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Fabrication of CNTs-Ag-TiO2 ternary structure for enhancing visible light photocatalytic degradation of organic dye pollutant
Materials Chemistry and Physics 248 (2020) 122873
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Fabrication of CNTs-Ag-TiO2 ternary structure for enhancing visible light photocatalytic degradation of organic dye pollutant Chen Zhao a, Jun Guo a, Chenlu Yu a, Zhejuan Zhang a, *, Zhuo Sun a, Xianqing Piao b a
Department of Physics and Electronic Science, Engineering Research Center for Nanophotonics &Advanced Instrument, Ministry of Education, East China Normal University, North Zhongshan Rd 3663, Shanghai, 200062, China b Shanghai Industrial Technology Institute, Jinsu Rd 200, Shanghai, 201206, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Research Highlights: � A facile procedure based on in-situ reduction method was developed to synthesize a uniformly coated CNTs-AgTiO2(CAT) ternary structure. � Ag nanoparticles were constructed be tween CNTs and TiO2 and served as a “bridge” in the CAT ternary structure, which exhibited an enhanced photo catalytic performance under visible light. � The detailed mechanism of silver as an intermediate layer has been clearly expounded based on the obviously improved photocatalytic efficiency. A R T I C L E I N F O
A B S T R A C T
Keywords: CNTs-Ag-TiO2 ternary structure Photocatalysis Visible light Degradation mechanism
In order to improve the catalytic efficiency of titanium dioxide (TiO2) under visible light, TiO2 nanoparticles was composed with the silver-covered carbon nanotubes (CA) to form CNTs-Ag-TiO2 (CAT) ternary structure via in situ reduction combined with sol-gel method. The composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra. Using methylene blue (MB) as the model pollutant, photocatalytic activity of CAT was studied to investigate the influence of the composited amount of CA on the effect of photocatalysis. The results revealed that the photocatalytic activity of TiO2 under both visible light and UV light were improved by compositing with CA. When the mass ratio of CA is 15%, the degradation rare of methylene blue (MB) by CAT reached to 80.8% under visible light in 3 h, which is 16.5 times higher than that of TiO2. Accordingly, the degradation rate reached to 99.2% under UV light in 40 min, which is nearly twice as fast as that of TiO2.
1. Introduction Since multi-walled carbon nanotubes (CNTs) were discovered by Japanese scientist IIJIMA ([1]) in 1991, their high electrical and thermal
conductivities, chemical stability and excellent mechanical properties have made them as ideal candidates for nano-sized electronic devices, catalysts, super-capacitors and reinforcement phase in polymer com posite ([2–5]).
* Corresponding author. E-mail address: [email protected] (Z. Zhang). https://doi.org/10.1016/j.matchemphys.2020.122873 Received 10 September 2019; Received in revised form 23 February 2020; Accepted 1 March 2020 Available online 8 March 2020 0254-0584/© 2020 Published by Elsevier B.V.
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In recent years, great interests have been aroused in the field of catalysis where CNTs can act as support for different kinds of catalyst particles, thus, many researchers were making attempts to deposit cat alytic materials onto the surface of CNTs. Among these catalytic mate rials, titanium dioxide (TiO2) has been one of the best candidate due to its’ unique photocatalytic properties. Li ([6]) et al. synthesized CNTs/TiO2 core-shell nanocomposites via sol-gel method, they found that depositing TiO2 onto the surface of CNTs can lead to an enhanced degradation rate (from 50% to 94%) of methylene blue (MB) compared to bare TiO2 under UV light irradiation. They also pointed out that the distance for electron transfer is affected by the thickness of TiO2 layer, and a thinner layer of TiO2 is preferred in MB photo-decomposition. Although CNTs can act as photosensitizer for photogenerated elec trons ([7]), this kind of TiO2 photocatalyst is still inactive under visible-light irradiation because the band edge of TiO2 lies in ultraviolet region which covers approximately 4% of the whole solar spectrum ([8]), leading to their limited applications. One promising way to solve the problem is to anchor noble metal particles (such as Au, Pt, Ag et al.) onto TiO2 nanostructures to extend the spectral response of TiO2. Ge ([9]) et al. anchored Ag particles uniformly on both inside and outside of vertically aligned TiO2 nanotubes using an ultrasonication-assisted in situ deposition technique. The results showed that the deposition of Ag nanoparticles enhanced the light absorption significantly, thus improving the photocatalytic activity notably under visible-light irra diation. Ag nanoparticles (AgNPs) can absorb visible light due to its’ LSPR effect, in AgNPs-semiconductor (TiO2, for instance) composite, AgNPs play a key role in harvesting visible-light ([10]), and the Schottky barrier formed at the interface of AgNPs-semiconductor prohibits the recombination of photo-generated electrons and holes effectively, thus improving its’ photocatalytic ability. On the basis of previous experiments, some reseachers pointed out that the photocatalytic activity of TiO2 can be increased further when the effects of CNTs and noble particles are combined. However, they focused too much on the influence of noble particles as an outermost layer in modifying CNTs-semiconductor composite and failed to explore the effect of noble particles have on the whole composite system when noble particles served as an intermediate layer. They also pointed out that the uniformity of the coverage is of great importance in determining photocatalytic activity. Electroplating, as one common method for get ting uniform covering, is limited due to its’ high cost. Chemical reduc tion method and sol-gel method are preferred in industrialization because of their low cost and easy approach, but obtaining a homoge neous coating via these methods is difficult. In this paper, the uniformly covered CNTs-Ag-TiO2(CAT) nano-composite with high photocatalytic efficiency under visible light was prepared by a simple two-step method that combined in situ chemical reduction method and sol-gel method together. Photocatalytic testing of the synthesized composite was per formed using MB(10 mg/L) as a model organic dye. Then, the mor phologies and photocatalysis mechanism of the CAT ternary composite structure were proposed based on a series of characterization techniques including electron microscopy, UV and visible light spectrum and elec trochemical impedance spectra et al.
0.5 M hydrochloric acid (HCl), 0.05 g CNTs were dispersed in the so lution by ultrasonicaction for 30 min. The above mixtures were then placed under room temperature for another 8 h. Finally, activated CNTs were collected through centrifugation. 2.2. Preparation of CNTs-Ag(CA) nano-composite 0.05 g of activated CNTs were added into 20 ml of de-ionized water and sonicated for 30 min 1 ml 0.1 M AgNO3 solution was added drop wise under magnetic stirring, then the solution was stirred for another 30 min. After the process was finished, 1 ml of 0.6 M Poly vinylpyrrolidone K-30(PVP–K30) and 1 ml 0.1 M NaBH4 was added dropwise under magnetic stirring. The CA samples were collected by centrifugation and washed by de-ionized water for three times and dried in an oven at 70 � C. 2.3. Preparation of CNTs-Ag-TiO2(CAT) nano-composite The TiO2 coated CNT-Ag nano-composites were synthesized via solgel method using tetrabutyl titanate (TBT) as precursor. Sodium dode cylbenzenesulfonate (SDBS) is involved as dispersing agent according to a method that was used by Gao et al. ([11]), which can improve the dispersion of CA samples significantly, providing a support to the growth of TiO2. Initially, certain amount of TBT was mixed with 15 ml of ethanol and 5 ml of glacial acetic acid under vigorous stirring for 30 min to form a TiO2 precursor solution. In parallel, 0.05 g of CA samples were dispersed in de-ionized water (the molar ratio of TBT:H2O was 1:220) with 2% wt of SDBS and sonicated for 30 min. Then 20 ml of ethanol was added and the suspension was stirred for another 30 min. The TiO2 precursor was added dropwise into the CA suspension under magnetic stirring that was kept for 2 h to finish the whole reaction process. Finally, the synthesized CAT samples (the mass ratios of CA are 5%, 10%, 15% and 20%) were calcined under 450 � C for 1 h (the heating rate is 3–4 � C/min) after they were collected. For comparison, CNTs-TiO2(CT) samples were obtained using a similar procedure using only CNTs as support for TiO2. 2.4. Characterization The morphologies of as synthesized CAT samples were examined by scanning electron microscopy (SEM, Hitachi S-4800) and highresolution transmission electron microscopy (HRTEM, JEOL JEM2100). The dried sample powder was loaded into the sample bin, then The X-ray diffraction (XRD) analysis of the nanoparticles was performed using a Holland Panalytical PRO (PW 3040/60) with Cu Kα radiation (V ¼ 30 kV, I ¼ 25 mA) and the scanning angle (2θ) from 20� to 80� . The chemical states of the elements inside were investigated by X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB) with a monochromatic Al Kα X-ray source (1486.6 eV Al radiation, 25 W beam power, 100 μm beam size and 45� measurement angle). The diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra of the powder samples were measured by an UV–Vis spectrophotometer (Hitachi U-3900) and fluorescence spectrophotometer (HORIBA Jobin Yvon fluoromax-4), respectively. The electrochemical impedance spec tral (EIS) measurement was carried out on an electrochemical work station (AUTOLAB PGSTAT302 N) for an applied open circuit potential with varying frequency range (1 MHz-0.01 Hz), and the photo-current measurement was carried out by the same instrument using a three electrode configuration with the CAT electrodes as working electrodes, Ag/AgCl electrode as the reference electrode and a standard platinum electrode as counter electrode, and the electrolyte was 0.1 M Na2SO4 aqueous solution.
