Accepted Manuscript Title: In2 S3 /carbon nanofibers/Au ternary synergetic system: hierarchical assembly and enhanced visible-light photocatalytic activity Author: Xin Zhang Changlu Shao Xinghua Li Na Lu Kexin Wang Fujun Miao Yichun Liu PII: DOI: Reference:
S0304-3894(14)00818-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.10.005 HAZMAT 16318
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
5-7-2014 29-9-2014 3-10-2014
Please cite this article as: X. Zhang, C. Shao, X. Li, N. Lu, K. Wang, F. Miao, Y. Liu, In2 S3 /carbon nanofibers/Au ternary synergetic system: hierarchical assembly and enhanced visible-light photocatalytic activity, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In2S3/carbon nanofibers/Au ternary synergetic system: hierarchical assembly and enhanced visible-light photocatalytic activity Xin Zhang, Changlu Shao,* Xinghua Li,* Na Lu, Kexin Wang, Fujun Miao and
cr
ip t
Yichun Liu
Ac ce p
te
d
M
an
us
Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
*Corresponding author: Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China Email: [email protected]; [email protected]; Tel. 8643185098803. 1
Page 1 of 37
Synthesis of In2S3/CNFs/Au ternary synergetic system.
cr
Enhanced visible-light photocatalytic activity.
ip t
Highlights:
an
us
Easy photocatalyst separation and reuse.
Abstract
M
In this paper, carbon nanofibers (CNFs) were successfully synthesized by electrospinning technique. Next, Au nanoparticles (NPs) were assembled on the
te
d
electrospun CNFs through in situ reduction method. By using the obtained Au NPs modified CNFs (CNFs/Au) as hard template, the In2S3/CNFs/Au composites were
Ac ce p
synthesized through hydrothermal technique. The results showed that the super long one-dimensional (1D) CNFs (about 306 nm in average diameter) were well connected to form a nanofibrous network; and, the Au NPs with 18 nm in average diameter and In2S3 nanosheets with 5-10 nm in thickness were uniformly grown onto the surface of CNFs. Photocatalytic studies revealed that the In2S3/CNFs/Au composites exhibited
highest visible-light photocatalytic activities for the degradation of Rhodamine B (RB) compared with pure In2S3 and In2S3/CNFs. The enhanced photocatalytic activity might arise from the high separation efficiency of photogenerated electron-hole pairs based on the positive synergetic effect between In2S3, CNFs and Au components in 2
Page 2 of 37
this ternary photocatalytic system. Meanwhile, the In2S3/CNFs/Au composites with hierarchical structure possess a strong adsorption ability towards organic dyes, which also contributed to the enhancement of photocatalytic activity. Moreover, the
ip t
In2S3/CNFs/Au composites could be recycled easily by sedimentation due to their
us
cr
nanofibrous network structure.
an
Keywords: Electrospinning; CNFs; In2S3; Au NPs; RB; photocatalysis; Recyclability. 1. Introduction
M
In recent years, photocatalytic technology has been extensively used for the treatment of polluted water [1-4]. Among various photocatalysts, TiO2 has been
te
d
extensively studied due to its exceptional optical and electronic properties, non-toxicity, chemical stability, and low cost [5, 6]. However, TiO2 can solely absorb
Ac ce p
the UV light due to its wide band gap of 3.2 eV. Since the UV region occupies only near 4% of the entire solar spectrum, while 45% of the energy belongs to visible light, most solar energy cannot be used [7-9]. Consequently, the development of efficient photocatalysts with visible light response has become a research focus [10]. Indium sulfide (In2S3), a typical III–VI group chalcogenide with a band gap of
2.0–2.2 eV, has attracted intense interest as a potential visible light photocatalyst due to its high photosensitivity and photoconductivity, stable chemical composition and physical characteristics at ambient conditions, and low toxicity [11-13]. However, similar to other narrow band gap visible light photocatalysts, poor quantum yield 3
Page 3 of 37
caused by the rapid recombination of photogenerated electron-hole pairs is still a challenge to enhance the photocatalytic efficiency of In2S3 to meet the practical application requirements. Therefore, several attempts have been made to reduce the
ip t
recombination of photogenerated electron–hole pairs by combining In2S3 with
cr
electron acceptor such as the matchable band structure semiconductors, noble metals
us
and carbon materials [14-18]. Among them, carbon materials attract great interest due to their superior electronic, physicochemical, mechanical character and high
an
absorption properties. In the carbon family, carbon nanofibers with one-dimensional (1D) structure have gradually become a rising star, due to their excellent electrical
M
conductivity [19, 20]. Especially, the electrospun carbon nanofibers (CNFs) with super long 1D nanostructure have been proved to possess enhanced electronic
te
d
transport properties compared with traditional carbon nanofibers obtained by other physical and chemical methods [21-23]. Moreover, their unique three-dimensional
Ac ce p
(3D) interconnected structure obtained from electrospinning technique also contributes to the charge transfer [24]. Therefore, one could expect that coupling In2S3 with electrospun CNFs to construct In2S3/CNFs would inhibit electron–hole pairs
recombination resulting in enhanced photocatalytic activity. On the other hand, many reports have shown that incorporating noble metal (e.g.
Au, Ag, Pt, and Pd) nanoparticles (NPs) into composite photocatalysts could further improve their photocatalytic activity through further extending electron–hole lifetime with the noble metal NPs acting as irreversible electron sinks. For instance, Chai et al. reported
that
the In2S3/(Pt-TiO2)
nanocomposites
exhibited
higher
visible
4
Page 4 of 37
photoactivity compared with In2S3/TiO2 [25]. Roy et al. reported that the photocatalytic efficiency of graphene-ZnO-Au nanocomposites was higher than graphene-ZnO [26]. These pioneering works promote us to envision that the
ip t
photocatalytic efficiency of In2S3/CNFs may be further improved by incorporating the
cr
noble metal NPs (just like Au NPs) to construct In2S3/CNFs/Au composites.
us
In our work, the CNFs were fabricated by electrospinning technique. Next, the Au NPs were assembled on the electrospun CNFs via in situ reduction method. Finally,
an
by using the obtained Au NPs modified CNFs (CNFs/Au) as hard template, In2S3/CNFs/Au composites were fabricated through a hydrothermal technique.
M
Utilizing the positive synergetic effect between the In2S3, CNFs and Au components in the obtained In2S3/CNFs/Au composites, the photogenerated electrons on the
te
d
conduction bands (CB) of In2S3 could easily transfer to CNFs, and then irreversibly captured by the Au NPs, realizing a more effective separation of photogenerated
Ac ce p
electron-hole pairs to get a much better performance in the photocatalytic processes compared with the pure In2S3 and In2S3/CNFs. Also, by utilizing their free-standing
nanofibrous network structure, the In2S3/CNFs/Au composites could be easily
recycled by sedimentation in the practical application.
2. Experimental section 2.1. Materials. Polyacrylonitrile (PAN) (Mw = 150 000), N,N-dimethylformamide (DMF) and tetrachloroauric acid (HAuCl4) were purchased from Tianjin Chemicals (Tianjin, 5
Page 5 of 37
China). HCl and SnCl2 were purchased from Beijing Chemicals (Beijing, China). In(NO3)3·5H2O and L-cysteine were purchased from sinopharm Chemicals (shanghai,
ip t
China). All materials were directly used as received without further purification. 2.2. Preparation of the CNFs
cr
In our experiments, the preparation process consisted of two steps. First, 1.5 g of
us
PAN powders were dissolved in 10 mL of DMF solution. After stirring at room temperature for 12 h, the above precursor solutions were drawn into a hypodermic
an
syringe for electrospinning. The positive voltage applied to the tip was 10 kV and the distance between the needle tip and the collector was 15 cm. The as-spun PAN
M
nanofibers were collected on aluminum foil, which was attached on the edge of the collecting wheel. For carbonization, the substrates were placed in a tube furnace and
d
stabilized in air for 60 min at 270 °C, then they were carbonized in nitrogen at 1000
te
°C at a ramp rate of 5 °C min-1, and finally cooled to room temperature. Thus,
Ac ce p
electrospun CNFs were obtained. 2.3. Preparation of CNFs/Au
In a typical experiment, 0.02 g of SnCl2 was dissolved in 20 mL of 5 mM HCl
solution. Then, 0.03 g of as-electrospun CNFs was added to the above solution and stirred for 6 h at room temperature. The precipitate was recovered by centrifugation, followed by washing with distilled water five times. After that, the activated CNFs were obtained. Then, the activated CNFs were added to 20 mL of 0.5 mM tetrachloroauric acid (HAuCl4) solution under vigorous stirring for about 5 min at room temperature. The obtained composite was washed with deionized water and 6
Page 6 of 37
ethanol to remove any ionic residual, then dried at 60 °C for 12 h. Thus, the expected CNFs/Au was obtained after those processes. 2.4. Preparation of the In2S3/CNFs/Au composites
ip t
In a typical procedure, 0.6 mmol In(NO3)3·5H2O and 1.8 mmol L-cysteine were
cr
dissolved in 20mL distilled water under magnetic stirring [9, 25]. After 10 min, the
us
obtained solution and 15 mg CNFs/Au was mixed together, followed by stirring for 10 min. The mixed solution was transferred into a 25 mL Teflon-lined stainless
an
autoclave, sealed and maintained at 180 °C for 24 h, and then cooled down to room temperature. The as-fabricated products were collected and washed several times with
M
ethanol and deionized water, respectively, then dried at 60 °C for 12 h. For contrast, the In2S3/CNFs was fabricated by the same method, while the pure In2S3 was obtained
2.5. Characterization
te
d
in the absence of CNFs during the preparation process.
Ac ce p
The morphologies of the as-prepared nanofibers were observed by scanning
electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100). Energy dispersive X-ray (EDX) spectroscope coupled to a scanning electron microscope (SEM) was used to analyze the composition of samples. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku, D/max-2500 X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was performed on a VG-ESCALAB LKII instrument with an Mg K ADES (hν = 1253.6eV) source at a
residual gas pressure of below 10-8 Pa. The UV-Vis diffuse reflectance spectra of the 7
Page 7 of 37
samples were recorded on a Cary 500 UV-Vis-NIR spectrophotometer. 2.6. Photocatalytic tests The photocatalytic activity of the samples was evaluated from the degradation of
ip t
Rhodamine B (RB). 0.04 g photocatalyst was suspended in 100 mL RB solution with
cr
the initial concentration of 10 ppm. The solution was stirred in the dark for 30 min to
us
obtain a good dispersion and to reach the adsorption–desorption equilibrium between the organic molecules and the catalysts surface. A 150 W xenon lamp with a cutoff
an
filter (λ > 420 nm) was used as the visible light source for visible-light photocatalysis. At given intervals of illumination, 3 mL reacting solutions in series were taken out
M
and analyzed. The concentrations of RB in the reacting solutions were analyzed by a Cary 500 UV-Vis-NIR spectrophotometer at λ = 554 nm.
te
d
2.7. Photoelectrochemical experiment
Photoelectrochemical measurements were performed using the conventional three
Ac ce p
electrodes setup connected to an electrochemical station (CHI600D Chenhua Shanghai, China). This assembly had pure In2S3/FTO, In2S3/CNFs/FTO and
In2S3/CNFs/Au/FTO (effective area was 1 cm2) as working electrodes, and a Pt wire and an Ag/AgCl (saturated KCl) electrode were used as the counter electrode and reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution.
