中文   |   English   |   Email Alert
Fig/Tab
Advanced Search
image

Carbon Dots in Lubrication Applications

Chuang He , Shuang E , Honghao Yan , Xiaojie Li

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (2) : 356 -369.

PDF
Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (2) : 356-369 DOI: 10.7536/PC210105
research-article
Scan to read & Share
Carbon Dots in Lubrication Applications
Author information +
History +
PDF

Abstract

Various forms of friction and wear not only consume more than 20% of the world’s total energy, but also cause an enormous amount of equipment damage. As a result, it is of great significance to develop friction-reducing and anti-wear lubricating materials for saving energy and prolonging the service life of mechanical equipment. Carbon dots (CDs), as a new kind of carbon nanomaterial, have been extensively used in chemical sensing, bioimaging, catalysis, optoelectronic devices and other fields. In recent years, a large number of studies have explored the application of CDs in the fields of industrial lubrication, micro/nano-electrical-mechanical systems lubrication and biological lubrication, proving that CDs have excellent tribological properties and great potential to become the next generation of green and efficient friction-reducing and anti-wear lubrication materials. However, it still lacks a systematic summary and discussion of the application of CDs in the field of lubrication. Consequently, in this paper, the research progress of CDs in lubrication applications is comprehensively and systematically summarized. Firstly, lubricating effects of CDs as nano-additives and lubricating coatings and three strategies (size and shape control, surface modification and heteroatom doping) to improve their lubrication performance are introduced in detail. Then, the lubrication mechanism of CDs is fully analyzed. Finally, the main challenges of the application of CDs in lubrication are outlined.

Contents

1 Introduction

2 Lubrication applications

2.1 Nano-additive

2.2 Lubricating coating

2.3 Other lubrication applications

3 Lubricating mechanism

3.1 Lubrication mechanism of size and shape-controlled carbon dots

3.2 Lubrication mechanism of surface-modified carbon dots

3.3 Lubrication mechanism of heteroatom-doped carbon dots

4 Conclusion and prospect

Graphical abstract

Keywords

carbon dots / lubrication / friction / lubricating mechanism / additives

Cite this article

Download citation ▾
Chuang He, Shuang E, Honghao Yan, Xiaojie Li. Carbon Dots in Lubrication Applications. Progress in Chemistry, 2022, 34(2): 356-369 DOI:10.7536/PC210105

登录浏览全文

4963

注册一个新账户 忘记密码

1 Introduction

The loss caused by various forms of friction and wear accounts for about 23% of the total global energy consumption, while causing a large number of equipment parts damage, making energy shortages, environmental pollution, carbon dioxide emissions and other issues more serious[1][2,3]. It has been estimated that reducing friction and wear in vehicles, machinery and other equipment through the use of new lubricating materials and technologies could reduce energy losses by up to 40% over the long term. These energy savings are equivalent to about 1.4% of global GDP per year[1]. Therefore, the development of anti-friction and anti-wear lubricating materials is of great significance for saving energy, developing economy and prolonging the service life of mechanical equipment.
In order to reduce friction and wear and improve energy efficiency, people have made great efforts to develop more efficient anti-friction and wear-resistant materials and improve lubrication technology. Traditional lubricating materials, such as zinc dialkyldithiophosphate (ZDDP), have good friction-reducing and anti-wear properties, but they contain sulfur, phosphorus, heavy metals and other elements, which can not meet the environmental requirements[4,5]. With the upsurge of nanotechnology research in the 1980s, a large number of nanomaterials, such as titanium dioxide, silicon dioxide, copper nanoparticles, molybdenum disulfide, tungsten disulfide, carbon nanotubes, nano-diamond, fullerene, graphene and nano-onion carbon, have been designed and developed as nano-additives or lubricating coatings.It has achieved excellent lubrication effect and greatly promoted the development of lubrication technology[6,7][8,9][10,11][12,13][14,15][16,17][18,19][20,21][22,23][24,25]. Among them, carbon-based nanomaterials have attracted great attention of scholars at home and abroad because of their excellent tribological properties and green environmental protection. However, the above carbon-based nanomaterials still face some problems in practical application, such as poor dispersion, complex preparation process, expensive raw materials, difficult surface modification and so on, which greatly limit their application and promotion. Fortunately, as a new type of carbon-based nanomaterials, carbon dots show excellent tribological properties, and have the advantages of adjustable surface groups, excellent dispersion stability, low toxicity, environmental friendliness and easy synthesis.As a nano additive or a lubricating coating, greatly improves the lubricating performance of a lubricant or a substrate material, and is a novel green and efficient lubricating material[26⇓⇓⇓~30].
Carbon dots were accidentally discovered by Scrivens et al. At the University of South Carolina in 2004 when they used electrophoresis to purify single-walled carbon nanotube products[31]. In 2006, Sun et al. First used transmission electron microscopy (TEM) and other means to characterize carbon dots[32]. At present, carbon dots are usually defined as carbon nanoparticles with fluorescence effect and size generally less than 10 nm[33⇓~35]. Most of the carbon dots consist of a core and surface groups, in which the core is generally composed of sp2 hybrid graphitic carbon or amorphous carbon, and the surface groups are mostly oxygen-containing or nitrogen-containing groups (Fig. 1A)[35⇓~37]. In 2015, Yang Bo et al. divided the carbon dots into graphene quantum dots, carbon nanodots and polymer dots based on the differences in their structures and fluorescence sources (Fig. 1b): the carbon core of graphene quantum dots has an obvious graphene structure, and its lateral size is significantly larger than its thickness, with anisotropy; The transverse size of the carbon nano dot is approximately equal to the thickness, and the carbon nano dot can be subdivided into a carbon nano particle without a lattice and a carbon quantum dot with an obvious lattice according to whether the carbon core has the lattice or not; The polymer point has a non-conjugated group crosslinking structure, and the carbon core is an amorphous structure or has a certain degree of carbonization[38]. Carbon dots have been widely used in chemical sensing, bioimaging, catalysis, optoelectronic devices, plant systems, functional materials, lubrication and other fields because of their unique fluorescence tunability, excellent photostability, diverse structures, good biocompatibility, excellent catalysis, low toxicity and environmental protection[39⇓~41][37,42,43][44⇓~46][47⇓~49][50⇓~52][53,54][26⇓⇓⇓~30]. Up to now, domestic and foreign scholars have made a comprehensive and detailed summary of the application research progress of carbon dots in biological imaging, energy storage, catalysis, sensing, plant growth and other fields, but the application research progress of carbon dots in lubrication, an important field, has not been systematically reviewed[51,55⇓⇓⇓⇓ ~60]. In 2015, Kang et al. First applied carbon dots as nano-additives for liquid paraffin in the lubrication field[28]. Subsequently, in order to further improve the tribological properties of carbon dots, a large number of scholars have carried out extensive and in-depth research, and made a series of significant progress[26,27,29,30,61,62]. Therefore, it is necessary to review the application and development of carbon dots in the field of lubrication, which will be conducive to the development of new efficient green lubricating materials.
In this paper, the research progress of the application of carbon dots in the field of lubrication was systematically summarized and analyzed in three parts. Firstly, the lubricating effect of carbon dots as nano-additives and lubricating coatings and three strategies to improve their lubricating performance (size and shape control, surface modification, heteroatom doping) were introduced in detail. Then, the mechanism of carbon spot lubrication is analyzed comprehensively. Finally, the main challenges faced by the application of carbon dots in the field of lubrication are summarized.

2 Lubrication application

At present, as nano-additives and lubricating coatings, carbon dots have greatly improved the lubricating properties of lubricants and substrate materials, and have shown broad application prospects in industrial lubrication, micro/nano-electromechanical system lubrication and other fields, and have great potential to become the next generation of green and efficient lubricating materials. Generally speaking, size and shape control, surface modification and heteroatom doping are used to improve the tribological properties of carbon dots, and further enhance their lubricating effect as nano-additives and lubricating coatings. The following will be described in detail with examples.

