Fast Sonochemical Exfoliation of Hematene type Sheets and Flakes from Hematite Nanoarchitectures Shows Enhanced Photocurrent Density
This report demonstrates a sonochemical exfoliation strategy to form a hematene nanosheet and flakes from hematite nanopowder and thin film. The optical properties of obtained hematene sheets and flakes show unusual features that differ from that of a pristine Hematite film, which allows it to further apply for photoelectrochemical functionality enhancement. Here, we found that the ultrasonicated Hematite film with Hematene flakes shows enhanced photocurrent density than the pristine Hematite film. The increased light absorption property of the Hematene flakes due to the scattering effect helps in achieving it. The exfoliation process is further confirmed by the X-ray diffraction of the Hematene sheet where (104) plane Bragg peak is split up after the ultrasonication process. It is corroborated with a qualitative predictive mechanism for the fast exfoliation of nanosheets and flakes. Here, surface energies and hydroxylation of crystal facets are proposed to be the factor responsible for the quick exfoliation process.
Introduction<!>Materials<!>Synthesis of Hematite nanoparticle and thin film:<!>Synthesis of Hematene Sheets and Flakes containing Hematite Films<!>Characterization of obtained powder and thin films:<!>Results and discussion<!>Sonochemical Exfoliation of Hematene sheets and flakes<!>Detail atomic structure investigation of Hematene sheet with HRTEM<!>How Hematene form?<!>Formation of Hematene Flakes<!>FESEM and EDX investigation of Hematene flakes sensitized thin film<!>Optical and Crystallographic properties of Hematene flakes and sheets<!>Surface properties of Hematene flakes and sheet from XPS study (X-Ray photoelectron spectroscopy)<!>Photoelectrochemical properties of Hematene flakes sensitized thin film<!>Conclusions
<p>The emergence of 2D materials by exfoliation techniques finds paramount importance in the current development of new semiconductors. In this regard, both graphene and metal chalcogenides are studied and applied broadly. The first scotch tape-based exfoliation of 2D materials from carbonbased allotrope Graphite leads to the discovery of Graphene, and it pushes a new regime of materials research into tremendous applications, primarily for semiconductor devices and quantum physics study [1]. Next, the liquid-based exfoliation of layered materials provides high surface area results in enhanced optical and electrochemical properties, and it is possible to exfoliate them into nanosheets due to the weak out-of-plane bonding [2]. The chemical exfoliation of metal carbides forms 2D transition metal carbide layers known as Mxene, and it finds various applications from electromagnetic shielding to energy conversion and storage application [3]. Other 2D materials such as phosphorene, transition metal chalcogenides are also well studied for their diverse application [4 -8]. Besides these, recently, interest rises to develop 2D metallene nanomaterials such as silicene, antimonene, germanene, stanene [9]. Within this category, the single layer of PdTe made by electrostatic doping shows polar metal characteristics and proven by first-principle calculations [10]. These materials are Van der Waals solids due to the force existing in between the interlayers in bulk material, as observed in the case of Graphene [11].</p><p>Metal oxides based 2D materials still need further research to decipher their role in various applications. Several metal oxides, such as TiO2, WO3, ZnO and Co3O4, have been rigorously</p><p>studied and converted to their two-Dimensional forms in the form of layered double hydroxide and shows many different electrochemical properties [12]. A recent candidate of 2D copper (II) oxide nanosheet [13] is prepared by the surface Sulphur functionalization, which shows semiconductorconductor transition. Besides these, Iron oxides and their analogues also possess widespread applications in the diverse fields of catalysis [14], magnetic storage media [15], gas sensing, Lithium-ion batteries [16] and other biological applications [17]. Hematite (α-Fe2O3), an n-type semiconductor, possesses excellent stability and an appropriate bandgap, making it well known for solar water splitting photosynthesis application [18]. However, iron oxide-based 2D materials such as layered double hydroxide, which is generally not photoactive, are used in electrocatalytic hydrogen production due to the high surface area, low overpotential and fast water oxidation kinetics. It shows a higher catalytic turnover process for the oxygen evolution reaction [19]. The photoactive 2D exfoliated layers of the Hematite semiconductor has not been explored. With the discovery of the ultrasonication method, various nanostructured materials with reasonable control over size, morphology and crystallographic structure can be formed. Cavitation and Nebulization are primary physical phenomena associated with the ultrasonic synthesis of nanomaterials [20].