Wenliang Wang , Haochun Zhang , Yigang Chen , Haifeng Shi ,3,＊

1 School of Science, Jiangnan University, Wuxi 214122, Jiangsu Province, China.

2 Department of General Surgery, The Affiliated Wuxi No. 2 People’s Hospital of Nanjing Medical University, Wuxi 214002, Jiangsu Province, China.

3 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China.

Abstract: Graphitic carbon nitride (g-C3N4) has been widely used as a potential photocatalytic material for the removal of tetracycline from water. However, the poor visible light absorption ability and high recombination rate of the photogenerated charge significantly inhibit the catalytic activity of g-C3N4.Therefore, facile methods to improve the photocatalytic efficiency of g-C3N4 need to be developed. Hematite (α-Fe2O3), which has a good visible light absorption and corrosion resistance, is often used for photocatalysis and photo-Fenton reactions. Therefore, a two-dimension/two-dimension (2D/2D) S-scheme heterojunction constructed of g-C3N4 and α-Fe2O3 nanosheets could be expected to improve the degradation efficiency of tetracycline. In this study, 2D/2D Sscheme α-Fe2O3/g-C3N4 photo-Fenton catalysts were prepared using a hydrothermal strategy. The photo-Fenton catalytic activity of α-Fe2O3/g-C3N4 (α-Fe2O3 50% (w)) was significantly improved by the addition of a small amount of H2O2,removing 78% of tetracycline within 20 min, which was approximately 3.5 and 5.8 times the removal achieved using α-Fe2O3 and g-C3N4, respectively. The high catalytic activity was attributed to the synergy between the photocatalysis and Fenton reaction promoted by the continuous Fe3+/Fe2+ conversion over the 2D/2D S-scheme heterojunction. The 2D/2D S-scheme heterojunction was crucial in the fabrication of the α-Fe2O3/g-C3N4 photocatalyst with a large surface area,adequate active sites, and strong oxidation-reduction capability. Furthermore, the photo-Fenton reaction provided additional hydroxyl radicals for the degradation of tetracycline with the aid of H2O2. The excess reaction product (Fe3+) was reduced to Fe2+ by the photogenerated electrons from the conduction band of α-Fe2O3. The resulting Fe2+ could participate in the photo-Fenton reaction. The morphological structures of α-Fe2O3/g-C3N4 were analyzed using transmission electron microscopy to demonstrate the formation of a 2D/2D structure with face-to-face contact, and the optical properties of the composites were measured using ultraviolet-visible diffuse reflectance spectroscopy. α-Fe2O3/g-C3N4 possessed a significantly improved visible light absorption compared to g-C3N4. Five sequential cyclic degradation tests and X-ray diffraction (XRD) patterns obtained before and after the reaction showed that the α-Fe2O3/g-C3N4 composites possessed stable photo-Fenton catalytic activity and crystal structures. Transient photocurrent responses of α-Fe2O3/g-C3N4 demonstrated that the prepared composites exhibited a higher charge transfer efficiency compared to that of single α-Fe2O3 and g-C3N4. In addition, according to the photoluminescence analysis and active species trapping experiments, a possible S-scheme heterojunction charge transfer process in the photo-Fenton catalytic reaction was proposed. This study provided a promising method for the construction of a high-performance photo-Fenton catalytic system to remove antibiotics from wastewater.

Key Words: Photocatalysis; Fenton reaction; Catalytic activity; S-scheme heterojunction; 2D/2D interface

1 Introduction

By view of the high toxicity, solubility, persistence, and carcinogenicity of persistent organic pollutants from industrial or urban areas in the aquatic ecosystem, the treatment of organic pollutants in water has become an urgent environmental problem1-3.As one of the most commonly used antibiotics in livestock and aquaculture, tetracycline is often detected in natural water4-6.Therefore, it is urgent to find a technique for efficient tetracycline degradation. It is reported that the traditional methods, such as physical adsorption and biodegradation, are not efficient to treat tetracycline due to the difficulties faced by adsorbents after treatment and the antibacterial properties of tetracycline. As a promising method, semiconductor photocatalysis technology can absorb optical energy to produce free radicals used for tetracycline decomposition in water7-20.Photocatalytic technology can degrade and mineralize organic pollutants into H2O and CO2under a mild condition21-31.Recently, g-C3N4received more and more in-depth research as a developed semiconductor because of its stability, special layered morphology, and favourable reduction properties. But for pure g-C3N4nanosheets, both the weak visible photoabsorption capacity and the rapid combination of photogenerated carriers in the photocatalytic reaction greatly inhibit the degradation efficiency of tetracycline32-38. Therefore, it is still a research hotspot to accelerate the carriers migration rates in g-C3N4photocatalysts.

