Polyacrylic Acid-Functionalized Graphene@Ca(OH)2 Nanocomposites for Mural Protection.

Murals are one of the precious legacies of our ancestors; however, they face severe damage along with archeological discoveries, which need urgent repair. Nowadays, nanotechnology provides new concepts and materials for the consolidation and protection of murals. In this work, an innovative method for the protection of murals was proposed with graphene-based nanomaterials through strategically synthesizing a polyacrylic acid-functionalized graphene/nano-Ca(OH)2 material (PAAG@Ca(OH)2) by a facile and economic aqueous method. As a result, the nanocomposite PAAG@Ca(OH)2 was demonstrated with high porosity, strong adsorption, appropriate hydrophilicity, and better permeability compared to the commercial AC33 sample according to the simulated tests. As expected, the nanocomposite PAAG@Ca(OH)2 displayed a promising application for the reinforcement of murals, which opens up a new avenue for the protection of murals.

Introduction

Murals are an important part of humankind’s excellent cultural heritage.[1] However, murals always face severe damage over the years, such as flaking (Figure A,B), hollowing (Figure C), fungus damage, and cracks. Therefore, the development of mural reinforcement materials becomes more and more indispensable for the protection of cultural heritage.[2,3] In the past decades, the diagnosis and conservation of cultural heritage have attracted broad attention in the field of materials science.[4] The reinforcement materials of murals are mainly divided into inorganic materials and organic materials. Inorganic materials mainly involve lime water, barium hydroxide, and alkaline earth silicate. In the early days, Dutkiewicz used lime water to reinforce the frescoes of Russian churches.[5] Subsequently, barium hydroxide has been applied gradually in the field of mural or stone reinforcement. However, the usage of inorganic materials will inevitably carry the aqueous solution into the mural, which will result in the crystallization of soluble salts.[6] Meanwhile, the large size of inorganic materials leads to poor permeability, which will limit the strength of mural reinforcement. Recently, owing to excellent permeation, good adhesion, and transparency, organic acrylic polymers have been applied to the reinforcement of murals, such as Paraloid B-72 and Primal AC33.[7] However, an organic polymer material exhibits an obvious aging problem after thermal oxidation and optical irradiation, which induces the degradation of mechanical strength. When the Cholula murals in Mexico were reinforced with polyvinyl acetate and Paraloid B-72, the paint layer had fallen away from the ground layer after being restored for about 20 years, resulting from the crystallization of soluble salts on the surface. The main reason is that the polymer blocks the surface pores and hinders the interaction between the mural and the environment, which generates mechanical stresses in the mural layer.

Figure 1

Digital pictures of murals from the Murongzhi tomb of the Tang dynasty with the problems of flaking on the painting surface (A,B) and hollowing phenomena (C). Synthetic method of PAAG@Ca(OH)2 nanocomposites (D).

In recent years, with the rise of various nanomaterials, their unique nanoparticle size and physicochemical properties have attracted attention in the field of heritage protection.[8−16] For example, Giorgi et al. carried out research on nanocalcium hydroxide in the mural restoration of Mexican murals and the La Antigua Ciudad Maya de Calakmul World Cultural Heritage Site.[17] Nevertheless, low mechanical strength and carbonization still exist in the murals. Since graphene has made giant progress in experiment, a lot of attention has been focused on the application of graphene due to its unique structure of a single planar sheet of sp2-bonded carbon atoms, which endows it with excellent physical and chemical properties, such as a high surface area and excellent electrical and thermal properties.[18−22] Graphene has been widely applied in the fields of optical electrodes, optoelectronic devices, sensors, and photocatalysis.[23−26] To date, graphene has been less studied in the field of heritage protection, particularly in the consolidation materials for murals.

In this work, we integrated polyacrylic acid-functionalized graphene with nano-Ca(OH)2 (PAAG@Ca(OH)2) for better protection of murals. Benefiting from the large specific surface area of graphene, the fabricated PAAG@Ca(OH)2 can easily capture and store CO2 under the ambient environment. In addition, the as-prepared nanocomposite PAAG@Ca(OH)2 possesses high porosity, strong adsorption, appropriate hydrophilicity, and better permeability compared with the commercial AC33 through the simulated tests. As a result, the PAAG@Ca(OH)2 exhibits a potential application for the reinforcement of murals.

Experimental Section

Chemicals and Reagents

Polyacrylic acid (PAA) (Mw = 25,000) was purchased from Aldrich (St. Louis, USA). Graphite powder was bought from Alfa Aesar. Sodium hydroxide (NaOH) and calcium chloride dihydrate (CaCl2·2H2O) were obtained from Beijing Chemical Reagent. The aqueous solutions were prepared with Millipore Milli-Q (18.2 MΩ cm) deionized water.