</p><p>Applying it for the exfoliation of the 2D single layer from metal oxides into sheets and flakes [21] is an exciting area of materials development. Balan et al. reported a sonochemical route for the exfoliation of 2D Hematite from geological Iron Ore, called Hematene. The Hematene layer forms by breaking covalent bonds between the Fe-O layers of Hematite crystal by the ultrasonication process [22]. They used the sheet-like layer for the TiO2 photoelectrode sensitization to measure the photocurrent density with an application toward water-splitting mediated hydrogen evolution.</p><p>The Hematene obtained also shows ferromagnetic order, while Hematite usually shows antiferromagnetic behaviour. Hematene is a non-Van der waal material, and it shows exotic magnetic properties with a magnetic moment of 4B per atom according to a recent DFT study followed by its surface functionalization on Stanene converts Hematene into ferrimagnets [23].</p><p>Hematene, as a non-van der Waal 2D material, is also chemically stable in the ambient conditions due to its oxygen passivation [24], and next, Hematene based heterostructure (Hematene / BaTiO3) is also developed to tune the magnetic properties [25]. Not only magnetism but Hematene also find suitable application in the biomedical sector and associated energy research with the development of it for the combined application of glucose sensing and electrocatalytic action followed by the development of a biofuel cell operated by the glucose [26]. For the oxygen evolution electrocatalysis study, recently defect enriched Hematene has a current density of 10 mA/cm 2 at 0.250 V overpotential [27]. The energy storage outlook of this material is also recently taken into consideration and is published elsewhere [28].</p><p>Hematene sheets are applied for solar water splitting reactions. However, the direct application of Hematene flakes on a pristine hematite photoanode for photoelectrochemical functionality enhancement is not carried out so far. The motivation of the current study is the ultrasonic treatment of Hematite film to get Hematene sheets and flakes and to understand its effect on photocurrent density enhancement to fill this gap. The next question is whether a short sonochemical period is enough to make such kinds of nanosheets? As the usual exfoliation time in the studies mentioned above, geological ores are long as the Fe-O strong bonding will not efficiently allow the exfoliation process [22]. However, exfoliation of Hematene from Hematite nanoparticle and their thin film can be an additional advantage concerning surface energies of different crystalline facts within different morphological variants of iron oxide [29]. Hence, the development of a fast exfoliation method is another motivation of the current study.</p><!><p>All chemicals used in the current study have been procured from commercial sources and possessed 97-99% purity. Ferric (III) Chloride hexahydrate (FeCl3.6H2O) and Urea (NH2CONH2) was purchased from Sigma-Aldrich. Absolute Ethanol used for washing the ochre yellow precipitate was of analytical grade and was obtained from Changshu (C.S.), while distilled water was distilled in our laboratory. For the deposition of Hematite, FTO glass slides were obtained from Ants Ceramic Limited, Mumbai, India.</p><!><p>A synthetic protocol similar to previously reported literature has been employed to synthesize the Hematite thin film using water as a solvent [30,51]. Here, 1 g of Ferric Chloride Hexahydrate and 2.2 g of Urea were mixed with distilled water in a round bottom flask and was stirred at 80° C. The resultant mixture forms an ochre yellow colour solution after 6 hours of the reaction. The solution was then centrifuged and washed with Ethanol. The residue obtained was dried at 70°C. Later, this mass was calcinated in the air at 500°C for 3 hours to get a reddish-brown colour powder, which is the characteristics colour of Hematite. FTO (Fluorine doped tin oxide) substrate was clean with isopropanol; soap water followed upon by dip-coated in the supernatant solution to obtain thin films of the same. It was then dried and further heat-treated at 500°C for 3 hours to get the Hematite film.</p><p>The thickness of the hematite film is shown in Figure S3, and it is 0.95µm. The RMS roughness of the hematite film before sonication is found to be 7.6 nm. We have used the centrifugation process in the isolation of akageneite powders. We have also used the centrifuged process in the isolation and calculation of sonicated Hematene sheet yield. It is a new amendment, as described below.