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Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG)
Materials Chemistry and Physics xxx (2016) 1e7
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG) Malik Amanullah, Qurat-ul-Ain Javed*, Syed Rizwan Department of Physics, School of Natural Sciences, National University of Science and Technology, Islamabad 44000, Pakistan
h i g h l i g h t s ZnO and C-ZnO was synthesized by PEG assisted post growth annealing process. At 5% and 10% of PEG successful synthesis of C-ZnO was found. XRD, SEM and EDX characterizations confirm the successful synthesis of ZnO and C-ZnO. Change in surface energy with respect to PEG molecule concentration was calculated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2016 Received in revised form 4 May 2016 Accepted 21 May 2016 Available online xxx
Zinc Oxide (ZnO) provides unique properties owing to its wide bandgap, large resistivity range and possibility to tune the physical properties. The surfactant assisted carbon doping was made possible due to the lowering of surface energy. The ZnO and carbon doped ZnO (C-ZnO) nanowires fabricated by hydrothermal process, Poly Ethylene Glycol (PEG) is used as surfactant in hydrothermal synthesis followed by post growth annealing treatment at 600 Ce700 C. At 5%e10% of diluted PEG carbon is doped in ZnO. The crystallinity, structural morphology and elemental composition analysis for ZnO and C-ZnO nanowires were carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques respectively. Carbon doping in ZnO nanowires in the presence of different percentage of surfactant is explained by calculating the change in surface energy with respect to change in PEG molecule concentration. It was found that the surface energy per molecule modulates from 3.92 108 J/m2 to 8.16 107 J/m2 in the PEG concentration range between 5% and 10%. Our results provides a new theoretical calculations, implemented on real system, to observe the details of PEGassisted Carbon doping in II-VI semiconductor nanowires. © 2016 Elsevier B.V. All rights reserved.
Keywords: Surface properties Crystal growth Electron microscopy Chemical synthesis
1. Introduction ZnO has various distinctive properties like direct wide band gap (3.37 eV), large exciton binding energy (60 meV) and wide resistivity range (1041012 U.cm) which attracts the attention of researchers to explore further interesting properties [1e5].One dimensional (1D) nanostructure is useful for the investigation of electrical, transport and mechanical properties [6e8]. And also play an important role for the nano-fabrication device [9]. Among 1D structure, ZnO has fundamental importance in recent research [10]. Further properties of ZnO can be explored by the introduction of
* Corresponding author. E-mail address: [email protected] (Q.-u.-A. Javed).
suitable dopant at appropriate percentage. Transition metal (TM) -ion doping in ZnO and GaN are considered to DMS materials and have room temperature (RT) ferromagnetism, which can be used for spintronics applications. TM-ion doping causes clustering problem in the host material [11,12]. So there is research to look for non-metallic doping in ZnO. There are also recent claims that the carbon-doped ZnO exhibits RT ferromagnetism [13e15]. So the challenge is the successful synthesis of C doped ZnO nanowires. Different doping techniques can be used such as ion implantation [16], magnetic sputtering [17], and high temperature calcinations [18] for C-doping in ZnO, and it has been observed that the successful C-doping in ZnO thin film is achieved by post growth annealing at 800 C [19]. Growth of ZnO nanowires can be made possible by hydrothermal process, and the diameter can be further tuned with the help of appropriate percentage of
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Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
suitable surfactant [20]. Since the surfactant is adsorbed at the surface and passivates the surface due to saturation in dangling bonds at the surface, resulting in lowering the surface energy and the diffusion rate is determined by the diffusion coefficient of the surfactant. The interface is created between surface atoms and surfactant atoms which limits further diffusion that decreases the surface energy. The extent of decrement can be quantified by taking the surface energy as a function of concentration of surfactant added. Surface tension or surface energy (s) between the molecules decreases as surfactant concentration ‘C’ in a solution is increased. Minimum achievable surface tension (smin) can be explained in such a way that when surfactant is added to the solution, the ‘C’ is distributed as individual molecule concentration in a bulk solution ‘Cbulk’ and surface molecules concentration G. As ‘C’ in a solution is increased, ‘Cbulk’ starts forming micelles. The value of ‘C’ at which micelles are formed is known as critical micelle concentration (cmc) and at cmc G also reaches to saturation level Gcmc. A further addition at saturation value will only cause micelle formation and no effect to G. Since the surface tension is due only to the surface concentration so, it also reaches to the saturation value i.e. smin ¼ scmc [21e23]. As surface energy changes with surfactant concentration, so surface energy can be tuned to such a value where dopant can make a bond with host material with the help of post growth annealing process. Doping is made possible by post-growth annealing in the surfactant assisted hydrothermal process. 1.1. C-doped ZnO
Fig. 1. XRD pattern for ZnO.
and temperature leakage. Finally the auto-calve is placed in drying oven for 14 h, and the suspension was rinsed with water and ethanol with the help of centrifugation. ZnO nanowires is obtained and color of the obtained product is white-yellowish.
