Improved photocatalytic hydrogen production property over Ni/NiO/N–TiO2−x heterojunction nanocomposite prepared by NH3 plasma treatment
Graphical abstract
Ni species are in intimate contact with TiO2 in Ni/NiO/N–TiO2−x nanocomposite, which cause the formation of heterostructure, thus improve the photogenerated charge separation and thereby promote the photocatalytic efficiency.
Introduction
Hydrogen energy is a storable, clean, and environmentally friendly fuel, which is identified as an attractive candidate to replace the fossil fuels in the future. The scientists are much interested in finding ways to produce hydrogen from renewable energy sources such as solar and wind. Photocatalytic water splitting into hydrogen and oxygen by photocatalysts has been considered as a promising approach to resolve this problem since it was discovered by Honda and Fujishima in 1972 [1]. In order to produce hydrogen gas effectively, the conduction band of semiconductor should be more negative than that of H2 production potential and the valence band should be more positive than that of oxygen oxidation potential. It is counted by Osterloh et al. that over 130 inorganic materials and their derivatives were utilized as catalysts for water splitting in the past decades [2] Among them, TiO2 has attracted extensive interest and considered to be most suitable for photocatalytic water splitting because of its low-cost, chemical inertness, opportune redox potential, non-toxicity, and long-term stability against photocorrosion and chemical corrosion [3]. However, the actual utilization of solar energy to produce hydrogen on TiO2 is still difficult, due to the following shortcomings: 1) low solar light utilization, 2) short life time of photogenerated electrons, 3) fast backward reaction (H2 + 1/2 O2 → H2O), and 4) large H2 production overpotential [4]. Till now, many efforts have been devoted to conquer these shortcomings, including metal or nonmetal doping [5], [6], combination with other semiconductors [7] or graphene [8], addition of sacrificial reagents [9], noble metals deposition [10], and formation of heterojunction [11].
N doping is considered to be the most effective approaches to enhance the visible light absorption since it was reported by Sato [12]. The doped N atoms narrowed the band gap of TiO2 by mixing N 2p and O 2p states, therefore demonstrating the visible light activity. [13]. Deposition of noble metals has been proven to be very effective method to prolong life time of photogenerated electrons, thus was intensively investigated for photocatalytic H2 production. The improved photocatalytic activity by noble metals deposition is due to the formation of Schottky barrier at the metal/semi-conductor interface, which leads to the efficient charge separation [14]. Many noble metals, including Au, Pt, Ag, and Pd, were used for photocatalytic H2 production [15], [16], [17], [18], [19]. The result is widely accepted that Pt/TiO2 system exhibited the best H2 production ability [17]. However, Pt is a rare and expensive noble metal which inhibits its practical application. Therefore, it is highly desirable to develop low-cost additives to replace Pt. More and more attentions have been focused on multicomponent photocatalysts. The intimate heterostructure which formed between components with different nanostructures can significantly improve the photogenerated charge separation and thereby promote the H2 production ability [11], [20], [21]. Li et al. prepared AgIn5S8/TiO2 heterojunction nanocomposite for H2 production. They considered that the enhanced photoactivity can be ascribed to some AgIn5S8 nanoparticles closely contacting the TiO2 nanoparticles to form heterojunction structure, which results in an efficient charge separation at the interface [11]. Brahimi et al. prepared CuAlO2/TiO2 heterojunction applied to visible light H2 production [20]. They reported that ideal configuration CuAlO2/TiO2 heterojunction showed efficiency six times greater than that of CuAlO2 under the same experimental conditions. Jang et al. prepared heterojunction photocatalyst TiO2/AgGaS2 for hydrogen production from water under visible light [21]. They suggested that the fabrication method was critical for the photocatalytic performance. This heterojunction configuration results in an efficient charge separation at the interface, followed by fast diffusion of photoelectrons generated in AgGaS2 towards surrounding TiO2, leading to high photocatalytic activity.
