Bandgap Engineering of Organic Semiconductors for Highly Efficient Photocatalytic Water Splitting
有机半导体的带隙工程用于高效光催化水分解
首次发表日期：2018年6月28日 https://doi.org/10.1002/aenm.201801084 引用次数：125
The copyright line for this article was changed on August 27, 2018 after original online publication.
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Abstract
The bandgap engineering of semiconductors, in particular low-cost organic/polymeric photocatalysts could directly influence their behavior in visible photon harvesting. However, an effective and rational pathway to stepwise change of the bandgap of an organic/polymeric photocatalyst is still very challenging. An efficient strategy is demonstrated to tailor the bandgap from 2.7 eV to 1.9 eV of organic photocatalysts by carefully manipulating the linker/terminal atoms in the chains via innovatively designed polymerization. These polymers work in a stable and efficient manner for both H2 and O2 evolution at ambient conditions (420 nm < λ < 710 nm), exhibiting up to 18 times higher hydrogen evolution rate (HER) than a reference photocatalyst g-C3N4 and leading to high apparent quantum yields (AQYs) of 8.6%/2.5% at 420/500 nm, respectively. For the oxygen evolution rate (OER), the optimal polymer shows 19 times higher activity compared to g-C3N4 with excellent AQYs of 4.3%/1.0% at 420/500 nm. Both theoretical modeling and spectroscopic results indicate that such remarkable enhancement is due to the increased light harvesting and improved charge separation. This strategy thus paves a novel avenue to fabricate highly efficient organic/polymeric photocatalysts with precisely tunable operation windows and enhanced charge separation.
半导体的带隙工程，特别是低成本的有机/聚合物光催化剂，可以直接影响它们在可见光子吸收中的行为。然而，有机/聚合物光催化剂带隙的逐步变化的有效和合理途径仍然非常具有挑战性。通过精心操纵链中的连接子/端原子，通过创新设计的聚合，展示了一种有效的策略，将有机光催化剂的带隙从2.7电子伏特调节至1.9电子伏特。这些聚合物在环境条件下（420 nm < λ < 710 nm）稳定且高效地工作，对H2和O2的发展均表现出高达参考光催化剂g-C3N4的18倍的氢进化速率（HER），引领到420 nm/500 nm时高明显量子产率（AQY）分别为8.6%/2.5% 。对于氧进化速率（OER），最佳聚合物显示出比g-C3N4高19倍的活性，且在420 nm/500 nm时具有出色的AQY为4.3%/1.0%。理论模拟和光谱结果表明，这种显著的增强是由于增加的光吸收和改善的电荷分离。这种策略为制造具有精确可调操作窗口和增强电荷分离的高效有机/聚合物光催化剂铺平了一条新的途径。
1 Introduction 介绍
Photocatalytic water splitting has attracted substantial attention over the past 40 years as a promising approach to mitigate key energy and environmental issues.1 As the photocatalytic activity is highly dependent on the electronic structure of the photocatalyst, it is crucial to adjust the bandgap in order to utilize the highest possible proportion of visible photons and achieve the target of 10% solar to fuel conversion efficiency.2 Moreover, bandgap tunable semiconductors are especially useful in the construction of a Z-scheme for water splitting, which is considered to be a more promising approach to solar H2 production than the single photocatalyst-based water splitting system.3-8 A Z-Scheme requires an appropriate match of redox potentials between two photocatalysts and two mediators. So far, the strategies that have been successfully applied to change the bandgaps and band positions of photocatalysts include heteroatom doping and the use of junctions between materials. For instance, the doping of cations (e.g., Rh) into SrTiO3 introduced new energy levels and narrowed its wide bandgap (3.1 eV) to the visible light region (2.4 eV).9 Another well-known exemplar is the solid solution of GaN: ZnO (2.5 eV), which forms a visible light responsive bandgap from two UV-responsive semiconductors (3.4 and 3.2 eV).10
光催化水分解在过去的40年里引起了相当大的关注，作为一种有望缓解重要能源和环境问题的方法。光催化活性高度依赖于光催化剂的电子结构，调整带隙是至关重要的，以利用尽可能高比例的可见光子并达到10%太阳能转化效率至燃料的目标。此外，可以调节带隙的半导体在构建适合水分解Z-方案时特别有用，这被认为比基于单光催化剂的水分解系统更有前景。Z-方案需要两个光催化剂和两个介质之间氧化还原电位的适当匹配。迄今已成功应用于改变光催化剂带隙和带位的策略包括杂原子掺杂和材料之间的结界。例如，在SrTiO中掺杂阳离子（如Rh）引入了新的能级，并将其宽带隙（3.1 eV）缩小到可见光区（2.4 eV）。另一个著名的例子是GaN：ZnO（2.5 eV）的固溶体，它将两个紫外光响应的半导体（3.4和3.2 eV）形成为可见光响应的带隙。
Compared with the progress in bandgap engineering of inorganic photocatalysts, there have been limited reports of the emerging organic photocatalysts (e.g., heptazine-based polymers and covalent organic frameworks), although they are known for their suitable band positions for water splitting, low-cost, chemical stability, and good tunability of their framework and electronic structures.11-17 Currently, the majority of organic/polymeric photocatalysts still suffer from an intrinsic wide bandgap (e.g., ≈2.7 eV for g-C3N4) and only responds to a limited region of the solar spectrum (<460 nm), not matching with the strongest portion of 450–700 nm in sunlight (2.7–1.8 eV photons).18, 19 Although approaches such as element doping, copolymerization, and introduction of defects have attempted to narrow the bandgap to ≈2.0 eV, the resultant operation window is either far from the target region or the apparent quantum yield (AQY) is still moderate, probably due to defect-induced recombination centers.20-22 The lack of facile bandgap engineering methods has inhibited the application of organic/polymeric photocatalysts to potential applications including the construction of Z-scheme water splitting systems. Therefore, an effective and rational polymerization strategy to stepwise engineer precisely the electronic structure of polymers is a real need scientifically and technically.
与无机光催化剂的带隙工程进展相比，有机光催化剂（例如基于若氮烷基聚合物和共价有机框架）的报道有限，尽管它们以适合的带位置、低成本、化学稳定性和框架和电子结构的良好可调性而闻名于水分解。目前，大多数有机/聚合光催化剂仍然存在固有的宽带隙（例如g-C 3 N 4 的≈2.7 eV），仅响应太阳光谱的有限区域（<460 nm），与450–700 nm之间最强的光部分（2.7–1.8 eV 光子）不匹配。虽然诸如元素掺杂、共聚合和缺陷引入等方法已尝试将带隙缩小至≈2.0 eV，但由于缺陷诱导的复合中心，其操作窗口要么远离目标区域，要么明显的量子产率仍然适中。缺乏简便的带隙工程方法已经限制了有机/聚合光催化剂应用于包括构建Z-型水分解系统在内的潜在应用。因此，一种有效且合理的聚合策略以逐步精确调控聚合物的电子结构在科学和技术上是真正需要的。
Taking graphitic carbon nitride (g-C3N4 or GCN), the most widely reported heptazine-based polymers, as an instance, the conduction band (CB) is considered to consist of conjugated C and N 2p orbitals in the heptazine while the VB is mostly composed of the 2p orbitals of edge N atoms, resulting in a π–π* excitation bandgap of over 2.7 eV.23, 24 A disordered framework could allow n–π* excitation of the lone pair electrons on edge N atoms that is forbidden in a planar structure, which would result in a photon energy that is smaller than 2.5 eV and was reported to aid the visible light absorption.25, 26 Very recently, we have theoretically and experimentally proved that the oxygen and nitrogen linker-controlled heptazine-based chains strongly influence the polymer's electronic structure.27 The band positions of O-linked chains are relatively more positive than those of N-linked chains, hence the hybrid polymer consists of a lower CB contributed by the introduced O-linked chains and a VB contributed by N-linked chains, resulting in a narrowed bandgap. More importantly, the electron acceptor–donor nature between O-chains and N-chains promotes the physical charge separation for enhanced efficiency.28 Also, the selective doping into the linker position instead of heptazine units in the polymer maintains an integrated conjugated framework and avoids additional defect-based recombination centers. However, this modified polymer is only active for H2 production with no activity for water oxidation under visible light irradiation.