2. Experimental 2.1. Functionalization and activation of CNTs CNTs (20–40 nm of outer diameter, Shenhen Nanotech Port Co. ltd) and the chemical agents (Sinopharm Chemical Reagent Co. Ltd)were used as received. To achieve a uniform coating, CNTs were functional ized via acid treatment. Briefly, 0.5 g CNTs were added in 40 ml of nitric acid (68%) and sulfuric acid (98%) (VHNO3:VH2SO4 ¼ 1:3) and refluxed at 80 � C for 4 h under magnetic stirring. After acid pre-treatment, the CNTs were washed in de-ionized water until PH ¼ 7 and dried at 70 � C for 10 h. Then functionalized CNTs were activated by Sn2þ as follows: after 0.1 g of tin (II) chloride dehydrate (SnCl2�2H2O) was dissolved in 20 ml of
2.5. Evaluation of photocatalytic activity The photocatalytic performance of the CAT samples was evaluated 2
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by using methylene blue (MB) as dye pollutant. In a typical procedure, 50 mg of the catalyst was dispersed into 50 ml of MB solution (10 mg/L). Then the suspension was stirred in dark for 30 min to obtain an adsorption-desorption equilibrium state. A metal halide lamp was used as a visible light source by equipping it with a cutoff filter (λ > 420 nm). The temperature was controlled at 25 � C by a cooling water recirculation system (SH-BILON-T-1000S). During the process, approximately 2 ml of suspension was withdrawn from the solution at given time intervals (30min) and centrifuged to separate residual catalyst from the solution. The absorbance intensity was measured by UV–Vis spectrophotometer and the degradation rates of the catalysts were calculated using the degradation efficiency equation by C/C0, where C0 is the initial con centration of MB and C is the concentration after light irradiation.
surface with larger size can be explained as follows: during the func tionalization process, a lot of functional groups were attached on the surface of CNTs after over-oxidized by sulfuric acid and nitric acid, leading to an over-sensitization effect for rapid growth of large Ag particles. The HRTEM image of CA (Fig. 1e) further reveals the structure of silver particles on the surface of CNT, the lattice spacing of the Ag particles is 0.234 nm, which is in agreement with the (111) plane of metallic face-centered cubic (FCC) Ag. Fig. 1b and f show the SEM and TEM image of 15% CAT sample, it can be obviously found that the surface of 15% CAT sample is different from that of CA. More particles with bigger diameter were wrapped outside the wall of the CNTs, indi cating the coverage of TiO2 on the surface of CA. The high resolution image of the area (marked by circle) in Fig. 1f is shown in Fig. 1g, the lattice spacing of 0.350 nm is in agreement with (101) plane of anatase TiO2, and 0.234 nm is for (111) plane of FCC Ag, which suggested that a dense layer of anatase TiO2 is formed on the surface of CA, and these two particles have a close contact with each other. The elemental mapping image of 15% CAT is presented in Fig. 1g, which demonstrates the presence and the dispersion of C, Ti, Ag and O. Moreover, when the mass ratio of CA increased from 15% to 20%, the relative amount of TiO2 is reduced, making part of CA surface unwrapped or unevenly wrapped (marked by red circle in Fig. 1c). It can be found that the coverage of 20% CAT is less uniform. The XRD patterns of the synthesized CA and CAT samples are shown in Fig. 2. CA sample exhibits sharp diffraction peaks at 38.1� ,44.2� ,64.4� and 77.4� , which are typical diffraction peaks for (111), (200), (220) and (311) planes of FCC silver (JCPDS No. 04–0783), and is in agree ment with HRTEM image shown in Fig. 1d. This means that metallic silver nano-particles with high crystallinity are formed. Fig. 2b shows the XRD spectra of CAT samples, the peaks located at 25.8� , 38.6� , 48.1� , 53.9� , 55.1� and 62.7� are indexed to (101), (004), (200), (105), (211) and (204) crystal planes of anatase TiO2(JCPDS No. 21–1272). It is obvious that TiO2 are anchored on the surface of CA. Meanwhile, the peaks for silver are not obvious in Fig. 2b. When the surface of CA is further composite with TiO2, the diffraction peaks of Ag and TiO2 are overlapped because they are so close, which leads to the decrease of the intensity of the characteristic peak and the increase of the peak width. Additionally, when the mass ratio of CA increases (from 5% to 20%), the diffraction peak of (101) shifts to smaller degree. One possible reason is that, during calcination, carbon atoms at the surface of functionalized CNTs may diffuse into the lattice gap of TiO2, resulting in an increase in the lattice parameters. To further identify the chemical states of the element inside the sample, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The fully scanned spectrum shown in Fig. 3a reveals that the sample only contains Ti, C, Ag and O. The high resolution spectrum of C 1s in Fig. 3b reveals that the C 1s peak is divided into four peaks centered at 284.6eV, 285.5eV, 286.7eV and 288.3eV which correspond to C–C sp2, – O, respectively. The peak of C–C at 284.6eV is C–C sp3, C–O and C– ascribed to the sp2 hybridized carbon atoms at the surface of CNTs ([12]), and another peak appeared at 285.5eV is attributed to the defects (sp3 hybridized carbon) on the nanotube structure ([13]). It should be noted that, during calcination, when brookite changes into anatase, the tension caused by the reconstruction of TiO2 will transfer to the surface of CNTs, leading to an increase in the distortion of the surface, and this may cause the appearance of the peak at 285.5eV. On the other hand, the spectrum also demonstrates that there exists no Ti–C bond in the com posite because no peaks around 281.5eV are found, which means C atoms do not substitute O atoms in TiO2 lattice ([14]). Fig. 3c shows the high resolution spectrum of O 1s, the peaks centered at 530.8eV and 532.6eV are in accordance with Ti–O and C–O bonds. However, compared to pure TiO2, the peak for Ti–O shift slightly to higher binding energy. This is possibly due to the existence of non-lattice oxygen ([15, 16]). Fig. 3d presents the high resolution spectrum of Ag 3 d, two peaks at 368.7 eV and 374.7eV are for Ag 3d5/2 and Ag 3d3/2, respectively. The slitting (6.0 eV) between these two peaks indicates the existence of
3. Results and discussion 3.1. Morphology and structure Fig. 1a and Fig. 1d show the SEM and TEM image of the as synthe sized CA sample. It can be seen that the silver-nanoparticles were deposited onto the surface of CNT uniformly with an average diameter of 2.12 � 0.42 nm. The small amount of Ag particles attached to the tube
Fig. 1. SEM images of (a) CA, (b)15% CAT, (c)20% CAT and TEM images of (d) (e) CA, (f) (g)15% CAT, (h) the elemental mapping image of 15% CAT. 3
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Fig. 2. XRD patterns of as synthesized (a)CA, (b)CAT samples.
