On-chip electrocatalytic microdevice: an emerging platform for expanding the insight into electrochemical processes

Huan Yang a, Qiyuan He b, Youwen Liu *a, Huiqiao Li a, Hua Zhang *cd and Tianyou Zhai *a
aState Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China. E-mail: ywliu@hust.edu.cn; zhaity@hust.edu.cn
bDepartment of Materials Science and Engineering, City University of Hong Kong, Hong Kong, P. R. China
cDepartment of Chemistry, City University of Hong Kong, Hong Kong, P. R. China
dHong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM), City University of Hong Kong, Hong Kong, P. R. China. E-mail: Hua.Zhang@cityu.edu.hk

Received 2nd December 2019

First published on 14th April 2020


Electrochemical conversion is an important process in renewable energy conversion, and electrocatalysts play a vital role since they can improve the rate and efficiency of chemical transformations. Thus, the continuing interest in electrocatalysis is fueled both in terms of mechanism exploration and performance optimization, and this field is continuously being updated. However, conventional electrochemical methods still have room to be explored, such as in situ dynamic monitoring, external field regulation, and single-entity electrocatalytic detection. Noteworthily, inspired by the recent success in nanoelectronic semiconductor devices, the emerging field of on-chip electrocatalytic microdevices, focusing on the electrochemical behaviors at individual nanowire/nanosheet as the working electrode, has emerged as a powerful alternative platform to the traditional techniques. This unique device configuration enables several advantages, such as in situ electronic/electrochemical measurements and adjustable microstructure of individual catalysts, which is constantly expanded to directly probe electrochemical processes to obtain previously inaccessible information. Hence, herein, we first introduce the device configuration and its advantages as an emerging platform. Subsequently, the attempts to expand the insight into electrochemical processes through this type of microdevice are explicitly analyzed and summarized including dynamic monitoring, external field regulation, identification of active sites, and single structural factor regulation. Finally, some personal perspectives on the challenges and future research directions in this promising area are also presented. We believe that this review will provide new insight into electrochemical processes, ranging from dynamic exploration to performance optimization.


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Huan Yang

Huang Yang received her BS Degree in Materials Science and Engineering from Central South University in 2018. She is currently a Postgraduate researcher in materials science and engineering under the supervision of Prof. Tianyou Zhai and Dr Youwen Liu at the Huazhong University of Science and Technology, China. Her research interests include on-chip electrochemistry.

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Qiyuan He

Dr Qiyuan He is currently an Assistant Professor in City University of Hong Kong. He obtained his PhD Degree from Nanyang Technological University in Singapore in 2013. He then joined the University of California, Los Angeles as a Postdoctoral Fellow before returning to Nanyang Technological University as a Research Fellow in 2016. Dr He's research scope is highly interdisciplinary, focusing on semiconductor interfaces in various applications including nanoelectronics, iontronics, chemical/biological sensors, catalysis, and on-chip electrochemistry.

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Youwen Liu

Dr Youwen Liu received his BS degree from China University of Geosciences in 2012, and then received his PhD Degree in Inorganic Chemistry from the University of Science and Technology of China (2017). After that he joined School of Materials Science and Engineering, Huazhong University of Science and Technology as an Associate Professor. His current research areas include the design and synthesis of 2D solids, and the regulating their intrinsic properties for energy storage and conversion applications.

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Huiqiao Li

Prof. Huiqiao Li received her BS Degree in Chemistry from Zhengzhou University in 2003, and then received her PhD Degree in physical Chemistry from Fudan University in 2008. Afterward, she worked as a Postdoctoral Fellow for 4 years at the Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Japan. Currently, she is a Full Professor at the School of Materials Science and Engineering, HUST. Her research interests include energy-storage materials and electrochemical power sources, such as lithium-ion batteries, sodium-ion batteries, and supercapacitors.

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Hua Zhang

Prof. Hua Zhang obtained his PhD at Peking University in 1998. As a Postdoctoral Fellow, he joined Prof. Frans C. De Schryver's group at Katholieke Universiteit Leuven (Belgium) in 1999, and then moved to Prof. Chad A. Mirkin's group at Northwestern University in 2001. Subsequently, he worked at NanoInk Inc. (USA) and Institute of Bioengineering and Nanotechnology (Singapore), and then joined Nanyang Technological University in July 2006. Now he is the Herman Hu Chair Professor of Nanomaterials at the Department of Chemistry in City University of Hong Kong. His current research interests focus on phase engineering of nanomaterials (PEN) and controlled epitaxial growth of heterostructures, including the synthesis of 2D, unconventional crystal phase and amorphous nanomaterials, for applications in catalysis, clean energy, (opto-)electronic devices, nano- and biosensors, and water remediation.

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Tianyou Zhai

Prof. Tianyou Zhai received his BS Degree in Chemistry from Zhengzhou University in 2003, and then his PhD Degree from the Institute of Chemistry, Chinese Academy of Sciences in 2008. Afterwards, he joined the National Institute for Materials Science as a JSPS Postdoctoral Fellow and then as an ICYS-MANA Researcher in NIMS. Currently, he is the Chief Professor of the School of Materials Science and Engineering, Huazhong University of Science and Technology. His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics, and optoelectronics.


1. Introduction

Storing electrical energy on a large scale is arguably the greatest challenge in transitioning from fossil fuels to clean and sustainable energy.1,2 Energy can be efficiently stored by electrochemically converting inert molecules (e.g., water, carbon dioxide, and nitrogen) into higher value chemicals (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia).3–5 The effectiveness of these chemical conversions depends on efficient catalysts, which play a key role in energy conversion technologies. Thus, the continuing interest in electrocatalysis is fueled by mechanism studies (such as identification of active sites6,7 and reaction dynamics8–10) and performance optimization.11–13 A series of structural tuning methods including doping,14–17 alloying,18–20 defects,21–23 and size24,25 have been demonstrated to enhance the catalytic activity of catalysts. However, most conventional electrochemical methods (material measured on glassy carbon electrode/thin film) still face limitations in both exploring reaction mechanisms and performance optimization. For instance, the addition of extra fields (electric, light and magnetic), which can tune the intrinsic energy band structure of the catalyst or cooperatively change the molecule/ion distribution at the solid–electrolyte interface,26,27 may provide a new direction and opportunities for performance optimization. However, the accurate application of an extra field in conventional electrochemical methods is nearly impossible, and therefore, has rarely been explored. In addition, the in situ study of electrocatalytic interfaces is particularly informative, but extremely challenging because these interfaces are generally buried between the solid support and liquid electrolyte, and thus are difficult to access via conventional spectroscopic techniques.28 Hence, it is of considerable significance to develop novel electrochemical testing platforms as alternatives to the traditional techniques, which will expand the insight into electrocatalytic processes for designing high-performance catalysts.

