TiO2 Photocatalysts: Spatially

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Phenylamine-Functionalized rGO/TiO2 Photocatalysts: Spatially Separated Adsorption Sites and Tunable Photocatalytic Selectivity Huogen Yu, Pian Xiao, Jing Tian, Fazhou Wang, and Jiaguo Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09903 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

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ACS Applied Materials & Interfaces

Phenylamine-Functionalized rGO/TiO2 Photocatalysts: Spatially Separated Adsorption Sites and Tunable Photocatalytic Selectivity

Huogen Yu,†,‡* Pian Xiao,‡ Jing Tian,‡ Fazhou Wang†, Jiaguo Yu§

†

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan 430070, People’s Republic of China ‡

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Wuhan 430070, PR China §

State Key Laboratory of Advanced Technology for Material Synthesis and

Processing, Wuhan University of Technology, Wuhan 430070, PR China

1

ACS Paragon Plus Environment


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ABSTRACT: The preferential adsorption of targeted contaminants on a photocatalyst surface is highly required to realize its photocatalytic selective decomposition in a complex system. To realize the tunable preferential adsorption, altering the surface charge or polarity property of photocatalysts has widely been reported. However, it is quite difficult for a modified photocatalyst to realize the simultaneously preferential adsorption for both cationic and anionic dyes. In this study, to realize the selective adsorption for both cationic and anionic dyes on a photocatalyst surface, the negative reduced graphene oxide (rGO) nanosheets and positive phenylamine (PhNH2) molecules are successfully loaded on the TiO2 surface (PhNH2/rGO-TiO2) with spatially separated adsorption sites, where the negative rGO and positive PhNH2 molecules work as the preferential adsorption sites for cationic and anionic dyes, respectively. It was interesting to find that although all the TiO2 samples (including the naked TiO2, PhNH2/TiO2, rGO-TiO2 and PhNH2/rGO-TiO2) clearly showed a better adsorption performance for cationic dyes than anionic dyes, only the PhNH2/rGO-TiO2 with spatially separated adsorption-active sites exhibited an opposite photocatalytic selectivity, namely the naked TiO2, PhNH2/TiO2 and rGO-TiO2 showed a preferential decomposition for cationic dyes while the resultant PhNH2/rGO-TiO2 exhibited an excellently selective decomposition for anionic dyes. In addition, the resultant PhNH2/rGO-TiO2 photocatalyst not only realizes the tunable photocatalytic selectivity but also can completely and sequentially decompose the opposite cationic and anionic dyes. KEYWORDS: Preferential adsorption, photocatalytic selectivity, adsorption active sites, graphene, photocatalysis 2


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INTRODUCTION Selective catalytic oxidation/elimination of targeted organic contaminants from a mixture is believed to be one of the most significance and promising solution to the environmental issues.1-3 Among various selective catalytic systems, photocatalytic selective degradation is especially attractive owning to its high catalytic activity, cost effectiveness, and environmental friendless, making it potentially suitable for purification of targeted organics.4-6 The well-known TiO2-based photocatalysts have been widely applied for effective and complete degradation of organic pollutants into CO2 and H2O under UV irradiation, primarily due to the strong oxidation power of photogenerated holes (ca. 2.7 eV, vs SHE).7-9 As a consequence, the photocatalytic selectivity of TiO2-based materials is usually far from satisfaction owning to the simultaneously and indistinguishably rapid oxidation of various organic compounds or their intermediate products. Therefore, it is highly crucial and a great challenge to tune the photocatalytic selectivity of TiO2-based photocatalysts for the complete mineralization of targeted organics in a complex mixture. To achieve a high photodegradation efficiency for organic substances, the pre-adsorption of targeted organic substance on a photocatalyst surface is highly required.10-12 Usually, the widely reported TiO2-based photocatalysts show a comparable adsorption performance for various organic substances such as cationic and anionic dyes (Figure 1A-a).13-15 In this case, various dyes can be randomly decomposed owing to their random adsorption, and selective photocatalytic degradation of a specific dye cannot be achieved. To realize the photocatalytic 3



