TiO2 Nanotubes for Photoelectrocatalytic Reduction

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Worm-like FeS2/TiO2 Nanotubes for Photoelectrocatalytic Reduction of CO2 to Methanol under Visible Light Ershuan Han,† Fengyun Hu,† Shuai Zhang,† Bo Luan,‡ Peiqiang Li,*,†,§ Hongqi Sun,∥ and Shaobin Wang*,§ †

Department of Chemistry and Material Science, Shandong Agricultural University, Tai’an, Shandong 271018, People’s Republic of China ‡ Chambroad Chemical Industry Research Institute of the Yellow River Delta, Binzhou, Shandong 256500, People’s Republic of China § Department of Chemical Engineering, Curtin University, General Post Office Box U1987, Perth, Western Australia 6845, Australia ∥ School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, Western Australia 6027, Australia ABSTRACT: Photoelectrocatalytic (PEC) reduction of CO2 to hydrocarbons provides a great technique for CO2 utilization, renewable energy transformation, and storage. Iron disulfide (FeS2), as an earth-abundant and nontoxic semiconductor, has narrow band gap energy, high photovoltaic conversion efficiency, and light absorption, making it very promising as a photoelectrode in a PEC cell. Herein, novel worm-like FeS2/TiO2 nanotubes (NTs) was prepared by introducing FeS2 on TiO2 NTs and exhibited excellent PEC performance for CO2 reduction to methanol. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) showed that worm-like FeS2 was densely packed on the TiO2 NT substrate. By introduction of FeS2 on TiO2 NTs, the visible light absorption was improved greatly and the energy band gap energy was narrowed to 1.70 eV, which significantly enhanced the photocatalytic performance under visible light. Furthermore, the resistance was reduced with increasing electrocatalytic ability. The major product of PEC reduction of CO2 was methanol, reaching 91.7 μmol h−1 L−1. PEC reduction of CO2 with promising FeS2.11,12 Herein, we prepared novel worm-like FeS2/TiO2 NTs by introducing FeS2 on TiO2 NTs and proved that the as-prepared materials exhibited excellent PEC performance for CO2 reduction to methanol.13−16

1. INTRODUCTION Carbon dioxide, a principal greenhouse gas, is an inexpensive, nontoxic, and abundant C1 resource.1 With the increasing worldwide attention of its greenhouse effect, efficient utilization of CO2 to manufacture commodity chemicals, fuels, and other materials has received considerable interest within the scientific community.2 In particular, effective conversion of CO2 to an environmentally friendly and non-fossil fuel has been becoming one of the most popular research hotspots, because transformation of CO2 to fuels not only decreases carbon emission to the atmosphere but also achieves resource recycling. Among various strategies, electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) reductions of CO2 employing electricity, solar energy, or both, respectively, are promising techniques.1,3,4 TiO2 is one of the hottest materials in the PC reductions. Titanium dioxide nanotubes (TiO2 NTs) are endowed with higher PC activity than TiO2 nanoparticles and other forms for the greater specific surface area and more active sites.1,5,6 However, ultraviolet (UV) light accounting for only 4% of the solar energy can be used by TiO2 for its wide band gap of 3.2 eV, which seriously limited its application in photocatalysis. In recent years, iron disulfide (FeS2) has been attracting much attention as an earth-abundant and nontoxic semiconductor as a result of its narrow band gap, high photovoltaic conversion efficiency, and high absorption coefficient.7−10 Furthermore, FeS2 displays great stability against photocorrosion in PEC applications. These valuable characteristics make FeS2 very promising as a photoelectrode in photoelectrochemical cells. However, few reports were focused on © XXXX American Chemical Society