A 150 W xenon lamp with a cutoff filter (λ > 420 nm) was used as the visible light source. The photocurrent response spectra were carried out at a constant potential of +0.6 V to the working photoanode. Electrochemical impedance spectra (EIS) were measured at open-circuit voltage. A sinusoidal ac perturbation of 5 mV was applied to 8
Page 8 of 37
the electrode over the frequency range of 100 mHz to 2 MHz.
ip t
3. Results and discussion 3.1. SEM of the samples
cr
The morphology of the samples was analyzed by SEM. The SEM image of
us
electrospun CNFs is shown in Fig. 1a, which shows that the average diameter (according to the size distribution showed in Fig. S1a) and length of the electrospun
an
CNFs are about 306 nm and several millimeters, respectively. The obtained CNFs with high aspect ratio and smooth surface are well connected to form a conductive 3D
M
free-standing network, which is favorable for electrical charge transport. The SEM image of CNFs/Au (Fig. 1b) shows that there is no essential change to the
te
d
morphology of CNFs during the growth of Au NPs. From the magnified image of CNFs/Au (inset of Fig. 1b), it is clearly that the Au NPs with an average diameter of
Ac ce p
18 nm (according to the size distribution showed in Fig. S1b) well dispersed on the surface of CNFs without aggregation. And, after hydrothermal treatment, the surface of CNFs/Au is uniformly covered by the ultrathin In2S3 nanosheets (Fig.1c). No
aggregation could be found in the SEM images of In2S3/CNFs/Au composites. The high-magnification SEM image (Fig. 1d) reveals that the In2S3 nanosheets with a
thickness of 5-10 nm are interconnected with each other, forming the 3D nanosheets networks. Moreover, the In2S3/CNFs composites are synthesized for contrast. The corresponding SEM image is shown in Fig. 1e, which shows that the morphology of In2S3/CNFs is almost no difference with In2S3/CNFs/Au composites. The formation of 9
Page 9 of 37
the uniform hierarchical structures may originate from the special characteristics of the electrospun CNFs, i.e. the 3D open structure and large surface areas, which could provide abundant active sites for the assembly of Au NPs and In2S3 nanosheets.
ip t
However, the pure In2S3 obtained by the same hydrothermal method without the
cr
presence of CNFs/Au, self-assemble to form 3D In2S3 microspheres (Fig. 1d). And,
photocatalytic activity.
an
3.2. TEM of the In2S3/CNFs/Au composites
us
the In2S3 microspheres aggregate with each other, which is detrimental to their
For further investigating the In2S3/CNFs/Au composites, TEM and HRTEM
M
measurements were performed. The typical TEM image of an individual hierarchical composite nanofiber is shown in Fig. 2a. It can be observed clearly that the Au NPs
te
d
(about 18 nm) are evenly dispersed on the surface of CNFs, which coincides with the results of the SEM observations. This indicates that Au NPs are not detached from the
Ac ce p
surface of CNFs during the hydrothermal process. In addition, the In2S3 nanosheets
had a narrow size distribution (the distance from the surface of CNFs to the outer edge of the In2S3 nanosheets is around 100-150 nm) uniformly covered on the surface of CNFs/Au. The HRTEM image (Fig. 2b) of the In2S3 nanosheets display two types
of clear lattice fringes ca. 0.62 nm and ca. 0.32 nm, which correspond to the (111) and (311) planes of the cubic In2S3, respectively. The TEM images of the In2S3/CNFs/Au
composites reveal that highly dense secondary In2S3 nanosheets are successfully grown on the CNFs/Au templates, coinciding with the results from SEM observations. 3.3. XRD patterns 10
Page 10 of 37
The X-ray diffraction (XRD) patterns of pure CNFs, Au-CNFs, In2S3/Au-CNFs and pure In2S3 are shown in Fig. 3. The broad peaks centered at around 25ºand 44º are attributed to the (002) and (100) plane of the carbon structure in CNFs (Fig. 3a), while
ip t
the four peaks at 38.51º, 43.91º, 64.71º, and 78.61º are attributed to (111), (200),
cr
(220), and (311) planes of face-centered-cubic (fcc) Au (JCPDS 65-2870) in Fig. 3b.
us
For the pure In2S3, the diffraction peaks at 2 values of 14.3º, 27.5º, 28.7º, 33.2º, 43.7º, 47.8º, 56.6º and 59.4º can be ascribed to the (111), (311), (222), (400), (511)
an
(440), (622) and (444) planes in the face-centered-cubic (fcc) -In2S3 (JCPDS 65-0459), which is in agreement with the literatures [18, 27]. As shown in Fig.1c, all
M
the peaks can be indexed while no diffraction peak for any other crystal phase is detected, indicating that there is no crystal phase impurity existing in the obtained
te
d
sample In2S3/CNFs/Au composites.
3.4. XPS spectra of the In2S3/CNFs /Au
Ac ce p
To determine the chemical composition and valence states of various elements in
the In2S3/CNFs/Au composites, XPS measurements were carried out. The high resolution XPS spectra for the C 1s region, around 285 eV, are shown in Fig. 4a. The binding energy peak at 284.6 eV can be attributed to C–C bonds and is identified as originating from the amorphous carbon phase or from adventitious carbon; the peak at 285.6 eV is characteristic of the combination of C–O groups [28]. Moreover, the peak of the carboxyl carbon (O–C=O) is located at 289 eV [29]. Interestingly, the peak of the carboxyl carbon (O–C=O) in our work is found to shift obviously to a lower binding energy compared with pure CNFs, suggesting that the carboxyl acts as a 11
Page 11 of 37
nucleation site for Au NPs and In2S3 nanosheets on CNFs. Fig. 4b shows the XPS spectrum of In 3d with two symmetrical peaks at the binding energies of 444.4 eV for In 3d5/2 and 452.0 eV for In 3d3/2 [30]. For the spectrum of S 2p in Fig. 4c, the peaks
ip t
at 160.7 and 162.1 eV can be attributed to S 2p3/2 and S 2p1/2 transitions, respectively
cr
[31]. The spin–orbit splitting of In 3d and S 2p are found to be 7.6 and 1.4 eV,
us
indicating that the valence state of In and S are +3 and -2 respectively in the In2S3/CNFs/Au composites [31-33]. Fig. 4d shows the Au 4f region of the XPS
an
spectrum of the In2S3/CNFs/Au composites. The Au 4f7/2 signal can be resolved into two components, which are centered at 83.5 and 87.2 eV, respectively. These are
M
assigned to the characteristic doublets of Au0 loaded on CNFs, suggesting that only elemental Au is formed on the CNFs surface [34]. The above XPS results confirm that
te
d
the composites are composed of CNFs, In2S3 and Au. 3.5. Photocatalytic activity
Ac ce p
The photocatalytic activity of the samples was evaluated by the decomposition of
RB under visible light irradiation. Temporal changes in the concentration of RB, as monitored by the maximal absorption of RB at 554 nm in the UV–Vis spectra over the as-prepared photocatalysts, are shown in Fig. 5. The degradation efficiency of all the samples is defined as C/C0, where C is the concentration of RB at the irradiation time t and C0 is the concentration of RB at the irradiation time t = 0, respectively. Before studying and comparing the activities of the samples, the control experiments were performed under different conditions: (1) in the presence of the photocatalysts but in the dark; (2) with visible light irradiation but in the absence of the photocatalysts. As 12
Page 12 of 37
shown in Fig. 5a, the adsorption–desorption equilibrium of RB in the dark is established within 30 min. And, there is no appreciable degradation of RB after 3 h in the absence of the photocatalysts. Interestingly, the In2S3/CNFs/Au composites show
ip t
strong adsorption ability towards the RB (Fig. 5a and Fig. S2). This can be partly
cr
attributed to the large specific surface area of the 2D In2S3 nanosheets and the high
us
absorption property of CNFs in the In2S3/CNFs/Au composites. After the adsorption process, the photocatalytic degradation activities of the different samples were
an
investigated. Fig. 5b shows the degradation curves of RB on pure CNFs, CNFs/Au, pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites. It can be seen that pure CNFs
M
and CNFs/Au have no photocatalytic activity under visible light irradiation. After visible light irradiation for 60 min, 95.5% of RB is decomposed by using the
te
d
In2S3/CNFs/Au composites as the photocatalyst, while approximately 85.5 and 75.2% degradation of RB are observed for the pure In2S3 and In2S3/CNFs samples,
Ac ce p
respectively. As expected, In2S3/CNFs/Au composites exhibited much higher photocatalytic efficiency compared with the pure In2S3 and In2S3/CNFs. For a better
comparison of the photocatalytic efficiency of In2S3/CNFs/Au composites,
In2S3/CNFs and pure In2S3, the kinetic analysis of degradation of RB was discussed. The kinetic linear simulation curves of the photocatalytic degradation of RB over the above catalysts shows that the above degradation reactions followed a Langmuir -Hinshelwood apparent first-order kinetics model due to the low initial concentrations of the reactants. The explanation is described below: [35]
13
Page 13 of 37
where r is the degradation rate of the reactant (mg/(L min)), C is the concentration of
ip t
the reactant (mg/L), t is the visible light irradiation time, k is the reaction rate constant (mg/(L min)), and K is the adsorption coefficient of the reactant (L/mg).
cr
When the initial concentration (C0) is very low (C0 < 10 mg/L) for RB in the present
an
us
experiment), eq.1 can be simplified to an apparent first-order model: [36]
where kαpp is the apparent first-order rate constant (min-1). The determined kαpp values
M
for degradation of RB with different catalysts are summarized in the Fig. 5c. The apparent first-order rate constants of In2S3/CNFs/Au composites, In2S3/CNFs and pure
te
d
In2S3 for the degradation of RB are 0.0529, 0.0322 and 0.0248 min−1, respectively. The photocatalytic reactivity order is well-consistent with the activity studies above.
