Boosting the Solar Water Oxidation Performance of Fe2O3 Photoanode via Embedding Laser‐Generated Pt Nanocrystals

α‐Fe2O3 with suitable band structure, good chemical stability, and easy preparation, is a potential photoanode material. However, the key to enhance the performance of α‐Fe2O3 photoanode is to improve the transport characteristics of bulk carriers. It is expected to form a Schottky barrier to improve the carrier separation efficiency by embedding metal nanoparticles into the matrix, but the process is still challenging. Herein, a strategy of forming the Schottky barrier is shown to improve bulk carrier transport dynamics by embedding laser‐generated Pt nanocrystals in α‐Fe2O3 photoanode, which achieves photocurrent densities of up to 1.16 mA cm−2 at 1.23 VRHE (from original 0.21 mA cm−2). This work provides another way to promote the carrier transfer and separation of α‐Fe2O3, which is of great significance to improve the photoelectrochemical water splitting performance.


Introduction
Photoelectrochemical (PEC) water splitting is the key technology in response to the rapid deterioration of the global climate and the shortage of fossil energy, [1][2][3] which is the ideal technology for conversion and storage of solar energy.19] Element doping can change the energy band structure of Fe 2 O 3 , [20][21][22][23][24][25] increase the carrier concentration and carrier lifetime, improve the electrical conductivity, and increase the electric field at the electrode/electrolyte interface to inhibit the carrier recombination. [15,26,27]Noticeably, doping with elements Pt can improve the charge transfer characteristics in the bulk of the Fe 2 O 3 , and thus resulted in a record-breaking performance at that time. [28]To promote the carrier separation and external transport, coating ultrathin oxide/nonoxide overlayer on Fe 2 O 3 is the best choice, [25,[29][30][31] based on which the obtained Fe 2 O 3 /TiO 2 /FeOOH photoanode by integration of the three modifications achieved a remarkable PEC performance. [14]Similarly, other oxide/nonoxide materials such as Cu:NiO x , [32] Al 2 O 3 , [33] CDots, [34,35] and metalorganic framework-derived p-Cu 2 O [23] were successfully employed to decorate the Fe 2 O 3 surface to enhance the carrier transfer.
However, most of the previous works were a specific modification to address a single specific issue, and the excellent photoelectrode results from the superimposition of multiple modification strategies.Here, inspired by the success of Pt element doping in Fe 2 O 3 photoanode, and the strategy of embedding laser-generated nanocrystals in material matrix proposed by Wang's group, [36][37][38][39][40] we demonstrate a strategy of embedding laser-generated Pt nanocrystals in Fe 2 O 3 photoanode to promote the carrier transport in bulk and carrier transfer at SCLJs for boosting its PEC performance, leading to the improved photocurrent density 1.16 mA cm À2 at 1.23 V RHE (from original 0.21 mA cm À2 ), and significantly increase of bulk charge separation efficiency about 2 times at 1.23 V RHE .

