Efficient visible-light photocatalytic degradation system assisted by conventional Pd catalysis

Different approaches like doping and sensitization have been used to develop photocatalysts that can lead to high reactivity under visible-light illumination, which would allow efficient utilization of solar irradiation and even interior lighting. We demonstrated a conceptually different approach by changing reaction route via introducing the idea of conventional Pd catalysis used in cross-coupling reactions into photocatalysis. The –O–Pd–Cl surface species modified on Ni-doped TiO2 can play a role the same as that in chemical catalysis, resulting in remarkably enhanced photocatalytic activity under visible-light irradiation. For instance, Pd/Ni-TiO2 has much higher activity than N-TiO2 (about 3 ~ 9 times for all of the 4-XP systems) upon irradiation with wavelength of 420 nm. The catalytically active Pd(0) is achieved by reduction of photogenerated electrons from Ni-TiO2. Given high efficient, stable Pd catalysts or other suitable chemical catalysts, this concept may enable realization of the practical applications of photocatalysis.

I n heterogeneous photocatalysis, the organic pollutants can be degraded via direct oxidization by the oxidative species generated upon photoexcitation of a photocatalyst. Many photocatalysts have been designed and prepared to achieve this. TiO 2 is the most widely studied one due to its nontoxicity and high photostability 1,2 . However, so far the photocatalytic efficiency for all of these catalysts is still low, which keeps them far away from practical applications. It is noted that the efficiency of such a photocatalytic system (PCS) can be improved by controllable modulation of light harvest and behavior of photogenerated charge carriers, which can be realized by different design concepts, such as modulation of crystallographic facets and morphology, doping, sensitization, surface treatment, and combination with other semiconductors and noble metals [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . The majority of these studies are focused on the modification of the photocatalysts themselves. However, the photodegration efficiency is still not high enough hitherto, especially for those under visible-light irradiation that accounts for about 44% of total solar energy. On the other hand, the efficiency can also be improved if the photodegradation route of pollutants can be changed beforehand. A different concept for the design of a PCS can be proposed accordingly.
It is known that the reaction rate can be promoted remarkably by a catalyst via changing the reaction route in a chemical reaction, as less free energy is required to reach the transition state (i.e., intermediate). Coupling reaction is one of the most famous catalytic chemical reactions, for which palladium (Pd) catalysts are often used [19][20][21][22] . The tetrakis(triphenylphosphine) palladium(0) is the common catalyst for homogeneous catalytic reaction, and in heterogeneous Pd catalysis, Pd is often fixed to a solid support in the form of dopant, particle, and complex 20,21 . Besides the coupling reactions, Pd catalysts can also be used in hydrogenation reaction 23 , hydrogen evolution 24 , and NO reduction 25 . Although the Pd species have been employed to improve the photocatalytic degradation too, the mechanism is different from the above Pd catalysis, for which the Pd species can act as the center for electron collection 26 , change active sites at the (grain) interface of two solids 27 , or absorb light due to surface plasma resonance effect 28 .
In this work, the concept of conventional Pd catalysis used in the cross-coupling reactions is thus introduced into the PCS so as to efficiently photodegrade the organic pollutant. The target molecules are the toxic 4halogeno-phenol (X-C 6 H 4 -OH, denoted as 4-XP, X 5 F, Cl, Br, and I, respectively), as it exhibits similarity in molecular structure to those used in cross-coupling reactions and is very difficult to be removed from the contaminated water. The Pd catalyst is the -O-Pd-Cl surface species modified on Ni-doped TiO 2 . The obtained PCS shows greatly enhanced photodegradation of 4-XP under visible-light irradiation, implying the feasibility of this novel design concept for efficient photocatalysis.

