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X-Shapedα-FeOOHwithEnhancedChargeSeparationforVisible-Light-DrivenPhotocatalyticOverallWaterSplitting

ArticleinChemSusChem·January2018DOI:10.1002/cssc.201800059

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X-Shaped a-FeOOH with Enhanced Charge Separation forVisible-Light-Driven Photocatalytic Overall Water SplittingTianqi Wang,[a] Zhifeng Jiang,*[a, b] Ka Him Chu,[a] Dan Wu,[a] Bo Wang,[a] Hongli Sun,[a]

Ho Yin Yip,[a] Taicheng An,[c] Huijun Zhao,[d, e] and Po Keung Wong*[a]

Introduction

Driven by the global energy crisis and environmental challeng-es, the development of efficient photocatalysts for exploringclean and sustainable energy has attracted increasing atten-tion. Photocatalytic overall water splitting (POWS) is consideredas one of the most effective methods of generating energy.[1]

In the past decades, various types of photocatalysts, such asmetal oxides and oxyhydroxides, have been synthesized andutilized for water splitting.[2] Admittedly, numerous photocata-lysts exhibit good performance for H2 or O2 generation, butmost only execute water-splitting half-reactions (i.e. , water oxi-dation or reduction),[3] with even fewer photocatalysts able todirectly split water to O2 and H2 under visible light (VL) irradia-tion.[4] In addition, to improve the POWS performance of pho-tocatalysts, the loading of co-catalysts such as Pt, Au, andRuO2 has been commonly adopted to lower the activationenergy/overpotential and/or assist the electron–hole separa-tion.[5] However, the adoption of co-catalysts significantly in-creases the cost and hinders the large-scale application of thephotocatalysts. Therefore, a cost-effective and VL-induced pho-tocatalytic system for POWS without using a co-catalyst is ur-gently required to meet the needs of terawatt solar energytransfer.

Among catalysts based on earth-abundant metals, iron oxy-hydroxides (e.g. , a-, b-, g- and d-FeOOH) are good alternativesfor water splitting under VL because of their narrow bandgaps and high stability.[6] For example, Seabold and Choi re-ported a BiVO4-coated FeOOH photocatalyst with enhancedwater oxidation performance.[7] Kim et al. reported a FeOOH/Fe2O3 thin film for efficient solar water splitting.[8] In addition,bare d-FeOOH and Ni(OH)2-loaded d-FeOOH were used aspromising photocatalysts for H2 production.[6d, 9] However, tothe best of our knowledge, the direct splitting of pure waterinto H2 and O2 by bare a-FeOOH has not been studied; there-fore, the feasibility of POWS over a-FeOOH is unknown.

Photocatalytic overall water splitting (POWS) is a promisingroute for converting solar energy into green and sustainableenergy. Herein, we report a facile hydrothermal approach forthe fabrication of x-shaped a-FeOOH photocatalysts containinghigh-index facets for POWS. The x-shaped a-FeOOH photocata-lysts exhibited enhanced visible-light-driven POWS activities incomparison with that of FeOOH without x-structures, with amaximum H2 and O2 evolution rate of 9.2 and 4.7 mmol h�1 g�1,respectively. The morphology and particle size of the a-FeOOHcould be controlled by adjusting the NH4F concentration inthe precursors. The photodeposition of Pt and RuO2 on the x-shaped a-FeOOH revealed the specially separated reduction

and oxidation centers on the surface of a-FeOOH, with the oxi-dation-active sites selectively located on the edges of the a-FeOOH x-structures. Electrochemical experiments further af-firmed the enhanced charge separation in the x-shaped a-FeOOH. The smaller particle size and unique x-shape of the a-FeOOH photocatalyst were shown to enhance the POWS per-formance owing to the large specific surface area, high propor-tion of exposed high-index facets, high electron-transfer effi-ciency and effective separation of the photogenerated elec-tron–hole pairs. The current study revealed that the x-shapeda-FeOOH products could serve as cost-effective and stablephotocatalysts for POWS.

