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Photoinduced Superparamagnetism in Nanostructured R-Fe2O3

I. Ross Macdonald,† Russell F. Howe,*,† Sina Saremi-Yarahmadi,‡ and K. G. U. Wijayantha‡ †

Chemistry Department, University of Aberdeen, AB24 3UE, U.K., and ‡Departments of Materials and Chemistry, Loughborough University, LE11 3TU, U.K.

ABSTRACT This Letter reports a remarkable influence of light on the magnetic properties of nanocrystalline R-Fe2O3. EPR spectroscopy shows that the R-Fe2O3 is superparamagnetic, showing an intense narrow EPR signal at room temperature which shifts to a lower field and broadens to be barely detectable at 77 K or below. In situ irradiation of samples held in vacuo at 4 or 77 K with near-UV or visible light causes the appearance of an intense new EPR signal, which decays immediately after the irradiation is stopped. We suggest that the generation of conduction band electrons by irradiation into the band gap of the R-Fe2O3 moderates the coupling between the magnetic domains in these nanocrystals.This effect is largely suppressed in the presence of adsorbed oxygen, which can scavenge conduction band electrons. SECTION Nanoparticles and Nanostructures

atalysis under light irradiation (photocatalysis) with oxide semiconductors has attracted intensive scientific research activities in the last few decades.1 A particularly promising application of such materials is the production of hydrogen from water using solar radiation, either through direct photocatalysis or via photoelectrochemistry.2,3 R-Fe2O3 (hematite) is a metal oxide semiconductor which could be used either as a sensitizer or a photocatalyst in its own right. It has a band gap of ∼2.1 eV, which means that much of the solar spectrum can be absorbed. It has good chemical and photoelectrochemical stability, low cost, and ease of fabrication. However, when used as a photoanode in photoelectrochemical cells, hematite shows relatively poor solar to chemical conversion efficiency. This is attributed to the relatively slow chargetransfer kinetics and the high rates of hole/electron recombination as a result of small hole diffusion length.4 Improvements in performance have been achieved by modifying the morphology of the hematite to reduce the surface state density,5 modifying the nanostructure,6 or doping with elements such as silicon.7 The photocatalytic properties of hematite are strongly affected by size, morphology, and surface area of iron oxide films and particles. For example, it has been suggested that the conduction band edge of hematite shifts to -0.62 V versus SCE at pH 12 for the colloidal particles smaller than 200 nm.8 This shift may facilitate the formation of OH• free radicals while inhibiting electron-hole recombination, resulting in facile photocatalytic oxidation of organic compounds. The photoactivity of hematite films has been found recently by Sivula et al. to depend strongly on the annealing temperature.9 These variations were shown however to be largely due to incorporation of dopant ions from the conducting tin oxide substrate. Despite this interest, there is currently little information

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available about the nature of trap sites and the dynamics of trapping versus recombination in hematite. In the case of titania photocatalysts, in situ EPR spectroscopy is a powerful technique for observing trapped electrons and trapped holes formed following band gap irradiation. At low temperatures (77 or 4 K), trapped electrons are identified as Ti3þ cations and trapped holes as O- radical anions,10 and the rates of formation and decay of these species have been reported.11 In the work presented here, we attempted to apply the same approach to a nanocrystalline hematite material and discovered the unusual photoinduced superparamagnetism described in this Letter. Our nanocrystalline hematite powder was prepared by the CVD method described in ref 6. XRD and TEM (Supporting Information) showed that the material comprises phase-pure crystals of hematite, mostly in the 5-7 nm size range. The UV-vis spectrum (Supporting Information) does not show a well-defined band gap. The reported band gap of 2.1 ev for bulk hematite corresponds to a wavelength of 560 nm, but in the nanocrystalline material, there is an extensive tailing into the near-infrared region of the spectrum. X-ray absorption spectroscopic studies on a similar material by Thurnauer et al. have shown that the surface Fe3þ cations have lower coordination numbers than the octahedral coordination characteristic of bulk hematite, and they estimate that up to 50% of the Fe3þ may be so distorted in 3 nm nanocrystallites.12 The tailing of the band gap absorption edge into the near-infrared may thus be attributed to the presence of surface states

