# Structural Analysis of Colloidal Titania-Based Ribbons and Their Self-Assembly upon Drying

Sébastien Boukhris, Alexandra Iacoban, Mohamed Ibrahim, Hussein Badr, Andrei Kuncser, Stefan Neatu, Florentina Neatu, Michel Barsoum, Mihaela Florea, Doru Constantin

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To cite this version:

Sébastien Boukhris, Alexandra Iacoban, Mohamed Ibrahim, Hussein Badr, Andrei Kuncser, et al. "Structural Analysis of Colloidal Titania-Based Ribbons and Their Self-Assembly upon Drying." Small Structures, 2025, 6 (8), pp.2500017. 10.1002/sstr.202500017. hal-05085474

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Distributed under a Creative Commons Attribution 4.0 International License
Structural Analysis of Colloidal Titania-Based
Ribbons and Their Self-Assembly Upon Drying

Sébastien Boukhris,† Alexandra C. Iacoban,‡,¶ Mohamed Ibrahim,§ Hussein

Badr,§,|| Andrei C. Kuncser,‡ Stefan Neatu,‡ Florentina Neatu,‡ Michel W.

Barsoum,§ Mihaela Florea,‡,¶ and Doru Constantin*,†

†Institut Charles Sadron, CNRS and Université de Strasbourg, 67034 Strasbourg, France

‡National Institute of Materials Physics, 405A Atomistilor Street, 077125 Magurele,
Romania

¶Interdisciplinary School of Doctoral Studies, University of Bucharest, 36-46 Mihail Kogalniceanu Street, 050107 Bucharest, Romania

§Department of Materials Science and Engineering, Drexel University, Philadelphia,
Pennsylvania 19104, United States

||Present address: Stanford University, Chemical Engineering Department.

E-mail: [constantin@unistra.fr](mailto:constantin@unistra.fr)

Abstract

Synchrotron-based small- and wide-angle X-ray scattering is used to elucidate the structure of low dimensional lepidocrocite titania-based nanofilaments (TiO₂). In the colloidal state they consist of quantum-confined one-dimensional nanofilaments, NFs, loosely associated into ribbons, one lepidocrocite sheet thick (about 4 Å), 30 to 40 Å wide (5 to 8 NFs) and more than 300 Å long. In the dry state, they reach a final state of extended sheets, stacked three to about twenty high, whose crystallinity increases with stack height, in parallel with a decrease in photocatalytic activity. These findings
suggest a kinetic pathway for the self-assembly of initially one-dimensional TiO₂ NFs
into two- and ultimately three-dimensional structures, providing context for a recent
body of work on these low-dimensional materials.

**Keywords**

colloids, photocatalysis, SAXS, self-assembly, TEM, titania

**Introduction**

Effective heterogeneous catalysis requires that the catalyst be finely divided, in order to
increase its specific surface area, SSA. Taking this idea to its logical limit implies that the
lower the dimensions of a material the better its catalytic efficiency¹. This often turns
out to be true, but not only due to the size of the objects: their shape and the exposed
crystal facets are equally important parameters². In particular, the functional difference
between one dimensional (1D) and two-dimensional (2D) objects and the corresponding
three-dimensional (3D) structures can be profound.

Additionally, the electronic properties of the catalyst are essential in determining its
efficiency, effectiveness, and stability of catalytic reactions. The band gap energy ($E_g$) plays
a key role in photocatalytic processes: a large value ensures strong redox potential but
limits absorption to UV light, diminishing the applicability of the photocatalyst, while a
small $E_g$ leads to better solar absorption but often results in insufficient driving force for
redox reactions, which reduces the effectiveness of the photocatalyst. Furthermore, the
position of the edges of the conduction and valence bands (CB and VB) in relation to
the redox potential of the reacting species is crucial for any specific oxidation or reduction
reactions. Regarding photocatalytic water splitting, it is necessary for the CB to have a
more negative potential than that required for hydrogen evolution, while the VB must have
a more positive potential than that necessary for oxygen evolution to enable these reactions.
Optimal catalytic performance is attained by combining suitable particle size in order to enhance the active surface area and reduce recombination losses³.