Synthesis of Pt Nanocrystals via Laser Irradiation
Figure 1a schematically illustrates the preparation of ligand-free Pt nanocrystals (Pt NCs) via the unfocused pulsed laser irradiation in liquid (PLIL).The Pt metal sheet was immersed in ethanol, and then the mixture was irradiated with the laser flux of 1.0 J pulse À1 cm À2 for 5 min.Subsequently, a yellow-brown transparent colloidal solution was obtained (Figure 1d-inset), and the concentration of Pt NCs was around 0.28 mg mL À1 (Table S1, Supporting Information).The transmission electron microscopy (TEM) image shows that the size of the particles generated by the laser is less than 2 nm (Figure 1b).Furthermore, high-resolution TEM (HRTEM) shows marked lattice fringes (0.23 nm) corresponding to the Pt (111) crystal planes.Particle size statistics (Figure 1d) shows that the size of Pt nanocrystals is almost less than 2 nm, mainly distributed in 1-3 nm.Compared with the traditional wet chemical synthesis, PLIL technology can prepare nanocrystals in situ in solution without any ligands, which is conducive to the transmission of electrons. [41,42]

Embedding Laser-Generated Pt Nanocrystals in Fe 2 O 3 Photoanode
In this article, the preparation process of Fe 2 O 3 film is in accordance with the spin-coating method previously reported.Figure S1a, Supporting Information, describes the preparation process of Fe 2 O 3 film.In short, a layer of FeOOH film is deposited on fluorine-doped tin oxide (FTO) substrate by spin-coating, and then FeOOH is transformed into Fe 2 O 3 .To construct Fe 2 O 3 @Pt thin films, Pt NCs were prepared in situ in ethanol, and then the Pt NCs colloidal solution was mixed with the Fe(NO 3 ) 3 precursor to form the Fe 2 O 3 @Pt precursor solution.The specific manufacturing process is shown in Figure S1b, Supporting Information.Then, we embed the Pt NCs with different concentrations in Fe 2 O 3 films, which are denoted as Fe 2 O 3 @Pt-1, Fe 2 O 3 @Pt-2, Fe 2 O 3 @Pt-3, Fe 2 O 3 @Pt-4, and Fe 2 O 3 @Pt-5.All films were prepared on FTO substrates followed by an annealing step at 500 and 800 °C in air, respectively.
The prepared Fe 2 O 3 films show worm-like nanoporous structure (Figure 2a), and the thickness of the film is about 100 nm (Figure 2c).TEM images show that the crystallinity of the particles in the Fe 2 O 3 film is good (Figure S2a, Supporting Information).The HRTEM (Figure S2b, Supporting Information) image shows the row spacing of 0.22 and 0.24 nm, which matches the (200) plane and (112) of α-Fe 2 O 3 , respectively. [39]After embedding Pt NCs, the particle size of the films decreased significantly (Figure 2b).With the increase of the concentration of Pt NCs, this trend becomes more obvious, which can be seen from the scanning electron microscopy (SEM) images of the films prepared with different concentrations of Pt NCs (Figure S3a,d, Supporting Information).However, too many Pt NCs would lead to an inhomogeneous film (Figure S3e,f, Supporting Information).There is no significant change in the thickness of the film (Figure 2c,d) because the concentration of the precursor solution does not change and the limited concentration of Pt NCs has little effect on the properties of the solution.Further, the optical microscopies of the Fe 2 O 3 precursor in Figure 2e,f imply that the embedded Pt NCs could serve as the nucleus to regulate the nucleation and growth kinetics of the Fe 2 O 3 growth, resulting in the denser and smaller morphology.Because of the worm-like structure becomes much denser and smaller, increasing the film surface roughness causes contact angle dropping from 38.0°to 27.1°(Figure S4, Supporting Information), resulting in a more hydrophilia Fe 2 O 3 @Pt surface, which is conducive to the full contact between the electrolyte and the reactive site on the film surface.XRD patterns and Raman spectroscopy (Figure S5, Supporting Information) showed that the crystal structure of Fe 2 O 3 @Pt was not affected by the incorporation of Pt NCs, and no new peak and obvious peak shift were found.To explore the position relationship between Pt NCs and Fe 2 O 3 film, TEM characterization was performed.Figure 2g,h shows the TEM and HRTEM of Fe 2 O 3 @Pt film.Figure 2g reveals that the existence of Pt NCs, and two lattice fringes can be seen from HRTEM (Figure 2h), corresponding to the (111) crystal plane of Pt and the (112) crystal plane of α-Fe 2 O 3 , respectively.

