WO2013072697A2

WO2013072697A2 - Method

Info

Publication number: WO2013072697A2
Application number: PCT/GB2012/052846
Authority: WO
Inventors: Jas Pal Singh Badyal; Thomas J. Wood
Original assignee: Surface Innovations Limited
Priority date: 2011-11-17
Filing date: 2012-11-16
Publication date: 2013-05-23
Also published as: US20140302257A1; GB201220620D0; CN104169464A; EP2780490A2; WO2013072697A3; GB201119867D0; GB2497182A; KR20140131907A; GB2497182B; TW201329285A; JP2014533776A

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.

Description

METHOD

Field of the invention

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

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, photocata lysis,

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.

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).

Electroless deposition of zinc oxide is potentially attractive given that it proceeds at mild temperatures (less than 50° 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(O) catalyst under aqueous conditions (where dimethylaminoborane reduces the nitrate). Effectively palladium(O) catalyses the oxidation of dimethylaminoborane:

leading to the corresponding reduction of nitrate ions (which causes a local rise in pH):

This increase in pH triggers the growth of zinc oxide according to the following acid- base reaction:

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

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:

(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.

In an embodiment, the agent comprising palladium(II) and/or platinum(II) is an agent comprising palladium(II). Figure 1 shows a reaction scheme for an embodiment of the invention.

Step (i) involves applying a nitrogen-containing aromatic heterocycle functionalised coating to the substrate.

Step (i) may be a solventless method for functionalising solid surfaces with nitrogen containing aromatic heterocyclic groups.

In an embodiment, step (i) of applying a nitrogen-containing aromatic heterocycle functionalised coating to the substrate is performed by plasma deposition.

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.

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. WO 2006/111711 Al 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.

In an embodiment, step (i) of the method of the invention uses a pulsed plasma deposition procedure.

In an embodiment, step (i) of the method of the invention uses a substantially continuous wave plasma deposition procedure.

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/cm³. Low average-power plasma polymerisation can potentially overcome the limitations of other techniques for the production of surfaces bearing nitrogen containing aromatic heterocyclic moieties.

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.

In an embodiment, step (i) comprises subjecting the substrate to a plasma discharge of a monomer possessing aromatic heterocyclic nitrogen functionality.

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 Al. 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:

In an embodiment, the nitrogen-containing aromatic heterocyclic groups in the nitrogen-containing aromatic heterocycle functionalised coating are derived from pyridine.

Step (i) of the method of the invention may use a plasma polymerisation procedure as described in WO 2006/111711 Al. 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 Al.

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).

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). 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).

Palladium and/or platinum centres can be coordinated to nitrogen-containing heterocycles such as pyridine via electron lone pair interaction. WO 2006/111711 Al 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. 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.

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 Al.

Step (iii) involves reducing the complexed palladium(II) and/or platinum(II) in the coating to palladium(O) and/or platinum(O).

In an embodiment, 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).

In an embodiment, step (iii) of the method of the invention entails reduction of complexed palladium(II) to palladium(O) by a reducing agent such as, for example, DMAB. Step (iv) involves 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.

In an embodiment, the zinc salt in step (iv) is zinc nitrate. In an embodiment, the reducing agent in step (iv) is DMAB.

In an embodiment, steps (iii) and (iv) of the method of the invention entail reduction of complexed palladium(II) centres to palladium(O) by DMAB, followed by the reaction between zinc nitrate and DMAB in the presence of the palladium(O) centres. 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(O) and/or platinum(O) in step (iii) may also act as the reducing agent in step (iv).

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(O) by DMAB, directly followed by the reaction between zinc nitrate and the DMAB in the presence of the resultant palladium(O) centres.

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.

In an embodiment, the zinc oxide coating is an antibacterial coating.

In an embodiment, the zinc oxide coating is for use in UV protection.

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. In an embodiment, the apparatus is a medical dressing.

In an embodiment, the apparatus is a thin film transistor.

In an embodiment, the apparatus is a dye-sensitized solar cell.

In an embodiment, the apparatus is a kinetic energy harvester.

In an embodiment, the apparatus is an electrode. In an embodiment, the apparatus is a transparent electrode, e.g. in a liquid crystal display. 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.

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.

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.

