# Synthesis of well-defined allophane nanospheres

Pierre Picot, Frédéric Gobeaux, Antoine Thill

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Pierre Picot, Frédéric Gobeaux, Antoine Thill. Synthesis of well-defined allophane nanospheres. Applied Clay Science, 2026, 279, pp.108056. 10.1016/j.clay.2025.108056. hal-05376129

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Short Communication

Synthesis of well-defined allophane nanospheres

Pierre Picot *, Frédéric Gobeaux, Antoine Thill

Université Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France

ARTICLE INFO

Keywords:

Allophane

Synthesis

Aluminosilicate

Nanosphere

SAXS

Cryo-TEM

A B S T R A C T

Well-defined allophane nanospheres, with internal and external diameters of 3.0 and 3.9 nm, respectively, were successfully synthesized and characterized by cryo-TEM, SAXS, and IR spectroscopy. In addition, we propose a refined structural framework, clarifying the coexistence of open and closed imogolite- local structures, which are commonly grouped together under the term *allophane*.

1. Introduction

Allophane was first observed by Riemann in 1809 at Gräfenthal and named by Stromeyer and Hausmann in 1816 (Hausmann and Stromeyer, 1816). It is a ubiquitous clay mineral found in volcanic soils, commonly associated with imogolite, halloysite, and kaolinite. Defined as “a group of clay-size[d] minerals with short-range order, containing silica, alumina and water in chemical combination” (Parfitt, 1990), allophane is typically described as hollow spherules of 3–5 nm with wall thicknesses of 0.7–1 nm, forming aggregates of variable size and morphology, from the nano- to the micro-scale (Kitagawa, 1971; Wada and Wada, 1977; Smalley, 1979; Brigatti et al., 2006).

The term *allophane* refers to clay minerals with variable compositions, generally represented as (SiO₂)₁⁻₂Al₂O₃(H₂O)s⁻₆. This compositional variability leads to the definition of a family of compounds classified by their Si/Al molar ratio: Al-rich allophane (Si/Al ≈ 0.5) and Si-rich allophane (Si/Al ≈ 1.0). Al-rich allophane shares the local structure of imogolite — a natural aluminosilicate nanotube often co-occurring in soils (Yoshinaga and Aomine, 1962; Cradwick et al., 1972). In this imogolite-local structure (ImoLS), isolated silicon tetrahedra are bound to a gibbsite-like sheet via three Si–O–Al bonds. By contrast, Si-rich allophane resembles kaolinite in composition and contains oligomerized silicate (Si–O–Si) bonds. According to the nanospherule model, the excess silicon is proposed to form oligomerized silicate inside the cavity (Goodman et al., 1985; Brigatti et al., 2006).

A perfect ImoLS-based sphere cannot form, and defects in the form of pores are always present. These pores, with estimated sizes of 0.3–2 nm, have been inferred from density measurements and deuterium exchange

experiments (Paterson, 1977; Wada and Wada, 1977). Allophane usually originates from the weathering of volcanic soils (Wada, 1987; Parfitt, 2009), but can also be synthesized in the laboratory (Wada et al., 1979; Denaix et al., 1999; Du et al., 2022). Classical synthesis protocols rely on the co-precipitation of an aluminum salt (perchlorate or chloride) slowly hydrolyzed by NaOH in the presence of a silicon precursor (tetraethoxysilane, TEOS), followed by a growth stage at elevated temperature (90–100 °C) over several days, and a final dialysis step to remove salts and alcohols.

The existence of well-defined spherical particles has often been debated. Considerable progress has recently been made in understanding their formation, and there is now consensus that intermediate nanostructures (proto-allophane, or nanotiles with an ImoLS) govern their final morphology. Using small-angle X-ray scattering (SAXS), we previously demonstrated that the size of these intermediates during co-precipitation strongly depends on synthesis conditions (Picot et al., 2023): high concentrations favor nanospherules formed from smaller intermediates, while low concentrations promote nanotubular structures via larger intermediates. Similarly, Du et al. (2022) investigated the effect of Si/Al molar ratio, showing that increasing the ratio from 0.5 to 1 reduces aluminum-rich by-products (bayerite) and enhances oligomeric silicate species, thus yielding a continuum from Al-rich to Si-rich allophanes (Du et al., 2022). Regardless of the ratio, proto-allophane (equivalent to proto-imogolite) intermediates were always detected. Careful TEM analysis suggests that most syntheses produce mixtures of proto-allophane fragments and well-defined allophane nanospheres, either isolated or aggregated (Yuan et al., 2019; Du et al., 2020, 2022). Frequently, the term *allophane* is used to describe both structures

<footnote>* Corresponding author.
E-mail address: pierre.picot@cea.fr (P. Picot).</footnote>
(intermediates and nanospheres).