PEC Water Splitting Performance of Fe 2 O 3 @Pt
Next, we test the photoelectrochemical performance of Fe 2 O 3 @Pt photoelectrode (Figure 3a) under AM 1.5G illumination (100 mW cm À2 ) to evaluate how the embedding of Pt NCs impacts the PEC water splitting performance of Fe 2 O 3 films.Figure S6, Supporting Information, shows the UV-vis absorption curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.It can be seen from the figure that the absorption limits of the two films are about 600 nm, In the range of 360-480 nm, two samples show certain optical absorption characteristics, and the difference is not big.Figure 3d  Figure 3e shows the applied bias photon-to-current efficiency (ABPE) curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.ABPE reflects the photoelectric conversion efficiency of the sample under applied bias.It can be seen from the figure that the maximum ABPE value of Fe 2 O 3 film without Pt NCs is 0.017%, and the maximum ABPE value of Fe 2 O 3 @Pt-2 could reach up to 0.062%, the IPCE and ABPE results are summarized in Table S2, Supporting Information.Therefore, the implantation of Pt NCs can improve the photoelectric conversion efficiency of the films, and further improve their photoelectrochemical properties.
To further characterize the effect of Pt NCs implantation on the properties of the samples, electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) tests were carried out.Figure 3f shows the electrochemical impedance spectra of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.Table S3, Supporting Information, shows the solution resistance R sol , the transfer resistance R bulk inside the film, and the charge transfer resistance R ct of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films. [43]It can be clearly seen that the solution resistance R sol of two films has no obvious change, while the interfacial properties (charge transfer resistance R ct ) change greatly.The charge transfer resistance R ct of Fe 2 O 3 films without Pt NCs is 8425 Ω, while R ct of Fe 2 O 3 @Pt-2 is 619 Ω, which indicates the decrease of recombination after embedding Pt NCs.Meanwhile, the significant decrease of R bulk suggests that more efficient carrier separation and transport inside Fe 2 O 3 @Pt-2 film. [44]Combined with the SEM image, the decrease of charge transfer resistance of Fe 2 O 3 @Pt-2 may be due to the worm-like structure becoming much denser and smaller, good contact between particles, and promoted charge transfers due to the addition of Pt NCs.MS curves (Figure S8, Supporting Information) reveal that the implantation of Pt NCs does not change the type of Fe 2 O 3 (n-type), but results in much shallower slopes, indicating higher carrier densities. [37]And the increase of carrier densities will shift the Fermi level of the bulk (measured potential) toward more cathodic potentials for Fe 2 O 3 @Pt films. [45]Therefore, the addition of Pt NCs can reduce the charge transfer resistance of Fe 2 O 3 thin films, increase the concentration of carriers, which is conducive to carrier transport, and improve the photoelectrochemical properties of Fe 2 O 3 thin films.
It can be found from the SEM images and contact angle results that the implantation of Pt NCs affects the size of worm-like particles and the surface state of the samples.Figure S9, Supporting Information, shows the cyclic voltammetry curves and estimated double-layer capacitance of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films at different scanning rates.As shown in Figure S9c, Supporting Information, the electrochemical active surface area (ECSA) value of Fe 2 O 3 @Pt-2 film is 5.1 μF cm À2 , which is higher than that of Fe 2 O 3 film (3.9 μF cm À2 ).Therefore, the implantation of Pt NCs is beneficial to improve the ECSA of the samples, provide more active sites in the oxygen evolution reaction, and improve the photoelectrochemical properties.
To analyze the effect of Pt nanocrystals implantation on carrier transfer, we measured the surface charge injection efficiency (η inj ) and the bulk charge separation efficiency (η sep ) of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.The values of η inj and η sep were based on the calculation according to Equation (S6) and (S7), Supporting Information, where J H 2 O , J H 2 O 2 , light harvesting efficiency, and J abs for each photoanode are shown in Figure 4a and S10, Supporting Information, and these values are summarized in Table S4, Supporting Information.It can be seen that the η inj of the Fe 2 O 3 @Pt-2 film increases (Figure 4b), at 1.23 V RHE , η inj increased from 19.4% (Fe 2 O 3 ) to 48.3% (Fe 2 O 3 @Pt-2).Furthermore, the above tendency was also observed for η sep at 1.23 V RHE for the Fe 2 O 3 @Pt-2 with respect to Fe 2 O 3 (Figure 4c).At 1.23 V RHE , η sep increased from 20.1% to 40.1%.The above η inj and η sep results indicate that the implantation of Pt NCs can promote the separation of bulk carriers and accelerate the more holes transfer to photoelectrode/electrolyte interface, resulting in the more efficient PEC water oxidation reaction.
