Charge Transfer between Triphenyl Phosphine and Colloidal Silver: A

May 27, 2007 - the red shift of CtN stretching frequency is found to increase with increasing the surface coverage of PPh3. This could be explained in...
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J. Phys. Chem. C 2007, 111, 8632-8637

Charge Transfer between Triphenyl Phosphine and Colloidal Silver: A SERS Study Combined with DFT Calculations Gengshen Hu, Zhaochi Feng, Difei Han, Jun Li, Guoqing Jia, Jianying Shi, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ReceiVed: January 17, 2007; In Final Form: April 17, 2007

The adsorption of triphenyl phosphine (PPh3) on colloidal silver has been investigated by means of surfaceenhanced Raman spectroscopy (SERS). On the basis of surface selection rule, it is deduced from SERS results that PPh3 adsorbs on silver surface via its P atom with three phenyl rings tilted with respect to the silver surface. The electron-donor effect of PPh3 can be sensitively probed by the coadsorbed SCN-. The Raman frequency of νCN of the adsorbed SCN- shifts to lower frequencies when PPh3 is coadsorbed with SCN-, and the red shift of CtN stretching frequency is found to increase with increasing the surface coverage of PPh3. This could be explained in terms of the electron-donor effect of PPh3: PPh3 adsorbed on silver surface donates its lone pair of electrons to silver surface, and then the negative charge of silver surface transfers to the π* orbital of CtN bond via S-Ag bond. Consequently, the CtN bond is weakened, and the frequency of νCN shifts to lower frequencies. Density functional theory (DFT) calculations further confirm the experimental results that the charge transfer is from PPh3 to silver surface rather than reversely. The information obtained from the adsorption of PPh3 on silver by SERS may be helpful to understand the mechanism of heterogeneous catalysis involving phosphine ligands coordinated on transition-metal surfaces.

1. Introduction Phosphine ligands, which are able to tune the electron density1 of the metal center of catalysts, are widely used in homogeneous catalysis.2-5 The properties of phosphine ligands, such as coordination ability with metal center, the steric effect, and the electron donor-acceptor ability, can dramatically affect the catalytic activity and selectivity.6-8 The electron donor-acceptor properties of phosphine ligands in homogeneous catalysis system have been studied on the basis of the correlation of IR frequencies of the co-coordinated CO.1,9 More recently, Rh/SiO2 catalyst modified with PPh3, one of the most widely used phosphine ligands, was used in heterogeneous hydroformylation of olefin.10-13 Compared with the conventional homogeneous hydroformylation, the heterogeneous catalytic process has several merits, for example, the catalyst can be easily separated from the reaction system and can be recyclable. Also, these merits will lead to the possibility for the application of the catalyst in industry. After the modification with PPh3, the Rh/SiO2 catalyst shows higher activity and selectivity than that of unmodified catalyst. This phenomenon implies that PPh3 ligand plays a significant role in enhancing the catalytic activity and selectivity of the Rh/SiO2 catalyst. However, the exact role of PPh3 is not clear. Furthermore, phosphine ligands have been used as stabilizers in the preparation of nanoparticles in recent years. For examples, BINAP was used to stabilize gold14 nanoparticles and PPh3 was served as a stabilizer in the syntheses of copper,15 gold,16 and palladium17 nanoparticles. However, up to now, the interaction of phosphine ligand with metal surface has rarely been investigated,18-20 and it is still unknown why phosphine ligands can prevent the aggregation of metal nanoparticles. * To whom correspondence should be addressed. E-mail: [email protected].

