Subscriber access provided by Kaohsiung Medical University
Article
Combinatorial studies on wet-chemical synthesized Ti-doped #-FeO: How does Ti improve photoelectrochemical activity? 2
3
4+
Yi-Hsuan Wu, Wei-Ru Guo, Mrinalini Mishra, Yen-Chen Huang, Jeng-Kuei Chang, and Tai-Chou Lee ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00316 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Combinatorial Studies on Wet-chemical Synthesized Ti-doped α-Fe2O3: How Does Ti4+ Improve Photoelectrochemical Activity? Yi-Hsuan Wu,† Wei-Ru Guo,‡ Mrinalini Mishra,‡ Yen-Chen Huang,‡ Jeng-Kuei Chang,*,† and Tai-Chou Lee*,‡ †
Institute of Materials Science and Engineering, National Central University, 300 Zhongda Rd., Zhongli District, Taoyuan City 320, Taiwan.
‡
Department of Chemical and Materials Engineering, National Central University, 300 Zhongda Rd., Zhongli District, Taoyuan City 320, Taiwan.
ABSTRACT: Hematite (α-Fe2O3)-based photoanode for photoelectrochemical water oxidation has been intensively studied for decades. Doping with isovalent or aliovalent ions is one way to mitigate several intrinsic drawbacks of bare hematite. While addition of Ti in the bulk has been reported for improving photoresponse, several aspects of effects of Ti impregnation have not been justified. In this work, Ti:Fe2O3 nano-ellipsoids synthesized by a facile one-pot hydrothermal process present improved photoelectrochemical response. Tuned symmetry of Fe-O and Fe-Fe (Fe-Ti) with less recombination during charge transportation, tuned electron configuration of O2p-Fe4s4p hybridization in Ti-adjoining regime with enhanced electron relaxation within Fe2O3 lattice, and suppressed O2/H2O back reaction (reduction of O2), and inhibit formation of surface defects during hydrothermal synthesis were attested by x-ray absorption
spectroscopy,
Mott-Schottky
analysis
and
photoelectrochemical
impedance
spectroscopy. Additionally, Ti:Fe2O3 showed enhanced light absorption and hydrogen evolution rate of 11.76 µmol h-1 cm-2 under illumination was observed while using Ti:Fe2O3 as the working electrode. Additional experiments on Mn4+ and In3+ incorporation showed mixed effects. This
ACS Paragon Plus Environment
1
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
study provides insights and clarification toward “Ti-doping” of the hematite photoanode for solar hydrogen production from water. KEYWORDS: photoelectrochemical water oxidation, hematite, Ti-doping, X-ray absorption spectroscopy, and surface defect
ACS Paragon Plus Environment
2
Page 2 of 40
Page 3 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
INTRODUCTION Hematite (α-Fe2O3) bears corundum structure, has a band gap energy (Eg) in the range of 1.9 - 2.2 eV (absorption wavelength: 650 - 560 nm), and is suitable for visible-light absorption. Various crystal structures can be prepared using different synthesis methods.1,2 In addition, outstanding photochemical stability, earth abundance, and low cost render this nontoxic transition-metal-oxide as a potential photoanode candidate for photoelectrochemical (PEC) water splitting applications. However, α-Fe2O3 photoanode is not able to practically demonstrate its theoretical water oxidation photocurrent (12.5 mAcm-2 under AM 1.5G solar illumination2). One of the probable reasons is extremely short hole diffusion length of 2 - 4 nm, which limits the oxidation reaction across the solid-liquid interface,3 although hematite has a light penetration depth of 118 nm at λ = 550 nm. Further, the lifetime of charge carriers is short (~3 ps) due to carrier trapping and phonon coupling as revealed by luminescence studies.4 Another drawback is poor electrical conductivity of hematite. A pure polycrystalline hematite exhibits ca. 10-14 Ω-1 cm1
conductivity and an electron mobility of 10-2 cm2/V/s.5 A common strategy to mitigate these problems and enhance photoresponse for better
oxygen-evolution reaction (OER) kinetics is doping by aliovalent ions.6-10 which has been deployed in hydrogen-evolution reaction (HER) side as well.7-8 Consequent enhancement and/or control of photoelectrode properties, such as morphology, crystal orientation, conductivity, optical property, and carrier concentration have been widely investigated.4-17 Some studies showed the formation of new compounds and heterojunction by aliovalent ion impregnation.12,13 Note that many papers reported more than 5 mol% doping, especially those using hydrothermal or other solution-based methods,13-17 whereas doping concentration higher than 2 mol% is not favorable based on thermodynamic considerations.6 One of the promising dopants for tuning the properties of hematite is Ti (4s23d2). Studies on Ti-doped hematite, attribute the enhanced PEC
ACS Paragon Plus Environment
3
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
performance to an increase in electron concentration, i.e. substitution of Fe3+ by Ti4+, leading to a higher charge carrier density.10,18,19 Some literature propose different functions of Ti dopants.20,21 For example, Zandi et al. ascribed the effect of Ti atoms to alleviating the “dead layer” which apparently results from the lattice mismatch between hematite and tin oxide.20 Further, Kim et al. reported improved hole collection efficiency (transport efficiency both in the bulk region and surface) in ultra-thin hematite film through Ti alloying.21 In this study, we implemented combinatorial material and electrochemical analysis on Tiimpregnated samples fabricated using a facile, low-cost, and effective wet chemical procedure. Nanoscaled titanium oxide formed in hydrothermal reaction decomposes during high-temperature sintering. Ti atoms, diffusing into hematite lattice from titanium-oxide nuclei, substitute Fe sites and generate Fe-Ti-O nanocomposites (Fe2-xTixO3) near the surface (Scheme 1), which tune lattice symmetry and contribute to electron affinity of Fe2p-O3d hybridized orbital. The MottSchottky (M-S) results suggest that Ti atoms introduced through our wet chemical process increase the donor density. Photoelectrochemical impedance spectroscopy (PEIS) data reveal bulk/surface recombination are inhibited by Fe-Ti-O nanocomposites. Effects of aliovalent ion on photoelectrochemical properties of iron oxide materials are provided to shed light on future design for doped-Fe2O3.
EXPERIMENTAL SECTION Fabrication of Pristine and Ti-doped Hematite Film Our synthesis method was based on Vayssieres et al.’s work published in 2001.22 However, urea was used instead of NaNO3. First, FTO-coated glass substrates were cleaned in acetone in an ultrasonic bath for 5 min. Then, 7.5 mmol iron(III) chloride hexahydrate and equimolar urea were dissolved in 50 mL DI water to form a homogeneous precursor solution.
