WS2具有类石墨烯的二维层状结构，钨离子夹在两层硫离子中间，形成类似三明治的密排六方结构[12,13]。钨离子与硫离子通过化学键相结合，而硫离子层间则通过van der Waals力相连接。根据钨离子的配位形式(包括八面体配位和三棱柱配位)，WS2可分为金属相和半导体相[14,15]。金属相的WS2成八面体配位，构成四方晶系(1T)型晶体结构。半导体相的WS2则通过三棱柱配位形成六方晶系(2H)或三方晶系(3R)型结构，其中2H型为稳定结构，1T与3R型均为亚稳定型。金属性质决定了1T相WS2 (1T-WS2)比2H相有更高的载流子浓度和更多的活性位点[16,17]。在许多应用中1T-WS2显著提高了器件的光电化学性能[16,18~20]，原因是其优越的导电性促进了基面和活性边缘位点之间电荷的转移[21]。但是，1T相亚稳[22]，因为在其单晶胞中1T相的生成能比2H相高约0.537 eV[18]。同时，单层1T-TMDs在室温有很强的重堆积和相变倾向[15]。
制备WS2二维材料的方法，有液相剥离法、锂离子插层法、水热合成法和化学气相沉积法等[22~24]。水热合成法采用“自底而上”的策略合成，更容易实现TMDs二维材料的可控制备。控制水热反应过程中的温度、前驱体比例、反应时间和生长衬底等参数，即可精确调控样品的物理特性[25]。以钼酸铵和硫脲为前驱体采用水热法制备1T-MoS2,随着制备温度的提高MoS2纳米花球的生成更充分且其直径随之增大。使用TEM、XRD及XPS对MoS2相转化表征结果表明，与2H-MoS2相比1T-MoS2具有更丰富的活性位点和更高的电导率,且随着制备温度的提高制备出的1T-MoS2增多[26]。迄今为止，化学/物理剥离[27]和胶体合成[28]是制备1T-WS2最常用的方法。但是，这些方法复杂、产率低且不稳定[17]。
本文用简单的一步溶剂热法制备WS2，控制前驱体中WCl6/TAA的比例和反应温度以调控1T相的含量以制备出1T相含量最佳的WS2。
1 实验方法
1.1 材料的制备
实验用溶液均用去离子水配制。
用简单的一步溶剂热法制备1T/2H相WS2。将0.25 mmol的WCl6 (0.9914 g)加入装有60 mL无水乙醇的烧杯中并搅拌30 min使钨前体完全分散，然后按WCl6/TAA摩尔比为1∶6、1∶9、1∶12、1∶15和1∶20分别加入1.5 mmol (1.1270 g)、2.25 mmol (1.6905 g)、3 mmol (2.2539 g)、3.75 mmol (2.8175 g)和5 mmol(3.7566 g)的TAA。继续搅拌30 min后超声30 min，使前驱体均匀分散在体系中。然后将溶液置于体积为100 mL温度为200℃的水热反应釜中加热24 h。选取WCl6/TAA最佳摩尔比1∶12的溶液分别在160、180、200和220℃加热24 h。然后使反应釜自然冷却至室温，收集釜内反应体系中生成的黑色产物并用去离子水和无水乙醇充分清洗，最后将产物在70℃真空干燥箱中干燥12 h，即得到1T/2H相WS2。将不同WCl6/TAA摩尔比的WS2分别命名为W-6、W-9、W-12、W-15和W-20。将在不同温度下制备的WS2分别命名为W-160、W-180、W-200 (W-12)和W-220。
1.2 性能表征
用X射线衍射仪(XRD，D/max-2400)分析WS2的晶相组成。用X射线光电子能谱仪(XPS，EscaLab 250xi USA)分析WS2中金属元素的价态和1T相含量。用扫描电镜(SEM，JSM-6710F)和透射电镜(TEM，TECNAI G2 TF20)表征WS2的形貌和晶格宽度。用能谱仪(EDS)分析WS2的表面元素。用拉曼光谱(Raman IRprestige-21，Japan)进一步分析WS2的晶相结构。
将适量的WS2加入到装有200 mL、20 mg/L的苯酚溶液的玻璃烧杯中，进行磁力搅拌并加入一定量的PMS，然后加入一定量的Fe(NO3)3引发类芬顿反应。定时提取4 mL反应悬浊液(包括0、2.5、5、7.5、10、15、20 min)后加入0.2 mL淬灭剂(甲醇)终止反应。将提取液在10000 r/min下离心10 min后，提取2 mL上清液。用四氨基安替比林分光光度法进行苯酚测定，波长为510 nm。
2 结果和讨论
2.1 前驱体比例对WS2中1T/2H相的影响
根据XRD谱(图1)分析了产物的晶相结构。结果表明，WCl6/TAA摩尔比为1∶6的WS2其TAA的含量较低，生成的H2S较少，使反应不充分，结晶度较差，因此XRD谱中WS2的特征峰不明显。随着TAA含量的提高谱中WS2的特征峰越来越清晰，与2H-WS2标准卡片PDF#08-0237相比其特征峰均向左偏移，且TAA含量越高偏移越显著。W-12的(002)晶面从14.32°向左偏移至12.28°，而谱中的特征峰向左偏移，表明其晶格宽度变大。发生了部分2H-WS2向1T-WS2转变[29]，证实已制备出1T相的WS2[30]。晶格宽度变大表明，活性位点增多，对助催化降解有利。这是过量的TAA中氨基的插层所致[17]。