2.2. Synthesis of carbon-doped-ZnO 10
2
The electronic configuration of Zn atom is [Ar]3d 4s and for oxygen atom is 1s22s22p4 and þ2 oxidation state of Zn atoms forms sp3 covalent bonding with four nearest neighbors of oxygen atoms. Since the ionicity of ZnO have substantial ionic character up to a greater extent, so it is expected that the electronic configuration of the Carbon atom ([He]2s22p2) will disturb the sp3 hybridization when doped. There are two lattice sites where carbon can reside itself, first at the Zn-site which is hardly to occur because Zn atom is screened by four oxygen atoms, also it is 5 times less than the atomic mass of Zn so it cannot replace the Zn-atom. In other words, the carbon atom has not enough momentum to replace Zn atoms. The Second lattice site is the oxygen site, where it can reside due to almost same atomic size. The substitution of C at oxygen lattice site is also responsible for room temperature ferromagnetism. A strong coupling between carbon 2p and Zn 4s, 3d exists due to the formation of spin split exchange interaction state. There exists a p-p exchange interaction in localized C2p and valence band electrons. Our focus is on the carbon doping in ZnO nanowires. As for the dopant material is concerned, it can alter the chemical and physical properties while retaining the same crystal structure [24].
Carbon is doped through post-growth annealing process, suspension obtained from autoclave was precipitated. 4% by weight carbon was added followed by annealing at 600 C to 700 C for 3 h then placed under normal room temperature condition for cooling. And the color of the final product changes to yellowish-black. As suspension was prepared under the different percentage of PEG (1%, 5%, 10%, 15%, 20%) and role of the PEG is to passivates the surface energy of ZnO. For different percentage of PEG we have different surface energy for ZnO nanowires. Carbon doping is only possible for specific surface energy of ZnO nanowires assisted by PEG in post growth annealing process. Obtained result is presented in Results and Discussion section.
2. Experimental section 2.1. Synthesis of ZnO and carbon-doped ZnO 10 ml of 1 M Zinc Nitrate Hexahydrate (Zn(NO3)2.6H2O) was dissolved in 20 ml of Ammonium Carbonate (NH4)2CO3) and stirred at 800 rpm (revolution per minute) with the help of hot plate followed by the centrifuge process, two times, at 1000 rpm to separate precipitate from residual reactant by using the centrifuge machine. Obtained precipitate was dispersed in 70 ml of different percentage of Poly Ethylene Glycol (PEG) by sonicator for 30 min without heating. The solution was added in Teflon cup and was placed to auto-calve and the screws were tightened so as to avoid pressure
Fig. 2. XRD pattern for C-ZnO.
Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7 Table 1 Crystalize size and lattice strain for ZnO and C-doped ZnO at different percentages of PEG. PEG
1% 5% 10% 15% 20%
Crystalize size(nm)
Lattice strain
ZnO
C-ZnO
ZnO
C-ZnO
41.58 41.98 43.23 42.4 39.70
14.27 22.68 22.98 17.19 14.68
0.0028 0.0028 0.0027 0.0027 0.0029
0.0082 0.0051 0.0051 0.0077 0.0079
3. Results and discussion 3.1. X-ray diffraction analysis Figs. 1 and 2 show the XRD pattern of ZnO and Carbon-doped ZnO respectively, obtained from Cu-source irradiated Karays with wavelength of 1.54 nm. The obtained pattern is analyzed with MDI Jade 6.5 using the PDF (Powder Diffraction File) card number 361451. ZnO is Wurtzite structure a b c (90 90 120 ) where a, b and c are lattice constants. By calculating the average lattice constant for the above obtained pattern, the values come out to be 3.2495 3.2495 5.2076 (90 90 120 ) deviating 0.009 % 0.009 % 0.019% with actual values for lattice parameters 3.2498 3.2498 5.2066 (measured in Å). Crystal size (CS) is calculated by using the Scherrer’s formula:
Dp ¼
kl bcosq
(1)
where, Dp is the radius of the highest intensity peak, k is a shape factor usually taken as 0.9, b is Full Width Half Maximum (FWHM) and q is Bragg’s angle taken in radian. By providing the above information, CS has been calculated for ZnO and C-ZnO at different
3
percentages of PEG as shown in Table 1. The CS increases in the range from 1% to 15% but decreases for 20%. We observed that as PEG concentration is increased from 1% to 10%, the CS also increases for both, ZnO and C-ZnO. This is due to the slow nucleation offered by PEG molecule as PEG molecules cover the bulk Zn and O atoms and allow lesser molecules to nucleate. Slow nucleation causes larger CS. In PEG concentration range between 5% and 10% the CS value goes higher for both, ZnO and C-ZnO. At this concentration, PEG-ZnO has lower surface energy because most of the surface is covered by PEG molecules, making it easier for carbon atoms to make bond with ZnO in post growth annealing at 600 C by doing less work. In other words, lesser energy is required for C to dope in ZnO and to avoid distorting the crystal. The concentration range between 10% and 15% decreases CS; this is due to the fact that the PEG molecule concentration is just enough for most of the molecules to remain nonnucleated.