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selectivity, many researchers focused on the surface modification of TiO2 photocatalysts by using specific charged species or polarity molecules to control their surface charge (Figure 1A-b).5,16-19 As a consequence, the oppositely charged dyes can be easily and preferentially adsorbed on the photocatalyst surface, resulting in desirable photocatalytic selectivity. In this case, however, only the oppositely charged dyes can be selectively decomposed for a given photocatalyst while the identically charged dyes are usually remained in this system. Therefore, it is quite interesting and a great challenge to develop a new strategy for the preparation of TiO2 photocatalysts with excellently preferential adsorption for both typical cationic and anionic dyes to realize their controllable photocatalytic selectivity. Considering the obvious advantages of simultaneous adsorption for various dyes in Figure 1A-a and selective adsorption for a specific dye in Figure 1A-b, it is expected that when the photocatalyst surface is simultaneously modified with two kinds of oppositely charged species which are located on a spatially separated position (Figure 1A-c), the resultant photocatalysts not only can exhibit the excellent photocatalytic selectivity, but also can easily and completely decompose the cationic and anionic dyes. In this study, the negative aniline-functionalized-reduced graphene oxide (rGO) nanosheets and positive phenylamine (PhNH2) molecules are successfully loaded on the TiO2 surface with spatially separated loading sites, and the resultant PhNH2-modified rGO-TiO2 (PhNH2/rGO-TiO2) photocatalysts exhibit obviously tunable photocatalytic selectivity. In this case, the negative rGO and positive PhNH2 molecules on the TiO2 surface work as the preferentially adsorption-active sites for 4


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cationic

and

anionic

dyes,

respectively.

More

important,

the

resultant

PhNH2/rGO-TiO2 photocatalyst not only realizes tunable photocatalytic selectivity but also can completely decompose the oppositely cationic and anionic dyes.

Figure 1．． (A) Schematic diagram illustrating the adsorption behaviors of cationic and anionic dyes on TiO2 surface with controllable microstructures. (B) Schematic diagram illustrating the controllable preparation of various photocatalysts and (C) their corresponding XPS N 1s spectra: (a) TiO2, (b) PhNH2/TiO2, (c) rGO-TiO2 and (d) PhNH2/rGO-TiO2.

EXPERIMETNAL SECTION Reagents. Graphene oxide (GO) was synthesized from natural graphite powder (>99.8%, Nanjing XFNANO Materials Tech Co., P.R. China) by a modified Hummer’s method shown in our previous studies,20, 21 and the brown GO solution (0.1 5



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wt%) was obtained by ultrasonic dispersion of GO (0.5 g) in deionized water (500 mL) for 2 h. Commercial Degussa TiO2 nanoparticles (P25 TiO2, a SBET of ca. 50 m2/g, 80% anatase and 20% rutile) was pre-treated at 550 oC for 3 h in order to obtain a clean TiO2 surface, which is suitable for the following preparation of rGO-TiO2 nanocomposite with a strongly coupling interface. All the other reagents (analytical grade) were supplied by Shanghai Chemical Ltd. (P.R. China) and used as received without further purification. Distilled water was used in all experiments. Preparation of rGO-TiO2 photocatalyst. The rGO-TiO2 composite was obtained through a simple hydrothermal method. According to our previous studies,20,21 it was found that when the GO amount (WGO/WTiO2) was controlled to be 1 wt%, the prepared rGO-TiO2 showed the highest photocatalytic performance. Therefore, in this case, the amount of GO in the rGO-TiO2 composite was controlled to be 1 wt%. Typically, 0.2 g of the calcined P25 power was first dispersed into 15 mL distilled water and then 2 mL of 1 mg/mL GO aqueous was added into the above suspension solution. After strong stirring for 3 h at room temperature, the mixed solution was transferred into a Teflon-lined stainless steel autoclave and then maintained at 160 oC for 8 h. After cooling down to the room temperature naturally, the synthesized rGO-TiO2 composites were washed through distilled water for several times. For comparison, naked TiO2 sample without rGO and pure rGO nanosheets were also synthesized by a hydrothermal procedure under the same reaction conditions. Preparation of phenylamine-functionalized rGO-TiO2 (PhNH2/rGO-TiO2) 6