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique that is able to detect molecules at trace concentration levels even at single-molecule21 level and to quench the fluorescence background by radiationless energy transfer between metal surface and adsorbed species.22 It can provide useful information about the adsorption orientation of molecule on metal surface and the interaction between adsorbate and metal surface including the coordination information. The metal surface modified by PPh3 could be conveniently studied by surface-enhanced Raman scattering spectroscopy with the aid of enormous enhancement. In the present work, the interaction between PPh3 and metal surface has been investigated, using silver as the model metal, by means of surfaceenhanced Raman spectroscopy and theoretical calculations. The information obtained will be helpful to understand the mechanism of hydroformylation involving the PPh3 ligand. 2. Experimental Section 2.1. Chemicals. Silver nitrate, hydroxylamine hydrochloride, and sodium hydroxide were purchased from Sigma-Aldrich. NaSCN was obtained from Merck. Triphenyl phosphine (PPh3) was purchased from Fluka. Ethanol was obtained from Beijing J&K Chemical Ltd. for the preparation of triphenyl phosphine solution. All the chemicals are of analytical grade and were used as received. Tridistilled water was used for the preparation of silver colloids. 2.2. Preparation of Colloidal Silver. The colloidal silver was prepared by reducing AgNO3 with hydroxylamine hydrochloride23 at room temperature. A 225 mL mixture solution of hydroxylamine hydrochloride (1.67 × 10-3 M) and sodium hydroxide (3.33 × 10-3 M) was vigorously stirred, and then 25 mL silver nitrate solution (0.01 M) was added. The transparent solution changed to dark green immediately after

10.1021/jp0703915 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/27/2007

Charge Transfer between PPh3 and Colloidal Silver

J. Phys. Chem. C, Vol. 111, No. 24, 2007 8633 TABLE 1: Assignments of Raman Bands of PPh3 and SERS Bands of Adsorbed PPh3 on Aga Raman

SERS

194

Figure 1. Raman spectra of (a) solid PPh3 and (b) PPh3 adsorbed on colloidal silver with a final concentration of 1.5 × 10-6 M. The asterisks denote the solvent bands.

adding the silver nitrate solution. After stirring for several minutes, the colloidal silver was available for use. 2.3. Methods. The UV-vis absorption spectra measurements were carried out on a JASCO V-550 UV-vis spectrophotometer. Surface-enhanced Raman scattering (SERS) spectra were recorded in backscattering geometry on an Acton Raman spectrometer equipped with a liquid-nitrogen-cooled CCD detector at a resolution of 4 cm-1. A 532 nm semiconductor laser was used as the excitation source, and the laser power at the sample was set as 60 mW. All SERS experiments were performed with a quartz tube at room temperature. For SERS measurements, PPh3 ethanol solution with appropriate concentration was added to the silver sol at the volume ratio of 1:1. For the SERS measurements of PPh3 and SCNmixture, the PPh3 ethanol solution was mixed with SCNsolution and colloidal silver at the volume ratio of 1:1:2. 2.4. Theoretical Calculations. To study the charge-transfer effect between PPh3 and silver surface, theoretical calculations were carried out by using the Gaussian 03 program package.24 Optimizations of the molecular structures were done by density functional theory (DFT) methods. All calculations were performed at B3LYP/Lanl2DZ level. 3. Results and Discussion 3.1. Raman and SERS Spectra of PPh3. Figure 1a and b shows the normal Raman and surface-enhanced Raman scattering (SERS) spectra of PPh3, respectively. The assignments of Raman bands are listed in Table 1, mainly on the basis of previous work19,25-28 on the vibrational frequencies of PPh3 and its derivatives. It is straightforward that the PPh3 molecule has two possible binding sites, aromatic ring and lone pair of electrons of P atom, available for the adsorption of the molecule on the metal surface. Comparing the SERS spectrum with the normal Raman spectrum, it is found that almost all in-plane vibrational modes of phenyl ring appear in the SERS spectrum while some of them are absent in the normal Raman spectrum. The peak at 1000 cm-1 in the SERS spectrum corresponds to the 997 cm-1 peak in the Raman spectrum. This peak is the strongest mode in the Raman spectra of monosubstituted benzene derivatives29 and is assigned to the phenyl ring breathing mode. No apparent shift in peak position or obvious broadening of peak shape was observed for the phenyl ring breathing mode before and after

