1.  Introduction {#sec1}
===============

Hematite has attracted great attention from many researchers because it can work as a photoanode for water splitting that has great potential for solar energy conversion.^[@ref1]−[@ref7]^ Even though it has been regarded as one of the most promising photoanodes for water splitting that can potentially deliver a high solar to hydrogen (STH) efficiency (∼15% theoretical value),^[@ref7]−[@ref9]^ its STH efficiency of ∼1--2% prohibits its implementation.^[@ref8],[@ref10]^ Multiple limiting factors have been proposed so far, such as poor conductivity,^[@ref11]^ short electron--hole lifetime,^[@ref12]^ and poor hole transfer efficiency^[@ref1],[@ref7],[@ref13],[@ref14]^ among others.  Particularly, the limitation on the poor hole transfer efficiency is the main deficiency of hematite because it is believed that the water oxidation reaction would be enhanced by facilitating hole transfer to H~2~O.^[@ref15],[@ref16]^

To overcome the problem, there have been many attempts, such as deposition of co-catalysts and passivation of the surface defects on hematite, which has been regarded as a way to improve the hole transfer efficiency of hematite by reducing the number of surface trap states.^[@ref1],[@ref7],[@ref17]−[@ref22]^ For example, Grätzel and co-workers applied Co^II^ onto the surface of hematite film and found that the overpotential for water oxidation was reduced, providing a better photo-electrochemical (PEC) performance.^[@ref2]^ In addition, Gamelin and his co-workers applied Co-phosphate oxygen evolution reaction catalyst and observed a higher PEC performance than that of the Co^II^ deposition case.^[@ref23]^

Recently, one of our co-authors (Wang) and co-workers investigated hematite films coated with amorphous NiFeO*~x~* and found that it also showed high PEC performance that was attributed to the Fermi level pinning effect.^[@ref18],[@ref22],[@ref24],[@ref25]^ However, this line of research was mainly focused on the investigation of the thermodynamic aspects of the system.  Although intensity modulated photocurrent spectroscopy has been later employed to determine the surface kinetics with and without NiFeO*~x~* at slower time scales,^[@ref18]^ the kinetic information on photoexcited carriers is still missing.  This missing piece may be provided by our recent success in studying carrier dynamics with a time-resolved technique, the heterodyne transient grating (HD-TG).  The principle of the technique is described in [Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf) in the Supporting Information.  For instance, using this technique, we recently observed the carrier dynamics in a titanium oxide film, clarified the electron and hole dynamics, and distinguished the reactive and nonreactive carriers at the solid/liquid interface.^[@ref26]^ As such, our primary goal for this study is to investigate the photoexcited carrier dynamics of a hematite film and to further understand the effect of the NiFeO*~x~* coating by the HD-TG technique.  Briefly, this technique is a time-resolved method in which the refractive index change induced by photo-chemical or -physical processes is monitored and is sensitive to the dipole change at the solid/liquid interface.^[@ref26]−[@ref35]^ We found that, indeed, the amorphous NiFeO*~x~* coating plays an important role in decreasing the surface recombination rate and increasing the number of long-lived charge carriers, which could be attributed to the reduction in the number of surface defects. 

2.  Results and Discussion {#sec2}
=========================

A HD-TG response for a hematite film with a NiFeO*~x~* coating in acetonitrile (ACN) is shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}.  Because the NiFeO*~x~*-coated film clearly exhibited all of the components described in this study, it was selected as the reference sample.  In addition, because ACN is known as an inactive solvent, meaning that there should be no electron or hole transfer to the solvent in the photocatalytic process,^[@ref26]^ it was used as a solvent to get a reference response of a hematite film.  As can be seen in the response, it featured two different components after a quick rise, a fast decay part from 10^--6^ to 10^--5^ s and a slow decay part from 10^--5^ to 10^--4^ s. The rising part is due to the trapping of charge carriers to the surface states and the following relaxations correspond to the decay or the recombination of charges.  As we clarified in a previous work,^[@ref26]^ because the decay responses of photoexcited carriers in particulate films of a photocatalyst cannot be fitted with a single exponential due to a variety of features of the carriers, such as defect or surface states, the response consists of the sum of exponential functions with different time constants.  To analyze the time constant distribution included in such responses, the maximum entropy method (MEM) was utilized because it yields a continuous distribution of lifetimes of discrete exponential responses by maximizing the entropy of the error between the observed and the fitting results.  The result is shown in the inset of [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}.  As can be seen, two decay components were clearly extracted, peaked at 10^--5.5^ (3 μs) and 10^--4^ s (0.1 ms), respectively.  The data support the fact that the response consists of two components.  To validate the fitting, the fitting curve obtained from the MEM analysis was overlapped with the response.  As shown in [Figure S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf), we obtained a good match of these two groups of data. 

