U.S. patent application number 14/358461 was filed with the patent office on 2014-10-09 for method.
The applicant listed for this patent is SURFACE INNOVATIONS LIMITED. Invention is credited to Jas Pal Singh Badyal, Thomas J. Wood.
Application Number | 20140302257 14/358461 |
Document ID | / |
Family ID | 45444288 |
Filed Date | 2014-10-09 |
United States Patent
Application |
20140302257 |
Kind Code |
A1 |
Badyal; Jas Pal Singh ; et
al. |
October 9, 2014 |
METHOD
Abstract
A method for applying a zinc oxide coating to a substrate,
comprising the steps of: (i) applying a nitrogen-containing
aromatic heterocycle functionalised coating to the substrate; (ii)
contacting the nitrogen-containing aromatic heterocycle
functionalised coating with an agent comprising palladium(II)
and/or platinum(II), resulting in a coating comprising complexed
palladium(II) and/or platinum(II); (iii) reducing the complexed
palladium(II) and/or platinum(II) in the coating to palladium(0)
and/or platinum(0); and (iv) contacting the coating comprising
complexed palladium(0) and/or platinum(0) with a zinc salt in the
presence of a reducing agent under aqueous conditions to form a
zinc oxide coating on the substrate.
Inventors: |
Badyal; Jas Pal Singh;
(Durham, GB) ; Wood; Thomas J.; (Durham,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SURFACE INNOVATIONS LIMITED |
Oxfordshire |
|
GB |
|
|
Family ID: |
45444288 |
Appl. No.: |
14/358461 |
Filed: |
November 16, 2012 |
PCT Filed: |
November 16, 2012 |
PCT NO: |
PCT/GB2012/052846 |
371 Date: |
May 15, 2014 |
Current U.S.
Class: |
427/576 ;
427/301 |
Current CPC
Class: |
C23C 18/2006 20130101;
C23C 18/30 20130101; C23C 18/2086 20130101; C23C 18/52 20130101;
C23C 18/1893 20130101 |
Class at
Publication: |
427/576 ;
427/301 |
International
Class: |
C23C 18/30 20060101
C23C018/30 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 2011 |
GB |
1119867.8 |
Claims
1. A method for applying a zinc oxide coating to a substrate,
comprising the steps of: (i) applying a nitrogen-containing
aromatic heterocycle functionalised coating to the substrate; (ii)
contacting the nitrogen-containing aromatic heterocycle
functionalised coating with an agent comprising palladium(II)
and/or platinum(II), resulting in a coating comprising complexed
palladium(II) and/or platinum(II); (iii) reducing the complexed
palladium(II) and/or platinum(II) in the coating to palladium(O)
and/or platinum(O); and (iv) contacting the coating comprising
complexed palladium(O) and/or platinum(O) with a zinc salt in the
presence of a reducing agent under aqueous conditions to form a
zinc oxide coating on the substrate.
2. The method of claim 1, wherein step (i) of applying a
nitrogen-containing aromatic heterocycle functionalised coating to
the substrate is performed by plasma deposition.
3. The method of claim 2, wherein step (i) comprises subjecting the
substrate to a plasma discharge of a monomer possessing aromatic
heterocyclic nitrogen functionality.
4. The method of claim 1, wherein the agent comprising
palladium(II) and/or platinum(II) in step (ii) comprises a salt of
palladium(II) and/or platinum(II).
5. The method of claim 4, wherein the salt of palladium(II) and/or
platinum(II) is a salt of palladium(II).
6. The method of claim 5, wherein the salt of palladium(II) is
palladium chloride.
7. The method of claim 1, wherein the reduction of the complexed
palladium(II) and/or platinum(II) to palladium(O) and/or
platinum(O) in step (iii) occurs in the presence of
dimethylaminoborane (DMAB).
8. The method of claim 1, wherein the zinc salt in step (iv) is
zinc nitrate.
9. The method of claim 1, wherein the reducing agent in step (iv)
is dimethylaminoborane (DMAB).
10. (canceled)
11. A zinc oxide coating obtainable by the method of claim 1.
12. (canceled)
13. An apparatus comprising a substrate and the zinc oxide coating
of claim 11 on the substrate.
14. A method for applying a zinc oxide coating to a substrate,
comprising the steps of: (i) applying a nitrogen-containing
aromatic heterocycle functionalised coating to the substrate by
plasma deposition; (ii) contacting the nitrogen-containing aromatic
heterocycle functionalised coating with a salt of palladium(II)
and/or a salt of platinum(II), resulting in a coating comprising
complexed palladium(II) and/or platinum(II); (iii) reducing the
complexed palladium(II) and/or platinum(II) in the coating to
palladium(O) and/or platinum(O); and (iv) contacting the coating
comprising complexed palladium(O) and/or platinum(O) with a zinc
salt in the presence of a reducing agent under aqueous conditions
to form a zinc oxide coating on the substrate.
15. A method for applying a zinc oxide coating to a substrate,
comprising the steps of: (i) applying a nitrogen-containing
aromatic heterocycle functionalised coating to the substrate by
plasma deposition; (ii) contacting the nitrogen-containing aromatic
heterocycle functionalised coating with a salt of platinum(II),
resulting in a coating comprising complexed platinum(II); (iii)
reducing the complexed platinum(II) in the coating to palladium(O)
and/or platinum(O); and (iv) contacting the coating comprising
complexed palladium(O) and/or platinum(O) with zinc nitrate in the
presence of a reducing agent under aqueous conditions to form a
zinc oxide coating on the substrate.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for applying a
zinc oxide coating to a substrate, a zinc oxide coating obtainable
by such a method, and an apparatus comprising a substrate and such
a zinc oxide coating on the substrate.