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photocatalysts. The PhNH2/rGO-TiO2 samples were prepared by a simple impregnation method. For a typical synthesis, 0.2 g of the resulting rGO-TiO2 was immersed in a PhNH2-C2H5OH solution (0.1 mol/L for phenylamine) and then stirred at room temperature for 5 h. After that, the as-prepared samples were washed with distilled water and ethanol for several times and dried at 60 oC for overnight. In this case, the weight ratio of PhNH2 to GO in the PhNH2/rGO-TiO2 composite is controlled to be 10:1, 20:1, 30:1, 40:1 and 50:1, and the resulting sample can be referred to as PhNH2/rGO-TiO2(X), where X represents 10:1, 20:1, 30:1, 40:1 and 50:1, respectively. According to our experimental results, it was found that when the PhNH2 amount (compared to GO) was controlled to be 30:1, the resultant PhNH2/rGO-TiO2(30:1) sample showed the highest photocatalytic degradation activity. Therefore, in this study, the PhNH2/rGO-TiO2(30:1) was used as the reference sample and was referred to be PhNH2/rGO-TiO2. It should be noted that though the amount of PhNH2 was obviously higher than that of rGO nanosheets in the PhNH2-C2H5OH solution during preparation, it was found that only a small amount of PhNH2 could be successfully loaded on the surface of PhNH2/rGO-TiO2, which can be determined by the XPS results. For comparison, the PhNH2/TiO2 was also prepared by an impregnation method under an identical reaction condition as that of PhNH2/rGO-TiO2(30:1) without the addition of GO nanosheets. Characterization. X-ray diffraction (XRD) patterns were obtained on a D/MX-

ⅢA X-ray diffractometer (Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) 7



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measurements were done on a KRATOA XSAM800 XPS system with Mg Kα source. Raman spectra were recorded at room temperature using an INVIA/INVIA spectrophotometer (Renishaw, China) with a 514.5nm Ar+ laser as an excitation source. Fourier Transform Infrared spectra (FTIR) were acquired using a Nexus FT-IR spectrophotometer (Thermo Nicolet, America). UV-vis absorption spectra were obtained using a UV-Visible spectrophotometer (UV-2450, SHIMADZU, Japan). The ζ-potential analysis was conducted in suspension solution (pH = 7) containing various samples (5 mg) on a Zeta potential Instrument (ZetaPALS/90plus, Brookhaven, USA). Photocatalytic activity. The evaluation of photocatalytic selectivity of mixed methyl orange (MO) and methylene blue (MB) dyes was conducted in aqueous solution containing the prepared samples at ambient temperature. Experimental details were shown as follows: 50 mg of the sample was dispersed into 20 mL of mixed solution of MO (10 mg/L) and MB (10 mg/L) in a cylindrical reaction vessel (25 mL in capacity) and a 365-nm LED lamp with a light intensity of 80 mWcm−2 was used as a light source. Before UV irradiation, the mixed suspension was placed in dark for 3 h without stirring, and then the suspension was exposed to UV-light irradiation under stirring condition at room temperature. At certain time intervals, the reaction solution was centrifuged to measure the concentration of various dyes. Owing to a low concentration of the dyes in this system, their photocatalytic reaction can be supposed to be a pseudo-first-order reaction. Therefore, its kinetics may be expressed as ln(ct/c0) = -kt, where k is the apparent rate constant, and c0 and ct are the dye concentrations at 8


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initial state and after irradiation for t min, respectively. In addition to the above MO-MB mixing system, other mixing dye solutions such as MO (10 mg/L) and RhB (10 mg/L), Orange II (10 mg/L) and RhB (10 mg/L), and Reactive brilliant red X-3B (10 mg/L) and RhB (10 mg/L), have also been used as the target dyes to investigate the selective photocatalytic performance. Adsorption-desorption

equilibrium

studies.