208 247 268

250

406

414

497

518

616 680 698 745 846 912 997 1024 1091

618 682

1154 1178 1430 1584 3054

746 846 919 1000 1027 1094 1141 1158 1185 1279 1339 1435 1530 1585 3062

assignments Ph X-sensitive mode, phenyl C-C out-of-plane def. Ph X-sensitive mode, P-Cring def. mode Ph X-sensitive mode, P-Cring def. mode Ph X-sensitive mode, phenyl C-C out-of-plane def. Ph X-sensitive mode, phenyl C-C in-plane def. Ph X-sensitive mode, phenyl C-C out-of-plane def. Ph R(C-C-C), phenyl ring in-plane def. Ph φ(C-C), phenyl ring out-of-plane def. C-C out-of-plane def. Ph γ(C-H), phenyl C-H out-of-pane def. Ph γ(C-H), phenyl C-H out-of-pane def. Ph γ(C-H), phenyl C-H out-of-pane def. phenyl ring breath. Ph β(C-H), phenyl C-H in-plane def. P-Cring stretch. mode Ph β(C-H), phenyl C-H in-plane def. Ph β(C-H), phenyl C-H in-plane def. Ph β(C-H), phenyl C-H in-plane def. Ph β(C-H), phenyl C-H in-plane def. Ph ν(C-C), phenyl CdC stretch. mode Ph ν(C-C), phenyl CdC stretch. mode Ph ν(C-C), phenyl CdC stretch. mode Ph ν(C-C), phenyl CdC stretch. mode Ph ν(C-C), phenyl C-H stretch. mode

a All values in cm-1; def., deformation; stretch., stretching; breath., breathing; Cring, the C atom link to substituted group in benzene ring; Ph X-sensitive mode, frequency sensitive to substituted group on phenyl ring.

the adsorption of PPh3, indicating that the adsorption orientation through aromatic ring can be excluded.30 The peak at 406 cm-1 in the Raman spectrum, which is assigned to the phenyl C-C in-plane deformation mode, shows remarkable enhancement and shifts to 414 cm-1 in the SERS spectrum. The shift for this mode is due to its substitute sensitivity. In the 1000-1600 cm-1 spectral region, the vibrations are mainly associated with the phenyl ring C-H in-plane deformation modes and the phenyl ring CdC in-plane stretching modes. The peak at 1027 cm-1 in the SERS spectrum, corresponding to the peak at 1024 cm-1 in the Raman spectrum and assigned to the phenyl ring C-H in-plane deformation vibration mode, is intensively enhanced. Some other phenyl ring C-H in-plane deformation modes, such as 1141, 1154, 1185, and 1279 cm-1 in the SERS spectrum, are also enhanced while some of them are absent in the Raman spectrum. The peak at 1584 cm-1 in the Raman spectrum, corresponding to the peak at 1585 cm-1 in the SERS spectrum, is assigned to the phenyl CdC in-plane stretching mode. Another two phenyl CdC stretching modes at 1339 cm-1 and 1530 cm-1, which do not appear in the Raman spectrum, are also enhanced. The intense enhancement of these in-plane modes indicates that the phenyl rings are perpendicular or at least tilted to the surface. Further evidence is provided by the observation of a SERS peak at 3062 cm-1, corresponding to phenyl ring in-plane C-H stretching mode at 3054 cm-1 in the Raman spectrum. On the other hand, the peak at 497 cm-1 in the Raman spectrum, assigned to phenyl ring C-C out-of-plane deformation mode, is intensely enhanced and shifts to 518 cm-1 in the SERS spectrum. The observed frequency shift arises from its substitute sensitivity. Another phenyl ring C-C out-of-plane deformation mode at 680 cm-1 in the Raman spectrum is also enhanced and shifts to 682 cm-1 in the SERS spectrum. As observed from

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Figure 2. UV-vis spectra of (a) pure silver colloid, (b) silver colloid after adding PPh3, and (c) silver colloid after adding NaCl solution.