Ag0, which is reduced from Agþ by NaBH4. However, the binding en ergies of Ag 3 d shift to higher values compared to bulk Ag, which are 368.2 eV for Ag 3d5/2 and 374.2 eV for Ag 3d3/2. The shift possibly stems from the loss of electrons in Ag, and this is often observed in depositing noble metals onto the surface of CNTs ([17,18]), indicating a strong interaction between the AgNPs and CNTs. Fig. 3e demonstrates the high resolution spectrum of Ti 2p. Two diffraction peaks at 459.5eV and 465.2eV corresponds to Ti 2p3/2 and Ti 2p1/2, respectively. The slitting (5.7eV) between these peaks are strong evidence for Ti4þ, which indicates Ti element can only exists in the composite in the form of TiO2. However, compared to standard binding energies of Ti 2p3/2 (458.6eV) and Ti 2p 1/2 (464.1eV), these peaks shift to higher values. This is considered to be the influence of CNTs on the electronic state of Ti element due to the combination. In other words, the relationship be tween CNTs and TiO2 is not a simple mixing one, but an internal one with electronic interactions between them.
and 2.30 eV (15% CAT), respectively. The shift could be attributed to the formation of Ti–O and C–O bonds [22,23], the photosensitizing effect of CNTs (24) and the LSPR effect of Ag particle ([25–27]), which have been certified. Photoluminescence (PL) spectra has always been used as an indirect tools to evaluate the recombination efficiency of photogenerated electron-hole pairs ([28]). As shown in Fig. 6b, the PL emission peak of 15% CAT has a lowest intensity, indicating that the recombination of e and hþ is inhibited greatly. The peak intensity of 15% CT is lower than that of TiO2, this can possibly attribute to the transfer of electrons during the excitation process ([24]), which will be explained later. Moreover, what shouldn’t be ignored is that, when the proportion of CA in CAT increases to 20%, the intensity of the PL spectra still increases slightly. In 20% CAT, TiO2, the active component that produces photogenerated electron-hole pairs, decreases with the increment of CA, suggesting that the photogenerated electron-hole pairs are more likely to be recombined internally using CA as recombination center. Fig. 6c shows the time-resolved fluorescence spectra of the samples. It can be seen that 15% CAT sample achieves a shorter fluorescence lifetime (1.41 ns) compared to 15% CT (1.65 ns) and TiO2(1.88ns). In 15% CAT, CA can act as a transmission agent and inhibit the fluores cence phenomenon caused by the recombination of electron-hole pairs, thus exhibits a shorter fluorescence lifetime, and accordingly, a lower recombination rate of the charge carriers. The results are in agreement with PL spectra. The electrochemical impedance spectra (EIS) of the as obtained samples, shown in Fig. 6d, are presented as Nyquist plots. The imped ance curves of the samples demonstrate a compressed semicircle in the high to medium frequency range, which could be assigned to the charge transfer resistance (Rct), and a near-45� inclined line in the lowfrequency range, which considered to be the Warburg impedance. The fitting of semi-arcs was carried out using NOVA software (equivalent circuit shown in the inset of Fig. 6d) to estimate the charge transfer resistance (Rct) in parallel with the double-layer capacitance (Cdl) at the electrode-electrolyte interface. The arc radii of 10% CAT, 15% CAT and 20% CAT ternary photoanode, corresponding fitting values of Rct are 32.7 Ω, 7.1 Ω and 76.2 Ω, respectively, are smaller than 15% CT (93.7 Ω) and TiO2(96.9 Ω) suggesting that the introduction of Ag nanoparticles is favorable to the separation of charge carriers ([29]). The smaller value of Rct for the CAT ternary photoanode confirms the faster charge transfer and separation rate within the ternary system compared to that of CT photoanode at the semiconductor-electrolyte interface. The results are in agreement with PL spectra (Fig. 6c). To further prove the results mentioned above, the transient photo current responses of the as obtained samples were conducted and the results are shown in Fig. 6d. The I-t curves of TiO2, CT and CAT samples
3.2. Formation mechanism of CA and CAT The schematic illustration of the fabrication process of CA compos ites is shown in Fig. 4. After treated by sulfuric acid and nitric acid, various functional groups such as hydroxyl and carboxyl are formed on the surface of CNTs. SnCl2 from SnCl2�2H2O can attach and form a layer in colloid form, as an activating layer, onto the surface of CNTs where functional groups are rich. When silver nitrate is added drop-wise, the small part of silver ions are reduced by SnCl2 layer at the surface of CNTs, forming numerous small Ag core. When the main reducing re agent NaBH4 is added, metallic silver is reduced and accumulate on those small particles and grow into bigger size. During this process, PVP can act as a shape control agent and prevent the aggregation of AgNPs via its’ steric hindrance effect. In the past decades, many researchers ([3,11,19–21]) synthesized CNTs/TiO2 via sol-gel method. They all pointed out that functional groups on the surface of CNTs can act as anchor points for TiO2. On the basis of these theories, we deduce that TiO2 can also anchor on the surface of CA (the schematic illustration of the fabrication process is shown in Fig. 5), and they may disperse into each other to have an “associated relationship”, which has been proved by TEM mapping shown in Fig. 1g. 3.3. Optical and photoelectric properties UV-VIS diffuse reflection spectra (UV-VIS DRS), shown in Fig. 6a, are used to compare the optical properties of TiO2, 15% CT and 15% CAT samples. It can be seen that the introduction of CNTs and CA results in a red shift of adsorption edge from 3.20 eV (TiO2) to 2.88 eV (15% CT) 4
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Fig. 3. XPS spectra of 15% CAT sample (a) XPS fully scanned spectrum, (b)C 1s, (c)O 1s, (d) Ag 3 d, (e)Ti 2p.
with four complete cycles of intermittent-light irradiation are shown in Fig. 6d. It is easy to find that all these responses are highly sensitive. The photocurrent responses of the samples are ranked in the following order: 15% CAT >10% CAT >20% CAT >5% CAT >15% CT > TiO2, the result indicate that the introduction of CNTs and Ag can separate the photogenerated electrons and holes further compared to bare TiO2, and a maximum value is found under the condition of 15% CAT. The photo current response results are in agreement with those of PL and EIS measurements.
3.4. Photocatalytic activity of the CAT ternary structure To study the photocatalytic properties of the obtained samples, degradation experiments of methylene blue (MB) under visible light and UV light are conducted and the results are shown in Fig. 7. Under visible light irradiation, TiO2 performed poorly in degrading organic pollutant, which may ascribe to its’ wide band gap energy. The degradation rate of MB by TiO2 is only 4.9% in 3 h. When CNTs were introduced, due to the incorporation of carbon atoms into the lattice of TiO2 and the formation of Ti–O and O–C bonds, the degradation rate of 5
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Fig. 4. Schematic illustration of the fabrication process of CA composites.
Fig. 5. Schematic illustration of the fabrication process of CAT.
