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The polyol synthesis is a well-established wet chemistry one-pot method employed for the synthesis of CoO nanostructures, yielding aggregates of tens to hundreds of nanometres in diameter, depending on the synthesis parameters. These aggregates are composed of smaller primary particles showing evidence of oriented attachment. Traditionally, the synthesis is carried out in di-ethylene glycol; however, recent investigations have demonstrated the advantages of employing tetra-ethylene glycol as an alternative solvent medium, which substantially raises the temperature at which the synthesis can be performed without significant formation of impurities, and offers notable operating benefits by acting on easily controllable synthesis parameters (e.g., water/cation ratio and temperature). These advantages, coupled with the importance of CoO in materials science research, have prompted a more comprehensive examination of the reaction mechanism. To this end, we have combined in situ and ex situ synchrotron radiation studies to monitor the reaction progression and elucidate the CoO formation processes. A scheme of the experimental setup. During the reaction, small aliquots are withdrawn, brought to the measurement cell, and then back to the reaction flask. The measurement cell is illuminated with X-ray synchrotron light. 2D-X-ray powder diffraction patterns are measured over time, integrated and collected. The panel with the resulting phase evolution is displayed on the right.
Introduction
The polyol synthesis is a bottom-up/one-pot method often employed to synthesize nanostructured inorganic materials [1]. It was originally developed for the synthesis of metallic nanoparticles [2], exploiting the reducing power of polyols (i.e., aliphatic chains with multiple hydroxide groups) and their solvent physical–chemical analogies with water, combined with higher boiling temperatures and viscosities. The method was later expanded to a plethora of inorganic materials [1, 3], including transition metal oxides, by promoting nucleophilic substitution reactions, like hydrolysis, instead of redox processes.
Focusing on the polyol synthesis of CoO, the typical synthesis protocol consists in (i) dissolving a metal acetate, here Co(CH3COO)2·4H2O, in di-ethylene glycol (DEG) in presence of a certain amount of water (usually expressed as hydrolysis ratio,) [4, 5, 6–7] and (ii) heating the reaction mixture until the precipitation of the face-centred cubic CoO phase. Under these conditions, water acts as the nucleophilic agent, and the reaction yields CoO aggregates of ~ 100 nm made of much smaller (~ 5 nm) primary particles showing evidence of oriented attachment [8]. In a previous study [9], we have found that the use of tetra-ethylene glycol (TTEG), instead of the more common choice of DEG, significantly expands the capabilities of the method, with the most important improvements being (i) the absence of unwanted massive reduction of CoO to metallic Co and Co-carbides when the temperature reaches ~ 200 °C, (ii) the possibility of performing the synthesis at much lower hydrolysis ratios, and (iii) of tuning the particle aggregation by changing the hydrolysis ratio [9]. These advances have proven useful in tuning the physical properties of the product by acting on easily controllable synthesis parameters. In the same study, awareness of the formation of cobalt based layered hydroxide solid intermediate (Co-LHS), was crucial for the correct interpretation of the magnetic properties [9]. For this reason, we decided to investigate the reaction to a further extent, focusing on its mechanism. Granular nanostructured CoO is an extremely important functional material, for fundamental studies in magnetism and various applications (e.g., water splitting, electrocatalysis, etc.) [10, 11, 12, 13, 14–15], making this investigation fascinating from a fundamental point of view and relevant for potential applications.
In this study, we combined in situ (IS) synchrotron radiation X-ray powder diffraction (IS-XRPD) experiments (MCX beamline, Elettra synchrotron [16]) to monitor the time resolved progression of the cobalt acetate reaction in TTEG in presence of water, with all its phase evolutions. IS experiments can reveal details of the reaction that might otherwise be hidden by analysing the synthesis intermediates ex situ (ES). All the phases detected with IS experiments were isolated separately and studied by ES laboratory observations, including XRPD, ultraviolet–visible-near infrared spectroscopy (UV–Vis-NIR), and Fourier-transformed infrared (FTIR) spectroscopy, to elucidate the chemical nature of each intermediate formed during the reaction progression. Insights into the Co2+ coordination chemistry in the synthesis intermediates were obtained through X-ray absorption spectroscopy (XAS) experiments (ASTRA beamline, Solaris synchrotron [17]). This comprehensive multi-technique approach provided a more complete understanding of the reaction mechanism and the formation of intermediate species, revealing detailed insights into the forced hydrolysis synthesis process in TTEG solvent.