图1
图1
不同WCl6/TAA制备条件下WS2的XRD谱
Fig.1
WS2 XRD patterns under different WCl6/TAA preparation conditions
图2
图2
不同WCl6/TAA比条件下WS2的SEM照片
Fig.2
SEM images of WS2 under different WCl6/TAA ratios (a) 1:6, (b) 1:9, (c) 1:12, (d) 1:15
用XPS研究了元素的化学状态。结果表明，WCl6/TAA摩尔比不同的WS2其XPS谱中的特征峰均发生了不同程度的偏移，其1T相的含量也不同[32]。图3a给出了用W元素调控不同相含量的分析。可以看出，随着TAA含量的提高其特征峰位均向低结合能偏移。例如，4价钨的W 4f5/2峰值从W-6的32.72°偏移至W-12的32.14°，W 4f7/2从W-6的35.48°偏移至W-12的34.28°。相应地，1T相的含量从W-6的41.32%提高到W-12的73.15%[17,33]。继续提高WCl6/TAA至1∶15其特征峰位置和1T相含量几乎不变，因为氨基的插层已经饱和。S的表征结果与W的变化趋势相同。
图3
图3
不同参比下WS2的XPS谱
Fig.3
XPS spectra of WS2 under different conditions XPS spectra of W (a) and XPS spectra of S (b)
为了验证所制备的纳米材料的性能，在WS2/PMS/Fe3+体系下对苯酚进行降解。在水热反应中TAA中氨基的插层使WS2中1T相的含量提高。前驱体中TAA的含量越高则WS2中1T相的含量越高，其共催化降解效果越好[26]。保持WS2的投加量为0.2 g/L、PMS的投加量为0.35 g/L、Fe3+投加量为0.1 mmol/L，对20 mg/L的苯酚进行降解。如图2a所示，前躯体中WCl6与TAA的摩尔比为1∶6时生成的WS2形貌较差，1T相的含量最低，使降解效果最差。随着TAA含量的提高其降解效果随之提高。WCl6与TAA的摩尔比达到1∶12时反应10 min即达到平衡，继续提高前驱体中TAA的含量降解效果没有明显的提高。其原因是，氨基的插层达到饱和[34]。由反应动力学常数k也可以发现，WCl6/TAA大于1∶12时降解效率没有明显的提高。
图4
图4
不同WCl6/TAA比条件下WS2对苯酚的助催化降解性能及其反应速率常数
Fig.4
Co-catalytic degradation performance (a) andreaction rate constant of WS2 for phenol (b) under different WCl6/TAA ratios
以上结果表明，随着前驱体中TAA含量的提高氨基插层的影响越来越明显，WS2的纳米花瓣结构也更清晰，1T相含量也随之提高。综合考虑经济成本和降解效果，后续实验将WCl6与TAA的摩尔比固定为1∶12。
2.2 改变温度调控WS2 中1T/2H的相
根据XRD谱分析产物的晶相结构(图5)。温度低于200℃时W-160和W-180的XRD谱中的特征峰并不明显，是温度过低结晶度较差所致。随着反应温度的提高WS2的特征峰越来越显著，表明温度的提高有利于WS2的生成[17]。与2H-WS2标准卡片PDF#08-0237比较可见，在各反应温度下制备的WS2其特征峰均向左偏移。在一定的范围内，反应温度越高偏移越显著，表明其晶格宽度越大。例如，W-200的(100) 晶面从32.76°偏移至31.63°。这表明，提高反应温度有利于2H-WS2向1T-WS2转化[29]。但是，反应温度达到220℃时，过高的温度对其结构产生了副作用。与W-200相比，其晶面向2H相发生了偏移，表明晶格宽度减小[26]。例如，W-220的(100)晶面相对于W-200的31.63°向右偏移到32.14°，这不利于1T-WS2的制备[35]。
图5
图5
在不同温度制备的WS2的XRD谱
Fig.5
XRD patterns of WS2 prepared at different temperatures
图6
图6
在不同温度制备的WS2的SEM照片
Fig.6
SEM images of WS2 at different temperatures (a) 160oC, (b) 180oC, (c) 200oC, (d~f) 220oC