![ HD-TG response of a hematite film coated by NiFeO*~x~* in ACN probed by 638 nm light.  The time constant distribution was analyzed using MEM analyses in the range of ∼10^--6^--10^--2^ s.](ao-2017-00021u_0006){#fig1}

In the HD-TG technique, due to the similarity of the optical setup to the transient absorption (TA) technique, TA responses sometimes overlap simultaneously with the response caused by the refractive index change.  To distinguish the TG response from TA responses, we compared the dependence of the response on the transmission grating--sample distance. ( The theory for the signal sign dependence on the distance is described in the [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf).)  Those data are presented in [Figure S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf).  Whereas the slower component clearly changed the sign of the signal, indicating that it was caused by the refractive index change, the faster component did not. ( The faster component remained a rising curve.)  This supports that the faster component originated from TA and the slower one was caused by the refractive index change. 

To confirm whether the faster and slower components were induced by a linear process, we also checked the pump laser intensity dependence, and the results clearly showed that both components did not depend on the pump laser intensity.  This means that they arose from a linear process ([Figure S4](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf)).  In addition, we also measured the responses by changing the grating spacing to confirm whether the components arose from an acoustic response induced by the heat ([Figure S5](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf)).  As can be seen in the figure, it implies that the acoustic response mixed in our result was quite small. 

According to a previous study of the electron--hole recombination dynamics of hematite,^[@ref1]^ TA spectra were measured and two components at 575 and ∼700 nm were assigned to the surface-trapped electrons and holes, respectively, and it is probable that the two components also appeared in the observed response in the present study because the probe wavelength (638 nm) was close to them. 

To specify the physical processes for each component, a hole scavenger, ethanol as a solvent, was employed, and a HD-TG response for the hematite film with a NiFeO*~x~* coating was measured (see [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}).  The black and blue responses correspond to the HD-TG responses observed in ACN and ethanol, respectively.  Because it is well known that ethanol works as a hole scavenger,^[@ref26]^ it is expected that the holes trapped at the hematite surface can be removed by ethanol.  Although two components in ACN (black) were observed at ∼3 μs and ∼0.1 ms, the response in ethanol (blue) showed only one component, the slower decay part was almost lost in ethanol.  This indicates that the slower decay component arose from the holes trapped at the surface.  MEM analyses were also employed to make the change clearer in the lifetime distribution, as shown in the inset of [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}.  Both of them showed two components with time constants of ∼3 μs and ∼0.1 ms.  Because the holes trapped at the surface are removed by the hole scavenger, ethanol, a portion of the holes should disappear.  Even though not only the slower component but also the faster component decreased in ethanol solvent, probably, due to the refractive index difference between the two solvents (the TG signal intensity depends not only on the refractive index change but also on the absolute refractive index of the sample), the intensity ratio of the slower one to the faster one in ACN obviously differs from that in ethanol (0.1 with ACN and 0.03 with ethanol).  Thus, we can assign the component having the time constant of 0.1 ms to the decay of holes trapped at the surface.  According to earlier studies,^[@ref1]^ the rate of the hole decay was reported on the millisecond order, which agreed with our results.  The fitting response was reconstructed from the lifetime distribution obtained from the MEM analysis and the fitting curve, and the responses are compared in [Figure S6](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf). 

