Disparate roles of doped metal ions in promoting surface oxidation of TiO2 photocatalysis

Journal of Photochemistry and Photobiology A: Chemistry 315 (2016) 59–66

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Disparate roles of doped metal ions in promoting surface oxidation of TiO2 photocatalysis Jiadong Xiaoa,c,d, Yongbing Xiea,b,d,* , Hongbin Caoa,b,d, Faheem Nawazc , Shanshan Zhanga , Yueqiu Wangb a

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, China Beijing Engineering Research Center of Process Pollution Control, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China d Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 June 2015 Received in revised form 18 September 2015 Accepted 23 September 2015 Available online xxx

The key standpoint favors the role of doped metal ions as electron traps in enhancing surface-charge separation during TiO2 photocatalysis. Besides that, this study demonstrated two disparate impacts of metal ion doping on reactive species generation during TiO2 photocatalysis. Ag+ and Cu2+ outperformed other dopants in photooxidation of oxalic acid (OA), while further increasing their doping amount from the optimum value produced a negligible impact on the photocatalytic performance. This implied other dominant roles of doped metal ions other than electron captors, because an excessive amount of traps can serve as charge-recombination centers and largely decrease the photocatalytic activity. N2 purging and NaF (a scavenger for surface-radicals) markedly reduced the mineralization rate after Ag+ doping, verifying the significance of dissolved oxygen (DO) in reactive species generation upon TiO2 surface. Non-
OH reactive oxygen species (ROS) were found to account for 75% in OA mineralization, attributed to enhanced DO adsorption towards TiO2 surface by Ag+ doping. However, Cu2+ doping could facilitate the oxidation by hvb+ plus
OHads, which took as dominant a part as 53%. Ag+-Cu2+ co-doping integrated these two positive sides and more inclined to the role of Ag+. Correspondingly, surface oxidation dominated whereas liquid-phase oxidation weakened. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Photocatalysis Metal ion doping TiO2 Oxalic acid Reactive oxygen species

1. Introduction With the rapid industrial development, a large amount of wastewater is emerged and discharged into the environment, while its complex composition increases the difficulties for water purification. Owing to an accelerating concern about the recalcitrant organics in wastewater, water quality control and regulations have become more stringent in many countries [1,2]. The exploration of efficient advanced treatment to mineralize these contaminants becomes extremely urgent. Advanced oxidation processes (AOPs) are impressed as highly competitive technologies for elimination of a wide variety of recalcitrant organics resistant to conventional approaches [3–11]. Among all AOPs, TiO2

* Corresponding author at: Chinese Academy of Sciences, Institute of Process Engineering, No. 1 Beiertiao, Zhongguancun, Beijing 100190, China. Fax: +86 10 82544844. E-mail address: [email protected] (Y. Xie). http://dx.doi.org/10.1016/j.jphotochem.2015.09.013 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.

photocatalysis is capable of converting photon energy into chemical energy, and proves to be a green, eco-friendly and efficient tool for wastewater treatment and water purification [8,9,12]. Unfortunately, the rapid recombination of photo-induced charge carriers could largely decrease the photonic efficiency and overall quantum yield of TiO2 photocatalysis. Three main strategies have been adopted to strengthen surface-charge separation or broadening the photoabsorption, including metal or non-metal (ion) doping [13–17], anatase-rutile [18] or p–n [19] heterojunction formation and compounding with charge transfer materials [20–22]. Among these attempts, metal ion doping has proven to be a simple and valid way to promote photonic efficiency due to the efficient charge transfer from the doped metal ions to Ti4 + ions [23,24]. While notable advances have been made, a controversy remains on how metal ion doping influences the reactivity and oxidation pathway during TiO2 photocatalytic decontamination. The positive influence of doped metal ions on the photocatalytic activity of TiO2