Several structural models of ImoLS nanospheres have been proposed (Abidin et al., 2006; Creton et al., 2008). Using the semi-empirical PM3 method, Abidin et al. predicted a nanosphere with a diameter/wall thickness of 4.25/0.7 nm and six pores of ~1.0 nm. Creton et al., using molecular dynamics, obtained a slightly larger nanosphere (4.98/1.0 nm) with six pores of ~0.7 nm. These predictions, however, remain unconfirmed experimentally. The synthesis of a pure allophane sample (composed exclusively of well-defined nanospheres) would be a decisive step toward validating such models. Natural allophane is most likely to be nanospherical (Parfitt, 2009; Wada and Wada, 2014) but it remains challenging to disperse it without damaging its structure for characterization (e.g., TEM and SAXS).

In this work, we report for the first time a synthesis protocol yielding predominantly ImoLS nanospheres with monodisperse diameters at high concentration. We compare this route with classical syntheses and propose a clearer structural framework for the various co-existing phases.

## 2. Experiments and discussion

All chemicals were used without further treatments and were provided by Sigma-Aldrich for aluminum-tri-sec butoxide (ASB, 97 %), TEOS (99 %), chlorohydric acid (HCl, 37 %), sodium hydroxide (NaOH, 98 %) and from Alfa Aesar for aluminums chloride hexahydrate (AlCl₃·(H₂O)₆, 99 %). MilliQ water was used for all experiments.

The classical synthesis is adapted from Du et al., 2022. Briefly, in a perfluoroalkoxy alkane (PFA) flask, TEOS was added under stirring to a 100 mM AlCl₃ solution to reach a Si/Al molar ratio of 0.575. After 10 min, NaOH pellets are added to reach a NaOH/Al molar ratio of 2. Next, the solution is left to age for 24 h. Then, it is kept for seven days at 90 °C in an oven. Finally, the solution is dialyzed against MilliQ water (renewed every 24 h) with 8–10 kDa membranes (Spectrumlabs) for 5 days. This synthesis will be called Allo-PD. For the new synthesis protocol, we have changed the type of aluminum precursor and used an alkoxide. Briefly, in a PFA flask, ASB is added to a 75 mM HCl solution to reach a H⁺/Al molar ratio of 0.5 and the solution is left to stir for 1 h. TEOS is then added to reach a Si/Al molar ratio of 0.525, and the subsequent aging and purification steps are the same as for Allo-PD. This synthesis will be called Allo-MD.

Fig. 1 displays SAXS diagrams and cryo-TEM images of Allo-PD (grey) and Allo-MD (black). For the former, even though a few spheres are visible in the cryo-TEM image, it is not the dominant shape. Other objects that correspond to proto-imogolite/allophane are curved, isolated, stacked, or aggregated to form structures of a few to tens of nanometers. The associated SAXS curve (grey), therefore unsurprisingly, does not exhibit well-defined oscillations, which would be typical of a monodisperse nanostructure. For Allo-MD, mostly isolated or aggregated spheres can be observed from cryo-TEM images with diameters around 4 nm. Not all objects are perfect spheres, some are deformed, others are coils, and proto-imogolites/allophanes are also visible (see Fig. S1 in SI for additional cryo-TEM images of Allo-MD). Nevertheless, compared to the Allo-PD synthesis, the objects here are better defined and more regular. The grey dashed curve corresponds to a fit using a core/shell spherical model with polydispersity (see section S3 in SI), a core radius of 1.5 nm and a shell thickness of 0.44 nm, i.e. spheres with an external diameter of 3.9 nm. The values of the fit are consistent with cryo-TEM observations and the model proposed by Abidin et al. (Abidin et al., 2006). Based on our results, we do not observe the presence of pores in the wall. Fig. 2a displays X-ray diffraction (XRD) of Allo-PD (grey) and Allo-MD (black). For XRD patterns, no differences are observed between the two protocols. The three broad reflections at 26° (3.4 Å), 40° (2.2 Å), and 66° (1.4 Å) 2θ are characteristic of these compounds (Henmi and Yoshinaga, 1981; Parfitt, 1990; Du et al., 2017) and are in fairly good agreement with Creton et al. who calculated the XRD pattern of their allophane model and predicted peaks at 26, 40 and

<figure><img src="image_3.png" /><figcaption>Fig. 1. SAXS diagrams and related cryo-TEM images of Allo-PD (grey) and Allo-MD (black). The concentration of both samples is 10 g·l⁻¹. For clarity, the grey curve is multiplied by 2. The dashed grey line corresponds to the core/shell spherical model with polydispersity in radius (see Table S1 for parameter values). The scale bars on cryo-TEM images are 25 nm.</figcaption></figure>