Within this context, Duan and co-worker proposed and designed, for the first time, a brand new on-chip electrocatalytic microdevice for in situ monitoring the electrochemical surface conditions of metal nanocatalysts.29 This on-chip microdevice has developed to be a platform technology and demonstrates unique advantages in the direct probing of many electrocatalytic processes to obtain previously inaccessible information. Firstly, a single nanowire or nanosheet is often used as a microscopic working electrode, avoiding the influence of binders and conductive carbon additives, which are commonly used in conventional electrochemical methods. Secondly, the morphology and structure of the catalyst in the microdevice can be precisely designed and manipulated, which are essential to eliminate experimental uncertainties and allow quantitative or semi-quantitative fundamental studies. Thirdly, it is considerably easier to monitor the change in the physical properties of individual nanowire/nanosheet (such as conductivity) in a nanodevice. Therefore, in the subsequent research processes, the structural advantages of this microdevice model were constantly expanded and enriched as a powerful tool to discover novel phenomena and clarify electrochemical fundamentals at the nanoscale (Fig. 1). Specifically, our group unraveled the universal self-gating of interfacial carrier concentration of semiconductors during electrocatalytic reactions.30 This dynamic study of the electrocatalytic surface/interface provided new insight into the electronic origin of the semiconductor–electrolyte interface during electrocatalysis, paving the way for the design of high-performance semiconductor catalysts. Meanwhile, by coupling with field-effect transistors, a on-chip electrocatalytic microdevice can allow modulation of the external electric field with controllable intensity and direction by applying a back gate voltage through the dielectric on the substrate. For instance, C. Frisbie's group fabricated analogous on-chip electrocatalytic devices based on ZnO and MoS2 ultrathin films to investigate the electrocatalytic behavior of thin films under the regulation of an applied electric field.31,32 Furthermore, the individual nanowire/nanosheet on the on-chip microdevice could be precisely designed and in situ controlled, which are essential for eliminating distractions from multiple factors and allow (semi-) quantitative electrochemical studies. This unrivalled advantage allowed us to explore phase-dependent electrocatalysis in an on-chip microdevice by realizing the co-existence of different crystal phases on the same MoS2 nanosheet by laser-induced phase transition.33 Furthermore, on-chip microdevice can be used to identify the catalytically active sites by selectively exposing the area of interest on a microscopic catalyst nanosheet using e-beam lithography, which provides a direct and precise strategy to probe the spatially resolved electrocatalytic activities. Manish Chhowalla et al. systematically studied the catalytic active sites distribution of MoS2 and the effect of electron injection on its catalytic activity, which further confirmed the high activity of the edge and defect sites of MoS2.34


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Fig. 1 Development history of on-chip electrocatalytic microdevices. These figures were adapted with permission from ref. 29, Copyright (2015) Nature Publishing Group; ref. 34, Copyright (2016) Nature Publishing Group; ref. 32 and 27, Copyright (2017) American Chemical Society; ref. 42 and 33, Copyright (2017 and 2018) Nature Publishing Group, respectively; ref. 30, Copyright (2019) Nature Publishing Group; ref. 69, Copyright (2019) John Wiley and Sons; ref. 86, Copyright (2019) Chinese Chemical Society; and ref. 83, Copyright (2019) John Wiley and Sons.

Recently, on-chip electrocatalytic microdevices have been gaining increasing interest as a platform to investigate electrochemical processes at the microscopic scale, which are often inaccessible by conventional electrochemical methods. Accordingly, it is necessary to summarize and update the budding work in a timely manner to allow more steady development in this emerging research field. In this review, we first introduce the device configuration and its unique advantages as a brand new electrochemical platform. Accordingly, we provide a comprehensive summary of this field, including the device configuration, structural features and obtained new insight into electrochemical processes through this microdevice. Notably, the new electrochemical phenomena and corresponding mechanisms will be highlighted. Finally, we provide some perspectives for the future of this emerging research field.

2. Device configuration and fabrication method

As a brand-new electrochemical measurement technique, a clear understanding of the device configurations across reported electrocatalytic microdevices is an important prerequisite for recognizing the detail studies in these works. In this section, the configuration and fabrication process of electrocatalytic microdevices and their advantages in electrochemical investigation will be introduced. In addition, according to the current experiments in this field, some matters that need attention in the testing process of this type of device will also be summarized.

2.1 Device configuration

A schematic of the on-chip electrocatalytic microdevice configuration is summarized in Fig. 2a. In general, the catalytical material can be individual nanowires and two-dimensional (2D) nanosheets, as well as their thin films, which can be placed on SiO2/Si substrates by means of drop-casting, dry/wet transfer, and chemical deposition. The metal electrodes, which serve as the current collector, are then deposited on the catalyst. Specially, the metal electrodes are generally protected by an insulating polymethyl methacrylate (PMMA) layer to avoid the interference of the signal generated by their electrochemical reactions. Unlike the two-electrode setup often used in iontronics,35 the three-electrode system is adopted in electrocatalytic microdevices, in which conventional carbon rods/Pt wire and Ag/AgCl/calomel can be used as the counter electrode (CE) and reference electrode (RE), respectively. To date, most of the studies have focused on conceptually simple electrocatalytic water splitting in aqueous electrolyte systems, such as H2SO4 and KOH. The small volume of electrolyte solution is either contained in a polymer reservoir or forms a droplet on the device chip held by surface tension. Notably, the size of a single electrocatalyst is generally on the micrometre scale; therefore, less than 1 mL of electrolyte is usually needed to cover the surface of the catalyst. Accordingly, the diameter of the CE and RE need to be within the millimeter scale to ensure their full immersion in the electrolyte for testing. In addition, the catalytic current signal generated by microscopic catalysts is usually at the nA level, and therefore, it is vital to maintain a stable test environment to avoid environmental influence from dust, vibration and electromagnetic interference. Markedly, to further confirm the device reliability, a calibration is often performed on the measurement system by performing the hydrogen evolution reaction (HER) on a microscopic Pt film. The HER results are comparable to that of commercial Pt electrodes measured in conventional electrochemical cells, proving that this approach is completely feasible and practical.36
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Fig. 2 Schematic of the device configuration and fabrication method. (a) On-chip microdevice configuration based on nanowires, nanosheet and film. (b) Schematic of the individual 2D nanosheet-based on-chip electrocatalytic microdevice fabrication process.

2.2 Device fabrication method

A remarkable feature of the on-chip electrocatalytic device is that it can be precisely designed and fabricated with nanometer precision using nanofabrication methods, which is critical for the accuracy of experiments. Taking nanosheet as an example (Fig. 2b), the fabrication process of the on-chip device includes the following steps: material preparation, electrode deposition, spin coating of PMMA resist, selective exposure, and building the system. Specifically, the catalyst material (nanowire, nanosheet and film) can be placed on the substrate via various deposition method such as drop casting, transferring and chemical depositing. Then, the metal electrode is fabricated by photo-/e-beam lithography followed by the metal deposition process. The Au/Cr electrode is commonly used since the adhesion of pure Au to SiO2 and catalysts is often very weak, and Cr generally provides excellent adhesion. Subsequently, mild treatment such as plasma, laser or e-beam can be introduced on the individual nanowire/nanosheet catalyst for specific experimental design, such as making intuitive structural changes to reveal the structure–activity relationship. Next, an insulated PMMA layer is spin-coated on the substrate to avoid leakage of the electrochemical current and enable the selective exposure of a specific reaction window on the material by e-beam lithography. In addition, the PMMA insulation layer can be replaced with the more robust Al2O3 by atomic layer deposition (ALD). However, it is more difficult to etch the oxide layer to expose the reaction windows. Finally, the device is connected to a customized measurement system to enable in situ measurement, collecting the electrical signal (Ids, conductance current) and electrochemical signal (Ic, reaction current) concurrently, which also allows back gate (VBG) modulation to regulate the reaction.

3. On-chip electrocatalytic microdevice for expanding the insight into electrochemical processes

On-chip electrocatalytic microdevices are great platforms to directly probe electrochemical processes in unprecedented ways, such as in situ dynamic monitoring, external-field regulation, identification of active sites and single-structural factor regulation (Fig. 3). In this section, we summarize the current research status in detail, with a focus on the mechanism exploration.
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Fig. 3 Emerging on-chip electrocatalytic microdevice for expanding the insight into electrochemical processes.