以石墨氮化碳（g-C 3 N 4 或GCN）作为例子，这是报道最广泛的以亚氨基苯环为基础的聚合物之一。将导带（CB）视为由亚氨基苯环中共轭的C和N 2p轨道组成，而价带（VB）则主要由边缘N原子的2p轨道组成，其结果为超过2.7电子伏的π-π*激发带隙。受规则结构禁止的n-π*激发影响，边缘N原子上的孤对电子可能导致光子能量小于2.5电子伏，从而增强可见光吸收。最近，我们在理论上和实验上证实氧和氮连接子控制的亚氨基苯链会强烈影响聚合物的电子结构。氧连接链的带位置相对正电于氮连接链的带位置，使混合聚合物包含一个由氧连接链贡献的较低导带和由氮连接链贡献的价带，从而导致带隙变窄。更重要的是，氧链和氮链之间的电子受体-给予者特性有助于物理电荷分离以增强效率。此外，选择性地将掺杂物掺入连接子位置而不是聚合物中的亚氨基苯单元可以保持一个统一的共轭框架并避免额外的基于缺陷的复合中心。然而，这种修改后的聚合物仅对H 2 的产生具有活性，但在可见光照射下对水氧化没有活性。
In this study, based on the previous work, we developed a new approach to control precisely the bandgap of organic photocatalysts, resulting into stepwise bandgap changes from 2.7 to 1.9 eV, by carefully tailoring the linker and terminal atoms among donor–acceptor domains. This fine control of band positions has been achieved by adding different amounts of formic acid as an important precursor and an innovative stoichiometry-tuned polymerization, which provides an effective way to synthesize a series of polymer photocatalysts with controlled electronic structures. Furthermore, this approach allows us to observe the correlation between the band positions and photocatalytic activities of polymer semiconductors. The resultant polymers work in a stable and efficient manner for H2 and more importantly O2 evolution at ambient conditions under visible light irradiation (420 nm < λ < 710 nm), representing not only up to 18 times higher hydrogen evolution rate (HER) than the widely reported pristine g-C3N4, but also nearly 20 times higher oxygen evolution rate (OER) activity. More importantly these activities are well correlated with the band position changes of the polymers.
在这项研究中，基于先前的工作，我们开发了一种新方法，通过精心设计给体-受体结构中的连接子和末端原子，精确控制有机光催化剂的带隙，使带隙从2.7 eV逐步变化到1.9 eV。通过添加不同量的甲酸作为重要前体和创新的化学计量调控聚合，我们实现了对带位的精细控制，提供了一种有效的方法来合成一系列具有可控电子结构的聚合物光催化剂。此外，这种方法使我们能够观察聚合物半导体的带位与光催化活性之间的相关性。由此产生的聚合物在可见光照射下的环境条件下（420 nm < λ < 710 nm）稳定高效地用于产氢和更重要的是产氧，不仅产氢速率（HER）高出广泛报道的原始g-C N，近20倍氧进化速率（OER）。更重要的是，这些活性与聚合物的带位变化密切相关。
2 Results and Discussion
2.结果和讨论
2.1 The Design of Polymerization Pathway
2.1 聚合途径的设计
In order to engineer the bandgap of the heptazine-based polymers, the novel polymerization pathway was carefully designed and controlled as shown in Scheme S1 (Supporting Information). As reported before, the band position shift originates from the existence of O-linked chains connected to N-linked domains, which have lower (more positively orientated) CB with an electron acceptor nature as illustrated in our recent study.27, 28 The total electronic structure of such hybrid polymers consists of CB contributed by O-chains and VB contributed by N-chains, resulting in a narrow bandgap. Therefore, it is crucial to control the O-terminated heptazine monomers during the polymerization to tailor the bandgap. Different from DCDA precursor that undergoes a widely accepted melamine-melem-melon-g-C3N4 pathway (Scheme S1, Supporting Information, the upper),11 the formic acid treated precursor involves additional intermediates including hydroxyl- and amine-terminated heptazine (Scheme S1, Supporting Information, the lower) as monomers,29, 30 resulting in O-linked heptazine in the following process. Also, some unreacted DCDA molecules will form ordinary N-linked heptazine, which polymerize together with O-linked ones. By stoichiometrically controlling the amount of formic acid in the pretreatment, the ratio between O-linked and N-linked chains and the extent of CB shift in the final produced polymers can be stepwise controlled; hence the properties including the electronic structure of polymers can be tuned reproducibly and reliably.
为了调控基于七氮杂环聚合物的带隙，新型的聚合途径被仔细设计并控制，如图S1（支持信息）所示。 正如之前报道的那样，能级位置的移动源于连接到N-链结构的O-链存在，这些O-链具有与电子受体性质更低（更正向取向）的导带，正如我们最近研究所示。 这种混合聚合物的总体电子结构由O-链贡献的导带和N-链贡献的价带组成，导致一个窄带隙。 因此，在聚合过程中控制O-端化七氮杂环单体以调节带隙至关重要。 与经历广泛接受的三聚蔓藤酸-蜜瓜-脲途径不同，(见图S1，支持信息，上)，在甲酸处理前体中包括羟基-和胺基-端化七氮杂环（图S1，支持信息，下）作为单体，从而在下一过程中形成O-链铵基七氮杂环。 此外，一些未反应的DCDA分子将形成普通的N-链七氮杂环，并与O-链的一起聚合。 通过在预处理中比例控制甲酸的数量，可以分阶段控制O-链和N-链之间的比例及最终产生的聚合物中的CB偏移程度； 因此，包括聚合物的电子结构在内的属性可以被可靠和可复制地调节。
Such a proposed polymerization routine has been examined in detail by characterization of intermediates. The structural differences are shown in Carbon-13 solid-state nuclear magnetic resonance (13C ssNMR) of intermediates obtained at different temperatures during the polymerization of acid-treated precursors (Figure S1a, Supporting Information). For example, at 130 °C, the two peaks at 120 (cyano group) and 164 ppm are from the residual DCDA while those at 170 and 158 ppm are related to carbon atoms in O containing intermediates.31-33 No polymerization was expected at 130 °C, thus NMR spectra show the mixture of unreacted and formic acid treated DCDA. This is further confirmed by FTIR, which shows the features of both DCDA and cyanuric acid. As the temperature rises from 130 °C to 350 °C, the precursor transforms from a triazine-like structure to a heptazine-like structure because the 158 ppm peak submerges in NMR at 350 °C, indicating the inner circle carbon formation in heptazine34 and the heptazine CN vibration peaks appear at 1350–1200 cm−1 in FT-IR.18, 35 In the final product formed at 450–550 °C, the oxygen characters are less obvious due to a low concentration and an overlap with other species but the broader peaks due to a disordered structure are similar to the previously reported.27
这种拟议的聚合例程已通过对中间体进行表征进行了详细研究。结构差异显示在从酸处理的前体聚合过程中，在不同温度下获得的中间体的碳-13固态核磁共振（ 13 C ssNMR）图谱中（参见支持信息的图S1a）。例如，在130℃时，位于120（氰基）和164ppm处的两个峰是源自残留的DCDA，而位于170和158ppm处的峰与含氧中间体中的碳原子有关。没有预期在130℃时发生聚合，因此核磁共振光谱显示未反应和甲酸处理的DCDA混合物。这进一步得到傅里叶变换红外光谱的证实，它显示了DCDA和异尿酸的特征。随着温度从130℃升至350℃，前体从嗪类结构转变为氮杂氮类结构，因为在350℃时，158ppm峰在核磁共振中消失，表明氮杂氮类结构中的内环碳形成，而氮杂氮类C-N振动峰在1350-1200 cm −1 处在傅里叶变换红外光谱中出现。在450-550℃形成的最终产物中，由于浓度较低且与其他物种重叠，氧特征不太明显，但由于无序结构而产生的更宽峰与以前报告的类似。
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis support the proposed pathway. With the DCDA representing the same as that reported during thermal condensation,11 the acid-treated precursor has two melting points at 130 °C and 190 °C corresponding to the mixture compositions in the acid-treated precursor. The FAT intermediates went through triazine ≈236 °C and formed heptazine after 300 °C.29 As observed, the polymerization behavior of FAT sample is similar to g-C3N4 but at a lower temperature, probably owing to the precyclization at a lower temperature (Scheme S1, Supporting Information, lower) which aids the next step of the polymerization.29 Notably, the color of the FAT intermediate starts changing from white to brown during polymerization ≈250 °C while DCDA remains white until the final step to yellow after 450 °C. Such an obvious color change of FAT intermediates indicates that the bandgap shift due to the oxygen-containing groups appears at relatively lower temperatures in the polymerization process.