用XPS研究了元素的化学状态。可以发现，在160~200℃范围内随着反应温度的提高，其XPS特征峰均向低结合能处偏移。4价钨的W 4f5/2峰值从W-6的32.49 eV偏移至W-12的32.14 eV，W 4f7/2从W-160的34.72 eV偏移至W-200的34.28 eV，其1T相的含量从W-160的50.75%提高到W-200的73.15%[27]。进一步提高反应温度，W-220的特征峰相对于W-200向高结合能偏移，4价钨的W 4f5/2偏移至32.29 eV，W 4f7/2偏移至34.43 eV，其1T相的含量降低到62.89%[26]。这个结果与XRD给出的结果一致，再次验证了反应温度提高到220℃不利于1T相的生成。S的表征结果，也与W的变化趋势相同。
图7
图7
在相同温度制备的WS2的XPS谱
Fig.7
XPS spectra of WS2 at the same temperature (a) XPS spectra of W, (b) XPS spectra of S
在水热反应过程中，温度的变化影响产物的生成。在一定范围内，反应温度的提高使WS2中1T相的含量提高，其共催化降解效果越好。保持WS2投加量为0.2 g/L、PMS投加量为0.35 g/L和Fe3+投加量为0.1 mmol/L，对20 mg/L的苯酚进行降解。图10a表明，反应温度从160℃提高到200℃时，其共催化效果随之提高。但是，反应温度到达220℃时其降解效果降低，因为过高的温度破坏了其内部结构，使1T相的含量降低。从其K值也可以发现，在反应温度为200℃的产物其降解速率最高。这个结果，与XRD和XPS谱的表征结果一致。
图8
图8
在不同温度制备的WS2对苯酚的助催化降解性能及其反应速率常数
Fig.8
Co-catalytic degradation performance (a) and reaction rate constant (b) of WS2 for phenol under different temperatures
图9
图10
图10
W-200的高分辨率投射图像和晶格间距
Fig.10
High resolution projection image of W-200 (a, b) and its lattice spacing (c)
以上结果表明，在160~200℃区间，随着反应温度的提高WS2的纳米花瓣结构越来越清晰，1T相的含量和共催化效果也随之提高。但是，反应温度继续提高到220℃则WS2发生大量团聚，部分结构被破坏，使其1T相的含量降低。因此，选择200℃为最佳反应温度。
选择WCl6/TAA比为1∶12为最佳参杂比，200℃作为最佳制备温度，则W-200 (W-12)是本文制备的最佳材料。
2.3 W-200中的1T相、元素分布和形貌
通过EDS能谱观察了W-200的元素分布。图9表明，W和S元素的分布较为均匀。计算结果表明，W和S元素的摩尔比接近1∶2，符合WS2的原子比，证实WS2的制备是成功的。
进一步用透射电镜(HRTEM)观察了W-200的形貌。从图10a、b可以看出，W-200的纳米花瓣结构更加清晰。相关文献报道，在水热法制备WS2的过程中，TAA中过多的氨基嵌入使WS2中某些晶面晶格条纹间距扩大，使2H相变为1T相[29]。晶格宽度大表明活性位点增加，有利于催化降解等[34]。进一步放大测量出的W-200其晶格宽度(图10c)，可见W-200是1T相和2H相的混合物[19]。在2H-WS2 (PDF#08-0237)中，(002)晶面在14.320°处的晶格宽度为0.618 nm，氨基的插层使其晶格间距变大而转化为1T相。测量结果表明，d = 0.624 nm和d = 0.645 nm处的晶格条纹属于2H-WS2(002)晶面(PDF#08-0237)，d = 0.961 nm和d = 1.046 nm处的晶格条纹属于1T-WS2[17]。
2.4 共催化降解反应前后WS2 的状态
为了进一步研究助催化降解实验前后WS2纳米材料的变化，用扫描电镜SEM、XRD谱、拉曼光谱和XPS谱分析了反应前后WS2的状态。
对比反应前后的扫描电镜照片，可见WS2的结构没有明显的改变，均为纳米花球状结构，进一步表明其可循环使用。
对比反应前后的XRD谱可见，各个峰和峰位没有明显的变化，反应后的WS2中也没有生成新相。与反应前的(002)晶面对比，反应后的(002)晶面强度明显减弱，表明在该体系的反应过程中主要损耗的是(002)晶面，大部分反应都发生在这个表面[38]。对比反应前后的Raman谱，可见具有1T相特征的J1,J2特征峰并未发生明显的改变，反应后处于349.32 cm-1位置的E
图11
图12
图12
W-200反应前后的SEM图像
Fig.12
