- Nano Express
- Open Access
Hierarchical Si/ZnO trunk-branch nanostructure for photocurrent enhancement
© Dee et al.; licensee Springer. 2014
- Received: 21 July 2014
- Accepted: 26 August 2014
- Published: 4 September 2014
Hierarchical Si/ZnO trunk-branch nanostructures (NSs) have been synthesized by hot wire assisted chemical vapor deposition method for trunk Si nanowires (NWs) on indium tin oxide (ITO) substrate and followed by the vapor transport condensation (VTC) method for zinc oxide (ZnO) nanorods (NRs) which was laterally grown from each Si nanowires (NWs). A spin coating method has been used for zinc oxide (ZnO) seeding. This method is better compared with other group where they used sputtering method for the same process. The sputtering method only results in the growth of ZnO NRs on top of the Si trunk. Our method shows improvement by having the growth evenly distributed on the lateral sides and caps of the Si trunks, resulting in pine-leave-like NSs. Field emission scanning electron microscope image shows the hierarchical nanostructures resembling the shape of the leaves of pine trees. Single crystalline structure for the ZnO branch grown laterally from the crystalline Si trunk has been identified by using a lattice-resolved transmission electron microscope. A preliminary photoelectrochemical (PEC) cell testing has been setup to characterize the photocurrent of sole array of ZnO NR growth by both hydrothermal-grown (HTG) method and VTC method on ITO substrates. VTC-grown ZnO NRs showed greater photocurrent effect due to its better structural properties. The measured photocurrent was also compared with the array of hierarchical Si/ZnO trunk-branch NSs. The cell with the array of Si/ZnO trunk-branch NSs revealed four-fold magnitude enhancement in photocurrent density compared with the sole array of ZnO NRs obtain from VTC processes.
- Zinc oxide
Homo- and hetero-hierarchical nanostructures (NSs) consist of two or more materials in the family of nanostructures have become one of the most intensively studied topics in the field of nanotechnology. Nanoparticles (NPs), nanowires (NWs) (including nanorods and nanowhiskers), nanolayers (NLs) (including nanoflakes and nanowalls), and other types of fundamental building blocks consist of a single material-NSs have been uncovered, synthesized, and studied for more than few decades ago. The next level of study based on hierarchical NSs is the combination/integration of more than one type of fundamental building blocks as mentioned above which may consist of more than one material. Many researchers' works for applications of hierarchical NSs actually show better performance compared with the primary building block NSs [1–3]. Those applications include hybrid nanoelectronic, nano-optoelectronic, nanomechanical, and electrochemical devices.
Recently, the characterization and implementation of hierarchical NSs in photoelectrochemical (PEC) cell has been widely explored [4, 5]. Hierarchical core-shell or trunk-branch NSs are expected to give better performance to the photocurrent. Those are commonly addressed as photoconductors. A photoconductor is a device which will conduct electricity when exposed to light. Infrared detectors, optical imaging devices, photodetectors, photovoltaics, optical switches, biological and chemical sensing photocopiers, and optical receivers for fiber-optic communication all rely on the characteristic of a photoconductor. In the scale of nanometer, scientists believe that photoconductors will provide better answer for nanoelectronics, nano, and molecular scaled optical-related devices.
Basically, photocurrent could be sourced from two major mechanisms, namely photovoltaic and PEC processes. In photovoltaic process, photon from sun light generates free electron-hole pairs where they are then collected at the electrode, and electrical power could be extracted at the external circuit. For PEC process, absorbed photons are used to excite electrons and the excited electrons will drive the chemical reaction. One of the common examples for the second process is water splitting to generate hydrogen.
For visible light detection, Si as a group IV semiconductor material, is well-established due to its compatibility with CMOS process. It has been well-understood and studied. Up to date, some numbers of Si-based nanowires photoconductive devices have been studied [6–10]. Metal oxide NWs are also another important type of photosensitive materials. One of the most intensively studied materials is zinc oxide (ZnO) nanostructure. Its unique properties on magnetic, mechanical, optical, and the recent spintronics provide further opportunities on a wide variety of applications. Due to its wide bandgap (E g = 3.37 eV at room temperature), applications as UV photodetector is possible. However, sparse literature showed photoresponse for a hierarchical NS consists both of Si and ZnO materials. In this work, hierarchical NS for a Si/ZnO trunk-branch array was fabricated and its initial photoactivity namely photocurrent was tested under one sun light irradiation.