Results
Structure and composition of as-prepared photocatalysts. The TiO 2 -based photocatalysts modified by Pd surface species are used in this work, which are prepared by a sol-gel method. Only anatase TiO 2 can be observed in the XRD patterns for all of the photocatalysts ( Fig. 1), indicating that no phase change among different samples. Moreover, no patterns ascribed to the oxides of Pd or Ni can be observed. The cell volume and lattice parameters can be derived from the diffraction peaks corresponding to crystal planes (101) and (200) in the XRD patterns using Scherrer's formula, for which of Ni-TiO 2 are larger than those of pure TiO 2 (Table 1). So Ni ions are doped in substitution mode as the ionic radius of Ni 21 (72 pm) is slightly larger than that of Ti 41 ions (68 pm). While the cell volume and lattice parameters remain almost unchanged upon Pd modification (i.e., almost no shift in XRD peak), implying that Pd ions are not present in substitution or interstitial mode since the ionic radius of Pd 21 (86 pm) is much larger than that of Ti 41 ions. This is further confirmed by the high-resolution TEM results (see Supplementary Fig. S1). The fringe spacing (d) of (101) crystallographic plane is determined to be 3.52 Å for both TiO 2 and Pd/TiO 2 , indicating that the modified Pd exists as surface species; while it increases to 3.59 Å for Pd/Ni-TiO 2 , suggesting that Ni is doped into the crystal lattice as the ionic radius of Ni 21 is larger than that of Ti 41 ions. In addition, the BET specific surface area for TiO 2 , Ni-TiO 2 , Pd/TiO 2 , and Pd/Ni-TiO 2 is determined to be 86.4, 77.0, 86.6, and 90.3 m 2 /g, respectively. That is to say, no big difference is found among different systems, which is consistent with the slight changes observed in the crystallite size ( Table 1).
The chemical states of elements in different photocatalysts are determined by XPS technique. Since the radius of Clions (182 pm) is much larger than that of O 2 ions (140 pm), it is impossible for Clions to be doped into TiO 2 lattice in substitution or interstitial mode. The peak at 198.1 eV for TiO 2 sample is slightly lower than that of Cl2p 3/2 in TiCl 4 ( Fig. 2A) 29 , which is ascribed to the Clions linked with unsaturated Ti sites on the surface of TiO 2 , i.e., -O-Ti-Cl structure 30,31 . Our previous work demonstrates that this -O-Ti-Cl cannot cause visible-light response and can hardly influence photocatalytic activity under visible-light irradiation 31 . For the Cl2p XPS spectra of Ni-TiO 2 , Pd/TiO 2 and Pd/Ni-TiO 2 samples, the binding energy (BE) of Cl2p 3/2 peaks (198. 3-198.5 eV) locates between that of TiCl 4 (198.2 eV) and NiCl 2 (199.2 eV) or PdCl 2 (198.9 eV) ( Fig. 2A). Hence, the Clions are linked with unsaturated Ti or other metal sites on the surface (i.e., -O-Me-Cl structure, Me 5 Ti, Ni, and Pd). The amount of such surface species increases greatly when TiO 2 is modified by Ni and/or Pd, as the XPS signal increases significantly upon the modification.
For both Ni-TiO 2 and Pd/Ni-TiO 2 samples, two pairs of doublet Ni2p peaks as well as the corresponding satellite peaks (around 862 and 880 eV) can be observed in the spectra (Fig. 2B). The peak located at around 855.8 eV for Ni2p 3/2 is attributed to the doped Ni ions in TiO 2 lattice, since the peak position is almost the same as -Ti-O-Ni-O-structure in the bulk NiTiO 3 (855.9 eV) 32 . Considering the above Cl2p XPS results, another one at around 856.7 eV is ascribed to the surface -O-Ni-Cl structure. Moreover, it seems that the total amount of Ni species in the catalyst decreases in the Pd/Ni-TiO 2 compared with that in Ni-TiO 2 , as the XPS signal decreases obviously for Pd/Ni-TiO 2 .