15% CT increased to 13%. The photocatalytic ability of CAT samples increased dramatically because Ag particles can absorb visible light due to its’ LSPR effect. Additionally, when the mass ratio of CA increased from 5% to 15%, the degradation rate increased from 39.4% to 80.8% in 180 min, this can possibly due to the higher uniformity of the TiO2 layer at the surface of CNTs. However, when the mass ratio of CA increased to 20%, the degradation efficiency decreased to 58.5% instead. This sug gested that a non-uniform coverage of TiO2 in 20% CAT can lead to a lower yield of photo-generated electrons and holes compared to 15% CAT (this was proved by the PL spectra and the photocurrent response of the samples), leading to the decrease in photodegradation efficiency. Under UV light irradiation, the degradation rate of TiO2 reached 95.4% in 60 min. The degradation rate of 15% CT is 97.4% in 60 min, the increment can ascribe to the outstanding conductivity of CNTs, which facilitates the transfer of electrons, thus separate the electrons and holes further, leading to a higher degradation rate. We can also find
in Fig. 7b that with the increase the amount of Ag, the photocatalytic ability raised at first and then decreased to a lower level. Among all these CAT samples, 15% CAT ternary structure exhibited an optimum degradation rate, which is 100% in 60 min (99.2% in 40 min). After the introduction of Ag, AgNPs can act as an acceptor of electrons and form a Schottky barrier when contact with TiO2, resulting in a lower recom bination rate, which is favorable for the enhancement of photocatalytic ability. 3.5. Photocatalytic stability of 15% CAT ternary structure To test the stability of the 15% CAT catalyst, a recycling experiment is conducted under the same conditions. The results shown in Fig. 8 indicated that the photocatalytic efficiency of the 15% CAT sample re mains stable in the first three cycles, then decreased slightly in the fourth and the fifth experiment, which may attribute to the slight detachment 6
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Fig. 6. (a) DRS spectra, (b) Photoluminescence spectra, (c) Time-resolved fluorescence spectra, (d) EIS spectra and (e) Transient photocurrent responses of the samples.
of TiO2 on the surface of CNTs.
amount of holes on the valence band (VB), as shown in Fig. 10a and b. Because the conduction band of TiO2 is higher than the equilibrated Fermi-level, excited electrons can be fed into to the lifted Fermi-level of Ag easily. While the electrons combine with the absorbed oxygen mol ecules to form superoxide radical anions (O2 ), the holes on the VB of TiO2 can promote the conversion from OH to ⋅OH, the high yield of these two particles can greatly improve the degradation of the pollutant. Moreover, CNTs, acting as the carrier of both TiO2 and Ag, can transform away the electrons to further prevent the recombination of electron-hole pairs, which is favorable for enhancing photocatalytic efficiency. Under visible light, the LSPR effect of Ag plays a key role. Upon resonance with incoming photons, some part of the electrons will have a higher energy than the previous conduction band of TiO2(before contact with Ag, ECB0), enabling the transmission of the electrons from Ag to
3.6. Photodegradation mechanism Generally, there exists many oxygen defects in TiO2, this means that there are many excess electrons inside TiO2, which leads to its’ n-type semiconductor properties ([30]). The Fermi-level (Ef) of TiO2 is higher than the work function(W) of Ag. After contact, electrons and holes will flow to build up a new thermal equilibrium state, in which Ef and W are equilibrated, seen in Fig. 9. Meanwhile, a space-charge region is formed, and can prohibit the recombination of photo-generated holes and elec trons in the degradation process. Under UV irradiation, shown in Fig. 10a and Fig. 10b, the electrons will migrate to the conduction band (CB) of TiO2, leaving the same 7
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Fig. 7. Photocatalytic degradation of MB over the samples under (a) visible light irradiation and (b) UV light irradiation.
electrons that have enough energy to migrate to the CB of TiO2. After the dissipation, the electrons will change back to standard Fermi-Dirac distribution ([31]). In CAT, as shown in Fig. 10e, the Fermi levels will be equilibrated due to the contact of TiO2 with both Ag and CNTs. After absorbing visible light, the LSPR effect of Ag will cause the oscillation of electrons, thus obtaining a higher energy than the energy of the CB of TiO2(ECB0). Then the electrons will transmit to the CB of TiO2, and will react with the absorbed O2 molecules to form superoxide radical anions (O2 ), which is vital in the whole degradation process. Additionally, electrons on the CB of TiO2 have a higher energy state than the equili brated Fermi level, thus the electrons can feed into the Fermi level of CNTs and can be transmitted immediately, leading to a lower recom bination rate. All these factors contribute to the advanced photocatalytic abilities of CAT samples. Anchoring TiO2 onto the surface of CA can result in an enhanced phtotocatalytic performance. The LSPR sensitizing effect of Ag particles, formation of Ti–O–C bonds and the high conductivity of CNTs can lead to an extended response range to visible light, a decrease in band gap and a further inhibited recombination of charge carriers, all these factors contribute to the excellent photocatalytic activity of CAT both under UV and visible light.
Fig. 8. Cycling tests for degradation of MB using 15% CAT.
4. Conclusion In this work, a facile procedure based on in-situ reduction method was developed to synthesize CAT nano-composite with a ternary struc ture. Ag nanoparticles were constructed between CNTs and TiO2 and served as a “bridge”. Combining the LSPR effect of AgNPs and the photosensitizing effect of CNTs, CAT achieved a longer life in photo generated electron-hole pairs, leading to a higher efficiency of photo catalysis for the degradation of organic pollutant in a large wavelength range, especially in visible light region. Additionally, the working mechanism of ternary structure was well suggested in this study, which could be further extended to some other semiconductor materials, such as ZnO and so on. In conclusion, this novel ternary structure based on large band gap photocatalysts could realize important value in photo catalytic purification both in the purification of water and air. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Chen Zhao: Conceptualization, Methodology, Software, Investiga tion, Formal analysis, Writing - original draft. Jun Guo: Software, Validation, Resources. Chenlu Yu: Resources. Zhejuan Zhang: Super vision, Data curation, Writing - review & editing. Zhuo Sun: Supervi sion. Xianqing Piao: Supervision.
Fig. 9. Energy diagrams in different states.
TiO2, seen in Fig. 10c. Through electron-electron relaxation, the energy state will change back to Fermi-Dirac distribution, and the Fermi level is higher. However, as shown in Fig. 10d, there are still small part of 8
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Fig. 10. Proposed photocatalytic mechanism of CAT under (a) (b) UV light irradiation; (c) (d) (e) visible light irradiation.
Acknowledgments [6]
This work is financially supported by National Natural Science Foundation of China of China (No. 11204082), Shanghai Natural Fund (No: 16ZR1410700), Fundamental Research Funds for the Central Uni versities, Project of Social Office of Shanghai Science and Technology Commission (No: 19DZ1205102) and the Direct Fund (No: 201902 ZR).