</p><!><p>For the synthesis of Hematene sheets, Hematite nanopowders obtained are ultrasonicated for 5 min in Ethanol in a glass vial. The obtained solution is further applied for TEM characterization to confirm the presence of Hematene. The Hematene flake is formed by the ultrasonication of as obtained Hematite film for 5 min in Ethanol. Films are taken out and characterized further with FESEM to confirm the presence of Hematene flakes.</p><p>The exfoliation yield of the process is calculated by following the protocol as described elsewhere [ 52]. Here, the weight of the empty centrifugation tube is first taken. Then the Hematene sheet solution is centrifuged for 10 min at a speed of 10000 rpm/hr. After this, the weight of the Hematene sheet along with the tube is retaken. The difference comes out as 1 mg, which is a low exfoliation yield. The measurement setup consists of the photoelectrochemical cell (custom built from Borosil) beaker having a 3-electrode system configuration with 1MKOH (pH = 13.6) as the electrolyte.</p><!><p>Ag/AgCl on sat. KCl and platinum wire were used as the reference and counter electrode, respectively. A LED lamp (100mW) from Tec. Inc. USA [tec light] with embedded electronic shutter option was used as the light source to measure the photocurrent density. The transient photocurrent density is taken using the same light source using the built-in light on-off shutter option with both fast and slow interval modes.</p><!><p>Before discussing Hematene sheets and flakes formation, we will first explain the force hydrolysis reaction to form hematite nanoparticles and films. Here, the forced hydrolysis is defined as a reaction of iron trichloride with water [Figure 1-I], and this yields many different morphologies.</p><p>In this case, we have also used Urea, whose role is to control nanoparticles' shape. Due to an acidic bi-product (carbonic acid) formation, Urea mainly controls nanoparticle morphology, as established in a separate study. It adsorbed on the specific crystal planes of growing nanocrystals [30]. From the SEM characterization of the as-prepared Hematite powders [Figure 1-II], we found that the morphology shows a rod-like pattern, which is a characteristic of the formation of akageneite rods on top of which Hematite nanoparticle nucleates [31]. The heat treatment at 500°C results in spherical shape nanoparticles. A similar morphological transition and its growth have been reported in earlier literature [32]. The mean nanoparticle size of rod-shaped particles found here is 34 nm, while hematite nanoparticles are 115 nm [Figure S5]. Next, from the XRD of Hematite nanoparticles, it is found that (104) Bragg planes have the maximum XRD peak intensity than that of (110) plane reveals it as the most exposed crystallographic plane [33]. The XRD patterns of the sample in their powder form have been illustrated in Figure 1-II C and matched well with the JCPDS no. 34-1266, which is usually indexed for akageneite (β-FeOOH) phase and, upon thermal treatment of it at 500°C, undergoes a phase transformation into Hematite (α-Fe2O3). The two prominent peaks (104) and ( 110), indexed to the JCPDS no. 33-0664, grow from ( 004) and (11-2) of the Akageneite XRD pattern (before calcination). [33].</p><!><p>The sonochemical exfoliation process has been carried out for both Hematite nanoparticle powders and the thin film to get Hematene sheets and flakes, as shown in Figure 1-III in a one-pot setup. We called it ultrasonic or sonochemical exfoliation because we observed such nano sheet-like materials after the ultrasonic preparation of nanopowder and film. On ultrasonication for 5 minutes in Ethanol, hematite surface layers are exfoliated, forming both flakes for thin film and sheet-like morphology for nanoparticles. While, in the regular LPE (liquid phase exfoliation) process, the materials to be exfoliated is kept in the ultrasonication for more than 24 hours and results in 2D layers or flakes [22,26,27]. In this case, exfoliation is accomplished with a short period of ultrasonication though it is not ideally a 2D layer material with a thickness range of 2-3 nm.</p><p>We will first discuss the HRTEM results obtained for the Hematene sheets to shed light on their structural and morphological characteristics. Afterwards, we will discuss the result obtained from the FESEM investigation of the Hematene flakes. Whether these materials possess any characteristics for photoelectrochemical functionality, we have further studied its detailed structural, topological, morphological, optical and surface composition investigation with advanced microscopy as well as spectroscopic and diffraction tools.