Letter pubs.acs.org/acscatalysis
Rationally Designed/Constructed CoOx/WO3 Anode for Efficient Photoelectrochemical Water Oxidation Jingwei Huang,† Yan Zhang,† and Yong Ding*,†,‡ †
State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China S Supporting Information *
ABSTRACT: Water oxidation catalysts have been usually used to enhance the charge injection efficiency of WO3 anodes. Herein, we simultaneously improve the charge separation efficiency and charge injection efficiency, as well as the oxidation selectivity of WO3 for photoelectrochemical water oxidation by loading CoOx nanoparticles in a neutral electrolyte. The produced oxygen is monitored by a Clark electrode, which shows a high Faradaic efficiency on CoOx/ WO3. In an effort to gain insight into the role of CoOx on the high Faradaic efficiency, the rotating ring−disk electrode system was used to detect the products of the CoOx/WO3 heterojunction in the photoelectrochemical water oxidation process. The results show that the loading of CoOx nanoparticles can improve oxidation selectivity of WO3 and therefore improve the Faradaic efficiency of WO3. KEYWORDS: photoelectrochemical water oxidation, tungsten oxide, cobalt oxide, p−n junction, rotating ring−disk electrode
P
Water oxidation catalyst is usually integrated to the anode semiconductor, to reduce the activation energy13,14 or improve oxidation selectivity.15 A rational choice of catalyst will play the role of “one stone, two birds”. That is, on the one hand, the reaction activation energy will be reduced by the water oxidation catalyst. On the other hand, the formation of heterojunctions (such as the p−n junction) between catalyst and semiconductor will induce the formation of an internal built-in electric field directing from an n-type semiconductor (light absorber) toward the p-type semiconductor catalyst. This built-in electric field is reported to promote charge separation in the process of PEC water splitting.15,16 In this work, CoOx nanoparticles (which are p-type semiconductors and are known to be excellent water oxidation catalysts)17−19 was loaded on WO3 nanoflakes to construct CoOx/WO3 p−n heterojunction anode. With CoOx loading, the performance of WO3 was improved greatly. A rotating ring−disk electrode (RRDE) system then was used to detect the products of PEC water splitting in situ, which showed a promoted oxidation selectivity by loading CoOx and gave a direct evidence for the improved Faradaic efficiency of WO3. The WO3 nanoflake electrode was prepared by a previously reported method (see the Supporting Information (SI)I for experimental details).20 The obtained nanoflakes with a height
hotoelectrochemical (PEC) water splitting into hydrogen and oxygen using semiconductors as photoelectrodes has been widely regarded as a promising approach to convert solar energy directly into fuel.1−3 Oxygen evolution at the photoanode, which requires the transfer of four electrons and the formation of O−O bonds, is usually considered to be kineticscontrolling step.4 So far, many works have focused on the fabrication of high-performance photoanodes.5 An n-type WO3 with a band gap of ∼2.6 eV is an attractive candidate for a photoanode, because of its quite positive valence band edge (3 V versus NHE),6,7 which provides enough overpotential to evolve O2. Furthermore, it has the advantage of good electron transport properties (∼12 cm2 V−1 s−1).8 The WO3 anode is known to be unstable in alkaline electrolytes. Furthermore, researchers previously discovered that acidic anions (Cl−, SO4−, and ClO4−) themselves in acid electrolytes (HCl, H2SO4, and HClO4) are oxidized rather than water oxidation by the WO3 anode.9,10 More efforts have been devoted to balance the above contradiction in neutral electrolytes for WO3.11,12 Kyoung-Shin Choi et al. reported that O2 evolution and the formation of peroxo species can occur in the phosphate solution and the photocurrent-tooxygen conversion efficiency increases gradually as the pH increases.10 The unfavorable H2O2 formation competes with oxygen evolution in the neutral electrolyte. Therefore, besides the promotion of kinetics, it is necessary to improve the oxidation selectivity for WO3 in neutral electrolytes and to quantitatively analyze the anode’s oxidation products. © 2017 American Chemical Society
Received: January 4, 2017 Revised: February 3, 2017 Published: February 3, 2017 1841
DOI: 10.1021/acscatal.7b00022 ACS Catal. 2017, 7, 1841−1845
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ACS Catalysis of ca. 2 μm grow vertically on a fluorine-doped tin oxide (FTO) substrate and interconnect with each other (see Figures 1A and
Figure 2. Current−potential curves of CoOx/WO3 and WO3 under AM 1.5 global irradiation in 0.1 M KPi buffer (pH 7) (A) without and (B) with 1 M Na2SO3 at a scan rate of 10 mV/s. Dark current is shown as a dashed line. (C) Calculated charge separation efficiency and (D) charge injection efficiency of CoOx/WO3 and WO3.
