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Fe2O3 NTs with dual synergistic effects for photoelectrocatalytic reduction CO2 into methanol
Journal of Colloid and Interface Science 486 (2017) 232–240
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Regular Article
One dimensional SnO2 NRs/Fe2O3 NTs with dual synergistic effects for photoelectrocatalytic reduction CO2 into methanol Zhongxue Yang a, Huying Wang a, Weijie Song a, Wei Wei a, Qingping Mu b, Bo Kong c, Peiqiang Li a,⇑, Hongzong Yin a,⇑ a b c
Department of Chemistry, Shandong Agricultural University, 61 Daizong Road, 271018 Tai’an, China Chambroad Chemical Industry Research Institute of the Yellow River Delta, China Department of Chemical and Biological Engineering, Iowa State University, Ames, United States
g r a p h i c a l a b s t r a c t The as prepared SnO2 NRs/Fe2O3 NTs with hexagon snowflake surface exhibits excellent photoelectrocatalytic ability for CO2. It achieves the largest methanol yield 2.05 mmol L1 cm2 and faradaic current efficiency 87.04%.
Faradic Current Efficiency (%)
100
-1.1V, η =87.04%
80
60
40
20
0
-0.9
-1.0
-1.1
-1.2
-1.3
Potential (V)
a r t i c l e
i n f o
Article history: Received 22 June 2016 Revised 19 September 2016 Accepted 23 September 2016 Available online 24 September 2016 Keywords: SnO2 NRs/Fe2O3 NTs PEC reduction Dual synergistic effects CO2 Methanol
a b s t r a c t The hydrothermal method was explored to prepare SnO2 nanorods (SnO2 NRs) with the special faces of (1 1 0) and (1 0 1) on the surface of Fe2O3 nanotubes (Fe2O3 NTs). According to the SEM and XRD results, the formation process of the hierarchically assembled SnO2 NRs was deduced. The SnO2 NRs/Fe2O3 NTs catalyst that had reached for 120 mins behaved the best photoelectrocatalytic properties. From the view of photocatalytic reduction, the conduction band (0.75 eV vs NHE) is negative enough to drive CO2 reduction, and the valence band (1.82 eV) is positive enough to oxidize H2O to generate proton, and then the proton is used for CO2 reduction. From the electrocatalytic reduction point, the net CO2 reduction current density of the composite is 7.48 times that of Fe2O3 NTs at 1.1 V, indicating that the electrocatalytic performance of Fe2O3 NTs is greatly enhanced by the introduction of 6-fold branched SnO2 NRs. The predominant reduction product is analyzed by GC was methanol. Herein, two synergistic effects are proved according to the methanol yields, one is the synergistic effect of the photocatalytic and electrocatalytic
⇑ Corresponding authors at: Institute of Chemistry and Material Science, Shandong Agricultural University, No. 61 Daizong Road, Tai’an, Shandong 271018, PR China. E-mail address: [email protected] (P. Li). http://dx.doi.org/10.1016/j.jcis.2016.09.055 0021-9797/Ó 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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reduction, the other is the synergistic effect between SnO2 NRs and Fe2O3 NTs. The results indicated that the composite catalyst behaves excellent photoelectrocatalytic activity for CO2 reduction. Ó 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Carbon dioxide (CO2) concentration has been growing since the fossil fuel combustion and respiration [1,2]. CO2 is widely recognized as not only one of the main greenhouse gas, but also a potential carbon resource. As long as CO2 capture is energy demanding and encompasses an additional cost to be forced by carbon tax incentives [3,4], the effective utilization of CO2 is of great significance to respond to the global environmental protection which has aroused much concern [5]. To date, significant efforts have been devoted to exploring various strategies and catalysts for the conversion of CO2 [6–11]. Photocatalytic (PC) reduction use a semiconducting photocathode with the assist of renewable solar energy and water to convert the waste CO2 into carbon-based fuels (e.g. CH3OH) [12–15], it is one of the most prominent and economical methods to reduce atmospheric CO2 and solve the energy crisis [16,17]. Numerous semiconductors such as TiO2 and GaP suffered from the shortage of visible light absorption for the large band gaps or the photo-corrosion process [18–23]. Hematite (a-Fe2O3) is undoubtedly one excellent visible light photocatalyst from the point of the effective utilization of solar energy. It can absorb the light of wavelength up to 600 nm due to its narrow band gap (2.20 eV), collect up to 40% energy of the solar spectrum [24–27]. Among all sorts of Fe2O3 nanomaterials, one-dimensional (1D) Fe2O3 nanotubes (Fe2O3 NTs) with peculiar and fascinating physicochemical properties, low processing cost, large surface areas, high active sites and efficient charge transferring capacity, can reduce the electron-hole pairs recombination during their participation in redox processes [13,27–29]. However, it is less stable in aqueous solution. Hematite can dissolve at 25 °C with the reaction of Fe2O3 + 3H2O = 2Fe3+ + 6OH [30]. Then the free aquatic Fe3+ ion has a powerful tendency to hydrolyze to form Fe(OH)3y (y 6 4) [31]. y On the other hand, tin oxide (SnO2) behaves better electrocatalytic (EC) activity among so many catalyst species [32–37], especially when it possesses some special crystal surfaces, though it cannot be excited by visible light for its wide gap (3.80 eV) [38]. As a kind of electrocatalyst, SnO2 not only can endow excellent EC performance to photocatalyst (Fe2O3 NTs), but can improve the stability of Fe2O3 NTs in aqueous solution as a protective layer. Furthermore, SnO2 will not hinder the light absorption of the photocatalyst for its high optical transparency [39]. At the same time, the visible photocatalysis can eliminate the toxic intermediate which is easily poison the catalyst surface in the electrocatalytic process [40,41], resulting in the improvement of the EC efficiency. Herein, the strategy of coupling SnO2 to Fe2O3 NTs permits the improvement of photoelectrocatalytic (PEC) activities during the process of CO2 reduction. Thus, a superior photoelectrocatalyst combining both merits of Fe2O3 and SnO2 might be expected. In this study, Fe2O3 NTs were successfully prepared on the Fe foils by the electrochemical anodization method and modified by SnO2 NRs through a surfactant-free hydrothermal method. SnO2 in the as-prepared SnO2 NRs/Fe2O3 NTs catalyst had special crystal surfaces of (1 1 0) and (1 0 1). The composite catalyst was utilized for PEC reduction of CO2 with water under visible light irradiation. We evaluate systematically the PEC reduction of CO2 querying the synergistic effect of PC and EC reduction, as well as the synergistic catalytic effect of the sole catalyst. The study of PEC reduction of CO2 not only paved a new path for exploring novel catalysts with
synergistic effects but also provided the experimental and theoretical basis for further research. 2. Experimental section 2.1. Preparation of catalysts The iron foils (20 mm 40 mm 1.5 mm) were firstly polished by sand papers and degreased by boiling in 10 wt.% oxalate and sonicated in ethanol. The anodization experiments were performed at 30 V for 2 h (20 °C). The ferrous foil was the working electrode, and titanium foil was the counter electrode. The electrolytes were consisted of 0.25 wt.% NH4F in a nonaqueous solution (Volume of water:glycol = 3:97). After sonicating for 5 min, the samples were placed in a muffle furnace (KSL-1100X) under oxygen atmosphere with the flow rate of 60 sccm (standard cubic centimeters min1), then heated to 500 °C with the rate of 3 °C min1 and maintained at 500 °C for 2 h. Lastly, they were cooled to room temperature with the rate of 3 °C min1. The SnO2 NRs was prepared through a surfactant-free hydrothermal method. In a typical experiment, the aqueous solution premade by dissolving 0.203 g SnCl45H2O and 0.623 g NaOH into 35 mL of deionized water was ultrasonically cleaned for 10 min, and then the in-suit growth Fe2O3 NTs precursors (20 mm 40 mm 1.5 mm) was immersed in it. They were transferred into a 50 mL Teflon-lined stainless autoclave and heated at 220 °C for various durations at the oven. The resulting SnO2 NRs/ Fe2O3 NTs was dried at 60 °C for one hour in air. 2.2. Characterization of catalysts The surface morphologies of the as-prepared samples were characterized by scanning electron microscopy (SEM, Philips XL30 FEG) with an accelerated voltage of 20 kV. The crystalline structures were characterized by X-ray diffraction (XRD, Rigaku D/MAX-rA, Japan) using a diffractometer with Cu Ka radiation, k = 1.54184 Å in the range of 2h = 20°–70°, the scan rate of 4° min1. Surface compositions were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromated X-ray source (Al Ka ht = 1486.6 eV). The UV–visible diffused reflectance spectrum (UV–vis DRS) was measured for photochemical properties using IS19-1 in combination with a single reflection internal accessory (Beijing Purkinje General Instrument Co., Ltd). XPS and UV–vis DRS measurements took SnO2 NRs/Fe2O3 NTs reacting for 120 min as a test object. The electrochemical properties were measured by CHI660D potentiostat (Shanghai Chen Hua Instrument Co., LTD). Properties of PEC CO2 reduction was measured by CHI660D potentiostat in 0.1 mol L1 KHCO3. The as-prepared electrode, Hg/ Hg2Cl2 in saturated KCl solution and platinum wire acted as the working electrode, the reference electrode, and the counter electrode, respectively. The potential was swept linearly at a scan rate of 50 mV s1. N2 was plunged into KHCO3 at the rate of 40 sccm for 20 min before the experiment. After that, CO2 was plunged for 20 min until it was saturated, then repeated the experiment. The samples were illuminated under visible light by a Xenon lamp with a band-pass filter (k P 420 nm, 100 mW cm2). The Faradic current efficiency (g) was calculated as followed:
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Fig. 1. SEM images of different samples. (a) pre-synthesized Fe2O3 NTs, inset showed magnified images of the Fe2O3 NTs. (b) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 60 min. (c) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 90 min. (d) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 120 min.
Faradaic efficiency ¼
moles product number of electrons needed for conversion moles of electrons passed ð1Þ
At the end of the electrolysis, the products were immediately analyzed by gas chromatography (GC-9A, Shimadzu). The GC was equipped with glass packed column (2 m, inner diameter 3 mm, Parapok Q, 80–100) and flame ionization detector. The column was kept at 80 °C and the detector 200 °C. High purity N2 worked as a carrier gas with a flow rate of 25 sccm.
