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Well aligned ZnO nanorods growth on the gold coated glass substrate by aqueous chemical growth method using seed layer of Fe3O4 and Co3O4 nanoparticles
Journal of Crystal Growth 368 (2013) 39–46
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Well aligned ZnO nanorods growth on the gold coated glass substrate by aqueous chemical growth method using seed layer of Fe3O4 and Co3O4 nanoparticles Z.H. Ibupoto a,n, K. Khun a, Jun Lu b, Xianjie Liu b, M.S. AlSalhi c, M. Atif d, Anees A. Ansari e, M. Willander a,c a
Physical Electronic and Nanotechnology Division, Department of Science and Technology, Campus Norrk¨ oping, Link¨ oping University, SE-60174 Norrk¨ oping, Sweden Department of Physics, Chemistry, and Biology (IFM), Link¨ oping University, 58183 Link¨ oping, Sweden Physics and Astronomy Department, College of Science, King Saud University, Riyadh, Saudi Arabia d Research Chair for Laser Diagnosis of Cancer, King Saud University, Riyadh, Saudi Arabia e King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, P.O. Box 2455, Saudi Arabia b c
a r t i c l e i n f o
abstract
Article history: Received 14 August 2012 Received in revised form 28 December 2012 Accepted 5 January 2013 Communicated by T. Nishinaga Available online 29 January 2013
In this study, Fe3O4 and Co3O4 nanoparticles were prepared by co-precipitation method and sol–gel method respectively. The synthesised nanoparticles were characterised by X-ray diffraction [XRD] and Raman spectroscopy techniques. The obtained results have shown the nanocrystalline phase of obtained Fe3O4 and Co3O4 nanoparticles. Furthermore, the Fe3O4 and Co3O4 nanoparticles were used as seed layer for the fabrication of well-aligned ZnO nanorods on the gold coated glass substrate by aqueous chemical growth method. Scanning electron microscopy (SEM), high resolution transmission electron microscopy [HRTEM], as well as XRD and energy dispersive X-ray techniques were used for the structural characterisation of synthesised ZnO nanorods. This study has explored highly dense, uniform, well-aligned growth pattern along 0001 direction and good crystal quality of the prepared ZnO nanorods. ZnO nanorods are only composed of Zn and O atoms. Moreover, X-ray photoelectron spectroscopy was used for the chemical analysis of fabricated ZnO nanorods. In addition, the structural characterisation and the chemical composition study and the optical investigation were carried out for the fabricated ZnO nanorods and the photoluminescence [PL] spectrum have shown strong ultraviolet (UV) peak at 381 nm for Fe3O4 nanoparticles seeded ZnO nanorods and the PL spectrum for ZnO nanorods grown with the seed layer of Co3O4 nanoparticles has shown a UV peak at 382 nm. The green emission and orange/red peaks were also observed for ZnO nanorods grown with both the seed layers. This study has indicated the fabrication of well aligned ZnO nanorods using the one inorganic nanomaterial on other inorganic nanomaterial due to their similar chemistry. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterisation A1. Crystal structure A2. Hydrothermal crystal growth A2. Seed crystals B1. Nanomaterials B1. Oxides B2. Semiconducting II–VI materials
1. Introduction Nowadays, various ZnO nanostructures such as nanorods, nanotubes, nanosheets, nanobelts, nanoflowers and nanowalls are widely used in the fabrication of various devices including ultraviolet photo detectors, field effect transistors, light emitting diodes, chemical and biosensing applications [1–3], due to their pronounced characteristics. It is highly demanded that the used ZnO nanostructures should be improved with better orientation, alignment and the ordered array of crystal for the fabrication of new devices. ZnO nanorods can be grown using different growth methods including vapour–liquid–solid (VLS) growth [4,5], chemical vapour deposition
n
Corresponding author. Tel.: þ46 11363119. E-mail address: [email protected] (Z.H. Ibupoto).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.01.009
[6], however the hydrothermal or electrodepositing techniques is promising method for the fabrication of ZnO nanostructures due to the growth simplicity and the cheapness [7,8]. The seed layer has significant effect on the orientation and crystal quality of grown ZnO nanostructures either by the hydrothermal or electrodeposition growth methods [9]. The solution based fabrication of ZnO nanorods is affected by the precursor’s concentration, growth time, growth temperature, different types of zinc sources, and the pH of growth solution. The decrease in growth temperature from 90 to 60 1C, resulting decrease in mean diameter of ZnO nanorods [10]. The increase in concentration of zinc salt precursor yields longer ZnO nanorods with larger diameter [11]. The pH of solution medium can change the morphology of growing ZnO nanostructures and also can alter the growth speed [12]. However, the length and diameter of ZnO nanorods can also be controlled by the growth process time [13].
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The ZnO nanorods with proper orientation are more important because it can enhance the surface per unit area of ZnO nanorods which in results will enhance the performance of nanoscale based devices. The density and the length of ZnO nanorods are used for the determination of surface area of nanorods and this can be achieved by keeping in mind these growth parameters. Moreover, the seed grains have also significant effect on the crystal quality of ZnO nanorods and the size of seed grains on growing substrate decides the size of grown ZnO nanorods. It is also found that the presence of seed layer before the growth of ZnO nanorods improves the growth rate of nanorods [14,15]. Due to the significant role of seed layer in the growth of ZnO nanostructures different seed methods such as radio frequency sputtering, atomic layer deposition and consequently the annealing have been used to produce the desired seed layer with the enhanced surface coverage of the substrate and the controllable seed layer. These methods for the treatment of seed layer employ the complicated process and work under the high vacuum conditions. The multiwalled carbon nanotube-composed seed layers have also been used for the improvement of structural and optical properties of ZnO nanorods [16]. The magnetic nanoparticles are considered as potential candidates for bringing change in the magnetic properties of other magnetic material when magnetic nanoparticles coating is carried out; due to availability of two different magnetic phases on the substrate that can give birth to novel nanohybrid with versatile possible applications. It is reported that ZnO nanorods exhibit magnetic and ferromagnetic properties at room temperature even in the absence of magnetic vicinity [17–19]. Recently, the organic–inorganic composite has been used as seed layer for the growth of well aligned ZnO nanorods on the silicon substrate by the hydrothermal method [20]. However, still more work is required to grow seed layer based well aligned ZnO nanorods on the cheap substrates. In this study, two different Fe3O4 and Co3O4 nanoparticles were synthesised by the co-precipitation method and sol–gel method respectively. The freshly prepared magnetic nanoparticles were further used as a seed layer for the growth of wellaligned ZnO nanorods. The ZnO nanorods were grown by the low temperature aqueous chemical growth method and their structural and optical characterisations were studied.
