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Sun, H., Yang, Y., Han, Y., Tian, M., Li, B., Han, L., Wang, A., Wang, W., Zhao, R., and He, Y. (2020). "X-ray photoelectron spectroscopy analysis of wood degradation in old architecture," BioRes. 15(3), 6332-6343.

Abstract

To investigate the decay mechanisms of red oak (Quercus rubra) and hemor (Schima spp.) woods in the old architectural structure of Xichuan Guild Hall, chemical composition changes were determined and analyzed with X-ray photoelectron spectroscopy (XPS). The results showed that decaying resulted in a noticeable decrease of the O/C from 0.59 to 0.42 in the red oak wooden components. The increase of C1 contribution, decrease of C4 contribution, increase of O1 and O3 contributions, and decrease of O2 contribution indicated that the carbohydrates in red oak wooden components can be easily degraded by fungi compared with lignin. Moreover, decaying resulted in a slight decrease of the O/C from 0.49 to 0.47 in the hemor wooden components. The results of increase of C1 contribution, decrease of C3 and C4 contributions, increase of O1, and decrease of O2 and O3 contributions indicated that carbohydrate and lignin were all degraded by fungi.


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X-ray Photoelectron Spectroscopy Analysis of Wood Degradation in Old Architecture

He Sun,b,1 Yan Yang,a,*,1 Yanxia Han,a Mingjin Tian,a Bin Li,a Li Han,c Aifeng Wang,a Wei Wang,a Rui Zhao,a and Yiming He a

To investigate the decay mechanisms of red oak (Quercus rubra) and hemor (Schima spp.) woods in the old architectural structure of Xichuan Guild Hall, chemical composition changes were determined and analyzed with X-ray photoelectron spectroscopy (XPS). The results showed that decaying resulted in a noticeable decrease of the O/C from 0.59 to 0.42 in the red oak wooden components. The increase of Ccontribution, decrease of C4 contribution, increase of O1 and Ocontributions, and decrease of O2 contribution indicated that the carbohydrates in red oak wooden components can be easily degraded by fungi compared with lignin. Moreover, decaying resulted in a slight decrease of the O/C from 0.49 to 0.47 in the hemor wooden components. The results of increase of Ccontribution, decrease of C3 and C4 contributions, increase of O1, and decrease of O2 and O3 contributions indicated that carbohydrate and lignin were all degraded by fungi.

Keywords: Xichuan Guild Hall, Old architectures; Wooden components; Degradation behavior; Chemical composition changes; XPS

Contact information: a: School of Architecture, Nanyang Institute of Technology, Nanyang City, Henan Province, 473000, P.R. China; b: College of Material Science and Engineering, Southwest Forestry University, Kunming City, Yunnan Province,650224, P.R. China; c: Henan Key Laboratory of Zhang Zhongjing Formulae and Herbs for Immunoregulation, Nanyang Institute of Technology, Nanyang City, Henan Province, 473000, P.R. China; 1: He Sun and Yan Yang contributed equally to this work; * Corresponding author: yangyanrainy@163.com

INTRODUCTION

The Xichuan Guild Hall, which was built in 1910, is located in Henan Province, China, and it is considered a key cultural relic protection unit of Nanyang City. However, its wooden components have changed greatly in the past hundred years. Alterations on their anatomical structure, chemical composition degradation, and the decreased quality of their physical and mechanical properties reduce their residual mechanical strength and eventually affect their quality and life span (Ferraz et al. 1995; Choi et al. 2006; Arias et al. 2010; Koyani et al. 2014; Bari et al. 2019; Brischke et al. 2019; Chang et al. 2019; Gao et al. 2019; Li et al. 2019). Analyzing the changes in their chemical compositions can provide reference and guidance for the research of degradation mechanisms, maintenance, and reinforcement of decayed wooden components in the future.

X-ray photoelectron spectroscopy (XPS) (Stark et al. 2004, 2007; Inari et al. 2011; Huang et al. 2012; Xu et al. 2013; Tomak et al. 2013; Fernández-Fernández et al. 2014; Banuls-Ciscar et al. 2016; Croitoru et al. 2018) is an effective technique for investigating the chemistry of woods, especially decayed ones (Dey et al. 1992; Ferraz et al. 1995). Its advantage is that it requires minimal sample preparation and quantity compared with conventional gravimetric techniques (Xu et al. 2013).

This study examined the changes of chemical compositions and investigated the degradation mechanisms of wooden components in the old wooden structure of Xichuan Guild Hall. XPS was used for chemical analyses to provide insights into the degradation process for decayed and non-decayed wooden components exposed to fungi in with long-term usage.