</p><!><p>We have performed the HRTEM (High-resolution transmission electron microscopy) investigation of ultrasonically dispersed nanoparticles [Figure 2A] to probe the morphologies. Nanoparticles</p><p>show an aggregate like feature resembling cauliflower in the first observation. Next, we have found a partially hexagonal sheet-like morphology at a 20 nm scale, which on close inspection at the higher resolution, shows the lattice structure of Hematite [Figure 2 B -E]. Comparing this morphology with the existing literature report [20], the terminology of Hematene comes from a similar lattice structure observed during the ultrasonic exfoliation of naturally available iron ores [20]. The atomic structure of the Hematene sheet formed is further investigated by lattice imaging of Hematene sheet edges, as shown in Figure 2F. Here, the interlayer spacing between two atomic planes of Hematene is 2.66 Å, corresponding to the (104) plane of Hematite crystallographic structure. The FFT (Fast Fourier Transform) of HRTEM lattice imaging results in the electron diffraction patterns of hexagonal symmetry, as shown in Figure 2G. A similar hexagonal symmetry is also observed for Hematene obtained by Balan et al. [22]. However, we do not have here direct evidence about the crystallographic projection to know the basal planes. Whereas in other studies, it is the (001) plane [22].</p><p>The indexing of diffraction spots reveal (131) planes with d -spacing value closely matched to the JCPDS pattern of Hematite with the rhombohedral crystal system. These observations indicate the crystallographic origin of Hematene nanosheet exfoliated from Hematite lattice with the ultrasonication procedure, and the formation of a free-standing 2-D Hematene sheet is well confirmed. We have made another exciting observation by looking at different edges (yellow rectangular portion) of the Hematene sheet lattice fringes. Here, we have obtained a superlattice arrangement of the atomic assembly for the nanocrystallites, as shown in Figure 2H. The d-spacing matches well with (012) Bragg planes, and the plane at a 75° angle is (104). A closer look at it reveals an evolutionary pattern of the atomic arrangement in a hexagonal order [ Figure 2I]. There are two types of atomic layers observed. The inward layer and outer layers are repeating in ordered manners refer to the formation of a single layer Hematene sheet [22,27].</p><p>Nevertheless, we strictly not confirmed it by molecular dynamic simulation as described elsewhere [22], which shows a thickness of 20 Å. The FFT of the corresponding lattice fringes [Figure 2J] also indicates the formation of hexagonal spots corresponding to each atom constituting the Hematene crystallographic structure and repeating in a similar pattern. The FFT diffraction spot corresponds to the (131) planes of Hematite. A similar atomic arrangement is observed for the Hematite nanocrystal with a bitruncated dodecahedron shape [29]. respectively. The presence of a diffraction spot further confirms that the base material from which</p><p>Hematene is exfoliated has a Hematite crystallographic structure.</p><!><p>The mechanism of Hematene exfoliation and temporal control in understanding the surface energies of most exposed crystallographic planes is shown in Figure 3. Note that, to understand the same, first we have to know the surface energies of (104) planes in our finding and the naturally obtain Hematite ores which have basal planes (0001) lying along the foliation plane [34]. Followed by this, we will emphasize the hydroxylation of the Hematite surface. It takes a minute to an hour for the hydroxylation, depending on the surface reactivity of each plane. It is a well-established fact that the Hematite surface usually contains Fe atoms with an unoccupied state, acts as Lewis acids, and reacts with Lewis bases such as hydroxyl ions via the available electron pair [33]. Due to the same, hydroxyl ions adsorb on the surface of iron oxide via coordination with Fe atoms. The heat treatment removes the surface hydroxyl group and forms water molecules due to the surface condensation, and the same is desorbed later. The dehydroxylated Hematite nanoparticle on exposure to ambient temperature forms three types of coordination based on the earlier study [30] due to water vapour adsorbed by the (104) exposed facet. The hydroxylation of the Hematite surface depends on the surface energies of exposed crystal facets. For instance, the surface energy of the (104) plane of nanoparticles described here is around 2.64 J / m 2 [36], while that of (001) basal planes in the case of naturally abundant Hematite ores are 1.53 J/m 2 [37]. So, the hydroxylation time of both cases will differ and will finally affect the exfoliation step run by a secondary process such as ultrasonication and solvent interaction with metal oxide surfaces. In this case, this hydroxylation step is faster due to the more exposed (104) plane, leaving the exposed hydroxyl group, which will further react with the absolute alcohol.