With the merits of photochemical stability, low cost, non-toxicity, and so on, TiO2 has given high hopes in the areas of energy, environment protection, and organic synthesis and their many applications [1, 2]. However, the efficiency of the photocatalyst is hindered by the high recombination rate of photo-generated electrons and holes and the narrow light response range. TiO2 can only be excited by light with wavelength less than 387.5 nm, which comprise only 3%-5% of the solar spectrum, due to the intrinsic band gap (3.2 eV, anatase), that is, much of visible and infrared light cannot be utilized [3, 4]. Because visible light response was achieved with N-doped TiO2 by Asahi et al. [4] in 2001, N-doping has been recognized as one of the best strategies to extend the light response range of TiO2. Meanwhile, the combination of nonmetal doping and co-catalyst deposition has been shown to give a better effect than the single modification [5, 6, 7]. CuxO is one of the best co-catalysts for TiO2 because of its nontoxicity, low cost, and unique optical and electrical properties [8, 9].
Conventional methods of N-doping include sol-gel synthesis [10], calcination in a N-containing atmosphere [4], TiN oxidation [11], electrochemical anodic oxidation [12], ion implantation [13], and ultrasonic [14] and plasma treatments [7, 15]. The plasma treatment method is easy to operate, there is no need for a high temperature, and it is rapid. So it has been widely used in the surface modification of materials. Pulsipher et al. [15] treated TiO2 films with a NH3/N2 radio frequency discharge and obtained a series of N-doped samples, in which the highest N content reached 31.5%. Trejo-Tzab et al. [7] also prepared N-doped TiO2 by N2 plasma. Plasma treatment can enhance the photocatalytic performance of TiO2 by both element doping and that it also induced the presence of oxygen vacancies and Ti3+ by the collision and spluttering of energetic species and materials during the plasma treatment [16, 17, 18]. Dielectric barrier discharge (DBD) is a kind of low temperature discharge plasmas. As compared with other kinds of discharges, the DBD apparatus is more compact in structure, more convenient to operate, and has a lower requirement of the reaction and low energy consumption [19, 20].
In this work, CuO nanoparticles were deposited on the surface of P25-TiO2, and the resulting CuO/TiO2 was treated by a N2/Ar DBD plasma. The samples were characterized by X-ray diffraction (XRD), ultraviolet-visible spectrophotometry (UV- Vis), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance spectroscopy (EPR). The effects of discharge parameters on the photocatalytic activity were evaluated by the photocatalytic degradation of methyl orange (MO). Mercury-containing wastewater was processed by the modified photocatalysts.
CuO was deposited on the surface of P25-TiO2 by the dipping method [21]. P25-TiO2 (1 g) was added into 100 mL of 0.25 mol/L NaOH aqueous solution, and a 0.05 mol/L Cu(NO3)2 solution was added dropwise under vigorous stirring until the mass ratio of Cu:Ti was 0.01. After further stirring for 6 h, the precipitates were filtrated and washed with deionized water until the pH was 7, then dried at 80 ℃ for 12 h and subsequently calcined at 400 ℃ for 2 h in air. The resulting CuO/TiO2 was then treated with N2/Ar plasma in a DBD plasma reactor.
The experimental set-up for the plasma treatment is presented in Fig. 1. A molybdenum wire that served as the outer electrode was compactly wound around a 60 mm outer diameter corundum tube. A stainless steel rod (40 mm in diameter) was placed in the center of the corundum tube to act as the inner electrode, which was connected to the power supply (CP-8000P, Nanjing Suman Electronics Co., Ltd, China). The discharge gap between the corundum tube and inner electrode, where a quartz boat with the sample was placed, was 7 mm. A DBD plasma was generated between the steel electrode and the outer electrode that was attached to ground. N2 (99.999%) and Ar (99.999%) were introduced into the reactor with the pressure and flow rates of N2 and Ar controlled by mass flow controllers. Experiments were carried out using changes in the power delivered to the plasma, plasma treatment time, reaction pressure, and gas composition. The plasma treated catalyst was denoted as Cu2O/N-TiO2.