The

adsorption-desorption

equilibrium studies were conducted under dark by stirring 20 mL mixing solution of MO (10 mg/L) and MB (10 mg/L) with 50 mg of the prepared sample in a 25 mL cylindrical reaction vessel at ambient temperature. At certain time intervals, the catalyst powder was separated from the mixture by centrifugation and then the amount of adsorption for the dye was determined by using a UV-Visible spectrophotometer (UV-2450, SHIMADZU, Japan).

RESULTS AND DISCUSSION The synthetic strategy of the PhNH2/rGO-TiO2 photocatalyst is illustrated in Figure 1B. Firstly, TiO2 nanoparticles (Figure 1B-a) were obtained through a high-temperature calcination method by using P25 TiO2 as the precursor. According to our previous studies, 20, 21 the rGO nanosheets could be strongly coupled on the TiO2 nanoparticle surface to from rGO-TiO2 composite (Figure 1B-c) via a facile hydrothermal method (see the Supporting Information for details). Subsequently, the resultant rGO-TiO2 composite was dispersed in a PhNH2 solution to prepare the PhNH2/rGO-TiO2 photocatalyst with spatially separated loading sites for negative rGO nanosheets and positive PhNH2 molecules (Figure 1B-d). In this case, the PhNH2 9



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molecules not only can be grafted on the rGO surface via the chemical interactions between the PhNH2 and oxygen-containing groups (such as -COOH and epoxy -C-O-C-) on the rGO surface, but also can be directly modified on the TiO2 surface as the amino groups in PhNH2 can easily be protonated and then be attracted on the TiO2 surface via a strong hydrogen bonding (Figure 1B-d).28,29 For comparison, the TiO2 nanoparticles were also dispersed in the PhNH2 solution to prepare the PhNH2/TiO2 (Figure 1B-b). The above surface modification progress can further be demonstrated by measuring their Zeta potential (ζ). After loading rGO nanosheets, the ζ-value of TiO2 (-9.26 mV) clearly decreased to -10.47 mV for rGO-TiO2 due to the addition of various oxygen-containing groups (such as R-COOH, R-OH, -C=O and -C-O-C-) and π electrons in rGO,20,30,31 while the resultant PhNH2/TiO2 exhibited a slightly increased ζ-value (-8.41 mV) owing to the presence of protonated PhNH3+ molecules.28,29 After the further addition of PhNH2 onto the rGO-TiO2 photocatalyst, the ζ-value of PhNH2/rGO-TiO2 further decreased to be -12.14 mV, which is mainly caused by the formation of negative -OH on the rGO surface via the epoxy ring opening reaction (Figure 1B-d).22 The X-ray diffraction (XRD) patterns (Figure S1) indicate that all the resulting samples mainly consist of well-crystallized anatase with minor amount of rutile, and no diffraction peaks corresponding to rGO nanosheets and PhNH2 molecules can be observed. The typical transmission electron microscopy (TEM) images of the resulting samples are clearly shown in Figure S2. It is found that the particle size of P25 TiO2 is mainly in the range of 10-50 nm (Figure S2A). As for the rGO-TiO2 10