3.2. Surface characterization The Scanning Electron Microscopy (SEM) images of ZnO and CZnO are shown in Fig. 3 for the PEG concentration of 5% & 10%, while for PEG concentration of 1% and 15% are shown in Fig. 4. The results obtained for ZnO and C-ZnO at 5% and 10% of PEG show that well defined shape of nanowires are achieved. This is due to the fact that the surface energy offered by Zn and O atom during nucleation is such that it allows the carbon atom to form a bond while at 1% and 15% PEG concentration, the regular ZnO shape is found, but the shape of a C-ZnO is distorted, this is attributed to the fact that carbon presents an environmental fluctuation which do not allow the particles of Zn and O to nucleate. Detail of diameter and average length is listed in Table 2. In all cases, it is observed that the diameter of nanowires increases with carbon doping due to dislocation of lattice sites in the crystal.
Fig. 3. SEM characterization for ZnO and C-ZnO at 5% and 10% of PEG.
Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
Fig. 4. SEM characterization for ZnO and C-ZnO at 1% and 15% of PEG.
3.4. Dependence of ZnO (a-phase) surface energy on PEG (b-phase) surfactant concentration and carbon doping
3.3. Energy dispersive X-ray (EDX) characterization The peak of oxygen is observed at 0.525 keV, while the peaks of Zinc are observed at 1.01, 8.63 and 9.58 keV; these peaks distribution corresponds to the pure ZnO. Fig. 5a shows that the Gaussian peaks for Zn and O exist at specific energies when 10% of PEG surfactant was used. The 4%, by weight, doping of carbon in C-ZnO gives an extra peak at 0.277 keV, while other peaks are observed at the same position as before but with different intensity, as shown in Fig. 5b. This difference in intensity is due to the free electrons provided by carbon which enhances the electron transition. All the above results are summarized in Table 3 where we can see that as carbon is doped, it replaces oxygen as well as Zn atoms. Carbon was doped in ZnO through post growth annealing process, carbon replaces the Zn and Oxygen lattice site. This is due to pp exchange interaction between localized C2p electrons and valence band electrons of Zn and Oxygen, results are listed in Table 3. Crystal distortion was observed for C-ZnO as shown by the XRD pattern (Fig. 2). Furthermore, In order to study the effect of carbon doping on ZnO surface morphology SEM technique was applied. SEM images were taken at 1%, 5%, 10% and 15% PEG concentrations (Figs. 3 and 4), results showed a diameter of nanowires increased with carbon doping at all PEG concentrations.
Table 2 Avg. diameter and length at different percentages of PEG for ZnO and C doped ZnO. PEG
1% 5% 10% 15%
Avg. diameter (nm)
Avg. length (m m)
ZnO
C-ZnO
ZnO
C-ZnO
63.91 50.44 54.29 117.7
Nil 98.6 98.97 141.54
25.99 14.63 0.64 324.6
Nil 32.48 16.67 18.23
For an ease, we assume that the surfactant is to be considered as
b phase and the host material as a phase. As the PEG molecules (Nb) are added during the nucleation of ZnO molecules (Na) they cover the surface available for nucleation, in result control the nucleation rate and lowers the surface energy. Lowering of surface energy is quantifiable in terms of measuring the change in Helmholtz energy with respect to changes in the surface area available for nucleation. Using the first law of thermodynamics we can write,
Dεab ¼ T Dsab W
(2)
where Dεab and Dsab are, respectively the change in internal energy Nb N and entropy when b molecule is added to a-phase. c ¼ Na þN ¼ Nba is b Na the fractional concentration of b phase and 1 c ¼ Na is the fractional concentration of a phase, the fractional concentration is taken to avoid nonlinearity in a system. Addition of surfactant reduces the surface energy of host material by doing the work which can be quantified as W ¼ sabDa; sab is the surface energy between respective phases and ‘Da’ is the change in surface area.
