study of blueshift of optical band gap in zinc oxide (zno) nanoparticles

Study of blueshift of optical band gap in zinc oxide (ZnO) nanoparticles prepared by low-temperature wet chemical method M.K. Debanath, S. Karmakar n Department of Instrumentation & USIC, Gauhati University, Guwahati 781014, India article info Article history: Received 29 July 2013 Accepted 16 August 2013 Available online 24 August 2013 Keywords: ZnO nanoparticles Semiconductors UV–vis absorption Blueshift Optical band gap abstract We report synthesis of zinc oxide (ZnO) nanoparticles via wet chemical method using molar solutions of zinc nitrate hexahydrate [Zn(NO 3 ) 2 Á 6H 2 O] and ammonium hydroxide (NH 4 OH) with polyvinylpyrolli- done (PVP) as capping agent. The synthesized ZnO nanoparticles are characterized by X-ray diffraction (XRD) technique and electron microscopy (TEM and SEM with EDX) for compositional analysis and surface morphology. The average crystallite size calculated from XRD pattern has been found to be on the order of 8.5 nm. In this work, the hyperbolic band model (HBM) and UV–vis absorption spectra have been utilized to calculate the band gap and nanoparticle size which was proposed by Meulenkamp. It was Meulenkamp who correlated the particle sizes with the wavelength at which the absorption becomes half of that at the shoulder (λ 1/2 ). The optical band gap value of the prepared ZnO nanoparticle is found to be on the order of 3.63 eV which indicates the presence of blueshift. In order to compare the size and distribution of ZnO nanoparticles, TEM has been utilized. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Nano-semiconductor materials that exhibit peculiar proper- ties which are not shown by their bulk counterparts have attracted much interest from both fundamental and technologi- cal researchers. ZnO is technologically an important material due to its wide range of optical and electrical properties; also it is a semiconductor crystal with a large binding energy (60 meV) and wide band gap (3.3 eV at 30 0 K). ZnO nanoparticles are used in a variety of applications such as U V absorption, antibacterial treatment [1], UV light emitter s [2], photocatalyst [3] and as an additive in many industrial products. It is also used in the fabrication of solar cells [4], gas sensors [5], luminescent materi- als [6], transparent conductor, heat mirrors and coatings. Differ- ent synthesis methods have been developed for the preparation of ZnO nanoparticles and among these, the wet chemical method is the most attractive one due to its perfect control of morphol- ogy, purity, crystallinity, composition and low cost for large-scale production. This work reports the synthesis of ZnO nanoparticle for investig ation of the blueshift of the optical band gap of nanocrystals at pH¼7.5. 2. Experimental Materials: All materials were purchased from the commercial market with highest purity (99.99%). Zinc nitrate hexahydrate [Zn (NO 3 ) 2 Á 6H 2 O] and ammonium hydroxide (NH 4 OH) as the starting materials, polyvinylpyrrolidone (PVP) as capping agent and double-distilled water as dispersing solvent were used to prepare ZnO nanoparticle. Preparation of ZnO nanoparticles: 100 ml of 0.1 mol solution of Zn(NO 3 ) 2 Á 6H 2 O was stirred constantly for 30 min at 60 1C (solu- tion A). 3 wt% of PVP was stirred constantly at 60 1C for 30 min (solution B). Now NH 4 OH was slowly added drop by drop into solution A and stirred at room temperature for 15 min and the pH of the solution was continuously measured. When the pH of the solution is 7.5 and a white solution (solution C) of ZnO is formed the addition of NH 4 OH is stopped. The final mixture (solution B and solution C) is stirred constantly for 1 h at 60 1C and allowed to cool down at room temperature till the white precipitate of ZnO is formed. The whole solution is allowed to settle overnight in a dark chamber. Finally, the precipitate is filtrated which is washed with distilled water to dissolve the impurities and dried at 60 1Cinan oven for 12 h. The basic chemical reaction governing the formation of ZnO has been reported in our earlier work [7]. Characterization methods: Powder X-ray diffraction(XRD) pat- tern of prepared ZnO nanoparticle is recorded by a Philips X-ray Diffractrometer (X'Pert Pro) with Cu K α1 radiation (λ¼1.5406 Å). A scanning electron microscope (SEM with EDX, JEOL JSM Model 6390LV) has been used for surface morphology and compositional Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.069 n Corresponding author. Tel.: þ 91 985 998 3361. E-mail addresses: mrinaldebanath@rediffmail.com (M.K. Debanath), sanjibkab@rediffmail.com (S. Karmakar). Materials Letters 111 (2013) 116–119 analysis of the prepared ZnO nanoparticles. The optical absorption spectra of ZnO dispersed in water are recorded by a UV–vis spectrometer (HITACHI model U-3210). Transmission electron microscopy (TEM) observations are carried out on a JEM-100 CX II electron microscope. The band gap of the ZnO nanoparticle is calculated from the UV absorption result. 