2.1 Nanometer additive

Throughout the history of human development, lubricants have always played an important role in reducing friction and wear and improving energy efficiency[63]. As early as about 1880 BC, the ancient Egyptians used water as a lubricant to lubricate wooden boards to move huge stone statues[64]. At present, lubricant is still one of the most commonly used and effective lubrication technologies in the field of industrial lubrication. Lubricating fluid is generally composed of base fluid and additives. Among them, anti-friction and anti-wear additives are indispensable additives. A large number of studies have shown that carbon dots, as an anti-friction and anti-wear additive, have achieved excellent lubricating effect[26⇓⇓⇓~30,61,62]. Compared with other carbon-based nanomaterials, carbon dots as nano-additives have the following significant advantages: (1) the polarity of carbon dots can be changed by a convenient and low-cost method to make them stably dispersed in polar or non-polar solutions, so carbon dots can be used as additives for water-based or oil-based lubricants at the same time[65,66]; (2) As a lubricant additive, carbon dots can not only effectively reduce the friction coefficient and wear, but also greatly improve the bearing capacity of the lubricant, so carbon dots can be used as a multi-functional additive[28,66,67]; And (3) that preparation method of the carbon dot is simple and convenient, the cost is low, and the raw material source is wide. Carbon dots can be prepared in one step by pyrolysis, hydrothermal, ultrasonic, microwave and gas detonation methods[27,67,68][69⇓~71][26,72,73][29,74,75][76⇓~78]. At the same time, citric acid, grapes, phenylenediamine, graphite, coal, soot, even leaves, peels, plastic waste and other materials can be used as raw materials for the preparation of carbon dots[28,39,57,72,75,79⇓ ~81].

2.1.1 Composite nano-particles containing carbon dots as nano-additive

In 2015, Kang and others first explored the application of carbon dots in the field of lubrication. Water-soluble carbon dots (WCDs) with average particle size of about 5 nm were prepared by electrochemical method with a voltage of 30 V applied on graphite electrode for 120 H. Then, carbon dots and Cu(NO3)2 were used as precursors to prepare carbon dots /CuSx composite nanoparticles with an average particle size of about 20 nm (Fig. 2a); The four-ball friction test shows that the composite nanoparticles as liquid paraffin additives can significantly reduce the friction coefficient and wear scar diameter compared with pure liquid paraffin under both low and high loads, and significantly improve the friction reduction and anti-wear performance of liquid paraffin (Fig. 2b)[28]. Subsequently, Liu et al., He et al., Cai et al. respectively prepared composite nanomaterials containing carbon dots, such as two-dimensional nanosheets modified by carbon dots, graphene quantum dots-graphene-sulfurized isobutylene composites, and carbon dot/graphene oxide mixtures, which showed excellent lubricating properties as additives for water or polyethylene glycol (PEG)[29][73][27]. It should be noted that Liu et al., He et al., and Cai et al. Control the shape and size of the carbon dot by a simpler method than Kang et al.[29][73][27]. Among them, Liu et al. Rapidly prepared spherical carbon dots with an average particle size of about 3 nm by microwave method[29]; He et al. Prepared graphene quantum dots in one step by ultrasonic method (Fig. 2C)[73]; Cai et al. Prepared uniformly dispersed carbon dots with a particle size of about 2.5 nm by high temperature pyrolysis[27]. Recently, Sarno et al. prepared poly (methyl methacrylate) (PMMA)/carbon dots composite nanoparticles in one step by pyrolysis, in which carbon dots are wrapped in PMMA molecular chains and released during friction to achieve the lubricating effect of friction reduction and wear resistance (Fig. 2D); The results of ball-on-disc friction experiments show that the friction coefficient and wear scar diameter of the composite nanoparticles can be reduced by 20. 2% and 41. 6%, respectively, when the amount of the composite nanoparticles is only 0. 3 wt% as an additive in commercial mineral oil[82].