[a] T. Wang, Z. Jiang, K. H. Chu, D. Wu, B. Wang, H. Sun, H. Y. Yip, P. K. WongSchool of Life SciencesThe Chinese University of Hong KongShatin, NT, Hong Kong (P.R. China)E-mail : ntjiangzf@sina.com

pkwong@cuhk.edu.hk

[b] Z. JiangInstitute for Energy Research, School of Chemistry and Chemical Engineer-ingJiangsu UniversityZhenjiang, Jiangsu 212013 (P.R. China)E-mail : ntjiangzf@sina.com

[c] T. AnInstitute of Environmental Health and Pollution Control, School of Environ-mental Science and EngineeringGuangdong University of TechnologyGuangzhou 510006, Guangdong (P.R. China)

[d] H. ZhaoCentre for Clean Environment and Energy, Griffith Scholl of EnvironmentGriffith UniversityQueensland 4222 (Australia)

[e] H. ZhaoLaboratory of Nanomaterials and NanostructuresInstitute of Solid State Physics, Chinese Academy of SciencesHefei 230031, Anhui (P.R. China)

Supporting Information and the ORCID identification number(s) for theauthor(s) of this article can be found under:https://doi.org/10.1002/cssc.201800059.

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The photocatalytic efficiency of micro-/nano-semiconductorsis closely interrelated with their shape, size, and exposedfacets.[10] For example, the degradation rate of rhodamine B byFeOOH nanorods was more than two times faster than that byFeOOH microrods.[11] a-FeOOH particles with various morphol-ogies (e.g. , rod-like, array-like, and pancake-like morphology)exhibited different photocatalytic efficiencies toward the de-composition of acetaldehyde, which was attributable to thedifferences in their shapes and specific surface areas.[12] In addi-tion, it has been widely reported that the high-index facetspromote the photocatalytic performance of semiconductorsowing to the high density of atomic steps, ledges, and kinks,which usually serve as active sites for surface catalytic reac-tions.[13] However, the high-index facets usually disappearduring crystal growth to minimize the total surface energy,which makes it difficult to fabricate photocatalyst with ex-posed high-index facets.[14] Thus, it is interesting to design andsynthesize high-index-faceted photocatalysts to promote theefficiency of POWS.

Herein, we report a facile method to fabricate a unique x-structured a-FeOOH photocatalyst enclosed by 16 high-index{31̄1̄} facets. The POWS performance and photostability of theas-prepared a-FeOOH photocatalysts were investigated underVL irradiation. The effect of the morphology, facet, and size onPOWS activity was also examined. The work presented herecan provide a simple yet effective approach to prepare a-FeOOH-based photocatalysts for POWS.

Results and Discussion

As shown in the X-ray diffraction (XRD) pattern of the FeOOHsample synthesized with 30 mm of NH4F (FeOOH-30F; Fig-ure 1 A), all diffraction peaks were indexed to a-FeOOH (PDFcard No.: 99-0055), which indicated that a-FeOOH was success-fully prepared in a pure phase. The unique x-like structure ofFeOOH-30F was clearly observed in the transmission electronmicroscopy (TEM) image (Figure 1 B), with an angle of approxi-mately 1178 between two branches. As shown in Figure 1 D–F,

Figure 1. (A) XRD pattern, (B) TEM image, and (C–F) SAED patterns (left) and HRTEM images (right) of FeOOH-30F (SAED and HRTEM were obtained from thepositions labeled in panel (B)).

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all the selected-area electron diffraction (SAED) patterns ob-tained from the different positions labeled in Figure 1 B re-vealed an angle of 58.98 between the (130) and (112) planes ofa-FeOOH, which was identical to the theoretical angle be-tween the (130) and (112) planes. The corresponding latticefringes of approximately 0.26 and 0.15 nm in the high-resolu-tion transmission electron microscopy (HRTEM) imagesmatched well with the interplanar spacings of the (130) and(112) planes of a-FeOOH, respectively. The predominantly ex-posed facets of the x-shaped a-FeOOH crystals could be calcu-lated to be high-index {31̄1̄} facets according to the crystallo-graphic structure of a-FeOOH. Therefore, the x-shaped a-FeOOH crystal was enclosed with 16 high-index {31̄1̄} facets.Based on a previous study,[15] the x-shaped a-FeOOH with mul-tiple high-index {31̄1̄} facets may serve as an efficient photoca-talyst for POWS.

