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.
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.
Results and discussion
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).
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