2. Materials and experimental section
and amines. The prepared black precipitates were dried in air at 100 1C and afterwards annealed at 300 1C for 2 h and dried in vacuum oven at 40 1C. 2.2. The fabrication of ZnO nanorods on the gold coated glass substrate The fabrication of ZnO nanorods on the gold coated substrate was as followed. Firstly, the glass substrate was coated with the gold layer by the evaporator Satis (725). The glass substrates were placed inside evaporation chamber and a 20 nm thickness of titanium layer was deposited then 100 nm thickness of gold was evaporated. The titanium was used prior to the gold deposition as adhesive layer for the gold layer. After the gold deposition on the glass substrate, the gold coated glass substrates were washed with isopropanol and cleaned with deionized water and dried by air flow at room temperature. The preparation of homogeneous and completely dissolved solution of magnetic metal oxide nanoparticles as a seed layer is critical task and in this work we prepared seed solution of iron ferrite and cobalt oxide nanoparticles of 0.070 M into 0.1 M hydrochloric acid respectively. The homogeneous seed solution was obtained in the ultrasonic bath for the period of 1.5 h. Then cleaned substrates were used for the deposition of magnetic nanoparticles seed layer using spin coating technique. Both the magnetic nanoparticles were spin coated 3 times on the substrates at 3000 rpm. The magnetic nanoparticles seed coated substrates were annealed at 120 1C for 20 min in the preheated oven. Then annealed substrates were affixed in Teflon sample holder and placed into 250 ml beaker containing equimolar solution of zinc nitrate hexahydrate and hexamethylenetetramine (HMT). The sample solution was kept in oven at less than 100 1C for 5–7 h. After the completion of growth time, the ZnO nanostructures grown substrates were taken out from the oven, cleaned with the deionized water and dried with air flow. The freshly prepared Fe3O4 and Co3O4 nanoparticles were characterised by XRD and Raman spectroscopy techniques. The structural characterisation of grown ZnO nanostructures was carried out using field emission electron microscopy, energy dispersive X-ray, X-ray diffraction technique (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence (PL) technique was used for the optical characterisation of nanoparticles seed layer coated ZnO nanorods.
2.1. The synthesis of magnetic nanoparticles 3. Results and discussion Two different types of magnetic nanoparticles were as synthesised: Fe3O4 magnetic nanoparticles were prepared by using the mixture of equimolar concentration of ferrous and ferric chlorides with volume of 10 ml of ferrous chloride and 20 ml of ferric chloride at room temperature. The mixture was left on stirring and 5 ml of 5 N triethylamine were added by dropwise for 5 h till the formation of black precipitates of Fe3O4 magnetic nanoparticles. The synthesised magnetic nanoparticles were separated from mixture with the help of powerful magnet, and then cleaned with the distilled water and methanol. After this, the collected particles were dried. For the synthesis of Co3O4 nanoparticles, the cobalt chloride and the urea were used as starting precursor without any further purification. 1.0 g of cobalt chloride was dissolved in 20 ml of distilled water after that 2 g of urea were dissolved separately in the distilled water and mixed in the solution with constant stirring at 100 1C for 2 h. The resulting mixture was removed from the hot plate and the black colour precipitate were separated with the help of centrifuged and washed several times with the deionized water in order to remove the excess chloride
3.1. XRD and Raman study of Fe3O4 nanoparticles and Co3O4 nanoparticles The crystal phase investigation of the nanocrystalline Fe3O4 and Co3O4 was performed with an X-ray powder diffractometer equipped with Cu Ka radiation (Rigaku). Fig. 1(a) shows the X-ray diffraction patterns of Fe3O4 nanoparticles, which are correlated to reflection planes of the standard pattern for Fe3O4 (ASTM 19-629) material. The broad reflection planes might be associated to the nano-size of Fe3O4 particles. The average particle size of Fe3O4 magnetic particles is about 6.0 72.0 nm using Debye– Scherrer formula from (311) peak. Fig. 1(b) shows the X-ray powder diffraction pattern of the sol–gel prepared Co3O4 nanoparticles. The results of XRD pattern indicates that the Co3O4 nanoparticles are well-crystalline and the patterns are according to the reported work. The principle reflection peaks of Co3O4 in diffraction pattern are measured, which correspond to the (220) and (311) planes. These reflection peaks
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Fig. 1. (a–d) The XRD pattern for (a) Fe3O4 nanoparticle, (b) Co3O4 nanoparticle, (c) the Raman spectroscopy for Fe3O4 nanoparticle and (d) Co3O4 nanoparticle.
Fig. 2. (a–c) The XRD pattern of grown ZnO nanorods (on log scale) (a) without seed, (b) with Fe3O4 seed, and (c) with Co3O4 seed on gold coated glass substrate.
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can be indexed to the pure cubic fluorite structure of Co3O4. The intensities and positions of the diffraction plane which are perfectly similar to the JCPDS card number and no difference between them and the reported data was observed. Raman study was used to find out the different modes of nanoparticles at the room temperature and the below room temperature 23 1C with exciton at 488 nm. It can be seen from Fig. 1(c) and (d), that Fe3O4 nanoparticles exhibit five peaks which corresponds to A1g, Eg, Eg, A1g, and Eg at 200, 300, 390, 500, 590 cm 1 respectively [21,22] and Co3O4 three peaks appeared at 485, 523, and 690 cm 1 which correlates to Eg, F2g and A1g modes of crystal phase of Co3O4 nanoparticles [23].