3. Results and discussion Fig. 1a shows that the as-prepared Fe2O3 NTs like volcanoes with uncertain sizes (The diameter from 357 nm to 1 lm), the volcanos has an uncertain number of nanotubes (approximate from 9 to 30). The nanotube structure is with a pore diameter of 25– 30 nm, nanowall thickness of 8–10 nm, and length of 1.7–2.3 lm, which is calculated from the inset view in Fig. 1a and the crosssectional view (Fig. S1). Fe2O3 NTs grow uniformly in arrays. The thin size enhances the surface area and active sites significantly [42]. Fig. 1b–d shows the SEM images of the samples of SnO2 NRs/Fe2O3 NTs at various reaction durations. For the sample having reacted for 60 min, SnO2 exhibited an irregular bar morphology (Fig. 1b) which was generally of ca. 500 nm in length and ca. 130 nm in diameter. When the reaction proceeded to 90 min, many SnO2 NRs were cross growth in arrays (Fig. 1c). After reacting for 120 min, the SnO2 NRs were further aggregated as shown in Fig. 1d, just like the hexagon snowflake. The XRD peak pattern of the annealed Fe2O3 NTs was shown in Fig. S2. The peaks in Fig. 2a could be well indexed to the rhombohedral symmetry of Fe2O3 with lattice constants of a = 5.0491 Å,
c = 13.6577 Å (JCPDS card No. 33-0664), which indicated the crystalline hematite phase [43]. Also, the XRD peak patterns of the as-prepared samples obtained at various reaction durations were presented in Fig. 2. The strong and sharp diffraction peaks indicated good crystallizations [44]. For the product reacting for 60 min, there was almost the Fe2O3 phase and only a little SnO2 phase, namely the crystal face of (1 1 0) matching with JCPDS card No. 41-1445 [45]. When the reaction was prolonged to 90 min, and finally 120 min, the tetragonal rutile crystalline phase of SnO2 with lattice parameters [46] of a = 4.7528 Å, c = 3.1639 Å increased continuously from less to more. At the same time, the intensity of Fe2O3 phases of (2 0 6), (1 1 0), (1 1 3), (2 0 2) weakened for a growing number of SnO2 had covered on the surface of Fe2O3 NTs. The increased crystal plane of SnO2 will be a favor to the improvement of the electrocatalysis [47]. On the basis of the SEM and XRD results stated in Fig. 2a and b, the formation process of the hierarchically assembled SnO2 NRs on the surface of Fe2O3 NTs in hydrothermal process was deduced, as illustrated schematically in Fig. 3. The SnO2 grew to irregular nanorods, and then they grow up into nanorod crossed arrays on the 4-fold symmetrical structure, taking the (1 1 0) and (1 0 1) planes as the preferential growth direction. As the reaction further proceeded, the adjacent SnO2 NRs arrays tended to gather together to form the 6-fold symmetrically branched state. To gain more information about the surface composition, XPS analysis was performed on the SnO2 NRs/Fe2O3 NTs catalyst. The wide spectrum showed the only emissions of C, Sn, O and Fe elements (Fig. S3). The peaks of Fe 2p emission (Fig. 4a) that centered at 711.6 and 724.8 eV were attributed to the Fe 2p3/2 and Fe 2p1/2, respectively, which confirmed the existence of Fe3+ [48]. The binding energies (Fig. 4b) at 486.3 and 494.7 eV for Sn 3d were assigned to the Sn 3d5/2 and Sn 3d3/2, indicating that the chemical state of
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(a) 110
JCPDS:41-1445 SnO2 101
Intensity (a.u.)
101
211
Intensity (a.u.)
120 min
90 min
(b)
JCPDS:41-1445 SnO2
110
211
SnO2 /Fe2O3 NTs
SnO2 /Fe
60min
206
JCPDS:33-0664 Fe 2O3
110
113202
JCPDS:33-0664 Fe2O3
110
214 012
122
202 113
206 30
40
50
60
70
30
40
214 122 50
012 60
70
2θ (degree)
2θ (degree)
(c) SnO2/Fe2O3 NTs
Intensity (a.u.)
before -1.1V PEC
after -1.1V PEC
20
30
40
50
60
70
2θ (degree) Fig. 2. XRD patterns of (a) SnO2 NRs/Fe2O3 NTs at various reaction durations, (b) SnO2 NRs at different carrier and (c) SnO2 NRs/Fe2O3 NTs contrasting before and after the PEC reduction of CO2. The standard SnO2 and Fe2O3 patterns are placed at the top and bottom, respectively.
Fig. 3. Schematic illustration of the formation process of hierarchically assembled SnO2 NRs based on the surface of Fe2O3 NTs: from irregular NRs to 6-fold symmetrical structure with their idealized states.
the element Sn was Sn4+ [46]. The results testified that the catalyst was exactly composed of Fe2O3 and SnO2, and it was consistent with the XRD results. The net current densities of CO2 reduction experiments were carried out to evaluate the electrochemical properties of different catalysts. The net current density was obtained from the different values between the current density with CO2 and the current density with N2. In comparison to Fe2O3 NTs catalyst shown in Fig. 5a, the net current density of CO2 reduction obtained from SnO2 NRs/ Fe2O3 NTs composite catalyst was much higher, indicating that the
composite behaved excellent EC activity of CO2 reduction. Especially for the catalyst (120 min), the net current density (1.57 mA cm2) was 7.48 times that of Fe2O3 NTs at 1.1 V (Under this potential, the current efficiency reached peak, Fig. 7b), it indicated that the EC performance of Fe2O3 NTs was greatly enhanced by the introduction of 6-fold branched SnO2 NRs. To verify the outstanding electrochemical performance of the composite, the EIS experiment was performed (Fig. 5b). The semicircle in the medium-frequency region was assigned to the charge transfer resistance [49]. When light irradiation was applied on the
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(a)
Sn 3d 3/2
Intensity (a.u.)
Intensity (a.u.)
Sn3d 5/2
(b)
Fe2 p3/2
Fe 2 p1/2 711.6 eV
494.7 eV 724.8 eV
730
486.3 eV
725
720
715
710
498
495
492
Binding Energy (eV)
489
486
483
Binding Energy (eV)
Fig. 4. The high resolution XPS spectra of Fe 2p (a) and Sn 3d (b) of the SnO2 NRs/Fe2O3 NTs.
160
(a)
(b)
Fe2O3 NTs SnO2 NRs/Fe 2O3 NTs-60min
4
120
SnO2 NRs/Fe 2O3 NTs -90min
Fe 2O3 NTs dark
SnO2 NRs/Fe 2O3 NTs -120min
-Z" (ohm)
The Net Current Density (mA cm -2)
5
3
-1.1 V
2
1.57 mA cm
Fe 2O3 NTs light SnO2 NRs/Fe 2O3 NTs dark
80
SnO2 NRs/Fe 2O3 NTs light
-2
40 1
0.21 mA cm 0
-1.0
-2
-1.2
-1.4
Potential (V vs.SCE)
0
0
100
200
300
Z' (ohm)
Fig. 5. (a) The net current density of CO2 reduction of different catalysts. (b) The impedance plots of different catalysts in dark or light.
Fe2O3 NTs, it can be seen from Fig. 5b that the impedance of the Fe2O3 NTs reduces from 310 X to 175 X. It illustrated that the conductivity of the Fe2O3 NTs was enhanced after irradiating, it further deduced that the recombination ratio of photogenerated electronhole is decreased. It shows that the Fe2O3 NTs have the peculiar and fascinating photocatalytic properties. However, the electrocatalytic property of Fe2O3 NTs is very poor. After loading SnO2NRs, it shows that the impedance of the SnO2 NRs/Fe2O3 NTs (56 X) is much smaller than that of Fe2O3 NTs. It indicated that the SnO2 NRs can greatly enhance the charge transfer of the material, which is in favor of the improvement of the electrocatalysis. The lower impedance confirms that the high conductivity and greatly enhances electron transport of the composite, resulting in the excellent catalytic performance [50]. The optical properties of SnO2 NRs/Fe2O3 NTs composite, SnO2 NRs/Fe, and Fe2O3 NTs were given by the UV–vis DRS (Fig. 6a). It could be clearly seen that SnO2 NRs which was on the surface of Fe2O3 NTs and Fe improved the light absorption of Fe2O3 NTs and Fe obviously, suggesting that SnO2 was optical transparency [39]
and Fe2O3 NTs has more excellent optical performance than Fe. Following a linear fit of the experimental (aht)2 versus ht, where a, h, and t are the absorption coefficient, Planck constant and light frequency, respectively, a band gap of 2.57 eV of the as-prepared catalyst was obtained. The two peaks that centered at 4.3 eV and 7.1 eV in the valence band photoemission spectrum were assigned to the electron emission from p (nonbonding) and r (bonding) O 2p orbitals, respectively [51] (Fig. 6c). At the same time, the position of the valence band maximum (VBM) was determined to be 1.82 eV from the onset of the valence band emission spectrum by a linear extrapolation. With the estimated band gap of 2.57 eV, the conduction band minimum (CBM) was calculated to be 0.75 eV (vs. NHE). It has been reported that the SnO2 has enough negative potential to catalyze reduction CO2 [52]. It indicated that the most positive VB potential had enough oxidation ability to oxide H2O, supplying protons for CO2 reduction. At the same time, the more negative CB potential had enough reduction ability to reduce CO2. Furthermore, the composite exhibited a much lower impedance
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2.4
(b)
(a)
Fe2O3 NTs (α hν)2 (10 4eV 2/cm 2)
Intensity (a.u.)
2.0
1.6
SnO2/Fe 2O3 NT
10.0k
Eg=2.57eV
5.0k
1.2
SnO2 /Fe 0.8
300
400
500
0.0
600
2.0
2.5
Wavelength (nm)
3.0
3.5
hυ (eV)
7.06 eV
(c)
Intensity (a.u.)
4.27 eV
1.82 eV
10
8
6
4
2
Binding Energy (eV) Fig. 6. (a) UV–vis DRS of different catalysts. (b) Plot analysis of optical band gap of SnO2 NRs/Fe2O3 NTs. (c) Valence band photoemission spectrum of SnO2 NRs/Fe2O3 NTs (solid line) and its Gaussian fit (dashed line). The spectrum was smoothed and baseline corrected.
(35 X) when it was placed under the illumination condition (Fig. 5b), suggesting that the photoexcited charge carriers decreased their charge transfer resistance [28]. Therefore, the composite had excellent photochemical performance. The CO2 reduction experiments were performed in a quartz cell with circulating cooling water. The predominant reaction product analyzed by GC was methanol. The PEC activity of SnO2 NRs/ Fe2O3 NTs was evaluated according to methanol production. All the methanol yields of SnO2 NRs/Fe2O3 NTs that increased with continuous visible light irradiation under different applied voltage (vs. SCE) were shown in Fig. 7a. For 6 h reaction time, the largest methanol yield was 2.05 mmol L1 cm2 (Table 1), which was attained under visible light irradiation and 1.1 V extra voltage. The peak efficiency 87.04% at 1.1 V (Fig. 7b) could clearly prove the largest methanol yield. It was explained as followed: The reactions of CO2 reduction and HER were competitive on the surface of the electrode [53]. When the extra voltage was lower than 1.1 V, the CO2 reduction was the main reaction, and it reached a peak at 1.1 V. When the extra voltage was higher than 1.1 V, the HER took the lead, which weakened the reduction of CO2.
