facile synthesis of surface-modified nanosized α-fe 2 o 3...

Facile Synthesis of Surface-Modified Nanosized α‑Fe2O3 as EfficientVisible Photocatalysts and Mechanism InsightWanting Sun, Qingqiang Meng, Liqiang Jing,* Dening Liu, and Yue Cao

Key Laboratory of Functional Inorganic Materials Chemistry,

Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China

*S Supporting Information

ABSTRACT: In this study, α-Fe2O3 nanoparticles with highvisible photocatalytic activity for degrading liquid-phase phenoland gas-phase acetaldehyde have been controllably synthesizedby a simple one-pot water-organic two-phase separatedhydrolysis-solvothermal (HST) method. Further, the visiblephotocatalytic activity is enhanced greatly after modificationwith a proper amount of phosphate. The enhanced activity isattributed to the increased charge separation by promotingphotogenerated electrons captured by the adsorbed O2 bymeans of the atmosphere-controlled surface photovoltagespectra, along with the photoelectrochemical I−V curves. Onthe basis of the O2 temperature-programmed desorption measurements, it is suggested for the first time that the promotion effectresults from the increase in the amount of O2 adsorbed on the surfaces of Fe2O3 by the partial substitution of −Fe−OH with−Fe−O−P−OH surface ends. Expectedly, the positive strategy would be also applicable to other visible-response nanosizedoxides as efficient photocatalysts. This work will provide us with a feasible route to synthesize oxide-based nanomaterials withgood photocatalytic performance.

1. INTRODUCTION

Hematite (α-Fe2O3), a kind of thermodynamically stable ironoxide phase under ambient conditions with virtues of low cost,good corrosion resistance, and excellent environmentalcompatibility, has become the focus of intensive research forwidespread potential applications in many fields includingcatalysis, pigments, gas sensors, field emission, and lithium ionbattery electrodes.1−8 As stimulated by the aforementionedpromising applications, much attention has been paid to thecontrolled synthesis of α-Fe2O3 by various methods, includingvapor−solid growth technique, high-energy ball milling,chemical precipitation, sol−gel process, hydrothermal approachand so forth.9−14 These techniques, however, usually exhibitmarked shortcomings, such as weak crystallinity, poormonodispersity, needed complicated synthesis, and post-treatment procedures. The shortcomings would greatlyinfluence the widely practical applications of Fe2O3. Therefore,it is desirable to develop a facile approach to controllablysynthesize α-Fe2O3 with ideal performance.Semiconductor photocatalysis has attracted much attention

in recent years owing to its applications to environmentalpurification and to produce solar fuel as sustainable energyresource, and TiO2 is taken as one of the ideal photocatalystsdue to its virtues.15−18 However, one critical drawback of TiO2is that its band gap is so large that it is active only under UVlight as a small portion of the solar spectrum. Presently, α-Fe2O3, as an n-type semiconductor with an indirect band gap of2.0−2.2 eV that allows for the absorption of substantial

amounts of the incident visible solar spectrum,19,20 hasattracted tremendous interest in its potential application forphotocatalytic degradation of organic contaminations in bothwater and air,21−23 and for photoelectrochemical water splittingto produce H2 as a popular solar fuel.24−26 However, itsperformance is not ideal for practical application. To improvethe performance of α-Fe2O3, a great deal of effort has beenmade up to day. For example, α-Fe2O3/SnO2 incorporatingsemiconductors,27,28 Fe3O4@Fe2O3 core/shell nanoparticles,

29

hybrid Fe2O3−Pd nanoparticles,20 and Fe2O3 doped withmetallic and nonmetallic elements,30,31 have been carried out toenhance the photocatalytic activity for degrading organicpollutants and water splitting with certain successes.Generally speaking, because photocatalytic reactions typically

occur at the surfaces of oxide photocalalysts,32 surfacemodification would influence the photocatalytic performanceby altering the electron- or hole-induced reaction paths.Inorganic nonmetal redox-inert anions, often used as surfacemodifiers, have been reported to improve the photocatalyticactivity of TiO2 under ultraviolet illumination.

33−36 Our grouphave recently demonstrated that the activity of TiO2 fordegrading pollutants and water splitting is enhanced obviouslyafter phosphate modification.37,38 The phosphate anions areabundant in nature with several advantages,39,40 such as strong

Received: September 27, 2012Revised: December 24, 2012Published: January 2, 2013

Article

pubs.acs.org/JPCC

© 2013 American Chemical Society 1358 dx.doi.org/10.1021/jp309599d | J. Phys. Chem. C 2013, 117, 1358−1365

pubs.acs.org/JPCC

bonding ability, high negative-charge, easy formation ofhydrogen bond, and chemical redox-inertness toward photo-generated electrons and holes. Thus, modified phosphategroups would significantly influence the surface chemistry ofnanosized oxides. Although surfactant phosphate anion as anefficient banding ligand has been found to determine the shapeof Fe2O3 nanoparticles,

41,42 to the best of our knowledge, fewstudies are involved with the phosphate modification toimprove the visible photocatalytic activity of Fe2O3. In addition,it is widely accepted that the step that the photogeneratedelectrons are captured by the adsorbed O2 is very crucial forefficient photocatalytic reactions by preventing the buildup ofnegative charges,37 which is very meaningful for us tounderstand the mechanisms on the enhanced photocatalyticactivity and to design high-activity oxide-based visible photo-catalytic nanomaterials by surface nanoengineering strategies.Surprisingly, this step is often neglected. This greatly spurs usto carry out this work.Herein, we first have successfully developed a one-pot water-

organic two-phase separated hydrolysis-solvothermal (HST)approach to controllably prepare α-Fe2O3 nanoparticles withhigh photocatalytic activity under visible illumination. Then, thevisible activity of the resulting α-Fe2O3 is further enhanced bymodification with a proper amount of phosphate. On the basisof the atmosphere-controlled surface photovoltage spectra,time-resolved surface photovoltage spectra, O2 temperature-programmed desorption measurement and photoelectrochem-ical I−V curves, it is clearly demonstrated for the first time thatthe surface modification with an appropriate amount ofphosphate improves the adsorption of O2 so as to promotethe photogenerated electrons captured, leading to the increasein the charge separation and then in the visible photocatalyticactivity. Moreover, the enhanced adsorption of O2 is closelyrelated to the substitution of −Fe−OH with −Fe−O−P−OHsurface ends. This work will provide us with a feasible route tosynthesize oxide-based nanomaterials with good photocatalyticperformance.