the SERS spectrum, three phenyl ring C-H out-of-plane deformation modes at 745, 846, and 912 cm-1 are also enhanced. On the basis of the surface selection rule,31,32 the enhancements of both in-plane and out-of-plane modes suggest that phenyl rings are tilted with respect to the surface. The vibrational mode associated with P atom is also enhanced. The peak at 1091 cm-1 assigned to the P-C stretching vibration is markedly enhanced in intensity and shifts to 1094 cm-1 in the SERS spectrum. The observed frequency shift may be associated with the change in electronic structure of PPh3 molecule as a result of charge-transfer effect between PPh3 and colloidal silver. Although the peak assigned to P-Ag stretching mode is not observed, the intense enhancement of P-Cring stretching mode indicates the existence of the strong interaction between P atom and silver surface. So, it is suggested that PPh3 adsorbs on silver surface via its P atom with three phenyl rings tilted with respect to the surface. This is consistent with the adsorption of monosulfonated triphenylphosphine on gold colloids.19 3.2. Charge Transfer between PPh3 and Silver Surface. To explore the interaction between PPh3 and colloidal silver, UV-vis spectra of silver colloids before and after adding one drop of saturated aqueous PPh3 solution were measured, as shown in Figure 2. After the addition of PPh3 to silver colloid solution, a new absorption band at ca. 706 nm appears in the longer wavelength region along with the decrease in the intensity of the plasma absorption band of silver colloids at ca. 413 nm. The decrease in the intensity of the plasma absorption band indicates that the aggregation of silver colloids takes place to some extent. The new band at 706 nm is due to the charge transfer between the adsorbate and metal colloids.33-36 Alternatively, such a band has been ascribed to the coagulation of colloidal silver particles in the presence of the foreign molecules.37-40 For comparison, UV-vis spectrum of silver colloids after adding one drop of aqueous NaCl solution (0.1 M), a well-known aggregation reagent for the aggregation of silver colloids, was also recorded as shown in Figure 2c. Apparently, no new band in the longer wavelength region was observed, which gives further support that the new band is not due to the aggregation of silver colloids. Substantial evidence regarding the ascription of the change-transfer band has been reported recently by Du and Fang.34 Another convincing evidence, which can also exclude the possibility that new band is due to the aggregation of silver colloids, is the interaction between adsorbate and silver island films. Yamada et al.41 observed a new absorption band at ca. 600 nm after the adsorption of pyridine on silver island films,

Hu et al. where the aggregation process cannot obviously occur, that is, the new band does not result from the aggregation of silver particles after adding foreign molecules. They associated the new band with the effect of strong chemisorption or the charge transfer between adsorbate and silver surface. Similarly, Thomas et al. observed a new band at ca. 520 nm after foreign molecule adsorption on silver film. They also attributed the new band to charge-transfer band.33 The above arguments strongly favor the assignment that the band at 706 nm is due to the charge transfer between PPh3 and silver surface. However, the direction of charge transfer is not clear. In other words, the charge transfer may be from PPh3 to silver or reverse. Some papers reported that the electron transfers from the Fermi level of metal into an empty π* orbital of the adsorbate,42,43 whereas other papers reported that the charge transfer is from the highest occupied molecular orbitals (HOMO) of adsorbate to the lowest unoccupied molecular orbitals (LUMO) of metal.33,44,45 To make clear the direction of charge transfer between PPh3 and metal surface, SCN- was first used as probe molecule to investigate the interaction between the adsorbate and metal surface. SCN- is selected as probe molecule because the frequency of νCN is very sensitive to the coordination environment, such as the applied potential20,46-48 and SERS substrate49 although SCN- adsorbs on silver surface via S atom.47 Furthermore, the frequency of νCN appears in the region 2000∼2200 cm-1 where there is no interference of the PPh3 signal. So, it is possible to detect the change of surface charge on the basis of the Raman frequency shift of the coadsorbed SCN-. The SERS spectra of SCN- (0.025 M) in the presence of PPh3 with different concentrations are shown in Figure 3A. The peak position of νCN is at 2133 cm-1 when the concentration of PPh3 is 7.5 × 10-7 M. Compared with the SERS spectrum of SCN-, no shift in frequency was observed. However, the frequency of νCN begins to red shift with increasing the concentration of PPh3. When increasing the concentration of PPh3 to 7.5 × 10-4 M, the frequency of νCN shifts to 2122 cm-1. The frequency of νCN further decreases to 2110 cm-1 with increasing the concentration of PPh3 up to 7.5 × 10-2 M. The higher the concentration of PPh3 is, the larger the red shift observed. This is consistent with the adsorption of SCN- on electrode with different applied potentials.20,46,47 The more negative potential was applied, the larger the red shift was observed. So, it is suggested that the red shift of the νCN frequency is due to the electron-donor effect of PPh3. To make sure that the red shift is caused by the electrondonor effect of PPh3, the SERS spectra of SCN- in the presence of P(OPh)3 with different concentrations were measured as shown in Figure 3B. No apparent shift for the νCN frequency was observed. This is consistent with Tolman’s results.1,9 Compared with PPh3, P(OPh)3 exhibits weaker σ-donor ability and better π-acceptor ability.50,51 Figure 3C shows the SERS spectra of SCN- with different concentrations. No apparent shift of νCN was observed, indicating that the red shift of the νCN frequency is not induced by the dipole-dipole coupling interaction52,53 with increasing the coverage of SCN-. So, it is valid and feasible using the coadsorbed SCN- as a probe molecule to investigate the charge transfer between the adsorbate and metal surface. The red shift of the νCN frequency could be explained in terms of the electron-donor effect of PPh3: PPh3 adsorbed on silver surface donates its lone pair of electrons to silver surface, and then the negative charge of silver surface transfers to SCN-

