Surface reconstruction of titania with g-C3N4 and Ag for promoting efficient electrons migration and enhanced visible light photocatalysis

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Surface reconstruction of titania with g-C3 N4 and Ag for promoting efficient electrons migration and enhanced visible light photocatalysis Kah Hon Leong a , Sze Ling Liu a , Lan Ching Sim b , Pichiah Saravanan a,c,∗ , Min Jang a,c , Shaliza Ibrahim a a

Environmental Engineering Laboratory, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia c Nanotechnology & Catalysis Research Center (NANOCAT), University of Malaya, 50603 Kuala Lumpur, Malaysia b

a r t i c l e

i n f o

Article history: Received 19 May 2015 Received in revised form 24 June 2015 Accepted 28 June 2015 Available online xxx Keywords: (g-C3 N4 )–Ag/TiO2 Reconstruction Solar photodeposition Visible light Amoxicillin Electron migration

a b s t r a c t The developments of heterogeneous photocatalysts are one among the competent reconstruction approach to enrich the visible light responsiveness of conventional TiO2 . In the present work the TiO2 was reconstructed with graphitic carbon nitride (g-C3 N4 ) and silver (Ag) to form a ternary (g-C3 N4 )–Ag/TiO2 . The graphitic carbon nitride an intriguing material was prepared through a facile pyrolysis by using urea as a precursor. The silver (Ag) that plays a role as electron-conduction mobiliser in the ternary was synthesised through solar mediated photodeposition method. The synthesised ternary composite characteristics were thoroughly investigated through various physical and chemical analyses. The presence of g-C3 N4 in the ternary photocatalysts promoted the formation of interface between the Ag/TiO2 and gC3 N4 and stimulated the electron transfer between them. These electrons migration acknowledged by the synergic effect prolonged the lifetime of charge carriers. The g-C3 N4 also significantly tuned the energy band of conventional TiO2 . The prepared ternary exhibited significantly high visible light photocatalytic performance by degrading Amoxicillin (AMX) a poor photosensitising pollutant at highest rate. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The hunt for high performance visible light responsive photocatalyst has becomes a vital in order to sustainably address the burning environmental issues. The hunt resulted an innovative carbon material; graphitic carbon nitride (g-C3 N4 ) as a visible light enhancer. It also recently engrossed increasing research attention as an abundance organic semiconductor with narrow band gap energy [1–7]. The heptazine ring present in its structure enables this metal free semiconductor to possess a good physicochemical stability [3,8,9]. It also possesses distinctive two-dimensional layered structure which favours the hybridising with other wide band gap semiconductor photocatalysts [3,10,11]. Therefore, gC3 N4 emerged as a best candidate to reconstruct the well-known conventional TiO2 in order to overcome its setback; poor visible light utilisation and high recombination rate of charge carriers.

∗ Corresponding author at: Environmental Engineering Laboratory, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (P. Saravanan).

Limited attempts have been performed since the foremost discovery by Wang et al., reported g-C3 N4 as an effective visible light enhancer for conventional photocatalysts and demonstrated its photoactivity through water split reaction under visible light [12]. Few studies showed the enhanced visible light photoactivities of reconstructed TiO2 with g-C3 N4 and most of them employed a good photosensitising compound i.e., dyes as a model compound or pollutant rather than the poor [1,13–15]. Though the reconstruction through g-C3 N4 promisingly enables the visible light utilisation but it still possesses a rapid recombination of charge carriers and further hinders the photocatalysis ability. This limitation is up beaten by doping/depositing noble metal with or onto the photocatalysts [16–18]. The presence of noble metal forms a junction between them and serves as an electron sinks that facilitates the effective migration of electrons across the junction. Apart the surface plasmon resonance (SPR), unique characteristic of noble metal catalyses the visible light absorption property [19–25]. Hence reconstructing the conventional TiO2 with noble metal and g-C3 N4 through a proper synthesis route significantly enhance the visible light characteristics along with efficient charge separation of electrons and holes. Very recently Chai and co-workers reconstructed the photocatalyst through the above said composite i.e., (g-C3 N4 )–PtTiO2 and evaluated its visible light performance for water splitting

http://dx.doi.org/10.1016/j.apsusc.2015.06.184 0169-4332/© 2015 Elsevier B.V. All rights reserved.