XRD analysis was carried out to verify the crystal structure of the photocatalysts using a Rigaku Smartlab3 diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA. The patterns were recorded in the range from 20° to 80° with a step size of 0.02°/s. UV-Vis absorption spectra were collected on a Perkin Elmer Lambda 950 scanning spectrophotometer using polytetrafluoroethene as the reference. The lattice structure was visualized by TEM using a JEOL JEM 2010F electron microscope equipped with a field emission gun electron source using an acceleration voltage of 200 kV. XPS analysis was performed on a PHI 5000 VersaProbe system to characterize the surface properties of the photocatalysts. All binding energies were referenced to the C 1s peak of adventitious carbon on the surface at 284.6 eV. EPR spectra were recorded at -196 ℃ on a Bruker EMX 10/12 spectrometer working in the X-band with 10 mg of the sample placed in an EPR quartz probe cell.
The photocatalytic activity was evaluated by monitoring MO degradation. An xenon light of 300 W (CEL-HXF 300) was used as light source, and visible light with wavelength above 400 nm was acquired by using a UVCUT-400 cut-off filter. A solution of 100 mL of MO (10 mg/L) and 0.10 g of catalyst were placed into a water-cooled jacket beaker (20 ℃), and adsorption- desorption equilibrium was attained by stirring using a magnetic stirrer for 30 min before irradiation. The suspension was then irradiated for 120 min. Aliquots (5 mL) before (C0) and after (C) irradiation was sampled. The aliquots were filtrated through a 0.45 μm syringe filter to remove the photocatalyst particles, and then the dye photodegradation was monitored by measuring the MO concentration using absorbance on a UV-Vis spectrometer (Spectrumlab 752s) at 464 nm. The decomposition efficiency was calculated as (1 - C/C0) × 100%.
An HgCl2-containing solution with a Hg2+ concentration of 50 mg/L was prepared, and three kinds of photocatalysts were used to process this simulated wastewater under visible light irradiation. The dosage of photocatalysts was 1 g/L, and the temperature was 20 ℃. Before irradiation, the system was stirred in the dark for 60 min to give adsorption-desorption equilibrium. After the specified reaction time, an aliquot was sampled and filtered by a 0.45 μm syringe filter to remove the photocatalyst particles. Then inductively coupled plasma emission spectrometry (ICP, iCAP 6300, Thermo Fisher) was used to monitor the concentration of Hg2+.
XRD patterns of the TiO2, CuO/TiO2, and Cu2O/N-TiO2 catalysts are shown in Fig. 2. P25-TiO2 is composed of 20% rutile and 80% anatase. Signals corresponding to Cu species were not detected in the pattern after the deposition of CuO, which would be due to the low content and high dispersion of the Cu species [22]. The pattern of the plasma treated sample was unchanged, which means that the plasma treatment had no effect on the crystal structures of the TiO2 phases.
As shown in Fig. 3, the UV-Vis absorption spectrum of TiO2 was affected by the deposition of CuO and plasma treatment. The absorption below 400 nm was attributed to the intrinsic band gap. After TiO2 modification, the absorption edges of CuO/TiO2 and Cu2O/N-TiO2 were extended to the visible light range. The absorption in the range of 600-800 nm was due to the d-d transition of Cu2+ ions [23]. The absorption of Cu2O was centered at 500-600 nm [24]. The small absorption near 400-500 nm originated from the interfacial charge transfer between TiO2 and CuxO [25]. It is noteworthy that the absorption in the range of 500-600 nm was clearly increased after the plasma treatment, which was caused by the doped N and induced surface vacancies [16].
The structure of Cu2O/N-TiO2 was observed by TEM (Fig. 4), which clearly demonstrated that particles with the size of about 3 nm were uniformly supported on the surface of TiO2. From the interplanar spacing (0.208 nm), the nanoparticles can be recognized as Cu2O [26], which meant that the deposited CuO was reduced to Cu2O during the plasma treatment.
Figure 5 shows the XPS spectra of Cu 2p, O 1s, N 1s, and Ti 2p of the Cu2O/N-TiO2 samples treated at different times. The ratio of N2:Ar and discharge current were kept at 8:2 and 1.0 A, respectively. The atomic compositions of the samples treated for different times are shown in Table 1.
We can see from Fig. 5(a) that the binding energy of the Cu 2p3/2 level was shifted from ~933 to ~931 eV, which illustrated that CuO was reduced to Cu2O by the plasma treatment. That was consistent with the TEM image. The O 1s and Ti 2p orbitals (Fig. 5(b) and (d)) showed the same trend as the Cu 2p3/2 orbital. This was explained by that N doping resulted from the plasma treatment, and electrons from the doped N species partially migrated to Ti and O. The electron density of Ti and O would increase, thus leading to the shifted binding energy [19].
As shown in Fig. 5(c), the peaks at 396, 400, and 407 eV appeared with the increase of treatment time in the N 1s spectra of the modified TiO2. The content of N also increased with time (Table 1). The signal at 400 eV was assigned to N species in the interstitial of the TiO2 lattice, such as NO and NHx [27]. The signal at 396 eV was attributed to the formation of Ti-N, which resulted from the replacement of O by N [4]. The signal at 407 eV was assigned to NO3 [28]. Asahi et al. [4] believed that the Ti-N bond formed led to the decrease of band gap and brought about the visible light response. However, this point of view has not been widely recognized, and the reason for induced photocatalytic activity is still under debate [29].
The EPR spectrum of Cu2O/N-TiO2 is shown in Fig. 6. A peak was shown at g = 1.975, which can be assigned to surface Ti3+ [16]. Surface Ti3+ can react with O and form O2- and/or O-, which would generate a signal around g = 2.00 [30], but this feature was not obvious in Fig. 6. The reason was that the signal was too weak and was screened by the signal of Ti3+ [31].
The effect of the modification depended on the discharge parameters. N containing gases such as N2 and NH3 are usually used in N doping by gas phase methods, but the presence of the gas was not sufficient for doping. Huang et al. [17] modified TiO2 by a N2 radio frequency plasma, but N was not found in the product obtained. The doping is mainly affected by the gas composition, flow rate, delivered power, and treatment time. In our work, samples modified under different conditions were used to degrade MO by visible light irradiation for 120 min, and appropriate reaction conditions were optimized using the degrading efficiency.
In the experiments, the total gas flow was controlled at 1 L/min, and the ratio of N2:Ar was selected as a variable. As shown in Fig. 7(a), the photocatalytic activity was enhanced with the increase of the proportion of N2, but the modified sample obtained at the ratio of 8:2 was superior to that when only N2 was used. The dissociation energy of N2 is higher than that of Ar, so Ar is easier to excite. By the Penning effect of the plasma discharge process, the metastable particles of Ar can react with N2 and promote the excitation and dissociation of N2, and more N intermediates would be formed, such as N* and N2+ [32, 33]. So the increase of the proportion of N2 increased the concentration of N, but the increase of the proportion of Ar led to the higher density of metastable species.
With regard to the treatment time, the concentration of doped N increased with the increase of treatment time. However, the results in Fig. 7(b) showed that an excessive treatment was detrimental to the activity of the modified sample. We can see from Table 1 that the Cu content decreased with increasing treatment time. During the discharge, energetic metastable species strip the surface of TiO2 continuously, so nano-sized Cu species may be stripped from the surface of the TiO2. The impairment of the p-n junction effect caused the loss of photocatalytic activity.
The power delivered to the reactor was crucial to the modification effect. Insufficient power cannot dissociate the gas, while excess power supplied to the system is converted into heat and causes overheating of the apparatus. In this work, the power was controlled by the adjustment of the discharge current. Figure 7(c) shows the influence of delivered power on the modification effect with the N2:Ar ratio of 8:2 and treatment time of 10 min. The results showed that the best discharge current was 1.0 A. In summary, the optimal operation conditions were N2:Ar at the ratio of 8:2, treatment time of 20 min, and discharge current of 1.0 A.
Figure 8 shows the results of the treatment of Hg2+- containing wastewater by the catalysts. The simulated wastewater had a neutral pH. Under this circumstance, the adsorption of Hg2+ on the surface of the photocatalyst was negligible in the dark. We can see from Fig. 8(a) that the concentration of Hg2+ during the adsorption step showed no trend of decrease. Under visible light irradiation, the concentration of Hg2+ decreased significantly compared to that in the dark. The pristine TiO2 showed a slight activity for Hg2+ treatment, but the photocatalytic activity of CuO/TiO2 was much higher. The co-modified Cu2O/N-TiO2 gave the best performance and complete conversion in 20 min.
During the photocatalytic reduction of Hg2+, photo-generated electrons reacted with Hg2+ adsorbed on the surface of the photocatalyst, and the heavy metal ions were reduced to a low valence state even to zero valence. The reactions are [34]:
The used photocatalyst was recovered with 5% HNO3 and washed by deionized water. Then the dried photocatalyst was reused in the treatment of Hg2+. As shown in Fig. 8(b), 4 recycling runs in the photocatalytic treatment of Hg2+ over Cu2O/N-TiO2 under visible light illumination were carried out. The reaction time was limited to 20 min for each run. The data showed that the photocatalyst exhibited almost no loss of activity after 4 cycles and was stable during the photocatalytic treatment of Hg2+.
In order to verify the experimental results, the Cu2O/N-TiO2 photocatalyst was collected after reaction, and the surface was analyzed by SEM (JSM-6510, JEOL) and energy dispersive X-ray spectroscopy (EDX) after drying at 30 ℃. As shown in Fig. 9, tiny particles were adsorbed on the surface of the photocatalyst. The EDX spectrum demonstrated the existence of Hg that was separate from O, Ti, and C. It can be concluded that the Hg2+ was reduced in the photocatalytic reaction and then adsorbed on the surface [35].
CuO/TiO2 was modified by a N2/Ar DBD plasma treatment. Characterization of the obtained material demonstrated that N doping was achieved, and there was also the formation of surface defect states, such as Ti3+. The modifications induced by the plasma treatment led to the enhancement of visible light photoactivity. The discharge parameters were optimized using the degradation of MO by the samples under visible light irradiation. The appropriate treatment conditions were N2:Ar at the ratio of 8:2, treatment time of 20 min, and discharge current of 1.0 A. The plasma-treated Cu2O/N-TiO2 catalyst showed superior performance to TiO2 and CuO/TiO2 for the treatment of Hg2+-containing wastewater, and it is expected to be a promising candidate for other applications.
TiO2具有光化学稳定性好、成本低、无毒等优点, 广泛应用于能源、环境、合成以及其他光催化领域[1, 2]. 但是, 其光催化效果受限于较高的光生电子与光生空穴复合率和过于狭窄的光响应范围, 其本征禁带宽度(~3.2 eV, 锐钛矿)限制了它只能被波长387.5 nm以下的光线所激发, 而这一部分仅占太阳光谱的3%-5%, 大量的可见光甚至红外光线无法被利用[3, 4]. 自2001年Asahi等[4]制备了可见光响应的N掺杂TiO2以来, N掺杂改性被视为扩展其光响应范围的最佳途径之一. 同时, 非金属元素掺杂与助剂负载比单一的改性方法更有效[5, 6, 7]. CuxO由于具有无毒、廉价和独特的光电特性已经成为TiO2最好的助剂之一[8, 9].
通常的N掺杂方法包括溶胶凝胶法[10]、含氮气氛焙烧[4]、TiN氧化法[11]、电化学阳极氧化法[12]、离子注入法[13]、超声法[14]以及等离子体法[7, 15]等. 其中, 等离子体处理法具有操作简便、无需高温和处理时间短等优点, 已经被广泛运用于材料表面改性. Pulsipher等[15]利用射频等离子放电分别在NH3和N2气氛中对制备的TiO2薄膜进行处理, 得到一系列N掺杂的样品, 最高含N量可达31.5%. Trejo-Tzab等[7]使用N2等离子放电同样实现了N掺杂. 等离子处理方法不仅能有效进行元素掺杂, 而且在处理过程中高能电子和基团与材料的表面碰撞、溅射所形成的氧空位和Ti3+缺陷, 也能显著提高TiO2的光催化性能[16, 17, 18]. 在多种等离子放电方式中, 介质阻挡放电(DBD)作为一种低温等离子放电方式, 相对于其他放电方式无需复杂的设备, 操作更简单, 反应条件要求更低, 耗能低, 更具有可操作性[19, 20].