ACS Paragon Plus Environment
4
Page 4 of 40
Page 5 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Thereafter, substrates were placed in 100 mL Teflon-lined autoclave, with FTO side facing the wall. The precursor solution was then poured into the autoclave container. The reaction was carried out at 95 ºC for 5 h in the oven, forming a uniform yellowish FeOOH film. The FeOOH film was heat treated in a tube furnace at 550 °C for 2 h in air with a heating rate of 10 °C/min, and then the temperature was further raised to 750 °C at a heating rate of 20 °C/min for 15 min. Consequently, a red-orange Fe2O3 film was obtained. To prepare Ti-doped Fe2O3 (Ti:Fe2O3), cooled 2 M TiCl4 (aq) was firstly diluted to 0.1 M. Diluted Ti precursor (0.5 mL) was then added dropwise into the FeCl3 precursor solution. The significance of cooled and diluted Ti precursor is explained in Supporting Information (Figure S1). Ratio of Ti/Fe was kept at 0.67 mol%. For the sake of comparison, other ratios maintained below 2 mol% were tried as well. The procedures for hydrothermal and heat treatments for the doped samples are identical to those for pristine Fe2O3 (p-Fe2O3). Additionally, TiO2 (Uniregion Biotech P90) was annealed at appropriate temperature and ambience to generate anatase, brookite, and rutile for performing a comparative surface chemical composition analysis with that of Ti:Fe2O3. Furthermore, MnCl2 and InCl3 were added in the FeCl3 precursors solution (while maintaining the Mn/Fe and In/Fe ratio at 0.67 mol%) separately and identical experimental procedures were conducted to perform a comparative study on the effects of other aliovalent ions. Material Characterization Morphologies of samples were examined with scanning electron microscopy (SEM; FEI Inspect F50). Energy Dispersive Spectroscopy (EDS) was performed on the same instrument as well. X-ray diffraction (XRD) patterns for crystal structure analysis were recorded in the 2θ range of 20° - 70° at a scan rate of 1.2° min−1 using Bruker D8 Advance (with a Cu-Kα radiation, λ = 0.15418 nm). X-ray photoelectron spectroscope (XPS; Thermo VG Sigma Probe) equipped with
ACS Paragon Plus Environment
5
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
a monochromatic Al Kα source (hν = 1486.6 eV) was employed to study surface chemical compositions. The pass energy/energy step size for survey and specific orbital spectra were 100 eV/1.0 eV and 50 eV/0.1 eV, respectively. The optical properties of the films were measured by a UV-vis spectrometer (Varian Cary 100) equipped with an integrating sphere, in the wavelength range of 300 - 800 nm at room temperature. The absorption spectra were obtained while using an identical FTO-coated glass substrate as the reference. Photoluminescence (PL) spectroscopy was conducted using Horiba Jobin Yvon LABRAM HR 800 UV spectroscope equipped with 532 nm Ar laser with a power of 50 mW as the light source. The MPL 100× objective lens had a spot size of 0.70 µm2. All the PL measurements were done at room temperature. The obtained spectra were then properly smoothed with all background signals subtracted and were normalized with respect to the thicknesses of the samples. X-ray absorption spectroscopy, including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS), for evaluating the structural and electronic configurations and coordination number of central atom of the prepared pristine and doped films, was conducted at National Synchrotron Radiation Research Center (NSRRC), Taiwan using the beamlines 17C and 20 A. Photoelectrochemical Measurements and Hydrogen Evolution All photoelectrochemical measurements were carried out in a three-electrode electrochemical system with the prepared semiconductor thin film as the working electrode, a Pt plate electrode as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode in 1.0 M NaOH (aq.) (pH 13.6) solution. The electrolyte was prepared using Milli-Q water (resistivity-18.2 MΩ.cm), degassed by purging nitrogen, and then ultrasonicated for 30 min before each experiment. The edges of samples, and contact between samples and copper wires were covered with epoxy resin to prevent current leakage.
ACS Paragon Plus Environment
6
Page 6 of 40
Page 7 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Current-voltage characteristics were obtained as a function of applied potential from -0.5 to +0.80 V vs. VSCE using a computer-controlled potentiostat (Autolab PGSTAT302). Potential was then converted into Reversible Hydrogen Electrode (RHE) using the relation: ERHE = ESCE + 0.059× pH + 0.1536 at 25 °C.23 Steady light illumination with a power density of 100 mW cm-2 was provided by a 300 W Xenon lamp (Perkin Elmer Model PE300UV). The power density was measured by a power meter (Newport 1918-R). Chronoamperometry and the electrochemical impedance spectroscopy (EIS) experiments were conducted at 1.186 VRHE under illumination (100 mW cm-2). The frequency of EIS was swept from 0.1 to 104 Hz. To retrieve Ti-doping density, M-S measurements were carried out at the frequency of 8 kHz in a potential window 0.6 ~ 1.4 V vs. VRHE with 0.05 V decrease at each step. Incident photon-to-current efficiency (IPCE) was measured with a series of monochromatic filters (Hard Coated OD 4 10 nm Bandpass Filters, Edmund Optics) for the wavelength range of 310 - 700 nm, under reduced incident intensity of light at 30 mW/cm2. Additionally, evolved gas was collected for 11 h using water displacement method from the vicinity of the Pt counter electrode while using Ti:Fe2O3 as the working electrode under illumination (100 mW cm-2) at 1.23 V vs RHE. The collected gas was then analyzed using a gas chromatograph (8700F, China Chromatography). The temperature was maintained at 25 °C during the PEC reaction.
RESULTS AND DISCUSSION As described in the Experimental section, pristine (p-Fe2O3) and Ti:Fe2O3 samples were synthesized using urea-assisted hydrothermal method. The reaction mechanism is described in the Supporting Information. During hydrothermal reaction in presence of urea, as the pH value of the solution decreases, the rate of hydrolysis of Fe3+, as well as Fe(OH)3 formation decreases. TiCl4 is a dense, colorless, and easy-hydrolysis Lewis acid. When mixed with FeCl3 aqueous
ACS Paragon Plus Environment
7
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 40
solution under the hydrothermal condition, the hydrolyzed byproduct, HCl will increase the acidity of the reaction bath and thus inhibit fast film growth. Images of as-prepared FeOOH, p-Fe2O3, and Ti:Fe2O3 are shown in Figure S2. Typical yellowish FeOOH is clearly seen. The Ti:Fe2O3 appears to be a lighter shade of orange than the pristine one. The lighter color can be attributed to reduced thickness. Figures 1a and 1b show the SEM images of the cross-section view of p-Fe2O3 and Ti:Fe2O3. The thicknesses of p-Fe2O3 and Ti:Fe2O3 are 380 nm and 210 nm, respectively. Various thicknesses of p-Fe2O3 were also obtained by varying the duration of hydrothermal treatment, as shown in Figure S3. The thickness of Ti:Fe2O3 is still larger than the average light penetration depth of bulk hematite (118 nm at λ = 550 nm). Nevertheless, a thinner film can be beneficial to the PEC performance because of a shorter charge-carrier transportation length. The doped film appears denser with notable necking among the nano-ellipsoids and coalescence (Figure S4). Both p-Fe2O3 and Ti:Fe2O3 samples are indexed to hematite (JCPDS card No. 86-550), shown in Figure S5. No other crystal phases were observed. X-ray photoelectron spectroscopy (XPS) was utilized to determine the oxidation states of p-Fe2O3 and Ti:Fe2O3. The survey scan clearly indicates the existence of Ti in Ti:Fe2O3 (Figure S6). Figure 1c shows the Fe 2p spectra. Both pristine and doped samples exhibit binding energy of Fe 2p1/2 and Fe 2p3/2 peaks at 725.0 eV and 711.6 eV, respectively. Presence of Fe3+ satellite peak as well as lack of Fe2+ peak (716 eV) corroborate the fact that satisfactory amount of O2 was obtained by two-step heat treatment. Several groups have reported that the formation of Fe2+ is induced after substitution of Fe3+ by M4+ ions (M: Si4+, Ti
4+
or Sn4+) upon similar high-
temperature treatment,24-26 while this can be inhibited by sufficient amount of O2, diffusion of Ti4+, and consequent fixation of Fe-Ti-O nanostructure in hematite. The doped sample shows clear Ti 2p peaks (Figure 1d) at 458.4 eV (2p3/2) and 464.2 eV (2p1/2) which is identical to Shen
ACS Paragon Plus Environment
8
Page 9 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