2. MATERIALS AND METHODS 2.1. Fabrication of a TiO2 NT Electrode. Highly ordered TiO2 NTs in situ grew on the Ti substrate by the anodic oxidation method. A mixture of NH4F (0.5 wt %), glycol (97.7 wt %), and distilled water (1.8 wt %) acted as electrolytes. A titanium sheet was used as the anode, and platinum foil was used as the counter electrode at the applied voltage of 60 V. After 60 min of anodization, the titanium sheets were rinsed with twice-distilled water and then dried. The samples were sonicated for 5 min and placed in a muffle furnace at 500 °C for 2 h by heating at a rate of 3 °C min−1 under an oxygen flow rate of 60 mL min−1. After that, the temperature was dropped to room temperature at a rate of 3 °C min−1. Finally, TiO2 NTs were obtained. 2.2. Preparation of FeS2/TiO2 NTs. In a typical synthesis, a homogeneous solution was obtained by dissolving 10 mL of poly(vinyl alcohol) (3 wt %) and 0.20 g of polyvinylpyrrolidone in 20 mL of deionized water, followed by the addition of 1 mmol of FeCl2·4H2O under stirring. The solution of 5 mL of NaOH (0.75 M), 0.20 g of sulfur powder, and the as-prepared TiO2 NTs was put into a Teflonlined stainless-steel reactor, stirring for 0.5 h. The closed reactor was kept under the condition of 180 °C for 12 h. Subsequently, the Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 21, 2017 Revised: January 21, 2018 Published: February 19, 2018 A

DOI: 10.1021/acs.energyfuels.7b03234 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Scheme 1. Schematic Strategy of the Electrode Preparation

samples were washed a few times by deionized water and ethanol successively. Ultimately, the FeS2/TiO2 NTs were acquired at 50 °C for 2 h in a vacuum drying oven.17,18 The average loading of FeS2 on TiO2 NTs is 1.2 mg cm−2 by calculating the mass of the catalyst before and after loading of FeS2. The detailed preparation procedures of FeS2/TiO2 NT electrodes are illustrated in Scheme 1. 2.3. Catalyst Characterizations. X-ray diffraction (XRD) of crystalline structures was measured by a diffractometer (Rigaku D/ MAX-rA, Japan). Using scanning electron microscopy (SEM, Philips XL30FEG) at an accelerated voltage of 20 kV, the surface morphology of the FeS2/TiO2 NTs was obtained. The surface properties and composition of the samples were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The photochemical properties of the FeS2/TiO2 NTs were measured by ultraviolet−visible (UV−vis) diffuse reflection spectroscopy (DRS, Beijing Purkinje General Instrument Co., Ltd.). The photoelectrochemical experiments were performed in a standard three-electrode system (CHI660D). In a 0.1 mol L−1 KHCO3 solution, electrochemical impedance spectroscopy (EIS) of different sensitized electrodes was recorded with the electrochemical workstation. The frequency range was from 10−2 to 10−5 Hz. FeS2/TiO2 NTs with different ratios were used as working electrodes. The electrochemical workstation recorded linear sweep voltammetry (LSV) under different voltages in the following test conditions: sweep speed of 50 mV s−1, scanning potential from 0 to −1.6 V, and light intensity of 100 mW cm−2. Also, 0.5 mol L−1 KHCO3 solution was exploited as the electrolyte, and before the detection, N2 or CO2 was used to degas for 20 min. 2.4. Photoelectrochemical Reduction. All of the experiments of the CO2 reduction reaction were carried out in a reactor with watercirculating double cells at 25 °C. The reference, working, and counter electrodes were saturated calomel electrode (SCE), FeS2/TiO2 NTs, and platinum sheet, respectively. The visible light was supplied by a 500 W xenon lamp (λ ≥ 420 nm; 100 mW cm−2). The product analysis was accomplished using gas chromatography (GC, 6890-N, Agilent). The column and detector temperatures were kept at 100 and 150 °C, respectively. The carrier gas of high-purity N2 was flowing at a rate of 30 mL min−1.

Figure 1. (a) SEM image of FeS2/TiO2 NTs and (inset) HRTEM image of FeS2 and (b) XRD patterns of FeS2/TiO2 NTs and TiO2 NTs.

