Preparation of Zirconium Phosphate Nanomaterials and Their Applications as Inorganic Supports for the Oxygen Evolution Reaction

Zirconium phosphate (ZrP) nanomaterials have been studied extensively ever since the preparation of the first crystalline form was reported in 1964. ZrP and its derivatives, because of their versatility, have found applications in several fields. Herein, we provide an overview of some advancements made in the preparation of ZrP nanomaterials, including exfoliation and morphology control of the nanoparticles. We also provide an overview of the advancements made with ZrP as an inorganic support for the electrocatalysis of the oxygen evolution reaction (OER). Emphasis is made on how the preparation of the ZrP electrocatalysts affects the activity of the OER.

ZrP acidity and porosity are desired traits for different applications, especially in heterogeneous catalysis [ 6 , 7 ]. Not only that, but these parameters are tunable, expanding the possible applications of ZrP [ 8 ]. Beyond that, the major attractiveness of ZrP comes from its ability to perform ion-exchange with its acidic phosphate groups. The composite materials that result after ion-exchange differ greatly in their properties when compared to α-ZrP, allowing them to be implemented for several different applications. Ion-exchange in ZrP occurs at the Brönsted acid groups (P-OH) which are also present at the surface of the nanoparticles, opening another pathway for the modification of this material: surface modification [ 9 , 10 , 11 ]. These composite materials have been used for several applications including photocatalysis [ 12 , 13 ], drug delivery [ 14 , 15 , 16 , 17 ], amperometric biosensors [ 18 , 19 , 20 ], catalysis [ 7 , 21 , 22 , 23 , 24 , 25 , 26 ], flame retardancy [ 27 , 28 , 29 ], and others. We encourage the reader to also see other recent reviews on the topic of the synthesis of ZrP and its applications [ 7 , 8 , 30 , 31 ].

The first report of a crystalline form of zirconium phosphate dates back to 1964 [ 1 ]. The chemical composition of this material was determined to be Zr(HPO 4 ) 2 ·H 2 O. Later, the first crystal structure of this crystalline ZrP material was reported in 1969 and then refined in 1977 [ 2 , 3 ]. Its structure consists of a layered arrangement ( ). Zr(IV) ions align in a near perfect plane bridged by orthophosphate groups, which are 5.3 Å apart from each other, above and below the Zr ions plane. Each Zr ion is coordinated by six oxygen atoms from six different phosphate groups, forming an octahedral coordination with the metal center. Three of the oxygen atoms from each phosphate group are coordinated to three different Zr ions. The fourth oxygen is bonded to a hydrogen atom and points above and below each ZrP layer. The stacking of these layers, which are 6.6 Å thick and 7.6 Å apart from each other, creates a zeolitic cavity with a diameter of 2.61 Å that is occupied by a water molecule [ 4 , 5 ]. This description corresponds to that of a phase that has been named α-ZrP, the most extensively studied phase of ZrP. Other phases have been achieved by varying the synthesis protocol and some of these phases will be explained later in this review.

2. Synthesis and Preparation of ZrP Nanomaterials

2.1. α-ZrP

Clearfield and Stynes were able to synthesize the first crystalline ZrP (α-ZrP) by refluxing amorphous gels in phosphoric acid [1]. Later, a hydrothermal method and a synthesis with HF were reported [32]. The method used for the synthesis of α-ZrP has a direct impact on the size and shape of the resulting nanoparticles. This is another advantage as we can adjust these parameters depending on the application while still retaining the main characteristics of α-ZrP. More specifically, the concentrations of reactants, temperature during synthesis, pressure, and use of complexing agents have been found to have an effect on the aspect ratios of the nanoparticles [33].

Sun et al. reported the synthesis of α-ZrP nanoparticles through three different methods and characterized the materials [34]. The first method consisted of mixing ZrOCl2·8H2O with phosphoric acid (H3PO4) at varying concentrations. Namely, 3, 6, 9, and 12 M H3PO4 at 100 °C and 24 h. The second method consisted of mixing ZrOCl2·8H2O with H3PO4 (at the same concentrations as in approach 1) in a Teflon® pressure vessel and heating each reaction at 200 °C for 24 h. The final approach consisted of mixing ZrOCl2·8H2O with 3.0 M H3PO4 in a Teflon® flask. HF was added so that the molar ratios of F/Zr were 1, 2, 3, and 4.

