左會文　陸春?！∪斡駱s　李　奕　章永凡　陳文凱,4,5,*

(1福州大學(xué)化學(xué)系，福州350116；2成都理工大學(xué)核技術(shù)與自動化工程學(xué)院，成都610059；3常州大學(xué)材料與工程學(xué)院，江蘇常州213164；4福建省理論與計(jì)算化學(xué)重點(diǎn)實(shí)驗(yàn)室，福建廈門361005；5福州大學(xué)，能源與環(huán)境光催化國家重點(diǎn)實(shí)驗(yàn)室，福州350002)

單層石墨相氮化碳負(fù)載Pt4團(tuán)簇吸附O2的第一性理論研究

左會文1陸春海2任玉榮3李奕1章永凡1陳文凱1,4,5,*

(1福州大學(xué)化學(xué)系，福州350116；2成都理工大學(xué)核技術(shù)與自動化工程學(xué)院，成都610059；3常州大學(xué)材料與工程學(xué)院，江蘇常州213164；4福建省理論與計(jì)算化學(xué)重點(diǎn)實(shí)驗(yàn)室，福建廈門361005；5福州大學(xué)，能源與環(huán)境光催化國家重點(diǎn)實(shí)驗(yàn)室，福州350002)

采用第一性原理密度泛函理論結(jié)合周期性平板模型模擬研究了Pt4團(tuán)簇吸附單層石墨相氮化碳(g-C3N4)的幾何結(jié)構(gòu)和電子性質(zhì)，以及氧氣在其表面上的吸附行為。同時(shí)，對比分析了氧氣在純凈的石墨相氮化碳和Pt4團(tuán)簇上的吸附行為。計(jì)算結(jié)果表明，Pt4團(tuán)簇吸附在3-s-三嗪環(huán)石墨相氮化碳表面，并與四個(gè)邊緣氮原子成鍵,形成兩個(gè)六元環(huán)時(shí)為最穩(wěn)定構(gòu)型。Pt4團(tuán)簇傾向于吸附在三嗪環(huán)石墨相氮化碳的空位并與鄰近三個(gè)氮原子成鍵。由于Pt與N原子較強(qiáng)的雜化作用，以及金屬與底物之間較多電子轉(zhuǎn)移增強(qiáng)了Pt4團(tuán)簇吸附g-C3N4的穩(wěn)定性。另外，對比分析了氧氣在純凈的g-C3N4和金屬吸附的g-C3N4上吸附行為，發(fā)現(xiàn)金屬原子的加入促進(jìn)了電子轉(zhuǎn)移，同時(shí)拉長了O―O鍵長。Pt4吸附3-s-三嗪環(huán)g-C3N4比Pt4吸附三嗪環(huán)g-C3N4表現(xiàn)出微弱的優(yōu)勢，表現(xiàn)出明顯的基底扭曲以及較大的吸附能。這些結(jié)果表明，化學(xué)吸附通過調(diào)節(jié)電子結(jié)構(gòu)和表面性質(zhì)增強(qiáng)催化性能的較好方法。

