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Most analytical techniques used to study the surface chemical properties of superparamagnetic iron oxide nanoparticles (SPIONs) are barely suitable for in situ investigations in liquids, where SPIONs are mostly applied for hyperthermia therapy, diagnostic biosensing, magnetic particle imaging or water purification. Magnetic particle spectroscopy (MPS) can resolve changes in magnetic interactions of SPIONs within seconds at ambient conditions. Herein, we show that by adding mono- and divalent cations to citric acid capped SPIONs, the degree of agglomeration can be utilized to study the selectivity of cations towards surface coordination motifs via MPS. A favored chelate agent, like ethylenediaminetetraacetic acid (EDTA) for divalent cations, removes cations from coordination sites on the SPION surface and causes redispersion of agglomerates. The magnetic determination thereof represents what we call a "magnetically indicated complexometric titration". The relevance of agglomerate sizes for the MPS signal response is studied on a model system of SPIONs and the surfactant cetrimonium bromide (CTAB). Analytical ultracentrifugation (AUC) and cryogenic transmission electron microscopy (cryo-TEM) reveal that large micron-sized agglomerates are required to significantly change the MPS signal response. With this work, a fast and easy-to-use characterization method to determine surface coordination motifs of magnetic nanoparticles in optically dense media is demonstrated.

Luminomagnetic composites have been synthesized that allow for an individual tuning of luminescence intensity, chromaticity and magnetization by combination of superparamagnetic, citrate-stabilized iron oxide nanoparticles with the luminescent MOFs 3∞[Ln2(BDC)3(H2O)4] (Ln=Eu, Tb; BDC2-=terephthalate). The components are arranged to a concept of inverse structuring compared to previous luminomagnetic composites with MOF@magnetic particle (shell@core) composition so that the luminescent MOF now acts as core and is covered by magnetic nanoparticles forming the satellite shell. Thereby, the magnetic and photophysical properties are individually tuneable between high emission intensity (1.2 ⋅ 106 cps mg-1) plus low saturation magnetization (6 emu g-1) and the direct opposite (0.09 ⋅ 106 cps mg-1; 42 emu g-1) by adjusting the particle coverage of the MOF. This is not achievable with a core-shell structure having a magnetic core and a dense MOF shell. The composition of the composites and the influence of different synthesis conditions on their properties were investigated by SEM/EDX, PXRD, magnetization measurements and photoluminescence spectroscopy.

Nickel oxide ((Formula presented.)) is a promising hole transport material for perovskite/Si tandem solar cells. Various silicon cell architectures may be used as bottom cells. The polycrystalline (poly-Si) (Formula presented.) tunnel diode is expected to be a high-efficiency interconnection scheme between the two subcells of monolithic tandems in p-i-n configuration with a high thermal budget, excellent passivation properties, and low contact resistivity. However, (Formula presented.) is then interfaced to poly-Si((Formula presented.)) and the chemical integrity of the interface due to the necessity of annealing treatments has to be questioned. For this purpose, the (Formula presented.) /poly-Si contact resistivity for different annealing temperatures is investigated between 100 and 500 °C, and two different (Formula presented.) deposition techniques, namely, wet-chemically applied and sputter-deposited (Formula presented.). The values of more than (Formula presented.) are obtained. The insertion of a nm-thin metallic Ni interlayer is shown to enable a tremendous decrease of the contact resistivity by 2-3 orders of magnitude. The formation of (Formula presented.) is proven by highly resolved (scanning) transmission electron microscopy ((S)TEM) coupled with energy-dispersive X-ray spectroscopy (EDXS). This interfacial engineering approach is expected to provide an effective way of improving the contact properties and integrability of (Formula presented.) into various tandem cell processes.