To date, a variety of methods have been studied to regulate and modify g-C3N4, like precious metal modification, iondoping, and building heterojunction. In these methods, the construction of heterojunctionviacombining g-C3N4with other semiconductors is preferable because it can stagger the energy level arrangement the transfer of photogenerated carriers. At present, α-Fe2O3is widely used in composite photocatalytic material systems due to its corrosion resistance, low cost, and good visible light absorption capacity39-45. Suppose α-Fe2O3is used to combine with g-C3N4to achieve α-Fe2O3/g-C3N4composite. In that case, it will not only play a key role as a catalyst component in heightening visible light response, but also be a promising heterogeneous iron-based photo-Fenton catalyst46. Photo-Fenton reaction is a kind of Fenton reaction assisted by ultraviolet or visible irradiation, which significantly enhances activity in antibiotic degradation. As an important advanced oxidation process, the photo-Fenton reaction can oxidize Fe2+to Fe3+with the assistance of H2O2and light energy,and produce strong oxidation hydroxyl radical (·OH). The advantages of the Fenton reaction and the photocatalytic reaction were combined by designing the α-Fe2O3/g-C3N4photocatalysis-Fenton system. In the photocatalytic reaction, the photogenerated electrons of the semiconductor promote the circulation of iron ions in the Fenton reaction, thereby increasing the tetracycline degradation activities over α-Fe2O347-50.Nevertheless, the traditional α-Fe2O3nanomaterials possessed the disadvantages of poor dispersion and the lack of active sites,limiting the enhancement of catalytic activity. Consequently, it is still arduous to look for superior photo-Fenton catalysts with abundant active sitesviacontrolling the morphology of α-Fe2O3nanomaterials.

Recently, 2D/2D S-scheme heterojunction produces more active sites for improving the activity of catalysts. For one thing,the 2D/2D nanostructure possesses abundant catalytic active sites and enhanced light absorption capacity. For another, the 2D/2D nanostructure displays a larger contact interface in comparison to 2D/0D or 2D/1D nanostructures, which creates more transmission channels for photogenerated carriers51-57.Furthermore, in the S-scheme charge transfer model, the photogenerated carriers are spatially separated on the two catalysts, which prolongs the separation time of photogenerated carriers and the preserves a strong oxidation-reduction capability of the catalysts58,59. Yanget al.designed S-scheme α-Fe2O3/g-C3N4heterojunction with observably increased water disinfection efficiency60. Inspired by the above strategies, the significantly improved catalytic activities of α-Fe2O3/g-C3N4composites were due to the photo-Fenton catalytic system based on the 2D/2D S-scheme heterojunction charge transfer mechanism. The 2D/2D S-scheme heterojunction could effectively promote the transmission and separation of carriers,provide large specific surface area and rich active sites, and maintain the best redox capacity of the composites. In addition,the photocatalytic reaction promoted the reduction of Fe3+,which accelerated the production of hydroxyl radicals in the Fenton reaction61-66.

In the manuscript, a two-dimension S-scheme α-Fe2O3/g-C3N4was reported. The degradation experiment showed that the degradation efficiency of α-Fe2O3/g-C3N4(50 : 100) to tetracycline within 20 min was better than that of α-Fe2O3, g-C3N4, α-Fe2O3/g-C3N4(30 : 100) and α-Fe2O3/g-C3N4(70 : 100)α-Fe2O3/g-C3N4(50 : 100). The crystal structure, microstructure,elemental state, optical properties, and other properties of composites were wholly studied. In addition, the experimental results of PL analysis andI-tanalysis indicated that the formation of α-Fe2O3/g-C3N4heterojunction enhances the migration rates of carriers. The reusability of the catalyst was studied by circulating experiments. This work provided a valuable solution for removing tetracycline by designing and preparing a two-dimensional photo-Fenton catalyst.