Charge Transfer between PPh3 and Colloidal Silver

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Figure 3. (A) SERS spectra of SCN- (0.025 M) in the presence of PPh3 with different concentrations, (a) 7.5 × 10-7 M, (b) 7.5 × 10-6 M, (c) 7.5 × 10-5 M, (d) 7.5 × 10-4 M, (e) 7.5 × 10-3 M, (f) 7.5 × 10-2 M; (B) SERS spectra of SCN- (0.025 M) in the presence of P(OPh)3 with different concentrations, (a) 7.5 × 10-7 M, (b) 7.5 × 10-6 M, (c) 7.5 × 10-5 M, (d) 7.5 × 10-4 M, (e) 7.5 × 10-3 M, (f) 7.5 × 10-2 M; (C) SERS spectra of SCN- with different concentrations, (a) 1 × 10-1 M, (b) 1 × 10-2 M, (c) 1 × 10-3 M.

SCHEME 1: A Model for the Electron-Donor Effect of PPh3 on Silver Surfacea

SCHEME 2: Optimized Structures of (a) PPh3-Ag and (b) PPh3-Ag+ at B3LYP/Lanl2dz Level

TABLE 2: Calculated Mulliken Charges of PPh3 before and after Its Coordination with Silver Atom and Silver Ion Mulliken chargea

a

The charge transfer is from PPh3 to the silver surface and then to SCN-.

via S-Ag bond. The conjugated effect between S atom and CtN bond makes it possible to increase the electron density of the π* orbital of CtN bond and consequently weaken the CtN bond and decrease the frequency of νCN. The process of charge transfer is shown in Scheme 1. It is well-known that PPh3 can donate its lone pair of electrons to metal and that its 3d-orbitals could accept the d electrons of a transition metal in homogeneous catalysis. Our result indicates that the σ-donor ability of PPh3 is larger than the π-acceptor ability and that PPh3 exhibits charge-transfer effect and could change the surface charge property of nanoparticles even in heterogeneous system. This is consistent with the fact that PPh3 is a poor π-acceptor.50,54-56 So, the net charge transfer is from PPh3 to metal rather than from metal to PPh3. This may explain why the addition of PPh3 to Rh/SiO2 catalyst could dramatically increase the catalytic activity and selectivity. 3.3. Density Functional Theory (DFT) Calculations. To gain further insight into the interaction between PPh3 and metal surface, DFT calculations were performed. The calculations were carried out at B3LYP/Lanl2DZ level on the basis of two different complexes models: one surface silver atom is bounded to the P atom (as shown in Scheme 2a, denoted as PPh3-Ag) and one surface silver ion is bounded to the P atom (as shown in Scheme 2b, denoted as PPh3-Ag+). The calculated Mulliken charges of PPh3 and its silver complexes are given in Table 2. The calculated results indicate

group

PPh3b

PPh3-Ag

Ph ring 1 Ph ring 2 Ph ring 3 P Ag

-0.226 -0.227 -0.226 0.682

-0.169 -0.168 -0.168 0.697 -0.197

Ph ring 1 Ph ring 2 Ph ring 3 P Ag

Change in Mulliken Atomic Chargeb 0.057 0.059 0.058 0.015 -0.197

PPh3-Ag+ 0.017 0.016 0.017 0.564 0.386 0.243 0.243 0.243 -0.118 -0.614

a Mulliken atomic charges (e) as estimated from the DFT analysis for PPh3, PPh3-Ag, and PPh3-Ag+ at B3LYP/Lanl2DZ level. b Changes in Mulliken atomic charges (e) for PPh3 before and after coordination with silver atom and ion.