热失重分析（TGA）和差示扫描量热（DSC）分析支持提出的途径。随着DCDA代表的与热缩聚过程中报告的相同，经酸处理的前体具有两个熔点，分别为130°C和190°C，对应于经酸处理的前体中的混合物成分。FAT中间体经过三嗪约236°C，形成庚氮三嗪在300°C后。观察到，FAT样品的聚合行为类似于g-C 3 N 4 ，但在较低温度下，可能是由于在较低温度下的预环化（如示例S1，支持信息，下）帮助了聚合的下一步骤。值得注意的是，FAT中间体的颜色在聚合过程中约250°C开始从白色变为棕色，而DCDA直到450°C后的最后一步才变为黄色。FAT中间体的明显颜色变化表明，由于含氧基团而导致的带隙移位在聚合过程中出现在相对较低的温度。
2.2 Characterizations of Novel Organic Semiconductors
2.2 新型有机半导体的表征
These bandgap tunable polymers are synthesized at 550 °C for 4 h and the final products are noted as FAT-X with X representing the stoichiometry ratio of formic acid to DCDA in the precursors. FAT-0 is the reference DCDA-derived g-C3N4. The properties of the synthesized polymers with different amounts of formic acid in precursors were investigated via extensive and thorough characterizations. From elemental analysis (EA), the bulk atomic stoichiometry of eight FAT samples was found to be: C3N4.51H1.6O0.07 (FAT-0), C3N4.50H1.7O0.07 (FAT-0.1), C3N4.49H1.6O0.07 (FAT-0.2), C3N4.48H1.6O0.07 (FAT-0.5), C3N4.44H1.7O0.09 (FAT-0.8), C3N4.43H1.8O0.15 (FAT-1.0), C3N4.40H2.0O0.25 (FAT-1.5) and C3N4.33H2.0O0.26 (FAT-2.0) (Table 1). As the amount of formic acid increases in the precursors, the polymers show a decreasing amount of nitrogen with increasing oxygen, indicating more O species in the polymers' bulk structure. Unlike some recently reported copolymerization polymers with slightly different features from g-C3N4, the FAT samples exhibit distinct and stepwise changes.
这些带隙可调的聚合物在550°C、4小时合成，最终产物标记为FAT-X，其中X表示前体中甲酸与DCDA的化学计量比。FAT-0是参照DCDA衍生的g-C 3 N 4 。通过广泛和深入的特性表征，对不同量甲酸的前体合成的聚合物性质进行了研究。从元素分析（EA）中，发现了八个FAT样本的总体原子化学计量：C 3 N 4.51 H 1.6 O 0.07 （FAT-0）、C 3 N 4.50 H 1.7 O 0.07 （FAT-0.1）、C 3 N 4.49 H 1.6 O 0.07 （FAT-0.2）、C 3 N 4.48 H 1.6 O 0.07 （FAT-0.5）、C 3 N 4.44 H 1.7 O 0.09 （FAT-0.8）、C 3 N 4.43 H 1.8 O 0.15 （FAT-1.0）、C 3 N 4.40 H 2.0 O 0.25 （FAT-1.5）和C 3 N 4.33 H 2.0 O 0.26 （FAT-2.0）（表1）。随着前体中甲酸量的增加，聚合物中氮的含量递减，氧含量递增，表明聚合物的总体结构中有更多的氧种类。与最近报道的带有不同特性的g-C 3 N 4 之间有些微不同的共聚聚合物不同，FAT样本呈现出独特而分阶段的变化。
表1. 大气压下测得的FAT样本性质摘要
| Samples 样本 | Composition 成分 | N content [%] N含量[%] | O content [%] O含量[%] | Surface area [m2 g−1] 表面积 [m 2 g −1 ] |
Bandgap [eV] 带隙 [eV] | HER [µmol g−1 h−1] |
|---|---|---|---|---|---|---|
| FAT-0 脂肪-0 | C3N4.51H1.6O0.07 C 氮 H 氧 |
49.1 | 0.76 | 5.5 | 2.74 | 44 |
| FAT-0.1 | C3N4.50H1.7O0.07 | 49 | 0.76 | 5.6 | 2.72 | 103 |
| FAT-0.2 | C3N4.49H1.6O0.07 C 氮 H 氧 |
49 | 0.76 | 6.7 | 2.69 | 114 |
| FAT-0.5 脂肪-0.5 | C3N4.48H1.6O0.07 C 氮 H 氧 |
49 | 0.77 | 6.9 | 2.66 | 192 |
| FAT-0.8 | C3N4.44H1.7O0.09 %% |
48.1 | 0.98 | 9.9 | 2.06 | 456 |
| FAT-1.0 脂肪-2.0 | C3N4.43H1.8O0.15 %% |
47.7 | 1.62 | 12.1 | 1.92 | 772 |
| FAT-1.5 脂肪-1.5 | C3N4.40H2.0O0.25 %% |
45.5 | 2.67 | 16.4 | 2.01 | 656 |
| FAT-2.0 C <b0></b0> N <b1></b1> H <b2></b2> O <b3></b3> | C3N4.33H2.0O0.26 %% |
45.2 | 2.71 | 40.0 | 2.05 | 556 |
To obtain the crystallinity information of FAT polymers, powder X-ray diffraction (PXRD) patterns of the samples were measured (Figure 1a). At first glance, the patterns of the FAT samples are similar to FAT-0 (g-C3N4) in shape, which has two peaks locating at 13.0° and 27.4° assigned to the (100) and (002) planes, respectively, corresponding to intralayer packing size of 6.82 Å and an interlayer distance of 3.26 Å.22 A close examination shows that the (100) peak of the other FAT samples becomes weaker and slightly right shifts to 13.4°, indicating a reduced crystallinity and a closer crystalline distance of 6.67 Å probably due to small polymer size and the shorter bond lengths in the presence of oxygen.33 It is more evident that (002) peaks shift left, suggesting an enlarged layer-to-layer distance up to 3.30 Å due to a variation in structure, such as distortions in the FAT samples or the oxygen species between layers.27 The width of the (002) peak becomes broadened as the amount of formic acid increases, which also suggests that the crystalline size of the polymers is smaller. The shifts of peak positions are consistent with the change in the formic acid equivalent, suggesting the higher amount of O-containing groups, the larger shift of peak positions. Both the highest peak density and largest shift distance is found on the sample FAT-1.0. Further increasing formic acid concentration causes the peak shift to decrease. One can see formic acid in the precursor first results in a decline of polymerization degree (Figure 1a, FAT-0.1). As the amount further increases, relatively well-crystallized product gradually forms because the XRD patterns become sharper from FAT-0.2 to FAT-1.0. After that, extra formic acid in the precursor leads to poor polymerization again. Therefore, FAT-1.0 and FAT-0 (g-C3N4) could be two relatively highly crystallized polymers.