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has been universally explained by their roles as electron traps to promote surface-charge separation [23–27]. Metal ions in TiO2 matrix could efficiently capture the photo-induced electrons and extend the life time of charge carriers, eventually to enhance the photocatalytic performance [23–27]. When the dopant excesses a certain amount, the metal ions turn into charge-recombination centers, generating a large decrease of photocatalytic efficiency [26,27]. Nevertheless, disparate opinions are proposed as well, making it difficult to draw a clear conclusion regarding this issue. For instance, Pouretedal et al. revealed that Cu+ could be generated from the reduction of doped Cu2+ by photo-induced electrons and act as a strong oxidative to promote the oxidation [28]. BET surface area was confirmed by Feng et al. as the most significant factor in enhancing TiO2 photocatalytic performance by metal ion doping [23]. Additionally, Xiong et al. proposed that the deposited Ag greatly increased dissolved oxygen (DO) adsorption, thereby leading to an enhanced production of reactive oxygen species (ROS) and higher photodegradation rate [29]. These also indicate that metal ion doping could influence the generation of reactive species involved in TiO2 photocatalytic decontamination thereupon to alter the oxidation pathways. The clarification on how metal ion doping influences reactive species generation is of great significance to reveal the high-activity mechanism, while it is seldom followed. Herein, a series of monometallic ion (Mn2+, Ni2+, Zn2+, Co2+, Fe3+, Ce3+, Cu2+, Ag+) doped TiO2 nanocrystals were fabricated by a facile double-layered hydrothermal method. Their photocatalytic activities were evaluated by oxalic acid (OA) decomposition. OA was selected as the targeted pollutant due to its resistance to advanced treatment, and it is also a detectable recalcitrant intermediate in the photocatalytic mineralization of many larger organic compounds [26,27]. Cu2+ and Ag+ were found to outperform other dopants, and paradoxically, further increasing the doping amount had an ignorable effect on the photocatalytic performance. It challenged the dominant role of doped metal ions as electron captors in inhibiting the recombination of electron-hole pairs. To clear up this confusion, various scavengers and N2-purging were performed to semiquantitatively determine the contribution of every oxidation pathway. Finally, disparate roles of doped metal ions in enhancing TiO2 photocatalytic activity were revealed. 2. Experimental 2.1. Materials and reagents Tetrabutyl titanate (TBT, 99%) was purchased from Sigma– Aldrich (Germany) as the source of titanium. OA was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Ni(NO3)26H2O, Zn(NO3)26H2O, Co(NO3)26H2O, Fe(NO3)39H2O, Cu(NO3)23H2O and AgNO3 were supplied by Xilong Chemical Co., Ltd., China as the precursors of metal ions, respectively. Mn(NO3)24H2O (98%) and Ce(NO3)36H2O (99%) were obtained from Alfa Aesar (USA). A commercial TiO2 (Degussa P25, 80% anatase and 20% rutile) was used as the catalyst for comparison. NaF, tert-butanol (tBA) were purchased from Xilong Chemical Co., Ltd., China. Nitrogen (purity 99.999%) was provided by Beijing Qianxi Gas Co. Ltd., China. All chemicals used in this study were at least in analytical grade without further purification. Ultra-pure water was used for all synthesis and treatment. 2.2. Preparation of metal ion doped TiO2 Metal ion (Mn2+, Ni2+, Zn2+, Co2+, Fe3+, Ce3+, Cu2+, Ag+) doped TiO2 were fabricated using a double-layered hydrothermal method modified from the reported works [30,31]. Typically, 3.52 mL TBT, 11.3 mL ethanol and a certain amount of metal nitrate were added

into a 40 mL Teflon cup. After stirring for 30 min, the cup was transferred into a 200 mL Teflon-lined stainless autoclave containing 20 mL of a mixture of ethanol and water (4.4% water, v/v). The solution was then heated in an oven at 180  C for 18 h. Finally, the resulting products were collected and washed several times with ethanol and water, and then dried at 100  C under vacuum. The obtained mono-doped TiO2 were named as M-X (M indicates the metal ion; X/% indicates the feeding molar ratio of the metal ion to Ti), and similarly, co-doped TiO2 were denominated as `M1/M2-X1/X2. 2.3. Characterization The crystal phase and crystallite size were analyzed by X-ray Diffraction (XRD) (X' PERT-PRO MPD) with a CuKa irradiation (l = 0.15406 nm). Diffraction patterns were recorded in a 2u range of 5–90 . The (1 0 1) peak (2u = 25.28 ) of anatase was used for analysis. The average crystallite sizes of nanoparticles were calculated according to the Scherrer formula [32]. The specific surface areas were obtained by the Brunauer–Emmett–Teller (BET) method using N2 physical adsorption (Autosorb-IQ, Quantachrome) at 77 K. Transmission Electron Microscopy (TEM, JEM 2100 microscope) was carried out at an acceleration voltage of 200 kV, to investigate the morphology and microstructure of the doped TiO2. X-ray photoelectron spectroscopy (XPS) data were obtained on an ESCALAB 250Xi instrument (Thermo Fisher Scientific, USA). The UV–vis diffuse reflectance spectroscopy (DRS) of samples was carried out using Varian Cary 5000. The determination of the optical band gap (Eg) of the catalysts was derived from Eq. (1):