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(Figure S2B) and PhNH2/rGO-TiO2 (Figure S2C,D) composites, they show very similar particle morphology with the pure TiO2. In addition, the rGO nanosheets are tightly covered on the TiO2 particle surface (shown in the red arrow), indicating a well interfacial interaction of rGO nanosheets with the TiO2 nanoparticles. X-ray photoelectron spectroscopy (XPS), Fourier transformation infrared spectroscopy (FTIR) and Raman spectrum are further used to investigate the microstructures of PhNH2/rGO-TiO2 composite. For comparison, pure GO (prepared by the modified Hummer’s method) and rGO (prepared from a hydrothermal treatment of GO solution) are also tested under identical experimental conditions. Figure S3 and Figure S4 show the XPS and FTIR spectra for various samples, respectively. According to the high-resolution XPS C 1s (Figure S3B) and FTIR spectra (Figure S4A), it is found that that compared with as-prepared GO sample, the intensity of all the oxygen-containing groups in rGO, rGO-TiO2 and PhNH2/rGO-TiO2 samples has a significant decrease, suggesting the effective reduction of GO to rGO via the present hydrothermal treatment.32-34 In addition, compared with the TiO2 and rGO-TiO2 photocatalysts (the XPS peak at ca. 399.20 eV is from the adventitious N element from XPS instrument itself), the resultant PhNH2/rGO-TiO2 and PhNH2/TiO2 samples (after grafting PhNH2 molecules) clearly show the new N 1s XPS peaks for C-N (399.64 eV)22,35 and C-NH3+ (401.57 eV)22,36 bonds (Figure 1C), suggesting the formation of covalent bonds between PhNH2 and rGO and a strong hydrogen-bonding between PhNH3+ and TiO2, respectively, in good agreement with our proposed formation mechanism in Figure 1B. Simultaneously, the corresponding FTIR spectra 11



(Figure S4B) also show an obvious absorption peak at 1377 cm-1 in both of the PhNH2/TiO2 and PhNH2/rGO-TiO2 samples, which can be attributed to the stretching vibration of C-N in the PhNH2 molecules,23,37 in good agreement with the XPS results. Figure S5A and B show the corresponding Raman spectrum of various samples. It is clear that the TiO2 and TiO2/PhNH2 photocatalysts only exhibit four Raman characteristic peaks of TiO2 nanoparticles, while in addition to the characteristic peaks of TiO2 nanoparticles, the rGO-TiO2 and PhNH2/rGO-TiO2 photocatalysts also show two typical D and G bands for the rGO nanosheets.20,21 Compared with the GO, rGO and rGO-TiO2, there is a notable increase in the intensity ratio of D/G for PhNH2/rGO-TiO2 (Figure S5B) due to the formation of a strong interfacial interaction (such as C-N-C bond) between rGO nanosheets and PhNH2 molecules. The corresponding UV-Vis spectra (Figure S6) show that compared with the pure TiO2 and rGO-TiO2, the PhNH2-modified samples (PhNH2/TiO2 and PhNH2/rGO-TiO2) clearly exhibit enhanced visible-light absorption in the range of 400-700 nm due to the surface loading of PhNH2 molecules, in good agreement with their corresponding

A 1.6

0min 1min 2min 3min 4min 5min 7min 12min 17min 27min 37min 47min

1.2

0.8

MO

B

1.6

Absorbance (a.u.)

color change from white to black (the inset of Figure S6).

Absorbance (a.u.)

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1.2

0min 1min 2min 3min 4min 5min 7min 12min 17min 27min 37min 47min

MB

0.4

0.0

0.8

MO

MB

0.4

0.0

400

500

600 Wavelength (nm)

700

400

500

600

Wavelength (nm) 12


700

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C

4

D

r = kMO/kMB= 0.23 -----MB: k = 0.357 3 -----MO: k = 0.083

2 1 0

4

r = kMO/kMB= 1.88 -----MB: k = 0.084 -----MO: K = 0.158

3 ln(C0/C)

ln(C0/C)

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2 1

0

2

4

6

8

10

0

0

2

Time (min)

4 6 Time (min)

8

10

Figure 2. (A,B) The selective decomposition progress, (C,D) the decomposition rate (k) and (E) their corresponding color-variation images of the MO-MB mixed solution for (A,C, left in E) rGO-TiO2 and (B,D, right in E) PhNH2/rGO-TiO2(30:1).