Dε T Dsab sab ¼ ab Da
sab ¼
vfab va
(3)
(4) NVT
Dεab is to be determined in terms of interatomic potential [25].
Dεab ¼ Pab εab
1 εaa þ εbb 2
(5)
Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
5
Fig. 5. EDX analysis for (a) ZnO and (b) C-ZnO at 10% of PEG.
εaa,εab and εbb are the interatomic potential or bond energies between the molecules of a and b phases. Pab are the number of bonds in a solution between respective phases which can be defined as;
Pab ¼ zNa cð1 cÞ
(6)
Dεab ¼ zNa cð1 cÞ εab
1 εaa þ εbb 2
(7)
Similarly sab is to be defined by Boltzmann relation sab ¼ klnWab/ W0; W0 and Wab are a number of ways in which the Na and Nb can
‘z’ is the nearest neighbors in the resultant phase (ab). So Equation (5) becomes;
Table 3 EDX for ZnO and C-ZnO. Element
E(keV)
OK 0.525 Zn K* 1.01,8.63,9.58 Total Carbon doped ZnO CK 0.277 OK 0.525 Zn K 1.01,8.63,9.58 Total *K-shell of Zn atom.
%Mass
%Error
%Atom
K
50.09 49.91 100
1.82 12.16
80.39 19.61 100
57.1171 42.8829
37.34 36.53 26.14 100
0.3 0.78 2.68
53.67 39.42 6.9 100
17.9963 39.9783 42.0255
Fig. 6. Surface energy as function of change in surface area.
Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
energy available for nucleation. Our experimental data show that Carbon doping is favorable from 5% to 10% of b concentration, this is due the fact that the available surface energy allows the Carbon atom to make bond with ZnO. The change in surface energy with respect to changes in concentration from 5% to 10% for surface area obtained in experiment at 5% and 10% is shown in figure. Upon Carbon doping by post growth annealing process, the stretching in surface energy with respect to temperature is shown in Fig. 8. At 5% of b concentration, the surface energy per molecule starches from 3.92 108 J/m2 to 3.98 108 J/m2, and for 10% of b concentration surface energy per molecule starches from 8.08 107 J/m2 to 8.16 107 J/m2, when the temperature is changed from 200 C to 700 C. This stretching in surface energy
Fig. 7. Variation in surface energy with concentration of b phase.
Fig. 8. Stretching in surface energy with respect to temperature at PEG at 5% and 10%.
be arranged before and after the mixing. Constraint W0 ¼ 1 for indistinguishable state is applied because particles are distributed ðN þN Þ! a! classically, and Wab ¼ NaN!N ¼ Naa !Nbb! , so; b!
DSab ¼ k ln
4. Conclusions
Na ! N0 !Ns !
(8)
Applying the Stirling formula ln N!zN ln N N to Equation (8) we get:
DSab ¼ R½ð1 cÞlnð1 cÞ þ clnc
(9)
whereas R ¼ kNa. Now inserting Equations (7) and (9) in Equation (3) we get;
1
with temperature is due to increase of thermal energy of surface atoms.
1
εaa þ εbb zNa cð1 cÞ εab sab ¼ 2 Da
References
þ TR½ð1 cÞlnð1 cÞ þ clnc
The Carbon doping in ZnO nanowires is achieved by post growth annealing process using surfactant assisted hydrothermal process. The Carbon makes a bond with ZnO when the surface energy per molecule of ZnO is in between 3.92 108 J/m2 and 8.16 107 J/ m2 at post-growth annealed temperature of 600e700 C. Such a surface energy of ZnO is obtained by changing the concentration of PEG in between 5% and 10% which resulted a change in nanowires morphology.
