Materials Chemistry and Physics 114 (2009) 382–386
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Materials Chemistry and Physics
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ailoring the photocatalytic activity of nanoparticulate zinc oxide by transitionetal oxide doping
aron Dodda,c,∗, Allan McKinleya, Takuya Tsuzukib, Martin Saundersc
School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, AustraliaAdvanced Nanotechnology Limited, 112 Radium Street, Welshpool, WA 6106, AustraliaCentre for Microscopy, Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
r t i c l e i n f o
rticle history:eceived 1 May 2008eceived in revised form 14 August 2008ccepted 14 September 2008
a b s t r a c t
The successful use of nanoparticulate ZnO in applications such as UV-screening agents or photocatalyst forthe destruction of chemical waste requires the development of techniques for controlling its photocatalyticactivity. In this study, we have investigated transition metal doping as a means of achieving this goal.
eywords:xidesemiconductorshemical synthesis
Powders of ZnO, MnxZn1−xO, and CoxZn1−xO were synthesised by a three-stage process consisting of high-energy mechanical milling, heat treatment, and washing. The photocatalytic activity of these powders wasevaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy. It wasfound that the photocatalytic activity of CoxZn1−xO progressively decreased with the doping level. Incontrast, the activity of MnxZn1−xO initially increased with doping up to a level of 2 mol% and thereafterdeclined. These results demonstrate that doping with transition metal oxides can be used to tailor the
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photocatalytic properties
. Introduction
Photocatalysis by inorganic semiconductors is considered toave significant potential for applications involving the destructionf organic pollutants [1]. In this process, irradiation of semicon-uctor particles with sufficiently energetic light results in theormation of electron–hole pairs, which can subsequently migrateo the particle surface and react with adsorbed molecules to gen-rate free radicals. These free radicals are highly reactive and canherefore mineralize organic pollutants into harmless compounds,uch as CO2, H2O, and simple mineral acids [1,2]. However, theidespread use of photocatalysis for this purpose is limited as theajority of photogenerated charge carriers recombine rather than
eacting with adsorbed molecules to generate free radicals [2].One strategy that has been investigated for improving the activ-
ty of particulate photocatalysts is to decrease the particle sizeown to the nanometric size regime [2–5]. Decreasing the aver-
ge particle size increases the specific surface area and thereforelso increases the number of active surface sites where photogen-rated charge carriers are able to react with adsorbed molecules toorm free radicals. However, decreasing the particle size of a photo-
∗ Corresponding author at: School of Biomedical, Biomolecular and Chemicalciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA009, Australia.
atalyst also increases the rate of surface charge recombination. Asresult, the activity of particulate photocatalysts does not increaseonotonically with decreasing particle size [4,5]. It is thus evident
hat attainment of high activity in nanoparticulate photocatalystsequires a means of inhibiting charge carrier recombination.
In addition to applications as photocatalysts for the destruc-ion of chemical waste, nanoparticulate powders of semiconductorxides are also of interest for use in ultraviolet (UV) light filters, suchs protective paints and topical sunscreens. In such applications,igh photocatalytic is undesirable as it can lead to degradation ofrganic components that are present in the formulation. Previoustudies have shown that photoinduced mineralization can lead toreduction in the efficacy of sunscreens that contain both physi-
al and chemical UV absorbers [6]. In addition, DNA and RNA haveeen shown to be damaged in vitro upon exposure to photogener-ted free radicals [7]. The successful exploitation of nanoparticulateemiconductors as UV-filters therefore necessitates the develop-ent of techniques for suppressing their photocatalytic activity.In this study, we have investigated whether doping with tran-
ition metal oxides can be used to control the photocatalyticctivity of nanoparticulate ZnO such that its properties can beptimized for either waste destruction (high activity) or UV-
hielding (low activity). Powders of doped ZnO were manufacturedy mechanochemical processing and their photocatalytic activityas characterised by measuring the hydroxyl radical productionnder UV-light using the spin-trapping technique with electronaramagnetic resonance (EPR) spectroscopy.