Recently, TiO2 coupled with Ni or its derivatives is found to be an effective multicomponent photocatalyst [22], [23], [24]. NiO is a p-type semiconductor and easily forms heterojunctions with n-type TiO2. Chen et al. prepared heterostructural photocatalyst comprising p-type NiO and n-type TiO2 for degradation of methylene blue [22]. They considered the photoactivity enhancement is attributed to P–N junction and cocatalyst effects. Oros-Ruiz et al. reported the photocatalytic degradation of trimethoprim by metallic Ni nanoparticles supported on TiO2–P25 [23]. The results show that the metallic Ni on the titania surface acts as a sink of electrons, promoting the formation of OH radicals, thus increases the activity for the mineralization of trimethoprim. Yu et al. prepared Ni(OH)2 cluster-modified TiO2 nanocomposite for photocatalytic H2 production [24]. They suggested that the function of metallic Ni is to help the charge separation and to act as cocatalyst for water reduction, thus enhancing the photocatalytic H2-production activity. Considering both N and Ni are effective dopants to improve TiO2 photocatalytic performance, combination of them should be a exciting modification method to improve TiO2 photocatalytic performance. Moreover, Zhang et al. theoretically calculated the electronic structure, deformation charge density, dipole moment and optical property of N and Ni codoped anatase TiO2 by density functional theory [25]. They found the codoping is helpful for enhancing the adsorption in visible-light region. However, only a few literature reported on N, Ni co-modified TiO2 photocatalyst [26], [27]. Zhang et al. prepare nitrogen and nickel co-doped TiO2 for photocatalytic degradation of formaldehyde [26]. Hu et al. prepared composite photocatalyst, N–TiO2 loaded with Ni2O3, and used for degradation of 2,4,6-trichlorophenol under visible light [27]. They suggested that the photocatalytic activity and stability of composite photocatalyst were much higher than that of catalyst modified with nitrogen or Ni2O3 alone. To our knowledge, there are few reports on the photocatalytic H2 production over N, Ni co-modified TiO2 photocatalyst.
Nonthermal plasma is composed of atoms, ions and electrons, which are much more reactive than their molecule precursors. Under plasma condition, a lot of reactions, which take place efficiently only at high temperature and pressure, can be initiated under mild conditions, thus is used frequently to prepare functional nanomaterials. Here, we report a convenient NH3 plasma method to prepare visible light responsive Ni/NiO/N–TiO2−x heterojunction nanocomposites. The photocatalytic H2 production ability of prepared catalysts were investigated under visible light.
Section snippets
Preparation and characterization
In a typical experiment, 0.1 mol of Ti(OC4H9)4 was dissolved in 100 mL ethanol to form solution A. Desired amount of Ni(NO3)2·6H2O (molar ratio Ni/Ti = 0.01, 0.03, and 0.05) was dissolved in a mixture of 50 mL deionized water adjusted to pH = 3 with nitric acid and 50 mL ethanol to prepare solution B. Then, the solution A was added dropwise into the solution B under vigorous stirring at room temperature, and aged for 48 h to form the gel. The obtained gel was dried for 10 h at 80 °C, followed
Results and discussion
The XRD patterns of prepared TiO2 catalysts are shown in Fig. 2. All samples exhibit the diffraction peaks of the anatase phase, indicating modification did not influence the crystal structure. The lattice parameters of prepared catalysts were measured according to the method of Shen [30]. The results shown in Table 1 indicate that the lattice parameters of all catalysts was unchanged along a- and b-axes, but c-axis parameter decreased obviously after NH3 plasma treatment. This is probably due
Conclusions
A series of visible light responsive Ni/NiO/N–TiO2−x heterojunction nanocomposites were prepared by nonthermal NH3 plasma treatment. Ni species deposition decreased the surface free energy of nanocomposites, reduced the thermodynamic driving force for particle growth, thus leading to the smaller particle sizes. NH3 plasma treatment did not influence the crystal structure but shift its absorption edges to the visible light region, reduce partial NiO to metallic Ni, and form high concentration of
Acknowledgment
This work was supported by Program for New Century Excellent Talents in University (No. NCET-11-1011), National Natural Science Foundation of China (No. 41071317, 30972418, 21103077), National Key Technology R & D Programme of China (No. 2007BAC16B07, 2012ZX07505-001), the Natural Science Foundation of Liaoning Province (No. 20092080).
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