The methanol yields were 2.05, 1.21, 0.69 mmol L1 cm2 under PEC, EC and PC reduction at 6 h (Table 2), respectively. It could be seen that the EC performance was much more excellent than PC performance. But when the illumination was applied to EC reduction, namely PC and EC were used together to reduce CO2, it attained the highest methanol yield (2.05 mmol L1 cm2). Also, the methanol yield under PEC conditions was higher than that the simple addition (1.90 mmol L1 cm2) of PC and EC reduction. Therefore, the synergistic effect of PC and EC reduction was obviously reflected in the product yields. The methanol yields on different catalysts (Fig. 7d and e) were attained under visible light illumination at 1.1 V. When the reaction time was 6 h, the methanol yield of SnO2 NRs/Fe2O3 NTs was 3.7, 3.3, 2.7 and 1.7 times that of Fe2O3 NTs, SnO2 NRs, SnO2 NRs/ Fe and Fe2O3 NTs + SnO2 NRs (simple addition) (Table 3), respectively. Not only the PEC reduction of CO2 was efficiently improved when coupling Fe2O3 NTs and SnO2 NRs, but the couple of Fe2O3 NTs and SnO2 NRs generated the synergistic effect. On the basis of the present work and our previous results, we propose the CO2 PEC reduction mechanism as follows (Fig. 8):
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100
(a) Faradic Current Efficiency (%)
-1
1.5
1.0
0.5
0.0
-1.1V, η=87.04%
(b) -0.9 V PEC -1.0 V PEC -1.1 V PEC -1.2 V PEC -1.3 V PEC
-2
Methanol Concentration (m mol L cm )
2.0
60
120
180
240
300
80
60
40
20
0
360
-0.9
-1.0
-1.1
Time (min)
2.0
Meth anol Concentration (mmol L cm )
(c)
-1.3
(d)
-2
-2
Methanol Concentration (mmol L cm )
2.0
-1.2
Potential (V)
1.5
1.0
0.5
0.0
60
120
SnO 2 NRs
-1
-1
-1.1V PEC -1.1V EC PC -1.1V EC+PC
180
240
300
Fe 2O3 NTs
1.5
SnO2 NRs/Fe 2O3 NTs SnO2 NRs+Fe 2O3 NTs
1.0
0.5
0.0
360
60
120
180
2.0
240
300
360
Time (min)
(e)
-1
-2
Methanol Concentration (mmol L cm )
Time (min)
SnO2 /Fe 1.5
Fe2 O3 NTs SnO2 /Fe2 O3 NTs
1.0
0.5
0.0
0
50
100
150
200
250
300
350
Time (min) Fig. 7. (a) Methanol concentration of SnO2 NRs/Fe2O3 NTs under PEC CO2 reduction with different potentials. (b) Faradic current efficiency of SnO2 NRs/Fe2O3 NTs as a function of applied potential in the process of PEC CO2 reduction. (c) Methanol concentration of SnO2 NRs/Fe2O3 NTs under different conditions of 1.1 V PEC, PC and 1.1 V EC CO2 reduction. (d) Methanol concentration of different catalysts under 1.1 V PEC CO2 reduction. (e) Methanol concentration of different catalysts base under 1.1 V PEC CO2 reduction. All the extra voltage mentioned above refer to SCE.
Z. Yang et al. / Journal of Colloid and Interface Science 486 (2017) 232–240 Table 1 Methanol concentration of the PEC process on the SnO2 NRs/Fe2O3 NTs in the 6th hour. 0.9 0.79
Potential (V) Methanol concentration (mmol L1 cm3)
1.0 0.70
1.1 2.05
1.2 1.40
1.3 0.66
Table 2 Methanol concentration of the different catalytic process on the SnO2 NRs/Fe2O3 NTs in the 6th hour under 1.1 V. Different conditions
PEC
PC
EC
PC + EC
Methanol concentration (mmol L1 cm3)
2.05
0.69
1.21
1.90
Table 3 Methanol concentration of the PEC process on the different catalysts in the 6th hour under 1.1 V. Semiconductor
SnO2 NRs
Fe2O3 NTs
SnO2 NRs + Fe2O3 NTs
SnO2 NRs/ Fe2O3 NTs
Methanol concentration (mmol L1 cm3)
0.61
0.55
1.16
2.05
239
4. Conclusions In summary, the SnO2 NRs/Fe2O3 NTs catalyst was prepared after SnO2 NRs was decorated on the surface of Fe2O3 NTs through a surfactant-free hydrothermal method. The SEM showed that the Fe2O3 NTs was volcanoes like and SnO2 was just like hexagon snowflake. According to the SEM and XRD results, the formation process of the hierarchically assembled SnO2 NRs was deduced. When the reaction time was prolonged from 60 min to 90 min and 120 min, SnO2 changed from irregular nanorods to nanorod crossed arrays on the 4-fold and 6-fold symmetrical structure ordinally. The SnO2 NRs/Fe2O3 NTs catalyst that had reacted for 120 min behaved the best photoelectrocatalytic properties. From the view of photocatalytic reduction, the composite had matched energy band. The conduction band was negative enough to drive CO2 reduction, and the valence band was positive enough to oxidize H2O for H+, which was used for CO2 reduction. From the electrocatalytic reduction point, the net CO2 reduction current density of the composite was 7.48 times that of Fe2O3 NTs at 1.1 V, indicating that the electrocatalytic performance of Fe2O3 NTs was greatly enhanced by the introduction of 6-fold branched SnO2 NRs. The predominant reduction product analyzed by GC was methanol. Two synergistic effects were proved according to the methanol yields, one was the synergistic effect of the photocatalytic reduction and the electrocatalytic reduction, the other was the synergistic effect between SnO2 NRs and Fe2O3 NTs. The largest methanol yield (2.05 mmol L1 cm2), which was attained under visible light irradiation and 1.1 V extra voltage, was far higher than that of solely electrocatalytic or photocatalytic reduction and solely SnO2 NRs or Fe2O3 NTs. The results indicated that the catalyst behaved excellent photoelectrocatalytic activity of CO2 reduction. Herein, the study not paved a new path for exploiting novel photoelectrocatalysts, but provided the experimental and theoretical basis for further research.
Acknowledgements
Fig. 8. The CO2 reduction mechanism on the SnO2 NRs/Fe2O3 NTs catalyst (The catalyst symbolizes by R).
The reduction of CO2 to methanol is six electrons reaction. And, the process of CO2 PEC reduction is concretely deduced. First, the catalyst R is excited from ground state (R) to singlet state (R⁄). Then R⁄ combines with the CO 2 radical (CO2 is reduced to CO2 radical by electron) and receives one proton to form the [R---O@CAOH]. Then the [R---O@CAOH] combines with three hydrogen protons and three electrons on the electrode surface, and gets the product [R---O@CH2], but the formed structure is unstable and vulnerable by electron and hydrogen proton, further to form the [R---OACH3]. Then the [R---OACH3] is vulnerable by electron leading to the break of [R---O] bond, then combines with hydrogen proton, forms CH3OH, and catalyst return to the ground state R attending the next reaction circle. Furthermore, the as-prepared catalyst has good stability after the catalysis. The information can be obtained from Fig. 2c, the SnO2 NRs/Fe2O3 NTs XRD pattern after the PEC reduction of CO2 was compared with the initiative XRD pattern. It can be seen that the diffraction peaks have no change before and after the PEC reduction CO2 for 1 h at 1.1 V. It indicates the metal oxide on the SnO2 NRs/Fe2O3 NTs surface is not reduced to metal. Thus, the SnO2 NRs/Fe2O3 NTs has good performance on the stability.
This research was supported by the National Natural Science Foundation of China (Grant No. 21203114), Planning Project of Science and Technology in Colleges of Shandong Province (Grant No. J14LC16). We are grateful to the foundation supported by Shandong Jingbo Holdings Corporation.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.055.
References [1] Y. Chen, W. Zhou, D. Ding, M. Liu, F. Ciucci, M. Tade, Z. Shao, Advances in cathode materials for solid oxide fuel cells: complex oxides without alkaline earth metal elements, Adv. Energy Mater. (2015) (DOI). [2] J.E. Miller, A.H. McDaniel, M.D. Allendorf, Considerations in the design of materials for solar-driven fuel production using metal-oxide thermochemical cycles, Adv. Energy Mater. 4 (2014). [3] S. Sen, D. Liu, G.T.R. Palmore, Electrochemical reduction of CO2 at copper nanofoams, ACS Catal. 4 (2014) 3091–3095. [4] Y. Liao, S.W. Cao, Y. Yuan, Q. Gu, Z. Zhang, C. Xue, Efficient CO2 capture and photoreduction by amine-functionalized TiO2, Chem. – Eur. J. 20 (2014) 10220–10222. [5] H. Zhai, Y. Ou, E.S. Rubin, Opportunities for decarbonizing existing US coalfired power plants via CO2 capture, utilization and storage, Environ. Sci. Technol. (2015) (DOI). [6] K. Sekizawa, K. Maeda, K. Domen, K. Koike, O. Ishitani, Artificial ZScheme constructed with a supramolecular metal complex and semiconductor for the photocatalytic reduction of CO2, J. Am. Chem. Soc. 135 (2013) 4596–4599.
