Gas-surface interactions on two-dimensional crystals

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Abstract

Two-dimensional (2D) crystals have developed into a popular mainstream research topic which is interesting for basic research and many applications. Gas-surface interactions, as reviewed here, are important for catalysis including noble metal-free catalysts, materials science, and surface science as well as environmental and energy technologies. Basic science concerns fundamental differences of 2D crystals and bulk materials as well as e.g. how the substrate of epitaxial 2D crystals affects their surface properties.

Most of the attention so far obtained (gas-phase) water adsorption which always was an evergreen in surface science. However, studies about small inorganic/organic molecule adsorption (CO, CO2, NOx, O2, H, rare gases, H2S, SO2, alkanes, benzene, alcohols, thiophene, etc.) and surface reactions on 2D crystals (CO oxidation, ethylene epoxidation, oxygen reduction reaction, SO2 and H2SO3 oxidation) started to appear in the literature as well. This review describes all of these probe molecules, but focuses on experimental and theoretical surface science model studies usually conducted at ultra-high vacuum (UHV).

The review focusses on graphene and functionalized graphene (graphene oxide, N-graphene, etc.) since the bulk of the literature deals with that system. However, included in fair detail are also many other 2D crystals such as silicatene, zeolite films (doped silicatene), metal dichalcogenides (such as MoS2, WS2), boron nitride, MXenes, germanene, silicene, TiO2, graphane, graphone, and portlandene.

As a prototypical example, in recent projects, the wetting properties of e.g. graphene for water were controversially discussed. Therefore, a long chapter is devoted to water on graphene. That dispute was originally based on contact angle measurements at ambient pressure. In the meanwhile detailed surface science works including theoretical modelling are available. Literature on other carboneous surfaces such as HOPG (see list of acronyms) will be considered as a reference. Related works are also visible for other inorganic 2D crystals such as silicatene, i.e., 2D-SiO2, or 2D-MoS2 as well as functionalized 2D crystals (i.e. graphene oxide, N-doped graphene, graphane, etc.). Hydrophobic systems also are interesting for applications.

Although included in this review, but not described in very detail are electro chemistry studies, projects in the liquid phase, photo-chemistry, high pressure catalysis, and pure engineering studies (membranes, separation, fuel cells). However, in comparison with 2D crystals and to perhaps motivate related UHV surface chemistry projects in the future many of these projects were included to some extent.

As a broader objective, this review summarizes the currently available knowledge needed to extend the use of 2D materials beyond the utilization of their remarkable electronic and mechanical properties.

Introduction

What is possibly the motivation of a physicist working on surface chemistry to start research about gas-surface interactions on two-dimensional (2D) crystals such as the most well-known one: graphene? For me there was a pragmatic and a more futuristic motivation. Not to scare anyone off already in the first paragraph, let's write down the obvious and pragmatic motivation first.

The use of traditional carbon materials in catalysis is not new [1]. Therefore, it is not too surprising that the application of new forms of carbon in catalysis and surface science has been vastly explored. Numerous reviews about graphene have appeared during the last few years [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. Exploring the catalytic properties of graphene also certainly has implications for sensor design [16] and even nanoelectronics [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. (Gas) sensors are based on variations of materials properties caused by interactions with adsorbates. That is surface science. Defects control graphene's properties for nanoelectronics, as defects (active sites) often control catalytic properties [8,[17], [18], [19], [20]]. Moreover, in another field (engineering), transparent graphene could be a component of displays (touch screens), solar cells, opto-electronics, and so on [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Again, the function of these devices will be strongly affected by the surface properties of graphene. Therefore, the further development of the materials science of graphene will be strongly linked to its surface science. Finally, for applications in microelectronics, supported graphene (graphene on a support) will likely be used rather than free-standing graphene, that is the exact type of graphene usually considered by surface chemists [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. The first step in any surface reaction or for any sensor application is the adsorption of the gas-phase species on the surface: the gas-surface interaction. Sure, in the meanwhile besides graphene many more two-dimensional crystals are known. Many of these are also included in this review. In summary, many studies are motivated by potential applications.