3.1 Dynamic monitoring

Electrocatalytic reactions are complex processes involving solid catalysts, liquid electrolytes, gas products, and so on,37 which impose limitations on the study of in-depth interfacial dynamics. The surfaces and interfaces involved are the most informative and play key roles in the electrocatalytic process.38In situ monitoring the catalytic interface between the catalyst and the electrolyte in real-time with other characterization techniques can propel a better understanding of the electrocatalytic processes. However, it is extremely difficult to incorporate proper spectroscopy techniques in conventional electrochemical cells, which are often incompatible with the electrolyte environment.39 In addition, the binders and conductive carbon additives used to prepare electrocatalysts have considerable effects on the accuracy of electrochemical interface measurements.40 Accordingly, the emerging on-chip electrical transport spectroscopy (ETS) creates a new path for in situ electrochemical surface studies. As the physical size of ultrafine metal nanostructures decreases along with their specific surface area increases, their electrical properties are greatly changed. Accordingly, Ding et al. designed a brand-new on-chip nanodevice and performed in-device cyclic voltammetry (CV) on Pt nanowires (Pt-NWs) with concurrent in situ conductance measurement (Fig. 4a).29 The changes in the conductivity of the metal nanostructures serve as a signal path to detect the dynamic electrochemical interface of the electrocatalyst. Interestingly, the electrical properties of the ultrafine platinum nanowires showed very high sensitivity and selectivity to the changes in electrochemical surface states. During the surface kinetics of Pt in the oxygen evolution reaction (OER) cycles (Fig. 4b), the coverage of surface oxide or the penetration of O species in the PtNWs could not reach saturation and the oxidation process continued (Fig. 4c), which overturns the prevailing assumption of a full surface coverage of oxygenated species in the initial stage of oxygen evolution.41 More importantly, ETS characterization of the PtNW surface allows quantitative analysis of the electrode surface, which can hardly be achieved by traditional CV measurement since the ETS signals are strictly surface-selective.
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Fig. 4 In situ electrocatalysis dynamic monitoring based on on-chip electrocatalytic microdevices. (a) Schematics of the PtNW-based on-chip device and concurrent in-device CV and ETS electronic circuit. (b) IG (gate current)–VG (gate voltage) (black curve) and normalized GSD (conductance)–VG (red curve) characteristics of the PtNW device. (c) Schematic of different Pt surface conditions with the sweeping electrochemical potentials and the corresponding conductivity changes. (d) Schematic of the double-layer model for the PtNW electrode with the specific adsorption of anions. (e) ETS characteristics of the PtNW device. (f) Resistance vs. electrochemical potentials. (g) IV characteristics and (h) OER polarization curves of Ni–graphene device. (i) Relative number density, ρ, of different electrolyte ions as a function of distance from the Ni cathode surface with an oxygen concentration of 0 (left) and 0.12 mmol cm−3 (right). (a–c) Adopted with permission from ref. 29, Copyright (2015) Nature Publishing Group; (d and e) ref. 42, Copyright (2018) American Chemical Society; and (f–i) ref. 45, Copyright (2017) Nature Publishing Group.

Thus, on-chip-based nanodevices combined with ETS can function as an effective in situ analysis technique to reveal important information about the electrochemical surface states in various complex electrochemical reactions. For instance, taking advantage of the same approach, the correlation between the oxygen reduction reaction (ORR) kinetics and surface adsorption of various anions on the Pt catalyst was deeply explored by Duan and co-workers (Fig. 4d and e).42 Their study quantitatively demonstrated that competitive anion adsorption had an inhibitory action on the adsorption of reactants or the formation of intermediates, which resulted in a poisoning effect on catalysts and hinders the ORR kinetics. During electrocatalytic reactions, the molecules in the solution adsorb on the metal nanostructures and act as diffusion scattering centers, which produce surface scattering effects, resulting in significant changes in resistivity or conductivity. Most of the previous OER studies focused on the rate-limiting step and catalytic activity from a thermodynamic point of view using conventional spectroscopy techniques.43,44 Thus, the understanding of the OER kinetic process, especially at the electrode/electrolyte interface, is still insufficient. Using an on-chip electrocatalytic device, Mai and co-authors discovered that the oxygen in the electrolyte had an inhibiting effect on the OER process.45 Using on-chip-based temporal IV measurement (Fig. 4f and g), the mechanism of oxygen molecules affecting the OER kinetics was revealed explicitly. The presence of oxygen in the electrolyte resulted in high initial resistance. However, the catalytic performance with in the presence of oxygen was obviously higher than that in the absence of oxygen, indicating that oxygen hinders the progress of the catalytic reaction (Fig. 4h). The fundamental reason for this is that the oxygen adsorption site on the surface is similar to the OH adsorption site, and the presence of oxygen seriously obstructs the accumulation of OH in the EDL (Fig. 4i). This leads to a notable reduction in the OH concentration at the catalyst interface, and consequently results in a decrease in the charge transfer and OER dynamics. In addition, the on-chip microdevice provides a convenient and practical platform for monitoring and analyzing the interfacial charge transport process of catalysts during electrocatalytic reactions. By using on-chip in situ measurements, He et al. unraveled a self-gating phenomenon in semiconductor electrocatalysis and clarified the modulation of the interfacial carrier concentration of semiconductors during electrocatalytic reactions.30 They found that the surface carrier concentration of a semiconductor catalyst was strongly modulated by the applied electrochemical potential during electrocatalytic reactions, similar to the gate modulation of field-effect transistors. For example, an n-type semiconductor catalyst is turned “on” (high conductance) at a negative electrochemical potential, thus promoting cathode reactions such as HER, while be turned “off” (surface insulating) at a positive electrochemical potential, thus preventing any anode reactions. On the other hand, a p-type semiconductor catalyst is turned on at a positive electrochemical potential, thus facilitating the anode reaction, such as the OER. However, the bipolar semiconductor catalyst can be turned on at both potential ranges, thus promoting both the anode and cathode reactions (Fig. 5a and b). This self-gating phenomenon leads to a charge transport pathway on the surface of the semiconductor, allows carrier transfer at the interface between the semiconductor and the electrolyte, and makes the semiconductor surface highly conductive (Fig. 5c). Therefore, the effective charge transport induced by the self-gating of the semiconductor is an essential condition for the occurrence of electrocatalytic reactions, and plays an important role in the charge transfer kinetics.


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Fig. 5 Demonstration of the self-gating phenomenon and modulation mechanism via in situ electronic/electrochemical measurements. (a) In situ measurements of three types of semiconductor electrocatalysts, n-type MoS2, p-type WSe1.8Te0.2, and bipolar WSe2. (b) Schematic of the correlation between the type of semiconductor and their preferred electrocatalytic activities. (c) Schematic of the surface conductance of the semiconductor electrocatalyst. (a–c) were adopted with permission from ref. 30, Copyright (2019) Nature Publishing Group.

The on-chip approach for in situ electrochemical surface monitoring and analysis provides new insight into electrochemical surface states and reaction mechanisms in complex electrochemical reactions, which makes up for the deficiency of conventional electrocatalytic measurements. Importantly, it promotes an in-depth and comprehensive understanding of electrocatalytic surfaces/interfaces, which is of great significance to the development of future catalysts.