2.1.2 Pure carbon dots as nano additive

Not only the composite nanomaterials containing carbon dots can be used as antifriction and antiwear additives, but also the pure carbon dots have excellent tribological properties and can be used as antifriction and antiwear additives for water-based or oil-based lubricating fluids. In addition, the tribological properties of carbon dots can be further improved by size and shape control, surface modification and heteroatom doping[62,69,77][30,68,83][61,65,84].
(1) Size and shape control
The size and shape of nanoparticles have a significant effect on their lubrication performance[62,85]. Therefore, it is necessary to study the relationship between the size and shape of carbon dots and their tribological properties. Generally speaking, the shape of the carbon nanodots and the polymer dots are spherical, and the shape of the carbon nanodots and the polymer dots cannot be controlled; And that graphene quantum dot are round only in the transverse direction, so that the number of the graphene layer in the longitudinal direction can be controlled to change the shape of the graphene quantum dots. In 2017, Fan et al. First compared the lubrication performance of nanomaterials such as layered molybdenum disulfide, graphene, graphene-like carbon/molybdenum disulfide composite, spherical molybdenum disulfide with particle size of 305.3 nm, carbon spheres with particle size of 279.9 nm and spherical graphene quantum dots with particle diameter of 3 nm as cyclopentane additives by ball-disc friction experiments.The results show that the spherical graphene quantum dots with a particle size of 3 nm not only show the best friction-reducing and anti-wear effects, but also have the best extreme pressure properties. Specifically, with the addition of graphene quantum dots at a concentration of 2 wt%, the friction coefficient and wear volume of cyclopentane lubricant are reduced by 60% and 91%, respectively, and the load-carrying capacity is increased to 500 N (Figure 3A)[62]. In 2018, our group synthesized graphene quantum dots in one step in milliseconds by gas detonation method, and prepared four types of graphene quantum dots with different sizes by adjusting the detonation parameters (the lateral sizes are 6.3, 28.3, 5.6 and 2.0 nm, respectively).The tribological properties of GQDs as mineral oil additives were studied in detail by changing the concentration and type of GQDs in four-ball friction experiments, which proved that the prepared GQDs were additives with excellent lubricating properties. Among them, graphene quantum dots (GQDs) with good dispersion, small size and abundant surface groups (lateral size of 5. 6 nm and thickness of 2. 5 nm) have the best lubricating effect, and the friction coefficient, wear scar diameter and depth of mineral oil are reduced by 65. 2%, 43.5% and 90. 9%, respectively, under the load of 392 N with the concentration of 0. 8 wt%[77]. In 2019, Zhao et al. used ammonium citrate as the carbon source and adopted the pyrolysis method to prepare carbon dots with a particle size of about 3 ~ 4 nm and graphene oxide with a larger lateral size by controlling the carbonization degree of the carbon source (Fig. 3B); Ball-on-disc friction experiments show that as an additive of water, carbon dots have better lubrication effect than graphene oxide, which can greatly reduce the friction coefficient and wear volume. In addition, carbon dots also show excellent corrosion inhibition performance, which can be adsorbed on the metal surface to form an adsorption film to prevent water and dissolved oxygen in water from diffusing to the metal surface, thus achieving corrosion inhibition effect[86]. In the same year, Wang et al. Prepared water-soluble graphene quantum dots with an average particle size of about 2 nm by hydrothermal method using graphene sheets as carbon sources, and then used them as water additives to study their tribological properties. The results show that the friction coefficient and wear rate can be reduced by 42. 5% and 58. 5%, respectively, when the concentration of the aqueous dispersion of graphene quantum dots is 4 mg/mL[69]. In the above literatures, scholars used electrochemical method, microwave method, gas detonation method, pyrolysis method and hydrothermal method to control the size and shape of carbon dots, and prepared spherical carbon dots with particle size less than 10 nm to improve their tribological properties. As a water-based or oil-based additive, the friction reduction and anti-wear properties of the base fluid were greatly improved.
(2) Surface modification
Surface modification is a common strategy to improve the tribological properties of nanoparticles. The surface groups can not only change the polarity of the nano-additive and increase its stability in the base fluid, but also make the nano-additive easily adsorbed on the friction surface to form a protective film and improve its lubrication performance[23,87⇓ ~89]. A large number of experiments have proved that carbon black, graphene and other nano-materials grafted with PEG or ionic liquid groups on the surface show better dispersion stability and lubrication performance[23,87]. Inspired by this, a large number of scholars use different surface groups to modify carbon dots to improve their tribological properties. In 2016, Wang et al. First studied the lubrication performance of carbon dots with specific groups on the surface. The method specifically comprises the following steps of: preparing the ionic liquid-coated carbon dots by adopting a two-step method including a pyrolysis method and an anion exchange method; The NTf2- of the anion group of the ionic liquid enables the carbon dots to have excellent dispersion stability in the polyethylene glycol; The tribological properties of the carbon dots as polyethylene glycol additives were studied in detail by changing the concentration, test load and action time of the prepared carbon dots using a four-ball friction model. The results show that the ionic liquid coated carbon dots have good friction-reducing and antiwear properties. When the concentration of ionic liquid is 0.3 wt% and the load is 392 N, the friction coefficient and wear scar diameter are reduced by 70% and 33%, respectively. At the same time, no matter increasing the load or prolonging the friction time, the lubrication performance of carbon dots was not weakened[30]. Subsequently, they further simplified the preparation process, using pyrolysis method, using gel (prepared from ionic liquid and lactic acid) or mixture (ionic liquid and citric acid) as raw materials, to prepare ionic liquid modified carbon dots in one step (Fig. 4A)[68][67]; Both of the two kinds of carbon dots as water-based additives showed good friction-reducing and anti-wear properties, and the friction coefficient and wear volume were reduced by more than 50%[67,68]. Recently, they used polyionic liquids instead of ionic liquids to synthesize polyionic liquid-modified carbon dots with better lubrication performance by surface-initiated atom transfer radical polymerization[90]. In 2018, Cai et al. Modified the surface of carbon dots with ionic liquids by covalent grafting, and improved the lubrication performance and bearing capacity of carbon dots[83]. In 2020, Bad Badía-La Laíño et al. prepared more environmentally friendly metal-free carbon dots modified by ionic liquids (Figure 4B) by using ionic liquids without metal elements as modification groups and hydrothermal method. The carbon dots can be stably dispersed in an organic solution for a long time, and as an additive of polyalphaolefin, the friction coefficient and the wear volume can be respectively reduced by more than 30% and 60% under extreme test conditions[71].
Besides ionic liquids, oleylamine, diphenylamine, PEG, polyethyleneimine (PEI), etc. Are also used as surface groups to modify carbon dots. In 2017, Wang et al. directly synthesized oleylamine modified carbon dots by pyrolyzing citric acid and oleylamine at 300 ℃ for 3 H as an additive for polyalphaolefin. When the additive concentration was 1.0 wt%, the friction coefficient and wear scar diameter were significantly reduced (Fig. 4C); More importantly, with the increase of friction time and load, the friction-reducing and anti-wear properties of these carbon dots are enhanced[79]. Subsequently, Cai et al. Pyrolyzed the hydrophilic carbon dots and oleylamine for 6 H, and successfully modified the oleylamine onto the surface of the carbon dots to form hydrophobic oleamine-coated carbon dots. The prepared carbon dots have the potential of multifunctional additives, which can not only be used as additives to improve the lubricating performance of commercial engine oil 5W-30, but also have excellent oxidation resistance[65,66]. In addition, they used pyrolysis to prepare carbon dots with diphenylamine groups on the surface in one step, which were used as additives for PEG or castor oil to improve their lubricating properties[91][66]. Compared with ionic liquids, oleylamine and other surface modifiers, PEGs have the advantages of low cost, environmental protection and degradability. At the same time, a large number of literatures have proved that PEG can be grafted onto the surface of carbon dots by simple treatment. Based on this, in 2019, our research group designed a simple ultrasonic method to directly prepare PEG-coated carbon dots in one step using cheap PEG, citric acid and urea as raw materials (Fig. 4D).
In the process of ultrasonic treatment, citric acid and urea dissolved in PEG are dehydrated and carbonized to form carbon dots, and PEG is not only used as a basic liquid, but also used as a passivating agent to modify the surface of the carbon dots to form PEG-coated carbon dots; The four-ball friction test shows that the friction coefficient and the wear scar diameter of the prepared lubricant are reduced by 67% and 29%, respectively, compared with those of pure polyethylene glycol at a load of 600 N[72]. Subsequently, Chen et al. Prepared PEG-modified carbon dots by pyrolysis in one step, and studied the tribological properties of PEG-modified carbon dots with different molecular weights (200-4000). Among them, PEG-200 modified carbon dots had the best lubrication performance, which reduced the friction coefficient and wear volume by 83. 5% and 90. 9%, respectively[92]. In addition, Wang et al. Directly prepared PEI modified carbon dots by hydrothermal treatment of citric acid and PEI, which were used as water-based additives to improve the lubricity of water[70]. Subsequently, they further reacted PEI-modified carbon dots with ionic liquids to synthesize branched polyelectrolyte-modified carbon dots, which have excellent friction-reducing and anti-wear properties as PEG additives (Fig. 4E)[93].
As mentioned above, because of its large specific surface area and a large number of active sites, it is easy to be modified by specific groups, so its polarity and stability can be adjusted, it can be used as an additive for different base fluids, and it has considerable competitive advantages in the field of lubrication. More importantly, preparation methods such as a pyrolysis method, a hydrothermal method, an ultrasonic method and the like can realize that specific groups are directly grafted or assembled on the surface of the carbon dot in the generation process, and the carbon dot coated by the specific groups can be conveniently synthesized in one step without a subsequent surface modification step, so that the preparation time is greatly saved.
(3) Heteroatom doping
Heteroatom doping inevitably leads to changes in the internal structure and electronic distribution of carbon dots, which provides a new way to adjust the electronic properties, elemental composition and surface chemistry, not only to change the fluorescence properties of carbon dots, but also to improve their lubrication properties[61,65,84,94]. Because of the small size of boron atoms, boron atoms are easy to penetrate into the metal friction interface in the friction process and promote the strengthening of the metal surface; The size of nitrogen atom is similar to that of carbon, and its valence electrons can form strong valence bonds with carbon atoms, and nitrogen-doped carbon dots have better water solubility, stronger electron-withdrawing ability and stability[42]. Therefore, it is necessary to study the lubrication performance of boron and nitrogen co-doped carbon dots. In 2017, Cai et al. First used boron-containing ionic liquid and glutamic acid as raw materials to prepare boron and nitrogen co-doped carbon dots in one step by pyrolysis (Fig. 5A); Different from the surface modified ionic liquid carbon dots, the carbon skeleton of the prepared carbon dots is doped with the ionic liquid containing boron element as an additive of the PEG, so that the friction coefficient and the wear volume are respectively reduced by 89.34% and 62.18% at a high load of 588 N, and the antiwear and friction-reducing performance of the PEG is remarkably improved[61]. Subsequently, Cai et al., Yan et al., Tomala et al. studied the tribological properties of nitrogen-doped carbon dots[65][76][94]. Cai et al. And Yan et al. Used citric acid and urea as carbon and nitrogen sources respectively, and prepared nitrogen-doped carbon dots with excellent water solubility in one step by pyrolysis method or gas phase detonation method.It shows long-term stability and excellent anti-friction and anti-wear performance, which can reduce the friction coefficient and wear volume by more than 50% at the same time (Fig. 5B). Tomala et al. Used bovine serum albumin as carbon and nitrogen source to synthesize nitrogen-containing carbon dots by hydrothermal method[65][76]. The prepared carbon dots have extraordinary extreme pressure performance and can improve the bearing capacity of the base liquid by more than 15 times[94].
In addition to boron, nitrogen and other elements, metal elements, sulfur, fluorine and other elements are also doped into the carbon dots to improve their lubrication performance. In 2019, Cai et al. First studied the tribological properties of metal-doped carbon dots. Molybdenum-doped carbon dots were prepared by pyrolyzing the mixture of ammonium molybdate, glutamic acid and ionic liquid at 200 ℃ for 20 min. The prepared carbon dots have strong interaction with the iron ions on the surface of the friction pair, and are easy to be adsorbed on the friction interface to form a lubricating film, thereby showing good friction performance[95]. In 2020, Wang et al. Successfully prepared nickel-doped carbon dots by hydrothermal method using citric acid, ethylenediamine and nickel acetate as raw materials, which showed better lubrication performance than carbon dots without metal elements[84]. In the same year, Tomala et al. Demonstrated that gallium-doped carbon dots have good wear resistance[94]. Wang et al., Xiao et al. Studied the lubrication performance of sulfur-doped carbon dots[96][97]. In 2018, Wang et al. used rice husk powder as raw material to synthesize sulfur-nitrogen co-doped carbon dots by hydrothermal reaction for 24 H. As an additive of PEG, the friction coefficient and wear rate could be reduced by 16.6% and 75.9% respectively when the addition amount was only 0.1 wt%[96]. In 2019, Xiao et al. Prepared water-soluble sulfur-doped carbon dots by hydrothermal method using sodium thiosulfate as sulfur source, which improved the friction-reducing and anti-wear properties of aqueous solution[97]. In addition, Cai et al. Synthesized fluorine-doped carbon dots by hydrothermal treatment using citric acid and fluorine-containing ionic liquids as raw materials. Compared with boron and nitrogen co-doped carbon dots, the prepared fluorine-doped carbon dots have better lubrication performance[98].
According to the above documents, boron, nitrogen, metal elements, sulfur, fluorine and other elements can be doped into the structure of carbon dots, thereby greatly improving the dispersion stability, friction reduction and anti-wear properties and extreme pressure properties of carbon dots. However, it should be noted that doping metal elements or sulfur elements can significantly improve the lubrication performance of carbon dots, but there are problems such as high preparation cost and environmental hazards.
Compared with lubricating fluid, lubricating grease can not only control friction and wear, but also has the functions of sealing and anti-corrosion, which is widely used in various valves and bearings. Therefore, it is of great significance to study the tribological properties of carbon dots as grease additives. In 2019, Sadeghalvaad et al. Prepared carbon dots by hydrothermal method and studied their properties as lithium grease additives. The results show that carbon dots can enhance the lubricating, sealing and anti-corrosion properties of lithium grease[99]. However, up to now, only this one article on the study of carbon dots as a grease additive can be retrieved.
To sum up, as a new nano-additive, carbon dots not only show excellent friction reduction and wear resistance, but also have excellent oxidation resistance and corrosion inhibition performance. In addition, by adjusting the polarity of the carbon point, the carbon point has good dispersion stability in water-based or oil-based lubricating fluid, and can be used as an additive of the water-based or oil-based lubricating fluid. More importantly, the tribological properties of carbon dots can be further improved by size and shape control, surface modification and heteroatom doping. Therefore, carbon dots have great potential to become the next generation of green, efficient and multi-functional nano-additives.