et al.’s result.15 Our results feature Ti at four-valence state in Fe2O3 after high-temperature treatment in air. The Ti 2p spectra of Ti:Fe2O3 was compared with that of anatase, brookite, and rutile. Positions of Ti 2p1/2 and Ti 2p3/2 peaks for the three phases of TiO2 are listed in Table S1. The comparison reveals that Ti ions at Ti:Fe2O3 surface bear a binding energy shift of at least 0.3 eV (a shift of 0.5 eV from anatase) toward lower binding energy. TiCl4 is easily hydrolyzed in an aqueous solution (TiCl4 + 2 H2O → TiO2 + 4 HCl). The TiO2 nuclei can be impregnated in Fe(OH)3 crystal. Considering that anatase TiO2 crystallizes at 300 °C,27 higher temperatures (550 °C and 750 °C) along with oxygen-containing atmosphere is suggested to facilitate Ti diffusion and substitution in hematite matrix. The Ti 2p feature of our doped sample bears similarity with that of Fe2TiO5 (an Fe-Ti-O semiconductor with 2.2 eV band gap) and FeTiO3 (Figures 1c and d).28,29 Relatively weak orbital-hybridization energy of Ti 2p O 1s can be altered by nearest Fe-O bonding, thereby resulting in low-binding-energy shift. The meticulous comparative analysis of surface chemical states of TiO2 and Ti:Fe2O3 imply an inference where Fe-Ti-O nanocomposites are primary phase in Τi:Fe2O3 film rather than TiO2Fe2O3 structure. Although the exact crystal phase could not be identified in the present study, it provides scope for the further work. The concentration of Ti with respect to the amount of Fe in Ti:Fe2O3 near the surface was also evaluated from the results of XPS analysis, as shown in Table S2. High Ti/Fe ratio indicates an essential thermodynamically favored phenomenon of accumulation (by diffusion) of dopants (solute) that form Fe-Ti-O nanocomposites in near-surface region for surface relaxation (i.e. reducing surface tension) during natural cooling process. This hypothesis is validated by the results of EDS analysis (Figure S7). Spatial resolution of EDS is governed by the penetration and spreading of the electron beam in the specimen. In contrast to back-scattered electrons and secondary electrons, the escape depth of X-ray (and penetration depth of EDS data) is far deeper,
ACS Paragon Plus Environment
9
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
which leads to a pear-shape interaction volume in specimen for SEM. At accelerating voltage of 15 keV used in our experiment, the penetration depth of X-ray can be 1.2 µm. Considering the thickness (< 400 nm) of our films, EDS is sufficient to evaluate the bulk composition. Nevertheless, the difference between the surface (Ti/Fe = 27.7 %) and bulk composition (Ti/Fe = 2.6 %) is significant enough to draw the conclusion that Ti atoms are distributed preferentially on the surface of Ti:Fe2O3, in spite of the fact that both XPS and EDS are semi-quantitative analysis. Local geometric and electronic structures of the samples is revealed by XAS. The measurements are performed over two main regions establishing an entire absorption spectrum: XANES and EXAFS. The former (typically starting ~ 50 eV above the absorption edge), dominated by core level electron transition, is related to oxidation valence state and filling ratio of d-band of a transition metal. The latter is correlated to coordination number of absorbing atoms, uniformity of adjoining atoms, and distance between center and bonding atoms. Thus, XAS is useful to study doped materials and local distortion of crystal lattice. Figure 2a shows Fe K-edge XANES spectra. The intensities of pre-edge and edge-peak are related to valence and unoccupied states of Fe atom in hematite, respectively. The intensities of the Fe K-edge pre-peaks, depicting octahedral-site features are affected by local oxygen coordination geometry. In transition-metal oxide system, a dopant with comparable ionic radius, appropriate doping concentration and valence tends to substitute metal sites rather than oxygen sites. The entire K-edge region, remain unchanged under effect of Ti-doping. This implies that Ti dopants have no influence on valence of Fe3+ ions, which is consistent with our anticipation. It is confirmed by the EXAFS spectra that Fe(III) remains in a 6-coordinated structure (Figure 2b). The bond lengths (radial distance) of the next-nearest neighbors: Fe-O and Fe-Fe for both the (pristine and doped) samples were 1.5 Å and 2.8 Å, respectively. These values are in agreement with those reported in literature.18 Amplitude (Fourier-transformed magnitude) of the scattered
ACS Paragon Plus Environment
10
Page 10 of 40
Page 11 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
photoelectron at the absorbing atom indicates symmetry of bonding atoms. When X-ray is absorbed by the central atom (Fe in this case), photoelectron passes to neighboring atoms: first are bonding oxygen and then nearest iron or substituted elements. The backscattered photoelectron passes back toward central atom, forming constructive interference for a crystal structure with good symmetry. The characteristic intensity of bonds for crystal structure with worse symmetry is lower. Peaks denoting Fe-O and Fe-M (M = element at adjoining iron position) increase slightly in Ti-doped samples. This evidence supports the fact that Ti-doping can improve the symmetrical structure of both Fe-O and Fe-M. There is an ample difference between Ti-O and surrounding Fe-O bonds as Ti4+ has smaller ionic radius (0.605 Å) than Fe3+ (0.645 Å) in Fe2O3.30 Hence, slight contraction of O-Ti-O bonding within Fe-O alignment tunes better symmetry of lattice that might benefit the transportation of charge carriers promoting less recombination in the crystal. Similarity in Fe K-edge XANES spectra and radial distribution function from EXAFS proved ionic bonds altered from Fe-O-Fe to Ti-O-Fe and confirmed that Ti substitutes at Fe octahedral sites. Ti impregnation into Fe2O3 host is also reflected as the red shift of Ti 2p spectra of Ti:Fe2O3 from that of TiO2 (Figure 1c). Fe L-edge spectra of Fe2O3 consists of L3 (~710.7 eV) and L2 (~723 eV) spin-orbit split components, which can be correlated to electronic transitions of Fe 2p3/2 and 2p1/2 core electrons split in range of Fe 2p core level to unoccupied 3d level that contribute to Fe 3d-O 2p hybridization.31 As shown in Figure 2c, L3 edge shows an obvious peak at 710.67 eV and a shoulder at around 713.5 eV. The edge features enable us to determine iron at +3 oxidation state in both undoped and doped samples.32 Furthermore, our results are in good agreement with the study by Gloter et al.31 The double-peak O K-edge spectra at 530.4 eV and 531.84 eV originate from crystal field splitting in Fe-O π and σ bonds.33 Region A indicates oxygen p character in the transition metal
ACS Paragon Plus Environment
11
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
3d band.34 As Fe remains in octahedral local coordination in α-Fe2O3, these two peaks (contained in region A) represent t2g and eg orbital in 3d-2p electronic configurations, respectively. Slightly lower t2g peak than the energy level of eg at which the wave function points toward oxygen ion, represents hematite nature. Normalized peak intensities are listed in Table 1. For Ti:Fe2O3, decreased intensity for Fe3d-O2p peak and distinguished difference in the intensity of t2g-eg peak characterize electron affinity or shift from Ti4+ to hybridized Fe3d-O2p. A shoulder at ~534.5 eV was ascribed to nano-heterogeneity of Ti:Fe2O3. According to molecular orbital theory, board peak in region B is able to describe O2p - Fe4s4p hybridization. In our opinion, addition of Ti4+ affects electronic configuration of O and leads to reduced intensity in this region. The regions C and D are explained by scattering in iron-oxygen clusters that represent minor contribution to variation.31 Overall similar spectra between p-Fe2O3 and Ti:Fe2O3 denotes Ti is localized at Fe site in Fe2O3 crystal. Ti:Fe2O3 has an apparent enhancement in absorption at λ < 550 nm, all the way to the UV light region (Figure S8a). These UV-vis spectra were also normalized with respect to thickness. The onset of the optical absorption is located at around 600 nm, corresponding to a band gap of 2.1 eV. Ti-incorporated composites hardly harvest long-wavelength region of light. The heterogeneous nanostructures on the surface can be effective light scattering centers within the crystal for enhancing absorption of light wavelength shorter than 550 nm. The enhancement is suggested to be Rayleigh-Gans scattering where Fe-Ti-O nanostructures, with much smaller size than incident light wavelength, act as the imbedded scattering centers. The Rayleigh-Gans approximation is valid for a scatterer whose dielectric constant close to that of the surrounding medium, which is applicable between dopant-induced nanostructures and the matrix.35 Incident-photon-to-current-efficiencies (IPCE) for p-Fe2O3 and Ti:Fe2O3 were measured at 1.23 VRHE as a function of incident light wavelength (Figure S8b). Enhancement was found in
