Synthesis of FeS2 and Co-doped FeS2 films with the aid of supercriticalcarbon dioxide and their photoelectrochemical properties{

Jiqing Jiao, Liuping Chen,* Daibin Kuang, Wei Gao, Huajie Feng and Jian Xia

Received 18th April 2011, Accepted 26th April 2011

DOI: 10.1039/c1ra00066g

The prepared FeS2 and Co-doped FeS2 films revealed better properties in photocurrent response and

photocatalysis of water photolysis under simulated sunlight. The films were synthesized on iron

substrates with the aid of supercritical carbon dioxide. The experimental results demonstrated that

the supercritical carbon dioxide could play an important role in evolution of phases and

morphologies during the reaction process. Moreover, the prepared films were characterized by XRD,

SEM, TEM and EDS. The UV-vis absorption spectroscopy indicated that the absorption edge has

obvious blue shift compared with bulk FeS2. Interestingly, characteristics of photoelectric response

and water photolysis were shown by photoelectrochemical experiments under sunlight. Therefore, the

prepared films would have an advantage in water photolysis utilizing sunlight.

1 Introduction

As a renewable and clean energy source, solar energy is one of

the most promising future energy resources. Sunlight can be

transformed into electrical energy by photovoltaic cells and

stored as chemical energy in a storage battery or in the form of

hydrogen by the electrolysis of water.1 Especially, water

oxidation driven by sunlight has received great attention because

of its potential as a technology for the production of green fuel.2

Following the discovery of light-induced water-splitting with a

TiO2 semiconductor photoanode under ultraviolet irradiation in

1972,3 there has been considerable interest in the photoelec-

trolysis of water with photoelectrochemical cells. This process

results in oxygen evolution at the semiconductor photoanode

and hydrogen evolution at the cathode.4 Up to now, a number of

semiconductor materials have been focused on utilization of

solar energy. Especially, TiO2 is a traditional material for the

photoanode, which has become a benchmark material for

understanding the photo-oxidation process in the water splitting

reaction.3–15 However, the wide band gap of TiO2 (3 eV) is out of

the visible light region and only a small fraction of the solar

spectra can be utilized, which makes the final photoenergy

conversion factor less than 1%.16 From the viewpoint of solar

energy utilization, the development of photoelectric materials

to split water efficiently under visible light is indispensable.

Thus, new classes of semiconductor materials, such as Fe2O3

(Eg = 2.1 eV), WO3 (Eg = 2.5 eV) and FeS2, are continuously

being tested in order to use the less energetic but more abundant

visible light.1,16–19

FeS2 has two crystal phases: pyrite and marcasite. Marcasite

(Eg = 0.34 eV) is a metastable phase, which isn’t applied to the

process of photoelectric translation due to its small energy band

gap. However, pyrite, as a cubic,20 has a suitable energy band

gap (Eg = 0.95 eV) for the solar spectra. It has excellent electron

mobility and high light absorption coefficient (a > 105 cm21 for

l ¡ 700 nm), and its optical absorption coefficient is two orders

of magnitude higher than that of crystalline silicon.21 These

characteristics make pyrite a potential candidate as an absorber

material for thin-film solar cells, photovoltaic cells and water

photooxidation.22–24 However, the structure of its interface and

the presence of bulk defects (point defects, dislocations) provide

pathways for recombination and transfer of electrons through

interfacial barriers. So extrinsic and intrinsic properties of pyrite

still remain to be improved.25 In order to obtain better

photoelectric properties, there are two main approaches, one is

to develop new methods for preparing pyrite with a pure phase.