performance of this system with other semiconductors loaded with CoOx is presented in Table S2 in the SI. The efficiency of an electrode in PEC water splitting is determined by the light absorption efficiency (ηabsorption), charge separation efficiency (ηseparation), and charge injection efficiency (ηinjection). To get insight into the promotion of the PEC water oxidation activity of WO3 by CoOx, the ηseparation and ηinjection of WO3 and CoOx/WO3 were calculated for analysis. First, 1 M Na2SO3 was added into the electrolyte as a hole scavenger to evaluate the photogenerated holes that reach the electrode/ electrolyte interface. As shown in Figure 2B, the photocurrent of CoOx/WO3 for Na2SO3 oxidation (JNa2SO3) is larger than that of WO3, indicating more photogenerated holes arrive at the CoOx/WO3/electrolyte interface. ηseparation is calculated by dividing JNa2SO3 by Jabs (the photocurrent density achievable, assuming 100% absorbed photon-to-current conversion efficiency for photons; the calculation details are available in Figure S8 in the SI). A higher ηseparation for CoOx/WO3 was obtained, as shown in Figure 2C. This can be ascribed to the formation of p−n heterojunctions between CoOx and WO3 (Figure S9 in the SI), in which an internal built-in electric field directed from WO3 toward CoOx. Under the conditions of PEC water oxidation, a reverse bias was applied on the p−n heterojunctions, leading to the enhancement of the internal built-in electric field. The photogenerated holes migrate to CoOx along the internal built-in electric field, while the photogenerated electrons transfer in the opposite direction. This leads to suppressed recombination of photogenerated electron−hole pairs, as proved by photoluminescence (PL) spectra (Figure S10 in the SI). It is known that a built-in electric field (or space charge) directed from the interior to the exterior of WO3 also exists near its surface when it is exposed to electrolytes.22 However, this space charge width is calculated to be only 1.6 nm (Figure S11 in the SI), which is far less than the thickness of the WO3 nanoflakes (Figure S12 in the SI). That is, this already existing space charge plays a fairly limited role in separating the photogenerated electrons and holes. ηinjection represents the yield of holes that are injected into the electrolyte to oxidize the water. It is calculated by dividing the photocurrent of water oxidation (Figure 2A) by JNa2SO3. The
Figure 1. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive spectroscopy (EDS) characterizations. (A) Top and (B) cross-sectional SEM images of WO3 nanoflake grown on an FTO substrate. TEM images of (C) WO3 and (D) CoOx/WO3. Inset in panel (D) shows a high-resolution transmission electron microscopy (HR-TEM) photomicrograph of CoOx. (E−H) EDS mapping images of the CoOx/WO3.
