Abstract
An enormous enhancement in the photo-to-current conversion efficiency over the nanocomposite material composed by BiVO4 on the surface of MWCNTs, with respect to electrode of pure BiVO4, was observed. The heterojunction formed between MWCNTs and nano-BiVO4 is beneficial for the separation of photogenerated electrons and holes, resulting in more electrons that are able to transport efficiently to the surface and therefore enhance the photoefficiency.
1. Introduction
Since the photoelectrochemical water splitting (the Honda-Fujishima effect) was reported in 1972 [1], great progresses have been made on the research and application of photocatalysis and photoelectrochemistry both in energy and environmental fields.
Up to date, the design and development of visible-light-responsive photocatalysts is one of the research directions, because the utilization of visible light, which accounts for more than half of the solar spectrum, is significant. For this goal, it is utmost important to develop photocatalysts with a narrow band gap. One of the efforts consists of creating an electron donor level between the valence band and conduction band of TiO2 by doping with metallic or nonmetallic elements such as such as Ag, Cu, Fe, Co, V, Cr, and Pd and rare earth element or N, S, and C [2–10]. However, although the doping of foreign elements extends the absorption to visible-light ranges, it increases the defects of semiconductor photocatalysts, which therefore is a part of the ultraviolet light-responsive performance that the titanium oxide originally possessed was occasionally ruined [11–17]. Another effort is the exploring of complex compounds that containing Bi3+, In3+, Sn2+ (s2 configuration), or Ag+ (d10 configuration) ions in an oxide system. Thus, it is able to elevate the valence band by means of the hybridization of their respective orbitals with the orbital, and also narrowing the band gap of the semiconductor [18].
BiVO4 is one of such complex oxides with narrow band gap. BiVO4 shows not only excellent visible-light photocatalytic properties, but also a high photo-to-current conversion efficiency [19–27]. Almost a decade ago, the high activity of BiVO4 for photocatalytic O2 evolution from aqueous suspensions containing Ag+ as a sacrificial electron acceptor under visible-light irradiation was observed [19]. It was then discovered that nanocrystalline BiVO4 thin-film electrodes show high photocurrent yields for oxygen evolution in neutral aqueous electrolytes [25–27]. Therefore, BiVO4 might be a good visible-light photovoltaic material. However, at the present stage, it is still necessary to explore strategies to improve the visible-light photoelectrochemical reactivity and efficiency of BiVO4.
The photoelectrochemical properties can be enhanced if the photogenerated electrons or holes are transmitted effectively, in which the recombination of electrons and holes is avoided. As we have known, the rate of electron-hole recombination is a decisive factor for photoelectrochemical properties. Thus, to form a heterojunction with semiconductor called a Schottky barrier, where there is a space-charge separation region, is an effective method of increasing recombination times for electron-hole pairs. Traditionally, this method of extending recombination times was established with platinum and other noble metal interfaces. CNTs have a variety of electronic properties, similar to the metals above mentioned, and they may also exhibit metallic conductivity as one of the many possible electronic structures [28]. The physical and chemical characteristics of semiconductor photocatalysts as well as the high conductivity along the tube axis of carbon nanotubes (CNTs) produced a great deal of incentive to disperse CNTs into the photoactive layer in order to obtain more efficient photoelectrochemical devices [29, 30]. However, the fabrication of nanosized BiVO4 on the surface of MWCNTs to form a nanocomposite of BiVO4@MWCNT was not reported in the literatures.
In this work, the photoelectrochemical property of the BiVO4@MWCNT thin-film photoelectrode was studied, and an enhancement in the photo-to-current conversion efficiency was observed. The heterojunction formed between BiVO4 and CNTs as well as the direct electron transportation effect of carbon nanotube is proposed to charge for the enhancement of photoefficiency.
2. Experimental Section
2.1. Fabrication of BiVO4@MWCNTs Nanocomposites
BiVO4-MWCNTs nanocomposite was prepared by coprecipitation method. First, required amounts of Bi(NO3)3·5H2O and NH4VO3 were separately dissolved in 2.0 mol/L of nitric acid solution. The pretreated MWCNTs (provided by Chengdu Organic Chemical Co. Ltd.) were dispersed into PEG in ultrasonic for 1 h. Then, the mixture of MWCNTs and PEG were added into the solution of Bi(NO3)3 and NH4VO3. Meantime, 5 g of urea was added into the above mixture. The solution was stirred at 353–363 K for 12 h. At last, the mixture was filtered, washed, and dried, and the BiVO4@MWCNTs nanocomposite was received. For comparison, pure BiVO4 was also prepared according to the procedure reported in the literature [23]. The electrodes were prepared using the above-synthesized powders by dip-coating on the ITO conductive glass.