Nontoxic, all-inorganic perovskite light absorbers have received a great deal of attention in recent years due to their potential to replace lead-containing perovskite materials in photovoltaics. Herein, the synthesis of the lead-free double perovskite Cs2AgBiBr6 as a powder, obtained via spray-drying, is reported to be capable of paving the way toward large scale production. Upon drying of the atomized precursor solution during spray-drying, the double-perovskite phase forms by in situ crystallization. The product that is obtained from spray-drying is compared to the one that is obtained from the so-called solution cooling method, which is often used for the absorber synthesis before dissolving the double perovskite powder for the thin film deposition. The absorber powder gained from the solution cooling method sets the benchmark for the spray-dried absorber powder. XRD and Raman analyses confirm the phase purity of the double perovskite, whereas UV-Vis spectroscopy confirms the desired bandgap. Furthermore, the absorber powders from both synthesis methods are successfully used to deposit and integrate absorber films in single-junction perovskite solar cells. Besides being introduced as an alternative synthesis route for the preparation of double perovskite absorber as dry powders independently from being applied onto a given substrate, the superior advantages of spray-drying compared to the conventional synthesis of the perovskite absorber via the solution cooling method are outlined: Spray-drying not only enables the synthesis of large amounts of perovskite absorber powders but also discards the usage of precarious solvents like concentrated hydrobromic acid and thus eliminates the need for extensive safety precautions. Hence, spray-drying may help the perovskite technology take the next step toward large-scale production and commercialization.

Tracking and tracing of objects are of paramount importance for realizing a Circular Economy and can be achieved by incorporating micrometer-scaled luminescent identification (ID) taggants. While established systems are passive, displaying merely an ID feature, for monitoring a products’ life cycle, conscious taggants with stimuli-triggered switchable ID signals, e.g., to record a recycling step, are desired. Herein, micrometer-scaled hybrid inorganic–organic taggants with a temperature-triggered switchable luminescent ID are reported. They are produced by assembling eight different types of organic and inorganic nanoparticles (NPs) via a two-step spray-drying process into supraparticles (SPs) with a core–shell structure. This hierarchical structure and well-balanced mass-ratios of the utilized nanosized building blocks are key to controlling the occurring coupling effects within SPs, thus, to obtain the switchable ID functionality. Initially, the temperature-inert inorganic luminescent ID-level, located in the core, is concealed by a mutable organic ID-level, emitted by the shell. Upon specific heat treatment, the ID is switched by irreversible quenching of the organic ID-level, revealing the inorganic ID-level. Furthermore, both ID-levels are read out ratiometrically using the same excitation wavelength for their detection. Based on the SPs’ toolbox-like manufacturing, the ID signals are adjustable to yield the demanded code variety on both ID-levels.

(Sub)micrometer-scaled identification (ID) taggants enable direct identification of arbitrary goods, thereby opening up application fields based on the possibility of tracking, tracing, and anti-counterfeiting. Due to their small dimensions, these taggants can equip in principle even the smallest subcomponents or raw materials with information. To achieve the demanded applicability, the mostly used optically encoded ID taggants must be further improved. Here, micrometer-scaled supraparticles with spectrally encoded luminescent and magnetically encoded signal characteristics are reported. They are produced in a readily customizable bottom-up fabrication procedure that enables precise adjustment of luminescent and magnetic properties on multiple hierarchy levels. The incorporation of commonly used magnetic nanoparticles and fluorescent dyes, respectively, into polymer nanocomposite particles, establishes a convenient toolbox of magnetic and luminescent building blocks. The subsequent assembly of selected building blocks in the desired ratios into supraparticles grants for all the flexibility to freely adjust both signal characteristics. The obtained spectrally resolved visible luminescent and invisible magnetic ID signatures are complementary in nature, thus expanding applicability and information security compared to recently reported optical- or magnetic-encoded taggants. Additionally, the introduced ID taggant supraparticles can significantly enhance the coding capacity. Therefore, the introduced supraparticles are considered as next-generation ID taggants.

Sb-substituted Cs2AgBiBr6 single crystals and powders are synthesized by various wet-chemical routes as well as by solvent-free mechanochemical synthesis. Phase purity and resulting optical properties of differently synthesized Sb-substituted Cs2AgBiBr6 absorbers are investigated and compared. X-ray diffraction confirms that Sb substitution results in an apparent single-phase formation with a unit cell shrinkage up to a certain substitution limit, which varies depending on the synthesis routes. Questions about the phase identification determined by X-ray diffraction are raised and thoroughly investigated by Raman spectroscopy. UV–vis spectroscopy reveals that Sb3+ substitution induces a reduction in the optical bandgap of Cs2AgBiBr6, whereas octahedral factor calculations provide that SbBr6 octahedra can be hardly stable in a Cs2AgBiBr6 double perovskite. The experimental results of the occurrence and evolution of Raman bands and theoretical calculations of the structural stability of SbBr6 octahedra in Cs2AgBiBr6 unambiguously raise doubts about the Sb-substitution feasibility in Cs2AgBiBr6, and a general substitution strategy in Cs2AgBiBr6 is discussed.