To better understand the formation mechanism of the x-shaped a-FeOOH, some conditional experiments were per-formed. As demonstrated in the field-emission scanning elec-tron microscopy (FESEM) images (Figure 2 and Figure S1 in the

Supporting Information), NH4F played an important role incontrolling the morphology of the products. In the absence ofNH4F (Figure S1 d), the product was composed of numerousnanospheres with diameters of approximately 50 nm. With aninitial NH4F concentration of 30, 60, and 120 mm, the resultantproducts displayed uniform x-shaped structures with smoothsurfaces; the particle sizes increased with increasing NH4F con-centration (Figure 2 A–C). However, when the NH4F concentra-tion was increased to 180 mm, some microrods and nano-spheres were observed (Figure 2 D). The results indicated thatthe x-shaped a-FeOOH could be produced only at suitable

NH4F concentrations. Details of the relationship between theparticle size of the x-shaped crystals and the NH4F concentra-tion are shown in Table 1. In addition, the hydrothermal tem-perature for the preparation of the x-shaped a-FeOOH was op-

timized. As shown in Figure S2 (Supporting Information), the x-shaped structures with some irregular particles were initiallyformed at a hydrothermal temperature as low as 100 8C. Thesample prepared at 120 8C (Figure S2 c) displayed uniform x-shaped structures. No noticeable changes in the morphologywere observed when the reaction temperature was increasedto 140 8C (Figure S2 d). Thus, the optimized hydrothermal tem-perature for the fabrication of x-shaped a-FeOOH was 120 8C.Moreover, the phase composition of the products was also de-pendent on the chemicals used. As observed in the FESEMimages (Figure S1) and the XRD patterns (Figure S3), Fe2O3

nanospheres were obtained in the absence of NH4F, b-FeOOH(akaganeite) was obtained with irregular shapes in the absenceof Fe2 + or urea, whereas hexagonal Fe3O4 disks with strongmagnetic properties and FeOOH microrods were obtainedwithout the addition of Fe3 + . Thus, all four chemicals (i.e. ,FeCl2, FeCl3, NH4F, and urea) were essential for the formation ofpure a-FeOOH with uniform x-shaped structures. To under-stand the effect of the ions (NH4

+ and F�) on the formation ofx-shaped morphology, NH4Cl or NaF were used instead ofNH4F. As illustrated in the FESEM images (Figure S1 e and f), anx-shaped morphology was observed in the product preparedwith NaF, whereas the morphology of the product preparedwith NH4Cl was similar to that synthesized in the absence ofNH4F. This suggested that F� rather than NH4

+ ions contribut-ed to the unique x-shaped morphology of a-FeOOH. The po-tential role of the F� ions in the formation of x-shaped a-FeOOH was inhibition of the crystallization growth rate in the[010] direction by the preferential adsorption of the F� ions onthe (010) faces of the a-FeOOH crystals during the crystalliza-tion growth process, leading to the formation of fourbranches.[16]

The diffraction peaks in the XRD patterns of the FeOOH sam-ples synthesized with 30, 60, and 120 mm NH4F (FeOOH-30F,FeOOH-60F, and FeOOH-120F) were all indexed to a-FeOOH(Figure 3; PDF card No.: 99-0055, marked with *), and no im-purity peaks were observed. However, some b-FeOOH peakswere observed in the XRD pattern of the FeOOH sample syn-thesized with 180 mm NH4F (FeOOH-180F; Figure 3, markedwith #), which indicated that a high NH4F concentration leads

Figure 2. FESEM images of (A) FeOOH-30F, (B) FeOOH-60F, (C) FeOOH-120F,and (D) FeOOH-180F.

Table 1. The average particle sizes, specific surface areas, pore volumesand pore sizes of the as-synthesized samples.