The select synthesis of uniform FeS2 octahedral and cubic

crystallites could be prepared by a facile surfactant-assisted

ethylene glycol-mediated solvothermal approach.26 More

Recently, J. Puthussery et al.27 used a simple hot-injection and

subsequent partial ligand exchange route to synthesize phase-

pure, single-crystalline, and well-dispersed colloidal pyrite

nanocrystals inks, which were then fabricated by sintering layers

at 500–600 uC under a sulfur atmosphere to obtain polycrystal-

line pyrite thin films. The other approach is to synthesize

transition metals doped pyrite. Several different dopants have

already been reported, such as Co,28 Ti,29 Ni,30 Cu31 or Zn.25

For these elements, both FeS2 and CoS2 crystallize in the pyrite

structure and constitute a mixed crystal system of general

composition Fe12xCoxS2 for (0 , x , 1).32–34 Recently, some

technologies have been developed to produce thin pyrite films,

such as evaporated iron layer,35 ion beam magnetron sputter-

ing,36 spray pyrolysis,37 electrodeposition38 and magnetron

School of Chemistry and Chemical Engineering, Sun Yat-Sen University,Guangzhou, 510275, P. R. China. E-mail: cesclp@mail.sysu.edu.cn;Fax: +86-020-84112245; Tel: +86-020-84115559{ Electronic supplementary information (ESI) available. See DOI:10.1039/c1ra00066g

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sputtering.39 For all those methods, not only is the reaction

temperature very high, but also the prepared samples are largely

dependent on the equipment. Moreover, many sulfides, such as

Fe3S4, Fe7S8, FeS, would be easily produced during the reaction

process. Thus, further efforts should be devoted to investigating

green and simple synthesis routes to produce pyrite and Co-

doped FeS2 films.

As a preferred green solvent to traditional organic solvents,

supercritical carbon dioxide (scCO2) has been used in materials

science and industrial processes owing to its low viscosity, high

diffusivity, nontoxicity, nonflammability, low cost and recycl-

ability, etc. In the system of scCO2 and water, CO2 dissolves in

water to form carbonic acid, thereby decreasing the pH value to

2.8–3.0 during the reaction process;40–42 and water–CO2 (W/C)

microemulsions have been formed with specially designed

surfactants.42 These properties are attractive for studying the

formation of nanoparticles using the system of water and scCO2

according to reports.40–43 Previously, our group has reported

nanosized materials synthesized with the aid of scCO2, which

showed a significant effect on the crystallization of products.40–42

Herein, FeS2 and Co-doped FeS2 films could be successfully

grown on iron substrates through the reaction of iron foil and

sulfur source with the mixed system. The detailed reaction

conditions, such as the pressure of the system, temperature and

time, were investigated, and the structure of the films obtained

was characterized. Furthermore, the prepared FeS2 and Co-

doped FeS2 films were utilized as photoanodes, which demon-

strated excellent properties for water splitting and photoelectric

response by sunlight. Interestingly, compared with conventional

dye-sensitized photoelectrodes, the films without dye sensitizer

had better photoelectric properties under simulated sunlight.

2 Experimental

2.1 Chemicals

Chemicals: All reagents were of analytical grade and used as

received. The chemicals used in the syntheses were sodium

tripolyphosphate (Na5P3O10, STPP), sodium thiosulfate penta-

hydrate (Na2S2O3?5H2O), sulfur (S), potassium hydroxide

(KOH), cobaltous chloride (CoCl2?6H2O), ethylene glycol and

methanol, all of which were purchased from the Shanghai

Reagent Company (P.R. China). The CO2 (purity: 99.95%) and

iron (Fe) foil (purity: 99.99%, thickness: 0.1 mm, 1.0 cm 60.5 cm) were provided by the Guangzhou Gas Company and

Alfa Aesar, respectively.

2.2 Synthetic method

Preparation of the FeS2 and Co-doped FeS2 films: In a typical

synthetic procedure, a certain amount of Na5P3O10 was completely

dissolved in 12 mL mixture solution (Vethylene glycol : Vdeionized water =

1 : 9) with stirring; 2.7919 g Na2S2O3?5H2O was added to the

solution after complete dissolution of Na5P3O10; 0.1800 g sulfur

powder was then injected into the solution with stirring. When the

Co-doped FeS2 film was prepared, 0.0134 g CoCl2?6H2O were

added to the solution. The prepared mixture was transferred into

the PTFE reactor with the top with a small hole, which allows CO2

to flow into the reactor, then a piece of Fe foil (1.0 cm 6 0.50 cm)

was placed in the solution. Finally, the PTFE reactor was putted

into the stainless steel autoclave which was then sealed. The

stainless steel container was filled up with CO2 with a compressor.