石墨相氮化碳；鉑簇；氧氣分子；吸附；光催化劑；密度泛函理論

1　Introduction

The catalytic activity of metal particles embedding has attracted broad attention due to the excellent electronic and catalytic applications,such as Pt1,2,Cu3,Au4,5,Fe6,7supported on graphene or graphene oxides,which exhibit high catalytic activity for CO oxidation.To find out highly active materials with lower cost is our main purpose.Furthermore,in recent years,another graphenelike material,g-C3N4,has been regarded as one of the most promising candidates for various applications due to their exceptional physical and chemical properties8－11.In particular,the tri-s-triazine ring structure and high condensation make the polymer exhibit super thermal and chemistry stability under ambient conditions because of the strong covalent coupling between carbon and nitrogen atoms.In addition,g-C3N4is made up of layered structure which can be synthesized by facile thermodynamics methods from affordable precursors such as melamine,1, 3,5-trichlorotriazine(C3N3Cl3),and NH4Cl12－15,suggesting that it is easy and inexpensive to obtain these materials.What′s more,the band gap of g-C3N4is around 2.7 eV according to experimental reports11,12,16,17,and it can be acted as an electron-rich non-toxic organic semiconductor material.These traits of g-C3N4have attracted intense interest and potential applications in photochemical reduction of CO218－21,photodecomposition of dye22,23,hydrogen production by water splitting17,24－26,oxygen reduction27－31,NO decomposition32and so on,have been reported.However,the catalytic performance of intrinsic g-C3N4remains unsatisfactory because of the low surface areas.So,some strategies were made to modify g-C3N4for enhanced catalytic performance.Wang et al.9have reported that introduction of mesoporous can enhance the light harvesting ability of the material due to its large specific surface area and multiple scattering effects.Moreover,nonmetallic doping of g-C3N4is not only an effective strategy to modify its electronic structure and surface properties but improve the electroconductibility and the photocatalytic activity.For instance,Ma et al.33reported that the doping with nonmetal impurities facilitates the increase of visible-light adsorption and alters the connectivity patter and topology of g-C3N4sheet.Finally,coupling with heterogeneous semiconductors and metal doped of g-C3N4could enhance the light absorbance in the range of UV-Vis region and improve the catalytic activity because of the high specific surface area,the good structural stability,and the highly dispersed metal nanoparticles.Mansor et al.31reported that Pt-doped g-C3N4exhibit better electrochemically stability and higher methanol oxidation reaction activity compared with conventional carbon black. However,reports on the application of Pt cluster supported on g-C3N4sheets in electrocatalytic oxygen reduction in proton exchange membrane fuel cells and other electrochemical application are rare and no theoretical calculation has been done to predict its unexplored oxidation capacity.

On the basis of the above consideration,studies related towards understanding the microscopic mechanism behind catalytic activity in metal deposited on g-C3N4are deemed as important. Hence,the structure,formation energy,band gap,charge population separation,and density of states(DOS)were discussed in detail.In this work,we present an extensive theoretical investigation of the geometries and electronic structure of the pure and metal atom deposited on TGCN and HGCN by using density functional theory(DFT).Besides,in order to better understand the interaction between O2molecule and the substrate,we should illustrate the behavior of the oxygen adsorption on Pt cluster primarily.In this article,our main purpose is to investigate the structure and electronic properties of Pt4nanoparticles adsorbed on the two species of g-C3N4and the interaction of metal atom with substrate.

2　Computational method

The spin-unrestricted was performed in this work using the Dmol3package to calculate the electronic structure34,geometry structure,and the properties of bare and Pt cluster supported g-C3N4.The calculations were done by Perdew-Wang(PW-91) exchange-correlation functional within the generalized gradient approximation(GGA)35.In this study,double-numerical basis with polarization functions basis set(DNP)were represented with the effective core potential(ECP).During the computation,the C,N, and O atoms were performed using the all electron basis set,while the ECP was chosen for relativistic effect when Pt metal was included in the systems.In the optimized process,the vacuum spaces of 1.8 nm was set for the g-C3N4monolayer to avoid any interaction of the layers with its periodic images.Since GGA underestimes the band gap of a semiconductor36,we re-examinedthe band structure of g-C3N4by using the HSE06 hybrid density functional37,38in CASTEP,which has been reported to be more accurate than GGAin predicting the electronic structure of g-C3N4.

