A facile morphology-controllable synthetic route to monodisperse K 3 PMo 12 O 40 ▪ nH 2 O crystals

Synthesis is the base of experimental chemistry. Herein, monodisperse K 3 PMo 12 O 40 ▪ nH 2 O poly-oxometalates (POMs) with different morphologies have been reached by tuning synthetic conditions of the K/POM ratio, stirring speed and time, and reaction temperatures. Among these factors, the K/POM ratio is identi ﬁ ed most critical in morphological controls of the K 3 PMo 12 O 40 ▪ nH 2 O particles, altering them from cubes to spheres. Additionally, morphological transformations were identi ﬁ ed through a self-assembly and Ostwald ripening process, setting a generic synthetic strategy for the POM systems. Such synthetic strategies have substantial applications in catalytic or surface-demanding ﬁ elds requiring POM materials with controlled morphology.


Introduction
Controlled synthesis of inorganic micro-nano functional materials is a constant focus in experimental chemistry since the physicochemical properties of materials are substantially affected by their morphologies and sizes [1e3].This also applies to the synthetic polyoxometalates (POMs), a class of anionic molecular transition metal oxides with diverse molecular structure which is widely used in various research domains [4e9].Among many POM types, the Keggin type, such as 12-phosphomolybdic acid (H 3 PMo 12 O 40 , denoted by PMo 12 ), is one of the most important and representative polyoxo-structures which have been intensively studied in recent decades [10e14].These heteropolyacids can easily react with other appropriate cations (NH 4þ , K þ , Cs þ , Ag þ , etc.) to form insoluble compounds with distinct properties and morphologies [15e22].Moreover, they can be synthesized by changing their counter cations or the preparation methods and conditions.For example, the Keggin-H 3 PMo 12 O 40 were catalytic active in aquathermolysis of extra-heavy oil [23] at the nanoparticle form and useful in dehydration of ethanol and hydration of ethylene but in combination with the K 2 H 1 PW 12 O 40 [24].Despite the progresses, the morphological evolutions and conditions to reach the dedicated POM shapes remain elusive in facile synthesis.A shape-controlled synthesis of insoluble POM compounds is thus needed to establish a material ground for extensive applications of the POMs in various fields.
One of the most efficient procedures to synthesize small-scale materials is through the self-assembly process [25,26].In recent years, the self-assembly method has played an important role in the development and research of nanotechnology and nanomaterials.As a spontaneous organization process, it is propelled by molecular hydrogen bonds, van der Waals forces, electrostatic interactions, and other mechanisms [27].For insoluble POM materials, the self-assembled POM particles have also been reported in literature [28e31].Despite these progresses, the corresponding controllable factors of morphologies and crystal growth mechanisms remain elusive.Therefore, a rational design and fabrication of such materials with special surface morphologies is needed to fully understand and utilize the unique processes of molecular assemblies and to count external influencing factors such as the stirring speed, time, and the reaction temperatures [32e34].Even though stirring reportedly shapes well morphologies and structures of synthetic crystals [2,3,35,36], it remains unclear what kind of impact the stirring speed and time will bring to the synthesis of POMs at room temperature.Therefore, it is necessary to develop a synthetic route to accurately control morphologies of the POM micro/nanostructured crystals and explore its formation and organization mechanism.