3.2. X-ray diffraction study of ZnO nanorods The crystal quality and crystal pattern study for ZnO nanorods using magnetic nanoparticles as a seed layer were done by XRD technique. Fig. 2(a)–(c) shows the XRD analysis in log scale for ZnO nanorods prepared by the hydrothermal method. XRD pattern study is shown in Fig. 2(a) for the seedless ZnO nanorods on the gold coated glass substrate, Fig. 2(b) ZnO nanorods with Fe3O4 as seed layer on the gold coated glass substrate and in Fig. 2(c) ZnO nanorods with Co3O4 as seed layer on the gold coated glass substrate. In addition, it can be observed from Fig. 2(a)–(c), for the seedless grown ZnO has shown very weak peak at the (002) plane
Fig. 3. (a–e) The FESEM image of grown ZnO nanorods (a) without seed, (b) with Fe3O4 seed, and (c) with Co3O4 seed, (d) the cross section with Fe3O4 seed, and (e) the cross section with Co3O4 seed on gold coated glass substrate.
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due to the improper orientation on the gold coated glass substrate as shown in Fig. 2(a) [24]. However for Fig. 2(b), the peak intensity for (002) plane is more increased. This confirms that iron oxide magnetic nanoparticles provided the proper nucleation sites for the nanorods; this could also be attributed to the patterned growth of one magnetic material over other magnetic nanomaterial. Similar XRD pattern was observed for the cobalt oxide seed layer coated ZnO nanorods with more intense peak for (002) planes. On the right side of Fig. 2(a)–(c), the standard values for (hkl) peaks and intensities are shown for the comparison with the obtained results of XRD. The experimental measured diffraction peak seems to be better in terms of intensity compared to the ZnO nanorods growth with KOH seed layer, and monoethanolamine seed layer [25]. All the obtained results have indicated that the seed coated layer of another magnetic nanomaterial can improve the crystal quality of ZnO nanorods and the orientation on the substrate due to the presence of already highly magnetic nanomaterial on the surface [26,27]. All the obtained diffraction peaks are reproducible with the reference JCPDS 80-0075, which can be assigned to the hexagonal phase of ZnO.
3.3. SEM and TEM characterisation of ZnO nanorods The morphology of fabricated ZnO nanorods using different magnetic nanoparticles as seed layer on the gold coated glass substrate was characterised by FESEM as shown in Fig. 3(a)–(c). Fig. 3(a) shows the seedless grown ZnO nanorods on the gold coated glass substrate, Fig. 3(b) ZnO nanorods with iron ferrite Fe3O4 as seed layer on the gold coated glass substrate, and Fig. 3(c) ZnO nanorods with cobalt oxide Co3O4 as seed layer on the gold coated glass substrate. During this analysis, it has been observed that the ZnO nanorods without seed layer were not so dense and also the orientation of nanorods remained as subject of matter on the seedless gold coated glass substrate [24] as clearly shown in Fig. 3(a). However, ZnO nanorods grown with magnetic nanoparticles as a seed layer were well aligned and dense as shown in Fig. 3(b) and (c). In addition to top view SEM images, cross sectional SEM images were taken for both the Fe3O4 and Co3O4 seed layers
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coated ZnO nanorods as shown in Fig. 3(d) and (e) respectively. It can be observed from Fig. 3(d) and (e) that the nanorods are well aligned and perpendicular to the substrate. The ZnO growth pattern was uniform with preferred c-axis orientation because the seed grains of magnetic nanoparticles provide better nucleation sites for the growth of ZnO nanorods [26,27]. These observations have shown that the metal oxide magnetic nanoparticles can be used as seed stuff for the growth of nanomaterials which exhibit magnetic and ferromagnetic properties. Due to magnetic characteristics of seed layer, it helps other growing material with better alignment and orientation as shown by two popular ferromagnetic materials in the present study including iron ferrite and cobalt oxide Fig. 3(b) and (c). Moreover, the presence of magnetic nanoparticles on the surface of gold coated glass substrate might act as catalyst for the growth of ZnO nanorods by providing rough surface to growing ZnO nanostructures. The energy dispersive X-ray study has shown that ZnO nanorods are composed of [Zn] and [O] atoms and no any other impurity was found except [Fe] atoms are observed due to the involvement from the surface of substrate as shown in Fig. 4(a). Similar results were obtained for Co3O4 seed layer coated ZnO nanorods as shown in Fig. 4(b). However, the gold peak was also observed in both cases due to the gold coating on the glass substrate. TEM study was carried out by using FEI Tecnai G2 TF20 UT with a field emission gun operated at 200 kV with a point resolution of ˚ equipped with energy dispersive spectrum (EDS). As can be 1.9 A, seen in Fig. 5(a) and (b), HRTEM and selective area electron diffraction SAED study for iron oxide seeded ZnO nanorod which demonstrates the nanorod as single crystal with hexagonal structure and preferred growth pattern was along [0001] direction. Similar types of results were obtained for cobalt oxide nanoparticles seed layer coated ZnO nanorods as shown in Fig. 5c.
3.4. X-ray photoelectron spectroscopy (XPS) analysis of Fe3O4 and Co3O4 nanoparticles seed layer coated ZnO nanorods The XPS technique was used for the determination of surface chemical properties of ZnO nanorods using seed layer of Fe3O4
Fig. 4. (a, b) The EDX pattern of ZnO nanorods grown with (a) Fe3O4 seed and (b) Co3O4 seed.