Titanium dioxide, TiO₂, is one of the most widely studied and used semiconductors for photocatalysis due to its stability, non-toxicity, abundance, wide availability and low cost. Under UV light, TiO₂ generates electron-hole pairs, e-h, that can initiate redox reactions, helping to break down organic molecules and pollutants. This is why TiO₂ plays a major role in environmental cleanup processes, such as degrading pollutants, purifying water, and breaking down harmful chemicals. TiO₂ has also been studied extensively for its potential in water splitting –a process that can generate hydrogen fuel by splitting water (H₂O) into hydrogen (H₂) and oxygen (O₂) using solar energy. However, the recombination of e-h pairs generated reduces the efficiency of the water splitting process. Therefore, many efforts have been invested in enhancing TiO₂'s efficiency under visible light with reduced recombination, either by doping with metals or non-metals, forming composites, or using different nanostructures that increase the surface area and improve charge separation, reducing recombination. With respect to the latter, it has been postulated that lower dimensionality in TiO₂ and especially 1D morphologies, can help in preventing e-h recombination.

Solid-state theory indicates that, when the dimensionality of a material is reduced, quantum confinement occurs and modifies the electronic band structure. This effect shifts the conduction and valence bands, improving charge separation and enhancing the photocatalytic response. Unlike nanoparticles or bulk TiO₂, 1D TiO₂ structures have fewer grain boundaries, which reduces charge trapping and minimizes recombination losses⁴. Therefore, a lot of effort has been dedicated in the past few years to reducing the dimensionality of semiconductor materials, such as TiO₂, in order to tune E_g⁵⁻⁷.

In summary, smaller particles perform better than larger ones and 1D and 2D morphologies are preferred to bulk samples⁸⁻¹¹. For example, Cargnello et al.¹⁰ controlled the e-h recombination rates by engineering the length of brookite nanorods. Esmat et al.¹¹ showed that 20 nm wide lepidocrocite TiO₂ ribbons enhanced charge separation. The reason in both
cases is that the 1D nature helps suppress e-h recombination.

In 2022, we reported on a scalable, one-pot, near-ambient process to convert low-cost, earth abundant, water-insoluble titanium precursors, such as TiB₂, TiC, TiN, etc., into 1D lepidocrocite titanate nanofilaments (NFs), henceforth referred to as 1DL¹²,¹³. Quite recently we showed that we can process 1DL starting with TiOSO₄¹⁴.

From our expanding body of work, we have shown that the fundamental repeat unit is the one shown in Figure 1A¹²,¹³,¹⁵. The NFs, with a cross-section of approximately 5 × 7Å² grow along the [100] or *a*-direction and stack along the *b*-direction¹⁶.

To put these values in perspective, if the 1DLs in just one gram of material were aligned end to end, they would span about 700 million kilometers. It is important to note that there are numerous literature studies that claim the formation of 1D titania¹⁷. However, to the best of our knowledge, none meet the true criterion for one-dimensionality, viz. quantum confinement. The fact that our 1DL's *E*<sub>g</sub> ≈ 4 eV is a record for a titania-based material strongly bolsters our claim for quantum confinement. In short, they constitute a new form of TiO₂ and are being patented as such¹⁸.

The production of these 1DLs is surprisingly simple: To make them we simply heat –at ambient pressures and temperatures below 100°C– Ti-precursors, e.g. TiB₂, TiC, TiN etc., in aqueous tetraalkylammonium hydroxide solutions for a few hours to days. Currently, we routinely fabricate 100 g batches of 1DLs with nothing more sophisticated than polyethylene bottles on hot plates. Our materials are thus quite surprisingly easy to make and inexpensive.

Beyond this step, the self-assembly of 1DL NFs is highly dependent on the solvent used during post-synthesis processing¹⁵. If the ethanol, EtOH, in the EtOH washed samples is allowed to simply evaporate, the NFs organize into porous mesostructured particles (PMPs) that are spherical-like (not shown)¹³, which results in free-flowing, non-agglomerated powders. Conversely, when the EtOH is replaced by water, the 1DLs form on filtration quasi-two-dimensional, q-2D flakes, or ribbons comprised of NFs. The dimensions of these q-2D flakes or ribbons are one of the main concerns of this work.
As a direct result of their extreme dimensionality, it is not surprising that the 1DLs possess some exceptional properties. For instance, they have superior adsorption capabilities for cationic organic dyes, such as rhodamine 6G (R6G) and crystal violet<sup>19</sup>, and malachite green<sup>20</sup>, with adsorption capacities of 1850 and 1930 and 2100 mmol.kg<sup>-1</sup>, respectively. Some of these values are more than double those achieved by highly adsorbent montmorillonite clays<sup>19</sup>. From the dye absorption results we can infer that the SSAs are above 1500 m<sup>2</sup>/g. We note in passing that these SSAs are not obtainable in BET measurements. 1DL PMPs also adsorb heavy metals such as uranium, U with absorption capacities reaching up to 424 mg of uranium per gram of 1DL, effectively converting contaminated water into potable water<sup>21</sup>. Additionally, composites incorporating 1DLs with a dynamic covalent thiol-yne network have shown a remarkable improvement in mechanical properties, leading to a 500-fold increase in modulus at 60 wt% filler when compared to the pure polymer<sup>22</sup>.