2 Experimental and computational section

2.1 Materials

The materials contained C2H4N4, NH4Cl, Fe(NO3)3·9H2O,CH3COONa, absolute ethanol, H2O2, and deionized water. All chemical reagents were of analytical grade. These reagents were used for the experiments without further purification.

2.2 Synthesis methods

2.2.1 Synthesis of g-C3N4

2.2.2 Synthesis of α-Fe2O3/g-C3N4

0.87 g Fe(NO3)3·9H2O was evenly mixed with 60 mL absolute ethanol at room temperature for 30 min. 3.5 mL H2O was poured into the solution (Scheme 1). Afterwards, 0.3 g g-C3N4and 1.71 g CH3COONa were successively introduced in the previous solution. 60 min later, the processed solution was poured into the hydrothermal kettle and insulated for 24 h at 180 °C39. The resulting mixture was washed multiple times. In addition, under the same conditions, α-Fe2O3hexagonal nanosheets were prepared without adding g-C3N4. The obtained products were named FO/CN (30), FO/CN (50), and FO/CN (70) according to α-Fe2O3/g-C3N4with 30%, 50%, 70% mass ratios of α-Fe2O3.

2.3 Characterization

XRD served as the main tool to test the crystal structure of catalysts prepared at a speed of 3 (°)·min-1through a DX2700 diffractometer (Haoyuan Instrument, China). Ultravioletevisible diffuse reflectance spectroscopy (UV-Vis DRS) was gained with a UV-2600 spectrophotometer (Shimadzu, Japan).transmission electron microscopy (TEM) of FO, CN, FO/CN were obtained using FEI Tecnai G2 F20. Fourier transform infrared (FTIR) transmittance spectra of FO, CN, FO/CN were gained with a Nicolet iS5 spectrometer (Thermo Fisher Scientific, USA). X-ray photoelectron spectroscopy (XPS)measurements (ESCALAB 250xi, Thermo, USA) estimated the chemical states of the FO, CN, FO/CN.

2.4 Photo-Fenton catalytic activity

Scheme 1 Procedure for the preparation of single CN,single FO, and FO/CN.

The tetracycline (TC 10 mg·L-1) degradation tests with 300 W Xenon lamp were used to evaluate the activities of FO, CN,FO/CN (λ＞ 420 nm). The 50 mg samples were introduced into 100 mL of tetracycline solution and evenly mixed under shading for 1 h to get a balance of desorption and absorption before Fig.1 XRD patterns of single CN, single FO, FO/CN (30),FO/CN (50), and FO/CN (70).visible light exposure. Then, before providing light energy to the solution, pour 30 μL of H2O2solution (30%) was poured into the mixture. During the catalytic reaction, a certain amount of liquid was collected every five minutes. After centrifugation, the supernatant was taken to test the concentration of TC.

3 Results and discussion

3.1 Morphology and structure

XRD was deployed to exhibit the crystalline phase of the photocatalysts. According to Fig. 1, diffraction peaks at 13.2°and 27.4° could be well indexed as (100) and (002) crystal planes of CN, representing the in-plane structural packing motif of tris-triazine units and linear stacking of conjugated aromatic systems, respectively. The peaks at 24.1°, 33.2°, 35.6°, 40.9°,49.5°, 54.1°, 57.6°, 62.4°, and 64.0° of FO could be corresponded to (012), (104), (110), (113), (024), (116), (018),(214), and (300) crystal planes (JCPDS. No 33-0664)9,39. It was worth noting that the XRD pattern of FO/CN displayed two parts of peaks that were consistent with FO and CN, respectively,which indicated that the FO/CN heterostructures were successfully constructed.

The chemical compositions of FO, CN, and FO/CN were analyzed using FTIR spectroscopy. The peaks at 546 cm-1and 473 cm-1were regarded as the stretching vibration of Fe－O.For CN, the main absorption band between 3000 and 3500 cm-1could be ascribed to the band of N－H and the stretch of O－H,while the peaks between 1639 cm-1and 1238 cm-1corresponded to C－N stretching, tensile vibration modes of C－N heterocyclic ring and tensile vibration of C－NH－C connecting units (Fig. 2). The sharp band at 808 cm-1could correspond to striazine unit mode31,46,47. Obviously, the peaks of CN and FO coexisted within the FO/CN, manifesting that the FO/CN composites were successfully prepared.