[7] [8]
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various functionalized carbon nanotubes-polyurethanes nanocomposites, J. Appl. Polym. Sci. 136 (14) (2019). Z. Li, B. Gao, G.Z. Chen, R. Mokaya, S. Sotiropoulos, G.L. Puma, Carbon nanotube/ titanium dioxide (CNT/TiO2) core-shell nanocomposites with tailored shell thickness, CNT content and photocatalytic/photoelectrocatalytic properties, Appl. Catal. B Environ. 110 (2011) 50–57. C. Han, M.-Q. Yang, B. Weng, Y.-J. Xu, Improving the photocatalytic activity and anti-photocorrosion of semiconductor ZnO by coupling with versatile carbon, Phys. Chem. Chem. Phys. 16 (32) (2014) 16891–16903. A.A. Ashkarran, M. Kavianipour, S.M. Aghigh, S.A.A. Afshar, S. Saviz, A.I. Zad, On the formation of TiO2 nanoparticles via submerged arc discharge technique: synthesis, characterization and photocatalytic properties, J. Cluster Sci. 21 (4) (2010) 753–766. M.-Z. Ge, C.-Y. Cao, S.-H. Li, Y.-X. Tang, L.-N. Wang, N. Qi, et al., In situ plasmonic Ag nanoparticle anchored TiO2 nanotube arrays as visible-light-driven photocatalysts for enhanced water splitting, Nanoscale 8 (9) (2016) 5226–5234. P. Wang, B. Huang, Y. Dai, M.-H. Whangbo, Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles, Phys. Chem. Chem. Phys. 14 (28) (2012) 9813–9825. B. Gao, C. Peng, G.Z. Chen, G.L. Puma, Photo-electro-catalysis enhancement on carbon nanotubes/titanium dioxide (CNTs/TiO2) composite prepared by a novel surfactant wrapping sol-gel method, Appl. Catal. B Environ. 85 (1–2) (2008) 17–23. K.A. Wepasnick, B.A. Smith, K.E. Schrote, H.K. Wilson, S.R. Diegelmann, D. H. Fairbrother, Surface and structural characterization of multi-walled carbon nanotubes following different oxidative treatments, Carbon 49 (1) (2011) 24–36.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Fabrication of CNTs-Ag-TiO2 ternary structure for enhancing visible light photocatalytic degradation of organic dye pollutant Chen Zhao a, Jun Guo a, Chenlu Yu a, Zhejuan Zhang a, *, Zhuo Sun a, Xianqing Piao b a
Department of Physics and Electronic Science, Engineering Research Center for Nanophotonics &Advanced Instrument, Ministry of Education, East China Normal University, North Zhongshan Rd 3663, Shanghai, 200062, China b Shanghai Industrial Technology Institute, Jinsu Rd 200, Shanghai, 201206, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Research Highlights: � A facile procedure based on in-situ reduction method was developed to synthesize a uniformly coated CNTs-AgTiO2(CAT) ternary structure. � Ag nanoparticles were constructed be tween CNTs and TiO2 and served as a “bridge” in the CAT ternary structure, which exhibited an enhanced photo catalytic performance under visible light. � The detailed mechanism of silver as an intermediate layer has been clearly expounded based on the obviously improved photocatalytic efficiency. A R T I C L E I N F O
A B S T R A C T
Keywords: CNTs-Ag-TiO2 ternary structure Photocatalysis Visible light Degradation mechanism
In order to improve the catalytic efficiency of titanium dioxide (TiO2) under visible light, TiO2 nanoparticles was composed with the silver-covered carbon nanotubes (CA) to form CNTs-Ag-TiO2 (CAT) ternary structure via in situ reduction combined with sol-gel method. The composite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra. Using methylene blue (MB) as the model pollutant, photocatalytic activity of CAT was studied to investigate the influence of the composited amount of CA on the effect of photocatalysis. The results revealed that the photocatalytic activity of TiO2 under both visible light and UV light were improved by compositing with CA. When the mass ratio of CA is 15%, the degradation rare of methylene blue (MB) by CAT reached to 80.8% under visible light in 3 h, which is 16.5 times higher than that of TiO2. Accordingly, the degradation rate reached to 99.2% under UV light in 40 min, which is nearly twice as fast as that of TiO2.
1. Introduction Since multi-walled carbon nanotubes (CNTs) were discovered by Japanese scientist IIJIMA ([1]) in 1991, their high electrical and thermal
conductivities, chemical stability and excellent mechanical properties have made them as ideal candidates for nano-sized electronic devices, catalysts, super-capacitors and reinforcement phase in polymer com posite ([2–5]).
* Corresponding author. E-mail address: [email protected] (Z. Zhang). https://doi.org/10.1016/j.matchemphys.2020.122873 Received 10 September 2019; Received in revised form 23 February 2020; Accepted 1 March 2020 Available online 8 March 2020 0254-0584/© 2020 Published by Elsevier B.V.
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In recent years, great interests have been aroused in the field of catalysis where CNTs can act as support for different kinds of catalyst particles, thus, many researchers were making attempts to deposit cat alytic materials onto the surface of CNTs. Among these catalytic mate rials, titanium dioxide (TiO2) has been one of the best candidate due to its’ unique photocatalytic properties. Li ([6]) et al. synthesized CNTs/TiO2 core-shell nanocomposites via sol-gel method, they found that depositing TiO2 onto the surface of CNTs can lead to an enhanced degradation rate (from 50% to 94%) of methylene blue (MB) compared to bare TiO2 under UV light irradiation. They also pointed out that the distance for electron transfer is affected by the thickness of TiO2 layer, and a thinner layer of TiO2 is preferred in MB photo-decomposition. Although CNTs can act as photosensitizer for photogenerated elec trons ([7]), this kind of TiO2 photocatalyst is still inactive under visible-light irradiation because the band edge of TiO2 lies in ultraviolet region which covers approximately 4% of the whole solar spectrum ([8]), leading to their limited applications. One promising way to solve the problem is to anchor noble metal particles (such as Au, Pt, Ag et al.) onto TiO2 nanostructures to extend the spectral response of TiO2. Ge ([9]) et al. anchored Ag particles uniformly on both inside and outside of vertically aligned TiO2 nanotubes using an ultrasonication-assisted in situ deposition technique. The results showed that the deposition of Ag nanoparticles enhanced the light absorption significantly, thus improving the photocatalytic activity notably under visible-light irra diation. Ag nanoparticles (AgNPs) can absorb visible light due to its’ LSPR effect, in AgNPs-semiconductor (TiO2, for instance) composite, AgNPs play a key role in harvesting visible-light ([10]), and the Schottky barrier formed at the interface of AgNPs-semiconductor prohibits the recombination of photo-generated electrons and holes effectively, thus improving its’ photocatalytic ability. On the basis of previous experiments, some reseachers pointed out that the photocatalytic activity of TiO2 can be increased further when the effects of CNTs and noble particles are combined. However, they focused too much on the influence of noble particles as an outermost layer in modifying CNTs-semiconductor composite and failed to explore the effect of noble particles have on the whole composite system when noble particles served as an intermediate layer. They also pointed out that the uniformity of the coverage is of great importance in determining photocatalytic activity. Electroplating, as one common method for get ting uniform covering, is limited due to its’ high cost. Chemical reduc tion method and sol-gel method are preferred in industrialization because of their low cost and easy approach, but obtaining a homoge neous coating via these methods is difficult. In this paper, the uniformly covered CNTs-Ag-TiO2(CAT) nano-composite with high photocatalytic efficiency under visible light was prepared by a simple two-step method that combined in situ chemical reduction method and sol-gel method together. Photocatalytic testing of the synthesized composite was per formed using MB(10 mg/L) as a model organic dye. Then, the mor phologies and photocatalysis mechanism of the CAT ternary composite structure were proposed based on a series of characterization techniques including electron microscopy, UV and visible light spectrum and elec trochemical impedance spectra et al.