(10)
The above equation is the change of surface energy density with respect to changes in surface area. Da is the change in surface area when PEG concentration is changed in a solution, the surfactant molecules are organic compounds and exist in the form of micelles or colloidal particles. For 1-dimensional cylindrical structure, the surface area is a ¼ 2prl þ 2pr2, where ‘r’ and ‘l’ are radius and length respectively which change with respect to b concentration. Fig. 6 shows the change in surface energy per molecule with respect to changes in changes in Da, Da is estimated from experimental data written in Table 2 for different percentages of PEG. This estimation can be varied for different shapes (see Fig. 7). This exponential decay corresponds to the fact that when b molecule is added, at first they start forming micelles and when the concentration is reached at cmc there is no further effect on surface
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG) Malik Amanullah, Qurat-ul-Ain Javed*, Syed Rizwan Department of Physics, School of Natural Sciences, National University of Science and Technology, Islamabad 44000, Pakistan
h i g h l i g h t s ZnO and C-ZnO was synthesized by PEG assisted post growth annealing process. At 5% and 10% of PEG successful synthesis of C-ZnO was found. XRD, SEM and EDX characterizations confirm the successful synthesis of ZnO and C-ZnO. Change in surface energy with respect to PEG molecule concentration was calculated.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 February 2016 Received in revised form 4 May 2016 Accepted 21 May 2016 Available online xxx
Zinc Oxide (ZnO) provides unique properties owing to its wide bandgap, large resistivity range and possibility to tune the physical properties. The surfactant assisted carbon doping was made possible due to the lowering of surface energy. The ZnO and carbon doped ZnO (C-ZnO) nanowires fabricated by hydrothermal process, Poly Ethylene Glycol (PEG) is used as surfactant in hydrothermal synthesis followed by post growth annealing treatment at 600 Ce700 C. At 5%e10% of diluted PEG carbon is doped in ZnO. The crystallinity, structural morphology and elemental composition analysis for ZnO and C-ZnO nanowires were carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques respectively. Carbon doping in ZnO nanowires in the presence of different percentage of surfactant is explained by calculating the change in surface energy with respect to change in PEG molecule concentration. It was found that the surface energy per molecule modulates from 3.92 108 J/m2 to 8.16 107 J/m2 in the PEG concentration range between 5% and 10%. Our results provides a new theoretical calculations, implemented on real system, to observe the details of PEGassisted Carbon doping in II-VI semiconductor nanowires. © 2016 Elsevier B.V. All rights reserved.
Keywords: Surface properties Crystal growth Electron microscopy Chemical synthesis
1. Introduction ZnO has various distinctive properties like direct wide band gap (3.37 eV), large exciton binding energy (60 meV) and wide resistivity range (1041012 U.cm) which attracts the attention of researchers to explore further interesting properties [1e5].One dimensional (1D) nanostructure is useful for the investigation of electrical, transport and mechanical properties [6e8]. And also play an important role for the nano-fabrication device [9]. Among 1D structure, ZnO has fundamental importance in recent research [10]. Further properties of ZnO can be explored by the introduction of
* Corresponding author. E-mail address: [email protected] (Q.-u.-A. Javed).
suitable dopant at appropriate percentage. Transition metal (TM) -ion doping in ZnO and GaN are considered to DMS materials and have room temperature (RT) ferromagnetism, which can be used for spintronics applications. TM-ion doping causes clustering problem in the host material [11,12]. So there is research to look for non-metallic doping in ZnO. There are also recent claims that the carbon-doped ZnO exhibits RT ferromagnetism [13e15]. So the challenge is the successful synthesis of C doped ZnO nanowires. Different doping techniques can be used such as ion implantation [16], magnetic sputtering [17], and high temperature calcinations [18] for C-doping in ZnO, and it has been observed that the successful C-doping in ZnO thin film is achieved by post growth annealing at 800 C [19]. Growth of ZnO nanowires can be made possible by hydrothermal process, and the diameter can be further tuned with the help of appropriate percentage of
http://dx.doi.org/10.1016/j.matchemphys.2016.05.051 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Amanullah, et al., Surfactant-assisted carbon doping in ZnO nanowires using Poly Ethylene Glycol (PEG), Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.05.051
2
M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
suitable surfactant [20]. Since the surfactant is adsorbed at the surface and passivates the surface due to saturation in dangling bonds at the surface, resulting in lowering the surface energy and the diffusion rate is determined by the diffusion coefficient of the surfactant. The interface is created between surface atoms and surfactant atoms which limits further diffusion that decreases the surface energy. The extent of decrement can be quantified by taking the surface energy as a function of concentration of surfactant added. Surface tension or surface energy (s) between the molecules decreases as surfactant concentration ‘C’ in a solution is increased. Minimum achievable surface tension (smin) can be explained in such a way that when surfactant is added to the solution, the ‘C’ is distributed as individual molecule concentration in a bulk solution ‘Cbulk’ and surface molecules concentration G. As ‘C’ in a solution is increased, ‘Cbulk’ starts forming micelles. The value of ‘C’ at which micelles are formed is known as critical micelle concentration (cmc) and at cmc G also reaches to saturation level Gcmc. A further addition at saturation value will only cause micelle formation and no effect to G. Since the surface tension is due only to the surface concentration so, it also reaches to the saturation value i.e. smin ¼ scmc [21e23]. As surface energy changes with surfactant concentration, so surface energy can be tuned to such a value where dopant can make a bond with host material with the help of post growth annealing process. Doping is made possible by post-growth annealing in the surfactant assisted hydrothermal process. 1.1. C-doped ZnO
Fig. 1. XRD pattern for ZnO.
and temperature leakage. Finally the auto-calve is placed in drying oven for 14 h, and the suspension was rinsed with water and ethanol with the help of centrifugation. ZnO nanowires is obtained and color of the obtained product is white-yellowish.
