Multiwalled Carbon Nanotube-TiO 2 Nanocomposite for Visible-Light-Induced Photocatalytic Hydrogen Evolution

Multiwalled carbon nanotube(MWCNT-) TiO 2 nanocomposite was synthesized via hydrothermal process and characterized by X-ray diffraction,UV-vis diffuse reflectance spectroscopy, field emission scanning electronmicroscope, thermogravimetry analysis, andN 2 adsorption-desorption isotherms. Appropriate pretreatment onMWCNTs could generate oxygen-containing groups, which is beneficial for forming intimate contact between MWCNTs and TiO 2 and leads to a higher thermal stability of MWCNT-TiO 2 nanocomposite. Modification with MWCNTs can extend the visible-light absorption of TiO 2 . 5 wt% MWCNT-TiO 2 derived from hydrothermal treatment at 140C exhibiting the highest hydrogen generation rate of 15.1μmol⋅h under visible-light irradiation and a wide photoresponse range from 350 to 475 nm with moderate quantum efficiency (4.4% at 420 nm and 3.7% at 475 nm).The above experimental results indicate that the MWCNT-TiO 2 nanocomposite is a promising photocatalyst with good stability and visible-light-induced photoactivity.


Introduction
Hydrogen resource obtained directly from water splitting using photocatalyst and solar light is under scrutiny as a clean energy resource; however it has been facing technical challenge to find a stable and efficient photocatalyst that can maximally utilize solar light [1].In addition to oxide semiconductors, a great variety of novel photoactive semiconductors have been developed in the last few years.Among these, mixed oxides of transition metal like Nb, V, or Ta or with main group elements such as Ga, In, Sb, or Bi have been extensively investigated as attractive candidates for visiblelight-induced photocatalysis [2].Also, sulfides and nitrides of different metals have been frequently selected to obtain materials with visible-light-driven photoactivity [3].Titania (TiO 2 ) is a widely used photocatalyst due to its high chemical stability, low cost, and nontoxic nature [4], but it can only absorb UV light with low quantum efficiency due to its wide bandgap (ca.3.2 eV), which limits its application in visiblelight-driven photocatalysis.The above problems have been partly resolved by many methods, such as surface modification via organic materials [5], semiconductor coupling [6], bandgap modification by nonmetals [7,8], and plasmonic metal (Ag and Au) doping [9][10][11][12][13][14], creating oxygen vacancies and disorder or dye sensitization [15][16][17].
Due to the special structures and extraordinary mechanical and unique electronic properties, carbon nanotubes (CNTs) have the potential to extend the photoresponse range of TiO 2 to visible-light region by modification of bandgap and/or sensitization and increase the photoactivity of TiO 2 by contribution to high surface area and inhibition of electronhole recombination [18].Single-walled carbon nanotubes (SWCNTs) have shown a synergy effect on enhancing photoactivity for H 2 evolution over a mixture of SWCNTs and TiO 2 [19].Ou et al. [20] have also demonstrated that multiwalled carbon nanotubes (MWCNTs) could enhance the visible-light-driven photoactivity of TiO 2 by acting as a photosensitizer in the MWCNT-TiO 2 : Ni composite.MWCNT-TiO 2 nanocomposite with visible-light-driven photoactivity was successfully synthesized via direct growth of TiO 2 nanoparticles on the surface of the functionalized MWCNTs by the hydrothermal treatment in our group [21].However, the effects of MWCNT pretreatment, MWCNT content, and

Hydrothermal treatment Calcination
Ion exchange Ti(SO 4 ) 2 and CTAB synthetic conditions of MWCNT-TiO 2 nanocomposite on its photocatalytic hydrogen evolution efficiency and quantum efficiency under monochromatic light irradiation are still beyond our knowledge.Herein, MWCNT-TiO 2 nanocomposite was synthesized, characterized, and employed for photocatalytic H 2 production from triethanolamine (TEOA) solution.The effects of pretreatment methods, content of MWCNTs, and hydrothermal temperature on the photocatalytic H 2 evolution efficiency over the Pt-loaded photocatalysts were studied.Moreover, the apparent quantum efficiency (AQE) of 5 wt% MWCNT-TiO 2 upon incident monochromatic light is also investigated.

