Finds documents with both search terms in any word order, permitting "n" words as a maximum distance between them. Best choose between 15 and 30 (e.g. NEAR(recruit, professionals, 20)).
Finds documents with the search term in word versions or composites. The asterisk * marks whether you wish them BEFORE, BEHIND, or BEFORE and BEHIND the search term (e.g. lightweight*, *lightweight, *lightweight*).
Activate our intelligent search to find suitable subject content or patents.
Select sections of text to find matching patents with Artificial Intelligence.
powered by
Select sections of text to find additional relevant content using AI-assisted search.
powered by
(Link opens in a new window)
Abstract
This study delves into the application of g-C3N4/poly(alkyl siloxane) coatings on porous quartz sandstone, focusing on enhancing water repellency and self-cleaning properties. The research involves the preparation of three types of g-C3N4 nanoparticles with varying degrees of exfoliation, which are then dispersed in a commercial poly(alkyl siloxane) agent at two concentrations. The coatings are applied to sandstone samples, and their interaction with water is assessed through static contact angle, water absorption by capillarity, and water vapor permeability measurements. The self-cleaning activity is evaluated by measuring the discoloration of Rhodamine B stained surfaces under simulated daylight radiation. The study finds that the higher the degree of exfoliation of g-C3N4 particles, the better the self-cleaning effect on the sandstone surface. The normative limits for self-cleaning properties are met for particles exfoliated for 45 and 60 minutes at a concentration of 4 grams per liter of poly(alkyl siloxane). The coatings significantly reduce water uptake into the pore system of the stone while maintaining water vapor permeability. The results indicate that g-C3N4 in poly(alkyl siloxane) coatings contributes to both preventing capillary water uptake and improving water vapor permeability. The study concludes that g-C3N4 is a promising photocatalyst for the protection of stone architectural and artistic objects, with potential applications in the conservation of cultural heritage.
AI Generated
This summary of the content was generated with the help of AI.
Abstract
Graphitic carbon nitride (g-C3N4) was synthesised by thermal polycondensation of melamine and subsequently exfoliated by calcination at 600 °C for 30, 45, and 60 min. The resulting nanoparticles—differing in morphology, size, and specific surface area—were dispersed in a poly(alkyl siloxane) hydrophobising agent at concentrations of 2 g·L⁻¹ and 4 g·L⁻¹ and applied to the surface of porous quartz sandstone. The effect of the degree of g-C3N4 exfoliation on the self-cleaning performance of the resulting coatings and their interaction with water was evaluated. Photodegradation tests performed on sandstone samples artificially stained with Rhodamine B confirmed a self-cleaning effect for coatings containing particles exfoliated for 45 and 60 min, at a concentration of 4 g of g-C3N4 per litre of poly(alkyl siloxane). The addition of nano-g-C3N4 enhanced the hydrophobicity of the sandstone surface. The coatings substantially reduced water uptake into the stone’s pore system while still allowing water vapour to evaporate freely into the surrounding environment. g-C3N4 thus appears to be a promising photocatalytically active nanomaterial for self-cleaning and water-repellent surface protection of building and sculptural stone—a field in which it has not yet been applied.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Introduction
The hydrophobisation of stone surfaces on architectural and sculptural objects is typically achieved by applying a thin layer of a suitable water-repellent polymer. This layer limits water penetration into the stone’s pore system while allowing moisture to evaporate freely from the pores into the surrounding environment. Nanoparticles of various materials can be incorporated into the polymer matrix to increase the surface roughness of the resulting polymer film, and thus further enhance its hydrophobicity (Tokarský et al. 2022; De Ferri et al. 2011; Manoudis et al. 2009a; Speziale et al. 2020). When particles with photocatalytic properties are used, the stone surface gains additional protective functionalities, most notably self-cleaning ability through the photodegradation of surface-bound pollutants, and increased resistance to biodeterioration (La Russa et al. 2016; Colangiuli et al. 2019; Roveri et al. 2020; Gherardi et al. 2018; van der Werf et al. 2015; Aldosari et al. 2019; Speziale et al. 2020).
Nano-sized TiO2 in its anatase form is the most widely used photocatalyst in the field of stone protection (Munafo et al. 2015), however the number of studies exploring nano-ZnO as an alternative to TiO2 is steadily increasing (Helmi and Hefni 2016; van der Werf et al. 2015; Tokarský et al. 2022; Manoudis et al. 2009a; Aldosari et al. 2019). TiO2 and ZnO have comparable band gap energies—approximately 3.2 eV and 3.37 eV, respectively (Ong et al. 2018) and the photocatalytic performance of both materials is activated by the irradiation with ultraviolet (UV) light at wavelengths of approximately 380 nm or lower. Both materials exhibit biocidal properties; in the case of ZnO, its antifungal activity is particularly notable as it is not dependent on light irradiation (van der Werf et al. 2015).
Advertisement
Since the UV portion of daylight accounts for only about 5% of the total spectrum (Li et al. 2022), efforts to extend the photoactivity of UV-light responsive photocatalysts into the visible (VIS) region have led scientists to explore various strategies (Khan and Shah 2023; Speziale et al. 2020). It is evident that approaches aimed at shifting photocatalytic activity from the UV to the VIS region tend to complicate synthesis procedures. The simplest way to overcome limitations of TiO2 and ZnO in light absorption is to replace them with alternative semiconductors that are active under visible-light irradiation.
Graphitic carbon nitride (g-C3N4) has emerged as a highly promising photocatalyst for application in the VIS light region (Dong et al. 2014a, b). This layered semiconductor exhibits a characteristic band gap of 2.7 eV (Molaei 2023), corresponding to a wavelength of 460 nm, which enables the effective harnessing of a substantial portion of daylight to activate its photocatalytic properties. A widely used method for g-C3N4 synthesis involves the thermal polycondensation of melamine at temperatures ranging from 500 °C to 600 °C within semi-closed crucibles, resulting in what is commonly referred to as bulk g-C3N4 (Yan et al. 2009). However, the photocatalytic performance of bulk g-C3N4 is limited due to its low specific surface area (SSA), typically below 10 m2∙g− 1 (Škuta et al. 2021), and the rapid recombination of photogenerated electron-hole pairs (Zhang et al. 2021). The SSA of bulk g-C3N4 can be increased by thermal, chemical or mechanical exfoliation methods (Dong et al. 2015; Xu et al. 2013; She et al. 2014). The application of an appropriate exfoliation method can result in a SSA greater than 100 m2∙g− 1 (Papailias et al. 2018). g-C3N4 is a next-generation photocatalyst that exhibits high photocatalytic efficiency, excellent chemical and thermal stability, tuneable surface functional groups, and a visible-light driven band gap. Moreover, it is an environmentally friendly and non-toxic material that does not require complex equipment for its synthesis (Wang et al. 2009; Liao et al. 2012; Yang et al. 2019).