Crystal Si (111) (c-Si)- and indium tin oxide (ITO)-coated glass were used as substrates for ZnO deposition. Prior to the growth of ZnO nanorods (NRs), ZnO seed layers were spin-coated on the substrates. The colloidal solution was prepared by dissolving 0.2 M zinc acetate dehydrate and 0.2 M diethanolamine in ethanol and stirred at 60°C for 30 min. The solution was spin-coated onto the substrates at a spinning speed of 2,000 rpm for 30 s. The samples were then heated at 100°C for 15 min. The spin coating process was repeated three times. Subsequently, the samples were annealed at 300°C for 1 h in a Carbolite furnace to yield the ZnO seeds.
Growth of ZnO NRs
For HTG process. ZnO seeded substrates were placed into a beaker filled with mixture of 0.04 M Zn(NO3)2 and 0.04 M HMTA aqueous solution, and heated inside a laboratory oven at 90°C for 2 h. The as-grown ZnO NR samples were rinsed with deionized water for several times to remove impurities.
For VTC growth process. ZnO NRs were deposited onto the ZnO seeded substrates using a quartz tube furnace. Mixture of ZnO and graphite powder (ratio of 1:1) with a total weight of approximately 0.2 g was placed inside the center hot zone of the quartz tube. The added graphite powder was used to form eutectic for reducing the vaporized temperature of ZnO [11, 12]. One end of the quartz tube was connected to N2 gas inlet, while the other end was remained open. The powder mixture was heated to 1,100°C for 1 h. The substrates were placed under a downstream of N2 flow, at about 12 cm from the powder boat. The substrate temperature was about 500°C at equilibrium.
Synthesis of Si/ZnO trunk-branch NSs
A Hitachi SU8000 field emission scanning electron microscopy (FESEM) was utilized for the morphological study of the samples. High-resolution transmission electron microscopy (HRTEM) micrographs of the samples were taken via a JEOL HRTEM (JEM-2100F), operating at an accelerating voltage of 200 kV. Characterization by X-ray diffraction and photoluminescence have been previously performed and published [17, 18] (see Additional file 1). A preliminary PEC cell testing has been carried out to characterize the photocurrent. The prepared NSs on ITO-coated glass substrate were used as working electrode. The test was done by using a VersaSTAT 3 potentiostat (Ametek Princeton Applied Research, Oak Ridge, TN). A solar light simulator (Oriel Instrument) was used to generate an equivalent intensity of one sunlight (100 mWcm−2) AM 1.5 G radiation. A conventional three-electrode cell was constructed with the samples as working electrode, a platinum wire as counter electrode, and Ag/AgCl (in 3 M KCl) as reference electrode. The electrodes were immersed in a 1 M KCl electrolyte solution throughout the test. Since it was a PEC cell, the area of illumination is the same as the area which was immersed in the electrolyte, which was 1 cm × 2 cm2 for the sample of ZnO NRs as working electrode. While for Si/ZnO sample, it was 1 × 1 cm2. Current density was calculated in each case for comparison purpose.
As shown in Figure 6, under constant light radiation, the Si/ZnO trunk-branch NSs' photocurrent is gradually reducing over a period of 50 s within the measurement time. This may due to a less stability of the NSs. The same result was obtained for a similar hierarchical NS namely ZnO/Si broom-like nanowires by Kargar and co-workers . The comparison is quiet relevant since both have the same materials and resemble the same structure. The only difference is that Kargar's NSs with the ZnO NRs is shown only on the top portion of the Si backbone NWs whereas our work shows NSs with ZnO NRs evenly distributed on the lateral side and cap of each Si trunk, although both researches show FESEM's images with quite similar number of density for Si trunk on the substrate and the similar HTG growth process for both our and Karger's experiments on the growth of ZnO NRs. Kargar's work produced broom-like nanowires whereas our work came out with the hierarchical nanostructures resembling the leaves of a pine tree. However, the seeding process for ZnO seeds was different. Kargar used sputtering process and we used spin coating method. The figure of merit by using spin coating process is the seeding could be evenly distributed in the whole lateral side of each Si trunk and resulted in the even growth of pine-leave-like NSs.