3.2 External field regulation

A series of widely accepted structural control strategies, such as doping,46 alloying,47 bandgap regulation,48 crystal plane modulation,49 and defect engineering,50,51 have been developed to optimize the electrocatalytic performance of catalysts. The addition of an external field (electric, light and magnetic), which can tune the intrinsic either energy band structure of catalysts or molecule/ion distribution on the electrolyte surface cooperatively,26,27 may provide a brand-new strategy towards performance optimization. However, it is difficult to apply external fields accurately with conventional testing methods (material measured on glassy carbon electrode/thin film). Accordingly, by coupling with a field-effect transistor device, the on-chip electrocatalytic microdevice provides a practical platform to study the electrocatalytic reactions of semiconductors under an external electric field by applying a back-gate voltage through a solid dielectric on the substrate,52–54 such as the commonly used SiO2 layer. The back gate modulation is usually applied through the heavily doped Si layer underneath.55 Alternatively, solid electrolytes such as LiF3 were also reported to enable solid-state ion gating through the active fluoride ion in LaF3.56 Recently, transition metal dichalcogenides (TMDs) have attracted intense attention due to their high specific surface area, abundant active sites and excellent stability.57 But they often require considerable performance optimization due to their low conductivity.58,59 Accordingly, the modulation of an external field on TMD electrocatalysts has been extensively studied by many researchers using electrocatalytic microdevices based on individual MoS2 nanosheets. Field-effect modulation can enhance the electrocatalytic performance by significantly reducing either the contact resistance or resistivity of the catalyst, which facilitates the charge transport process. Thus, on-chip gate-dependent electrochemical measurement can serve as a reliable method to substantiate the correlation between catalyst conductivity and HER activity.60 For example, with an increase in the VBG from 0 V to 20 V during the HER process with 2H-MoS2, the overpotential decreased and the current increased dramatically (Fig. 6a and b).60 In another example from Mai's group, the overpotential of MoS2 reached 38 mV at the gate voltage of 5 V, which is comparable to the performance of Pt catalysts (Fig. 6c and d).26 The conductance of MoS2 increased with an increase in the gate voltage, and increased much faster in the positive gate range, which was more favorable for the HER (Fig. 6e). In addition, Daniel V. Esposito suggested that the role of the electrolyte should not be ignored. 0.5 M H2SO4 electrolyte can act as a strong p-dopant and result in high contact resistance. This doping phenomenon was ameliorated by top-gate modulation of n-doped monolayer MoS2.61
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Fig. 6 Field-effect regulation of the catalytic performance of semiconductor catalysts based on the on-chip microdevice. (a) Optical image of 2H-MoS2 flake-based on-chip microdevice. (b) Polarization curves of a single MoS2 flake. (c) Optical image of the individual MoS2 nanosheet-based HER device. (d) Polarization curves of the HER at different back gate voltages. (e) Channel conductance of MoS2 at different gate voltages. (f) Energy band diagrams of the MoS2 nanosheet device at different gate bias. (g) Schematic of the back-gated electrochemical on-chip device configuration and optical image of the ZnO film electrode. (h) Sheet conductance (σSD) vs. back gate bias VBG. (i) Electron transfer at ZnO/electrolyte interface. (j) CV curves measured at VBG = 0 V, >0 V, and <0 V, respectively. (k) Energy band diagrams of the monolayer MoS2 working electrode at different back-gate biases. (a and b) Reproduced with permission from ref. 60, Copyright (2016) Nature Publishing Group. (c–f) Reprinted with permission from ref. 26, Copyright (2016) John Wiley and Sons. (g–i) Modified with permission from ref. 31, Copyright (2016) American Chemical Society. (j and k) Adopted with permission from ref. 32, Copyright (2017) American Chemical Society.

On the other hand, the applied electric field can regulate the intrinsic Fermi level and band structure of semiconductor catalysts, thereby affecting the electron transfer kinetics and optimizing the performance of the catalyst. From the energy band theory point of view (Fig. 6f), there is a high energy barrier between MoS2 and Au, which impedes charge carrier transport without external gate modulation.26 Applying a positive gate voltage will bring the Fermi level closer to the bottom of the conduction band to lower the energy barrier and increase the conductance. On the contrary, a negative gate voltage will make the energy barrier even higher.26 C. Daniel Frisbie's group reported the field-effect modulation of a 5 nm-thick ZnO film obtained by ALD, which behaved like a 2D semiconductor (Fig. 6g–i), and then investigated the heterogeneous charge transfer kinetics between MoS2 and the redox species (Fig. 6j and k).31,32 Additionally, back-gate modulation through the SiO2 dielectric can adjust the electron occupation in the semiconductor conduction band and give rise to a band edge shift in the electrode, which consequently affects the charge transfer kinetics on the electrode. Subsequently, they proposed a perspective on the field-effect enhancement of the catalytic activity of monolayer MoS2. Under positive gate bias, the enhancement of the intrinsic reactivity of the active sites in monolayer MoS2 promotes an improvement in catalytic activity.62 Specifically, the field effect triggers an increase in the charge of the Mo metal center near the active sites (S vacancy), thereby enhancing the Mo–H bond strength. As discussed above, a field-tuned on-chip electrocatalytic device provides a practicable and flexible platform to understand the various electrocatalytic reaction mechanisms.

Besides the regulation of the band structure and electron transfer kinetics in semiconductor catalysts, the applied electric field also affects the electric double layer and the diffusion process.63 Yan et al. demonstrated that an external electric field could effectively regulate the H+ distribution and optimize the adsorption process of H+ on the catalyst surface during the HER.27 They presented a VSe2 nanosheet-based electrocatalytic device (Fig. 7a) and achieved improved catalytic performances under negative back-gate modulation (Fig. 7b). Different from the aforementioned semiconductor catalyst, VSe2 is metallic with a high intrinsic carrier concentration, and its electrical characteristics can hardly be modulated by an external electric field.27 Instead, the back gate electric field can modulate the ion concentration in the electrolyte to regulate the adsorption and desorption processes in HER. They further conducted a continuum simulation to verify the redistribution of ions in the electrolyte and the increase in H+ concentration on the catalyst surface under back-gate modulation (Fig. 7c and d).27 Moreover, the adsorption of H ions affects the electrical transport property and charge transfer barrier, which further influences the absorption dynamics and enhances the catalytic activity. Thus, external-field regulation based on on-chip electrocatalytic microdevices not only provides a new direction for the optimizing performance but also stimulates attention on the molecule/ion distribution between solid/liquid interfaces.


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Fig. 7 Field-effect modulation of the ion redistribution based on the back-gated on-chip device. (a) Schematic of the VSe2 nanosheet-based HER device. (b) Polarization curves of VSe2 measured at different back gate voltages. (c) Stationary state of net charge concentration distribution in electrolyte region under different potential biases. (d) Concentration of positive and negative charges (line represents H+ and dots represent SO42−) and the net charge (inset) under different potential biases. Reproduced with permission from ref. 27, Copyright (2017) American Chemical Society.

Furthermore, the electric field can be combined with the photoelectric effect to tune the electrocatalytic performance of electrode materials.64 Simulated sunlight and back-gate voltage were adopted in an HER device based on individual MoS2 nanosheets to explore the influence of catalyst conductivity and the contact resistance of the electrode material. At a gate voltage of 3 V and sunlight illumination of 60 mW cm−2, a low onset overpotential of 89 mV was achieved. Both the electric field and the photoelectric effect improved the conductivity of MoS2; nevertheless, the back-gate voltage accelerated the electron injection into its active sites, which could achieve faster charge transfer kinetics than the generation of photoelectrons by sunlight illumination in the MoS2 layer.64

Thus, based on the above studies, the on-chip back-gate platform provides a powerful complement to clarify how the catalytic performance can be affected by the band structure of the catalyst and the molecular distribution on the catalyst surface. Notably, it expands the current catalytic performance optimization strategies and opens up new avenues for designing high-efficiency electrocatalysts. However, it is noteworthy that currently the field regulation is still insufficiently explored. The introduction of more external fields (such as magnetism) and the applications in other catalytical applications (e.g., CO2 reduction, N2 reduction, and CH4 oxidation) need further in-depth investigation.

