The conversion of perhydropolysilazane to silica is a fundamental step in preceramic polymer chemistry and is of great importance for the development of functional inorganic materials via solution-based methods. Perhydropolysilazane (PHPS) is an inorganic polymer that can be converted to amorphous silica (SiOₓ) without the use of traditional alcoholic silane precursors or acidic/basic catalysts.[1] This makes the process safer, more environmentally friendly and applicable to temperature-sensitive substrates.
The conversion of PHPS into SiOx with only UV-light[2]
A special aspect of PHPS is that it is a reactive, highly soluble polymer, with Si–H groups that readily react with water and oxygen to form siloxane bonds (Si–O–Si).[3] This allows the formation of silica networks under relatively mild conditions, such as room temperature, humidity, UV light or ozone.[4] Furthermore, the resulting silica material is chemically inert, transparent, porous and thermally stable – properties that make it suitable for a wide range of applications, from microelectronics to medical coatings and catalysts. Furthermore, recent research shows that it is possible to obtain PHPS-encapsulated particles or structures with nanometer resolution via patterning, further strengthening the role of PHPS in nanotechnology.
Perhydropolysilazane (PHPS) is an inorganic polymer, which consist of repeatedly varied of Si–N and Si–H. Due to the presence of reactive Si–H bonds, the polymer is particularly suitable for further functionalisation and conversion to silica. The general formula can be represented as [HSi(NH)O]ₙ or similar derivatives, depending on synthesis and processing.[5] A unique feature of PHPS is the presence of these Si–H groups, which are highly reactive and therefore form the basis for hydrolysis and further condensation. This makes PHPS particularly suitable for conversion to silica and functionalisation with other groups or nanoparticles.
The molecular weight distribution and degree of polymerisation of PHPS can vary depending on the synthesis conditions. Due to the incomplete crosslinking of the structure, PHPS remains soluble in apolar solvents, making it attractive for wet processing methods. The presence of Si-H groups makes the material not only reactive, but also potentially useful for further functionalisation. PHPS is usually supplied as a viscous, colorless to pale yellow liquid, dissolved in solvents such as dibutyl ether, toluene or hexane.[6] In commercial formulations, a typical composition is 20 wt% PHPS in dibutyl ether. Storage under inert gas (e.g. nitrogen or argon) is essential to prevent premature hydrolysis by atmospheric moisture. Processing should ideally take place in a drying cabinet or under glove box conditions.[7]
Important features of the inorganic polymer perhydropolysilazane are[8]:
High reactivity: The Si–H bonds readily undergo hydrolysis and oxidation.
Solubility: Soluble in organic solvents, which facilitates film formation via wet-processing.
The first step in the conversion of PHPS to silica is the hydrolysis of the Si–H groups. This usually occurs under the influence of air humidity or by direct addition of water vapour or aqueous solutions. The formed silanol groups (Si–OH) then react with each other or with the remaining Si–H groups to form Si–O–Si bonds and split off water. This leads to a gradual network formation of silica.[9]
This reaction shows how the inorganic polymer PHPS is converted into silica after coming in touch with water (direct water or indirectly with water in the air)
The extent of hydrolysis and condensation is influenced by environmental parameters such as air humidity, temperature, the nature of the substrate and the layer thickness. For thin films (<1 µm), the hydrolysis usually proceeds completely within a few hours at room temperature and a humidity of 40–90%. At thicker films or at low water diffusion rates, the conversion may be incomplete, resulting in residual groups that affect chemical stability and porosity.[10]
Alternatively, silanol groups can react with residual Si–H groups:
HO–Si–O + H–Si–O → Si–O–Si + H₂
These successive reactions produce a three-dimensional network of Si–O–Si bonds. This hydrolysis-condensation reaction chain can occur at room temperature, provided there is sufficient humidity. The rate of conversion is influenced by the layer thickness, moisture access, substrate interaction, and the diffusion rate of water. The presence of catalytic traces (e.g. acids or bases) from the substrate or the environment can increase the rate, although this can also affect the morphology and uniformity of the final material.[10]
Intermediate species may exist, such as silanol-terminated chains, cyclic oligomers, or partially cross-linked networks. These species can be identified using spectroscopic techniques such as FTIR, which show a characteristic Si–OH stretching around 950 cm⁻¹ and Si–O–Si vibrations around 1080–1100 cm⁻¹.[11] A decreasing Si–H stretching around 2250 cm⁻¹ is an indicator of ongoing conversion. [2]
Conversion can occur at room temperature but is more complete under elevated temperature or UV light (e.g., 185 nm). UV exposure for 15 minutes can lead to near-complete conversion, as evidenced by FTIR and XPS.[2]
Whereas room temperaturehydrolysis and condensation is feasible, thermal post-treatment is typically necessary to force the condensation to completion, remove residual groups and densify the silica network.