First, the reactor was purged with a flow of CO2 to remove any

entrapped air from the autoclave and then filled with liquid CO2

to a desired amount using a high-pressure compressor. After

charging, the autoclave was slowly heated to the desired

temperature (FeS2: 160 uC; Co-doped FeS2: 180 uC), the pressure

of reaction system should be controlled under 11 MPa. The

reaction was maintained for 24 h. At the end of reaction, the

autoclave was cooled to room temperature, and then CO2 was

slowly vented through a pressure valve. The Fe foil was taken out

of solution, washed with deionized water followed by ethanol

three times, and finally air-dried for characterization. The detailed

experimental conditions for the synthesis of the samples are

listed in Table 1.

2.3 Characterization

The X-ray diffraction (XRD) pattern was recorded on a D8

(Bruker, Germany) X-ray diffractometer with graphite mono-

chromator Cu Ka radiation (l = 1.54178) operating at 40 kV

and 40 mA. Scanning electron microscopy (SEM) and energy-

dispersive X-ray spectroscopy (EDS) were carried out with thermal

field emission environmental SEM–EDS–EBSD (Quanta 400F,

Holand). The EDS and transmission electron microscopy (TEM)

image, selected area electron diffraction (SAED) pattern and high-

resolution transmission electron microscopy (HRTEM) image

were performed on a JEM-2010HR field emission transmission

electron microscope (JEOL, Japan). UV–vis absorption spectrum

was measured on UV–vis–NIR Spectrophotometer UV-3150

(SHIMADZU, Japan). All the measurements were performed at

room temperature.

2.4 Photoelectrochemical characterization

Photocurrents were measured in a three-electrode configuration

with 1 mol?L21 KOH (Containing 10% methanol, 25 C, pH =

13.6) as electrolyte, Ag/AgCl/ sat. KCl as reference, and a

platinum wire as counter electrode, separated by glass frits.

Contact to the Fe substrate of the FeS2 film was made with a Cu

clip wire above the electrolyte. The current of photoelectro-

chemical and photoelectric response were measured by an

Table 1 The experimental parameters for the synthesis of FeS2 and Co-doped FeS2 films on the iron foila

No. mSTPP mCoCl2T p t Vegw Phase

S1 2.0693 0 160 11 24 1 : 9 FeS2

S2 2.0693 0 160 6 24 1 : 9 FeS2

S3 2.0693 0 160 0 24 1 : 9 FeS2 + Fe7S8

S4 2.0693 0 160 11 24 0 : 10 FeS2 + XS5 2.0693 0 160 11 24 1 : 4 FeS2 + XS6 0 0 160 11 24 1 : 9 FeS2 + Fe7S8

S7 1.0347 0 160 11 24 1 : 9 FeS2 + Fe7S8

S8 2.0693 0 160 11 36 1 : 9 FeS2 + Fe7S8

S9 2.0693 0 160 11 48 1 : 9 FeS2 + Fe7S8 + XS10 2.0693 0 180 11 24 1 : 9 FeS2 + XS11 2.0693 0.0107 160 11 24 1 : 9 FeS2 + Fe7S8

S12 2.0693 0.0107 180 11 24 1 : 9 Co-doped FeS2

a mSTPP and mCoCl2in g, mCoCl2

= mCoCl2?6H2O; T in oC; p in MPa, p= 0

means No CO2 in the reaction system; t in hours; Vegw =Vethylene glycol:Vdeionized water; X = unknown phases.

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electrochemical workstation (CHI750B) under simulated sun-

light illumination. The potential of the photoelectrode is

reported against the reversible hydrogen electrode (RHE):

ERHE = EAgCl + 0.059pH + EuAgCl with EuAgCl = 0.1976 V at 25 uC

Sunlight was simulated using a Keithley 2400 source meter under

simulated AM 1.5 G illumination (100 mW?cm22) provided by

solar simulator (69920, 1 kW Xe lamp with optical filter, Oriel).

