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Electroless deposition of multi-functional zinc oxide surfaces displayingphotoconductive, superhydrophobic, photowetting, and antibacterialproperties

T. J. Wood,a G. A. Hurst,a W. C. E. Schofield,a R. L. Thompson,a G. Oswald,a J. S. O. Evans,a G. J. Sharples,b

C. Pearson,c M. C. Pettyc and J. P. S. Badyal*a

Received 30th August 2011, Accepted 3rd January 2012

DOI: 10.1039/c2jm14260k

Electroless growth of zinc oxide has been accomplished using palladium catalysts coordinated to pulsed

plasma deposited poly(4-vinylpyridine) nanolayers. Four different and concurrent functional

properties have been identified, which encompass photoconductivity, superhydrophobicity, photo-

switchable wetting, and bacterial killing.

1. Introduction

Zinc oxide is a transparent semiconductor with wurtzite

(hexagonal close packed) crystal structure and a bandgap of�3.3

eV. It exhibits many desirable properties, which include ultravi-

olet light absorption,1 photoconductivity,2–10 photocatalysis,11–15

photowettability,16,17 piezoelectricity,18 antibacterial behav-

iour,19,20 and wound-healing.21 These find technological appli-

cation in thin film transistors,22–26 dye-sensitized solar cells,27–32

kinetic energy harvesters,33–35 transparent electrodes in liquid

crystal displays,36 sunblock,37 fabric protection,38 and medical

dressings.39 Often zinc oxide is utilised as thin films, which have

been produced by RF sputtering,40–43 chemical vapour deposi-

tion,44–48 vapour diffusion catalysis,49 spray pyrolysis,50–53 elec-

trodeposition,54–56 sol–gel synthesis,57–60 or pulsed laser

deposition.61–63 Inherent limitations of such methods can include

their substrate-dependence (e.g. requirement for conducting or

physically robust substrates),54,61 and often harsh process

conditions (e.g. high temperatures40,44,49,50,57 or oxidative chem-

ical environments54,56). 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 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), isinexpensive, and produces highly crystalline films.64 It entails

aDepartment of Chemistry, Science Laboratories, Durham University,Durham, DH1 3LE, England, UKbSchool of Biological and Biomedical Sciences, Biophysical SciencesInstitute, Science Laboratories, Durham University, Durham, DH1 3LE,England, UKcSchool of Engineering and Computing Sciences, Science Laboratories,Durham University, Durham, DH1 3LE, England, UK

This journal is ª The Royal Society of Chemistry 2012

reduction of palladium(II) centres to palladium(0) by

dimethylaminoborane:65–67

Pd2+ + (CH3)2NHBH3 / Pd0 + 2H+ + (CH3)2NBH2

which is followed by the reaction between zinc nitrate and

dimethylaminoborane in the presence of the palladium(0) cata-

lyst (where dimethylaminoborane reduces the nitrate).64,68

Effectively palladium(0) catalyzes the oxidation of

dimethylaminoborane:

(CH3)2NHBH3 + 2H2O / HBO2 + (CH3)2NH2+ + 5H+ + 6e�

leading to the corresponding reduction of nitrate ions (which

causes a local rise in pH):

NO3� + H2O + 2e� / NO2

� + 2OH�

This increase in pH triggers the growth of zinc oxide according to

the following acid–base reaction:69

Zn2+ + 2OH� / ZnO + H2O

Previously, the catalytic palladium centres were introduced by

the application of Pd/Sn colloids onto the surface followed by

acid washing for the removal of tin64—this harsh process is not

suitable for many substrates (e.g. textiles). Instead, given that

palladium centres can be coordinated to nitrogen-containing

heterocycles such as pyridine via electron lone pair interac-

tion,70–72 the aforementioned solution phase chemistry should in

principle be transferrable to a surface by utilizing immobilized

pyridine rings. Pulsed plasmachemical deposition of poly(4-

vinylpyridine) is one potential way for tethering pyridine groups

onto solid surfaces.72 This comprises modulating an electrical

discharge in the presence of gaseous precursors containing

polymerizable carbon-carbon double bonds.73 Mechanistically,

J. Mater. Chem., 2012, 22, 3859–3867 | 3859

Scheme 1 Palladium catalyst seeding of pulsed plasma deposited

poly(4-vinylpyridine) films followed by electroless growth of zinc oxide.

