CoO

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Boosting Electrocatalytic Hydrogen-Evolving Activity of Co/CoO Heterostructured Nanosheets via Coupling Photogenerated Carriers with Photothermy Zhaojun Sun,† Yu Liang,† Yongmeng Wu,† Yifu Yu,*,† and Bin Zhang*,†,‡

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†

Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University, No. 135 Yaguan Road, Haihe Education Park, Jinnan District, Tianjin 300354, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, No. 92 Weijin Road, Nankai District, Tianjin 300072, China S Supporting Information *

ABSTRACT: Electrocatalytic hydrogen evolution from water splitting holds great promise for renewable energy conversion and usage, but its application is limited by high energy consumption. The development of a facile strategy to efficiently improve the efficiency of energy conversion and the sluggish reaction kinetics using low-cost and stable electrocatalysts is crucial but still highly challenging. Recently, light irradiation is demonstrated to be an efficient external driving force for improving the hydrogen evolution reaction (HER) activities of electrocatalysts. The enhancement of activities arise from either light-excited hot electrons/carriers or photothermy, while the integrating of two action mechanisms is rarely reported. Herein, we present a synergetic effect between light-excited carriers and photothermy to enhance electrocatalytic HER activities of a Co/CoO heterostructured ultrathin nanosheet array supported on Ni foam (denoted as Co/CoO-NF). After exposure to light irradiation, the overpotential at 10 mA cm−2 decreased from 232 mV (dark) to 140 mV (light), and the Tafel slope decreased from 151 mV dec−1 (dark) to 85 mV dec−1 (light) for Co/CoO-NF. The coupling effect between photogenerated carriers and photothermy is demonstrated for the improvement of electrocatalytic activities through a series of characterizations, revealing a new avenue for developing a novel electrocatalytic system with high efficiency of energy conversion. KEYWORDS: Photogenerated carriers, Photothermy, Synergetic effect, HER, Co/CoO nanosheets

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boosted HER performance of Ni.20 The other mechanism is that solar energy is absorbed and converted into thermal energy to increase the temperature of catalysts and improve sluggish kinetics, thus enhancing HER acticity.21,22 Two kinds of action mechanisms both demonstrated the positive effect of light irradiation on electrocatalysis, indicating a promising strategy for increasing the conversion efficiency from electric energy to chemical energy through the utilization of sustainable solar energy.23 However, present research has only been focused on the standalone function of incident light, and integrating light-excited hot electrons/carriers and photothermy to improve electrocatalytic performance is highly challenged. Herein, a Co/CoO heterostructured nanosheet array supported on Ni foam (denoted as Co/CoO-NF) is prepared. After exposure to light irradiation, the HER activities of Co/

INTRODUCTION Electrocatalytic hydrogen evolution is considered as a promising alternative energy source to fossil fuels.1,2 Precious metals show excellent electrocatalytic performance for hydrogen evolution reaction (HER), but the rarity and their high price hinder their practical application.3 In the past decades, lots of earth-abundant and inexpensive alternatives have been developed to replace noble metal-based catalysts, including transition metal carbides,4 oxides,5,6 chalcogenides,7−11 selenides,12 phosphides,13−15 metal alloys,16 and perovskite oxides.17 Although considerable achievement has been made, the efficiency of overall energy conversion still needs to be improved. Recently, it has been found that light irradiation can improve the electrocatalytic performance through two kinds of mechanisms.18−22 One is that incident light can excite hot electrons and photogenerated carriers, both of which can modulate the electron density of active sites, thus optimizing HER activities.18−20 For example, plasmon-excited hot electrons over gold nanostructures can facilitate HER activity of MoS2.19 Photogenerated carriers over a NiO semiconductor © XXXX American Chemical Society

Received: June 7, 2018 Revised: July 18, 2018 Published: August 2, 2018 A

DOI: 10.1021/acssuschemeng.8b02676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

In the XRD pattern (Figure 2a), the diffraction peaks of Co/ CoO powders at 2θ = 44.2°, 51.5°, and 75.8° can be assigned

CoO-NF can be efficiently improved (an overpotential decrease of 92 mV for the benchmark current density of 10 mA cm−2 and a decrease of 85 mV dec−1 for Tafel slope). Through a series of characterizations, a synergetic effect between photogenerated carriers and photothermy is demonstrated for the improvement of electrocatalytic activities.