Open circuit voltage (OCP) is the difference between the quasi-Fermi level of the electron-hole.The difference between the OCP under the condition of on light and off light is called photovoltage, which is the force to inject the photogenerated hole into the electrolyte.Figure 4d and S11, Supporting Information, show the OCP of Fe 2 O 3 films with different concentrations of Pt NCs.After embedding the Pt NCs, the photovoltage of the films increased significantly, which indicates that the driving force of electron-hole separation is enhanced after adding Pt NCs.The photovoltage of the Fe 2 O 3 film is 104 mV, while that of the Fe 2 O 3 @Pt-2 film is 189 mV, and the OCPs are repeatable under intermittent irradiation (Figure S11b and Table S5, Supporting Information).This indicates that the driving force of electron-hole separation is enhanced after adding Pt NCs.

Unravelling the Roles of Pt Nanocrystals of Fe 2 O 3 Photoanode
The efficient charge transfer inside Fe 2 O 3 @Pt-2 film was further confirmed by intensity modulated photo-current spectroscopy (IMPS).Figure S12a,b, Supporting Information, shows typical IMPS responses of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.And according to Equation (S10) and (S11), Supporting Information, we can extract the values of carrier transport time τ d , charge recombination constants k rec , transport rate constants k trans , and charge transfer efficiency (k tran /(k tran þ k rec )), and these values are summarized in Table S6, Supporting Information.As shown in Figure S12c, Supporting Information, the value of k trans is 111.8 s À1 for Fe 2 O 3 @Pt-2 film, which is about 12 times larger than that of Fe 2 O 3 (9.09s À1 ), while the carrier transport time τ d is lesser (0.05 ms) for Fe 2 O 3 @Pt-2 than that for Fe 2 O 3 (0.184 ms), which is in good agreement with the EIS and OCP measurement.Meanwhile, the charge transfer efficiency (k tran /(k tran þ k rec )) can be improved from 0.17 (Fe 2 O 3 ) to 0.87 (Fe 2 O 3 @Pt-2) (Figure S12d, Supporting Information).These results indicate that the implantation of Pt NCs in during photocatalysis process, not only is prone to speed up the carrier transfer inside photoelectrode, but also reduces charge recombination. [46,47]o explain this phenomenon, we build a model of Fe 2 O 3 @Pt, as shown in Figure 3a, Pt NCs were uniformly embedded in Fe 2 O 3 particles.Because the work function of Pt is larger than that of Fe 2 O 3 , the Schottky barrier can be formed when Pt and Fe 2 O 3 contact (Figure S13a,b, Supporting Information). [45]t this time, photogenerated electrons can be transferred from Fe 2 O 3 to Pt, which can reduce the recombination of electrons and holes.Different from the Schottky barrier formed by two semiconductors, the electrons transferred from Fe 2 O 3 to Pt metal will not accumulate, and usually drift current is formed to transfer away, which is because Pt metal has excellent conductivity.In addition, the formed build-in electric field between Fe 2 O 3 and Pt, as shown schematically in Figure S13c, Supporting Information, which provides an extra channel to facilitate the photogenerated holes transfer and collection.Therefore, the implantation of Pt nanocrystals can not only promote the separation of photogenerated electrons and holes, but also transfer the photogenerated holes quickly, which improves the carrier transport dynamics and improves the photoelectrochemical properties of the films.
