Detailed insights into the formation pathway of CdS and ZnS in solution: a multi-modal in situ characterisation approach.

The high stability, high availability, and wide size-dependent bandgap energy of sulphidic semiconductor nanoparticles (NPs) render them promising for applications in optoelectronic devices and solar cells. However, the tunability of their optical properties depends on the strict control of their crystal structure and crystallisation process. Herein, we studied the structural evolution during the formation of CdS and ZnS in solution by combining in situ luminescence spectroscopy, synchrotron-based X-ray diffraction (XRD) and pair distribution function (PDF) analyses for the first time. The influence of precursor type, concentration, temperature and heating program on the product formation and on the bandgap or trap emission were investigated in detail. In summary, for CdS, single-source precursor (SSP) polyol strategies using the dichlorobis(thiourea)cadmium(II) complex and double-source precursor approaches combining Cd(CH3COO)2·2H2O and thiourea led to the straightforward product at 100 °C, while the catena((m2-acetato-O,O')-(acetate-O,O')-(m2-thiourea)-cadmium) complex was formed at 25 and 80 °C. For ZnS, the reaction between Zn(CH3COO)2·2H2O and thiourea at 100 °C led to the product formation after the crystallisation and dissolution of an unknown intermediate. At 180 °C, besides an unknown phase, the acetato-bis(thiourea)-zinc(II) complex was also detected as a reaction intermediate. The formation of such reaction intermediates, which generally remain undetected applying only ex situ characterisation approaches, reinforce the importance of in situ analysis for promoting the advance on the production of tailored semiconductor materials.

From ligand exchange to reaction intermediates: what does really happen during the synthesis of emissive complexes?

In situ monitoring of the formation of emissive complexes is essential to enable the development of rational synthesis protocols, to provide accurate control over the generation of structure-related properties (such as luminescence) and to facilitate the development of new compounds. In situ luminescence analysis of coordination sensors (ILACS) utilizes the sensitivity of the spectroscopic properties of lanthanide ions to their coordination environment to detect structural changes during crystallization processes. Here, ILACS was utilized to monitor the formation of [Eu(bipy)2(NO3)3] (bipy = 2,2'-bipyridine) during co-precipitation synthesis. Validity of the ILACS results was ensured by concomitant utilization of in situ monitoring of other reaction parameters, including in situ measurements of pH value, ionic conductivity, and infrared spectra, as well as ex situ and synchrotron-based in situ X-ray diffraction analyses. Gradual desolvation of the Eu3+ ions and attachment of ligands were detected by an exponential increase of the intensity of the 5D0 → 7FJ (J = 0-4) transitions in the emission spectrum. Additionally, the in situ emission spectra show a decrease in the crystallization rate and an increase in the induction time in response to a reduction in the concentration of the starting solutions from 12 mM until crystallization ceased at starting reactant concentrations <6 mM. An increase to a three-fold higher concentration leads to the formation of a reaction intermediate, and its stability was determined to be highly concentration-dependent. The in situ luminescence measurements also demonstrated the existence of a ligand exchange process within the [Eu(bipy)2(NO3)3] complex upon addition of a phen (phen = 1,10'-phenanthroline) solution and the generation of a new phen-containing emissive complex. In attempting to solve the structure of this new phen-containing complex, a different, but nevertheless previously unsynthesized complex, [Eu(phen)2(NO3)3]bipy, was obtained, which shows characteristic Eu3+ luminescence in the red spectral range.

Real-time monitoring of CdTe quantum dots growth in aqueous solution.