3.3 Identification of active sites

The surface of a catalyst usually has a plurality of possible catalytically active sites with different adsorption/desorption properties.65 Based on the Sabatier principle, the ideal active site should have a moderate adsorption capacity for intermediates, which can activate the intermediate and facilitate its desorption.5 Thus, in-depth understanding of the active sites of catalysts is instructive for the design and synthesis of high-performance catalysts. In conventional electrocatalysis, sophisticated characterizations such as scanning tunneling microscopy (STM) and X-ray absorption fine structure (XAFS) are often required due to the complicated chemical reaction environments and interference factors, which pose great difficulties for the precise identification and regulation of active sites. Alternatively, the on-chip microdevice is a powerful complementary tool to identify the catalytically active sites by selectively exposing the area of interest on a microscopic catalyst nanosheet. It provides a direct and precise strategy to probe the spatially resolved electrocatalytic activity.

Based on the advantages of the optional catalytic zone in an on-chip electrocatalytic device, the catalytic active site distribution of MoS2 has been extensively studied. Through the selective exposure of the edge and basal plane, as depicted in Fig. 8a and b, Zhang et al. studied the in-plane and edge catalytic activity of 1T′- and 2H-MoS2. Their results showed that the catalytic activity of the edge was much higher than that of the in-plane, irrespective of the phases (Fig. 8c) and the in-plane catalytic activity of 1T′-MoS2 was superior to that of 2H-MoS2.66 Moreover, the method of selective exposure allowed quantitative analysis of the turnover frequencies (TOFs) at the active sites (including the basal plane and the edge) (Fig. 8d). The improved performance originated from the Mo-terminated edge active sites and phase transition from the 2H phase to the 1T phase.66 This quantitative local measurement also provides an effective method to measure the catalytic activity of nanomaterials down to a single atom. For example, Chhowalla's group investigated the catalytic activity of the MoS2 basal plane at different vacancy concentrations and extracted the activity of a single vacancy, demonstrating that the vacancy activity increased with the vacancy concentration.67 Similarly, they found that by reducing the contact resistance, the catalytic activity of the 2H-MoS2 basal plane was comparable to that of 1T-MoS2 (Fig. 8e–h).34 S-vacancy active sites also exist on the MoS2 basal plane (Fig. 8i), but are often deactivated due to the high contact resistance between the catalyst and the substrate.34 Phase transition is also an effective way to reduce the contact resistance and then facilitate the injection of charge from the substrate to the catalyst, thereby enhancing the catalytic performance of the basal plane.34 On the other hand, the development of reasonable and efficient methods for generating active sites is of great significance for the regulation of catalyst performance. Dong et al. reported a novel method, selective steam etching, which can create high-density active sites on the MoS2 basal plane and result in a significant performance improvement.68


image file: c9cs00601j-f8.tif
Fig. 8 Identification of active sites through on-chip electrocatalytic microdevice. Optical images of single-layer MoS2 with its edge covered (a) or exposed (b). (c) Polarization curves of MoS2. (d) Volcano plot based on the ΔGH* and TOF of metal catalysts (●) for the HER and MoS2 (★) data fitted in. Optical images of an MoS2 flake with its basal plane exposed (e) and both its basal plane and edge exposed (f). (g and h) Current density and Tafel slope values with the contact resistance of single-layer MoS2, respectively. (i) STEM image of a single sulfur vacancy (orange circle) and double sulfur vacancies (yellow circles) in an MoS2 nanosheet. (j) Schematic of the PtSe2-based HER microdevice. (k) Polarization curves of PtSe2. Inset: Optical image of a PMMA-covered electrocatalytic microdevice. (l) Calculated free energy diagram for HER at different sites of 1 L PtSe2. Inset: Top view of optimized structures of different adsorption sites. (m) Optical images of different types of Td-WTe2-based microdevices. (n) Different types of HER active sites of Td-WTe2. (o) Polarization curves measured from different devices. (p) Volcano plot of the exchange current density as a function of the ΔGH* of the three different catalytic sites. (a–d) Adopted with permission from ref. 66, Copyright (2016) Nature Publishing Group. (e–i) Modified with permission from ref. 34, Copyright (2017) John Wiley and Sons. (j–l) Adopted with permission from ref. 69, Copyright (2019) John Wiley and Sons. (m–p) Reproduced with permission from ref. 36, Copyright (2018) John Wiley and Sons.

This method of identifying the active sites based on on-chip devices is convenient and efficient and offers a versatile platform for other 2D materials. For example, a PtSe2 nanosheet-based on-chip electrochemical device was proposed to identify the active sites of PtSe2 by selectively exposing its edge and the basal plane (Fig. 8j–l).69 The results showed that the active sites on the edge had higher activity, as was the case with MoS2, and the ΔGH* value indicated that the catalytic activity of PtSe2 mainly originated from the unsaturated atoms at its edges rather than its basal plane. Furthermore, the electrocatalytic activity of two-dimensional PtSe2 is related to its thickness. As the number of layers increases, the active sites on its edge sites multiply and the energy barrier for electron transfer is reduced, which promotes the electrocatalytic activity. For bulk or few-layer TMD materials, vertical electron conduction is encumbered by the interlayer transition of electrons due to weak interlayer electron coupling.70 Prasad V. Sarma et al. adopted screw dislocation-driven (SDD) growth in the chemical vapor deposition (CVD) process of WS2 and achieved a spiral WS2 domain. In the vertical direction, the dislocation lines serve as a joint to connect numerous active edge sites, which contributes to the vertical electron conduction and enhances the catalytic performance of WS2.71 Besides, Lou et al. compared the basal activity of various 2D TMDs and found that 3R-NbS2 had outstanding basal activity.72 This is due to the self-nanostructure phenomenon observed in local measurements, which increases the specific surface area and accelerates electron transfer.72 As is well known, the exposed crystal planes of nanomaterial catalysts have a profound effect on their performance in catalytic reactions.73 Thus, selectively exposing different crystal planes to study the electrocatalytic reactions of semiconductors using in situ on-chip devices can be a promising approach for investigating the correlation between crystal plane and properties in electrocatalysis. WTe2 is a semi-metal with high electron mobility and different types of hydrogen absorption sites, which has attracted the attention of researchers. Based on the catalyst model of the on-chip electrochemical device, different types of catalytic sites of WTe2 have been studied intensively (Fig. 8m–p).36 For both WTe2 and MoS2, the active sites at the edges are highly active while the plane are relatively inert.36 The WTe2 nanosheet-based on-chip device provides a platform to directly explore the correlation between ΔGH and HER activity by selectively exposing the basal, (010) edge, and (100) edge. From the polarization curve, it can clearly be seen that the active sites at the (100) edge have the highest activity, and that of the basal plane is the worst.36 Then, plotting the exchange current density as a function of the Gibbs reaction energy (ΔGH*), a direct relationship between catalytic activity and thermodynamic ΔGH* can be constructed. Apparently, the (100) edge has the optimal ΔGH* value and the highest exchange current density among the different types of active sites, coinciding with the tendency in the electrochemical results.36

Briefly, on-chip-based in situ electrochemical measurement gives new insight into the active sites of nanocatalysts. The characteristic of selective exposure of catalyst materials enables the study of the active sites of interest accurately. Undoubtedly, nanosheet/film-based on-chip nanodevices provide a convenient and versatile platform to identify the active sites and study the catalytic performance of various 2D materials, which are very instructive for the in-depth understanding of the catalytic reaction mechanism.

