Nano high-entropy materials: Controllable synthesis, characterization techniques and thermal/photo catalytic applications

High entropy materials (HEMs) are emerging materials, which originated from high entropy alloys (HEAs). Compared to traditional alloys, HEAs exhibited extraordinary performance in the fields of mechanical, mechanics, magnetism, and thermo-electric response. According to the first report which was independently proposed by Cantor et al. and Yeh et al. respectively in 2004, HEAs are defined as a solid solution alloy consisting of five or more metal elements with an approximate equimolar ratio and have single phase structure [1,2]. The concept of HEAs provides a new idea for the design of novel alloys materials. With the continuous development of HEAs, the atomic ratio of metal elements composing HEAs can be expanded to 5∼35 at% [3,4]. Based on the definition of high entropy, formation of HEAs can be identified by the value of mixed configuration entropy (∆S_mix), which could be expressed as:

Eq. (1) originated from Boltzmann’s hypothesis on the relationship between entropy and system complexity [1]. Where R is the molar gas constant, x_i represents the mole fraction of anyone metal component, and S represents the value of the configurational entropy of mixing of the alloy system. According to Eq. (1), when the alloy is composed of metal elements approaching the equimolar ratio (x_i=1/n), the equation can be changed to the Eq. (2):

Where n represents the number of components in the alloy. According to the Eq. (2), when the n ≥ 5, the configurational entropy of mixing can be expressed as ∆S_mix>1.61R. Generally, the alloy with mixed ∆S_mix>1.5R can be defined as HEAs [[3], [4], [5], [6]]. With the progress of HEAs research, the alloy with ∆S_mix>1.36R is also identified as a HEAs for quaternary alloys [3,4,6]. Beside of Eq. (2), we also find that a more accurate expression of ∆S_mix, as show in Eq. (3) [7].

In the Eq. (3) originated from Boltzmann and Gibbs interpretation of entropy. sl1 and sl2 represent different sub-lattices in the structure. x_i and x_j represent the molar fraction of the i_th and j_th component of the respective sub-lattice, N and M are corresponding to the number of elements in the sub-lattices, respectively. Eq. (3) can be used to analyze the more local microscopic configuration entropy in the microstructure.

For the purpose of formation of single-phase solid solution in HEAs, components of HEAs can be discussed using thermodynamic criteria. In generally, the phase stability of multiple component systems can be evaluated by the follow equation:

Where ΔG_mix, ΔH_mix, and ΔS_mix represent the changes of Gibbs free energy, mixing enthalpy, and mixing configuration entropy, respectively. T is the thermodynamic temperature [8,9]. According to the Eq. (4), the value of ΔG_mix is directly determined by the relative values of ΔH_mix and TΔS_mix. The increase of T and ΔS_mix can lead to the decrease of ΔG_mix, thus promoting the formation of simple single-phase solution structure with uniformly mixed elements in an alloy system. In general, the crystal structures of typical HEAs are mainly include face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal close-packed (hcp) phases [10,11]. The judgmental basis of HEAs is proposed in Fig. 1.

Early researches proved that HEAs as bulk structural materials have great prospects due to the unique physical and chemical properties [[1], [2], [3],[12], [13], [14]]. HEAs contain four important effects which is different from the traditional alloys: High entropy effect, lattice distortion effect, sluggish diffusion effect and the “cocktail” effect. Specifically, (i) high entropy effect means the multiple elements (≥ 4) consisted HEAs can increase value of the configuration entropy of mixing in the system and promote the materials tend to form stable single phase solid solution structure [1,15]. (ii) The effect of lattice distortion refers to the different metal atoms with different atom size, which are confine into one crystal cell and generate strong lattice distortion. This results in the lattice obviously shift from ideal points compared to perfect crystal [6]. (iii) The sluggish diffusion effect refers to the lattice distortion, increasing the diffusion active energy of atoms in the lattice and thus showing excellent thermodynamic stability for HEAs [6,16,17]. (iv) The “cocktail” effect means that the synergistic effect between elements in HEAs greatly improves some properties of materials to generate various unexpected properties [3,6,16].