Fig. 2 FT-IR spectra of CN, FO, FO/CN (30), FO/CN (50),and FO/CN (70).

The morphology and nanostructure of catalysts were studiedviaTEM and High Resolution Transmission Electron Microscope (HRTEM). The prepared CN nanosheets possessed a loose fold morphology, which indicated that the CN was 2D ultra-thin nanosheets (Fig. 3a). TEM image (Fig. 3b) showed that the FO possessed innumerable uniform hexagonal nanosheets with the size of about 100 nm. According to Fig. 3c,after FO and CN were combined, a 2D/2D structure with face-to-face contact was formed. This two-dimensional heterojunction structure with a large specific surface area could significantly improve the migration rates of photogenerated carriers and provide more abundant active sites. In the HRTEM(Fig. 3d), the crystal plane spacings were 0.27 nm and 0.37 nm,pointing to (104) and (012) lattice planes of FO, respectively67.Moreover, the Brunauer-Emmett-Teller (BET) surface areas of FO, CN, and FO/CN (50) were 19.31, 36.27, and 27.49 m2·g-1,respectively. The above data proved that FO/CN (50) possessed a larger specific surface area than FO.

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Fig. 3 TEM images of (a) CN, (b) FO, (c) FO/CN (50), and (d) HRTEM image of FO/CN (50).

Fig. 4 XPS spectra of (a) survey, (b) C 1s, (c) N 1s, (d) O 1s, and (e) Fe 2p.

The surface chemical states of FO, CN, and FO/CN (50) were further analyzed. X-ray photoelectron spectroscopy (XPS)survey spectrum indicated that FO/CN (50) contains C, N, O,and Fe elements (Fig. 4a). According to Fig. 4b, for pure CN, the binding energies of C 1sspectra at 288.1 eV and 284.5 eV could be deconvoluted into two peaks, which was identified as thesp2-bonded nitrogen in triazine rings (N―C＝N) and graphitic carbon (C―C) species, respectively. In N 1sspectra (Fig. 4c),three peaks in the samples could be found. The peaks at 404.6 eV, 400.9 eV, and 398.6 eV were well fitted by the N 1sspectra of CN. The main peaks could be regarded asπ-excitations,tertiary nitrogen C―(N)3groups, andsp2-hybridized N (C―N＝C) species. The O 1sspectra in FO were fitted into peaks at 531.9 eV and 529.0 eV, corresponding to the surface hydroxyl groups (OH) and lattice oxygen in Fe―O―Fe (Fig. 4d). In Fig.4e, two well-resolved peaks at 712.4 eV and 726.2 eV could be related to Fe 2p3/2and Fe 2p1/2, respectively, exhibiting the formation of FO9,68. Furthermore, O 1sand Fe 2pelement in FO/CN (50) exhibited palpable shifts in contrast to the FO sample, while C 1s, N 1selement in FO/CN (50) witnessed palpable shift contrast to the CN sample, proving the strong interaction between FO and CN in FO/CN (50).

The visible-light response capabilities of semiconductors were closely related to the degradation activity of TC. UV-Vis DRS were implemented to research the optical properties of FO,CN, FO/CN. The optical absorption edge of CN was located at 450 nm, while the FO had a visible absorption edge at 580 nm(Fig. 5). Base on the calculation ofEg= 1240/λ(eV), theEgof CN and FO are 2.76 and 2.10 eV31,50. In the calculation process,λwas the light absorption edge. The absorption edges of FO/CN(30), FO/CN (50), and FO/CN (70) were on the right of CN,indicating that the composite possessed better light absorption ability. In addition, when the content of FO increased gradually,the absorption range of FO/CN was further expanded.

3.2 Photo-Fenton catalytic activity under visible light

Fig. 5 UV-Vis spectra of CN, FO, FO/CN (30), FO/CN (50),and FO/CN (70).

Fig. 6 (a) Photo-Fenton catalytic degradation efficiencies of TC, (b) kinetic curves for the degradation of TC using different catalysts,(c) histogram of degradation rate after 20 min, (d) degradation experiments under other reaction conditions.

