulted in the structured silver film.
Figure 9. (a) Photo of the silver lines made by using an optical
microscope. (b) EDX mapping on the structured substrate to show
silver only in the printed area.
Scheme 3. Schematical description of (i) the microcontact printing
and (ii) the embossing process.
With both these soft printing processes, we were able to
To conclude, patterned silver films could be obtained
obtain structured silver coatings. To avoid crystallization of with both methods. With the embossing process, we were
the complex during the printing process, a small amount able to obtain homogeneous structuring. The compactness
(10 wt.-%) of an alkyl derivative of the ligand, 1-tert-butyl- in the silver lines was included by using SEM analysis. EDX
pyrrolidin-2-one, was added to the reaction mixture. Fig- mapping shows silver only in the printed area. Minor
ure 8a shows an optical microscope picture of silver lines amounts of silver between the lines could be due to an in-
with a line width of 20 μm and a distance of 20 μm between complete displacement or takeup of the ink. When using
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J. Fritsch, B. Schumm, R. Biedermann, J. Grothe, S. Kaskel
FULL PAPER
NO3) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
the μcp technique, there is no silver between the lines but,
as shown in Figure 9, the smearing of the ink is enlarged
relative to the embossing process. With regards to the fabri-
cation of transparent electrodes, the printing process has to
be optimized in terms of the nano dimension of the line
width.
Table 1. Crystallographic data for [Ag(Pyl)2]NO3.
[Ag(Pyl)2]NO3
Empirical formula
Mr
Ag1C8H14N3O5
340.09
Conclusion
Space group (no.)
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C2/c (15)
5.3583(10)
15.217(3)
14.986(3)
90
99.296(6)
90
1205.9(4)
4
5080/1364
27.50
A simple reaction was used for the synthesis of a silver–
pyrrolidone complex. The preparation of colorless crystals
of [Ag(Pyl)2]NO3 was possible, and the corresponding crys-
tal structure was solved and refined. Photoreduction by
using UV irradiation and a subsequent thermal treatment
at moderate temperatures allowed for the formation of ele-
mental silver. This precursor–metal transformation was
used in the preparation of thin compact films as well as
structured silver films. For the compact silver coatings, we
measured sheet resistances down to 6.5 Ω on films irradi-
ated with UV light for 20 min and treated at 220 °C for
60 min. These values correspond to specific electrical resist-
ances of down to 6.5ϫ10–5 Ωcm by a calculated film thick-
ness of about 100 nm. Structured silver coatings were ob-
tained by using soft printing techniques.
γ [°]
V [Å3]
Z
Reflections collected/independent
θmax.
GoF on F2
1.098
0.0207
0.0299, 0.0854
0.727, –0.471
Rint
R1, wR2
Max., min. peaks [eÅ–3]
Preparation of [Ag(Pyl)2]NO3: Silver nitrate (0.34 g, 2 mmol,
99.5%, Sigma Aldrich) was dissolved in water (1 mL). 2-Pyrrol-
idone (0.34 g, 4 mmol, 99.0%, Sigma Aldrich) was added to the
solution, and the mixture was stored under exclusion of light. The
product was crystallized by slowly evaporating the solvent after a
few days at room temperature.
Experimental Section
Reduction of [Ag(Pyl)2]NO3 and Preparation of Thin Films: To re-
duce the silver complex, solutions of [Ag(Pyl)2]NO3 in ethanol/
water (4:1) were prepared. Photoreduction was carried out by UV
irradiation for 20 min. Subsequent treatment at T = 220 °C yielded
highly crystalline elemental silver. Compact films of silver were pre-
pared by means of dip coating complex solutions (0.8 m) on pre-
treated glass substrates and subsequent reduction.