The photocatalytic selectivity of PhNH2/rGO-TiO2 sample was first evaluated by the photocatalytic decomposition of mixed aqueous solution for cationic methylene blue (MB) and anionic methyl orange (MO) under UV-light irradiation. A typical photocatalytic selectivity of rGO-TiO2 before and after PhNH2 modification is shown in Figure 2. Before PhNH2 modification, it is found that the rGO-TiO2 photocatalyst shows an obviously preferential degradation for MB dye (Figure 2A), indicating that the degradation rate of MB is much faster than that of MO. However, after PhNH2 modification, the resultant PhNH2/rGO-TiO2 photocatalyst exhibits a much higher decomposition rate towards MO than MB dyes (Figure 2B), suggesting its excellent 13



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photocatalytic selectivity for anionic MO dye. According to previous reports,6,38,39 the photocatalytic selective ability for a given photocatalyst can usually be quantified as r =k

anionic/k cationic

(k is the apparent rate constant for a specific dye), and when the r

value is larger (or lower) than 1, indicating the selective decomposition for anionic (or cationic) dye. Therefore, the r values of rGO-TiO2 and PhNH2/rGO-TiO2 can be calculated to be 0.23 (Figure 2C) and 1.88 (Figure 2D), respectively, suggesting the tunable photocatalytic selectivity of rGO-TiO2 via a facile PhNH2 modification. In fact, the above tunable photocatalytic-selective degradation can directly and clearly be observed via the color change of MO-MB mixing solution during UV-light irradiation (Figure 2E). It was clear that the color of MO-MB mixing solution for rGO-TiO2 photocatalyst first changed from the original green (0 min) to orange color (7-37 min) of MO dye and finally become colorless (47 min), while the characteristic blue color of MB dye for the PhNH2/rGO-TiO2 photocatalyst was well remained along with the MO selective degradation, which is in good agreement with the above preferential degradation results. To further investigate the performance repeatability of the PhNH2/rGO-TiO2 photocatalyst, five cycles were carried out for the photocatalytic decomposition of MO-MB dyes. It is found that the PhNH2/rGO-TiO2 photocatalyst can maintain an efficient photocatalytic-selective performance and the corresponding r value of PhNH2/rGO-TiO2 from the first to the fifth time are 1.88, 1.80, 1.68, 1.60 and 1.58, respectively. In addition, the photocatalytic selective decomposition from cationic MB to anionic MO dyes (r value ranging from 0.23 to 2.30) can be gradually controlled in the PhNH2/rGO-TiO2 photocatalysts only by adjusting the PhNH2 14


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amount (Figure 3 and Figure S7). In fact, the above phenomenon for the selective decomposition of anionic MO dye by the addition of PhNH2 molecules can further be demonstrated in the naked TiO2 photocatalyst (Figure S8). It was found that the PhNH2/TiO2 (r = 0.29) also contributed to the enhanced photocatalytic decomposition for anionic MO dye compared with the naked TiO2 photocatalyst (r = 0.20). Therefore, it is very clear that the PhNH2-modified rGO-TiO2 can promote the preferentially photocatalytic decomposition of anionic MO over the cationic MB. r = 2.30

Ratio of the kMO to kMB

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2

1

r = 1.88

r = 1.11

r = 1.00 r = 0.78 r = 0.52 r = 0.23

0

a

b

c

d

e

f

Figure 3. The ratio (r) of the apparent rate constants (k) for various photocatalysts in the MO-MB mixed solution: (a) rGO-TiO2, (b) PhNH2/rGO-TiO2(10:1), (c) PhNH2/rGO-TiO2(20:1), (d) PhNH2/rGO-TiO2(30:1), (e) PhNH2/rGO-TiO2(40:1), (f) PhNH2/rGO-TiO2(50:1).