1B). The mass of WO3 nanoflakes was determined to be ca. 0.35 mg/(cm2 FTO) by scraping it off from the FTO substrates, using eight samples. Via a hydrothermal deposition method (see the SI for experimental details), CoOx nanoparticles 5 ± 1 nm in size were loaded on WO3 nanoflakes (see Figure S1 in the SI, as well as Figure 1D). The integration of WO3 and CoOx leads to the formation of p−n heterojunctions between them.21 X-ray diffraction (XRD) (Figure S2 in the SI), X-ray photoelectron spectroscopy (XPS) (Figure S3 in the SI), and energy-dispersive X-ray (EDX) (Figures S4 and S5 in the SI) analyses confirm the formation of CoOx nanoparticles. ICP−AES measurement gives a CoOx loading amount of 1.5 μg/(cm2 FTO). With the loading of CoOx nanoparticles, the microstructure of WO3 nanoflakes remains unchanged (see Figures S6 and S7 in the SI). Element mapping images reflect that CoOx nanoparticles are distributed evenly on the surface of WO3 nanoflakes (Figures 1E−H). The activities of WO3 and CoOx/WO3 for PEC water oxidation were examined by measuring the current−potential curves in 0.1 M KPi buffer (pH 7) under simulated air mass 1.5 global (AM 1.5G) irradiation at 100 mW cm−2 (Figure 2A). The loading of CoOx nanoparticles significantly increases the photocurrent of WO3 over the entire potential range. The photocurrent density of CoOx/WO3 at 1.23 V vs RHE (the water oxidation potential) reaches 1.55 mA/cm2, which is 40% of its theoretical maximum photocurrent,5 and almost twice the photocurrent density of WO3 (0.80 mA/cm2). This is among the biggest enhancement of photocurrent by loading catalysts on WO3 in the reported literatures (Table S1 in the SI). Furthermore, a cathodic shift of the onset potential from 0.70 V to 0.55 V can be observed, indicating the positive catalytic water oxidation effect of CoOx. The comparison of the 1842
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ACS Catalysis results show that the ηinjection value of CoOx/WO3 reaches 80% at 1.2 V vs RHE, which is 1.6 times greater than the efficiency of WO3. Because of the excellent catalytic property of CoOx and the formation of p−n heterojunctions between CoOx and WO3, the ηinjection and ηseparation values of CoOx/WO3 are respectively enhanced. As a consequence, the photocurrent is improved by loading CoOx nanoparticles on WO3 nanoflakes. The strategies to improve ηseparation and ηinjection always need different methods. Manipulating the morphology23 and constructing heterojunctions24,25 are the most commonly used methods to improve ηseparation, while catalyst loading26 is the main way to promote ηinjection. Here, we achieved both goals by simply loading CoOx nanoparticles. The produced oxygen is quantified by a sensitive Clark electrode. Before the experiment, the oxygen in both the liquid phase and the headspace of the reaction flask is removed by bubbling Ar gas and the electrolyte is sealed by a layer of cyclohexane to prohibit the diffusion of produced oxygen from electrolyte to headspace. As shown in Figure 3, CoOx/WO3
Figure 4. Chopped current−potential curves of CoOx/WO3 and WO3 used to study the H2O2 formation by the RRDE system in 0.1 M KPi buffer (pH 7) under the illumination of xenon lamp. The ring currents were observed with the ring potential kept at 1.6 V vs RHE. Rotation rate = 1500 rpm. The ring current data were normalized to have the same dark current.
CoOx/WO3 is lower than that of WO3, indicating that the formation of H2O2 is obviously suppressed by loading CoOx on WO3, that is, CoOx improves the oxidation selectivity of WO3. To investigate whether the unsuppressed H2O2 originates from CoOx nanoparticles, the products of RRDE modified with CoOx nanoparticles in electrochemical water oxidation was analyzed since CoOx catalyst shows the same catalytic behavior in electrochemical and PEC water oxidation. In the process of electrochemical water oxidation, a high valence state of Co in CoOx forms (Figure S16B in the SI) and is responsible for water oxidation, as reported by Liu et al.28 Similarly, the valence of Co in CoOx/WO3 changes to a high state at the potential range of 0.6−0.8 V vs RHE (this oxidation peak is very clear in the initial LSV scan) in PEC water oxidation (Figure 2A and Figure S17 in the SI). This phenomenon also occurs in PEC water oxidation catalyzed by Co-Pi, in which the valence state of Co recycles from +2 to +4 for collecting photogenerated holes and backs to +2 again after oxidizing water.29 As shown in Figure S16 in the SI, the ring current is almost unchanged, even above 1.776 V vs RHE, at which H2O could be oxidized to H2O2. This confirms that H2O2 does not form on CoOx nanoparticles in the electrochemical water oxidation process, i.e., no H2O2 will form on CoOx nanoparticles in PEC water oxidation. The electron−transfer number (n) and yield of H2O2 were then calculated based on the current−potential curves (Figure S18 in the SI), according to the following equations (the derivation process is shown in the SI):
Figure 3. Generated oxygen monitored by a Clark electrode in the PEC experiment. The photocurrent is controlled at 0.8 mA.