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DISCUSSION Self-template conversion of a Co(OH)2 nanosheet is adopted to prepare the Co/CoO nanosheet (Figure 1a). The Co(OH)2

Figure 2. (a) XRD patterns of the Co/CoO powder and Co/CoO500°C powder. (b) HRTEM image of Co/CoO-NF-500°C. XPS spectra of (c) Co 2p core level and (d) O 1s core level in the sample of Co/CoO-NF.

to the (111), (200), and (220) planes of Co (JCPDs No. 150806). Note: To exclude the effect of a Ni foam substrate on a XRD pattern, the sample for the XRD test is in powder form (see details in the Supporting Information). CoO was not detected in the XRD pattern, which may be due to the presence of the amorphous domain observed from the HRTEM image (Figure 1e). To identify the amorphous species in the nanosheet, the Co/CoO powder and Co/CoONF were further calcined at 500 °C under a high-purity Ar atmosphere to improve their crystallinity. Notably, three diffraction peaks at 36.5°, 42.4°, and 61.5° corresponding to CoO (JCPDS No. 43-1004) appeared in the Co/CoO-500°C powder (Figure 2a). Furthermore, the HRTEM image (Figure 2b) of Co/CoO-NF-500°C showed the fringe spacing of 0.21 and 0.20 nm, corresponding to CoO (200) and Co (111), respectively. In the Co 2p XPS spectrum of Co/CoO-NF (Figure 2c), two peaks at 781.4 and 797.6 eV with two satellite peaks at 786.2 and 803.4 eV confirmed the presence of Co2+ in the nanosheets.24−26 The peaks at 779.3 and 794.6 eV in Co 2p spectrum could be assigned to metallic Co0.27 Moreover, the O 1s XPS spectrum contained two peaks centered at 529.1 and 531.3 eV were attributed to surface lattice oxygen and surface adsorption oxygen, respectively, further demonstrating the existence of cobalt-based oxide (Figure 2d).28 All the aforementioned results demonstrated the successful preparation of a Co/CoO heterostructured nanosheet array with an amorphous CoO domain and crystallographic Co region. To gain more insight into the response of the Co/CoO-NF electrode to light, UV−vis-infrared absorption and infrared thermography were adopted. As shown in Figure 3a, the sample of Co/CoO-NF exhibited a strong absorption at the whole ultraviolet, visible, and infrared region. Notably, there is an excitation absorption at the ultraviolet region, indicating the formation of photogenerated carriers by CoO when the sample is irradiated. To investigate the photothermy of Co/CoO-NF, incident light was shined on Co/CoO-NF, and the infrared camera was used to probe the temperature of the samples in water. As displayed in the time-dependent temperature curve

Figure 1. (a) Schematic illustration of the preparation of Co/CoONF. (b) Low-magnification SEM image, (c) SEM image, (d) TEM image, and (e) HRTEM image of Co/CoO-NF.

nanosheet array supported on a Ni foam (denoted as Co(OH)2-NF) precursor was obtained by a hydrothermal method (Figure S1a,b). The subsequent calcination treatment enabled the conversion of Co(OH)2-NF into Co/CoO-NF. First, the calcination in air induced the decomposition of Co(OH)2-NF into a Co3O4 nanosheet array supported on Ni foam (Co3O4−NF), as shown in Figures S1c−f and S2. Then, the as-obtained Co3O4-NF was further annealed under the H2/ Ar (containing 3 vol % H2) atmosphere to produce Co/CoONF. The as-obtained Co/CoO-NF after two-step calcinations was systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The SEM images in Figure 1b and c show that crossed nanosheet array vertically grown on the Ni foam can be maintained well after a two-step annealing treatment. As displayed in Figure 1d, a typical TEM image verified the two-dimensional morphology of Co/CoO, which is peeled off from Co/CoO-NF. The HRTEM image (Figure 1e) showed that the nanosheet was composed of an amorphous domain and crystallographic region. It can be clearly seen that the domains marked by a yellow dotted line match well with the (111) lattice plane of metal Co (JCPDS No. 15-0806). This indicates the in-plane heterogeneous structure in as-obtained Co/CoO nanosheets. B

DOI: 10.1021/acssuschemeng.8b02676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 3. (a) UV−vis-infrared absorption spectra of Co/CoO-NF. (b) Time-dependent temperature curve of Co/CoO-NF in the water under the irradiation of light. Inset shows infrared images of Co/CoO-NF in water before irradiation (left) and after irradiation for 25 min (right).

Figure 4. (a) iR-corrected HER polarization curves and (b) Tafel plots of Co/CoO-NF under the conditions of darkness, irradiation, and water bath (48 °C) in 1 M KOH electrolyte. (c) iR-corrected HER polarization curves with different wavelengths of irradiation: (1) without irradiation, (2) 550 nm, (3) 500 nm, (4) 450 nm, (5) 400 nm, (6) 365 nm, and (7) full wavelength. (d) J−t curve of Co/CoO-NF electrode at a potential of −0.29 V vs RHE for HER under intermittent illumination. (e) Time-dependent current density curves of Co/CoO-NF without and with irradiation at −0.23 V vs RHE for HER. (f) Schematic illustration of the synergetic effect between photogenerated carriers and photothermy for enhancing electrocatalytic HER over Co/CoO-NF.

under light irradiation (Figure 3b), the temperature of Co/ CoO-NF quickly reached to 40 °C in 2 min. Then, it took 20 min to increase the temperature from 40 to 48 °C. Finally, the temperature of Co/CoO-NF is maintained at 48 °C. The quick temperature increase at the initial stage revealed the high efficiency from light energy to thermal energy over the sample of Co/CoO-NF. The slow increase in temperature after 40 °C can be attributed to the heat transfer from Co/CoO-NF to water. The results of UV−vis absorption and infrared thermography proved that incident light can not only excite