It is very interesting and worthwhile to investigate that whether the PhNH2/rGO-TiO2 composite can work as a general photocatalyst to selectively decompose the anionic azo dyes in various mixing system. In addition to the above 15



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MO-MB mixing system, other mixed dye solutions such as MO-RhB, Orange II-RhB, and Reactive brilliant red X-3B-RhB (Table S1), have also been used as the target dyes to investigate their selective photocatalytic performance and the corresponding results are shown in Figures S9 and S10. Obviously, it was found that the resultant PhNH2/rGO-TiO2 photocatalyst clearly showed an obviously higher photocatalytic performance for the degradation of anionic MO, Orange II, and Reactive brilliant red X-3B than the cationic RhB dye, while the rGO-TiO2 always exhibited a preferential decomposition for cationic dye, further strongly demonstrating the photocatalytic selectivity of PhNH2/rGO-TiO2 for the anionic azo dyes. As photocatalytic reactions usually occur on the photocatalyst surface during light irradiation, the prerequisite adsorption of targeted dye molecules is highly required for an effective decomposition.40,41 In this sense, the photocatalytic selectivity of a photocatalyst largely depends on its adsorption selectivity for specific dyes. As a consequence, the surface modification of photocatalysts by using specifically charged species or polarity molecules to control their surface charges for the enhanced selective adsorption has been widely reported.19 Figure 4A and Figure S11 show the adsorption behavior of various photocatalysts towards cationic MB and anionic MO dyes, and the corresponding selective-adsorption mechanisms are also illustrated in Figure 4B. Firstly, it can be seen that the calcined P25 TiO2 exhibits preferential adsorption for MB over MO (Figure 4A-a), which can be attributed to the strong electrostatic force between the positive MB and negative TiO2 with a

ζ potential of −9.26 mV (Figure 4B-a), resulting in the selective degradation for MB 16


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dye (Figure S8). After the PhNH2 is loaded on the TiO2 surface, the resultant PhNH2/TiO2 shows an increased ζ potential (-8.41 mV) owing to the successful grafting of protonated PhNH3+ molecules. In this case, the adsorption capacity of PhNH2/TiO2 for MO has an obvious increase while that for MB shows a slight decrease (Figure 4A-b), leading to an improved r value from 0.20 to 0.29 (Figure S8). It has been widely reported that the rGO nanosheets can work as good adsorption active sites for aromatic dyes due to the presence of a large amount of conjugate π-electrons.42,43 Hence, it is quite reasonable to deduce that the incorporation of rGO onto TiO2 surface can significantly improve its adsorption performance for both of the aromatic MO and MB (Figure 4A-c). Most importantly, the resultant rGO-TiO2 sample shows a much higher selective adsorption towards cationic MB than anionic MO dye owing to a strong electrostatic attraction between the positive MB dyes and negative rGO nanosheets (Figure 4B-c). In this case, the rGO-TiO2 shows a more negative ζ potential (−10.47 mV) owing to the presence of various oxygen-containing groups on rGO nanosheets. Therefore, the rGO-TiO2 photocatalyst clearly shows an excellently preferential adsorption and selective decomposition for MB dye (Figure 2C), similar to the TiO2 and PhNH2/TiO2 photocatalysts. After further surface modification by PhNH2 molecules, the adsorption capacity of PhNH2/rGO-TiO2 for MB can further be enhanced while its corresponding adsorption capacity for MO has a slight decrease (Figure 4A-d), which can be attributed to the formation of more negative R–O- groups and π-electrons on the rGO nanosheets via an epoxide ring-opening and amidation reactions between PhNH2 and rGO (Figure 1B and Figure 17



4B-d), resulting in a further decreased ζ potential (-12.14 mV).

A 100

MB MO

0

-Aeq) / A0 (%)

80 60 40

(A

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20 0

a

b

c

18


d

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Figure 4. (A) The adsorption equilibrium value of MO and MB dyes and (B) the preferentially selective absorption process of cationic and anionic azo dyes for various photocatalysts: (a) TiO2, (b) PhNH2/TiO2, (c) rGO-TiO2 and (d) PhNH2/rGO-TiO2.