Preparation and Characterization of MWCNT-TiO 2
Nanocomposites.All other chemical reagents used in the study were of analytical grade and used without further purification, unless stated otherwise.The preparation procedure for MWCNT-TiO 2 nanocomposites is demonstrated in Figure 1.In a typical experiment, pretreated MWCNTs were ultrasonically dispersed in deionized water, and then titanium sulfate (Ti(SO 4 ) 2 ) was added into the dispersion under stirring.The obtained mixture was added into cetyltrimethylammonium bromide (CTAB) solution under the molar ratio of Ti(SO 4 ) 2 : CTAB : H 2 O is 1 : 0.12 : 100.After stirring, the resulting mixture (pH 0.2) was aged at room temperature for 12 h and then transferred into an autoclave for 72 h hydrothermal treatment at 100 ∘ C. The resulting materials were collected using the centrifugation technique and mixed with a water and ethanol (molar ratio 1 : 1) solution of sodium chloride under stirring at 40 ∘ C for 5 h.Resultant sample was washed with water and ethanol, dried at 80 ∘ C overnight, and calcined at 400 ∘ C for 5 h.
X-ray diffraction (XRD) patterns were obtained on a XRD-6000 diffractometer using Cu K as radiation ( = 0.15418 nm).Scanning electron microscope (SEM) observation was conducted on a JEOL-6700F electron microscope.Diffuse reflectance spectra (DRS) were recorded on a Cary 5000 UV-Vis-NIR spectrophotometer equipped with an integrating sphere (Varian, USA).Thermogravimetry (TG) curves were recorded on a STA 449C thermal analyzer (Netzsch, Germany).The Brunauer-Emmett-Teller (BET) surface areas were analyzed by nitrogen adsorption-desorption measurement using a Micromeritics ASAP 2020 apparatus after the samples were degassed at 180 ∘ C.

Photocatalytic Activity Measurement.
The obtained MWCNT-TiO 2 nanocomposite was loaded with Pt and tested for its photocatalytic activity through a photocatalytic hydrogen evolution system as described in our previous publication [21,23].A 300 W Xe-lamp (PLS-SXE300C, Beijing Trusttech Co., Ltd., China) was applied as the light source, which was collimated and focalized into 5 cm 2 parallel faculae.A cut-off filter (Kenko, L-42;  > 420 nm) was employed to obtain the visible-light irradiation ( > 420 nm).The reaction was performed in a water suspension which contains 85 mL water, 15 mL triethanolamine (TEOA), and 40 mg photocatalyst.The suspension was irradiated from top of the system after thoroughly removing air.H 2 production rate was analyzed with a gas chromatograph (GC, SP-6800A, TCD detector, 5 Å molecular sieve columns, and Ar carrier).
The apparent quantum efficiency (AQE) was measured under the same photocatalytic reaction condition except for the incident monochromatic light wavelength.The hydrogen yields of 1 h photocatalytic reaction under visible-light with different wavelengths of 350, 365, 380, 420, 435, 450, 475, 500, 520, and 550 nm were measured.Each run was carried out three times and the average value was taken (the relative errors are under 10%).The band-pass and cut-off filters and a calibrated Si photodiode (SRC-1000-TC-QZ-N, Oriel, USA) were used in measurement.Apparent quantum efficiencies at different wavelengths were calculated by the following equation: The number of reacted electrons The number of incident photons × 100 = 2 × The number of evolved H 2 molecules The number of incident photons × 100. (1)

Results and Discussion
Figure 2 shows the XRD patterns of MWCNT, TiO 2 , and various MWCNT-TiO 2 nanocomposites.As can be seen from Figure 2(a), the main diffraction peaks of various products can be ascribed to anatase TiO 2 , whereas a decrease in the crystallinity of anatase can be found after the introduction of MWCNTs, indicating the decrease in the grain size of TiO 2 .The characteristic peaks for CNTs at 2 = 26.0∘ and 43.4 ∘ were not observed for 1.25-10 wt% MWCNT-TiO 2 , which is different from the previous observation [24].This phenomenon could be attributed to the good dispersion of MWCNTs in the nanocomposite after surface functionalization in nitrate solution [21].Average crystal sizes calculated using Scherrer equation [25] from the broadening of the (101) peaks of the anatase are 18.1, 12.8, 12.1, 12.9, 12.6, and 11.8 nm for TiO 2 and 1.25∼20 wt% MWCNT-TiO 2 , respectively (see Table 1).The small grain of TiO 2 nanoparticles in MWCNT-TiO 2 may be attributed to restricted direct contact of grains due to the presence of MWCNTs.As can be observed from Figure 2(b), even for MWCNT-TiO 2 calcined at 800 ∘ C for 5 h, the XRD pattern shows that all of the crystal phases are still anatase; no peak of rutile appears.Average crystal sizes calculated from the broadening of the (101) peaks of the anatase are 11.6, 12.9, 13.6, 17.8, and 32.9 nm for the 5 wt% MWCNT-TiO 2 as-synthesized and calcined at 400, 500, 600, and 800 ∘ C, respectively.These results suggest that MWCNTs in the nanocomposite probably inhibit the phase transformation of TiO 2 from amorphous phase to anatase phase and lead to a higher thermal stability.
