CommunicationCrystalline Fe2O3/Fe2TiO5 heterojunction nanorods with efficient charge separation and hole injection as photoanode for solar water oxidation
Introduction
The possibility of solar fuel production using Photoelectrochemical Cell (PEC) has developed considerably since Fujishima and Honda demonstrated in 1972, for the first time, solar water splitting using Titanium Dioxide (TiO2) as photoanode [1]. After that, efforts have been made to employ cheap, abundant and non-toxic semiconducting materials as photoanodes and to improve overall efficiency and performance of PEC devices. In the last decade, hematite (α-Fe2O3) based PEC devices have attracted many scientists in exploiting its advantages such as its good visible light absorption and stability but the reported Solar-to-Hydrogen (STH) efficiency (4% or less) is still far from the theoretical limit of 16.8% [2]. This is due to the high bulk/surface recombination in hematite owing to its small minority carrier diffusion length, poor charge transport properties, slow water oxidation kinetics etc. Bulk recombination and surface recombination could be tackled by using nanostructuring, doping or surface treatments [2], [3], [4]. One way to improve the charge separation/transport in the bulk of photoanode is by forming a staggered gap (type II) band alignment based heterojunction with offsets in conduction band (ΔEC) and valence band (ΔEV) allowing facile transfer of electrons and holes and increased charge separation due to built-in potential [5].
To fabricate such heterojunctions, the key is to explore other stable and photo absorbing semiconductors to couple with hematite, which are crystalline in nature to avoid charge trapping on lattice disordering and share favorable band alignment and interface properties with hematite. Heterojunction of hematite with binary metal oxides have earlier been reported by various groups. WO3/Fe2O3 composite photoanodes [6], ZnO/Fe2O3 core-shell nanowires [7] and Fe2O3/TiO2 photoelectrodes [8] with n/n heterojunction structure yielded lower onset potential and improved performance of the PEC cell device as compared to their constituents. Heterojunction formed with ternary oxides like ferrites have also been reported. Branched array of Co-doped Fe2O3 nanorods with MgFe2O4 [9] and Fe2O3:Ti/ZnFe2O4 films [10] demonstrated good charge separation resulting in appreciable photocurrent density. These heterojunctions involved expensive element based oxides like V, Bi, W etc. which have to replace by abundant elements for a long sustainable solution. There is clearly a need to explore the heterojunction approach for large scale, cost-effective and efficient solar water splitting technology employing abundant materials with favorable band alignment and thermal/aqueous stability.
We have previously reported about synthesis of an abundant mineral, Fe2TiO5, with a band gap of 2.1 eV which is stable in aqueous solutions [11]. In that work, we performed XPS/UPS measurements on pristine Fe2TiO5 and discovered that the work function (Ef) was around 4.77 eV whereas difference between work function and valence band (Ev−Ef) was approximately 1.5 eV as extracted from XPS which helped us evaluate conduction band position since band gap of 2.1 eV is known from Tau plot analysis. Using this, band levels possessed by Fe2O3 and Fe2TiO5 integrate are joined as heterojunction in the schematic representation (in yellow) in Fig. 1. It shows easy electron injection to Fe2O3 owing to the conduction band alignment and hole transport to electrolyte due to valence band alignment between both materials. This band alignment has also been recently reported by Deng et al. [12]. There exists structural coherency between pseudobrookite and hematite which allows the overgrowth of pseudobrookite on hematite following the relation: 〈110〉 hematite || 〈101〉 pseudobrookite [13]. It also possesses the physical and chemical properties that are ideal for the construction of photoanodes for water oxidation using PEC.
Liu et al. reported TiO2 based photoanodes with amorphous Fe2TiO5 overlayer acting as a visible light absorber which resulted in high performance and low onset potential [14]. Recently, Deng et al. demonstrated surface treatment of Fe2O3 with Fe2TiO5 for a surface passivating effect which enhanced photocurrent [12]. In both cases, the ultrathin Fe2TiO5 overlayer, fabricated by a solid state reaction between Fe and Ti based phases which limits the thickness of Fe2TiO5, was amorphous in nature. In present paper, we report Fe2O3/Fe2TiO5 heterojunction comprising of crystalline Fe2TiO5 overlayer capable of providing more distinct electrical junction as compared to amorphous layer. This heterojunction fabricated using low cost hydrothermal technique was characterized using impedance spectroscopy and bulk/surface charge separation efficiencies to investigate charge dynamics of the system. They were then coupled with CoOx cocatalyst to achieve enhanced performance for Fe2O3/Fe2TiO5 heterojunction based PEC device.
Section snippets
Results and discussion
The heterojunctions were synthesized on FTO substrates using the sequential hydrothermal synthesis of Fe2O3 nanorods followed by fabrication of nanoporous Fe2TiO5 films as mentioned in Section 2 (SI). A schematic representation of complete methodology is shown in Fig. 2. Reaction conditions to synthesize Fe2O3 nanorods were as earlier reported [15] whereas Fe2TiO5 films were fabricated under different reaction temperatures of 120 °C, 135 °C and 150 °C with a fixed reaction time of 12 h. Pristine Fe2
Conclusion
In summary, Fe2O3/Fe2TiO5 heterojunctions consisting of crystalline hematite and pseudobrookite showed superior PEC performance compared to pristine Fe2O3 nanorods. The photocurrent density at 1.23 V vs RHE for heterojunction was enhanced to 1.4 mA/cm2 without co-catalyst as compared to infinitesimally low photocurrent of 0.01 mA/cm2 exhibited by pristine Fe2O3 nanorods. High surface charge separation efficiency of 85% indicated the role of Fe2TiO5 in enhancing hole injection into the electrolyte.