240
Z. Yang et al. / Journal of Colloid and Interface Science 486 (2017) 232–240
[7] J. Qu, X. Zhang, Y. Wang, C. Xie, Electrochemical reduction of CO2 on RuO2/TiO2 nanotubes composite modified Pt electrode, Electrochim. Acta 50 (2005) 3576–3580. [8] S.C. Yan, S.X. Ouyang, J. Gao, M. Yang, J.Y. Feng, X.X. Fan, L.J. Wan, Z.S. Li, J.H. Ye, Y. Zhou, A room-temperature reactive-template route to mesoporous ZnGa2O4 with improved photocatalytic activity in reduction of CO2, Angew. Chem. 122 (2010) 6544–6548. [9] J.M. Bockris, J. Wass, The photoelectrocatalytic reduction of carbon dioxide, J. Electrochem. Soc. 136 (1989) 2521–2528. [10] K.K. Sakimoto, C. Liu, J. Lim, P. Yang, Salt-induced self-assembly of bacteria on nanowire arrays, Nano Lett. 14 (2014) 5471–5476. [11] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G. Mikami, S. Okamoto, Y. Izumi, Photocatalytic conversion of carbon dioxide into methanol in reverse fuel cells with tungsten oxide and layered double hydroxide photocatalysts for solar fuel generation, Catal. Sci. Technol. 4 (2014) 1644–1651. [12] C. Das Neves Gomes, E. Blondiaux, P. Thuéry, T. Cantat, Metal-free reduction of CO2 with hydroboranes: two efficient pathways at play for the reduction of CO2 to methanol, Chem. – Eur. J. 20 (2014) 7098–7106. [13] Z. Yang, J. Xu, C. Wu, H. Jing, P. Li, H. Yin, New insight into photoelectric converting CO2 to CH3OH on the one-dimensional ribbon CoPc enhanced Fe2O3 NTs, Appl. Catal. B 156 (2014) 249–256. [14] P. Li, J. Xu, H. Jing, C. Wu, H. Peng, J. Lu, H. Yin, Wedged N-doped CuO with more negative conductive band and lower overpotential for high efficiency photoelectric converting CO2 to methanol, Appl. Catal. B 156 (2014) 134–140. [15] S. Sfaelou, M. Antoniadou, V. Dracopoulos, K. Bourikas, D.I. Kondarides, P. Lianos, Quantum dot sensitized titania as visible-light photocatalyst for solar operation of photofuel cells, J. Adv. Oxidat. Technol. 17 (2014) 59–65. [16] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C.P. Kubiak, Photochemical and photoelectrochemical reduction of CO2, Annu. Rev. Phys. Chem. 63 (2012) 541–569. [17] C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G. Centi, A novel photoelectrochemical approach for the chemical recycling of carbon dioxide to fuels, Chem. Eng. Trans. 25 (2011) 683–688. [18] C.D. Windle, E. Pastor, A. Reynal, A.C. Whitwood, Y. Vaynzof, J.R. Durrant, R.N. Perutz, E. Reisner, Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2, Chem. – Eur. J. 21 (2015) 3746–3754. [19] H. Abdullah, D.-H. Kuo, Photocatalytic performance of Ag and CuBiS2 nanoparticle-coated SiO2@TiO2 composite sphere under visible and ultraviolet light irradiation for azo dye degradation with the assistance of numerous nano p–n diodes, J. Phys. Chem. C 119 (2015) 13632–13641. [20] C.X. Kronawitter, M. Lessio, P. Zhao, C. Riplinger, J.A. Boscoboinik, D.E. Starr, P. Sutter, E.A. Carter, B.E. Koel, Observation of surface-bound negatively charged hydride and hydroxide on GaP (1 1 0) in H2O environments, J. Phys. Chem. C (2015) (DOI). [21] P. Li, J. Zhang, H. Wang, H. Jing, J. Xu, X. Sui, H. Hu, H. Yin, The photoelectric catalytic reduction of CO2 to methanol on CdSeTe NSs/TiO2 NTs, Catal. Sci. Technol. 4 (2014) 1070–1077. [22] P. Li, H. Hu, J. Xu, H. Jing, H. Peng, J. Lu, C. Wu, S. Ai, New insights into the photo-enhanced electrocatalytic reduction of carbon dioxide on MoS2-rods/ TiO2 NTs with unmatched energy band, Appl. Catal. B 147 (2014) 912–919. [23] P. Li, X. Sui, J. Xu, H. Jing, C. Wu, H. Peng, J. Lu, H. Yin, Worm-like InP/TiO2 NTs heterojunction with unmatched energy band photo-enhanced electrocatalytic reduction of CO2 to methanol, Chem. Eng. J. 247 (2014) 25–32. [24] E.E. Barton, D.M. Rampulla, A.B. Bocarsly, Selective solar-driven reduction of CO2 to methanol using a catalyzed p-GaP based photoelectrochemical cell, J. Am. Chem. Soc. 130 (2008) 6342–6344. [25] Y.-S. Hu, A. Kleiman-Shwarsctein, A.J. Forman, D. Hazen, J.-N. Park, E.W. McFarland, Pt-doped a-Fe2O3 thin films active for photoelectrochemical water splitting, Chem. Mater. 20 (2008) 3803–3805. [26] I. Cesar, A. Kay, J.A. Gonzalez Martinez, M. Grätzel, Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping, J. Am. Chem. Soc. 128 (2006) 4582–4583. [27] D.K. Zhong, D.R. Gamelin, Photoelectrochemical water oxidation by cobalt catalyst (‘‘CoPi”)/a-Fe2O3 composite photoanodes: oxygen evolution and resolution of a kinetic bottleneck, J. Am. Chem. Soc. 132 (2010) 4202–4207. [28] W. Zhang, X. Li, Q. Zhao, Y. Hou, Y. Shen, G. Chen, Uniform a-Fe2O3 nanotubes fabricated for adsorption and photocatalytic oxidation of naphthalene, Mater. Chem. Phys. 129 (2011) 683–687. [29] P. Li, H. Wang, J. Xu, H. Jing, J. Zhang, H. Han, F. Lu, Reduction of CO2 to low carbon alcohols on CuO FCs/Fe2O3 NTs catalyst with photoelectric dual catalytic interfaces, Nanoscale 5 (2013) 11748–11754.
[30] A. Mao, K. Shin, J.K. Kim, D.H. Wang, G.Y. Han, J.H. Park, Controlled synthesis of vertically aligned hematite on conducting substrate for photoelectrochemical cells: nanorods versus nanotubes, ACS Appl. Mater. Interfaces 3 (2011) 1852– 1858. [31] V. Parker, I. Khodakovskii, Thermodynamic properties of the aqueous ions (2+ and 3+) of iron and the key compounds of iron, J. Phys. Chem. Ref. Data 24 (1995) 1699–1745. [32] A. Stefánsson, Iron (III) hydrolysis and solubility at 25°C, Environ. Sci. Technol. 41 (2007) 6117–6123. [33] E. Guinea, F. Centellas, E. Brillas, P. Cañizares, C. Sáez, M.A. Rodrigo, Electrocatalytic properties of diamond in the oxidation of a persistant pollutant, Appl. Catal. B 89 (2009) 645–650. [34] Y.-H. Cui, X.-Y. Li, G. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968–1976. [35] G. Zhao, X. Cui, M. Liu, P. Li, Y. Zhang, T. Cao, H. Li, Y. Lei, L. Liu, D. Li, Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/Sb-doped SnO2 electrode, Environ. Sci. Technol. 43 (2009) 1480–1486. [36] S.Y. Yang, W. Choi, H. Park, TiO2 nanotube array photoelectrocatalyst and Ni– Sb–SnO2 electrocatalyst bifacial electrodes: a new type of bifunctional hybrid platform for water treatment, ACS Appl. Mater. Interfaces 7 (2015) 1907–1914. [37] R.A. Hameed, R.M. El-Sherif, Ni–P–SnO2/C composite: synthesis, characterization and electrocatalytic activity for methanol oxidation in KOH solution, Int. J. Hydrogen Energy 40 (2015) 10262–10273. [38] H. Xia, H. Zhuang, T. Zhang, D. Xiao, Visible-light-activated nanocomposite photocatalyst of Fe2O3/SnO2, Mater. Lett. 62 (2008) 1126–1128. [39] M. Niu, F. Huang, L. Cui, P. Huang, Y. Yu, Y. Wang, Hydrothermal synthesis, structural characteristics, and enhanced photocatalysis of SnO2/a-Fe2O3 semiconductor nanoheterostructures, ACS Nano 4 (2010) 681–688. [40] J. Pan, J. Li, Z. Yan, B. Zhou, H. Wu, X. Xiong, SnO2@ CdS nanowire-quantum dots heterostructures: tailoring optical properties of SnO2 for enhanced photodetection and photocatalysis, Nanoscale 5 (2013) 3022–3029. [41] N. Karanasios, J. Georgieva, E. Valova, S. Armyanov, G. Litsardakis, S. Sotiropoulos, Photoelectrocatalytic oxidation of organics under visible light illumination: a short review, Curr. Org. Chem. 19 (2015) 512–520. [42] G. Zhao, P. Li, F. Nong, M. Li, J. Gao, D. Li, Construction and high performance of a novel modified boron-doped diamond film electrode endowed with superior electrocatalysis, J. Phys. Chem. C 114 (2010) 5906–5913. [43] S.K. Mohapatra, S.E. John, S. Banerjee, M. Misra, Water photooxidation by smooth and ultrathin a-Fe2O3 nanotube arrays, Chem. Mater. 21 (2009) 3048–3055. [44] Z. Sun, H. Yuan, Z. Liu, B. Han, X. Zhang, A highly efficient chemical sensor material for H2S: a-Fe2O3 nanotubes fabricated using carbon nanotube templates, Adv. Mater. 17 (2005) 2993–2997. [45] Z. Zhang, M.F. Hossain, T. Takahashi, Self-assembled hematite (a-Fe2O3) nanotube arrays for photoelectrocatalytic degradation of azo dye under simulated solar light irradiation, Appl. Catal. B 95 (2010) 423–429. [46] F. Gu, X. Ren, J. Zhang, W. Shao, G. Huang, C. Li, Morphological evolution from SnO2 quantum dots to quantum chains inside channels of mesoporous spheres, Ind. Eng. Chem. Res. 50 (2011) 12542–12547. [47] P. Li, G. Zhao, M. Li, T. Cao, X. Cui, D. Li, Design and high efficient photoelectricsynergistic catalytic oxidation activity of 2D macroporous SNO2/1D TiO2 nanotubes, Appl. Catal. B 111 (2012) 578–585. [48] H. Wang, F. Sun, Y. Zhang, L. Li, H. Chen, Q. Wu, C.Y. Jimmy, Photochemical growth of nanoporous SnO2 at the air–water interface and its high photocatalytic activity, J. Mater. Chem. 20 (2010) 5641–5645. [49] M. Muhler, R. Schlögl, G. Ertl, The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene 2. Surface chemistry of the active phase, J. Catal. 138 (1992) 413–444. [50] G.-Y. Zhang, Y.-Y. Xu, D.-Z. Gao, Y.-Q. Sun, Α-Fe2O3 nanoplates: PEG-600 assisted hydrothermal synthesis and formation mechanism, J. Alloy. Compd. 509 (2011) 885–890. [51] S. Wang, X. Jiang, H. Zheng, H. Wu, S.-J. Kim, C. Feng, Solvothermal synthesis of MoS2/Carbon nanotube composites with improved electrochemical performance for lithium ion batteries, Nanosci. Nanotechnol. Lett. 4 (2012) 378–383. [52] S. Zhang, P. Kang, T.J. Meyer, Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate, J. Am. Chem. Soc. 136 (2014) 1734–1737. [53] T. Nikolay, L. Larina, O. Shevaleevskiy, B.T. Ahn, Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells, Energy Environ. Sci. 4 (2011) 1480–1486.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
One dimensional SnO2 NRs/Fe2O3 NTs with dual synergistic effects for photoelectrocatalytic reduction CO2 into methanol Zhongxue Yang a, Huying Wang a, Weijie Song a, Wei Wei a, Qingping Mu b, Bo Kong c, Peiqiang Li a,⇑, Hongzong Yin a,⇑ a b c
Department of Chemistry, Shandong Agricultural University, 61 Daizong Road, 271018 Tai’an, China Chambroad Chemical Industry Research Institute of the Yellow River Delta, China Department of Chemical and Biological Engineering, Iowa State University, Ames, United States
g r a p h i c a l a b s t r a c t The as prepared SnO2 NRs/Fe2O3 NTs with hexagon snowflake surface exhibits excellent photoelectrocatalytic ability for CO2. It achieves the largest methanol yield 2.05 mmol L1 cm2 and faradaic current efficiency 87.04%.