wMn3O4 synthesis systems. In contrast to those systems, mechan-ical milling did not result in chemical reaction of the precursors.Milling merely resulted in amorphisation and overall microstruc-tural refinement of the reactant mixture. As shown in Fig. 2, the
A. Dodd et al. / Materials Chemi
. Experimental techniques
.1. Powder synthesis
Aqueous slurries of nanoparticulate ZnO, Mn3O4, and Co3O4 powder were manu-actured by a three-stage process consisting of mechanical milling, low temperatureeat treatment, and washing. In the first stage of processing, 10 g of reactant mixturesere milled for 6 h within a hardened steel vial using a Spex 8000 mixer/mill with
wenty 9.5 mm stainless steel balls as the grinding media. The reactant mixtureshat were used corresponded to the following equations:
nCl2 + Na2CO3 + 4NaCl → ZnCO3 + 6NaCl (1)
nCl2 + Na2CO3 + 4NaCl → MnCO3 + 6NaCl (2)
oCl2 + Na2CO3 + 4NaCl → CoCO3 + 6NaCl (3)
ollowing milling, the reactant mixtures were heat-treated in air for 1 h at 400 ◦C. Inhe final stage of processing, the NaCl reaction by-product was removed by repeatedashing with deionised water. Powders of CoxZn1−xO and MnxZn1−xO were synthe-
ised by the same basic method using the ZnCl2 + Na2CO3 + 4NaCl reactant mixturesor which a given amount of the ZnCl2 was substituted with either MnCl2 or CoCl2.
All of the reactant mixtures employed in this study contained NaCl diluent. Therimary purpose of the NaCl diluent was to improve the milling characteristics of theeactant powder charge. In the absence of any NaCl diluent, the reactant mixturesecame caked onto the interior of the milling vial, which inhibited chemical reactionf the precursors. The secondary purpose of the NaCl diluent was to lower the volumeraction of the oxide particles in the salt by-product so as to promote the formationf dispersed particles [8].
The washed slurries of MnxZn1−xO and CoxZn1−xO were divided into two por-ions. The first portion of slurry was dried so as to allow analysis of the powderarticles by X-ray diffraction (XRD) and BET gas adsorption. The second portionas used in the undried state to prepare samples for UV–vis spectroscopy andhotocatalytic testing.
.2. Powder characterisation
The chemical evolution of the reactant mixtures during processing was followedy X-ray diffraction using a Siemens D5000 diffractometer with Cu K� radiation.he ‘c’ lattice parameter of the washed MnxZn1−xO and CoxZn1−xO powders wasalculated from the position of the (0 0 2) diffraction peak.
The specific surface area of the MnxZn1−xO and CoxZn1−xO powders was mea-ured by five-point BET gas adsorption using a Micromeritics Tristar instrument. Allowders were vacuum degassed at 150 ◦C for 1 h prior to analysis.
A Cary 2E UV-visible spectrophotometer was used to measure the absorptionpectra of the washed MnxZn1−xO and CoxZn1−xO powders. The spectra were mea-ured over the wavelength range of 200–800 nm with an optical path length of0 mm. Samples were prepared by diluting the washed slurries down to 0.02 wt.%ith Milli-Q water. Dispex N400 was added 10 wt.% relative to the solids content.
rior to measurement, the diluted suspensions were subjected to intense ultrason-cation for 15 min.
.3. Photocatalytic testing
Samples for photocatalytic testing were prepared by diluting the washednxZn1−xO and CoxZn1−xO slurries down to a solids content of 0.02 wt.% with Milli-Qater. Following dilution, the slurries were subjected to intense ultrasonication for
5 min in order to disperse the particles within the suspension. Immediately prioro measurement of the photocatalytic activity, 1 mL of a 30 mM solution of 5,5-imethyl-1-pyrroline N-oxide (DMPO) spin-trap was added to 10 mL of the powderuspension.
The photocatalytic activity of the washed powders was characterised by mea-uring the hydroxyl radical concentration as a function of irradiation time usinghe spin-trapping technique with electron paramagnetic resonance spectroscopy9–11]. Samples were irradiated with 300 nm light whilst within a quartz flat cellhat was located in the cavity of the EPR spectrometer. Irradiation was providedy a 1 kW Hg–Xe lamp. A monochromator was attached to the lamp to select therradiation wavelength. The concentration of photogenerated hydroxyl radicals was
easured as a function of time by recording the intensity of the first central line ofhe first derivative EPR spectrum corresponding to the DMPO-OH spin adduct.