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reaction. Their findings demonstrated the migration of electrons from the g-C3 N4 to Pt-TiO2 and retarded the charge carrier recombination [26]. However limited findings are available on developing an innovative ternary composite that possesses superior visible light utilisation and effective electrons migration for prolonging the life time of charge carriers. In the present study the conventional TiO2 was reconstructed with the support of Ag and g-C3 N4 . The fabrication of g-C3 N4 was performed with the simple pyrolysis of urea while Ag/TiO2 was achieved through a solar photodeposition approach [17]. Urea being chosen as the precursor due to its nontoxic nature, low cost and also an active molecular under thermal treatment [27]. The visible light photocatalysis of prepared ternary composite was evaluated through the photodegradation of poor photosensitising pollutant Amoxicillin (AMX). 2. Material and methods Titanium (IV) chloride (TiCl4 , 99.9%, Merck), Silver Nitrate (AgNO3 , 99.9%, Sigma Aldrich), tetrahydrofuran (THF, Fluka), urea, nitric acid, ethylene glycol and benzyl alcohol (99.8% anhydrous) were purchased from R&M Chemical and Milli-Q water (18.2 M cm). All chemicals were analytical grade and used as received without any further purification. 2.1. Synthesis of g-C3 N4 and (g-C3 N4 )–Ag/TiO2 A facile thermal heating method as reported by Liu et al. was adopted for synthesising graphitic carbon nitride [27]. The syntheses of conventional TiO2 and Ag/TiO2 were obtained from our previous studies [17,28]. In a typical preparation of ternary composite, 0.012 g prepared g-C3 N4 sheet was well dispersed in Milli-Q water (18.2 M cm) ultrasonically. 0.4 g of freshly prepared Ag/TiO2 was then added into the solution and subjected to 70 ◦ C for 1 h. The resulting suspension was then centrifuged and washed repeatedly with Milli-Q water (18.2 M cm) for few times and dried overnight at 60 ◦ C. 2.2. Characterisation The phase compositions were analysed with Bruker D8 advance X-ray powder diffractometer with Cu K␣ radiation  = 0.154 nm. Raman and photoluminescence (PL) spectra were acquired by using Renishaw, inVia Raman Microscope with the excitation wavelength at 514 nm and 325 nm. The Hitachi SU-8000, FESEM with an accelerating voltage of 20 kV and HRTEM, JEM-2100F, Jeol with 200 kV was utilised for obtaining the morphologies of the prepared samples. The functional group spectrum was obtained through Perkin Elmer 400 IR spectrophotometer with scan range of 4000–450 cm−1 . Axis Ultra DLD instrument of Kratos using monochromatic AlK␣ radiation (225 W, 15 mA, 15 kV) and C1s binding energy of adventitious carbon (284.9 eV) as reference was employed for obtaining X-ray photoelectron spectra (XPS). Shimadzu UV-2600 spectrophotometer equipped with an integrating sphere attachment and BaSO4 as a reference was utilised for obtaining diffuse reflectance spectra of prepared samples.

Fig. 1. XRD pattern of (a) g-C3 N4 , (b) TiO2 , (c) (g-C3 N4 )–TiO2 , (d) Ag/TiO2 and (e) (g-C3 N4 )–Ag/TiO2 .

high-pass UV filter (FSQ-GG400, Newport Corp). All experiments were performed under identical condition for a period of 6 h with 1 h dark experiment. Control experiment was carried out with zero photocatalyst condition to ensure the photocatalytic activity. The residual concentrations of AMX in the samples were quantified through UPLC, Acquity H-Class, Waters mounted with C18 column (2.1 mm × 50 mm, 1.7 ␮m) using KH2 PO4 (pH 1.8): methanol (80:20) as mobile phase at 0.4 mL min−1 . The column was maintained at 40 ◦ C. 3. Results and discussion 3.1. XRD and Raman The phase composition of (g-C3 N4 )–Ag/TiO2 together with binary and pure is presented in Fig. 1. The diffraction obtained for pure g-C3 N4 shows the presence of binary peaks at 13.1◦ and 27.4◦ , attributed to the interlayer stacking of g-C3 N4 that indexed as (0 0 1) and (0 0 2) respectively [29,30]. The diffraction obtained at 38.1◦ (1 1 1), 44.3◦ (2 0 0), 64.4◦ (2 2 0) and 77.4◦ (3 1 1) for binary and ternary signifies the existence of Ag NPs and is also well supported with JCPDS no. 04-0783. It is noted that no typical