Results and discussion
The morphology and structure of the product were analyzed by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD).Fig. 2 depicts XRD patterns of KPMo-1-n sequence.A number of peaks can be well-indexed to the standard diffraction peaks of cubic K 3 PMo 12 O 40 ▪4H 2 O (JCPDS No.09-0408), and no obvious impurity peaks are observed, confirming the high purity of the products.Furthermore, the XRD patterns undergo subtle changes with the increase of the rate of 3À ratio has little influences on their crystal structure.
Fig. 3 showed the SEM images of the samples (KPMo-1-n) with micrometer-sized crystals synthesized under the conditions of different molar ratio of [PMo 12 O 40 ] 3À /K þ in reaction solution.As shown in Fig. 3aed, when K þ /[PMo 12 O 40 ] 3À < 7, the irregular polyhedral particles are composed of dodecahedral and cube crystals to form hollow vesicles with 1e2 mm.Furthermore, KPMo-1-4 and KPMo-1-5 morphologies did not undergo significant mutations, probably due to the small excess of K þ concentration in the solution.When K þ /[PMo 12 O 40 ] 3À ¼ 7, the irregular polyhedral particles were transformed to uniform dodecahedral crystals with 1e2 mm in diameter (shown in Fig. 3e).However, when K þ / [PMo 12 O 40 ] 3À > 7, the dodecahedral crystals were altered to On the basis of the KPMo-1-8, the influences of the stirring time and speed on the shapes and sizes of the products were studied on KPMo-1-8-m samples.Fig. 4aef exhibit the SEM images of the samples whose stirring times were adjusted from 0 to 60 min under a stirring speed at 300 r$min-1.As the stirring time increased from 0 min to 60 min, the KPMo were gradually transformed from monodisperse solid microspheres to hollow microspheres.However, as shown in Fig. 4feh, the hollow structure of microspheres gradually disappeared and re-evolved into smaller solid microspheres (500e600 nm) along with the enhancing of the stirring speed from 300 to 900 r$min-1 under the condition of the stirring time similar to KPMo-1-8-5.Furthermore, the XRD distinctive diffraction peak of sample in Figure S1 corresponds to  K3PMo12O40▪4H2O (JCPDS No. 09e0408), and the position of the characteristic diffraction peak is identical to KPMo-1-8, with no obvious impurity peaks, implying that KPMo was effectively synthesized.This suggests appropriate stirring speed and time contributed to the formation of hollow structures.Significantly, the surfaces of these samples (Fig. 4bef) became rougher, compared with that of the sample before stirring (Fig. 4a).Therefore, the stirring time and speed in the reaction played a vital role on the shape and size of KPMo.
It is well known that the reaction temperatures and time have an important impact on the growth and shape of the crystals.The XRD spectra of the samples generated by altering the reaction temperature and duration are shown in Figure S2.It is obvious that we successfully prepared high-purity K3PMo12O40▪nH2O samples even the reaction parameters were changed.Fig. 5 showed SEM images of the samples prepared by tuning the reaction temperature and time in thermostat water bath on the basis of KPMo-1-9 without agitation.In Fig. 5bed, when the reaction temperature was maintained within a certain range from 323 to 363 K, a new type of core-shell-structured microspheres (about 5e6 mm) were obtained, in analogue to multi-shell onions.However, such a coreshell structure can be not formed at lower temperatures below 323 K (shown in Fig. 5a) or higher temperature where irregular maintaining the time of constant temperature treatment in 343 K water bath for 6 h, 12 h, or 24 h, the core-shell-structured microspheres evolved to the multi-shell microspheres as shown in Fig. 5eeh.Therefore, appropriate reaction temperature and time can promote the formation of the core-shell-structured microspheres with multi-shells.
According to these results of SEM observation above and the self-organization mechanism of POMs salts [28e30], the formation and growth of KPMo microparticles undergo a process of nucleationeformationegrowth.(i) The KPMo nanocrystallites were formed through an electrostatic interaction of [PMo 12 O 40 ] 3À with K þ and self-assembly aggregated.(ii) Then, the aggregation later grew to form cubes by 6 equivalent planes of (100) or dodecahedron by 12 equivalent planes of (110) when K þ /[PMo 12 O 40 ] 3À 7 (Fig. 3aee).(iii) As the K/POM ratio of the synthetic solution increased from 3 to 7, the crystal growth rate perpendicular to the (110) planes was accelerated by the attachment of K þ , and the morphology of the KPMo particles gradually became dodecahedrons.(iv) When the concentration of K þ in solution was enough high (K þ /[PMo 12 O 40 ] 3À > 7), the crystal growth rate of other crystallographic plane was also accelerated, and thus, the morphology of the particle changed from dodecahedral to spherical due to the disappearance of the anisotropic crystal growth.(v) In order to decrease the surface energy of particles, the spherical particles would further grow to form bigger solid microspheres in diameter by attachment of the residual nanocrystallites in solution (Fig. 3feh).
The formation of the hollow microspheres can be explained through the Oswald ripening mechanism.According to the results of spherical KPMo BET measurement (shown in Fig. 6), the solid microspheres formed by self-organization are porous, so that the  water molecules in solution could enter the interior through these porous channels.The internal nanocrystallites of these microspheres were scoured by water molecules and then desorbed into the aqueous solution under appropriate stirring speeds.The desorbed nanocrystallites were attached on the surface of the microspheres and continued to participate in the growth of the microspheres, resulting in the formation of hollow microspheres along with the prolonging of the stirring time (Fig. 4f).
However, the formation of the core-shell-structured microspheres with multi-shells might undergo two processes of layer-bylayer self-assembly and Oswald ripening.These solid spherical particles formed by non-anisotropic crystal growth were used as a template where multilayered shells gradually formed through layer-by-layer self-assembly under the appropriate reaction temperature about 343 K (Fig. 5e).This kind of microspheres was again eroded by water molecules with higher kinetic energy subjected to the heating.Due to the weak binding forces between these shells, some nanocrystallites between these shells were desorbed into the aqueous solution.As the reaction time at this temperature was extended to 24 h, the distance between these multilayered shells would be gradually increased, resulting in the formation of the core-shell-structured microspheres with multi-shells (Fig. 5g).