The pyrolysis reaction to convert PHPS into silica
Important processes during heating:
Removal of unreacted Si–H and Si–OH groups
Densification of the silica network to a glassy structure
Removal of organic residues and traces of solvent
Temperatures between 150 and 350 °C are common for this.[12] Between 200 and 250 °C exothermic reactions are observed indicating accelerated network formation. The atmosphere plays an important role during heating:
In air: complete oxidation to amorphous silica and elimination of hydrogen and nitrogen-containing fragments
Thermal UV-curing at 120 °C greatly improves the material's conversion degree, as reflected by a rise in the O/Si ratio from approximately 1.75 to 1.89, compared to curing at room temperature.[2] The improved conversion results in improved material properties such as increased optical transparency, increased chemical resistance, and improved mechanical strength.
However, the thermal treatment is also associated with a high volumetric shrinkage, typically in the range of 20% to 40%, depending on the film thickness and the applied temperature ramp. The shrinkage can introduce internal stresses that may result in cracking or delamination of the coating or film. To reduce such an effect, there is a need for a properly controlled thermal profile. This is achievable through stepwise heating protocol and close monitoring of the reaction by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).[13][14]
To monitor the progress of the conversion and assess the quality of the silica formed, several advanced characterisation techniques are applied:
The left spectrum is the conversion of silica before & after UV-irradiation for different time at room temperature. The right spectrum show what happens with silica UV-irradiation at different temperatures for 15 min[2]
FTIR spectroscopy (Fourier transform infrared): Analysis of Si–H stretching (~2250 cm⁻¹), Si–OH (~950 cm⁻¹) and asymmetric Si–O–Si stretching (~1080–1100 cm⁻¹). The decrease of the Si–H peak and the increase of the Si–O–Si band confirm the conversion.[2][15]
UV-Vis spectroscopy: Changes in optical absorption or transparency provide information about film integrity and network growth.[2]
XPS: Shows uniform Si and O distribution; nitrogen absence confirms complete conversion.[2]
SEM/TEM (scanning/transmission electron microscopy): Visualisation of morphology, fiber structure (in electrospinning), porosity, and composition at the nanoscale.[2]
The broad applicability of PHPS conversion to silica is due to the versatile processing methods and the ceramic character of the end product:
Antibacterial coatings: By adding silver nanoparticles to PHPS solutions, hybrid coatings with antimicrobial properties can be formed after conversion. These coatings are interesting for medical devices and food packaging.[16]
Printable electronics: PHPS is compatible with techniques such as inkjet printing, spray coating and spin coating. The solutions can be applied to flexible substrates and then converted to electrically insulating silica at low temperature.[17]
Here is shown how the inorganic polymer (poly ethylene oxide) transforms from a solid fibre after dissolving the polymer powder in deionised waterElectrospinning: Blends of PHPS with poly(ethylene oxide) can be electrospun into fibers, which are then thermally or chemically converted to porous silica fibers. These structures have applications in filtration, catalysis and sensory.[18][19]Poly(ethylene oxide) in water after electrospun which turned the solution into solid fibre
Barrier layers and optical coatings: The high optical transparency and chemical inertness of SiOₓ make PHPS-derived films suitable for use as protective layers in displays or as gas barriers in packaging materials.[20]
While PHPS offers a safer alternative to alkoxysilanes, it remains highly reactive. Moisture sensitivity can lead to premature polymer degradation or uncontrolled gelation. Proper storage (under argon or nitrogen, low humidity) is essential. Thermal treatment can release ammonia or hydrogen; ventilation is required during processing.[20]
Additionally, PHPS-derived silica is free of residual organic contaminants and is suitable for eco-friendly coatings. The conversion route does not require toxic solvents or catalysts, aligning with green chemistry principles.[21]
The conversion of PHPS to silica offers both unique advantages and some limitations, depending on the intended application. This dual nature requires balancing performance, processing conditions, and cost-effectiveness when selecting PHPS for specific end uses.
The advantages to convert PHPS into SiOx are:
Low processing temperatures: In contrast to classical sol-gel, the conversion of PHPS can take place at room temperature. This makes it compatible with polymeric or sensitive substrates (room temperature to 120 °C with UV) and catalyst-free conversion.