3.4 Single structural factor regulation

There are plenty of regulation measures to optimize the catalytic performance in the literature.74–76 However, in conventional electrocatalysis, the catalysts are often influenced by a combination of several factors, such as thickness, size, and defects.77 Thus, it is very difficult to isolate the effects from individual factors (such as vacancy, phase, and interface), which hinders the development of high- performance catalysts.

Accordingly, on-chip electrocatalytic microdevices can be specifically designed to focus on a single structural factor, while eliminating disturbances from multiple factors and allows (semi-) quantitative electrochemical studies.

The crystal structure of a material has a significant effect on its physicochemical properties.78,79 As is widely accepted, TMDs exhibit several crystal phases according to their atomic coordination mode.80 The metallic phase usually exhibits better catalytic performance than the semiconductor phase due to faster electron transport.81 However, measuring the performance of different phases on the same nanosheet is a huge obstacle in conventional electrocatalysis. In this regard, by exploiting in situ on-chip-based measurements, we realized the simultaneous existence of different phases on the same nanosheet to explore the electrocatalytic properties of different phases (Fig. 9).3 Treated by laser irradiation and thermal annealing, MoS2 nanosheet-based HER devices can perfectly achieve controlled local phase patterning. Thus, we fabricated three different phase composition types for comparison to measure the phase-dependent electrocatalytic performance. The basal plane of 1T′-MoS2 exhibited a remarkable electrocatalytic performance with an onset overpotential of 65 mV, which was much more efficient than 2H-MoS2. This outstanding performance is ascribed to the high catalytic activity of the 1T′-MoS2 basal plane and excellent charge transport capability.33 Meanwhile, Mai's group made a comparison between 1T-MoS2 and 2H-MoS2 based on electrocatalytic activity under different electric fields, demonstrating that field-modulated 2H-MoS2 exhibited excellent HER performance, surpassing that of 1T-MoS2.82


image file: c9cs00601j-f9.tif
Fig. 9 In situ probing phase-dependent electrocatalytic performance by on-chip microdevice. (a) Schematic showing the fabrication of three types of electrochemical microdevices. (b and c) Raman mapping images of a 1T′-MoS2 flake before (a) and after (b) laser irradiation, showing the phase transition at the irradiation spots. (d and e) Polarization curves and corresponding Tafel plots of the three types of electrochemical microdevices. Inset is the optical image of EM-1. Reproduced with permission from ref. 33, Copyright (2018) Nature Publishing Group.

Especially, on account of the unique zigzag chain structure of 1T′-MoS2, we found a unique in-plane anisotropy phenomenon that correlates electrocatalytic activity with anisotropic charge transport.83 An obvious angle-dependent HER performance was observed in a multi-terminal device. In this on-chip device, electrodes with different angles share the same active area, which makes charge injection the only variable in the HER performance. Therefore, the influence of the charge transfer process on the electrocatalytic interface could be ruled out, and the catalytic performance is only affected by the lattice orientation of 1T'-MoS2 and its anisotropic charge transport.83

In addition, due to their large specific surface area and atomic thickness, two-dimensional TMDs tend to oxidize due to the adsorption of oxygen and water molecules in the air,84,85 which can alter their intrinsic physical properties. Therefore, it is necessary to establish the structure–activity relationship between oxidation and HER performance. In this regard, we evaluated the effect of a single oxidation factor on the HER performance through on-chip devices (Fig. 10).86 The degree of oxidation was precisely controlled by the different processing times with O2 plasma. The oxidized MoTe2 had a lower initial overpotential and activation energy, demonstrating that some degree of oxidation can improve the HER performance.86 The lattice insertion of oxygen atoms will change the electronic structure of a material, which will affect its chemical properties. The oxygen introduced during the oxidation process modulates the electron density, which increases the surface charge and reduces ΔGH*.86


image file: c9cs00601j-f10.tif
Fig. 10 In situ probing of the oxidation-dependent electrocatalytic performance using an on-chip microdevice. (a) Schematic of the on-chip electrocatalytic microdevice. (b–d) Polarization curves, corresponding Tafel slope and temperature-dependent polarization curves of an individual MoTe2 flake with different degrees of oxidation, respectively. (e) Calculated ΔGH* of the different active sites. (f) Kelvin probe force microscopy image of an individual MoTe2 flake showing different degrees of oxidation. Reproduced with permission from ref. 86, Copyright (2019) Chinese Chemical Society.

Moreover, the electrochemical molecular intercalation process of two-dimensional layered materials can also be monitored based on on-chip microdevices, combining optics, electrochemistry and in situ electronic characterization. We conducted an intensive study on intercalation with hydrazine hydrate for tantalum disulphide (TaS2) nanosheets based on an on-chip microdevice.87 Through the solution-based molecular spontaneous intercalation approach, a TaS2–N2H4 hybrid superlattice was obtained and the interlayer spacing increased by about 1.5 times.87 The inserted N2H4 molecule, as a strong electron donor, provided electrons to the S–Ta–S lattice, and regulated the electronic state of the catalyst. The adsorption site activity of the catalyst was activated so that the electrocatalytic activity increased. Meanwhile, He et al. reported that organic hexadecyl trimethylammonium bromide (CTAB) intercalation induced a phase transition in MoS2 from the 2H phase to the 1T phase, leading to a significant increase in conductivity.88 In contrast, the insertion of tetrachromium bromide (THAB) resulted in less charge injection to avoid the phase transition from the 2H phase to 1T phase.88

Interface design is an indispensable part of the development of high-efficiency catalysts, providing more catalytic active sites and affecting the electron transport in the catalytic reaction.89,90 Accordingly, Zhu et al. induced boundaries in the monolayer MoS2 basal plane, including 2H–2H domain boundaries and 2H-1T phase boundaries (Fig. 11a and b).91 These boundaries provide a high density of highly active catalytic sites. They used in situ on-chip devices to expose the boundaries and study their effects on the catalytic process. The performance of MoS2 with 2H–2H domain boundaries (pristine type III) was significantly better than that without domain boundaries (pristine type I). Moreover, the 2H-1T phase boundaries (heterophase type I) were superior to the 2H–2H domain boundaries in HER performance (Fig. 11d and e).91 Specifically, the boundaries played a positive role in the catalytic process and accelerate the HER reaction. For further understanding the mechanism of the phase boundaries affecting the HER process, the surface of the heterophase sample was hydrogenated to simulate the hydrogen adsorption process. After hydrogen treatment, an atomic-size width depression was observed clearly at the phase boundary (Fig. 11c), indicating that the absorption of atomic hydrogen tends to occur on the S sites at the phase boundary.91 Theoretical calculations demonstrated that the density of S atoms around the Fermi level was significantly reduced due to the formation of S–H covalent bonds (Fig. 11f). Notably, the ΔGH* at the 2H-1T phase boundary was comparable to that of the Pt (111) surface.91 Therefore, the phase boundary of the monolayer MoS2 basal plane acts as effective active sites for reducing the energy barrier and accelerating the kinetics in the hydrogen evolution reaction.


image file: c9cs00601j-f11.tif
Fig. 11 Interface design based on the on-chip electrocatalytic microdevice. (a) Transmission electron microscopy image of type-III MoS2 with high-density 2H–2H domain boundaries. (b and c) Scanning tunnelling microscopy images of MoS2 before and after hydrogenation, respectively. (d and e) Polarization curves and related Tafel slope of MoS2 with different types of domain boundaries, respectively. (f) Projected density of states (PDOS) of the S and H atoms at the phase boundary before and after hydrogenation. (g) Schematic of the hydrogen evolution pathway determined using the thermodynamic ΔGH and interfacial barrier (ΔΦsc). (h) PDOS for Au(111)/2H-MoS2 and Au(111)/Td-WTe2. (i) Polarization curves of the MoS2 nanoflake microdevice with a graphene contact and the MoS2/graphene heterostructure. Both were measured from the basal plane. (a–f) Adopted with permission from ref. 91, Copyright (2019) Nature Publishing Group. (g–i) Reproduced with permission from ref. 36, Copyright (2018) John Wiley and Sons.