aðhnÞ ¼ Bðhn  Eg Þ1=2

ð1Þ

where a is the optical adsorption coefficient, B is a constant dependent on the transition probability, h is the Plank’s constant and y is the frequency of the irradiation. The Eg values were calculated by plotting (ahy)2 versus hy, followed by extrapolation of the linear part of the spectra to the energy axis [33]. 2.4. Photocatalytic degradation The photocatalytic activity of the synthesized materials was evaluated through the OA degradation under UV radiation (200– 400 nm). The reactions were carried out at 25  C in a 400 mL cylindrical borosilicate glass reactor with a quartz cap, containing 300 mL of 1 mmol/L (mM) OA solution (initial pH was about 3) and 0.2 g/L of catalyst. The solution was irradiated under a 300 W Xenon lamp, with a radiant flux of about 360 mW/cm2 on the surface. During each run, the solution was magnetically stirring in dark for 30 min to obtain an adsorption/desorption equilibrium. For the N2-purging experiments, N2 was introduced from a gas diffuser at the bottom of the reactor for 30 min to remove DO in the reaction system. The concentration of OA was analyzed by high performance liquid chromatography (HPLC, Agilent series 1200, USA) equipped with a Zorbax SB-Aq column and a UV–vis detector qualified at 210 nm. The mobile phase was a mixture of methanol and water containing 10 mM H3PO4 (20/80%, v/v). 3. Results and discussion 3.1. Textural and photocatalytic properties of monometallic ion doped TiO2 Fig. S1a displays XRD patterns of the pure and monometallic ion doped TiO2 nanoparticles prepared via the double-layered hydrothermal manner. It is revealed that diffraction peaks of all

J. Xiao et al. / Journal of Photochemistry and Photobiology A: Chemistry 315 (2016) 59–66

the samples match well with those of a standard anatase TiO2 (JCPDS card No. 04-0477) with high crystallinity. No characteristic peaks of anatase TiO2 disappeared after metal ion doping, demonstrating that the crystallographic structures of the doped samples were similar to pure TiO2. No characteristic peaks of metal/metal oxides were observed, and no shift of peak positions occurred on M-1. This was possibly attributed to the low concentration and well dispersion of metal species on TiO2. As shown in Fig. S2a, pure TiO2 exhibited stronger photoabsorption in the range of 200–325 nm but weaker absorption at between 325 and 400 nm compared to M-1. There were no big distinctions among M-1 in terms of the photoabsorption at between 200 and 325 nm. The photoabsorption intensities of Ag-1 and Cu-1 were quite close to that of pure TiO2 but lower than that of other M1 materials (Mn-1, Ni-1, Zn-1, Co-1, Fe-1 and Ce-1) at between 325 and 400 nm. Fig. S2b presents the calculated Eg values. Zn-1, Cu-1 and Ag-1 had nearly the same Eg values as that of the pure TiO2 (3.31 eV), while Mn-1, Ni-1, Co-1, Fe-1 and Ce-1 possessed slightly lower Eg values. The valance states of metal species in the doped TiO2 were further investigated by XPS. Fig. S3 shows the Ag3d XPS spectrum for Ag-1 and Cu2p XPS spectrum for Cu-1. Two symmetrical peaks at 367.7 and 373.7 eV were corresponded to Ag3d5/2 and Ag3d3/2, confirming the only existence of Ag+ in Ag-1 [25]. The representative peaks at 932.6 and 952.4 eV for Cu2p3/2 and Cu2p1/2 revealed the only form of Cu2+ in Cu-1. Hence, the valance state of the doped metal ions kept the same with the feeding nitrates, which was in accordance with our previous work [31]. The metal ions are likely incorporated into the interstitial positions or substitutional sites of TiO2 crystallite structure. According to the effective radii of doping metal ions [34], Zn2+ (0.74 Å), Ce3+ (1.01 Å), Cu2+ (0.75 Å) and Ag+ (1.15 Å) have much larger ionic radii than Ti4+. Therefore, it is difficult for these four ions to enter the lattice of TiO2 to replace the Ti4+ ion, but they may

Table 1 Textural properties and adsorption performance of the fabricated photocatalysts. Photocatalyst

Feeding ratioa (mol%)

Dpb (nm)

SBET (m2/g)

Adsorption of OAc (mmol/g catalyst)