Faradic Current Efficiency (%)
100
-1.1V, η =87.04%
80
60
40
20
0
-0.9
-1.0
-1.1
-1.2
-1.3
Potential (V)
a r t i c l e
i n f o
Article history: Received 22 June 2016 Revised 19 September 2016 Accepted 23 September 2016 Available online 24 September 2016 Keywords: SnO2 NRs/Fe2O3 NTs PEC reduction Dual synergistic effects CO2 Methanol
a b s t r a c t The hydrothermal method was explored to prepare SnO2 nanorods (SnO2 NRs) with the special faces of (1 1 0) and (1 0 1) on the surface of Fe2O3 nanotubes (Fe2O3 NTs). According to the SEM and XRD results, the formation process of the hierarchically assembled SnO2 NRs was deduced. The SnO2 NRs/Fe2O3 NTs catalyst that had reached for 120 mins behaved the best photoelectrocatalytic properties. From the view of photocatalytic reduction, the conduction band (0.75 eV vs NHE) is negative enough to drive CO2 reduction, and the valence band (1.82 eV) is positive enough to oxidize H2O to generate proton, and then the proton is used for CO2 reduction. From the electrocatalytic reduction point, the net CO2 reduction current density of the composite is 7.48 times that of Fe2O3 NTs at 1.1 V, indicating that the electrocatalytic performance of Fe2O3 NTs is greatly enhanced by the introduction of 6-fold branched SnO2 NRs. The predominant reduction product is analyzed by GC was methanol. Herein, two synergistic effects are proved according to the methanol yields, one is the synergistic effect of the photocatalytic and electrocatalytic
⇑ Corresponding authors at: Institute of Chemistry and Material Science, Shandong Agricultural University, No. 61 Daizong Road, Tai’an, Shandong 271018, PR China. E-mail address: [email protected] (P. Li). http://dx.doi.org/10.1016/j.jcis.2016.09.055 0021-9797/Ó 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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reduction, the other is the synergistic effect between SnO2 NRs and Fe2O3 NTs. The results indicated that the composite catalyst behaves excellent photoelectrocatalytic activity for CO2 reduction. Ó 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Carbon dioxide (CO2) concentration has been growing since the fossil fuel combustion and respiration [1,2]. CO2 is widely recognized as not only one of the main greenhouse gas, but also a potential carbon resource. As long as CO2 capture is energy demanding and encompasses an additional cost to be forced by carbon tax incentives [3,4], the effective utilization of CO2 is of great significance to respond to the global environmental protection which has aroused much concern [5]. To date, significant efforts have been devoted to exploring various strategies and catalysts for the conversion of CO2 [6–11]. Photocatalytic (PC) reduction use a semiconducting photocathode with the assist of renewable solar energy and water to convert the waste CO2 into carbon-based fuels (e.g. CH3OH) [12–15], it is one of the most prominent and economical methods to reduce atmospheric CO2 and solve the energy crisis [16,17]. Numerous semiconductors such as TiO2 and GaP suffered from the shortage of visible light absorption for the large band gaps or the photo-corrosion process [18–23]. Hematite (a-Fe2O3) is undoubtedly one excellent visible light photocatalyst from the point of the effective utilization of solar energy. It can absorb the light of wavelength up to 600 nm due to its narrow band gap (2.20 eV), collect up to 40% energy of the solar spectrum [24–27]. Among all sorts of Fe2O3 nanomaterials, one-dimensional (1D) Fe2O3 nanotubes (Fe2O3 NTs) with peculiar and fascinating physicochemical properties, low processing cost, large surface areas, high active sites and efficient charge transferring capacity, can reduce the electron-hole pairs recombination during their participation in redox processes [13,27–29]. However, it is less stable in aqueous solution. Hematite can dissolve at 25 °C with the reaction of Fe2O3 + 3H2O = 2Fe3+ + 6OH [30]. Then the free aquatic Fe3+ ion has a powerful tendency to hydrolyze to form Fe(OH)3y (y 6 4) [31]. y On the other hand, tin oxide (SnO2) behaves better electrocatalytic (EC) activity among so many catalyst species [32–37], especially when it possesses some special crystal surfaces, though it cannot be excited by visible light for its wide gap (3.80 eV) [38]. As a kind of electrocatalyst, SnO2 not only can endow excellent EC performance to photocatalyst (Fe2O3 NTs), but can improve the stability of Fe2O3 NTs in aqueous solution as a protective layer. Furthermore, SnO2 will not hinder the light absorption of the photocatalyst for its high optical transparency [39]. At the same time, the visible photocatalysis can eliminate the toxic intermediate which is easily poison the catalyst surface in the electrocatalytic process [40,41], resulting in the improvement of the EC efficiency. Herein, the strategy of coupling SnO2 to Fe2O3 NTs permits the improvement of photoelectrocatalytic (PEC) activities during the process of CO2 reduction. Thus, a superior photoelectrocatalyst combining both merits of Fe2O3 and SnO2 might be expected. In this study, Fe2O3 NTs were successfully prepared on the Fe foils by the electrochemical anodization method and modified by SnO2 NRs through a surfactant-free hydrothermal method. SnO2 in the as-prepared SnO2 NRs/Fe2O3 NTs catalyst had special crystal surfaces of (1 1 0) and (1 0 1). The composite catalyst was utilized for PEC reduction of CO2 with water under visible light irradiation. We evaluate systematically the PEC reduction of CO2 querying the synergistic effect of PC and EC reduction, as well as the synergistic catalytic effect of the sole catalyst. The study of PEC reduction of CO2 not only paved a new path for exploring novel catalysts with
synergistic effects but also provided the experimental and theoretical basis for further research. 2. Experimental section 2.1. Preparation of catalysts The iron foils (20 mm 40 mm 1.5 mm) were firstly polished by sand papers and degreased by boiling in 10 wt.% oxalate and sonicated in ethanol. The anodization experiments were performed at 30 V for 2 h (20 °C). The ferrous foil was the working electrode, and titanium foil was the counter electrode. The electrolytes were consisted of 0.25 wt.% NH4F in a nonaqueous solution (Volume of water:glycol = 3:97). After sonicating for 5 min, the samples were placed in a muffle furnace (KSL-1100X) under oxygen atmosphere with the flow rate of 60 sccm (standard cubic centimeters min1), then heated to 500 °C with the rate of 3 °C min1 and maintained at 500 °C for 2 h. Lastly, they were cooled to room temperature with the rate of 3 °C min1. The SnO2 NRs was prepared through a surfactant-free hydrothermal method. In a typical experiment, the aqueous solution premade by dissolving 0.203 g SnCl45H2O and 0.623 g NaOH into 35 mL of deionized water was ultrasonically cleaned for 10 min, and then the in-suit growth Fe2O3 NTs precursors (20 mm 40 mm 1.5 mm) was immersed in it. They were transferred into a 50 mL Teflon-lined stainless autoclave and heated at 220 °C for various durations at the oven. The resulting SnO2 NRs/ Fe2O3 NTs was dried at 60 °C for one hour in air. 2.2. Characterization of catalysts The surface morphologies of the as-prepared samples were characterized by scanning electron microscopy (SEM, Philips XL30 FEG) with an accelerated voltage of 20 kV. The crystalline structures were characterized by X-ray diffraction (XRD, Rigaku D/MAX-rA, Japan) using a diffractometer with Cu Ka radiation, k = 1.54184 Å in the range of 2h = 20°–70°, the scan rate of 4° min1. Surface compositions were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromated X-ray source (Al Ka ht = 1486.6 eV). The UV–visible diffused reflectance spectrum (UV–vis DRS) was measured for photochemical properties using IS19-1 in combination with a single reflection internal accessory (Beijing Purkinje General Instrument Co., Ltd). XPS and UV–vis DRS measurements took SnO2 NRs/Fe2O3 NTs reacting for 120 min as a test object. The electrochemical properties were measured by CHI660D potentiostat (Shanghai Chen Hua Instrument Co., LTD). Properties of PEC CO2 reduction was measured by CHI660D potentiostat in 0.1 mol L1 KHCO3. The as-prepared electrode, Hg/ Hg2Cl2 in saturated KCl solution and platinum wire acted as the working electrode, the reference electrode, and the counter electrode, respectively. The potential was swept linearly at a scan rate of 50 mV s1. N2 was plunged into KHCO3 at the rate of 40 sccm for 20 min before the experiment. After that, CO2 was plunged for 20 min until it was saturated, then repeated the experiment. The samples were illuminated under visible light by a Xenon lamp with a band-pass filter (k P 420 nm, 100 mW cm2). The Faradic current efficiency (g) was calculated as followed:
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Fig. 1. SEM images of different samples. (a) pre-synthesized Fe2O3 NTs, inset showed magnified images of the Fe2O3 NTs. (b) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 60 min. (c) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 90 min. (d) SnO2 of the SnO2 NRs/Fe2O3 NTs with reacting for 120 min.
Faradaic efficiency ¼
moles product number of electrons needed for conversion moles of electrons passed ð1Þ
At the end of the electrolysis, the products were immediately analyzed by gas chromatography (GC-9A, Shimadzu). The GC was equipped with glass packed column (2 m, inner diameter 3 mm, Parapok Q, 80–100) and flame ionization detector. The column was kept at 80 °C and the detector 200 °C. High purity N2 worked as a carrier gas with a flow rate of 25 sccm.