On the other hand, the interface design exerts a considerable impact on the electrocatalytic performance of catalysts, especially for semiconductor catalysts. The interface between the catalyst material and the contact electrode has a Schottky barrier due to the semiconductor–metal contact, which leads to the obstruction of electron transport between the catalyst and contact electrode.92 Therefore, the HER activity of the semiconductor catalyst can be greatly modified by adjusting the Schottky barrier to regulate electron transport (Fig. 11g). As mentioned before, WTe2 exhibits a semi-metallic nature while MoS2 is a semiconductor. Comparing the density of states of Au/2H-MoS2 and Au/Td-WTe2 (Fig. 11h), Td WTe2 and gold are metal–metal interfaces with a strong overlapping metal state, and thus there is no interfacial barrier.36 However, the transfer of electrons from Au to MoS2 must overcome a certain energy barrier. Remarkably, when using graphene contacts as an alternative to gold contacts, the HER performance of MoS2 has a notable improvement. Moreover, the catalytic activity of the MoS2/graphene heterostructure was better than that of MoS2 with graphene contact (Fig. 11i).36 This is because in the case of heterostructures, the interface barrier is reduced and the electron transport distance is shortened, thereby significantly enhancing the efficiency of electron transport.36 Namely, efficient charge injection is the key to promoting electrocatalytic activity. When the Schottky barrier is large, its negative effect may even exceed the contribution of thermodynamics ΔGH on catalytic activity. Additionally, many studies have shown that the performance of TMDs is markedly improved after a phase transition from the semiconductor phase to the metallic phase.34,66,93 This phenomenon can be partially explained by the changes in the properties of the interface between the catalyst and the electrode and the removal of the Schottky barrier. Based on this, another study from Zhou et al. unveiled the interfacial effects of an MoS2/WTe2 hybrid catalyst.94 Efficient charge injection between MoS2 and WTe2 dominated in the electrocatalytic process and the effect of effective screening of mirror charges due to semi-metallic WTe2 was proven to be minor.94 This also proves the importance of interface design for the catalytic process. The on-chip microdevice can eliminate distractions from multiple factors, and thus, a single influence factor can be extracted and an in-depth and comprehensive study of its impact on the catalytic reaction and mechanism be performed. Thus, obviously, it is a convenient and effective device platform for investigating the effects of individual factors and establishing reliable structure–activity relationships.

4. Conclusions and outlook

In this review, we provided a comprehensive summary of the appealing on-chip electrocatalytic microdevice, including the device configuration, characteristics and obtained new insights into electrochemical processes. We gave an overview of the device configuration, fabrication method and unique device advantages. A variety of nanocatalysts including 1D nanowires, 2D nanosheets and thin films were investigated in the on-chip electrocatalytic microdevices, and in principle there was no restriction in terms of the types of catalysts used, which makes this platform technology greatly versatile. In addition, we highlighted the current achievements in the use of electrocatalytic devices to pursue new electrochemical phenomena and optimize electrocatalytic performances (Table 1). Specifically, the self-gating phenomenon and oxygen shielding effect during the electrocatalytic process were discovered by in situ electrical measurement on an on-chip device. Inspired by the electrostatic modulation of semiconductors in FETs, the performance of electrocatalysts can be dramatically improved by applying an external electric field, which provides a new route towards performance optimization. On-chip microdevices can be used to identify catalytic active sites by selectively exposing specific regions of a catalyst material (e.g. edge and basal plane) to the electrolyte, providing a direct and precise strategy to probe the spatially resolved electrocatalytic activities, while avoiding sophisticated characterization and theoretical simulation. Meanwhile, the individual nanowire/nanosheet on the on-chip microdevice can be precisely designed and controlled, which are essential to isolate the effect of a single factor from a combined effect of multiple factors such as crystal phase, active sites, and oxidation states. Besides, the sustained research efforts in this field will surely result in the discovery of more interesting new phenomena in electrocatalysis, for instance, the anisotropy of electrocatalytic properties on the basal plane of 1T′-MoS2 flakes.
Table 1 Summary of on-chip electrocatalytic devices
Material Method Measurement characteristics Highlights Ref.
Pt nanowires Hydrothermal method On-chip-based in situ electrical transport spectroscopy Reveal electrochemical interface information 29
Ni–graphene nanosheet Hydrothermal method On-chip electrochemical impedance spectroscopy and temporal IV measurement Design on-chip electronic circuit and monitor OER dynamics in situ 45
MoS2 nanosheet CVD Single-layer MoS2 nanosheet with edge or basal plane exposed Reveal the role of contact resistance on electron transport 34
1T′-MoS2 nanosheet CVD Local exposure measurement of the basal plane and edge of 2H and 1T′ phase MoS2 The optimum performance at the Mo-terminated edge of 2H- and 1T′-MoS2 66
MoS2 nanosheet CVD Helium ions irradiation on exposed basal plane of MoS2 Measure the activity of individual vacancies in MoS2 67
MoS2 nanosheet Mechanical exfoliation Selective steam etching and exposure through PMMA Create active edge sites on MoS2 basal plane by selective steam etching 68
MoS2 nanosheet CVD Gate-dependent HER measurements In situ observation of the effect of surface electron concentration on HER catalysis 60
MoS2 nanosheet Mechanical exfoliation Field-tuned individual MoS2 nanosheet-based HER device Report the role of different electric field states on HER performance 26
MoS2 nanosheet CVD Monolayer MoS2 as back-gated working electrodes Field effect modulation of the charge transfer kinetics from irreversible to reversible 32
MoS2 film CVD Monolayer MoS2 as back-gated working electrodes The positive bias increases the intrinsic activity of the active site 62
MoS2 nanosheet Mechanical exfoliation Induce electrical field and sunlight Facilitated electron injection process contributes to faster charge transfer kinetics than generated photo electrons 64
MoS2 nanosheet Mechanical exfoliation Use laser irradiation and thermal annealing to achieve controlled phase transformation Reveal crystal phase-dependent properties in electrochemical devices and catalysis 33
1T′-MoS2 nanosheet Mechanical exfoliation Eight-terminal device for angle-resolved HER measurements Report the in-plane anisotropic properties of 1T′-MoS2 layers 83
1T-MoS2 nanosheet CVD 2H–2H domain boundaries and 2H-1T-phase boundaries in basal plane Domain boundaries act as new highly active and tunable catalytic sites for HER 91
1T-MoS2 nanosheet Mechanical exfoliation Field-tuned device with 1T-MoS2 contact and 2H-MoS2 channel Superior HER Performance in 2H-MoS2 to that of the 1T phase 82
MoS2/WTe2 nanosheet CVD/Mechanical exfoliation Different exposed forms of MoS2 and WTe2 Reveal the site-dependent activities and the effect of interfacial barrier on HER 36
1T’-MoTe2 nanosheet Mechanical exfoliation Oxidized MoTe2 with basal plane exposed Unravel the role of oxidation behavior in the HER 86
ZnO film ALD In situ modulation of electrochemical reaction kinetics with back gate applied Field-effect modulation of band edge alignment and carrier concentration 31
PtSe2 nanosheet Chemical vapor transport PtSe2 with different number of layers and exposed areas Unveil the layer-dependent HER activity of PtSe2 69
WS2 nanosheet CVD Local exposure measurement of spiral WS2 domains Report an edge-rich spiral WS2 by SDD growth 71
VSe2 nanosheet Mechanical exfoliation Pristine VSe2 nanosheet as back-gated working electrodes Field-effect modulation of the redistribution of ions at the electrolyte-interface 27
Semiconducting TMD nanosheet CVD/Mechanical exfoliation On-chip-based in situ electronic/electrochemical measurements Unravel the universal self-gating phenomenon in semiconductor electrocatalysis 30