To determine the stable graphitic carbon nitride,two types of networks with different connection patterns are studied in this work:one is heptazine structural(C6N7)motified with hexagonal system(denoted as heptazine/g-C3N4),and the other is triazine (C3N3)structure unit(denoted as s-triazine/g-C3N4),as show in Fig.1.To explore this feature as well as its structural and electronic implications for metal atom supported and oxygen molecules adsorption on it,a 2×2×1 monolayer HGCNand a 3×3×1 single layer TGCNwere introduced as the models for extended g-C3N4sheets and used in all of the theory calculations.The Brillouin zone integration was performed with 9×9×1 k-point sampling.For geometric optimization,the convergence tolerance energy was 1×10－5hartree,the maximum force converged was less than 2×10－2hartree?nm－1and the maximum displacement convergence was 5×10－4nm.Charge transfers were calculated with the Mulliken charge analysis method.To study the stability of Pt4nanoparticles on two kinds of g-C3N4surface,the adsorption energy of the system was calculated using the following formula:

where Eadsis the adsorption energy of the Pt4cluster adsorbed g-C3N4system,Etotalis the total energy of the system,Eg-C3N4is the total energy of bare g-C3N4，and Eadsorbateis the total energy of free Pt4cluster in gas phase,respectively.The negative adsorption energy indicates that the reaction is exothermic(stable),and the positive value denotes an endothermic(unstable)reaction.

Fig.1　Two types of chemical structures of g-C3N4after optimization(a)heptazine phase,(b)s-triazine phase.Gray and blue spheres represent the carbon and nitrogen atoms,respectively,and different carbon and nitrogen atoms are labeled here.dringand dbridgerepresent the average C―N bond lengths of adsorption system.color online

Table 1　Cell structure parameters and average C―N bond length of HGCN and TGCN

3　Results and discussion

3.1Geometric structures and electronic properties of the bare g-C3N4

In this paper,two kinds of metal-free g-C3N4sheets were explored to understand their structures and electronic properties.One of the metal-free monolayer g-C3N4sheet is constructed with three fused tri-s-triazine rings connected by nitrogen atoms,referred to as heptazine/g-C3N4(Fig.1(a)),and the other one is constructed with a hexagonal unit cell,denoted as s-triazine/g-C3N4(Fig.1(b)). The optimized lattice parameters and other experimental values of these two types of g-C3N4are listed in Table 1.For HGCNunit cell,the calculated distance in-planner between two nitrogen pores is 0.712 nm,which is consistent with the other theory result(0.713 nm)33,39.In addition,Bojdys et al.40reported that the co-planar arrangement distance between two nitrogen pores at the center of adjacent heptazine rings would be 0.730 nm by high resolution transmission electron microscopy(TEM).From our calculation, the inter-planar distance is 0.319 nm,good agree with the values reported by Ma et al.33Furthermore,single layer heptazine based of g-C3N4sheet containsthree species of N atoms(labeled as N1, N2,N3)sites,two types of C atoms(labeled as C1,C2)sites (labeled in(Fig.1(a)).For TGCN unit cell,its contains 14 atoms and three-coordinated nitrogen atoms connecting the triazine rings,as well as two types of N atoms(denoted as N1,N2)and one type of C atom.The optimized lattice parameters of TGCN are a=0.476 nm and c=0.669 nm,which is in good agreement with experimental values(a=0.474 nm,c=0.672 nm)41and calculated values(a=0.475 nm,c=0.659 nm)42.

Since HGCN and TGCN have the same building blocks,a futher calculation of the bond length is shown in Fig.1.The dringand dbridgemark two varieties of C―N bonds,dringrepresents bond length in the aromatic rings and dbridgerepresents bond length of the bonds that connect the rings as bridges.The optimized bond length of dringand dbridgeare 0.133 and 0.148 nm in the HGCN,which is slightly larger than those bond lengths in the TGCN(0.132 and 0.145 nm)because of less π-conjugated rings.Moreover,the heptazine ring contains a nitrogen atom with sp2hybridization in the center,thus making it the most stable phase.

Table 2　Calculated band gaps(eV)in this paper and other theoretical or experimental band gaps of HGCN and TGCN

Fig.2　(a)Band structure and(b)density of states of pure HGCNThe Fermi level is set to zero.