Of more relevance to this work is the exceptional photocatalytic efficiency that surpasses that of widely used TiO<sub>2</sub> variants, such as P25, particularly in water-splitting applications, where 1DLs not only sustain a significantly high quantum yield of 11.7% but also maintain this elevated level of activity without any signs of degradation, even under rigorous operational conditions over extended periods-more than six months, including 300 hours, h, of illumination<sup>23</sup>. P25 is a commercially available nanoparticulate titania made by Evonik.

One of the most commonly employed characterization technique of nanomaterials is transmission electron microscopy, but its advantages (mainly, the direct and detailed visualization of individual NPs) also come with significant drawbacks: only a small number of objects can be imaged at the same time, making ensemble measurements tedious or, even, impossible. The limited penetration of electron beams precludes the study of large particles and of the inner structure of materials. Moreover, the stringent sample preparation requirements mean that following the formation kinetics of the NPs is out of the question.

Scattering techniques, such as small- and wide-angle X-ray scattering (SAXS and WAXS), are often better suited: they cover huge numbers of particles at once, are highly penetrating,
non-invasive and thus amenable to in situ and time-resolved techniques, so that they reveal
not only the particle morphology in the final state, but also its evolution during growth.
However, the interpretation of the results generally requires complex data treatment and
modeling.

In this work, mainly using SAXS and WAXS, we explore the structure of 1DL and their
self-assembly, in the colloidal state and after drying, as well as the correlation between
structure and photocatalytic activity. Although static, these measurements are indicative of
a complex process that starts with 1D objects and ends with well-ordered 3D crystals.

## Results and Discussion

The structure of a single NF that grows along the *a*-axis, with a cross-section of roughly
5 × 7 Å² is shown in **Figure 1A**. Figure 1B is a view of a ribbon –a few NFs associated
along the *c* direction (or, equivalently, a strip cut from a 2D layer). Figure 1C shows the
structure of bulk 2D TiO₂ lepidocrocite. The lattice parameters (used in all models below)
are: *a* = 3.8 Å, *b* = 23.06 Å and *c* = 2.95 Å. The data were acquired on colloidal solutions as
well as on dried powders.

## Colloidal suspensions

Stock colloidal suspensions were obtained as described in the Experimental Section. On
optical inspection they were transparent and homogeneous, except for the occasional pres-
ence of micron-sized grains (see **Figure S1**), probably consisting of unreacted precursor.
Considering their low volume fraction and compact morphology, they should not affect the
overall organization of the solution. Aside from this impurity sedimentation, the solutions
are stable for months.
<figure><img src="image_3.png" /><figcaption>Figure 1: Structure of A) a single 1DL NF, B) a ribbon, and C) bulk 2D TiO₂ lepidocrocite, in two orientations (left and right). The *a*, *b* and *c* crystallographic directions are indicated, as well as the approximate cross-section of an NF. Ti atoms are shown in blue and O atoms in red.</figcaption></figure>

## Overall morphology

The solutions were studied by SAXS and WAXS, at their initial concentration and after dilution by factors of approximately 3 and 10. The scattering patterns are isotropic, and the corresponding integrated intensities *I*(*q*) are shown in **Figure 2A**. The model (shown as solid lines) consists of: i) the form factor of a rigid parallelepiped *I*<sub>para</sub>(*q*) (which dominates at high *q*), and ii) a power-law contribution *I*<sub>aggr</sub>(*q*) ~ *q*<sup>-4</sup> (visible at low *q*), describing the large-scale aggregates. The parallelepipeds are characterized by their three side lengths: thickness, *T*, width, *W*, and length, *L*. Extensive modeling shows that the best fits are obtained for *T* below 4.5 Å so in the following we fix this parameter to *T* = 3.83 Å, corresponding to the effective width of one lepidocrocite sheet (along direction *b*), given by the two Ti planes (see Supporting Information). This is the relevant parameter for SAXS, as the Ti atoms overwhelmingly contribute to the scattering signal. The length *L* is at least 300 Å and we fix that parameter at this value, since the small-angle signal of the aggregates precludes resolving higher *L* values. Finally, the only morphological parameter to be fitted is *W*. The model agrees very well with the data, yielding *W* = 28 to 35 Å for the TiN precursor and *W* ~ 50 Å for TiB<sub>2</sub>, corresponding to assemblies (in the *c* direction) of about 5 and 8 NFs,
respectively. Attempts to prepare colloids from samples based on TiC precursor (studied below in the powder state) were unsuccessful due to low yields.