Sample Length[mm]

Width[mm]

Surface area[m2 g�1]

Pore volume[cm3 g�1]

Pore size[nm]

FeOOH-30F 0.60 0.35 29.60 0.10 2.37FeOOH-60F 1.10 0.56 19.51 0.05 1.32FeOOH-120F 3.60 1.82 11.30 0.03 1.32FeOOH-180F 5.85 3.20 19.07 0.05 1.32plate-like FeOOH 0.43 0.35 40.19 0.09 4.08

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to the formation of b-FeOOH. The FESEM image of the FeOOHwithout x-shaped structures (Figure S4 a) revealed a hierarchi-cal plate-like morphology, with an average particle size of ap-proximately 0.32 mm and a thickness of approximately 0.04 mm.According to the XRD pattern (Figure S4 b), the as-synthesizedplate-like sample was indexed to pure a-FeOOH.

The UV/Vis diffuse reflectance spectra (DRS) of the x-shapedFeOOH products are shown in Figure 4 A. The four FeOOHsamples shared almost the same light absorption profiles. Allthe absorption edges were centered in the VL region at ap-proximately 600 nm. The FeOOH-30F sample exhibited thehighest light adsorption capacity almost over the entire wave-length range, which indicated a relatively stronger light captur-ing ability. The band gap energy (Eg) of semiconductors wasestimated by using the Kubelka–Munk function [Eq. (1)]:

ahu ¼ Aðhu�EgÞn=2 ð1Þ

in which a is the absorption coefficient, hu is the photoener-gy, and n = 1 for a-FeOOH as a direct semiconductor.[17] Asshown in the plots of transformed Kubelka–Munk functionversus light energy (Figure 4 A, inset), the Eg values of FeOOH-30F, FeOOH-60F, FeOOH-120F, and FeOOH-180F samples were2.06, 2.12, 2.10, and 2.00 eV, respectively, which are consistentwith previous studies.[11, 18] To better understand the bandstructure of x-shaped FeOOH, the position of the valence bandmaximum (VBM) of FeOOH-30 as an example was estimatedusing the valance band X-ray photoelectron spectroscopy(XPS) spectrum (Figure 4 B) to be + 1.68 eV (vs. NHE).

Figure 3. XRD patterns of FeOOH-30F, FeOOH-60F, FeOOH-120F and FeOOH-180F.

Figure 4. (A) UV/Vis DRS patterns of x-shaped FeOOH photocatalysts (inset shows the plots of the transformed Kubelka–Munk function versus light energy),(B) valance band XPS spectrum, (C) Mott–Schottky plot and (D) band structure of FeOOH-30F.

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The conduction band minimum (CBM) of FeOOH-30F wascalculated to be �0.38 eV (vs. NHE) by using Equation (2):

ECB ¼ EVB�Eg ð2Þ

in which ECB is the CBM potential and EVB is the VBM poten-tial.[19] This value is consistent with that determined from theMott–Schottky plot (Figure 4 C). Based on the results, a dia-gram of the band structure of FeOOH-30F is presented in Fig-ure 4 D.

The elemental composition and valence state of the as-pre-pared a-FeOOH photocatalyst were characterized by XPS(Figure 5). In the XPS spectra of Fe 2p of FeOOH-30F (Fig-ure 5 A), two major peaks of FeIII together with two shake-upsatellites (sat.) were observed at 711.3 (Fe 2p3/2), 725.0 (Fe 2p1/2),719.8 (Fe 2p3/2 sat.), and 733.9 eV (Fe 2p1/2 sat.), respectively, in-dicating that the chemical state of Fe in the x-shaped a-FeOOH was 3 + .[20] In the region of O 1s in the XPS spectrum(Figure 5 B), there were two strong peaks, which could be de-convoluted into three peaks located at 529.7, 531.0, and532.3 eV; these peaks were attributed to Fe�O�Fe bond, Fe�O�H bond, and H�O�H bond, respectively .[21]