2. EXPERIMENTAL SECTIONAll of the reagents were of analytical grade and used as receivedwithout further purification, and deionized water was employedthroughout.2.1. Synthesis of Materials. Similar to the HST method

by which nanocrystalline anatase TiO2 with high photocatalyticactivity under UV illumination has been synthesized usingTi(OBu)4 and toluene as Ti resource and organic solvent in ourgroup, respectively.43 A modified HST method is developed tocontrollably synthesize nanosized α-Fe2O3 by choosing Fe-(NO3)3·9H2O as Fe resource and n-butanol as the organicphase. The key of the controlled synthetic method is to selectn-butanol with a little higher boiling point than water as organicphase to dissolve Fe(NO3)3 and also as hydrothermal solvent,and to introduce volatile ammonia into the water system tomodulate the hydrolysis of Fe ions in the organic phase.In a typical experiment, 10 mL of water phase containing a

planned amount of ammonia and 8 mL of n-butanol phasecontaining 0.8 g of Fe(NO3)3·9H2O were respectively placed ina 30 mL Teflon lined stainless-steel vessel, in which a 10 mL ofweighing bottle is installed to contain the organic n-butanol.Then, the sealed device is kept at a certain temperature (120−160 °C) for 6 h. Under the solvothermal conditions, thehydrolysis and nucleation process would take place at theinterfaces between the water and n-butanol, and the subsequent

crystallization process would happen in the organic phase,according to the boiling point of n-butanol, which is higher thanthat of water. Thus, the resulting Fe2O3 is collected in the n-butanol after the autoclave was allowed to cool naturally toroom temperature, and subsequently washed with distilledwater and absolute ethanol in turn, and dried at 80 °C in air.The samples obtained are represented by a-N-b, in which Nmeans NH3·H2O, a is the reaction temperature, and b is themolar ratio of NH3·H2O to Fe3+. To compare, a conventionalprecipitation-hydrothermal route (PHR) was used to synthesizeFe2O3 as follows. A certain amount of NH3·H2O (the molarratio of NH3·H2O to Fe3+ was 3) was dropped into 0.2 mol/LFe(NO3)3 aqueous solution gradually and then was kept at 140°C for 6 h in a Teflon-lined stainless-steel vessel to carry outhydrothermal reactions. After cooling naturally to roomtemperature, washing with distilled water and absolute ethanolin turn, and drying at 80 °C in air, Fe2O3 was produced. For theFe2O3, it is denoted as Fe2O3−PHR.The resulting Fe2O3 was further modified with different

amount of phosphate as follows. A 0.2 g sample of 140-N-3powders was impregnated in a 50 mL of planned-concentrationorthophosphoric acid solution under violent stirring for 2 h.Subsequently, the resulting suspension was centrifuged, andthen washed with water for several times and dried at 80 °C,along with the thermal treatment at 400 °C for 1 h. Thephosphate-modified sample is defined as XP-F, in which Pmeans phosphoric acid, F is used to represent Fe2O3 and Xindicates the concentration of phosphoric acid solution used.To carry out photoelectrochemical (PEC) measurements,

unmodified and phosphate-modified Fe2O3 film electrodeswere prepared. First, the nanocrystalline Fe2O3 paste wasprepared as follows. A 0.5 g sample of α-Fe2O3 powders wasdispersed in 2 mL of isopropyl alcohol and then treated by anultrasonic process for 30 min and stirred for 30 min. After that,0.25 g of Macrogol-6000 was added to the diluted powders, andthen the mixture had an ultrasonic treatment and was stirred for30 min. Finally, 0.1 mL of acetylacetone was introduced to themixture above, which still had an ultrasonic treatment and wasstirred for 1 day. Conductive fluorine-doped tin oxide (FTO)-coating glasses, used as the substrates for the Fe2O3 films, werecleaned by ultrasonic processing in acetone for 0.5 h and thenin deionized water for another 0.5 h prior to use. The Fe2O3film was prepared by the doctor blade method using Scotchtape as the spacer. After being dried in air for 0.5 h, the film wassintered at 450 °C for 0.5 h. Subsequently, the Fe2O3 film wasimmersed into a planned-concentration orthophosphoric acidsolution for 1 h. After that, the film was rinsed with deionizedwater, dried naturally in air for 0.5 h, and then sintered at 450°C for 0.5 h. The film on FTO glass was cut into 1.7 × 3.0 cm2

pieces with an exposed Fe2O3 surface area of 1.7 × 1.5 cm2. Tomake a photoelectrode, an electrical contact was made with theFTO substrate by using silver conducting paste connected to acopper wire which was then enclosed in a glass tube. Theunmodified and phosphate-modified Fe2O3 films are designatedas FF and YP-FF, respectively, in which FF means Fe2O3 filmand Y indicates the concentration of ortho-phosphoric acidsolution used.