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high binding energy located at 531.11 and 532.50 eV respectively. The low binding energy peak can be attributed to oxide O2 ions enveloped by Zn in the crystal phase of ZnO substance which shows the concentration of oxygen atoms in a complete oxidised chemical proportion [28]. The high binding energy component of O 1s peak can be assigned to oxygen deficiency inside the bulk of ZnO material [29]. However, peak at 532.50 eV might also be associated to chemically adsorbed oxygen, free oxygen atom or OH 1 radicals on the surface of nanorods [30,31]. Fig. 6(b) shows the Zn 2p peaks at around 1022.3 and 1045.35 eV which are close to known values of ZnO [32,33]. Fig. 6(c) shows the XPS spectrum around Zn 3p, two sets of peaks can be clearly observed, two peaks at lower binding energy ( 87.8 and 84 eV) can be assigned to Au 4f from Au substrate, other two peaks at higher binding energy (92.1 and 89 eV) are due to Zn 3p. The appearance of Au 4f means the ZnO film does not fully cover the gold substrate since the thickness of ZnO film is far beyond XPS technique reach. Besides, this for Fe 2p the weak peaks were observed at 704.0 and 718.0 eV binding energies respectively which are close to known values for Fe 2p [34] and it appeared due to very thin seed layer of Fe3O4 nanoparticles prior to growth of ZnO nanorods as shown in Fig. 6d. The appearance of Fe 2p is consistent with that ZnO film is not fully covered on gold substrate. Similar results were obtained for ZnO nanorods grown on the seed layer of Co3O4 nanoparticles as shown in Fig. 7(a)–(d). Fig. 7(a) shows the observed peaks for O 1s at 531.11, and 532.60 eV respectively. For Zn 2p the XPS spectra peaks are measured at 1022.33 and 1045.4 eV respectively as shown in Fig. 7(b). The appearance of Zn 2p at the same position of ZnO nanorods using seed layer of Fe3O4 and Co3O4 nanoparticles is very similar to the known values of ZnO [32], which means our ZnO nanorods have the same characteristics of bulk ZnO. Similar to spectral feature of Zn 3p was also observed, as shown in Fig. 7(c). Compared to spectrum in Fig. 6c, the signal from gold substrate is strongly suppressed; it indicates a higher coverage of ZnO film on gold substrate. Fig. 7(d) shows that no Co 2p peak is observed because of the possible very thin layer of these particles and better coverage of ZnO film. XPS study has shown that due to very thin layer of Fe3O4 nanoparticles Fe signal is observed; however in case of Co3O4 nanoparticles no signal of Co is detected and Co only exist interface between Au and ZnO. It can be concluded that Fe3O4 and Co3O4 nanoparticles only involved as seed layer rather than inside the ZnO. 3.5. Photoluminescence study of Fe3O4 and Co3O4 nanoparticles seed layer coated ZnO nanorods
Fig. 5. (a–c) The TEM image for synthesised ZnO nanorods (a) with Fe3O4 seed, (b) SAED for Fe3O4 and (c) Co3O4 seed.
and Co3O4 nanoparticles. The XPS spectra of O 1s and Zn 2p for ZnO nanorods are shown in Fig. 6. Fig. 6(a) shows the O 1s peaks with two components, one at low binding energy and other at
The photoluminescence study was carried out in order to locate the defect states in the single crystalline ZnO nanorods. Fig. 8(a) shows that a strong narrow UV peak was found at 381 nm, apart from that; green and orange/red emission peaks are also appeared which reflected the some defect states present in the ZnO nanorods grown with the seed layer of Fe3O4 nanoparticles. The strong narrow UV peak could be assigned to the recombination of free exciton by exciton to exciton collisions [19] because of 380 nm, which reflects the wide bandgap transition of ZnO nanorods. This indicated a good crystal quality of synthesised ZnO nanorods by the hydrothermal method using Fe3O4 nanoparticles as seed layer. However, green and orange/red peaks were also observed, which showed some defect states in the prepared ZnO nanorods. As the hydrothermal growth method is highly populated with Zn atoms relative to that of oxygen, therefore the enhanced green emissions peak which appeared can be attributed to the oxygen vacancies [35–37]. In addition to green emission peak, orange/red was also observed which might be correlated to the oxygen interstitial in prepared ZnO nanorods [20]. Fig. 8(b) shows the photoluminescence investigation for the Co3O4 nanoparticles seed layer coated ZnO nanorods. These synthesised ZnO
Relative intensty
Relative intensty
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O1s
540
535
530
Zn 2p
1060
525
1050
Zn 3p
95
90
1040
1030
1020
1010
Binding energy (eV)
Relative intensty
Relative intensty
Binding energy (eV)
100
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85
80
Fe 2p
735 730 725 720 715 710 705 700 695
Binding energy (eV)
Binding energy (eV)
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Relative intensty
Fig. 6. (a–d) The XPS analysis of ZnO nanorods growth with Fe3O4 seed.
O1s
540
535
1060
530
Zn 2p
1050
Zn 3p
100
95
90
85
1040
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Relative intensty
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Binding energy (eV)
Co 2p
800
790
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770
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Fig. 7. (a–d) The XPS analysis of ZnO nanorods growth with Co3O4 seed.
nanorods also revealed the similar photoluminescence response with respect to strong UV peak at 382 nm along with the green and orange/read emission peaks as shown in Fig. 8(b). The obtained results also indicate the good crystallinity of ZnO nanorods and the presence of some defects due to oxygen vacancies for the green emission peak and the oxygen interstitial for orange/red emission peak.
4. Conclusion In this present research work, ZnO nanorods were fabricated on the gold coated glass substrate by the hydrothermal growth
method using seed layer of freshly prepared Fe3O4 and Co3O4 nanoparticles. Fe3O4 and Co3O4 nanoparticles were characterised by XRD and Raman spectroscopy techniques and the prepared nanoparticles exhibit nano crystal phase structure. The FESEM, HRTEM and XRD techniques were used for the structural analysis of the prepared ZnO nanorods. This analysis indicated that the ZnO nanorods are highly dense, well aligned, uniform; the growth pattern was along 0001 plane and possessed good crystal quality. XPS study revealed that the ZnO nanorods contain no impurity except the involvement of Fe due to thin layer of Fe3O4 nanoparticles. Photoluminescence technique was used for the optical characterisation and we observed the strong narrow UV peak and ZnO crystals exhibit good crystal quality. This approach towards
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Fig. 8. (a, b) The PL pattern of ZnO nanorods (a) grown with Fe3O4 seed and (b) grown with Co3O4 seed.