According to the previous work, the investigation of HEAs mainly focused on their thermal, electrical, magnetic, and mechanical properties [3,[18], [19], [20], [21]]. About the various unique properties presented in HEAs, we can reasonably predict that when the size of the HEAs is reduced to the size for obtaining nano high entropy alloys (NHEAs), the combination of the singularity of the nano effect and the unique characteristics of the NHEAs can make this material having a large application prospect in the field of heterogeneous catalysis. Generally, the multiple active sites of NHEAs can provide near-continuous adsorption energies which help to optimize for superior persistent catalytic activity, and the excellent stability of NHEAs can make them be suitable into some extreme reaction conditions (high temperature, high pressure, and strong corrosive conditions). In 2018, Hu and his group implemented the concept of “high entropy” in the field of heterogeneous catalysis for the first time [22]. Discovery of more novel materials inject fresh blood into the high-entropy material family, and further expanded NHEMs applications. Since then, NHEAs and their various of derivatives such as nano high entropy oxides (NHEOs) [[23], [24], [25]], sulfides (NHESs) [[26], [27], [28], [29]], phosphide (NHEPs) [30,31], nitrides (NHENs) [32,33], fluoride (NHEFs) [34,35], metal carbides (NHEMCs) [[36], [37], [38]] etc. (all denoted as nano high-entropy materials NHEMs) were emergingly developed in the field of heterogeneous catalysis. Whereas, due to the complex composition of NHEMs and the difference in the properties (electronegativity, valence electron concentration, atomic size) of various element atoms, the material is prone to element segregation and phase separation during the synthesis of NHEMs. Limited by experimental conditions and other factors, the successful synthesis of NHEMs is still challenging. Furthermore, the complex composition of NHEMs brings open challenges to analyze the depended on their characterizations and catalytic mechanisms. Specifically, clearly confirming the role of multiple metallic sites in catalytic process is difficult but necessary. The complex metal composition makes the identification of catalyst active sites full of uncertainty. In recent years, most investigations of NHEMs focused on electrocatalysis reactions include electrocatalytic water splitting, oxygen reduction, alcohol oxidation and CO₂ reduction, etc. [[39], [40], [41], [42]]. Thermocatalytic and photocatalytic were two general strategies for renewable energy conversion and environmental protection (biomass conversion, green hydrogen production, pollutant degradation etc.) Heterogeneous catalysts in thermocatalytic and photocatalytic reactions usually need high temperature resistance, superior light responsive, and adjustable band gap. Based on above, superior thermal stability, synergistic effect between multi-metal sites, and suitable band gap of NHEMs can be considered as their potential advantages in thermocatalytic and photocatalytic. Therefore, it is of significance to pay attention on applications of NHEMs in thermocatalysis and photocatalysis. More importantly, most of the previous reviews only focused on the application of NHEMs in electrocatalysis and lacked a systematic summary and review of NHEMs in the field of thermocatalysis and photocatalysis. This review summarizes and reviews application and catalytic mechanism of NHEMs in thermocatalysis and photocatalysis for the first time, which can make up for the gap in the previous reviews of NHEMs heterogeneous catalysis and is conducive to the development of more novel and efficient NHEMs heterogeneous catalysts.

In this review, we firstly listed the classification of NHEMs, typical controllable synthesis methods, and discussed in their advantages and suitability for synthesis of different types of NHEMs. Subsequently, we reasonably estimated advantages and disadvantages of different controllable synthesis strategies. Furthermore, the characterization techniques of NHEMs were also summarized. Applications of NHEMs in heterogeneous thermocatalysis and photocatalysis are systematically addressed and their catalytic performance and structure-activity relationship are also discussed (Fig. 2). Finally, the conclusion and perspective based on the rational synthesis strategies, advanced characterization technologies and possibility of expanding applications of NHEMs are presented. Different from most of the focus on the application of NHEMs in electrocatalysis. This review mainly proposes some reasonable insights focus on the synthesis and characterization of NHEM, as well as its research progress in thermal catalysis and photocatalysis.