2.2. Characterization of the Samples. The crystalstructure of the samples were characterized by X-ray powderdiffraction (XRD) with a Rigaku D/MAX-rA powderdiffractometer (Japan), using Cu Kα radiation (λ = 0.154 18nm), and an accelerating voltage of 30 kV and emission currentof 20 mA were employed. Transmission electron microscopy

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp309599d | J. Phys. Chem. C 2013, 117, 1358−13651359

(TEM) observation was carried out on a JEOL JEM-2010EXinstrument operated at 200 kV accelerating voltage. The UV−vis diffuse reflectance spectra of the samples were recorded witha Model Shimadzu UV-2550 spectrometer. The specific surfaceareas of the samples were tested by BET instrument(Micromeritics automatic surface area analyzer Gemini 2360,Shimadzu), with nitrogen adsorption at 77 K. The Fouriertransform infrared spectra (FT-IR) of the samples werecollected with a Bruker Equinox 55 spectrometer, using KBras diluents. X-ray photoelectron spectroscopy (XPS) wasequipped with a Kratos-AXIS ULTRA DLD apparatus withAl (Mono) X-ray source to gain further insight into the surfacecomposition and elemental chemical state of the samples, andthe binding energies were calibrated with respect to the signalfor adventitious carbon (binding energy = 284.6 eV). Thesurface photovoltage spectroscopy (SPS) measurements ofsamples were conducted with a home-built apparatus that hadbeen described in detail elsewhere,44−46 the powder sample wassandwiched between two ITO glass electrodes by which theouter electric field could be employed, and the sandwichedelectrodes could be arranged in an atmosphere-controlledcontainer with a quartz window.A study on O2 temperature-programmed desorption (TPD)

is available to probe the interaction of O2 with oxide surfaces,which is carried out in a flow apparatus built by ourselves. In atypical O2-TPD experiment, the sample (about 50 mg) wasplaced in a quartz tube (i.d. 6 mm) with a small amount ofquartz wool plugging at two sides and pretreated at 275 °C for30 min in an ultrahigh-purity He flow of 20 mL/min. After thesample was cooled to ambient temperature, ultrahigh-purity O2was continuously passed over the sample for 90 min.Subsequently, the sample was flushed with the He flow forremoval of residual O2 in the quartz tube and a part of O2adsorbed physically on the sample. Finally, the O2-TPD profileof the sample was recorded by increasing the temperature from30 °C to the desired temperature at a heating rate of 10 °C/min under 20 mL/min of He flow, using a gas chromatograph(GC-2014, Shimadzu) with a TCD detector to monitor thedesorbed O2 amount.2.3. Evaluation of Visible Photocatalytic Activities.

Phenol is a typical recalcitrant contaminant without sensitizingas a dye, and acetaldehyde, as a kind of volatile toxic organiccompounds widely existing in industrial production, is harmfulto our health and environment. Thus, phenol and acetaldehydeare taken as liquid-phase and gas-phase pollutant representa-tives to evaluate the photocatalytic activity of the synthesizedFe2O3-based samples under visible light irradiation, respec-tively. The liquid-phase photocatalytic experiments were carriedout in a 100 mL of open photochemical glass reactor equippedwith an optical system provided from a side of the reactor byusing a 150 W GYZ220 high-pressure Xenon lamp made inChina with a cutoff filter (λ > 420 nm), which was placed atabout 10 cm from the reactor. During the evaluation ofphotocatalytic degradation of phenol, 0.15 g of photocatalystand 60 mL of 10 mg/L phenol solution were mixed by amagnetic stirrer for 2 h under visible light irradiation. Afterphotocatalytic reactions, the phenol concentrations wereanalyzed by the colorimetric method of 4-aminoantipyrine atthe characteristic optical adsorption of 510 nm with a ModelShimadzu UV2550 spectrophotometer after centrifugation.Photocatalytic degradation of gas-phase acetaldehyde was

conducted in 640 mL of cylindrical quartz reactor for 3 mouthsfor introducing a planned amount of photocatalyst powders and

a planned concentration of acetaldehyde gas. The reactor wasplaced horizontally and irradiated from the top side by using a150 W xenon lamp with a cutoff filter (λ > 420 nm). In a typicalphotocatalytic process, 0.15 g of photocatalyst was used, and apremixed gas system, which contained 810 ppm acetaldehyde,20% of O2, and 80% of N2, was introduced into the reactor. Toreach adsorption saturation, the mixed gas continuously movedthrough the reactor for 0.5 h prior to the irradiation. Thedetermination of acetaldehyde concentrations at different timeintervals in the photocatalysis was performed with a gaschromatograph (GC-2014, Shimadzu) equipped with a flameionization detector.

2.4. Photoelectrochemical (PEC) Experiments. PECexperiments were performed in a glass cell using a 150 Wxenon lamp with a cutoff filter (λ > 420 nm) and a stabilizedcurrent power supply as the illumination source, and 0.5 mol/LNaClO4 solution as the electrolyte. The working electrode wasthe prepared Fe2O3 film (1.7 × 1.5 cm2), vertically illuminatedfrom the FTO glass side. Platinum wire (99.9%) was used asthe counter electrode, and an Ag/AgCl (saturated KCl)electrode was used as the reference electrode to which all thepotentials in the paper were referred at 25 °C. For themeasurement in the presence and absence of O2, oxygen andoxygen-free nitrogen gas were bubbled through the electrolytebefore and during the experiments, respectively. Appliedpotentials were controlled by a commercial computer-controlled potentiostat (LK2006A made in China). Forcomparison, the I−V curves were also measured in the dark.