本文采用浸渍法在TiO2 (P25)表面负载CuO, 然后利用DBD放电装置在N2/Ar气氛中对CuO/TiO2进行处理, 运用X射线衍射(XRD)、紫外-可见吸收光谱(UV-Vis)、X射线光电子能谱(XPS)、透射电镜(TEM)和电子自旋共振(EPR)等手段对所得样品进行了表征, 并且通过它们对甲基橙的光催化降解考察了不同放电条件对其可见光光催化性能的影响. 最后将改性后的光催化剂用于处理模拟含汞废水.
通过浸渍法将Cu负载在TiO2 (P25, Degussa)表面[21]. 将1 g TiO2加入到100 mL NaOH溶液(0.25 mol/L)中搅拌均匀, 然后在剧烈搅拌下逐滴加入一定量Cu(NO3)2溶液(0.05 mol/L), 使得体系中Cu与Ti的质量比为1%. 搅拌6 h后, 过滤沉淀, 用水洗至pH = 7, 经80 ℃干燥12 h后于40 0 ℃焙烧2 h制得CuO/TiO2.
将CuO/TiO2置于自制的DBD等离子发生器中进行处理. 如图1所示, 与地电极相连的钼丝紧密缠绕在外径60 mm的刚玉管外作为外电极, 直径40 mm的不锈钢与高压电源(CP-8000P, 南京苏曼电子有限公司)相连, 置于刚玉管中作为内电极, 内电极与刚玉管的间隙约为7 mm. 将样品盛放在厚约1 mm的石英舟上, 然后放置在刚玉管与内电极的间隙中, 通过电极之间的介质阻挡放电对TiO2进行处理. N2 (99.999%)与Ar (99.999%)通入反应器, 气体的压力和流速由压力表和流量计控制. 在处理过程中, 通过改变N2与Ar的比例、等离子处理时间和等离子处理功率等考察不同处理条件对光催化剂的改性效果.
采用日本理学Smartlab3型X射线衍射仪(Cu Kα靶, λ = 1.5418 Å)测定催化剂的晶型结构, 工作电压与电流分别为40 kV和30 mA, 扫描速度为0.02°/s, 扫描角度从20°到80°. 催化剂的UV-Vis光谱通过Perkin Elmer Lambda 950型分光光度计测得, 以聚四氟乙烯作为参比. 晶格结构利用JEOL JEM 2010F型TEM表征, 扫描电压为200 kV. 使用PHI 5000 VersaProbe型电子能谱仪分析催化剂中各元素组分, 扫描结果以284.6 eV处的C 1s峰校正. 使用Bruker EMX-10/12型顺磁共振仪波谱仪测试10 mg样品在-196 ℃条件下的信号.
将甲基橙溶液作为目标底物检验制备光催化剂的活性. 以氙灯作为光源(300 W, CEL-HXF300), 并且用UVCUT-400截止型滤光片滤掉λ < 400 nm的光线得到可见光. 将0.1 g光催化剂加入100 mL浓度为10 mg/L的甲基橙溶液中, 在光照前搅拌30 min以达到吸附-脱附平衡, 在整个反应过程中通过夹套中的循环水控制反应体系温度为20 ℃. 达到一定反应时间后, 取样经0.45 μm的滤头过滤, 滤液使用紫外可见分光光度计(Spectrumlab 752s)在464 nm处测试吸光值, 反应前的吸光值记为C0, 反应后的吸光值为C, 光催化降解效率表示为(1 - C/C0) × 100%.
将Hg2+浓度为50 mg/L的HgCl2溶液作为模拟含汞废水, 在波长> 400 nm可见光条件下分别用未改性TiO2, CuO/TiO2和等离子改性的Cu2O/N-TiO2进行处理. 光催化剂投加量为1 g/L, 反应温度维持在20 ℃. 在光照反应之前避光搅拌60 min以达到吸附脱附平衡. 光照反应一定时间后, 取一定体积反应液经0.45 μm滤头滤去催化剂颗粒, 用电感耦合等离子发射光谱(iCAP 6300, Thermo Fisher)检测滤液中Hg2+含量.