Ac ce p
Moreover, the photograph in Fig. 5d shows that the In2S3/CNFs/Au composites can
be easily separated from the solution by sedimentation due to their nanofibrous network structure while the pure In2S3 could only be separated by centrifugation. The above experiment results indicate that the In2S3/CNFs/Au composites display an
efficient photocatalytic activity for the degradation of RB under visible light irradiation and can be easily separated for reuse. 3.6. Postulated photocatalytic mechanism Photoelectrochemical measurements are often used to qualitatively study the excitation and transfer of photogenerated charge carriers in photocatalysts. The 14
Page 14 of 37
transient photocurrent responses of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites are recorded for three on-off cycles under visible light irradiation and plotted in Fig. 6a. As expected, In2S3/CNFs/Au composites show the highest
ip t
photocurrent intensity of the three samples, whereas In2S3/CNFs and pure In2S3 have
cr
the middle and lowest values, respectively. Thus, the sample In2S3/CNFs/Au
us
composites may achieve the highest quantum yield based on the effective charge separation, which is consistent with the photocatalytic activity measurements and our
an
above discussions [37]. Furthermore, the better separation of photogenerated electrons-holes pairs in the In2S3/CNFs/Au composites is confirmed by PL emission
M
spectra. In Fig. 6b, the In2S3/CNFs/Au composites exhibit much lower emission intensity than In2S3/CNFs and pure In2S3. The quenching of PL emission spectra of
te
d
the In2S3/CNFs/Au can attributed to efficient electron transfer between In2S3, CNFs and Au NPs, which indicate that the recombination of the photogenerated charge
Ac ce p
carriers is inhibited greatly in the In2S3/CNFs/Au composites [38, 39]. The efficient charge separation could increase the lifetime of the charge carriers and enhance the efficiency of the interfacial charge transfer to adsorbed substrates, and then account for the higher activity of In2S3/CNFs/Au photocatalyst. To further understand the role of Au NPs in the In2S3/CNFs/Au composites, EIS
Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites without and with visible-light irradiation were carried out. As shown in Fig. 7, the Nyquist impedance plots of In2S3/CNFs and In2S3/CNFs/Au composites electrode materials cycled in 0.5 M Na2SO4 electrolyte solution both exhibit semi cycles at high frequencies. 15
Page 15 of 37
Considering that the preparation of the electrodes and electrolyte used are identical, the high frequency semicircle is relevant to the resistance of the electrodes [40]. In electrochemical spectra, the high frequency are corresponds to the charge transfer
ip t
limiting process and can be attributed to the charge transfer resistance at the contact
cr
interface between the electrode and the electrolyte solution [18, 41]. The smaller are
us
radius of the EIS Nyquist plot corresponds to the lower electric charge transfer resistance. Clearly, the introduction of Au NPs leads to a significantly decreased
an
diameter of the semicircular Nyquist plot as compared to In2S3/CNFs, in the cases of both with and without light irradiation, suggesting a faster charge transfer rate. This
M
result implies that the introduction of Au NPs can obviously favor the separation and
te
the photocatalytic activity.
d
transfer of photogenerated carriers in In2S3/CNFs/Au composites and then enhance
On the basis of the above analysis, a proposed mechanism is discussed to explain
Ac ce p
the enhancement of the photocatalytic properties of the In2S3/CNFs/Au composites
(Scheme 1). Under visible light illumination, In2S3 with narrow band gap energy can be easily excited and generates a large number of free electrons in the conduction band (CB) and holes in the valence band (VB). In this ternary synergetic system, the CB energy level of In2S3 is ca. 3.57 eV (CB potential = -0.93 V vs. NHE; E (eV) = E(V vs. NHE) + 4.5 (V)) [42], whereas the work function of CNFs and Au NPs are ca.4.6 eV and 5.1 eV, respectively [43, 44]. Such energy levels suggest that the electrons in the CB of In2S3 may move freely toward the surface of the CNFs, and might further transfer to Au NPs due to the work function of CNFs is higher than CB 16
Page 16 of 37
energy level of In2S3 but lower than work function of Au NPs. Meanwhile, if In2S3 nanosheets contact with Au NPs directly, the electrons in the CB of In2S3 might also be entrapped by Au NPs due to its high Schottky barriers at the metal–semiconductor
ip t
interface, which reduces the recombination with holes. Those electron transfer routes
cr
could enhance the separation of photoinduced holes and electrons, thus improve the
us
photocatalytic properties. We note that the decoration of Au NPs on the surface of CNFs obviously enhanced the charge separation efficiency compared to that of
an
In2S3/CNFs composites, as evidenced by the photocurrent and PL experiments, suggesting that Au/CNFs have better photoelectrons collection ability than pure CNFs.
M
Furthermore, The photogenerated holes left in VB transfer to the surface of In2S3 to oxidize water into O2(VB), which have been confirmed in our previous study on the
d
One-dimensional In2S3/TiO2 hierarchical heterostructures [9]. Then, the O2(VB) as well
te
as the dissolved O2 in solution capture the photogenerated electrons to yield O2− and
Ac ce p
subsequently produce •OH, which is a strong oxidizing agent to decompose the organic dyes. The mechanism for the photocatalytic degradation of RB in our experiment is proposed as follows (3)-(11): In2S3 + hν → In2S3(e– + h+)
(3)
In2S3(e–) + CNFs/Au → CNFs(e–) + Au(e–)
(4)
CNFs(e–) + Au → Au(e–)
(5)
In2S3(h+) + 2H2O → In2S3 + O2(VB) + 4H+ Au(e–) + O2(VB
+ dissolved)
→ Au + O2–
O2– + H2O → H2O• + OH–
(6) (7) (8)
17
Page 17 of 37
H2O• + H2O → H2O2 + •OH
(9)
H2O2 → 2•OH
(10) (11)
ip t
•OH + RB → CO2 + H2O 3.7. Stability of In2S3/CNFs/Au Photocatalyst
cr
The stability of a photocatalyst is very important for industrial application. To test
us
the stability of RB photodegradation on In2S3/CNFs/Au composites, we reused the catalyst for three times. As shown in Fig. 8a, each experiment is carried out under
an
identical conditions; after a three cycle experiment, the photocatalytic activity of the In2S3/CNFs/Au composites remain almost unchanged. Fig. 8b shows the XRD
M
patterns and SEM images of the In2S3/CNFs/Au composites before and after the three times photocatalytic reaction. After three photocatalytic runs, the position and the
te
d
ratio of peaks are nearly the same as that of fresh photocatalyst. And the secondary In2S3 nanostructures are still very complete, clearly indicating the stability of the
Ac ce p
In2S3/CNFs/Au photocatalyst.
4. Conclusions
In summary, we described herein an effective route to synthesize the
In2S3/CNFs/Au
ternary
synergetic
system.
The
as-prepared
In2S3/CNFs/Au
composites exhibited enhanced photocatalytic activity in the decomposition of RB. In the ternary synergetic system, CNFs/Au serving as electron collectors can more efficiently suppress charge recombination, improve interfacial charge transfer, and provide a greater number of photocatalytic reaction centers, resulting in a high 18
Page 18 of 37
quantum yield to get a much better performance in the photocatalytic processes. Moreover, the In2S3/CNFs/Au composites could be recycled easily by sedimentation due to their free-standing nanofibrous network structure. This study provides a new
ip t
strategy to construct the semiconductor/CNFs/Au composites with high photocatalytic
us
cr
activity and good recyclable performance.
Acknowledgements
an
The present work is supported financially by the National Basic Research Program of China (973 Program) (Grant No.2012CB933703), the National Natural Science
M
Foundation of China (No. 91233204, 51272041, and 61201107), the 111 Project (No.B13013), the Fundamental Research Funds for the Central Universities
Ac ce p
Province (20121802).
te
d
(14ZZ1511 and 12SSXM001), and the Program for Young Scientists Team of Jilin
19
Page 19 of 37
ip t cr us an M
References
[1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor
te
d
electrode, Nature 238 (1972) 37–38.
[2] M. Batzill, Fundamental aspects of surface engineering of transition metal oxide
Ac ce p
photocatalysts, Energy Environ. Sci. 4 (2011) 3275–3286. [3] X. Zhang, C. Shao, Z. Zhang, J. Li, P. Zhang, M. Zhang, J. Mu, Z. Guo, P. Liang, Y. Liu, In situ generation of well-dispersed ZnO quantum dots on electrospun silica nanotubes with high photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 785–790.
[4] S. Park, S. Kim, H.J. Kim, C.W. Lee, H. J. Song, S.W. Seo, H.K. Park, D.W. Kim, K.S. Hong, Hierarchical assembly of TiO2–SrTiO3 heterostructures on conductive SnO2 backbone nanobelts for enhanced photoelectrochemical and photocatalytic performance, J. Hazard. Mater. 275 (2014) 10–18. 20
Page 20 of 37
[5] S.U. Khan, M. Al-Shahry, W.B. Ingler Jr., Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243–2245. [6] M. Miljevic, B. Geiseler, T. Bergfeldt, P. Bockstaller, L. Fruk, Enhanced
ip t
Photocatalytic Activity of Au/TiO2 Nanocomposite Prepared Using Bifunctional
cr
Bridging Linker, Adv. Funct. Mater. 24 (2014) 907–915.
us
[7] A.L. Linsebigler, G. Lu and J.T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 95 (1995) 735-758.
an
[8] M.A. Fox and M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341-357.
M
[9] X. Zhang, X. Li, C. Shao, J. Li, M. Zhang, P. Zhang, K.Wang, N. Lu, Y. Liu, One-dimensional hierarchical heterostructures of In2S3 nanosheets on electrospun
(2013) 892-900.
te
d
TiO2 nanofibers with enhanced visible photocatalytic activity, J. Hazard. Mater. 260
Ac ce p
[10] Y.X. Li, G. Chen, Q. Wang, X. Wang, A.K. Zhou, Z.Y. Shen, Hierarchical ZnS-In2S3-CuS Nanospheres with Nanoporous Structure: Facile Synthesis, Growth Mechanism, and Excellent Photocatalytic Activity, Adv. Funct. Mater. 20 (2010) 3390–3398.
[11] W.T. Kim and C.D. Kim, Optical energy gaps of β-In2S3 thin films grown by
spray pyrolysis, J. Appl. Phys. 60 (1986) 2631-2633. [12] W. Qiu, M. Xu, X. Yang, F. Chen, Y. Nan, J. Zhang, H. Iwai and H. Chen, Biomolecule-assisted hydrothermal synthesis of In2S3 porous films and enhanced photocatalytic properties, J. Mater. Chem. 21 (2011) 13327-13333. 21
Page 21 of 37
[13] Y. Xing, H. Zhang, S. Song, J. Feng, Y. Lei, L. Zhao and M. Li, Hydrothermal synthesis and photoluminescent properties of stacked indium sulfide superstructures, Chem. Commun. (2008) 1476-1478.
ip t
[14] C. Gao, J. Li, Z. Shan, F. Huang and H. Shen, Preparation and visible-light
cr
photocatalytic activity of In2S3/TiO2 composite, Mater. Chem. Phys. 122 (2010) 183-187.
us
[15] V. Štengl, F. Opluštil and T. Němec, In3+-doped TiO2 and TiO2/In2S3
an
nanocomposite for photocatalytic and stoichiometric degradations photochem. photobiol. 88 (2012) 265-276.
M
[16] X. An, J. C. Yu, F. Wang, C. Li and Y. Li, One-pot synthesis of In2S3 nanosheets/graphene composites with enhanced visible-light photocatalytic activity,
te
d
Appl. Catal. B 129 (2013) 80-88.
[17] M. Yang, B, Weng and Y. Xu, Synthesis of In2S3–CNT nanocomposites for
Ac ce p
selective reduction under visible light, J. Mater. Chem. A 2 (2014) 1710-1720. [18] M.Q. Yang, B. Weng and Y.J. Xu, Improving the Visible Light Photoactivity of In2S3−Graphene Nanocomposite via a Simple Surface Charge Modification Approach Langmuir 29 (2013) 10549-10558. [19] L. Chen, X. Zhang, H. Liang, M. Kong, Q. Guan, P. Chen, Z. Wu and S. Yu, Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors, ACS Nano 6 (2012) 7092–7102. [20] R.L. McCreery, Advanced Carbon Electrode Materials for Molecular Electrochemistry, Chem. Rev. 108 (2008) 2646–2687. 22
Page 22 of 37
[21] M. Zhang, C. Shao, J. Mu, X. Huang, Z. Zhang, P. Zhang and Y. Liu, Hierarchical heterostructures of Bi2MoO6on carbon nanofibers: controllable solvothermal fabrication and enhanced visible photocatalytic properties, J. Mater.
ip t
Chem. 22 (2012) 577–584.
cr
[22] P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, C. Su, N. Lu, K. Wang and Y.
us
Liu, An electron-rich free-standing carbon@Au core–shell nanofiber network as a highly active and recyclable catalyst for the reduction of 4-nitrophenol, Phys. Chem.
an
Chem. Phys. 15 (2013) 10453-10458.