BACKGROUND TO THE INVENTION
[0002] Zinc oxide is a transparent semiconductor with wurtzite
(hexagonal close packed) crystal structure and a bandgap of about
3.3 eV. It exhibits many desirable properties, which include
ultraviolet light absorption, photoconductivity, photocatalysis,
photowettability, piezoelectricity, antibacterial behaviour, and
wound-healing. These find technological application in thin film
transistors, dye-sensitized solar cells, kinetic energy harvesters,
transparent electrodes in liquid crystal displays, sunblock, fabric
protection, and medical dressings. Often zinc oxide is utilised as
thin films, which have been produced by RF sputtering, chemical
vapour deposition, vapour diffusion catalysis, spray pyrolysis,
electrodeposition, sol-gel synthesis, or pulsed laser
deposition.
[0003] Inherent limitations of such methods can include their
substrate-dependence (e.g. the requirement for conducting or
physically robust substrates), and often harsh process conditions
(e.g. high temperatures or oxidative chemical environments).
Therefore a strong demand exists for a more universal approach
towards generating zinc oxide surfaces, particularly with a view
towards future adoption of the material's multifunctional
attributes for application in, for example, the emerging field of
wearable electronics (fibertronics).
[0004] Electroless deposition of zinc oxide is potentially
attractive given that it proceeds at mild temperatures (less than
50.degree. C.), is relatively inexpensive, and produces highly
crystalline films. Izaki, M. et al., J. Electrochem. Soc., 1997,
144, L3, and Shinagawa, T. et al., Electrochim. Acta, 2007, 53,
1170, describe a reaction between zinc nitrate and
dimethylaminoborane (DMAB) in the presence of a palladium(0)
catalyst under aqueous conditions (where dimethylaminoborane
reduces the nitrate). Effectively palladium(0) catalyses the
oxidation of dimethylaminoborane:
(CH.sub.3).sub.2NHBH.sub.3+2H.sub.2O.fwdarw.HBO.sub.2+(CH.sub.3).sub.2NH-
.sub.2.sup.++5H.sup.++6e.sup.-
leading to the corresponding reduction of nitrate ions (which
causes a local rise in pH):
NO.sub.3.sup.-+H.sub.2O+2e.sup.-.fwdarw.NO.sub.2.sup.-+2OH.sup.-.
[0005] This increase in pH triggers the growth of zinc oxide
according to the following acid-base reaction:
Zn.sup.2++2OH.sup.-.fwdarw.ZnO+H.sub.2O.
[0006] It is an aim of the present invention to provide a method
for applying a zinc oxide coating to a substrate via electroless
deposition, embodiments of which can enhance the ease and/or
efficiency with which such zinc oxide coatings can be produced, and
can also enhance their performance characteristics.
SUMMARY OF THE INVENTION
[0007] According to a first aspect of the present invention there
is provided a method for applying a zinc oxide coating to a
substrate, comprising the steps of: [0008] (i) applying a
nitrogen-containing aromatic heterocycle functionalised coating to
the substrate; [0009] (ii) contacting the nitrogen-containing
aromatic heterocycle functionalised coating with an agent
comprising palladium(II) and/or platinum(II), resulting in a
coating comprising complexed palladium(II) and/or platinum(II);
[0010] (iii) reducing the complexed palladium(II) and/or
platinum(II) in the coating to palladium(0) and/or platinum(0); and
[0011] (iv) contacting the coating comprising complexed
palladium(0) and/or platinum(0) with a zinc salt in the presence of
a reducing agent under aqueous conditions to form a zinc oxide
coating on the substrate.
[0012] In an embodiment, the agent comprising palladium(II) and/or
platinum(II) is an agent comprising palladium(II).
[0013] FIG. 1 shows a reaction scheme for an embodiment of the
invention.
[0014] Step (i) involves applying a nitrogen-containing aromatic
heterocycle functionalised coating to the substrate.
[0015] Step (i) may be a solventless method for functionalising
solid surfaces with nitrogen containing aromatic heterocyclic
groups.
[0016] In an embodiment, step (i) of applying a nitrogen-containing
aromatic heterocycle functionalised coating to the substrate is
performed by plasma deposition.
[0017] Plasmachemical deposition is an established technique for
the functionalization of surfaces. Film thickness can be easily
controlled, and the process is solventless, conformal, as well as
being substrate-independent, thereby making it well-suited for
application to three-dimensional substrates such as textiles.
[0018] For example, pulsed plasmachemical deposition of, e.g.,
poly(4-vinylpyridine) is one potential way for tethering pyridine
groups onto solid surfaces. This comprises modulating an electrical
discharge in the presence of gaseous precursors containing
polymerizable carbon-carbon double bonds. Mechanistically, there
are two distinct reaction regimes corresponding to the plasma duty
cycle: on- and off-periods (typical timescales are of the order of
microseconds and milliseconds respectively). Namely, monomer
activation and reactive site generation at the surface occur during
each short burst of plasma (via VUV irradiation, ion, or electron
bombardment) followed by conventional carbon-carbon double bond
polymerization proceeding in the subsequent extended off-time (in
the absence of any VUV-, ion-, or electron-induced damage to the
growing film). Extremely high levels of precursor structural
retention within the deposited nanolayer can be achieved, thereby
yielding specific functionalities at the surface. Furthermore, by
programming the pulsed plasma duty cycle, it is possible to control
(i.e. tailor) the surface density of desired chemical groups. The
obtained functional films are covalently attached to the underlying
substrate via free radical sites generated at the interface during
the onset of plasma exposure. Other advantages include the fact
that the plasmachemical approach is quick (single-step),
solventless, energy-efficient, and the reactive gaseous nature of
the electrical discharge provides conformality to a whole host of
substrate materials and complex geometries (e.g. microspheres,
fibres, tubes, etc.). Effectively, any surface which relies on a
specific functionality for its performance can, in principle, be
produced by the aforementioned pulsed plasmachemical methodology.