that three phenyl rings of the free PPh3 molecule have negative charge and that the P atom has positive charge. This could be explained in terms of the conjugation effect between the P atom and the phenyl rings. The lone-pair electrons of P atom are delocalized to the phenyl rings; as a consequence, the P atom has positive charge while the phenyl rings have negative charge. Table 2 also shows the change of Mulliken charges before and after PPh3 coordinated to silver atom or ion, and it is very straightforward that the charge transfer is from PPh3 to silver in both models. For example, PPh3 donates 0.197e to silver surface for PPh3-Ag mode while more charge (0.614e) is transferred from PPh3 to silver for PPh3-Ag+ model. Another interesting phenomenon is that the charge transfer arises mainly from the contribution of phenyl rings rather than from P atom (see Scheme 1) although PPh3 adsorbs on silver surface via its

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Hu et al. of surface negative charge leads to the red shift of CtN stretching mode for the coadsorbed SCN- on Ag. The information obtained from the adsorption of PPh3 on silver could be helpful to understand the heterogeneous and homogeneous catalysis involving phosphine ligand coordinated to transition-metal surface. In the hydroformylation reaction, for example, σ-donation of the phosphine ligands can increase the electron density of the metal center and thus can increase the π-back-donation from metal to CO and then can activate CO molecule by weakening the C-O bond. In the hydrogenation reaction,57 the PPh3 in RhCl(PPh3)3 catalyst can promote the activation of the molecular hydrogen by breaking down the H-H bond and can promote the activation of olefin by donating electron to the π* orbital of CdC and then can increase the catalytic activity. This makes the reaction take place under mild conditions. 4. Conclusions The adsorption of triphenyl phosphine on colloidal silver has been investigated by means of surface-enhanced Raman spectroscopy. It has been found that PPh3 adsorbs on colloidal silver surface via P atom with three phenyl rings tilted to the Ag surface. The electron-donor effect of PPh3 can be effectively detected by using the coadsorbed SCN- as a probe molecule. The Raman frequency of νCN of the adsorbed SCN- shifts to lower frequencies in the presence of PPh3 owing to the electrondonor effect of PPh3. DFT calculations further confirm that the charge transfer is from PPh3 to silver surface rather than the reverse direction and also indicate that the major contribution of charge-transfer effect is from the phenyl rings rather than from P atom although the coordination between PPh3 and silver happens through the P atom with lone pair electrons coordinating to a surface silver atom.

Figure 4. (a) Frontier orbital theory analysis (B3LYP/Lanl2DZ, isocontour ) 0.02) and (b) hybrid orbital theory analysis of coordination of PPh3 with Ag atom.

P atom. For instance, about 0.174e transfers from the phenyl rings while only 0.015e transfers from P atom for PPh3-Ag model. The electronic structure analysis of the interaction between PPh3 and Ag (as shown in Figure 4a) shows that the most favorable interaction should occur between the highest occupied molecular orbital (HOMO) of the electron donor (PPh3) and the lowest unoccupied molecular orbital (LUMO) or singly occupied molecular orbital (SOMO) of the electron acceptor (Ag or Ag+) because of the smallest energy gap between these two molecular orbitals. For the adsorption of PPh3 on silver surface, the charge transfer is from the HOMO of PPh3 to the LUMO or SOMO of silver cluster. Thus, the energies of the complexes (PPh3-Ag and PPh3-Ag+) can be decreased, and the complexes are quite stable after coordination. As shown in Figure 4b, silver atom has 4d105s1 valence electron configuration. It is well-known that silver coordinates through the sp hybrid orbital. For the adsorption of PPh3 on Ag surface, P atom provides the lone pair of electrons to occupy the vacant sp hybrid orbitals of silver atom. Because of the poor π-acceptor ability of PPh3 and the weak d-feedback property of silver, the net charge transfer is from PPh3 to silver surface. So, the calculated results further confirm the conclusion that the charge transfer is from PPh3 to silver surface. The increase

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