Furthermore, the ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) and ultraviolet photoelectron spectroscopy (UPS) analysis of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films are shown in Figure S14, Supporting Information.The UV-vis DRS in Figure S14a, Supporting Information, revealed that the optical bandgap of the Fe 2 O 3 and Fe 2 O 3 @Pt-2 samples is 2.11 and 2.10 eV, respectively.The much narrower bandgap of Fe 2 O 3 @Pt-2 will increase carrier densities, resulting the highest PEC performance of Fe 2 O 3 @Pt-2.The positions of the valence band maximum (VBM) and conduction band minimum (CBM) were estimated to be 2.05 and -0.06 V RHE for Fe 2 O 3 , and 1.99 and -0.11V RHE for Fe 2 O 3 @Pt-2 by UPS spectra in Figure S14b,c, Supporting Information, which is calculated on the previously published. [48]Using VBM, CBM, Fermi levels, and bandgap, as shown in Figure 4e,f and Table S7, Supporting Information, we constructed band diagrams of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films before and after contacting with the electrolytes.It can be seen that the implantation of Pt NCs has a positive effect on the band structure of Fe 2 O 3 films, the lower Fermi level of Fe 2 O 3 @Pt-2 is resulted from the increase of carrier densities. [45]e 2 O 3 @Pt-2 has lower Fermi level than that of Fe 2 O 3 , showing the largest initial energy difference (Fermi level difference between Fe 2 O 3 and electrolyte), resulting the widest depletion width to efficiently separate electron-hole pairs.[48,49]

Discussion
We further prepared sub-5 nm ligand-free Au nanocrystals (Au NCs) in ethanol (precursor solvents) through the PLIL (Figure 5a and S15a, Supporting Information), and then introduced Au NCs into the Fe 2 O 3 film via spin-coating method (Figure 5b and S15d, Supporting Information).The prepared Fe 2 O 3 @Au films show worm-like nanoporous structure, as shown in Figure S15b,c, Supporting Information, the worm-like structure becomes much denser and smaller than Fe 2 O 3 (Figure 2a and 4c).The crystal structure of Fe 2 O 3 @Au film shows no obvious change from XRD and Raman results after the implantation of Au NCs (Figure S16, Supporting Information).The photocurrent density of Fe 2 O 3 @Au could enhance up to 0.62 mA cm À2 at 1.23 V RHE , and the IPCE and ABPE spectra of Fe 2 O 3 @Au film also show obvious enhancement compared with that of Fe 2 O 3 films (Figure 4d and S17, Supporting Information).And the above increase tendency was also observed for the surface charge injection efficiency (η inj ) and the bulk charge separation efficiency (η sep ) of Fe 2 O 3 @Au (Figure S18, Supporting Information), indicating that the implantation of Au NCs also could promote the separation of bulk carriers and more holes can arrive at the photoelectrode/electrolyte interface.Further, the EIS, MS, IMPS, OCP, and ECSA results of Fe 2 O 3 @Au film also demonstrate that the implantation of Au NCs could accelerate the carrier separation and transfer (Figure 5f, S19-S22, and Table S2-S6, Supporting Information), which is consistent with the results of the implantation of Pt NCs.

Figure 1 .
Figure 1.a) Schematic illustration of Pt nanocrystals generated by PLIL.b) TEM image and c) HRTEM image of Pt nanocrystals.d) Size distribution of Pt nanocrystals (insert: photographs of Pt solution before and after PLIL).