3. Results and discussion Fig. 1a shows that the as-prepared Fe2O3 NTs like volcanoes with uncertain sizes (The diameter from 357 nm to 1 lm), the volcanos has an uncertain number of nanotubes (approximate from 9 to 30). The nanotube structure is with a pore diameter of 25– 30 nm, nanowall thickness of 8–10 nm, and length of 1.7–2.3 lm, which is calculated from the inset view in Fig. 1a and the crosssectional view (Fig. S1). Fe2O3 NTs grow uniformly in arrays. The thin size enhances the surface area and active sites significantly [42]. Fig. 1b–d shows the SEM images of the samples of SnO2 NRs/Fe2O3 NTs at various reaction durations. For the sample having reacted for 60 min, SnO2 exhibited an irregular bar morphology (Fig. 1b) which was generally of ca. 500 nm in length and ca. 130 nm in diameter. When the reaction proceeded to 90 min, many SnO2 NRs were cross growth in arrays (Fig. 1c). After reacting for 120 min, the SnO2 NRs were further aggregated as shown in Fig. 1d, just like the hexagon snowflake. The XRD peak pattern of the annealed Fe2O3 NTs was shown in Fig. S2. The peaks in Fig. 2a could be well indexed to the rhombohedral symmetry of Fe2O3 with lattice constants of a = 5.0491 Å,
c = 13.6577 Å (JCPDS card No. 33-0664), which indicated the crystalline hematite phase [43]. Also, the XRD peak patterns of the as-prepared samples obtained at various reaction durations were presented in Fig. 2. The strong and sharp diffraction peaks indicated good crystallizations [44]. For the product reacting for 60 min, there was almost the Fe2O3 phase and only a little SnO2 phase, namely the crystal face of (1 1 0) matching with JCPDS card No. 41-1445 [45]. When the reaction was prolonged to 90 min, and finally 120 min, the tetragonal rutile crystalline phase of SnO2 with lattice parameters [46] of a = 4.7528 Å, c = 3.1639 Å increased continuously from less to more. At the same time, the intensity of Fe2O3 phases of (2 0 6), (1 1 0), (1 1 3), (2 0 2) weakened for a growing number of SnO2 had covered on the surface of Fe2O3 NTs. The increased crystal plane of SnO2 will be a favor to the improvement of the electrocatalysis [47]. On the basis of the SEM and XRD results stated in Fig. 2a and b, the formation process of the hierarchically assembled SnO2 NRs on the surface of Fe2O3 NTs in hydrothermal process was deduced, as illustrated schematically in Fig. 3. The SnO2 grew to irregular nanorods, and then they grow up into nanorod crossed arrays on the 4-fold symmetrical structure, taking the (1 1 0) and (1 0 1) planes as the preferential growth direction. As the reaction further proceeded, the adjacent SnO2 NRs arrays tended to gather together to form the 6-fold symmetrically branched state. To gain more information about the surface composition, XPS analysis was performed on the SnO2 NRs/Fe2O3 NTs catalyst. The wide spectrum showed the only emissions of C, Sn, O and Fe elements (Fig. S3). The peaks of Fe 2p emission (Fig. 4a) that centered at 711.6 and 724.8 eV were attributed to the Fe 2p3/2 and Fe 2p1/2, respectively, which confirmed the existence of Fe3+ [48]. The binding energies (Fig. 4b) at 486.3 and 494.7 eV for Sn 3d were assigned to the Sn 3d5/2 and Sn 3d3/2, indicating that the chemical state of
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(a) 110
JCPDS:41-1445 SnO2 101
Intensity (a.u.)
101
211
Intensity (a.u.)
120 min
90 min
(b)
JCPDS:41-1445 SnO2
110
211
SnO2 /Fe2O3 NTs
SnO2 /Fe
60min
206
JCPDS:33-0664 Fe 2O3
110
113202
JCPDS:33-0664 Fe2O3
110
214 012
122
202 113
206 30
40
50
60
70
30
40
214 122 50
012 60
70
2θ (degree)
2θ (degree)
(c) SnO2/Fe2O3 NTs
Intensity (a.u.)
before -1.1V PEC
after -1.1V PEC
20
30
40
50
60
70
2θ (degree) Fig. 2. XRD patterns of (a) SnO2 NRs/Fe2O3 NTs at various reaction durations, (b) SnO2 NRs at different carrier and (c) SnO2 NRs/Fe2O3 NTs contrasting before and after the PEC reduction of CO2. The standard SnO2 and Fe2O3 patterns are placed at the top and bottom, respectively.
Fig. 3. Schematic illustration of the formation process of hierarchically assembled SnO2 NRs based on the surface of Fe2O3 NTs: from irregular NRs to 6-fold symmetrical structure with their idealized states.
the element Sn was Sn4+ [46]. The results testified that the catalyst was exactly composed of Fe2O3 and SnO2, and it was consistent with the XRD results. The net current densities of CO2 reduction experiments were carried out to evaluate the electrochemical properties of different catalysts. The net current density was obtained from the different values between the current density with CO2 and the current density with N2. In comparison to Fe2O3 NTs catalyst shown in Fig. 5a, the net current density of CO2 reduction obtained from SnO2 NRs/ Fe2O3 NTs composite catalyst was much higher, indicating that the
composite behaved excellent EC activity of CO2 reduction. Especially for the catalyst (120 min), the net current density (1.57 mA cm2) was 7.48 times that of Fe2O3 NTs at 1.1 V (Under this potential, the current efficiency reached peak, Fig. 7b), it indicated that the EC performance of Fe2O3 NTs was greatly enhanced by the introduction of 6-fold branched SnO2 NRs. To verify the outstanding electrochemical performance of the composite, the EIS experiment was performed (Fig. 5b). The semicircle in the medium-frequency region was assigned to the charge transfer resistance [49]. When light irradiation was applied on the
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(a)
Sn 3d 3/2
Intensity (a.u.)
Intensity (a.u.)
Sn3d 5/2
(b)
Fe2 p3/2
Fe 2 p1/2 711.6 eV
494.7 eV 724.8 eV
730
486.3 eV
725
720
715
710
498
495
492
Binding Energy (eV)
489
486
483
Binding Energy (eV)
Fig. 4. The high resolution XPS spectra of Fe 2p (a) and Sn 3d (b) of the SnO2 NRs/Fe2O3 NTs.
160
(a)
(b)
Fe2O3 NTs SnO2 NRs/Fe 2O3 NTs-60min
4
120
SnO2 NRs/Fe 2O3 NTs -90min
Fe 2O3 NTs dark
SnO2 NRs/Fe 2O3 NTs -120min
-Z" (ohm)
The Net Current Density (mA cm -2)
5
3
-1.1 V
2
1.57 mA cm
Fe 2O3 NTs light SnO2 NRs/Fe 2O3 NTs dark
80
SnO2 NRs/Fe 2O3 NTs light
-2
40 1
0.21 mA cm 0
-1.0
-2
-1.2
-1.4
Potential (V vs.SCE)
0
0
100
200
300
Z' (ohm)
Fig. 5. (a) The net current density of CO2 reduction of different catalysts. (b) The impedance plots of different catalysts in dark or light.
Fe2O3 NTs, it can be seen from Fig. 5b that the impedance of the Fe2O3 NTs reduces from 310 X to 175 X. It illustrated that the conductivity of the Fe2O3 NTs was enhanced after irradiating, it further deduced that the recombination ratio of photogenerated electronhole is decreased. It shows that the Fe2O3 NTs have the peculiar and fascinating photocatalytic properties. However, the electrocatalytic property of Fe2O3 NTs is very poor. After loading SnO2NRs, it shows that the impedance of the SnO2 NRs/Fe2O3 NTs (56 X) is much smaller than that of Fe2O3 NTs. It indicated that the SnO2 NRs can greatly enhance the charge transfer of the material, which is in favor of the improvement of the electrocatalysis. The lower impedance confirms that the high conductivity and greatly enhances electron transport of the composite, resulting in the excellent catalytic performance [50]. The optical properties of SnO2 NRs/Fe2O3 NTs composite, SnO2 NRs/Fe, and Fe2O3 NTs were given by the UV–vis DRS (Fig. 6a). It could be clearly seen that SnO2 NRs which was on the surface of Fe2O3 NTs and Fe improved the light absorption of Fe2O3 NTs and Fe obviously, suggesting that SnO2 was optical transparency [39]
and Fe2O3 NTs has more excellent optical performance than Fe. Following a linear fit of the experimental (aht)2 versus ht, where a, h, and t are the absorption coefficient, Planck constant and light frequency, respectively, a band gap of 2.57 eV of the as-prepared catalyst was obtained. The two peaks that centered at 4.3 eV and 7.1 eV in the valence band photoemission spectrum were assigned to the electron emission from p (nonbonding) and r (bonding) O 2p orbitals, respectively [51] (Fig. 6c). At the same time, the position of the valence band maximum (VBM) was determined to be 1.82 eV from the onset of the valence band emission spectrum by a linear extrapolation. With the estimated band gap of 2.57 eV, the conduction band minimum (CBM) was calculated to be 0.75 eV (vs. NHE). It has been reported that the SnO2 has enough negative potential to catalyze reduction CO2 [52]. It indicated that the most positive VB potential had enough oxidation ability to oxide H2O, supplying protons for CO2 reduction. At the same time, the more negative CB potential had enough reduction ability to reduce CO2. Furthermore, the composite exhibited a much lower impedance
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2.4
(b)
(a)
Fe2O3 NTs (α hν)2 (10 4eV 2/cm 2)
Intensity (a.u.)
2.0
1.6
SnO2/Fe 2O3 NT
10.0k
Eg=2.57eV
5.0k
1.2
SnO2 /Fe 0.8
300
400
500
0.0
600
2.0
2.5
Wavelength (nm)
3.0
3.5
hυ (eV)
7.06 eV
(c)
Intensity (a.u.)
4.27 eV
1.82 eV
10
8
6
4
2
Binding Energy (eV) Fig. 6. (a) UV–vis DRS of different catalysts. (b) Plot analysis of optical band gap of SnO2 NRs/Fe2O3 NTs. (c) Valence band photoemission spectrum of SnO2 NRs/Fe2O3 NTs (solid line) and its Gaussian fit (dashed line). The spectrum was smoothed and baseline corrected.
(35 X) when it was placed under the illumination condition (Fig. 5b), suggesting that the photoexcited charge carriers decreased their charge transfer resistance [28]. Therefore, the composite had excellent photochemical performance. The CO2 reduction experiments were performed in a quartz cell with circulating cooling water. The predominant reaction product analyzed by GC was methanol. The PEC activity of SnO2 NRs/ Fe2O3 NTs was evaluated according to methanol production. All the methanol yields of SnO2 NRs/Fe2O3 NTs that increased with continuous visible light irradiation under different applied voltage (vs. SCE) were shown in Fig. 7a. For 6 h reaction time, the largest methanol yield was 2.05 mmol L1 cm2 (Table 1), which was attained under visible light irradiation and 1.1 V extra voltage. The peak efficiency 87.04% at 1.1 V (Fig. 7b) could clearly prove the largest methanol yield. It was explained as followed: The reactions of CO2 reduction and HER were competitive on the surface of the electrode [53]. When the extra voltage was lower than 1.1 V, the CO2 reduction was the main reaction, and it reached a peak at 1.1 V. When the extra voltage was higher than 1.1 V, the HER took the lead, which weakened the reduction of CO2.