the fabrication of ZnO nanorods may be useful for the development of nanoscale based optoelectronics devices.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Well aligned ZnO nanorods growth on the gold coated glass substrate by aqueous chemical growth method using seed layer of Fe3O4 and Co3O4 nanoparticles Z.H. Ibupoto a,n, K. Khun a, Jun Lu b, Xianjie Liu b, M.S. AlSalhi c, M. Atif d, Anees A. Ansari e, M. Willander a,c a
Physical Electronic and Nanotechnology Division, Department of Science and Technology, Campus Norrk¨ oping, Link¨ oping University, SE-60174 Norrk¨ oping, Sweden Department of Physics, Chemistry, and Biology (IFM), Link¨ oping University, 58183 Link¨ oping, Sweden Physics and Astronomy Department, College of Science, King Saud University, Riyadh, Saudi Arabia d Research Chair for Laser Diagnosis of Cancer, King Saud University, Riyadh, Saudi Arabia e King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, P.O. Box 2455, Saudi Arabia b c
a r t i c l e i n f o
abstract
Article history: Received 14 August 2012 Received in revised form 28 December 2012 Accepted 5 January 2013 Communicated by T. Nishinaga Available online 29 January 2013
In this study, Fe3O4 and Co3O4 nanoparticles were prepared by co-precipitation method and sol–gel method respectively. The synthesised nanoparticles were characterised by X-ray diffraction [XRD] and Raman spectroscopy techniques. The obtained results have shown the nanocrystalline phase of obtained Fe3O4 and Co3O4 nanoparticles. Furthermore, the Fe3O4 and Co3O4 nanoparticles were used as seed layer for the fabrication of well-aligned ZnO nanorods on the gold coated glass substrate by aqueous chemical growth method. Scanning electron microscopy (SEM), high resolution transmission electron microscopy [HRTEM], as well as XRD and energy dispersive X-ray techniques were used for the structural characterisation of synthesised ZnO nanorods. This study has explored highly dense, uniform, well-aligned growth pattern along 0001 direction and good crystal quality of the prepared ZnO nanorods. ZnO nanorods are only composed of Zn and O atoms. Moreover, X-ray photoelectron spectroscopy was used for the chemical analysis of fabricated ZnO nanorods. In addition, the structural characterisation and the chemical composition study and the optical investigation were carried out for the fabricated ZnO nanorods and the photoluminescence [PL] spectrum have shown strong ultraviolet (UV) peak at 381 nm for Fe3O4 nanoparticles seeded ZnO nanorods and the PL spectrum for ZnO nanorods grown with the seed layer of Co3O4 nanoparticles has shown a UV peak at 382 nm. The green emission and orange/red peaks were also observed for ZnO nanorods grown with both the seed layers. This study has indicated the fabrication of well aligned ZnO nanorods using the one inorganic nanomaterial on other inorganic nanomaterial due to their similar chemistry. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterisation A1. Crystal structure A2. Hydrothermal crystal growth A2. Seed crystals B1. Nanomaterials B1. Oxides B2. Semiconducting II–VI materials
1. Introduction Nowadays, various ZnO nanostructures such as nanorods, nanotubes, nanosheets, nanobelts, nanoflowers and nanowalls are widely used in the fabrication of various devices including ultraviolet photo detectors, field effect transistors, light emitting diodes, chemical and biosensing applications [1–3], due to their pronounced characteristics. It is highly demanded that the used ZnO nanostructures should be improved with better orientation, alignment and the ordered array of crystal for the fabrication of new devices. ZnO nanorods can be grown using different growth methods including vapour–liquid–solid (VLS) growth [4,5], chemical vapour deposition
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Corresponding author. Tel.: þ46 11363119. E-mail address: [email protected] (Z.H. Ibupoto).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.01.009
[6], however the hydrothermal or electrodepositing techniques is promising method for the fabrication of ZnO nanostructures due to the growth simplicity and the cheapness [7,8]. The seed layer has significant effect on the orientation and crystal quality of grown ZnO nanostructures either by the hydrothermal or electrodeposition growth methods [9]. The solution based fabrication of ZnO nanorods is affected by the precursor’s concentration, growth time, growth temperature, different types of zinc sources, and the pH of growth solution. The decrease in growth temperature from 90 to 60 1C, resulting decrease in mean diameter of ZnO nanorods [10]. The increase in concentration of zinc salt precursor yields longer ZnO nanorods with larger diameter [11]. The pH of solution medium can change the morphology of growing ZnO nanostructures and also can alter the growth speed [12]. However, the length and diameter of ZnO nanorods can also be controlled by the growth process time [13].
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The ZnO nanorods with proper orientation are more important because it can enhance the surface per unit area of ZnO nanorods which in results will enhance the performance of nanoscale based devices. The density and the length of ZnO nanorods are used for the determination of surface area of nanorods and this can be achieved by keeping in mind these growth parameters. Moreover, the seed grains have also significant effect on the crystal quality of ZnO nanorods and the size of seed grains on growing substrate decides the size of grown ZnO nanorods. It is also found that the presence of seed layer before the growth of ZnO nanorods improves the growth rate of nanorods [14,15]. Due to the significant role of seed layer in the growth of ZnO nanostructures different seed methods such as radio frequency sputtering, atomic layer deposition and consequently the annealing have been used to produce the desired seed layer with the enhanced surface coverage of the substrate and the controllable seed layer. These methods for the treatment of seed layer employ the complicated process and work under the high vacuum conditions. The multiwalled carbon nanotube-composed seed layers have also been used for the improvement of structural and optical properties of ZnO nanorods [16]. The magnetic nanoparticles are considered as potential candidates for bringing change in the magnetic properties of other magnetic material when magnetic nanoparticles coating is carried out; due to availability of two different magnetic phases on the substrate that can give birth to novel nanohybrid with versatile possible applications. It is reported that ZnO nanorods exhibit magnetic and ferromagnetic properties at room temperature even in the absence of magnetic vicinity [17–19]. Recently, the organic–inorganic composite has been used as seed layer for the growth of well aligned ZnO nanorods on the silicon substrate by the hydrothermal method [20]. However, still more work is required to grow seed layer based well aligned ZnO nanorods on the cheap substrates. In this study, two different Fe3O4 and Co3O4 nanoparticles were synthesised by the co-precipitation method and sol–gel method respectively. The freshly prepared magnetic nanoparticles were further used as a seed layer for the growth of wellaligned ZnO nanorods. The ZnO nanorods were grown by the low temperature aqueous chemical growth method and their structural and optical characterisations were studied.