Examples devised in the past include: anhydride, carboxylic acid,
amine, cyano, epoxide, hydroxyl, halide, thiol, furfuryl,
perfluoroalkyl, perfluoromethylene, and trifluoromethyl
functionalized surfaces.
[0019] WO 2006/111711 A1 describes a method for applying a coating
containing reactive nitrogen functionality contained within an
aromatic heterocyclic structure to a substrate, which method
includes subjecting said substrate to a plasma discharge of a
monomer possessing said heterocyclic nitrogen functionality.
[0020] In an embodiment, step (i) of the method of the invention
uses a pulsed plasma deposition procedure.
[0021] In an embodiment, step (i) of the method of the invention
uses a substantially continuous wave plasma deposition
procedure.
[0022] In an embodiment, step (i) of the method of the invention
uses a low average-power plasma deposition procedure. In an
embodiment, such a procedure occurs at a power density of up to 10
mW/cm.sup.3. Low average-power plasma polymerisation can
potentially overcome the limitations of other techniques for the
production of surfaces bearing nitrogen containing aromatic
heterocyclic moieties.
[0023] However, not many surface sites are required for palladium
catalyst attachment, since the electroless deposition process is
autocatalytic, so the conventional high structural retention
criteria for the deposited plasma polymer layer are not compulsory,
and the electroless deposition process can work at higher average
powers as well.
[0024] In an embodiment, step (i) comprises subjecting the
substrate to a plasma discharge of a monomer possessing aromatic
heterocyclic nitrogen functionality.
[0025] Step (i) of the method of the invention may use monomers
possessing at least one conventionally polymerisable unsaturated
functional group (e.g. selected from acrylate, methacrylate,
alkene, styrene, alkyne and/or derivatives thereof) that is
substantially distinct from the nitrogen containing aromatic ring
structure desired at the substrate surface (e.g. selected from
pyridine, pyrrole, quinoline, isoquiniline, purine, pyrimidine,
indole and/or derivatives thereof). Suitable monomers are described
in WO 2006/111711 A1. In an embodiment, the nitrogen containing
aromatic ring structures are derived from pyridine. A particular
example of a suitable monomer is a vinyl pyridine such as, for
example, 4-vinyl pyridine:
##STR00001##
[0026] In an embodiment, the nitrogen-containing aromatic
heterocyclic groups in the nitrogen-containing aromatic heterocycle
functionalised coating are derived from pyridine.
[0027] Step (i) of the method of the invention may use a plasma
polymerisation procedure as described in WO 2006/111711 A1.
[0028] Step (i) of the method of the invention may result in a
product wholly coated in a polymer coating possessing nitrogen
containing aromatic heterocyclic functionality. Alternatively, the
nitrogen containing aromatic heterocycle functionalised polymer
coating is only applied to one or more selected surface domains of
the substrate. The applications of such patterned substrates
include fields where the spatial control of, for example, surface
wettability is a consideration. The restriction of the nitrogen
containing aromatic heterocycle coating to specific surface domains
may be achieved by the methods described in WO 2006/111711 A1.
[0029] Instead of using plasma deposition, step (i) of applying a
nitrogen-containing aromatic heterocycle functionalised coating to
the substrate may also be performed by a technique selected from,
for example, spin coating, solvent casting, UV induced graft
polymerization, and the use of self-assembled monolayers
(SAMs).
[0030] Once the nitrogen-containing aromatic heterocycle
functionalised coating has been applied to the substrate, the
nitrogen containing aromatic heterocyclic groups are further
complexed in step (ii).
[0031] Step (ii) involves contacting the nitrogen-containing
aromatic heterocycle functionalised coating with an agent
comprising palladium(II) and/or platinum(II), resulting in a
coating comprising complexed palladium(II) and/or platinum(II).
[0032] Palladium and/or platinum centres can be coordinated to
nitrogen-containing heterocycles such as pyridine via electron lone
pair interaction. WO 2006/111711 A1 describes a method where, after
the application of a nitrogen containing aromatic heterocycle
functionalised coating to a surface as described above, the surface
is contacted with a solution of a metal salt, such as palladium
chloride, under conditions such that the metal salt complexes with
the surface heterocyclic groups.
[0033] In an embodiment, the agent comprising palladium(II) and/or
platinum(II) in step (ii) comprises a salt of palladium(II) and/or
platinum(II). In an embodiment, the salt of palladium(II) and/or
platinum(II) is a salt of palladium(II). In an embodiment, the salt
of palladium(II) is a halide such as, for example, palladium
chloride.
[0034] Step (ii) of the method of the invention may use the
procedure for complexation of palladium chloride with a nitrogen
containing aromatic heterocycle functionalised coating as described
in WO 2006/111711 A1.
[0035] Step (iii) involves reducing the complexed palladium(II)
and/or platinum(II) in the coating to palladium(0) and/or
platinum(0).
[0036] In an embodiment, the reduction of the complexed
palladium(II) and/or platinum(II) to palladium(0) and/or
platinum(0) in step (iii) occurs in the presence of
dimethylaminoborane (DMAB).
[0037] In an embodiment, step (iii) of the method of the invention
entails reduction of complexed palladium(II) to palladium(0) by a
reducing agent such as, for example, DMAB.
[0038] Step (iv) involves contacting the coating comprising
complexed palladium(0) and/or platinum(0) with a zinc salt in the
presence of a reducing agent under aqueous conditions to form a
zinc oxide coating on the substrate.
[0039] In an embodiment, the zinc salt in step (iv) is zinc
nitrate.
[0040] In an embodiment, the reducing agent in step (iv) is
DMAB.
[0041] In an embodiment, steps (iii) and (iv) of the method of the
invention entail reduction of complexed palladium(II) centres to
palladium(0) by DMAB, followed by the reaction between zinc nitrate
and DMAB in the presence of the palladium(0) centres.
