PHOTOLUMINESCENCE AND PHOTOCATALYTIC ACTIVITY OF Mn- DOPED ZnO NANOPARTICLES
G. VOICU, O. OPREA *, B. S. VASILE, E. ANDRONESCU
University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Romania
Nanocrystalline ZnO particles doped with Mn(II) ions were prepared by a forced hydrolysis method of zinc acetate dihydrate and manganese acetate tetrahydrate, under reflux, in buthanol. The precipitate obtained was separated by centrifugation at 9.000 rpm and purified by refluxing in water. The dopant percentage was 1, 2.5 and 5%. The qualitative composition of the nanopowder has been evidenced in the elemental EDS maps. Optical investigation shows that the Mn doping in ZnO lattice leads to a decrease in the near band edge position due to the introduction of new unoccupied states by Mn 3d electrons. The luminescence of ZnO is quenched by increasing the dopant ions percentage.
At doping rate of 1% Mn in the ZnO lattice a tenfold decrease in intensity of luminescence was observed, along with modification of the luminescence pattern. Further increases of dopant percent from 1% to 5% had as result a decrease of only 30% in the luminescence intensity. The photocatalytic activity was investigated against methylene blue. The increase of Mn percentage leads to a better photocatalytic activity.
(Received March 11, 2013; Accepted April 18, 2013)
Keywords: Mn-doped zinc oxide, nanomaterials, photocatalysis
1. Introduction
In the last decade various oxide semiconductors [1-4] have been used as catalyst, phosphor, gas sensor, photocatalyst, UV-photoprotector, varistor or in dye-sensitized solar cells [5- 10]. The field of applications is determined by the electrical, optical and structural properties of the semiconductors. Among them, ZnO offers some unique optoelectronic properties due to its wide bandgap of 3.3 eV and large excitonic binding energy of 60 meV.
Due to the special optical properties, high transparencies in the visible domain coupled with high absorbance of the UV radiation, ZnO has been toughly investigated as a coating material, from paints [11], to sunscreens [12] and fabric coating [13].
The photocatalytic activity of ZnO is well known and has been investigated versus a wide variety of pollutants. There are some application where a high photocatalytic activity is desirable [14], but there are also some application where ZnO capacity to degrade various organic substrate is an impediment, like f abric or paper coating.
Doping of ZnO with transition metals (TM) can lead to ferromagnetic properties at room temperature (RT FM), might form dilute magnetic semiconductors (DMSs) and is an effective way to tune the properties of ZnO [15,16]. The doping TM will create some unoccupied states that will consequently alter of the band gap energy [17]. The presence of a dopant ion in the ZnO lattice can influence also the photocatalytic capacity of the nanoparticles, in both directions, giving an easy method to tune this property to suit various applications [18-20].
In our previous works, we have prepared pure and doped ZnO by force hydrolysis, sol-gel or pyrosol methods [21-23], and we manage to tune the luminescent properties of ZnO by thermal treatment [24].
* Corresponding author: [email protected]
In the case of Mn-doped ZnO it is known that the solubility of the dopant in ZnO lattice depends on many factors (the preparation method and conditions, the annealing temperature, the doping concentration, and even the grain size). All factors are related to the structural defects that may exist in the ZnO nanoparticles, which will influence also the electronic and optoelectronic properties. The intrinsic defects commonly found in ZnO are zinc interstitials (Zni), zinc vacancies (VZn), oxygen interstitials (Oi), oxygen vacancies (VO), oxygen antisites (OZn), and zinc antisites (ZnO), as were detailed described by Kohan et al [25].
Samples with different Mn(II) content (1-5%) were synthesized by adapting the previously reported forced hydrolysis method in alcohol [21], from zinc acetate dihydrate and manganese acetate tetrahydrate, under reflux. The samples were structurally characterized by means UV-Vis and PL spectrometry, XRD, TEM, HRTEM and SAED. The photocatalytic activity was investigated against methylene blue.