A second and perhaps more futuristic motivation for research on 2D crystals such as graphene, which for some colleagues just results in head-scratching, is the following. One overarching question probably is, does graphene's highly praised unique electronic structure (i.e. quantum electrodynamics on a benchtop) translate into a catalytically useful substrate/material? What is the connection of quantum electrodynamics and gas-surface interactions, if any? (If you are a head-scratching colleague, jump over this single paragraph – no worries.)

Presently nobody really knows a definite answer to that question, I guess. However, I am aware of two concrete examples that clearly motivate studies on graphene (and other two-dimensional crystals) for catalysis, opposed to e.g. HOPG (highly oriented pyrolytic graphite), which are related to its unique electronic-structure properties.

The first example is fuel cell applications where Pt clusters on graphene could be used in connection with traditional Nafion [21]. Recently, it was demonstrated that graphene membranes allow for proton transfer, i.e., graphene is permeable for protons (similar to Nafion) [21]. However, graphene is impermeable to atoms and molecules (in contrast to Nafion) [21]. Therefore, Pt/graphene systems would eliminate the fuel cross-over problem in fuel cells, boosting significantly the efficiency of fuel cells. That does not work with HOPG and clearly shows that basic science studies on water-Pt/graphene and Hydrogen-Pt/graphene make sense. Water is also required in direct liquid fuel cells. The permeability of graphene which affects chemistry and surface science applications is certainly related to graphene's unique electronic properties. Yes, fuel cells usually use liquids, but water and Hydrogen will adsorb on the membrane/electrode surfaces, that is closely related to the surface science of gas-surface interactions.

The second example is small-molecule gas sensors. The high carrier mobility, conductivity and low intrinsic noise (all electronic properties) promise a high signal-to-noise ratio of sensors which can result in high sensitivity [16,22]. Therefore, graphene can be advantageous as component of small-molecule gas sensors. Thus, studying gas-surface interactions in that regard makes sense.

In the light of these two briefly described examples, one would respond with “yes” to the question “does graphene's unique electronic structure also make it a catalytically interesting substrate”. One may consider this second motivation as also driven by applications. Therefore, I should mention that there are, of course, many basic science motivations to studies of 2D materials which are simply related to their reduced dimensionality.

Why is ultra-high vacuum (UHV) surface science work described in this review? First of all, “Surface Science Reports” is a surface science journal. More importantly, I believe that UHV works still provide the best and easiest access to a mechanistic understanding of surface processes, even when applications may require high pressures or the liquid phase. (Note that some colleagues doubt the strict existence of a pressure gap and have proven that there is none for some processes [23].) In particular, studies about water adsorption on graphene are a prototypical example to illustrate that ambient pressure studies are severely restricted in characterizing the intrinsic materials properties (see Section 2). A review also always reflects the scientific opinion of the one who did write it, which we have to accept and acknowledge.

The topic summarized here has certainly many dimensions with many cross-connections of research subareas which makes it difficult to find a unique structure for the review. Should one structure it according to the surface considered (graphene, silicatene, graphene oxide, etc.), or the probe molecules used (CO, CO2, NOx, alkanes, benzene, etc.), or according to reaction types (CO oxidation, hydrogenations..), or based on the functionalization, or based on topics (wetting properties, intercalation, ..), or etc. ? I chose to organize the text by emphasizing the probe molecules rather than the surfaces. (A fairly complete list of 2D materials can be found in Ref. [24]).

Realistically most of this review deals anyhow with graphene and its functionalization, as the most studied system so far. (If a reader may prefer a different structure, PDF files of published papers are searchable by keywords.)

Therefore, I divided the text in basically two chapters about the gas-surface interactions and surface reactions of small inorganic molecules (Section 3) and organic molecules (Section 4). The adsorption of water (Section 2) and noble metal-free heterogeneous catalysis (Section 5) become separate chapters due to the extent of the literature related to these topics.

