Nanotextured Spikes of α ‐ Fe 2 O 3 / NiFe 2 O 4 Composite for E ffi cient Photoelectrochemical Oxidation of Water

Content Provider	Semantic Scholar
Author	Hussain, Shabeeb Tavakoli, Mohammad Mahdi Waleed, Aashir Virk, Umar Siddique Yang, Shihe Waseem, Amir Fan, Zhiyong Nadeem, Muhammad Arif
Copyright Year	2018
Abstract	We demonstrate for the first time the application of p-NiFe2O4/nFe2O3 composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe2O4/n-Fe2O3 composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe2O4 and Fe2O3) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe2O4/n-Fe2O3 composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured pNiFe2O4/n-Fe2O3 device with equal molar 1:1 ratio of NiFe2O4 and Fe2O3 was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm at 1.23 VRHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm at 1.23 VRHE). The performance of p-NiFe2O4/n-Fe2O3 composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm at 1.23 VRHE was obtained for the 1:1 molar ratio p-NiFe2O4/n-Fe2O3 composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe2O4/n-Fe2O3 composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities. ■ INTRODUCTION Day-by-day increase in consumption of fossil fuel is causing depletion of crude oil reservoirs and rapid increase in atmospheric carbon dioxide (CO2). This has dragged the attention of scientists toward green technology, for example, solar energy, which is environmentally friendly and is a clean source of energy. The solar flux irradiating the earth surface (1.3 × 10 TW) exceeds the global energy consumption (1.6 × 10 TW in 2010) by about four orders of magnitude. The conversion of solar energy into a clean form of chemical energy such as hydrogen via photoelectrochemical (PEC) splitting of water could meet the desired target of a cheap and clean source of energy. Since the first report of successful PEC water splitting by Fujishima and Honda, various semiconducting materials exhibiting photocatalytic activity have been discovered. In particular, materials with narrow band gap, such as WO3, 5 BiVO4, 6,7 Fe2O3, 8,9 and NiFe2O4 10 are suitable choices for PEC water splitting and are of greater interest. Because of the suitable band gap (2.2 eV), band position, and chemical stability, hematite (Fe2O3) is considered to be the most suitable Received: August 8, 2017 Revised: February 2, 2018 Article pubs.acs.org/Langmuir Cite This: Langmuir XXXX, XXX, XXX−XXX © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.7b02786 Langmuir XXXX, XXX, XXX−XXX potential candidate to work as photoanode for solar-driven electrolysis. Theoretically, a photoconversion efficiency of 16% can be achieved for pure hematite in the conversion process of solar energy to hydrogen via PEC water-splitting reaction. However, practically, maximum photoconversion efficiency of 0.6% has been achieved for pure hematite, which is too low when compared with the theoretical value. It is now well established that this lower solar to hydrogen conversion efficiency of hematite is due to short lifetime of photogenerated charges (<10 ps), short diffusion length of photogenerated holes (2−4 nm), and slow kinetics of water oxidation. Thus efforts were being made to overcome these problems and to enhance the efficiency of hematite photoanode by various methods. Specifically, the strategies adopted for circumventing the above-mentioned drawbacks involve the doping of hematite with the transition metals and appropriate design of nanoarchitecture of hematite particles. These methods altogether increase the light absorption capacity of material, promote the oxygen evolution reaction, and decrease the charge recombination rate. Transition-metal ferrites with the general formula of MFe2O4 (M = Ni, Cu, Zn, Co) have intriguing properties of narrow band gap and visiblelight absorbance with high efficiency. MFe2O4 has higher conductivity of electric charges due to the hopping process between metal ions of variable oxidation states at O-sites. This property is beneficial for transferring the charge carriers. In addition, MFe2O4 has many other advantages such as stability against photocorrosion, low toxicity, low cost, high adsorption ability, and easy preparation. For the first time, we are reporting the coupling of p-Fe2O3 with the n-NiFe2O4 to address the certain issues associated with hematite, that is, short diffusion length of photogenerated electrons and holes. The conduction band of hematite is more negative as compared with the valence band of nickel ferrite, so the photoexcited electrons in the conduction band of hematite can travel toward the