[0042] In an embodiment, steps (iii) and (iv) of the method of the
invention occur together as a one-pot reaction. In an embodiment,
the reducing agent which reduces the complexed palladium(II) and/or
platinum(II) in the coating to palladium(0) and/or platinum(0) in
step (iii) may also act as the reducing agent in step (iv).
[0043] In an embodiment, steps (iii) and (iv) of the method of the
invention entail the in situ reduction of complexed palladium(II)
centres to palladium(0) by DMAB, directly followed by the reaction
between zinc nitrate and the DMAB in the presence of the resultant
palladium(0) centres.
[0044] According to a second aspect of the present invention there
is provided a zinc oxide coating obtainable by, or which has been
produced using, a method according to the first aspect.
[0045] In an embodiment, the zinc oxide coating is an antibacterial
coating.
[0046] In an embodiment, the zinc oxide coating is for use in UV
protection.
[0047] According to a third aspect of the present invention there
is provided an apparatus comprising a substrate and a zinc oxide
coating according to the second aspect.
[0048] In an embodiment, the apparatus is a medical dressing.
[0049] In an embodiment, the apparatus is a thin film
transistor.
[0050] In an embodiment, the apparatus is a dye-sensitized solar
cell.
[0051] In an embodiment, the apparatus is a kinetic energy
harvester.
[0052] In an embodiment, the apparatus is an electrode. In an
embodiment, the apparatus is a transparent electrode, e.g. in a
liquid crystal display.
[0053] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", mean "including but not
limited to", and do not exclude other moieties, additives,
components, integers or steps. Moreover the singular encompasses
the plural unless the context otherwise requires: in particular,
where the indefinite article is used, the specification is to be
understood as contemplating plurality as well as singularity,
unless the context requires otherwise.
[0054] Preferred features of each aspect of the invention may be as
described in connection with any of the other aspects. Other
features of the invention will become apparent from the following
examples. Generally speaking the invention extends to any novel
one, or any novel combination, of the features disclosed in this
specification (including any accompanying claims and drawings).
Thus features, integers, characteristics, compounds, chemical
moieties or groups described in conjunction with a particular
aspect, embodiment or example of the invention are to be understood
to be applicable to any other aspect, embodiment or example
described herein unless incompatible therewith. Moreover unless
stated otherwise, any feature disclosed herein may be replaced by
an alternative feature serving the same or a similar purpose.
[0055] Where upper and lower limits are quoted for a property, for
example for the concentration of a component or a temperature, then
a range of values defined by a combination of any of the upper
limits with any of the lower limits may also be implied.
[0056] In this specification, references to properties such as film
thicknesses, contact angles and the like are--unless stated
otherwise--to properties measured under ambient conditions, i.e. at
atmospheric pressure and at a temperature of from 18 to 25.degree.
C., for example about 20.degree. C.
DETAILED DESCRIPTION
[0057] Embodiments of the present invention will now be further
described with reference to the following non-limiting examples and
the accompanying figures, of which:
[0058] FIG. 1 shows a reaction scheme for an embodiment of the
invention, namely the palladium catalyst seeding of pulsed plasma
deposited poly(4-vinylpyridine) films followed by electroless
growth of zinc oxide.
[0059] FIG. 2 shows XPS spectra of: (a) pulsed plasma deposited
poly(4-vinylpyridine); (b) pulsed plasma deposited
poly(4-vinylpyridine) seeded with palladium(II) chloride; (c)
electroless zinc oxide growth onto palladium(II) chloride seeded
pulsed plasma deposited poly(4-vinylpyridine).
[0060] FIG. 3 shows infrared spectra of: (a) 4-vinylpyridine
monomer; and (b) pulsed plasma deposited poly(4-vinylpyridine). *
denotes polymerizable alkene bond absorbances in precursor.
[0061] FIG. 4 shows X-ray diffraction analysis of 500 nm thick zinc
oxide film electrolessly grown onto palladium seeded pulsed plasma
deposited poly(4-vinylpyridine).
[0062] FIG. 5 is an optical microscope image of zinc oxide film
electrolessly grown onto palladium seeded pulsed plasma deposited
poly(4-vinylpyridine).
[0063] FIG. 6 shows 500 nm thick zinc oxide grown by electroless
deposition onto non-conducting glass: (a) electrical conductivity
and equilibrium water contact angle following UV irradiation in air
(switched off at 750 s); and (b) equilibrium water contact angle
recovery of zinc oxide film following UV light extinction at 750 s
(offset to time=0 h).
[0064] FIG. 7 shows XPS C(1s) envelope of electrolessly deposited
zinc oxide onto silicon wafer: (a) no UV exposure, and (b) 750 s UV
exposure.
[0065] FIG. 8 shows a mechanism illustrating change in adsorbed
species on zinc oxide surface during UV irradiation followed by
subsequent oxygen readsorption over time.
[0066] Experiments were carried out to illustrate embodiments of
the invention.
[0067] Experimental Methods
[0068] Film thickness measurements were carried out using a
spectrophotometer (nkd-6000, Aquila Instruments Ltd.).
Transmittance and reflectance curves across the 300-1000 nm
wavelength range were fitted to a Cauchy model for dielectrics
using a modified Levenberg-Marquardt method. The pulsed plasma
poly(4-vinylpyridine) deposition rate was measured to be 15.+-.2 nm
min.sup.-1.
[0069] X-ray photoelectron spectroscopy (XPS) characterization of
the functionalized substrates was carried out using a VG Escalab
spectrometer equipped with an unmonochromated Mg-K.alpha. X-ray
source (1253.6 eV) and a concentric hemispherical analyzer
operating in constant analyzer energy mode (pass energy=20 eV) with
the photoelectrons collected at a take-off angle of 12.degree. from
the substrate normal.
[0070] Elemental compositions were calculated using sensitivity
factors derived from chemical standards:
C(1s):O(1s):N(1s):Pd(3d):Zn(2p) equals 1.00:0.36:0.57:0.05:0.05.