Thus, the colloidal state of the TiO₂ lepidocrocite NFs is that of *ribbons*, quite elongated along direction *a*, one sheet thick along *b* and about 5 to 8 NFs wide in the *c* direction. This conclusion is confirmed by TEM images of the colloidal suspensions (in the dried state, after deposition on a grid), see Figure 2B: they exhibit a fiber-like texture consisting of ribbons a few nm wide, in qualitative agreement with the SAXS results. Note that during the drying process the ribbons can start to assemble, as discussed below.

## Ribbon structure

To acquire more information about the internal structure of the nanoribbons, we allowed the most concentrated colloidal solutions for each precursor to concentrate by slow evaporation of water over about two months within their capillaries, reaching concentrations of 62.5 g/L for TiB₂ and 51.9 g/L for TiN. More details about the process are provided in SI.

In-house WAXS patterns (Figure 2C for the TiB₂ sample) are anisotropic, indicative of an orientationally ordered phase. This conclusion is supported by polarized optical microscope, POM, micrograph observations showing that the samples are birefringent and display schlieren textures, indicative of a nematic-like anisotropic phase (Figure S1). This conclusion is in accord with those of Ref. ¹⁶, where we showed that beyond a certain colloidal suspension concentration the 1DLs formed liquid crystals.

The anisotropy of the small-angle signal shows that the NFs are oriented with their long axis roughly along the first bisector, as shown in Figure 2C. The strong peak at $q = 3.3 \AA^{-1}$ shown in Figure 2D centered around this same direction is the [200] Bragg reflection of lepidocrocite, confirming that the NFs grow along the *a* crystalline axis. This sharp signal is clearly visible in the integrated intensity profile $I(q)$ shown in Figure 2D (as solid red dots for TiB₂ and as black open squares for TiN). Another distinctive feature is a wide correlation bump around $q = 2 \AA^{-1}$. The curves also exhibit several small and sharp peaks (marked by
<figure><img src="image_4.png" /><figcaption>Figure 2: A) SAXS patterns and fits with a “parallelepiped plus power law” model for colloidal suspensions as a function of concentration and precursor chemistry. Data shifted vertically for clarity (higher is more concentrated). The components of the model are shown individually as green dashed lines for the lowermost pattern. B) Typical TEM image of the TiB₂ sample. C) WAXS image for partially oriented TiB₂ sample. Diagonal line indicates average NF orientation. D) Integrated WAXS intensity, *I(q)*, for the TiB₂ and TiN samples and comparison with the atomic model (see text for details). Top axis marks the position of the Bragg reflections expected for a perfectly crystalline ribbon: only the [200] peak is present in the experimental data. Small peaks marked with asterisks are due to residual precursor.</figcaption></figure>

asterisks on the graph) due to residual precursor.

In the same graph we show (by a solid black curve) the signal expected for a perfectly
crystalline ribbon of the same size (obtained as detailed in the SI): This model captures quite well the position and shape of the [200] reflection and roughly reproduces the correlation bump. However, it also predicts a strong and sharp [101] reflection at $q = 2.7 \AA^{-1}$, which is clearly absent from our data. It follows that the assembly is crystalline along its length, but not along its width. Said otherwise, the ribbons are only loosely structured, without a well-defined periodicity along *c*. The most likely explanation is that the 1DLs associate via hydrogen bonds, rather than creating a crystalline sheet by polycondensation or olation, while the TMA⁺ cations and their water of hydration occupy the interfilamentous space. As recently shown²⁴, the competition between these two kinds of bonds can be modulated by changing the nature of the solvent.