When the photocatalytic particles are irradiated with light of equal or greater energy than their band gap, negatively charged free electrons (e−) are generated in conduction band (CB), while positively charged holes (h+) are formed in the valence band (VB) (Fig. 1).
Photogenerated holes can sequentially react with water molecules to produce reactive oxygen species (ROS), including superoxide radicals (•O2−), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radicals (•OH) (Li et al. 2014). Electrons promoted to the conduction band reduce oxygen molecules to form •O2−, H2O2, and •OH. The •OH and •O2− radicals (Fig. 1) are the most stable and are considered the principal ROS responsible for photodegradation processes (Li et al. 2018). Unlike TiO2, the VB edge of g-C3N4 is low to produce •OH radicals directly; instead, these species are generated indirectly through the reduction of O2, which can be schematically represented as O2 → H2O2 → •OH (Cui et al. 2017). During photodegradation, ROS react with the molecules of polluting substances forming carbon dioxide, water, and—depending on the contaminant—various smaller organic compounds.
Advertisement
Although g-C3N4 possesses outstanding properties, its application in inorganic building materials has been investigated only in a limited number of recent studies (Yang et al. 2019, 2021; Lu et al. 2023; Peng et al. 2018). The incorporation of g-C3N4 nanoparticles into cementitious materials has been explored primarily to enhance self-cleaning performance and mitigate air pollution. The potential use of g-C3N4 as a component of protective coatings on the surface of stone objects has not yet been reported.
The present paper evaluates the effect of adding nano-g-C3N4 on the water repellency and self-cleaning activity of poly(alkyl siloxane)-based hydrophobic coatings applied to the surface of porous quartz sandstone. Three types of g-C3N4 nanoparticles with varying degree of exfoliation were prepared. The particles were dispersed in a commercial poly(alkyl siloxane) agent at two concentrations and sprayed onto the surface of sandstone samples. The interaction of the coated surfaces with water was assessed by measuring the static contact angle, water absorption by capillarity, and water vapor permeability. Discolouration of Rhodamine B stained surfaces was assessed after 4 h and 26 h of exposure to simulated daylight radiation.
Materials and methods
g-C3N4 nanoparticles
Three types of g-C₃N₄ nanoparticles with varying size, morphology, and SSA were prepared for the experiment. The synthesis of nano-g-C₃N₄ was conducted in two steps. Initially, bulk g-C3N4 was synthesised via thermal polycondensation of melamine. In a typical procedure, 10 g of melamine (Carl Roth Co. GmbH, Germany) was placed in a ceramic crucible covered with a ceramic lid and subjected to calcination at 550 °C for 4 h in an LH09/13 muffle furnace (LAC s.r.o., Czech Republic). The resulting yellow brittle solid was subsequently ground for 30 s using a vibration mill and manually sieved to collect particles smaller than 100 μm. The resulting material was designated as Bulk-g-C3N4.
In the subsequent exfoliation step, 0.5 g of the Bulk-g-C3N4 was evenly dispersed over the bottom of a ceramic bowl and thermally treated at 600 °C in air for 30, 45–60 min using the same furnace. The exfoliated samples were labelled TEX30, TEX45, and TEX60, corresponding to the respective calcination duration. Figure 2 presents a volume comparison of these materials.
Fig. 2
Volume comparison of Bulk-g-C3N4 and exfoliated g-C3N4 powders (50 mg each)
The morphology of the g-C₃N₄ particles was examined using a JEOL JSM-7610FPlus scanning electron microscope (Jeol Ltd., Japan) operating in high vacuum at an accelerating voltage of 10 kV. Prior to imaging, the samples affixed to conductive carbon tape were sputter-coated with a thin platinum layer using a Q150V ES plus coater (Quorum Technologies Ltd., UK). The scanning electron microscopy (SEM) images taken using a secondary electron detector are shown in Fig. 3.
Fig. 3
SEM images of Bulk-g-C3N4 (a), TEX30 (b), TEX45 (c), and TEX60 (d) particles (JSM-7610 F Plus scanning electron microscope; Jeol Ltd., Japan)
In the powder state, the particles tend to form agglomerates. After 30 min of exfoliation (TEX30, Fig. 3b), the particle morphology remains largely comparable to that of the original Bulk-g-C3N4 (Fig. 3a). However, whereas the Bulk-g-C₃N₄ particles mostly present a smooth and compact surface, the TEX30 particles already display partial surface disruption induced by exfoliation, with the onset of flake separation. Prolonged exfoliation for 45 min results in the disintegration of most of the material into smaller, flake-like particles (Fig. 3c, d).
The structure integrity of g-C3N4 is maintained following thermal exfoliation, as demonstrated by the presence of (002) and (100) diffraction lines in the XRD patterns of all samples (Fig. 4a), recorded using a MiniFlex600 X-ray powder diffractometer (Rigaku, Japan). The prominent diffraction peak located near 32° (CoKα), corresponding to the (002) plane, reflects the interlayer stacking of aromatic structures. In contrast, the low-intensity diffraction peak around 15° (CoKα), assigned to the (100) planes, indicates the interplanar spacing (Cao et al. 2015). A slight shift of the (002) peak maximum towards higher angles is observed for TEX45 and TEX60 samples, indicating tighter layer stacking in g-C3N4, likely caused by the flattering of previously undulated layers during thermal exfoliation (Papailias et al. 2018).
Fig. 4
XRD patterns (a) and FTIR spectra (b) of Bulk-g-C3N4, TEX30, TEX45, and TEX60 samples
The Fourier-transform infrared (FTIR) spectra, recorded in ATR mode using a Nicolet Summit spectrometer (ThermoFisher Scientific, Netherlands), are presented in Fig. 4b. All samples exhibit characteristic absorption bands in the 1000–1700 cm− 1 region, attributed to the stretching vibrations of aromatic C–N heterocycles (Chen et al. 2015; Pan et al. 2015). The absorption band at ⁓891 cm− 1 corresponds to out-of-plane bending vibrations of C–H bonds in aromatic domains, while the sharp peak centred at 809 cm− 1 is assigned to the breathing mode of s-triazine units (Lotsch et al. 2007; Han et al. 2015). The broad bands observed above 3000 cm− 1 arise from N–H and O–H stretching vibrations (Yang et al. 2013; Dong et al. 2014a, b). The presence of these characteristic g-C3N4 bands in all analysed samples confirms that the thermal exfoliation process preserves the fundamental structural framework of g-C3N4.