SEM images of W-200 before (a) and after (b) reaction
图13给出了W-200反应前后的XRD谱和Raman谱。与使用前的WS2相比，使用后的WS2的W(VI)的含量变化很小，只增加了2.48%。这表明，在PMS活化过程中PMS氧化产生的Fe(III)先将W(IV)氧化为W(VI)，而W(VI)有被PMS还原为W(IV)的趋势。这是一个循环促进的过程。原始WS2的S 2p XPS谱表明，在161.76和162.81 eV处S2-的两个特征峰分别归属于1T相WS2的S 2p3/2和S 2p1/2。在163.37和164.40 eV处的S2-的两个特征峰分别归属于2H相WS2的S 2p3/2和S 2p1/2[41]。使用后的WS2中峰值结合能的降低说明产生了S空位，与拉曼谱的结果一致。使用后的WS2有极少一部分4价W转化为6价W，而硫空位的形成使更多的4价W暴露出来成为新的助催化活性中心，弥补了连续使用过程中损失的4价钨，使其可循环使用效果提高。
图13
图13
W-200反应前后的XRD谱和Raman谱
Fig.13
XRD patterns (a), Raman spectra (b) and XPS (c, d) of W-200 before and after reaction
3 结论
(1) 改变水热反应中前驱体的比例和温度可调控WS2中1T相的含量。随着前驱体中WCl6/TAA摩尔比的提高产物中1T相的含量随之提高，但是WCl6/TAA比例大于1∶12后产物中1T相含量的变化不明显。水热温度为160~200℃时，随着反应温度的提高产物中1T相含量随之提高，但是水热温度达到220℃后1T相的含量降低。W-200的1T相含量最高，其助催化效果最佳。
(2) 用溶剂热法制备的WS2纳米材料结构稳定，反应主要发生在WS2的(002)晶面，使用过程中产生的硫空位使其具有优异的可循环使用性。
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Electrochemical properties of nitrogen-doped DLC films deposited by PECVD technique
[J].Diamond-like carbon (DLC) and Nitrogen-doped diamond-like carbon (N-DLC) films were synthesized by glow discharge plasma enhanced chemical vapor deposition (PECVD) using a hybrid ion beam system. The microstructure and surface topography of films were characterized by Raman spectroscopy, X-ray photoemission spectroscopy and scanning probe microscopy. The electrochemical performance of the films was examined by an electrochemical workstation. Results indicate that the surface of all the deposited films is very smooth with low roughness. Nitrogen doping enhances the clustering of sp2 sites and results in the formation of C-N bond, meanwhile, increases the amount of C-O bond and the surface active sites of films. N-DLC film electrode shows a wide potential window range over 4.5 V, lower background current (0.3±0.2 μA/cm2) in H2SO4 solution. N-DLC film electrode with good repeatability and stability displays significantly current response and nearly reversible electrode reaction in K3Fe (CN) 6 solution. Furthermore, the electrode reaction is controlled by diffusion process.