Pure end product: Since PHPS does not contain any organic residues, it produces a virtually carbon-free silica after complete conversion.
Precise patterning: Due to its compatibility with UV and lithographic techniques, patterns on a micron to nanometer scale can be realized.
There are also some disadvantages to perform this reaction and those are:
Sensitivity to moisture: PHPS is highly reactive and requires strict moisture-free storage and handling, otherwise uncontrolled hydrolysis occurs. Furthermore it is required to work with PHPS in a controlled humidity or UV for full conversion. [7]
Limited long-term stability: With long-term storage, the viscosity and reactivity may decrease due to slow polymerisation.[7]
Shrinkage and Cracking: Significant volume shrinkage occurs during thermal post-treatment, which can induce stresses in the material.[7]
Limited Commercial Availability: PHPS is not widely available and often requires laboratory synthesis. [2]
These properties make PHPS particularly suitable for applications in research, prototyping and niche production of functional layers, sensors and coatings.
^ abcdefghijklmnYue, Shuangshuang; Wang, Shuanjin; Han, Dongmei; Huang, Sheng; Xiao, Min; Meng, Yuezhong (October 2022). "Perhydropolysilazane-derived-SiOx coated cellulose: a transparent biodegradable material with high gas barrier property". Cellulose. 29 (15): 8293–8303. doi:10.1007/s10570-022-04746-9.
^Krüger, H. (2003). "Conversion Reactions of Polysilazanes to Silicon Dioxide". Chemistry of Materials. 15: 398–405.[verification needed]
^Hetemi, Dardan; Pinson, Jean (2017). "Surface functionalisation of polymers". Chemical Society Reviews. 46 (19): 5701–5713. doi:10.1039/C7CS00150A. PMID28766657.
^Gelain, T. (2020). "Processing Conditions for PHPS". ACS Applied Materials & Interfaces. 12: 50783–50791.[verification needed]
^Monti, Matteo; Dal Bianco, Barbara; Bertoncello, Renzo; Voltolina, Stefano (December 2008). "New protective coatings for ancient glass: Silica thin-films from perhydropolysilazane". Journal of Cultural Heritage. 9: e143 –e145. doi:10.1016/j.culher.2008.08.002.
^ abNaganuma, Yasuhiro; Kato, Chihiro; Watanabe, Toshiyuki; Kaneko, Satoru; Tanaka, Satomi (August 2024). "Physicochemical and structural properties of silica films prepared from perhydropolysilazane using vacuum ultraviolet irradiation". Thin Solid Films. 802 140453. doi:10.1016/j.tsf.2024.140453.
^Morrison, Christine N.; Ahn, Seokhoon; Schnobrich, Jennifer K.; Matzger, Adam J. (February 2011). "Two-Dimensional Crystallization of Carboxylated Benzene Oligomers". Langmuir. 27 (3): 936–942. doi:10.1021/la103794j. PMID21207984.
^Wei, W. "Thermal Behavior of Preceramic Polymers". Journal of Thermal Analysis and Calorimetry. 147: 1885–1895.
^Han, Deshang; Wang, Yu; Pan, Yi; Qiu, Jian; Wang, Jing; Wang, Chuansheng; Liu, Haichao (November 2022). "Influence of spraying perhydropolysilazane coating on friction and wear of end-face metal during the mixing process". Polymer Testing. 115 107743. doi:10.1016/j.polymertesting.2022.107743.
^Zhang, Yulin; Wang, Wenyue; Li, Pengfei; Wang, Li-Ming; Zhang, Junrong; Li, Xiao; Zhang, Zongbo; Xu, Caihong (2 August 2023). "Thermal Conversion Process of Tetraethyl Orthosilicate-Based Silica Sol and Perhydropolysilazane into Inorganic Silica Films". Crystal Growth & Design. 23 (8): 5965–5974. doi:10.1021/acs.cgd.3c00539.
^Morones, Jose Ruben; Elechiguerra, Jose Luis; Camacho, Alejandra; Holt, Katherine; Kouri, Juan B; Ramírez, Jose Tapia; Yacaman, Miguel Jose (October 2005). "The bactericidal effect of silver nanoparticles". Nanotechnology. 16 (10): 2346–2353. doi:10.1088/0957-4484/16/10/059. PMID20818017.
^"Zilververband". www.startwondverzorging.nl (in Dutch). Retrieved 2025-07-08.
