Dual Excitation Transient Photocurrent Measurement for Charge Transfer Studies in Nanocarbon Hybrids and Composites
Dual Excitation Transient Photocurrent Measurement for Charge Transfer Studies in Nanocarbon Hybrids and Composites
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DOI：
10.1002/admi.201600244
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发表时间：
2016-08
影响因子：
5.4
通讯作者：
A. Cherevan;D. Eder
中科院分区：
文献类型：
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作者：
A. Cherevan;D. Eder
DOI：10.1002/adm.201600244纳米碳本身。事实上，已知CNT [23，24]和石墨烯[ 25 ]的电性能受到光的强烈影响，因此对于基于纳米碳的混合物和复合材料上的TPM，需要考虑纳米碳的这种固有行为。在过去的几十年里，碳纳米管的光响应产生了相当大的争论，直到最近，它的测辐射热性质才得到实验证实。[ 26 ]该理论意味着CNT的光照射产生具有异常高结合能的激子，[ 27 ]这不能直接有助于光电导。在这种情况下，它们必须热解离，因此局部加热纳米碳基质。温度的增加导致CNT的电导率增加（与3D金属相反，如由用于1D导体的Luttinger液体模型所解释的。[28，29]），并导致额外的光电流。然而，这方面的知识，尚未实施到实践中，标准的光电流测量纳米碳混合物和复合材料仍然进行不考虑裸碳纳米管的光响应。在这里，我们报告了一种新的技术，我们称之为双激发瞬态光电流测量（DETPM）。该技术允许区分测热效应（即，由于光照射产生的热而导致的固有电导率变化）。我们已经测试了这种技术的两个高性能的混合动力车，CNT-TiO 2和CNT-Ta 2 O 5，并明确证明存在的电荷转移从半导体的纳米碳在这两种情况下。我们通过改进的艾德溶胶-凝胶法合成了CNT-Ta 2 O 5杂化材料，通过原子层沉积法合成了CNT-TiO 2杂化材料（合成的细节在实验部分和ESI中表征）。我们的目标是创建具有高度保形涂层和CNT与金属氧化物之间的扩展界面的混合物，以最大化界面电荷转移。图S1（支持性信息）中两种混合物的SEM图像证实了纳米碳表面上存在均匀涂层，并证明了样品的均匀性。如图1 B、c中的高分辨率透射电子显微镜（HRTEM）进一步揭示的，在两种情况下，在两种组分之间建立了紧密的界面。重要的是，用于TPM的所有样品均以自立式宏观膜的形式制备，以消除任何底物贡献。在典型的DETPM实验中，使用含有纯CNT（即，参考）或基于CNT的混合物连接到如图2a所示的两个电极，并施加小偏压以建立恒定电流。纳米碳-无机杂化物-其中功能化合物以纳米颗粒、层或薄膜的形式沉积在纳米碳上-构成了一类新型材料，其已经在各种光-燃料和光-电转换应用如光催化中展示了其潜力。[ 1-3 ]杂化物的益处来自纳米碳从光催化剂中提取、[4，5]储存[ 6 ]和传导[ 7-9 ]光激发电子的强能力，如图1a所示。这样的电荷转移过程导致电子与空穴的有效空间分离，减慢复合速率，并且因此通常导致杂化物的更好的光催化性能，因为更多的电荷可用于随后的氧化还原反应。然而，这一关键过程是相当困难的记录和评估，只有有限数量的技术已被应用于获得纳米碳混合物中的界面电荷转移的信息。光致发光（PL）已被用来监测PL猝灭，这已被归因于光激发电荷的转移或半导体和纳米碳之间的能量转移。[ 10-13 ]例如，Kamat及其同事向ZnO纳米颗粒溶液中加入了浓度不断增加的碳纳米管（CNT）[ 14 ]和石墨烯[ 15 ]，并观察到PL信号的减少，从中他们推断出存在从ZnO到纳米碳的电子转移。然而，这种PL信号的损失也可能源于例如由于纳米碳的逐渐增加的分数引起的额外的光吸收以及由颗粒尺寸和形状差异引起的贡献。除此之外，PL仅适用于有限数量的化合物，即，荧光材料。瞬态光电流测量（TPM）构成了一种替代方法，其中固体样品的计时电流法数据（电流与时间）在用光照射时在外部偏压下记录。该方法依赖于这样的事实，即当光激发电荷转移到纳米碳中时，会产生完成电路的额外载流子，从而导致所谓的额外光电流。TPM已经应用于纳米碳混合物和复合材料; [6，16 -22 ]然而，这些工作都没有考虑到这种光响应也可能源于
DOI: 10.1002/admi.201600244 the nanocarbon itself. In fact, it is known that the electrical properties of CNTs [ 23,24 ] and graphene [ 25 ] are strongly infl uenced by light, hence this inherent behavior of the nanocarbons needs to be considered for TPM on nanocarbon-based hybrids and composites. The photoresponse of CNTs has generated considerable debate during the last few decades and only recently its bolometric nature has been experimentally confi rmed. [ 26 ] The theory implies that light illumination of CNTs creates excitons with unusually high binding energies, [ 27 ] which cannot directly contribute to the photoconductivity. In this scenario, they have to dissociate thermally, therefore locally heating up the nanocarbon matrix. The increase in temperature leads to an increase in conductivity of the CNTs (in contrast to 3D metals, as explained by the Luttinger liquid model for 1D conductors. [ 28,29 ] ) and results in additional photocurrent. This knowledge, however, has not yet been implemented into practice and standard photocurrent measurements on nanocarbon hybrids and composites are still conducted without considering the photoresponse of bare CNTs. Here we report a new technique, which we describe as dual excitation transient photocurrent measurement (DETPM). This technique allows for distinguishing the bolometric effect (i.e., intrinsic conductivity change due to heat produced by light irradiation) from a potential photoexcited charge transfer in CNTs-based hybrids. We have tested this technique on two high-performance hybrids, CNT-TiO 2 and CNT-Ta 2 O 5 , and explicitly demonstrate the