## Dried Powders

In the case of the powders obtained after drying in air for 24 hours, h, at 50°C and 80°C, the scattering signal changes drastically (as shown in **Figure 3**), with the appearance of a peak around $0.55 \AA^{-1}$ revealing the presence of sheets stacked along the *b* direction. The peak shape depends on the average number, $N_0$, of sheets in the stack (the model includes a dispersity $\delta N = \sqrt{N_0}$ of this parameter) and on their degree of order, quantified by the Gaussian standard deviation (GSD). See the SI for details. Figure 3A shows scattering intensities $I(q)$ for three samples obtained under various experimental conditions: precursor and treatment (colored dots). The curves (black lines) are given by a continuous model in the *a* – *c* plane (no inner layer structure) 3.83 Å thick, with about 3, 15 and 20 layers per stack and GSDs of 0.2, 0.2 and 0.05, respectively. The interlayer spacing $d = 11.6 \AA \simeq b/2$ (the unit cell contains two layers along the *b*-direction) is constant from one sample to the next and reflects the fact that the interlayer space is occupied by TMA⁺ cations and their water of hydration. The lateral size of the sheets exceeds the detection limit of our setup (200 Å).

The continuous model captures the signal increase at small angles and the [0*k*0] peaks
corresponding to layering along the *b* direction but not the inner sheet structure, which yields
[hkl] peaks with *h* > 0 or *l* > 0, appearing at *q* > 1.5 Å⁻¹. For the two bottom curves in
Figure 3A, they are clearly separated and can be indexed as shown in Figure 3B for the
middle red curve, to [110], [130], [150] and [011] (the two latter peaks are superposed). Note
that in the top curve in Figure 3A (for thinner stacks) these peaks are not distinct, but
rather fuse into one broad hump.

<figure><img src="image_5.png" /><figcaption>Figure 3: SAXS and WAXS *I*(*q*) characterization. A) SAXS/WAXS signal in log-log representation for various samples in powder form (colored dots) with a stack model (black lines) with an interlayer spacing *d* = 11.6 Å. From top to bottom, models correspond to stacks of 3, 15 and 20 layers. Curves shifted vertically for clarity. B) Middle curve in A (TiB₂ precursor at 80°C) shown in lin-log representation. Aside from the [0k0] peaks accounted for by the model, reflections [110], [130], [150] and [011] are also present.</figcaption></figure>

To get a fuller image of the in-plane structure, we obtained powder X-ray diffraction
(XRD) patterns, which allow us to extend the accessible *q* range up to 5.5 Å⁻¹. This data is shown in **Figure 4**, using the same color scheme shown in Figure 3.

<figure><img src="image_6.png" /></figure>

Figure 4: XRD patterns for samples shown in Figure 3A using same color scheme and atomic model (black lines) for stacks consisting of three (top) and 15 layers (bottom). Bragg reflections are indexed below the patterns. Some residual precursor (TiC and TiB₂, respectively) yield the peaks labeled by asterisks in the top curve and the sharp reflections in the center and bottom curves. Patterns shifted vertically for clarity's sake.

The two bottom patterns display clear positional order along the three space directions and are in good agreement with the 15-layer atomic model (shown by solid black line). The corresponding Bragg reflections are indexed at the bottom of the graph and agree with our more recent work²⁵. The top pattern, however, is quite different from the crystalline 3-layer model: the [200] peak is clearly visible, so the structure along the NF length is preserved, but their lateral order is quite weak, as shown by the weak signals for the [101], [002] and [202] reflections.
Photocatalytic activity and band gap

<table><caption>Table 1: Hydrogen production rates and AQY under UV-Vis light (2 suns irradiation for 6 h) for the three samples in Figures 3 and 4 as well as a TiB₂ sample washed with LiCl and a P25 powder. The average number of layers corresponding to the models in Figure 3 is also shown. Last three columns report *E*<sub>g</sub> (in eV and in nm) for all samples and the VBM, in eV, for some of them.</caption><thead><tr><th>Photocatalyst</th><th>H<sub>2</sub> prod.<br>(µmol g<sup>-1</sup> h<sup>-1</sup>)<br>0d, 6h</th><th>AQY<br>(%)<br>0d, 6h</th><th>Avg. number<br>of layers</th><th><i>E</i><sub>g</sub><br>(eV)</th><th>Wavelength<br>(nm)</th><th>VBM<br>(eV)</th></tr></thead><tbody><tr><td>TiC 80°C</td><td>631</td><td>11.7</td><td>3</td><td>~ 4 *</td><td>310</td><td></td></tr><tr><td>TiB<sub>2</sub> 50°C</td><td>261</td><td>2.43</td><td>20</td><td>3.69</td><td>336.0</td><td>1.71</td></tr><tr><td>TiB<sub>2</sub> 80°C</td><td>221</td><td>2.05</td><td>15</td><td>3.75</td><td>330.6</td><td>1.33</td></tr><tr><td>TiB<sub>2</sub> 80°C LiCl</td><td>412</td><td>3.83</td><td>5 to 8</td><td>3.61</td><td>343.4</td><td>2.22</td></tr><tr><td>TiO<sub>2</sub> P25</td><td>180</td><td>1.02</td><td></td><td>3.2 †</td><td>385</td><td>1.8 ‡</td></tr></tbody></table>