3. RESULTS AND DISCUSSION3.1. Characterization and Photocatalytic Activity of

the Resulting Fe2O3. The X-ray diffraction (XRD) is used toanalyze the crystal structure and crystallization degree ofsamples. Figure 1 displays XRD patterns of Fe2O3 samplessynthesized by the HST method with different experimentalconditions. All diffraction peaks of the samples can be indexedas the hexagonal-phase α-Fe2O3 (hematite) according to thestandard card JCPDS 33-0664. It can be seen that, as theammonia concentration and hydrothermal temperature in-crease, the intensities of XRD peaks gradually become strong,indicating that the sample’s crystallization and correspondingcrystallite size increase gradually according to the Scherrerformula.47 However, as the reaction temperature rises to 160°C and the ammonia concentration is greater than 3 (the molarratio of NH3·H2O to Fe3+), the diffraction peak intensities ofthe samples do not nearly change a little. Thus, when thereaction temperature is lower than 160 °C and the ammoniaconcentration is lower than 3, there are suitable conditions tocontrollably synthesize nanosized α-Fe2O3. In addition,compared with the α-Fe2O3 prepared by the PHR method, asshown in the Supporting Information (Figure S1), the 140-N-3sample exhibits wide and weak diffraction peaks, indicating thatit has small crystallite size. On the basis of the TEM imagesshown in Figure S2, Supporting Information, one can see thatthe Fe2O3−PHR with an irregular sphere form has an averagediameter of about 80 nm, whereas the 140-N-3 with a similarsphere form does an average size of about 15 nm. This isresponsible for the about 4-time larger BET surface area (83.98m2·g−1) of the 140-N-3 than that of the Fe2O3−PHR (18.42m2·g−1). Expectedly, as the nanoparticle size decreases, the blueshift in the DRS absorption spectra (Figure S3, SupportingInformation) can be seen. From the above analysis, it isdemonstrated that α-Fe2O3 nanoparticles with small size and



large surface area could be controllably synthesized by the HSTmethod, much superior to the traditional PHR one.In particular, it is seen from Figure S4 (Supporting

Information) that the photocatalytic activity of the Fe2O3−PHR for degrading liquid-phase phenol and gas-phaseacetaldehyde is very low under visible light illumination,whereas the Fe2O3 obtained by the HST process exhibits muchhigh activity, which is attributed to its small nanoparticle sizeand high surface area. Moreover, among the as-prepared Fe2O3samples, the 140-N-3 one displays the highest activity, whichdepends on the comprehensive results of nanoparticle size,surface area, and crystallinity. Thus, we choose the 140-N-3 forfurther modification with phosphate in the next work.3.2. Structural Characterization and Surface Compo-

sition of Modified Fe2O3. The XRD patterns of unmodifiedand phosphate-modified Fe2O3 are shown in Figure S5,Supporting Information. It is confirmed that all the samplesare pure hematite, demonstrating that the phosphatemodification has nearly no effect on the crystalline-phasecomposition, crystallite size, and crystallinity of Fe2O3. Andalso, one can see from the representative TEM photographs ofdifferent Fe2O3 samples shown in Figure 2 that the sphericalmorphology and nanoparticle size (about 20 nm) do notchange after phosphate modification. Compared with theunmodified Fe2O3, the modified one exhibits a good dispersion,

which is attributed to the roles of the phosphate modificationeffectively inhibiting the agglomerations and contacts amongFe2O3 nanoparticles.35,36 Expectedly, the phosphate modifica-tion does not influence the optical absorption of α-Fe2O3 onthe basis of the DRS spectra (Figure S6, SupportingInformation).As seen from the FT-IR spectra of different Fe2O3 samples

shown in Figure S7 (Supporting Information), the strong peakranging from 460 to 570 cm−1 corresponds to the Fe−Ostretching vibration mode in crystal α-Fe2O3.

42,48 The IR peaksat about 1630 and 3400 cm−1 are generally assigned to hydroxylgroups and adsorbed water molecules, respectively.49,50 A newIR band at 940−1108 cm−1 emerges in the modified Fe2O3 andits intensity is proportional to the used phosphate amount,which is attributed to the characteristic absorption peaks ofphosphate groups,51−53 or to the characteristic frequencies ofantisymmetric/symmetric stretching of −P−O and −P−OHgroups,54,55 suggesting that the formation of P−OH groups onthe surfaces of modified Fe2O3. Compared with the unmodifiedFe2O3, the characteristic stretching vibration band of modifiedα-Fe2O3 displays a slight shift to high wavenumber and thestretching vibration absorption peak of Fe−O−P centering at1048 cm−1 appears,56 indicating that the phosphate groups aremodified on the surfaces of Fe2O3 via chemical bonding. This is

Figure 1. XRD patterns of different Fe2O3 samples. (A) (a) 120-N-0;(b) 120-N-1; (c) 120-N-3; (d) 120-N-5; (e) 120-N-7. (B) (a) 140-N-0; (b) 140-N-1; (c) 140-N-3; (d) 140-N-5; (e) 140-N-7. (C) (a) 160-N-0; (b) 160-N-1; (c) 160-N-3; (d) 160-N-5; (e) 160-N-7. Figure 2. TEM images of unmodified and phosphate-modified Fe2O3

samples: (A) F; (B) 0.15P-F.



further supported by the XPS spectra shown in Figure S8,Supporting Information. As seen from the Fe2p XPS, one cansee that there are two main peaks at about 711 and 725 eV,corresponding to Fe2p3/2 and Fe2p1/2, respectively. Noticeably,the shakeup satellite structures as the fingerprints of electronicstructure of Fe3+ are also observed at the higher binding energysides of the main peaks.57,58 This is in agreement with the XRDresult that the resulting samples are pure α-Fe2O3 phase.Compared with the unmodified Fe2O3, the phosphate-modifiedone exhibits a slight high binding energy for Fe2p,demonstrating that the phosphate modification would havecertain effects on the surface properties of Fe2O3. For themodified Fe2O3, it is seen that the binding energy of P2p iscentered at about 133 eV, which is characteristic for P elementin the phosphate.59 This is in good agreement with the IRresults. In addition, the isoelectric point (IEP) of Fe2O3 ischanged from pH 8.5 to about pH 6.5 after phosphatemodification shown in Figure S9 (Supporting Information),implying that the −P−OH groups exist on the modified Fe2O3surfaces.60