Two pairs of doublet Pd3d peaks can be observed in the spectra for both Pd/TiO 2 and Pd/Ni-TiO 2 samples (Fig. 2C). The peak at 336.2 eV for Pd3d 5/2 is ascribed to -O-Pd-O-structure at the surface (i.e., one Pd 21 ion is linked with two unsaturated oxygen ions). Considering the aforementioned Cl2p XPS results as well as the BE of Pd3d 5/2 for PdO (336.3 eV) and PdCl 2 (337.9 eV), another Pd3d 5/2 peak at 337.7 eV is attributed to -O-Pd-Cl structure (i.e., one Pd 21 ion is linked with one Clion and one unsaturated oxygen ion) at the surface. Furthermore, the ratio of -O-Pd-Cl structure to -O-Pd-Ois much higher for Pd/Ni-TiO 2 than Pd/TiO 2 , implying that the major surface species is -O-Pd-Cl for Pd/Ni-TiO 2 while -O-Pd-O-for Pd/TiO 2 . Since the total amount of Ni species in Pd/Ni-TiO 2 catalyst is less than that in Ni-TiO 2 , the utilization of Ni species during catalyst synthesis may promote the formation of surface -O-Pd-Cl species, possibly because the structure containing Ni 21 ions can change into -O-Pd-Cl in the presence of Pd 21 ions. So the synergistic interactions are present between Pd and Ni species to some degree during the catalyst synthesis.
In addition, NaCl, instead of NiCl 2 , has been used during the synthesis too, which can also provide Clions. The XPS signal of -O-Pd-Cl structure can be observed too. Moreover, the ratio of -O-Pd-Cl structure to -O-Pd-O-for the Pd/TiO 2 -NaCl catalyst is still   Photocatalytic activity. Photodegradation of X-C 6 H 4 -OH (4-XP, X 5 F, Cl, Br, and I, respectively), a hazardous and toxic pollutant, has been used as target molecules to evaluate the photocatalytic activity of the resultant PCS under visible-light irradiation. The nitrogen doped TiO 2 (N-TiO 2 ) is also given here for comparison, as it shows relatively good visible-light photocatalytic activity. The photocatalytic activity upon visible-light irradiation (l . 420 nm) falls in the series TiO 2 , Ni-TiO 2 , N-TiO 2 , Pd/TiO 2 , Pd/Ni-TiO 2 for all of the systems (Fig. 3), especially in initial stage of the photodegradation. That is to say, TiO 2 and Ni-TiO 2 exhibit very low activity; while Pd/TiO 2 shows a remarkably improved activity, and Pd/Ni-TiO 2 has the highest activity. When compared Pd/Ni-TiO 2 with Pd/TiO 2 for the photodegradation in the first two hours, the enhancement amplitude is about 40.7%, 47.5%, 49.1%, and 23.3% for 4-FP, 4-ClP, 4-BrP, and 4-IP, respectively. Furthermore, Pd/Ni-TiO 2 has much higher activity than N-TiO 2 (about 3 , 9 times for all of the 4-XP systems), and is even much higher than Ni-TiO 2 (about 15.3, 34.6, 67.9, and 2.2 times for 4-FP, 4-ClP, 4-BrP, and 4-IP, respectively). Thus, the presence of Pd species can greatly improve the visible-light photocatalytic activity of the reported PCS, while Ni species just improve it slightly. To further confirm this, in addition, 4-ClP has been photodegraded with less amount of photocatalyst (5 mg, instead of 10 mg in the above experiments) under illumination with slightly higher energy (l . 400 nm). Exactly the same trend is observed (see Supplementary Fig. S2). Since no big difference in the BET specific surface area is found among TiO 2 , Ni-TiO 2 , Pd/TiO 2 , and Pd/Ni-TiO 2 , the effect of surface area on the photocatalytic activity can be ignored here. It is noted that the photocatalytic activity of a PCS is closely related to its optoelectronic properties. Pure TiO 2 shows no visible-light absorption due to its large band gap of 3.1 eV, while the other three have relatively strong absorption in the visible-light region due to the formation of Ni doping energy levels and/or -O-Me-Cl surface species (Fig. 4A). It is noted that Pd/TiO 2 has the strongest visible-light absorption, followed by Pd/Ni-TiO 2 . As discussed above, the major surface spe- Furthermore, the Pd/TiO 2 exhibits higher visible-light surface photovoltaic spectroscopic (SPS) response throughout 400 , 600 nm than Pd/Ni-TiO 2 , although both of them are relatively low (Fig. 4B). Ni-TiO 2 exhibits very little visible-light SPS response (only a small bump around 400 , 430 nm) and no visible-light SPS response at all for TiO 2 . Such a SPS response corresponds to the visiblelight phtotocatalysis, i.e., only a PCS exhibits SPS response that can show photocatalysis, though maybe with different activity due to the influences from other factors. Besides the above photodegradation results, indeed, about 28.6% of 4-ClP are photodegraded for Pd/TiO 2 and 33.3% for Pd/Ni-TiO 2 under irradiation with a wavelength longer than 450 nm for 8 h, while no photodegradation is observed for Ni-TiO 2 . Then, the question arises. Why the PCS with Pd/Ni-TiO 2 shows lower visible-light absorption and SPS response than Pd/TiO 2 , but has higher photocatalytic activity?