To quantify the photocatalytic performance of the ribbon stacks, we present in **Table 1** H₂ production rates, the apparent quantum yield (AQY) and the band gap energy (with corresponding light wavelength) for the three samples whose structure is discussed in Figures 3 and 4, as well as for a TiB₂-based sample washed with LiCl solution (see SI for more details) and for commercial TiO₂ P25. We also show the maximal energy of the valence band (VBM) for some samples.

First of all, there is a significant difference (more than a factor of four) in photocatalytic activity between the TiC-based sample, which is less well structured, and the two TiB₂-based ones, that consist of thicker and better-ordered stacks. This effect could be possibly due to the better dispersion of the former (which leads to higher surface-to-volume area and enhanced light absorption), but also to its lack of crystalline order, which could facilitate the access of molecules in solution to the solid phase. The difference among the two TiB₂ samples is small, both in AQY and in average number of layers, and other effects (better dispersion state, optimized *E*<sub>g</sub>, and intrinsic defects/internal states) most probably come

<footnote>*Results from<sup>23</sup>.</footnote>

<footnote>†Results from<sup>26</sup>.</footnote>

<footnote>‡Results from<sup>27</sup>.</footnote>
into play.

We can also compare the EtOH-washed TiB₂ dried at 80°C sample with a similar one washed with LiCl solution before drying. The latter contains a mixture of partially and fully lithiated stacks, containing on the average 5 and 8 layers respectively (see SI for more details). Both the stack height and the H₂ production are intermediate between those of the EtOH-washed TiB₂ samples and of the TiC-based one, supporting an inverse correlation between these parameters. However, the intercalation of Li⁺ ions could also enhance photocatalytic activity by introducing shallow traps for electrons or holes, improving charge separation by capturing and slowing down their recombination²⁸.

Finally, all our samples have better H₂ production rates than TiO₂ P25, although their band gap is higher than for the latter.

## Sample stability

To highlight the significance of our materials for water splitting, we evaluated their stability for one month in a water-methanol mixture and for over 18 h under UV-Vis irradiation. The cumulative hydrogen production for the samples washed with EtOH and LiCl, as shown in **Figure 5**, reaffirming the high stability of these materials in agreement with our initial report²³.

## Conclusions

The most important result of this work is our conclusion that the 1DL NFs are well dispersed in the colloidal state, in the form of ribbons 4 Å thick, 30 to 50 Å wide along *c*, and at least 300 Å long along the *a* crystallographic direction. The absence of a [101] peak in the colloidal WAXS patterns (Figure 2C and D) implies that order along the *c*-direction is *weak*. This in turn implies the NFs are bonded together along that direction with TMA cations and/or H-bonds. This is an important result because it implies that in the colloidal state, the 1DLs
<figure><img src="image_7.png" /><figcaption>Figure 5: Cumulative hydrogen production under UV-Vis irradiation and photocatalytic stability (where *h* represents the hours under UV-Vis irradiation, and *d* the days in water: methanol mixture).</figcaption></figure>

are surrounded on two sides by TMA cations. Recently we were able to wrap individual 1DL NFs with polymers and thus isolate them¹⁶. This feat would not have been possible had the NFs not been weakly bound/isolated.

Upon drying, the SAXS patterns change significantly and now the samples can be described as extended sheets in the *a*–*c* plane that stack along the *b*-direction. The number of sheets in the stacks is a function of precursor chemistry and processing temperatures. Herein, the number of layers varies from 3 in the case of TiC reacted at 80°C, to 15 to 20 layers for dried powders obtained after reaction of TiB₂ with TMAH at 80°C and 50°C, respectively. In the latter case, the existence of [101] and [002] peaks implies that, during the drying process, the NFs fuse–by polycondensation, or dehydroxylation– along the *c* crystallographic direction to yield extended sheets that assemble along the *b* direction in stacks of different heights¹².

