Accessing the spatiotemporal heterogeneities of single nanocatalysts by optically imaging gas nanobubbles

Abstract

Catalysts that catalyze the generation of products in the gas phase, especially those involved in the hydrogen evolution reaction (HER), hold great promise for ecofriendly and sustainable energy development. In general, gas chromatography is widely used to measure catalytic activity. Unfortunately, it gives an averaged output that washes out the heterogeneities among individuals. To assess the unique catalytic properties at the single nanoparticle level, various methods based on single particle catalysis have been proposed. Over the past fifteen years, tremendous breakthroughs have been achieved, which uncovered hidden spatial and temporal heterogeneities. Although powerful, effectively quantifying the activities of single HER nanocatalysts remains challenging because of the fast diffusion of hydrogen (H₂). In 2017, a novel approach based on a nanobubble indicator was proposed to correlate the kinetics of gas bubble evolution with the catalytic activities of individual nanoentities during the HER process. Since then, a plethora of optical microscopy techniques have been utilized to monitor dynamically evolved nanobubbles and to measure the catalytic activities of single HER catalysts. In this minireview, we summarized state-of-the-art optical microscopy for in operando imaging of dynamic nanobubbles involved in gas-generating reactions while highlighting some important discoveries, including the blinking photocatalytic activity and heterogeneous distribution of active sites. Finally, challenges and future perspectives in this promising field were identified.

Introduction

Catalysts that catalyze the generation of products in the gas phase, especially those involved in the hydrogen evolution reaction (HER) [1,2] and oxygen evolution reaction (OER) [3], hold great promise for ecofriendly and sustainable energy development. Traditionally, the catalytic activities of these catalysts are usually evaluated by gas chromatography at an ensemble level. However, the averaged output from quantitative samples washes out the heterogeneities (differences) among individuals and hampers researchers from identifying the relationship between the structure and activity/function of catalysts in a bottom-up manner [4]. Recent advancements in nanotechnology provide opportunities to prepare nanoscale gas-generating catalysts with ultrahigh activities. Concomitantly, more significant heterogeneities are also introduced among individuals with respect to the structure (shape, size, crystal facet, etc.) and catalytic efficiency of nanocatalysts. Thus, it is highly desirable to develop a tool to accurately measure the chemical activity of individual nanocatalysts, which can promote the rational design of HER/OER catalysts with high performance [3,5]

To overcome the abovementioned difficulties, single particle catalysis has been proposed in operando to reveal the hidden spatiotemporal heterogeneities of catalysts at the single nanoparticle (NP) level [4,6, 7, 8∗]. Since the pioneering work done by Hofkens's group and Chen's group, single molecule fluorescence (SMF)-based single particle catalysis has become one of the most widely explored technologies to measure the catalytic activities of single NPs [9, 10∗, 11]. This method utilizes nonfluorescent precursors as model substrates and detects SMF signals from fluorescent product molecules during catalytic reactions. For instance, by this method, the intraparticle distribution of catalytic sites could be accessed with a spatial resolution of ca. 40 nm [12]. Despite this advantage, the SMF method only detects the efficiencies of model reactions rather than the real activities of HER/OER catalysts. Moreover, the introduction of dyes into the HER/OER reaction might interfere with the inherent catalytic behaviors of NPs. Dark-field scattering spectroscopy [13,14] has also been proposed to monitor the catalytic processes of single NPs. However, suitable types of materials used for this detection are generally required to express a strong scattering nature, e.g., metal nanoobjects with a localized surface plasmon resonance (LSPR) effect [15]. Considering that most catalysts used in photocatalysis with H₂ generation are semiconductors (pure dielectrics), dark-field scattering is not applicable. Although metal–semiconductor hybrid nanostructures have been reported [14], the introduction of metal NPs might make the catalytic processes more complicated.

Despite many breakthroughs in interrogating and even manipulating the activities of single catalysts [16], there is still a major challenge to directly monitor the catalytic kinetics of single HER and OER catalysts, particularly photocatalytic H₂ generation catalysts. Indeed, the extremely small numbers of gas molecules generated around a single catalyst NP and the highly diffusive/relatively inert nature of gas molecules in these complicated reaction systems make it very difficult to design a fluorogenic reaction with high specificity and sensitivity to detect gas molecule products [17,18]. Thankfully, gas bubbles are usually generated during HER and OER reactions, rendering it possible to measure the catalytic efficiency of these catalysts by monitoring the evolution of gas bubbles [5]. In addition, in the field of interface science, the physical properties of nanometer-sized bubbles (so-called nanobubbles) have been extensively explored in recent years [19,20], which provides a theoretical background to study the catalysts involved in gas-generating reactions based on the growth kinetics of nanobubbles. Inspired by relevant works, our group utilized surface plasmon resonance microscopy (SPRM) to monitor the evolution of nanobubbles in situ during photocatalytic H₂ generation on single cadmium sulfide (CdS) NPs in 2017 [21]. This allowed for in operando measurement of the photocatalytic HER activities of single semiconductor catalysts. Afterward, our group and other groups demonstrated the power of utilizing nanobubbles as an indicator to quantify the activities of single catalysts involved in gas-generating reactions by optical microscopy. These studies include photocatalytic water splitting, formic acid decomposition, and electrocatalytic HER process [22∗, 23∗∗, 24∗∗]. Compared with traditional gas chromatography-based methods, these methods based on nanobubble indication by optical imaging focused on the size, location, and growth kinetics of nanobubbles during catalytic reactions rather than the amounts and compositions of gas products. Although nucleation-related factors such as the geometric structure of catalysts might compromise their capability in measuring catalytic activities, these methods enabled direct detection of gaseous molecule products under practical conditions. Note that this is hard to achieve through other techniques, such as the SMF method, and opens up a new dimension for the fundamental study and application of nanobubbles.

In this minireview, we first introduce optical microscopy for in operando imaging of nanobubbles in gas-generating reactions and present a representative progress. Subsequently, we emphasize some important discoveries with respect to single gas-generating catalysts via optical imaging of nanobubbles, especially the blinking photochemical activity of single semiconductor catalysts and heterogeneous distribution of active sites. Finally, we conclude with our opinions on the challenges and perspectives in this promising field.