. Results and discussion
.1. Synthesis of ZnO, Mn3O4, and Co3O4
The synthesis of nanoparticulate ZnO powder byechanochemical reaction of ZnCl2 + Na2CO3 + 4NaCl has pre-
iously been reported [5,12]. In these earlier studies, it was foundhat chemical reaction of the precursors occurred during milling
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ig. 1. XRD pattern of the MnCl2 + Na2CO3 + 4NaCl reactant mixture following (a)illing for 6 h, (b) heat treatment at 400 ◦C for 1 h, and (c) washing.
ith the consequent formation of a composite powder consistingf nanocrystalline ZnCO3 grains embedded within a matrix ofaCl. Subsequent heat treatment and washing yielded an aqueous
lurry of well dispersed ZnO nanoparticles. The same basic methodas employed in the present study.
The reactant mixture for the synthesis of Mn3O4 was found toxhibit a chemical evolution during processing similar to that dis-layed by the ZnO synthesis. As shown in Fig. 1, the XRD pattern ofhe MnCl2 + Na2CO3 + 4NaCl reactant mixture after milling consistsf broadened diffraction peaks corresponding to MnCO3 and NaCl,hich indicates that chemical reaction of the precursors occurreduring milling. Subsequent heat treatment of the as-milled powderesulted in the disappearance of the MnCO3 peaks and the appear-nce of new diffraction peaks corresponding to Mn3O4. Washingesulted in the disappearance of the NaCl diffraction peaks.
CoCl2 + Na2CO3 + 4NaCl was found to behave in a manner thatas significantly different to that exhibited by both the ZnO and
ig. 2. XRD pattern of the CoCl2 + Na2CO3 + 4NaCl reactant mixture following (a)illing for 6 h, (b) heat treatment at 400 ◦C for 1 h, and (c) washing.
384 A. Dodd et al. / Materials Chemistry and Physics 114 (2009) 382–386
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oocoactof the CoxZn1−xO powders. Three absorption bands are evident inthe optical wavelength region for the suspensions of the dopedpowders. The presence of these characteristic absorption bandsprovides additional evidence that the dopant was incorporated intothe ZnO as Co2+.
Fig. 3. XRD patterns of MnxZn1−xO and CoxZn
RD pattern of the as-milled powder consists solely of diffrac-ion peaks corresponding to NaCl. Although no other diffractioneaks are evident, the presence of CoCl2 could be inferred from thelue coloration of the as-milled powder, which is characteristic ofnhydrous CoCl2. Following heat treatment, the diffraction patternonsists of peaks corresponding to Co3O4 and NaCl, which indicateshat chemical reaction occurred during heat treatment. Followingashing only the Co3O4 diffraction peaks remained, indicating the
uccessful removal of the NaCl by-product phase.
.2. Synthesis of MnxZnO1−x and CoxZnO1−x
The successful preparation of Mn3O4 and Co3O4 indicates thatt should be possible to manufacture nanoparticulate powders of
nxZn1−xO and CoxZn1−xO using the basic ZnO synthesis systemor which a given amount of the ZnCl2 precursor is substituted withmolar equivalent of MnCl2 or CoCl2. Powders that were synthe-
ised from these combined reactant mixtures were found to giveRD patterns characteristic of ZnO. No other diffraction peaks werevident, which indicates that the powders were single phase to ateast the detection limit of the XRD technique.
Fig. 3 shows XRD patterns of the washed MnxZn1−xO andoxZn1−xO powders. It can be seen that the diffraction peaks of thenxZn1−xO progressively shifted to higher angles with the dopant
ontent. In contrast, the peaks of the CoxZn1−xO powders shifted toower angles with dopant content. These peak shifts indicate thathe lattice constants increased with dopant concentration for the
nxZn1−xO and decreased for the CoxZn1−xO. This is illustrated inig. 4, which shows the ‘c’ lattice constant as a function of dopantoncentration for the MnxZn1−xO and CoxZn1−xO powders. Bothystems show a near linear variation with doping level, as woulde expected if the dopant cations were incorporated into a ZnO solidolution. Similar behaviour has been observed in other experimen-al studies regarding transition metal-doped ZnO powders [13] andhin films [14–17].