[23] L. Peng, L.F. Hu and X.S. Fang, Energy Harvesting for Nanostructured
M
Self-Powered Photodetectors, Adv. Funct. Mater. 24 (2014) 2591–2610. [24] W. Li, L. Zeng, Z. Yang, L. Gu, J. Wang, X. Liu, J. Cheng and Y. Yu,
te
d
Free-standing and binder-free sodium-ion electrodes with ultralong cycle life and high rate performance based on porous carbon nanofibers, Nanoscale 6 (2014) 693-698.
Ac ce p
[25] B. Chai, T. Peng, P. Zeng, J. Mao, Synthesis of floriated In2S3 decorated with
TiO2 nanoparticles for efficient photocatalytic hydrogen production under visible light, J. Mater. Chem. 21 (2011) 14587–14593. [26] P. Roy, A. Periasamy, C. Liang and H. Chang, Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene, Environ. Sci. Technol. 47 (2013) 6688−6695. [27] B. Chai, P. Zeng, X. Zhang, J. Mao, L. Zan and T. Peng, Walnut-like In2S3 microspheres: ionic liquid-assisted solvothermal synthesis, characterization and formation mechanism, Nanoscale 4 (2012) 2372–2377. 23
Page 23 of 37
[28] Y.H. Liu and X.L. Jing, Pyrolysis and structure of hyperbranched polyborate modified phenolic resins, Carbon 45 (2007) 1965-1971. [29] A. Francois, J. P. B. Christophe, A. Yasmine, J. Philippe, T. Philippe, P. Michel
ip t
and G. R. Paul, XPS analysis of chemical functions at the surface of Bacillus subtilis,
cr
J. Colloid Interface Sci. 309 (2007) 49-55.
us
[30] L.Y. Chen, Z.D. Zhang and W.Z. Wang, Self-assembled porous 3D flowerlike β-In2S3 structures: synthesis, characterization, and optical properties, J. Phys. Chem.
an
C 112 (2008) 4117-4123.
[31] X. Fu, X. Wang, Z. Chen, Z. Zhang, Z. Li, D. Y. C. Leung, L. Wu and X. Fu,
M
Photocatalytic performance of tetragonal and cubic β-In2S3 for the water splitting under visible light irradiation, Appl. Catal. B 95 (2010) 393-399.
te
d
[32] N. Revathi, P. Prathap, Y.P.V. Subbaiah and K.T.R. Reddy, Substrate temperature dependent physical properties of In2S3 films, J. Phys. D: Appl. Phys. 41
Ac ce p
(2008) 155404-155414.
[33] S.-H. Yu, L. Shu, Y.-S. Wu, J. Yang, Y. Xie and Y.T. Qian, Organo thermal synthesis and characterization of nanocrystalline β-Indium sulfide, J. Am. Ceram. Soc. 82 (1999) 457-460.
[34 P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, Y. Sun, Y. Liu, In situ assembly of well-dispersed Au nanoparticles on TiO2/ZnO nanofibers: A three-way synergistic
heterostructure with enhanced photocatalytic activity, J. Hazard. Mater. 237-238 (2012) 331-338. [35] C.S. Turchi and D.F.J. Ollis, Photocatalytic degradation of organic water 24
Page 24 of 37
contaminants: Mechanisms involving hydroxyl radical attack, J. Catal., 122 (1990) 178–192. [36] M.S. Lee, S.S. Park, G.-D. Lee, C.-S. Ju and S.-S. Hong, Synthesis of TiO2
ip t
particles by reverse microemulsion method using nonionic surfactants with different
cr
hydrophilic and hydrophobic group and their photocatalytic activity, Catal. Today 101
us
(2005) 283-290.
[37] Q.J. Xiang, J.G. Yu and M. Jaroniec, Synergetic Effect of MoS2 and Graphene as
an
Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578.
M
[38] S.C. Han, L.F. Hu, Z.Q. Liang, S. Wageh, A.A. Al-Ghamdi, Y.S. Chen, X.S. Fang, One-Step Hydrothermal Synthesis of 2D Hexagonal Nanoplates of α-Fe2O3/Graphene
te
d
Composites with Enhanced Photocatalytic Activity, Adv. Funct. Mater. (2014) DOI: 10.1002/adfm.201401279.
Ac ce p
[39] S.C. Han, L.F. Hu, N. Gao, A.A. Al-Ghamdi, X.S. Fang, Efficient Self-Assembly Synthesis of Uniform CdS Spherical Nanoparticles-Au Nanoparticles Hybrids with Enhanced Photoactivity, Adv. Funct. Mater. 24 (2014) 3725–3733. [40] D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, D. Hu, C. Wang, L.V. Saraf, J. Zhang, I.A. Aksay, J. Liu, Self-assembled TiO2-graphene hybrid nanostructures for enhanced Li-ion insertion, ACS Nano 3 (2009) 907–914. [41] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653–2659. [42] Semiconductor Electrodes, ed. H. O. Finklea, Elsevier, New York, 1988. 25
Page 25 of 37
[43] P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo and Y. Liu, In situ assembly of well-dispersed Ag nanoparticles (AgNPs) on electrospun carbon nanofibers (CNFs) for catalytic reduction of 4-nitrophenol, Nanoscale, 3 (2011)
ip t
3357–3363.
cr
[44] O. Bratus, A. Evtukh, E. Kaganovich, A. Kizjak, I. Kizjak, E. Manoilov, Charge
us
storage characteristics of gold nanoparticles embedded in alumina matri,
an
Semiconductor Physics, Quantum Electronics & Optoelectronics, 12 (2009) 53-56.
M
Figure Captions:
Fig. 1 SEM images of the CNFs (a); CNFs/Au (b); In2S3/CNFs/Au composites (c and
(insets).
te
d
d)；In2S3/CNFs (e) and pure In2S3 (f) at low magnification and high magnification
Ac ce p
Fig. 2 TEM (a) and HRTEM (b) images of In2S3/CNFs/Au composites. Fig. 3 XRD patterns of pure CNFs (a), CNFs/Au (b), In2S3/CNFs/Au composites (c)
and pure In2S3 (d).
Fig. 4 (a) XPS spectra of C 1s for the sample In2S3/CNFs/Au composites and the pure CNFs; (b-d) XPS spectrum of In 3d, S 2p and Au 4f for the sample In2S3/CNFs/Au composites. Fig. 5 (a) Degradation profiles of RB in the presence of the different photocatalysts
but in the dark and with visible light irradiation but in the absence of the photocatalyst; (b) Degradation curve of RB over different photocatalysts under visible light; (c) 26
Page 26 of 37
Kinetic linear simulation curves of RB degradation over the different photocatalysts (1-3) under visible light irradiation: (1) In2S3/CNFs/Au composites; (2) In2S3/CNFs; (3) pure In2S3; (d) Photographs of RB solutions following sedimentation for 20 min
ip t
with the photocatalysts of In2S3/CNFs/Au and pure In2S3 after visible light irradiation
cr
of 60 min.
us
Fig. 6 (a) Transient photocurrent response of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au electrodes under the irradiation of visible light (λ > 420 nm) [Na2SO4
an
= 0.5 M]; (b) PL emission spectra of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites.
M
Fig. 7 EIS Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites in 0.5 M Na2SO4 aqueous solution.
te
d
Scheme 1. Postulated mechanism of the visible-light-induced photodegradation of RB with In2S3/CNFs/Au composites. On the right-hand side, the reported work functions
Ac ce p
of selected materials are given.
Fig. 8 (a) Degradation curves of RB over sample In2S3/CNFs/Au catalyst for reusing
3 times; (b) XRD patterns and SEM images of the In2S3/CNFs/Au catalyst before and
after the photocatalytic reaction.
27
Page 27 of 37
ip t cr us an M d te Ac ce p
Fig. 1 SEM images of the CNFs (a); CNFs/Au (b); In2S3/CNFs/Au composites (c
and d)；In2S3/CNFs (e) and pure In2S3 (f) at low magnification and high magnification (insets).
29
Page 28 of 37
ip t cr us an M d te Ac ce p
Fig. 2 TEM (a) and HRTEM (b) images of In2S3/CNFs/Au composites.
30
Page 29 of 37
ip t cr us an M d te Ac ce p
Fig. 3 XRD diffraction patterns of pure CNFs (a), CNFs/Au (b), In2S3/CNFs/Au
composites (c) and pure In2S3 (d).
31
Page 30 of 37
ip t cr us an M d te Ac ce p
Fig. 4 (a) XPS spectra of C 1s for the sample In2S3/CNFs/Au composites and the
pure CNFs; (b-d) XPS spectrum of In 3d, S 2p and Au 4f for the sample In2S3/CNFs/Au composites.
32
Page 31 of 37
ip t cr us an M d te
Fig. 5 (a) Degradation profiles of RB in the presence of the different
Ac ce p
photocatalysts but in the dark and with visible light irradiation but in the absence of the photocatalyst; (b) Degradation curve of RB over different photocatalysts under visible light; (c) Kinetic linear simulation curves of RB degradation over the different photocatalysts (1-3) under visible light irradiation: (1) In2S3/CNFs/Au composites; (2)
In2S3/CNFs; (3) pure In2S3; (d) Photographs of RB solutions following sedimentation for 20 min with the photocatalysts of In2S3/CNFs/Au and pure In2S3 after visible light irradiation of 60 min.
33
Page 32 of 37
ip t cr us an M d te Ac ce p
Fig. 6 (a) Transient photocurrent response of the pure In2S3, In2S3/CNFs and
In2S3/CNFs/Au electrodes under the irradiation of visible light (λ > 420 nm) [Na2SO4= 0.5 M]; (b) PL emission spectra of the pure In2S3, In2S3/CNFs and
In2S3/CNFs/Au composites.
34
Page 33 of 37
ip t cr us an M d te Ac ce p
Fig. 7 EIS Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites in 0.5 M
Na2SO4 aqueous solution.
35
Page 34 of 37
ip t cr us an M d te Ac ce p
Scheme 1. Postulated mechanism of the visible-light-induced photodegradation of RB
with In2S3/CNFs/Au composites.
36
Page 35 of 37
ip t cr us an M d te Ac ce p
Fig. 8 (a) Degradation curves of RB over sample In2S3/CNFs/Au catalyst for
reusing 3 times; (b) XRD patterns and SEM images of the In2S3/CNFs/Au catalyst before and after the photocatalytic reaction.
37
Page 36 of 37
te
d
M
an
us
cr
ip t
Graphical abstract
Ac ce p
We describe a route to synthesize In2S3/CNFs/Au ternary synergetic system with high efficiency visible light photocatalytic activity.