3.3. Photogenerated Charge Properties of ModifiedFe2O3. The photocatalytic activity of material is closely relatedto the behavior of photogenerated charges.61 And, also, thesurface photovoltaic spectroscopy (SPS), with its very highsensitivity, is a well-suited and direct method to explore theproperties of photogenerated charges of solid semiconductingmaterials.62 In light of the SPS principle,44,63 the surfacephotovoltage signal of semiconductor materials mainlyoriginates from the creation of electron−hole pairs, followedby the separation under the built-in electrical filed in the spacecharge region and/or at the aid of the diffusion process.Nevertheless, for nanoparticles, band bending in bulk semi-conductors would not occur due to the limited size. In this case,the SPS response should mainly derive from the photo-generated charge separation via the diffusion process becausethe built-in electric fields are neglected.29 Figure 3 shows theSPS responses of unmodified and modified Fe2O3 in differentO2-concentration atmosphere. For F and 0.15P-F, if there is nooxygen (in pure N2 atmosphere), the photoelectrons andphotoholes would easily recombine, leading to no SPSresponse. And, the SPS response gradually becomes strong asO2 content increases, indicating that the presence of O2 is anessential condition for the SPS occurrence of α-Fe2O3 becauseof its ability to capture photogenerated electrons. Thus, it isdeduced that the positive photogenerated holes can preferen-tially diffuse to the surfaces of testing electrode in the presenceof O2, leading to an obvious SPS response. This is furtherconfirmed by the increased SPS responses at the aid of outerpositive field in air shown in Figure S10, SupportingInformation.Although the phosphate modification would not change the

SPS attributes, it could greatly affect the SPS intensity as shownin Figure 4. One can see that the SPS response graduallybecomes strong as the used phosphate amount increases,indicating that the separation of photogenerated charges ofFe2O3 is enhanced in the presence of O2 after phosphatemodification. In fact, this is also preliminarily supported by thetime-resolved photovoltage spectroscopy, as shown in FigureS11 (Supporting Information), indicating that phosphatemodification could be beneficial to promote the separation ofphotogenerated carries. However, if the phosphate amount istoo large, the SPS intensity begins to go down, even lower thanthat of unmodified Fe2O3, such as 0.5P-F. This is possibly

because that the excess phosphate would be unfavorable totransport photogenerated charges so as to influence the chargeseparation, which is also supported by the following PECresults. Therefore, it is clearly demonstrated that themodification with an appropriate amount of phosphate wouldobviously enhance the SPS response of nanosized Fe2O3 in thepresence of O2, leading to the marked increase in thephotogenerated charge separation.

3.4. Visible Photocatalytic Activities of ModifiedFe2O3. The photocatalytic activities for degrading liquid-phase phenol solution and gas-phase acetaldehyde have beenevaluated, as shown in Figure 5. As seen here, the photo-catalytic activity of as-prepared unmodified Fe2O3 is very low,whereas the Fe2O3 modified with an appropriate amount ofphosphate exhibits remarkably high activity. Among thephosphate-modified Fe2O3 samples, 0.15P-F with surface

Figure 3. SPS responses of unmodified (A) and phosphate-modifiedFe2O3 samples (B) in N2 (a), air (b), and O2 (c).

Figure 4. SPS responses of unmodified (a) and phosphate-modifiedFe2O3 (b)−(f) in air. Concentration (mol/L) of phosphoric acidsolution used: (b) 0.05, (c) 0.1, (d) 0.15, (e) 0.3, and (f) 0.5.



atomic number ratio of P to Fe is 0.06 based on the XPS resultdisplays the highest activity. Noticeably, the high photocatalyticactivity of the as-prepared Fe2O3 corresponds to its strong SPSresponse. Widely accepted, the separation and recombinationof photoinduced charge carriers are in competitive processes,and the photocatalytic reaction is effective only whenphotoinduced electrons and holes are separated.61,64 And,also, the step that the photogenerated electrons are captured bythe adsorbed O2 is crucial for charge separation and further forphotocatalytic reactions. In this case, it is reasonable that thephotocatalytic activity is consistent with the SPS response. Onthe basis of the above SPS results, it is confirmed that themodification with a proper amount of phosphate greatlyimprove the charge separation of the as-prepared Fe2O3, whichis very responsible for the enhanced activity. Because thephosphate modification would not influence the crystalstructure, nanoparticle size and optical adsorption of Fe2O3, itis assumed that the phosphate groups should play importantroles in the production of SPS response and then in thephotocatalytic reactions in the presence of O2. In addition, it isworth noting that the modification with excess phosphate isunfavorable for the SPS response and the photocatalyticreactions. This is possibly attributed to the point that the excessphosphate modified on the surfaces would suppress the chargetransportation or transfer.37

3.5. Mechanism Insight. Although the phosphatemodification does not change the SPS attribute of Fe2O3, itcould greatly influence its SPS intensity in the presence of O2.Thus, it is assumed that the phosphate modification is favorablefor the adsorption of O2. To prove this assumption, the curvesof O2 temperature-programmed desorption (TPD) of un-modified and phosphate-modified Fe2O3 were recorded, asshown in Figure 6. For the F, as the desorption temperaturerises, the amount of desorbed O2 gradually increases; however,

it begins to come down when the temperature goes up to about390 °C, until no desorbed O2 at 500 °C. Compared with theunmodified Fe2O3, the phosphate-modified one shows muchslow desorption of O2, especially at the high temperature (over350 °C) corresponding to the chemically adsorbed form.65 Andalso, for the same temperature, the amount of desorbed O2 ofthe phosphate-modified Fe2O3 is much larger than that of F,demonstrating that the phosphate modification remarkablypromotes the adsorption of O2 on the surfaces of Fe2O3.Naturally, the increase in the amount of adsorbed O2, especiallyfor the chemically adsorbed form, would be beneficial forcapturing the photogenerated electrons of Fe2O3, leading to theincreased SPS response. Thus, it is expected that the phosphatemodification would be favorable for the photoelectrochemicalreduction of O2.The photoelectrochemical properties of unmodified and

phosphate-modified Fe2O3 photoanodes were studied bymeasuring I−V plots in the absence or presence of O2 systems,as shown Figure S12, Supporting Information. It can be seenthat the I−V curve of the unmodified FF under visibleillumination nearly overlaps with the one in the dark, indicatingthat the photogenerated electron−hole recombination of Fe2O3easily take place in the absence of O2 in itself, which is inagreement with the literature.19,20,31 This is greatly differentfrom TiO2.