Previous studies of Mn-doped ZnO thin films have taken thebserved lattice parameter expansion as evidence that the dopantation is incorporated into ZnO solid solution in the divalent state15–17]. The Mn2+ ion is characterised by a slightly larger size0.066 nm) compared to Zn2+ (0.060 nm). As a consequence, sub-titution of Zn2+ ions with Mn2+ would be expected to cause anxpansion of the unit cell. Further evidence that Mn ions in ZnOdopt the divalent state has been provided by quantitative electron
nergy loss spectroscopy [18] and electron paramagnetic resonancepectroscopy [19].
The CoxZn1−xO powders ranged in color from pale to dark greenepending on the dopant level, which is consistent with previ-us experimental observations of CoO–ZnO solid solutions [20].
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powders, where x = 0.00, 0.01, 0.02, and 0.05.
he MnxZn1−xO powders exhibited a brown color similar to thateported by Zhou et al. who prepared Mn0.05Zn0.95O powders byhemical precipitation in alcohol [19].
Experimental studies regarding the optical properties of dopednO powders have identified characteristic absorption bands fornxZn1−xO and CoxZn1−xO solid solutions. Rao and Deepak [21]
eported that MnxZn1−xO powders exhibit a broad absorption bandn the range of 400–450 nm as a result of the 6A1(S) → 4T1(G) tran-ition. CoxZn1−xO samples have been found to give three absorptionands in the 550–700 nm region due to the 4A2(F) → 2E(G)659 nm), 4A2(F) → 4T1(P) (615 nm) and 4A2(F) → 2A1(G) (568 nm)ransitions, which are characteristic of Co2+ ions in tetrahedral coor-ination [22].
Fig. 5 shows transmission as a function of wavelength for aque-us suspensions of the MnxZn1−xO powders. It can be seen that theptical transparency of the suspension decreased with the dopantoncentration. The difference spectra indicates that this decrease inptical transparency is the result of an additional absorption banddjacent to the band-gap, which can be attributed to either dopantations in the ZnO or the presence of a secondary phase. Fig. 6 showsransmission as a function of wavelength for aqueous suspensions
ig. 4. Lattice constant (c) as a function of dopant concentration for the MnxZn1−xOnd CoxZn1−xO powders.
A. Dodd et al. / Materials Chemistry and Physics 114 (2009) 382–386 385
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Fig. 7. DMPO-OH concentration as a function of exposure time for the undoped ZnO.The inset shows the characteristic 1:2:2:1 spectra of the DMPO-OH spin adduct.
ig. 5. Transmission as a function of wavelength for aqueous suspensions of thenxZn1−xO powders, where x = 0.00 (�), 0.01 (�), 0.02 (�), and 0.05 (�). The inset
hows the transmission of the doped powders relative to the undoped ZnO.
.3. Photocatalytic activity
For all of the MnxZn1−xO and CoxZn1−xO suspensions, irradiationith 300 nm light resulted in the appearance of a growing 1:2:2:1
PR spectrum characteristic of the expected DMPO-OH spin adduct.he concentration of the DMPO-OH spin adduct was measured asfunction of time by recording the intensity of the first central linef the first derivative EPR spectrum. As a representative example,ig. 7 shows the DMPO-OH concentration as a function of exposureime for the aqueous suspension of the undoped ZnO.
It was found that the spectral growth rate was different forach ZnO sample, which indicates that they were characterisedy different levels of photocatalytic activity. This is illustrated inigs. 8 and 9, which show the DMPO-OH yield following 50 s of irra-
iation for the suspensions of MnxZn1−xO and CoxZn1−xO. It can beeen that the addition of up to 2 mol% of MnO resulted in a progres-ive increase in the DMPO-OH yield. A further increase in the dopingevel up to 5 mol% resulted in a reduction in the DMPO-OH yield,
ig. 6. Transmission as a function of wavelength for aqueous suspensions of theoxZn1−xO powders, where x = 0.00 (�), 0.01 (�), 0.02 (�), and 0.05 (�). The insethows the transmission of the doped powders relative to the undoped ZnO. Arrowsark absorption bands at 568, 615, and 659 nm.