2.2. The Photoelectrochemical Measurements
The photoelectrochemical properties were carried by CHI760C electrochemical work station (Shanghai Chenhua instrument Co., Ltd.) in a three-electrode cell setup with a flat circular quartz window (diameter = 2 cm) opposite the working electrode, and platinum wire and Ag/AgCl were used as counter and reference electrodes, respectively. The light source was a 300-W Xe arc lamp (PLS-SXE300, Beijing Changtuo Co. Ltd.), and the electrodes were illuminated from the back electrode of the ITO side with a 420 nm bandpass filter in front of a quartz window to remove the light of wavelength less than 420 nm. The electrolyte was 0.5 mol/L Na2SO4 solution without pH control.
3. Results and Discussions
3.1. The Enhanced Photoefficiency of BiVO4@MWCNTs Nanocomposites
Figure 1 shows the photoelectrochemical behaviors of the thin-film photoelectrodes fabricated by pure BiVO4, a mixture of BiVO4 and MWCNTs, and BiVO4@MWCNTs nanocomposite, which were measured by transient photocurrent-time curves. As can be seen from the current-time (I-t) curves, pure BiVO4 thin-film electrode showed an intensive photocurrent density of about 30 μA/cm2 under visible light ( nm) irradiation. The photocurrent over the photoelectrode prepared from a mixture of BiVO4 and CNT is increased in a certain extent with comparison to that over pure BiVO4 photoelectrode, getting to 42 μA/cm2. While it is interesting that the photocurrent intensity on BiVO4@MWCNTs nanocomposite electrode is much higher than that over the above two electrodes, reaching to over 140 μA/cm2. It should be noted that the amount of photoactive BiVO4 in BiVO4@MWCNTs nanocomposite and mixture of BiVO4-MWCNTs is much lower than that of pure BiVO4. So, it is unambiguously that the photocurrent over BiVO4@MWCNTs nanocomposite photoelectrode was enhanced greatly with comparison to the pure BiVO4 as well as BiVO4-MWCNTs electrodes. As all of the semiconductors were prepared by coprecipitation method, while the only difference is just whether or not formation of nanocomposites of BiVO4 with the MWCNTs, the enhanced photocurrents over photoelectrodes of BiVO4@MWCNTs suggested that the formation of nanocomposite structure of BiVO4@MWCNTs is the major factor for improving the photo-to-current conversion efficiency.
3.2. The Structure of BiVO4@MWCNTs Nanocomposites
The crystalline structure of BiVO4 and the morphologies of the formatted composites with carbon nanotube were characterized by X-ray diffraction pattern and electron microscopy. Figure 2 shows the XRD patterns of pure BiVO4, BiVO4@MWCNTs nanocomposite, and MWCNTs, respectively. The diffraction peaks of pure BiVO4 prepared by a coprecipitation process are in good agreement with the standard JCPDS card no. 14-0688. This structure is the typical monoclinic scheelite BiVO4 (Figure 2(a)). No impurity peaks were observed, indicating that the sample is a pure phase BiVO4 with a monoclinic scheelite structure. The XRD patterns of the BiVO4@MWCNTs composites exhibit diffraction peaks much similar to the pure BiVO4 powders except that two small peaks at 26.06° and 44.46° (indicated by dots in Figure 2), which are attributed to the characteristic peaks of MWCNTs (Figure 2(c)). These results suggested that the BiVO4 particles in BiVO4@MWCNTs composites sample are also of monoclinic scheelite phase.
In the three main crystal forms of BiVO4 (tetragonal zircon type and monoclinic and tetragonal scheelite structure), monoclinic scheelite BiVO4 is much more active than the tetragonal scheelite for O2 evolution from aqueous AgNO3 solution under visible-light irradiation [21]. The photo-to-current properties of monoclinic scheelite BiVO4 was also found much intensive than the other two structures. This is because the tension in the microstructure of monoclinic scheelite BiVO4 is more intensive than that of tetragonal scheelite, due to the presence of a 6s2 lone pair of Bi3+ resulted in the lone-pair distortion of the former one is much higher than the later one [21, 23]. These distinctive differences in the structure can be conveniently distinguished by XRD patterns, in which the monoclinic scheelite BiVO4 with a high distortion generally shows well splitting of peaks at 18.5°, 35°, and 46° of 2θ. The widened part of XRD patterns near 19° of pure BiVO4 and BiVO4@MWCNTs composites are distinguished shown in the section of Figure 2(b). It can be observed that both samples showed two peaks with well resolution at this area. This observation suggested that both of the pure BiVO4 and the BiVO4@MWCNTs composites are of monoclinic scheelite type with a high distortion.