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°C, for example about 20°C. Detailed description

Embodiments of the present invention will now be further described with reference to the following non-limiting examples and the accompanying figures, of which:

Figure 1 shows a reaction scheme for an embodiment of the invention, namely the palladium catalyst seeding of pulsed plasma deposited poly(4-vinyl pyridine) films followed by electroless growth of zinc oxide. Figure 2 shows XPS spectra of: (a) pulsed plasma deposited poly(4-vinylpyridine); (b) pulsed plasma deposited poly(4-vinyl pyridine) seeded with palladium(II) chloride; (c) electroless zinc oxide growth onto palladium(II) chloride seeded pulsed plasma deposited poly(4-vinylpyridine). Figure 3 shows infrared spectra of: (a) 4-vinyl pyridine monomer; and (b) pulsed plasma deposited poly(4-vinylpyridine). * denotes polymerizable alkene bond absorbances in precursor.

Figure 4 shows X-ray diffraction analysis of 500 nm thick zinc oxide film electrolessly grown onto palladium seeded pulsed plasma deposited poly(4-vinylpyridine). Figure 5 is an optical microscope image of zinc oxide film electrolessly grown onto palladium seeded pulsed plasma deposited poly(4-vinylpyridine).

Figure 6 shows 500 nm thick zinc oxide grown by electroless deposition onto nonconducting 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).

Figure 7 shows XPS C(ls) envelope of electrolessly deposited zinc oxide onto silicon wafer: (a) no UV exposure, and (b) 750 s UV exposure.

Figure 8 shows a mechanism illustrating change in adsorbed species on zinc oxide surface during UV irradiation followed by subsequent oxygen readsorption over time.

Experiments were carried out to illustrate embodiments of the invention.

Experimental Methods

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-vinyl pyridine) deposition rate was measured to be 15 ± 2 nm min^"1. X-ray photoelectron spectroscopy (XPS) characterization of the functionalized substrates was carried out using a VG Escalab spectrometer equipped with an unmonochromated Mg-Κα 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° from the substrate normal.

Elemental compositions were calculated using sensitivity factors derived from chemical standards: C(ls) : O(ls) : N(ls) : Pd(3d) : Zn(2p) equals 1.00 : 0.36 : 0.57 : 0.05 : 0.05. All binding energies were referenced to the C(ls) hydrocarbon peak at 285.0 eV. The core level spectra were fitted to a linear background. 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° 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^"1.

Depth profiling measurements were undertaken by the Rutherford backscattering technique using a He⁺ ion beam (5SDH Pelletron Accelerator). Backscattered He⁺ ions were detected with 19 keV resolution using a PIPS detector.

X-ray diffraction patterns of electrolessly deposited zinc oxide layers (1 μιτι thick, mounted on a silicon (100) substrate) were collected using a powder diffractometer (Bruker d8) equipped with a Cu tube (1.5418 A wavelength), and a linear position- sensitive detector (Lynx Eye with a Ni filter). Data were collected from 5-65° 2Θ with a step size of 0.02°.

The electrolessly deposited zinc oxide layers were imaged with an optical microscope (Olympus BX40) fitted with a x50 magnification lens.

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. 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.

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 μΙ_ droplet size. High purity water (B.S. 3978 grade 1) was employed as the probe liquid.

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 A₆5onm 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 °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 °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

Pulsed plasmachemical deposition was undertaken in a cylindrical glass reactor (4.5 cm diameter, 500 cm³ volume, 1 x 10 ³ mbar base pressure, leak rate better than 1.7 x 10^"9 mol s^"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 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 ps 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.

The poly(4-vinyl pyridine) functionalized surfaces were then immersed into an aqueous catalyst solution containing 2 μΜ 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.

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.

XPS characterization of pulsed plasma deposited poly(4-vinyl pyridine) 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-vinyl pyridine) 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_{5 2}) and Pd(3d_{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 PdCI₂ complexation to the poly(4-vinylpyridine) surface (the presence of the O(ls) peak at 532.7 eV is due to water absorption from the aqueous palladium(II) chloride solution), see Figure 1 and Figure 2.

For the 4-vinyl pyridine monomer, the following infrared band assignments can be made: vinyl C=C stretching (1634 cm^"1), aromatic quadrant C=C stretching (1597 cm ¹ and 1548 cm^"1), aromatic semicircle C=C and C=N stretching (1495 and 1409 cm^"1 respectively), and =CH₂ wag (927 cm^"1), see Figure 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.