To estimate the real content of MWCNTs in composites, 1.25-10 wt% MWCNT-TiO 2 were analyzed by TGA technique.The results shown in Table 1 suggest that the MWCNT/TiO 2 ratios estimated before the synthesis of the MWCNT-TiO 2 were consistent with the results obtained from TGA analysis.Therefore, negligible losses of MWCNTs occurred during the nanocomposite preparation procedure, which is in accordance with the results of Raman analysis on 5 wt% MWCNT-TiO 2 [21].The BET specific surface areas and pore volumes of the samples are summarized in Table 1.Compared with mesoporous TiO 2 nanoparticles prepared via hydrothermal processes [26], there is a rapid decrease in both BET surface area (from 318 to 111 m 2 /g) and total volume (from 0.61 to 0.35 cm 3 /g) of TiO 2 nanoparticles after modification with 1.25 wt% MWCNT, which could be due to the close-packed structure between TiO 2 nanoparticles and MWCNTs.With increasing MWCNT content, the BET surface area of MWCNT-TiO 2 decreased firstly and then increased, which can be attributed to the destruction of closepacking due to the self-agglomeration of MWCNTs or TiO 2 nanoparticles at high content.
The UV-vis spectra of MWCNT-TiO 2 display a similar absorption edge to TiO 2 (Figure 3), but an apparent enhancement of absorption throughout the visible-light region can be observed even for the nanocomposite containing 1.25 wt% MWCNTs.A correlation between the MWCNTs amount and absorption changes in the UV-vis spectra obviously features the enhancement of visible-light absorption upon increasing the MWCNT content; that is, the adsorption intensity of the present MWCNT-TiO 2 continuously increased with enhancing MWCNT content owing to its good dispersion.
Control experiments showed no appreciable H 2 evolution in the absence of either photocatalyst or irradiation under visible-light or full spectra irradiation.H 2 generation rates over 5 wt% MWCNT + TiO 2 (a simple mixture of MWCNTs and TiO 2 nanoparticles), 5 wt% MWCNT-TiO 2 , TiO 2 , and MWCNTs under the visible-light and full spectra irradiation are shown in Figure 4.The pristine TiO 2 and MWCNTs as well as the MWCNT + TiO 2 demonstrate no appreciable H 2 evolution under the visible-light irradiation, whereas  MWCNT-TiO 2 (A, B, and C) exhibits various H 2 generation rates, suggesting the importance of chemical linking between MWCNTs and TiO 2 for their visible-light-driven photoactivity [27].The pretreatment of MWCNTs has a strong effect on the photocatalytic H 2 evolution efficiency over MWCNT-TiO 2 .MWCNT-TiO 2 (B) demonstrates the highest H 2 evolution rate under both visible-light and full spectra irradiation.Different oxidizing reagents possess different degrees of oxidation power, which would purify and decorate MWCNTs with oxygen-containing groups or even destroy their nanotube structure [28].It is reported that MWCNT bundles appear exfoliated and curled after treatment with strong oxidative environment such as refluxing in nitric acid or stirring in piranha (mixture of sulphuric acid 96 wt% and hydrogen peroxide 30 wt% in ratio 70 : 30) [ the method of refluxing in nitric acid with that of sonicating in a mixture of sulfuric acid and nitric acid, the method of refluxing in nitrate solution provides a moderate oxidation that would generate oxygen-containing groups but would not destroy the MWCNTs structure.Therefore, MWCNT-TiO 2 (B) exhibited the highest photoactivity.As demonstrated in Figure 5, MWCNT-TiO 2 exhibits no photocatalytic activity until the MWCNT content enhanced to 3.5 wt% under visible-light irradiation.After that, its photocatalytic H 2 evolution efficiency increases firstly  and then decreases slightly with further enhancement of the MWCNT content.The maximum efficiency is achieved at 5 wt% MWCNT-TiO 2 under visible-light irradiation.