2.3. Photocatalytic activity The photocatalytic performance of the prepared composites was studied by adopting Amoxicillin (AMX), a poor photosensitising pollutant. All the photocatalysis experiments were performed in a simple 500 mL borosilicate beaker with a working volume of 250 mL. The initial concentration of the AMX was set at 20 mg L−1 with photocatalyst loading of 1 g under stirring conditions. The visible light was generated by 500 W tungsten–halogen lamps with

Fig. 2. Raman spectra of (a) TiO2 , (b) (g-C3 N4 )–TiO2 , (c) Ag/TiO2 and (d) (gC3 N4 )–Ag/TiO2 .

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Fig. 3. (a) FESEM image, (b-f) HRTEM images of (g-C3 N4 )–Ag/TiO2 .

diffraction peaks of g-C3 N4 appeared for both composites due to the low amount of loading on the surface of the nanocomposite [31]. Fig. 2 displays the presence of anatase phase of the prepared TiO2 through Raman spectrum. The obtained spectrum well correlated with diffraction analysis and further validated the presence of pure anatase phase of TiO2 . 3.2. Morphology The morphology of the (g-C3 N4 )–Ag/TiO2 is illustrated in Fig. 3 (a–f). It is clearly seen that the g-C3 N4 is well distributed onto the surface of Ag/TiO2 . The high resolution image of Fig. 3d clarifies an uniform photodeposition of Ag NPs onto the surface of TiO2 similar to our earlier findings [17]. The average particles size of Ag NPs captured through HRTEM images are in range of 4–6 nm. Fig. 3f depicts the lattice fringes that further signify the presence of Ag NPs (0.24 nm) and TiO2 (0.35 nm) in the prepared ternary composite. 3.3. FT-IR spectra The functional group analyses of the prepared nanocomposites are illustrated in Fig. 4. An intense band recorded at 1641, 1570,

Fig. 4. FTIR spectra of (a) g-C3 N4 , (b) (g-C3 N4 )–TiO2 , (c) (g-C3 N4 )–Ag/TiO2 , (d) TiO2 and (e) Ag/TiO2 .

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Fig. 5. X-ray photoelectron spectra of (a) full spectrum (b) C 1s, (c) N 1s, (d) O 1s, (e) Ti 2p and (f) Ag 3d of ternary.

and 1410 cm−1 was assigned to the stretching vibration of aromatic C N and proved the successful synthesis of g-C3 N4 [27]. The sharp band at 807 cm−1 is attributed to the out-of plane bending vibration characteristics of triazine units [27,31]. Meanwhile, the bands at 1320 and 1240 cm−1 resembled the stretching vibration of C N ( C) C or C NH C. This is also well correlated with the stretching vibration of hydrogen bonding interaction with a broad band in-between 3100 and 3300 cm−1 [27]. An additional broad band at 3350 cm−1 represents the O H group of water molecule. The presence of wide absorption band at 500–700 cm−1 is ascribed to Ti O Ti bonding [32,33]. Similarly the Ag/TiO2 also expressed an absorption bands as that of TiO2 .

3.4. XPS analysis Fig. 5 shows the surface elemental species present in the prepared ternary composite through photoelectron spectra. The C 1s spectrums displayed two distinct peaks at binding energy 284.9 and 288.1 eV. The peak at 284.9 eV is attributed to both C C coordination of sp2 carbon contributed by g-C3 N4 nanosheets and the reference carbon [34,35], while the peak at 288.1 eV is ascribed to N C N coordination. The N 1s spectrum displayed three peaks that denotes C N C (398.6 eV), N (C)3 (399.8 eV) and C N H (401.1 eV). The main peak at 398.6 eV arises from N involved sp2 bond of triazine rings present in g-C3 N4 [36,37]. The O 1s

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Fig. 6. UV-Vis absorption spectra of (a) TiO2 , (b) (g-C3 N4 )–TiO2 , (c) g-C3 N4 , (d) Ag/TiO2 and (e) (g-C3 N4 )–Ag/TiO2 .