2.2 Lubricating coating

Lubricating oils/greases are widely used in industrial and engineering lubrication fields, but in the fields of micro/nano-electromechanical systems, bio-lubrication and so on, lubricating coatings are more commonly used and effective lubrication means. In recent years, with the rapid development of micro robots, precision positioning equipment and nano-sensors, micro/nano electro-mechanical systems have attracted more and more attention. However, frictional wear caused by contact between moving mechanical parts limits the performance and service life of the system. To control friction and wear, one of the most effective methods is to apply a lubricating coating between the contact surfaces[100]. Therefore, it is necessary to study the tribological properties of carbon dots as lubricating coatings.
In 2017, Zhang et al. First studied the lubricating effect of carbon dots as lubricating coatings. Firstly, glucose and hydrochloric acid were used as raw materials to synthesize carbon dots by acid-assisted ultrasonic method. Subsequently, ionic liquid modified carbon dots with a core-shell structure were prepared by grinding the carbon dots and the ionic liquid (fig. 6a); Finally, the prepared carbon dots are modified on the surface of a silicon wafer serving as a substrate to form a lubricating coating; The ball-on-disc friction test showed that the coating had excellent tribological properties, and the friction coefficient and wear rate were 0.006 and 7×10-6mm3/Nm, respectively[26]. In 2018, Wilhelm et al. Synthesized dodecylamine functionalized carbon dots by multi-step method including preparation of graphene quantum dots and surface functionalization, and then coated the prepared graphene quantum dots on a steel substrate by spray deposition to form a lubricating coating. The prepared lubricating coating not only reduces the friction coefficient from 0.17 to 0.11, but also has remarkable corrosion inhibition performance[101]. In 2020, Yang et al. prepared water-soluble graphene quantum dots with a particle size of 1 ~ 3 nm by a hydrothermal method, and then used its aqueous solution as a raw material to obtain a dense graphene quantum dot lubricating coating on the surface of a silicon wafer by a simple electrophoretic deposition method (Fig. 6B), and the thickness and roughness of the coating can be changed by adjusting the deposition voltage; The friction coefficient and wear volume can be reduced by 85% and 97%, respectively, which is helpful to improve the lubrication performance and prolong the service life of silicon-based devices[100].
As an important means of lubrication, lubricating coatings have broad application prospects in micro/nano electromechanical systems. At present, nano-materials such as diamond-like carbon, molybdenum disulfide, tungsten disulfide, graphene and carbon dots are used as lubricating coatings to enhance the lubricating effect of the substrate. Compared with other nano-materials, carbon dots (CDs) are one of the most promising and effective lubricating coating materials due to their environmental friendliness, simple preparation, low cost and excellent lubricating performance. However, the existing literatures mainly focus on the tribological properties of carbon dots prepared by surface modification strategy as lubricating coatings, and there is still a lack of research on carbon dots prepared by size and shape control, heteroatom doping and other strategies as lubricating coatings.

2.3 Other Lubrication Applications

As a new type of lubricant material, carbon dots have shown great potential applications not only in industrial lubrication and micro/nano-electromechanical systems lubrication, but also in bio-lubrication due to their good biocompatibility and degradability. In the study of biological tribology, the problem of artificial joint lubrication has attracted great attention. After artificial joint replacement, the synovial tissue of the joint is excised, which makes the body fluid the only way to lubricate. Its limited lubrication performance makes the artificial joint material produce more wear debris, which seriously shortens the service life of the artificial joint[102]. Conventional lubricants are diluted by body fluids and absorbed by the human body. Therefore, it is necessary to develop new lubricating materials to control the wear of artificial joints. In 2017, Dong et al. Successfully prepared carbon dot/chitosan/PEG composite gel, in which PEG was grafted onto the surface of carbon dot through hydrogen bonding to form PEG-modified carbon dot (Fig. 7A); When the gel was added to the laser texture, it showed excellent slow release performance, and the release solution contained PEG-modified carbon dots, which could significantly reduce the friction coefficient and wear between the friction interfaces (Fig. 7B); Therefore, the composite gel is expected to be used as a lubricating material for artificial joints[102]. Subsequently, they further prepared other types of composite gels containing PEG-modified carbon dots[103,104]. The addition of carbon dots can enhance the rheology and mechanical properties of the composite gel, and make it have better slow-release and lubrication properties. At the same time, the safety evaluation experiment proves that this type of composite gel has good biocompatibility and is safe to use in vivo, which further proves the feasibility of the composite gel containing carbon dots as a lubricating material for artificial joints[103].
In addition, Hajalilou et al also used carbon dots as additives for magnetorheological fluids, which greatly improved the shear stress and sedimentation rate of magnetorheological fluids, enabling them to be used in mechanical systems requiring active control of vibration or transmission of torque[105].
As mentioned above, carbon dots have been widely used in industrial lubrication, micro/nano electromechanical system lubrication, bio-lubrication and other lubrication fields as nano-additives, lubricating coatings, lubricating gels and so on because of their excellent friction reduction, wear resistance, corrosion inhibition and oxidation resistance, as well as good biocompatibility and environmental friendliness. However, up to now, there are only more than 40 research literatures on the application of carbon spot lubrication. Compared with the research on the application of carbon dots in sensing, bio-imaging and catalysis, the research on the application of carbon dots in lubrication is still in its infancy.