The POWS performance of the x-shaped a-FeOOH sampleswas investigated under VL irradiation without using any sacrifi-cial agent or co-catalyst. Figure 6 A shows a typical time courseof POWS over FeOOH-30F. Nearly stoichiometric evolution of

hydrogen and oxygen was observed within 12 h, with a H2/O2

molar ratio of approximately two, indicating that overall watersplitting did occur. As demonstrated in Figure 6 B, FeOOH-30Fshowed the highest H2 and O2 evolution rates of 9.2 and4.7 mmol h�1 g�1, respectively. For the FeOOH-60F, FeOOH-120Fand FeOOH-180F samples, the generated amounts of H2/O2

were 6.1/3.2, 5.4/2.7 and 1.9/1.0 mmol h�1 g�1, respectively.Considering the photocatalytic performance, the x-shaped a-FeOOH photocatalysts prepared in this work had the highestphotocatalytic performance for VL water splitting comparedwith those of recently reported studies (see details in Table S1).The results also suggested that the gas amounts generated in-creased with decreasing particle size, which was mainly attri-buted to the change in the specific surface area of the sample.As shown in Figure 7 and Table 1, the FeOOH-30 sample exhib-ited the largest specific surface area of 29.60 m2 g�1, followedby the FeOOH-60, FeOOH-180, and FeOOH-120 samples in se-quence. The larger surface area may provide more active sites,which can contribute to the adsorption of molecules and lightharvesting, and thus enhance the photocatalytic efficiency.[17a]

Despite the large specific surface area, a smaller particle sizenormally favors the photogenerated electrons reaching thesurface for better utilization in the photochemical process,[6c]

which was also reported by Wang and co-authors.[22] Therefore,the small particle size of the a-FeOOH catalysts was crucial forthe photocatalytic activity.Figure 5. XPS spectra of (A) F 2p and (B) O 1s of FeOOH-30F.

Figure 6. (A) Time courses of the photocatalytic H2 and O2 evolution frompure water over FeOOH-30F under VL irradiation and (B) amount of H2 andO2 generated photocatalytically over different FeOOH photocatalysts underVL irradiation.

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As photogenerated electrons have an important influenceon the POWS process, transient photocurrent responses weremeasured by performing several on–off light irradiation cyclesto understand the photophysical behavior of the photoexcitedcharge carriers. As compared in Figure 8, the FeOOH-30F elec-trode exhibited the highest photocurrent intensity, followed bythe FeOOH-60, FeOOH-120F and FeOOH-180F electrodes.Because the photocurrent density is related to the separation

efficiency of photogenerated electron–hole pairs within elec-trodes,[23] the higher photocurrent density implies a lower elec-tron–hole pair recombination rate and a higher electron trans-fer efficiency. The photocurrent experimental results providedanother potential reason for the different POWS efficiencies ofx-shaped a-FeOOH photocatalysts.

To demonstrate the importance of the special x-like struc-ture, the POWS efficiencies of the x-shaped FeOOH and plate-like FeOOH are compared (Figure 9 A ). Clearly, the x-shapedFeOOH-30F showed a much higher gas evolution rate thanplate-like FeOOH. More importantly, for the plate-like FeOOHsample, no H2 and O2 were generated after 4 h, indicating arelatively low photocatalytic activity. As shown in Figure 9 B,the photocurrent of the x-shaped FeOOH was significantlyhigher than that of plate-like FeOOH, indicating a higher pho-togenerated electron-transfer efficiency of the x-shapedFeOOH sample. The low recombination and improved separa-tion of the photogenerated electron–hole pairs of the x-shaped FeOOH could be also evidenced using the room-tem-perature photoluminescence (PL) spectra (Figure 9 C): it had amuch lower PL intensity compared to that of plate-like FeOOH.In addition, in the Nyquist plot (Figure 9 D) the x-shapedFeOOH exhibited a semicircle with a smaller diameter thanthat of plate-like FeOOH, indicating a faster interfacial chargetransfer to the electron acceptor.[24] Therefore, the x-like struc-tures with multiple exposed high-index facets can facilitate thecharge transfer and suppress the recombination of charge car-riers. In addition, although the plate-like FeOOH exhibited alarger specific surface area (Table 1 and Figure 7), its POWS effi-ciency was still lower than that of the x-shaped FeOOH. Thisobservation suggested that the morphological evolution of x-shaped FeOOH contributes more to the POWS performancethan the specific surface area.