![ HD-TG response of a hematite film coated by NiFeO*~x~* in ACN (black) and ethanol (blue) probed by 638 nm.  The time constant distribution was analyzed using MEM analyses in the range of ∼10^--6^--10^--2^ s.](ao-2017-00021u_0002){#fig2}

As mentioned before, it was reported that the two components in TA spectra at 575 and at ∼700 nm were assigned to the surface-trapped electrons and holes, respectively, and we found that the faster component was caused by the TA signal.  Because we used 638 nm for the probe laser, it is probable that the TA signal due to the surface-trapped electrons was observed because of its broad peak width.  Thus, to measure only the component due to the holes, which was caused by the refractive index change, we changed the wavelength of the probe laser to 785 nm and measured the HD-TG responses, as shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}.  The intensity of the responses was adjusted to overlap each other for easy comparison.  As can be seen in the figure, it is noticeable that the intensity of the component at ∼3 μs was almost lost when it was probed by the 785 nm laser.  This indicates that the faster component observed at 638 nm was caused by the absorbance change of the surface-trapped electrons.  Thus, we can assign the component having the time constant of 3 μs to the decay of the surface-trapped electrons.  In addition, because the rising part of the slower component clearly appeared in the response probed by 785 nm, it can be assigned to the trapping of the holes.  Indirectly, it also means that the rising part of the faster component might correspond to the trapping of the electrons. 

![ HD-TG response of hematite probed by 638 nm (black) and 785 nm (red).  The time constant distribution was analyzed using MEM with the NLS method in the range of ∼10^--6^--10^--2^ s.](ao-2017-00021u_0003){#fig3}

We were able to observe the component from the electrons by probing at 638 nm, whereas it was not measured in the previous report by probing at 700 nm without applying potentials;^[@ref17]^ thus, our result is consistent with that of the earlier study.  However, because they measured it in water, it cannot be ruled out that the disappearance of the component arising from the electrons was caused by the difference of the solvent.  It will be clarified in our future work, photoexcited carrier dynamics of a hematite film in water, depending on potential and pH.

In summary, the observed carrier dynamics of the hematite film is illustrated in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}.  By the irradiation of UV light (355 nm), electrons and holes were generated; electrons were trapped in the surface/defect states in \<1 μs, as observed in the rise time of the HD-TG response, and holes were trapped in the surface/defect states in ∼10 μs, as observed in the rise time at the wavelength of 785 nm in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}.  The trapped electrons decayed due to the recombination process (τ: 3 μs), and the rest of the holes were decayed by the intrinsic recombination (τ: 0.1 ms), which can be scavenged by ethanol. 

![ Schematic illustration of the carrier dynamics of the hematite film after UV irradiation.](ao-2017-00021u_0001){#fig4}

To clarify the role of the NiFeO*~x~* coating in the carrier dynamics of hematite, HD-TG responses with and without the NiFeO*~x~* coating were compared in ACN and are shown in [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}.  The black and red responses correspond to those for the coated and noncoated hematite in ACN, respectively.  For the coated film, the response showed two components arising from the trapped electrons and holes, whereas both components decayed faster and the slower component became weaker in intensity.  MEM analyses were also employed, as shown in the inset of [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}.  The black and red ones correspond to the time constant distributions of a hematite film with and without the NiFeO*~x~* coating, respectively.  As can be seen in the inset of the figure, the sample with NiFeO*~x~* showed two peaks at 3 μs and 0.1 ms, whereas the sample without NiFeO*~x~* showed two peaks at 2 μs and 0.05 ms.  The increase in the intensity of the slower component indicates that the number of trapped holes increased for the NiFeO*~x~* coating, whereas it did not affect the number of trapped electrons.  Therefore, it was demonstrated that the amorphous NiFeO*~x~* coating on a hematite film plays a role in increasing the number of trapped holes at the surface because the surface recombination was retarded.  We believe that the large number of holes can contribute to the acceleration of the water oxidation process if the trap states exist shallowly at the surface.  Even though there is no quantitative information, we hypothesize that the reduced recombination could be a result of reduced surface defect density.  Because of the NiFeO*~x~* coating, there should be surface states favorable for the stabilization of the trapped holes.  This is consistent with the fact that the water oxidation is enhanced by this coating. 