The particle size distribution (PSD) curves of the individual g-C3N4 samples measured in aqueous suspensions using a Zetasizer Ultra (Malvern Pananalytical, UK) via dynamic light scattering technique (DLS), are compared in Fig. 5a. These curves illustrate the effect of exfoliation time on PSD by showing the differential distributions of particle size weighted by the number of particles. The PSD curve of Bulk-g-C3N4 reveals a broad size distribution, with the maximum centred at ⁓530 nm. Progressive reductions in the peak maxima of the exfoliated samples are observed, corresponding to hydrodynamic diameters of 370, 260, and 50 nm for TEX 30, TEX45, and TEX60, respectively. The bimodal character of the PSD curves for TEX30 and TEX45 indicates that exfoliation of Bulk-g-C3N4 was incomplete in these samples.
Fig. 5
PSD curves of Bulk-g-C3N4, TEX30, TEX45, and TEX60 samples (a) and time- dependent degradation of RhB under VIS irradiation (b)
The SSA of the g-C3N4 particles was determined by nitrogen physisorption using a Sorptomatic 1990 analyser (Thermo Finnigan, USA) within a relative pressure range of 0.05–0.25. The obtained data were evaluated using the Brunauer-Emmett-Teller (BET) method. The SSA values for Bulk-g-C3N4, TEX30, TEX45, and TEX60 were 10, 38, 78, and 154 m2·g− 1, respectively, indicating a significant influence of exfoliation time on the development of surface area. Notably, the particles exfoliated for 60 min exhibited approximately a 15-fold increase in SSA compared to Bulk-g-C3N4. To assess the effect of exfoliation on pore characteristics, nitrogen adsorption-desorption isotherms were recorded over the relative pressure range of 0–1 for Bulk-g-C3N4 and TEX60, as presented in Fig. 6a and b. Both samples displayed hysteresis loops in the relative pressure range of approximately 0.5–1, indicating the presence of mesopores. The higher nitrogen uptake at low relative pressures observed for TEX60 suggests the presence of micropores. Despite this, both samples can be classified predominantly as mesoporous.
Fig. 6
Nitrogen adsorption-desorption isotherms of Bulk-g-C3N4 (a) and TEX60 (b) samples (Va…adsorbed volume of nitrogen per 1 g of sample; P/P0 …relative pressure)
The photocatalytic activity of g-C3N4 particles was assessed through visible-light-assisted degradation of Rhodamine B (RhB) in aqueous solution. The results, obtained following the procedure published by Praus et al. (2021), are presented in Fig. 5b. The Bulk-g-C3N4 sample exhibited the lowest photocatalytic performance, achieving only ~ 20% RhB degradation after 120 min of irradiation. Consequently, this sample was excluded from further self-cleaning tests. In contrast, all exfoliated samples displayed progressively enhanced photocatalytic activity, reaching 90–99% RhB degradation within 120 min. The increased exfoliation time positively influenced the degradation rate, which is primarily attributed to the progressive increase in SSA associated with prolonged exfoliation. As shown in Fig. 5b, the TEX60 sample exhibited the highest photocatalytic performance among all tested materials.
g-C3N4/poly(alkyl siloxane) dispersions
Nano-g-C3N4 was incorporated into the Porosil VV plus hydrophobising agent. This commercially available poly(alkyl siloxane)-based product is designed for the hydrophobisation of silicate materials and has been applied to several important historical monuments in the Czech Republic, including St. Vitus Cathedral within the Prague Castle complex and the Holy Trinity Column in Olomouc, which is listed as a UNESCO World Heritage Site (Tokarský et al. 2022; Pavlík et al. 2008).
Porosil VV plus (hereafter referred to as Porosil) is colourless, transparent product supplied as a two-component, low-viscosity system. Solution A contains the active siloxane component, while Solution B contains the catalyst. The preparation is commercially available with active component concentrations of 5, 10, or 15%. In this study, a concentration of 5% was used.
g-C3N4/Porosil suspensions were prepared using g-C3N4 particles obtained accordingly to the procedure described above. The nano-g-C3N4 was dispersed into solution A and homogenised using an Omni Sonic Ruptor 400 Ultrasonic Homogeniser (Omni International, USA) for 15 min at an ultrasonic power of 320 W. Solution B was added immediately prior to spray application. Suspensions with g-C3N4 loadings of 2 g and 4 g per litre of Porosil were prepared in this manner (see Table 1).
Table 1
Denotation of the g-C3N4 dispersions in Porosil
Dispersion
code
Exfoliation time
[min]
Concentration of g-C3N4 in Porosil [g·L− 1]
PorTEX 30/2
30
2
PorTEX 30/4
30
4
PorTEX 45/2
45
2
PorTEX 45/4
45
4
PorTEX 60/2
60
2
PorTEX 60/4
60
4
Substrate material
Porous fine-grained quartz sandstone from the locality of Mšené-lázně (Czech Republic; N50°21’06”, E14°07’50”) was used as the substrate material in these experiments. This sculptural and building stone has been historically employed in the construction, completion, and restoration of significant Czech monuments, such as St. Vitus Cathedral, the President’s House at Prague Castle, and numerous statues in towns and cities across the Czech Republic (Pavlík et al. 2008; Tokarský et al. 2022).
The rock is very well sorted, consisting predominantly (⁓95%) of stable angular to sub-oval quartz and quartzite clasts, with an average grain size of 0.2 to 0.3 mm (Fig. 7b, d). The surfaces of some quartz grains exhibit local corrosion and are overgrown by authigenic quartz. A minor proportion (⁓2%) on unstable components includes sericitised feldspars, lithic fragments containing feldspars, and micas. The remaining ⁓3% comprises a matrix primarily located along grain contacts or occasionally filling intergranular pores, consisting of kaolinite, illite, hydromuscovite, and limonite. Pyrite and limonite occur as accessory minerals.
Fig. 7
Sandstone from the locality of Mšené-lázně: photograph of ground specimen surface (a); SEM image of the ground surface (b); CT scan of the primary pore system (c); optical micrograph of the thin section (d) (Q – quartz; F – feldspar; M - muscovite; white arrows - mixture of clay minerals and limonite on the grain surfaces; black arrows – authigenic quartz overgrowths on the grain surface)
The primary pore system of the rock is formed by the mutual arrangement of clastic grains, which are in contact at points or areas (Fig. 7b, c). The rock predominantly contains large pores with sizes of ⁓10–30 μm (Fig. 8). Pores of both smaller and larger dimensions occur only in negligible quantities. In thin section, the rock exhibits no microfissures.