Received Date: May 18, 2010 Accepted Date: July 27, 2010 Published on Web Date: August 04, 2010

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DOI: 10.1021/jz100650w |J. Phys. Chem. Lett. 2010, 1, 2488–2492

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Figure 2. EPR spectra recorded at 4 K during irradiation of R-Fe2O 3 in vacuo with 320-900 (narrowest signal), 500, or 700 nm light (broadest signal). The dashed trace shows the spectrum of the same sample in the dark measured at 295 K (the intensity of this signal has been adjusted to match the 4 K spectra; the 4 K spectra were all recorded with the same signal gain).

ture below which the anisotropic interactions dominate and superparamagnetism is lost. For nanoparticles of maghemite (γ-Fe2O3) embedded in a silicate glass, Berger et al.18 determined the blocking temperature to be the temperature where the doubly integrated signal intensity reached a maximum, in their case, about 90 K. A much lower blocking temperature is seen in static magnetization measurements since these relate to relaxation times of seconds rather than the subnanosecond scale of EPR.17 For the data shown in Figure 1, we estimate the blocking temperature to be below 130 K, but an accurate value cannot be obtained because of the inaccuracies in integrating such broad signals. Superparamagnetism has also been seen in naturally occurring hematite by both EPR and static magnetization measurements, although in this case, the extreme heterogeneity of particle sizes makes interpretation of the data more difficult.19 Irradiation of the hematite sample in vacuo at 4 K with broad-band (320-900 nm) radiation caused the appearance of an intense EPR signal at g = 2.00 with a line width of 30 mT. As shown in Figure 2, this signal is significantly narrower than that measured from the same sample in the dark at 295 K. The signal decayed immediately when the irradiation was stopped. The growth and decay of the signal upon irradiation and in the dark could be reproduced many times at 4 K. Figure 3 shows typical kinetic data obtained by monitoring the time dependence of the EPR signal intensity at a fixed magnetic field (corresponding to the negative maximum in the first derivative signal). The intensity and line width of the new signal appearing upon irradiation at 4 K depended on the wavelength range of the radiation employed. Irradiation with the 500 nm filter in place gave a signal with line width comparable to that measured at room temperature in the dark (the total intensity transmitted through this filter was 31% of that of the 320900 nm radiation), while the 700 nm filter gave a weaker and broader signal (intensity 19% of the broad-band radiation). The kinetics of growth and disappearance of the new signal were similar with broad-band and selected wavelength irradiation, with one notable difference. As seen in Figure 3,

Figure 1. EPR spectra of R-Fe2O3 in vacuo recorded in the dark at temperatures between 295 (most intense) and 75 K (least intense), in approx 20 K steps.

associated with lower coordination Fe3þ within the band gap of the hematite. Details of the in situ EPR experiment are given in the Supporting Information. Figure 1 shows EPR spectra of the hematite in vacuo in the dark following room-temperature outgassing to 10-5 mbar measured at temperatures between 295 and 75 K (at approximately 20 K intervals). The intense signal seen at room temperature broadens and shifts to lower field as the temperature is lowered. These effects are completely reversible. At temperatures below 75 K (not shown), the signal becomes even less distinct and can scarcely be detected at 4 K. The temperature-dependent EPR spectra shown in Figure 1 are quite similar to those reported by Zysler et al. for ultrafine R-Fe2O3 grains (d ≈ 3 nm) dispersed in an alumina matrix, although the lowest temperature measured in that case was 120 K.13 As pointed out by these authors, such temperature dependence is characteristic of superparamagnetic behavior. Crystalline R-Fe2O3 is antiferromagnetic. Hematite has the rhombohedral corundum structure containing a single octahedral Fe3þ site, and the spins are coupled antiferromagnetically.14 The magnetic behavior of nanocrystalline R-Fe2O3 is the result of spin canting away from the antiferromagnetic axis, which dominates in nanocrystals with a large surface to volume ratio.15 Ferromagnetism then results. At low temperatures, the anisotropic interactions between coupled ferromagnetic domains in the nanocrystals or between nanocrystals result in an extremely broad ferromagnetic resonance signal, which can scarcely be detected. At high temperatures, thermal fluctuations remove any alignment between the magnetic moments associated with individual magnetic domains, and an intense narrow signal is seen, which is described as superparamagnetic resonance. A theoretical explanation of this phenomenon has been given in several papers by Berger et al.16-18 The so-called blocking temperature is the tempera-