Notes
Electronic supplementary information (ESI) available: Experimental Section, Morphological Characterization including FESEM/TEM images, Photoelectrochemical Characterization plots.
Acknowledgments
Financial support from the NTU Start Up (Grant Number: M4080297) Grant “Nanomaterials for Energy Harvesting” is kindly acknowledged. The authors are thankful to Mr. Mengyuan Zhang for TOC graphic.
Prince Saurabh Bassi is currently a PhD candidate, who is expected to be graduated soon, from School of Materials Science and Engineering at Nanyang Technological University (NTU), Singapore. He graduated with a B.Tech degree (Materials Science) in 2009 from Indian Institute of Technology (IIT), Kanpur, India. Prior to joining NTU for Ph.D., he worked as research associate in the field of organic electronics at Samtel Centre for Display Technologies, IIT Kanpur, India. His research interest
References (26)
- et al.
Nature
(1972) - et al.
ChemSusChem
(2011) - et al.
Energy Environ. Sci.
(2013) - et al.
Phys. Chem. Chem. Phys.
(2014) - et al.
Acc. Chem. Res.
(2013) - et al.
Phys. Chem. Chem. Phys.
(2014) - et al.
ACS Appl. Mater. Interfaces
(2015) - et al.
Nanoscale
(2015) - et al.
Angew. Chem.
(2013) - et al.
ACS Appl. Mater. Interfaces
(2012)
ACS Appl. Mater. Interfaces
ACS Nano
Rock-Forming Minerals: Non-Silicates: Oxides, Hydroxides and Sulphides
Cited by (105)
Exploring the photoelectrochemical process through surface states of plasmonic Ag-loaded NiFe-LDH-modified CuWO<inf>4</inf> photoanode
2024, Colloids and Surfaces A: Physicochemical and Engineering AspectsEnhanced photoelectrochemical performance of ZnO/α-Fe<inf>2</inf>O<inf>3</inf> heterojunction photoelectrode fabricated by facile hydrothermal and spin-coating method
2024, International Journal of Hydrogen Energy1D metal telluride heterostructure: A review on its synthesis for multifunctional applications
2023, Journal of Alloys and Compounds
Prince Saurabh Bassi is currently a PhD candidate, who is expected to be graduated soon, from School of Materials Science and Engineering at Nanyang Technological University (NTU), Singapore. He graduated with a B.Tech degree (Materials Science) in 2009 from Indian Institute of Technology (IIT), Kanpur, India. Prior to joining NTU for Ph.D., he worked as research associate in the field of organic electronics at Samtel Centre for Display Technologies, IIT Kanpur, India. His research interest focusses on synthesis of photocatalysts and investigation into charge carrier dynamics of semiconductor systems. Recently, he has been concentrating on exploration of novel systems like iron-titanates, especially Fe2TiO5 nanoporous films, for application in solar water oxidation.
Dr. Rajini P Antony is currently working as a K S Krishnan Research Fellow in Bhabha Atomic Research Center, Mumbai. She worked as a postdoctoral research fellow with Prof. Lydia Helena Wong, in School of Materials Science and Engineering, Nanyang Technological University Singapore during the period of 2013–2015. She obtained her Doctoral degree in Chemical sciences from Indira Gandhi Center for Atomic Research, Kalpakkam, India. Her research interests include Photoelectrochemical water splitting, Photocatalysis, Nano and porous material development by various synthesis routes for clean energy applications and electrocatalysis for water oxidation and reduction reactions.
Dr. Pablo P. Boix received his PhD. from the Universitat Jaume I (2012, Castelló, Spain). During this period, he analyzed the physical processes of optoelectrical devices including DSC, QDSC, organic photovoltaics and water splitting systems by impedance spectroscopy. In 2012 he joined ERI@N, where his research focuses on the electrochemical characterization and development of water splitting systems and perovskite solar cells, unveiling the working mechanisms which determine the performance of these optoelectrical devices.
Dr. Yanan Fang received her Ph.D. degree from Nanyang Technological University in Singapore, and a Bachelor of Science degree from Southeast University in Nanjing, China. She joined Energy Research Institute @NTU working on perovskite solar cell in 2012. She is currently holding a position of research fellow in School of Materials Science & Engineering, NTU. Her research interests include eco-materials, energy-related materials and characterization of microstructures and defects in functional materials.
Dr. James Barber is Professor of Biochemistry, Senior Research Fellow at Imperial College London and the Cannon Visiting Professor to NTU in Singapore. He is a Fellow of the Royal Society, Member of European Academy and Foreign Member of the Swedish Royal Academy of Sciences. He has Honorary Doctorates of Stockholm University and the University of East Anglia and awarded several medals and prizes including Flintoff Medal of RSC, Novartis Medal (UK Biochem. Soc), Wheland Medal (Univ. of Chicago), Eni-Ital gas Prize and Interdiciplinary Prize Medal of the RSC. In 2009 he was the Lee Kuan Yew Distinguished Visitor to Singapore.
Dr. Lydia Helena Wong is currently an Assistant Professor at the School of Materials Science and Engineering, Nanyang Technological University (NTU) Singapore. She graduated with a Bachelor Degree of Applied Science (with Hons) in 2002 and Doctor of Philosophy in Materials Science and Engineering from NTU in 2007. She was also a Visiting Scholar in Department of Chemical Engineering in Stanford University. After her PhD, she was working as a Senior Engineer in Chartered Semiconductor (now: Global Foundries) before coming back to serve at NTU. Her research group currently focuses on the investigation of non-toxic and abundant metal oxides and chalcopyrite materials for clean energy application.