</p><p>Next, the metal oxide is well known for the dehydration of alcohol and beta elimination reaction, in which unsaturated organic compounds such as ethylene get produced according to the literature described elsewhere [38]. Here, the alcohol hydrogen atom intercalates to the Fe-O-Fe linkage's oxygen, forming a hydroxyl moiety (O-H) covalent bond between two metal atoms. It takes place on (104) crystal facets leaving stress on the planes, which will finally lead to the formation of Hematene sheets. It is bound initially by non-van der Waal forces. Besides this, this hydroxyl moiety acts as a knob to control the magnetoelectric coupling in ferroelectrics such as Ca3Mn2O7.</p><p>It is also formed by hydrogen intercalation and, due to the same antiferromagnetic metal oxides, shows ferrimagnetic properties [39]. It is also evident in the case of Hematene, which shows ferrimagnetic behaviour, as stated above [23].</p><!><p>So far, we have described the formation of a free-standing Hematene nanosheet. The free-standing Hematene sheets are challenging to apply directly for device application. Nevertheless, to use them as a functional material, we need a handful of systems where their influence can be observed towards a specific functionality. In this regard, our goal is to use this type of 2D material for photoelectrochemical water splitting applications. So, to achieve the same, as described above, we ultrasonically transform Hematite nanoparticle film [Figure 1-III] into an exfoliated film with the generation of flakes, which we called Hematene flakes by the 2D morphological characteristics as observed by the FESEM. Here, Hematite film is sensitized by Hematene flakes after the ultrasonication process. From here on, we will designate both Hematite films before and after ultrasonication as B. S. and A. S. The homogenous solution mixture is used for thin-film coating, as shown in Figure 1-I A, which on calcination at 500C results in a Hematite film by following the procedure described elsewhere [40].</p><!><p>We have performed field emission scanning electron microscopy (FESEM) studies of Hematene flakes to observed their morphology. It is followed by a detailed compositional characteristic of the Hematene flakes to confirm the presence of Fe and O only, which composed the usual iron oxide bulk counterpart. The morphology of calcined Hematite thin films (A.S. and B.S.), before and after ultrasonication, has been studied, and their images are presented in Figures 4 A and B, respectively.</p><p>The FESEM image of the B.S. [Figure 4 A] obtained by the dip-coating methodology described in our earlier study [41] shows anisotropic nanoparticles with the fusion of two nanoparticles in an elongated pattern. The A.S. morphology is shown in the FESEM image [ Figure 4 B]. Here, the particle morphology remains the same but the exfoliation results in more porous like features. In Next, we have also carried out the AFM measurement of the Hematene sheet and flakes to determine their thicknesses. However, the result obtained [Figure S1] does not reveal the image found during the TEM and FESEM investigation. To determine the thickness of corresponding hematite nanoparticles, sheets and films, we have plotted the RMS roughness of the surface for each sample.</p><p>The result obtained is shown in Figure S2. For the Hematene sheet, the surface roughness decreases, while for Hematene flakes, it increases after the ultrasonication process. We have compared the FESEM of free-standing Hematene flakes (high magnification) with the one obtained on the thin film [Figure S 11] at a bulk level. Here, the surface roughness in both cases does not show any significant difference, and it appeared to be smooth on close inspection of FESEM images.</p><!><p>We resulting in a broader band gap value and blue shift of the absorption edge [43]. The same result agrees with the band gap value of the Hematene obtained by the ultrasonication of natural Hematite ores [22].</p><p>Next, we have presented the photoluminescence spectrum, as shown in Figure 5D. Here, on excitation with 320 nm wavelength, we have observed the emission spectra at 640 nm. Hematite film A.S. and B.S. indicate that the emission peak shows a doublet pattern while the Hematene sheet shows singlet. The size of the iron oxide nanoparticle controls the changes in peak shape, a common observation found in the X-ray emission spectra [44].