38
Page 37 of 37
S0304-3894(14)00818-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.10.005 HAZMAT 16318
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
5-7-2014 29-9-2014 3-10-2014
Please cite this article as: X. Zhang, C. Shao, X. Li, N. Lu, K. Wang, F. Miao, Y. Liu, In2 S3 /carbon nanofibers/Au ternary synergetic system: hierarchical assembly and enhanced visible-light photocatalytic activity, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.10.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
In2S3/carbon nanofibers/Au ternary synergetic system: hierarchical assembly and enhanced visible-light photocatalytic activity Xin Zhang, Changlu Shao,* Xinghua Li,* Na Lu, Kexin Wang, Fujun Miao and
cr
ip t
Yichun Liu
Ac ce p
te
d
M
an
us
Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
*Corresponding author: Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China Email: [email protected]; [email protected]; Tel. 8643185098803. 1
Page 1 of 37
Synthesis of In2S3/CNFs/Au ternary synergetic system.
cr
Enhanced visible-light photocatalytic activity.
ip t
Highlights:
an
us
Easy photocatalyst separation and reuse.
Abstract
M
In this paper, carbon nanofibers (CNFs) were successfully synthesized by electrospinning technique. Next, Au nanoparticles (NPs) were assembled on the
te
d
electrospun CNFs through in situ reduction method. By using the obtained Au NPs modified CNFs (CNFs/Au) as hard template, the In2S3/CNFs/Au composites were
Ac ce p
synthesized through hydrothermal technique. The results showed that the super long one-dimensional (1D) CNFs (about 306 nm in average diameter) were well connected to form a nanofibrous network; and, the Au NPs with 18 nm in average diameter and In2S3 nanosheets with 5-10 nm in thickness were uniformly grown onto the surface of CNFs. Photocatalytic studies revealed that the In2S3/CNFs/Au composites exhibited
highest visible-light photocatalytic activities for the degradation of Rhodamine B (RB) compared with pure In2S3 and In2S3/CNFs. The enhanced photocatalytic activity might arise from the high separation efficiency of photogenerated electron-hole pairs based on the positive synergetic effect between In2S3, CNFs and Au components in 2
Page 2 of 37
this ternary photocatalytic system. Meanwhile, the In2S3/CNFs/Au composites with hierarchical structure possess a strong adsorption ability towards organic dyes, which also contributed to the enhancement of photocatalytic activity. Moreover, the
ip t
In2S3/CNFs/Au composites could be recycled easily by sedimentation due to their
us
cr
nanofibrous network structure.
an
Keywords: Electrospinning; CNFs; In2S3; Au NPs; RB; photocatalysis; Recyclability. 1. Introduction
M
In recent years, photocatalytic technology has been extensively used for the treatment of polluted water [1-4]. Among various photocatalysts, TiO2 has been
te
d
extensively studied due to its exceptional optical and electronic properties, non-toxicity, chemical stability, and low cost [5, 6]. However, TiO2 can solely absorb
Ac ce p
the UV light due to its wide band gap of 3.2 eV. Since the UV region occupies only near 4% of the entire solar spectrum, while 45% of the energy belongs to visible light, most solar energy cannot be used [7-9]. Consequently, the development of efficient photocatalysts with visible light response has become a research focus [10]. Indium sulfide (In2S3), a typical III–VI group chalcogenide with a band gap of
2.0–2.2 eV, has attracted intense interest as a potential visible light photocatalyst due to its high photosensitivity and photoconductivity, stable chemical composition and physical characteristics at ambient conditions, and low toxicity [11-13]. However, similar to other narrow band gap visible light photocatalysts, poor quantum yield 3
Page 3 of 37
caused by the rapid recombination of photogenerated electron-hole pairs is still a challenge to enhance the photocatalytic efficiency of In2S3 to meet the practical application requirements. Therefore, several attempts have been made to reduce the
ip t
recombination of photogenerated electron–hole pairs by combining In2S3 with
cr
electron acceptor such as the matchable band structure semiconductors, noble metals
us
and carbon materials [14-18]. Among them, carbon materials attract great interest due to their superior electronic, physicochemical, mechanical character and high
an
absorption properties. In the carbon family, carbon nanofibers with one-dimensional (1D) structure have gradually become a rising star, due to their excellent electrical
M
conductivity [19, 20]. Especially, the electrospun carbon nanofibers (CNFs) with super long 1D nanostructure have been proved to possess enhanced electronic
te
d
transport properties compared with traditional carbon nanofibers obtained by other physical and chemical methods [21-23]. Moreover, their unique three-dimensional
Ac ce p
(3D) interconnected structure obtained from electrospinning technique also contributes to the charge transfer [24]. Therefore, one could expect that coupling In2S3 with electrospun CNFs to construct In2S3/CNFs would inhibit electron–hole pairs
recombination resulting in enhanced photocatalytic activity. On the other hand, many reports have shown that incorporating noble metal (e.g.
Au, Ag, Pt, and Pd) nanoparticles (NPs) into composite photocatalysts could further improve their photocatalytic activity through further extending electron–hole lifetime with the noble metal NPs acting as irreversible electron sinks. For instance, Chai et al. reported
that
the In2S3/(Pt-TiO2)
nanocomposites
exhibited
higher
visible
4
Page 4 of 37
photoactivity compared with In2S3/TiO2 [25]. Roy et al. reported that the photocatalytic efficiency of graphene-ZnO-Au nanocomposites was higher than graphene-ZnO [26]. These pioneering works promote us to envision that the
ip t
photocatalytic efficiency of In2S3/CNFs may be further improved by incorporating the
cr
noble metal NPs (just like Au NPs) to construct In2S3/CNFs/Au composites.
us
In our work, the CNFs were fabricated by electrospinning technique. Next, the Au NPs were assembled on the electrospun CNFs via in situ reduction method. Finally,
an
by using the obtained Au NPs modified CNFs (CNFs/Au) as hard template, In2S3/CNFs/Au composites were fabricated through a hydrothermal technique.
M
Utilizing the positive synergetic effect between the In2S3, CNFs and Au components in the obtained In2S3/CNFs/Au composites, the photogenerated electrons on the
te
d
conduction bands (CB) of In2S3 could easily transfer to CNFs, and then irreversibly captured by the Au NPs, realizing a more effective separation of photogenerated
Ac ce p
electron-hole pairs to get a much better performance in the photocatalytic processes compared with the pure In2S3 and In2S3/CNFs. Also, by utilizing their free-standing
nanofibrous network structure, the In2S3/CNFs/Au composites could be easily
recycled by sedimentation in the practical application.
2. Experimental section 2.1. Materials. Polyacrylonitrile (PAN) (Mw = 150 000), N,N-dimethylformamide (DMF) and tetrachloroauric acid (HAuCl4) were purchased from Tianjin Chemicals (Tianjin, 5
Page 5 of 37
China). HCl and SnCl2 were purchased from Beijing Chemicals (Beijing, China). In(NO3)3·5H2O and L-cysteine were purchased from sinopharm Chemicals (shanghai,
ip t
China). All materials were directly used as received without further purification. 2.2. Preparation of the CNFs
cr
In our experiments, the preparation process consisted of two steps. First, 1.5 g of
us
PAN powders were dissolved in 10 mL of DMF solution. After stirring at room temperature for 12 h, the above precursor solutions were drawn into a hypodermic
an
syringe for electrospinning. The positive voltage applied to the tip was 10 kV and the distance between the needle tip and the collector was 15 cm. The as-spun PAN
M
nanofibers were collected on aluminum foil, which was attached on the edge of the collecting wheel. For carbonization, the substrates were placed in a tube furnace and
d
stabilized in air for 60 min at 270 °C, then they were carbonized in nitrogen at 1000
te
°C at a ramp rate of 5 °C min-1, and finally cooled to room temperature. Thus,
Ac ce p
electrospun CNFs were obtained. 2.3. Preparation of CNFs/Au
In a typical experiment, 0.02 g of SnCl2 was dissolved in 20 mL of 5 mM HCl
solution. Then, 0.03 g of as-electrospun CNFs was added to the above solution and stirred for 6 h at room temperature. The precipitate was recovered by centrifugation, followed by washing with distilled water five times. After that, the activated CNFs were obtained. Then, the activated CNFs were added to 20 mL of 0.5 mM tetrachloroauric acid (HAuCl4) solution under vigorous stirring for about 5 min at room temperature. The obtained composite was washed with deionized water and 6
Page 6 of 37
ethanol to remove any ionic residual, then dried at 60 °C for 12 h. Thus, the expected CNFs/Au was obtained after those processes. 2.4. Preparation of the In2S3/CNFs/Au composites
ip t
In a typical procedure, 0.6 mmol In(NO3)3·5H2O and 1.8 mmol L-cysteine were
cr
dissolved in 20mL distilled water under magnetic stirring [9, 25]. After 10 min, the
us
obtained solution and 15 mg CNFs/Au was mixed together, followed by stirring for 10 min. The mixed solution was transferred into a 25 mL Teflon-lined stainless
an
autoclave, sealed and maintained at 180 °C for 24 h, and then cooled down to room temperature. The as-fabricated products were collected and washed several times with
M
ethanol and deionized water, respectively, then dried at 60 °C for 12 h. For contrast, the In2S3/CNFs was fabricated by the same method, while the pure In2S3 was obtained
2.5. Characterization
te
d
in the absence of CNFs during the preparation process.
Ac ce p
The morphologies of the as-prepared nanofibers were observed by scanning
electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100). Energy dispersive X-ray (EDX) spectroscope coupled to a scanning electron microscope (SEM) was used to analyze the composition of samples. X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku, D/max-2500 X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was performed on a VG-ESCALAB LKII instrument with an Mg K ADES (hν = 1253.6eV) source at a
residual gas pressure of below 10-8 Pa. The UV-Vis diffuse reflectance spectra of the 7
Page 7 of 37
samples were recorded on a Cary 500 UV-Vis-NIR spectrophotometer. 2.6. Photocatalytic tests The photocatalytic activity of the samples was evaluated from the degradation of
ip t
Rhodamine B (RB). 0.04 g photocatalyst was suspended in 100 mL RB solution with
cr
the initial concentration of 10 ppm. The solution was stirred in the dark for 30 min to
us
obtain a good dispersion and to reach the adsorption–desorption equilibrium between the organic molecules and the catalysts surface. A 150 W xenon lamp with a cutoff
an
filter (λ > 420 nm) was used as the visible light source for visible-light photocatalysis. At given intervals of illumination, 3 mL reacting solutions in series were taken out
M
and analyzed. The concentrations of RB in the reacting solutions were analyzed by a Cary 500 UV-Vis-NIR spectrophotometer at λ = 554 nm.
te
d
2.7. Photoelectrochemical experiment
Photoelectrochemical measurements were performed using the conventional three
Ac ce p
electrodes setup connected to an electrochemical station (CHI600D Chenhua Shanghai, China). This assembly had pure In2S3/FTO, In2S3/CNFs/FTO and
In2S3/CNFs/Au/FTO (effective area was 1 cm2) as working electrodes, and a Pt wire and an Ag/AgCl (saturated KCl) electrode were used as the counter electrode and reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution.