Discussion
It is noted that the formation of some intermediates (such as hydroxyphenyl radicals) upon photoexcitation from the 4-XP molecules adsorbed on the photocatalyst surface is the first and critical step in the photodegradation, which will further undergo a sequence of reactions, eventually leading to a complete photodegradation of 4-XP into CO 2 and H 2 O 33,34 . Thus, the photocatalytic efficiency can be improved significantly if the high free energy required for this critical process can be reduced via chemical catalysis. Considering the formation of a stable trans-s-Pd(II) intermediate via reduction of a Pd(0) complex catalyst is the key step in the cross-coupling reactions [20][21][22] , it is suggested that the -O-Pd-Cl surface species reported here can play a similar role, i.e., acting as the catalyst too (actually -ORPd(0) rCl). The catalytically active Pd(0) complex is formed in-situ in the cross-coupling reactions, typically through oxidation of  Since no such reducing agent is present in our PCS, it is suggested that here the active -ORPd(0) rCl is formed via the reduction of -O-Pd(II)-Cl by photogenerated electrons upon irradiation, which can react with the target molecules to form organopalladium intermediates that can further produce the short-lived hydroxyphenyl radicals (HPR) via reaction with the photogenerated electrons and/ or holes (Fig. 5). The degradation occurring thereafter is similar to those in conventional photocatalysis. It is believed that the conventional photodegradation mechanism plays a trivial role in this initial stage, even if it does play a role.
The formation of HPR has been confirmed previously 35 , which will undergo further rapid oxidation to the final mineralization products carbon dioxide and water. The regeneration of -O-Pd(II)-Cl from the active -ORPd(0) rCl can be realized by the oxidization of oxygen or holes, though it is noted that the amount of -O-Pd(II)-Cl decreases to some degree according to the signal intensity of Cl2p XPS (see Supplementary Fig. S3). Except the 4-IP system, the relative reactivity in terms of photodegradation decreases in the order of Br . Cl . F, which is the same as that for the cross-coupling reactions. Moreover, in a moderate range, the photocatalytic activity increases with the increasing amount of Pd and Ni introduced during catalyst synthesis (see Supplementary Fig. S4), due to the increased amount of -O-Pd-Cl; while the activity decreases when the amount of Ni is too high, possibly because the surface Ni species can also act as recombination centers. Thus, the proposed concept is successfully realized, which may afford a feasible approach for design of an efficient PCS, even in the case of inferior visible-light absorption because the critical step for formation of the degradable intermediates is via Pd catalysis instead of via direct interactions with the photogenerated electrons or holes. This mechanism can also explain why the TiO 2 modified by Ni and/or Pd has shorter photoluminescence lifetime than TiO 2 (see Supplementary Fig. S5), while exhibits higher visiblelight photodegradation activity on 4-XP as discussed above.