All binding energies were referenced to the C(1s) hydrocarbon peak
at 285.0 eV. The core level spectra were fitted to a linear
background.
[0071] Fourier transform infrared (FTIR) analysis of the deposited
films was undertaken using a Perkin-Elmer Spectrum One spectrometer
equipped with a liquid nitrogen cooled MCT detector.
Reflection-absorption (RAIRS) measurements utilized a variable
angle accessory (Specac Ltd) set at 66.degree. fitted with a KRS-5
polarizer to remove the s-polarized component. All spectra were
averaged over 128 scans at a resolution of 4 cm.sup.-1.
[0072] Depth profiling measurements were undertaken by the
Rutherford backscattering technique using a .sup.4He.sup.+ ion beam
(5SDH Pelletron Accelerator). Backscattered .sup.4He.sup.+ ions
were detected with 19 keV resolution using a PIPS detector.
[0073] X-ray diffraction patterns of electrolessly deposited zinc
oxide layers (1 .mu.m thick, mounted on a silicon (100) substrate)
were collected using a powder diffractometer (Bruker d8) equipped
with a Cu tube (1.5418 .ANG. wavelength), and a linear
position-sensitive detector (Lynx Eye with a Ni filter). Data were
collected from 5-65.degree. 2.theta. with a step size of
0.02.degree..
[0074] The electrolessly deposited zinc oxide layers were imaged
with an optical microscope (Olympus BX40) fitted with a .times.50
magnification lens.
[0075] For the photochemical studies, ultraviolet light from a low
pressure Hg--Xe arc lamp running at 100 W (Oriel Corporation, model
6136, emitting a strong line spectrum in the 240-600 nm region) was
focused onto deposited zinc oxide films at a focal length of 30
cm.
[0076] Electrical conductivity measurements were made using a pair
of parallel silver electrodes (6 mm length and separated by 1 mm)
painted onto the zinc oxide film which had been deposited onto a
non-conducting glass substrate. The electrical conductivity
behaviour of the zinc oxide films was found to be ohmic both prior
to and following UV exposure (0-200 V range). In the case of
UV-response curves, a constant voltage of 10 V was applied and the
electric current measured with a Keithley 2400 Source Meter.
[0077] Sessile drop water contact angle measurements were performed
at ambient temperature using a video capture apparatus in
combination with a motorized syringe (VCA2500XE, A.S.T. Products
Inc.) dispensing a 2 .mu.L droplet size. High purity water (B.S.
3978 grade 1) was employed as the probe liquid.
[0078] Antibacterial testing was carried out according to a
modified form of the Japanese Industrial Standard Protocol. A
bacterial cell culture (wild-type Escherichia coli K-12 lab strain
W3110) grown to an A.sub.650nm of 0.4 was applied in minimal salts
buffer onto zinc oxide coated nonwoven polypropylene cloth and
uncoated controls. Samples were incubated for 24 h at 37.degree. C.
in a moist, dark environment. Excess culture and cloth were
transferred to a 2 ml spin column and centrifuged at 9000 rpm for 2
min to maximise recovery. The recovered culture was vortexed to
resuspend cells; tenfold dilutions were performed and spotted onto
LB agar. Plates were incubated overnight at 30.degree. C., after
which the number of colonies was counted. To control for cell
absorption onto samples, the above procedure was repeated with 1
min exposure of bacteria on coated and uncoated cloth.
EXAMPLES
[0079] Pulsed plasmachemical deposition was undertaken in a
cylindrical glass reactor (4.5 cm diameter, 500 cm.sup.3 volume,
1.times.10.sup.-3 mbar base pressure, leak rate better than
1.7.times.10.sup.-9 mol s.sup.-1). A copper coil (4 mm diameter, 10
turns) wound around the reactor was attached to a 13.56 MHz radio
frequency (RF) power supply via an L-C matching unit. The whole
apparatus was enclosed in a Faraday cage. The chamber was evacuated
using a 30 L min.sup.-1 rotary pump attached to a liquid nitrogen
cold trap and the system pressure monitored with a Pirani gauge. A
pulse signal generator was used to trigger the RF power generator
and an oscilloscope monitored the pulse shape. Prior to deposition,
the glass reactor was cleaned by scrubbing with detergent, rinsing
in acetone, oven drying, and then running a 40 W continuous wave
air plasma for 30 min. Next, silicon (100) wafers (Silicon Valley
Microelectronics Inc.), glass coverslips (VWR International Ltd),
or nonwoven polypropylene cloth pieces (Corovin GmbH) were inserted
into the chamber, and the system pumped back down to base pressure.
At this stage, the reactor was purged with 4-vinylpyridine
precursor (+95%, Sigma-Aldrich, further purified with three
freezepump-thaw cycles) at a pressure of 0.2 mbar for 5 min
followed by ignition of the electrical discharge. The optimum duty
cycle for pyridine ring retention was on-period=100 .mu.s and
off-period=4 ms in combination with peak power=40 W. Upon
completion of deposition, the precursor was allowed to continue to
flow through the system for a further 5 min in order to quench any
trapped reactive sites contained within the deposited film.
[0080] The poly(4-vinylpyridine) functionalized surfaces were then
immersed into an aqueous catalyst solution containing 2 .mu.M
palladium(II) chloride (+99.999%, Alfa Aesar), 3.0 M sodium
chloride (+99.5%, Sigma), and 0.5 M sodium citrate dehydrate (+99%,
Aldrich) (which had been adjusted to pH 4.5 with citric acid
monohydrate (+99%, Aldrich)) for 12 h, and subsequently washed in
deionized water.
[0081] Next, the palladium(II) chloride immobilized surfaces were
placed into an aqueous chemical bath containing 0.05 M zinc nitrate
(+98%, Sigma-Aldrich) and 0.05 M dimethylaminoborane (+97%,
Sigma-Aldrich) at a temperature of 323 K for 2 h. Following zinc
oxide growth, the surface was rinsed with deionized water.