</p><p>The scattering contribution and its quantitative analysis are also understood here to consider the scattering contribution to optical properties. It is carried out by measuring the diffuse reflectance spectrum of Hematite Film B.S. and A.S., followed by the Hematene sheet. Here we have applied the Kubelka -Munk function to the diffuse reflectance spectrum with the following equation:</p><p>R is the diffuse reflectance of the layer relative to a standard such as magnesium oxide, k is the molar absorption coefficient, and s is the scattering coefficient of the sample. The diffuse reflectance spectra of all samples are shown in figure 6 A. We have observed a similar type of diffuse reflectance pattern for both Hematite film A.S. and Hematene sheet while Hematite film B.S. shows high reflectance till 600 nm, afterwards the reflectance lowers to 400 nm. To understand the same, we have plotted the scattering coefficient (s) and found that it varies significantly for pristine Hematite film, but for Hematite film A.S. and Hematene sheet, the pattern looks almost the same [Figure 6B]. It further confirms that the morphology of the Hematene sheet and flakes are the same as the scattering contribution, which is almost similar for both cases, whether film or dispersion. For pristine hematite film, the scattering gets higher from 550 nm onwards. Hence the absorbance changes due to different scattering coefficients for all samples. It is also found that the scattering and absorption coefficients of Hematite changes for morphology and size of the nanoparticle [30].</p><p>Our previous study [40] shows that the forced hydrolysis of iron chloride salts in the presence of Urea always results in maximum peak intensity for (104) Bragg planes, as confirmed by XRD.</p><p>Here, we have studied the detailed crystallographic properties of the Hematene flakes. We have carried out the thin film XRD investigation of all three materials such as Hematene sheet, hematite film B.S. and hematite film A.S. As shown in figure 6 C, we found that all the Bragg diffraction peak shows the typical crystallographic structure of Hematite according to the JCPDS pattern. The diffracted intensity of (110) plane decreases after the ultrasonication process for the Hematite Film A.S. Next, on expanding the (104) and (110) peak [Figure 6D], it is evident that a new peak splitting has been observed for (104) plane in case of Hematite film A.S. The (104) peak intensity after the ultrasonication process decreases while a new peak appeared at 0.6˚ from the original Bragg angle.</p><p>It further signifies the hydroxyl ion intercalation along the (104) peak described in the mechanism [Figure 3]. A similar type of peak splitting is observed in the in-situ X-ray diffraction pattern of chloroaluminate ion intercalation in Aluminium batteries [45].</p><!><p>The XPS (X-Ray photoelectron spectroscopy) is performed for hematite nanoparticle film obtained from the heat treatment of rod-shaped particle, the hematite film A.S. and Hematene sheet (dispersion) followed by a control measurement of substrate FTO (Fluorine doped tin oxide) on which hematite film is deposited. The survey scan of all samples is shown in Figure S6. The composition shows the variation in atomic ratio both before and after ultrasonication.</p><p>The O1s core level spectrum of all samples is shown in Figure 7 A. The first peak at 529 eV comes from the hematite lattice, while a second peak at 532 eV is considered a defect peak, which is usual of Hematene electronic structure as mentioned elsewhere [22]. The oxygen 1s core level scan of FTO reveals a peak at around 531.70 eV [46], and this peak deviates itself from the defect mediated</p><!><p>We have applied Hematene flakes sensitized Hematite film for the photoelectrochemical response study due to its light absorption properties, as discussed above. We have not performed any measurement for the sheet-like Hematene materials as it is challenging to make a stable dispersion.</p><p>There is a way to sensitize any photoanode materials with these Hematene materials like Graphene sensitized photoanode encountered in many studies. With this regard, Hematene coated titanium dioxide is published elsewhere as a sensitizing material [22]. Also, its application for the photoelectrochemical water splitting reaction is worth investigating due to the direct photoanode functionality of both bare Hematite and doped counterpart in terms of PEC water oxidation and matching of the valence band edge position with water oxidation potential [18]. Here, our goal is to see any difference in the photoelectrochemical response of this Hematene flakes sensitized photoanode from that of the Hematite film. For the same, we have measured the photocurrent density of both B.S. and A.S. in 1M KOH, as shown in Fig. 8A.