Results and discussion
We employed various complementary techniques to study the evolution of CoO phases during polyol synthesis, both in the laboratory and using synchrotron radiation. The samples were synthesized following the procedure described in the Experimental Methods section. The morphology of the nanoparticles produced at the end of the synthesis was investigated by transmission electron microscopy (TEM), while the analysis of the XRPD patterns provided insights into the crystallographic structure and crystallite sizes.
XRPD [Fig. 1(a)] and Selected area electron diffraction (SAED) (Fig. S2) demonstrate the successful precipitation of the fcc-CoO phase, with no impurities detected. TEM images reveal aggregates of spherical particles with a diameter of approximately 50 nm [Fig. 1(c)]. Higher magnification images [Fig. 1(d)] show 8–9 nm nanoparticles at the edges of the aggregates, although the Rietveld analysis indicates a crystallite size of 17 nm, which is larger. The discrepancy suggests that the particles initially form with a size around 8–9 nm and that, after the aggregation, the high temperature induces the crystal growth in the core of the aggregates, so that the size we obtain with Rietveld refinement is higher. It is known that, at the nanoscale, the critical temperature of magnetically ordered materials regularly decreases as the size decreases [18, 19, 20]–21. Recently [9], we investigated the size dependence of Néel temperature (TN) in CoO nanoaggregates. According to the previous result, a TN of ~ 255 K [Fig. 1(b)], ) is compatible with a size of 17 nm, reinforcing the validity of our findings. The 255 K value is obtained by measuring the magnetization vs. temperature curve (zero-field cooled [22]), presenting the maximum in magnetization at the TN typical of antiferromagnets [23]. The low T increase of magnetization is due to the paramagnetic susceptibility contribution originating from uncompensated surface magnetic moments, often observed in nano-sized CoO [9, 24, 25].
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Figure 1
Structural, morphological, and magnetic characterizations of CoO aggregates at the end of a synthesis (ES-CoO). (a) Ex situ XRPD diffraction pattern with experimental data (black dots), fitting curve (red line) and residual plot (blue). (b) Zero-field cooled magnetization vs. temperature curve, TN in evidence. (c) TEM picture and (d) higher magnification picture, with the size of the surface particles put in evidence. (e) Histogram of the statistical distribution of TEM nanoparticles (red) and log-normal fitting (black line).
The IS-XRPD experiment was carried out as described in the Experimental Methods section to follow the phase evolution in the ~ 100–200 °C temperature range (Fig. 2). The diffraction pattern measured at 100 °C shows an intense peak at , followed by dense series of weaker reflections extending until . This first intermediate phase is detectable in IS-XRPD patterns until up to a temperature of approximately 180 °C. Its formation is visibly associated with a colour change, shifting from pink/fuchsia to beige. Subsequently, a close inspection of the pattern reveals the emergence of a second intermediate phase, characterized by two distinct sawtooth peaks at and , observable within the reaction temperature range of 164 °C to 177 °C [Fig. 2(b and c)]. Both intermediate phases disappear around 180 °C. The rapid nucleation of the fcc-CoO phase in a narrow temperature range can be appreciated in Fig. 2(b) and (c) starting from 164 °C, after which diffraction lines from the CoO phase quickly grow with further heating. When the synthesis reaches 200 °C, no residues of other phases can be detected in the IS-XRPD patterns, demonstrating the advantage of using TTEG, considering that much longer times are required using the DEG synthesis [5, 6]. Hence, IS-XRPD data provided the time resolution required to accurately probe the evolution of intermediate phases during the synthesis and allowed to recognize the formation of three phases during the synthesis process. While the identity of CoO is already well established and needs no further clarification, ES laboratory scale syntheses and characterizations can serve the purpose of recognizing the two intermediates. With this aim, three intermediate samples were synthetized and isolated by halting the reaction at 120, 165 and 180 °C, respectively, and are referred to as Int120, Int165, Int180, as described in the Experimental Methods section. Their ES-XRPD patterns are shown in Fig. 3, alongside the diffraction pattern of the precursor, cobalt acetate tetrahydrate (CA-4H).