Fig. 7 (a) Successive cycles photo-degradation of TC. (b) XRD patterns of FO/CN (50) photocatalyst before and after reusing five times.

The decomposition rates of TC by the prepared catalysts were used as the standard to test the activities of composites and single component materials. After providing the light energy for the catalysts for 20 min, the decomposition rates of single FO and single CN in the presence of 30 μL H2O2on TC were only 57%and 47%, respectively, which was obviously not ideal. On the contrary, the photo-Fenton activities of FO/CN were effectively higher than that of FO and CN (Fig. 6a). The degradation efficiencies of TC by FO/CN (30), FO/CN (50), and FO/CN (70)in the presence of H2O2were 76%, 78%, and 72%, respectively.Furthermore, the experiments displayed that the activities of FO/CN improved with the augment of FO content. When the mass ratio of FO to CN reached 50 : 100, the activities of the composites hit a peak. Further increasing the mass ratio of FO in the composites would lead to the decrease of the catalytic activity of FO/CN69. Fig. 6b and 6c show the pseudo-secondorder model of the activities of single catalysts and FO/CN composites. The apparent rate constants of FO, CN, FO/CN (30),FO/CN (50), and FO/CN (70) were 0.203, 0.121, 0.434, 0.705,and 0.476 L·mg-1·min-1, respectively. Base on the above experimental results, the degradation efficiency of TC by FO/CN(50) in the photo-Fenton reaction was significantly higher than single catalysts, which was 3.5 times as the FO, and 5.8 times higher than the CN. According to Table S1 (in Supporting Information), compared with the degradation efficiency of TC by other catalysts, the activity of 2D/2D S-scheme FO/CN (50)heterojunction designed in this work was relatively satisfactory.The influences of light energy, H2O2, and catalysts on TC decomposition were discussed through degradation experiments under conditions without H2O2or light. Without the presence of H2O2, the degradation rate of TC over FO and FO/CN (50)showed a significant decrease, which proved that the Fenton reaction accelerated the decomposition of TC (Fig. 6d). The experimental results displayed that H2O2could hardly degrade TC without adding catalyst. In addition, degradation rates of TC by composites were still higher than those of FO and CN even without adding H2O2, which was because 2D/2D S-scheme heterojunction accelerated the transfer of charge and increased the active reaction sites on the catalyst. Under the condition of no light, the degradation efficiency of TC over FO/CN (50) only assisted by H2O2was poor, which proved that the activity of Fenton reaction was not ideal. According to Fig. 7a, FO/CN (50)was reused for five TC degradation experiments under the same reaction conditions. TC degradation rate of the composites remained unchanged, indicating that FO/CN (50) composite possessed good stability. Fig. 7b displays the XRD patterns of FO/CN (50) composite before and after photodegradation. It was not difficult to find that the XRD spectra of FO/CN (50)collected after multiple reactions were basically consistent with the samples not involved in the reaction, which proved that the crystal structure of FO/CN (50) was stable.

3.3 Photo-Fenton catalytic reaction mechanisms

Fig. 8 (a) Photoluminescence spectra for CN, and FO/CN (50). (b) Transient photocurrent responses for the FO, CN, and FO/CN (50).

In Fig. 8a, to investigate the charge transfer efficiency of composites, CN and FO/CN (50) were analyzed by photoluminescence (PL) spectroscopy. In contrast with pure FO and CN, the PL intensity of FO/CN (50) was significantly reduced, which displayed that FO/CN (50) possessed a faster charge transfer rate70. Since the distance and time of electron transmission were shortened, 2D/2D heterojunction was considered to be the main reason for the accelerated photogenerated charge transfer efficiency in composites. In addition, transient photocurrent responses (I-t) of FO, CN, and FO/CN (50) were performed to explore the charge excitation and separation in the samples. In Fig. 8b, it was not difficult to find that FO/CN exhibited significantly enhanced photocurrent compared with FO and CN, which displayed that FO/CN possessed more efficient charge transfer46.