ACS Paragon Plus Environment
12
Page 12 of 40
Page 13 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Ti:Fe2O3. Both pristine and doped samples exhibit decay in efficiency, dropping to near zero at 600 nm. To further investigate the influence of light absorption, absorbed-photon-to-currentefficiency (APCE) was also calculated (inset of Figure S8b). The converted result confirms contribution of light scattering within crystal and improvement in generation of effective electrons. However, it is not the only factor for the enhanced solar-to-hydrogen conversion. Figure 3 shows the PL spectra of pristine and doped Fe2O3 samples measured using 532 nm laser. The PL signal at ~572 nm corresponds to the band-edge emission and coincides with location of absorption edge of the UV-visible spectra (Figure S8a). The inset shows the enlarged view of the peak at ~572 nm depicting reduced intensity for the doped sample implying reduced recombination of charge carriers. Although the reduction was not large in this case, it does suggest better photoactivity for the doped sample. Existence of peaks other than the band edge emission in the shorter wavelength region entails the existence of variation in band gap originating from the intermolecular potentials between the Fe2O3 molecules. Peaks other than band edge emission in the longer wavelength region represent defect levels in the forbidden energy gap of the crystal.36 It is to be noted that two small peaks are observed at ~594 nm and ~617 nm which also exhibit reduced intensity upon doping indicating alleviation of defects. Initially, we employed various Ti/Fe ratios of precursors, restricted below 2 mol% to prepare the doped samples, which in turn exhibited negligible influence on photoelectrochemical performance under light irradiation (Figure S9). Hence, further experiments were carried out with doped samples prepared with a Ti/Fe ratio of 0.67 mol%. Linear sweep voltammetry in the dark and under illumination, shown in Figure 4a, were conducted in a potential window between 0.4 and 1.85 VRHE. Unlike typical onset potential (0.70 VRHE) of pristine hematite, that of Ti-doped sample shifted anodically to 0.82 VRHE. Photocurrent rises rapidly, indicating a low recombination rate, reaches 0.87 mA/cm2 at 1.23 VRHE and 1.05 mA/cm2 at 1.40 VRHE for
ACS Paragon Plus Environment
13
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Ti:Fe2O3. Interestingly, the “breakdown potential” (onset potential of electrolysis of water) is delayed from 1.5 VRHE to 1.7 VRHE, which can be attributed to a good coverage of the deposited semiconductor on FTO and also the Ti-doping. The site for electron-injection from FTO to electrolyte is limited, resulting in delay of onset potential of dark current.37 This finding is in accordance with that of Malviya et al.’s work.38 When both electrodes achieve their saturation photocurrent (after 1.20 VRHE in our case), the trend of photocurrent increase is consistent. As holes are delivered through the depletion region, they either fill surface states (nonFaradaic current) or contribute to water oxidation. Although both processes reflect photocurrent response, the plateau of photocurrent is associated with the latter. At this stage, the surface states are filled. Finally, when the light is blocked or turned off, remaining electrons in depletion region will firstly recombine with surface-state trapped holes. This recombination results in cathodic current spike. Under larger applied bias, charge carriers experience the electric field to facilitate their transportation in bulk material, thus the transient current is saturated under illumination. Severe spiking transient current in chopping test (Figure 4b) was found at potential lower than 1.0 VRHE in pristine hematite. Ti-doped electrode exhibits a smooth transient current in chopped diagram right over onset potential. Distinct transient photocurrent attests impeded PEC performance by surface states in pristine hematite electrode. The stability of electrodes, illustrated in Figure 4c was examined at 1.2 VRHE under 1 sun illumination in identical environment for 12 h. Inset of Figure 4c presents measured hydrogen evolution at counter electrode while Ti:Fe2O3 was the working electrode (filled marker). The calculated hydrogen evolution based on the recorded photocurrent is denoted by unfilled marker on the same plot. The rate of gas evolution was evaluated as 11.76 µmol h-1 cm-2, with a Faradaic efficiency of 82.3 %, which is close to that of calculated value. Hence, addition of Ti does not hamper stability of crystal or photoelectrochemical reaction at surface. Figure S10 shows the photocurrent density as
ACS Paragon Plus Environment
14
Page 14 of 40
Page 15 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
a function of applied potential for p-Fe2O3 with various thicknesses. It is observed that the photoelectrochemical performance of the p-Fe2O3 increases with a decrease in thickness. However, the 150 nm thick p-Fe2O3 photoelectrode exhibits the lowest photocatalytic activity, perhaps due to a lower absorption, and/or lower crystallinity. Furthermore, all p-Fe2O3 samples (0.50 mA/cm2 at 1.23 VRHE of Fe2O3 film with 220 nm thickness) show lower photocurrent density than Ti:Fe2O3 (0.87 mA/cm2 at 1.23 VRHE). Most of the previous works state that aliovalent dopants donate free electrons into hematite crystal where donor density is enhanced and improved photoresponse is observed. Doping density as well as flat-band potential of the samples can be determined using M-S plot. Frequency dependent analysis should be applied to non-planar surfaces, where surface states play an important role.39 Figure 4d presents M-S plots obtained from EIS data measured from 10000 Hz to 1 Hz in the dark. The potential window was set at -0.4 - 0.4 VSCE with 0.05 V sweeping step. All the EIS data were fitted with simplest R(RC) equivalent circuit, where RS is series resistance, RCT is charge-transfer resistance in the bulk, and Csc is the space-charge-layer capacitance. The M-S equation is:
=
V − V −
(1)
where AS is surface area of electrode, C is the specific capacitance of space-charge layer (F/cm2), e0 is the equivalent electron charge (1.602×10-19 C), ε is the dielectric constant of hematite (32), ε0 is the permittivity of vacuum (8.854×10-12 CV-1m-1), Nd is the carrier density, V is the applied bias, VFB is the flat-band potential, and kT/e0 is a temperature-dependent correction term with k the Boltzmann constant (1.38×10-23 JK-1) and T the experimental temperature.40,41 The reported
ACS Paragon Plus Environment
15
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 40
dielectric constant (ε) of hematite varies widely from 12 to 120. In this work, we select 32 based on a report by Glasscock et al.40 Doping density and VFB potential are 2.3×1017 /cm3 and 0.733 VRHE for p-Fe2O3, and 4.68×1017 /cm3 and 0.795 VRHE for Ti:Fe2O3, respectively, as denoted in Figure 4d. It is to be noted that M-S plot is closely related to Csc, donor density, and applied bias at constant temperature. The linear part of M-S plot fails in the vicinity of flat band potential as the majority carriers are accumulating at the interface; The straight-line feature also fails in large potential region where the effective separation distance of the space-charge layer decreases. In addition, significant bias potential can lead to electrochemical reaction with species in the solution. Furthermore, Zandi et al. reported a M-S plot after 3% Ti-doping.20 A more anodic flatband potential, ~0.85 VRHE, was observed, agreed with our observation in this study. In theory, each Ti atom donates one free electron to hematite (or applicable metal-oxide) lattice. For instance, a 3 % Ti should generate ~1021 cm-3 free electrons based on theoretical calculation. In addition, some previous works declared more than five-fold enhancement in donor density with similar Ti-incorporation concentration.12, 37,41-43 Five-fold increase in donor density in the order of ~1021 cm-3 is considered as “well-doped” hematite electrode. Our result (twice increase of donor density in the order of 1017 cm-3) is identical to Zandi et al.’s work where Ti contributes little to free electrons.20 In contrast to those reported in the literature, our PEIS results combined with the inferences from XAS and light-absorption-and-transmittance study suggest that Ti may serve more as crystal-symmetry tuner and short-wavelength-light scattering centers. Additionally, density of surface states (DOSss) versus applied bias under illumination was measured to exclusively investigate the effects of surface-state trapping (Figure 4a). The density of surface states is proportional to measured Cns and is given by the relation:
=
!"