presence of the charge transfer from the semiconductor to the nanocarbon in both the cases. We synthesized the CNT-Ta 2 O 5 hybrid via a modifi ed sol–gel process and the CNT-TiO 2 hybrid via atomic layer deposition process (details of synthesis are in the Experimental Section and characterization in ESI). We aimed to create hybrids with a high degree of conformal coating and an extended interface between the CNTs and the metal oxide to maximize the interfacial charge transfer. SEM images of both hybrids in Figure S1 (Supporting Information) confi rm the presence of uniform coating on the nanocarbon surface and demonstrate homogeneity of the samples. As further revealed by high-resolution transmission electron microscopy (HRTEM) in Figure 1 b,c a tight interface between the two components was established in both the cases. Importantly, all samples used for the TPM were prepared in the form of free-standing macroscopic membranes to eliminate any substrate contribution. In a typical DETPM experiment, a macroscopic membrane, containing either pure CNTs (i.e., reference) or the CNT-based hybrids, was connected to two electrodes as shown in Figure 2 a and a small bias was applied to establish a constant current. Nanocarbon-inorganic hybrids – where a functional compound is deposited onto a nanocarbon in the form of nanoparticles, layers, or thin fi lms – constitute a novel class of materials that have already demonstrated their potential in various lightto-fuel and light-to-electricity conversion applications such as photo catalysis. [ 1–3 ] Benefi ts of the hybrids arise from the strong ability of the nanocarbons to withdraw, [ 4,5 ] store, [ 6 ] and conduct [ 7–9 ] photoexited electrons from the photocatalyst as depicted in Figure 1 a. Such a charge transfer process leads to effi cient spatial separation of the electrons from the holes, slows down recombination rates, and thus often results in a better photocatalytic performance of the hybrids as more charges are available for subsequent redox reactions. This key process, however, is rather diffi cult to record and assess and only a limited number of techniques have been applied to obtain information on interfacial charge transfer in nanocarbon hybrids. Photoluminescence (PL) has been used to monitor PL quenching, which has been attributed to either transfer of photoexcited charges or energy transfer between semiconductor and nanocarbon. [ 10–13 ] For example, Kamat and co-workers have added increasing concentrations of both carbon nanotubes (CNTs) [ 14 ] and graphene [ 15 ] to ZnO nanoparticle solution and observed a decrease in PL signal, from which they deduced the presence of electron transfer from ZnO to the nanocarbon. However, such a loss of PL signal could also originate, for example, from additional light absorption due to the gradually increasing fraction of the nanocarbon as well as contributions arising from particle size and shape differences. In addition to that, PL is only applicable to a limited number of compounds, i.e., fl uorescent materials. Transient photocurrent measurements (TPM) constitute an alternative method, in which chronoamperometry data (current vs time) of a solid sample are recorded under an external bias upon illumination with light. The method relies on the fact that when the photoexcited charges are transferred into the nanocarbon, additional carriers completing the electric circuit are created therefore resulting in a so-called additional photocurrent. TPM has already been applied to nanocarbon hybrids and composites; [ 6,16–22 ] however, none of these works have considered that this photoresponse could also originate from