The methanol yields were 2.05, 1.21, 0.69 mmol L1 cm2 under PEC, EC and PC reduction at 6 h (Table 2), respectively. It could be seen that the EC performance was much more excellent than PC performance. But when the illumination was applied to EC reduction, namely PC and EC were used together to reduce CO2, it attained the highest methanol yield (2.05 mmol L1 cm2). Also, the methanol yield under PEC conditions was higher than that the simple addition (1.90 mmol L1 cm2) of PC and EC reduction. Therefore, the synergistic effect of PC and EC reduction was obviously reflected in the product yields. The methanol yields on different catalysts (Fig. 7d and e) were attained under visible light illumination at 1.1 V. When the reaction time was 6 h, the methanol yield of SnO2 NRs/Fe2O3 NTs was 3.7, 3.3, 2.7 and 1.7 times that of Fe2O3 NTs, SnO2 NRs, SnO2 NRs/ Fe and Fe2O3 NTs + SnO2 NRs (simple addition) (Table 3), respectively. Not only the PEC reduction of CO2 was efficiently improved when coupling Fe2O3 NTs and SnO2 NRs, but the couple of Fe2O3 NTs and SnO2 NRs generated the synergistic effect. On the basis of the present work and our previous results, we propose the CO2 PEC reduction mechanism as follows (Fig. 8):
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100
(a) Faradic Current Efficiency (%)
-1
1.5
1.0
0.5
0.0
-1.1V, η=87.04%
(b) -0.9 V PEC -1.0 V PEC -1.1 V PEC -1.2 V PEC -1.3 V PEC
-2
Methanol Concentration (m mol L cm )
2.0
60
120
180
240
300
80
60
40
20
0
360
-0.9
-1.0
-1.1
Time (min)
2.0
Meth anol Concentration (mmol L cm )
(c)
-1.3
(d)
-2
-2
Methanol Concentration (mmol L cm )
2.0
-1.2
Potential (V)
1.5
1.0
0.5
0.0
60
120
SnO 2 NRs
-1
-1
-1.1V PEC -1.1V EC PC -1.1V EC+PC
180
240
300
Fe 2O3 NTs
1.5
SnO2 NRs/Fe 2O3 NTs SnO2 NRs+Fe 2O3 NTs
1.0
0.5
0.0
360
60
120
180
2.0
240
300
360
Time (min)
(e)
-1
-2
Methanol Concentration (mmol L cm )
Time (min)
SnO2 /Fe 1.5
Fe2 O3 NTs SnO2 /Fe2 O3 NTs
1.0
0.5
0.0
0
50
100
150
200
250
300
350
Time (min) Fig. 7. (a) Methanol concentration of SnO2 NRs/Fe2O3 NTs under PEC CO2 reduction with different potentials. (b) Faradic current efficiency of SnO2 NRs/Fe2O3 NTs as a function of applied potential in the process of PEC CO2 reduction. (c) Methanol concentration of SnO2 NRs/Fe2O3 NTs under different conditions of 1.1 V PEC, PC and 1.1 V EC CO2 reduction. (d) Methanol concentration of different catalysts under 1.1 V PEC CO2 reduction. (e) Methanol concentration of different catalysts base under 1.1 V PEC CO2 reduction. All the extra voltage mentioned above refer to SCE.
Z. Yang et al. / Journal of Colloid and Interface Science 486 (2017) 232–240 Table 1 Methanol concentration of the PEC process on the SnO2 NRs/Fe2O3 NTs in the 6th hour. 0.9 0.79
Potential (V) Methanol concentration (mmol L1 cm3)
1.0 0.70
1.1 2.05
1.2 1.40
1.3 0.66
Table 2 Methanol concentration of the different catalytic process on the SnO2 NRs/Fe2O3 NTs in the 6th hour under 1.1 V. Different conditions
PEC
PC
EC
PC + EC
Methanol concentration (mmol L1 cm3)
2.05
0.69
1.21
1.90
Table 3 Methanol concentration of the PEC process on the different catalysts in the 6th hour under 1.1 V. Semiconductor
SnO2 NRs
Fe2O3 NTs
SnO2 NRs + Fe2O3 NTs
SnO2 NRs/ Fe2O3 NTs
Methanol concentration (mmol L1 cm3)
0.61
0.55
1.16
2.05
239
4. Conclusions In summary, the SnO2 NRs/Fe2O3 NTs catalyst was prepared after SnO2 NRs was decorated on the surface of Fe2O3 NTs through a surfactant-free hydrothermal method. The SEM showed that the Fe2O3 NTs was volcanoes like and SnO2 was just like hexagon snowflake. According to the SEM and XRD results, the formation process of the hierarchically assembled SnO2 NRs was deduced. When the reaction time was prolonged from 60 min to 90 min and 120 min, SnO2 changed from irregular nanorods to nanorod crossed arrays on the 4-fold and 6-fold symmetrical structure ordinally. The SnO2 NRs/Fe2O3 NTs catalyst that had reacted for 120 min behaved the best photoelectrocatalytic properties. From the view of photocatalytic reduction, the composite had matched energy band. The conduction band was negative enough to drive CO2 reduction, and the valence band was positive enough to oxidize H2O for H+, which was used for CO2 reduction. From the electrocatalytic reduction point, the net CO2 reduction current density of the composite was 7.48 times that of Fe2O3 NTs at 1.1 V, indicating that the electrocatalytic performance of Fe2O3 NTs was greatly enhanced by the introduction of 6-fold branched SnO2 NRs. The predominant reduction product analyzed by GC was methanol. Two synergistic effects were proved according to the methanol yields, one was the synergistic effect of the photocatalytic reduction and the electrocatalytic reduction, the other was the synergistic effect between SnO2 NRs and Fe2O3 NTs. The largest methanol yield (2.05 mmol L1 cm2), which was attained under visible light irradiation and 1.1 V extra voltage, was far higher than that of solely electrocatalytic or photocatalytic reduction and solely SnO2 NRs or Fe2O3 NTs. The results indicated that the catalyst behaved excellent photoelectrocatalytic activity of CO2 reduction. Herein, the study not paved a new path for exploiting novel photoelectrocatalysts, but provided the experimental and theoretical basis for further research.
Acknowledgements
Fig. 8. The CO2 reduction mechanism on the SnO2 NRs/Fe2O3 NTs catalyst (The catalyst symbolizes by R).
The reduction of CO2 to methanol is six electrons reaction. And, the process of CO2 PEC reduction is concretely deduced. First, the catalyst R is excited from ground state (R) to singlet state (R⁄). Then R⁄ combines with the CO 2 radical (CO2 is reduced to CO2 radical by electron) and receives one proton to form the [R---O@CAOH]. Then the [R---O@CAOH] combines with three hydrogen protons and three electrons on the electrode surface, and gets the product [R---O@CH2], but the formed structure is unstable and vulnerable by electron and hydrogen proton, further to form the [R---OACH3]. Then the [R---OACH3] is vulnerable by electron leading to the break of [R---O] bond, then combines with hydrogen proton, forms CH3OH, and catalyst return to the ground state R attending the next reaction circle. Furthermore, the as-prepared catalyst has good stability after the catalysis. The information can be obtained from Fig. 2c, the SnO2 NRs/Fe2O3 NTs XRD pattern after the PEC reduction of CO2 was compared with the initiative XRD pattern. It can be seen that the diffraction peaks have no change before and after the PEC reduction CO2 for 1 h at 1.1 V. It indicates the metal oxide on the SnO2 NRs/Fe2O3 NTs surface is not reduced to metal. Thus, the SnO2 NRs/Fe2O3 NTs has good performance on the stability.
This research was supported by the National Natural Science Foundation of China (Grant No. 21203114), Planning Project of Science and Technology in Colleges of Shandong Province (Grant No. J14LC16). We are grateful to the foundation supported by Shandong Jingbo Holdings Corporation.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.055.
References [1] Y. Chen, W. Zhou, D. Ding, M. Liu, F. Ciucci, M. Tade, Z. Shao, Advances in cathode materials for solid oxide fuel cells: complex oxides without alkaline earth metal elements, Adv. Energy Mater. (2015) (DOI). [2] J.E. Miller, A.H. McDaniel, M.D. Allendorf, Considerations in the design of materials for solar-driven fuel production using metal-oxide thermochemical cycles, Adv. Energy Mater. 4 (2014). [3] S. Sen, D. Liu, G.T.R. Palmore, Electrochemical reduction of CO2 at copper nanofoams, ACS Catal. 4 (2014) 3091–3095. [4] Y. Liao, S.W. Cao, Y. Yuan, Q. Gu, Z. Zhang, C. Xue, Efficient CO2 capture and photoreduction by amine-functionalized TiO2, Chem. – Eur. J. 20 (2014) 10220–10222. [5] H. Zhai, Y. Ou, E.S. Rubin, Opportunities for decarbonizing existing US coalfired power plants via CO2 capture, utilization and storage, Environ. Sci. Technol. (2015) (DOI). [6] K. Sekizawa, K. Maeda, K. Domen, K. Koike, O. Ishitani, Artificial ZScheme constructed with a supramolecular metal complex and semiconductor for the photocatalytic reduction of CO2, J. Am. Chem. Soc. 135 (2013) 4596–4599.