ACS Paragon Plus Environment
16
(2)
Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
where Cns is extracted by fitting the equivalent circuit (Figure 4e). It represents contribution from the trapped charges at the surface.43,44 Onset potential resides at near half of surface states that are occupied, suggesting a shift in DOS maxima.39,44 Fe-Ti-O distributed in the near-surface region, with four-valence Ti, can affect energetic distribution of the trap states. In addition to innerbandgap surface states, surface disorder layer37 or microscopic cracks are capable of deleterious recombination which bars charge carriers from further contribution. A lower DOSss maxima indicates less surface defects and disorder layers on the Ti:Fe2O3 surface (which will be discussed in the following paragraph) and a decrease in transient photocurrent of Ti:Fe2O3 is evident. Thus, the anodic shift of onset potential correlates to flat-band potential as well as probable interference in energetic distribution of the trap states. In fact, experimental results of doped hematite are not always consistent in the literature, perhaps due to different preparation methods, different doping concentration, dopant distribution, microstructures, etc. The onset potential of Ti-doped Fe2O3, prepared using hydrothermal method deposition
38
45
and using paused-laser
, also observed an anodic shift. Most of co-catalyst systems reduce the onset
potential with reduced thermodynamic barrier and less surface states.47,48 However, anodic/cathodic shift of onset potential cannot be wholly ascribed to surface states. In phenomenological observation, hematite-involved tandem system with non-planar structure also exhibits shift of onset potential.49,50 Further, O2 reduction resulting from electron injection would severely reduce photoelectrochemical performance as well.51 We measured the light/dark current density of pristine and Ti-doped hematite electrodes with bubbled air (Figure 5). In both conditions, distinguishable reduction in cathodic current is observed for Ti:Fe2O3. This attests suppression of O2 reduction by electron-injection from conduction band (back reaction) after Tidoping.
ACS Paragon Plus Environment
17
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Furthermore, PEIS was utilized to probe the charge transfer pathway and understand the rate-limiting step of water oxidation as well. The frequency of the sinusoidal signal applied in this experiment was varied from 0.1 to 100k Hz at a bias voltage of 1.2 VRHE to simulate the water oxidation reaction. Dual-arc feature appears in pristine hematite under illumination (Figure 4e). The equivalent model used for analysis is a good approximation of surface-state dominated charge transfer during water oxidation reaction.39 The elements used for constructing the equivalent circuit for p-Fe2O3 are series resistance (Rs) composed of the resistances from FTO and the external wiring contacts, Rct,bulk denoting the bulk semiconductor character for charge transfer, Rct,ns accounting for charge transfer within space-charge layer and near-surface imperfection, Cbulk as space charge capacitance of the bulk hematite, and Cns representing capacity of near-surface states. Near-surface parameters represent roughness and surfaceimperfection of p-Fe2O3 that interfere with the electrochemical-physical behavior of semiconductor-liquid-interface charge transfer. This can be the consequence of (1) nature of surface disorder layer of Fe2O3,37 (2) –OH formation in humid air or water,6 and/or (3) distribution of surface defects due to quick ramping rate (~ 20 °C /min) at second-stage of heat treatment. During hydrothermal reaction and subsequent heat treatment, Ti-containing nuclei probably mediate hematite crystal growth and inhibit formation of surface disorder layer. In Figure 4e the second-arc (low frequency region presents slow recombination at surface compared to that in the bulk) is clearly erased which corroborates domination of near-surface electrochemical behavior of Ti-doped sample. Slightly increased Cbulk implies that more holes could be trapped in the intermediate states to avoid quick recombination with electrons and facilitate their transportation. Besides, Rct,ns of Ti:Fe2O3 is much smaller compared to that of pFe2O3. This further confirms the positive effect of Ti-incorporation in the bulk and the near-
ACS Paragon Plus Environment
18
Page 18 of 40
Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
surface region, with better charge-transfer and surface-modification (Table 2). Increased Cns value indicates that more trapped holes at intermediate state participate in oxidation reaction.26 To ensure appropriate analysis of the electrochemical behavior of the electrode, Bode phase plot was introduced (Figure 4f). In low-frequency region (1-200 Hz), a plateau attributable to relatively slower transition in surface region was eliminated upon doping with Ti4+. Here, PEIS of both pristine and doped samples were fitted using the same model, as shown in Figure 4e. The slight mismatch between the data and fitted curve, in particular for Ti:Fe2O3, indicates the deviation from the behavior of ideal capacitor, perhaps due to the inhomogeneous composition, non-uniform reaction rate, etc. All the analyses regarding Ti:Fe2O3 mentioned above are a part of a broader investigation. In fact, we applied Mn and In as dopant for additional aliovalent-ion study. However, performances of both Mn:Fe2O3 and In:Fe2O3 are not comparable with that of Ti:Fe2O3 (Figure 6). The drawbacks of In:Fe2O3 can be attributed to mismatch in crystal structures between In2O3 (Octahedral) and α-Fe2O3 (trigonal), and also ionic radius (In3+ 0.8 Å and Fe3+ 0.645 Å in αFe2O3).30 For Mn:Fe2O3, oxidization of Mn2+ to Mn4+ during heat treatment is suggested by Chiam et al.16 Consequently, relatively small ionic radius (0.53 Å) of Mn4+ hinders ideal substitution devoid of excess crystal stress. Plus, incongruous three-coordinate oxide and octahedral metal centers may result in adverse influence. It is evident that additional free electron is not the key criterion for effective doping. A comparison among various dopants and iron is shown in Table 3. It is essential to understand and engineer dopants in transition-metal oxide. Formation of favorable oxide phase (MOx, where M denotes doping element), similarity of crystal structure between MOx and matrix, ionic radius and electronegativity of dopant, most stable valence of dopant, and dopant concentration are crucial for further study.