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Figure 2
The figure reports the in situ XRPD curves at different q intervals. (a) 0.75–1.25 Å−1 interval, showing the dehydrated acetate main peak. (b) 2.0–5.5 Å−1interval, where the peaks of all the phases are visible. (c) Comparison of the in situ XRPD curves in the164–177 °C temperature range with the XRPD of Int180 powders.
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Figure 3
Ex situ XRPD patterns (from bottom to top) of the precursor (CA-4H, pink), and intermediates isolated at three different temperatures (Int120, yellow; Int165, purple; Int180, blue). Int120 is a dehydrated cobalt acetate, and Int180 is a layered phase. Int165 consists of a mere mixture of the two materials (see main text).
The IS-XRPD patterns shown in the early stages of the reaction match the ex situ pattern of Int120 (Fig. S3), meaning that we successfully isolated the intermediate, which has the appearance of a beige powder. However, despite the salt precipitation, the washing supernatant of Int120 is still pink/fuchsia, meaning that part of the cations remains dissolved in the medium. Since water is soluble in TTEG, we can interpret this fact as TTEG partially dissolving Co2+ and subtracting water from undissolved CA-4H. FTIR spectroscopy proves valuable in this case. A few studies were dedicated to the decomposition of CA-4H in inert atmosphere [26, 27, 28–29], which is an acceptable approximation of our non-oxidizing environment. Despite some differences, these studies agree with CA-4H undergoing a dehydration process before any other transformation. The FTIR spectra of our intermediate [Fig. 4(b)] matches those published in refs.[26, 28]: the two peaks at 1567 cm−1 and 1420 cm−1 can be attributed to the asymmetric and symmetric stretching modes of the CO bonds in acetate groups, while the broad band at 3400 cm−1 can be attributed to OH stretching in crystallization water molecules. This suggests that our first intermediate is not completely dehydrated. Since its profile is remarkably different from the one of the dihydrate salt [30], we can infer that our compound probably retains either one or half water molecules per Co2+ cation.
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Figure 4
Spectroscopical characterization of (from bottom to top) CA-4H (pink), Int120 (yellow), Int180 (blue), and ES-CoO (red). (a) UV–Vis-NIR, putting in evidence the tetrahedral (Td) and octahedral (Oh) transitions of CA-4H and Int180; Oh transitions are in the same position for both samples. (b) FTIR spectra, evidencing the OH and CO stretching modes relevant for out interpretation.