The authors of this study observed that, for method 1, the crystallinity and aspect ratio increase as the H3PO4 concentration increases. The resulting nanoplatelets had sizes ranging from ~60 nm to ~200 nm. The nanoplatelets that resulted from method 2 had an enhanced crystallinity when compared to those from method 1. Moreover, the aspect ratio increases and the size of the nanoplatelets now range from ~400 to ~1200 nm as the concentration of H3PO4 increases. Finally, the α-ZrP platelets obtained from method 3 have an even higher crystallinity than the nanoplatelets from method 2. An increase in the aspect ratio is also observed and the sizes range from ~2000 to over ~4000 nm. The scanning electron microscopy (SEM) images of these samples are shown in . Recently, Contreras-Ramirez et al. made a detailed study on how the synthesis method and reaction conditions affect the structural order and crystallinity of α-ZrP. In their study, the authors used synchrotron X-ray atomic pair distribution function (PDF) analysis to observe the changes in the structural order caused from the different methods and conditions [35].

The reflux, hydrothermal, and HF method for the synthesis of α-ZrP have been the most widely used methods throughout the decades. However, other methods have been reported in the literature such as precipitation with oxalic acid and by liquid-phase deposition [36,37,38]. Pica et al. reported the synthesis via an alcohol intercalation/deintercalation method [39]. α-ZrP was prepared by dissolving zirconium propionate in different anhydrous alcohols (ethanol, propanol, and butanol) and adding H3PO4 at different H3PO4/Zr molar ratios (2, 4, and 6). This synthesis procedure results in transparent gels. Characterization of these gels by X-ray powder diffraction (XRPD) shows only one well defined peak in all diffractograms corresponding to (002) planes of ZrP. This corresponds to the interlayer distance and it was found to vary depending on the alcohol used during synthesis. More specifically, the interlayer distance was found to be 14.4 Å when ethanol was used, 16.1 Å for propanol, and 18.6 Å for butanol. This highlights that the alcohols are contained within the layers of ZrP. The TEM images of these samples reveal that the resulting nanoparticles are hexagonal shaped and have regular planar sizes of ~40 nm, independent of which alcohol was used. When the gels are dried at 60 °C until complete solvent evaporation, a white powder is obtained. The XRPD diffractograms for all samples shows a shift in the first peak to d = 7.56 Å, indicating the formation of α-ZrP. The authors found that the crystallinity of the resulting α-ZrP nanoparticles was dependent on the H3PO4/Zr molar ratio and increases with increasing ratio. The transmission electron microscopy (TEM) images for the samples prepared with an H3PO4/Zr ratio of 6 show that the nanoparticles kept a more or less hexagonal morphology and the distribution of planar sizes increased to a ~30–200 nm range.

Highly crystalline α-ZrP was reported with a new method by using a minimal solvent synthesis procedure [40]. By adding H3PO4 to powder ZrOCl2·8H2O with a H3PO4/Zr molar ratio of two, α-ZrP was successfully obtained. Moreover, if the H3PO4/Zr molar ratio is increased to three, α-ZrP with enhanced crystallinity is obtained. The SEM images of α-ZrP prepared with a H3PO4/Zr molar ratio of three show that the cross-sectional dimensions of the nanoplatelets are in the range of ~100–500 nm with a thickness of ~40–66 nm. This method introduces a greener and milder option for the synthesis of highly crystalline α-ZrP as no excess of H3PO4 is required and/or the use of hazardous HF.

2.2. ZrP Exfoliation

The process of separating the layers of a bulk layered material is known as exfoliation. This process can be achieved by a variety of methods and has been extensively studied for several layered materials [41]. The two-dimensional materials (2D) of nanosheets or few layered nanosheets that result after exfoliation have been shown to have several advantages over their bulk systems [42]. Enhancement in conductivity, surface area, mechanical flexibility, optical transparency, and other properties make these exfoliated materials suitable for application in different areas, such as electrocatalysis, electronics, energy storage, and others [42].

ZrP has been successfully exfoliated and its nanosheets used for different applications [6,21,43,44,45,46,47,48,49,50]. The most widely used strategy for ZrP exfoliation consists of the intercalation of small amine cations that can easily displace the protons from the phosphate groups in an acid-base reaction and enter the interlayer space [51]. The mechanism of this process is said to be the formation of an amine double layer in the interlayer space, leading to exfoliation due to cation–cation repulsions [44]. Kaschak et al. studied the exfoliation of ZrP with tetra-n-butylammonium hydroxide (TBA+OH−) [52]. The authors estimated the TBA+ diffusion rate within the ZrP galleries from the time required to achieve a constant expansion rate and the distance travelled in this time. The intercalation rates determined suggests a first-order process that depends on the opening of the interlayer at the edges of the nanoplatelets. As the exfoliation of ZrP is known to produce hydrolysis products, Kaschak et al. studied this process by atomic force microscopy (AFM) and TEM. Results show that the hydrolysis reaction occurs from the edges inward and the percent hydrolysis increases as a function of time. The hydrolysis process after 1 h was monitored by varying the temperature of the reaction. It was found that the hydrolysis percent varied drastically with temperature. For both semi-crystalline and micro-crystalline ZrP, the rate of hydrolysis during exfoliation is essentially zero at 0 °C. However, at higher temperatures, the percent hydrolysis is noticeable and increases with increasing temperature.