ACS Paragon Plus Environment
19
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
As reported above, among all factors, it is hard to distinguish the predominating one as present study does not exclusively focus on one of these elements. In nanoparticles system, the particle-and-space-charge-layer relation affects the performance a lot, while some reports conclude that increase of donor density is the deciding factor to enhance IPCE and photocurrent.42 However, they do not show results of optical measurement. Cho et al. reported simultaneous reduction of bulk, interface, and surface recombination of hematite photoanode by flame engineering.37 A titania-iron oxide system proposed by Monllor-Satoca et al. summarizes the interplay of the different factors involved in the photoactivity improvement.46 Among the factors identified in the context, our current findings imply a combined effect.
CONCLUSION In this combinatorial study, Ti-doping through facile hydrothermal process was carried out. Ti atoms were found to distribute preferentially near the surface of Fe2O3 matrix. Contribution of Ti-doping was clarified: (1) tune symmetry of Fe-O and Fe-Fe (Fe-Ti) with less recombination during charge transportation, (2) tune electron configuration of O2 - Fe4s4p hybridization in Ti-adjoining regime with enhanced electron relaxation within Fe2O3 lattice, (3) suppress O2/H2O back reaction (reduction of O2), (4) scatter light at short wavelength of light within the crystal and enhance in-crystal light absorption, and (5) inhibit formation of surface defects during hydrothermal synthesis. As applied bias increases (> 0.92 VRHE), aforementioned advantages expedite PEC performance of Ti:Fe2O3 over p-Fe2O3. It is to be noted that, from practical point of view, the anodic shift of photocurrent onset potential may degrade the effectiveness of the PEC cell owing to the demand for large bias. Reduction of onset potential can be achieved by further detailed engineering.
ACS Paragon Plus Environment
20
Page 20 of 40
Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Conductivity of Fe2O3 was improved slightly through Ti doping, as revealed from M-S plot. However, the enhancement of photoelectrochemical activity is thought to be associated with the decrease in charge transfer resistance across the semiconductor-electrolyte interface, derived from PEIS analysis. A hydrogen evolution rate of 11.76 µmol h-1 cm-2 under illumination (100 mW cm-2) at 1.23 vs RHE was noted upon using Ti:Fe2O3 as the working electrode. Preliminary investigations on Mn- and In-doped α-Fe2O3, however, showed negative results. Affinity of ionic radius, valence difference, electron negativity, and crystal structure of metal oxide of the dopant shall be taken into account before deploying further experiments. It is apparent that different preparation methods may lead to different results, even for identical ions.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Reaction Mechanism of hydrothermal synthesis in presence of urea, Ti 2p peak positions of various samples evaluated from XPS spectra, surface composition and atomic ratio of dopant for Ti:Fe2O3, image showing fresh and hydrolyzed TiO2 precursor solution, digital photo of deposits, SEM images of pristine hematite synthesized by changing the duration of hydrothermal treatment, top view SEM images and XRD pattern of p-Fe2O3 and Ti:Fe2O3, XPS survey scan spectra of pristine and doped Fe2O3, EDS spectrum of Ti:Fe2O3 with inset table denoting composition of each detected element, light absorption behavior and IPCE (APCE) of pristine and doped Fe2O3, J-V diagram for samples prepared with various ratios of Ti/Fe of precursor solution, and J-V curves for p-Fe2O3 synthesized by varying the hydrothermal reaction time. (PDF)
ACS Paragon Plus Environment
21
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected] *E-mail:
[email protected]
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
ACKNOWLEDGEMENT We appreciate the financial support from the Ministry of Science and Technology of Taiwan. We are also grateful to NSRRC of Taiwan and the Valuable Instrument Center, National Central University, Taiwan for XAS, XPS, XRD, and SEM analyses.
REFERENCES (1) Leland, J. K.; Bard, A. J. Photochemistry of Colloidal Semiconducting Iron Oxide Polymorphs. J. Phys. Chem. 1987, 91, 5076-5083. (2) Prévot, M. S.; Sivula, K. Photoelectrochemical Tandem Cells for Solar Water Splitting. J. Phys. Chem. C 2013, 117, 17879-17893. (3) Lindgren, T.; Vayssieres, L.; Wang, H.; Lindquist, S. E. In Chemical Physics of Nanostructured Semiconductors; Kokorin, A. I., Bahnemann, D. W., Eds.; Taylor & Francis, 2003; Chapter 3, pp 83-110. (4) Bohn, C. D.; Agrawal, A. K.; Walter, E. C.; Vaudin, M. D.; Herzing, A. A.; Haney, P. M.; Talin, A. A.; Szalai, V. A. Effect of Tin Doping on α-Fe2O3 Photoanodes for Water Splitting,
ACS Paragon Plus Environment
22
Page 22 of 40
Page 23 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
J. Phys. Chem. C 2012, 116, 15290-15296. (5) Joly, A. G.; Williams, J. R.; Chambers, S. A.; Xiong, G.; Hess, W. P.; Laman, D. M. Carrier Dynamics in α-Fe2O3 (0001) Thin Films and Single Crystals Probed by Femtosecond Transient Absorption and Reflectivity. J. Appl. Phys. 2006, 99, 053521. (6) van de Krol, R.; Principles of Photoelectrochemical Cells. In Photoelectrochemical Hydrogen Production, van de Krol, R., Grätzel, M., Eds.; Vol. 90; Springer, 2012; Chapter 2, pp 13-68. (7) Lu Y. H.; Lin W. H.; Yang C. H.; Chiu Y. H.; Pu Y. C.; Lee M. H.; Tseng Y. H.; Hsu Y. J.; A Facile Green Antisolvent Approach to Cu2+-doped ZnO Nanocrystals with Visible-LightResponsive Photoactivities. Nanoscale, 2014, 6, 8796-8803. (8) Chiu Y. H.; Lai T. H.; Chen C. Y.; Hsieh P. Y.; Ozana K.; Niinomi M.; Okada K.; Chang T. F. M.; Matsushita N.; Sone M.; Hsu Y. J.; Fully-Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production. ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b00727. (9) Souza, F. L.; Lopes, K. P.; Nascente, P. A.; Leite, E. R. Nanostructured Hematite Thin Films Produced by Spin-coating Deposition Solution: Application in Water Splitting. Sol. Energy Mater. Sol. Cells 2009, 93, 362-368. (10) Cao, D.; Luo, W.; L,i M.; Feng, J.; Li, Z.; Zou, Z. A Transparent Ti4+ Doped Hematite Photoanode Protectively Grown by a Facile Hydrothermal Method. CrystEngComm. 2013, 15, 2386-2391. (11) Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Sn-doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 2119-2125. (12) Miao, C.; Ji, S.; Xu, G.; Liu, G.; Zhang, L.; Ye, C. Micro-Nano-Structured Fe2O3:Ti/ZnFe2O4 Heterojunction Films for Water Oxidation. ACS Appl. Mater. Interfaces ACS Paragon Plus Environment