3. Results and discussion X-ray diffraction analyses: In or der to identify the p hase and t o know the structure of ZnO nanoparticles, X-r a y diffraction pattern is recorded in the range of 25–751 (2 θ) a t a scanning rate o f 0.021/s and 1s/step.Fig. 1(a) s how s the p resence of only Z nO phase in the sample which corresponds to (100), (002), (1 0 1 ), (1 02), (110 ), (1 03), (200), (112), (20 1), and (004) planes in the hexagonal phase of ZnO with space grou p P6 3 mc and u nit cell par ameters are a¼b¼3.257 Å and c¼ 5.213 Å (PDF N o. 79 -0207). T he b roaden ing o f peaks i s obse rved which is mainly due to the finite size (D) of t he crystallite and some contribution of strain. T he following procedure is adopted prior to XRD observation as r eported ear lier [7]: (1). The diffractometer is calibrated by a standard silicon sample. (2). Correction of instrumental broadening of β 2θ arising due to slit width of the K α1 and K α2 lines is also made. The diffraction broadening only due to grain size β g is given by the Warren rule [8] β 2 g ¼ β 2 2θ Àβ 2 where β is FWHM of a line produced under similar geometrical conditions by the standard material (silicon) with crystallite size $54 nm. The crystallite size (D) for the chemically prepared ZnO nanoparticles is then evaluated for the preferred planes (hkl) using Debye Scherrer's formula [9] D ¼ Kλ=β g cos θ where λ is the wavelength of radiation used, θ is the Bragg angle and K¼0.9 for spherical shape. The average crystallite size is found to be $8.5 nm in the directions perpendicular to (101) and (102) planes. XRD is widely used to determine the particle size of nanopar- ticles but TEM is the best way for the measurement of nanoparticle size. The Scherrer method for calculating particle size gives an average value of the entire particle responsible for diffraction. However, by TEM, besides directly measuring particle size, the morphology of the particles can also be observed. The typical HRTEM result in Fig. 1(b) shows almost spherical shapes with smooth surface. Almost non-dispersed, individual nanoparticles are identified as bulk dots in the HRTEM micrograph having average diameter of nanoparticle is$ 6.5 nm. It is to be noted that the HRTEM photograph pattern is recorded using selected area observations and the particle size is determined in higher magni- fication ($ 2  10 5 ). The grain boundaries (GBs) of nanograined ZnO can drastically change the physical properties [10–12].Itis observed that the ferromagnetism (FM) at room temperature (RT) of the pure and doped ZnO nanoparticles is because of GBs and related oxygen vacancies or defects arranged in GBs and the RT ferromagnetism of the microstructure in ZnO and doped ZnO depends on the specific grain boundary (GB) area, i.e. the ratio of GB area to grain volume S GB [13]. FM of ZnO is Mn-doped ZnO are observed only if the grain boundary area in the unit volume of the material is greater than a certain threshold value S th (S th ¼(773)  10 7 m 2 /m 3 for pur e ZnO a nd S th ¼(274)  10 5 m 2 /m 3 for Mn-doped ZnO) [14].ThusnanograinedZnOisconsideredtobeanimportant material for applications in solar cells,sensors,photovoltaics,photo- catalysis and sprint onics [15]. UV–vis characterization:UV–vis spectra are taken from the colloidal solution of ZnO. The UV absorption spectra in Fig. 2(a) show broad peak at λ $298.8 nm (4.16 eV) which indicates that the presence of blueshift is observed with decrease in particle size with respect to bulk ZnO (λ$376 nm; 3.3 eV) and this could be attributed to the confinement effects [16]. Fig. 1(a) and Fig. 3 indicate that ZnO nanoparticles are successfully synthesized via the wet chemical method. The direct band gap of ZnO colloid is estimated from the graph of hν versus (αhν) 2 through the absorp- tion coefficient α which is related to the band gap E g as (αhν) 2 ¼k (hνÀE g ), where hν is the incident light energy and k is a constant. The extrapolation of the straight line in Fig. 2(b) to (αhν) 2 ¼0 gives the value of band gap energy E g . The optical band gap (E g ) is found to be size dependent and there is an increase in the band gap of the semiconductor with a decrease in particle size. The optical band gap value obtained for ZnO nanoparticle at pH¼ 7.5 is 3.63 eV. The dependence of the absorption band gap as shown in Fig. 2(a) on the size of ZnO nanoparticle is used to determine 25 30 35 40 45 50 55 60 65 70 75 0 20 40 60 80 100 120 140 160 180 200 220 004 201 112 200 103 110 102 101 002 100 