[0082] XPS characterization of pulsed plasma deposited
poly(4-vinylpyridine) layers confirmed the presence of only carbon
and nitrogen at the surface, with no Si(2p) signal showing through
from the underlying silicon substrate, see Table 1. Furthermore, a
good correlation was found to exist between the atomic percentages
calculated for the precursor (theoretical) and pulsed plasma
deposited poly(4-vinylpyridine) films, which is consistent with a
high level of structural retention. Immersion into palladium(II)
chloride solution gave rise to the appearance of Pd(3d.sub.5/2) and
Pd(3d.sub.3/2) signals at 338.3 eV and 343.5 eV respectively and a
Cl(2p) peak at 198.8 eV. This can be taken as being indicative of
PdCl.sub.2 complexation to the poly(4-vinylpyridine) surface (the
presence of the O(1s) peak at 532.7 eV is due to water absorption
from the aqueous palladium(II) chloride solution), see FIG. 1 and
FIG. 2.
TABLE-US-00001 TABLE 1 XPS Elemental Compositions. Surface % C % N
% Pd % Cl % O % Zn 4-Vinylpyridine(theoretical) 87.5 12.5 0.0 0.0
0.0 0 Pulsed plasma poly(4-vinylpyridine) 87.3 .+-. 0.5 11.8 .+-.
0.5 0.0 0.0 0.9 .+-. 0.5 0 Palladium(II) seeded pulsed plasma 63.3
.+-. 0.5 8.3 .+-. 0.5 2.9 .+-. 0.2 5.5 .+-. 0.5 20.0 .+-. 0.5 0
poly(4-vinylpyridine) Deposited zinc oxide.sup.a 5.0 .+-. 0.9 0.0
0.0 0.0 63.0 .+-. 0.8 32 .+-. 1 Deposited zinc oxide after 750 s UV
5.5 .+-. 0.9 0.0 0.0 0.0 62.5 .+-. 0.8 32 .+-. 1 exposure.sup.a
.sup.a(No discemible difference was observed in the XPS cone level
peak shapes)
[0083] For the 4-vinylpyridine monomer, the following infrared band
assignments can be made: vinyl C.dbd.C stretching (1634 cm.sup.-1),
aromatic quadrant C.dbd.C stretching (1597 cm.sup.-1 and 1548
cm.sup.-1), aromatic semicircle C.dbd.C and C.dbd.N stretching
(1495 and 1409 cm.sup.-1 respectively), and .dbd.CH.sub.2 wag (927
cm.sup.-1), see FIG. 3. All of these bands were discernible
following pulsed plasma deposition apart from the vinyl
carbon-carbon double bond features (which disappear during
polymerization). This is consistent with the high level of
structural retention associated with pulsed plasma deposition.
[0084] Control samples of pulsed plasma deposited
poly(4-vinylpyridine) without palladium(II) chloride seeding gave
rise to the absence of electroless zinc oxide growth, which
highlights the key role of the immobilized palladium catalyst. In
contrast, zinc oxide films were clearly visible to the naked eye
for the palladium catalyst seeded pulsed plasma deposited
poly(4-vinylpyridine) films. Only zinc, oxygen and a trace amount
of carbon (due to atmospheric adsorption) were detectable by XPS,
see FIG. 2 and Table 1. The absence of N(1s) and Pd(3d) signals
confirmed complete coverage of the catalyst seeded
poly(4-vinylpyridine) layer by zinc oxide. Ion beam analysis
determined the zinc oxide film growth rate to be 230.+-.20 nm
h.sub.-1.
[0085] X-ray diffraction characterisation showed peaks at
31.9.degree., 34.5.degree., 36.3.degree., 47.6.degree.,
56.6.degree., and 62.9.degree., which are consistent with zinc
oxide in the wurtzite structure (hexagonal close packed), see FIG.
4. Rietveld refinement confirmed that the ratio of peak intensities
matches that expected for wurtzite zinc oxide. Therefore the films
are polycrystalline and randomly oriented. Peak widths measured for
the powder diffraction patterns suggest a minimum crystallite size
of 25 nm; although a number of other parameters, including lattice
strain, can also be contributing factors.
[0086] Optical microscopy showed a roughened surface corresponding
to the different crystalline faces, see FIG. 5.
[0087] During UV irradiation, zinc oxide films deposited onto flat
non-conducting glass pieces exhibited a marked increase in
electrical conductivity rising from a dark conductivity value of
10.sup.-7 mS cm.sup.-1 up to 1.5 mS cm.sup.-1 after 750 s, see FIG.
6. The electrical conductivity was observed to slowly decay
following termination of UV exposure. In the case of storage under
ultra high vacuum (pressure <10.sup.-8 mbar), photoconductivity
was retained following a period of weeks, whilst UV irradiation
under vacuum gave rise to an increase in electrical
conductivity.
[0088] High water contact angle values of 150.degree., but with a
large contact angle hysteresis, were measured for zinc oxide coated
flat silicon substrates, see Table 2. Exposure of these surfaces to
UV radiation in air resulted in a significant drop in the
equilibrium water contact angle attributable to surface
hydrophilicity, see Table 2 and FIG. 6. Exposure to higher
intensity UV light over the same period of time caused the contact
angle to drop below 20.degree.. Following termination of UV
exposure, the contact angle slowly recovers to its original value
of 150.degree. over a period of around 3 weeks, FIG. 6. However,
when such zinc oxide coated silicon wafers, which had been exposed
to UV in air, were stored under ultra high vacuum conditions
(<10.sup.-8 mbar), there was no recovery in contact angle (i.e.
remained at 60.degree. over a period of 4 weeks). Also, UV
irradiation of zinc oxide coated samples under ultra high vacuum
conditions or pure O.sub.2 (rather than in air) produced no
discernible change in the contact angle (i.e. remained at
150.degree.). These control experiments highlight that contact
angle decay during UV exposure and subsequent hydrophobic recovery
upon UV termination involve surface reaction with air. The XPS
C(1s) envelope corresponding to small amounts of adsorbed
hydrocarbon species (285.0 eV) did not change, see FIG. 7.