3. RESULTS AND DISCUSSION Figure 1a shows the surface morphology of electrode material, FeS2/TiO2 NTs. It can be seen that FeS2 grew in a worm-like morphology and densely packed on the TiO2 NT substrate. The length of worm-like FeS2 ranged from 80 to 120 nm. The crystal structure of FeS2/TiO2 NTs was studied by XRD in Figure 1b. The strong and sharp diffraction peaks suggested that FeS2/TiO2 NTs had excellent crystallization. The XRD pattern of TiO2 NTs is also shown in Figure 2b. For the FeS2/ TiO2 NTs, the diffraction peaks of TiO2 NTs can be observed, suggesting that TiO2 NTs maintain the internal structure without destruction. The peaks at 2θ of 75.90° and 77.58° were indexed to the crystal planes of (421) and (420) of FeS2, respectively. In addition, all of the diffraction peaks of FeS2/ TiO2 NTs have no change after 3 time reuses for CO2 PEC

reduction, and it is indicated that the prepared FeS2/TiO2 NT material has excellent stability and reusability. The highresolution transmission electron microscopy (HRTEM) image of FeS2 for more crystalline characteristics of FeS2 is presented in the inset of Figure 1a. The interplanar spacing of 0.14 nm refers to the (420) lattice plane of the pyrite phase of FeS2 in the HRTEM image. Furthermore, the XRD peaks (Figure 1b) indicated no formation of new phases. These results confirmed that a FeS2/TiO2 NT hybrid was successfully prepared. On the basis of UV−vis DRS, FeS2/TiO2 had a stronger absorption of visible light than that of TiO2 NTs, which means that the modification by FeS2 greatly improved the light absorption of TiO2 NTs (Figure 2a).11,15 The band gap energy (Eg) can be obtained from the plot of (αhν)2 versus hν, as illustrated in panels b and c of Figure 2. The Eg value of TiO2 B


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Figure 2. (a) UV−vis DRS spectra of TiO2 NTs and FeS2/TiO2 NTs, (b) optical band gap of TiO2 NTs, (c) optical band gap of FeS2/TiO2 NTs, and (d) Mott−Schottky plot of FeS2/TiO2 NTs.

From the literature, the semiconductor level band is equal to the Fermi level. The conduction potential of a n-type semiconductor is about 0.2−0.3 eV above the flat belt.21−23 Therefore, the conduction band maximum of FeS2/TiO2 NTs is 0.29−0.39 eV. The Eg value of FeS2/TiO2 NTs was narrowed to 1.70 eV. It was further deduced that the valence band of FeS2/TiO2 NTs is 1.99−2.09 eV. The elemental composition and chemical state of O, Ti, Fe, and S in the FeS2/TiO2 NTs were analyzed by XPS. The XPS survey spectrum of the FeS2/TiO2 NTs confirms the existence of Ti, O, Fe, S, and C elements (Figure 3a). The detection of C is due to the substrate for XPS analysis. The bonding energies of Ti 2p peaks are located at 464.26 and 458.44 eV, Fe 2p peaks are found to be at 719.40 and 707.10 eV, while the S 2p peak is appearing at 163.88 eV. Those XPS binding energies indicated that the prepared samples were comprised of Fe2+, S1−, and Ti4+, without other impurity. EIS was conducted on TiO2 NTs and FeS2/TiO2 NTs in an aqueous solution of 0.1 mol L−1 KHCO3 (Figure 4a). It can be seen that the EIS value of FeS2/TiO2 NTs (1.9 kΩ) was significantly reduced in comparison to TiO2 NTs (25.0 kΩ), which indicated that the worm-like structure of FeS2/TiO2 NTs has an excellent electric transmission performance. The resistance of the substrate was decreased when FeS2 was introduced, which improved the electron transfer and offered adequate electrons for CO2 reduction. Subsequently, the LSV was tested in the same solution purging with N2 or CO2 at a rate of 40 mL min−1. It is well-known that the increase of the current density is ascribed to two reactions: CO2 conversion