A 150 W xenon lamp with a cutoff filter (λ > 420 nm) was used as the visible light source. The photocurrent response spectra were carried out at a constant potential of +0.6 V to the working photoanode. Electrochemical impedance spectra (EIS) were measured at open-circuit voltage. A sinusoidal ac perturbation of 5 mV was applied to 8
Page 8 of 37
the electrode over the frequency range of 100 mHz to 2 MHz.
ip t
3. Results and discussion 3.1. SEM of the samples
cr
The morphology of the samples was analyzed by SEM. The SEM image of
us
electrospun CNFs is shown in Fig. 1a, which shows that the average diameter (according to the size distribution showed in Fig. S1a) and length of the electrospun
an
CNFs are about 306 nm and several millimeters, respectively. The obtained CNFs with high aspect ratio and smooth surface are well connected to form a conductive 3D
M
free-standing network, which is favorable for electrical charge transport. The SEM image of CNFs/Au (Fig. 1b) shows that there is no essential change to the
te
d
morphology of CNFs during the growth of Au NPs. From the magnified image of CNFs/Au (inset of Fig. 1b), it is clearly that the Au NPs with an average diameter of
Ac ce p
18 nm (according to the size distribution showed in Fig. S1b) well dispersed on the surface of CNFs without aggregation. And, after hydrothermal treatment, the surface of CNFs/Au is uniformly covered by the ultrathin In2S3 nanosheets (Fig.1c). No
aggregation could be found in the SEM images of In2S3/CNFs/Au composites. The high-magnification SEM image (Fig. 1d) reveals that the In2S3 nanosheets with a
thickness of 5-10 nm are interconnected with each other, forming the 3D nanosheets networks. Moreover, the In2S3/CNFs composites are synthesized for contrast. The corresponding SEM image is shown in Fig. 1e, which shows that the morphology of In2S3/CNFs is almost no difference with In2S3/CNFs/Au composites. The formation of 9
Page 9 of 37
the uniform hierarchical structures may originate from the special characteristics of the electrospun CNFs, i.e. the 3D open structure and large surface areas, which could provide abundant active sites for the assembly of Au NPs and In2S3 nanosheets.
ip t
However, the pure In2S3 obtained by the same hydrothermal method without the
cr
presence of CNFs/Au, self-assemble to form 3D In2S3 microspheres (Fig. 1d). And,
photocatalytic activity.
an
3.2. TEM of the In2S3/CNFs/Au composites
us
the In2S3 microspheres aggregate with each other, which is detrimental to their
For further investigating the In2S3/CNFs/Au composites, TEM and HRTEM
M
measurements were performed. The typical TEM image of an individual hierarchical composite nanofiber is shown in Fig. 2a. It can be observed clearly that the Au NPs
te
d
(about 18 nm) are evenly dispersed on the surface of CNFs, which coincides with the results of the SEM observations. This indicates that Au NPs are not detached from the
Ac ce p
surface of CNFs during the hydrothermal process. In addition, the In2S3 nanosheets
had a narrow size distribution (the distance from the surface of CNFs to the outer edge of the In2S3 nanosheets is around 100-150 nm) uniformly covered on the surface of CNFs/Au. The HRTEM image (Fig. 2b) of the In2S3 nanosheets display two types
of clear lattice fringes ca. 0.62 nm and ca. 0.32 nm, which correspond to the (111) and (311) planes of the cubic In2S3, respectively. The TEM images of the In2S3/CNFs/Au
composites reveal that highly dense secondary In2S3 nanosheets are successfully grown on the CNFs/Au templates, coinciding with the results from SEM observations. 3.3. XRD patterns 10
Page 10 of 37
The X-ray diffraction (XRD) patterns of pure CNFs, Au-CNFs, In2S3/Au-CNFs and pure In2S3 are shown in Fig. 3. The broad peaks centered at around 25ºand 44º are attributed to the (002) and (100) plane of the carbon structure in CNFs (Fig. 3a), while
ip t
the four peaks at 38.51º, 43.91º, 64.71º, and 78.61º are attributed to (111), (200),
cr
(220), and (311) planes of face-centered-cubic (fcc) Au (JCPDS 65-2870) in Fig. 3b.
us
For the pure In2S3, the diffraction peaks at 2 values of 14.3º, 27.5º, 28.7º, 33.2º, 43.7º, 47.8º, 56.6º and 59.4º can be ascribed to the (111), (311), (222), (400), (511)
an
(440), (622) and (444) planes in the face-centered-cubic (fcc) -In2S3 (JCPDS 65-0459), which is in agreement with the literatures [18, 27]. As shown in Fig.1c, all
M
the peaks can be indexed while no diffraction peak for any other crystal phase is detected, indicating that there is no crystal phase impurity existing in the obtained
te
d
sample In2S3/CNFs/Au composites.
3.4. XPS spectra of the In2S3/CNFs /Au
Ac ce p
To determine the chemical composition and valence states of various elements in
the In2S3/CNFs/Au composites, XPS measurements were carried out. The high resolution XPS spectra for the C 1s region, around 285 eV, are shown in Fig. 4a. The binding energy peak at 284.6 eV can be attributed to C–C bonds and is identified as originating from the amorphous carbon phase or from adventitious carbon; the peak at 285.6 eV is characteristic of the combination of C–O groups [28]. Moreover, the peak of the carboxyl carbon (O–C=O) is located at 289 eV [29]. Interestingly, the peak of the carboxyl carbon (O–C=O) in our work is found to shift obviously to a lower binding energy compared with pure CNFs, suggesting that the carboxyl acts as a 11
Page 11 of 37
nucleation site for Au NPs and In2S3 nanosheets on CNFs. Fig. 4b shows the XPS spectrum of In 3d with two symmetrical peaks at the binding energies of 444.4 eV for In 3d5/2 and 452.0 eV for In 3d3/2 [30]. For the spectrum of S 2p in Fig. 4c, the peaks
ip t
at 160.7 and 162.1 eV can be attributed to S 2p3/2 and S 2p1/2 transitions, respectively
cr
[31]. The spin–orbit splitting of In 3d and S 2p are found to be 7.6 and 1.4 eV,
us
indicating that the valence state of In and S are +3 and -2 respectively in the In2S3/CNFs/Au composites [31-33]. Fig. 4d shows the Au 4f region of the XPS
an
spectrum of the In2S3/CNFs/Au composites. The Au 4f7/2 signal can be resolved into two components, which are centered at 83.5 and 87.2 eV, respectively. These are
M
assigned to the characteristic doublets of Au0 loaded on CNFs, suggesting that only elemental Au is formed on the CNFs surface [34]. The above XPS results confirm that
te
d
the composites are composed of CNFs, In2S3 and Au. 3.5. Photocatalytic activity
Ac ce p
The photocatalytic activity of the samples was evaluated by the decomposition of
RB under visible light irradiation. Temporal changes in the concentration of RB, as monitored by the maximal absorption of RB at 554 nm in the UV–Vis spectra over the as-prepared photocatalysts, are shown in Fig. 5. The degradation efficiency of all the samples is defined as C/C0, where C is the concentration of RB at the irradiation time t and C0 is the concentration of RB at the irradiation time t = 0, respectively. Before studying and comparing the activities of the samples, the control experiments were performed under different conditions: (1) in the presence of the photocatalysts but in the dark; (2) with visible light irradiation but in the absence of the photocatalysts. As 12
Page 12 of 37
shown in Fig. 5a, the adsorption–desorption equilibrium of RB in the dark is established within 30 min. And, there is no appreciable degradation of RB after 3 h in the absence of the photocatalysts. Interestingly, the In2S3/CNFs/Au composites show
ip t
strong adsorption ability towards the RB (Fig. 5a and Fig. S2). This can be partly
cr
attributed to the large specific surface area of the 2D In2S3 nanosheets and the high
us
absorption property of CNFs in the In2S3/CNFs/Au composites. After the adsorption process, the photocatalytic degradation activities of the different samples were
an
investigated. Fig. 5b shows the degradation curves of RB on pure CNFs, CNFs/Au, pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites. It can be seen that pure CNFs
M
and CNFs/Au have no photocatalytic activity under visible light irradiation. After visible light irradiation for 60 min, 95.5% of RB is decomposed by using the
te
d
In2S3/CNFs/Au composites as the photocatalyst, while approximately 85.5 and 75.2% degradation of RB are observed for the pure In2S3 and In2S3/CNFs samples,
Ac ce p
respectively. As expected, In2S3/CNFs/Au composites exhibited much higher photocatalytic efficiency compared with the pure In2S3 and In2S3/CNFs. For a better
comparison of the photocatalytic efficiency of In2S3/CNFs/Au composites,
In2S3/CNFs and pure In2S3, the kinetic analysis of degradation of RB was discussed. The kinetic linear simulation curves of the photocatalytic degradation of RB over the above catalysts shows that the above degradation reactions followed a Langmuir -Hinshelwood apparent first-order kinetics model due to the low initial concentrations of the reactants. The explanation is described below: [35]
13
Page 13 of 37
where r is the degradation rate of the reactant (mg/(L min)), C is the concentration of
ip t
the reactant (mg/L), t is the visible light irradiation time, k is the reaction rate constant (mg/(L min)), and K is the adsorption coefficient of the reactant (L/mg).
cr
When the initial concentration (C0) is very low (C0 < 10 mg/L) for RB in the present
an
us
experiment), eq.1 can be simplified to an apparent first-order model: [36]
where kαpp is the apparent first-order rate constant (min-1). The determined kαpp values
M
for degradation of RB with different catalysts are summarized in the Fig. 5c. The apparent first-order rate constants of In2S3/CNFs/Au composites, In2S3/CNFs and pure
te
d
In2S3 for the degradation of RB are 0.0529, 0.0322 and 0.0248 min−1, respectively. The photocatalytic reactivity order is well-consistent with the activity studies above.