Fig. 8
Pore size distribution in Mšené sandstone – incremental pore volume vs. pore diameter. Results of mercury intrusion porosimetry on three specimens performed on AutoPore 9500 porosimeter (Micromeritics, USA) in pressure range of 0–228 MPa
Thanks to its open pore structure, the Mšené sandstone exhibits relatively fast water transport and remains permeable to water vapour (Tokarský et al. 2022; Pavlík et al. 2008).
The sandstone is typically pigmented (marbled) with limonite, ochre to brownish in colour. However, it contains light beige, colour-homogeneous parts which were selected for preparation of test specimens in the experiment (Fig. 7a).
The basic physical properties of the Mšené sandstone are as follows (Tokarský et al. 2022): bulk density 1769 ± 31 kg.m− 3 (EN 1936:2006), skeletal density 2660 ± 3 kg.m− 3 (ISO 12154:2014), total porosity 33.6 ± 1.3% (EN 1936:2006), and water absorption at atmospheric pressure 14.9 ± 0.3% (EN 13755:2008).
Rectangular and cylindrical test specimens were prepared from sandstone blocks by drilling and cutting. The specimens intended for self-cleaning tests and static contact angle (SCA) measurements were prepared with dimensions of 50 × 50 × 5 mm. The upper surfaces were ground with 500-grit abrasive paper to ensure flatness, and a 25 × 25 mm measuring field was marked for subsequent colorimetric analysis. The characteristics of the ground sandstone surface are illustrated in Fig. 7a, b. Specimens for vapour permeability tests measured 25 × 25 × 10 mm. Cylinders for capillary water absorption measurements had a diameter of 50 mm and an identical height. For all specimens, the vertical axis was oriented perpendicular to the sedimentary layers.
Coating application and self-cleaning tests
The g-C3N4/poly(alkyl siloxane) dispersions were applied to the specimen surfaces using a spray-coating technique. A Fengda BD-182 K airbrush equipped with a 0.8 mm nozzle, in conjunction with a Fengda AS-196 compressor manufactured by Fenghua Bida Machinery Manufacture Co., China, was employed for application. The application of each coating layer was executed from a distance of 20 cm, under a pressure of 100 kPa, for a duration of two seconds. Three layers were applied sequentially, with each subsequent layer sprayed after the previous one had cured (for 30 min at 20 ± 1 °C). For each type of dispersion, three coated specimens were prepared.
The self-cleaning performance of the coated stone surfaces was assessed via the RhB photodegradation test, following the UNI 11259:2008 standard. Since the procedure was originally developed to evaluate the photocatalytic activity of hydraulic binders, slight modifications were made to adapt it for porous polymer-coated sandstone, particularly concerning the application method of the RhB dye. The RhB solution (250 mg·L⁻¹ in water) was applied using the same Fengda airbrush setup, in two spraying passes, with a nozzle-to-surface distance of 30 cm, and spraying pressure of 200 kPa.
Following application, the specimens were dried for 24 h under laboratory conditions before being placed in a ICH110L controlled climate chamber (Memmert, Germany) equipped with an illumination unit comprising three fluorescent lamps. The samples were conditioned in the dark for 24 h at 20 °C and a 60% relative humidity. Afterwards, the lamps were switched on to simulate the daylight-conditions (standard illuminant D65, 6500 K) and relative humidity was increased to 70%.
The specimens were photographed and colorimetrically characterised prior to, and following the application of the g-C3N4/poly(alkyl siloxane) coatings. Thereafter, the RhB-staining procedure was carried out, and colour measurements of the RhB-stained surfaces were conducted before illumination (0 h), as well as after 4 and 26 h of exposure.
The colour changes observed on the test specimens were measured with the help of a MiniScan EZ 4500 S spectrophotometer (HunterLab, USA). The system is equipped with xenon lamps, and the light reflection intensities fall within the range of the visible light spectrum (λ = 400–700 nm). The L*, a*, and b* values of the CIELAB colour space - representing brightness, the red-green component, and the yellow-blue component, respectively - were measured at 5 points within the square measuring field. The subsequent processing of the colour data was conducted using EasyMatchQC software (HunterLab, USA).
The a* chromatic coordinate was the sole variable utilised to ascertain the photocatalytic discolouration of RhB dye (D) over time. The percentage was calculated using the following equation:
in which the a*(0) represents the mean red-green value of the RhB-stained specimen surface at time 0, i.e. before exposure to light, and a*(t) represents the mean red-green value of the same surface after t hours (i.e. 4 h and 26 h) under illumination.
The total colour difference (ΔE*) between the coated and non-coated stone surfaces was calculated prior to the RhB tests. The following equation was used to determine the ΔE*:
wherein ΔL*, Δa*, and Δb* represent the average changes in the values of the colour components L*, a*, and b*.
Evaluation of sandstone-water interaction
The effect of protective coatings on the interaction of the stone surface with water was evaluated by measuring the SCA for water, water absorption by capillarity, and water vapour permeability.
The hydrophobicity of the coatings was quantified using SCA values, determined according to the sessile drop method. This was performed using an Ossila contact goniometer (Ossila Ltd, GB). The SCA analyser was employed in combination with an Intralux 6000-1 double arm fibre optic illuminator (VOLPI AG, Switzerland) and the NIS Elements image processing and analysis software (NIKON, Japan). Ten droplets of demineralised water were placed to different points of each specimen using a Transferpette S automatic pipette. The volume of each droplet was 6 µL.
In order to assess the effect of g-C3N4 particle exfoliation degree on the hydrophobicity of coatings, Porosil with and without nano-g-C3N4 was sprayed onto laboratory slides in three layers for SCA measurements. In the next step, SCA values were measured directly on selected surfaces of coated sandstone.