photogenerated holes in the valence band of nickel ferrite. Thus lifetime and diffusion length of electrons and holes increase by this double absorbing mechanism (Scheme 1). During the process, photoexcited electrons from the conduction band of hematite travel through the outer circuit via valence and conduction bands of nickel ferrite. The p-NiFe2O4/n-Fe2O3 composite thin films were deposited on planar (fluorinated tin oxide (FTO)-coated glass) as well as on perfectly ordered 3D-nanospike (NSP) substrates. The PEC water-splitting studies showed that planar p-NiFe2O4/n-Fe2O3 as well as the 3D-NSP p-NiFe2O4/n-Fe2O3 devices outperformed the pure hematite. Later devices, however, showed a remarkable increase (208%) in photoconversion efficiency compared with hematite device. It is worth mentioning that 3D-NSP devices showed increased PEC activity as compared with planar FTO device and pure hematite device due to the higher surface area and light absorption. The increased photoconversion efficiencies upon deposition of the composite material on 3D-NSP are essentially due to the diffraction effect because the length, width, and distance between neighboring nanospikes are comparable to the wavelength of visible light. This work also summarizes the PEC water splitting results for two different compositions (1:1 and 1:2 molar ratio) of p-NiFe2O4/n-Fe2O3 composite, of which composite with equal molar ratio (1:1) between NiFe2O4 and Fe2O3 showed the highest activity. ■ EXPERIMENTAL SECTION Fabrication of Highly Ordered 3D-NSP Arrays Substrate. Aluminum foil was cut into pieces of 2.5 cm length and 1.5 cm width and then pressed between two glass slides to flatten the aluminum chips. The chips were sonicated for ∼10 min in acetone, isopropanol (IPA), and deionized water, respectively, for removing dirt and impurities on the surface. Aluminum chips were then polished electrochemically in 1:3 v/v solution (perchloric acid in ethanol) by applying DC potential of 12 V for ∼2 min at 10 °C. For growing symmetric NSP array, the polished aluminum chips were imprinted by 1 × 1 cm2 homemade silicon mold (consisting of squarely ordered arrays of pillars with 200 nm height and 1200 nm pitch). After that, aluminum chips were anodized under DC voltage of 480 V at 5 °C in the acidic solution (120 mL of 2% aqueous solution of citric acid, 120 mL of ethylene glycol, and 9 mL of 0.1% phosphoric acid) for 6 h. After completion of anodization process the aluminum chips were removed and washed with DI water. For exposing perfectly ordered arrays of NSP, the chips were dipped in the etching solution (1.5% (w/w) of chromic acid and 6% (w/w) of phosphoric acid in water) at 98 °C for 60 min to etch anodized aluminum oxide (AAO). After complete etching of AAO, the exposed 3D nanostructured aluminum chips were rinsed with DI water and dried by compressed air blower. For protection against NaOH solution, a protection layer of Al2O3 was grown over nanospikes by low-voltage anodization at 20 V for 2 h in 3.4% aqueous solution of H2SO4. Then, 100 and 50 nm thick layers of Ti and Pt were deposited, respectively, by using magnetron sputtering for more protection against basic electrolyte solution. Finally, a 100 nm-thick FTO was deposited on top of 3D-NSP substrates by using ultrasonic spray pyrolysis (UPS) of ethanolic solution of 0.2 M SnCl4 and 0.04 M of NH4. Compressed air was used as a carrier gas for transferring the mist of precursor solution into the decomposition chamber of USP setup. The temperature of the decomposition chamber was maintained at 450 °C. Deposition of NiFe2O4/Fe2O3 Composite Layer on 3D-NSP Substrate. The USP method was employed for the deposition of photoactive NiFe2O4/Fe2O4 composite thin films on 3D nanostructured multilayered substrate. The calculated amounts of FeCl3· 6H2O and NiCl2·6H2O in different molar ratios were mixed and dissolved in ethanol. It was followed by the addition of an appropriate amount of acetyl acetone as a complexing agent for making the metallic ion more volatile. A small amount of prepared precursor solution was transferred to the evaporation chamber of USP setup, and the resulting mist was transferred to the decomposition chamber. The temperature in the chamber was kept at 500 °C. Table 1 shows the Scheme 1. Schematic Representation of Energy Band Positions of NiFe2O4 and α-Fe2O3, Dual-Absorbing Mechanism Langmuir Article DOI: 10.1021/acs.langmuir.7b02786 Langmuir XXXX, XXX, XXX−XXX B molar concentration
File Format	PDF HTM / HTML
Alternate Webpage(s)	https://eezfan.home.ece.ust.hk/Papers/paper%20142.pdf
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article