3 Results and discussion

3.1 Synthesis of the FeS2 and Co-doped FeS2 films in scCO2

The phases of the iron sulfide products were characterized by

XRD. Fig. 1 (S1 and S2) shows the XRD patterns of films

prepared in the presence of CO2 at 160 uC. All peaks match quite

well with the Joint Committee on Powder Diffraction Standards

(JCPDS) card 42-1340, and no additional peaks were found. The

result confirmed the formation of pure FeS2 crystals. S2 was

obtained under 6 MPa, the peaks of S2 indicated that the

product was pyrite. But S1 showed a good crystallinity compared

with S2. However, peaks of FeS2 and Fe7S8 (JCPDS: 25-0411)

were found when the reaction was conducted at 160 uC in the

absence of CO2 (Fig. 1 S3). When deionized water without

ethylene glycol was used as the solvent, an unknown phase is

observed in the product (S4); when the proportion of ethylene

glycol was increased (Vethylene glycol:Vdeionized water = 1 : 4), more

peaks of unknown phase could be seen in Fig. 1 (S5).

Indubitably, these results demonstrated that the CO2 and

ethylene glycol showed a significant effect on the crystallization

of pyrite.

Furthermore, CO2 and ethylene glycol not only have an

important effect on the phase formation, but also exert a key

influence on the morphology of product. The diverse morpho-

logies of Samples 1–5 can be obtained at different experimental

conditions (Fig. 2). A low-magnification SEM image of the

product obtained under 11 MPa is shown in Fig. 2a. The FeS2

film consists of a nanorod, a uniform nanorod with a length in

the range 0.5–1 mm and less than 100 nm in diameter can be

observed. Fig. 2b is an enlarged SEM image of the grain part of

Fig. 2a, from which the number of ridges on the film can be

clearly seen. The thickness of ridge top is less than 100 nm, much

thinner than the bottom thickness. As seen from Fig. 2c, when

the reaction pressure was reduced to 6 MPa, particles with

different sizes and morphologies could be found. When the CO2

pressure decreased to 0 MPa, namely CO2 wasn’t employed,

irregular particles including large size grains and rods were

prepared, which can be clearly observed in Fig. 2d. Similarly,

when ethylene glycol wasn’t added into the solvent, Sample 4

was made of polyhedron and small particles (Fig. 2e); when

2.4 mL ethylene glycol was injected into the solvent, irregular

grains are obtained and the size was larger than 1 mm (Fig. 2f).

The evident differences in the phase and morphology obtained

suggest that the CO2 and ethylene glycol could play an important

role in both the crystal phase and alignment of samples. This

could be interpreted below: the release rates of H+ ions and Fe2+

ions from iron substrates were possibly affected by the pH value

of the solution, since scCO2 dissolves into water to form carbonic

acid. Furthermore, the pH value can be controlled by changing

the pressure of CO2, namely, the release rates of both H+ ions

and Fe2+ ions can be easily adjusted by varying the CO2

pressure, and therefore the morphology of FeS2 was controlled

by regulating the pressure of the reaction system. A similar result

was reported previously.40 As for ethylene glycol, on the one

hand, it dissolved in water and changed the solubility of CO2 in

the solution mixture, and then the pH value of the solution was

adjusted; on the other hand, it could affect the solubility of S in

Fig. 1 The XRD patterns of samples: (S1) Sample 1; (S2) Sample 2; (S3)

Sample 3; (S4) Sample 4; (S5) Sample 5. *, + and X denote Fe, Fe7S8 and

unknown phase, respectively.

Fig. 2 The SEM images of samples. (a) SEM images of Sample 1; (b) is

enlarged picture of (a); (c) Sample 2; (d) Sample 3; (e) Sample 4; (f)

Sample 5.

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the solution mixture. The formation of FeS2 is represented by the

following equations:

CO2zH2O'H2CO3'HzzHCO3{ (1)

Fez2Hz~Fe2zzH2: (2)

Fe2zzS2O32{zSzH2O~FeS2zSO4

2{z2Hz (3)

The results demonstrate that variations in reaction conditions

including the additive, pressure, reaction temperature and doped

reagent, result in dramatic differences in the form and crystal

phases of the FeS2 and Co-doped FeS2 films (Table 1). In the

current synthetic system, the STPP played also an important role

in crystallization of FeS2. As shown in Fig. 3 S6, mixed phases

including FeS2, Fe7S8, etc., were prepared without STPP; when

1.0347 g STPP was injected into the solution, the diffraction

peaks of Fe7S8 were still found in the XRD pattern (Fig. 3 S7).