The discussion are extended to compare photocurrent effect of our Si/ZnO trunk-branch NSs with other popular photosensitive nanomaterials, for instance, TiO2[24, 25] and InGaN . Hwang et al.  synthesized high density Si/TiO2 core-shell NWs, and the photocurrent density is about 0.25 mA/cm2 under the illumination of 100 mWcm−2 full spectrum in a solar simulator, which has the same value as our Si/ZnO trunk-branch NSs. Our Si/ZnO trunk-branch NSs showed fairly higher photocurrent density compared to the Si/InGaN core-shell NW arrays (0.05 to 0.12 mA/cm2) demonstrated by Hwang et al. .
An improved method has been used for the growth of Si/ZnO trunk-branch NSs where the ZnO NRs could be distributed more evenly on the lateral side and cap of each Si trunk. The photocurrent of the NSs have been measured and compared to the sole ZnO NRs. Significant improvement was recorded for this hierarchical Si/ZnO NS array.
This work was supported in part by the Fundamental Research Grant Scheme (FRGS/1/2013/SG06/UKM/02/1), High Impact Research Grant by Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/MOHE/SC/06), Funding for Higher Institutions' Centre of Excellence (HICOE AKU95), and Prototype Research Grant Scheme (PRGS/1/13/SG07/UKM/02/1).
- Gao P-X, Shimpi P, Gao H, Liu C, Guo Y, Cai W, Liao K-T, Wrobel G, Zhang Z, Ren Z, Lin H-J: Hierarchical assembly of multifunctional oxide-based composite nanostructures for energy and environmental applications. Int J Mol Sci 2012, 13(6):7393–7423.View ArticleGoogle Scholar
- Alenezi MR, Henley SJ, Emerson NG, Silva SRP: From 1D and 2D ZnO nanostructures to 3D hierarchical structures with enhanced gas sensing properties. Nanoscale 2014, 6: 235–247. 10.1039/c3nr04519fView ArticleGoogle Scholar
- Lee J-H: Gas sensors using hierarchical and hollow oxide nanostructures: overview. Sensors Actuators B 2009, 140: 319–336. 10.1016/j.snb.2009.04.026View ArticleGoogle Scholar
- Hwang YJ, Wu CH, Hahn C, Jeong HE, Yang P: Si/InGaN core/shell hierarchical nanowire arrays and their photoelectrochemical properties. Nano Lett 2012, 12(3):1678–1682. 10.1021/nl3001138View ArticleGoogle Scholar
- Kim H, Yong K: Highly efficient photoelectrochemical hydrogen generation using a quantum dot coupled hierarchical ZnO nanowires array. ACS Appl Mater Interfaces 2013, 5(24):13258–13264. 10.1021/am404259yView ArticleGoogle Scholar
- Ahn Y, Dunning J, Park J: Scanning photocurrent imaging and electronic band studies in silicon nanowire field effect transistors. Nano Lett 2005, 5: 1367–1370. 10.1021/nl050631xView ArticleGoogle Scholar
- Servati P, Colli A, Hofmann S, Fu YQ, Beecher P, Durrani ZAK, Ferrari AC, Flewitt AJ, Robertson J, Milne WI: Scalable silicon nanowire photodetectors. Physica E: Low-dimensional Systems and Nanostructures 2007, 38: 64–66. 10.1016/j.physe.2006.12.054View ArticleGoogle Scholar
- Kim KH, Keem K, Jeong DY, Min BD, Cho KA, Kim H, Moon B, Noh T, Park J, Suh M, Kim S: Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition. Jpn J Appl Phys 2006, 45: 4265–4269. Part 1 Part 1 10.1143/JJAP.45.4265View ArticleGoogle Scholar
- Choi HG, Choi YS, Jo YC, Kim H: A low-power silicon-on-insulator photodetector with a nanometer-scale wire for highly integrated circuit. Jpn J Appl Phys 2004, 43: 3916–3918. Part 1 Part 1 10.1143/JJAP.43.3916View ArticleGoogle Scholar
- Park J-H, Kim H, Wang I-S, Shin J-K: Quantum-wired MOSFET photodetector fabricated by conventional photolithography on SOI substrate. In 4th IEEE Conference on Nanotechnology (NANO-04). Munich, Germany: IEEE New York; 2004:425–427.Google Scholar
- Fu DC, Majlis BY, Yahaya M, Salleh MM: Electrical characterization of cross-linked ZnO nanostructures grown on Si and Si/SiO2 substrate. Sains Malays 2008, 37(3):281–283.Google Scholar