2. Experimental procedure
Zinc acetate dihydrate, Zn(CH3COO)2·2H2O and Mn(CH3COO)2·4H2O with 99.9% purity, were obtained from Merck. Buthanol was used as received from Sigma without further purification.
2.1 Mn(II) (1%) doped ZnO synthesis
2.173g (9.9 mmoles) Zn(CH3COO)2 2H2O and 0.0245g (0.1 mmoles) Mn(CH3COO)2·4H2O were dissolved in 50 mL buthanol. The solution was then kept for 12h on a thermostatic bath at 120oC under magnetic stirring. The light brown colloidal precipitate formed was then separated by centrifugation at 9.000 rpm and washed several times with ethanol. The light-brown powder was dried at 105oC for 30 min in the ambient atmosphere following the removal of supernatant.
We have prepared three doped ZnO samples and a pure control sample by this method (0%, 1%, 2.5% and 5%).
2.2 Experimental techniques
a) Electron Microscope Images. The transmission electron images were obtained on ultrasonated powdered samples using a TecnaiTM G2 F30 S-TWIN high resolution transmission electron microscope from FEI, equipped with STEM/HAADF detector, EDX (Energy dispersive X-ray Analysis) and EFTEM - EELS (Electron energy loss spectroscopy) operated at an acceleration voltage of 300 KV obtained from a Shottky Field emitter with a TEM point resolution of 2 Å and line resolution of 1.02 Å.
b) X-ray Diffraction. X-ray powder diffraction patterns were obtained with a Shimadzu XRD6000 diffractometer, using Cu Kα (1.5406 Å) radiation operating with 30 mA and 40 kV in the 2θ range 10–70o. A scan rate of 1o min-1 was employed.
c) Photoluminescence spectra. Photoluminescence spectra (PL) were measured with a Perkin Elmer P55 spectrometer using a Xe lamp as a UV light source at ambient temperature, in the range 200-800 nm, with all the samples in solid state. The measurements were made with scan speed of 200 nm·min-1, slit of 10 nm, and cut-off filter of 1% for ZnO sample and without any cut- off filter for Mn doped ZnO samples. An excitation wavelength of 320 nm was used.
d) Diffuse reflectance spectra measurements were made with a JASCO V560 spectrophotometer with solid sample accessory, in the domain 200-800nm, with a speed of 200nm·min-1.
e) Photocatalytic activity was determined against methylene blue (MB) solution, 10-4 %, by irradiation with an Hg lamp. Samples of 0.0250 g powder were inserted in 20 mL solution of MB. Samples were allowed to stay 30 min in dark to reach the adsorption equilibrium. After that, at defined time intervals a sample of 2mL was taken out and its UV-Vis spectra was recorded.
hydrol [ASTM doping crystal larger.
D = 0.
and β (101) w Mn do slightly the ch parame smalle compr
Table
Sampl ZnO ZnO/ M ZnO/
2.5%
ZnO/ M
reveals
3. Resul The figure lysis. All dif M 80-0075].
g did not ch lline particle The cryst 89·λ/β·cosӨ are the diffr was used to c
The lattice oped ZnO sa y with the in hange is sma
eter, which i The Bragg er values re ressive micro
e 1. Lattice pa e
Mn 1%
Mn Mn 5%
Fig. 1. XR
The TEM s that the
lts and dis e 1 shows th ffraction pea The XRD hange the w e size of Mn tallite size Ө, where D is action angle calculate the e constant cal
mples the va ncreased inc all, the conc is an indicato g angle of th
lative to th o stress [26].