CoO to generate electron/hole pairs but can also heat Co/ CoO-NF to 48 °C in water. The HER activities of the Co/CoO-NF electrocatalyst were performed using a three-electrode setup with Co/CoO-NF as the cathode electrode, a graphite rod as the counter electrode, and a Hg/HgO electrode as the reference electrode in a 1 M KOH solution at a scan rate of 10 mV s−1. As shown in Figure 4a, Co/CoO-NF required an overpotential of 232 mV to achieve a current density of 10 mA cm−2 without illumination, denoted as Co/CoO-NF-(dark). After being exposed to the C

DOI: 10.1021/acssuschemeng.8b02676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

surface of catalysts can disturb the double layer, facilitate the reaction kinetics, and improve desorption of hydrogen from active sites. On the other hand, the photogenerated electron transferred from the excited CoO can increase the electronic density of the HER-active Co and thus improve HER performance. Moreover, the synergetic-effect-induced enhancement for HER showed excellent stability. We believe that the synergetic-effect-induced enhancement can not only enrich the HER catalysts but can also pave a new avenue for diverse applications (oxygen evolution reaction, CO2 reduction, electrosynthesis, etc.).

illumination at room temperature, denoted as Co/CoO-NF(light), the electrode only needed an overpotential of 140 mV to achieve a current density of 10 mA cm−2, which is competitive for the reported electrocatalyst (Table S1). When light irradiation was replaced by a water bath of 48 °C, denoted as Co/CoO-NF-(48°C), the overpotential was 181 mV for a current density of 10 mA cm−2. Tafel plots were also performed in the same activity order: Co/CoO-NF-(light) > Co/CoO-NF-(48°C) > Co/CoO-NF-(dark) (Figure 4b). The activity difference between Co/CoO-NF-(48°C) and Co/ CoO-NF-(dark) revealed that heat did enhance electrocatalytic HER performance through disturbing the double layer, facilitating the reaction kinetics, and improving the desorption of gaseous products.29−31 On the other hand, the activity difference between Co/CoO-NF-(light) and Co/CoO-NF(48°C) indicated that the effect of light irradiation was not limited to photothermy. To reveal the reason for enhanced electrocatalytic activities of Co/CoO-NF-(light) than Co/ CoO-NF-(48°C), monochromatic light and chopped illumination were applied to the Co/CoO-NF electrode. As for monochromatic incident light, it is known that the shorter wavelength can motivate more photogenerated carriers. The HER activity increased with the decrease in wavelength (Figure 4c), indicating the effect of photogenerated carriers on HER performance. The response of a Co/CoO-NF electrode to intermittent irradiation was fast (Figure 4d). When the illumination was applied to the electrode, the current density obviously increased. When the illumination was turned off, the current density dropped rapidly. These results clarified that the photogenerated electron transferred from excited CoO can enrich electrons of HER-active Co and thus improve HER performance.20 Therefore, a synergetic effect between photogenerated carriers and photothermy is demonstrated for the improvement of electrocatalytic HER activities (Figure 4f). In addition, the HER performance of Co/CoONF-(light) can be maintained well, indicating the high durability of the irradiation-driven enhancement of electrocatalytic activity (Figure 4e). SEM images clearly show that the morphology and structure of Co/CoO-NF can be well maintained after electrocatalysis, indicating its good mechanical stability during electrocatalysis process (Figure S3a,b). However, the chemical states of cobalt present changes as confirmed by XPS results (Figure S3c,d). After deconvolution of the peak of Co 2p, the ratio of peak areas between metallic Co and Co 2+ is 0.72 and 2.13 for Co/CoO-NF before and after stability test, respectively (Figure S3c). The surface lattice oxygen is consumed during the HER process (Figure S 3d). Therefore, the reason for the gradual equilibrium of Co/CoONF-(light) in the HER performance could be ascribed to the partial reduction from Co2+ to metallic Co. Notably, the final sample of Co/CoO-NF after the stability test is also composed of Co and CoO, guaranteeing the stability of the catalysts after a long-time reaction (Figure 4e).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02676. Experimental details, material characterizations, electrochemical measurements, and additional characterizations and measurements. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (B.Z.). ORCID

Bin Zhang: 0000-0003-0542-1819 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 (Grant No. 21422104) and the Natural Science Foundation of Tianjin City (Grants No. 17JCJQJC44700 and No. 16JCZDJC30600).

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CONCLUSIONS In summary, the Co/CoO-NF was constructed to demonstrate a synergetic effect strategy between photogenerated carriers and photothermy for enhancing electrocatalytic HER activities. Under irradiation, HER activities can be significantly improved (92 mV decrease of overpotential at 10 mA cm−2 and 85 mV dec−1 decrease of Tafel slope). On the one hand, the Co/CoONF absorbed incident light and converted solar energy to thermal energy. The local high temperature (48 °C) on the D

DOI: 10.1021/acssuschemeng.8b02676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.8b02676 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX