Controllable synthesis porous Ag2CO3 nanorods for efficient photocatalysis
© Guo et al.; licensee Springer. 2015
Received: 29 January 2015
Accepted: 7 April 2015
Published: 21 April 2015
The novel porous Ag2CO3 nanorods were facilely synthesized via a one-pot aqueous solution reaction at room temperature. The crystalline phase and size distribution of the nanorods were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. In addition, the porous feature of nanorods was confirmed by transmission electron microscopy (TEM) and nitrogen adsorption-desorption. The morphology and size of the Ag2CO3 crystal can be regulated via the choice of dispersing agents and adding approaches of reactants. Photocatalytic results show that the porous Ag2CO3 nanorods exhibit excellent photodegradation of rhodamine B (RhB) under visible-light irradiation, particularly the photoactivity performance and stability can be further improved in the presence of sodium bicarbonate (NaHCO3). It is indicated that NaHCO3 can prevent effectively the photocorrosion and promote the probability of electron-hole separation.
Semiconductor photocatalysts have attracted intense attention expecting to apply in the fields of pollution removal and fuel production by utilizing abundant sunlight [1-3]. Over the past years, TiO2 as the most universal used photocatalyst has been widely studied owing to its high photocatalytic activity, stability, nontoxicity, and low cost [4-7]. However, TiO2 is a wide bandgap (approximately 3.2 eV) semiconductor and difficult to be activated in visible-light region, only can be utilized under UV light which is a small fraction (about 4%) of the entire solar spectrum. In addition, TiO2 quantum yield of photoactivated processes is frequently lower due to its high recombination of photogenerated electron-hole pairs. Such clear drawback is the main motivation for searching a new, active under visible-light-driven and more efficient photocatalysts [8-10].
Recently, it has been found that silver-containing complex oxide semiconductors show great promise for improving photocatalytic performance owning to their tops of valence band can form a new higher-energy valence band consisting of the hybrid orbital of Ag 4d and O 2p, which make the bandgap narrower [11,12]. As a result, a series of visible-light-responsive novel silver-containing complex oxide semiconductor photocatalysts have been developed, such as AgNbO3 [13,14], AgSbO3 [12,15], Ag2CrO4 [16,17], Ag2SO3 , Ag3AsO4 , AgMO2 (M = Al,Ga,In) , and AgIO4 , and their active visible-light-driven photocatalysts for the degradation of organic pollutants have also been explored. More recently, it was reported that Ag2CO3 showed high-efficiency visible-light activity and exhibited universal degradation ability for typically several organic dyes [22-24]. However, the photocorrosion behavior of Ag2CO3 exposed to the light irradiation cannot be ignored. Hence, the addition of light stabilizer in reaction solution is critical to Ag2CO3 photocatalyst for its practical application. Dai et al.  prepared Ag2CO3 short rods by a simple precipitation reaction, and it showed that high visible-light photocatalytic activity for the photodegradation of rhodamine B (RhB). The authors stated that the silver nitrate (AgNO3) is beneficial to the stability during the photocatalytic degradation reaction process because it can act as an electron acceptor to avoid photocorrosion of Ag2CO3 photocatalyst. Moreover, it has been reported that the photocatalytic efficiency can be further improved by rational design to achieve porous structures, in that, the porous structures avail the adsorption of reactant molecules and provide multiple accessible passages which reduce the reactant diffusion distance due to their large specific surface area (SSA). Significantly, porous structure can produce more isolated and separated active sites after photoirradiation and provide special channels for charge transport, which results in high efficiency of charge separation and transport in under photoirradiation [25-27].
As far as we know, the synthesis of porous silver-containing complex oxide photocatalysts by one-pot aqueous solution reaction at room temperature has rarely reported. Herein, in the present work, we prepared a novel porous Ag2CO3 nanorod photocatalyst by one-pot aqueous solution reaction using PVP-K90 dispersing agent at room temperature. The as-prepared samples showed efficient photocatalytic activity for the degradation RhB aqueous solution by utilizing sodium bicarbonate (NaHCO3) as a light stabilizer under visible-light irradiations. Furthermore, the growth behavior of Ag2CO3 and photocatalysis enhanced mechanism of NaHCO3 were also discussed.
All the chemicals were analytic grade purity and were used without further purification. AgNO3, NaHCO3, polyvinylpyrrolidone (PVP-K30, PVP-K90) and RhB were purchased from Shanghai Chemical Regent Factory of China (Shanghai, China).
Synthesis of porous Ag2CO3 nanorods
The porous Ag2CO3 nanorods were synthesized by a typically simple aqueous solution reaction at room temperature. In a typical synthesis, AgNO3 (0.025 M) and PVP-K90 (0.45 M) were dissolved in 40 mL deionized water to form a clear solution by magnetic stirring, then, 40 mL aqueous solution of NaHCO3 (0.05 M) was dropwise added to the obtained solution. The reaction was carried out at room temperature for 2 h under magnetic stirring, and the precipitate was collected by centrifugation, washed three times with deionized water and absolute ethyl alcohol, and dried at 50°C for 12 h. Furthermore, the synthesis of Ag2CO3 thin nanorods was similar to the above description except that PVP-K90 was replaced by PVP-K30. The cube-like Ag2CO3 was achieved by one-time injection of the NaHCO3 solution using PVP-K90 as the dispersing agent. N-doped TiO2, which is good photocatalytic activity under visible-light irradiation, was obtained as a reference to compare with our sample according to the reported literature .
Scanning electron microscopy (SEM) images were taken using a field-emission scanning electron microscope (JSM-6701 F, JEOL Ltd., Akishima-shi, Japan) and equipped with an energy-dispersive (ED) detector with this field-emission scanning electron microscope (FE-SEM) operated at 15 kV. Energy-dispersive X-Ray (EDX) analysis was also performed on the JSM-6701 F instrument during SEM. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 electron microscope (JEOL Ltd., Akishima-shi, Japan) at an accelerating voltage of 200 kV. X-ray diffraction (XRD) data for the finely ground samples were collected at 298 K using a Bruker D8 X-ray diffractometer (Bruker AXS, Inc., Madison, WI, USA) with Cu-Kα radiation source (λ = 1.5406 Å). It was operated at 40 kV in the 2θ range of 10° to 80° in the continuous scan mode with the step size of 0.01°. The changes in the oxidation state of Ag were recorded though an AXIS-ULTRA DLD-600 W photoelectron spectrometer (Shimadzu Corporation, Kyoto, Japan) (XPS) with Al K1 radiation. Nitrogen adsorption-desorption isotherms were collected on an Autosorb-iQ sorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA) and analyzed followed by the Brunauer-Emmett-Teller (BET) equation. The pore size distribution plots were obtained by using the Barret-Joyner-Halenda (BJH) model.
Photocatalytic performance measurements
The photocatalytic performance of the as-prepared samples was evaluated by measuring the degradation of RhB. In all catalytic activity of experiments, the samples (0.05 g) were put into a solution of RhB dyes (50 mL, 10 mg/L), which was then irradiated with a 300-W Xe arc lamp to provide visible light with λ ≥ 420 nm by an ultraviolet cutoff filter. Before the suspensions were irradiated, they were magnetically stirred for 30 min in the dark to complete the adsorption-desorption equilibrium between dyes and photocatalysts. The degradation of RhB was monitored by UV-vis spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan) every 5 min. Before the spectroscopy measurement, these photocatalysts were removed from the photocatalytic reaction systems by centrifugation. The relative concentrations (C/C 0) of the RhB solutions were determined by the absorbance (A/A 0) at 553 nm.
Results and discussion
To avoid the disadvantages of photocorrosion, we employ NaHCO3 as a stabilizer to inhibit the photocorrosion and decrease the solubility of Ag2CO3 in aqueous solution. So, to further evaluate the photostability of Ag2CO3 in the presence of NaHCO3, the recycled experiments for the photodegradation of RhB were performed, and the results are shown in Figure 5C. After four cycles, the Ag2CO3 still gives 70% degradation rate of RhB after 40-min visible-light irradiation and that the Ag2CO3 in the absence of NaHCO3 almost lost their activity. It indicates that the presence of NaHCO3 is helpful to enhance the stability and photocatalytic activity of Ag2CO3. On the basis of experimental results, two possible reasons are proposed to explain the significantly enhanced photocatalytic activity and stability of the presence of NaHCO3. Firstly, the NaHCO3 may effectively prevent the dissolution of the Ag2CO3 in aqueous solution. More importantly, when the presence of NaHCO3, it can facilitate reaction (Equation 2) equilibrium shift to the left and decrease photogenerated electrons reduce Ag+ ions in Ag2CO3. So, it avoids the formation of large amounts of Ag particles, which lead to the photocatalyst inactivate. However, a small amount of Ag particles on the surface of Ag2CO3 can become electron-rich collective. These electrons will participate in the degradation of pollutants. Thus, it promotes effective separation of electron-hole pairs.
In summary, the novel porous Ag2CO3 nanorods were successfully synthesized by using a facile, simple, effective method. The morphology and size of the as-prepared samples can be controlled by adjusting the dispersing agent category and means of adding to reactant. The obtained porous Ag2CO3 nanorods exhibit the capability to efficiently catalyze the degradation of organic pollutants under visible-light irradiation. Furthermore, adding an appropriate concentration of NaHCO3 solution can effectively improve photoactivity and stability of Ag2CO3. Consequently, our work provides a one-pot aqueous solution reaction at room temperature of strategy which may be useful to extend to the synthesis of porous nanorods of other inorganic materials.
This work was supported by the Yunnan Provincial Science and Technology Innovation Talents scheme -Technological Leading Talent (NO. 2013HA002) and State International Joint Research Center of Advanced Technology for Superhard Materials (NO.2013B01038).
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