38 Similar to the FF, the phosphate-modified FFexhibits nearly the same current under irradiation as that in thedark, implying that the phosphate modification would notinfluence the photogenerated charge separation of Fe2O3 in theabsence of O2 in the neutral (pH7) system, which is also unlikethe phosphate-modified TiO2. For TiO2, the photocurrentdensity could be enhanced markedly after phosphatemodification, which is attributed to the formed strong negativeelectric field at the surfaces of TiO2.

35,38,66 It is different fromTiO2 that the surfaces of Fe2O3 should be charged by a smallamount of negative charges after phosphate modification,because its IEP is changed from pH 8.5 to about pH 6.5, whichis in agreement with the literatures.67,68 Thus, it is speculatedthat the phosphate modification would display weak effects onthe photogenerated charge separation of Fe2O3. Although thephotocurrent of unmodified or phosphate-modified FF is thesame as its current in the dark in the presence of O2 (as shownin Figures S13 and S14, Supporting Information), themodification with a proper amount of phosphate could enhancethe current of FF as shown in Figure 7. However, if the amountof used phosphate is excess, the current of modified FF beginsto go down, even lower than the unmodified FF, which is

Figure 5. Photocatalytic degradation rates of liquid-phase phenol (A)and gas-phase acetaldehyde (B) on different Fe2O3 samples.

Figure 6. Curves of O2 temperature-programmed desorption onunmodified and phosphate-modified Fe2O3 samples.



because that the excess phosphate groups are unfavorable forcharge transportation or transfer processes. This further provesthe previous expectation.Obviously, the increase in the amount of chemically adsorbed

O2 after phosphate modification, which would contribute tocapturing the photogenerated electrons, should result from thesurface state with surface-bound phosphate groups. Accordingto the above discussion, there are a certain amount ofphosphate groups (−Fe−O−P−OH) on the surfaces ofphosphate-modified Fe2O3 besides plentiful hydroxyl groups(−Fe−OH) in comparison with naked Fe2O3. Thus, it isexpected that the enhanced amount of O2 adsorption should beattributed to the partial substitution of −Fe−OH with −Fe−O−P−OH groups, especially for the attribute change of H inthe −OH group. That is to say that the −Fe−O−P−OHgroups, which can act as acid sites due to the acidic character ofphosphate groups, are possibly favorable for O2 adsorptioncompared with −Fe−OH groups. For this, a detailedcomparative experiment is designed to carry out, as shown inFigure S15, Supporting Information.It can be seen that the SPS response of the phosphate-

modified Fe2O3 becomes weak after subsequent KNO3treatment, however, still stronger than that of the unmodifiedone. This is in good agreement with the amount of adsorbedO2 on the basis of the O2-TPD curves. As expected, the visiblephotocatalytic activity of the phosphate-modified Fe2O3 fordegrading acetaldehyde is decreased after the KNO3 treatment,whereas still higher than that of the unmodified one. Becausethe substitution of the H in the −Fe−O−P−OH group with Kbased on the XPS results, it is concluded that the H change inthe surfaces of Fe2O3 would greatly influence the adsorption ofO2, and then charge separation and photocatalytic activity.

4. CONCLUSIONSIn this paper, we report a facile one-pot water−organic two-phase separated hydrolysis-solvothermal method for synthesisof α-Fe2O3 nanoparticles with high photocatalytic activity fordegrading liquid-phase phenol and gas-phase acetaldehydeunder visible illumination for the first time. Moreover, thephotocatalytic activity of the resulting α-Fe2O3 nanoparticles isgreatly improved by modification with a proper amount ofphosphate. Mainly based on the atmosphere-controlled SPSresponses, time-resolved photovoltage spectra and the O2-TPDcurves, along with the photoelectrochemical reduction of O2, itis confirmed that the enhanced activity of α-Fe2O3 after

phosphate modification is attributed to the effective separationof photogenerated charge carriers resulting from the increase inthe amount of adsorbed O2. And also, it is suggested for thefirst time that the change of surface ends by the substitution of−Fe−OH with −Fe−O−P−OH groups after phosphatemodification would greatly promote the adsorption of O2,which is responsible for the enhanced charge separation andthen visible photocatalytic activity, and the increase in thesurface acidity would be favorable to promote the adsorption ofO2. Naturally, it is speculated that the surface acidity-increasedmodification method would be applicable to improve thephotocatalytic activity of other oxide-based semiconductorphotocatalysts greatly, such as WO3, BiVO4, LaFeO3, and so on.This work would facilitate our deep understanding about themechanism of enhanced photocatalytic activity on surfacemodification and might provide feasible strategies to furtherdesign and synthesize oxide-based semiconductors withexcellent photocatalytic performance.

■ ASSOCIATED CONTENT*S Supporting InformationXRD patterns, TEM images, DRS spectra and photocatalyticdegradation data of the Fe2O3−PHR and 140-N-3 samples;XRD patterns, DRS spectra, FT-IR spectra, XPS spectra, ζpotential, SPS responses, time-resolved SPS response and I−Vcurves of unmodified Fe2O3 and phosphate-modified Fe2O3samples; XRD patterns, XPS spectra, SPS responses, O2-TPDcurves and photocatalytic degradation data of H2O−F, H2O−0.15P−F and KNO3−0.15P−F samples. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is financially supported by the National NatureScience Foundation of China (21071048), the Program forInnovative Research Team in University, the Chang JiangScholar Candidates Programme for Provincial Universities inHeilongjiang (2012CJHB003), the Science Foundation ofHarbin City of China (No. 2011RFXXG001), and the Programfor Innovative Research Team in Heilongjiang University(Hdtd2010-02), for which we are very grateful.