Pure TiO2 P25 Mn-1 Ni-1 Zn-1 Co-1 Fe-1 Ce-1 Cu-1 Ag-1 Cu-3 Cu-5 Cu-7 Ag-0.1 Ag-0.4 Ag-0.7 Ag-3 Ag-5 Ag-7 Ag/Cu-0.1/0.9 Ag/Cu-0.2/0.8 Ag/Cu-0.33/0.67 Ag/Cu-0.5/0.5 Ag/Cu-0.67/0.33 Ag/Cu-0.75/0.25 Ag/Cu-0.8/0.2 Ag/Cu-0.9/0.1

  1 1 1 1 1 1 1 1 3 5 7 0.1 0.4 0.7 3 5 7 Ag:0.1; Cu:0.9 Ag:0.2; Cu:0.8 Ag:0.33; Cu:0.67 Ag:0.5; Cu:0.5 Ag:0.67; Cu:0.33 Ag:0.75; Cu:0.25 Ag:0.8; Cu:0.2 Ag:0.9; Cu:0.1

12.7 21.0 11.3 10.2 10.2 12.7 9.7 10.7 10.7 12.7 11.3 11.3 9.2 12.0 12.7 12.7 12.7 12.7 12.7 12 11.3 11.3 12.0 12.7 12.7 12.7 12.7

120.7 50.0 148.6 130.7 151.6 140.9 139.1 156.8 156.3 98.7 143.2 142.2 151.1 113.6 111.3 106.5 112.7 116.2 104.1 132.3 134.0 137.4 136.5 130.3 140.0 136.4 145.7

579.6 155.2 740.7 908.2 846.6 821.1 682.9 664.9 513.1 761.1 159.4 161.7 382.1 696.9 820.1 679.5 580.0 627.6 463.8 1062.2 574.6 810.3 988.5 922.4 771.7 653.7 950.7

a b c

The feeding molar ratio of the metal ion to Ti. Calculated by Scherrer equation based on the (1 0 1) peak (2u = 25.28 ). Determined after an adsorption–desorption equilibrium for 30 min.

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reside in the interstitial sites of TiO2 lattice [14,23,26,34]. On the other hand, Mn2+ (0.67 Å), Ni2+ (0.69 Å), Co2+ (0.65 Å) and Fe3+ (0.55 Å) have comparable or smaller radii than Ti4+ (0.61 Å). It is thermodynamically favorable for these four ions to substitute Ti4+ in the TiO2 matrix [14,23,26,34]. The textural properties and adsorption performance of all the fabricated materials are exhibited in Table 1. The transitional metal ion doping slightly decreased the crystallite size of TiO2, except for Co-1. This reduction can be attributed to the segregation of the dopant cations at the grain boundary, which inhibits grain growth by restricting the coalescence of some smaller neighboring grains. And the inverse effect of Co2+ was also reported in other papers [24,35,36]. The surface area of pure TiO2 was 120.7 m2/g, which is larger than that of Ag-1 (98.7 m2/g), but lower than those of Mn-1, Ni-1, Zn-1, Co-1, Fe-1, Ce-1, Cu-1 (130.7–156.8 m2/g) due to their smaller crystallite sizes. This is consistent with the XRD result. Correspondingly, some M-1 (M: Mn, Ni, Zn, Co, Fe, Ce) materials showed more potent adsorption than the pure TiO2. However, Cu1 with larger surface area showed weaker adsorption, while Ag1 with smaller surface area exhibited stronger adsorption compared to the pure TiO2. Different surface charge properties may contribute to this phenomenon [37]. Ag-1 probably exhibited more puissant electrostatic interaction towards the target compound compared to Cu-1. Fig. 1 shows the total photocatalytic decomposition of OA and their apparent rate constants with suspension of various metal ion doped TiO2 under UV irradiation. The process followed clear pseudo-zero-order kinetics according to the linear trends of C/C0 versus irradiation time (Fig. 1 and Table S1). The rate constants decrease in the following order: Ag-1 > Cu-1 >> Ce-1 > Fe-1 > pure TiO2 > P25 > Co-1 > Zn-1 > Ni-1 > Mn-1. Ag-1 showed 2.97-fold and 3.10-fold higher mineralization rate of OA compared with pure TiO2 and Degussa P25, respectively. And the degradation rate with Cu-1 was 2.61-fold and 2.71-fold higher compared to that with pure TiO2 and P25, respectively. However, Ag-1 and Cu-1 had negligible photocatalytic efficiency in OA degradation under visible-light irradiation (Fig. S4b), mainly due to their extremely low photoabsorption intensities in visible-light region (420– 800 nm) (Fig. S4a). It is well known that the surface adsorption, photoabsorption and charge separation over the catalyst surface significantly affect the photocatalytic efficiency. The photoabsorption intensities of Ag-1 and Cu-1 were quite close to that of pure TiO2 but lower than that of other M-1 materials (Mn-1, Ni-1, Zn-1, Co-1, Fe-1 and Ce-1) especially at between 325 and 400 nm (Fig. S2a). The effect of photo-absorption capability of Ag-1 and Cu1 can be excluded from the possible reasons of enhanced photocatalytic activity. The adsorption of OA on these two samples was smaller or comparable to other M-1 (Table 1). The positive influence of Ag+ and Cu2+ on the photocatalytic activity of TiO2 may be explained by their capacity to modify the interfacial electron transfer into electron acceptors such as oxygen and also their roles as electron traps. 3.2. Influence of doping concentration of Ag+ and Cu2+ To further evaluate the effect of Ag+ and Cu2+ doping amounts on the photocatalytic activity, we carried out a set of tests to degrade OA in the presence of Ag-X and Cu-X, respectively. Fig. 2 shows the effect of doping contents of Ag+ and Cu2+ on OA destruction. The photocatalytic efficiency increased with the doping content of Ag+ from a molar ratio of 0.1–1%. Further increasing the Ag+ content slightly weakened the photocatalytic performance. Similarly, 5 mol% Cu2+ doped TiO2 stood out among Cu-X samples. As the content of the doped metal ion increases, the surface barrier turns higher, and hence the space charge region becomes narrower, and the electron-hole couples within the