ZrP has also been successfully exfoliated via a melt-compounding method [53]. Here, ZrP is first intercalated with diglycolamine (DGA). After intercalation, the product was dried and ground into fine powders and subsequently mixed with maleic anhydride grafted polyolefin elastomer (POE-g-MA). The mixtures were then exfoliated with melt compounding.

Another method that has been reported for the exfoliation of ZrP is the assisted exfoliation with ionic liquids [54]. Herein, α-ZrP is first intercalated with small molecules such as DGA. Then, the DGA intercalated ZrP is mixed with the ionic liquid 1-methyl-3-n-octylimidazolium bromide [OMIm]Br and ultrasonicated for 30–60 min. After product collection and characterization, the authors noticed the successful exfoliation of ZrP. The initial intercalation step is crucial as it helps in expanding the interlayer spacing and weakens the H-bonding between the layers. DGA is thought to attach to the ZrP through its amine end while the rest of the molecule is a small polar chain that can attract OMIm cations. Hence, it is believed that OMIm enters the interlayer through a cation-lone pair attraction, forming highly charged surfaces, which leads to the complete exfoliation of α-ZrP and stabilization of the nanosheets in ionic liquids.

Recently, an ZrP exfoliation procedure using Tris-(hydroxymethyl)-aminomethane (Tris) was reported [48]. For this, α-ZrP is added to a Tris buffer solution and the mixture is ultrasonicated for 20 min. This method introduces the substitution of commonly used organic strong bases to a more environmentally friendly, less basic, and non-toxic Tris exfoliating agent.

After exfoliation, some exfoliating agents can be displaced with another cationic species if the latter is put in contact with a suspension of the exfoliated ZrP nanoparticles. To obtain the protonated ZrP nanosheets, producing the Brönsted acid groups, a follow-up method with an acid can be performed [21]. The exfoliation of ZrP has also been accomplished by using alkanol amines and other wet methods [55,56].

2.4. Intercalation of Guest Species into ZrP

Whittingham defined intercalation (in chemistry) as “the reversible insertion of guest species into a lamellar host structure with maintenance of the structure features of the host” [66]. The zeolitic cavity with a diameter of 2.61 Å in α-ZrP impedes the direct intercalation of species with larger dimensions, hence the intercalation of these species is not significant and/or they are exchanged at very slow rates [5,67,68,69]. To circumvent this problem, pre-intercalation methods such as the intercalation of sodium cations or small alkyl amines into α-ZrP (producing expanded ZrP phases) are commonly performed as the first step for the intercalation of the intended guest species [69,70]. However, these pre-intercalation methods typically result in the co-intercalation of various species, hindering the analysis of experimental results.

To address this, Martí and Colón reported in 2003 a new method for the direct intercalation of large metal complexes that does not require a pre-intercalation step [13]. This method consists of using a highly hydrated phase of zirconium phosphate, namely, Zr(HPO4)2·6H2O (θ-ZrP) [13]. θ-ZrP can be prepared by using a reflux method that results in a material with the same type of layers as α-ZrP, but with an interlayer distance of 10.4 Å, instead of 7.6 Å [71]. The increased interlayer distance is due to the six water molecules per formula unit in θ-ZrP, in contrast with α-ZrP that only has one [72]. When θ-ZrP is allowed to dry, it converts back to α-ZrP. This can be confirmed with XRPD as the first diffraction peak at 2θ = 8.6° (d002 = 10.4 Å) for θ-ZrP, which corresponds to the interlayer distance, shifts towards 11.6° (d002 = 7.6 Å) when this material dehydrates. Hence, determining if an intercalation was successful is trivial, as the XRPD analysis of a dry intercalation product should yield a first diffraction peak with a distance greater than 7.6 Å [14].

In their study, Martí and Colón intercalated the luminescent metal complex tris(2,2’-bipyridyl ruthenium(II) ([Ru(bpy)3]2+) by direct ion exchange into ZrP, using θ-ZrP ( ) [13]. The intercalated product shows an increase in the interlayer distance to 15.2 Å, and [Ru(bpy)3]2+ retains its structural integrity. This method of intercalation via direct ion exchange in ZrP is by an ion exchange reaction with the protons in the Brönsted acid groups (P-OH). As the intercalation proceeds, the intercalant enters the interlayer space though the edges and diffuses into the center of the solid [73]. The direct intercalation of several guest species into θ-ZrP has been successfully used to produce composite materials for different applications [18,20,74,75,76,77,78,79].