■ REFERENCES(1) Yu, C. C.; Dong, X. P.; Guo, L. M.; Li, J. T.; Qin, F.; Zhang, L. X.;Shi, J. L.; Yan, D. S. J. Phys. Chem. C 2008, 112, 13378−13382.(2) Cao, S. W.; Zhu, Y. J. J. Phys. Chem. C 2008, 112, 6253−6257.(3) Pailhe, N.; Wattiaux, A.; Gaudon, M.; Demourgues, A. J. SolidState Chem. 2008, 181, 2697−2704.(4) Hu, X. L.; Yu, J. C.; Gong, J. M.; Li, Q.; Li, G. S. Adv. Mater.2007, 19, 2324−2329.(5) Li, L.; Koshizaki, N. J. Mater. Chem. 2010, 20, 2972−2978.(6) Zhou, W.; Lin, L.; Wang, W.; Zhang, L.; Wu, Q.; Li, J.; Guo, L. J.Phys. Chem. C 2011, 115, 7126−7133.(7) Wang, Z. Y.; Luan, D. Y.; Madhavi, S.; Hu, Y.; Lou, X. W. EnergyEnviron. Sci. 2012, 5, 5252−5256.(8) Zhu, X. J.; Zhu, Y. W.; Murali, S.; Stoller, M. D.; Ruoff, R. S. ACSNano 2011, 5, 3333−3338.(9) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A. V.;Zhang, H. Chem. Phys. Lett. 2003, 379, 373−379.

Figure 7. I−V curves of different Fe2O3 electrodes in the dark.Potentials are measured against a Ag: AgCl (saturated KCl solution)reference electrode in 0.5 mol/L NaClO4 solution under O2.



http://pubs.acs.org

mailto:[email protected]

(10) Wang, L. L.; Jiang, J. S. Physica B 2007, 390, 23−27.(11) Zysler, R. D.; Vasquez-Mansilla, M.; Arciprete, C.; Dimitrijewits,M.; Rodriguez-Sierra, D.; Saragovi, C. J. Magn. Magn. Mater. 2001,224, 39−48.(12) Liu, X. Q.; Tao, S. W.; Shen, Y. S. Sens. Actuators, B 1997, 40,161−165.(13) Jia, C. J.; Sun, L. D.; Yan, Z. G.; You, L. P.; Luo, F.; Han, X. D.;Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44,4328−4333.(14) Jing, Z. H.; Wu, S. H. Mater. Lett. 2004, 58, 3637−3640.(15) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Appl. Catal., B2003, 40, 271−286.(16) Matsuoka, M.; Kitano, M.; Takeuchi, M.; Tsujimaru, K.; Anpo,M.; Thomas, J. M. Catal. Today 2007, 122, 51−61.(17) Zhang, Z. H.; Yuan, Y.; Shi, G. Y.; Fang, Y. J.; Liang, L. H.; Ding,H. C.; Jin, L. T. Environ. Sci. Technol. 2007, 41, 6259−6263.(18) Kumar, S. G.; Devi, L. G. J. Phys. Chem. A 2011, 115, 13211−13241.(19) Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.;Sivula, K.; Gratzel, M.; Durrant, J. R.; Klug, D. R. J. Am. Chem. Soc.2011, 133, 10134−10140.(20) Wei, Y. H.; Han, S. B.; Walker, D. A.; Warren, S. C.;Grzybowski, B. A. Chem. Sci. 2012, 3, 1090−1094.(21) Qurashi, A.; Zhong, Z. H.; Alam, M. W. Solid State Sci. 2010, 12,1516−1519.(22) Liu, G.; Deng, Q.; Wang, H. Q.; Ng, D. H. L.; Kong, M. G.; Cai,W. P.; Wang, G. Z. J. Mater. Chem. 2012, 22, 9704−9713.(23) Zhang, Z. H.; Hossain, M. F.; Miyazaki, T.; Takahashi, T.Environ. Sci. Technol. 2010, 44, 4741−4746.(24) Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N. J.Phys. Chem. C 2007, 111, 16477−16488.(25) Boudjemaa, A.; Boumaza, S.; Trari, M.; Bouarab, R.; Bouguelia,A. Int. J. Hydrogen Energy 2009, 34, 4268−4274.(26) Tahir, A. A.; Wijayantha, K. G. U.; Saremi-Yarahmadi, S.;Mazhar, M.; McKee, V. Chem. Mater. 2009, 21, 3763−3772.(27) Niu, M. T.; Huang, F.; Cui, L. F.; Huang, P.; Yu, Y. L.; Wang, Y.S. ACS Nano 2010, 4, 681−688.(28) Wu, W.; Zhang, S. F.; Ren, F.; Xiao, X. H.; Zhou, J.; Jiang, C. Z.Nanoscale 2011, 3, 4676−4684.(29) Wei, X.; Xie, T. F.; Peng, L. L.; Fu, W.; Chen, J. S.; Gao, Q.;Hong, G. Y.; Wang, D. J. J. Phys. Chem. C 2011, 115, 8637−8642.(30) Cesar, I.; Kay, A.; Martinez, J. A. G.; Gratzel, M. J. Am. Chem.Soc. 2006, 128, 4582−4583.(31) Hu, Y. S.; Kleiman-Shwarsctein, A.; Forman, A. J.; Hazen, D.;Park, J. N.; McFarland, E. W. Chem. Mater. 2008, 20, 3803−3805.(32) Zhang, J. L.; Wu, Y. M.; Xing, M. Y.; Leghari, S. A. K.; Sajjad, S.Energy Environ. Sci. 2010, 3, 715−726.(33) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086−4093.(34) Mohapatra, P.; Parida, K. M. J. Mol. Catal. A: Chem. 2006, 258,118−123.(35) Zhao, D.; Chen, C. C.; Wang, Y. F.; Ji, H. W.; Ma, W. H.; Zang,L.; Zhao, J. C. J. Phys. Chem. C 2008, 112, 5993−6001.(36) Qin, X.; Jing, L. Q.; Tian, G. H.; Qu, Y. C.; Feng, Y. J. J. Hazard.Mater. 2009, 172, 1168−1174.(37) Cao, Y.; Jing, L. Q.; Shi, X.; Luan, Y. B.; Durrant, J. R.; Tang, J.W.; Fu, H. G. Phys. Chem. Chem. Phys. 2012, 14, 8530−8536.(38) Jing, L. Q.; Zhou, J.; Durrant, J. R.; Tang, J. W.; Liu, D. N.; Fu,H. G. Energy Environ. Sci. 2012, 5, 6552−6558.(39) Connor, P. A.; McQuillan, A. J. Langmuir 1999, 15, 2916−2921.(40) Henderson, M. A. J. Phys. Chem. C 2011, 115, 23527−23534.(41) Almeida, T. P.; Fay, M.; Zhu, Y. Q.; Brown, P. D. J. Phys. Chem.C 2009, 113, 18689−18698.(42) Pradhan, G. K.; Parida, K. M. ACS Appl. Mater. Interfaces 2011,3, 317−323.(43) Xie, M. Z.; Jing, L. Q.; Zhou, J.; Lin, J. S.; Fu, H. G. J. Hazard.Mater. 2010, 176, 139−145.(44) Jing, L. Q.; Sun, X. J.; Shang, J.; Cai, W. M.; Xu, Z. L.; Du, Y. G.;Fu, H. G. Sol. Energy Mater. Sol. Cells 2003, 79, 133−151.