The pattern of the second intermediate appeared in the IS-XRPD matches that of Int180 [Figs. 2(c) and 3]. The reflections at and can be attributed to the formation of the turbostratic disordered single layered cobalt hydroxy-acetate salt (Co-LHS), already observed in our previous study [9]. The characteristic sawtooth shape of these peaks is typical of layered structures with turbostratic disorder (often observed in various transition metal brucite and/or hydrozincite-type hydroxide derivatives [31, 32–33], consisting of a particular class of stacking faults in which the single layers keep a repetitive distance but arrange twisted or shifted between each other. The two asymmetric reflections can be assigned to the (100) and (110) planes constituting the two-dimensional layers [34], while the reflections at q lower than ~ 2.3 Å−1 can be associated with the (00 l) vertical stacking of the layers. The layers are often separated by small intercalation molecules, likely water and acetate anions in our case. A minor fraction of tetrahedrally coordinated Co2+ ions (CoO4, Td), within a majority of octahedrally coordinated Co2+ (CoO6, Oh) is characteristic of hydrozincite-type cobalt(II) hydroxide derivative, which belongs to the α-hydroxide compound family [31, 33, 35]. The presence of these two geometries finds confirmation in our data. The electronic d-d transitions of metal complexes are observable in the UV–Vis-NIR spectral range. Their energies are tied to the electronic configuration, which, in turn, is connected to the coordination chemistry. Since Int180 (as well as the other compounds discussed in the present work) includes Co2+ coordinated by oxygen atoms, we can compare its UV–Vis-NIR spectrum with compounds possessing the same structural element. On the basis of Tanabe-Sugano diagrams, d7 Co2+ ion in Oh field is expected to have three spin allowed d-d transitions [36, 37]: 4T2g(F) ← 4T1g(F) (ν1(Oh)), 4A2g(F) ← 4T1g(F) (ν2(Oh)), 4T1g(P) ← 4T1g(F) (ν3(Oh)). Notably, all three transitions are also present in the spectrum of CA-4H [Fig. 4(a)], which is known also from structural data to have Co2+ in Oh coordination [38], and which colour is pink, as it typically happens for Oh Co2+ oxygen-coordinated complexes (e.g., [Co(H2O)6]2+ [39]). In Int180, ν1(Oh) transition can be found at ~ 500 nm, and ν3(Oh) at ~ 1275 nm [Fig. 4(a)], while ν2(Oh) is obscured by the intense doublet at ~ 600 nm. On the other hand, Td coordination is expected to have three spin-allowed transitions: 4T2(F) ← 4A2(F) (ν1(Td)), 4T1(F)← 4A2(F) (ν2(Td)), 4T1(P)← 4A2(F) (ν3(Td)). Excluding the transitions already assigned, the ~ 600 nm doublet, and the ~ 1575 nm broadband remain. The first of the two can be assigned with ν3(Td), which is also responsible for the blue/grey colour of the powder, and its splitting is likely due to Jahn–Teller distortions or spin–orbit coupling [40]. The other two transitions are expected to be found in the NIR range. According to spectroscopic studies conducted on Co2+-doped wurtzite ZnO, the broad band at ~ 1575 nm can be assigned with the ν2(Td) transition, while the ν1(Td) transition is supposed to appear in the 2300–2400 nm range, beyond our investigation range [5, 41, 42]. This interpretation is consistent with the increased intensity of the pre-edge X-ray Absorption Near Edge Structure (XANES) peak of Int180 [Fig. 5(a)], and with those already present in literature [31, 35].
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Figure 5
(a) The normalized expanded Co-K pre-edge region of the investigated samples. The intensity of the pre-edge peak is sensitive to the Co-coordination symmetry, and it is weaker for regular octahedral coordination but enhanced for tetrahedrally coordinate Co sites (see text). (b) The Co-K XANES region for the investigated samples. The derivative of the absorption signal is reported in the inset showing no edge shift between the samples, demonstrating the same Co2+ valence state in all the investigated samples. (c) Modulus of the Fourier Transform of the k2χ(k) experimental data (coloured symbols) and best fit curves (black lines), vertically shifted for clarity. The same colour scheme is applied to individuate the samples in the three panels.
Int165 (Fig. 3) is a mere mixture of the dehydrated salt and the α-hydroxide, hence, its UV–Vis-NIR and FTIR spectra are only shown in the supplementary material. Small discrepancies in temperature between the in situ and ex situ experiments are probably related to small experimental setup differences required to adapt the synthesis to the synchrotron beamlines. Since a high hydrolysis ratio promotes the formation of Co-LHS [9], a lesser amount of water in the IS experiments, evaporated at higher rates due to the setup used in IS experiments, is likely at the origin of the quicker hydroxide decomposition.
The analysis of X-ray absorption fine structure (XAFS) spectroscopy data provides details about the Co valence state and local coordination chemistry of the ES powders. Specifically, the near edge (XANES) region of XAFS spectra is sensitive to the average valence state and coordination symmetry of the absorber atoms [43].