arameters and Lattice con
a=b 0.3249 0.3258 0.3261 0.3267
RD patterns of p
bright field i powder is
scussions he XRD patt aks from the patterns sho wurtzite struc n-doped ZnO of the sam s the average
and FWHM average cry lculated for p alues are pre
orporation o entration of or for the Mn he intense ( hat of pure
d the structura nstants (nm)
c 0.5187 0.5193 0.5195 0.5196
pure ZnO powd doping levels
image, figure composed f
tterns of ZnO e samples c ow that ther cture of ZnO O is larger, 1
mples was e grain size, M of an obser
ystallite size ( pure ZnO are esented in tab of Mn in the f the dopant n doping in Z (101) reflect
ZnO. This
al analysis res
2
7 3
3 3
5 3
6 3
der (d) and Mn- 1% (c), 2.5% (
e 2a, of Mn- from polyhe
O and Mn d orrespond to re was no se O. Compare 19.7 nm, and
estimated λ is the X-ra rved peak, re (D) of ZnO p e a = b = 0.3 ble 1. Both t
ZnO lattice plays a role ZnO.
ion showed is evidence
sults for the pu Ө(101)
6.363 6.358 6.324 6.319
-doped ZnO pow (b) and 5% (a).
-doped ZnO edral particl
doped ZnO p o the hexago econd-phase d with pure d also its latt
from the S ay wavelengt
espectively.
particles.
249 nm, c = the ‘a’ and ‘ as shown in e in the vari
a slight shi e for the cr
ure and Mn do FWHM (101)
0.59 0.48 0.45 0.42
wder samples a
nanoparticle les, with an
prepared by onal ZnO st peak and t e ZnO, 14 n ttice paramet Scherrer eq gth (0.15405 The stronge 0.5187 nm.
‘c’ values inc n table 1. Al iation in the ift ∆(2Ө101) reation of i
oped ZnO pow
M D
1 1 1 1
at different
es (5%) as ob n average s
forced tructure that the nm, the ters are quation, nm), Ө est peak For the creased lthough e lattice toward internal
wders.
(nm) 14.0 17.2 18.4 19.7
btained, size of
approximately 10nm, which have a slight tendency to aggregates as nanorods with dimensions of 10nm x50 nm.
Additional information about the structures of the nanoparticles was found through detailed analysis with HRTEM. The HRTEM image, figure 2b, shows clear lattice fringes of interplanar distances of d = 1.62 Å and d = 2.59 Å/(0 0 2) for nanocrystalline ZnO, corresponding to Miller indices (1 1 0) and (0 0 2) respectively, of crystallographic planes of hexagonal ZnO. In addition, the regular succession of the atomic planes indicates that the nanocrystalites are structurally uniform and crystalline with no amorphous phase present.
Fig. 2. (a) TEM images of 5% Mn doped ZnO polyhedral shaped particles - SAED pattern of planes of hexagonal structure ZnO [ASTM 80-0075]; (b) HRTEM with the (1 1 0) and (0 0 2) crystallographic planes of ZnO
From the selected area diffraction pattern obtained on ZnO nanopowder, we can state that the only phase identified is the crystalline hexagonal form of ZnO [ASTM 80-0075]. Moreover, the SAED image of 5% Mn doped ZnO nanoparticles confirms the Miller indices of characteristic crystalline structures identified by XRD (inset of figure 2a).
In order to correlate the microstructure and the distribution of O, Zn and Mn, qualitative analyses, e.g. elemental map and spectrum of the same region were recorded. Elemental mapping of the 5% Mn-doped sample revealed a uniform distribution of various ions in the sample (Fig.3).
Zn O Mn Fig. 3. Elemental mapping of 5%Mn-doped ZnO sample showing the presence of (a) Zn (b) O and (c) Mn
ions, respectively.
UV an and the
the lum anneal the nan nanopa a weak 447 nm emissi the do impuri eV (48 respect and vi occurr blue em or the insertio peak a
The photol nd visible ran e visible emi
Fig. 4. Pho (r
High inten minescence s ling or annea
It has been nostructures articles [32]
The results ker UV emis m) and a seco
on from the opant percen ities introduc The PL sp 83 nm). The tively [35].