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Fig. 1. Pseudo-zero-order kinetics of photocatalytic decomposition of OA with various metal ion doped TiO2 photocatalysts (a) and the comparison of their apparent rate constants (b).

region are separated effectively by the strong electric force before recombination [27,38]. Nevertheless, when the dopant exceeds a certain amount, the space charge region becomes too narrow and the penetration depth of UV irradiation greatly exceeds the space charge layer, consequently resulting in higher possibility of charge recombination [27]. An excessive number of metal ions can instead serve as recombination centers for electron-hole couples, and the quenching of the captured carriers thereby occurred upon the doped metal ions [13].Therefore, there theoretically exists an optimum doping concentration for Ag+ or Cu2+, and an excessive amount of doping would greatly decrease the overall photonic efficiency [27]. Paradoxically, when the doping amount further increased from the optimum one, the influence on photocatalytic performance was slight and even negligible. When increasing the Ag+ concentration from 1% (optimum) to 7%, the rate constant decreased to a quite limited extent from 0.088 mM/min to 0.074 mM/min with a loss of 0.014 mM/min (Table S1), and the removal rate decreased by 7% and 14% at the half and terminal reaction time, respectively. However, a reduce of oxidation rate from 0.154 min1 to 0.132 min1 was found by Behnajady et al. when the doping content increased from 1% (optimum) to 1.5% [13]. A 35% loss of rhodamine B removal rate was also revealed by Xin et al. when improving the dopant amount from 5% (optimum) to 7% [39]. The photocatalytic activity of Cu-X was not sensitive to the dopant amount either. This refuted a large possibility of charge recombination when excessive metal ions were present. And it also indirectly challenged the dominant role of doped metal ions as electron captors in facilitating surface-charge separation, because

its negative impact was scarcely presented when the doping amount was excessively above the optimum one. The XRD results of Ag-X and Cu-X are shown in Fig. S1b and S1c, respectively. The intensity of anatase peak at 2u = 38.08 (0 0 4) gradually increased with increasing amount of Ag+ from 3 mol% to 7 mol% (Fig. S1b). Meanwhile, three Ag peaks were notably observed at 44.25 (2 0 0), 64.41 (2 2 0) and 77.36 (3 11) when the Ag+ content augmented from 3% to 7%. Pham et al. also revealed the simultaneous occurrence of Ag and Ag2O in the lattice of TiO2 when the doping content is higher than 5% using AgNO3 as the source of Ag+ dopant [25]. Metallic Ag was formed by the thermal decomposition from Ag2O [25]. However, nearly no change was observed in the characteristic peaks of anatase TiO2 when increasing the doped Cu2+ from 0.4 mol% to 7 mol% (Fig. S1c). This indicated that the crystalline structure of Cu-X was similar to that of pure TiO2. Ag+ doped TiO2 with different Ag/Ti ratios had comparable crystallite sizes with pure TiO2 according to Table 1. This finding is consistent with other reported papers [23,40], showing that the doped Ag+ has quite a limited effect on inhibiting the growth of titania grain. Interestingly, the Cu2+ doped TiO2 (Cu1, Cu-3, Cu-5 and Cu-7) with larger surface area showed weaker adsorption (Table 1), while the Ag+ doped TiO2 (Ag-0.1, Ag-0.4, Ag0.7, Ag-1, Ag-3 and Ag-5) with smaller surface area exhibited stronger adsorption compared to the pure TiO2. This behavior possibly resulted from varied surface charge properties of the photocatalysts in the acidic condition, which can also determine the adsorption of OA in the aspect of electrostatic interaction