electrode produces more oxygen than that of a bare WO3 one under identical conditions. The Faradaic efficiencies of WO3 and CoOx/WO3 electrode are calculated to be 71.5% and 92.1%, respectively. To shed light on the improved Faradaic efficiency by loading CoOx nanoparticles, the RRDE system was used to detect the products of PEC water splitting (Figure S13 in the SI). The collection efficiency of the RRDE was measured to be 0.353 (see Figure S14 and Table S3 in the SI). The disk was scanned from 0.6 V to 1.8 V (vs RHE) at a scan rate of 10 mV s−1 to photoelectrochemically oxidize H2O under the chopped illumination of a xenon lamp. Therefore, the disk current represents the oxidation of H2O. In this process, water can be oxidized to O2 (four electrons transfer product) and H2O2 (two electrons transfer product). The latter is unwanted for WO3, as mentioned earlier, although the oxidation of H2O to O2 can be realized by a 2e− /2e− two-step pathway, using other materials.27 The potential of the ring was kept at 1.6 V vs RHE, at which the formed H2O2 can be oxidized rapidly (Figure S15 in the SI). Therefore, the value of ring current represents the amount of H2O2 generated on the disk electrode. As demonstrated in Figure 4, CoOx/WO3 shows higher disk photocurrent than that of WO3. Despite this, the ring current of
n=
4(ID,light − ID,baseline) (ID,light − ID,baseline) +
IR,light − IR,dark
(1)
N
H 2O2 yield (%) =
2(IR,light − IR,dark ) N (ID,light − ID,baseline) + (IR,light − IR,dark )
(2)
where ID,light is the disk current with illumination, ID,baseline is the baseline of the disk current without illumination, and IR,light and 1843
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IR,dark are the ring currents with and without illumination, respectively. N is the collection efficiency of the RRDE. As demonstrated in Figure 5, WO3 has an electron transfer number of 3.61−3.96 from 0.8 V to 1.8 V, while CoOx/WO3
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00022. Experimental details, materials characterization, and supporting data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yong Ding: 0000-0002-5329-8088 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 21173105 and 21572084), Fundamental Research Funds for the Central Universities (Nos. lzujbky-2016-k08 and lzujbky-2016-210) and the Natural Science Foundation of Gansu (No. 1506RJZA224).
Figure 5. Electron transfer number and H2O2 yield, based on RRDE measurements.
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has an electron transfer number of 3.81−3.98 in the same potential range. In the previous literatures,30,31 the electrodes having a calculated electron transfer number within the range of 3.85−3.96 are regarded to favor a four-electron transfer reaction. By loading CoOx nanoparticles, the electron transfer number of WO3 is improved to the values that are considered to be the four-electron transfer reaction. Meanwhile, the yield of H2O2 decreases from 16% to 5% at 1.23 V vs RHE by loading CoOx nanoparticles on WO3. Note that the yield of H2O2 at high potential is smaller than that at low potential. This can be ascribed to the predominant electrochemical water oxidation reaction at high potential. Incident photon-to-current conversion efficiency (IPCE) spectra show higher IPCE values of CoOx/WO3 electrode than that of the WO 3 electrode (Figure S19 in the SI), demonstrating that the formation of p−n junctions facilitates the separation of photoexcited electrons and holes. Finally, the stability of the electrodes was investigated (Figure S20 in the SI). CoOx nanoparticles can suppress the rapid decay photocurrent of WO3 photoanodes. However, more protection measures should be taken in future research to enhance the stability of WO3 photoanodes in neutral electrolytes. In summary, we have shown that the formation of p−n heterojunctions in CoOx/WO3 can simultaneously improve the charge separation efficiency and charge injection efficiency of WO3 for photoelectrochemical water oxidation. Furthermore, for the first time, the rotating ring−disk electrode (RRDE) system was used to disclose the suppression of H2O2 formation by loading CoOx on WO3, i.e., CoOx nanoparticles improve the oxidation selectivity of WO3. This gives direct evidence that the improved Faradaic efficiency of the CoOx/WO3 electrode (up to 92.1%) can be ascribed to the improved oxidation selectivity. Our work gives a “one stone, two birds” strategy to load a water oxidation catalyst on the photoanode and the study of other p− n junction composites is underway. This RRDE system technique can also be applied to other metal oxide photoanodes that are capable of H2O2 formation in water oxidation.