As for the 4-IP system, similar to that for the cross-coupling reactions, it is believed that the -O-Pd(II)-Cl may still exhibit very high catalytic activity for the formation of organopalladium intermediate and short-lived HPR, possibly higher than that for the 4-BrP too. Unlike the cross-coupling reactions, however, some of the resultant HPR can react with each other to form a dimer or other oligomer due to the absence of other reactants in the system for addition reactions with these intermediates. These oligomers are more difficult to be photodegraded than the corresponding monomer. Although no suitable facilities are available to probe the resultant intermediates or oligomers, this speculation can be proved indirectly by the UV-vis absorption spectra after irradiation (see Supplementary Fig. S6). The absorption corresponding to the benzene ring (,230 nm) decreases with increasing irradiation time. Meanwhile a new absorption bump appears at around 255 nm, corresponding to the absorption of oligomers. For the 4-IP system, the intensity of this new absorption reaches a maximum value in a relatively short time (2-3 h), while it keeps increasing with irradiation time for the 4-BrP system. This may suggest the rapid formation of oligomers for the 4-IP PCS due to high catalytic effect on the 4-IP system, resulting in the abnormal low photodegradation of 4-IP.
Here both the -O-Pd(II)-O-and -O-Ni(II)-Cl species cannot play such a role, or very little even if they could. For the former, it may be because it is difficult to form the organopalladium intermediate or HPR, possibly because both the oxygen ions linked to the Pd atom are also connected with another ion. Furthermore, PdO shows very low activity for the photocatalytic degradation of 4-XP (see Supplementary Fig. S7). On the other hand, no PdO is observed from the XRD patterns. For the -O-Ni(II)-Cl, it is possibly because it cannot be reduced efficiently by photogenerated electrons. Or, even if organonickel intermediate could be formed, calculation results indicate that the dissociation energy of Ni-X in the corresponding intermediate is larger than that of Pd-X (see Supplementary Table S1). So the X atom linked to the Pd atom can be more easily dropped out than that linked to Ni. So the -O-Ni(II)-Cl would not be an efficient catalyst in the 4-XP degradation, even it could act as a chemical catalyst. In addition, although no direct evidence shows the existence of the active -ORPd(0) rCl intermediate under visible-light irradiation, the signal of Pd(0) can be observed in the XPS spectrum (,334.5 eV for Pd3d5/2) after UV-light photodegradation of target molecules when the same photocatalysts are used (see Supplementary Fig. S8). This may also imply the viability of the proposed mechanism.
Moreover, as discussed above, the presence of Clions during the catalyst synthesis can be in favor of the formation of surface -O-Pd-Cl species, especially via the conversion of -O-Pd-O-into -O-Pd-Cl. Thus, the photocatalysts have also been prepared with the same protocol but without the presence of Cl species in the starting materials so as to further study the effect of Clions. It is found that all the obtained photocatalysts still exhibit visible-light response (see Supplementary Fig. S9). Similar to the catalysts prepared with the presence of Cl species, Pd/TiO 2 has the strongest visible-light absorption, followed by Pd/Ni-TiO 2 , and Ni-TiO 2 is the weakest. It can be seen from Figure 6 that the photodegradation activity of the obtained catalysts is much lower than the catalysts prepared with the presence of Cl species (Fig. 3b). For instance, the obtained Ni-TiO 2 shows almost no activity on 4-ClP photodegradation. When the same amount of Pd is used in the precursor (1.5%), only about 35% of 4-ClP is decomposed after 4 hours of photodegradation for the catalyst prepared without Cl (1.5Pd/Ni-TiO 2 ); while almost all of the 4-ClP is decomposed after 4 hours for the catalyst prepared with Cl (Fig. 3b). The same is true for the Pd/TiO 2 catalyst. Therefore, the presence of Cl species in the precursor is very important. These results can further confirm the feasibility of the proposed mechanism.

Conclusion
In summary, we have demonstrated a new design concept for efficient visible-light photocatalysis, i.e., a conventional Pd catalysis study for developing suitable PCS so as to enable realization of the practical applications of photocatalysis. The Pd may be replaced by Ni, Fe or Cu in a specific system. We envision that the future investigations may even develop a PCS that can work efficiently in the case of inferior visible-light absorption, as the critical step for formation of the intermediates is via Pd catalysis instead of via direct interactions with the photogenerated electrons or holes. We believe that not only the field of photocatalytic degradation, but also other photocatalytic fields like photosynthesis may be benefited from this too since it is developed from coupling reactions.