To further understand the mechanism of efficient chargeseparation for x-shaped FeOOH, the photodeposition of Pt andRuO2 on x-shaped FeOOH was performed. The location of thereduction- and oxidation-active sites of the photocatalysts canbe indicated by the distribution of photodeposited Pt andRuO2 particles, respectively. As shown in Figure 10 A, a largenumber of Pt particles were evenly deposited on the surfaceof FeOOH, indicating that the abundant reduction-active siteswere uniformly distributed on FeOOH. Furthermore, the RuO2

particles were mainly concentrated at the top edges of FeOOHx-structures, whereas the facets showed nearly smooth surfa-ces (Figure 10 B). The selective deposition of RuO2 implied thatthe oxidation-active sites were mainly distributed on theedges. It could be induced that under irradiation the photo-generated electrons efficiently and evenly transfer to the sur-face of the x-shaped FeOOH crystals, whereas the holes pre-ferred to migrate to the edges. Therefore, efficient electrontransfer and charge-carrier separation were realized, whicheventually improved the POWS performance of x-shapedFeOOH.

The proportion of exposed facets has a significant influenceon the photocatalytic activity.[25] A comparison study of thePOWS performance between the FeOOH with a low proportionof exposed {31̄1̄} facets (FeOOH-LP) and FeOOH-120F was con-

Figure 7. (A) N2 adsorption–desorption isotherms and (B) pore-size distribu-tions of the as-prepared samples.

Figure 8. Transient photocurrent responses of FeOOH samples (conditions:potential = + 0.6 V; reference electrode: Ag/AgCl; counter electrode: Pt;electrolyte solution: Na2SO4, 0.1 m).

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ducted. As shown in Figure S5 a and b, the FeOOH-LP andFeOOH-120F showed similar particle sizes of 3.9 and 3.6 mm inlength, respectively. However, the branches of FeOOH-LP weremuch shorter, leading to a lower proportion of exposed high-index {31̄1̄} facets. The morphological structures of FeOOH-LPand FeOOH-120F are illustrated in Figure S5 c and d. The pro-portions of exposed {31̄1̄} facets of FeOOH-LP and FeOOH-120F were approximately 77 % and 96 %, respectively. ThePOWS efficiency of FeOOH-120F was higher than that ofFeOOH-LP (Figure S6) because the exposed {31̄1̄} facets provid-ed a high number of well-separated reduction- and oxidation-active sites, which is crucial for POWS activity, as discussedearlier.

The cycling tests for the VL-driven photocatalytic activitiesof x-shaped a-FeOOH photocatalyst were conducted to investi-gate its stability and reusability. After every 12 h of irradiation,the system was evacuated and irradiated again without the re-covery of photocatalyst. As illustrated in Figure 11, no noticea-ble decrease in the H2 and O2 evolution rate was detectedafter five consecutive cycles (60 h in total), revealing excellentphotostability and reusability of the x-shaped a-FeOOH. Fig-ure S7 shows the XPS spectrum and XRD pattern of FeOOH-30F after the POWS reaction. The results indicated that the

Figure 10. FESEM images of FeOOH-30F after the photodeposition of (A) Ptand (B) RuO2. Scale bars correspond to 500 nm.

Figure 11. Time courses of the H2 and O2 evolution from pure water overFeOOH-30F under VL irradiation.

Figure 9. (A) Photocatalytic H2 and O2 evolution from pure water over FeOOH-30F and plate-like FeOOH under VL irradiation, (B) transient photocurrent re-sponses (the conditions were the same as those shown in Figure 8), (C) room-temperature photoluminescence spectra (lex = 365 nm), and (D) Nyquist plots ofFeOOH-30F and plate-like FeOOH.

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chemical state of Fe and the phase and structure of a-FeOOHremained intact, further suggesting that the a-FeOOH photo-catalyst was stable during the POWS process. Therefore, the x-shaped a-FeOOH samples could serve as cost-effective andhighly stable photocatalysts for VL-driven overall watersplitting.