Table 2 shows the band gaps of HGCN and TGCN calculated by DFT-GGA and hybrid functional HSE06.It has been widly known that the standard DFT based on the GGAunderestimes the band gap of a semiconductor36.Fig.2(a)shows that HGCN has an indirect gap with the maximum of the valence band(VBM)at the G point and the minimum of the conduction band(CBM)at the M point.The minimum band gap by DFT-GGAis 1.25 eV,which is consistent with other previous calculation reported43,44.We also calculated the band structure by the HSE06 hybrid functional, which shows that the band gap is 2.68 eV,closer to the experimental resultes estimated by UV-Vis spectra11,16.Interestingly, different from the HGCN,the TGCN is a direct band gap semiconductor with minimum value at the G point,shown in Fig.3(a). We obtain the GGA band gap of 1.50 eV closer to 1.48 eV from Mattesini et al.45.From the HSE06 hybrid functional,the band gap is 2.88 eV,which is about 0.22 eV smaller than the experimental result(3.10 eV)46.The results above show that HSE06 hybrid method gives larger band gap energy compared to the DFT-GGA. Nevertheless,the trend for the change of band gaps by DFT-GGA is consistent with those by HSE06 hybrid method.

In order to clearly analyze the chemical bonding characteristic, the partially density of states(PDOS)of HGCNand TGCN by DFT-GGA function are described in Fig.2(b)and Fig.3(b).In general,the Fermi level(Ef)is localized at the VBM.Fig.2(b) shows that the energy band states near the Efare mainly occupied by electrons in 2p orbital of N2 atoms and also have some contribution from the N3 atoms.In addition,C atoms have little contribution to the valance band(VB),which presents the little hybridization with the adjacen N atoms.The conduction band (CB)mainly consists of 2p states of N3 and C atoms.And,C atom has a somewhat large contribution to the CB and is strongly hybridized with the N3 atom.For TGCN,the VBM is mainly composed of N2-2p atomic orbital,and the CBM is mainly dominated by the 2p electrons of the N2 and C atoms.Additionally,despite the N1 atoms contribution to either the VBM or CBM is insignificant,its occupied states are mainly located－2.00 eV below the Efand unoccupied states are located 3.75 eV above the Ef.Our calculations suggest that electron transition from the valence band populated by N-2p orbitals to the conduction band formed by C-2p orbitals makes both HGCN and TGCN have visible-light-respones as photocatalysts.And these photocatalysts contributes to the visible-light-driven O2reduction.

Fig.3　(a)Band structure and(b)density of states of pure TGCNThe Fermi level is set to zero.

3.2Interaction of Pt4cluster on g-C3N4

In this paper,two kinds of metal-free single-layered g-C3N4sheet were introduced as a model to investigate the interaction of Pt cluster with substrate.For Pt4cluster adsorbed on HGCN sheet, among many initial structures,three types of relatively stable optimized configurations are chosen depended on the number of nanoparticles that adsorb on substrate.The optimized configurations and the corresponding geometric parameters are indicated in Fig.4 and Table 3,respectively.It is gratifying that two nanoparticles interact with four edge nitride atoms at the cavity site and form two hexagonal rings,showing exceptional stability compared to other two modes where one or three nanoparticle atoms placed the cavity site.The adsorption energy for the most stable structure is found to be－5.18 eV,which is 2.17 and 0.33 eV lower than the other two adsorption configurations,respectively.Upon adsorption,the initial planar surface of g-C3N4shows an obvious deformation,in which the distance of nitrogen atoms to the nearest Pt atom are found to move up slightly and the structure of substrate becomes corrugated shape.The average dringbond length for the Pt4/HGCN is elongated to 0.138 nm as compared to the HGCNbond length(0.133 nm).And the dbridgeis 0.148 nm,showing the metal deposition has little influence for dbridgebond length.The Mulliken charge analyses show there is a main tendency charge transfers from Pt4cluster to HGCN substrate.For the most stable structure,the amount of Mulliken charge donated by the two Pt atoms on the HGCN surface(0.23e)is large than that one Pt atom on surface(0.12e),and less than that three Pt atoms on surface(0.40e).Therefore,the charge transfer is in proportion to the increasing number of Pt atoms that interact with the substrate structure.For Pt4/TGCN,among all the optimized structures, we found that one of Pt atoms bonding at the vacancy site and other Pt atoms without connecting to the TGCN surface is the most stable structure,as shown in Fig.4(d).The adsorption energy of Pt4/TGCN is－5.38 eV,slightly larger that of the Pt4/HGCN. The adsorbed planar configuration of TGCN is distorted and strongly reconstructed.The average dringand dbridgebond lengths are 0.138 and 0.148 nm,respectively,clearly more elongate than bare TGCN.The shortest distance between Pt4cluster and g-C3N4becomes 0.131 nm and the average Pt―Pt bond length becomes 0.267 nm.Meanwhile,there is about 0.25e charge transferring from metal to substrate according to the Mulliken charge population analyses.