EXPERIMENTAL

Materials

Samples (Table 1) were collected from the decayed wooden components of the Xichuan Guild Hall in Nanyang City, Henan Province, China. Among the wooden component samples, sample No. 3, Quercus rubra (Fagaceae), which was identified by Yan Yang et al. (2020), was obtained from the surface of wooden beam’ tops, and sample No. 1, Schima spp. (Theaceae) which was identified by Yang et al. (2020), was obtained from the surface of wooden column’ roots. Samples of the non-decayed red oak and hemor woods were obtained from model specimens of the Southwest Forestry University. Radial sections (Entrapment treatment of the sample seen from the paper conducted by Yang et al. (2020) were sliced using a microtome (SM2000R, Leica company) for XPS analysis.

Table 1. Information about the Materials

XPS Analysis

XPS analyses were performed using a VG MKII system (MultiPak V9.3, Kanagawa-ken, Japan) with a Mg Kα X-ray source. The samples were analyzed at a 10−6 Pa pressure with a 20 eV pass energy, 8 kV operating voltage, 30 mA operating current, and 0.1 eV resolution at a temperature of 20 °C to 400 °C and then thoroughly cleaned and degreased before the removal of wood water. All samples were removed immediately before examination to minimize contact with bare hands. After preparation, the samples were immediately placed in a vacuum chamber.

The following three types of spectra were collected: a low-resolution spectrum (survey spectrum) from 0 eV to 1100 eV, a high-resolution spectrum of the C1s region from 278 eV to 298 eV, and a high-resolution spectrum of the O1s region from 523 eV to 543 eV (Yang et al. 2018; Stark et al. 2007). The oxygen to carbon ratio (O/C) was determined from the low-resolution spectra. The C1s peak from the high-resolution spectra was deconvoluted into four subpeaks, namely, an unoxidized carbon, i.e., C1, and oxidized carbons, i.e., C2, C3, and C4 by using Origin 8.5 software (OriginLab, Northampton, MA, USA).

The oxygenated to unoxygenated carbon ratio (Cox/Cunox) was calculated using the sum of C2, C3, and C4 to C1 ratio (Matuana et al. 2002; Stark et al. 2007, 2004). The O1s peak from the high-resolution spectra was deconvoluted into three subpeaks, that is, O1, O2, and O3 by using Origin 8.5 software.

RESULTS AND DISCUSSION

XPS Analysis of Red Oak Wooden Components

XPS survey spectrum analysis of the wood surfaces

The survey spectra of C1s and O1s were obtained to evaluate the chemical structures of the surfaces of the red oak wooden components in decayed and non-decayed samples, and the results are presented in Fig. 1. The experimental atomic compositions, atomic percentages, and O/C of wood surfaces are shown in Table 2.

Survey spectrum analysis revealed that C and O atoms are the major elements located at the binding energy (BE) values from 284 eV to 290 eV and from 531 eV to 534 eV, respectively. Small amounts of nitrogen (N), silicon (Si), and calcium (Ca) atoms were also found on the wood surfaces at BE values of approximately 396.91, 98.91 (or 149.91), and 345 eV, respectively (Fig. 1 and Table 2) (Popescu et al. 2009; Xu et al. 2013). In principle, the O/C values are 0.83 for cellulose, approximately 0.8 for hemicellulose, and 0.33 for lignin (Inari et al. 2006; Kocaefe et al. 2013). A high O/C usually indicates a high relative content of carbohydrates. Conversely, a low O/C indicates a high relative content of lignin (Li et al. 2005; Kocaefe et al. 2013).

The relative content of the C atom of red oak increased from 61.4 in the non-decayed sample to 67.8 in the decayed sample, whereas that of O atom decreased from 36.5 to 28.2 (Fig. 1 and Table 2).

Decaying reduced the O/C from 0.59 to 0.42 at a decrease percentage of approximately 28.8%. This finding indicates reduction in the predominant oxygen-containing functional groups, such as carboxyl, acetyl, and hydroxyl groups on the surfaces of the decayed woods. Therefore, the relative content of lignin increased, whereas that of carbohydrates decreased. These findings were in good agreement with those for Pinus massoniana decayed by brown-rot fungi (Li et al. 2018).