Preliminary results suggest that the lower the number of sheets in a stack, the higher
the H-production rates during photocatalysis of water/methanol mixtures. This conclusion, however, is tentative and requires much more work. These comments notwithstanding, we have recently shown that $E_g$ of 1DL colloidal suspensions, as well as filtered films made of these colloids, is a strong function of colloidal concentration and varies from 3.5 to 4.5 eV as the concentration changes from 40 to 0.01 mg/L, respectively$^7$. While at this time we explain these results as due to a 1D to 2D transition, this argument is qualitative. The results obtainable by SAXS, especially the ribbon morphology and number of layers per stack, constitute important information that is hard to access otherwise and will be invaluable in theoretical modeling and further understanding.

This work shows that scattering techniques provide useful information on the static structure of low-dimensional titanate nanostructures and their various morphologies. Among its perspectives is the time-resolved study of the drying process, which could validate the pathway proposed above and provide more detail about the evolution.

# Experimental Details

## Synthesis

The samples were prepared by reacting TiB₂ (99.9%, 325 mesh, Thermo Fisher Scientific), TiC (99.9%, ≤ 2µm, Alfa Aesar) or TiN (99.7%, ≤ 4µm, Thermo Fisher Scientific) precursor powders in an aqueous solution containing 25 wt.% tetramethyl ammonium hydroxide, TMAOH (as-received, 25% [w/w] aqueous, 99.9999%, Thermo Scientific) in a polyethylene bottle that was then heated and shaken at 80°C for 5 days in a temperature-controlled shaking incubator. The reaction bottle was vented with two 23-gauge needles to prevent pressure built-up. The weights of each reactant were chosen to keep the Ti:TMAOH molar ratio at 0.6:1. This ratio was used because it is the one used in most of our previous work and it is one whose structure and properties we best understand.

In the case of TiB₂, the post-reaction sediments were collected into a beaker and washed
with 200 proof EtOH for 3-4 cycles until neutral conditions were achieved (pH = 7). The powders were then dried at 50°C and 80°C in open air for 24 hours. In this case, the samples were never exposed to water, and the end result is PMPs in which the 1DL NFs were intercalated with TMA cations. These represent the powder samples presented in Section and denoted as TiC 80°C; TiB₂ 80°C and TiB₂ 50°C.

For ion exchange, the sediments, while being wet in ethanol, were further stirred in a 0.5M LiCl solution for 2 h. The powders were then allowed to settle, and the supernatant was discarded. This step was repeated 2-3 times to ensure complete ion exchange of TMA with Li ions. Subsequently, the powder was thoroughly washed with DI water to remove any excess LiCl salt. Finally, the powders were dried at 50°C in open air. In this case, the end result is PMPs in which the 1DL NFs were intercalated with Li cations.

In the cases of TiN and TiC, the post-reaction sediments were similarly washed with ethanol until pH was reached. To prepare a colloidal suspension, in all three cases (TiB₂, TiN, and TiC), the resulting sediment, while being wet in ethanol, was transferred into centrifuge tubes and mixed with DI water. The mixture was then vortex-shaken and centrifuged at 3500 rpm for 20 minutes to settle any unreacted precursors. The resulting supernatants are the target colloids and they were collected for further testing. Only for the TiC sample, the resulting colloid was further filtered via vacuum-assisted filtration.

## X-ray scattering experiments

SAXS and WAXS (small- and wide-angle X-ray scattering) measurements were performed on the SWING beamline of the SOLEIL synchrotron (Saint-Aubin, France) over several sessions, at a beam energy *E* = 12 or 16 keV. The sample-to-detector distances were about 6.2 m and 0.5 m, respectively. The beam size was approximately 500 × 200 µm² (H × V). All measurements were performed at room temperature (T = 22°C).

The colloidal suspensions were held in sealed glass capillaries with an outer diameter of 1.6 mm and 10 µm thick walls, purchased from WJM-Glas (Berlin, Germany), placed
vertically in a motorized holder. The dried powders were confined between two sheets of
adhesive tape and mounted onto the same holder. In each case, we subtracted from the raw
signal the contribution of a reference sample (water-filled capillary or empty adhesive tape,
respectively).

The scattered signal was recorded by an Eiger 4M detector (Dectris Ltd., Switzerland) with pixel size 75 µm. Preliminary data treatment (angular averaging and normalization) was done using the software Foxtrot developed at the beamline, which yielded the intensity as a function of the scattering vector *I*(*q*) in absolute units.