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Figure 4. Comparison of EPR spectra obtained when R-Fe2O3 is irradiated at 77 K with light of the wavelength indicated in vacuo (dotted trace) or in oxygen (50 mbar).

electrons into the conduction band upon absorption of light will populate previously vacant Fe 3d orbitals, effectively reducing Fe3þ to Fe2þ and thereby reducing the extent of coupling between ferromagnetic domains. This has a similar effect on the EPR spectra to raising the temperature (although the line shapes obtained are by no means identical). If the promoted electrons originate from the top of the valence band, the resulting positive holes may be written as Fe4þ cations. The overall process may thus be represented as

Figure 3. Time dependence of the appearance and disappearance of signal intensity at 338 mT when R-Fe2O3 in vacuo is exposed to light of the wavelengths indicated at 4 K.

when broad-band irradiation was stopped, there was an initial transient (less than 1 s) further increase in signal intensity before it decayed. This transient increase was reproducible in successive exposures, but the time scale was too short to allow a field scan of the spectrum during its occurrence. The effect was not seen when selective wavelength excitation was employed. Similar experiments were performed at 75 K (not shown). The rates of appearance and decay of the EPR signals at this temperature were not noticeably different from those at 4 K, although no transient increase in intensity was seen when broad-band irradiation was stopped at 75 K. Quantification of the signal intensity is not possible since not all parts of the powder sample within the EPR cavity can be exposed uniformly to the radiation (and shadowing effects may mean that parts of the sample receive no irradiation at all). The possibility of direct heating from the light source inducing superparamagnetism in the hematite can be dismissed. The infrared and far-UV components of the light source have been removed by filters. Comparison of the line widths obtained upon broad-band irradiation at 4 or 75 K with those measured between 75 and 298 K in the dark would suggest that the apparent sample temperatures are raised to greater than 350 K in less than 1 s. Furthermore, the line width of the signal obtained upon broad-band irradiation at 4 K is even narrower than that obtained upon irradiation at 75 K. In both experiments, the sample is held in a flow of liquid helium or nitrogen, respectively (the cryostat uses a vacuum pump to draw a liquid cryogen flow over the sample). The alternative explanation is that the effects observed are a consequence of electronic excitation of the R-Fe2O3 semiconductor. R-Fe2O3 is an extrinsic semiconductor with an indirect band gap of 2.1 eV.20 The valence band comprises 2p orbitals of O2- and occupied Fe 3d orbitals, and the conduction band corresponds to vacant Fe 3d orbitals. Promotion of