3 Lubrication Mechanism

It is of great theoretical significance to study the role of nano-materials in friction system and explain its lubrication mechanism for the design and development of nano-materials with more efficient lubrication performance. At present, the lubrication applications of carbon dots are mostly concentrated in the field of industrial lubrication, that is, carbon dots are used as nano-lubricants to improve the tribological properties of lubricants. Therefore, the research on the lubrication mechanism of carbon dots is mainly focused on how to achieve friction reduction and anti-wear as nano-additives.
A large number of scholars proposed the lubrication mechanism of nanoparticles as additives based on the morphology characterization of nanoparticles and the analysis of friction interface. The existing lubrication mechanisms can be roughly divided into four types: (1) lubrication protective film: preventing the friction interface from direct contact[6,16⇓⇓⇓⇓⇓ ~22]; (2) roll effect: it acts as a rolling bear to convert sliding friction into rolling friction; (3) fil effect: compensating wear Los and reducing that roughness of a friction interface; (4) Polishing effect: reduce the roughness of the friction interface. According to the existing lubrication mechanism of nanoparticles, the morphology characteristics of carbon dots and the analysis of friction interface, scholars have expounded the lubrication mechanism of carbon dots. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and other modern detection methods are generally used to analyze the friction interface.

3.1 Lubrication Mechanism of Size and Shape Controlled Like Carbon Dot

Up to now, Fan et al, Yan et al, Zhao et al and Wang et al have respectively studied the lubrication mechanism of the size and shape control carbon dots prepared by them[62][77][86][69]. In 2017, Fan et al. Used XPS, Raman and other means to analyze the friction interface lubricated by graphene quantum dots, and found that there was no Fe 2p peak in XPS and only 1516 cm-1 peak in Raman, indicating that graphene quantum dots formed a lubricating protective layer on the friction interface. Furthermore, TEM and Raman were used to analyze the microstructure of the graphene quantum dot debris after the friction experiment, and TEM showed that the debris was "Ganoderma lucidum cloud", while Raman contained D and G peaks.It is indicated that the graphene quantum dots participate in the friction, and the structure after friction becomes a graphene-like structure (Fig. 8A), so the lubrication mechanism of the graphene quantum dots is attributed to the synergistic effect of the lubricating protective film on the friction interface and the graphene-like debris[62]. In 2018, our research group used SEM and EDS to study the friction interface morphology and carbon element distribution, and found that the wear spot diameter of graphene quantum dots after lubrication was smaller and the carbon content was more, which proved that graphene quantum dots were adsorbed on the friction surface, and speculated that the lubrication mechanism was the synergistic effect of lubrication protective film, polishing effect and repair effect (Fig. 8B)[77]. In 2019, Zhao et al. used TEM to observe the cross section of the wear scar lubricated by carbon dots, and clearly observed the lubricating protective film with carbon dots inside (Fig. 8 C), while Raman showed D and G peaks. The above characterization shows that carbon dots can not only be adsorbed on the friction interface to form a lubricating protective film, but also achieve friction reduction and anti-wear effects through rolling effect and filling effect (Fig. 8 d)[86]. In the same year, Wang et al. analyzed the friction interface through SEM, EDS, Raman, etc., and considered that the main reason why carbon dots as a water-based lubricant play a role in reducing friction and anti-wear is that small carbon dots are easy to gather on the surface of the contact zone to form an effective lubricating protective film[69].

3.2 Lubrication mechanism of surface modified carbon dots

Surface-modified carbon dots have abundant surface groups, which make them more easily adsorbed on the friction interface to form a lubricating protective film. Wang et al, Cai et al, Chen et al and Yan et al studied the lubrication mechanism of different kinds of surface-modified carbon dots. In 2016, Wang et al. Used SEM, EDS and other means to analyze the friction interface, and found that the types and contents of elements on the friction surface are quite different after lubrication with different lubricants (containing carbon points and not containing carbon points).Therefore, the lubrication mechanism of ILs modified carbon dots was proposed, namely synergistic lubrication effect, including the protective film lubrication of ILs and the nano-lubrication of carbon nuclei (rolling, repairing and polishing effects). In particular, in the friction process, the contact salient points of the friction pair emit electrons and are positively charged in the friction process; Ionic liquid carbon dots with negative charge are more easily adsorbed on the friction interface under the action of electrostatic attraction to form a physical adsorption lubricating film; At low load, the lubricating film reduces wear by preventing direct contact of the friction interface, and the rolling effect of the carbon core reduces friction, while at high load, the tribochemical reaction between the ionic liquid group and the friction interface forms a chemical reaction film to control friction and wear, and the polishing and repairing effect of the carbon core reduces scratches on the friction surface (Fig. 9a)[30]; For the carbon dots without surface modification, although they can also be adsorbed on the friction interface, they have poor embedding stability and are easily squeezed out, resulting in poor lubrication effect (Fig. 9b)[68]. Subsequently, they further analyzed the friction interface by Raman, XPS, TEM and other means, and verified that the lubrication mechanism of surface-modified carbon dots was attributed to the synergistic effect of surface groups and carbon nuclei[90,93]. In 2018, Cai et al. Used SEM, EDS, Raman and other means to analyze the friction interface of diphenylamine-modified carbon dots after lubrication, and proposed that the lubrication mechanism of diphenylamine-modified carbon dots was the combined action of lubrication protective film and rolling effect[91]. In 2019, Chen et al. attributed the lubrication mechanism of PEG-modified carbon dots to the composite lubrication film composed of carbon dots and Fe3O4 by analyzing the elements of friction interface[92]. Compared with other nanoparticles, carbon dots have unique fluorescence characteristics, which provide a new way to trace their final position and help to clarify their role in lubrication. Inspired by this, our research group not only analyzed the friction interface, but also compared the photoluminescence intensity of the carbon dot-based lubricating fluid before and after the friction experiment, and found that the intensity value was almost constant, indicating that most of the carbon dots were not transferred from the lubricating oil to the friction interface, so the prepared PEG-modified carbon dots improved the lubrication performance mainly through the rolling effect (Fig. 9c)[72].

