Multiwalled carbon nanotube- (MWCNT-) TiO2 nanocomposite was synthesized via hydrothermal process and characterized by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, field emission scanning electron microscope, thermogravimetry analysis, and N2 adsorption-desorption isotherms. Appropriate pretreatment on MWCNTs could generate oxygen-containing groups, which is beneficial for forming intimate contact between MWCNTs and TiO2 and leads to a higher thermal stability of MWCNT-TiO2 nanocomposite. Modification with MWCNTs can extend the visible-light absorption of TiO2. 5 wt% MWCNT-TiO2 derived from hydrothermal treatment at 140°C exhibiting the highest hydrogen generation rate of 15.1
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 [
Due to the special structures and extraordinary mechanical and unique electronic properties, carbon nanotubes (CNTs) have the potential to extend the photoresponse range of TiO2 to visible-light region by modification of bandgap and/or sensitization and increase the photoactivity of TiO2 by contribution to high surface area and inhibition of electron-hole recombination [
Herein, MWCNT-TiO2 nanocomposite was synthesized, characterized, and employed for photocatalytic H2 production from triethanolamine (TEOA) solution. The effects of pretreatment methods, content of MWCNTs, and hydrothermal temperature on the photocatalytic H2 evolution efficiency over the Pt-loaded photocatalysts were studied. Moreover, the apparent quantum efficiency (AQE) of 5 wt% MWCNT-TiO2 upon incident monochromatic light is also investigated.
MWCNTs (diameter < 8 nm; length 10–30
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-TiO2 nanocomposites is demonstrated in Figure
Schematical formation model for MWCNT-TiO2 nanocomposite.
X-ray diffraction (XRD) patterns were obtained on a XRD-6000 diffractometer using Cu K
The obtained MWCNT-TiO2 nanocomposite was loaded with Pt and tested for its photocatalytic activity through a photocatalytic hydrogen evolution system as described in our previous publication [
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:
Figure
Summary of the physicochemical properties of MWNT and MWNT-TiO2 nanocomposite.
Sample | Crystal sizea (nm) |
|
|
Mean pore sizec (nm) | Total volumed (cm3/g) |
---|---|---|---|---|---|
1.25 wt% MWNT-TiO2 | 12.8 | 1.5 | 110.8 | 10.9 | 0.35 |
2.5 wt% MWNT-TiO2 | 12.1 | 3.2 | 97.5 | 12.4 | 0.36 |
5 wt% MWNT-TiO2 | 12.9 | 5.7 | 79.8 | 12.1 | 0.28 |
10 wt% MWNT-TiO2 | 12.6 | 9.9 | 110.1 | 11.8 | 0.37 |
20 wt% MWNT-TiO2 | 11.8 | 18.5 | 166.7 | 9.1 | 0.42 |
MWNT | — | — | 392.5 | 7.6 | 0.74 |
aCalculated by the Scherrer equation.
bCarbon content determined by TGA-DSC analyses.
cAverage pore diameter calculated from BJH desorption average pore width (4 V/A).
dSingle point total pore volume at the relative pressure of
XRD patterns of MWCNT, TiO2, and MWCNT-TiO2 with (a) different MWCNT content; (b) 5 wt% MWCNT and calcination at different temperatures.
As can be observed from Figure
To estimate the real content of MWCNTs in composites, 1.25–10 wt% MWCNT-TiO2 were analyzed by TGA technique. The results shown in Table
The UV-vis spectra of MWCNT-TiO2 display a similar absorption edge to TiO2 (Figure
DRS patterns of MWCNT, TiO2, and MWCNT-TiO2 with different MWCNT content. (a) TiO2; (b) 1.25 wt%; (c) 2.5 wt%; (d) 5 wt%; (e) 10 wt%; (f) 20 wt%; (g) MWCNT.
Control experiments showed no appreciable H2 evolution in the absence of either photocatalyst or irradiation under visible-light or full spectra irradiation. H2 generation rates over 5 wt% MWCNT + TiO2 (a simple mixture of MWCNTs and TiO2 nanoparticles), 5 wt% MWCNT-TiO2, TiO2, and MWCNTs under the visible-light and full spectra irradiation are shown in Figure
H2 evolution efficiency over MWCNT, TiO2, and various photocatalysts containing MWCNT and TiO2 under visible-light (
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 [
As demonstrated in Figure
The rate of H2 evolution on MWCNT-TiO2 nanocomposites with different MWCNT content under visible-light (
It is believed that TiO2 nanoparticles could directly grow on the surface of the functionalized MWCNTs through the hydrothermal treatment [
SEM micrographs of 2.5 wt% (a) and 5 wt% (b) MWCNT-TiO2 nanocomposite.
Under full spectra irradiation, MWCNT-TiO2 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-TiO2 demonstrate the better and the best photoactivity, respectively. Carbon materials such as graphene, C60, and CNT can act as electron traps due to their high electron affinities [
It is well understood that the hydrothermal temperature plays an important role in the morphology, crystallinity, and particle size of TiO2 [
Effect of hydrothermal temperature on photocatalytic H2 production over 5 wt% MWCNT-TiO2.
The above experimental results could be rationalized by the following discussions. As can be seen from Figure
XRD patterns of 5 wt% MWCNT-TiO2 derived from different hydrothermal temperatures for 72 h.
Under full spectra irradiation, photocatalytic H2 generation over TiO2 irradiated by UV light would make the best part comparing to that over MWCNT-TiO2 irradiated by visible-light. 5 wt% MWCNT-TiO2 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-TiO2 and low electron-hole recombination rate.
The photocatalytic activities for H2 production over 5 wt% MWCNT-TiO2 upon incident light with different wavelength were investigated. MWCNT-TiO2 shows a relatively wide photoresponse under monochromatic light of wavelength ranging from 350 to 475 nm (Figure
The dependence of photocatalytic activities for H2 production over 5 wt% MWCNT-TiO2 nanocomposite upon wavelength (controlled via cut-off filters).
A series of MWCNT-TiO2 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-TiO2 nanocomposite were investigated. MWCNTs have a good dispersion in the nanocomposite, which inhibit grain growth of TiO2 and improve its thermal stability. Appropriate pretreatment on MWCNTs could generate oxygen-containing groups, which would become the anchoring sites with TiO2 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-TiO2 nanocomposite with a 5% weight ratio under visible-light irradiation. The photocatalytic activity of MWCNT-TiO2 is related to the crystallinity of TiO2, link type between MWCNT and TiO2, 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-TiO2 nanocomposite is a promising photocatalyst with good thermal stability, chemical stability under UV light irradiation, and visible-light-induced photoactivity.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research was financially supported by the Natural Science Foundation of China (20973128, 21271146, and 21307035), Natural Science Foundation of Hubei Province (2011CDB139), and Fundamental Research Funds for Central Universities of China (2013PY112).