On the basis of the above results, it is interesting to find that although the PhNH2/rGO-TiO2 shows a much better selective adsorption toward cationic MB than anionic MO, it exhibited a completely different photocatalytic selectivity compared with the TiO2, PhNH2/TiO2 and rGO-TiO2 photocatalysts. The TiO2, PhNH2/TiO2 and rGO-TiO2 photocatalysts showed a preferential decomposition for cationic dyes due to their preferential adsorption, in good agreement with the well-known mechanism about preferential-adsorption-induced preferential-degradation.41,42 However, the resultant PhNH2/rGO-TiO2 exhibited an excellently selective decomposition for anionic dyes, suggesting that the photocatalytic selectivity of photocatalytic materials can be well controlled by adjusting their surface microstructures. As for the PhNH2/rGO-TiO2, the PhNH2 molecules not only can be grafted on the rGO surface, but also can be directly loaded on the TiO2 surface due to the excessive PhNH2 molecules during impregnation progress, a similar grafting with the PhNH2/TiO2 sample. In this case, the negative rGO nanosheets and positive PhNH3+ molecules with spatially separated loading sites have been clearly loaded on the TiO2 surface (Figure 5), where the rGO nanosheets and PhNH2 clusters work as the effective active sites for the preferential adsorption of cationic MB and anionic MO, respectively. Under UV-light irradiation, the photogenerated electrons can easily transfer to the

19



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PhNH2-modified rGO nanosheets to reduction oxygen, while the photogenerated holes on the valence band of TiO2 can easily transfer to the adsorbed anionic dyes, causing their rapid decomposition. After the effective oxidation of adsorbed anionic molecules, the dissociative anionic dyes can further be adsorbed on the PhNH2-loaded active sites, resulting in the selective photodegradation of anionic azo dyes. After the complete decomposition of anionic dyes, the produced hydroxyl radicals from H2O or -OH oxidation by photogenerated holes can easily diffuse onto the adsorbed cationic dyes on rGO surface, leading to the further decomposition of cationic dyes.

Figure 5. The proposed preferential adsorption and selective decomposition mechanism for anionic azo dyes on the PhNH2/rGO-TiO2 with spatially separated adsorption sites.

CONCLUSION

In conclusion, the negative rGO nanosheets and positive PhNH2 molecules are 20


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successfully loaded on the TiO2 surface with spatially separated loading sites, where the negative rGO and positive PhNH2 molecules work as the preferential adsorption sites for cationic and anionic azo dyes, respectively. In this case, both of cationic and anionic azo dyes can simultaneously and preferentially be adsorbed on the PhNH2/rGO-TiO2 photocatalysts. Although all the TiO2 samples (including the naked TiO2, PhNH2/TiO2, rGO-TiO2 and PhNH2/rGO-TiO2) clearly showed a better adsorption

performance

for

cationic

dyes

than

anionic

dyes,

only

the

PhNH2/rGO-TiO2 with spatially separated adsorption-active sites realized the opposite photocatalytic selective decomposition, which exhibited a better decomposition performance

for

anionic

than

cationic

dyes.

In

addition,

the

resultant

PhNH2/rGO-TiO2 photocatalyst not only realizes tunable photocatalytic selectivity but also can completely decompose the opposite cationic and anionic dyes.

21



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ASSOCIATED CONTENT Supporting Information The tunable photocatalytic selectivity for different cationic and anionic azo dyes; XRD patterns; TEM images; XPS spectra; FTIR spectra; Raman spectra; UV-Vis spectra; photocatalytic selectivity for different dyes and photocatalysts; adsorption behavior of various photocatalyst. This material is available free of charge on the ACS Publication website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]; Tel: 0086-27-87756662, Fax: 0086-27-87879468

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51472192, 21477094, and 21277107) and 973 Program (2013CB632402). This work was also financially supported by program for new century excellent talents in university (NCET-13-0944), and the Fundamental Research Funds for the Central Universities (WUT 2015IB002).

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TOC

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TiO2 Photocatalysts: Spatially

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