Due to the characteristic delocalized π electrons, pristine and free standing graphene is largely inert, which results in weak non-covalent adsorption (physisorption) of the most common gas-phase species. Therefore, the functionalization fabrication procedures (such as doping or adsorption of functional groups) are probably of more general interest than specific applications since naturally one of the first steps in many subsequent project is a functionalization of the raw nanomaterials under consideration. To serve this interest, rather detailed sections about graphene oxide synthesis, N-doping, S-doping, etc. are included as well as gas-surface interactions with these functionalized 2D crystals. In fact, the functionalization usually is a gas-surface interaction by itself. However, the synthesis of 2D crystals is already described in a number of reviews [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15],25] and therefore it is not included here in detail.

Note that, at least according to my literature search, experimental ultra-high vacuum (UHV) surface science studies on basic surface properties of 2D crystals are still relatively rare, despite the fact that the field of 2D nanomaterials is exploding since the discovery of graphene. The literature is dominated by high pressure/ambient pressure works on powders and porous materials, many synthetic works (!), liquid phase studies, electro-chemistry works, DFT theory, etc. Therefore, many of these references are indeed included, but usually only in comparison with more traditional surface science works. I am a surface chemist/physicist myself. Similarly, I did not apply the term two-dimensional (2D) crystals too strictly, meaning that related studies on supposedly polycrystalline thin films also are included. This is difficult to avoid since the crystallinity of samples is often not perfectly characterized. My own work has a focus on kinetics and dynamics (i.e. gas-surface adsorption/scattering), the title of this review “… gas-surface interactions …” highlights the topic described. Therefore, mostly tables with binding energies of adsorbates are included rather than vibrational frequencies or XPS peak positions. Every review is certainly influenced by the own work of the author. I do not review electronic structure properties of 2D materials. Finally, I did not touch many of the hundreds of publications about metal–organic frameworks (MOFs) or covalent organic frameworks (COFs) or porous granulate-like surfaces or biomass derived 2D materials (e.g. Nanocellulose [26]) some of which may also be considered as 2D crystals. This appears to me as an own and separate field of study. Interesting works certainly have also been done on even lower dimensional systems such as nanoparticles, fullerenes [27], nanotubes [28,29],4 etc. which are not included here. Doing so would have added another 500 references which was simply not doable.

This review focusses on graphene, its functionalization and includes some sections about other 2D materials. Described is the basic surface science of gas-surface interactions.

For example, in 2018, an entire issue of chemical reviews was devoted to two-dimensional nanomaterials, see Ref. [30]. However, as most reviews on this subject, it focusses solely on synthesis and electrochemistry applications. In other words, the bare synthesis of 2D crystals is already described in a large number of reviews [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15],25,31,32] and therefore it is not included here in detail. Discussions are included regarding functionalization of these systems, but standard and well-known synthesis procedures are omitted. Still missing is, however, a comprehensive text about characterizations of chemical surface properties, gas-surface interactions, and surface reactions (heterogeneous catalysis) on model systems which is what you are reading now.

There is certainly a connection of traditional surface science works (on truncated bulk crystal surfaces) and 2D crystals. In particular, for adsorption/reactions of CO2 see the reviews listed as Refs., [7,[33], [34], [35], [36]], for water surface chemistry see Refs., [[37], [38], [39], [40]], NO interactions are reviewed in Refs., [[41], [42], [43]], and traditional surface science projects on small organics are reviewed in e.g. Ref., [44,45], in particular for alkanes, see Refs. [46,47]. For an introduction to nanocatalysis also see e.g. the reviews/books in Refs. [[48], [49], [50], [51], [52]]; for single atom catalysts try Refs. [53,54] and for supported cluster systems read e.g. Refs. [50,55]. More “general public style” introductory texts about graphene can be found here: Refs. [[56], [57], [58]].