3.3. The Morphologies of BiVO4@MWCNTs Nanocomposites
Figure 3 shows the typical morphologies of the BiVO4@MWCNTs, which were examined by SEM. It is seen that most of BiVO4 particles are nanosized that attached to the MWCNTs tightly. On the other hand, there are some bulks BiVO4 particles that were observed among BiVO4@MWCNTs nanoparticles, revealing that the particles free from the MWCNTs grow more readily than those on the MWCNTs (shown in the inset of Figure 3), and the diameter bulk particles reach to 1 μm. In the mean time, some of the MWCNTs attached tightly to the surface of the bulk BiVO4.
Figure 4 shows the typical TEM photographs of pure MWCNTs and BiVO4@MWCNTs. It is seen that BiVO4 nanoparticles are attached on the wall of MWCNTs (Figure 4(b)), contrasting to the relative smooth surface of pure MWCNT (Figure 4(a)). The size of BiVO4 nanoparticles is less than 10 nm, in the range of 5–10 nm. In addition, the BiVO4 nanoparticles on the MWCNTs are significantly smaller than the particles of bulk BiVO4, which should be attributed to the restriction effect of MWCNTs on the growth of attached nanoparticles. To investigate the composition of these tiny particles, EDX spectra analysis was applied to probe the composition of the coated nanoparticles (Figure 4(c)). It was found that V, Bi, O, C, and Cu elements are present on the surface of the nanotubes, revealing the existence of V and Bi on MWCNTs. The Cu signal arises from the copper grid and the C signal comes from the MWCNTs. Furthermore, the interface between MWCNT and BiVO4 is clearly seen, indicating that BiVO4 nanoparticles are well attached on the outermost shell of MWCNTs. Besides attaching on the surface of MWCNTs, BiVO4 nanoparticles were also observed in the inner hollow cavity of nanotubes (shown in Figure 4(d)). It shows that nanosize cylinder in cavity of nanotubes is about 10 nm for the diameter and 40 nm for the length. However, the diameter of MWCNTs is just about 7-8 nm. The expansion force of the crystal growth of BiVO4 led to the distortion of the nanotubes, while the suppression of the carbon wall resulted in the growth of crystal toward the tube axis, which is free of such hindrance.
(a)
(b)
(c)
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3.4. The Structural Effect of Nanocomposites on the Photoefficiency
As we have observed, the photo-to-current efficiency of BiVO4@MWCNTs nanocomposites has been enhanced enormously. The enhancement should be related to the structure of BiVO4@MWCNTs nanocomposites. For a semiconductor thin-film electrode, the photocurrent should be controlled by the recombination probability of photogenerate electrons and holes. The higher rate of e--h+ recombination will lead to a low photocurrent, and thus a lower photoefficiency. Because the MWCNTs had a high electron affinity and excellent conductivity, the exited electrons from photoactive BiVO4 were trapped easily by MWCNTs and transferred quickly along CNT to the ITO conductive glass substrate. Consequently, an intensive current was observed. Moreover, the observations in the TEM micrographs of BiVO4@MWCNTs suggested that the BiVO4 nanoparticles are tightly attached with the surface of MWCNTs, and this tight attachment can form a heterojunction between the photoactive materials with CNT and lead to less boundaries exist between the two phases; therefore, less electrons losses occurred in the boundary. The heterojunction formed in the device can achieve charge separation and collection due to the tightly attachment of BiVO4 with MWCNTs. Due to the introduction of internal junctions of BiVO4/nanotube within the nanocomposite matrix, the high electric field at these junctions can split up the electrons and holes. Therefore, in addition to the MWCNTs acting as a pathway for the electrons collection, electrons and holes can travel toward their respective contacts, avoiding the recombination, and thus, the photo-to-current efficiency is enhanced. However, although in the mixture of BiVO4 and MWCNTs, the MWCNTs were also participated in the electrons transfer, the heterojunction between BiVO4 and MWCNTs is not formed and electric field does not exist, so the main photogenerated electrons were lost in the boundary between BiVO4 and the MWCNTs. Therefore, the photocurrent was not enhanced greatly as expect.
4. Conclusions
The BiVO4@MWCNTs nanocomposites were successfully fabricated by a soft-chemistry coprecipitation approach. An enhancement in the photocurrent over BiVO4@MWCNTs composite electrode with respect to electrode of mixture of BiVO4/MWCNTs as well as pure BiVO4 was observed. It is concluded that the participation of MWCNTs acts as the electrons transfer media and the combination of BiVO4 with MWCNTs tightly is beneficial for the electrons collection, resulting in that more electrons are able to transport quickly to the surface, which thus enhance the photo-to-current conversion efficiency.
Acknowledgments
This work was financial supported by the National Foundation of Natural Sciences (no. 20973097) and The National Basic Research Program of China (973 Program, 2009CB220000). The authors also thank Scientific Research Foundation for Returned Overseas Chinese Scholars for the financial support.