For the accelerated vapour permeability test (Manoudis et al. 2009a; Alessandrini et al. 2000), measurement cells were prepared. The cell comprised a sandstone specimen (25 × 25 × 10 mm) fixed on the open top of a cylindrical PVC container that had been partially filled (two-thirds full) with demineralised water. The measurement cells, sealed by a teflon tape, were placed in a climatic chamber, maintained at a relative humidity of 20% and a temperature of 40 ± 0.5 °C. The cells were weighted at 24-hour intervals. The vapour flow through the stone’s pore structure was considered stable once the difference between two consecutive 24-hour weight changes, ΔMi−1 and ΔMi, fell below 5%. This variation was counted as follows:
The water vapour permeability was evaluated under constant vapour flow from three consecutive daily measurements as the mass of water vapour passing through a unit surface area (m2) in 24 h:
$$\Delta M_i\;=\;(M_i\;-\;M_0)/A$$
(4)
A: upper surface area of the sandstone specimen (m2),
M0: initial mass of the measuring cell (g).
The test was carried out on samples of uncoated sandstone, sandstone coated with Porosil, and sandstone coated with PorTEX 60/4 dispersion. The stone surfaces were treated with three layers of coating and placed in the PVC container with the treated side facing upwards. The specimens were prepared in triplicates.
Capillary water absorption measurement was performed by the gravimetric sorption technique following the European standard EN 1925:1999. Cylindrical test specimens of 50 mm diameter and the same height were drilled from the sandstone block perpendicular to the sediment layers. The dried specimens were placed in a glass container on a plastic grid, with base immersed in water to a depth of 3 ± 1 mm.
The amount of water absorbed by capillarity forces was determined by weighing the specimen after 1, 3, 5, 10, 15, 30, and 60 min. The reduction of water capillary absorption by coatings (RCA) is defined as (Manoudis et al. 2009b):
Mu: the mass of the water absorbed by the uncoated stone specimen,
Mt: the mass of the water absorbed by the coated stone specimen.
As in the vapour permeability test, the capillary water absorption was tested on samples of uncoated sandstone, sandstone coated with Porosil, and sandstone coated with PorTEX 60/4 dispersion. The coating was applied in three layers to the bottom base and lateral surface of the specimens, while the top base remained uncoated. The specimens were prepared in triplicates.
Results and discussion
The application of Porosil on sandstone surface, both with and without the incorporation of g-C3N4 particles, results in the formation of an ultrathin, transparent coating that uniformly envelops clastic grains without blocking the interconnected pore network. This observation is supported by SEM images of the sandstone surfaces presented in Fig. 9. A comparison between the untreated sandstone surface (Fig. 9a) and the surface treated with Porosil alone (Fig. 9b) reveals no substantial visual difference. In contrast, the sample treated with PorTEX 60/4 exhibits a discernible increase in surface roughness of the clastic grains, caused by the deposition of g-C3N4 particles and their clusters (Fig. 9c). Intergranular spaces at the surface of the sample remain open even after treatment. It can thus be stated that the application of the coatings does not cause significant changes to the sandstone surface.
Fig. 9
SEM images of the tested sample surfaces: uncoated sandstone (a), sandstone coated with Porosil (b), and sandstone coated with PorTEX 60/4 (c). EDS spectrum (d) of the detail of the surface shown in (e) overlapped with corresponding elemental maps of OKα1 (f), SiKα1 (g), and N Kα1,2 (h) (JSM-7610FPlus scanning electron microscope; Jeol Ltd., Japan)
Elemental analysis via energy dispersive X-ray spectroscopy (EDS), as shown in Fig. 9d-h, confirms that g-C3N4 is homogeneously distributed across the grain surfaces when embedded within the poly(alkyl siloxane) matrix. This is especially evident in the EDS map of N Kα1,2 overlaying the detail of a quartz grain (Fig. 9e, h), which reveals dense coverage by g-C3N4 clusters of varying size on the region of the grain accessible for the EDS detector.
The total colour difference (ΔE*) between the sandstone surface before and after application of Porosil was 3.7. The incorporation of light-yellow g-C3N4 particles into the Porosil matrix partially compensated for this colour change. The ΔE* values observed after dispersions application ranged from 1.5 to 2.2. Therefore, the use of C3N4/poly(alkyl siloxane) dispersions does not result in a colour difference, exceeding the perception threshold (ΔE* = 5) (García and Malaga 2012), and consequently does not lead to any notable alteration in the stone surface’s macroscopic appearance.
The results of the RhB photodegradation test on sandstone surfaces coated with g-C3N4/poly(alkyl siloxane) dispersions are presented in Figs. 10 and 11. Although g-C3N4 particles are hydrophilic and require moisture access for photocatalysis, they still exhibit self-cleaning properties when embedded within a poly(alkyl siloxane) matrix. According to the UNI 11259:2008 standard, a surface is considered to exhibit self-cleaning properties when it achieves at least a 20% reduction in the red-green (a*) component after 4 h of illumination and at least a 50% reduction after 26 h in the RhB test. This criterion was fulfilled by the PorTEX 45/4 and PorTEX 60/4 dispersions. The most significant self-cleaning performance was observed for the PorTEX 60/4 dispersion (containing particles exfoliated for 60 min at a concentration of 4 g per litre of Porosil), reaching up to 64% decrease in the red-green component after 26 h of illumination. This dispersion was therefore further tested with respect to the interaction between the modified sandstone surface and water. Other dispersions also exhibited varying degrees of photocatalytic activity; however, the extent of RhB photodegradation remained below the normative thresholds. Similar to the results observed in the liquid phase (Fig. 5b) the photocatalytic performance of g-C3N4 within poly(alkyl siloxane) coatings on the sandstone surface rises with decreasing particle size and increasing SSA.
Fig. 10
Results of the RhB photodegradation test on sandstone surfaces coated with g-C3N4/poly(alkyl siloxane) dispersions (surface area: 25 × 25 mm)
Self-cleaning effect of treated sandstone surfaces during 26-hour exposure to daylight-simulating radiation. The surfaces were coated with g-C3N4/poly(alkyl siloxane) dispersions at concentrations of 2 g·L⁻¹ (a) and 4 g·L⁻¹ (b). Dx (%) represents photocatalytic discolouration of RhB dye after 4 and 26 h
The graph in Fig. 12 illustrates the influence of the degree of exfoliation and the concentration of g-C3N4 in Porosil on SCA values of coatings applied to laboratory glass slides. The SCA values increase slightly in the order: Porosil ˂ PorTEX 30 ˂ PorTEX 45 ˂ PorTEX 60. The lowest SCA value was recorded for pure Porosil (110.4 °), while the highest was observed for the PorTEX 60/4 dispersion (114.7 °), representing a 4° increase compared to pure Porosil. It was observed that the higher the degree of exfoliation of particles in the dispersion (i.e. the higher the proportion of fine, flake-like particles and their SSA), the more uniform the resulting surface nanostructure and the higher the final SCA values.