2Fe3þ þ hν f Fe4þ þ Fe2þ This explanation would require the Bohr radius of the hole/ electron exciton to be greater than the iron-iron distance in hematite, but to our knowledge, this value has not been determined for hematite. Alternatively, excitation of electrons from O2- 2p states deeper within the valence band would leave positive holes as O- species, which are paramagnetic.10 Any EPR signal of O- is however completely obscured by the intense superparamagnetic resonance signal appearing upon irradiation. The extent of this photoinduced superparamagnetism is strongly dependent on the steady-state flux of photons with energies greater than or equal to the band gap. Thus, the effect is largest with broad-band or narrow-band radiation of wavelengths below 600 nm. The 700 nm light has a lesser effect due to poor overlap of this energy with the band gap. The fact that the effect is still seen with 700 nm light does however imply that surface states within the band gap are also involved. Support for our suggested explanation comes from the observed effects of adsorbed oxygen. Oxygen is known to be a scavenger of conduction band electrons in oxide semiconductors. Figure 4 compares the EPR spectra obtained upon irradiation in vacuo and in the presence of oxygen with light of different wavelengths. These experiments were conducted at 77 K in order to ensure that physisorbed oxygen on the surfaces of the nanocrystals was freely mobile (which is not the case at 4 K). The removal of conduction band electrons through reaction with adsorbed oxygen appears to largely suppress the superparamagnetic resonance signal appearing upon irradiation with 700 or 400 nm light. The largest effects would be expected for states closest to the surface of the

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AUTHOR INFORMATION

nanocrystals, which are those responsible for longer wavelength light absorption. The effect is less pronounced with broad-band radiation. The larger superparamagnetic signal obtained with broad-band radiation contains a major contribution from bulk states corresponding to the band gap of ∼2.1 eV. In this case, quenching with oxygen removes only the contribution from the surface states. The superoxide ion is the expected primary product of electron trapping by adsorbed oxygen, but its characteristic EPR signal would also be hidden by the superparamagnetic resonance signal. Reference 15 proposes a core/shell model for the superparamagnetism in isolated nanoparticles of hematite, where the magnetic behavior is mainly related to surface effects. (A very recent Mossbauer and magnetic study confirms that bulk and surface iron in nanocrystalline hematite have different magnetic properties.21) This model would also be consistent with our observations on photoinduced superparamagnetism, that is, photoexcitation moderates the coupling between ferromagnetic domains within individual nanocrystals. We cannot however rule out the possibility that the superparamagnetism results from moderation of coupling between adjacent nanocrystals. The size of the ferromagnetic domains cannot be deduced from the EPR studies. The dynamics of conduction band electron creation, trapping, and recombination will be complex in highly defective materials such as nanostructured R-Fe2O3. Under steady-state irradiation, the rate at which the superparamagnetic resonance signal appears will result from a balance between rates of electron promotion into the conduction band, trapping by Fe3þ sites, and recombination. The initial transient increase in signal intensity when broad-band irradiation is first stopped may suggest that the superparamagnetic resonance signal results from trapping of conduction band electrons. When irradiation is stopped, there is a transient increase in trapping of delocalized conduction band electrons prior to subsequent recombination. Further studies of these effects are now in progress in our laboratories, including, for example, the variation of light intensity, state of hydration of the R-Fe2O3 surface, crystal size, and doping with ions which are known to enhance the photocatalytic performance. Our suggestion of the origins of the superparamagnetic resonance signal must also be confirmed by static magnetization measurements. The initial studies reported here show nevertheless that light-induced superparamagnetism in R-Fe2O3 provides a new approach to following the behavior of conduction band electrons in this important photocatalyst. Factors influencing the rates of formation and decay of conduction band electrons, which exert a strong influence on the catalytic properties, may be monitored by observing the light-induced superparamagnetism. The alteration of magnetic properties in nanostructured hematite by exposure to light may also suggest other applications in the area of magnetic materials.

Corresponding Author: *To whom correspondence should be addressed.

ACKNOWLEDGMENT R.F.H. and I.R.M. acknowledge financial support in Aberdeen from EPSRC. S.S.-Y. and K.G.U.W. acknowledge financial support from the Materials Research School at Loughborough University. Collaboration on this project was made possible through the EPSRC UK Photochemistry Network, led by Professor Andrew Mills.

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SUPPORTING INFORMATION AVAILABLE X-ray powder (15)

diffraction pattern, TEM images, and UV-vis reflectance spectrum of hematite; description of experimental procedure for in situ EPR experiments. This material is available free of charge via the Internet at http://pubs.acs.org.


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