It is believed that TiO 2 nanoparticles could directly grow on the surface of the functionalized MWCNTs through the hydrothermal treatment [21].As shown in Figure 6, no MWCNTs were found in the SEM micrograph of 2.5 wt% MWCNT-TiO 2 , which can be attributed to the fact that MWCNTs are embedded inside the nanocomposite by TiO 2 nanoparticles, resulting from the direct growth of TiO 2 on the surface of MWCNTs.For 5 wt% MWCNT-TiO 2 , MWCNTs can be observed on the surface of nanocomposite owing to the increase of MWCNT content.These observations can fairly explain the effect of MWCNT content on the visiblelight-induced photoactivity.For the MWCNT-TiO 2 with MWCNT content less than 3.5 wt%, MWCNTs embedded inside the nanocomposite cannot be irradiated and excited by visible-light, and thus no visible-light-induced photoactivity was observed [30].As the visible-light absorbent and sensitizer, higher MWCNT content means more efficient visiblelight absorption, and more photogenerated electrons can be transferred to TiO 2 ; TiO 2 also plays an important role in the separation of photogenerated carriers: the electrons can be transferred from TiO 2 to the loaded Pt.It is reported [31] that the electrical conductivity in the interfacial contact between graphene and photocatalyst components is vital to the overall photocatalytic H 2 production.In addition, unintentional doping of TiO 2 may happen under annealing at 400 ∘ C [32,33], which can increase the visible-light absorbance of TiO 2 and MWCNT-TiO 2 .Therefore, there exists an optimal ratio of MWCNT to TiO 2 for achieving excellent electrical conductivity in the nanocomposites and significant photoactivity for H 2 evolution [30].
Under full spectra irradiation, MWCNT-TiO 2 with small MWCNT contents (1.25 wt% and 2.5 wt%) exhibits lower photoactivity than the pure titania, whereas 5 wt% and 3.5 wt% MWCNT-TiO 2 demonstrate the better and the best photoactivity, respectively.Carbon materials such as graphene, C 60 , and CNT can act as electron traps due to their high electron affinities [31,34,35], which would be functionalized as charge separators and enhance the photoactivity [30,36].However, as discussed above, for the MWCNT-TiO 2 with MWCNT content less than 3.5 wt%, MWCNTs embedded inside the nanocomposite could act as recombination center of electron and holes, resulting in a reduced photoactivity under full spectra irradiation.
It is well understood that the hydrothermal temperature plays an important role in the morphology, crystallinity, and particle size of TiO 2 [37], which are related to the photoactivity.The effect of hydrothermal temperature on the H 2 generation rate was evaluated under visible-light irradiation.As demonstrated in Figure 7, the H 2 generation rate over 5 wt% MWCNT-TiO 2 increases with the enhanced hydrothermal temperature before 140 ∘ C and decreases afterwards under both visible-light and full spectra irradiation.The highest H 2 generation rate of 15.1 mol⋅h −1 under visiblelight irradiation was obtained with 140 ∘ C hydrothermal treatment, whereas that of 323.7 mol⋅h −1 under full spectra irradiation was obtained with 100 ∘ C hydrothermal treatment.
The above experimental results could be rationalized by the following discussions.As can be seen from Figure 8, the XRD patterns of 5 wt% MWCNT-TiO 2 derived from different hydrothermal temperatures confirm the fact that the crystallinity of MWCNT-TiO 2 increases with enhancing hydrothermal temperature.It can be calculated using Scherrer equation [25] that average crystal sizes of the anatase are 8.3, 12.9, 15.7, and 26.7 nm for the 5 wt% MWCNT-TiO 2 with hydrothermal temperature of 80, 100, 140, and 180 ∘ C, respectively [38].Under visible-light irradiation, MWCNTs, as a photosensitizer, can absorb visible-light and the photogenerated electrons (e − ) can be excited from the VB to CB of the MWCNT [39].TiO 2 , as the "bridge" between MWCNTs and loaded Pt, would transfer the photogenerated electrons from MWCNT to Pt nanoparticles to generate H 2 from water reduction [40].As the electron acceptor and transfer station, TiO 2 with high crystallinity and tight combination with MWCNTs could benefit the electron injections and suppress the electron-hole recombination.On the other hand, high crystallinity of TiO 2 would inevitably lead to large crystal size and particle size, which would lead to a high electron-hole recombination rate owing to the long distance for electron transfer.As a result, 5 wt% MWCNT-TiO 2 derived from hydrothermal treatment at 140 ∘ C has the highest photoactivity under visible-light irradiation.