TABLE-US-00002 TABLE 2 Water Contact Angle Measurements for Zinc
Oxide Coated Substrates. Water Contact Angle/.degree. Substrate
Equilibrium Advancing Receding Hysteresis Silicon wafer 150 .+-. 1
152 .+-. 1 35 .+-. 1 117 .+-. 2 Silicon wafer after 60 .+-. 1 65
.+-. 1 7 .+-. 1 58 .+-. 2 750 s UV in air Silicon wafer after 150
.+-. 1 152 .+-. 1 35 .+-. 1 117 .+-. 2 750 s UV UHV Silicon wafer
after 149 .+-. 1 151 .+-. 1 35 .+-. 1 116 .+-. 2 750 s UV in
O.sub.2 Nonwoven 154 .+-. 1 154 .+-. 1 154 .+-. 1 0 .+-. 2 Nonwoven
after 154 .+-. 1 154 .+-. 1 154 .+-. 1 0 .+-. 2 750s UV in air
[0089] Electroless growth of zinc oxide onto pulsed plasma
poly(4-vinylpyridine) coated non-woven polypropylene substrates
gave rise to superhydrophobicity (high equilibrium water contact
angles, exceeding 150.degree., in combination with low contact
angle hysteresis), see Table 2. In this case, the water repellency
of the zinc oxide surface was not found to be perturbed by exposure
to UV radiation, see Table 2.
[0090] Zinc oxide coated polypropylene cloth pieces also displayed
significant antibacterial activity (up to a log kill of 2.9)
towards the Gram-negative bacterium, Escherichia coli, see Table 3.
Control samples of the polypropylene cloth pieces exhibited no
antibacterial activity, whereas a reduction of only log 0.2 was
observed following exposure to the pulsed plasma
poly(4-vinylpyridine) coated cloth. This small drop can be
attributed to the absorption (rather than killing) of cells onto
the hydrophilic layer, since a similar result was obtained
following a 1 min incubation period (as opposed to 24 h).
TABLE-US-00003 TABLE 3 Antibacterial Activity Against the
Gram-negative Bacterium, Escherichia coli for nonwoven cloth.
Fraction of Cells Log Substrate Recovered After 24 h Kill Uncoated
1 0 Pulsed plasma poly(4-vinylpyridine) 0.6 .+-. 0.3 0.2 Zinc oxide
0.0012 .+-. 0.0009 2.9 Zinc oxide irradiated with UV light 0.009
.+-. 0.005 2.0
[0091] In these experiments, XPS and infrared analyses have shown
that a variety of substrates can be coated with structurally
well-defined poly(4-vinylpyridine) layers (in marked contrast to
earlier high power continuous wave plasma polymers derived from
4-vinylpyridine). Subsequent seeding with catalytic palladium
centres can provide for the localised electroless growth of zinc
oxide. An additional benefit of this approach is that the
functional polymer nanolayer can serve to protect the underlying
substrate material from subsequent chemical processing steps such
as, for example, the oxidising and reducing agents contained within
the zinc oxide electroless deposition solution.
[0092] The semiconducting nature of zinc oxide stems from natural
doping (n-type) in the form of interstitial singly charged zinc
cations (Zn.sup.+), which lie close to the conduction band, and so
can be easily thermally ionized (to Zn.sup.2++e), thus supplying
electrons to the conduction band. The aforementioned excess
electrons leaving behind interstitial Zn.sup.+ can become trapped
at the surface by adsorbed oxygen (O.sub.2(ads)) to give
O.sub.2.sup.-.sub.(ads) species. Zinc oxide coated pulsed plasma
poly(4-vinylpyridine) films display photoconductivity during UV
light exposure, see FIG. 6. Contributions to the shape of the
photoconductivity curve can be split into fast reversible (electron
promotion from valence to conduction band), and slow irreversible
(surface chemistry of adsorbed species). In addition, when zinc
oxide is exposed to UV photon radiation with energy greater than or
equal to its bandgap (3.3 eV) electron promotion from the valence
band to the conduction band leads to electron-hole pair formation.
These electrons can also become trapped by physisorbed oxygen at
the surface to form chemisorbed O.sub.2.sup.-.sub.(ads) up to a
self-limiting concentration of O.sub.2.sup.-.sub.(ads) due to
electrostatic repulsion between O.sub.2.sup.-.sub.(ads) at the
surface. Such O.sub.2.sup.-.sub.(ads) species are capable of
attracting holes from the bulk, which migrate towards the surface
to combine with the O.sub.2.sup.-.sub.(ads) species leading to the
formation of a surface vacancy and photodesorption of molecular
oxygen:
ZnO+hv.fwdarw.ZnO+e-h (electron-hole pair)
e+O.sub.2(ads).fwdarw.O.sub.2.sup.-.sub.(ads)
h+O.sub.2.sup.-.sub.(ads).fwdarw.O.sub.2(g).uparw.+.quadrature.
[0093] Following the loss of molecular oxygen from the surface via
desorption, further electrons promoted from the valence band to the
conduction band during UV irradiation are no longer able to become
trapped by adsorbed oxygen, and instead contribute to electrical
conductivity. Conversely, readsorption of oxygen onto the surface
for instance after the termination of photodesorption leads to a
decay in conductivity, see FIG. 6. Therefore in the case of zinc
oxide films, the shift in equilibrium between oxygen desorption and
readsorption processes will dominate the shape of the
photoconductivity rise and decay curves due to the inherent high
surface area of the deposited material. In the case of storage
under vacuum following termination of UV irradiation,
photoconductivity was retained over a period of weeks, which is
consistent with the electrical conductivity decay mechanism being
governed by molecular oxygen adsorption.