Thirteen metal hydroxide adsorbers were synthesized via precipitation by systematically varying different two-, three- and four-valent metal precursors, namely Mg2+, Ca2+, Zn2+, Fe3+, Al3+ and Zr4+. The resulting materials were classified in four groups based on their structure, morphology and BET surface area. Two of the groups could be classified as layered double hydroxides (LDH). The rest of the materials could be either only partially related to a LDH-like structure or formed non-layered precipitates. The phosphate removal performance of each adsorber (dose 200 mg/L) was tested in spiked distilled water and municipal wastewater (10 mg P/L) at pH 7-8. The phosphate adsorption capacity of the materials after 1 h varied between 32 mg P/g (for a rod-like-sample, lacking a layered structure) and 51 mg P/g (for a sample appearing as a mixture of particulate matter covered with a web-like structure). Longer contact time (24 h) did not increase significantly the adsorption efficiency. None of the materials was ideally selective for phosphate, especially the ones with a clear LDH structure and highest surface area (110 m2/g) which adsorbed up to 25 mg/g Cl− and other competing anions. The adsorbed phosphate could be desorbed (>90% in most of the cases) by treatment with an alkaline solution (0.5 M NaOH or 1 M NaOH + 1 M NaCl) which turned out to be the best option for regeneration among many other tested combinations.

An innovative mechanochemical method is reported for the in situ formation of crystalline metal-complexes on the surface of superparamagnetic nanocomposite microparticles. The process is demonstrated for coating Fe3O4 multicore-silica matrix particles with the 1,2,4-1H-triazole complex [ZnCl2(TzH)2]. The use of mechanochemistry demonstrates a flexible process to obtain functional shells on magnetic particle cores without the need for complicated surface-functionalisation reactions in solution. Simple mixing of the desired shell-precursors ZnCl2 and 1,2,4-1H-triazole (TzH) with the magnetic particles in a ball mill is sufficient to tailor the particle surfaces with novel functionalities while retaining the superparamagnetic behaviour.

A process is reported to obtain a nanoparticle sol from co-precipitated iron oxide particles without using any surfactant. The sol - a true ferrofluid - is not only stable over a wide range of pH but also in physiological solutions. This is a decisive step towards biomedical applications where nanoparticle agglomeration could so far only be prevented by using unwanted surfactants.

Iron oxide nano octahedra were synthesized from iron sulfate in a facile, fast and inexpensive way. Exposure to ultrasound during precipitation of the iron salt with sodium hydroxide in combination with addition of nitric acid was found to predetermine the shape of the particles, which grow in a subsequent natural ageing process. Simply by controlling the ageing time, the size of well-defined iron oxide nano octahedra can be controlled in the range from ~80 to 120. nm, which ultimately influences the magnetic properties of the particles.

Superparamagnetic microparticles modified with an ion exchange system are reported for the recovery of phosphate from waste water by a magnetic separation technique. Layered double hydroxides (LDH), anionic clays, are precipitated from an aqueous solution and deposited by an ultrasonic treatment on superparamagnetic microparticles consisting of Fe3O4 multicores embedded in a SiO2 matrix. Deposition can be conducted in a batch process as well as in a continuous way, using an ultrasonic flow cell. The obtained composite particles show good magnetic separability and have a specific surface area of around 100 m(2) g(-1). Zr doped LDH shows improved phosphate adsorption in waste water. After magnetic separation and regeneration, the composite particles are re-used which is demonstrated for four cycles. Phosphate ions are concentrated in the regeneration solution. Simple, low cost, and a fast continuous synthesis of the composite particles paves the way for application beyond lab scale in real waste water treatment plants.