Fig. 1. XPS survey spectra of the red oak wooden components

Table 2. The Experimental Atomic Compositions, Atomic Percentage, and Ratio of Oxygen to Carbon (O/C) of Wood Surfaces Obtained by XPS Analysis

Sample descriptions: NDROW-the non-decayed red oak wood; DROW- the decayed red oak wood; NDHW- the non-decayed hemor wood; DHW- the decayed hemor wood. “+”- increase percentage; “-“- decrease percentage. M- Mean value; S- Standard deviation.

C1s spectra analysis of the wood surfaces

The deconvoluted high-resolution XPS spectra of C1s peak in lignocellulosic materials are usually assigned to four classes of carbon atoms expressed as C1, C2, C3, and C4 (Inari et al. 2006; Popescu et al. 2009). The deconvoluted peaks with corresponding theoretical BE values and bond type for the high-resolution XPS scan of C1s are presented in Table 3. The Cclass of carbon atoms is ascribed to carbon atoms that are bonded only to carbon or hydrogen atoms, i.e., C-H or C-C and originates mainly from phenyl propane structures in the lignin component of the wood and extractives consisting of fatty acids, fats, waxes, and terpenoids. The BE value of C1 is low at approximately 284.8 eV (Inari et al. 2006; Popescu et al. 2009; Wang et al. 2009). C1 is referred to as unoxygenated carbon atom (Cunox). The Cclass of carbon atom contains carbon atoms that are bonded to one noncarbonyl oxygen atom, i.e., C-OH, C-O-C and is mainly derived from cellulose and hemicellulose containing a large quantity of hydroxyl groups (-OH). Accordingly, the BE value of C2 is higher than that of C1 at approximately 286.5 eV (Inari et al. 2006; Popescu et al. 2009; Wang et al. 2009). The C2 class is referred to as oxygenated carbon atom (Cox). The C3 class of carbon atom corresponds to carbon atoms that are bonded to one carbonyl oxygen atom or two non-carbonyl oxygen atoms, i.e., C=O or O-C-O and is derived from the acetal structure (O-C-O) of cellulose and hemicellulose and the carbonyl structure (C=O) of lignin. The high oxidation states of carbon atoms in C=O and O-C-O lead to high BE values of approximately 288 eV to 288.5 eV (Inari et al. 2006; Popescu et al. 2009; Wang et al. 2009). The C3 class is referred to as oxygenated carbon atom (Cox). The C4 class of carbon atom is associated with carbon atoms that are bonded to one carbonyl oxygen atom and one non-carbonyl oxygen atom, i.e., O-C=O. These atoms are acetyl groups and glucuronic acid present in hemicellulose and extractives, such as resin acids, fatty acids, and other substances. C4 has the highest BE value (289 eV) among carbon atom classes because it has the highest oxidation state (Inari et al. 2006; Popescu et al. 2009). The C4 class is also referred to as oxygenated carbon atom (Cox).

The high-resolutions of C1s of the surfaces of red oak wooden components in decayed and non-decayed samples were deconvoluted into four subpeaks by using Origin 8.5 software as shown in Fig. 2. The BE values, peak area, and Cox/Cunox for decomposed carbon peaks for all samples are presented in Table 4. The C1 contributions increased noticeably from 22.4% to 35.8% with an increase percentage of approximately 59.9%. This finding indicates that the carbohydrates were considerably decayed by fungi, leading to an increase in C-C bonds and lignin and extractive contents. Meanwhile, the contributions of C2 and C3 increased from 51.3% to 53.8% and 6.2% to 9.1%, respectively, after decay. These results implied an increase in the hydroxyl groups (-OH) and acetal structure (O-C-O) mainly derived from cellulose and hemicellulose. However, the C4 contributions decreased rapidly from 20.0% to 1.28% with a decrease percentage of approximately 93.6%. This finding indicates that the hemicellulose was considerably decayed by fungi, leading to a sharp decrease in the acetyl groups and glucuronic acid. These changes in C1, C2, C3, and C4 relative contents indicated that carbohydrates, especially cellulose, in red oak wooden components can be easily degraded by fungi compared with lignin. These findings were in good agreement with the results presented by Xu et al. (2013) and Li et al. (2018).

The Cox/Cunox ratio decreased from 3.46 to 1.79 with decrease percentage of approximately 48.3% after decay (Table 4). This result indicates that the carbohydrates were considerably decayed by fungi and suggests the decrease in oxygen-containing functional groups during the decay process. These findings were in good agreement with those by Xu et al. (2013).

In general, brown-rot fungi degrade cellulose and hemicellulose but do not affect lignin and extractives (Fardim et al. 2006; Tomak 2013; Guo et al. 2015). Therefore, it is speculated that the red oak wooden components in the old wooden structures of Xichuan Guild Hall were easily affected by brown-rot fungi during their long-term use.