(45) Xin, B. F.; Jing, L. Q.; Ren, Z. Y.; Wang, B. Q.; Fu, H. G. J. Phys.Chem. B 2005, 109, 2805−2809.(46) Jing, L. Q.; Xin, B. F.; Yuan, F. L.; Xue, L. P.; Wang, B. Q.; Fu,H. G. J. Phys. Chem. B 2006, 110, 17860−17865.(47) Zhang, Q. H.; Gao, L.; Guo, J. K. Appl. Catal. B: Environ. 2000,26, 207−215.(48) Zhao, B.; Wang, Y.; Guo, H.; Wang, J.; He, Y.; Jiao, Z.; Wu, M.Mater. Sci. Poland 2007, 25, 1143−1148.(49) Ding, Z.; Lu, G. Q.; Greenfield, P. F. J. Phys. Chem. B 2000, 104,4815−4820.(50) Luan, Y. B.; Jing, L. Q.; Xie, M. Z.; Shi, X.; Fan, X. X.; Cao, Y.;Feng, Y. J. Phys. Chem. Chem. Phys. 2012, 14, 1352−1359.(51) Piras, F. M.; Rossi, A.; Spencer, N. D. Langmuir 2002, 18,6606−6613.(52) Klahn, M.; Mathias, G.; Kotting, C.; Nonella, M.; Schlitter, J.;Gerwert, K.; Tavan, P. J. Phys. Chem. A 2004, 108, 6186−6194.(53) Joseph Antony Raj, K.; Ramaswamy, A. V.; Viswanathan, B. J.Phys. Chem. C 2009, 113, 13750−13757.(54) Stanghellini, P. L.; Boccaleri, E.; Diana, E.; Alberti, G.; Vivani, R.Inorg. Chem. 2004, 43, 5698−5703.(55) Wang, Q. F.; Zhong, L.; Sun, J. Q.; Shen, J. C. Chem. Mater.2005, 17, 3563−3569.(56) Mal, N. K.; Bhaumik, A.; Matsukata, M.; Fujiwara, M. Ind. Eng.Chem. Res. 2006, 45, 7748−7751.(57) Fujii, T.; Groot, F. M. F. de; Sawatzky, G. A.; Voogt, F. C.;Hibma, T.; Okada, K. Phys. Rev. B 1999, 59, 3195−3202.(58) Chen, Y. J.; Zhang, F.; Zhao, G. G.; Fang, X. Y.; Jin, H. B.; Gao,P.; Zhu, C. L.; Cao, M. S.; Xiao, G. J. Phys. Chem. C 2010, 114, 9239−9244.(59) Wang, Q. F.; Zhong, L.; Sun, J. Q.; Shen, J. C. Chem. Mater.2005, 17, 3563−3569.(60) Yee, C.; Kataby, G.; Ulman, G.; Prozorov, T.; White, H.; King,A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15,7111−7115.(61) Zhou, H. L.; Qu, Y. Q.; Zeid, T.; Duan, X. F. Energy Environ. Sci.2012, 5, 6732−6743.(62) Kronik, L.; Shapira, Y. Surf. Sci. Rep. 1999, 37, 1−206.(63) Lin, Y. H.; Wang, D. J.; Zhao, Q. D.; Yang, M.; Zhang, Q. L. J.Phys. Chem. B 2004, 108, 3202−3206.(64) Hu, X. L.; Li, G. S.; Yu, J. C. Langmuir 2010, 26, 3031−3039.(65) Cvetanovic, R. J.; Amenomiya, Y. Catal. Rev. 1972, 6, 21−48.(66) Kimling, M. C.; Caruso, R. A. J. Mater. Chem. 2012, 22, 4073−4082.(67) Liang, L.; Morgan, J. J. Aquat. Sci. 1990, 52, 32−55.(68) Arai, Y.; Sparks, D. L. J. Colloid Interface Sci. 2001, 241, 317−326.



facile synthesis of surface-modified nanosized α-fe 2 o 3...

Documents