23
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
2012, 4, 4428-4433. (13) Ida, S.; Yamada, K.; Matsunaga, T.; Hagiwara, H.; Matsumoto, Y.; Ishihara, T. Preparation of p-Type CaFe2O4 Photocathodes for Producing Hydrogen from Water. J. Am. Chem. Soc. 2010, 132, 17343-17345. (14) Deng, J.; Zhong, J.; Pu, A.; Zhang, D.; Li, M.; Sun, X.; Lee, S.-T. Ti-doped Hematite Nanostructures for Solar Water Splitting with High Efficiency. J. Appl. Phys. 2012, 112, 084312. (15) Shen, S.; Kronawitter, C. X.; Wheeler, D. A.; Guo, P.; Lindley, S. A.; Jiang, J.; Zhang, J. Z.; Guo, L.; Mao, S. S. Physical and Photoelectrochemical Characterization of Ti-doped Hematite Photoanodes Prepared by Solution Growth. J. Mater. Chem. A 2013, 1, 1449814506. (16) Chiam, S. Y.; Kumar, M. H.; Bassi, P. S.; Seng, H. L.; Barber, J.; Wong, L. H. Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese. ACS Appl. Mater. Interfaces 2014, 6, 5852-5859. (17) Sartoretti, C. J.; Alexander, B. D.; Solarska, R.; Rutkowska, I. A.; Augustynski, J.; Cerny, R. Photoelectrochemical Oxidation of Water at Transparent Ferric Oxide Film Electrodes. J. Phys. Chem. B 2005, 109, 13685-13692. (18) Yang, T.-Y.; Kang, H.-Y.; Jin, K.; Park, S.; Lee, J.-H.; Sim, U.; Jeong, H.-Y.; Joo, Y.-C.; Nam, K. T. An Iron Oxide Photoanode with Hierarchical Nanostructure for Efficient Water Oxidation. J. Mater. Chem. A 2014, 2, 2297-2305. (19) Pu, A.; Deng, J.; Li, M.; Gao, J.; Zhang, H.; Hao, Y.; Zhong, J.; Sun, X. Coupling Ti-Doping and Oxygen Vacancies in Hematite Nanostructures for Solar Water Oxidation with High Efficiency. J. Mater. Chem. A 2014, 2, 2491-2497. (20) Zandi, O.; Klahr, B. M.; Hamann, T. W.; Highly Photoactive Ti-doped α-Fe2O3 Thin Film
ACS Paragon Plus Environment
24
Page 24 of 40
Page 25 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Electrodes: Resurrection of the Dead Layer. Energy Environ. Sci. 2013, 6, 634-642. (21) Kim, D. W.; Riha, S. C.; DeMarco, E. J.; Martinson, A. B.; Farha, O. K.; Hupp, J. T. Greenlighting Photoelectrochemical Oxidation of Water by Iron Oxide. ACS Nano 2014, 8, 12199-12207. (22) Vayssieres, L.; Beermann, N.; Lindquist, S.-E.; Hagfeldt, A.; Controlled Aqueous Chemical Growth of Oriented Three-Dimensional Crystalline Nanorod Arrays: Application to Iron (III) Oxides. Chem. Mater. 2001, 13, 233-235. (23) Kahlert H.; In Electroanalytical Method, 2nd ed; Scholz F., Eds.; Springer, 2010; Chapter 3.2, pp 291-308. (24) Morrish, R.; Rahman, M.; MacElroy, J.; Wolden, C. A. Activation of Hematite Nanorod Arrays for Photoelectrochemical Water Splitting. ChemSusChem 2011, 4, 474-479. (25) Glasscock, J. A.; Barnes, P. R.; Plumb, I. C.; Savvides, N. Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. J. Phys. Chem. C 2007, 111, 16477-16488. (26) Shinde, P. S.; Choi, S. H.; Kim, Y.; Ryu, J.; Jang, J. S. Onset Potential Behavior in α-Fe2O3 Photoanodes: the Influence of Surface and Diffusion Sn Doping on the Surface States. Phys. Chem. Chem. Phys. 2016, 18, 2495-2509. (27) Lin, C.-P; Chen, H.; Nakaruk, A.; Koshy, P.; Sorrell, C. Effect of Annealing Temperature on the Photocatalytic Activity of TiO2 Thin Films. Energy Procedia 2013, 34, 627-636. (28) Courtin, E.; Baldinozzi, Sougrati, M. T.; Stievano, L.; Sanchez, C.; Laberty-Robert. C. New Fe2TiO5-Based Nanoheterostructured Mesoporous Photoanodes with Improved Visible Light Photoresponses. J. Mater. Chem. A, 2014, 2, 6567-6577. (29) Gambhire, B.; Lande, M. K.; Rathod, S. B.; Arbad, B. R.; Vidhate, K. N.; Gholap, R. S.; Patil, K. R. Synthesis and Characterization of FeTiO3 Ceramics. Arab J. Chem. 2016, 9,
ACS Paragon Plus Environment
25
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
S429-S432. (30) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Cryst. 1976, A32, 751-767. (31) Gloter, A.; Ingrin, J.; Bouchet, D.; Colliex, C. Composition and Orientation Dependence of the O K and Fe L2,3 EELS Fine Structures in Ca2(AlxFe1-x)2O5. Phys. Rev. B Condens. Matter 2000, 61, 2587-2594. (32) Fu, Y.; Dong, C. L.; Lee, W. Y.; Chen, J.; Guo, P.; Zhao, L.; Shen, S. Nb-Doped Hematite Nanorods for Efficient Solar Water Splitting: Electronic Structure Evolution versus Morphology Alteration. ChemNanoMat. 2016, 2, 704-711. (33) Wu, Z.; Gota, S.; Jollet, F.; Pollak, M.; Gautier-Soyer, M.; Natoli, C. Characterization of Iron Oxides by X-ray Absorption at the Oxygen K Edge Using a Full Multiple-Scattering Approach. Phys. Rev. B Condens. Matter 1997, 55, 2570-2577. (34) De Groot, F. M. F.; Grioni, M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H. Oxygen 1s X-ray-absorption Edges of Transition-metal Oxides. Phys. Rev. B Condens. Matter 1989, 40, 5715-5723. (35) Martorell, J.; Vilaseca, R.; Corbalán, R. Scattering of Second-Harmonic Light from Small Spherical Particles Ordered in a Crystalline Lattice. Phys. Rev. A 1997, 55, 4520-4525. (36) Thomas, P.; Pannala S.; Abraham K. E. Nanosecond and Ultrafast Optical Power Limiting in Luminescent Fe2O3 Hexagonal Nanomorphotype.. J. Appl. Phys. 2015, 117, 053103. (37) Cho, I. S.; Han, H. S.; Logar, M.; Park, J.; Zheng, X. Enhancing Low-Bias Performance of Hematite Photoanodes for Solar Water Splitting by Simultaneous Reduction of Bulk, Interface, and Surface Recombination Pathways. Adv. Energy Mater. 2015, 6, 1501840. (38) Malviya, K. D.; Dotan, H.; Shlenkevich, D.; Tsyganok, A.; Mor, H.; Rothschild, A. Systematic Comparison of Different Dopants in Thin Film Hematite (α-Fe2O3) Photoanodes ACS Paragon Plus Environment
26
Page 26 of 40
Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
for Solar Water Splitting. J. Mater. Chem. A 2016, 4, 3091-3099. (39) Steier, L.; Herraiz-Cardona, I.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Tilley, S. D.; Grätzel, M. Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes. Adv. Funct. Mater. 2014, 24, 7681-7688. (40) Glasscock, J.; Barnes, P.; Plumb, I.; Bendavid, A.; Martin, P. Structural, Optical and Electrical Properties of Undoped Polycrystalline Hematite Thin Films Produced Using Filtered Arc Deposition. Thin Solid Films 2008, 516, 1716-1724. (41) Sivula, K.; Formal, F. L.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (αFe2O3) Photoelectrodes. ChemSusChem. 2011, 4, 432-449. (42) Wang, G.; Ling, Y.; Wheeler, D. A.; George, K. E.; Horsley, K.; Heske, C.; Zhang, J. Z.; Li, Y. Facile Synthesis of Highly Photoactive α-Fe2O3-Based Films for Water Oxidation. Nano Lett. 2011, 11, 3503-3509. (43) Wang, D.; Chen, H.; Chang, G.; Lin, X.; Zhang, Y.; Aldalbahi, A.; Peng, C.; Wang, J.; Fan, C. Uniform Doping of Titanium in Hematite Nanorods for Efficient Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2015, 7, 14072-14078. (44) Brug, G. J.; van den Eeden, A. L. G.; Sluyters-Rehbach, M.; Sluyters, J. H. The Analysis of Electrode Impedances Complicated by the Presence of a Constant Phase Element. J. Electroanal. Chem. Interfacial Electrochem. 1984, 176, 275-295. (45) Yuan Y.; Gu J.; Ye K. H.; Chai Z.; Yu X.; Zhao C.; Zhang Y.; Mai W.; ACS Appl. Mater. Interfaces 2016, 8, 16071−16077. (46) Monllor-Satoca, D.; Bärtsch, M.; Fàbrega, C.; Genç, A.; Reinhard, S.; Andreu, T.; Arbiol, J.; Niederberger, M.; Morante, J. R. What Do You Do, Titanium? Insight into The Role of Titanium Oxide as a Water Oxidation Promoter in Hematite-Based Photoanodes. Energy Environ. Sci. 2015, 8, 3242-3254. ACS Paragon Plus Environment