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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 irra-

diation, 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 functional-

ities 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 distinct 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, fibers,

tubes, etc.).74 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,75 carboxylic

acid,76 amine,77 cyano,78 epoxide,79 hydroxyl,80 halide,81 thiol,82

furfuryl,83 perfluoroalkyl,84 perfluoromethylene,85 and tri-

fluoromethyl86 functionalized surfaces. In this article we describe

the utilisation of pulsed plasmachemical deposition of poly(4-

vinylpyridine) films for the immobilization of catalytic palladium

species required for the electroless growth of zinc oxide with

a view to providing multifunctionality (photoconductivity,

superhydrophobicity, photo-switchable wetting, and antibacte-

rial activity), Scheme 1.

2. Experimental

Pulsed plasmachemical deposition was undertaken in a cylin-

drical glass reactor (4.5 cm diameter, 500 cm3 volume, 1 � 10�3

mbar base pressure, leak rate better than 1.7 � 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 puri-

fied with three freeze-pump-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-

3860 | J. Mater. Chem., 2012, 22, 3859–3867

period ¼ 100 ms and off-period ¼ 4 ms in combination with peak

power ¼ 40 W.72 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 pulsed plasma poly(4-vinylpyridine) functionalized

surfaces were then immersed into an aqueous catalyst solution

containing 2 mM palladium(II) chloride (+99.999%, Alfa Aesar),

3.0 M sodium chloride (+99.5%, Sigma), and 0.5 M sodium

citrate dihydrate (+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.72 Next, the palla-

dium(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 pH 6.5 and a temperature of 323 K for 2 h.64

Following zinc oxide growth, the surface was rinsed with

deionized water. Control samples of pulsed plasma poly(4-

vinylpyridine) placed into zinc nitrate–dimethylaminoborane

solution at slightly alkaline pH (pH 8.5, adjusted with sodium

hydroxide solution) showed no deposition. The deposited zinc

oxide films passed the Scotch tape adhesion test for all of the

substrates employed.

Film thickness measurements were carried out using a spec-

trophotometer (nkd-6000, Aquila Instruments Ltd.). Trans-

mittance and reflectance curves across the 300–1000 nm

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wavelength range were fitted to a Cauchy model for dielectrics

using a modified Levenberg-Marquardt method.87 The pulsed

plasma poly(4-vinylpyridine) 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-Ka X-

ray source (1253.6 eV) and a concentric hemispherical analyzer

operating in constant analyzer energy mode (pass energy ¼ 20

eV) with the photoelectrons collected at a take-off angle of 12�

from the substrate normal. Elemental compositions were calcu-

lated 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.

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 Ruth-

erford backscattering technique using a 4He+ ion beam (5SDH

Pelletron Accelerator). Backscattered 4He+ ions were detected

with 19 keV resolution using a PIPS detector.

X-ray diffraction patterns of electrolessly deposited zinc oxide

layers (1 mm 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� 2q with a step size of 0.02�.The electrolessly deposited zinc oxide layers were imaged with

an optical microscope (Olympus BX40) fitted with a �50

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

region88) 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

SourceMeter.

Sessile drop water contact angle measurements were per-

formed at ambient temperature using a video capture apparatus

in combination with a motorized syringe (VCA2500XE, A.S.T.

Products Inc.) dispensing a 2 mL 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.89 A bacterial

cell culture (wild-type Escherichia coli K-12 lab strain W3110)

grown to an A650nm of 0.4 was applied in minimal salts buffer

This journal is ª The Royal Society of Chemistry 2012

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 were counted. To

control for cell absorption onto samples, the above procedure

was repeated with 1 min exposure of bacteria on coated and

uncoated cloth.

3. Results

3.1 Zinc oxide deposition

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, 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.72 Immersion into pal-

ladium(II) chloride solution gave rise to the appearance of

Pd(3d5/2) and Pd(3d3/2) 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 PdCl2 complexation to the poly(4-vinyl-

pyridine) surface (the presence of the O(1s) peak at 532.7 eV is

due to water absorption from the aqueous palladium(II) chloride

solution90), Fig. 1 and Scheme 1.

For the 4-vinylpyridine monomer, the following infrared band

assignments can be made:91 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), Fig. 2. 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-vinyl-

pyridine) 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, Fig. 1 and

Table 1. The O:Zn value is higher than the expected stoichio-

metric ratio, this can be attributed to adsorbed oxygen on the

surface as well as due to oxygen content present in any adsorbed

hydrocarbon impurities.92–94 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�1.

The thickness of the ZnO films could be controlled by varying the

period of substrate immersion into the electroless deposition

solution.