Figure 5(a and b) displays the normalized Co XANES spectra of the five samples (CA-4H, CA-4H-sol, Int120, Int180, ES-CoO). In the pre-edge region, a weak absorption is observed at ~ 7708 eV. This feature is attributed to dipole forbidden 1s → 3d transition. However, p-d hybridization can enable the transition from 1s orbitals to the p component of hybridized pd orbitals, increasing the absorption cross-section. For symmetry reasons [44, 45], the p-d hybridization is forbidden by the octahedral coordination, but favoured in tetrahedral symmetry. The evident raise of the pre-edge peak intensity in the Int120 and especially in Int180 [Fig. 5(a)] suggests a fraction of tetrahedrally coordinated Co, in agreement with previous UV–Vis findings. The pre-edge peak intensity drops in ES-CoO spectra as expected for the cubic Cobalt oxide phase.
The edge energy, defined as the maximum of the first derivative of the absorption spectrum (inset, [Fig. 5(a)]), remains constant across all samples, indicating an unchanged Co2+ oxidation state. The main peak at the Co K-edge (also called “white line”) corresponds to the 1s-4p transitions in the dipolar approximation. Changes in metal–ligand distances and structural deformations affect the antibonding (empty) 4p states, thereby altering the white line intensity. The precursor CA-4H and the CA-4H-sol samples have very similar features, indicating that even after heating at 80° C in the liquid phase, the hexa-coordination of the precursor is unchanged. The white line of Int120 undergoes a dramatic decrease, showing evident attenuation, likely related to increased structural disorder (as discussed below). Furthermore, in the Int180 sample, the white line also shifts to higher energies. These spectral changes indicate a progressive modification of the Co local structure correlating with the crystallographic changes observed by XRPD. Finally, the XANES spectrum of ES-CoO closely resembles that of cobalt oxide confirming the successful synthesis of the target compound.
The analysis of the EXAFS region provides further details about the local Co2+ coordination. The fitting of the EXAFS data is performed fitting the k2-weighted Fourier transforms FT (real and imaginary parts) of experimental EXAFS data to the model curves. The modulus of the FT of experimental data and best fit curves are reported in Fig. 5(c). All the data were fitted considering a first oxygen shell (Co–O) and the Co nearest neighbour shell (Co–Co). Following preliminary analyses, the energy shift, which represents the discrepancy between the theoretical and experimental onset of the photoelectron energy scale, was fixed at − 5 eV. Additionally, the amplitude reduction factor (), which accounts for many-body losses in the single-electron approximation, was set to . The Co–O and coordination number was fixed at 6 in all the samples and the Co–Co coordination number was fixed to 12 in ES-CoO sample. These constraints allow to minimize correlation among the fitting parameters and to enhance sensitivity to subtle structural variations between samples. In principle, the average coordination number in nanoparticles depends on particle size due to under-coordinated atoms at the surface [46]. However, regarding the first coordination shell, XANES shape confirms that Co is coordinated with six oxygen atoms. Additionally, TEM and XRD data for the ES-CoO sample (which is obtained at the end of the synthesis) show quite large particles, where surface effects are negligible. This justifies our use of 12 Co–Co neighbors. This approach allows for a more robust comparison of the local atomic environments across the different stages of the synthesis process. The best fit parameters are reported in Table 1, for the refined parameters, the standard uncertainties on the last digit are reported in parenthesis.
TABLE 1. Results of the EXAFS data analysis for the five samples investigated.
Co–O | Co–Co | |||||
|---|---|---|---|---|---|---|
Sample | N | R(Å) | N | R(Å) | ||
CA-4H | 6 | 2.06(1) | 7.3(5) | – | – | – |
CA-4H_sol | 6 | 2.05(1) | 9.1(5) | – | – | – |
Int120 | 6 | 2.05(1) | 15(1) | 1.4(1) | 3.08(1) | 5.1(6) |
Int180 | 6 | 2.03(1) | 14(1) | 4.9(6) | 3.09(1) | 9.1(7) |
ES-CoO | 6 | 2.09(1) | 8.2(5) | 12 | 3.01(1) | 8.2(5) |
Standard uncertainties on the last digit of the refined parameters are shown in parenthesis.