- Karamdel J, Dee CF, Saw KG, Varghese B, Sow CH, Ahmad I, Majlis BY: Synthesis and characterization of well-aligned catalyst-free phosphorus-doped ZnO nanowires. J Alloys Compd 2012, 512: 68–72. 10.1016/j.jallcom.2011.09.018View ArticleGoogle Scholar
- Chong SK, Dee CF, Yahya N, Rahman SA: Control growth of silicon nanocolumns’ epitaxy on silicon nanowires. J Nanopart Res 2013, 15: 1571.View ArticleGoogle Scholar
- Chong SK, Goh BT, Apanut Z, Muhamad MR, Dee CF, Rahman SA: Synthesis of indium-catalyzed Si nanowires by hot-wire chemical vapor deposition. Mater Lett 2011, 65: 2452–2454. 10.1016/j.matlet.2011.04.100View ArticleGoogle Scholar
- Chong SK, Goh BT, Dee CF, Rahman SA: Effect of substrate to filament distance on formation and photoluminescence properties of indium catalyzed silicon nanowires using hot-wire chemical vapor deposition. Thin Solid Films 2013, 529: 153–158.View ArticleGoogle Scholar
- Chong SK, Goh BT, Dee CF, Rahman SA: Study on the role of filament temperature on growth of indium-catalyzed silicon nanowires by the hot-wire chemical vapor deposition technique. Mater Chem Phys 2012, 135: 635–643. 10.1016/j.matchemphys.2012.05.037View ArticleGoogle Scholar
- Chong SK, Dee CF, Rahman SA: Structural and photoluminescence investigation on catalytic growth of Si/ZnO heterostructure nanowires. Nanoscale Res Lett 2013, 8: 174. 10.1186/1556-276X-8-174View ArticleGoogle Scholar
- Chong SK, Lim EL, Yap CC, Chiu WS, Dee CF, Rahman SA: Hierarchical-oriented Si/ZnO heterostructured nanowires. Sci Adv Mater 2014, 6: 782–792. 10.1166/sam.2014.1768View ArticleGoogle Scholar
- Dhara S, Giri PK: Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires. Nanoscale Res Lett 2011, 6: 504. 10.1186/1556-276X-6-504View ArticleGoogle Scholar
- Game O, Singh U, Gupta AA, Suryawanshi A, Banpurkar A, Ogale S: Concurrent synthetic control of dopant (nitrogen) and defect complexes to realize broadband (UV–650 nm) absorption in ZnO nanorods for superior photo-electrochemical performance. J Mater Chem 2012, 22: 17302–17310. 10.1039/c2jm32812gView ArticleGoogle Scholar
- Humayun Q, Kashif M, Hashim U, Qurashi A: Selective growth of ZnO nanorods on microgap electrodes and their applications in UV sensors. Nanoscale Res Lett 2014, 9: 29. 10.1186/1556-276X-9-29View ArticleGoogle Scholar
- Kao CY, Hsin CL, Huang CW, Yu SY, Wang CW, Yeh PH, Wu WW: High-yield synthesis of ZnO nanowire arrays and their opto-electrical properties. Nanoscale 2012, 4: 1476. 10.1039/c1nr10742aView ArticleGoogle Scholar
- Ma DDD, Lee CS, Au FCK, Tong SY, Lee ST: Small-diameter silicon nanowire surfaces. Science 2003, 299: 1874–1877. 10.1126/science.1080313View ArticleGoogle Scholar
- Devarapalli RR, Debgupta J, Pillai VK, Shelke MV: C@SiNW/TiO2 core-shell nanoarrays with sandwiched carbon passivation layer as high efficiency photoelectrode for water splitting. Scientific Reports 2014, 4: 4897.View ArticleGoogle Scholar
- Hwang YJ, Boukai A, Yang P: High density n-Si/n-TiO2 core/shell nanowire arrays with enhanced photoactivity. Nano Lett 2009, 9(1):410–415. 10.1021/nl8032763View ArticleGoogle Scholar
- Um HD, Moiz SA, Park KT, Jung JY, Jee SW, Ahn CH, Kim DC, Cho HK, Kim DW, Lee JH: Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes. Appl Phys Lett 2011, 98(3):033102. 10.1063/1.3543845View ArticleGoogle Scholar
- Kargar A, Sun K, Kim SJ, Lu D, Jing Y, Liu Z, Pan X, Wang D: Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion. Phys Status Solidi A 2013, 210(12):2561–2568. 10.1002/pssa.201329214View ArticleGoogle Scholar
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