Intensity (arb. unit) 2 θ (degree) Fig. 1. (a) XRD pattern of ZnO nanoparticles prepared at 60 1C and (b) TEM image of ZnO nanoparticles. M.K. Debanath, S. Karmakar / Materials Letters 111 (2013) 116–119 117 the particle size. By considering the strong absorption edge in the absorption spectra of the sample, average particle size has been estimated by using the following hyperbolic band model [17]: R ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2π 2 h 2 E gb m n ðE 2 gn ÀE 2 gb Þ v u u t where R is the radius , m* is the effectiv e mass of the specimen (m*¼29.15  10 À31 kg for ZnO), E gb is bulk band gap, h is Planck's constant (6.626 10 À34 Js)andE gn is the b and gap at strong absorp- tion edge. E gn can b e calculated by t he formula E gn ¼hc/λ gn ,whereh is Planck's c onstant, c is the v elocity o f light (3  10 8 ms À1 )andλ gn is the str ong absorption edg e ($ 298.8 nm) obtained from the a bsorption spectra as shown in Fig. 2(a). The av erage particle size is calculated and found to be 6.2 nm . We have calculated particle size of ZnO nanoparticle from another method which was proposed by Meulenkamp who introduced the equation below which correlates the particle sizes to λ 1/2 [18]. The value (2.247 0.09) nm for the diameter of the ZnO nanoparticle is calculated from the following equation: 1240=λ 1=2 ¼ aþ b=D 2 Àc=D where a ¼3.301, b¼ 294.0 and c¼À1.09, λ 1/2 (nm) is the wave- length at which absorption becomes half of that at the shoulder, and D (Å) is the diameter of the particle. It is observed that there are discrepancies among the sizes estimated from above men- tioned methods. To obtain the exact size of the particle using hyperbolic band model, the particle should be exactly circular in shape. From the HRTEM image it is observed that the particles are not exactly circular; hence discrepancies occur. Particle morphology and elemental study: From the SEM image in Fig. 3, it has been found that all the capped crystals of the prepared ZnO nanoparticles are nearly spherical in shape. It is also observed that the surface of the prepared sample is smooth and uniform with no cracks on it. The result of EDX analysis of ZnO nanoparticles indicates that the ZnO nanoparticle contains 100% ZnO void of template. 4. Conclusions W e ha ve successfull y s ynthesized ZnO nanoparticles b y a simple wet c hemical method a t l o w t emper ature. The nanostructure of the prepared ZnO nanoparticle h as been confirmed using XRD, SEM, U V– vis absorption a nd TEM m icrograph analyses. XRD r esult shows that the obtained ZnO nanoparticles are composed of ZnO phase in the hexag onal s y stem with s pace g r oup P 6 3 mc and unit c ell parameters are a¼b¼3.257 Å and c¼5.2 13 Å with proper crystallinity. The av erage cry stallite size obtained b y XRD is $8.5 nm using the Scherrer formula. The estimated optical b and gap of ZnO i s found to be 3.63 eV which also clearly indicates the presence of blueshift a t a growth temperature of 60 1C. The UV abso rpt ion resul t also show s an incr ease in optical band gap which e xhibits a quantum c onfinement effect. Acknowledgments The authors thank the S AIF, Department of Instrumentation & USIC, Gauhati University; T ezpur University ; SAIF, NEHU Shillong; and Department of Chemistry, Gauhati University ; for providing XRD, SEM, TEM and UV–vis spectroscopy measurements respectively . Fig. 3. SEM image of ZnO nanoparticles and EDX spectra of the prepared ZnO nanoparticles with composition of elements. 250 260 270 280 290 300 310 320 330 340 350 1.00 1.25 1.50 1.75 2.00 2.25 λ 1/2 Absorption (arb. unit) Wavelength (nm) 298.8 318.9 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 0 25 40 35 30 20 15 10 ( α h ν ) 2 x 10 6 (eV 2 m -2 ) h ν (eV) 5 3.63 Fig. 2. (a) Absorption spectra of ZnO colloid and (b) optical band gap of ZnO colloid. 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Photoemission spectroscopy of size selected zinc sul fide nanocrystallites. Journal of Applied Physics 2001;90:2504–11. [18] Meulenkamp EA. Synthesis and growth of ZnO nanoparticles. Journal of Physical Chemistry B 1988;102:5566–72. M.K. Debanath, S. Karmakar / Materials Letters 111 (2013) 116–119 119 . Study of blueshift of optical band gap in zinc oxide (ZnO) nanoparticles prepared by low-temperature wet chemical method M.K. Debanath, S. Karmakar n Department of Instrumentation. absorption Blueshift Optical band gap abstract We report synthesis of zinc oxide (ZnO) nanoparticles via wet chemical method using molar solutions of zinc nitrate hexahydrate [Zn(NO 3 ) 2 Á 6H 2 O] and ammonium hydroxide. E g . The optical band gap (E g ) is found to be size dependent and there is an increase in the band gap of the semiconductor with a decrease in particle size. The optical band gap value obtained