</p><p>It depicts a comparison between the light and dark current densities for the B.S and A.S. In all photocurrent density patterns, the usual photocurrent plateau is not observed, which mainly seen for the nanostructured Hematite photoanode. In this case, the Hematite from which Hematene flakes are exfoliated is the direct conversion of Akageneite, crystal which usually results in more resistive action, while doped Hematite already well established to show higher photocurrent density [44].</p><p>Despite not having a Plateau-like feature, the photocurrent density of film A.S is higher than that of B.S. and displayed a photocurrent density of 50 μA/cm 2 . Here, the photocurrent density of bare Hematite film before sonication is very low in the 30 A/cm 2 at 0.6 V. The slight increase in photocurrent density can be attributed to the more opening in the porosity of the photoanode materials (A.S.) due to the ultrasonication process. It is evident from the morphology discussion with the FESEM study, which shows a rougher surface for A.S. At the same time, the dark current density for A.S. is lower than that of B.S. Our earlier studies show that the Hematite films show considerable dark current density due to the presence of Fe 2+ site, which on electrochemical oxidation result in extra electrons with a direct effect of Faradaic current density in the dark condition [41]. While in the A.S. film, as the ultrasonication process removes surface layers as exfoliated ones, the dark current density is low. The photocurrent density mentioned here is also not that high, but the ultrasonication process leads to a better photocurrent density, a new finding in this domain. We believe the direct role of Hematene flakes in the overall photocurrent density is due to the maximum absorbance shown by these flakes, followed by the A.S. film absorbance is higher than that of B.S. Besides these, Hematene also shows extra panchromatic light-harvesting above 550 nm, and the additional increase in the photocurrent density is due to the collective action of the same. Besides these, the IPCE (incident photon to current conversion efficiency) of Hematene coated TiO2 photoelectrode shows increase efficiency in this region [22]. However, these ultrasonication methods will give rise to more efficient photoanodes, which are already well established. So, we keep it as the next objective for our future study with this new material. The basic understanding and observation of a transient photocurrent spike need to be understood regarding the photoinduced charge transfer effect. As described elsewhere [49,50], the photocurrent transient spike reaches a peak in the upward direction followed upon by decay to the steady-state when light is turned on. After 10 seconds of photon illumination, if the light is turned off, the zero current occurs and re-spiking when the light is turned back on again. The physical significance of the falling photocurrent density is the conduction band electrons rapid removal into the bulk, and spike formation occurs due to the recombination of electron holes by the surface state.</p><p>The sub-spike feature we discussed above is assigned as a sub-surface state generated peak. Now constraint. Our goal is to report here only the formation of Hematene sheets and flakes with their photoelectrochemical properties. Also, the Hematene formation for oleic acid-derived Hematite nano octahedron has not been studied. However, it will be the subject of further study.</p><!><p>The sonochemical treatment of hematite film leads to the formation of Hematene nanosheet and flakes as supported by the detailed HRTEM investigation with the observation of both inward and outward layers arrangement of atoms nanocrystallites. A thorough atomic structure understanding with the d-spacing calculation reveals that the Hematene originated from the Hematite as it also</p><p>shows the crystallographically more exposed (104) plane. Based on the same, its formation mechanism is proposed and predictive that hydroxylation is faster due to more reactive surface due to the high surface energy of (104) planes. The XPS study validates the usual oxygen defect signature of Hematene at 532 eV, which further confirms the enhanced photoelectrochemical functionality of the Hematite film after ultrasonication treatment and the formation of the subsurface state. The photocurrent density increased up to 50A/cm 2 for Hematene flakes sensitized Hematite photoanode due to its enhanced light-harvesting action, while the pristine shows current density up to 30A/cm 2 . Though it is not a fantastic performance, the future outlook of such sonochemical modification and making of Hematene flakes on champion photoanode will boost its efficiency further for the solar water-splitting process.</p>