The increa iolet emissio rence of dopi mission is ca existence of on of Mn(II) also correlate
luminescenc nges [27]. Th
ission is com
otoluminescen (right) samples
nsity of the v spectra is re aling in prese n reported tha
. [31-33]. A and emission s, figure 4, s ssion, and tw ond at 482 n
wide band g ntage. The s
cing an impu pectrum also
ese PL signa ase in the Mn on. The dec
ing depende aused by the f nonradiative
) ions on the es with the pa
e spectra of he UV emiss mmonly referr
nce spectra of p s at different d
visible lumine corded on p ence of reduc
at the sub-ba blue-green ns at 421 nm show that the wo blue-gree nm. The weak
gap of ZnO shift in the urity level in exhibits two als are attrib n(II) ions pe rease in inte nce on the in apparition o e decay chan e surface of article size [3
ZnO powder sion correspo rred to as a d
f pure ZnO pow doping levels
escence vers powders as o cing atmosph and-gap emis emission at m, 482 nm, an e as-synthesi en emissions, k UV emissi (NBE). This band edge p the band gap o additional buted to ban ercentage has ensity takes intensity. Ac of electron ca nnels [36, 37 the nanopart 38] and shap
rs usually pr onds to the n
eep-level or
wder (left) and (a) 1%, (b) 2.
sus NBE are obtained, wit
here) [29, 30 ssion in ZnO 453 nm is f nd 532 nm fo ized samples , one centred ion at 400 nm s peak is shif
position is p of ZnO [34
weak peaks nd-edge free s a direct eff place syste ccording to li apture centre
]. This behav ticles. The in e [39].
esent two em near band-ed
trap-state em
d Mn-doped Z 5% and (c) 5%
reported in hout further ].
O depends on found in lite or nanocones s have a wurt d at 456 nm m is assigned
fted to 405 n suggested to 4].
at 2.95 eV e excitons an
fect on the in ematically, w iteratures, th s on the surf viour can be ntensity of th
mission peak dge emission mission [28].
ZnO powder
%
literature wh treatments ( n the morpho erature for n
s [33].
rtzite structur (with a shou d to the free nm upon inc o be caused (421 nm) an nd bound ex ntensity of th which confir he quenching face of nanoc e correlated w the visible em
ks in the (NBE)
henever (simple ology of est like re, with ulder at exciton creasing d byMn
nd 2.56 xcitons, he blue rms the g of the crystals with the mission
figure the ab transpa region band to the nan related band-g modifi band g the M [F(R)·
that fo electro
an abso
Fig. 5. Diffu vs. the energ
The electr 5a. As it wa sorbance of arent to visi
.
The funda o conduction noparticles [ d to the Kube gap energies ied Kubelka- gap energy. A Mn-doped Zn
hν]2 = 0. Th or pure ZnO, onic levels in
Fig. 6. De doped ZnO
Like many orption maxi
A fuse reflectanc rgy (B) for the
ronic spectra as expected, f the nanopo ble light, th amental abso n band and c [40, 41]. For elka-Munk fu s (Eg) for th -Munk functi Adopting the nO powder he calculated
, which is in nside the ZnO
etermination o O powder (rig after y thiazine dy imum at 614
ce spectra (A) Mn-doped Zn b) 1%; c)
a recorded f the introduc wder in the e sample do rption refers an be used to r analysis pu
unction F(R) he Mn-dope ion vs. the en e method pro
samples are band gap fo n good agree
O band gap.
of photocataly ght) samples v r 120 min; d) yes, MB has 4 nm [44] vs
and plot of th nO powder sa
2.5%; d) 5%.
for Mn-dope ction of the M
visible regi oped with 5%
s to the optic o determine urposes the d
) by the relat ed ZnO pow nergy is pres oposed by Ca e determined for the Mn-d ement with th
ytic activity of versus methyle after 180 min a tendency 662 nm for t
he transforme mples at diffe .