0.5 M hydrochloric acid (HCl), 0.05 g CNTs were dispersed in the so lution by ultrasonicaction for 30 min. The above mixtures were then placed under room temperature for another 8 h. Finally, activated CNTs were collected through centrifugation. 2.2. Preparation of CNTs-Ag(CA) nano-composite 0.05 g of activated CNTs were added into 20 ml of de-ionized water and sonicated for 30 min 1 ml 0.1 M AgNO3 solution was added drop wise under magnetic stirring, then the solution was stirred for another 30 min. After the process was finished, 1 ml of 0.6 M Poly vinylpyrrolidone K-30(PVP–K30) and 1 ml 0.1 M NaBH4 was added dropwise under magnetic stirring. The CA samples were collected by centrifugation and washed by de-ionized water for three times and dried in an oven at 70 � C. 2.3. Preparation of CNTs-Ag-TiO2(CAT) nano-composite The TiO2 coated CNT-Ag nano-composites were synthesized via solgel method using tetrabutyl titanate (TBT) as precursor. Sodium dode cylbenzenesulfonate (SDBS) is involved as dispersing agent according to a method that was used by Gao et al. ([11]), which can improve the dispersion of CA samples significantly, providing a support to the growth of TiO2. Initially, certain amount of TBT was mixed with 15 ml of ethanol and 5 ml of glacial acetic acid under vigorous stirring for 30 min to form a TiO2 precursor solution. In parallel, 0.05 g of CA samples were dispersed in de-ionized water (the molar ratio of TBT:H2O was 1:220) with 2% wt of SDBS and sonicated for 30 min. Then 20 ml of ethanol was added and the suspension was stirred for another 30 min. The TiO2 precursor was added dropwise into the CA suspension under magnetic stirring that was kept for 2 h to finish the whole reaction process. Finally, the synthesized CAT samples (the mass ratios of CA are 5%, 10%, 15% and 20%) were calcined under 450 � C for 1 h (the heating rate is 3–4 � C/min) after they were collected. For comparison, CNTs-TiO2(CT) samples were obtained using a similar procedure using only CNTs as support for TiO2. 2.4. Characterization The morphologies of as synthesized CAT samples were examined by scanning electron microscopy (SEM, Hitachi S-4800) and highresolution transmission electron microscopy (HRTEM, JEOL JEM2100). The dried sample powder was loaded into the sample bin, then The X-ray diffraction (XRD) analysis of the nanoparticles was performed using a Holland Panalytical PRO (PW 3040/60) with Cu Kα radiation (V ¼ 30 kV, I ¼ 25 mA) and the scanning angle (2θ) from 20� to 80� . The chemical states of the elements inside were investigated by X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB) with a monochromatic Al Kα X-ray source (1486.6 eV Al radiation, 25 W beam power, 100 μm beam size and 45� measurement angle). The diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra of the powder samples were measured by an UV–Vis spectrophotometer (Hitachi U-3900) and fluorescence spectrophotometer (HORIBA Jobin Yvon fluoromax-4), respectively. The electrochemical impedance spec tral (EIS) measurement was carried out on an electrochemical work station (AUTOLAB PGSTAT302 N) for an applied open circuit potential with varying frequency range (1 MHz-0.01 Hz), and the photo-current measurement was carried out by the same instrument using a three electrode configuration with the CAT electrodes as working electrodes, Ag/AgCl electrode as the reference electrode and a standard platinum electrode as counter electrode, and the electrolyte was 0.1 M Na2SO4 aqueous solution.
2. Experimental 2.1. Functionalization and activation of CNTs CNTs (20–40 nm of outer diameter, Shenhen Nanotech Port Co. ltd) and the chemical agents (Sinopharm Chemical Reagent Co. Ltd)were used as received. To achieve a uniform coating, CNTs were functional ized via acid treatment. Briefly, 0.5 g CNTs were added in 40 ml of nitric acid (68%) and sulfuric acid (98%) (VHNO3:VH2SO4 ¼ 1:3) and refluxed at 80 � C for 4 h under magnetic stirring. After acid pre-treatment, the CNTs were washed in de-ionized water until PH ¼ 7 and dried at 70 � C for 10 h. Then functionalized CNTs were activated by Sn2þ as follows: after 0.1 g of tin (II) chloride dehydrate (SnCl2�2H2O) was dissolved in 20 ml of
2.5. Evaluation of photocatalytic activity The photocatalytic performance of the CAT samples was evaluated 2
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by using methylene blue (MB) as dye pollutant. In a typical procedure, 50 mg of the catalyst was dispersed into 50 ml of MB solution (10 mg/L). Then the suspension was stirred in dark for 30 min to obtain an adsorption-desorption equilibrium state. A metal halide lamp was used as a visible light source by equipping it with a cutoff filter (λ > 420 nm). The temperature was controlled at 25 � C by a cooling water recirculation system (SH-BILON-T-1000S). During the process, approximately 2 ml of suspension was withdrawn from the solution at given time intervals (30min) and centrifuged to separate residual catalyst from the solution. The absorbance intensity was measured by UV–Vis spectrophotometer and the degradation rates of the catalysts were calculated using the degradation efficiency equation by C/C0, where C0 is the initial con centration of MB and C is the concentration after light irradiation.
surface with larger size can be explained as follows: during the func tionalization process, a lot of functional groups were attached on the surface of CNTs after over-oxidized by sulfuric acid and nitric acid, leading to an over-sensitization effect for rapid growth of large Ag particles. The HRTEM image of CA (Fig. 1e) further reveals the structure of silver particles on the surface of CNT, the lattice spacing of the Ag particles is 0.234 nm, which is in agreement with the (111) plane of metallic face-centered cubic (FCC) Ag. Fig. 1b and f show the SEM and TEM image of 15% CAT sample, it can be obviously found that the surface of 15% CAT sample is different from that of CA. More particles with bigger diameter were wrapped outside the wall of the CNTs, indi cating the coverage of TiO2 on the surface of CA. The high resolution image of the area (marked by circle) in Fig. 1f is shown in Fig. 1g, the lattice spacing of 0.350 nm is in agreement with (101) plane of anatase TiO2, and 0.234 nm is for (111) plane of FCC Ag, which suggested that a dense layer of anatase TiO2 is formed on the surface of CA, and these two particles have a close contact with each other. The elemental mapping image of 15% CAT is presented in Fig. 1g, which demonstrates the presence and the dispersion of C, Ti, Ag and O. Moreover, when the mass ratio of CA increased from 15% to 20%, the relative amount of TiO2 is reduced, making part of CA surface unwrapped or unevenly wrapped (marked by red circle in Fig. 1c). It can be found that the coverage of 20% CAT is less uniform. The XRD patterns of the synthesized CA and CAT samples are shown in Fig. 2. CA sample exhibits sharp diffraction peaks at 38.1� ,44.2� ,64.4� and 77.4� , which are typical diffraction peaks for (111), (200), (220) and (311) planes of FCC silver (JCPDS No. 04–0783), and is in agree ment with HRTEM image shown in Fig. 1d. This means that metallic silver nano-particles with high crystallinity are formed. Fig. 2b shows the XRD spectra of CAT samples, the peaks located at 25.8� , 38.6� , 48.1� , 53.9� , 55.1� and 62.7� are indexed to (101), (004), (200), (105), (211) and (204) crystal planes of anatase TiO2(JCPDS No. 21–1272). It is obvious that TiO2 are anchored on the surface of CA. Meanwhile, the peaks for silver are not obvious in Fig. 2b. When the surface of CA is further composite with TiO2, the diffraction peaks of Ag and TiO2 are overlapped because they are so close, which leads to the decrease of the intensity of the characteristic peak and the increase of the peak width. Additionally, when the mass ratio of CA increases (from 5% to 20%), the diffraction peak of (101) shifts to smaller degree. One possible reason is that, during calcination, carbon atoms at the surface of functionalized CNTs may diffuse into the lattice gap of TiO2, resulting in an increase in the lattice parameters. To further identify the chemical states of the element inside the sample, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The fully scanned spectrum shown in Fig. 3a reveals that the sample only contains Ti, C, Ag and O. The high resolution spectrum of C 1s in Fig. 3b reveals that the C 1s peak is divided into four peaks centered at 284.6eV, 285.5eV, 286.7eV and 288.3eV which correspond to C–C sp2, – O, respectively. The peak of C–C at 284.6eV is C–C sp3, C–O and C– ascribed to the sp2 hybridized carbon atoms at the surface of CNTs ([12]), and another peak appeared at 285.5eV is attributed to the defects (sp3 hybridized carbon) on the nanotube structure ([13]). It should be noted that, during calcination, when brookite changes into anatase, the tension caused by the reconstruction of TiO2 will transfer to the surface of CNTs, leading to an increase in the distortion of the surface, and this may cause the appearance of the peak at 285.5eV. On the other hand, the spectrum also demonstrates that there exists no Ti–C bond in the com posite because no peaks around 281.5eV are found, which means C atoms do not substitute O atoms in TiO2 lattice ([14]). Fig. 3c shows the high resolution spectrum of O 1s, the peaks centered at 530.8eV and 532.6eV are in accordance with Ti–O and C–O bonds. However, compared to pure TiO2, the peak for Ti–O shift slightly to higher binding energy. This is possibly due to the existence of non-lattice oxygen ([15, 16]). Fig. 3d presents the high resolution spectrum of Ag 3 d, two peaks at 368.7 eV and 374.7eV are for Ag 3d5/2 and Ag 3d3/2, respectively. The slitting (6.0 eV) between these two peaks indicates the existence of
3. Results and discussion 3.1. Morphology and structure Fig. 1a and Fig. 1d show the SEM and TEM image of the as synthe sized CA sample. It can be seen that the silver-nanoparticles were deposited onto the surface of CNT uniformly with an average diameter of 2.12 � 0.42 nm. The small amount of Ag particles attached to the tube
Fig. 1. SEM images of (a) CA, (b)15% CAT, (c)20% CAT and TEM images of (d) (e) CA, (f) (g)15% CAT, (h) the elemental mapping image of 15% CAT. 3
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Fig. 2. XRD patterns of as synthesized (a)CA, (b)CAT samples.