![ HD-TG responses of a hematite film coated with NiFeO*~x~* (black) and without coating (red).  The time constant distribution was analyzed using MEM analyses in the range of ∼10^--6^--10^--2^ s.](ao-2017-00021u_0004){#fig5}

3.  Conclusions {#sec3}
==============

We investigated the excited carrier dynamics of a hematite film with and without NiFeO*~x~* coating using the HD-TG method.  We assigned two components observed in HD-TG responses to the decay of the surface-trapped electrons and holes, respectively.  We found that the number of surface-trapped holes of NiFeO*~x~* coated hematite was larger than that of the film without the NiFeO*~x~* coating, and their lifetime on the surface was also longer.  Therefore, we concluded that the amorphous NiFeO*~x~* coating on hematite increases the number of trapped holes at the hematite surface and decreases the surface recombination rate, which can improve the overall photocatalytic oxidization of water. 

4.  Experimental Section {#sec4}
=======================

In this study, we prepared a hematite sample based on the solution-derived method, which has a lower degree of structural disorder than that of a film prepared by the atomic layer deposition method and appears to favor growth of (110) planes.  The synthesis method has already been described previously.^[@ref22]^ Briefly, first, β-FeOOH was grown on a fluorine-doped tin oxide substrate (∼7 Ω/sq, Sigma) in a solution of 0.15 M iron(III) chloride hexahydrate (FeCl~3~, 97%, Alfa Aesar) and 1 M sodium nitrate (NaNO~3~, 99%, Alfa Aesar).  The deposition was conducted at 100 °C for 1 h. After rising, the annealing procedure was carried out on the electrodes in a tube furnace at 650 °C for 2 h to convert FeOOH into hematite.  To deposit NiFeO*~x~*, iron(III) 2-ethylhexanoate (50% w/w in mineral spirits, Strem Chemicals) and nickel(II) 2-ethylhexanoate (78% w/w in 2-ethylhexanoic acid, Strem Chemicals) were mixed in hexane to make a 15% w/w metal complex solution.  The solution was diluted 10 times with hexane and 10 μL/cm^2^ was drop-cast onto the hematite electrode.  After drying in air for 5 min, the sample was treated with ozone for 3 h to remove the organic precursors. 

For the HD-TG measurement, the third harmonic output of a Nd:YAG (YAG) laser (355 nm, ∼1 mJ/pulse, 10 Hz, pulsewidth: 5 ns) and a semiconductor laser (638 or 785 nm) were used as pump and probe lasers, respectively.  The grating spacing used in the present measurements was 60 μm.  The sample was pumped by the irradiation of UV light from the YAG laser, and the response was measured via the diffraction of the probe light.  The signal was detected by a photodiode (Thorlabs, DET110, rise time: 14 ns) and amplified by a voltage amplifier (Femto, DHPVA).  To eliminate the scattered pump light entering the detector, a filter for cutting 355 nm light was also placed in front of the entrance of the detector.  It was monitored and recorded by an oscilloscope (Agilent, DSO8104A, 4GSa/s). 

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.7b00021](http://pubs.acs.org/doi/abs/10.1021/acsomega.7b00021).Principle of the HD-TG method; fitting curve for the HD-TG response of a hematite film coated with NiFeO*~x~* in acetonitrile; phase dependence of the HD-TG responses and the TA response; HD-TG responses of a hematite film coated with NiFeO*~x~*, measured by changing the pump laser power intensity; HD-TG responses of a hematite film coated with NiFeO*~x~*, measured by changing the grating spacing; fitting curve for the HD-TG response of a hematite film coated with NiFeO*~x~* in ethanol; fitting curve for the HD-TG response of a bare hematite film in acetonitrile ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00021/suppl_file/ao7b00021_si_001.pdf))

Supplementary Material
======================

###### 

ao7b00021_si_001.pdf

The authors declare no competing financial interest. 

The research was financially supported by JSPS KAKENHI Grant (\#15K05549), Institute of Science and Engineering, Chuo University, JST PRESTO, and Basic Science Research Projects from The Sumitomo Foundation.