Table 3　Calculation results of the relative stabilities and adsorption energies associated with the various configurations

Fig.4　Several different adsorption modes of nanoparticles(NPs)on monolayer g-C3N4Adsorption configurations are represented by the number of nanoparticle atoms that adsorb on HGCN. (a)one atom,(b)two atoms,(c)three atoms,(d)Pt4cluster adsorption on TGCN

Fig.5　Partial density of states(PDOS)for Pt4cluster adsorption on HGCN surface

Fig.6　Partial density of states for Pt4cluster adsorption on TGCN surface

To better understand the details of the interaction between the Pt4nanoparticles and monolayer graphitic carbon nitride system, the spin-polarized local density of states(PDOS)are computed and shown in Fig.5 and Fig.6,respectively.For Pt4/HGCN,the VBM is dominated by Pd-5d and has partial contribution from 2p electrons of N atoms,with CBM by the Pt-5d electrons.Besides, compared with PDOS of bare HGCN,the Pt4cluster adsorption should induce a downshifting of the conduction band edge.Fig.5 (a)shows that much stronger hybridization is observed around －7.0 eV,－5.5 eV,and－4.5 eV to－1.0 eV energy band zone between 2p orbitals of N and 5d orbitals of Pt,as well as from 0 to 2.0 eV above Ef.Fig.5(b,c)clearly shows that there is small hybridization between the 6s orbitals of Pt and 2p orbital of N,6p orbitals of Pt and 2s orbitals of N around－7.0,－5.5,－4.5,and1.0 eV.In addition,the most of N-2s orbitals located at lower level and minor hybridization around Efwas observed between N-2s and Pt-6p orbitals.The strong hybridization of Pt―N bonds as well as the charge transfer from Pt to substrate makes Pt4cluster on HGCN better binding.For TGCN adsorbated system,due to the formation of Pt―N bonds,the 6s,6p,and 5d orbitals of Pt are partially occupied.Meanwhile,the height of Pt-5d peak is reduced and splited into four sharp peaks,a widen peak below the Ef.The stronger hybridization between N-2p states and Pt-5d states was observed which is similar to HGCN adsorbate.In brief,both of systems illustrate the stronger d-orbital characteristics of the Pt atom.

Fig.7　Side and top views of the most energetically favorable configuration of O2adsorbed on the pure(a)HGCNand(b)TGCN,(c)Pt4clusterThe bond distances are in nanometer.

Fig.8　The most favorable configurations of O2adsorbed on g-C3N4supported by Pt4nanoparticles(a)HGCN and(b)TGCNThe bond distances are given in nanometer.