Apparatuses

The transmission electron microscopy (TEM) image, high-resolution transmission electron microscopy (HRTEM) image, high-angle annular dark-field–scanning transmission electron microscopy (HAADF–STEM) image, selected-area electron diffraction (SAED) pattern, energy-dispersive X-ray (EDX) spectra, and elemental mapping images were tested by a TECNAI G2 high-resolution transmission electron microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. Scanning electron microscope (SEM) images were recorded with an XL30 ESEM FEG SEM (Philips, The Netherlands) operating with an accelerating voltage of 20 kV. X-ray photoelectron spectroscopy (XPS) analysis was recorded with an ESCALAB-MKII X-ray photoelectron spectrometer (VG Scientific, UK). Powder X-ray diffraction (XRD) patterns were obtained from a D8 ADVANCE diffractometer (Germany) using Cu Kα radiation (λ = 1.5406 Å).

Synthesis of Polyacrylic Acid-Functionalized Reduced Graphene Oxide (PAAG)

For the synthesis of graphene oxide (GO), graphite oxides were obtained from natural graphite powder by the Hummers method.[27] To exfoliate the graphite oxide into graphene oxide, the sample was processed under ultrasonication for 60 min (1000 W, 27% amplitude) and centrifugation at 5000 rpm for 30 min. Finally, a homogeneous graphene oxide (GO) dispersion with a concentration of 0.4 mg mL–1 was prepared.

For the preparation of polyacrylic acid-functionalized reduced graphene oxide (PAAG), 172 mg of polyacrylic acid (PAA) was added into 40 mL of the as-prepared exfoliated GO dispersion (0.4 mg mL–1) and then stirred for 20 min. After stirring for 2 h at 90 °C, the resulting stable black mixture was centrifuged at 15,000 rpm and washed with DI water five times to remove the excess polymer.

Preparation of the PAAG@Ca(OH)2 Samples

PAAG (30 mg) and 3.0 g of NaOH were dissolved in 100 mL of DI water. At the same time, 13.23 g of CaCl2·2H2O was also dissolved in 100 mL of DI water. Then, the two solutions were both heated to 90 °C and mixed quickly. When the solution was cooled to room temperature, the product PAAG@Ca(OH)2 was obtained and washed with DI water five times to remove the impurity adsorbed on the surface of the product.

Preparation of Simulated Samples

The plaster layer of simulated wall painting samples was mainly wet slaked lime, and the hemp fibers were in a silicone mold having a volume of 10 × 10 × 1 cm3. A small amount of short hemp fibers was added into the wet slaked lime in order to ease the shrinkage of the plaster layer during drying. All the samples were then left in an ambient room for a month to reach carbonation before further tests. The pigments were painted after the carbonation of the samples, and the pigments used here were cinnabar (red), ultramarine (blue), yellow ochre (yellow), and malachite (green).

Results and Discussion

Characterization of PAAG@Ca(OH)2 Nanocomposites

As shown in Figure D, the PAAG@Ca(OH)2 nanocomposites were fabricated by an economical and facile method. Initially, PAAG was fabricated by a hydrothermal method (see Section ) followed by mixing of the CaCl2 solution, which contained a certain amount of NaOH at an elevated temperature. Then, the PAAG@Ca(OH)2 nanocomposites were obtained after washing several times with DI water. Figure A shows the TEM image of the PAAG@Ca(OH)2 nanocomposites, and the afforded PAAG@Ca(OH)2 exhibits a hollow nanostructure, which is beneficial to mural protection. Moreover, the HRTEM image (Figure B) was investigated to better understand the PAAG@Ca(OH)2 nanocomposites. The lattice distance of 0.263 nm should be assigned to the (101) crystal plane of the Ca(OH)2 phase, which is consistent with the SAED pattern of the nanocomposites (Figure C). Figure D–H shows the HAADF–STEM and the elemental mapping images of the PAAG@Ca(OH)2 nanocomposites. It could be observed that C, O, and Ca elements were evenly distributed over the selected area, implying a good mixture of PAAG and Ca(OH)2. The morphologies of fabricated PAAG@Ca(OH)2 nanocomposites were also studied with SEM. As shown in Figures S1 and S2, the Ca(OH)2 nanoparticles were uniformly distributed on the PAAG surface at different magnifications. EDX spectroscopy (Figure S3) was also conducted to measure the elemental contents of the prepared PAAG@Ca(OH)2 nanocomposite, which is made up of C (35.25 at. %), O (47.13 at. %), and Ca (17.62 at. %).

Figure 2

TEM image (A), HRTEM image (B), SAED pattern (C), HAADF–STEM image (D), and elemental mapping images (E–H) of PAAG@Ca(OH)2.