Fig. 2. Influence of Ag+ (a) and Cu2+ (b) doping amounts on the resultant apparent rate constants and OA removal rate at 5 min and 10 min with Ag-X (a) and Cu-X (b).

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photo-induced electrons and suppress charge recombination and consequently improve its photocatalytic activity. However, the abnormal behavior regarding the increase of dopant content implied other dominant roles of doped metal ions other than electron traps. 3.4. Mechanism of enhanced photocatalytic property

Fig. 3. Pseudo-zero-order rate constants of photocatalytic degradation of OA on Ag+-Cu2+/TiO2 with different Ag+/Cu2+ molar ratios.

among the semiconductor surface, degraded substrate and charged radicals formed in the reactions [37]. 3.3. Photocatalytic activity of Ag+-Cu2+/TiO2 Fig. 3 reveals the total OA mineralization in the presence of Ag+Cu /TiO2 under UV. These co-doped samples were fabricated with varied molar ratios of Ag/Cu and a total molar ratio at 1%. Similarly to the mono-doped TiO2, the degradation rate constant was not sensitive to the doping amount, which increased from 0.072 mM/ min to 0.090 mM/min with the augment of Ag+/Cu2+ from 0.1/0.9 (mol/mol) to 0.8/0.2 (mol/mol). When the doping proportion of Ag+/Cu2+ exceeded 0.8/0.2, the photocatalytic activity very slightly decreased. Ag/Cu-0.8/0.2 (containing 0.8 mol% Ag+ and 0.2 mol% Cu2+) outperformed other co-doped samples, while its photocatalytic activity is just comparable to that of Ag-1 and Cu-5. No synergistic effect of Ag+-Cu2+ co-doping was observed. These three materials were superior to many reported TiO2 composites in photocatalytic destruction of OA under UV, even though more expensive Au or Pt were modified on TiO2 (Table 2). Fig. S5 exhibits the TEM micrographs of the pure TiO2, Ag-1, Cu5 and Ag/Cu-0.8/0.2. These images display the particles with uniform size distribution, spherical or cubic morphology and very slight agglomeration. The particle sizes of these samples were in the range of 8–15 nm, which were in agreement with the crystallite sizes calculated from the XRD patterns. The anatase phase was further confirmed by the lattice spacing of 0.35 nm from the inlet of Fig. S5a. Most of the electrons and holes are formed closed to TiO2 surface under UV, and the surface recombination of charge carriers is the main factor inhibiting the photonic efficiency [14]. In this case, Ag+ and Cu2+ upon TiO2 surface can separately capture the 2+

Table 2 Reaction rate constants in photocatalytic degradation of OA with the studied catalysts and other reported materials. Photocatalyst

kapp (mM/min)

Calcination

Reference

Ag-1 Cu-5 Ag/Cu-0.8/0.2 1% Ag/TiO2 1% Pt/TiO2 1% Au/TiO2 Au/TiO2 Au/TiO2 Au/N-TiO2

0.088 0.088 0.090 0.059 0.065 0.069 0.077 0.078 0.085

W/O W/O W/O W/O W/O W/O 420  C/2 h 420  C/3 h 420  C/3 h

This study This study This study [41] [41] [42] [36] [15] [15]