Ac ce p
Moreover, the photograph in Fig. 5d shows that the In2S3/CNFs/Au composites can
be easily separated from the solution by sedimentation due to their nanofibrous network structure while the pure In2S3 could only be separated by centrifugation. The above experiment results indicate that the In2S3/CNFs/Au composites display an
efficient photocatalytic activity for the degradation of RB under visible light irradiation and can be easily separated for reuse. 3.6. Postulated photocatalytic mechanism Photoelectrochemical measurements are often used to qualitatively study the excitation and transfer of photogenerated charge carriers in photocatalysts. The 14
Page 14 of 37
transient photocurrent responses of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites are recorded for three on-off cycles under visible light irradiation and plotted in Fig. 6a. As expected, In2S3/CNFs/Au composites show the highest
ip t
photocurrent intensity of the three samples, whereas In2S3/CNFs and pure In2S3 have
cr
the middle and lowest values, respectively. Thus, the sample In2S3/CNFs/Au
us
composites may achieve the highest quantum yield based on the effective charge separation, which is consistent with the photocatalytic activity measurements and our
an
above discussions [37]. Furthermore, the better separation of photogenerated electrons-holes pairs in the In2S3/CNFs/Au composites is confirmed by PL emission
M
spectra. In Fig. 6b, the In2S3/CNFs/Au composites exhibit much lower emission intensity than In2S3/CNFs and pure In2S3. The quenching of PL emission spectra of
te
d
the In2S3/CNFs/Au can attributed to efficient electron transfer between In2S3, CNFs and Au NPs, which indicate that the recombination of the photogenerated charge
Ac ce p
carriers is inhibited greatly in the In2S3/CNFs/Au composites [38, 39]. The efficient charge separation could increase the lifetime of the charge carriers and enhance the efficiency of the interfacial charge transfer to adsorbed substrates, and then account for the higher activity of In2S3/CNFs/Au photocatalyst. To further understand the role of Au NPs in the In2S3/CNFs/Au composites, EIS
Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites without and with visible-light irradiation were carried out. As shown in Fig. 7, the Nyquist impedance plots of In2S3/CNFs and In2S3/CNFs/Au composites electrode materials cycled in 0.5 M Na2SO4 electrolyte solution both exhibit semi cycles at high frequencies. 15
Page 15 of 37
Considering that the preparation of the electrodes and electrolyte used are identical, the high frequency semicircle is relevant to the resistance of the electrodes [40]. In electrochemical spectra, the high frequency are corresponds to the charge transfer
ip t
limiting process and can be attributed to the charge transfer resistance at the contact
cr
interface between the electrode and the electrolyte solution [18, 41]. The smaller are
us
radius of the EIS Nyquist plot corresponds to the lower electric charge transfer resistance. Clearly, the introduction of Au NPs leads to a significantly decreased
an
diameter of the semicircular Nyquist plot as compared to In2S3/CNFs, in the cases of both with and without light irradiation, suggesting a faster charge transfer rate. This
M
result implies that the introduction of Au NPs can obviously favor the separation and
te
the photocatalytic activity.
d
transfer of photogenerated carriers in In2S3/CNFs/Au composites and then enhance
On the basis of the above analysis, a proposed mechanism is discussed to explain
Ac ce p
the enhancement of the photocatalytic properties of the In2S3/CNFs/Au composites
(Scheme 1). Under visible light illumination, In2S3 with narrow band gap energy can be easily excited and generates a large number of free electrons in the conduction band (CB) and holes in the valence band (VB). In this ternary synergetic system, the CB energy level of In2S3 is ca. 3.57 eV (CB potential = -0.93 V vs. NHE; E (eV) = E(V vs. NHE) + 4.5 (V)) [42], whereas the work function of CNFs and Au NPs are ca.4.6 eV and 5.1 eV, respectively [43, 44]. Such energy levels suggest that the electrons in the CB of In2S3 may move freely toward the surface of the CNFs, and might further transfer to Au NPs due to the work function of CNFs is higher than CB 16
Page 16 of 37
energy level of In2S3 but lower than work function of Au NPs. Meanwhile, if In2S3 nanosheets contact with Au NPs directly, the electrons in the CB of In2S3 might also be entrapped by Au NPs due to its high Schottky barriers at the metal–semiconductor
ip t
interface, which reduces the recombination with holes. Those electron transfer routes
cr
could enhance the separation of photoinduced holes and electrons, thus improve the
us
photocatalytic properties. We note that the decoration of Au NPs on the surface of CNFs obviously enhanced the charge separation efficiency compared to that of
an
In2S3/CNFs composites, as evidenced by the photocurrent and PL experiments, suggesting that Au/CNFs have better photoelectrons collection ability than pure CNFs.
M
Furthermore, The photogenerated holes left in VB transfer to the surface of In2S3 to oxidize water into O2(VB), which have been confirmed in our previous study on the
d
One-dimensional In2S3/TiO2 hierarchical heterostructures [9]. Then, the O2(VB) as well
te
as the dissolved O2 in solution capture the photogenerated electrons to yield O2− and
Ac ce p
subsequently produce •OH, which is a strong oxidizing agent to decompose the organic dyes. The mechanism for the photocatalytic degradation of RB in our experiment is proposed as follows (3)-(11): In2S3 + hν → In2S3(e– + h+)
(3)
In2S3(e–) + CNFs/Au → CNFs(e–) + Au(e–)
(4)
CNFs(e–) + Au → Au(e–)
(5)
In2S3(h+) + 2H2O → In2S3 + O2(VB) + 4H+ Au(e–) + O2(VB
+ dissolved)
→ Au + O2–
O2– + H2O → H2O• + OH–
(6) (7) (8)
17
Page 17 of 37
H2O• + H2O → H2O2 + •OH
(9)
H2O2 → 2•OH
(10) (11)
ip t
•OH + RB → CO2 + H2O 3.7. Stability of In2S3/CNFs/Au Photocatalyst
cr
The stability of a photocatalyst is very important for industrial application. To test
us
the stability of RB photodegradation on In2S3/CNFs/Au composites, we reused the catalyst for three times. As shown in Fig. 8a, each experiment is carried out under
an
identical conditions; after a three cycle experiment, the photocatalytic activity of the In2S3/CNFs/Au composites remain almost unchanged. Fig. 8b shows the XRD
M
patterns and SEM images of the In2S3/CNFs/Au composites before and after the three times photocatalytic reaction. After three photocatalytic runs, the position and the
te
d
ratio of peaks are nearly the same as that of fresh photocatalyst. And the secondary In2S3 nanostructures are still very complete, clearly indicating the stability of the
Ac ce p
In2S3/CNFs/Au photocatalyst.
4. Conclusions
In summary, we described herein an effective route to synthesize the
In2S3/CNFs/Au
ternary
synergetic
system.
The
as-prepared
In2S3/CNFs/Au
composites exhibited enhanced photocatalytic activity in the decomposition of RB. In the ternary synergetic system, CNFs/Au serving as electron collectors can more efficiently suppress charge recombination, improve interfacial charge transfer, and provide a greater number of photocatalytic reaction centers, resulting in a high 18
Page 18 of 37
quantum yield to get a much better performance in the photocatalytic processes. Moreover, the In2S3/CNFs/Au composites could be recycled easily by sedimentation due to their free-standing nanofibrous network structure. This study provides a new
ip t
strategy to construct the semiconductor/CNFs/Au composites with high photocatalytic
us
cr
activity and good recyclable performance.
Acknowledgements
an
The present work is supported financially by the National Basic Research Program of China (973 Program) (Grant No.2012CB933703), the National Natural Science
M
Foundation of China (No. 91233204, 51272041, and 61201107), the 111 Project (No.B13013), the Fundamental Research Funds for the Central Universities
Ac ce p
Province (20121802).
te
d
(14ZZ1511 and 12SSXM001), and the Program for Young Scientists Team of Jilin
19
Page 19 of 37
ip t cr us an M
References
[1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor
te
d
electrode, Nature 238 (1972) 37–38.
[2] M. Batzill, Fundamental aspects of surface engineering of transition metal oxide
Ac ce p
photocatalysts, Energy Environ. Sci. 4 (2011) 3275–3286. [3] X. Zhang, C. Shao, Z. Zhang, J. Li, P. Zhang, M. Zhang, J. Mu, Z. Guo, P. Liang, Y. Liu, In situ generation of well-dispersed ZnO quantum dots on electrospun silica nanotubes with high photocatalytic activity, ACS Appl. Mater. Interfaces 4 (2012) 785–790.
[4] S. Park, S. Kim, H.J. Kim, C.W. Lee, H. J. Song, S.W. Seo, H.K. Park, D.W. Kim, K.S. Hong, Hierarchical assembly of TiO2–SrTiO3 heterostructures on conductive SnO2 backbone nanobelts for enhanced photoelectrochemical and photocatalytic performance, J. Hazard. Mater. 275 (2014) 10–18. 20
Page 20 of 37
[5] S.U. Khan, M. Al-Shahry, W.B. Ingler Jr., Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243–2245. [6] M. Miljevic, B. Geiseler, T. Bergfeldt, P. Bockstaller, L. Fruk, Enhanced
ip t
Photocatalytic Activity of Au/TiO2 Nanocomposite Prepared Using Bifunctional
cr
Bridging Linker, Adv. Funct. Mater. 24 (2014) 907–915.
us
[7] A.L. Linsebigler, G. Lu and J.T. Yates, Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results, Chem. Rev. 95 (1995) 735-758.
an
[8] M.A. Fox and M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341-357.
M
[9] X. Zhang, X. Li, C. Shao, J. Li, M. Zhang, P. Zhang, K.Wang, N. Lu, Y. Liu, One-dimensional hierarchical heterostructures of In2S3 nanosheets on electrospun
(2013) 892-900.
te
d
TiO2 nanofibers with enhanced visible photocatalytic activity, J. Hazard. Mater. 260
Ac ce p
[10] Y.X. Li, G. Chen, Q. Wang, X. Wang, A.K. Zhou, Z.Y. Shen, Hierarchical ZnS-In2S3-CuS Nanospheres with Nanoporous Structure: Facile Synthesis, Growth Mechanism, and Excellent Photocatalytic Activity, Adv. Funct. Mater. 20 (2010) 3390–3398.
[11] W.T. Kim and C.D. Kim, Optical energy gaps of β-In2S3 thin films grown by
spray pyrolysis, J. Appl. Phys. 60 (1986) 2631-2633. [12] W. Qiu, M. Xu, X. Yang, F. Chen, Y. Nan, J. Zhang, H. Iwai and H. Chen, Biomolecule-assisted hydrothermal synthesis of In2S3 porous films and enhanced photocatalytic properties, J. Mater. Chem. 21 (2011) 13327-13333. 21
Page 21 of 37
[13] Y. Xing, H. Zhang, S. Song, J. Feng, Y. Lei, L. Zhao and M. Li, Hydrothermal synthesis and photoluminescent properties of stacked indium sulfide superstructures, Chem. Commun. (2008) 1476-1478.
ip t
[14] C. Gao, J. Li, Z. Shan, F. Huang and H. Shen, Preparation and visible-light
cr
photocatalytic activity of In2S3/TiO2 composite, Mater. Chem. Phys. 122 (2010) 183-187.
us
[15] V. Štengl, F. Opluštil and T. Němec, In3+-doped TiO2 and TiO2/In2S3
an
nanocomposite for photocatalytic and stoichiometric degradations photochem. photobiol. 88 (2012) 265-276.
M
[16] X. An, J. C. Yu, F. Wang, C. Li and Y. Li, One-pot synthesis of In2S3 nanosheets/graphene composites with enhanced visible-light photocatalytic activity,
te
d
Appl. Catal. B 129 (2013) 80-88.
[17] M. Yang, B, Weng and Y. Xu, Synthesis of In2S3–CNT nanocomposites for
Ac ce p
selective reduction under visible light, J. Mater. Chem. A 2 (2014) 1710-1720. [18] M.Q. Yang, B. Weng and Y.J. Xu, Improving the Visible Light Photoactivity of In2S3−Graphene Nanocomposite via a Simple Surface Charge Modification Approach Langmuir 29 (2013) 10549-10558. [19] L. Chen, X. Zhang, H. Liang, M. Kong, Q. Guan, P. Chen, Z. Wu and S. Yu, Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors, ACS Nano 6 (2012) 7092–7102. [20] R.L. McCreery, Advanced Carbon Electrode Materials for Molecular Electrochemistry, Chem. Rev. 108 (2008) 2646–2687. 22
Page 22 of 37
[21] M. Zhang, C. Shao, J. Mu, X. Huang, Z. Zhang, P. Zhang and Y. Liu, Hierarchical heterostructures of Bi2MoO6on carbon nanofibers: controllable solvothermal fabrication and enhanced visible photocatalytic properties, J. Mater.
ip t
Chem. 22 (2012) 577–584.
cr
[22] P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, C. Su, N. Lu, K. Wang and Y.
us
Liu, An electron-rich free-standing carbon@Au core–shell nanofiber network as a highly active and recyclable catalyst for the reduction of 4-nitrophenol, Phys. Chem.
an
Chem. Phys. 15 (2013) 10453-10458.