J. Mater. Chem., 2012, 22, 3859–3867 | 3861

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 0Pulsed plasma poly(4-vinylpyridine) 87.3 � 0.5 11.8 � 0.5 0.0 0.0 0.9 � 0.5 0Palladium(II) seeded pulsed plasma poly(4-vinylpyridine)

63.3 � 0.5 8.3 � 0.5 2.9 � 0.2 5.5 � 0.5 20.0 � 0.5 0

Deposited zinc oxidea 5.0 � 0.9 0.0 0.0 0.0 63.0 � 0.8 32 � 1Deposited zinc oxide after 750 s UVexposurea

5.5 � 0.9 0.0 0.0 0.0 62.5 � 0.8 32 � 1

a No discernible difference was observed in the XPS core level peak shapes.

Fig. 1 XPS spectra of: (a) pulsed plasma deposited poly(4-vinyl-

pyridine); (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).

Fig. 2 Infrared spectra of: (a) 4-vinylpyridine monomer; and (b) pulsed

plasma deposited poly(4-vinylpyridine) (* denotes polymerizable alkene

bond absorbances in precursor).

Fig. 3 X-ray diffraction analysis of 500 nm thick zinc oxide film elec-

trolessly grown onto palladium seeded pulsed plasma deposited poly(4-

vinylpyridine).

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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), Fig. 3. Rietveld

refinement confirmed that the ratio of peak intensities matches

that expected for wurtzite zinc oxide.95 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 correspond-

ing to the different crystalline faces, Fig. 4.

3.2 Surface multifunctionality

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, Fig. 5. The electrical

conductivity was observed to slowly decay following termination

of UV exposure. In the case of storage under ultra high vacuum

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 Optical microscope image of zinc oxide film electrolessly grown

onto palladium seeded pulsed plasma deposited poly(4-vinylpyridine).

Fig. 5 500 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 ¼ 0 h).

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(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.3

High water contact angle values of 150�, but with a large

contact angle hysteresis were measured for zinc oxide coated flat

silicon substrates, 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, Table

2 and Fig. 5. 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, Fig. 5. 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 O2 (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(1s) envelope corresponding to

small amounts of adsorbed hydrocarbon species (285.0 eV) did

not change, Fig. 6.

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

hysteresis96), Table 2. In this case, the water repellency of the zinc

oxide surface was not found to be perturbed by exposure to UV

radiation, 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, Table 3. Control

samples of the polypropylene cloth pieces exhibited no antibac-

terial 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).

4. Discussion

Pulsed plasmachemical deposition is an established technique for

the functionalization of surfaces.73 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. In

this study, 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-vinyl-

pyridine97). Subsequent seeding with catalytic palladium centres

provides for the localised electroless growth of zinc oxide. An

additional benefit of this approach is that the functional polymer

nanolayer serves to protect the underlying substrate material

from subsequent chemical processing steps (in this case the

J. Mater. Chem., 2012, 22, 3859–3867 | 3863

Table 2 Water contact angle measurements for zinc oxide coated substrates

Substrate

Water Contact Angle/�

Equilibrium Advancing Receding Hysteresis

Silicon wafer 150 � 1 152 � 1 35 � 1 117 � 2Silicon wafer after 750 s UV in air 60 � 1 65 � 1 7 � 1 58 � 2Silicon wafer after 750 s UV in UHV 150 � 1 152 � 1 35 � 1 117 � 2Silicon wafer after 750 s UV in O2 149 � 1 151 � 1 35 � 1 116 � 2Nonwoven 154 � 1 154 � 1 154 � 1 0 � 2Nonwoven after 750 s UV in air 154 � 1 154 � 1 154 � 1 0 � 2

Fig. 6 XPS C(1s) envelope of electrolessly deposited zinc oxide onto

silicon wafer: (a) no UV exposure, and (b) 750 s UV exposure.

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oxidising and reducing agents contained within the zinc oxide

electroless deposition solution). The growth of zinc oxide in the

hexagonal wurtzite structure is not unexpected, since it is the

most thermodynamically stable form at ambient pressure and

temperature.98

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.99,100 The aforementioned excess elec-

trons leaving behind interstitial Zn+ can become trapped at the

surface by adsorbed oxygen (O2(ads)) to give O2�(ads) species.

101–103

Zinc oxide coated pulsed plasma poly(4-vinylpyridine) films

display photoconductivity during UV light exposure, Fig. 5.