2. Materials and experimental section
and amines. The prepared black precipitates were dried in air at 100 1C and afterwards annealed at 300 1C for 2 h and dried in vacuum oven at 40 1C. 2.2. The fabrication of ZnO nanorods on the gold coated glass substrate The fabrication of ZnO nanorods on the gold coated substrate was as followed. Firstly, the glass substrate was coated with the gold layer by the evaporator Satis (725). The glass substrates were placed inside evaporation chamber and a 20 nm thickness of titanium layer was deposited then 100 nm thickness of gold was evaporated. The titanium was used prior to the gold deposition as adhesive layer for the gold layer. After the gold deposition on the glass substrate, the gold coated glass substrates were washed with isopropanol and cleaned with deionized water and dried by air flow at room temperature. The preparation of homogeneous and completely dissolved solution of magnetic metal oxide nanoparticles as a seed layer is critical task and in this work we prepared seed solution of iron ferrite and cobalt oxide nanoparticles of 0.070 M into 0.1 M hydrochloric acid respectively. The homogeneous seed solution was obtained in the ultrasonic bath for the period of 1.5 h. Then cleaned substrates were used for the deposition of magnetic nanoparticles seed layer using spin coating technique. Both the magnetic nanoparticles were spin coated 3 times on the substrates at 3000 rpm. The magnetic nanoparticles seed coated substrates were annealed at 120 1C for 20 min in the preheated oven. Then annealed substrates were affixed in Teflon sample holder and placed into 250 ml beaker containing equimolar solution of zinc nitrate hexahydrate and hexamethylenetetramine (HMT). The sample solution was kept in oven at less than 100 1C for 5–7 h. After the completion of growth time, the ZnO nanostructures grown substrates were taken out from the oven, cleaned with the deionized water and dried with air flow. The freshly prepared Fe3O4 and Co3O4 nanoparticles were characterised by XRD and Raman spectroscopy techniques. The structural characterisation of grown ZnO nanostructures was carried out using field emission electron microscopy, energy dispersive X-ray, X-ray diffraction technique (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence (PL) technique was used for the optical characterisation of nanoparticles seed layer coated ZnO nanorods.
2.1. The synthesis of magnetic nanoparticles 3. Results and discussion Two different types of magnetic nanoparticles were as synthesised: Fe3O4 magnetic nanoparticles were prepared by using the mixture of equimolar concentration of ferrous and ferric chlorides with volume of 10 ml of ferrous chloride and 20 ml of ferric chloride at room temperature. The mixture was left on stirring and 5 ml of 5 N triethylamine were added by dropwise for 5 h till the formation of black precipitates of Fe3O4 magnetic nanoparticles. The synthesised magnetic nanoparticles were separated from mixture with the help of powerful magnet, and then cleaned with the distilled water and methanol. After this, the collected particles were dried. For the synthesis of Co3O4 nanoparticles, the cobalt chloride and the urea were used as starting precursor without any further purification. 1.0 g of cobalt chloride was dissolved in 20 ml of distilled water after that 2 g of urea were dissolved separately in the distilled water and mixed in the solution with constant stirring at 100 1C for 2 h. The resulting mixture was removed from the hot plate and the black colour precipitate were separated with the help of centrifuged and washed several times with the deionized water in order to remove the excess chloride
3.1. XRD and Raman study of Fe3O4 nanoparticles and Co3O4 nanoparticles The crystal phase investigation of the nanocrystalline Fe3O4 and Co3O4 was performed with an X-ray powder diffractometer equipped with Cu Ka radiation (Rigaku). Fig. 1(a) shows the X-ray diffraction patterns of Fe3O4 nanoparticles, which are correlated to reflection planes of the standard pattern for Fe3O4 (ASTM 19-629) material. The broad reflection planes might be associated to the nano-size of Fe3O4 particles. The average particle size of Fe3O4 magnetic particles is about 6.0 72.0 nm using Debye– Scherrer formula from (311) peak. Fig. 1(b) shows the X-ray powder diffraction pattern of the sol–gel prepared Co3O4 nanoparticles. The results of XRD pattern indicates that the Co3O4 nanoparticles are well-crystalline and the patterns are according to the reported work. The principle reflection peaks of Co3O4 in diffraction pattern are measured, which correspond to the (220) and (311) planes. These reflection peaks
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Fig. 1. (a–d) The XRD pattern for (a) Fe3O4 nanoparticle, (b) Co3O4 nanoparticle, (c) the Raman spectroscopy for Fe3O4 nanoparticle and (d) Co3O4 nanoparticle.
Fig. 2. (a–c) The XRD pattern of grown ZnO nanorods (on log scale) (a) without seed, (b) with Fe3O4 seed, and (c) with Co3O4 seed on gold coated glass substrate.
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can be indexed to the pure cubic fluorite structure of Co3O4. The intensities and positions of the diffraction plane which are perfectly similar to the JCPDS card number and no difference between them and the reported data was observed. Raman study was used to find out the different modes of nanoparticles at the room temperature and the below room temperature 23 1C with exciton at 488 nm. It can be seen from Fig. 1(c) and (d), that Fe3O4 nanoparticles exhibit five peaks which corresponds to A1g, Eg, Eg, A1g, and Eg at 200, 300, 390, 500, 590 cm 1 respectively [21,22] and Co3O4 three peaks appeared at 485, 523, and 690 cm 1 which correlates to Eg, F2g and A1g modes of crystal phase of Co3O4 nanoparticles [23].