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 Figure 2 and Table 1. The absence of N(ls) and Pd(3d) signals confirmed complete coverage of the catalyst seeded poly(4-vinyl pyridine) layer by zinc oxide. Ion beam analysis determined the zinc oxide film growth rate to be 230 ± 20 nm h^"1.

X-ray diffraction characterisation showed peaks at 31.9°, 34.5°, 36.3°, 47.6°, 56.6°, and 62.9°, which are consistent with zinc oxide in the wurtzite structure (hexagonal close packed), see Figure 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.

Optical microscopy showed a roughened surface corresponding to the different crystalline faces, see Figure 5. 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^"7 mS cm ¹ up to 1.5 mS cm ¹ after 750 s, see Figure 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^"8 mbar), photoconductivity was retained following a period of weeks, whilst UV irradiation under vacuum gave rise to an increase in electrical conductivity.

High water contact angle values of 150°, 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 Figure 6. Exposure to higher intensity UV light over the same period of time caused the contact angle to drop below 20°. Following termination of UV exposure, the contact angle slowly recovers to its original value of 150° over a period of around 3 weeks, Figure 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^~8 mbar), there was no recovery in contact angle (i.e. remained at 60° over a period of 4 weeks). Also, UV irradiation of zinc oxide coated samples under ultra high vacuum conditions or pure 0₂ (rather than in air) produced no discernible change in the contact angle (i.e. remained at 150°). 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(ls) envelope corresponding to small amounts of adsorbed hydrocarbon species (285.0 eV) did not change, see Figure 7.

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°, 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.

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-vinyl pyridine) 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).

In these experiments, XPS and infrared analyses have shown that a variety of substrates can be coated with structurally well-defined poly(4-vinyl pyridine) 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.

The semiconducting nature of zinc oxide stems from natural doping (n-type) in the form of interstitial singly charged zinc cations (Zn⁺), which lie close to the conduction band, and so can be easily thermally ionized (to Zn²⁺ + e), thus supplying electrons to the conduction band. The aforementioned excess electrons leaving behind interstitial Zn⁺ can become trapped at the surface by adsorbed oxygen (0₂(ads)) to give 0₂ ^~ _(acis) species. Zinc oxide coated pulsed plasma poly(4-vinylpyridine) films display

photoconductivity during UV light exposure, see Figure 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 0₂ ^~ _(acis) up to a self-limiting concentration of 0₂ ^~ _(acis) due to electrostatic repulsion between 0₂ ^~ _(acis) at the surface. Such 0₂ ^" _(acis) species are capable of attracting holes from the bulk, which migrate towards the surface to combine with the 0₂ ^" _(acis) species leading to the formation of a surface vacancy and photodesorption of molecular oxygen:

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 Figure 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.

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°, 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(ls) 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 Figure 7. Photodesorption of 0₂₍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 Figure 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 Figure 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 Figure 8.

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 0₂ ^~ _(acis₎ species. In a similar fashion to zinc oxide, such 0₂ ^" _(acis₎ species undergo photodesorption as 0_2(g) from Ti0₂ 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.

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, Figure 5. Furthermore, by changing the substrate from two-dimensional (flat) to porous threedimensional (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 Figure 8), there is sufficient Cassie-Baxter behaviour for superhydrophobicity to be sustained.

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.

Palladium catalyst seeded pulsed plasma poly(4-vinyl pyridine) 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.

Claims

1. A method for applying a zinc oxide coating to a substrate, comprising the steps of:

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 any one of the preceding claims, 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 any one of the preceding claims, 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 any one of the preceding claims, wherein the zinc salt in step (iv) is zinc nitrate.

9. The method of any one of the preceding claims, wherein the reducing agent in step (iv) is dimethylaminoborane (DMAB).

10. A method for applying a zinc oxide coating to a substrate, the method being substantially as herein described and illustrated.

11. A zinc oxide coating obtainable by a method according to any one of the preceding claims.

12. A zinc oxide coating substantially as herein described and illustrated.

13. An apparatus comprising a substrate and a zinc oxide coating according to claim 11 or 12 on the substrate.