Fig. 12
Influence of the addition of TEX30, TEX45 a TEX60 particles to Porosil at concentrations of 2 g·L− 1 and 4 g·L− 1 on the SCA values (coatings applied to laboratory glass slides)
The coating of pure Porosil on the surface of the tested sandstone yielded an average SCA of 135 ± 5°. When Porosil modified with nano-g-C3N4 was applied, a rough two-length-scale hierarchical structure was formed on the sandstone surface (Manoudis et al. 2009a), which significantly enhanced the surface’s water repellency. The highest hydrophobising effect was again achieved with the PorTEX 60/4 dispersion, producing an SCA of 145 ± 5° on the sandstone surface—10° higher than that of pure Porosil (Fig. 13). This value approaches the threshold of 150°, above which surfaces are classified as superhydrophobic.
Fig. 13
SCA on the sandstone surface coated with Porosil (a) and PorTEX 60/4 (b). The volume of the water droplet is 6 µL
The water vapour permeability values of coated and uncoated sandstone samples are presented in Table 2. A 10 mm thick plate of uncoated sandstone exhibited a water vapour permeability of 4125 ± 82 g/m2 over 24 h under the given laboratory conditions. For the coated samples, the permeability was 3978 ± 47 g/m2 for Porosil and 4041 ± 43 g/m2 for PorTEX 60/4. These values represent a reduction in water vapour permeability of 3.6% for Porosil and 2% for PorTEX 60/4. This reduction in water vapour permeability is markedly lower than that reported for siloxane treatments on other sandstone types (Manoudis et al. 2009a; Tsakalof et al. 2007). This is evidently due to the high porosity (> 33%) of the Mšené sandstone, which is characterised by a low proportion of rock matrix and large configurational-type pores that remain open even after the film formation. This is evidently due to the high porosity (> 33%) of the Mšené sandstone, which is characterised by a low proportion of rock matrix and large configurational-type pores that remain open even after the film formation. The addition of g-C3N4 nanoparticles to Porosil has a positive effect on vapour permeability, which is consistent with the findings of Manoudis et al. (2009a), who observed a similar trend on Hořice sandstone coated with poly(alkyl siloxanes) (including Porosil) containing SiO2 nanoparticles.
Table 2
Water vapor permeability reduction upon coating of stone specimens
Samples
Water vapour permeability
[g/m2. 24 h]
Reduction after treatment
[%]
Sandstone
4125 ± 82
-
Sandstone + Porosil
3978 ± 47
3.6
Sandstone + PorTEX 60/4
4041 ± 43
2
Capillary water uptake into the specimens of uncoated sandstone was very rapid, with most of the water being absorbed within the first minute, during which the samples reached a moisture content of approximately 10 wt% (Fig. 14). By the end of the experiment, the moisture content increased by only an additional 0.15 wt% (the increase is visually suppressed in the graph as a result of the logarithmic scaling of the y-axis). In contrast, for the samples coated with Porosil and PorTEX 60/4, the amount of absorbed water remained at only a few hundredths of percent throughout the entire experiment. The differences in the shape of the curves indicate a slightly higher reduction in water absorption for PorTEX 60/4 during the first 10 min of uptake. The reduction in capillary water absorption into the pore system of Mšené sandstone after 1 h of water uptake is approximately 99% for both tested coatings.
Fig. 14
Water capillary absorption in the Mšené sandstone specimens with and without Porosil and PorTEX 60/4 coatings
A commercially available hydrophobising poly(alkyl siloxane)-based agent was mixed with photocatalytically active g-C3N4 nanoparticles in concentrations of 2 g·L⁻¹ and 4 g·L⁻¹. The nano-g-C3N4 was prepared by thermal polycondensation of melamine followed by thermal exfoliation for 30, 45, or 60 min. The dispersions were then applied to the surface of porous quartz sandstone. The differences between the uncoated and coated sandstone surfaces were assessed, as was the effect of the degree of g-C3N4 exfoliation on the surfaces’ photocatalytic and water-repellent properties. The experimental results can be summarised as follows:
1)
Spraying of poly(alkyl siloxane), without and with the addition of nano-g-C3N4, produces a very thin transparent coating on the surface of the sandstone, which coherently covers the surface of the clastic grains without filling the interconnected pores. The g-C3N4 particles are well anchored in the poly(alkyl siloxane) matrix and are uniformly dispersed in it in the form of differently sized clusters. The application of coatings does not significantly change the appearance of the sandstone surface.
2)
The higher the degree of exfoliation of g-C3N4 particles in poly(alky siloxane) dispersion, the higher the self-cleaning effect on the sandstone surface these particles exhibit. The normative limits were met for TEX45 and TEX60 particles at a concentration of 4 g per litre of poly(alkyl siloxane). TEX60 particles showed the highest self-cleaning performance, achieving 64% discolouration of an artificially polluted surface after 26 h of visible light irradiation.
3)
Coatings with g-C3N4 provided significantly higher SCA values than pure poly(alkyl siloxane). The higher the degree of exfoliation of g-C3N4, the higher the SCA values. A coating containing TEX60 particles in concentration of 4 g per litre of poly(alkyl siloxane) gave an SCA of 145°. The coatings significantly reduce the water uptake into the pore system of the stone while not preventing the evaporation of water from the pores into the surrounding environment.
The results of the tests indicate that g-C3N4 in poly(alkyl siloxane) coating contributes both to prevent capillary water uptake of the sandstone and at the same time improve the water vapour permeability of the coating. However, the differences in these parameters between the poly(alkyl siloxane) with and without the addition of g-C3N4 are relatively small. More detailed research using a broader range of laboratory tests is needed.
This study presents g-C3N4 as a promising photocatalyst for the protection of stone architectural and artistic objects. Future research should focus on the behaviour of g-C3N4/poly(alkyl siloxane) coatings on stone surfaces under long-term exposure to real atmospheric conditions. In particular, attention should be focused on the stability of g-C3N4 particles and their clusters within the polymer matrix, as well as on the durability of the coatings and their long-term performance in terms of water repellency, photocatalytic efficiency, and resistance to various types of pollution.
Acknowledgements
This work was funded by the Czech Science Foundation under the project No. 24–10949 S.