60° 2θ (Creton et al., 2008). In addition, their calculated diffractogram has intense peaks related to the crystallinity of the allophane model they used. The absence of these peaks on their experimental diffractogram and our measurement for Allo-MD (black curve on Fig. 2a) suggests that once dried, the spherical structure tends to deform and/or form aggregates, as already observed for imogolite (Amara et al., 2014; D'Angelo et al., 2023). Fig. 2b shows IR spectra of both syntheses. In the 1200–1000 cm⁻¹ range, the dominant band at 965 cm⁻¹ is characteristic of Si-O-Al bonds in spherical ImoLS (Du et al., 2022). For Allo-PD (grey curve), additional bands are present, which are compatible with the presence of oligomeric silicates on the inner surface of the objects (Du et al., 2020, 2022). The Si/Al molar ratio measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) after dialysis corroborates the excess of silicon for the Allo-PD (0.60) compared to Allo-MD (0.5). In the 800–500 cm⁻¹ area (associated with Al—O vibrations), both curves have the same bands (680, 605, 560, 500 and 425 cm⁻¹) (Liao et al., 2018). However, for Allo-PD, they are less well defined due to the polydispersity in the shape and size of the objects.

The main difference between the two protocols is the ions to aluminum ratio: for the Allo-MD there is 0.5 Cl⁻ per Al whereas for the Allo-PD there are 3 Cl⁻ and 2 Na⁺ per Al. It is known that ions influence the quality of the synthesis for allophane and imogolite and their formation (Wada et al., 1979; Farmer et al., 1983; Abidin et al., 2007; Nakanishi et al., 2008). We can hypothesize that the ions adsorb on the edges of proto-allophanes, and therefore affect their size and shape. In addition, during the heating step, these adsorbed ions could limit or block the growth process from open ImoLS to close-ImoLS.

## 3. Conclusion

We present the first synthesis protocol for producing well-defined ImoLS nanospheres. As noted by Adams et al., many structures reported as allophane are in fact proto-allophane, unless verified by TEM (Adams et al., 2024). We support this view. Allophane is best described as an ill-defined material encompassing both reactive open ImoLS which are curved tile-like units that can evolve in shape and metastable closed
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** XRD patterns a) and IR spectra b) of Allo-PD (grey curves) and Allo-MD (black curves). For clarity, the grey curve on chart b) is shifted upwards.</figcaption></figure>

ImoLS of various forms. Among the latter, we confirm hollow nano-
spheres with internal and external diameters of 3.0 and 3.9 nm,
respectively. We highlight that IR and SAXS are effective tools for
identifying these nanospheres. We propose distinguishing “open ImoLS”
(reactive proto-imogolite/allophane) from “closed ImoLS” (metastable
structures including imogolite and allophane), the latter exhibiting
greater stability. Allophane is a 0D structure and, unlike imogolite,
cannot grow once closure occurs. This synthesis protocol opens new
perspectives for understanding the detailed structure of allophane
nanospheres. Future studies, particularly high-resolution cryo-TEM, will
be essential to confirm or disprove the presence of pores within their
walls and determine their size.

CRediT authorship contribution statement

**Pierre Picot:** Visualization, Validation, Supervision, Project admin-
istration, Methodology, Investigation, Formal analysis, Conceptualiza-
tion, Writing – original draft. **Frédéric Gobeaux:** Visualization,
Validation, Formal analysis, Writing – review & editing, Writing –
original draft. **Antoine Thill:** Visualization, Validation, Supervision,
Project administration, Methodology, Investigation, Formal analysis,
Data curation, Conceptualization, Writing – review & editing, Writing –
original draft.

**Declaration of competing interest**

The authors declare the following financial interests/personal re-
lationships which may be considered as potential competing interests:

Frédéric Gobeaux reports financial support was provided by Paris
Saclay University. Antoine Thill is member of editorial board of
Applied Clay Science.

Acknowledgements

The SWAXS Lab platform of NIMBE and LLB is acknowledged for
supporting SAXS measurements. We want to thank Olivier Taché and
Emeline Cournède for their help with the SAXS experiments in the
SWAXS Lab. The Xeuss 3.0 SAXS/WAXS instrument was financed with
state support managed by the French National Research Agency (ANR)
under the France 2030 framework (FastNano, ANR-22-PEXD-0006).
Cryo-TEM observations were made thanks to "Investissements d'Avenir"
LabEx PALM (ANR-10-LABX0039-PALM). This work was also funded by
the ANR grant BENALOR (ANR-20-CE09-0029). The authors thank Dr.
Mark Levenstein for reading the manuscript and making insightful
comments.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.clay.2025.108056.

Data availability

Data will be made available on request.

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