In-house WAXS experiments were performed on the X-ray scattering platform DiffériX of the Institut Charles Sadron (Strasbourg) using a rotating anode generator MicromaxTM-007 HF (Rigaku, Japan), operating at 40 kV and 30 mA, with a wavelength $\lambda = 1.54184$ Å, at room temperature. The beam was monochromated and collimated with a confocal Max-Flux OpticsTM mirror (Osmic, Inc., Japan) combined to a system of three pinholes. The scattered intensity was collected on an image plate (Molecular Dynamics, Sunnyvale, CA, USA) with a sample-detector distance of 6 cm. The colloidal suspensions were filled into glass capillaries as above. Data reduction was done in Igor Pro 7, according to usual procedures for isotropic scattering (angular averaging, electronic background, sample transmission and thickness corrections). *q* vectors were calibrated using a silver behenate powder standard. Intensities were converted to absolute scale using a calibrated Lupolen standard.

XRD

Structural investigations were performed at the NIMP (Bucharest) using a powder X-ray
diffraction (XRD) diffractometer (D8 Advance, Bruker-57AXS GmbH, Germany) with Ni-
filtered Cu radiation (λ = 1.54184 Å), at room temperature, in the Bragg-Brentano geometry,
in the 2θ range from 5° to 70° at a scanning rate of 0.02°/min. Additional measurements were
performed at the IPCMS (Strasbourg) on a D8 Discover diffractometer (Bruker, Germany)
with a quartz front monochromator and a motorized anti-scatter screen, using Cu radiation
($\lambda$ = 1.54184 Å), in the Bragg-Brentano geometry.

## TEM

A transmission electron microscope, TEM, (JEOL ARM 200F Japan) equipped with Field Emission Gun and ultra-high resolution polar piece. The TEM was operated at 200 kV. TEM specimens were prepared using a standard powder method, involving sample dispersion in ethanol followed by drop-casting the resulting suspension on a TEM grid (Cu 400mesh, lacey C).

## Photocatalytic tests

The photocatalytic tests for hydrogen production through water splitting were conducted in 25 mL of water-methanol mixture (4:1 volumetric ratio) in a cylindrical quartz reactor with a total volume of 55 mL, where 25 mg of the powders were immersed. The suspensions were magnetically stirred under nitrogen flow ($N_2$ purity 99.999% Linde) for 15 minutes before pressurizing to 2 atmospheres and irradiating for 6 h using an UV-Vis light with Asahi Spectra high-power illuminator MAX-303 system designed with a heat blocking, a 300 W Xenon light source and UV-Vis mirror module equipped with a one-meter-long quartz light guide and collimator lens (×1.0 (STD) Type RLQL80-1) that ensured the quasi-uniform two suns irradiation at a working distance of 4 cm. The resulting products were analyzed using a gas chromatograph (GC- Shimadzu, Japan) connected online to the reactor through the gas valve. The GC has a thermal conductivity detector (TCD) for hydrogen detection and other gases and a flame ionization detector (FID) for the identification of organic compounds. The GC has two parallel-coupled gas chromatography columns, one a MolSieve 5A (with a length of 30 m and an inner diameter of 0.32 mm) and the other Rt-Q-Bond (with a length of 30 m and an inner diameter of 0.53 mm).
## DR-UV-Vis

Diffuse reflectance UV-Vis spectroscopy (DR-UV-Vis) measurements were performed in the wavelength range of 200-800 nm utilizing an AvaLight-Xe pulsed light source linked to an AvaSpec-ULS 2048L spectrometer, which was equipped with an AvaSphere 80-REFL integration sphere. The spectra were calibrated using BaSO₄ as a reference. The DR-UV-Vis spectra of the catalysts were obtained in reflectance units and subsequently converted into the Kubelka-Munk remission function F(R∞). The spectra of the samples were acquired at ambient conditions.

## Acknowledgement

The authors acknowledge SOLEIL for the provision of synchrotron radiation facilities and Thomas Bizien for help using the SWING beamline. The DiffériX platform of the Institut Charles Sadron was used for in-house WAXS and we acknowledge Guillaume Fleith for performing the measurements and data treatment. We also thank the XRD platform of the IPCMS. This work was partially funded by NSF (DMR-2211319). MF, ACI and SN acknowledge the Romanian Ministry of Research, Innovation, and Digitization through the Core Program 2023-2026 (contract PC3-PN23080303) and project PN-III-P4-PCE-2021-1461.

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TOC Graphic

<figure><img src="image_8.png" /></figure>

Synchrotron-based X-ray scattering reveals the structure of 1DL lepidocrocite titania and their self-assembly. In the colloidal state they consist of one-dimensional nanofilaments loosely associated into thin and elongated ribbons. In the dry state, they reach a final state of extended sheets, stacked three to about twenty high. The stack height and crystallinity vary together and are inversely correlated with the photocatalytic activity.