3.3 Lubrication mechanism of carbon-like dots doped with heteroatom

Cai et al., Wang et al., Xiao et al. studied the lubrication mechanism of different kinds of heteroatom-doped carbon-like dots. In 2016, Cai et al. proposed that the lubrication mechanism of boron and nitrogen co-doped carbon dots was lubrication protective film based on the analysis of friction interface and the comparison of photoluminescence intensity of carbon dot solution before and after friction experiment. The friction interface was analyzed by SEM, EDS and Raman, and it was found that boron, nitrogen, carbon and other elements were distributed on the friction interface after carbon point lubrication. With the increase of load and friction time,The structure of carbon dots on the friction interface changes from disorder to order, and the photoluminescence intensity of the carbon dot solution decreases significantly after the friction test, which indicates that the carbon dots are transferred from the lubricating liquid to the friction interface and form a lubricating protective film[61]. Subsequently, they used the same analysis method to propose the lubrication mechanism of molybdenum-doped carbon dots and fluorine-doped carbon dots, both of which are lubrication protective films[95][98]. In 2018, Wang et al. used SEM, EDS, Raman, XPS and other means to analyze the friction interface and found that: in the case of high load and high-speed friction, when only the basic liquid is used for lubrication, the wear caused by friction makes the surface of the friction pair scratch each other and form many furrows; After sulfur and nitrogen co-doped carbon dots are added, these carbon dots are deposited in the furrow of the wear surface, and then tribochemical reaction occurs to form a lubricating protective film, which greatly reduces the friction coefficient and wear rate. Therefore, it is considered that the lubricating mechanism of sulfur and nitrogen co-doped carbon dots is the lubricating protective film (Fig. 10a)[96]. In 2019, Xiao et al. Used SEM and EDS to analyze several friction interfaces, and proposed that the lubrication mechanism of sulfur-doped carbon dots as water-based nano-additives is rolling effect and sliding effect. At the beginning of the friction experiment, most of the carbon dots remained spherical and showed rolling effect. As the experiment progresses, some carbon dots are crushed under the action of normal stress and shear stress to form a flaky structure, and shear sliding is more likely to occur under the action of shear stress (Fig. 10B)[97].
In addition, the lubrication mechanism of the composite nanoparticles containing carbon dots is mainly the synergistic effect of carbon dots and other nanoparticles. For example, Kang et al. attributed the lubrication mechanism of carbon dots /CuSx composite nanoparticles to the morphological characteristics of carbon dots (small size and multilayer graphite structure) and the high chemical activity of CuSx nanoparticles; The former promotes the formation of small and dispersed composite nanoparticles, while the latter is easy to form a wear resistant protective film of CuFeySx-y alloy on the steel substrate of the friction interface, thereby improving the lubrication performance[28].
There are many preparation methods of carbon dots and wide sources of raw materials, resulting in great differences in their structure, morphology, size and so on. Therefore, there is no unified lubrication mechanism. However, it can be seen from the above documents that the lubrication mechanism of carbon dots synthesized by any strategy can be attributed to one or more of lubrication protective film, rolling effect, filling effect and polishing effect.

4 Conclusion and prospect

In this paper, the applications of carbon dots as nano-additives, lubricating coatings and lubricating gels in industrial lubrication, micro/nano-electromechanical systems lubrication, bio-lubrication and other lubrication fields are introduced in detail, and three strategies to improve the tribological properties of carbon dots are summarized, including size and shape control, surface modification and heteroatom doping. Then, the lubrication mechanism of carbon dots as nano-additives is comprehensively analyzed, including lubrication protective film, rolling effect, filling effect, polishing effect and so on, which provides theoretical guidance for the design and development of carbon dots with better lubrication performance. A large number of studies have shown that carbon dots have great potential as a new type of green and efficient friction-reducing, antiwear and lubricating materials due to their small size, adjustable surface groups, excellent dispersion stability, good biocompatibility, low toxicity, environmental friendliness and convenient low-cost preparation method.
Great progress has been made in the application of carbon dots in the field of lubrication, but there are obvious shortcomings compared with its application in the fields of biological imaging, sensing, catalysis and optoelectronic devices. In 2015, the first literature on the lubrication performance of carbon dots appeared[30]. Up to now, there are less than 50 literatures about the tribological properties and lubrication applications of carbon dots on the Web of Science. Therefore, the application research of carbon spot lubrication is still in its infancy, and there are still a lot of problems to be solved. (1) There is a lack of rapid and efficient methods to prepare high-purity carbon dots in large quantities. Although many methods, such as hydrothermal method, pyrolysis method, ultrasonic method and microwave method, can synthesize carbon dots at low cost and in large quantities, it takes several days of dialysis to obtain carbon dots with high purity. Some scholars have designed and developed a new method to prepare carbon dots, which avoids the time-consuming dialysis purification process, but there is a lack of research on the tribological properties of the prepared carbon dots. Recently, our research group prepared a lubricant containing carbon dots by ultrasonic method in one step, which provides a new idea for the preparation of lubricants containing carbon dots, but it is not suitable for all types of basic fluids[72]. (2) Lack of quantitative relationship between the tribological properties of carbon dots and their structure (morphology, size, surface groups). Although the strategies of size and shape control, surface modification and heteroatom doping can be used to improve the lubricating properties of carbon dots, there is a lack of relevant research to establish the quantitative relationship between the structural characteristics of carbon dots, such as morphology, size, surface groups, and their lubricating properties. (3) The anti-wear performance of carbon dots still needs to be further improved. Compared with copper nanoparticles, carbon dots as nano-additives show more excellent friction-reducing properties, but more general anti-wear properties. Therefore, the anti-wear performance of carbon dots still needs to be further improved. (4) The lubrication mechanism of carbon dots as nano-additives is still unclear. Although various modern detection technologies such as TEM, SEM, Raman, EDS and XPS have been used to study the friction interface in detail, and the lubrication mechanism of carbon dots as nano-additives has been speculated, there is still a lack of corresponding theoretical support. A large number of literatures have explained the lubrication mechanism of nano-diamond, nano-onion carbon and other nano-additives by molecular dynamics and other computer simulation methods. Therefore, the lubrication mechanism of carbon dots as nano-additives can be further determined by computer simulation. (5) There is insufficient research on carbon dots as grease additives, lubricating coatings and lubricating gels. The vast majority of the literature on the application of carbon dots in lubrication is on the use of carbon dots as nano-additives in lubricating fluids. Therefore, it is necessary to vigorously study the application of carbon dots as grease additives, lubricating coatings and lubricating gels, and further adopt strategies such as size and shape control, surface modification and heteroatom doping to improve their lubricating performance. (6) Lack of compatibility evaluation of carbon dots as nano-additives. In the existing literature, only the lubricating properties of carbon dots as a single nano-additive in lubricating fluids or greases have been studied, and the compatibility of carbon dots with other additives in lubricating systems has not been studied. (7) There is no unified evaluation standard to effectively evaluate the tribological properties of carbon dots. Under different test conditions, the lubrication States of friction pairs can be divided into hydrodynamic lubrication, mixed lubrication and boundary lubrication. The different test methods for the tribological properties of carbon dots in the existing literatures may lead to different lubrication States, which makes it impossible to compare the tribological properties of carbon dots in different literatures. Therefore, it is necessary to establish a unified evaluation standard to effectively evaluate the tribological properties of carbon dots.
In this paper, the research progress in the application of carbon dots in lubrication was systematically introduced, so as to provide reference for the design and development of carbon dots or other nanoparticles with better lubrication performance, and promote the further development of lubrication materials and lubrication technology.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]

Holmberg K, Erdemir A. Friction, 2017, 5(3): 263.
[2]

Holmberg K, Erdemir A. Tribol. Int., 2019, 135: 389.
[3]

Holmberg K, Kivikytö-Reponen P, Härkisaari P, Valtonen K, Erdemir A. Tribol. Int., 2017, 115: 116.
[4]

Lin Y C, So H. Tribol. Int., 2004, 37(1): 25.
[5]

Barnes A M, Bartle K D, Thibon V R A. Tribol. Int., 2001, 34(6): 389.
[6]

Wu H, Zhao J W, Xia W Z, Cheng X, He A, Yun J H, Wang L, Huang H, Jiao S, Huang L, Zhang S, Jiang Z. Tribol. Int., 2017, 109: 398.
[7]

Gu K C, Chen B S, Chen Y. J. Rare Earths, 2013, 31(6): 589.
[8]

Jiao D, Zheng S H, Wang Y Z, Guan R F, Cao B Q. Appl. Surf. Sci., 2011, 257(13): 5720.
[9]