2.2. Synthesis of carbon-doped-ZnO 10
2
The electronic configuration of Zn atom is [Ar]3d 4s and for oxygen atom is 1s22s22p4 and þ2 oxidation state of Zn atoms forms sp3 covalent bonding with four nearest neighbors of oxygen atoms. Since the ionicity of ZnO have substantial ionic character up to a greater extent, so it is expected that the electronic configuration of the Carbon atom ([He]2s22p2) will disturb the sp3 hybridization when doped. There are two lattice sites where carbon can reside itself, first at the Zn-site which is hardly to occur because Zn atom is screened by four oxygen atoms, also it is 5 times less than the atomic mass of Zn so it cannot replace the Zn-atom. In other words, the carbon atom has not enough momentum to replace Zn atoms. The Second lattice site is the oxygen site, where it can reside due to almost same atomic size. The substitution of C at oxygen lattice site is also responsible for room temperature ferromagnetism. A strong coupling between carbon 2p and Zn 4s, 3d exists due to the formation of spin split exchange interaction state. There exists a p-p exchange interaction in localized C2p and valence band electrons. Our focus is on the carbon doping in ZnO nanowires. As for the dopant material is concerned, it can alter the chemical and physical properties while retaining the same crystal structure [24].
Carbon is doped through post-growth annealing process, suspension obtained from autoclave was precipitated. 4% by weight carbon was added followed by annealing at 600 C to 700 C for 3 h then placed under normal room temperature condition for cooling. And the color of the final product changes to yellowish-black. As suspension was prepared under the different percentage of PEG (1%, 5%, 10%, 15%, 20%) and role of the PEG is to passivates the surface energy of ZnO. For different percentage of PEG we have different surface energy for ZnO nanowires. Carbon doping is only possible for specific surface energy of ZnO nanowires assisted by PEG in post growth annealing process. Obtained result is presented in Results and Discussion section.
2. Experimental section 2.1. Synthesis of ZnO and carbon-doped ZnO 10 ml of 1 M Zinc Nitrate Hexahydrate (Zn(NO3)2.6H2O) was dissolved in 20 ml of Ammonium Carbonate (NH4)2CO3) and stirred at 800 rpm (revolution per minute) with the help of hot plate followed by the centrifuge process, two times, at 1000 rpm to separate precipitate from residual reactant by using the centrifuge machine. Obtained precipitate was dispersed in 70 ml of different percentage of Poly Ethylene Glycol (PEG) by sonicator for 30 min without heating. The solution was added in Teflon cup and was placed to auto-calve and the screws were tightened so as to avoid pressure
Fig. 2. XRD pattern for C-ZnO.
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7 Table 1 Crystalize size and lattice strain for ZnO and C-doped ZnO at different percentages of PEG. PEG
1% 5% 10% 15% 20%
Crystalize size(nm)
Lattice strain
ZnO
C-ZnO
ZnO
C-ZnO
41.58 41.98 43.23 42.4 39.70
14.27 22.68 22.98 17.19 14.68
0.0028 0.0028 0.0027 0.0027 0.0029
0.0082 0.0051 0.0051 0.0077 0.0079
3. Results and discussion 3.1. X-ray diffraction analysis Figs. 1 and 2 show the XRD pattern of ZnO and Carbon-doped ZnO respectively, obtained from Cu-source irradiated Karays with wavelength of 1.54 nm. The obtained pattern is analyzed with MDI Jade 6.5 using the PDF (Powder Diffraction File) card number 361451. ZnO is Wurtzite structure a b c (90 90 120 ) where a, b and c are lattice constants. By calculating the average lattice constant for the above obtained pattern, the values come out to be 3.2495 3.2495 5.2076 (90 90 120 ) deviating 0.009 % 0.009 % 0.019% with actual values for lattice parameters 3.2498 3.2498 5.2066 (measured in Å). Crystal size (CS) is calculated by using the Scherrer’s formula:
Dp ¼
kl bcosq
(1)
where, Dp is the radius of the highest intensity peak, k is a shape factor usually taken as 0.9, b is Full Width Half Maximum (FWHM) and q is Bragg’s angle taken in radian. By providing the above information, CS has been calculated for ZnO and C-ZnO at different
3
percentages of PEG as shown in Table 1. The CS increases in the range from 1% to 15% but decreases for 20%. We observed that as PEG concentration is increased from 1% to 10%, the CS also increases for both, ZnO and C-ZnO. This is due to the slow nucleation offered by PEG molecule as PEG molecules cover the bulk Zn and O atoms and allow lesser molecules to nucleate. Slow nucleation causes larger CS. In PEG concentration range between 5% and 10% the CS value goes higher for both, ZnO and C-ZnO. At this concentration, PEG-ZnO has lower surface energy because most of the surface is covered by PEG molecules, making it easier for carbon atoms to make bond with ZnO in post growth annealing at 600 C by doing less work. In other words, lesser energy is required for C to dope in ZnO and to avoid distorting the crystal. The concentration range between 10% and 15% decreases CS; this is due to the fact that the PEG molecule concentration is just enough for most of the molecules to remain nonnucleated.