Superparamagnetic nanoparticles of magnetite were coprecipitated from iron salts, dispersed with nitric acid and stabilised either by lactic acid (LA) or by a polycarboxylate-ether polymer (MELPERS4343, MP). The differently stabilised nanoparticles were incorporated into a silica matrix to form nanocomposite microparticles. The silica matrix was prepared either from tetraethylorthosilicate (TEOS) or from an aqueous sodium silicate (water glass) solution. Stabilisation of nanoparticles had a crucial influence on microparticle texture and nanoparticle distribution in the silica matrix. Magnetic measurements in combination with transmission electron microscopy (TEM) investigations suggest a uniform magnetic interaction of nanoparticles in case of LA stabilisation and magnetically interacting nanoparticle clusters of different sizes in case of MP stabilisation. Splitting of blocking temperature (TB) and irreversible temperature (Tir) in zero field cooled (ZFC) and field cooled (FC) measurements is discussed in terms of nanoparticle clustering.

Superparamagnetic nanocomposite microparticles, compromised of magnetite nanoparticles in a silica matrix, have been synthesised and surface-modified to act as adsorbers for substances (e.g. toxic heavy metals or valuable resources) dissolved in fluids like water. The particles can be used for a magnetic-extraction-assisted separation process of these target substances which thereby can be recovered from the fluid., In der vorliegenden Arbeit wurden superparamagnetische Nanokomposit-Mikropartikel, bestehend aus Magnetitnanopartikeln in einer Silica-Matrix, hergestellt. Die Oberfläche der Partikel wurde modifiziert, so dass die Partikel als Adsorber für gelöste Substanzen (z.B. giftige Schwermetalle oder Wertstoffe) in Fluiden wie z.B. Wasser eingesetzt werden können. Mit Hilfe der Partikel kann eine Abtrennung und damit Rückgewinnung der Zielstoffe auf Basis eines Magnetseparationsprozesses durchgeführt werden.

The synthesis of a reusable, magnetically switchable nanocomposite microparticle, which can be modified to selectively extract and recover HgII or CuII from water, is reported. Superparamagnetic iron oxide (magnetite) nanoparticles act as the magnetic component in this system, and these nanoparticles were synthesized in a continuous way, allowing their large-scale production. A new process was used to create a silica matrix, confining the magnetite nanoparticles using a cheap silica source [sodium silicate (water glass)]. This results in a well-defined, filigree micrometer-sized nanocomposite via a fast, simple, inexpensive, and upscalable process. Hence, because of the ideal size of the resulting microparticles and their comparably large magnetization, particle extraction from fluids by low-cost magnets is achieved.

Superparamagnetic iron oxide nanoparticles (magnetite (Fe3O4) and/or maghemite (?-Fe2O3)) were precipitated with ammonium hydroxide from a solution of Fe(II)- and Fe(III)-chlorides in water under ambient conditions. The agglomerated nanoparticles were dispersed by addition of nitric acid to a sol and stabilised with a polycarboxylate backbone polyethylene oxide side chain combpolymer (MELPERS4343, BASF, Germany). In this work focus was put on studying the colloids stability in great detail which is of uttermost importance for technical applications of ferrofluids. It was possible to demonstrate that the resulting nanoparticles showed a good colloidal stability in water over a wide pH range from 2 to 12 as well as in ethanol or ethanol/water/ammonium hydroxide mixtures. Remarkable is the stability of the ferrofluid even at very high electrolyte concentrations present, such as NaCl. The stabilisation is steric or electrosteric, depending on the pH. Superparamagnetism is preserved in all cases. At pH > 12 the ester bonds to the polyethylene oxide side chains were hydrolysed, steric stabilisation was lost. Electrostatic stabilisation was hampered due to neutralisation by Na+ where particles agglomerate. From the results of this study, an innovative binding scheme of the stabilising polymer to the nanoparticle, showing the changes of the polymer at different pH values leading to different stabilisation mechanisms, is proposed. The work describes an elegant way towards the synthesis of a very pH and electrolyte indifferent, stable ferrofluid. A ferrofluid, stable in such a remarkable way has not yet been reported to be obtained with such ease and in such a short time using mild, non-toxic, and cheap synthesis conditions. Highlights: Coprecipitated superparamagnetic iron oxide particles form ?m-sized agglomerates. ? Particle treatment with nitric acid and MELPERS4343 leads to nanodispersions. ? Sols stable: from pH 2 to 12 in water; in saline solutions and in ethanol. ? Particle stabilisation: mainly steric; changes to electrosteric with increasing pH. ? At pH > 12: degradation of the stabilising polymer by hydrolysis of ester bonds.