Fig. 2. The high resolution of C1s of the red oak wooden components

Table 3. Deconvoluted Peak Assignments with Corresponding Theoretical BE and Bond Type for High-Resolution XPS Scan Of C1s and O1s

Table 4. Detailed Values of BE, Peak Area, and Cox/Cunox for Decomposed Oxygen Peaks (C1s) for All Samples

O1s spectra analysis of the wood surfaces

The deconvoluted peak assignments with corresponding theoretical BE and bond type for high-resolution XPS scan of O1s are presented in Table 5. Low BE values of 531.6 ± 0.4eV are attributed to oxygen atoms between two phenolic groups or an oxygen double-bonded to a carbon, i.e., C=O, O-C=O denoted as O1 (Hua et al. 1993). These components are associated to lignin, and an increase in Ocontribution indicates a relative increase in lignin and extractive contents but a decrease in carbohydrate contents on wood surfaces (Hua et al. 1993). All oxygen atoms bonded to carbon atoms with a single bond (C-O), i.e., C-O, C-O-C, and O-C=O, are attributed to O2 component with a high BE value of 533.2 eV (Hua et al. 1993). These component are suggested to be associated to carbohydrates, and a decrease in O2 contribution indicates an increase in lignin and extractive contents but a decrease in carbohydrate contents on wood surfaces (Hua et al. 1993). The phenolic oxygen, i.e., C=O is attributed to the Ocomponent with the highest BE value of 534.3 ± 0.4 eV and is mainly associated with lignin in wood. Consequently, the O3 component in XPS spectra indicates the presence of lignin on the wood surfaces (Kocaefe et al. 2013).

The high-resolutions of O1s of the surfaces of red oak wooden components in decayed and non-decayed samples were deconvoluted into three subpeaks by using Origin 8.5 software as shown in Fig. 3. O1, O2, and Owere contained in both samples. BE values, peak area, and (O1+O3)/O2 for decomposed carbon peaks (O1s) from all samples are presented in Table 5. The O1 and Ocontributions increased noticeably from 2.55% to 5.99% and from 22.4% to 26.9%, respectively, with increase percentages of approximately 20.3% and 134.9%, respectively. These findings indicate that the carbohydrates were considerably decayed by fungi, leading to an increase in lignin content. O2 contributions decreased from 75.1% to 67.1% with decreased percentage of approximately 10.61% after decay, indicating a decrease in carbohydrate contents.

The (O1+O3)/O2 increased noticeably from 0.33 to 0.49 with increase percentage of approximately 48.5%. This finding indicates an increase in oxygen atoms that were double-bonded to carbon atoms, i.e., an increase in the carbonyl groups of lignin and the oxidation states of carbon atoms. This result was in good agreement with the study by Xu et al. (2013).

Basing on the O/C analysis for C1, C2, C3, and C4 and O1, O2, and O3 contents of cellulose, hemicellulose, and lignin of the red oak wooden components, it was concluded that cellulose and hemicellulose were easily attacked by fungi compared with lignin.

Fig. 3. The high resolution of O1s of the red oak wooden components

Table 5. Detailed Values of BE, Peak Area and (O1+O3)/O2 for Decomposed Carbon Peaks (O1s) for All Samples

XPS Analysis of Hemor Wooden Components

XPS survey spectra analysis of the wood surfaces

The survey XPS spectra of the surfaces of the hemor wooden components in decayed and non-decayed samples are presented in Fig. 4. The experimental atomic compositions, atomic percentages, and O/C of wood surfaces were obtained by XPS analysis and are presented in Table 2.

The relative content of C on the surface of the decayed hemor wood increased, whereas that of O decreased as shown in Fig. 4 and Table 2. Decaying slightly reduced the O/C from 0.49 in the non-decayed wood to 0.47 in the decayed wood, and the decrease percentage was approximately 4.08%. This result indicated a reduction in the predominant oxygen-containing functional groups on the decayed wood surface, that is, the relative content of lignin increased, whereas that of carbohydrates decreased. These findings were in good agreement with those of Li et al. (2018).