When STPP was increased to 2.0693 g, pure pyrite FeS2 was

successfully synthesized at last. The result indicates that the

STPP may be a favourable factor for the formation of pyrite,

which hasn’t been reported previously in the literature.

Furthermore, to investigate the evolution of morphology with

time, synthetic experiments with different reaction times were

conducted. When 36 h was adopted, one can see the peaks of

Fe7S8 in the pattern (Fig. 3 S8); when the reaction time was

increased to 48 h, the peaks of Fe7S8 and unknown phase can be

found (Fig. 3 S9). Thus, 24 h was appropriate to prepare the

rod-like nanostructured pyrite in this work. As an important

experimental parameter, different reaction temperatures were

adopted to synthesize the samples. Sample 10 was prepared at

180 uC. It is evident in S10 that the peaks at 2h of 23.2, 37.8 and

56.3 can’t be indexed to correspond to the sulfide from Fig. 3.

Thus, unknown compounds were prepared in S10 besides pyrite.

It is well known that pure pyrite films show p-type conduction,

and doping is necessary to obtain n-type conduction. The

improvement of electronic properties directly relates to the

distribution of dopant across the pyrite film.32 Therefore,

experiments were designed to produce n-type conduction in

pyrite through doping cobalt in scCO2 and water. When 0.0107 g

CoCl2 was injected into the solution, Co-doped FeS2 film could

be prepared at 160 uC. However, as shown in Fig. 3 S11, more

peaks of Fe7S8 could be clearly observed besides peaks of FeS2.

Surprisingly, only the doped pyrite film was obtained at 180 uC(Fig. 3 S12). This fact indicated that the doping procedure for

pyrite by Co needs higher temperature in the same reaction

medium. The element component of prepared Sample 12 was

recorded by energy-dispersive X-ray spectroscopy (EDS). As

seen in Fig. 4a, the EDS demonstrates that the product consists

of three elements. The Fe peaks at about 0.7 and 6.4 keV, Co

peak at about 0.7 keV and the S peak at 2.4 keV are observed.

The Fe, Co and S atomic percentages are analyzed to be 30.39%,

1.49% and 68.12%, respectively. The EDS analysis of the

compound reveals that the product is mainly composed of Fe,

Co and S and its atomic ratio is about 0.985 : 0.015 : 2. Fig. 4b

shows a low-magnification SEM image of the Co-doped FeS2

film (S12), indicating the close layer consists of lager particles,

the top of which has plenty of subulate ridges. An enlarged SEM

image shown in Fig. 4c suggests that the surface of the film

scatters lots of ridges and its top of thickness is less than 100 nm.

Compared with S1 prepared at 160 uC, the thickness of ridge

(S12) is much larger. The above results indicate that the higher

temperature is necessary for the Co doping into the pyrite crystal

lattice. But their products have a similar morphology.

The FeS2 and Co-doped FeS2 films were further investigated

by TEM shown in Fig. 5 displaying a more detailed structure of

the crystal. Fig. 5a confirms the taper morphology of FeS2, and

lots of the particles assemble into the film. The thickness of the

top is much thinner than that of the bottom. The result is in good

agreement with structures obtained from SEM observation

mentioned above in Fig. 2a and b. The top inset SAED pattern

corresponding to the tip (1 region), the (111), (200) and (111)

planes of FeS2 (JCPDS 42-1340) are determined from the sharp

spots due to the [022] zone, which demonstrates it has a single

crystal structure. The corresponding HRTEM image shows the

clear crystal lattice planes (Fig. 5b), which also have been

obtained from [022]–projected. The fringe space is determined to

Fig. 3 The XRD patterns of other samples: (S6) Sample 6; (S7) Sample

7; (S8) Sample 8; (S9) Sample 9; (S10) Sample 10; (S11) Sample 11; (S12)

Sample 12. *, + and X denote Fe, Fe7S8 and unknown phase,

respectively.

Fig. 4 (a) The energy-dispersive X-ray spectrum for S12; (b) the SEM

image of S12; and (c) is the enlarged picture of (b).