3.2. X-ray diffraction study of ZnO nanorods The crystal quality and crystal pattern study for ZnO nanorods using magnetic nanoparticles as a seed layer were done by XRD technique. Fig. 2(a)–(c) shows the XRD analysis in log scale for ZnO nanorods prepared by the hydrothermal method. XRD pattern study is shown in Fig. 2(a) for the seedless ZnO nanorods on the gold coated glass substrate, Fig. 2(b) ZnO nanorods with Fe3O4 as seed layer on the gold coated glass substrate and in Fig. 2(c) ZnO nanorods with Co3O4 as seed layer on the gold coated glass substrate. In addition, it can be observed from Fig. 2(a)–(c), for the seedless grown ZnO has shown very weak peak at the (002) plane
Fig. 3. (a–e) The FESEM image of grown ZnO nanorods (a) without seed, (b) with Fe3O4 seed, and (c) with Co3O4 seed, (d) the cross section with Fe3O4 seed, and (e) the cross section with Co3O4 seed on gold coated glass substrate.
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due to the improper orientation on the gold coated glass substrate as shown in Fig. 2(a) [24]. However for Fig. 2(b), the peak intensity for (002) plane is more increased. This confirms that iron oxide magnetic nanoparticles provided the proper nucleation sites for the nanorods; this could also be attributed to the patterned growth of one magnetic material over other magnetic nanomaterial. Similar XRD pattern was observed for the cobalt oxide seed layer coated ZnO nanorods with more intense peak for (002) planes. On the right side of Fig. 2(a)–(c), the standard values for (hkl) peaks and intensities are shown for the comparison with the obtained results of XRD. The experimental measured diffraction peak seems to be better in terms of intensity compared to the ZnO nanorods growth with KOH seed layer, and monoethanolamine seed layer [25]. All the obtained results have indicated that the seed coated layer of another magnetic nanomaterial can improve the crystal quality of ZnO nanorods and the orientation on the substrate due to the presence of already highly magnetic nanomaterial on the surface [26,27]. All the obtained diffraction peaks are reproducible with the reference JCPDS 80-0075, which can be assigned to the hexagonal phase of ZnO.
3.3. SEM and TEM characterisation of ZnO nanorods The morphology of fabricated ZnO nanorods using different magnetic nanoparticles as seed layer on the gold coated glass substrate was characterised by FESEM as shown in Fig. 3(a)–(c). Fig. 3(a) shows the seedless grown ZnO nanorods on the gold coated glass substrate, Fig. 3(b) ZnO nanorods with iron ferrite Fe3O4 as seed layer on the gold coated glass substrate, and Fig. 3(c) ZnO nanorods with cobalt oxide Co3O4 as seed layer on the gold coated glass substrate. During this analysis, it has been observed that the ZnO nanorods without seed layer were not so dense and also the orientation of nanorods remained as subject of matter on the seedless gold coated glass substrate [24] as clearly shown in Fig. 3(a). However, ZnO nanorods grown with magnetic nanoparticles as a seed layer were well aligned and dense as shown in Fig. 3(b) and (c). In addition to top view SEM images, cross sectional SEM images were taken for both the Fe3O4 and Co3O4 seed layers
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coated ZnO nanorods as shown in Fig. 3(d) and (e) respectively. It can be observed from Fig. 3(d) and (e) that the nanorods are well aligned and perpendicular to the substrate. The ZnO growth pattern was uniform with preferred c-axis orientation because the seed grains of magnetic nanoparticles provide better nucleation sites for the growth of ZnO nanorods [26,27]. These observations have shown that the metal oxide magnetic nanoparticles can be used as seed stuff for the growth of nanomaterials which exhibit magnetic and ferromagnetic properties. Due to magnetic characteristics of seed layer, it helps other growing material with better alignment and orientation as shown by two popular ferromagnetic materials in the present study including iron ferrite and cobalt oxide Fig. 3(b) and (c). Moreover, the presence of magnetic nanoparticles on the surface of gold coated glass substrate might act as catalyst for the growth of ZnO nanorods by providing rough surface to growing ZnO nanostructures. The energy dispersive X-ray study has shown that ZnO nanorods are composed of [Zn] and [O] atoms and no any other impurity was found except [Fe] atoms are observed due to the involvement from the surface of substrate as shown in Fig. 4(a). Similar results were obtained for Co3O4 seed layer coated ZnO nanorods as shown in Fig. 4(b). However, the gold peak was also observed in both cases due to the gold coating on the glass substrate. TEM study was carried out by using FEI Tecnai G2 TF20 UT with a field emission gun operated at 200 kV with a point resolution of ˚ equipped with energy dispersive spectrum (EDS). As can be 1.9 A, seen in Fig. 5(a) and (b), HRTEM and selective area electron diffraction SAED study for iron oxide seeded ZnO nanorod which demonstrates the nanorod as single crystal with hexagonal structure and preferred growth pattern was along [0001] direction. Similar types of results were obtained for cobalt oxide nanoparticles seed layer coated ZnO nanorods as shown in Fig. 5c.
3.4. X-ray photoelectron spectroscopy (XPS) analysis of Fe3O4 and Co3O4 nanoparticles seed layer coated ZnO nanorods The XPS technique was used for the determination of surface chemical properties of ZnO nanorods using seed layer of Fe3O4
Fig. 4. (a, b) The EDX pattern of ZnO nanorods grown with (a) Fe3O4 seed and (b) Co3O4 seed.