图2为TiO2, CuO/TiO2和Cu2O/N-TiO2的XRD图. P25的晶相组成约为80%的锐钛矿和20%的金红石相, 负载CuO之后, 并未出现Cu物种晶相, 可能是由于Cu含量较低且分散很均匀的缘故[22]. 还可以看出, 等离子处理也没有改变催化剂的晶相结构, 说明等离子处理对TiO2的晶体结构没有影响.
图3为等离子处理对TiO2的UV-Vis光谱的影响. P25在400 nm处的吸收属于TiO2本征吸收; 而负载了Cu和经过等离子处理之后, 其吸收边向可见光区域发生偏移. 其中600-800 nm范围内的吸收是由于Cu2+上d-d轨道电子发生跃迁[23]; 而Cu2O的吸收光谱在500-600 nm处[24]; 400-500 nm处的微弱吸收则源自于TiO2与CuxO之间的界面载流子迁移[25]. 等离子处理后, 掺杂的N元素和形成的氧空 位使得样品在500-600 nm范围内的吸收明显增强[16].
图4为等离子处理后Cu2O/N-TiO2的TEM照片. 可以看出, 大小约为3 nm的微粒均匀地负载在TiO2表面, 其晶面间距为0.208 nm, 可以确定为Cu2O [26]. 这说明经过等离子处理后, TiO2表面负载的CuO被活性中间产物和高能电子还原成Cu2O.
图5为不同时间等离子处理后样品的Cu 2p, O 1s, N 1s和Ti 2p XPS谱. 我们比较了气氛组成为N2:Ar = 8:2和电流1.0 A条件下不同处理时间对光催化剂表面元素组成的影响. 表1为等离子处理前后催化剂表面的元素组成与比例.
从图5(a)可见, 经等离子体处理后, Cu 2p3/2信号峰向低结合能位移, 由~933 eV移至~931 eV, 说明TiO2表面的CuO在等离子处理过程中被还原为Cu2O, 与TEM结果一致. 图5(b)和(d)分别为O 1s和Ti 2p谱图. 可以看出, O 1s和Ti 2p的峰都向低结合能偏移. 这可能是由于等离子处理造成N掺杂, 掺杂后N原子的部分电子转移给Ti和O, 引起Ti和O电子密度增加, 从而导致结合能偏移[19].
如图5(c)所示, 随着处理时间的增加, 改性催化剂的N 1s在约396, 400和407 eV都出现峰, 另外其表面N含量逐渐增加(表1). 一般认为, 400 eV处峰的形成是由TiO2晶格间隙的N化合物引起的, 如NHx和NO等[27]; 而396 eV处的峰则是TiO2晶格中的O被N取代后所形成的Ti-N键导致的[4]; 407 eV处峰说明可能存在NO3 [28]. Asahi等°[4]首先认为, N取代晶格中的O后形成的Ti-N导致禁带宽度降低, 造成可见光响应. 但是也有许多不同观点°[29], 目前尚无定论.
图6为TiO2等离子处理之后的EPR谱. 可以看出, TiO2在g = 1.975处出现了一个峰, 可归属于表面Ti3+ [16]. 一般情况下, 由于表面的Ti3+会与氧分子反应, 生成O2-或者O-, 二者都可在g = 2.00左右产生信号响应[30]. 但是在图6中并不明显, 可能是由于氧空位信号太弱, 被Ti3+的信号峰掩盖[31].
不同反应条件对掺杂效果影响很大. 一般在气相掺杂方法中都是以含氮气体作为反应物, 如N2和NH3或其他一些易挥发的有机含氮物质. 但是仅仅在氮源存在的情况下, 并不一定能够实现N掺杂, 如Huang等[17]利用N2射频放电等离子体处理TiO2后发现, 处理后的样品中并无N元素. 掺杂过程受到气体组分、气体流速、放电功率以及处理时间等条件的影响. 本文用不同条件下改性的光催化剂在可见光条件下降解甲基橙溶液, 通过比较改性后光催化剂的活性优化等离子改性条件.