Ag0, which is reduced from Agþ by NaBH4. However, the binding en ergies of Ag 3 d shift to higher values compared to bulk Ag, which are 368.2 eV for Ag 3d5/2 and 374.2 eV for Ag 3d3/2. The shift possibly stems from the loss of electrons in Ag, and this is often observed in depositing noble metals onto the surface of CNTs ([17,18]), indicating a strong interaction between the AgNPs and CNTs. Fig. 3e demonstrates the high resolution spectrum of Ti 2p. Two diffraction peaks at 459.5eV and 465.2eV corresponds to Ti 2p3/2 and Ti 2p1/2, respectively. The slitting (5.7eV) between these peaks are strong evidence for Ti4þ, which indicates Ti element can only exists in the composite in the form of TiO2. However, compared to standard binding energies of Ti 2p3/2 (458.6eV) and Ti 2p 1/2 (464.1eV), these peaks shift to higher values. This is considered to be the influence of CNTs on the electronic state of Ti element due to the combination. In other words, the relationship be tween CNTs and TiO2 is not a simple mixing one, but an internal one with electronic interactions between them.
and 2.30 eV (15% CAT), respectively. The shift could be attributed to the formation of Ti–O and C–O bonds [22,23], the photosensitizing effect of CNTs (24) and the LSPR effect of Ag particle ([25–27]), which have been certified. Photoluminescence (PL) spectra has always been used as an indirect tools to evaluate the recombination efficiency of photogenerated electron-hole pairs ([28]). As shown in Fig. 6b, the PL emission peak of 15% CAT has a lowest intensity, indicating that the recombination of e and hþ is inhibited greatly. The peak intensity of 15% CT is lower than that of TiO2, this can possibly attribute to the transfer of electrons during the excitation process ([24]), which will be explained later. Moreover, what shouldn’t be ignored is that, when the proportion of CA in CAT increases to 20%, the intensity of the PL spectra still increases slightly. In 20% CAT, TiO2, the active component that produces photogenerated electron-hole pairs, decreases with the increment of CA, suggesting that the photogenerated electron-hole pairs are more likely to be recombined internally using CA as recombination center. Fig. 6c shows the time-resolved fluorescence spectra of the samples. It can be seen that 15% CAT sample achieves a shorter fluorescence lifetime (1.41 ns) compared to 15% CT (1.65 ns) and TiO2(1.88ns). In 15% CAT, CA can act as a transmission agent and inhibit the fluores cence phenomenon caused by the recombination of electron-hole pairs, thus exhibits a shorter fluorescence lifetime, and accordingly, a lower recombination rate of the charge carriers. The results are in agreement with PL spectra. The electrochemical impedance spectra (EIS) of the as obtained samples, shown in Fig. 6d, are presented as Nyquist plots. The imped ance curves of the samples demonstrate a compressed semicircle in the high to medium frequency range, which could be assigned to the charge transfer resistance (Rct), and a near-45� inclined line in the lowfrequency range, which considered to be the Warburg impedance. The fitting of semi-arcs was carried out using NOVA software (equivalent circuit shown in the inset of Fig. 6d) to estimate the charge transfer resistance (Rct) in parallel with the double-layer capacitance (Cdl) at the electrode-electrolyte interface. The arc radii of 10% CAT, 15% CAT and 20% CAT ternary photoanode, corresponding fitting values of Rct are 32.7 Ω, 7.1 Ω and 76.2 Ω, respectively, are smaller than 15% CT (93.7 Ω) and TiO2(96.9 Ω) suggesting that the introduction of Ag nanoparticles is favorable to the separation of charge carriers ([29]). The smaller value of Rct for the CAT ternary photoanode confirms the faster charge transfer and separation rate within the ternary system compared to that of CT photoanode at the semiconductor-electrolyte interface. The results are in agreement with PL spectra (Fig. 6c). To further prove the results mentioned above, the transient photo current responses of the as obtained samples were conducted and the results are shown in Fig. 6d. The I-t curves of TiO2, CT and CAT samples
3.2. Formation mechanism of CA and CAT The schematic illustration of the fabrication process of CA compos ites is shown in Fig. 4. After treated by sulfuric acid and nitric acid, various functional groups such as hydroxyl and carboxyl are formed on the surface of CNTs. SnCl2 from SnCl2�2H2O can attach and form a layer in colloid form, as an activating layer, onto the surface of CNTs where functional groups are rich. When silver nitrate is added drop-wise, the small part of silver ions are reduced by SnCl2 layer at the surface of CNTs, forming numerous small Ag core. When the main reducing re agent NaBH4 is added, metallic silver is reduced and accumulate on those small particles and grow into bigger size. During this process, PVP can act as a shape control agent and prevent the aggregation of AgNPs via its’ steric hindrance effect. In the past decades, many researchers ([3,11,19–21]) synthesized CNTs/TiO2 via sol-gel method. They all pointed out that functional groups on the surface of CNTs can act as anchor points for TiO2. On the basis of these theories, we deduce that TiO2 can also anchor on the surface of CA (the schematic illustration of the fabrication process is shown in Fig. 5), and they may disperse into each other to have an “associated relationship”, which has been proved by TEM mapping shown in Fig. 1g. 3.3. Optical and photoelectric properties UV-VIS diffuse reflection spectra (UV-VIS DRS), shown in Fig. 6a, are used to compare the optical properties of TiO2, 15% CT and 15% CAT samples. It can be seen that the introduction of CNTs and CA results in a red shift of adsorption edge from 3.20 eV (TiO2) to 2.88 eV (15% CT) 4
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Fig. 3. XPS spectra of 15% CAT sample (a) XPS fully scanned spectrum, (b)C 1s, (c)O 1s, (d) Ag 3 d, (e)Ti 2p.
with four complete cycles of intermittent-light irradiation are shown in Fig. 6d. It is easy to find that all these responses are highly sensitive. The photocurrent responses of the samples are ranked in the following order: 15% CAT >10% CAT >20% CAT >5% CAT >15% CT > TiO2, the result indicate that the introduction of CNTs and Ag can separate the photogenerated electrons and holes further compared to bare TiO2, and a maximum value is found under the condition of 15% CAT. The photo current response results are in agreement with those of PL and EIS measurements.