为了获得FAT聚合物的结晶度信息，我们测量了样品的粉末X射线衍射（PXRD）图谱（图1a）。乍一看，FAT样品的图谱与FAT-0（g-CNb）的形状相似，其具有两个峰分别位于13.0°和27.4°处，分别对应（100）和（002）晶面，相应的层间包装尺寸为6.82埃，层间距为3.26埃。进一步观察发现，其他FAT样品的（100）峰变得较弱，并稍微右移至13.4°，表明了结晶度降低，晶体距离更近达6.67埃，可能是由于聚合物尺寸较小，在氧的存在下键长较短。更明显的是，（002）峰向左移动，表明层间距增加至3.30埃，这可能是由于FAT样品中的结构变化，如层间氧类物质引起的畸变。随着甲酸量的增加，（002）峰变得宽广，这也表明了聚合物的晶粒尺寸较小。峰位置的变化与甲酸当量的变化一致，说明氧含量较高时，峰位置的移动较大。在样品FAT-1.0上找到了最高的峰密度和最大的偏移距离。继续增加甲酸浓度会导致峰位置的偏移减少。可以看到，前驱体中的甲酸首先导致了聚合度的下降（图1a，FAT-0.1）。随着甲酸量的进一步增加，相对结晶良好的产物逐渐形成，因为FAT-0.2到FAT-1.0的XRD图谱变得更加清晰。 然后，前体中额外的甲酸再次导致聚合物化不好。因此，FAT-1.0和FAT-0（g-C 3 N 4 ）可能是两种相对高结晶度的聚合物。
Raman spectroscopy was used to detect the backbone of the FAT polymers (Figure 1b). From the spectra, the heptazine-based structure could be confirmed as most of these characteristic peaks appear at the same positions. The peaks in the 1200–1700, 980, and 690 cm−1 regions represent the disordered graphitic carbon–nitrogen vibrations, the symmetric N-breathing mode of heptazine and the in-plane bending, respectively.36, 37 Notably, the peak at 1406 cm−1 in FAT-0 (g-C3N4) becomes negligible while the one at 1170 cm−1 emerges as the amount of formic acid goes up. These two modes are both in-pane CH rock and semicircle stretching but in different directions.37 Such a change toward disordered structure could be assigned to the oxygen-linked chains. Similar to the intensity trend of XRD patterns, the intensity of signals from FAT-0 to FAT-1.0 and to FAT-2.0 shows a volcano curve. As discussed above, this results from the differences in the degree of crystallinity induced by oxygen amount.
拉曼光谱用于检测FAT聚合物的主链（图1b）。从光谱中可以确认以七氮杂环为基础的结构，大多数特征峰出现在相同位置。在1200-1700、980和690 cm区域的峰代表无序石墨炭氮振动、七氮杂环的对称N呼吸模式和平面弯曲。值得注意的是，FAT-0（g-CN）中1406 cm峰变得微不足道，而1170 cm处的峰随着甲酸含量的增加而出现。这两种模式都是平面C-H摇摆和半圆拉伸，但方向不同。这种朝向无序结构的变化可以归因于氧连接链。类似于XRD图案的强度趋势，从FAT-0到FAT-1.0再到FAT-2.0的信号强度呈现出火山曲线。正如前面讨论的，这是由氧量引起的结晶度差异造成的。
In order to confirm the difference of structure, FT-IR spectra of FAT polymers were also compared. Generally, FAT samples have similar but broadened and less sharp peaks, owing to the incorporation of oxygen species into the structure (Figure 1c).27 As no peaks show up in around 2200 and 1700 cm−1 region, the existence of CO from formic acid and cyano groups from DCDA has been ruled out, indicating these precursors were converted during polymerization.38 As the amount of formic acid increases, peaks of all other FAT samples at 1207 and 1455 cm−1 (indicated by dotted line in Figure 1c) related to C-NHx are gradually getting weaker than those of FAT-0 (g-C3N4) (Figure S2a, Supporting Information),35 while identical peaks of NHx ≈3000–3300 cm−1 in other FAT samples decline and an additional peak assigned to OH emerges at 3340 cm−1 (Figure S2b, Supporting Information), altogether verifying the decline of NHx groups and the formation of OH species.18, 39
为了确认结构上的差异，还比较了脂肪聚合物的FT-IR光谱。一般而言，脂肪样品的峰值相似但更加宽泛和不那么尖锐，这是由于氧类物质结构中的引入（图1c）。由于在2200和1700 cm区域未出现峰值，可以排除甲酸和DCDA中的氰基带来的羰基（C=O）的存在，表明这些原料在聚合过程中已被转化。随着甲酸含量的增加，所有其他脂肪样品在1207和1455 cm处的峰值（图1c中虚线指示）与与 C-NH有关的峰逐渐减弱，与FAT-0（g-CN）的峰值相比（图S2a，补充信息），而其他脂肪样品中NH≈3000-3300 cm的相同峰值也减少，并在3340 cm处出现一个说明OH的额外峰（图S2b，补充信息），共同验证NH基团的减少和OH类物质的生成。
13C ssNMR was used to illustrate such a structural change in detail (Figure 1d). The characteristic peaks of FAT-0 (g-C3N4) around 166, 163, and 157 ppm are assigned to external circle carbons (Ce: CN2NH and CN2NH2) and inner circle carbons (Ci: CN3), respectively (Figure 1d and Figure S3, Supporting Information).31, 34 The overall NMR chemical shift suggests that FAT polymers consist of a mixture of N-linked heptazine (melem) and O-linked heptazine. The observed right-shifted peaks in FAT samples by about 1 ppm with weakening NHx related signals result from the substitution of N linkers by oxygen bridges and terminals, which also matches the trend obtained from the NMR prediction software (Figure S3, Supporting Information).40-42
C ssNMR用于详细展示这种结构变化（图1d）。 FAT-0（g-C 3 N 4 ）的特征峰约166、163和157 ppm被归属于外环碳（C e ：CN 2 NH和CN 2 NH 2 ）和内环碳（C：CN 3 ），分别如图1d和支持信息中的图S3所示。整体核磁化学位移表明，FAT聚合物由N-连接的癸烷（Melem）和O-连接的癸烷混合而成。FAT样品中观察到的整体NMR化学位移向右偏移约1 ppm，并且由于N连接物被氧桥和末端所取代而导致了NH x 相关信号的减弱，这也与NMR预测软件得到的趋势相匹配（图S3，支持信息）。
X-ray photoelectron spectroscopy (XPS) was also undertaken to investigate the chemical state in these materials (FAT-1.0 as one example was shown in Figure S4, Supporting Information). According to the depth profile of O 1s XPS spectra, the spectrum peak shifts from 531.8 to 533.2 eV as the etching goes deeper from 0 to 130 nm (Figure S4a, Supporting Information). As observed, there are four types of O species on the surface of FAT-1.0, which are surface oxides CO (530 eV),43 COH bond (531.5 eV),44 adsorbed water (532.5 eV),45, 46 and COC bond (533.2 eV) (Figure S4b, Supporting Information).47 After etching to 130 nm, only two obvious peaks of COH and COC are detected (Figure S4c, Supporting Information), confirming the existence of oxygen linkers and OH terminals in the bulk materials. Therefore, the peak shift during the etching is assigned to the decline of surface species (CO, some CO and water). According to C1s XPS spectra, the ratio of compositions of CO to CN (CO/CN) keeps increasing with an increasing amount of formic acid (Table S1, Figure S5, Supporting Information), indicating a decreased amount of N and an increased amount of O species in these polymers.27 These results confirm oxygen linkers (and OH terminals) in the structure, which should affect the properties of FAT polymers.