On-chip devices are a momentous step forward in the electrocatalysis field with substantial progress achieved recently. However, in general, they are still in their infancy, with their true potential yet to come. Much work remains to be done to attain a comprehensive fundamental understanding of on-chip electrocatalytic microdevices and to realize their full potential, with the following opportunities and challenges (Fig. 12).


image file: c9cs00601j-f12.tif
Fig. 12 Future challenges and perspective for on-chip electrocatalytic microdevices. TERS graph is adapted with permission from ref. 95, Copyright (2017) Nature Publishing Group; AFM-IR graph is adapted with permission from ref. 96, Copyright (2017) American Chemical Society; SECM graph is adapted with permission from ref. 98, Copyright (2019) National Academy of Sciences; and The graph of more structure regulation is adapted permission from ref. 83, Copyright (2019) John Wiley and Sons.

Firstly, some limitations and uncertainties in the information obtained from nanodevices, such as undersized current signals, unusual Tafel slopes and limited catalytic systems, need to be solved. Specifically, the electrochemical current of a single catalyst collected on the on-chip device is at the nA level. In this regard, the device current is susceptible to interference and its repeatability and reliability appear especially important. Small current signals require us to explore amplification techniques to obtain more-refined signals on on-chip microdevices. Furthermore, careful analysis of the reported on-chip devices reveals that the Tafel slope of HER tends to be more than 100 mV dec−1, which is significantly larger than that from traditional tests, such as for single-layer WS2 (120 mV dec−1),71 2H-MoS2 (180 mV dec−1),33 1T′-WTe2 (110 mV dec−1),36 1T′-MoTe2 (185 mV dec−1).86 It is well known the Tafel slope is a critical parameter to judge the catalytic reaction kinetics and path. In view of the large deviations between individual nanosheet devices and conventional testing, rigorous mechanistic micro-kinetic analysis needs to be conducted in future research. Meanwhile, the current on-chip devices are still limited to catalysts with sizes greater than several μm, such as single 2D nanosheets and 1D nanowires, which is just the tip of the iceberg compared to the variety of reported nanostructured catalytic systems, including most of the nanostructures synthesized by solution methods (such as hierarchical compositions, nanocrystals, and quantum dots). Therefore, one of the major future tasks is to expand the current device fabrication process and improve the device configuration to expand the electrocatalytic systems.

Secondly, as stated above, on-chip devices remain in their infancy and the current research ideas are still quite preliminary. For instance, current research on on-chip devices is mainly focused on catalytic reactions in aqueous electrolytes such as the HER, OER, ORR. However, it can be expected that other smaller molecule (e.g., CO2, N2, and CH4) catalytic processes in more complex electrolyte systems will be explored by on-chip devices. On the other hand, the highly efficient external field modulation is limited by the extra power consumption and the difficulty in operation in practical applications. In this regard, other outfield regulation strategies (magnetic field, optical field, and force field) deserve attention. In addition, on-chip devices provide a perfect platform to investigate the effects of individual factors (such vacancy, phase, and interface) and establish reliable structure–activity relationships. However, more structural control strategies (such as  sudden cold, corrosion, and intercalation) need to be developed in this regard.

Third, in situ monitoring of the conversion pathways of small-molecule catalytic systems will certainly improve our understanding of the catalytic reaction mechanism. The incorporation of in situ spectrographic characterization (such as infrared spectroscopy, Raman spectroscopy, and X-ray techniques) with high spatial resolution is highly anticipated to understand the evolution of catalytic molecules (the adsorption, activation and fracture of chemical bonds) on an individual nanomaterial. Various modifications should be designed to enable operando electrochemical measurements for this new on-chip platform and improve the detection the sensitivity. For instance, tip-enhanced Raman spectroscopy (TERS) can significantly improve the intensity and spatial resolution of Raman signals and allow the probing of adsorbates at nanometre resolution.95 Atomic force microscopy-based infrared spectroscopy (AFM-IR) using the tip of the AFM probe provides chemical analysis and compositional mapping with spatial resolution much lower than the conventional optical diffraction limits.96 More importantly, the characterization of catalyst materials to gain insight into their structural, morphological, compositional, chemical and physical properties is another important perspective. Similarly, the key is to improve the spatial and temporal resolution of typical structural characterization techniques, including XAFS, AFM, and XPS. Inspired by the inventive electrochemical techniques in single/cluster nanoparticles, more and more innovative in situ characterization methods have emerged for microstructure characterization in electrocatalysis. Notably, identical location transmission electron microscopy (IL-TEM) can monitor the structure evolution of nanoparticles in the electrocatalysis process,97 and scanning electrochemical microscopy (SECM) actualizes the direct detection of edge activity in a single nanosheet.98,99 These delicate characterizations also hold immense potential to be a wonderful tool for application in on-chip devices.

Finally, besides electrocatalysis, this on-chip platform can be potentially expanded to other electrochemical applications such as, photovoltaics, photoelectrochemical and batteries. For instance, as early as in 2007, the Lieber Group reported a single p–i–n silicon nanowire solar cell capable of driving an on-chip nanosystem.100 The Geim Group used a similar configuration to construct on-chip photoelectrochemical devices to study the electrochemical properties of MoS2 with different layers.101 The Cui Group also described an MoS2 on-chip battery and recorded the intercalation process of Li ions.102 Considering the interconnectedness of these devices, we extend the exploration methods in electrocatalytic devices to other electrochemical devices, which will have a broader potential, and in this way, extend the understanding of various electrocatalytic processes. For example, the active centers of photoelectrocatalytic materials can be identified by selective exposure to electrochemical regions. We also can explore the dynamic process of photovoltaic electron migration under an external electric field. Similarly, we can explore the dynamics of the lithium-ion migration process and the evolution law of electrode structure through in situ IV tests in on-chip batteries.

Ultimately, we believe that the full cooperation of chemists, materials scientists, and semiconductor scholars will promote the rapid development of on-chip electrocatalytic microdevices to discover long-neglected electrocatalytic laws.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21805102, 21825103, and 51727809), and the Fundamental Research Funds for the Central University (2019kfyXMBZ018). H. Z. thanks the financial support from ITC via Hong Kong Branch of National Precious Metals Material Engineering Research Center (NPMM), and the start-up grant (Project No. 9380100) and grants (Project No. 9610478 and 1886921) in City University of Hong Kong. Q. H. thanks the support from the start-up Grant (Project No. 7200656) from City University of Hong Kong.

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