3.2. Surface characterization The Scanning Electron Microscopy (SEM) images of ZnO and CZnO are shown in Fig. 3 for the PEG concentration of 5% & 10%, while for PEG concentration of 1% and 15% are shown in Fig. 4. The results obtained for ZnO and C-ZnO at 5% and 10% of PEG show that well defined shape of nanowires are achieved. This is due to the fact that the surface energy offered by Zn and O atom during nucleation is such that it allows the carbon atom to form a bond while at 1% and 15% PEG concentration, the regular ZnO shape is found, but the shape of a C-ZnO is distorted, this is attributed to the fact that carbon presents an environmental fluctuation which do not allow the particles of Zn and O to nucleate. Detail of diameter and average length is listed in Table 2. In all cases, it is observed that the diameter of nanowires increases with carbon doping due to dislocation of lattice sites in the crystal.
Fig. 3. SEM characterization for ZnO and C-ZnO at 5% and 10% of PEG.
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
Fig. 4. SEM characterization for ZnO and C-ZnO at 1% and 15% of PEG.
3.4. Dependence of ZnO (a-phase) surface energy on PEG (b-phase) surfactant concentration and carbon doping
3.3. Energy dispersive X-ray (EDX) characterization The peak of oxygen is observed at 0.525 keV, while the peaks of Zinc are observed at 1.01, 8.63 and 9.58 keV; these peaks distribution corresponds to the pure ZnO. Fig. 5a shows that the Gaussian peaks for Zn and O exist at specific energies when 10% of PEG surfactant was used. The 4%, by weight, doping of carbon in C-ZnO gives an extra peak at 0.277 keV, while other peaks are observed at the same position as before but with different intensity, as shown in Fig. 5b. This difference in intensity is due to the free electrons provided by carbon which enhances the electron transition. All the above results are summarized in Table 3 where we can see that as carbon is doped, it replaces oxygen as well as Zn atoms. Carbon was doped in ZnO through post growth annealing process, carbon replaces the Zn and Oxygen lattice site. This is due to pp exchange interaction between localized C2p electrons and valence band electrons of Zn and Oxygen, results are listed in Table 3. Crystal distortion was observed for C-ZnO as shown by the XRD pattern (Fig. 2). Furthermore, In order to study the effect of carbon doping on ZnO surface morphology SEM technique was applied. SEM images were taken at 1%, 5%, 10% and 15% PEG concentrations (Figs. 3 and 4), results showed a diameter of nanowires increased with carbon doping at all PEG concentrations.
Table 2 Avg. diameter and length at different percentages of PEG for ZnO and C doped ZnO. PEG
1% 5% 10% 15%
Avg. diameter (nm)
Avg. length (m m)
ZnO
C-ZnO
ZnO
C-ZnO
63.91 50.44 54.29 117.7
Nil 98.6 98.97 141.54
25.99 14.63 0.64 324.6
Nil 32.48 16.67 18.23
For an ease, we assume that the surfactant is to be considered as
b phase and the host material as a phase. As the PEG molecules (Nb) are added during the nucleation of ZnO molecules (Na) they cover the surface available for nucleation, in result control the nucleation rate and lowers the surface energy. Lowering of surface energy is quantifiable in terms of measuring the change in Helmholtz energy with respect to changes in the surface area available for nucleation. Using the first law of thermodynamics we can write,
Dεab ¼ T Dsab W
(2)
where Dεab and Dsab are, respectively the change in internal energy Nb N and entropy when b molecule is added to a-phase. c ¼ Na þN ¼ Nba is b Na the fractional concentration of b phase and 1 c ¼ Na is the fractional concentration of a phase, the fractional concentration is taken to avoid nonlinearity in a system. Addition of surfactant reduces the surface energy of host material by doing the work which can be quantified as W ¼ sabDa; sab is the surface energy between respective phases and ‘Da’ is the change in surface area.