27
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(47) Zhong, D. K.; Gamelin, D. R. Photoelectrochemical Water Oxidation by Cobalt Catalyst (“Co-Pi”)/Α-Fe2O3 Composite Photoanodes: Oxygen Evolution and Resolution of a Kinetic Bottleneck. J. Am. Chem. Soc. 2010, 132, 4202-4207. (48) Yu, Q.; Meng, X.; Wang, T.; Li, P.; Ye, J. Hematite Films Decorated with Nanostructured Ferric Oxyhydroxide as Photoanodes for Efficient and Stable Photoelectrochemical Water Splitting. Adv. Funct. Mater. 2015, 25, 2686-2692. (49) Lin, Y.; Zhou, S.; Sheehan, S. W.; Wang, D. Nanonet-Based Hematite Heteronanostructures for Efficient Solar Water Splitting. J. Am. Chem. Soc. 2011, 133, 2398-2401. (50) Mayer, M. T.; Du, C.; Wang, D. Hematite/Si Nanowire Dual-Absorber System for Photoelectrochemical Water Splitting at Low Applied Potentials. J. Am. Chem. Soc. 2012, 134, 12406-12409. (51) Cao, D.; Luo, W.; Feng, J.; Zhao, X.; Li, Z.; Zou, Z. Cathodic Shift of Onset Potential for Water Oxidation on a Ti4+ Doped Fe2O3 Photoanode by Suppressing the Back Reaction. Energy Environ. Sci. 2014, 7, 752-759.
ACS Paragon Plus Environment
28
Page 28 of 40
Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Table 1. Normalized peak intensity in O K-edge spectra. (Peak D is set as zero) t2g
eg
B
D
α-Fe2O3
1.1
1.1
1.6
0
Ti:Fe2O3
0.8
0.9
1.4
0
ACS Paragon Plus Environment
29
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 30 of 40
Table 2. EIS parameters obtained from the equivalent circuits of the pristine and Ti-doped hematite film Rs
Rtrap
Cbulk
Rct,ns
Cns
(Ω cm2)
(Ω cm2)
(µF)
(Ω cm2)
(µF)
Fe2O3
26.7
838
3.45
912
56.9
Ti:Fe2O3
33.2
796
3.57
230
106
ACS Paragon Plus Environment
30
Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Table 3. Comparison of the three dopant ions with Fe3+ in α-Fe2O3
Primary phase in
α-Fe2O3
scope of the study
(trigonal, corundum
(crystal structure)
structure)
Favored valence
3+
TiO2
MnO2
In2O3
(anatase or rutile)
(rutile for β-MnO2)
(Octahedral)
4+
4+
3+
0.605 Å
0.530 Å
~ 0.800 Å
1.83
1.54
1.55
1.73
(3d)
(3d)
(3d)
(5p)
Yes
Yes
Yes*
0.645 Å Ionic radius (High spin, Fe2O3)
Electronegativity
Experiences decomposition (diffusion) during
--(Matrix)
heating process?
*Well-crystallized In2O3 is not likely to distort or decompose within heat-treatment condition of this work. However, considering the characteristic of precursor (InCl3) under hydrothermal condition, crystalized In2O3 is hardly formed before being embedded in FeOOH (consequently in hematite).
ACS Paragon Plus Environment
31
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Scheme 1. Schematic illustration of synthesis process and growth mechanism.
ACS Paragon Plus Environment
32
Page 32 of 40
Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Figure 1. Cross-section SEM images of (a) p-Fe2O3 and (b) Ti:Fe2O3. (c) Fe 2p spectra of undoped (black) and Ti-doped (red) samples. (d) Ti 2p spectra of anatase TiO2 (green), brooktie TiO2 (orange), rutile TiO2 (blue), and Ti:Fe2O3 (red).
ACS Paragon Plus Environment
33
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 2. XAS Spectra at (a) Fe K-edge (XANES), (b) Fe K-edge (EXAFS), (c) Fe L-edge, and (d) O K-edge for pristine and doped samples.
ACS Paragon Plus Environment
34
Page 34 of 40
Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Figure 3. PL emission spectra measured for p-Fe2O3 (black) and Ti:Fe2O3 (red) using 532 nm laser. Inset presents enlarged figure of peak maxima.
ACS Paragon Plus Environment
35
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 4. (a) DOS extracted from Cns (depicted by dots) and measured photocurrent density (depicted by lines) versus applied potential for pristine (black) and Ti-doped (red) samples. (b) Chopping test of pristine (black) and Ti-doped (red) hematite electrodes. (c) Stability test of pristine hematite (black) and Ti-doped one (red). Inset exhibits measured hydrogen evolution of Ti:Fe2O3 (filled marker) and theoretical values calculated by observed current (unfilled marker). The entire experiment was carried out at 1.2 VRHE in 1.0 NaOH (aq.) under illumination (100 mA cm-2). (d) M-S plots of pristine (black) and Ti-doped (red) sample. (e) Nyquist plots (real vs. ACS Paragon Plus Environment
36
Page 36 of 40
Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
imaginary impedance) and equivalent circuits for simulation photoelectrochemical mechanism in undoped/doped electrode, and (f) Bode phase plots of pristine and Ti-doped hematite.
ACS Paragon Plus Environment
37
ACS Applied Nano Materials
0.06 2
Current density (mA/cm )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 38 of 40
0.04 0.02
Ti:Fe2O3 (I)
Ti:Fe2O3 (D)
0.00 -0.02 -0.04 -0.06 0.45
Fe2O3 (I) Fe2O3 (D)
0.50 0.55 0.60 Potential (V vs. RHE)
0.65
Figure 5. Reduction dark/photo-current of pristine and Ti-doped Fe2O3 electrodes. The experiment was carried out with air bubbling to induce back reaction.
ACS Paragon Plus Environment
38
1.4
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Nano Materials
Current Density (mA/cm )
Page 39 of 40
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Potential Applied (V vs. RHE) Figure 6. Current density-voltage (J-V) curve of p-Fe2O3 (black), Ti-doped (red), Mn-doped (green), and In-doped hematite (dark blue) in 1.0 M NaOH (aq.) under illumination.
ACS Paragon Plus Environment
39
ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Table of Contents Graphic
“Ti-doped” nanostructures generated through post-sintering with facile hydrothermal synthesis are identified primarily improving crystal symmetry and hence charge-transfer, and thus inhibiting formation of surface defects compared to pristine hematite.
ACS Paragon Plus Environment
40
Page 40 of 40