Table 4　Calculation results of adsorption energy Eads,Mulliken charge,and do―ofor O2molecule adsorption on Pt4/HGCN and Pt4/TGCN substrate

3.3Adsorption of O2on Pt4/g-C3N4

We select two most favorable g-C3N4-supported by Pt cluster, pure g-C3N4and free Pt cluster as substrates to interact with oxygen molecules.All possible adsorption sites were examined to find configurations with relative lower adsorption energy.The final optimized structures are listed in Fig.7 and Fig.8.The most stable structure of O2adsorption on HGCN is that the O―O bond is parallel to the surface,with the adsorption energy of－0.59 eV, as shown in Fig.7(a).Meanwhile,there is the separation distance of 0.216 nm away from the O2to the substrate.Similar to the case of TGCN,the O―O bond is parallel to the surface with a distance of 0.231 nm,and the adsorption energy of－0.65 eV.The calculated results show that the interaction of O2molecules with both of pure g-C3N4is relatively low and the O―O bond length is less changed compared to its gas-phase value of 0.122 nm.What′s more,we also found that the O2molecule gains very small amount of electron charges from the surface,as shown in Table 4.Subsequently,we take account of O2adsorption on the pure Pt cluster. The optimized structures of O2adsorbed on the clusters in their ground states are shown in Fig.7(c).The results show that the O―O bond is parallel to the Pt―Pt bond and oxygen atoms are bonding with platinum atoms to form a four-member ring structure.The adsorption energy of O2on the Pt cluster is－1.14 eV.

The g-C3N4supported by Pt cluster has a remarkable change in the electronic structures compared to the pure g-C3N4sheet and free Pt cluster.And this change makes great effect on the adsorption of O2molecule on it.For the Pt4/HGCN surface,O2molecule prefers to bind with the Pt4model in a bridge configuration with an Eadsof－1.69 eV,the Eadspresented here are calculated for the lowest-energy states we found after testing variousadsorption sites,as shown in Fig.8(a)and Table 4.Interesting,the O―O bond length(0.139 nm)is significantly elongated from that free gas phase molecule.The elongation of the O―O bond is due to electrons populated to the antibonding 2π*orbital of O2molecule mainly from metal cluster.This also means that the O2molecule will be easy to be dissociated in the reaction.The Mulliken charge population analysis shows that the electrons are transferred to the O2molecule from substrate,due to the relatively large electronegativity of oxygen.On Pt4/TGCN,the Eadsis slightly less than that of Pt4/HGCN system,resulting in the less distortion. Lim and Wilcox47pointed out that stronger adsorption leads to a greater distortion in adsorbate-substrate system.According to Mulliken charge population analyses,the oxygen molecule gains 0.42e from the Pt4/TGCN substrate.The accumulated charge of O2adsorption on Pt4/HGCN and Pt4/TGCN is larger than that of free Pt cluster,as shown in Table 4.It is obvious that the existing of the g-C3N4promotes charge transfer and enlarges the total negative charge of the oxygen molecule.Moreover,the negative charged O2molecule plays a positive role in the oxygen reduction reaction. In conclusion,the interactions of oxygen molecule with Pt4/HGCN or Pt4/TGCN are enhanced than that with free Pt cluster and bare g-C3N4.

Fig.9　The corresponding PDOS of O2adsorbed on g-C3N4supported by Pt4nanoparticles(a)HGCN and(b)TGCNSolid and dashed curves represent the Pt-5d and O2-2p orbits,respectively.

In order to gain some insight into the detail of the interaction between oxygen molecules and the two kinds of g-C3N4supported by Pt nanoparticles,the PDOS plots for the adsorption systems are shown in Fig.9.Observing the PDOS overlap of orbital,we found that the bonds formation between O atoms with Pt4/HGCN mainly due to Pt-5d states and O-2p states.Meanwhile,Pt-5d states are splited into a few sharp peaks and weak peak below the Eflevel. The sililar phenomenon is observed in PDOS of Pt4/TGCN. Hence,the enhanced bonding interaction of Pt-5d and O-2p states improves the ability of O2molecular adsorption on Pt4/g-C3N4.