通过X射线光电子能谱（XPS）研究这些材料中的化学状态（例如，图S4显示了FAT-1.0）。根据O 1s XPS光谱的深度剖析，随着蚀刻深度由0至130纳米，光谱峰由531.8 eV移动到533.2 eV。在FAT-1.0的表面观察到四种O种类：表面氧化物C=O（530 eV）、C-OH键（531.5 eV）、吸附水（532.5 eV）和C-O-C键（533.2 eV）。蚀刻到130纳米后，只检测到C-OH和C-O-C的峰，证实材料中存在氧连接物和OH端基。蚀刻过程中峰值的变化被归因于表面物种（C=O、一些C-O和水）的减少。根据C1s XPS光谱，C=O与C=N的组成比（C=O/C=N）随着甲酸的增加而增加，表明这些聚合物中氮的量减少、氧的量增加。这些结果证实结构中存在氧连接物（和OH端基），应该影响FAT聚合物的性质。
Comparing the scanning electron microscopy (SEM) images between FAT-0 (g-C3N4) and FAT-1.0 (Figure 2a), it is observed that the former has a stacking layered structure while the latter shows a ribbon-like structure.48 This is consistent with the less crystallized structure inferred from XRD patterns and Raman spectra. The hierarchical network/ribbon like structure of FAT-1.0 samples is also different from the previously reported polymer due to a new synthetic protocol.27 The distortions in FAT samples allow more n–π* transitions from the edge N atoms to the CB, which might also aid the visible photon absorption.24 The fiber-like structure of FAT-1.0, instead of the packing plane-like FAT-0, should possess a higher surface area and a better contact with water.49
将FAT-0（g-CN）和FAT-1.0（图2a）的扫描电子显微镜（SEM）图像进行比较，发现前者具有层状堆积结构，而后者显示为带状结构。这与XRD图样和拉曼光谱推断的结晶较少的结构一致。FAT-1.0样品中的等级网络/带状结构也与先前报道的聚合物不同，这归因于一种新的合成协议。FAT样品中的扭曲让边缘N原子到CB的n-π*跃迁更多，这也可能有助于可见光子吸收。与FAT-0的堆积平面不同，FAT-1.0的纤维状结构应该具有更大的表面积并与水接触更好。
a) FAT-1.0和FAT-0（插图中的g-C b0 ）的SEM图像。b) UV-vis光谱。c) FAT样品的带间隙对齐（V/NHE，pH 0）（顶部：彩色照片）。d) FAT样品的N含量，O含量和可见光辐照下（420 nm < λ < 710 nm）的氢进化率（HER）。
In order to investigate the influence of surface area, Brunauer–Emmett–Teller (BET) surface area measurements were carried out (Table 1). Consistent with the differences in SEM images, the surface area continuously increases from 5.5 to 40 m2g−1 from FAT-0 to FAT-2.0 samples, probably due to the releasing of oxygen-containing species during the polymerization of oxygen precursors, similar to the thermal exfoliation of graphene oxide.50-52 Therefore, the characterizations of PXRD, Raman, SEM, FT-IR NMR, and XPS have confirmed the proposed framework of FAT polymers composed of distorted oxygen and nitrogen colinked heptazine units.
为了调查表面积的影响，进行了Brunauer–Emmett–Teller（BET）比表面积测量（表1）。与SEM图像的差异一致，从FAT-0到FAT-2.0样品，表面积持续增加，从5.5增加至40 m²/g，可能是由于氧前体聚合物化过程中氧含物种的释放，类似于氧前体热剥离氧化石墨烯。因此，PXRD、拉曼、SEM、FT-IR、NMR和XPS的表征已证实了由畸变的氧和氮共连的七元环单元组成的FAT聚合物的拟定框架。
An apparent visual difference among FAT-0 (g-C3N4) to FAT-1.0 samples is the color changes step-by-step from pale yellow to dark brown (Figure 2b and photos in Figure S6a, Supporting Information). As mentioned above, the bandgap engineering of photocatalysts could directly influence the harvest of visible photons, the driving force as well as the charge transfer behavior. The bandgaps11 of FAT samples were determined as a Tauc plot calculated from UV–vis spectra (Figure 2b and Figure S6a, Supporting Information). Such a stepwise bandgap change from 2.7 eV (FAT-0) to final 1.9 eV (FAT-1.0) is clearly due to increasing oxygen amount in the polymers (Table 1, Figure 2c). A gap between the absorption curves of FAT-0.5 and FAT-0.8 divides the samples into two groups. Further increasing the amount of acid in the precursor does not result in a narrower bandgap. On the contrary, the bandgaps of FAT-1.5 and 2.0 are slightly wider than FAT-1.0, which might be due to the excess amount of introduced O atoms.
FAT-0（g-C 3 N 4 ）到FAT-1.0样品之间显著的视觉差异是颜色从浅黄色逐步变为深棕色（图2b和支撑信息中的照片，见图S6a）。如上所述，光催化剂的带隙工程可以直接影响可见光子的吸收、驱动力以及电荷转移行为。FAT样品的带隙11是通过UV-Vis光谱的Tauc图确定的（图2b和图S6a，见支撑信息）。这种从2.7电子伏特（FAT-0）到最终1.9电子伏特（FAT-1.0）的逐步带隙变化显然是由于聚合物中氧含量的增加。FAT-0.5和FAT-0.8之间的吸收曲线之间存在差距将样品分成两组。进一步增加前体中酸的量并不会导致更窄的带隙。相反，FAT-1.5和2.0的带隙略宽于FAT-1.0，这可能是由于引入的氧原子过量导致的。
To investigate the band positions, XPS valence band spectra (Figure S6b, Supporting Information), Mott-Schottky plots (Figure S6c,d, Supporting Information) along with the Tauc plot (Figure S6a, Supporting Information) were used to determine the band alignment as shown in Figure 2c and Table S2 (Supporting Information). With the VB of FAT samples slightly changing, the narrow bandgaps of FAT samples mainly result from the down shift of CB. From FAT-0 to FAT-0.5, the CB position only moves moderately. From FAT-0.8 to FAT-2.0, the CB apparently moves downward (more positive) but is still sufficient to drive proton reduction, attributed to the cooperation of O-linked chains. While O-linked chains create defects and distortions in the low concentration polymers (FAT-0 to FAT-0.5), which allows n–π* transition, the CB shift is more distinct in samples with more O-linked chains (FAT-0.8 to FAT-2.0).