Under full spectra irradiation, photocatalytic H 2 generation over TiO 2 irradiated by UV light would make the best part comparing to that over MWCNT-TiO 2 irradiated by visible-light. 5 wt% MWCNT-TiO 2 derived from hydrothermal treatment at 100 ∘ C possesses moderate crystal size of anatase, resulting in the highest photoactivity due to the large surface area of MWCNT-TiO 2 and low electron-hole recombination rate.The photocatalytic activities for H 2 production over 5 wt% MWCNT-TiO 2 upon incident light with different wavelength were investigated.MWCNT-TiO 2 shows a relatively wide photoresponse under monochromatic light of wavelength ranging from 350 to 475 nm (Figure 9), which is consistent with the experimental results obtained with visible-light irradiation (see Figure 4).The apparent quantum efficiency (AQE) of MWCNT-TiO 2 as a function of the wavelength of the incident light was calculated according to the data in Figure 9. MWCNT-TiO 2 demonstrates high quantum efficiency upon irradiation with wavelengths of 420 and 475 nm (4.4% and 3.7%, resp.).With respect to the wide adsorption band in the entire visible-light region (Figure 3), a panchromatic photocatalytic activity could be expected for 5 wt% MWCNT-TiO 2 .However, no appreciable H 2 evolution is showed upon irradiation with wavelength longer than 500 nm.As demonstrated in our earlier study, not all MWCNTs in the nanocomposite are bound with TiO 2 nanoparticles, owing to the limited hydroxyl and carboxyl groups generated on the MWCNTs after the functionalization [21].Therefore, the absorption band between 400 and 800 nm of MWCNT-TiO 2 nanocomposite probably is a combination of the absorption spectrum of both MWCNT-TiO 2 and uncoupled MWCNT, which would absorb light with wavelength longer than 500 nm but cannot be excited to generate electron-hole pairs.On the other hand, the energy of the incident light at longer wavelength dramatically decreased, which would inevitably depress the photoactivity over MWCNT-TiO 2 .Furthermore, the photoactivity of TiO 2 under UV light irradiation is enhanced after coupling with MWCNT.The quantum efficiencies for H 2 production over P25 TiO 2 , mesoporous TiO 2 , and MWCNT-TiO 2 at 350 nm were measured to be 1.9, 2.5, and 3.8%, respectively.These phenomena were consistent with the previous discussion that MWCNTs could increase the carrier separation efficiency during photocatalytic process under full spectra irradiation.

Conclusions
A series of MWCNT-TiO 2 nanocomposites with different types of functionalized MWCNTs were synthesized through hydrothermal process and characterized by XRD, DRS, SEM, TGA, and BET techniques.The effects of pretreatment methods on MWCNT, MWCNT content, and hydrothermal temperature on the photocatalytic hydrogen evolution efficiency over MWCNT-TiO 2 nanocomposite were investigated.MWCNTs have a good dispersion in the nanocomposite, which inhibit grain growth of TiO 2 and improve its thermal stability.Appropriate pretreatment on MWC-NTs could generate oxygen-containing groups, which would become the anchoring sites with TiO 2 nanoparticles in the nanocomposite.However, chemical pretreatment with strong oxidative agents would destroy the intrinsic structure of MWCNTs, which is not beneficial for a high photocatalytic activity.The best photocatalytic activity was observed for the MWCNT-TiO 2 nanocomposite with a 5% weight ratio under visible-light irradiation.The photocatalytic activity of MWCNT-TiO 2 is related to the crystallinity of TiO 2 , link type between MWCNT and TiO 2 , and MWCNT content.A wide range of photoresponse from 350 to 475 nm is observed with high quantum efficiency.Upon irradiation with wavelengths of 420 and 475 nm, the quantum efficiency is 4.4% and 3.7%, respectively.The above experimental results indicate that the present MWCNT-TiO 2 nanocomposite is a promising photocatalyst with good thermal stability, chemical stability under UV light irradiation, and visible-light-induced photoactivity.

Table 1 :
Summary of the physicochemical properties of MWNT and MWNT-TiO 2 nanocomposite.
a Calculated by the Scherrer equation.b Carbon content determined by TGA-DSC analyses.c Average pore diameter calculated from BJH desorption average pore width (4 V/A).d Single point total pore volume at the relative pressure of ca.0.995. 29].Comparing