4　Conclusions

By means of DFT calculation,we frist investigated the structural and electronic properties of Pt cluster supported on HGCN and TGCN sheet,then analyzed the adsorption of oxygen molecule on pure g-C3N4,free Pt cluster,Pt4/HGCN and Pt4/TGCN sheet.The results show that the electronic structure of the Pt4nanoparticles can be effectively modified due to the influence of different g-C3N4substrates,causing different effects on the catalytic activities.For the HGCN sheet,the calculated formation energies for different adsorption modes indicate that Pt cluster prefers to bond with four edge N atoms on HGCN surface.In this structure,there is the relatively strong adsorption energy of－5.18 eV due to the strong hybridization of Pt-5d with N-2p orbitals. Meanwhile,there is about 0.23e charge transferring from Pt cluster to HGCN sheet according to the Mulliken charge population analysis.For Pt4nanoparticles adsorbed on TGCN sheet, one Pt atom prefers to bond with three nitride atoms at the vacancy site showing exceptional stability,and the corresponding Ebis－5.38 eV.Compared to oxygen molecule adsorption on free Pt cluster and bare g-C3N4,both Pt4/HGCN and Pt4/TGCN increase the electrons flowing from substrates to oxygen molecules and elongate the O―O bond length.In addition,the interactions of oxygen molecule with Pt4/HGCN or Pt4/TGCN also have been strengthened correspondingly.

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Pt4Clusters Supported on Monolayer Graphitic Carbon Nitride Sheets for Oxygen Adsorption:A First-Principles Study

ZUO Hui-Wen1LU Chun-Hai2REN Yu-Rong3LI Yi1ZHANG Yong-Fan1CHEN Wen-Kai1,4,5,*
(1Department of Chemistry,Fuzhou University,Fuzhou 350116,P.R.China;2College of Nuclear Technology and Automation Engineering,Chengdu University of Technology,Chengdu 610059,P.R.China;3School of Marterials Science and Engineering,Changzhou University,Changzhou 213164,Jiangsu Province,P.R.China;4Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry,Xiamen 361005,Fujian Province,P.R.China;5State Key Laboratory of Photocatalysis on Energy and Environment,Fuzhou University,Fuzhou 350002,P.R.China)

The structural and electronic properties of Pt4nanoparticles adsorbed on monolayer graphitic carbon nitride(Pt4/g-C3N4),as well as the adsorption behavior of oxygen molecules on the Pt4/g-C3N4surface have been investigated through first-principles density-functional theory(DFT)calculations with the generalized gradient approximation(GGA).The interaction of the oxygen molecules with the bare g-C3N4and the Pt4clusters was also calculated for comparison.Our calculations show that Pt nanoparticles prefer to bond with four edge N atoms on heptazine phase g-C3N4(HGCN)surfaces,forming two hexagonal rings.For s-triazine phase g-C3N4(TGCN)surfaces,Pt nanoparticles prefer to sit atop the single vacancy site,forming three bonds with the nearest nitrogen atoms.Stronger hybridization of the Pt nanoparticles with the sp2dangling bonds of neighboring nitrogenatoms leads to the Pt4clusters strongly binding on both types of g-C3N4surface.In addition,the results from Mulliken charge population analyses suggest that there are electrons flowing from the Pt clusters to g-C3N4. According to the comparative analyses of the O2adsorbed on the Pt4/HGCN,Pt4/TGCN,and pure g-C3N4systems,the presence of metal clusters promotes greater electron transfer to oxygen molecules and elongates the O―O bond.Meanwhile,its greater adsorbate-substrate distortion and large adsorption energy render the Pt4/HGCN system slightly superior to the Pt4/TGCN system in catalytic performance.The results validate that being supported on g-C3N4may be a good way to modify the electronic structure of materials and their surface properties improve their catalytic performance.

November 30,2015;Revised:March 1,2016;Published on Web:March 3,2016.

Graphitic carbon nitride;Pt cluster;Oxygen molecule;Adsorption;Photocatalyst; Density functional theory

O641

10.3866/PKU.WHXB201603032

*Corresponding author.Email:wkchen@fzu.edu.cn;Tel:+86-591-22866162.

The project was supported by the National Natural Science Foundation of China(21203227,51574090).國家自然科學(xué)基金(21203227,51574090)資助項(xiàng)目