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be 0.31 nm, which is in accordance with the (111) lattice spacing of

FeS2. The TEM image (Fig. 5c) clearly shows the size and

morphology of Co-doped FeS2 particles. The EDS shows that the

components of product are Fe, Co and S (Fig. 5 in the ESI{). As

seen from the corresponding HRTEM image of the 2 region

(Fig. 5d), the top of the inset corresponding to fast fourier trans-

form (FFT), and the clear lattice fringes reveal a good crystallinity.

3.2 The properties of photoelectric response and water photolysis

The optical property of FeS2 and Co-doped FeS2 films was

determined by UV-vis absorption spectroscopy. As shown in

Fig. 6, the larger absorption of S1 and S12 starts from around

600 nm and it reaches saturation at about 450 nm. As we know,

when the size of the product grains is considerably larger,

quantum-confinement effects seem inexistent. However, com-

pared with the direct band gap 0.9 eV (l = 1375 nm) of bulk

FeS2,44 the current experimental results show that the absorption

edge has an obvious blue shift, indicating the strong quantum

confinement of FeS2 and Co-doped FeS2 films. It can be

concluded that the reason would be that quantum-confinement

effects possibly occur near the peak edge of the tips, which are

thinner, since the optical properties of non-spherical nanocrys-

tals are controlled by the lowest dimension of nanocrystals.42

To investigate the electron-transfer process, several sets of

photoelectrochemical experiments were carried out to measure

the photocurrent response of the films. Potentiostatic (current

and time) experiments were conducted to check the photo-

response properties of the films. Fig. 7a shows the photocurrent

and dark current transients obtained using the films as photo-

anodes. The experimental equipment ran for 100 s firstly, then

the illuminating time was taken for 10 s each time. When the

Fig. 5 (a) TEM image of sample 1. The top inset is SAED pattern

corresponding to 1 region; (b) HRTEM image corresponding to 1 region;

(c) TEM image of sample 12; (d) HRTEM image corresponding to 2

region, the top inset is its FFT.

Fig. 6 UV-vis absorption spectra of S1 and S12.

Fig. 7 (a) Photocurrent response and (b) Current–voltage behavior of

Fe, FeS2 (S1) and Co-doped FeS2 (S12) films under the darkness and

illumination of simulated sunlight. Light intensity: 100 mW?cm22;

electrolyte: 1 mol?L21 KOH containing 10% methanol.

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electrodes were illuminated by simulated sunlight, the increase

in the current density for FeS2 film was 0.50 mA?cm22, but it

was about 1.0 mA?cm22 for Co-doped FeS2 film; when the

illumination was interrupted, the current density rapidly

dropped to the original value of steady state. The current

responded with the turn-on and turn-off of the light quickly.

Compared with the photoelectric response of synthesized films,

the photocurrent wasn’t produced when the iron film was

illuminated. Up to now, the photoelectric response of lots of

materials have been reported in the literature, such as

Ru(dcbpy)2(NCS)2 sensitized ZnO, polyoxophosphotungstate-

TiO2/Ti under UV pulse illumination and Tl2O3 films in the

1 mol?L21 KOH containing 0.18 mol?L21 TlAc.45–47 Although

the FeS2 and Co-doped FeS2 films weren’t sensitized, their

photoelectric response could be displayed under simulated

sunlight. And these films could be suitable for application

under sunlight.

Fig. 7b displays the current–voltage (I–V) curves of Fe, FeS2

and Co-doped FeS2 films under darkness and simulated sunlight,

respectively. The current of Fe film wasn’t measured under both

of darkness and sunlight up to about 1.4 VRHE. The current of

FeS2 film could be determined at 0.94 VRHE under darkness at

first, then the current rises slowly to 19.5 mA?cm22 at 1.1 VRHE

and it remains almost constant until 1.4 VRHE. Corresponding to

the potential of the reversible oxygen electrode, the current

density is 21.6 mA?cm22 at 1.23 VRHE; the photocurrent of FeS2

film would be produced at 0.90 VRHE under sunlight and the

cathodic shift of voltage value is about 0.04 V. The photocurrent

rapidly increased and the current density reached 42.5 mA?cm22.