REFERENCES
(1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (2) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Chem. Rev. 2014, 114, 9987−10043. (3) Hisatomi, T.; Kubota, J.; Domen, K. Chem. Soc. Rev. 2014, 43, 7520−7535. (4) Tang, J.; Durrant, J. R.; Klug, D. R. J. Am. Chem. Soc. 2008, 130, 13885−13891. (5) Li, J.; Wu, N. Catal. Sci. Technol. 2015, 5, 1360−1384. (6) Bignozzi, C. A.; Caramori, S.; Cristino, V.; Argazzi, R.; Meda, L.; Tacca, A. Chem. Soc. Rev. 2013, 42, 2228−2246. (7) Solarska, R.; Bienkowski, K.; Zoladek, S.; Majcher, A.; Stefaniuk, T.; Kulesza, P. J.; Augustynski, J. Angew. Chem., Int. Ed. 2014, 53, 14196−14200. (8) Berak, J. M.; Sienko, M. J. J. Solid State Chem. 1970, 2, 109−133. (9) Mi, Q.; Zhanaidarova, A.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. Energy Environ. Sci. 2012, 5, 5694−5700. (10) Hill, J. C.; Choi, K.-S. J. Phys. Chem. C 2012, 116, 7612−7620. (11) Hou, Y.; Zuo, F.; Dagg, A. P.; Liu, J.; Feng, P. Adv. Mater. 2014, 26, 5043−5049. (12) Wang, G.; Ling, Y.; Wang, H.; Yang, X.; Wang, C.; Zhang, J. Z.; Li, Y. Energy Environ. Sci. 2012, 5, 6180−6187. (13) Ran, J.; Zhang, J.; Yu, J.; Jaroniec, M.; Qiao, S. Z. Chem. Soc. Rev. 2014, 43, 7787−7812. (14) Yang, J.; Wang, D.; Han, H.; Li, C. Acc. Chem. Res. 2013, 46, 1900−1909. (15) Huang, J.; Ding, Y.; Luo, X.; Feng, Y. J. Catal. 2016, 333, 200− 206. (16) Chang, X.; Wang, T.; Zhang, P.; Zhang, J.; Li, A.; Gong, J. J. Am. Chem. Soc. 2015, 137, 8356−8359. (17) Deng, X.; Tüysüz, H. ACS Catal. 2014, 4, 3701−3714. (18) Gujral, S. S.; Simonov, A. N.; Higashi, M.; Fang, X.-Y.; Abe, R.; Spiccia, L. ACS Catal. 2016, 6, 3404−3417. (19) Chen, S.; Shen, S.; Liu, G.; Qi, Y.; Zhang, F.; Li, C. Angew. Chem., Int. Ed. 2015, 54, 3047−3051. (20) Su, J.; Feng, X.; Sloppy, J. D.; Guo, L.; Grimes, C. A. Nano Lett. 2011, 11, 203−208. (21) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices; John Wiley & Sons: Hoboken, NJ, 2007. (22) van de Krol, R.; Grätzel, M. Photoelectochemical Hydrogen Production; Springer: New York, 2012. 1844
DOI: 10.1021/acscatal.7b00022 ACS Catal. 2017, 7, 1841−1845
Letter
ACS Catalysis (23) Han, J.; Zong, X.; Wang, Z.; Li, C. Phys. Chem. Chem. Phys. 2014, 16, 23544−23548. (24) Xie, M.; Fu, X.; Jing, L.; Luan, P.; Feng, Y.; Fu, H. Adv. Energy Mater. 2014, 4, 1300995. (25) Singh, A. P.; Kodan, N.; Mehta, B. R.; Held, A.; Mayrhofer, L.; Moseler, M. ACS Catal. 2016, 6, 5311−5318. (26) Huang, J.; Hu, G.; Ding, Y.; Pang, M.; Ma, B. J. Catal. 2016, 340, 261−269. (27) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z. Science 2015, 347, 970−974. (28) Wang, H.-Y.; Hung, S.-F.; Chen, H.-Y.; Chan, T.-S.; Chen, H. M.; Liu, B. J. Am. Chem. Soc. 2016, 138, 36−39. (29) Zhong, D. K.; Choi, S.; Gamelin, D. R. J. Am. Chem. Soc. 2011, 133, 18370−18377. (30) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Nat. Mater. 2011, 10, 780−786. (31) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. J. Am. Chem. Soc. 2014, 136, 13925−13931.
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