XRD measurements were used to understand the crystal structure of the obtained PAAG@Ca(OH)2 nanocomposites. As observed in Figure A, the presence of the peaks located at 18.05, 28.60, 33.94, 46.97, 50.71, 54.28, 59.30, 62.56, 64.24, 71.85, and 84.59° correspond to the (001), (100), (101), (102), (110), (111), (200), (201), (103), (202), and (104) planes of Ca(OH)2 (JCPDS file no. 441481), respectively. Meanwhile, the peak at around 43.60° corresponds to the (100) plane of the PAAG, suggesting that PAAG@Ca(OH)2 nanocomposites were successfully prepared.

Figure 3

XRD pattern of PAAG@Ca(OH)2 (A). High-resolution C 1s XPS spectra (B), O 1s spectra (C), and Ca 2p spectra (D) of the PAAG@Ca(OH)2.

XPS tests were conducted to understand the chemical state of the elements in the PAAG@Ca(OH)2 nanocomposites. As shown in Figure S4, the three elements C, O, and Ca were observed in the XPS survey spectra of PAAG@Ca(OH)2. Figure B shows the C 1s XPS spectra of PAAG@Ca(OH)2 with the peaks located at 284.8 and 289.48 eV, which should correspond to the C–C group along with the O–C=O group, indicating the carbon elemental environments of PAAG@Ca(OH)2 nanocomposites. Moreover, the peaks located at 530.69, 531.49, and 532.28 eV in the high-resolution O 1s XPS spectra (Figure C) were assigned to the C–O, Ca–O, and C–OH/O–C–O groups, respectively. The Ca 2p spectrum exhibited in Figure D could be divided into two peaks (346.92 and 350.41 eV), which were related to the 2p3/2 of Ca2+ and 2p1/2 of Ca2+, which indicates the presence of Ca(OH)2 crystals in the PAAG@Ca(OH)2 nanocomposites and further implies the successful preparation of the PAAG@Ca(OH)2 nanocomposites.

Reinforcement Efficiency of PAAG@Ca(OH)2 Nanocomposites for Murals

In order to evaluate the reinforcement efficiency of the PAAG@Ca(OH)2 nanocomposite for murals, we first carried out a test with the simulated murals. Specifically, a 5 mg/mL PAAG@Ca(OH)2 nanocomposite was dissolved in ethanol and then brushed on the surface of the mural simulated sample until it cannot penetrate. Afterward, the sample blocks were placed in a constant-temperature humidity chamber with an open hole diameter of 50 mm for 15 days to allow carbon dioxide to carbonize calcium hydroxide in PAAG@Ca(OH)2. The temperature and humidity were set to 25 °C and 65%, respectively. For comparison, the commercial AC33 was used as a reference material. In general, the color change is an important factor to evaluate the rationality of the reinforcement material.[28] Based on the chromatic aberration measurement, the effect of PAAG@Ca(OH)2 and AC33 on the mural has been evaluated. By contrasting the color changes after reinforcement, the chromatic aberration ΔE was calculated as follows:where L represents illuminance, which is equivalent to brightness. a represents the range from magenta to green, and b represents the range from yellow to blue. ΔL, Δa, and Δb are the differences of L, a, and b before and after the measurement, respectively.

As shown in Table , the L, a, and b of the standard sample are L = 96.12, a = 0.33, and b = 2.66, respectively. We selected three points to measure the L, a, and b values before and after the reinforcement of PAAG@Ca(OH)2 and AC33. The calculation shows that the ΔE values of PAAG@Ca(OH)2 are less than 5 (Table ), while the ΔE values of AC33 are larger than 5. Therefore, the color of simulated samples had no significant changes after being reinforced with PAAG@Ca(OH)2, which demonstrates that PAAG@Ca(OH)2 has less effect on the color of the mural.

Table 1

The Change of Color after Mural Reinforcement by AC33 and PAAG@Ca(OH)2

sample type	ΔL	Δa	Δb	ΔE
AC33	–6.20	–0.97	1.13	6.38
	–5.41	–1.21	1.02	5.64
	–6.19	–1.67	0.97	6.48
PAAG@Ca(OH)2	–2.37	1.00	3.37	4.24
	–1.25	0.96	2.15	2.67
	–1.56	1.13	2.43	3.10

As far as we know, the porosity test is another important means to evaluate the reinforcement performance for murals. A mercury porosimeter was used to measure the pore size distribution and porosity. In Figure , the pore size of the mural sample was distributed over 0–2000 nm, and the pore size diameter was mainly located at about 1000 nm. The porosity of the untreated sample was 53.64%. As shown in Table S1, the porosity has a slight reduction after reinforcement by AC33 (53.2182%), while the porosity has a slight increase after reinforcement by PAAG@Ca(OH)2 (54.10%). These results indicate that the internal structure of the mural will not change greatly after the reinforcement with PAAG@Ca(OH)2. Therefore, it can be inferred that PAAG@Ca(OH)2 is an excellent material for the reinforcement of murals.