Declarations
Competing interests
The authors declare that they have no knowncompeting financial interests or personal relationships that could haveappeared to influence the work reported in this paper.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Aldosari MA, Darwish SS, Adam MA, Elmarzugi NA, Ahmed SM (2019) Using ZnO nanoparticles in fungal inhibition and self-protection of exposed marble columns in historic sites. Archaeol Anthropol Sci 11:3407–3422. https://doi.org/10.1007/s12520-018-0762-zCrossRef
Chen R, Zhang J, Wang Y, Chen X, Zapiena JA, Lee C-S (2015) Graphitic carbon nitride nanosheet@metal–organic framework core–shell nanoparticles for photo-chemo combination therapy. Nanoscale 7:17299–17305. https://doi.org/10.1039/C5NR04436GCrossRef
Colangiuli D, Lettieri M, Masieri M, Calia A (2019) Field study in an urban environment of simultaneous self-cleaning and hydrophobic nanosized TiO2based coatings on stone for the protection of building surface. Sci Total Environ 650:2919–2930. https://doi.org/10.1016/j.scitotenv.2018.10.044CrossRef
Cui W, Li J, Cen W, Sun Y, Lee SC, Dong F (2017) Steering the interlayer energy barrier and charge flow via bioriented transportation channels in g-C3N4: enhanced photocatalysis and reaction mechanism. J Catal 352:351–360. https://doi.org/10.1016/j.jcat.2017.05.017CrossRef
De Ferri L, Lottici PP, Lorenzi A, Montenero A, Salvioli-Mariani E (2011) Study of silica nanoparticles – polysiloxane hydrophobic treatments for stone-based monument protection. J Cult Heritage 12:356–363. https://doi.org/10.1016/j.culher.2011.02.006CrossRef
Dong G, Aia Z, Zhang L (2014a) Efficient anoxic pollutant removal with oxygen functionalized graphitic carbon nitride under visible light. RSC Adv 4:5553–5560. https://doi.org/10.1039/C3RA46068ACrossRef
Dong G, Zhang Y, Pan Q, Qiu J (2014b) A fantastic graphitic carbon nitride (g-C3N4) material: Electronic structure, photocatalytic and photoelectronic properties. J Photochem Photobiol C Photochem Rev 20:33–50. https://doi.org/10.1016/j.jphotochemrev.2014.04.002CrossRef
Dong F, Li Y, Wang Z, Ho W-K (2015) Enhanced visible light photocatalytic activity and oxidation ability of porous graphene-like g-C3N4nanosheets via thermal exfoliation. Appl Surf Sci 358:393–403. https://doi.org/10.1016/j.apsusc.2015.04.034CrossRef
EN 13755: 2008 (2008) Natural stone test methods - determination of water absorption at atmospheric pressure, European Committee for Standardization, Brussels
EN 1925:1999 (1999) Natural stone test Methods - Determination of water absorption coefficient by capillarity. CEN, Brussels
EN 1936:2006 (2006) Natural stone test Methods - Determination of real density and apparent density, and of total and open porosity. CEN, Brussels
García O, Malaga K (2012) Definition of the procedure to determine the suitability and durability of an anti-graffiti product for application on cultural heritage porous materials. J Cult Herit 13:77–82. https://doi.org/10.1016/j.culher.2011.07.004CrossRef
Gherardi F, Roveri M, Goidanich S, Toniolo L (2018) Photocatalytic nanocomposites for the protection of European architectural heritage. Materials 11:65. https://doi.org/10.3390/ma11010065CrossRef
Han Q, Hu C, Zhao F, Zhang Z, Chena N, Qu L (2015) One-step preparation of iodine-doped graphitic carbon nitride nanosheets as efficient photocatalysts for visible light water splitting. J Mater Chem A 3:4612–4619. https://doi.org/10.1039/C4TA06093HCrossRef
Helmi FM, Hefni YK (2016) Using nanocomposites in the consolidation and protection of sandstone. Int J Conserv Sci 7:29–40
ISO 12154:2014 (2014) Determination of density by volumetric Displacement - Skeleton density by gas pycnometry. ISO, Geneva
Khan H, Shah MUH (2023) Modification strategies of TiO2 based photocatalysts for enhanced visible light activity and energy storage ability: a review. J Environ Chem Eng 11:111532. https://doi.org/10.1016/j.jece.2023.111532CrossRef
La Russa MF, Rovella N, de Buergo MA, Belfiore CM, Pezzino A, Crisci GM, Ruffolo SA (2016) Nano-TiO2 coatings for cultural heritage protection: the role of the binder on hydrophobic and self-cleaning efficacy. Prog Org Coat 91:1–8. https://doi.org/10.1016/j.porgcoat.2015.11.011CrossRef
Li J, Yan P, Li K, Cen W, Yu X, Yuan S, Chu Y, Wang Z (2018) Generation and transformation of ROS on g-C3N4 for efficient photocatalytic NO removal: a combined in situ DRIFTS and DFT investigation. Chin J Catal 10:1695–1703. https://doi.org/10.1016/S1872-2067(18)63097-9CrossRef
Li Z, Wang S, Wu J, Zhou W (2022) Recent progress in defective TiO2 photocatalysts for energy and environmental applications. Renew Sustain Energy Rev 156:111980. https://doi.org/10.1016/j.rser.2021.111980CrossRef
Liao G, Chen S, Quan X, Yu H, Zhao H (2012) Graphene oxide modified g-C3N4 hybrid with enhanced photocatalytic capability under visible light irradiation. J Mater Chem 22:2721–2726. https://doi.org/10.1039/C1JM13490FCrossRef
Lotsch BV, Döblinger M, Sehnert J, Seyfarth L, Senker J, Oeckler O, Schnick W (2007) Unmasking melon by a complementary approach employing electron diffraction, solid-state NMR spectroscopy, and theoretical calculations—structural characterization of a carbon nitride polymer. Chem Eur J 13:4969–4980. https://doi.org/10.1002/chem.200601759CrossRef
Lu J, Yang Z, Huang M, Xu J, Zhou J, Briseghella B, Marano GC (2023) Microstructural and mechanical properties of photocatalytic cement mortar with g-C3N4/CoAl-LDH nanoflowers. J Build Eng 74:106900. https://doi.org/10.1016/j.jobe.2023.106900CrossRef
Manoudis PN, Karapanagiotis I, Tsakalof A, Zuburtikudis I, Kolinkeová B, Panayiotou C (2009a) Superhydrophobic films for the protection of outdoor cultural heritage assets. Appl Phys A 97:351–360. https://doi.org/10.1007/s00339-009-5233-zCrossRef