Under similar conditions, the current of Co-doped FeS2 film is

produced at about 0.94 VRHE under darkness. The current

density rapidly goes up to 42.5 mA?cm22 at 1.23 VRHE, which is

almost twice of FeS2 film. Surprisingly, the photocurrent would

be determined at 0.72 VRHE under sunlight and the cathodic shift

is about 0.22 V which is much larger than that of FeS2. The

photocurrent steeply climbed and the current density arrived to

56.8 mA?cm22 under sunlight, which is larger than that of FeS2

under illumination condition. The photocurrent density of FeS2

and Co-doped FeS2 is higher than that of reported materials

such as Fe2O3 and TiO2.1,48,49

Based on the current experimental results and reports,49,50 the

possible schematic of water splitting would be deduced to

understand how the prepared films help to split water. Step I, the

films absorbs photon energy which is greater than the band gap

energy of the material, then photoexcited electrons and holes

pairs are generated on the films; step II, electrons and holes

would immediately reach the surface of the films; Step III,

CH3OH, which was adsorbed by the surface of the films, would

be easily oxidized by holes to CO2 in the surface (CH3OH can be

easily oxidized than water). The electrons flow toward the

substrate (Fe) and pass to the cathode (Pt) through the external

circuit, which induced H2O to be reduced, and then H2 would be

generated. The first step is strongly dependent on the structure

and electronic properties of the materials. Especially, a number

of photoexcited electrons and holes can recombine in the second

step, which largely reduces the photooxidation ability. In this

study, on the one hand, CH3OH, a sacrificial reagent, promoted

H2 evolution by its preferential photooxidation. And it relatively

decreases the rate of the surface recombination between the

surface hole and electron pairs because the concentration of

holes on the surface decreases as to capture holes; on the other

hand, the iron substrate has better electric conductivity than the

prepared films. The electrons can easily reach the Fe substrate,

which reduce the electrons and holes recombination loss.

Consequently, the presence of CH3OH and the use of substract

(Fe) promote H2 evolution under illumination because of

decrease in the surface recombination rate of the electron and

hole. Thus, compared with the literature reported,1,48,49 the

photocurrent density of FeS2 and Co-doped FeS2 is higher.

In addition, electron paramagnetic resonance suggested that

Co2+ substituting Fe2+ in the pyrite structure introduces defect

states at different energy levels within the forbidden zone.51 The Co

defect state with a donor electron occurs in the band gap, near or

possibly overlapping the conduction band energy, so the extra

electrons easily move into the conduction band. Furthermore,

cobalt would play a catalytic role during the photocurrent process.

The electrocatalytic activity of cobalt and iron/cobalt oxides for

water oxidation is well established and involves the CoII/CoIII and

CoIII/CoIV couples.1,52,53 The distinct photocurrent response and

photocurrent density of the FeS2 and Co-doped FeS2 can be

attributed to the cobalt doped in the pyrite.

Conclusions

Pyrite FeS2 and Co-doped FeS2 films were synthesized with the

aid of scCO2. Diverse phases and morphologies could be

obtained by controlling different reaction conditions. The

reaction conditions including the pressure, precursor, STPP,

reaction temperature and doped reagent, result in dramatic

differences in the crystal phases. The UV-vis absorption

spectroscopy of these films indicated that the absorption

wavelength has an obvious blue shift compared with bulk

FeS2. Photoelectrochemical experiments were conducted to

measure the photocurrent response of the films. The current

increase of FeS2 films was 0.5 mA?cm22, and the current increase

of Co-doped FeS2 films was about 1 mA?cm22 under sunlight.

As shown in the current–voltage (I–V) curve, the current density

of FeS2 achieved 42.5 mA?cm22 at 1.23 VRHE and the cathodic

shift of voltage value was about 0.04 V under sunlight. The

doped Co2+ electrode resulted in a 0.22 V cathodic shift, and

the photocurrent increased to 56.8 mA.cm22 at 1.23 VRHE. The

present study has demonstrated that the FeS2 and Co-doped

FeS2 films prepared display excellent properties for photoelectric

response and water photolysis under sunlight.

Acknowledgements

We gratefully appreciate the assistance with the photoelectro-

chemical characterization of Mr. Bingxin Lei and Mr. Xihong Lu.

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