Dε T Dsab sab ¼ ab Da
sab ¼
vfab va
(3)
(4) NVT
Dεab is to be determined in terms of interatomic potential [25].
Dεab ¼ Pab εab
1 εaa þ εbb 2
(5)
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
5
Fig. 5. EDX analysis for (a) ZnO and (b) C-ZnO at 10% of PEG.
εaa,εab and εbb are the interatomic potential or bond energies between the molecules of a and b phases. Pab are the number of bonds in a solution between respective phases which can be defined as;
Pab ¼ zNa cð1 cÞ
(6)
Dεab ¼ zNa cð1 cÞ εab
1 εaa þ εbb 2
(7)
Similarly sab is to be defined by Boltzmann relation sab ¼ klnWab/ W0; W0 and Wab are a number of ways in which the Na and Nb can
‘z’ is the nearest neighbors in the resultant phase (ab). So Equation (5) becomes;
Table 3 EDX for ZnO and C-ZnO. Element
E(keV)
OK 0.525 Zn K* 1.01,8.63,9.58 Total Carbon doped ZnO CK 0.277 OK 0.525 Zn K 1.01,8.63,9.58 Total *K-shell of Zn atom.
%Mass
%Error
%Atom
K
50.09 49.91 100
1.82 12.16
80.39 19.61 100
57.1171 42.8829
37.34 36.53 26.14 100
0.3 0.78 2.68
53.67 39.42 6.9 100
17.9963 39.9783 42.0255
Fig. 6. Surface energy as function of change in surface area.
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M. Amanullah et al. / Materials Chemistry and Physics xxx (2016) 1e7
energy available for nucleation. Our experimental data show that Carbon doping is favorable from 5% to 10% of b concentration, this is due the fact that the available surface energy allows the Carbon atom to make bond with ZnO. The change in surface energy with respect to changes in concentration from 5% to 10% for surface area obtained in experiment at 5% and 10% is shown in figure. Upon Carbon doping by post growth annealing process, the stretching in surface energy with respect to temperature is shown in Fig. 8. At 5% of b concentration, the surface energy per molecule starches from 3.92 108 J/m2 to 3.98 108 J/m2, and for 10% of b concentration surface energy per molecule starches from 8.08 107 J/m2 to 8.16 107 J/m2, when the temperature is changed from 200 C to 700 C. This stretching in surface energy
Fig. 7. Variation in surface energy with concentration of b phase.
Fig. 8. Stretching in surface energy with respect to temperature at PEG at 5% and 10%.
be arranged before and after the mixing. Constraint W0 ¼ 1 for indistinguishable state is applied because particles are distributed ðN þN Þ! a! classically, and Wab ¼ NaN!N ¼ Naa !Nbb! , so; b!
DSab ¼ k ln
4. Conclusions
Na ! N0 !Ns !
(8)
Applying the Stirling formula ln N!zN ln N N to Equation (8) we get:
DSab ¼ R½ð1 cÞlnð1 cÞ þ clnc
(9)
whereas R ¼ kNa. Now inserting Equations (7) and (9) in Equation (3) we get;
1
with temperature is due to increase of thermal energy of surface atoms.
1
εaa þ εbb zNa cð1 cÞ εab sab ¼ 2 Da
References
þ TR½ð1 cÞlnð1 cÞ þ clnc
The Carbon doping in ZnO nanowires is achieved by post growth annealing process using surfactant assisted hydrothermal process. The Carbon makes a bond with ZnO when the surface energy per molecule of ZnO is in between 3.92 108 J/m2 and 8.16 107 J/ m2 at post-growth annealed temperature of 600e700 C. Such a surface energy of ZnO is obtained by changing the concentration of PEG in between 5% and 10% which resulted a change in nanowires morphology.
(10)
The above equation is the change of surface energy density with respect to changes in surface area. Da is the change in surface area when PEG concentration is changed in a solution, the surfactant molecules are organic compounds and exist in the form of micelles or colloidal particles. For 1-dimensional cylindrical structure, the surface area is a ¼ 2prl þ 2pr2, where ‘r’ and ‘l’ are radius and length respectively which change with respect to b concentration. Fig. 6 shows the change in surface energy per molecule with respect to changes in changes in Da, Da is estimated from experimental data written in Table 2 for different percentages of PEG. This estimation can be varied for different shapes (see Fig. 7). This exponential decay corresponds to the fact that when b molecule is added, at first they start forming micelles and when the concentration is reached at cmc there is no further effect on surface
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