3.4. Photocatalytic activity of the CAT ternary structure To study the photocatalytic properties of the obtained samples, degradation experiments of methylene blue (MB) under visible light and UV light are conducted and the results are shown in Fig. 7. Under visible light irradiation, TiO2 performed poorly in degrading organic pollutant, which may ascribe to its’ wide band gap energy. The degradation rate of MB by TiO2 is only 4.9% in 3 h. When CNTs were introduced, due to the incorporation of carbon atoms into the lattice of TiO2 and the formation of Ti–O and O–C bonds, the degradation rate of 5
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Fig. 4. Schematic illustration of the fabrication process of CA composites.
Fig. 5. Schematic illustration of the fabrication process of CAT.
15% CT increased to 13%. The photocatalytic ability of CAT samples increased dramatically because Ag particles can absorb visible light due to its’ LSPR effect. Additionally, when the mass ratio of CA increased from 5% to 15%, the degradation rate increased from 39.4% to 80.8% in 180 min, this can possibly due to the higher uniformity of the TiO2 layer at the surface of CNTs. However, when the mass ratio of CA increased to 20%, the degradation efficiency decreased to 58.5% instead. This sug gested that a non-uniform coverage of TiO2 in 20% CAT can lead to a lower yield of photo-generated electrons and holes compared to 15% CAT (this was proved by the PL spectra and the photocurrent response of the samples), leading to the decrease in photodegradation efficiency. Under UV light irradiation, the degradation rate of TiO2 reached 95.4% in 60 min. The degradation rate of 15% CT is 97.4% in 60 min, the increment can ascribe to the outstanding conductivity of CNTs, which facilitates the transfer of electrons, thus separate the electrons and holes further, leading to a higher degradation rate. We can also find
in Fig. 7b that with the increase the amount of Ag, the photocatalytic ability raised at first and then decreased to a lower level. Among all these CAT samples, 15% CAT ternary structure exhibited an optimum degradation rate, which is 100% in 60 min (99.2% in 40 min). After the introduction of Ag, AgNPs can act as an acceptor of electrons and form a Schottky barrier when contact with TiO2, resulting in a lower recom bination rate, which is favorable for the enhancement of photocatalytic ability. 3.5. Photocatalytic stability of 15% CAT ternary structure To test the stability of the 15% CAT catalyst, a recycling experiment is conducted under the same conditions. The results shown in Fig. 8 indicated that the photocatalytic efficiency of the 15% CAT sample re mains stable in the first three cycles, then decreased slightly in the fourth and the fifth experiment, which may attribute to the slight detachment 6
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Fig. 6. (a) DRS spectra, (b) Photoluminescence spectra, (c) Time-resolved fluorescence spectra, (d) EIS spectra and (e) Transient photocurrent responses of the samples.
of TiO2 on the surface of CNTs.
amount of holes on the valence band (VB), as shown in Fig. 10a and b. Because the conduction band of TiO2 is higher than the equilibrated Fermi-level, excited electrons can be fed into to the lifted Fermi-level of Ag easily. While the electrons combine with the absorbed oxygen mol ecules to form superoxide radical anions (O2 ), the holes on the VB of TiO2 can promote the conversion from OH to ⋅OH, the high yield of these two particles can greatly improve the degradation of the pollutant. Moreover, CNTs, acting as the carrier of both TiO2 and Ag, can transform away the electrons to further prevent the recombination of electron-hole pairs, which is favorable for enhancing photocatalytic efficiency. Under visible light, the LSPR effect of Ag plays a key role. Upon resonance with incoming photons, some part of the electrons will have a higher energy than the previous conduction band of TiO2(before contact with Ag, ECB0), enabling the transmission of the electrons from Ag to
3.6. Photodegradation mechanism Generally, there exists many oxygen defects in TiO2, this means that there are many excess electrons inside TiO2, which leads to its’ n-type semiconductor properties ([30]). The Fermi-level (Ef) of TiO2 is higher than the work function(W) of Ag. After contact, electrons and holes will flow to build up a new thermal equilibrium state, in which Ef and W are equilibrated, seen in Fig. 9. Meanwhile, a space-charge region is formed, and can prohibit the recombination of photo-generated holes and elec trons in the degradation process. Under UV irradiation, shown in Fig. 10a and Fig. 10b, the electrons will migrate to the conduction band (CB) of TiO2, leaving the same 7
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Fig. 7. Photocatalytic degradation of MB over the samples under (a) visible light irradiation and (b) UV light irradiation.
electrons that have enough energy to migrate to the CB of TiO2. After the dissipation, the electrons will change back to standard Fermi-Dirac distribution ([31]). In CAT, as shown in Fig. 10e, the Fermi levels will be equilibrated due to the contact of TiO2 with both Ag and CNTs. After absorbing visible light, the LSPR effect of Ag will cause the oscillation of electrons, thus obtaining a higher energy than the energy of the CB of TiO2(ECB0). Then the electrons will transmit to the CB of TiO2, and will react with the absorbed O2 molecules to form superoxide radical anions (O2 ), which is vital in the whole degradation process. Additionally, electrons on the CB of TiO2 have a higher energy state than the equili brated Fermi level, thus the electrons can feed into the Fermi level of CNTs and can be transmitted immediately, leading to a lower recom bination rate. All these factors contribute to the advanced photocatalytic abilities of CAT samples. Anchoring TiO2 onto the surface of CA can result in an enhanced phtotocatalytic performance. The LSPR sensitizing effect of Ag particles, formation of Ti–O–C bonds and the high conductivity of CNTs can lead to an extended response range to visible light, a decrease in band gap and a further inhibited recombination of charge carriers, all these factors contribute to the excellent photocatalytic activity of CAT both under UV and visible light.
Fig. 8. Cycling tests for degradation of MB using 15% CAT.
4. Conclusion In this work, a facile procedure based on in-situ reduction method was developed to synthesize CAT nano-composite with a ternary struc ture. Ag nanoparticles were constructed between CNTs and TiO2 and served as a “bridge”. Combining the LSPR effect of AgNPs and the photosensitizing effect of CNTs, CAT achieved a longer life in photo generated electron-hole pairs, leading to a higher efficiency of photo catalysis for the degradation of organic pollutant in a large wavelength range, especially in visible light region. Additionally, the working mechanism of ternary structure was well suggested in this study, which could be further extended to some other semiconductor materials, such as ZnO and so on. In conclusion, this novel ternary structure based on large band gap photocatalysts could realize important value in photo catalytic purification both in the purification of water and air. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Chen Zhao: Conceptualization, Methodology, Software, Investiga tion, Formal analysis, Writing - original draft. Jun Guo: Software, Validation, Resources. Chenlu Yu: Resources. Zhejuan Zhang: Super vision, Data curation, Writing - review & editing. Zhuo Sun: Supervi sion. Xianqing Piao: Supervision.
Fig. 9. Energy diagrams in different states.
TiO2, seen in Fig. 10c. Through electron-electron relaxation, the energy state will change back to Fermi-Dirac distribution, and the Fermi level is higher. However, as shown in Fig. 10d, there are still small part of 8
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Fig. 10. Proposed photocatalytic mechanism of CAT under (a) (b) UV light irradiation; (c) (d) (e) visible light irradiation.
Acknowledgments [6]
This work is financially supported by National Natural Science Foundation of China of China (No. 11204082), Shanghai Natural Fund (No: 16ZR1410700), Fundamental Research Funds for the Central Uni versities, Project of Social Office of Shanghai Science and Technology Commission (No: 19DZ1205102) and the Direct Fund (No: 201902 ZR).
[7] [8]
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