控制气体总流量为1 L/min, 通过改变N2:Ar比考察了气氛的影响. 由图7(a)可见, 随着N2比例的增加, 改性后的光催化剂活性增大, 但是纯N2改性后光催化剂可见光活性反而不如N2:Ar为8:2条件下改性的催化剂. 在N2与Ar的混合气体中, Ar的离解能低于N2, 所以Ar更容易被激发. 由于潘宁效应的影响, 在等离子放电过程中, Ar的亚稳态粒子能够与N2碰撞反应, 促进N2的激发和离解, 更多的N中间化合态如N和N2+能够提高表面处理及掺杂效率[32, 33]. 因此, N2:Ar比增加能够提高N活性中间物质的浓度, 而N2:Ar比降低则提高了N2处理的效率.
同样, 等离子处理时间也会影响改性效果. 如图7(b)所示, 随着处理时间的增加, TiO2中掺杂的N含量和表面的Ti3+以及氧空位增加, 因此光催化效果逐渐增加, 但是处理时间超过20 min之后, 光催化活性开始降低. 从表1可以看出, Cu含量随着等离子处理时间增加而逐渐降低. 在等离子处理过程中, 高能亚稳态粒子不断剥蚀材料表面, 由于Cu粒径较小, 可能会逐渐从TiO2表面脱落, TiO2与Cu2O所形成的p-n半导体异质结的作用会慢慢降低, 影响整体复合结构的光催化活性.
输出功率对于整个反应至关重要, 输出功率太低则气体离解程度不够, 而功率太大则会导致过多的能量转化为热能, 造成整个反应装置温度升高. 本文通过控制电流大小来调节等离子发生装置的输出功率. 图7(c)为N2:Ar比为8:2, 处理时间为10 min时不同输出电流对处理后光催化剂性能的影响, 可见输出电流1.0 A时最佳. 综上可确定最佳反应条件为N2:Ar = 8:2, 放电处理时间20 min, 反应电流1.0 A.
图8示出了改性催化剂处理Hg2+废水实验结果. 实验中模拟含Hg2+废水是用HgCl2配制的中性pH溶液, 在此环境中避光搅拌状态下TiO2对Hg2+吸附性能不佳. 从图8(a)可以看出, 在-60到0 min时间段内, 反应体系中Hg2+浓度不稳定且无明显降低趋势. 结果表明, 未改性TiO2对汞的处理效果很低, CuO的负载使其光催化效果明显增加, 而经等离子处理后的Cu2O/N-TiO2的处理效果达到最佳, 20 min左右即处理完全. 在光催化还原Hg2+的反应中, 光催化剂吸收光子产生光生电子, 然后光生电子将吸附在催化剂表面的Hg2+还原[34]:
将反应之后的光催化剂用5% HNO3回收Hg2+, 再经去离子水清洗和干燥, 重新用于Hg2+的处理. 如图8(b)所示, 将Cu2O/N-TiO2在可见光照射条件下重复使用4次, 反应时间为20 min, 经4次反应后光催化剂活性保持不变, 表现出良好的稳定性.
为了验证实验效果, 我们将反应结束后的光催化剂过滤收集, 30 ℃烘干后用扫描电镜(SEM, JSM-6510, JEOL)观察催化剂表面形貌, 并且通过能量色散X射线光谱仪(EDX)分析表面的元素组成, 结果如图9所示. 由SEM照片可以看出催化剂表面有微小颗粒, 除了含有O, Ti以及C元素之外, 还存在Hg元素, 说明反应液中Hg2+通过光催化还原之后吸附在光催化剂表面[35].
采用N2/Ar介质阻挡放电等离子体处理负载CuO的TiO2. 结果表明, 等离子体不仅能够实现N掺杂, 而且能够在催化剂表面生成Ti3+以及氧空位等缺陷态, 因而催化剂在可见光区的光吸收率与光催化活性增加. 通过可见光下甲基橙溶液的光催化降解实验, 优化了等离子体处理参数, 得到最佳处理条件为N2与Ar比例为8:2、处理时间为20 min、放电电流为1.0 A. 最后, 模拟汞污染废水的处理实验表明, 光催化剂经等离子改性后性能明显改善, 可以应用于更多领域.
2014, Vol. 35 Issue (10): 1752-1760
PDF (1130 KB)
2014, Vol. 35 Issue (10): 1752-1760
PDF (1130 KB)