Methods
Chemicals. All chemicals used were of analytical grade and the water was deionized water (.18.2 MV?cm). At room temperature, certain volume of NiCl 2 (0.5 mol/L) and PdCl 2 (0.1169 mol/L) solution were mixed with 40 mL of ethanol. Then 1 mL of HCl solution (12 mol/L) and 12 mL of Ti(OC 4 H 9 ) 4 was added dropwise into the mixture under vigorous stirring. The mixture was stirred until the formation of TiO 2 gel, followed by being aged for 24 h. The obtained gels were dried at 373 K for 10 h and annealed at 723 K in a muffle for 2.5 h. The resultant samples were denoted as Pd/Ni-TiO 2 . Pure TiO 2 , Ni doped TiO 2 (Ni-TiO 2 ) and Pd modified TiO 2 (Pd/TiO 2 ) were prepared using the same procedure, while without the addition of corresponding precursor. To study the effect of Clions, PdSO 4 and Ni(NO 3 ) 2 were also used as the precursors, for which the photocatalysts were prepared with the same protocol. Unless stated otherwise, the nominal molar ratio of Pd 21 to Ti 41 is fixed at 1.5% in the precursor and the one for Ni 21 to Ti 41 is 15%. For comparison, other molar ratio was also used for both Pd 21 to Ti 41 (such as 0.3%, 1.0%, and 1.3%) and Ni 21 to Ti 41 (such as 5%, 10%, and 20%).
Characterization. The XRD patterns were acquired using a Rigaku D/max 2500 Xray diffraction spectrometer (Cu Ka, l 5 1.54056 Å ). The average crystallite size was calculated according to the Scherrer formula (D 5 k l/B cosh). XPS measurements were carried out by using a Thermo ESCALAB 250 spectrometer with an Al Ka monochromator source and all the binding energies were calibrated to the adventitious C1s peak at 284.8 eV. The specific surface area of the obtained catalysts were determined by Brunauer-Emmett-Teller (BET) analysis (Micromeritics, tristar II 3020). Diffuse reflectance UV-visible (UV-vis) absorption spectra were recorded on a UV-vis spectrometer (U-4100, Hitachi). The surface photovoltage spectroscopy (SPS) measurements were performed with a solid sandwich structure via a homemade system using a light source-monochromator lock-in detection technique.
Calculation. All DFT calculations were performed using the DMol3 package 36,37 . The generalized gradient approximation (GGA) for the exchange-correlation potential prescribed by Perdew-Burke-Ernzerhof (PBE) and an all-electron double numerical basis set (DND) with polarization functions are used in spin-unrestricted density-functional-theory (DFT) based calculations 38 . For Pd atom, the relativistic effects were also considered. Geometry optimization was performed without symmetry constraint using a convergence criterion of 1.0 3 10 25 Hartree on the maximum energy gradient and 0.005 Å on the maximum displacement for each atom. Selfconsistent field (SCF) electronic structure calculations were carried out with a convergence criterion of 1.0 3 10 26 Hartree on the total energy.
Photocatalysis. The photocatalytic activity of all photocatalysts were determined by visible-light photodegradation of 4-XP solution (5 3 10 25 mol L 21 , 40 mL) with a sunlamp (Philips HPA 400/30S, Belgium) and a cutoff filter for the removal of UV light. The reactor was perpendicular to the light beam and located 10 cm away from the light source. All the suspensions were magnetic stirred at (25 6 2)uC in the dark for 30 min to reach adsorption equilibrium before photocatalysis, and oxygen gas was continuously bubbled through the suspension at a flux of 5 mL min 21 . The change in concentration of 4-ClP was monitored by a UV-visible spectrometer (UV-1061PC, SHIMADZU) using 4-aminoantipyrine as the chromogenic reagent, while the degradation of 4-FP, 4-BrP and 4-IP was monitored directly by the spectrometer. The reproducibility of the photocatalytic degradation was evaluated by repeating experiments at least three times with different batches of photocatalysts prepared by the same procedure.