PECVD制备掺氮类金刚石薄膜的电化学特性
[J].
Graphdiyne-WS2 2D-Nanohybrid electrocatalysts for high-performance hydrogen evolution react-ion
[J].
Highly ambient-stable 1T-MoS2 and 1T-WS2 by hydrothermal synthesis under high magnetic fields
[J].The phase-controlled synthesis of metallic and ambient-stable 2D MX (M is Mo or W; X is S) with 1T octahedral coordination will endow these materials with superior performance compared with their semiconducting 2H coordination counterparts. We report a clean and facile route to prepare 1T-MoS and 1T-WS through hydrothermal processing under high magnetic fields. We reveal that the as-synthesized 1T-MoS and 1T-WS are ambient-stable for more than 1 year. Electrochemical measurements show that 1T-MoS performs much better than 2H-MoS as the anode for sodium ion batteries. These results can provide a clean and facile method to prepare ambient-stable 1T-phase MX.
Whiskey-phase exfoliation: exfoliation and printing of nanosheets using Irish whiskey
[J].
Exfoliated transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2): an electrochemical impedance spectroscopic investigation
[J].
Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniques
[J].
Efficient degradation of Phenol by 1T/2H-MoS2/CuFe2O4 activated peroxymonosulfate and mechanism research
[J].
Metallic 1T-WS2 for selective impedimetric vapor sensing
[J].
Colloidal synthesis of 1T-WS2 and 2H-WS2 nanosheets: applications for photocatalytic hydrogen evolution
[J].
Stable metallic 1T-WS2 nanoribbons intercalated with ammonia ions: the correlation between structure and electrical/optical properties
[J].
Modulation of morphology and phase of magnetically separable 1T-WS2/CuFe2O4 heterojunctions for acceleration of peroxymonosulfate decomposition for rapid degradation of phenol
[J].
Fe and P doped 1T-phase enriched WS2 3D-dendritic nanostructures for efficient overall water splitting
[J].
Enhanced catalytic conversion of polysulfides using high-percentage 1T-phase metallic WS2 nanosh-eets for Li-S batteries
[J].
The preparation and characterization of WS2 nanomaterials with different morphologies and the mechanistic study of peroxymonosulfate activation for phenol degradation
[J].
Vertically aligned 1T-WS2 nanosheets supported on carbon cloth as a high-performance flexible photocatalyst
[J].
Hydrothermal synthesis of stable metallic 1T phase WS2 nanosheets for thermoelectric application
[J].
One-step synthesis of MoS2/WS2 layered heterostructures and catalytic activity of defective transition metal dichalcogenide films
[J].Transition metal dichalcogenides (TMDCs) are a promising class of two-dimensional (2D) materials for use in applications such as 2D electronics, optoelectronics, and catalysis. Due to the van der Waals (vdW) bonding between layers, vdW heterostructures can be constructed between two different species of TMDCs. Most studies employ exfoliation or co-vapor growth schemes, which are limited by the small size and uneven distribution of heterostructures on the growth substrate. In this work we demonstrate a one-step synthesis procedure for large-area vdW heterostructures between horizontal TMDCs MoS2 and WS2. The synthesis procedure is scalable and provides patterning ability, which is critical for electronic applications in integrated circuits. We demonstrate rectification characteristics of large-area MoS2/WS2 stacks. In addition, hydrogen evolution reaction performance was measured in these horizontal MoS2 and WS2 thin films, which indicate that, in addition to the catalytically active sulfur edge sites, defect sites may serve as catalyst sites.
Supercritical CO2-assisted reverse-micelle-induced solution-phase fabrication of two-dimensional metallic 1T-MoS2 and 1T-WS2
[J].
Excellent catalytic activity and water resistance of UiO-66-supported highly dispersed Pd nanoparticles for toluene catalytic oxidation
[J].
Magnetron sputtered WS2; optical and structural analysis
[J].
Enhancement of H2O2 decomposition efficiency by the co-catalytic effect of iron phosphide on the Fenton reaction for the degradation of methylene blue
[J].
WS2 nanosheets with highly-enhanced electrochemical activity by facile control of sulfur vacancies
[J].