Manoudis PN, Tsakalof A, Karapanagiotis I, Zuburtikudis I, Panayiotou C (2009b) Fabrication of super-hydrophobic surfaces for enhanced stone protection. Surf Coat Technol 203:1322–1328. https://doi.org/10.1016/j.surfcoat.2008.10.041CrossRef
Molaei MJ (2023) Graphitic carbon nitride (g-C3N4) synthesis and heterostructures, principles, mechanisms, and recent advances: a critical review. Int J Hydrog Energy 48:2708–32728. https://doi.org/10.1016/j.ijhydene.2023.05.066CrossRef
Ong CB, Ng LY, Mohammad AW (2018) A review of ZnO nanoparticles as solar photocatalysts: synthesis, mechanisms and applications. Renew Sustain Energy Rev 81:536–551. https://doi.org/10.1016/j.rser.2017.08.020CrossRef
Pan H, Zhang H, Liu H, Chen L (2015) Interstitial Boron doping effects on the electronic and magnetic properties of graphitic carbon nitride materials. Solid State Commun 203:35–40. https://doi.org/10.1016/j.ssc.2014.11.017CrossRef
Papailias I, Todorova N, Giannakopoulou T, Ioannidis N, Boukos N, Athanasekou CP, Dimotikali D, Trapalis C (2018) Chemical vs thermal exfoliation of g-C<Subscript>3</Subscript>N<Subscript>4</Subscript>for NO<Subscript>x</Subscript>removal under visible light irradiation. Appl Catal B Environ 239:16–26. https://doi.org/10.1016/j.apcatb.2018.07.078CrossRef
Pavlík Z, Michálek P, Pavlíková M, Kopecká I, Maxová I, Černý R (2008) Water and salt transport and storage properties of Mšené sandstone. Constr Build Mater 22:1736–1748. https://doi.org/10.1016/j.conbuildmat.2007.05.010CrossRef
Peng F, Ni Y, Zhou Q, Kou J, Lu C, Xu Z (2018) New g-C3N4 based photocatalytic cement with enhanced visible-light photocatalytic activity by constructing Muscovite sheet/SnO2 structures. Constr Build Mater 179:315–325. https://doi.org/10.1016/j.conbuildmat.2018.05.146CrossRef
Praus P, Smýkalová A, Foniok K, Novák V, Hrbáč J (2021) Doping of graphitic carbon nitride with oxygen by means of cyanuric acid: properties and photocatalytic applications. J Environ Chem Eng 9:105498. https://doi.org/10.1016/j.jece.2021.105498CrossRef
Roveri M, Goidanich S, Toniolo L (2020) Artificial ageing of photocatalytic nanocomposites for the protection of natural stones. Coatings 10:729. https://doi.org/10.3390/coatings10080729CrossRef
She X, Xu H, Xu Y, Yan J, Xia J, Xu L, Song Y, Jiang Y, Zhang Q, Li H (2014) Exfoliated graphene-like carbon nitride in organic solvents: enhanced photocatalytic activity and highly selective and sensitive sensor for the detection of trace amounts of Cu<Superscript>2+</Superscript>. J Mater Chem A 2(8):2563–2570. https://doi.org/10.1039/C3TA13768FCrossRef
Škuta R, Matějka V, Foniok K, Smýkalová A, Cvejn D, Gabor R, Kormunda M, Smetana B, Novák V, Praus P (2021) On P-doping of graphitic carbon nitride with hexachlorotriphosphazene as a source of phosphorus. Appl Surf Sci 552:149490. https://doi.org/10.1016/j.apsusc.2021.149490CrossRef
Speziale A, González-Sánchez JF, Tasci B, Pastor A, Sánchez L, Fernández-Acevedo C, Oroz-Mateo T, Salazar C, Navarro-Blasco I, Fernández JM, Alvarez JI (2020) Development of multifunctional coatings for protecting stones and lime mortars of the architectural heritage. Int J Architect Herit 14:1008–1029. https://doi.org/10.1080/15583058.2020.1728594CrossRef
Tokarský J, Ščučka J, Martinec P, Mamulová Kutláková K, Peikertová P, Lipina P (2022) Long-term effect of weather in Dfb climate subtype on properties of hydrophobic coatings on sandstone. J Build Eng 52:104383. https://doi.org/10.1016/j.jobe.2022.104383CrossRef
Tsakalof A, Manoudis P, Karapanagiotis I, Chryssoulakis I, Panayiotou C (2007) Assessment of synthetic polymeric coatings for the protection and preservation of stone monuments. J Cult Herit 8:69–72. https://doi.org/10.1016/j.culher.2006.06.007CrossRef
UNI 11259 (2008) Determination of the photocatalytic activity of hydraulic binders - Rodammina test method. UNI, Milan, 2008
van der Werf ID, Ditaranto N, Picca RA, Sportelli MC, Sabbatini L (2015) Development of a novel conservation treatment of stone monuments with bioactive nanocomposites. Herit Sci 3:29. https://doi.org/10.1186/s40494-015-0060-3CrossRef
Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, Domen K, Antonietti M (2009) A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat Mater 8:76–80. https://doi.org/10.1038/nmat2317CrossRef
Xu J, Zhang L, Shia R, Zhu Y (2013) Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis. J Mater Chem A 1(46):14766–14772. https://doi.org/10.1039/C3TA13188BCrossRef
Yan SC, Li ZS, Zou ZG (2009) Photodegradation performance of g-C3N4 fabricated by directly heating melamine. Langmuir 25:10397–10401. https://doi.org/10.1021/la900923zCrossRef
Yang S, Gong Y, Zhang J, Zhan L, Ma L, Fang Z, Vajtai R, Wang X, Ajayan PM (2013) Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light. Adv Mater 25:2452–2456. https://doi.org/10.1002/adma.201204453CrossRef
Yang Y, Ji T, Su W, Yang B, Zhang Y, Yang Z (2019) Photocatalytic NOx abatement and self-cleaning performance of cementitious composites with g-C3N4 nanosheets under visible light. Constr Build Mater 225:120–131. https://doi.org/10.1016/j.conbuildmat.2019.07.189CrossRef
Yang Y, Ji T, Lin Y, Su W (2021) Effect of adhesive on photocatalytic NOx removal and stability over polymeric carbon nitride coated cement mortars. J Clean Prod 295:126458. https://doi.org/10.1016/j.jclepro.2021.126458CrossRef
Zhang Y, Yuan J, Ding Y, Liu B, Zhao L, Zhang S (2021) Research progress on g–C3N4–based photocatalysts for organic pollutants degradation in wastewater: from exciton and carrier perspectives. Ceram Int 47:31005–31030. https://doi.org/10.1016/j.ceramint.2021.08.063CrossRef