Conclusions

An efficient and stable photocatalyst, a-FeOOH with a uniformx-shaped morphology and multiple exposed high-index facets,was successfully synthesized through a simple hydrothermalmethod for photocatalytic overall water splitting (POWS) appli-cation. The effects of various physicochemical parameters onthe formation of x-shaped a-FeOOH were investigated. Effi-cient POWS was achieved by this newly prepared photocata-lyst, with a maximum H2 and O2 generation rate of approxi-mately 9.2 and 4.7 mmol h�1 g�1, respectively, from pure waterunder visible-light (VL) irradiation. The good POWS per-formance of the x-shaped a-FeOOH photocatalyst was ascribedto the large surface area, exposure of a high proportion ofhigh-index facets, high electron transfer rate, and effectivecharge separation. The recycling experiments indicated excel-lent stability of the x-shaped a-FeOOH photocatalyst. Thiswork provides an alternative approach for designing a cost-ef-fective, stable, size controllable, co-catalyst free and VL-drivenphotocatalyst for solar-to-fuel conversion.

Experimental Section

Materials preparation

Typically, FeCl2 (60 mmol), FeCl3·6 H2O (60 mmol), urea (300 mmol),and NH4F (30 mmol) were dissolved in deionized water (70 mL) byvigorous stirring. Subsequently, the mixed solution was transferredto a 100 mL Teflon-lined stainless-steel autoclave, hydrothermallyheated at 120 8C for 12 h in a forced-air oven (DNG-9036A, Shang-hai Jinghong Manufacturing Company, China), and then allowed tocool to room temperature. After that, the slurry was mixed with200 mL ethanol (95 %) under ultrasonication, and purified using amagnet several times to completely remove the magnetic impuri-ties. The resulting yellowish–brown products were obtained by fil-tering the mixture through a membrane filter (0.2 mm, Millipore),washing several times with deionized water and ethanol, and final-ly drying at 60 8C for 12 h. To investigate the effect of the NH4Fcontent on the morphology of the products, NH4F concentration-dependent experiments were conducted by using different initialconcentrations of NH4F (i.e. , 30, 60, 120, and 180 mmol), the prod-ucts were denoted as FeOOH-30F, FeOOH-60F, FeOOH-120F andFeOOH-180F. For comparison, the plate-like a-FeOOH structureswere prepared according to a previously reported method.[26] Inbrief, 5 mmol of FeSO4·7 H2O and 10 mmol of CH3COONa were dis-solved in 50 mL of deionized water and then vigorously stirred at40 8C for 6 h. The slurry was centrifuged and thoroughly washedwith deionized water and ethanol and finally dried at 60 8C for12 h. The product was denoted as plate-like FeOOH. Moreover, tobetter understand the effect of the proportion of the dominantlyexposed {31̄1̄} facets on the photocatalytic activity, FeOOH with alow proportion of exposed high-index facets (FeOOH-LP) was pre-pared. The synthesis procedure was similar as that of FeOOH-120F,

except that the concentration of urea was decreased to 200 mmoland the reaction temperature was decreased to 110 8C. All chemi-cals used in the experiments were of reagent grade and used as re-ceived without further purification.

Characterization

The morphology of the samples was investigated using a Quanta400F field-emission scanning electron microscope (FEI Company,USA) and a Tecnai F20 high-resolution transmission electron micro-scope (FEI Company, USA). The UV/Vis DRS were recorded using aVarian Cary 100 UV/Vis spectrophotometer (Agilent TechnologiesInc. , USA) equipped with a labsphere diffuse reflectance accessory.The XRD patterns were obtained on a SmartLab X-ray diffractome-ter (Rigaku Corporation, Japan) operating at 40 mA and 40 kV withCuKa radiation. XPS measurements were performed using aThermo Scientific ESCALAB 250XI X-ray photoelectron spectrome-ter (Thermo Scientific, USA). The specific surface areas and pore-size distributions were determined using a Quadrasorb-SI SurfaceArea and Pore Size Analyzer (Quantachrome Instruments, USA).Room-temperature PL spectra were recorded with an FP-6500 fluo-rescence spectrometer (Jasco Corporation, Japan) using and excita-tion wavelength (lex) of 365 nm.