The reactive species and photocatalytic reaction pathway were determined through the influence of various scavengers and N2 purging on the degradation rate of OA as elucidated in Fig. 4. Fluoride has demonstrated to have strong adsorption on TiO2 surface [27,43]. The surface-bound or adsorbed hydroxyl radicals (
OHads) and other ROSads can be almost substituted by F–. Hence, NaF could distinguish the contribution of surface and liquid-phase oxidation as demonstrated in literatures [26,27,43]. As shown in Fig. 4a, OA photo-degradation was greatly suppressed by 100 mM NaF, indicating surface reaction functioned crucially in OA mineralization. Quantitatively, the removal rate of OA with Ag/Cu-0.8/0.2 decreased from 89% to 13% when 100 mM NaF was added. This revealed 76% of OA was oxidized over Ag/Cu0.8/0.2 surface with a prior adsorption, while 13% of the pollutant was destroyed by the reactive species in bulk solution. Comparatively, surface/liquid-phase oxidation accounted for 67/20 (%), 47/30 (%) and 53/36 (%) in OA mineralization in the UV/Ag-1 (Fig. 4b), UV/Cu-1 (Fig. 4c) and UV/pure TiO2 (Fig. 4d) systems, respectively. Hydroxyl radicals in bulk solution (
OHbulk) were widely reported as the dominant reagent in TiO2 photocatalytic reactions [44]. Its role in the photo-degradation of OA was evaluated through the addition of tBA (an efficient
OHbulk scavenger). Fig. 4a shows the addition of 100 mM tBA hardly changed the degradation rate of OA. This implies that
OH in bulk solution was not involved in OA photo-destruction with Ag/Cu-0.8/0.2 as catalyst, which was similar to that with Cu-1 (Fig. 4c) as catalyst. The 13% of OA oxidized in bulk solution may be caused by the ROS generated through the trapping of photo-induced electrons by DO. In terms of UV/Ag-1 and UV/pure TiO2 systems, oxidation by
OHbulk was also not important, and contributed as low as 12% (Fig. 4b) and 6% (Fig. 4d) to OA oxidation, respectively. This meant 8% of OA was oxidized by the ROS in bulk solution (ROSbulk) with Ag-1 whereas that was 30% in presence of pure TiO2, for 20% and 36% of OA were removed in bulk solution in total, respectively. Photocatalytic reaction with continuous N2 purging was carried out to determine the role of DO in OA removal. In the N2-purging system with Ag/Cu-0.8/0.2, when the DO concentration decreased from 5.99 ppm to 0.27 ppm, the OA removal decreased by 71%. This indicated that the influence of DO could not be ignored, and ROS both in bulk solution and upon the catalyst surface was the uppermost oxidant in the photocatalytic oxidation of OA. Despite 13% of OA was destroyed by ROSbulk, ROSads was more significant and it contributed to 58% of OA decomposition. The left 18% of OA destructed over the catalyst surface is possibly caused by the photo-induced holes (hvb+) and
OHads. As to photocatalysis with Ag-1, Cu-1 and pure TiO2, 65%, 6% and 26% of OA were mineralized by ROSads, while 2%, 41% and 27% of OA were oxidized by hvb+ plus
OHads. Table 3 elucidates the proportions of surface and liquid-phase oxidation to the whole OA mineralization in presence of Ag/Cu0.8/0.2, Ag-1, Cu-1 and pure TiO2, respectively. The contribution of ROSads was 75% with Ag-1, while that was 65%, 29% and 8% with Ag/Cu-0.8/0.2, pure TiO2 and Cu-1. Accordingly, the oxidation by ROSbulk varied in the opposite trend. The contribution increased from 9% upon Ag-1 to 15% over co-doped TiO2, to 34% over pure TiO2, and further to 39% over Cu-1. Moreover, the ratio of ROSads/ROStotal involved in photocatalysis with Ag-1 and

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Fig. 4. Photocatalytic degradation of OA with Ag/Cu-0.8/0.2 (a), Ag-1 (b), Cu-1 (c) and pure TiO2 (d) in N2-purging system or with different scavengers in air equilibrium system.

Ag/Cu-0.8/0.2 were 89% and 81%, and it was 43% and 35% higher than that in UV/pure TiO2, respectively. Moreover, valance band (VB) XPS was performed to determine the VB edges of Ag-1, Cu1 and pure TiO2, as shown in Fig. 5. It was found that the VB maximum of Cu-1 were equal to that of pure TiO2, while Ag-1 possessed 0.28 eV up-shifted VB level. The bandgap values of Ag-1 (3.32 eV), Cu-1 (3.31 eV) and pure TiO2 (3.31 eV) were nearly the same (Fig. S2b). This indicated that Ag-1 also exhibited 0.29 eV up-shifted conduction band (CB) minimum compared to Cu-1 or pure TiO2. Hence, the photoinduced electrons upon the CB of Ag1 were more reducing, which were more easily captured by DO to form ROSads, while the hvb+ became less oxidative. This was in agreement with the trapping experiments, confirming that Ag+ doping could promote the electron capture by DO yet weaken the oxidation by hvb+. Considering also into account the negligible influence of excessive dopant amount, it was concluded that the doped Ag+ could promote the adsorption of DO towards TiO2 surface further with facilitated capture of photo-generated electrons, ultimately accelerating ROSads generation. It functioned