It was well known that predominant active species were the keys during TC catalytic decomposition. In order to reveal the photocatalytic mechanism,tert-Butanol (BuOH), benzoquinone(BQ), and ethylenediaminetetraacetic acid (EDTA) were used to trap ·OH, ·O2-, and h+. The photodegradation rate of TC was 78%without adding trapping agent, while the BQ had a slight effect on the catalytic activity (60%) of FO/CN (50) (Fig. 9). The addition of EDTA and BuOH significantly reduced the catalytic activity of FO/CN (50), which was 25% and 38%, respectively.The experimental results showed that h+and ·OH made major contributions to the catalytic degradation of TC.

The following formula was utilized to obtain the conduction band (CB) and valence band (VB) of the prepared materials:

Fig. 9 Effect of different quencher agents on TC degradation by FO/CN (50) photocatalyst.

XandEe(4.5 eV) were the electronegativity and free electron energy, respectively. On the basis of previous experimental results, theEgof FO and CN were 2.10 eV and 2.76 eV,respectively. Hence, theEVB/ECBof FO and CN were 2.43/0.33 eV and 1.43/-1.33 eV, respectively9,42. In the traditional type-II heterojunction, FO/CN could not produce ·O2-, because the CB position of FO was more positive than the standard redox potential of E (O2/·O2-), while the existence of ·O2-has been testified by the test of scavenger. Consequently, it was not difficult to infer that the carrier migration path between FO/CN heterojunctions synthesized in this work conformed to the Sscheme charge transfer mechanism, which completely retained the redox performance of the catalyst.

Fig. 10 The reaction mechanism of TC degradation over FO/CN.

According to the above analysis results, the possible FO/CN photo-Fenton catalytic degradation mechanism was speculated in Fig. 10. Both FO and CN could absorb light energy. The photogenerated electrons on VB of FO and CN were transported to CB, and the holes on VB still existed. With the contact between CN and FO, a built-in electric field directed from CN to FO was formed at the interface of CN and FO. The relatively useless e-in FO and the relatively useless h+in CN combine rapidly driven by the built-in electric field, which effectively prolonged the separation time of e-and h+in the FO/CN. The eon the CB of CN reduced oxygen to ·O2-which could decompose TC. Similarly, the h+in the VB of FO oxidized H2O to ·OH with strong oxidizing. Therefore, the construction of S-scheme heterojunction promoted the separation efficiency of photogenerated carriers in the photo-Fenton catalytic reaction,and retained the strong reduction ability of CN and the strong oxidation ability of FO. In addition, the part photogenerated electrons from FO could promote the photo-Fenton catalytic reaction by participating in the Fe3+/Fe2+cycle process, and activating H2O2to produce ·OH, which improved the degradation efficiency of TC over FO/CN60,42. Since the oxidation process of ferrous ions generated hydroxyl radicals with strong oxidizing properties, the photo-Fenton reaction possessed higher catalytic activity in degrading TC than the pure photocatalytic reaction47. Furthermore, the addition of H2O2, as the key to triggering the photo-Fenton reaction, could maintain the continuous conversion of Fe3+/Fe2+. In the present study, only traces of H2O2were needed to enhance the catalytic activity of FO/CN, which reduced the cost of using H2O268. Furthermore,the significant improvement in the catalytic activity of FO/CN also benefited from the 2D/2D structures in face-to-face contact,which shortened the transmission distance of charges and provided abundant catalytic active reaction sites.

4 Conclusions

To sum up, 2D/2D S-scheme FO/CN photo-Fenton catalysts with different mass fractions of FO were hydrothermally synthesized at 180 °C. The characterization results of the morphology and physicochemical properties of FO/CN proved that FO nanosheets and CN nanosheets were successfully compounded to form 2D/2D S-system heterojunction composites. The activity test results showed that due to the synergistic effect of photocatalytic reaction and Fenton reaction,FO/CN displayed a significantly higher catalytic activity than simple materials and FO/CN composites without H2O2assistance, FO/CN (50) catalysts also displayed high stability and outstanding activity. The 2D/2D S-scheme heterojunction not only reduced the charge transport time and distance,provided ample active reaction sites, but also maintained good redox ability. In addition, the fast Fe3+/Fe2+conversion assisted by H2O2promoted the generation of ·OH, which also significantly improved the degradation efficiency. Consequently,this work provided a feasible blue print for designing other 2D/2D S-scheme photo-Fenton catalysts.

Supporting Information:available free of chargeviathe internet at http://www.whxb.pku.edu.cn.