[0094] Reversible wettability was also observed following UV
irradiation of zinc oxide coated flat substrates. Clean zinc oxide
surfaces are hydrophilic (equilibrium water contact angles
<35.degree., along with many other metal oxides), but in ambient
conditions they are known to attract amphiphilic, carbon-containing
contaminants present in the atmosphere. These adspecies act like
surfactants with the hydrophilic domain attracted towards the zinc
oxide surface (bonding via electron lone pair interaction with the
electron depletion layer at the zinc oxide surface); the
hydrophobic domain of the carbon-containing species is therefore
responsible for the widely observed hydrophobicity of zinc oxide
surfaces in ambient conditions. Two mechanisms are proposed in the
literature for the UV-induced switch to hydrophilicity of zinc
oxide surfaces: firstly, the UV light induces a photocatalytic
reaction at the zinc oxide surface resulting in removal of carbon
contaminants (as in the case of titanium dioxide self-cleaning
surfaces); or secondly, the UV light leads to desorption of
molecular oxygen from the zinc oxide surface followed by
dissociative water adsorption. The lack of change in
carbon:oxygen:zinc XPS elemental ratios and C(1s) envelope shapes
of the zinc oxide surface before and after UV irradiation suggests
that the first mechanism is unlikely to be the predominant factor,
see Table 1 and FIG. 7. Photodesorption of O.sub.2(g) from the zinc
oxide surface creates vacancies, which can allow water molecule
(present in the ambient air) adsorption. The necessity for water to
be present during UV exposure (as demonstrated by the control
experiments, where contact angle did not drop for zinc oxide
surfaces which were UV irradiated under ultra high vacuum or pure
oxygen and subsequently exposed to air, see Table 2) indicates a
photo-assisted dissociative water adsorption mechanism, see FIG. 2.
Where the photogenerated hydroxide groups are responsible for the
surface switching from hydrophobic to hydrophilic. The contact
angle fully recovers over a matter of weeks following extinction of
UV irradiation (the exact duration depending upon UV intensity),
see FIG. 6. Verification of oxygen readsorption processes
underpinning the reversible surface wetting behaviour back towards
hydrophobic recovery was achieved by observing the lack of contact
angle increase for when zinc oxide was UV irradiated in air (on
flat substrate) and then stored under ultra high vacuum for
extended periods of time. Therefore, in air, the water adsorbed at
the surface during photoirradiation is thermodynamically displaced
by oxygen species over a period of time. The speed at which this
happens (leading to recharged hydrophobicity) is slow, and
determines the long hydrophobicity recovery times seen for zinc
oxide. This photochemical contact angle decay observed for UV
exposure in air and reversal afterwards shows no correlation to the
fast (purely electronic) bulk electron photoconduction processes,
due to the far slower oxygen desorption and readsorption surface
chemistry processes, see FIG. 8.
[0095] Zinc oxide displays similar surface chemistry during UV
irradiation to that reported for titanium dioxide (another metal
oxide semiconductor with a comparable bandgap). It is postulated
that molecular oxygen adsorbs at titanium dioxide surface defect
sites and it then traps a photogenerated electron to become
O.sub.2.sup.-.sub.(ads) species. In a similar fashion to zinc
oxide, such O.sub.2.sup.-.sub.(ads) species undergo photodesorption
as O.sub.2(g) from TiO.sub.2 surfaces during UV irradiation.
Therefore, the same behaviour is observed for both zinc oxide and
titanium dioxide surfaces in relation to photoconductivity and
photo-switchable wetting.
[0096] By depositing zinc oxide onto a roughened surface (such as
nonwoven polypropylene), the aforementioned reversible wettability
changes for flat substrates (such as silicon wafers or glass
coverslips) were not observed; water contact angle hysteresis
became negligible, see Table 2. Theoretical studies predict that
for ideal surfaces (where the water droplet is in contact with all
of the surface) water contact angle hysteresis should increase with
roughness reaching a maximum beyond which the liquid is unable to
completely wet the whole surface. At this point, surface wetting
obeys the Cassie-Baxter relationship, where the roughness is so
great that air becomes trapped during liquid-surface contact giving
rise to incomplete wetting. In the present experiments, it is the
rough texture (visible to the naked eye) of deposited zinc oxide
films, which leads to the high equilibrium water contact angle,
FIG. 5.
[0097] Furthermore, by changing the substrate from two-dimensional
(flat) to porous three-dimensional (nonwoven), there is an
enhancement of Cassie-Baxter behaviour, culminating in very low
contact angle hysteresis, see Table 2. Although in the case of zinc
oxide coated nonwoven polypropylene the surface energy will
increase during UV exposure (as shown in FIG. 8), there is
sufficient Cassie-Baxter behaviour for superhydrophobicity to be
sustained.
[0098] Significant bactericidal effects were also measured for zinc
oxide coated polypropylene cloth substrates. Escherichia coli was
tested because it is renowned for being resistant to killing by
many conventional antibacterial surfaces. The observed
antibacterial activity in the present study is most likely
attributable to the presence of oxygenated radical species on the
zinc oxide surface. For instance, oxygen radical species can be
precursors to the formation of molecules such as hydrogen peroxide,
which are toxic towards bacteria by causing damage to the bacterial
cell wall, proteins and nucleic acids. This antibacterial mechanism
does not appear to be closely linked to the UV induced
photoconduction and photowettability mechanism, as seen by the
continued killing of bacteria by zinc oxide surfaces which had been
UV irradiated beforehand, see Table 3.
[0099] Palladium catalyst seeded pulsed plasma
poly(4-vinylpyridine) nanolayers have been employed for the
electroless growth of crystalline zinc oxide thin films. These are
found to display photoconductivity, photo-switchable wetting,
superhydrophobicity, and antibacterial properties.
* * * * *