Contributions to the shape of the photoconductivity curve can be

split into fast reversible (electron promotion from valence to

Table 3 Antibacterial activity against the Gram-negative bacterium, Escher

Substrate Fraction

Uncoated 1Pulsed plasma poly(4-vinylpyridine) 0.6 � 0.Zinc oxide 0.0012 �Zinc oxide irradiated with UV light 0.009 �

3864 | J. Mater. Chem., 2012, 22, 3859–3867

conduction band), and slow irreversible (surface chemistry of

adsorbed species).94 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 O2�(ads) up to a self-limiting

concentration of O2�(ads) due to electrostatic repulsion between

O2�(ads) at the surface.103 Such O2

�(ads) species are capable of

attracting holes from the bulk, which migrate towards the surface

to combine with the O2�(ads) species leading to the formation of

a surface vacancy and photodesorption of molecular

oxygen:3,103,104

ZnO + hn / ZnO + e-h (electron-hole pair)

e + O2 (ads) / O2�(ads)

h + O2�(ads) / O2 (g)[ + ,

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 photo-

desorption leads to a decay in conductivity,3 Fig. 5. 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.6 In the case

of storage under vacuum following termination of UV irradia-

tion, photoconductivity was retained over a period of weeks,

which is consistent with the electrical conductivity decay mech-

anism being governed by molecular oxygen adsorption.3

Reversible wettability was also observed following UV irra-

diation of zinc oxide coated flat substrates. Clean zinc oxide

ichia coli, for nonwoven cloth

of Cells Recovered After 24 h Log Kill

03 0.20.0009 2.9

0.005 2.0

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surfaces are hydrophilic105,106 (equilibrium water contact angles <

35�, along with many other metal oxides107), but in ambient

conditions they are known to attract amphiphilic, carbon-con-

taining contaminants present in the atmosphere.16,105,106 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 surface108,109); the hydrophobic domain of the carbon-

containing species is therefore responsible for the widely

observed hydrophobicity of zinc oxide surfaces in ambient

conditions.16 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 surfaces110); or

secondly, the UV light leads to desorption of molecular oxygen

from the zinc oxide surface followed by dissociative water

adsorption.16 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, Table 1 and

Fig. 6. Photodesorption of O2(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 experi-

ments, 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, Table 2) indicates

a photo-assisted dissociative water adsorption mechanism,

Scheme 2. Where the photogenerated hydroxide groups are

responsible for the surface switching from hydrophobic to

hydrophilic.16,17,111 The contact angle fully recovers over a matter

of weeks following extinction of UV irradiation (the exact

duration depending upon UV intensity), Fig. 5. 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

Scheme 2 Mechanism illustrating change in adsorbed species on zinc

oxide surface during UV irradiation followed by subsequent oxygen

readsorption over time (literature references numbered in brackets).

This journal is ª The Royal Society of Chemistry 2012

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.16 The speed at which this happens

(leading to recharged hydrophobicity) is slow, and determines

the long hydrophobicity recovery times seen for zinc oxide.16

This photochemical contact angle decay observed for UV expo-

sure 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, Scheme 2.

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 postu-

lated that molecular oxygen adsorbs at titanium dioxide surface

defect sites,112,113 and it then traps a photogenerated electron to

become O2�(ads) species.

114,115 In a similar fashion to zinc oxide,

such O2�(ads) species undergo photodesorption as O2(g) from

TiO2 surfaces during UV irradiation.116 Therefore, the same

behaviour is observed for both zinc oxide and titanium dioxide

surfaces in relation to photoconductivity and photo-switchable

wetting.117–119

By depositing zinc oxide onto a roughened surface (nonwoven

polypropylene), the aforementioned reversible wettability

changes for flat substrates (silicon wafers or glass coverslips)

were not observed; water contact angle hysteresis became negli-

gible, 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.120,121 At this point, surface

wetting obeys the Cassie-Baxter relationship,122 where the

roughness is so great that air becomes trapped during liquid-

surface contact giving rise to incomplete wetting. In the present

study, 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. 4. Furthermore, by changing the

substrate from two-dimensional (flat) to porous three-dimen-

sional (nonwoven), there is an enhancement of Cassie-Baxter

behaviour, culminating in very low contact angle hysteresis,

Table 2. Although in the case of zinc oxide coated nonwoven

polypropylene the surface energy will increase during UV

exposure (as described in Scheme 2), there is sufficient Cassie-

Baxter behaviour for superhydrophobicity to be sustained.123,124

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.125,126 The observed

antibacterial activity in the present study is most likely attrib-

utable to the presence of oxygenated radical species on the zinc

oxide surface.127–129 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.130,131 This

antibacterial mechanism does not appear to be closely linked to

the UV induced photoconduction and photowettability mecha-

nism, as seen by the continued killing of bacteria by zinc oxide

surfaces which had been UV irradiated beforehand, Table 3.

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5. Conclusions

Palladium catalyst seeded pulsed plasma poly(4-vinylpyridine)

nanolayers have been employed for the localised electroless

growth of crystalline zinc oxide thin films whilst concurrently

protecting the underlying substrate from the reactive chemical

reagents. These zinc oxide coatings are found to display photo-

conductivity, photo-switchable wetting, superhydrophobicity,

and antibacterial properties.

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