ed ZnO pow Mn ions in th ion of the sp
% Mn has a cal transition
the nature an diffuse reflec tion F(R)=(1 wder samples
sented in the ao et al.,[43]
d to be 3.13 doped ZnO p
he fact that d
f pure ZnO po ene blue: a) in n; e) after 240 to dimerise.
the monomer B ed Kubelka-Mu
rent doping le
wder sample he lattice of pectrum. Wh
60% absorp n of electron nd values of ctance, R, of -R)2/2R, [42 s, a plot of figure 5b. Th the band-ga 3 eV, by th powder samp doping ions
wder (left) an nitial; b) after
min.
The dimer o r MB [45].
Munk function evels a) 0%;
es are presen f ZnO has inc hile ZnO is ption in the ns from the v optical band f the sample 2]. To determ f the square This yields th
ap energies ( he extrapola ples is small will introdu
nd 5% Mn- r 60 min; c)
of MB, (MB
nted in creased
almost visible valence d gap of can be mine the of the e direct (Eg) for ation to
ler than uce new
B)2, has
The decreases of both absorption maximums, figure 6, indicate that all samples have a photocatalytic activity. Nevertheless, the degradation pathway of MB is different for pure ZnO and Mn–doped ZnO.
While in the case of ZnO sample, both MB and (MB)2 maximums are decreasing at the same rate, in the case of Mn doped ZnO, the MB absorption maximum falls abruptly, while the (MB)2 maximum decrease at a slower rate than in the case of ZnO. This indicates a rapid degradation of MB in the presence of Mn-doped ZnO, part of it being transformed in the dimer (MB)2. The dimer is also degraded, after 4h of irradiation, the sample with Mn-doped ZnO containing slightly more (MB)2 than the sample with ZnO, but no MB.
Because of the dimerization, the size of the molecule increases, this having a direct influence on photocatalysis process, through diffusion step. The decrease of the absorption maximum at 614 nm indicates that the Mn-doped ZnO possesses a photonic efficiency for degradation [46].
0 1 2 3 4
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
lg (Co/C)
Time (h) ZnO
ZnO Mn 1%
ZnO Mn 2.5%
ZnO Mn 5%
d
c b a
Fig. 7. Comparative evolution of lg(C0/C) vs irradiation time
The study shows that although both samples (pure and doped ZnO) have photocatalytic activity, the mechanism is different. While ZnO equally degrades MB and (MB)2, the doped Mn- ZnO transforms MB to (MB)2 and afterwards strongly degrades the dimer. Overall, the Mn-doped samples have a better photocatalytic activity than pure ZnO, figure 7.
4. Conclusions
A synthetic method for the pure ZnO and Mn-doped ZnO nanocrystalline powder using non-basic hydrolysis has been presented. The forced hydrolysis produces a nanopowder that once dried at 120oC, contains ZnO, with no detectable secondary phases. The Mn ions were homogenously incorporated in the ZnO lattice, XRD analysis indicating existence of a single- phase compound, wurtzite. TEM and XRD data sustain the formation of a single phase, monodisperse crystalline Mn-doped ZnO nanopowder.
The band gap value of doped samples is smaller than the band gap value of the bulk ZnO, and it is decreasing with as the Mn dopant percent increases.
On UV excitation with 320nm the doped samples exhibited an overall diminished emission, the blue-green photoluminescence being much stronger than the excitonic one.
Also with increasing of Mn percent, we notice an increase in the photocatalytic activity.
Both effects, the quenched luminescence and better photocatalytic activity can be explained by an increasing in the surface defects associated with lattice distortion induced by Mn ions.
Acknowledgments
Authors recognize financial support from the European Social Fund through POSDRU/89/1.5/S/54785 project: "Postdoctoral Program for Advanced Research in the field of nanomaterials"
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