Photodeposition of Pt and RuO2 on x-shaped FeOOH

The photodeposition of Pt and RuO2 was conducted using H2PtCl6

and RuCl3 as metal precursors, respectively. In a typical procedure,to achieve the photodeposition of 10 wt % of Pt or RuO2, 5 mgFeOOH-30F and a calculated amount of H2PtCl6 or RuCl3·x H2O weredispersed in 20 mL of methanol (20 %) or NaIO3 (0.02 m) solution,respectively. Then, the suspension was illuminated with a 300 WXeon lamp (l�420 nm) for 1 h under continuous stirring. Theproducts were filtered, washed thoroughly with deionized waterand ethanol, and dried at 60 8C for 12 h.

Photoelectrochemical measurements

The transient photocurrent responses and Nyquist plots were ob-tained from an electrochemical workstation (CHI 660D, ShanghaiChenhua Instrument Company, China) in a three-electrode quartzcell with Na2SO4 (0.1 m) electrolyte solution. Ag/AgCl and Pt wereused as the reference and the counter electrodes, respectively. Typ-ically, 5 mg of the as-prepared photocatalyst and 10 mL ofNafion 117 solution (5 wt %) were well dispersed in a 0.5 mL water/isopropanol mixed solvent (3:1 v/v) by sonication to form a homo-geneous colloid. Subsequently, 100 mL of the colloid was depositedon a fluorinated-tin-oxide (FTO) glass with an area of 1 cm2 anddried in air at room temperature. The electrodes were held at a po-tential of + 0.6 V. A 300 W Xeon lamp was used as the light sourcein the photocurrent measurements.

Photocatalytic reactions

Photocatalytic reactions were conducted at approximately 25 8C ina Labsolar-IIIAC closed gas circulation system (Beijing Perfect LightTechnology Co., Ltd China). Typically, 100 mg photocatalyst wasdispersed in 100 mL of pure water in a 400 mL Pyrex reactor byusing a magnetic stirrer. Prior to irradiation, the suspension wasevacuated several times to completely remove air. Subsequently,the suspension was irradiated using a 300 W Xeon lamp (PLS-SXE300C, Beijing Perfect Light Technology Co., Ltd China) with a

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420 nm UV cut-off filter. At given time intervals, the evolved gaseswere analyzed using an online GC-7990II gas chromatograph (Zhe-jiang Fuli Analytical Instrument Co., Ltd. , China) equipped with a5 � molecular sieve column and a thermal conductivity detector(TCD). Argon was used as the carrier gas.

Acknowledgements

This research was financially supported by the research grant(GRF14100115) from Research Grant Council and ITSP Tier 3Scheme (ITS/216/14) from Innovation and Technology Commis-sion of Hong Kong SAR Government. H.J.Z. and P.K.W. were alsosupported by the CAS/SAFEA International Partnership Programfor Creative Research Teams (2015HSC-UE004) of Chinese Acade-my of Sciences, China. Z.F.J. was supported by the National Natu-ral Science Foundation (21706102), Natural Science Foundationof Jiangsu (BK20170527), Postdoctoral Research Funding ofJiangsu Province (1701102B) and Hong Kong Scholar Program(XJ2016034), China. The authors would also like to acknowledgethe technical supports from the Prof. Jimmy Yu Laboratory andTianhe Scientific Collaboration Center, China.

Conflict of interest

The authors declare no conflict of interest.

Keywords: charge separation · high-index facets · ironoxyhydroxide · photocatalysis · water splitting

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Manuscript received: January 9, 2018

Accepted manuscript online: January 29, 2018Version of record online: && &&, 0000

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T. Wang, Z. Jiang,* K. H. Chu, D. Wu,B. Wang, H. Sun, H. Y. Yip, T. An, H. Zhao,P. K. Wong*

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X-Shaped a-FeOOH with EnhancedCharge Separation for Visible-Light-Driven Photocatalytic Overall WaterSplitting

X marks the spot! An x-shaped a-FeOOH photocatalyst with multiplehigh-index facets and enhanced chargeseparation is fabricated and used forphotocatalytic overall water splittingunder visible-light irradiation. The mor-phology and particle size of a-FeOOH iscontrolled by adjusting the NH4F con-centration in the precursors. The x-shaped a-FeOOH photocatalysts exhib-its higher water-splitting activity thanFeOOH without an x-structure.

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