为了研究带位置，使用XPS价带光谱（图S6b，支持信息），莫特-舒特基图（图S6c，d，支持信息）以及Tauc图（图S6a，支持信息）来确定带对齐，如图2c和表S2所示（支持信息）。FAT样品的VB轻微变化，FAT样品的窄禁带主要来自CB的向下移动。从FAT-0到FAT-0.5，CB位置只有轻微移动。从FAT-0.8到FAT-2.0，CB明显向下移动（更正），但仍足以促使质子还原，归因于O-链的协同作用。虽然O-链在低浓度聚合物（FAT-0到FAT-0.5）中创建缺陷和畸变，允许n-π*转变，但在具有更多O-链的样品（FAT-0.8到FAT-2.0）中，CB的移动更加明显。
2.3 The Evaluation of Photocatalytic Performance
2.3 光催化性能评估
After successful synthesis of bandgap tunable FAT polymers and illustration of their framework and electronic structures, the photocatalytic capabilities of FAT samples were fully evaluated. The H2 evolution rates (HERs) were measured in the presence of 3 wt% Pt co-catalyst (Figure S6e, Supporting Information) and 10% TEOA solution under 1 bar pressure and the visible irradiation (420 nm < λ < 710 nm) of 300 W light source (Newport-66485-300XF-R1, ≈100 mW cm−2). No activity was detected in the absence of light, photocatalysts, or electron donor. As the amount of nitrogen decrease and that of oxygen increases due to an increased amount of formic acid in the precursors, the HERs show a volcano-like trend peaking at FAT-1.0 (Figure 2d). From FAT-0 (g-C3N4) to FAT-1.0, the HER gains an 18-fold increase from 44 to 772 µmol g−1 h−1 and then decreases from FAT-1.0 to FAT-2.0. Such dramatically enhanced performance of FAT-1.0 is believed to be related to the narrowest bandgap, which utilizes maximum visible photons and the HER trend supports such relationship (Figure 2c,d). A further excess amount of formic acid probably shields the amine groups in precursors too much and results in the low degree of polymerization (e.g., FAT-1.5 and FAT-2.0 samples), thus slightly decreasing the activity due to defects as recombination centers.18 The HER of FAT-0 to FAT-0.5 only increases moderately but exhibits a sharper increment after FAT-8.0, indicating a good correlation between the bandgap and photocatalytic activity.24 The synthesized FAT polymers also maintain a highly reproducible activity during the 30 h run (or seven cycles, Figure 3a), proving the incorporation of O at the linker/terminal positions instead of into heptazine units does not affect the stability. The apparent quantum yield (AQY) measured on the optimum FAT-1.0 sample was determined to be 8.6% at 420 nm and 2.5% at 500 nm and it also shows activity at 600 and 700 nm, following the same profile of the UV–vis absorption spectrum and indicating a light driven reaction (Figure 3b). As FAT-0 does not work beyond 460 nm, the superior activity of FAT-1.0 in the whole visible region is again attributed to the narrowed bandgap.
成功合成带隙可调FAT聚合物并解释其框架和电子结构后，完全评估了FAT样品的光催化能力。在有3%重量Pt共催化剂（图S6e，支持信息）和10%TEOA溶液存在、1个大气压下和可见光照射（420 nm<λ<710 nm）的300瓦光源（纽波特-66485-300XF-R1，≈100 mW cm −2 ）下测量了H 2 的产率（HER）。在没有光、光催化剂或电子供体的情况下未检测到活性。由于氮含量降低、氧含量增加，原料中甲酸的增加，HER显示出类似火山的趋势，在FAT-1.0达到峰值（图2d）。从FAT-0（g-C 3 N 4 ）到FAT-1.0，HER从44增加到772 μmol g −1 h −1 ，然后从FAT-1.0降至FAT-2.0。FAT-1.0的性能显著提升被认为与最窄的带隙有关，利用了最大可见光子，HER趋势支持了这种关系。进一步过量的甲酸可能会过多地掩盖原料中的胺基团而导致聚合度低（例如FAT-1.5和FAT-2.0样品），因此由于缺陷作为复合中心，活性略微降低。从FAT-0到FAT-0.5的HER仅略有增加，但在FAT-8.0后出现更快的增长，表明带隙与光催活性之间存在良好的关联。 24 合成的FAT聚合物在30小时的运行中(或者七个循环，见图3a)也能保持高度可重复的活性，证明在连接剂/末端位置而非七呋喃烯单元中进行氧的掺杂并不影响稳定性。在最佳FAT-1.0样品上测得的视量子产额(AQY)分别为420nm处为8.6%，500nm处为2.5%，并且也在600nm和700nm处显示活性，遵循紫外-可见吸收光谱的相同图谱并指示为光驱动反应(见图3b)。由于FAT-0在460nm以后无法工作，FAT-1.0在整个可见区域的卓越活性再次归因于缩小的带隙。
a) 进行为期30小时的氢气析出率（HER）稳定性测试（420 nm < λ < 710 nm） b) 以10% TEOA作为空穴捕获剂，在Pt-FAT-1.0样品上在不同波长下进行H的明显量子产率（AQY）测量。 c) 进行为期25小时的氧气析出率（OER）稳定性测试（420 nm < λ < 710 nm） d) 以5×10^(-3) m NaIO作为电子捕获剂，在PtO-FAT-1.0样品上在不同波长下进行O的明显量子产率（AQY）测量。
As N-linked heptazine polymers (FAT-0 or g-C3N4) have been also reported to achieve water oxidation although its activity is very moderate,11 we also examined whether the highly active FAT samples would show an enhanced performance of the water oxidation reaction. Surprisingly, the PtOx-FAT-1.0 exhibited OER of 145 µmol g−1 h−1, which is 19 times higher than that of PtOx-FAT-0 (7.5 µmol g−1 h−1) (Figure 3c) under 5 × 10−3 m IO3− aqueous conditions (Figure 3c and Figure S6f, Supporting Information) under ambient conditions and visible light irradiation (420 nm < λ < 710 nm). The electron acceptor used here is IO3− because the commonly used Ag+ would shield the surface of a photocatalyst hence its activity cannot be continuously tested.53 In Figure 3d, the long period stability also validates that the oxygen is produced from water. Moreover, the oxygen content contained in 20 mg FAT-1.0 is 14.4 µmol according to the elemental analysis (Table 1), while the total amount of oxygen produced during the three cycles reached 57.5 µmol (Figure 3d), which is much more than the oxygen amount contained in materials, thus clearly demonstrating that the oxygen production is derived from water oxidation.11 FAT-1.0 also displays a high apparent quantum yield (AQY) of 4.3% and nearly 1.0% at 420 and 500 nm for water oxidation (Figure 3d), respectively, surpassing the previous OER on g-C3N4 (1.1% and 0 at 420 and 500 nm in 10 × 10−3 m Ag+).53 The superior performance for both H2evolution and O2 evolution on the bandgap tunable FAT samples makes them promising candidates for the construction of Z-scheme water splitting.