240
Z. Yang et al. / Journal of Colloid and Interface Science 486 (2017) 232–240
[7] J. Qu, X. Zhang, Y. Wang, C. Xie, Electrochemical reduction of CO2 on RuO2/TiO2 nanotubes composite modified Pt electrode, Electrochim. Acta 50 (2005) 3576–3580. [8] S.C. Yan, S.X. Ouyang, J. Gao, M. Yang, J.Y. Feng, X.X. Fan, L.J. Wan, Z.S. Li, J.H. Ye, Y. Zhou, A room-temperature reactive-template route to mesoporous ZnGa2O4 with improved photocatalytic activity in reduction of CO2, Angew. Chem. 122 (2010) 6544–6548. [9] J.M. Bockris, J. Wass, The photoelectrocatalytic reduction of carbon dioxide, J. Electrochem. Soc. 136 (1989) 2521–2528. [10] K.K. Sakimoto, C. Liu, J. Lim, P. Yang, Salt-induced self-assembly of bacteria on nanowire arrays, Nano Lett. 14 (2014) 5471–5476. [11] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G. Mikami, S. Okamoto, Y. Izumi, Photocatalytic conversion of carbon dioxide into methanol in reverse fuel cells with tungsten oxide and layered double hydroxide photocatalysts for solar fuel generation, Catal. Sci. Technol. 4 (2014) 1644–1651. [12] C. Das Neves Gomes, E. Blondiaux, P. Thuéry, T. Cantat, Metal-free reduction of CO2 with hydroboranes: two efficient pathways at play for the reduction of CO2 to methanol, Chem. – Eur. J. 20 (2014) 7098–7106. [13] Z. Yang, J. Xu, C. Wu, H. Jing, P. Li, H. Yin, New insight into photoelectric converting CO2 to CH3OH on the one-dimensional ribbon CoPc enhanced Fe2O3 NTs, Appl. Catal. B 156 (2014) 249–256. [14] P. Li, J. Xu, H. Jing, C. Wu, H. Peng, J. Lu, H. Yin, Wedged N-doped CuO with more negative conductive band and lower overpotential for high efficiency photoelectric converting CO2 to methanol, Appl. Catal. B 156 (2014) 134–140. [15] S. Sfaelou, M. Antoniadou, V. Dracopoulos, K. Bourikas, D.I. Kondarides, P. Lianos, Quantum dot sensitized titania as visible-light photocatalyst for solar operation of photofuel cells, J. Adv. Oxidat. Technol. 17 (2014) 59–65. [16] B. Kumar, M. Llorente, J. Froehlich, T. Dang, A. Sathrum, C.P. Kubiak, Photochemical and photoelectrochemical reduction of CO2, Annu. Rev. Phys. Chem. 63 (2012) 541–569. [17] C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G. Centi, A novel photoelectrochemical approach for the chemical recycling of carbon dioxide to fuels, Chem. Eng. Trans. 25 (2011) 683–688. [18] C.D. Windle, E. Pastor, A. Reynal, A.C. Whitwood, Y. Vaynzof, J.R. Durrant, R.N. Perutz, E. Reisner, Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2, Chem. – Eur. J. 21 (2015) 3746–3754. [19] H. Abdullah, D.-H. Kuo, Photocatalytic performance of Ag and CuBiS2 nanoparticle-coated SiO2@TiO2 composite sphere under visible and ultraviolet light irradiation for azo dye degradation with the assistance of numerous nano p–n diodes, J. Phys. Chem. C 119 (2015) 13632–13641. [20] C.X. Kronawitter, M. Lessio, P. Zhao, C. Riplinger, J.A. Boscoboinik, D.E. Starr, P. Sutter, E.A. Carter, B.E. Koel, Observation of surface-bound negatively charged hydride and hydroxide on GaP (1 1 0) in H2O environments, J. Phys. Chem. C (2015) (DOI). [21] P. Li, J. Zhang, H. Wang, H. Jing, J. Xu, X. Sui, H. Hu, H. Yin, The photoelectric catalytic reduction of CO2 to methanol on CdSeTe NSs/TiO2 NTs, Catal. Sci. Technol. 4 (2014) 1070–1077. [22] P. Li, H. Hu, J. Xu, H. Jing, H. Peng, J. Lu, C. Wu, S. Ai, New insights into the photo-enhanced electrocatalytic reduction of carbon dioxide on MoS2-rods/ TiO2 NTs with unmatched energy band, Appl. Catal. B 147 (2014) 912–919. [23] P. Li, X. Sui, J. Xu, H. Jing, C. Wu, H. Peng, J. Lu, H. Yin, Worm-like InP/TiO2 NTs heterojunction with unmatched energy band photo-enhanced electrocatalytic reduction of CO2 to methanol, Chem. Eng. J. 247 (2014) 25–32. [24] E.E. Barton, D.M. Rampulla, A.B. Bocarsly, Selective solar-driven reduction of CO2 to methanol using a catalyzed p-GaP based photoelectrochemical cell, J. Am. Chem. Soc. 130 (2008) 6342–6344. [25] Y.-S. Hu, A. Kleiman-Shwarsctein, A.J. Forman, D. Hazen, J.-N. Park, E.W. McFarland, Pt-doped a-Fe2O3 thin films active for photoelectrochemical water splitting, Chem. Mater. 20 (2008) 3803–3805. [26] I. Cesar, A. Kay, J.A. Gonzalez Martinez, M. Grätzel, Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping, J. Am. Chem. Soc. 128 (2006) 4582–4583. [27] D.K. Zhong, D.R. Gamelin, Photoelectrochemical water oxidation by cobalt catalyst (‘‘CoPi”)/a-Fe2O3 composite photoanodes: oxygen evolution and resolution of a kinetic bottleneck, J. Am. Chem. Soc. 132 (2010) 4202–4207. [28] W. Zhang, X. Li, Q. Zhao, Y. Hou, Y. Shen, G. Chen, Uniform a-Fe2O3 nanotubes fabricated for adsorption and photocatalytic oxidation of naphthalene, Mater. Chem. Phys. 129 (2011) 683–687. [29] P. Li, H. Wang, J. Xu, H. Jing, J. Zhang, H. Han, F. Lu, Reduction of CO2 to low carbon alcohols on CuO FCs/Fe2O3 NTs catalyst with photoelectric dual catalytic interfaces, Nanoscale 5 (2013) 11748–11754.
[30] A. Mao, K. Shin, J.K. Kim, D.H. Wang, G.Y. Han, J.H. Park, Controlled synthesis of vertically aligned hematite on conducting substrate for photoelectrochemical cells: nanorods versus nanotubes, ACS Appl. Mater. Interfaces 3 (2011) 1852– 1858. [31] V. Parker, I. Khodakovskii, Thermodynamic properties of the aqueous ions (2+ and 3+) of iron and the key compounds of iron, J. Phys. Chem. Ref. Data 24 (1995) 1699–1745. [32] A. Stefánsson, Iron (III) hydrolysis and solubility at 25°C, Environ. Sci. Technol. 41 (2007) 6117–6123. [33] E. Guinea, F. Centellas, E. Brillas, P. Cañizares, C. Sáez, M.A. Rodrigo, Electrocatalytic properties of diamond in the oxidation of a persistant pollutant, Appl. Catal. B 89 (2009) 645–650. [34] Y.-H. Cui, X.-Y. Li, G. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968–1976. [35] G. Zhao, X. Cui, M. Liu, P. Li, Y. Zhang, T. Cao, H. Li, Y. Lei, L. Liu, D. Li, Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/Sb-doped SnO2 electrode, Environ. Sci. Technol. 43 (2009) 1480–1486. [36] S.Y. Yang, W. Choi, H. Park, TiO2 nanotube array photoelectrocatalyst and Ni– Sb–SnO2 electrocatalyst bifacial electrodes: a new type of bifunctional hybrid platform for water treatment, ACS Appl. Mater. Interfaces 7 (2015) 1907–1914. [37] R.A. Hameed, R.M. El-Sherif, Ni–P–SnO2/C composite: synthesis, characterization and electrocatalytic activity for methanol oxidation in KOH solution, Int. J. Hydrogen Energy 40 (2015) 10262–10273. [38] H. Xia, H. Zhuang, T. Zhang, D. Xiao, Visible-light-activated nanocomposite photocatalyst of Fe2O3/SnO2, Mater. Lett. 62 (2008) 1126–1128. [39] M. Niu, F. Huang, L. Cui, P. Huang, Y. Yu, Y. Wang, Hydrothermal synthesis, structural characteristics, and enhanced photocatalysis of SnO2/a-Fe2O3 semiconductor nanoheterostructures, ACS Nano 4 (2010) 681–688. [40] J. Pan, J. Li, Z. Yan, B. Zhou, H. Wu, X. Xiong, SnO2@ CdS nanowire-quantum dots heterostructures: tailoring optical properties of SnO2 for enhanced photodetection and photocatalysis, Nanoscale 5 (2013) 3022–3029. [41] N. Karanasios, J. Georgieva, E. Valova, S. Armyanov, G. Litsardakis, S. Sotiropoulos, Photoelectrocatalytic oxidation of organics under visible light illumination: a short review, Curr. Org. Chem. 19 (2015) 512–520. [42] G. Zhao, P. Li, F. Nong, M. Li, J. Gao, D. Li, Construction and high performance of a novel modified boron-doped diamond film electrode endowed with superior electrocatalysis, J. Phys. Chem. C 114 (2010) 5906–5913. [43] S.K. Mohapatra, S.E. John, S. Banerjee, M. Misra, Water photooxidation by smooth and ultrathin a-Fe2O3 nanotube arrays, Chem. Mater. 21 (2009) 3048–3055. [44] Z. Sun, H. Yuan, Z. Liu, B. Han, X. Zhang, A highly efficient chemical sensor material for H2S: a-Fe2O3 nanotubes fabricated using carbon nanotube templates, Adv. Mater. 17 (2005) 2993–2997. [45] Z. Zhang, M.F. Hossain, T. Takahashi, Self-assembled hematite (a-Fe2O3) nanotube arrays for photoelectrocatalytic degradation of azo dye under simulated solar light irradiation, Appl. Catal. B 95 (2010) 423–429. [46] F. Gu, X. Ren, J. Zhang, W. Shao, G. Huang, C. Li, Morphological evolution from SnO2 quantum dots to quantum chains inside channels of mesoporous spheres, Ind. Eng. Chem. Res. 50 (2011) 12542–12547. [47] P. Li, G. Zhao, M. Li, T. Cao, X. Cui, D. Li, Design and high efficient photoelectricsynergistic catalytic oxidation activity of 2D macroporous SNO2/1D TiO2 nanotubes, Appl. Catal. B 111 (2012) 578–585. [48] H. Wang, F. Sun, Y. Zhang, L. Li, H. Chen, Q. Wu, C.Y. Jimmy, Photochemical growth of nanoporous SnO2 at the air–water interface and its high photocatalytic activity, J. Mater. Chem. 20 (2010) 5641–5645. [49] M. Muhler, R. Schlögl, G. Ertl, The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene 2. Surface chemistry of the active phase, J. Catal. 138 (1992) 413–444. [50] G.-Y. Zhang, Y.-Y. Xu, D.-Z. Gao, Y.-Q. Sun, Α-Fe2O3 nanoplates: PEG-600 assisted hydrothermal synthesis and formation mechanism, J. Alloy. Compd. 509 (2011) 885–890. [51] S. Wang, X. Jiang, H. Zheng, H. Wu, S.-J. Kim, C. Feng, Solvothermal synthesis of MoS2/Carbon nanotube composites with improved electrochemical performance for lithium ion batteries, Nanosci. Nanotechnol. Lett. 4 (2012) 378–383. [52] S. Zhang, P. Kang, T.J. Meyer, Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate, J. Am. Chem. Soc. 136 (2014) 1734–1737. [53] T. Nikolay, L. Larina, O. Shevaleevskiy, B.T. Ahn, Electronic structure study of lightly Nb-doped TiO2 electrode for dye-sensitized solar cells, Energy Environ. Sci. 4 (2011) 1480–1486.