Figure 4

Aperture dimensions of the untreated samples and those consolidated by the as-prepared AC33 and PAAG@Ca(OH)2.

The Bonding Strength of PAAG@Ca(OH)2 Nanocomposites for Murals

The “Scotch tape test” (STT) was also performed to evaluate the bonding strength of the PAAG@Ca(OH)2. The weight loss taken up by the Scotch tape is shown in Figure A,B. For the untreated sample, which was not coated with any reinforcing material, the weight loss is 3.21 ± 0.5 mg cm–2. The weight loss of the simulated samples consolidated with the AC33 is 1.54 ± 0.5 mg cm–2. However, the weight loss is only 0.71 ± 0.3 mg cm–2 for the simulated samples consolidated with the PAAG@Ca(OH)2 nanohybrids, which also proves that PAAG@Ca(OH)2 is an effective consolidation material for murals. The carbonation process transforming Ca(OH)2 to CaCO3 is the crucial reason for the better STT result.[29−31] It is well-known that graphene has a large specific surface area, which can constantly capture, store, and release CO2 to adjacent Ca(OH)2.[32] Thus, the fabricated PAAG@Ca(OH)2 could easily capture and store CO2 even under an ambient environment. Furthermore, the mural samples reinforced with AC33 and PAAG@Ca(OH)2 were also tested by SEM. It can be seen from Figure S5A that the untreated mural sample was porous and loose, which is consistent with the porosity and strength tests. After being reinforced with AC33 and PAAG@Ca(OH)2, the mural samples became compact (Figure S5B,C). In particular, after the sample block was reinforced by PAAG@Ca(OH)2, the apparent cavity almost disappeared. The results further reveal that the strength of murals can be improved by PAAG@Ca(OH)2.

Figure 5

Digital picture of the “Scotch tape test” (STT) (A) and weight loss of the simulated murals (B) with the untreated samples and consolidated with the as-prepared AC33 and PAAG@Ca(OH)2 materials (the untreated samples denote that the samples were not treated with any consolidation material); contact angles of the different materials, namely, the untreated sample (C) and the as-prepared AC33 (D) and PAAG@Ca(OH)2 (E) consolidated murals.

The Hydrophilicity of PAAG@Ca(OH)2 Nanocomposites for Murals

The exchange of moisture has an important impact on the mural pigment between air and the base layer. Therefore, it is important to explore the hydrophilic performance of different consolidated materials. It is mainly hydrophilic when the contact angle is less than 90° on the surface of the mural. Compared with the untreated samples (Figure C), the hydrophobicity was increased slightly after reinforcement with AC33 (Figure D) and PAAG@Ca(OH)2 (Figure E). The hydrophobicity change is not significant. So, PAAG@Ca(OH)2 has less effect of moisture exchange on the mural, which can be used to strengthen the pigment layer of the mural.

The Permeability of PAAG@Ca(OH)2 Nanocomposites for Murals

Permeability also plays a significant role in mural consolidation.[27] The penetration depth can be compared based on the Raman spectral test of AC33 and PAAG@Ca(OH)2 consolidated murals. As shown in Figure A, the D and G bands are attributed to graphene in the PAAG@Ca(OH)2. Therefore, the shifts from the surface to the location of the D and G of PAAG@Ca(OH)2 are not detectable along the depth of penetration. For the as-prepared AC33 consolidated mural sample, the Raman peaks of the AC33 (Figure B) were around 1457 and 1683 cm–1, and the intensity appeared from 0 to 200 μm and decreased from 200 to 350 μm, demonstrating that the penetration depth was about 200 μm. Meanwhile, for the PAAG@Ca(OH)2 consolidated mural sample, the intensity of the D band and the G band gradually decreased from 0 to 300 μm. At 350 μm, the intensity of the two bands can be negligible, which means that the penetration depth is about 350 μm. The results show that the PAAG@Ca(OH)2 nanocomposite can permeate much deeper in the mural than AC33, which demonstrates an important application of PAAG@Ca(OH)2 as a promising material for murals.

Figure 6

Raman spectra of the as-prepared AC33 (A) and PAAG@Ca(OH)2 (B) consolidated mural cross sections at different depths.

Conclusions

In this work, we demonstrate an unprecedented concept of mural protection enabled by polyacrylic acid-functionalized graphene through strategically synthesizing nanocomposites (PAAG@Ca(OH)2) using a facile and economic aqueous method. Through the simulated mural, the PAAG@Ca(OH)2 nanocomposite displays high porosity, strong adsorption, appropriate hydrophilicity, good permeability, and strong adhesion to mural pigments, which is a potential candidate to consolidate murals. As a result, the PAAG@Ca(OH)2 nanocomposite shows superior conservation efficiency to the AC33, which demonstrates an important application of graphene as a protective material for reinforcement of murals.