Fig. 8. Specific surface and DMPO-OH yield for the MnxZn1−xO powders. The DMPO-OH yield is shown in arbitrary units.
Fig. 9. Specific surface and DMPO-OH yield for the CoxZn1−xO powders. The DMPO-OH yield is shown in arbitrary units.
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86 A. Dodd et al. / Materials Chemi
hich indicates that there is an optimum doping level for whichhe photocatalytic activity is maximised. For aqueous suspensionsf CoxZn1−xO, the DMPO-OH yield decreased with the doping level.
Also shown in Figs. 8 and 9 is the specific surface area of thenxZn1−xO and CoxZn1−xO powders. For both systems, the specific
urface area increased with the dopant concentration. This couldeasonably be expected to enhance the photocatalytic activity as aigher specific surface area would increase the number of activeites where the photogenerated charge carriers could react withdsorbed molecules to generate •OH radicals.
Doping of ZnO with Mn2+ and Co2+ is known to introduce deepnergy levels between the valence and conduction bands [23,24].n general, such energy levels have large capture cross-sections forharge carriers and are therefore able to act as efficient recombina-ion centers. This is expected to suppress photocatalytic activity byeducing the number of photogenerated charge carriers availableor reaction with adsorbed molecules. Consistent with this expec-ation, the photocatalytic activity of CoxZn1−xO decreased with theopant concentration. The reason behind the contrary behaviourf the MnxZn1−xO system is presently unclear.
A possible explanation for the heightened photocatalytic activ-ty of the MnxZn1−xO powders relative to the undoped ZnO could behe presence of a secondary phase in addition to the ZnO solid solu-ion. Previous experimental studies have demonstrated that theddition of a suitable secondary phase can significantly increasehe activity of a semiconductor photocatalyst by inhibiting chargearrier recombination [25,26]. Although no secondary phase wasetected by XRD analysis of the MnxZn1−xO powders, it is neverthe-
ess possible that another phase, such as Mn3O4 or a Mn3−xZnxO4pinel [27], was present in proportions below the detection limit ofhe XRD technique.
The effect of transition metal doping on the photocatalytic activ-ty of nanoparticulate ZnO has previously been investigated byasey et al. [28]. In their study, a series of undoped and transitionetal (Fe, Ni, Co, and Mn) doped powders of nanoparticulate ZnOere synthesised using an unspecified proprietary method. The
ctivity of these powders was evaluated using the photobleachingf 1,1-diphenyl-2-picrylhydrazyl as the probe reaction. In contrasto the present study, Casey et al. found that doping decreased thehotocatalytic activity irrespective of the dopant cation. The differ-nt effect of Mn-doping in their study could perhaps be attributedo a difference in how the dopant was incorporated.
. Summary and conclusions
In this study, it has been demonstrated that mechanochemicalrocessing of anhydrous chloride precursors with Na2CO3 can besed to manufacture nanoparticulate powders of ZnO, Mn3O4, and
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d Physics 114 (2009) 382–386
o3O4. In addition, powders of doped ZnO can be manufacturedy simply combining the basic reaction system for the synthesisf ZnO with that for the dopant. Doping increased the specificurface area of the powder and also resulted in coloration. Thehotocatalytic activity of CoxZn1−xO progressively decreased withhe doping level. In contrast, the activity of MnxZn1−xO initiallyncreased with doping and thereafter declined. These results indi-ate that the photocatalytic activity of nanoparticulate ZnO canptimized for different applications through the addition of transi-ion metal oxide dopants.
cknowledgments
This research was supported by the Australian Research Councilnder Linkage Project LP0349177 and was carried out using facil-
ties at the Centre for Microscopy, Characterisation and Analysis,hich is supported by University, State and Federal Government
unding.
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