Quantum dots (QDs) are remarkable semiconductor nanoparticles, whose optical properties are strongly size-dependent. Therefore, the real-time monitoring of crystal growth pathway during synthesis gives an excellent opportunity to a smart design of the QDs luminescence. In this work, we present a new approach for monitoring the formation of QDs in aqueous solution up to 90 °C, through in situ luminescence analysis, using CdTe as a model system. This technique allows a detailed examination of the evolution of their light emission. In contrast to in situ absorbance analysis, the in situ luminescence measurements in reflection geometry are particularly advantageous once they are not hindered by the concentration increase of the colloidal suspension. The synthesized particles were additionally characterized using X-ray diffraction analysis, transition electron microscopy, UV-Vis absorption and infrared spectroscopy. The infrared spectra showed that 3-mercaptopropionic acid (MPA)-based thiols are covalently bound on the surface of QDs and microscopy revealed the formation of CdS. Setting a total of 3 h of reaction time, for instance, the QDs synthesized at 70, 80 and 90 °C exhibit emission maxima centered at 550, 600 and 655 nm. The in situ monitoring approach opens doors for a more precise achievement of the desired emission wavelength of QDs.

How do layered double hydroxides evolve? First in situ insights into their synthesis processes.

Despite the importance of layered double hydroxides (LDHs) in catalysis, medicine and water treatment, the crystallisation process of these materials is seldom investigated. In this study, in situ characterisation techniques granted unprecedented experimental access to the formation dynamics of carbonate-intercalated Mg2+/Al3+ LDHs as model system when applying the most relevant co-precipitation approaches by exploring the effects of temperature and concentration of reactants. For this purpose, a combinatorial multi-modal characterisation approach was applied involving in situ measurements of pH, ion conductivity and light scattering, as well as synchrotron-based in situ X-ray diffraction (XRD). Shortly after beginning the addition of basic solutions (i.e., sodium carbonate and sodium hydroxide) to the solutions of magnesium nitrate hexahydrate and aluminium nitrate nonahydrate, a stable pH was reached due to the uptake of hydroxyl ions for nuclei formation. Shortly after, crystal growth phase was detected by an increase in the light scattering signal and confirmed via in situ XRD. Increasing the concentration of reactants accelerated the onset of crystal growth by 70% without significantly changing the crystallite size. On the other hand, increasing the temperature up to 65 °C showed a smaller influence on the reaction kinetics but resulted in a two-fold increase in crystallite size. Adding the solution of metal precursors to the basic solution, saturation was rapidly reached, without an induction period, favouring the formation of very small crystallites of approximately 10 nm.

Synthesis of M-UiO-66 (M = Zr, Ce or Hf) employing 2,5-pyridinedicarboxylic acid as a linker: defect chemistry, framework hydrophilisation and sorption properties.

Metal-organic frameworks of general composition [M6(OH)4(O)4(PDC)6-x(Cl)2x(H2O)2x] with M = Zr, Ce, Hf; PDC2- = 2,5-pyridinedicarboxylate and 0 ≤ x ≤ 2 were obtained under reflux using formic, nitric or acetic acid as an additive. Rietveld refinements carried out using a fixed occupancy of the linker molecules according to the results of thermogravimetric measurements confirmed that the MOFs crystallize in the UiO-66 type structure and demonstrate that the structural models describe the data well. Further characterization was carried out by NMR spectroscopy, thermogravimetric analysis, Zr K-edge EXAFS- and Ce L3-edge XANES measurements. To highlight the influence of the additional nitrogen atom of the pyridine ring, luminescence and vapour sorption measurements were carried out. The hydrophilisation of the MOFs was shown by the adsorption of water at lower p/p0 (<0.2) values compared to the corresponding BDC-MOFs (0.3). For water and methanol stability cycling adsorption experiments were carried out to evaluate the MOFs as potential adsorbents in heat transformation applications.

Synthesis and structure of Zr(iv)- and Ce(iv)-based CAU-24 with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene.

Two new MOFs denoted as M-CAU-24 (M = Zr, Ce) based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under mild reaction conditions within 15 min. The MOFs with composition [M6(µ3-O)4(µ3-OH)4(OH)4(H2O)4(TCPB)2] crystallise in the scu topology, a connectivity hitherto unreported for Zr-MOFs with tetracarboxylate linker molecules. Zr-CAU-24 exhibits UV/blue ligand-based luminescence.