Synthesis of 1-tert-Butylpyrrolidin-2-one:[26] THF (20 mL), triethyl-
amine (3.5 g, 34.5 mmol, Acros Organics), and tert-butylamine
(2.52 g, 34.5 mmol, 99.5%, Sigma Aldrich) were mixed in an argon-
flushed flask and cooled to 0 °C. 4-Chlorobutyryl chloride (4.85 g,
34.5 mmol, 99.0%, Sigma Aldrich) was added very slowly under
vigorous stirring, and the mixture was stirred at 0 °C for 2 h. After-
wards, deposited triethylamine hydrochloride was filtered off,
washed two times with THF (10 mL), and the filtrate was concen-
trated under reduced pressure. The raw product was mixed with
ethyl acetate (90 mL), transferred into a separating funnel, and
washed with HCl (1 m, 10 mL) and two times with brine (10 mL).
The organic layer was dried with MgSO4, and the solvent was re-
moved under reduced pressure.
General Remarks: Triethylamine for the synthesis of tert-butyl-
pyrrolidone was dried with CaH2. THF was dried with an M.
Braun MB SPS-800 instrument. All other reagents and solvents
were used as received without further purification. PXRD data
were collected in transmission geometry with a Stoe Stadi-P pow-
der diffractometer and Cu-Kα1 radiation. XRD patterns of thin
films were recorded in Bragg–Brentano geometry with a Panalyt-
ical X’Pert Pro diffractometer, also with Cu-Kα1 radiation [λ =
0.154(05) nm]. For crystallite size determinations, the single-line
size/strain analysis (WinXPow, Stoe) was used. For correction of
the instrumental broadening, a LaB6 standard sample was used.
The sheet resistance of compact silver coatings was measured by
means of a four-point method with a Keithley 2400 source meter
coupled with a four-point probe cascade by Cascade Microtech.
For scanning electron microscopy (SEM) and energy-dispersive X-
ray spectroscopy, a ZEISS DSM-982 Gemini was used. EDX evalu-
ations of the films were performed by applying an acceleration volt-
age of 12 kV at a magnification of 1000ϫ. The surface morphology
and thickness of the silver films were evaluated with a Dimension
3100 atomic force microscope controlled by a Nanoscope IV SPM
controller from Veeco Digital Instruments. The measurements were
performed in tapping mode with a Veeco Nanoprobe RTESP7 tip.
NMR spectra were recorded with a Bruker DRX 500 P (1H:
500 MHz). Elemental analysis for the elements C, H, and N was
performed with a CHNS 932 analyzer from Leco. The respective
metal content was determined with an ICP-OES Vista RL appara-
tus from Varian Inc.
The obtained tert-butyl-4-chlorobutanamide (6 g, 33.8 mmol) dis-
solved in THF (10 mL) was added to a solution of potassium tert-
butoxide (3.9 g, 35 mmol, 99.99% sublimated grade, Sigma Ald-
rich) in THF (25 mL) with vigorous stirring very slowly at 0 °C
(Ar). After 2 h of stirring in an ice bath, the mixture was transfer-
red into a separating funnel, mixed with ethyl acetate (50 mL), and
washed two times with brine (20 mL). The organic layer was dried
with MgSO4, and the solvent was removed under reduced pressure.
After distillation under reduced pressure (14 mbar) at 75 °C, the
X-ray Crystallography: Single-crystal X-ray diffraction data for
[Ag(Pyl)2]NO3 were recorded with a Bruker APEX-II CCD and
Mo-Kα1 radiation. The structure was solved and refined with the
help of SHELX-97 software.[25] Further details of the structural
1
product was obtained as a colorless liquid (yield: 60%). H NMR
spectroscopic analysis provided evidence of the purity of the prod-
analysis are summarized in Table 1. CCDC-842454 ([Ag(Pyl)2]- uct compared to the data in the literature.[26]
882
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A New Silver-Based Precursor as Ink for Soft Printing Techniques
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Acknowledgments
This work was fully funded by the European Social Fund (ESF)
and the Free State of Saxony (project no. 080940507).
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Received: September 2, 2011
Published Online: January 3, 2012
Eur. J. Inorg. Chem. 2012, 878–883
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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