similarly to a silver-modified mesoporous TiO2 reported by Xiong et al. [29] and a F-residual 001-facet-exposed TiO2 reported by Luan et al. [45]. However, the doped Cu2+ played a disparate role from Ag+. Cu2+ doping resulted in the weakest ROSads generation, while it could facilitate the oxidation by
OHads and hvb+ upon TiO2 surface. The contribution of
OHads plus hvb+ was 53% with Cu-1, which was 33% and 51% larger than that with Ag/Cu-0.8/0.2 and Ag-1, respectively. Ag+-Cu2+ co-doping integrated these two kinds of positive impacts, and inclined more towards the role of higherdoped Ag+. Though Ag+-Cu2+ co-doping contributed to less ROSads generation than Ag-1 and inferior oxidation by
OHads plus hvb+ than Cu-1, it caused as strong surface oxidation in total as 85%, which was 8% and 24% higher than that of Ag-1 and Cu-1, respectively. Fig. 6 illustrates the three oxidation pathways

Table 3 Normalized contribution of different oxidation pathways in photocatalytic destruction of OA under UV irradiation. Photocatalyst

Ag/Cu-0.8/0.2 Ag-1 Cu-1 Pure TiO2 a b

Surface oxidation ROSadsa




65% 75% 8% 29%

20% 2% 53% 30%

Liquid-phase oxidationb

OHads and hvb+ ROSbulk 15% 9% 39% 34%




ROSads/ROStotal

OHbulk

0% 14% 0% 7%

81% 89% 17% 46%

ROS here excludes hydroxyl radicals and mainly refers to superoxide radicals. Oxidation in bulk solution.

Fig. 5. VB XPS of Ag-1, Cu-1 and pure TiO2.

J. Xiao et al. / Journal of Photochemistry and Photobiology A: Chemistry 315 (2016) 59–66

Fig. 6. Reaction mechanism of photocatalytic decomposition of OA with Ag+-Cu2 + /TiO2 under UV irradiation.

involved in photocatalytic decomposition of OA over Ag/Cu0.8/0.2 under UV irradiation. Oxidation by ROSads,
OHads plus hvb+, and ROSbulk accounted for a 65%, 20% and 15% proportion in OA destruction into carbon dioxide and water, respectively.
OHbulk made no difference during photocatalysis. 4. Conclusion Two disparate impacts of metal ion doped TiO2 on reactive species generation during photocatalysis were revealed in this study. Other than as electron captors, Ag+ doping could augment DO adsorption affinity and generate more ROS upon TiO2 surface, while the doped Cu2+ greatly facilitated the oxidation by hvb+ and
OHads. Ag+-Cu2+ co-doping integrated these two positive sides and was thereby very dependent of the surface oxidation. The photoactivity of co-doped TiO2 is 3.04-fold and 3.17-fold higher than that of pure TiO2 and Degussa P25, respectively, and is thereby of great potential for other photoreactions in environmental area. The study may help to extend the high-activity mechanism of widely investigated metal ion doped TiO2 and also provide feasible routes to further improve photocatalytic removal of refractory organics from water. Acknowledgements The authors greatly appreciate the financial support from National Natural Science Foundation of China (No. 21207133) and the National Key Technology R&D Program (No. 2011BAC06B09). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2015.09.013. References [1] M. Pera-Titus, V. Garcia-Molina, M.A. Banos, J. Gimenez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal. B: Environ. 47 (2004) 219–256. [2] J.D. Xiao, Y.B. Xie, H.B. Cao, Organic pollutants removal in wastewater by heterogeneous photocatalytic ozonation, Chemosphere 121 (2015) 1–17. [3] L.L. Xing, Y.B. Xie, H.B. Cao, D. Minakata, Y. Zhang, J.C. Crittenden, Activated carbon-enhanced ozonation of oxalate attributed to HO. oxidation in bulk solution and surface oxidation: effects of the type and number of basic sites, Chem. Eng. J. 245 (2014) 71–79.

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