Fig. 4. XPS survey spectra of the hemor wooden components (“cps”s in all ordinates of the figures have been corrected by “c/s”s )

C1s spectra analysis of the wood surfaces

The high-resolution C1s of the surfaces of red oak wooden components in decayed and non-decayed samples were deconvoluted into four subpeaks by using Origin 8.5 software (Fig. 5). The detailed BE values and content of each carbon signal group are presented in Table 4. The C1 contributions increased substantially from 20.2% to 28.7% with an increase percentage of approximately 42.2%. This finding indicates that the carbohydrates were considerably decayed by fungi, and the C-C bonds and contents of lignin and extractives were consequently increased. Meanwhile, the C2 contributions increased weakly from 38.7% to 40.4%, indicating an increase in hydroxyl groups (-OH) mainly derived from cellulose and hemicellulose. On the contrary, C3 contributions decreased remarkably from 38.3% to 30.8% after decay with a decrease percentage of approximately 19.5%. This finding indicates a decrease in the acetal structure (O-C-O) derived from cellulose and hemicellulose and in the carbonyl structure (C=O) of lignin. However, the C4 contributions decreased rapidly from 2.80% to 0%, indicating that the hemicellulose was considerably decayed by fungi, leading to a sharp decrease in the acetyl groups and glucuronic acid present in the hemicellulose. These results revealed that the carbohydrates in hemor wooden components can be degraded by fungi as implied by the changes of the relative contents of C1, C2, C3, and C4. These findings were in good agreement with the results presented by Xu et al. (2013). By comparison, the decadent degree of carbohydrates by fungi was lower in the hemor wooden components than in the red oak wooden components.

The Cox/Cunox decreased from 3.95 to 2.48 after decay, suggesting a decrease in the oxygen-containing functional groups during this process. These findings were in good agreement with the study by Xu et al. (2013).

Fig. 5. The high resolution of C1s of the hemor wooden components

O1s spectra analysis of the wood surfaces

The high-resolutions of O1s of the surfaces of hemor wooden components in the decayed and non-decayed woods were deconvoluted into three subpeaks by using Origin 8.5 software. Figure 6 shows that O1, O2, and Owere present in the decayed wood and the model specimen. The detailed BE values and content of each oxygen signal group are presented in Table 5. The Ocontributions increased noticeably from 2.75% to 15.9%, a percentage increase of approximately 479%. This finding indicates that lignin was attacked by fungi, and its content was consequently increased. The contributions of O2 decreased from 51.6% to 49.3% after decay with a decrease percentage of approximately 5.34%, indicating a reduction in the cellulose content. However, the Ocontributions decreased from 45.7% to 34.8% with a decrease percentage of approximately 23.9%, indicating a decadent of lignin and a decrease in its content.

The (O1+O3)/O2 increased from 0.94 to 1.03, indicating an increase in the oxygen atoms double-bonded to carbon atoms, i.e., an increase was observed for the carbonyl groups in the lignin and the oxidation states of the carbon atoms. This result was in good agreement with the study by Xu et al. (2013).

Fig. 6. The high resolution of O1s of the hemor wooden components

The O/C analysis revealed the C1, C2, C3, and C4 contents and, O2, O3 levels of cellulose, hemicellulose, and lignin of hemor wooden components. Based on these results, cellulose, hemicellulose, and lignin were all attacked by fungi.

CONCLUSIONS

  1. Decaying reduced the O/C ratio from 0.59 to 0.42 in the red oak wooden components, indicating that the oxygen-containing functional groups and carbohydrates decreased, whereas the relative lignin content increased. The contribution of C1 increased, those of C2 and C3 increased slightly, and that of C4 decreased. The contributions of O1 and Oincreased substantially, and that of O2 contributions decreased remarkably, indicating that the carbohydrates, especially cellulose, in red oak wooden components can be easily degraded by fungi compared with lignin.
  2. Decaying reduced the O/C from 0.49 to 0.47 in the hemor wooden components, indicating a slight decrease in the oxygen-containing functional groups and a reduction in the relative carbohydrate content. The contribution of C1 increased, that of C2 increased slightly, and those of C3 and C4 decreased. The contribution of O1 increased noticeably, those of O2 decreased substantially, but that of O3 decreased remarkably, indicating that carbohydrate and lignin were all degraded by fungi.
  3. In comparison, the decay was lower in the hemor wooden components than in the red oak wooden components.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial supports from Natural National Science Foundation of China (31700481), Cross-science Research Project of Nanyang Institute of Technology (presented by Yan Yang), Scientific Research Start-up Projects of Nanyang Institute of Technology (presented by Yan Yang and Wei Wang). He Sun and Yan Yang contributed equally to this work, and they are the first authors.

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Article submitted: April 27, 2020; Peer review completed: June 21, 2020; Revised version received and accepted: June 24, 2020; Published: July 2, 2020.

DOI: 10.15376/biores.15.3.6332-6343