[23] L. Peng, L.F. Hu and X.S. Fang, Energy Harvesting for Nanostructured
M
Self-Powered Photodetectors, Adv. Funct. Mater. 24 (2014) 2591–2610. [24] W. Li, L. Zeng, Z. Yang, L. Gu, J. Wang, X. Liu, J. Cheng and Y. Yu,
te
d
Free-standing and binder-free sodium-ion electrodes with ultralong cycle life and high rate performance based on porous carbon nanofibers, Nanoscale 6 (2014) 693-698.
Ac ce p
[25] B. Chai, T. Peng, P. Zeng, J. Mao, Synthesis of floriated In2S3 decorated with
TiO2 nanoparticles for efficient photocatalytic hydrogen production under visible light, J. Mater. Chem. 21 (2011) 14587–14593. [26] P. Roy, A. Periasamy, C. Liang and H. Chang, Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene, Environ. Sci. Technol. 47 (2013) 6688−6695. [27] B. Chai, P. Zeng, X. Zhang, J. Mao, L. Zan and T. Peng, Walnut-like In2S3 microspheres: ionic liquid-assisted solvothermal synthesis, characterization and formation mechanism, Nanoscale 4 (2012) 2372–2377. 23
Page 23 of 37
[28] Y.H. Liu and X.L. Jing, Pyrolysis and structure of hyperbranched polyborate modified phenolic resins, Carbon 45 (2007) 1965-1971. [29] A. Francois, J. P. B. Christophe, A. Yasmine, J. Philippe, T. Philippe, P. Michel
ip t
and G. R. Paul, XPS analysis of chemical functions at the surface of Bacillus subtilis,
cr
J. Colloid Interface Sci. 309 (2007) 49-55.
us
[30] L.Y. Chen, Z.D. Zhang and W.Z. Wang, Self-assembled porous 3D flowerlike β-In2S3 structures: synthesis, characterization, and optical properties, J. Phys. Chem.
an
C 112 (2008) 4117-4123.
[31] X. Fu, X. Wang, Z. Chen, Z. Zhang, Z. Li, D. Y. C. Leung, L. Wu and X. Fu,
M
Photocatalytic performance of tetragonal and cubic β-In2S3 for the water splitting under visible light irradiation, Appl. Catal. B 95 (2010) 393-399.
te
d
[32] N. Revathi, P. Prathap, Y.P.V. Subbaiah and K.T.R. Reddy, Substrate temperature dependent physical properties of In2S3 films, J. Phys. D: Appl. Phys. 41
Ac ce p
(2008) 155404-155414.
[33] S.-H. Yu, L. Shu, Y.-S. Wu, J. Yang, Y. Xie and Y.T. Qian, Organo thermal synthesis and characterization of nanocrystalline β-Indium sulfide, J. Am. Ceram. Soc. 82 (1999) 457-460.
[34 P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, Y. Sun, Y. Liu, In situ assembly of well-dispersed Au nanoparticles on TiO2/ZnO nanofibers: A three-way synergistic
heterostructure with enhanced photocatalytic activity, J. Hazard. Mater. 237-238 (2012) 331-338. [35] C.S. Turchi and D.F.J. Ollis, Photocatalytic degradation of organic water 24
Page 24 of 37
contaminants: Mechanisms involving hydroxyl radical attack, J. Catal., 122 (1990) 178–192. [36] M.S. Lee, S.S. Park, G.-D. Lee, C.-S. Ju and S.-S. Hong, Synthesis of TiO2
ip t
particles by reverse microemulsion method using nonionic surfactants with different
cr
hydrophilic and hydrophobic group and their photocatalytic activity, Catal. Today 101
us
(2005) 283-290.
[37] Q.J. Xiang, J.G. Yu and M. Jaroniec, Synergetic Effect of MoS2 and Graphene as
an
Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578.
M
[38] S.C. Han, L.F. Hu, Z.Q. Liang, S. Wageh, A.A. Al-Ghamdi, Y.S. Chen, X.S. Fang, One-Step Hydrothermal Synthesis of 2D Hexagonal Nanoplates of α-Fe2O3/Graphene
te
d
Composites with Enhanced Photocatalytic Activity, Adv. Funct. Mater. (2014) DOI: 10.1002/adfm.201401279.
Ac ce p
[39] S.C. Han, L.F. Hu, N. Gao, A.A. Al-Ghamdi, X.S. Fang, Efficient Self-Assembly Synthesis of Uniform CdS Spherical Nanoparticles-Au Nanoparticles Hybrids with Enhanced Photoactivity, Adv. Funct. Mater. 24 (2014) 3725–3733. [40] D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, D. Hu, C. Wang, L.V. Saraf, J. Zhang, I.A. Aksay, J. Liu, Self-assembled TiO2-graphene hybrid nanostructures for enhanced Li-ion insertion, ACS Nano 3 (2009) 907–914. [41] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653–2659. [42] Semiconductor Electrodes, ed. H. O. Finklea, Elsevier, New York, 1988. 25
Page 25 of 37
[43] P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo and Y. Liu, In situ assembly of well-dispersed Ag nanoparticles (AgNPs) on electrospun carbon nanofibers (CNFs) for catalytic reduction of 4-nitrophenol, Nanoscale, 3 (2011)
ip t
3357–3363.
cr
[44] O. Bratus, A. Evtukh, E. Kaganovich, A. Kizjak, I. Kizjak, E. Manoilov, Charge
us
storage characteristics of gold nanoparticles embedded in alumina matri,
an
Semiconductor Physics, Quantum Electronics & Optoelectronics, 12 (2009) 53-56.
M
Figure Captions:
Fig. 1 SEM images of the CNFs (a); CNFs/Au (b); In2S3/CNFs/Au composites (c and
(insets).
te
d
d)；In2S3/CNFs (e) and pure In2S3 (f) at low magnification and high magnification
Ac ce p
Fig. 2 TEM (a) and HRTEM (b) images of In2S3/CNFs/Au composites. Fig. 3 XRD patterns of pure CNFs (a), CNFs/Au (b), In2S3/CNFs/Au composites (c)
and pure In2S3 (d).
Fig. 4 (a) XPS spectra of C 1s for the sample In2S3/CNFs/Au composites and the pure CNFs; (b-d) XPS spectrum of In 3d, S 2p and Au 4f for the sample In2S3/CNFs/Au composites. Fig. 5 (a) Degradation profiles of RB in the presence of the different photocatalysts
but in the dark and with visible light irradiation but in the absence of the photocatalyst; (b) Degradation curve of RB over different photocatalysts under visible light; (c) 26
Page 26 of 37
Kinetic linear simulation curves of RB degradation over the different photocatalysts (1-3) under visible light irradiation: (1) In2S3/CNFs/Au composites; (2) In2S3/CNFs; (3) pure In2S3; (d) Photographs of RB solutions following sedimentation for 20 min
ip t
with the photocatalysts of In2S3/CNFs/Au and pure In2S3 after visible light irradiation
cr
of 60 min.
us
Fig. 6 (a) Transient photocurrent response of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au electrodes under the irradiation of visible light (λ > 420 nm) [Na2SO4
an
= 0.5 M]; (b) PL emission spectra of the pure In2S3, In2S3/CNFs and In2S3/CNFs/Au composites.
M
Fig. 7 EIS Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites in 0.5 M Na2SO4 aqueous solution.
te
d
Scheme 1. Postulated mechanism of the visible-light-induced photodegradation of RB with In2S3/CNFs/Au composites. On the right-hand side, the reported work functions
Ac ce p
of selected materials are given.
Fig. 8 (a) Degradation curves of RB over sample In2S3/CNFs/Au catalyst for reusing
3 times; (b) XRD patterns and SEM images of the In2S3/CNFs/Au catalyst before and
after the photocatalytic reaction.
27
Page 27 of 37
ip t cr us an M d te Ac ce p
Fig. 1 SEM images of the CNFs (a); CNFs/Au (b); In2S3/CNFs/Au composites (c
and d)；In2S3/CNFs (e) and pure In2S3 (f) at low magnification and high magnification (insets).
29
Page 28 of 37
ip t cr us an M d te Ac ce p
Fig. 2 TEM (a) and HRTEM (b) images of In2S3/CNFs/Au composites.
30
Page 29 of 37
ip t cr us an M d te Ac ce p
Fig. 3 XRD diffraction patterns of pure CNFs (a), CNFs/Au (b), In2S3/CNFs/Au
composites (c) and pure In2S3 (d).
31
Page 30 of 37
ip t cr us an M d te Ac ce p
Fig. 4 (a) XPS spectra of C 1s for the sample In2S3/CNFs/Au composites and the
pure CNFs; (b-d) XPS spectrum of In 3d, S 2p and Au 4f for the sample In2S3/CNFs/Au composites.
32
Page 31 of 37
ip t cr us an M d te
Fig. 5 (a) Degradation profiles of RB in the presence of the different
Ac ce p
photocatalysts but in the dark and with visible light irradiation but in the absence of the photocatalyst; (b) Degradation curve of RB over different photocatalysts under visible light; (c) Kinetic linear simulation curves of RB degradation over the different photocatalysts (1-3) under visible light irradiation: (1) In2S3/CNFs/Au composites; (2)
In2S3/CNFs; (3) pure In2S3; (d) Photographs of RB solutions following sedimentation for 20 min with the photocatalysts of In2S3/CNFs/Au and pure In2S3 after visible light irradiation of 60 min.
33
Page 32 of 37
ip t cr us an M d te Ac ce p
Fig. 6 (a) Transient photocurrent response of the pure In2S3, In2S3/CNFs and
In2S3/CNFs/Au electrodes under the irradiation of visible light (λ > 420 nm) [Na2SO4= 0.5 M]; (b) PL emission spectra of the pure In2S3, In2S3/CNFs and
In2S3/CNFs/Au composites.
34
Page 33 of 37
ip t cr us an M d te Ac ce p
Fig. 7 EIS Nyquist plots of In2S3/CNFs and In2S3/CNFs/Au composites in 0.5 M
Na2SO4 aqueous solution.
35
Page 34 of 37
ip t cr us an M d te Ac ce p
Scheme 1. Postulated mechanism of the visible-light-induced photodegradation of RB
with In2S3/CNFs/Au composites.
36
Page 35 of 37
ip t cr us an M d te Ac ce p
Fig. 8 (a) Degradation curves of RB over sample In2S3/CNFs/Au catalyst for
reusing 3 times; (b) XRD patterns and SEM images of the In2S3/CNFs/Au catalyst before and after the photocatalytic reaction.
37
Page 36 of 37
te
d
M
an
us
cr
ip t
Graphical abstract
Ac ce p
We describe a route to synthesize In2S3/CNFs/Au ternary synergetic system with high efficiency visible light photocatalytic activity.
38
Page 37 of 37