photocurrent density of 1.16 mA cm À2 at 1.23 V RHE .Further, the transient photocurrent measurements of pristine Fe 2 O 3 and Fe 2 O 3 @Pt-2 films were performed at 1.23 V RHE under chopped light illumination (Figure3c) to assess the charge recombination behavior at the semiconductor-electrolyte junction.Compared with the Fe 2 O 3 film, Fe 2 O 3 @Pt-2 films can achieve the steady state photocurrent faster, manifesting the significantly improved charge separation efficiency of photoelectrode caused by the embedding of Pt NCs.FigureS6, Supporting Information, shows the UV-vis absorption curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.It can be seen from the figure that the absorption limits of the two films are about 600 nm, In the range of 360-480 nm, two samples show certain optical absorption characteristics, and the difference is not big.Figure3dand S7, Supporting Information, show the incident photon-to-current conversion efficiency (IPCE) curves of the Fe 2 O 3 films with different concentrations of Pt NCs.It is obvious from the figure that the IPCE values of all Fe 2 O 3 films are very low (less than 3%) in the wavelength range of 520-600 nm, and the IPCE values increase rapidly with the decrease of wavelength in the wavelength range of 520-360 nm.The champion efficiency of Fe 2 O 3 @Pt-2 film could reach up to 15.1%, while the IPCE of the Fe 2 O 3 is lower than 5% through the entire responsive region.Figure3eshows the applied bias photon-to-current efficiency (ABPE) curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.ABPE reflects the photoelectric conversion efficiency of the sample under applied bias.It can be seen from the figure that the maximum ABPE value of Fe 2 O 3 film without Pt NCs is 0.017%, and the maximum ABPE value of Fe 2 O 3 @Pt-2 could reach up to 0.062%, the IPCE and ABPE results are summarized in TableS2, Supporting Information.Therefore, the implantation of Pt NCs can improve the photoelectric conversion efficiency photocurrent density of 1.16 mA cm À2 at 1.23 V RHE .Further, the transient photocurrent measurements of pristine Fe 2 O 3 and Fe 2 O 3 @Pt-2 films were performed at 1.23 V RHE under chopped light illumination (Figure3c) to assess the charge recombination behavior at the semiconductor-electrolyte junction.Compared with the Fe 2 O 3 film, Fe 2 O 3 @Pt-2 films can achieve the steady state photocurrent faster, manifesting the significantly improved charge separation efficiency of photoelectrode caused by the embedding of Pt NCs.FigureS6, Supporting Information, shows the UV-vis absorption curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.It can be seen from the figure that the absorption limits of the two films are about 600 nm, In the range of 360-480 nm, two samples show certain optical absorption characteristics, and the difference is not big.Figure3dand S7, Supporting Information, show the incident photon-to-current conversion efficiency (IPCE) curves of the Fe 2 O 3 films with different concentrations of Pt NCs.It is obvious from the figure that the IPCE values of all Fe 2 O 3 films are very low (less than 3%) in the wavelength range of 520-600 nm, and the IPCE values increase rapidly with the decrease of wavelength in the wavelength range of 520-360 nm.The champion efficiency of Fe 2 O 3 @Pt-2 film could reach up to 15.1%, while the IPCE of the Fe 2 O 3 is lower than 5% through the entire responsive region.Figure3eshows the applied bias photon-to-current efficiency (ABPE) curves of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.ABPE reflects the photoelectric conversion efficiency of the sample under applied bias.It can be seen from the figure that the maximum ABPE value of Fe 2 O 3 film without Pt NCs is 0.017%, and the maximum ABPE value of Fe 2 O 3 @Pt-2 could reach up to 0.062%, the IPCE and ABPE results are summarized in TableS2, Supporting Information.Therefore, the implantation of Pt NCs can improve the photoelectric conversion efficiency

Figure 4 .
Figure 4. a) The calculated current density flux and integrated current density ( J abs ) of Fe 2 O 3 and Fe 2 O 3 @ Pt-2 films.b) Surface charge injection efficiency (η inj ) of Fe 2 O 3 and Fe 2 O 3 @ Pt-2 films.c) Bulk charge separation efficiency (η sep ) of Fe 2 O 3 and Fe 2 O 3 @ Pt-2 films.d) Open-circuit potential of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films.Schematic diagram of band structures of Fe 2 O 3 and Fe 2 O 3 @Pt-2 films: e) before and f ) after contacting with the electrolytes.

Figure 5 .
Figure 5. a) TEM image of Au nanocrystals (insert: HRTEM image).b) HRTEM images of Fe 2 O 3 @Au films.c) J-V curve of Fe 2 O 3 and Fe 2 O 3 @Au films.d) IPCE spectra.e) EIS curves under irradiation of Fe 2 O 3 and Fe 2 O 3 @Au film.f ) Open-circuit potential of Fe 2 O 3 and Fe 2 O 3 @Au films.