NTs is 3.20 eV (Figure 2b), which suggests that prepared TiO2 presents the structure of anatase. After modification by FeS2, the Eg value was narrowed to 1.70 eV (Figure 2c), indicating that FeS2/TiO2 can be excited by low-energy light (λ ≤ 730 nm) in the visible range. To further study the PC properties of FeS2/TiO2 NTs, Mott−Schottky curves were explored with the Mott−Schottky equation below19 ⎛ 1 2 kT ⎞ ⎜E − E ⎟ =− FB − 2 2⎝ e ⎠ Csc ε0εrNAA

(1)

where Csc, ε0, εr, NA, A, E, EFB, k, T, and e are the space charge region capacity, permittivity of vacuum (8.854 × 10−12 F m−1), passive film relative dielectric constant under room temperature (FeS2 is 10.9),20 acceptor concentration, contact area between the electrode and solution (6 cm2), applied potential, flat band potential, Boltzmann constant (1.38 × 10 −23 J K −1 ), thermodynamic temperature (K), and electronic charge (1.602 × 10−19 C), respectively. At room temperature, the value of kT/e is about 25 mV, which could be ignored in general. The Mott−Schottky plot was obtained by Csc−2 versus E. With construction of the tangent line of the longest linear part, the FeS2/TiO2 NTs exhibit a n-type semiconductor at less than 0.94 V, while the catalyst is a p-type semiconductor at greater than 0.94 V. It is indicated that the as-prepared hybrid electrode had a p−n heterojunction structure. The point of the tangent line and the horizontal ordinate gives EFB, which is 0.59 eV. C


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Figure 3. XPS spectra of FeS2/TiO2 NTs: (a) wide survey, (b) Ti 2p, (c) O 1s, (d) Fe 2p, and (e) S 2p.

density at the under-illumination or no illumination condition further demonstrates the excellent photocatalytic CO2 reduction of FeS2/TiO2 NTs (Figure 4c). The existence of the p−n heterojunction improved the separation efficiency of photogenerated electrons and holes, resulting in a higher activity of PC reduction of CO2. Therefore, these results as well as the Eg value prove that FeS2/TiO2 NTs own excellent PC and EC reduction performances, exhibiting promising PEC reduction of CO2. Using GC, the reaction products on FeS2/TiO2 NTs were detected and methanol was found to be the predominant product. As illustrated in Figure 5a, under different applied voltages (versus SCE), the methanol yield continued to increase with increasing visible light irradiation. The highest

and hydrogen evolution reaction (HEC). Without visible light, the current density in CO2-saturated solution was higher than that in a N2 atmosphere (Figure 4b), suggesting that FeS2/TiO2 NTs possessed excellent electrochemical CO2 reduction ability. Under a CO2 atmosphere, a significant increase of the current density was observed under visible light irradiation compared to that of no illumination (Figure 4b), showing the excellent PC reduction ability of FeS2/TiO2 NTs. The photocurrent and dark current on the FeS2/TiO2 NTs in a CO2 atmosphere are shown in Figure 4c. It can be seen that the photocurrent of TiO2 is almost unchanged. This is because the band gap of TiO2 is too wide and can only be excited by UV light. However, it is hard to be excited by a xenon lamp light source (after filtering out UV light). The significant change of the current D


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Figure 4. (a) EIS of the prepared TiO2NTs and FeS2/TiO2NTs, (b) LSV of the prepared FeS2/TiO2 NTs under different conditions, and (c) amperometric current density with time change curve with or without illumination of simulated sunlight. Figure 5. (a) Variations of the methanol concentration with time under different potentials, (b) current transformed efficiency of TiO2 NTs and FeS2/TiO2 NTs during PEC reduction of CO2, and (c) methanol concentration at varying times for different catalysts under −1.2 V PEC CO2 reduction.