Shortenings are quite common in nanoscience which makes it often hard to read papers out of the own research corner. Therefore, I do avoid shortenings and will often spell out (the same) terms numerous times since reviews are usually not read from the front to back. So, if you start e.g. with Section 3, fine, you won't get lost because of abbreviations. Similarly, I avoid shortenings in figure captions since many colleagues scan through a review by looking at the figures first. In addition, a long list of shortenings is included at the front of this review.

For a similar reason the titles of the references are given. Also, in addition to using chemical formulas, I do spell out the names of even simple chemical compounds just for the sake of catching these easier when searching the PDF document (Thus, one will find e.g. CO2 and carbon dioxide used in parallel.).

As probably already noted, I use many sub-headlines and rather short chapter which makes it in my opinion easier to read and search a review. Some keywords are printed in bold face throughout the text.

Tables and figures are usually placed in a text just before these are referenced the first time. However, my tables are often summaries/overviews of a given chapter. Therefore, I layout the tables at the start of a (sub)chapter. If you are looking for a reference for adsorption of “abc” on surface “xyz” then look at the first table in chapter “abc” and find Refs. for 2D crystal “xyz”.

One could include more figures showing actual experimental/theoretical data, as done in some reviews. However, this often results in very specialist descriptions of a single system. Therefore, I did prefer to include generally applicable schematics rather than pure data figures.

Finally, a trivial note, perhaps: the field of 2D materials is “exploding” which makes it impossible to keep up with the literature on all topics. Even during the review and publication phase of this draft dozen more publications were released. I added some of these references in the last minute to the review, but without too much discussion.

Section snippets

Water adsorption on two-dimensional (2D) crystals

Although several reviews [37,38,40,77] and comprehensive papers [39,40] about water adsorption have been published over the decades, including those addressing specifically hydrophobic surfaces [77], most of these texts were published before nanomaterials and nanocatalysts were in the limelight. Therefore, these reviews include all the basics and detailed descriptions of work done on single crystals, however, the works on water adsorption on nanostructures has to the best of my knowledge not

Carbon monoxide (CO) adsorption on graphene

Given the expected small reactivity of carboneous surfaces for CO adsorption [155] (only physisorption on HOPG at 35 K), the extent of literature on this subject is quite impressive. However, CO adsorption is the prototypical model system in surface science [156] and still most studies are theoretical works (Table 6).

Gas-phase adsorption of small organic molecules on 2D crystals

Dozens of organic reactions catalyzed by GO are already known. However, nearly all of these studies have been conducted in the liquid phase. For a recent review see Ref. [314]. In this report I focus on studies applying traditional UHV surface science techniques to characterize the gas-surface interaction.

Motivation

Carbon materials are inexhaustible and as such sustainable, in contrast to precious metals. Therefore, (precious) metal-free heterogeneous catalysis using functionalized Carbon is an intriguing alternative to traditional chemical synthesis [[399], [400], [401], [402], [403], [404], [405], [406], [407]]. The concept of noble metal-free catalysis dates back many years [[399], [400], [401], [402], [403], [404], [405]], but was so far mostly explored in reactions with liquid phase reactants [217,314

Summary

Rather than specializing on a single system (e.g. water adsorption on graphene, Sect. 2), this review provides a quick overview of gas-surface interactions and surface reactions on numerous two-dimensional (2D) crystals. The today most commonly studied 2D materials (graphene, functionalized graphene) and their interaction with a variety of different inorganic (Sect. 3, CO, CO2, O2, H2, H2O, NOx, SOx, H2S) and organic (Sect. 4, benzene, alkanes, thiophene, alcohols) gasphase species is described

Acknowledgements

Some prior published results of my group related to the PhD research of Ashish Chakradhar, Mudivans Tilan Nayakasinghe, and Nilushni Sivapragasam at NDSU are described as well as prior published collaborative work with Dmitri Kilin and his group at NDSU. Their support, discussions, and work effort of the prior projects are acknowledged.

Part of our own (more recent) work which also is included in this review was funded by The Donors of the American Chemical Society (Petroleum Research Fund,

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