The results of the EXAFS data fitting demonstrate that the Co–O local structure is stable for all the samples with a Co–O distance close to 2.05 Å, while the formation of CoO stabilizes the longer Co–O distance (2.09 Å) closer to the Co–O distance in bulk CoO phase. The intermediate phases, Int120 and Int180 samples, demonstrate large disorder, likely due to different configurations of the Co coordination and the presence of tetrahedrally coordinated Co sites. Owing to the strong correlation between coordination numbers and parameters we found it more reliable to keep the coordination numbers fixed in the fitting. The Co–Co next neighbour distances in Int120 and Int180 samples are consistent (3.5% compressed) with those expected in α-Co(OH)2, parent of the Co-LHS compound (being 3.19 Å [35]). The low Co–Co coordination number in Int120 (1.4 instead of 6) is consistent with the formation of dimers and chains, suggesting that the structure of Int120 shares similarities with that of the chain-like [Co(ac)2H2O]·H2O compound published by Jiao and coworkers [47]. If the second shell Co–Co coordination number (Table 1) is used to calculate number of polyhedral composing a chain, one can deduce that the chains are composed by 3–4 polyhedra.
The FT of the EXAFS spectra [Fig. 5(c)], reveals distinct structural evolution across the synthesis stages. The FT of precursor (CA-4H) sample exhibits weak signals around 4 Å (uncorrected for phase shift), indicative of next neighbor coordination shells from acetate ligands. Upon dissolution (CA-4H-sol), the second shell signal diminishes (FT), while the pre-edge XANES features [Fig. 5(a)] and the intense Co–O peak remain consistent with octahedrally coordinated Co sites. In the Int120 intermediate, a slight decrease in the Co–O nearest neighbor shell intensity, coupled with an enhanced pre-edge XANES peak, suggests the emergence of tetrahedral Co coordination. Concurrently, the appearance of a Co–Co shell at approximately 3 Å indicates the onset of CoOx unit aggregation. EXAFS analysis yields an average Co–Co coordination number of ~ 2, consistent with the formation of edge-sharing CoOx octahedral chains coherently with the model proposed in ref. [47]. In the Int180 sample, the CoOx units aggregation progresses further, as evidenced by the intensification of the FT peak at 3 Å. The ES-CoO sample marks the completion of the transition to the cobalt oxide phase, characterized by the emergence of pronounced and closer Co–Co peak and evident structural features in the 3–6 Å region, indicative of well-ordered and relatively large nanoparticles.
These observations collectively elucidate the progressive structural transformation from the molecular precursors to the final Co-oxide networks and point out a peculiar attribute of this polyol synthesis: the progressive dimensional evolution of CoOx units throughout three distinct reaction stages. Such evolution can be categorized as follows: zero-dimensional isolated CoO6 units (CA-4H-sol), one-dimensional chains (Int120) and two-dimensional networks (Int180) consisting of CoO6/4 units, and ultimately, three-dimensional nanocrystalline structures (ES-CoO).
Conclusion
In this study, we employed a comprehensive suite of complementary in situ and ex situ techniques to elucidate the intricate evolution of polyol-dissolved cobalt acetate throughout the forced hydrolysis process, from the initial molecular precursor to the formation of CoO nanoparticles. This multifaceted approach, combining synchrotron and laboratory-based methods, allowed us to delineate a stepwise dimensional growth mechanism of Co2+-oxide phases with increasing dimensionality.
Our findings reveal a progression from isolated CoO6 octahedra (zero-dimensional) in the solvated precursor state, to one-dimensional chains and two-dimensional networks of CoO6/4 units in the intermediate stages, culminating in the formation of three-dimensional CoO nanoparticles. This dimensional evolution is accompanied by significant changes in the local coordination environment of cobalt, as evidenced by XANES and EXAFS analyses and supported by laboratory UV–Vis and IR spectroscopies.