Guo Y X, Zhang L G, Zhang G, Wang D A, Wang T M, Wang Q H. J. Mater. Chem. A, 2018, 6(6): 2817.
[10]

Liu G, Li X, Qin B, Xing D, Guo Y, Fan R. Tribol. Lett., 2004, 17(4): 961.
[11]

Gusain R, Khatri O P. J. Mater. Chem. A, 2013, 1(18): 5612.
[12]

Wu X H, Gong K L, Zhao G Q, Lou W J, Wang X B, Liu W M. Adv. Mater. Interfaces, 2018, 5(1): 1700859.
[13]

Ju C, Zheng D D, Zhao Q, Wang X B. Tribol. Lett., 2020, 68(3): 1.
[14]

Ma L, Liu Z, Cheng Z L. Ceram. Int., 2020, 46(3): 3786.
[15]

Gong K L, Lou W J, Zhao G Q, Wu X H, Wang X B. Friction, 2019, 8: 674.
[16]

Pei X W, Hu L T, Liu W M, Hao J C. Eur. Polym. J., 2008, 44: 2458.
[17]

Meng Y, Su F H, Chen Y Z. Tribol. Int., 2018, 118: 180.
[18]

Raina A, Anand A. Appl. Nanosci., 2017, 7(7): 371.
[19]

Ivanov M, Shenderova O. Curr. Opin. Solid State Mater. Sci., 2017, 21(1): 17.
[20]

Lee J, Cho S, Hwang Y, Cho H J, Lee C, Choi Y, Ku B C, Lee H, Lee B, Kim D, Kim S H. Tribol. Int., 2009, 42: 440.
[21]

Ku B C, Han Y C, Lee J E, Lee J K, Park S H, Hwang Y J. Int. J. Precis. Eng. Manuf., 2010, 11(4): 607.
[22]

Eswaraiah V, Sankaranarayanan V, Ramaprabhu S. ACS Appl. Mater. Interfaces, 2011, 3(11): 4221.
[23]

Hu Y W, Wang Y X, Zeng Z X, Zhao H C, Ge X, Wang K, Wang L, Xue Q. Carbon, 2018, 137: 41.
[24]

Luo N, Xiang J X, Shen T, Liang H L, Xin S. Diam. Relat. Mater., 2019, 97: 107448.
[25]

He C, Yan H H, Li X J, Wang X H. J. Mater. Sci., 2021, 56(2): 1286.
[26]

Ma W, Gong Z B, Gao K X, Qiang L, Zhang J Y, Yu S R. Mater. Lett., 2017, 195: 220.
[27]

Shang W J, Cai T, Zhang Y X, Liu D, Liu S G. Tribol. Int., 2018, 118: 373.
[28]

Huang H, Hu H L, Qiao S, Bai L, Han M M, Liu Y, Kang Z H. Nanoscale, 2015, 7(26): 11321.
[29]

Zhang W L, Cao Y L, Tian P Y, Guo F, Tian Y, Zheng W, Ji X, Liu J. ACS Appl. Mater. Inter., 2016, 8: 32440.
[30]

Wang B G, Tang W W, Lu H S, Huang Z Y. J. Mater. Chem. A, 2016, 4(19): 7257.
[31]

Xu X Y, Ray R, Gu Y L, Ploehn H J, Gearheart L, Raker K, Scrivens W A. J. Am. Chem. Soc., 2004, 126(40): 12736.
[32]

Sun Y P, Zhou B, Lin Y, Wang W, Fernando K A S, Pathak P, Meziani M J, Harruff B A, Wang X, Wang H, Luo P G, Yang H, Kose M E, Chen B, Veca L M, Xie S Y. J. Am. Chem. Soc., 2006, 128: 7756.
[33]

Kang Z H, Lee S T. Nanoscale, 2019, 11(41): 19214.
[34]

Baker S N, Baker G A. Angew. Chem. Int. Ed., 2010, 49: 6726.
[35]

Yao B W, Huang H, Liu Y, Kang Z H. Trends Chem., 2019, 1(2): 235.
[36]

Liu M L, Yang L, Li R S, Chen B B, Liu H, Huang C Z. Green Chem., 2017, 19: 3611.
[37]

Shuang E, Mao Q X, Yuan X L, Kong X L, Chen X W, Wang J H. Nanoscale, 2018, 10(26): 12788.
[38]

Zhu S J, Song Y B, Zhao X H, Shao J R, Zhang J H, Yang B. Nano Res., 2015, 8(2): 355.
[39]

Hu C, Yu C, Li M Y, Wang X, Yang J, Zhao Z, Eychmuller A, Sun Y P, Qiu J. Small, 2014, 10: 4926.
[40]

Dong Y Q, Li G L, Zhou N N, Wang R, Chi Y, Chen G. Anal. Chem., 2012, 84: 8378.
[41]

Zhou L, Lin Y H, Huang Z Z, Ren J S, Qu X G. Chem. Commun., 2012, 48: 1147.
[42]

Hai X, Mao Q X, Wang W J, Wang X F, Chen X W, Wang J H. J. Mater. Chem. B, 2015, 3(47): 9109.
[43]

E S, Mao Q X, Wang J H, Chen X W. Nanoscale, 2020, 12: 6852.
[44]

Yu S, Zhong Y Q, Yu B Q, Cai S Y, Wu L Z, Zhou Y. Phys. Chem. Chem. Phys., 2016, 18(30): 20338.
[45]

Li H T, He X D, Kang Z H, Huang H, Liu Y, Liu J, Lian S, Tsang C H, Yang X, Lee S T. Angew. Chem. Int. Ed., 2010, 49: 4430.
[46]

Yan Y B, Chen J, Li N, Tian J Q, Li K, Jiang J, Liu J, Tian Q, Chen P. ACS Nano, 2018, 12: 3523.
[47]

Zhang F, Feng X T, Zhang Y, Yan L P, Yang Y Z, Liu X G. Nanoscale, 2016, 8(16): 8618.
[48]

Guo X, Wang C F, Yu Z Y, Chen L, Chen S. Chem. Commun., 2012, 48(21): 2692.
[49]

Mao L H, Tang W Q, Deng Z Y, Liu S S, Wang C F, Chen S. Ind. Eng. Chem. Res., 2014, 53(15): 6417.
[50]

Qu S N, Wang X Y, Lu Q P, Liu X Y, Wang L J. Angew. Chem. Int. Ed., 2012, 51: 12215.
[51]

Li Y D, Xu X K, Wu Y, Zhuang J, Zhang X, Zhang H, Lei B, Hu C, Liu Y. Mater. Chem. Front., 2020, 4: 437.
[52]

Li H, Huang J, Liu Y, Lu F, Zhong J, Wang Y, Li S M, Lifshitz Y, Lee S T, Kang Z H. Nano Res., 2019, 12(7): 1585.
[53]

Zhu C, Fu Y J, Liu C G, Liu Y, Hu L, Liu J, Bello I, Li H, Liu N, Guo S, Huang H, Lifshitz Y, Lee S T, Kang Z. Adv. Mater., 2017, 29: 1701399.
[54]

Wu S S, Li W, Zhou W, Zhan Y, Hu C F, Zhuang J L, Zhang H R, Zhang X J, Lei B F, Liu Y L. Adv. Opt. Mater., 2018, 6(7): 1701150.
[55]

Yan Y B, Gong J, Chen J, Zeng Z P, Huang W, Pu K Y, Liu J Y, Chen P. Adv. Mater., 2019, 31(21): 1808283.
[56]

Sun X C, Lei Y. Trend. Aanl. Chem., 2017, 89: 163.
[57]

Liu M L, Chen B B, Li C M, Huang C Z. Green Chem., 2019, 21(3): 449.
[58]

Hu C, Li M Y, Qiu J S, Sun Y P. Chem Soc Rev, 2019, 48: 2315.
[59]