作为一种N-连接的七哌唑聚合物（FAT-0或g-C 3 N 4 ）也被报道能够实现水氧化反应，尽管其活性非常适中，我们也研究了高活性的FAT样品是否会显示出增强的水氧化反应性能。令人惊讶的是，PtO x -FAT-1.0的OER为145µmol g −1 h −1 ，比PtO x -FAT-0（7.5µmol g −1 h −1 ）高出19倍（图3c），在5×10 −3 m IO 3 − 水溶液条件下（图3c和支撑信息的图S6f）在常温和可见光照射（420 nm < λ < 710 nm）条件下。这里使用的电子受体是IO 3 − ，因为常用的Ag + 会遮挡光催化剂的表面，因此其活性无法进行连续测试。在图3d中，长时间稳定性也验证了氧气是从水中产生的。此外，根据元素分析，20毫克的FAT-1.0含有14.4µmol的氧（表1），而在三个周期内产生的氧总量达到57.5µmol（图3d），远远超过材料中所含氧气的数量，从而清楚地证明氧气的产生来源于水氧化。FAT-1.0在水氧化方面显示出较高的表观量子产率（AQY），分别为420和500 nm处的4.3%和接近1.0%（图3d），超过了以前关于g-C 3 N 4 的OER（在10×10 −3 m Ag + 时为1.1%和0）。 53消除带隙可调FAT样品对H 2 和O 2 的卓越性能使其成为构建Z-方案水分解的有前途的候选者。
2.4 The Origin of Superior Performance
2.4 优异性能的起源
To interpret the trend of enhanced activity as well as the structural change of FAT polymers, we refer to both experimental and theoretical approaches. Photoelectrochemical properties were investigated to compare the significant enhancement in photocatalytic performance on the FAT samples. As shown in Figure 4a, the photocurrent performance on the FAT-1.0 sample and FAT-0 shows a nearly 26 fold difference, consistent with the measured HER.54 Photoluminescence (PL) measurements were used to compare the charge separation capability on the FAT samples using a UV laser (325 nm, Figure 4b). The peak near 420 nm is assigned to emissions from band edges involving π-conjugated states. While the larger portion of recombination signals are those ≈500–600 nm corresponding to the intra states including n–π* transition and defect-based states.55, 56 From FAT-0 to FAT-2.0, PL peaks gradually decreases and the peak intensity trend reversely agrees with the HER on FAT polymers, indicating that a stepwise enhanced charge separation was obtained on the doped samples. The optimum sample FAT-1.0 shows about two magnitudes lower intensity than FAT-0. Moreover, with the bandedge peaks anchoring at 420 nm, the intra state peaks move from 500 to 600 nm as the amount of formic acid goes up, verifying the process of the band structure narrowing due to additional oxygen-linked chains.55 Therefore, less radiative electron–hole recombination and extensive light harvest ability due to narrow band states of FAT samples would promote their photoactivity. Other factors might influence the performance to some extent, such as surface area. However, FAT-1.0 (12.1 m2 g−1) instead of the samples with the larger surface area (FAT-1.5, 16.4 m2 g−1 and FAT-2.0, 40.0 m2 g−1), achieves the highest HER, so the enhancement in activity cannot be directly attributed to surface area.18 It should be noted that the PL signals slightly increase again in samples from FAT-1.0 to FAT-2.0. Similarly, the trend of structural characterizations and gas evolution measurements also peak at FAT-1.0 while more oxygen content does not contribute further to the enhancement.
为了解增强活性趋势以及FAT聚合物结构变化，我们参考实验和理论方法。研究了光电化学性质，比较了FAT样品的光催化性能显著提高。如图4a所示，FAT-1.0和FAT-0样品的光电流性能相差近26倍，与测得的HER一致。使用光致发光（PL）测量比较了FAT样品的电荷分离能力，使用紫外激光（325 nm，图4b）。420 nm附近的峰值被指派给包含π共轭状态的能带边缘发射。而大部分的复合信号是位于500-600 nm的内部态，对应于n-π*过渡和基于缺陷的态。从FAT-0到FAT-2.0，PL峰值逐渐减弱，峰值强度趋势与FAT聚合物上的HER相反，表明在掺杂样品上获得了逐步增强的电荷分离。最佳样品FAT-1.0的强度比FAT-0低大约两个数量级。此外，随着甲酸量增加，带边峰位于420 nm，内部态峰从500 nm移至600 nm，证实了由于额外的氧链的连接而导致的带结构变窄的过程。因此，FAT样品的窄带态会促进其光活性，减少相对较少的辐射电子-空穴复合，具有广泛的光收集能力。其他因素可能在一定程度上影响性能，如表面积。但是，FAT-1.0（12.1 m²/g）而不是具有较大表面积的样品（FAT-1.5，16.4 m²/g和FAT-2.0，40...Forty percent (wt. %)的FAT-0.5在HER方面得到最高的提高，因此活性的增加不能直接归因于表面积。值得注意的是，PL信号在FAT-1.0和FAT-2.0的样品中略有增加。类似地，结构表征和气体释放测量的趋势也在FAT-1.0处达到峰值，而更多的氧含量并不进一步促进提高。
a) FAT-0 和 FAT-1.0 电极在0.1 m NaSO中的周期性开/关光电流响应，与Ag/AgCl相比有0.6 V偏压。 b) FAT样品的光致发光光谱（由325 nm激光激发）。
Further explanation for this phenomenon comes from the results of computational work conducted on the system, which show a direct relationship between the incorporation of the oxygen within the framework and the electronic structure and hence the predicted performance of polymers. As previously showed, the oxygen content is incorporated in the framework outside the heptazine (tri-s-triazine) rings, replacing an NH group either by linking two rings or by terminating a chain (Figure S7, Table S3, Supporting Information). The models, shown in detail in Figure S7 (Supporting Information), are composed of four linear chains organized in two layers, with a total of eight tris-triazine rings. In g-C3N4, this configuration would correspond to 8 NH2 terminal groups and 8 NH linker groups in a unit cell. To model the high oxygen content FAT polymers (FAT-0.8 and over), 4 of those groups are substituted with either OH terminals or O linkers. This set up allowed us to investigate the different oxygen configurations that can be found locally within the polymer, which will be proved to have a major effect on the performance of the material.
进一步解释这一现象的原因源于对该体系进行的计算工作的结果，显示出氧在框架中的掺入与电子结构之间存在直接关系，从而影响聚合物的性能。之前的研究表明，氧含量被掺入到框架中，出现在三氮五环旁边，通过连接两个环或终止链来代替一个NH官能团（见图S7，表S3，支持信息）。详细显示于图S7中的模型（支持信息），由4条线性链组成，分为两层，共有8个三氮五环。在g-C 3 N 4 中，这种配置对应于在一个单元格中的8个NH 2 末端基团和8个NH连接基团。为了模拟高氧含量的FAT聚合物（FAT-0.8及以上），用OH末端基团或O连接基团替代了其中的4个基团。这样的设置允许我们研究在聚合物中局部可能发现的不同氧配置，这将被证明对材料性能产生重大影响。
The density functional theory (DFT) calculations show how the substitution of NH groups is favorable, as the formation energy goes from +13.05 to −10.33 kJ mol−1 for the modeled O-linked polymers. The stability increases even further when oxygen is incorporated as OH linkers (Table S4, Supporting Information), but the electronic effect of the two substitutions appears to be similar. Density of state (DOS) calculations have also been performed. The calculations present a smaller bandgap than experiment for g-C3N4 (see the Computational Methodology section for more detail), although this underestimation of the bandgap is expected. Interestingly, the calculated gaps for the FAT samples compare reasonably with experiment but in view of the known inadequacy of Perdew–Burke–Ernzerhof (PBE)–DFT in calculating bandgaps, no further use of calculated bandgaps is made in our analysis.