methanol yield was 91.7 μmol h−1 L−1 achieved at −1.2 V extra voltage. The dependence of the methanol yield upon applied voltage was in a parabolic form, increasing to the maximum at −1.2 V and thereafter decreasing. Figure 5b shows the current transformed efficiency curves of TiO2 NTs and FeS2/TiO2 NTs. A slight visible light absorption occurred on TiO2 NTs with a red shift as a result of trace carbon from the precursors of TiO2 NTs at calcination. The efficiency of TiO2 NTs for the conversion of CO2 to methanol was only 0.27%. While the efficiency peak on FeS2/TiO2 NTs reached 39.8% at an extra voltage of −1.2 V (Figure 5b). This interesting phenomenon could be due to the competitive CO2 reduction and hydrogen evolution reaction (HER). In the range from −1.0 to −1.2 V, the main reaction was the CO2 reaction, with little HER. With the negative potential increase, the methanol yield increased accordingly, indicating that the CO2 PEC reduction was

strengthened gradually and reached the peak yield at −1.2 V. The HER took place as the main reaction, while the CO2 reduction decreased after −1.2 V. Thus, the reduction efficiency of CO2 reached a maximum under −1.2 V at the same time with the methanol yield. Figure 5c shows the methanol yields on different catalysts with time under visible light irradiation at the optimized potential (−1.2 V). As seen, the methanol concentration on FeS2/TiO2 NTs after 6 h of reaction was 5.7 and 4.9 times as high as that of FeS2 and TiO2 NTs, respectively. E


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The improved efficiency of PEC reduction of CO2 on FeS2/ TiO2 NTs is closely related to its energy band matching. The energy gap of FeS2/TiO2 NTs is 1.7 eV, which means that they can strongly absorb visible light and be excited by sunlight below 730 nm. Meanwhile, the FeS2/TiO2 NTs have a more positive valence band (1.99−2.09 eV) compared to the oxidation potential of H2O/O2 (0.82 V). This means that the targeted catalyst has enough ability to oxidize H2O and supply plenty of protons for CO2 reduction. From this investigation and previous reports, the mechanism of PEC reduction of CO2 is proposed and shown in Scheme 2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongqi Sun: 0000-0003-0907-5626 Shaobin Wang: 0000-0002-1751-9162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant 21203114), the Natural Science Foundation of Shandong Province (Grant ZR2017MB018), and the Planning Project of Science and Technology in Colleges of Shandong Province (Grant J14LC16) and partially supported by the Australian Research Council (DP170104264). The authors are also grateful to the support from the Shandong Jingbo Holdings Corporation.

Scheme 2. Mechanism of PEC Reduction of CO2

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REFERENCES

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EIS of TiO2 NTs and FeS2/TiO2 NTs shows the resistance at 1.9 and 25.0 kΩ, respectively, indicating that the introduction of FeS2 increased the interfacial charge transfer, facilitated the charge separation, and refrained the electron/hole recombination. The Mott−Schottky plot (Figure 2d) showed two space charge regions: the low-potential p type and the high-potential n type. The total capacity presented the characteristics of a ptype semiconductor (the tangent line slope is negative). When the potential shifted negatively, a p-type semiconductor transformed to the gathering state from the exhausted state and a n-type semiconductor turned to the exhausted state. At this time, the total capacity presented the characteristics of a ntype semiconductor (the tangent line slope is positive). It indicated that the as-prepared hybrid had a p−n heterojunction structure,24 which further improved the interfacial charge transfer. The PC activity and the photocurrent density of FeS2/TiO2 NTs were relatively low. In comparison to the PC system, PEC has excellent advantages because the p−n heterojunction and the synergistic effect of photo- and electroreduction lead to high PEC efficiency.

4. CONCLUSION Worm-like FeS2/TiO2 NTs were prepared through a hydrothermal method, and they exhibited an improved visible light absorption, a narrowed energy band gap, and a reduced electrical resistance. It also presented an efficient PEC performance in CO2 reduction to methanol, achieving a methanol yield at 91.7 μmol h−1 L−1 at −1.2 V under visible light. This low-cost and efficient photoelectrocatalyst provides significant applications in PEC reduction of CO2 to methanol. F


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TiO2 Nanotubes for Photoelectrocatalytic Reduction

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