Fig. 7. Photoluminescence spectra of (a) g-C3 N4 , (b) (g-C3 N4 )–TiO2 , (c) TiO2 , (d) Ag/TiO2 and (e) (g-C3 N4 )–Ag/TiO2 .

spectrum exhibited two peaks at 530.1 and 531.8 eV corresponds to Ti O bond and O H bond respectively [38]. The occurrence of O H bond was due to the presence of water molecule on the surface of the ternary. Furthermore, the Ti 2p spectrum also showed two distinct peaks at 458.9 eV (Ti 2p3/2 ) and 464.6 eV (Ti 2p1/2 ). These two distinct peaks designate the presence of strong Ti4+ oxidation state in the synthesised sample [28]. Two classical peaks appeared at 368.6 eV (Ag 3d3/2 ) and 374.6 eV (Ag3d5/2 ) were attributed to Ag 3d with a spin energy separation of 6.0 eV, proved the predominance existence of metallic Ag0 state [17].

for potential redox reactions. Thus this heterojunction repels the recombination of electrons and holes and prolongs its lifetime.

3.5. UV–vis DRS and photoluminescence Fig. 6 portraits the obtained optical absorbance spectra of studied samples ranging from pure, binary to ternary composite. The pure graphitic carbon nitride demonstrated a significant shift leaning towards the visible light spectrum. Thus by incorporating g-C3 N4 onto the surface of conventional TiO2 drastically promoted the light absorption intensity with an evident visible light shift at 460 nm. As a result the g-C3 N4 greatly improved the absorption ability of TiO2 in the visible light region. In addition the incorporation of noble metal (Ag) onto the surface of TiO2 also enhanced the absorption ability in the visible spectrum. The absorption curve of ternary composite (g-C3 N4 )–Ag/TiO2 displayed an additional distinct shift towards the visible light spectrum. The estimated approximate band gap energies of the prepared photocatalysts were depicted in Fig. S1. Fig. 7 portraits the separation of charge carrier evolved during photocatalysis for all prepared samples. The g-C3 N4 demonstrated a rapid recombination of charge carriers as compared to the rest by showing the highest light emission after the absorption of illuminated photons. However the phenomenon was significantly reduced after incorporating g-C3 N4 with TiO2 where the g-C3 N4 prolonged the lifespan of the electron by allowing them to be transferred from g-C3 N4 to the conduction band of TiO2 [39]. In comparison, the photoluminescence intensities of Ag/TiO2 and (g-C3 N4 )–Ag/TiO2 are lower than that of the pure TiO2 indicates the good separation of charge carriers in Ag/TiO2 and (g-C3 N4 )–Ag/TiO2. The photoluminescence results clearly demonstrated the formation of heterojunction between (g-C3 N4 ) and Ag/TiO2 . This junction promoted the transfer of electrons from gC3 N4 to Ag NPs and then finally to the conduction band of TiO2

3.6. Photocatalysis evaluation The visible light photocatalytic degradation profile of the AMX with aid of prepared photocatalysts is illustrated in Fig. 8. Poor photodegradation efficiency was exhibited by TiO2 owing to its well know limitation, while reconstruction with g-C3 N4 showed an increase in photodegradation efficiency to 1.7 times than TiO2 . This was attributed to the characteristics of g-C3 N4 which stimulated the visible light absorption. Further a remarkable increase of photocatalytic degradation in the order of 3.2 times was successfully achieved for the ternary. This clearly established the addition of Ag NPs to the (g-C3 N4 )–TiO2 triggered the visible light absorption characteristics through an intrinsic surface plasmon resonance and simultaneously extended repulsion of charge carriers. The photocatalytic performance followed an order of (g-C3 N4 )–Ag/TiO2 (73.4%) > Ag/TiO2 (56.2%) > (g-C3 N4 )–TiO2 (38.9%) > g-C3 N4 (35.1%) > TiO2 (22.7%).

Fig. 8. Visible light photocatalysis performance of prepared photocatalysts.

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Fig. 9. Schematic explanation of (g-C3 N4 )–Ag/TiO2 interaction.