Fernando K A S, Sahu S, Liu Y M, Lewis W K, Guliants E A, Jafariyan A, Wang P, Bunker C E, Sun Y P. ACS Appl. Mater. Interfaces, 2015, 7(16): 8363.
[60]

Hai X, Feng J, Chen X W, Wang J H. J. Mater. Chem. B, 2018, 6(20): 3219.
[61]

Zhang Y X, Cai T, Shang W J, Liu D, Guo Q, Liu S G. Dalton Trans., 2017, 46(36): 12306.
[62]

Fan X Q, Li W, Fu H M, Zhu M, Wang L, Cai Z, Liu J, Li H. ACS Sustain. Chem. Eng., 2017, 5: 4223.
[63]

Zhou Y, Qu J. ACS Appl. Mater. Interfaces, 2017, 9(4): 3209.
[64]

Li J, Chen H, Stone H A. Proc Natl Acad Sci U.S.A., 2013, 110: 20023.
[65]

Shang W J, Ye M T, Cai T, Zhao L N, Zhang Y, Liu D, Liu S. J. Mol. Liq, 2018, 266: 65.
[66]

Ye M T, Cai T, Zhao L, Liu D, Liu S G. Tribol. Int., 2019, 136: 349.
[67]

Liu X, Huang Z Y, Tang W W, Wang B G. Nano, 2017, 12: 1750108.
[68]

Tang W W, Wang B G, Li J T, Li Y Z, Zhang Y, Quan H P, Huang Z Y. J. Mater. Sci., 2019, 54(2): 1171.
[69]

Qiang R B, Hu L F, Hou K M, Wang J Q, Yang S R. Tribol. Lett., 2019, 67: 64.
[70]

Mou Z H, Wang B G, Huang Z Y. Fuller. Nanotub. Car. N., 2019, 27: 899.
[71]

Chimeno-Trinchet C, Pacheco M E, Fernández-González A, Díaz-García M E, Badía-Laíño R. J. Int. Eng. Chem., 2020, 87: 152.
[72]

He C, Yan H H, Li X J, Wang X H. Green Chem., 2019, 21: 2279.
[73]

Zhang R H, Xiong L P, Pu J B, Lu Z B, Zhang G G, He Z Y. Adv. Mater. Interfaces, 2019, 6(24): 1901386.
[74]

Xu M H, He G L, Li Z H, He F, Gao F, Su Y, Zhang L, Yang Z, Zhang Y. Nanoscale, 2014, 6: 10307.
[75]

Tang L, Ji R, Cao X, Lin J, Jiang H, Li X, Teng K S, Luk C M, Zeng S, Hao J, Lau S P. ACS Nano, 2012, 6: 5102.
[76]

He C, Yan H H, Li X J, Wang X H. Diam. Relat. Mater., 2019, 91: 255.
[77]

He C, Yan H H, Wang X H, Bai M L. Diam. Relat. Mater., 2018, 89: 293.
[78]

Yan H H, He C, Li X J, Zhao T J. Diam. Relat. Mater., 2018, 87: 233.
[79]

Lu H S, Tang W W, Liu X, Wang B G, Huang Z Y. J. Mater. Sci., 2016, 52: 4483.
[80]

Liu H P, Ye T, Mao C D. Angew. Chem. Int. Ed., 2007, 46: 6473.
[81]

Tian L, Ghosh D, Chen W, Pradhan S, Chang X J, Chen S W. Chem. Mater., 2009, 21(13): 2803.
[82]

Sarno M, Abdalglil Mustafa W A, Senatore A, de Scarpa D. Tribol. Int., 2020, 148: 106311.
[83]

Shang W J, Cai T, Zhang Y X, Liu D, Sun L, Su X, Liu S. Tribol. Int., 2018, 121: 302.
[84]

Tu Z Q, Hu E Z, Wang B B, David K D, Seeger P, Moneke M, Stengler R, Hu K H, Hu X G. Friction, 2020, 8(1): 182.
[85]

Tang J Z, Chen S Q, Jia Y L, Ma Y, Xie H M, Quan X, Ding Q. Carbon, 2020, 156: 272.
[86]

Hu Y W, Wang Y X, Wang C T, Ye Y W, Zhao H, Li J, Lu X, Mao C, Chen S, Mao J, Wang L, Xue Q. Carbon, 2019, 152: 511.
[87]

Ruiz V, Yate L, Langer J, Kosta I, Grande H J, Tena-Zaera R. Tribol. Int., 2019, 137: 228.
[88]

Seymour B T, Fu W X, Wright R A E, Luo H, Qu J, Dai S, Zhao B. ACS Appl. Mater. Inter., 2018, 10: 15129.
[89]

Gusain R, Mungse H P, Kumar N, Ravindran T R, Pandian R, Sugimura H, Khatri O P. J. Mater. Chem. A, 2016, 4(3): 926.
[90]

Mou Z H, Wang B G, Lu H S, Dai S S, Huang Z Y. Carbon, 2019, 154: 301.
[91]

Ye M T, Cai T, Shang W J, Zhao L, Zhang Y, Liu D, Liu S. Tribol. Int., 2018, 127: 557.
[92]

Liu X, Chen Y G. Fuller. Nanotub. Car. N., 2019, 27: 400.
[93]

Mou Z H, Wang B G, Lu H S, Quan H P, Huang Z Y. Carbon, 2019, 149: 594.
[94]

Tomala A M, Kumar V B, Porat Z E, Michalczewski R, Gedanken A. Lubricants, 2019, 7(4): 36.
[95]

Cai T, Zhang Y X, Liu D, Tong D Y, Liu S G. Mater. Lett., 2019, 250: 20.
[96]

Wang B B, Hu E Z, Tu Z Q, David K D, Hu K, Hu X, Yang W, Guo J, Cai W, Qian W, Zhang H. Appl. Surf. Sci., 2018, 462: 944.
[97]

Xiao H P, Liu S H, Xu Q, Zhang H. Sci. China Technol. Sci., 2019, 62(4): 587.
[98]

Zhao L N, Cai T, Ye M T, Liu D, Liu S G. Carbon, 2019, 150: 319.
[99]

Sadeghalvaad M, Dabiri E, Afsharimoghadam P. SN Appl. Sci., 2019, 1(3): 1.
[100]

Qiang R B, Hou K M, Wang J Q, Yang S R. Appl. Surf. Sci., 2020, 509: 145338.
[101]

Wolk A, Rosenthal M, Neuhaus S, Huber K, Brassat K, Lindner J K N, Grothe R, Grundmeier G, Bremser W, Wilhelm R. Sci. Rep., 2018, 8(1): 1.
[102]

Lu H L, Ren S S, Zhang P P, Guo J D, Li J, Dong G. RSC Adv., 2017, 7: 21600.
[103]

Lu H L, Lv L, Ma J, Ban W R, Ren S S, Dong G N, Li J H, Dang X Q. J. Mech. Behav. Biomed. Mater., 2018, 88: 261.
[104]

Guo J D, Mei T J, Li Y, Hafezi M, Lu H L, Li J H, Dong G N. J. Biomater. Sci. Polym. Ed., 2018, 29(13): 1549.
[105]

Hajalilou A, Abouzari-Lotf E, Abbasi-Chianeh V, Shojaei T R, Rezaie E. J. Alloys Compd., 2018, 737: 536.

Funding

National Natural Science Foundation of China(11672068)

National Natural Science Foundation of China(10872044)

National Natural Science Foundation of China(11672067)

PDF (21859KB)

2497

Accesses

0

Citation

Detail
Sections
Recommended

/

〈 〉

Address of Editorial Office: No. 33 Beisihuan Xilu, Zhongguancun , Beijing 100190, China
Tel: +86 10 8262 7757
Fax: +86 10 8262 7757
E-mail: progchem@mail.las.ac.cn
http://www.progchem.ac.cn
    • 　
    Copyright © Editorial Office of Progress in Chemistry