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high binding energy located at 531.11 and 532.50 eV respectively. The low binding energy peak can be attributed to oxide O2 ions enveloped by Zn in the crystal phase of ZnO substance which shows the concentration of oxygen atoms in a complete oxidised chemical proportion [28]. The high binding energy component of O 1s peak can be assigned to oxygen deficiency inside the bulk of ZnO material [29]. However, peak at 532.50 eV might also be associated to chemically adsorbed oxygen, free oxygen atom or OH 1 radicals on the surface of nanorods [30,31]. Fig. 6(b) shows the Zn 2p peaks at around 1022.3 and 1045.35 eV which are close to known values of ZnO [32,33]. Fig. 6(c) shows the XPS spectrum around Zn 3p, two sets of peaks can be clearly observed, two peaks at lower binding energy ( 87.8 and 84 eV) can be assigned to Au 4f from Au substrate, other two peaks at higher binding energy (92.1 and 89 eV) are due to Zn 3p. The appearance of Au 4f means the ZnO film does not fully cover the gold substrate since the thickness of ZnO film is far beyond XPS technique reach. Besides, this for Fe 2p the weak peaks were observed at 704.0 and 718.0 eV binding energies respectively which are close to known values for Fe 2p [34] and it appeared due to very thin seed layer of Fe3O4 nanoparticles prior to growth of ZnO nanorods as shown in Fig. 6d. The appearance of Fe 2p is consistent with that ZnO film is not fully covered on gold substrate. Similar results were obtained for ZnO nanorods grown on the seed layer of Co3O4 nanoparticles as shown in Fig. 7(a)–(d). Fig. 7(a) shows the observed peaks for O 1s at 531.11, and 532.60 eV respectively. For Zn 2p the XPS spectra peaks are measured at 1022.33 and 1045.4 eV respectively as shown in Fig. 7(b). The appearance of Zn 2p at the same position of ZnO nanorods using seed layer of Fe3O4 and Co3O4 nanoparticles is very similar to the known values of ZnO [32], which means our ZnO nanorods have the same characteristics of bulk ZnO. Similar to spectral feature of Zn 3p was also observed, as shown in Fig. 7(c). Compared to spectrum in Fig. 6c, the signal from gold substrate is strongly suppressed; it indicates a higher coverage of ZnO film on gold substrate. Fig. 7(d) shows that no Co 2p peak is observed because of the possible very thin layer of these particles and better coverage of ZnO film. XPS study has shown that due to very thin layer of Fe3O4 nanoparticles Fe signal is observed; however in case of Co3O4 nanoparticles no signal of Co is detected and Co only exist interface between Au and ZnO. It can be concluded that Fe3O4 and Co3O4 nanoparticles only involved as seed layer rather than inside the ZnO. 3.5. Photoluminescence study of Fe3O4 and Co3O4 nanoparticles seed layer coated ZnO nanorods
Fig. 5. (a–c) The TEM image for synthesised ZnO nanorods (a) with Fe3O4 seed, (b) SAED for Fe3O4 and (c) Co3O4 seed.
and Co3O4 nanoparticles. The XPS spectra of O 1s and Zn 2p for ZnO nanorods are shown in Fig. 6. Fig. 6(a) shows the O 1s peaks with two components, one at low binding energy and other at
The photoluminescence study was carried out in order to locate the defect states in the single crystalline ZnO nanorods. Fig. 8(a) shows that a strong narrow UV peak was found at 381 nm, apart from that; green and orange/red emission peaks are also appeared which reflected the some defect states present in the ZnO nanorods grown with the seed layer of Fe3O4 nanoparticles. The strong narrow UV peak could be assigned to the recombination of free exciton by exciton to exciton collisions [19] because of 380 nm, which reflects the wide bandgap transition of ZnO nanorods. This indicated a good crystal quality of synthesised ZnO nanorods by the hydrothermal method using Fe3O4 nanoparticles as seed layer. However, green and orange/red peaks were also observed, which showed some defect states in the prepared ZnO nanorods. As the hydrothermal growth method is highly populated with Zn atoms relative to that of oxygen, therefore the enhanced green emissions peak which appeared can be attributed to the oxygen vacancies [35–37]. In addition to green emission peak, orange/red was also observed which might be correlated to the oxygen interstitial in prepared ZnO nanorods [20]. Fig. 8(b) shows the photoluminescence investigation for the Co3O4 nanoparticles seed layer coated ZnO nanorods. These synthesised ZnO
Relative intensty
Relative intensty
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O1s
540
535
530
Zn 2p
1060
525
1050
Zn 3p
95
90
1040
1030
1020
1010
Binding energy (eV)
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Relative intensty
Binding energy (eV)
100
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85
80
Fe 2p
735 730 725 720 715 710 705 700 695
Binding energy (eV)
Binding energy (eV)
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Relative intensty
Fig. 6. (a–d) The XPS analysis of ZnO nanorods growth with Fe3O4 seed.
O1s
540
535
1060
530
Zn 2p
1050
Zn 3p
100
95
90
85
1040
1030
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Relative intensty
Binding energy (eV)
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Binding energy (eV)
Co 2p
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770
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Fig. 7. (a–d) The XPS analysis of ZnO nanorods growth with Co3O4 seed.
nanorods also revealed the similar photoluminescence response with respect to strong UV peak at 382 nm along with the green and orange/read emission peaks as shown in Fig. 8(b). The obtained results also indicate the good crystallinity of ZnO nanorods and the presence of some defects due to oxygen vacancies for the green emission peak and the oxygen interstitial for orange/red emission peak.
4. Conclusion In this present research work, ZnO nanorods were fabricated on the gold coated glass substrate by the hydrothermal growth
method using seed layer of freshly prepared Fe3O4 and Co3O4 nanoparticles. Fe3O4 and Co3O4 nanoparticles were characterised by XRD and Raman spectroscopy techniques and the prepared nanoparticles exhibit nano crystal phase structure. The FESEM, HRTEM and XRD techniques were used for the structural analysis of the prepared ZnO nanorods. This analysis indicated that the ZnO nanorods are highly dense, well aligned, uniform; the growth pattern was along 0001 plane and possessed good crystal quality. XPS study revealed that the ZnO nanorods contain no impurity except the involvement of Fe due to thin layer of Fe3O4 nanoparticles. Photoluminescence technique was used for the optical characterisation and we observed the strong narrow UV peak and ZnO crystals exhibit good crystal quality. This approach towards
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Fig. 8. (a, b) The PL pattern of ZnO nanorods (a) grown with Fe3O4 seed and (b) grown with Co3O4 seed.
the fabrication of ZnO nanorods may be useful for the development of nanoscale based optoelectronics devices.
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