Conclusions
In conclusion, the monodisperse micron-sized K 3 PMo 12 O 40 ▪nH 2 O (KPMo) particles have been successfully prepared through a facile synthetic method and their morphologies controlled by changing preparation conditions.Based on experimental results, we systematically studied the formation and assembling mechanisms of the KPMo particles.It is found that the morphological transformation was mainly driven by a combined self-assembly and Oswald ripening process.Such a growth mechanism opens a generic synthetic strategy to reach precisely controls of morphologies for the synthetic POMs.Considering the wide application scopes of the materials, the present work is hoped to sever as a milestone in synthesis that will debut future studies and support large-scale utilities of the POMs in the future.

Credit author statement
Chunhong Yu: Data analysis and Writing original draft; Zhuomin Qiang: Data analysis, paper revision; Shuangwen Yu: Experiment, Data analysis; Taohai Li: Writing e Reviewing and Editing; Feng Li: Supervision, Writing e Reviewing and Editing; Marko Huttula and Wei Cao: Supervision.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
PMo 12 O 40 ▪nH 2 O crystals and studied the self-assembly mechanisms of this new type of micro/nanostructured materials.The monodisperse K 3 PMo 12 O 40 ▪nH 2 O (KPMo) morphologies were found substantially influenced by the synthetic conditions, such as the molar ratio of [PMo 12 O 40 ]3 À /K þ , stirring speed and time, and reaction temperature and time.A transformation process from the self-assembly to Ostwald ripening was identified.

Table 1
The specific synthetic condition parameters of all samples.
uniform smooth-faced solid microspheres with 1e2 mm diameter.And, it is clearly seen that KPMo has been changed into smooth microspheres when K þ /[PMo 12 O 40 ] 3À is elevated to 8. Continuing to increase the K þ /[PMo 12 O 40 ] 3À ratio, the morphology does not alter greatly.The K þ /[PMo 12 O 40 ] 3À ratio increase gradually enlarges the diameters of microspheres as shown in Fig. 3feh.The above results suggest that the molar ratio of K þ /[PMo 12 O 40 ] 3À significantly influences the morphologies of KPMo which evolved from irregular polyhedral crystals to monodisperse microspheres.