The use of tetra-ethylene glycol as the reaction medium has proven instrumental in achieving higher synthesis temperatures without substantial impurity formation, allowing for finer control over aggregate dimensions through modulation of water content. These original findings not only enhance our understanding of the mechanistic pathways underlying CoO nanoparticle formation in polyol synthesis but also offer valuable guidance for the rational design of synthesis protocols for other metal oxide nanostructures. This advance in synthetic methodology, alongside mechanistic insights, may open new possibilities for tailoring the size, morphology, and properties of CoO nanostructures.
The insights gained from this work have broad implications for materials science, potentially enabling the development of more efficient and precisely controlled nanoparticle synthesis methods for applications in catalysis, energy storage, and other applications.
Experimental methods
Synthesis
All the chemicals were purchased from Sigma-Aldrich and used without further purification. The synthesis was performed by dissolving cobalt acetate tetrahydrate Co(CH3COO)2·4H2O (CA-4H, ≥ 98.0% purity) in tetra-ethylene glycol (TTEG, 99% purity), in presence of deionized water, and heating the solution up to 200 °C. Intermediates were isolated by halting the synthesis at 120, 165, and 180° C. The nominal concentration of Co2+ was set to , while the hydrolysis ratio was set to (precursor hydration molecules included). The glassware consists of a three necked flask connected to an air condenser tube (~ 46 cm of effective length), a mechanical stirrer, and a temperature sensor. The heating was performed under the control of a PID system. The reaction was cooled at room temperature in a water bath. For the ES experiments, the products were washed three times by ethanol addition, sonication, and centrifugation (20′000 rpm, 15 min). The powders were then oven-dried overnight at 60° C. These samples are referred to as Int120, Int165, Int180, and ES-CoO. These intermediates were characterized with laboratory routinary techniques.
Instrumental techniques
In situ XRPD experiments (IS-XRPD) were conducted at the MCX beamline of the Elettra synchrotron in Trieste (Italy) [16] with a 17 keV X-ray beam energy. The experimental set-up consists of a closed circuit with a peristaltic pump, which withdraws a fraction of the fluid from the solution, passes it through a quartz capillary (1 mm diameter), and returns it to the reaction flask. Neoprene pipes (50 + 50 cm) are used for the connections. Angle dispersed 2D-XRPD patterns were measured using an area detector (marCCD-SX-165) in the 100° C to 200° C temperature range, with 3 s exposure time every 30 s. The IS-XRPD pattern of the solvent-and-water solution (without the Co-precursor) at 100 °C was measured and used as background to be subtracted to the data (original patterns can be found in the supplementary material). The 2D patterns were integrated to obtain standard intensity vs 2θ diffraction patterns.
Ex situ X-ray powder diffraction patterns (ES-XRPD) were measured on powders with a PANalytical X’pert Pro, (Bragg–Brentano θ–θ geometry, multichannel X’celerator detector, Co Kα X-ray tube, λ = 1.789 Å, 40 kV, 40 mA). For the sake of comparison, the ES-XRPD data are presented as a function of the scattering vector . Rietveld refinement was conducted with the MAUD software [48, 49] on ES-CoO to assess the crystal coherence size in the aggregates. Transmission electron microscopy (TEM) pictures were obtained with a Jeol 2100 Plus (200 kV, LaB6 source, EDS Si(Li) detector, CDD GATAN Multiscan Camera). TEM is also employed to perform selected area electron diffraction (SAED). Ultraviolet–Visible-near infrared spectroscopy (UV–Vis-NIR) was performed on the powders with a Perkin Elmer-Lambda 1050 spectrometer equipped with a polytetrafluoroethylene (PTFE) coated integration sphere, with emphasis on the 400–1800 nm range. Fourier transformed infrared spectroscopy (FTIR) was performed in transmission mode on solid powders diluted in KBr, using a Perkin-Elmer 1750 spectrophotometer between 4000 and 400 cm−1. The same characterizations were also performed on pristine CA-4H.