3.7. Photocatalytic mechanism With insight accomplishment of materials chemistry and photocatalytic performance the mechanism behind the prepared ternary photocatalyst is metaphorized in Fig. 9. The figure describes the edge potential of conduction band (CB) and valence band (VB) of a semiconductor estimated based on the following equations: EVB =  − E e + 0.5Eg

(1)

ECB = EVE − Eg

(2)

where EVB and ECB are the valence band and conduction band edge potential respectively,  is the electronegativity of the semiconductor; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV vs NHE) and Eg is the band gap energy of the semiconductor. The electronegativity of g-C3 N4 and TiO2 are 4.64 and 5.81 eV, respectively [31,40]. The CB and VB edge potential of g-C3 N4 were calculated at −1.21 and 1.49 eV respectively. Meanwhile for TiO2 , the CB was calculated as −0.29 eV and the VB as 2.91 eV. During the visible light illumination, g-C3 N4 dominates TiO2 owing to its tendency to absorb the visible light photons and produces electrons and holes [41]. The prevailing negative edge potential of g-C3 N4 (−1.21 eV) allows the photogenerated electrons to be transferred to the lower negative edge potential of CB TiO2 (−0.29 eV). It is well-known that the redox potential of O2 /• O2 − (−0.33 eV) is more negative than the CB of TiO2 (−0.29 eV) and hence the transferred electrons in the CB of TiO2 are not favourable to reduce the O2 . Instead, these electrons can reduce O2 to H2 O2 (O2 /H2 O2 is 0.695 eV) and the formed H2 O2 would further transformed into • OH by capturing an electron [42,43]. Meanwhile, the holes in the VB of g-C3 N4 can directly oxidise the AMX, but not the OH− to generate • OH radicals due to lower positive potential of VB of g-C3 N4 (1.49 eV) against standard redox potential of • OH/OH− (1.99 eV) [31,42,44]. In this ternary composite the presence of Ag NPs onto the surface of TiO2 played a key role as an electron-conduction bridge. This enabled a notable electron–holes separation in g-C3 N4 and also enhanced the electron transfer towards TiO2 through Schottky barrier between Ag and TiO2 . Therefore the excited electrons from g-C3 N4 easily migrate to Ag/TiO2 and thereby retarding the recombination of electron and holes pairs and promote more promising photocatalysis performance. The formation of Schottky barrier between Ag and TiO2 occurs due to the higher Fermi level (Ef ) of TiO2 than Ag. The formation hinders the transfer of electrons from Ag to TiO2 . However the

barrier was shattered and the transfer was possible due to strong electron oscillating on the SPR excitation and leads to the interband excitation. Thus, it triggers the energetic electron to be transfer to the TiO2 conduction band [17]. Moreover the SPR effect of Ag NPs also significantly contributed for the visible light absorption of the ternary [25]. The presence of visible light excites the electrons below the Fermi level of Ag NPs to the surface plasmon states. Thus, this excitation contributed to the high generation of energetic electrons. These electrons migrates to TiO2 which then reduce the O2 to H2 O2 and further transformed into • OH radicals. This aggressive formation of electrons contributed to the better photocatalytic performance. The kinetics of the photocatalytic activities well fitted with pseudo first-order reaction kinetics and the results are depicted in Fig. S2 and Table S1. The recyclability experiments were performed by recovering the photocatalyst and depicted in Fig. S3. The photocatalysis of (g-C3 N4 )–Ag/TiO2 for AMX declined marginally after three cycles. However the photocatalytic efficiency was not much affected. 4. Conclusions The reconstruction of conventional TiO2 was successfully achieved with the support of g-C3 N4 and Ag trough a simple route. The reconstructed titania resulted in a ternary composite (gC3 N4 )–Ag/TiO2 and revealed its superior visible light photocatalysis performance by successfully eliminating AMX from aqueous solution. The photocatalysis showed a significant improvement in the degradation efficiency than conventional TiO2 . The present experimental findings designate the interfacial interaction existed in the (g-C3 N4 )–Ag/TiO2 composite. The presence of g-C3 N4 notably contributed for tuning the band gap energy and prolonging the lifetime of the charge carriers. Where else the presence of Ag NPs in the composite acted as an electron-conduction bridge and contributed for the suppression of electrons and holes. The obtained synergetic effects through g-C3 N4 and Ag promoted the visible light performance of the TiO2 . The obtained ternary composite is expected to play a vital role as an effective high visible light responsive photocatalysts for efficient removal of lethal aquatic pollutants. Acknowledgements The first author is thankful to University of Malaya for the Bright Spark Fellowship. This research work was supported by

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Please cite this article in press as: K.H. Leong, et al., Surface reconstruction of titania with g-C3 N4 and Ag for promoting efficient electrons migration and enhanced visible light photocatalysis, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.184