The Co K edge X-ray Absorption spectroscopy (XAS) experiments were carried out at the ASTRA beamline [17] at room temperature using standard transmission geometry. The Co K edge spectra were measured on 5 samples labelled CA-4H, CA-4H-sol, Int120, Int180, and ES-CoO. The CA-4H is the Co-acetate precursor, Int120, Int180 ES-CoO are dry powders obtained from the laboratory synthesis and oven-dried. Powders were finely grounded, mixed with pure cellulose matrix (1:8 weight ratio) and pressed to obtain thin solid pellets. The CA-4H-sol sample was the liquid solution treated at 80 °C until complete solution (visual inspection) of the precursor in TTEG, deposited onto filter paper slices. Several slices were piled up to obtain suitable absorption signal.
Incident and transmitted X-ray intensities were measured using I0 and I1 ionization chambers. The X-ray absorption from a Co-metal foil (Exafs Materials Company) placed after I1 was measured simultaneously to monitor the X-ray beam energy calibration. Several (up to 7) spectra were measured, checked for energy calibration, and averaged to improve the data statistics. Raw XAS spectra were treated using the standard procedures [50, 51] which include the pre-edge background subtraction, glitches removal, post-edge normalization. Quantitative analysis was carried fitting the k2-weighted experimental EXAFS data to the theoretical curves in real space using the Artemis program [52]. Atomic clusters based on crystallographic structures were used to individuate the photoelectron scattering paths and calculate the photoelectron scattering amplitude and phase functions.
Acknowledgments
We would like to acknowledge Tom Chevry (Université Paris Cité), who crafted the glassware necessary for the in situ experiments. The authors acknowledge the CERIC-ERIC Consortium for the access to the experimental facilities at ELETTRA (MCX) and SOLARIS (ASTRA). M.B. and C.M. Rome Technopole Project (CUP:F83B22000040006). M.B. and C.M. acknowledge the MUR (IT) support (Grant: Dipartimenti di Eccellenza 2023–2027, Art. 1, 314-337 Legge 232/2016).
Author contributions
The paper and the Supplementary Materials were written basing on the contributions of all authors. M.B., C.M., S.A. and D.P. designed the experiments. M.B. and C.M. coordinated the data analysis and discussion. M.B. performed the lab-scale ex situ syntheses and characterizations. M.B., R.M., P.C. and C.M. performed the synchrotron light experiments. J.P. and L.G. supervised the execution of the in situ experiments at the ELETTRA synchrotron. A.M. and L.A. supervised the execution of the XAFS experiments. All authors contributed to the revision of the article, which was mainly written by M.B. and C.M..
Funding
D.P. thanks Project funded under the Italian National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.3—Call for tender No. 1561 of 11.10.2022 of Ministero dell’Università e della Ricerca (MUR); funded by the European Union—NextGenerationEU ⋅ Award Number: Project code PE0000021, Concession Decree No. 1561 of 11.10.2022 adopted by Ministero dell’Università e della Ricerca (MUR), CUP D33C22001330002-Project title “Network 4 Energy Sustainable Transition—NEST”. The synchrotron radiation experiments at ELETTRA (IT) and SOLARIS (PL) have been supported by the CERIC-ERIC consortium (Proposal Numbers: 20232125, 20237173). M.B., R.M. and C.M. acknowledge the Grant of Excellence Departments MIUR (ARTICOLO 1, COMMI 314 − 337 LEGGE 232/2016). A.M. and L.A. acknowledge the Polish Ministry and Higher Education project: “Support for research and development with the use of research infrastructure of the National Synchrotron Radiation Centre SOLARIS” under contract nr 1/SOL/2021/2. S.A. thanks the French National Research Agency (ANR) for its LABEX SEAM (ANR 11 LABX 086) grant.
Data availability
The data analysed and presented in the current study are available from the corresponding author on reasonable request.
Declarations
Conflict of interest
There are no conflicts of interest to declare.
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