密度泛函理论（DFT）计算显示，NH基团的取代是有利的，因为对于建模的O链聚合物，形成能从+13.05降至-10.33 kJ/mol。当氧被合并为OH连接体时稳定性进一步增加（请参见表S4，支持信息），但两种取代物的电子效应似乎是相似的。也进行了态密度（DOS）计算。计算得到的能带隙比g-C N的实验值更小（有关更多详细信息，请参阅计算方法部分），尽管预期会低估能带隙。有趣的是，对于FAT样品，计算得到的能隙与实验相比较合理，但考虑到Perdew-Burke-Ernzerhof（PBE）-DFT计算能隙的已知不足，我们的分析中不再使用计算得到的能隙。
However, a subtle difference between the oxygen arrangements seems to drive the electronic structure, as shown by the decomposed charge density of the valence and conduction bands. According to the EA (Table 1) of FAT samples, the oxygen content increases and the nitrogen content declines as more formic acid is added in the precursor, corresponding to the gradual substitution of NH groups with oxygen in the polymer. The coexistence of N-containing chains and O-containing chains in the FAT polymers can drastically improve the charge separation due to the N-chains' electron donor nature and O-chains' electron acceptor nature: when the oxygen is distributed in a well ordered fashion—at least locally—with adjacent N-containing and O-containing chains, the difference in electronic behavior results in a spatial separation of the VB and CB, which appear to be located on the two chain types, respectively (Figure 5a). Such spatial separation might possibly arise from a change in the electrostatic potential of the O-containing cells due to the presence of polar covalent bonds, as shown in previous literature,57, 58 although our modeling could not find solid evidence due to the complexity of the problem and the polymeric nature of the system. However, this finding is in good agreement with experiment, as it relates the increase in performance up to FAT-1.0 to an increase in charge separation, as photoexcited electrons are gathering on O-containing chains and holes on N-containing ones, therefore slowing down the charge recombination and improving the performance. However, little control can be achieved in how the oxygen is arranged, and a more uniform distribution of oxygen among the chains improves the charge separation only marginally with respect to pristine g-C3N4 (Figure 5b,c).
然而，在氧原子排列之间存在微妙的区别似乎可以推动电子结构，如导带和传导带的分解电荷密度所示。根据FAT样品的EA（表1）, 随着前体中甲酸的增加，氧含量增加，氮含量下降，对应于聚合物中NH基团逐渐被氧替代的过程。FAT聚合物中含N链和含氧链的共存可以显著提高电荷分离，因为N链具有电子给体的性质，而O链具有电子受体的性质：当氧以一定有序的方式分布时——至少在局部区域内——相邻的N含量链和O含量链，电子行为的不同导致价带和导带的空间分离，它们分布在两种链类型上（图5a）。这种空间分离可能源自由于存在极性共价键而导致的O含量细胞的静电势变化, 如先前文献中所示，尽管我们的建模由于问题的复杂性和系统的聚合性质，无法找到实证证据。然而，这一发现与实验结果相一致，因为它将FAT-1.0的性能提升与电荷分离的增加联系起来，光激发的电子集中在O含量链上，空穴集中在N含量链上，从而减慢电荷复合速度并提高性能。 然而，在氧的排列方面，很难实现较好的控制，相对于原始的g-C b0 N b1，让氧在链条之间更加均匀地分布只能稍微提高电荷分离效果（图5b、c）。
It is therefore reasonable to propose that the performance trend as the amount of formic acid in the precursor increases depends on the local distribution as well as on the amount of oxygen introduced in the g-C3N4 chains: while a reasonable amount of formic acid will benefit the performances of the polymer by reducing the bandgap and, in some areas of the polymer, create a spatial separation between the VB and the CB that improves the e−–h+ lifetime, a concentration of O-linkers and OH terminals that is too high would lower the probability of achieving this separation, leading to the decreased performances of FAT-1.5 and FAT-2.0. Therefore, a peak in performance is to be expected, as the control of the arrangement of oxygen within the framework is very difficult.
因此，可以合理地提议，随着前体中甲酸含量的增加，性能趋势取决于局部分布以及引入的氧气量。适量的甲酸会通过降低带隙、在某些区域内创建VB和CB之间的空间分离，从而改善e-h寿命，有益于聚合物的性能；而O连接剂和OH端基的浓度过高则会降低实现这种分离的概率，导致FAT-1.5和FAT-2.0的性能下降。因此，预计会出现性能峰值，因为在框架内控制氧的排列方式非常困难。
3 Conclusion 3 结论
In summary, a novel strategy has been successfully developed to synthesize bandgap tunable, highly efficient, and robust organic semiconductor photocatalysts with enlarged optical window and suppressed charge recombination, thus addressing the two major challenges in photocatalysis. By controlling the polymerization process, the framework and electronic structure could be carefully tailored via incorporation of oxygen and nitrogen linkers as well as terminals between heptazine units. Moreover, the optimum FAT-1.0 sample exhibits 18 times higher H2 evolution activity than reference g-C3N4 under visible light, resulting in high AQY of 8.3% and 2.5% at 420 and 500 nm. Surprisingly, it achieves 19 fold enhancement in O2 evolution and exhibits AQY of 4.3% and about 1.0% at 420 and 500 nm. The excellent performance and band structure flexibility make the FAT polymers a group of promising semiconductors for potential applications including the construction of Z-scheme water splitting and photoelectrodes. Experimental and theoretical results have rationalized the observations in terms of narrowed bandgaps and enhanced charge separation, which are due to the oxygen incorporation into the linker/terminal position and reasonably higher amount of oxygen, narrower bandgap, leading to higher activity. Further increasing the oxygen content will result into bandgap increase. These findings pave a new approach to modifying the properties of polymers for efficient solar energy conversion through careful control of the polymerization process.
总结来看，已成功开发了一种新型策略，能够合成带隙可调、高效且稳定的有机半导体光催化剂，拓展了光学窗口并抑制了电荷复合，从而解决了光催化中的两大挑战。通过控制聚合过程，可以通过在氮氧化合物和未定型氧基之间的六氮氮异三聚氰胺基团中的掺入精心调节骨架和电子结构。此外，最优FAT-1.0样品在可见光下的H＜b0＞发展活性比参考g-C＜b1＞N＜b2＞高出18倍，导致在420和500 nm处高AQY为8.3％和2.5％。令人惊讶的是，它实现了19倍的O＜b3＞发展，并在420和500 nm处表现出4.3％和约1.0％的AQY。出色的性能和带结构灵活性使FAT聚合物成为一个有潜力的半导体群体，可用于构建Z方案水分解和光电极。实验和理论结果已经用较窄的带隙和增强的电荷分离来合理解释这些现象，这是由于氧在连接剂/末端位置的掺入以及合理更高数量的氧，带宽更窄，导致活性更高。进一步增加氧含量将导致能隙增加。这些发现为通过精心控制聚合过程修改聚合物性能以实现高效太阳能转换铺平了道路。
4 Experimental Section 4 实验部分
Materials Preparation: In a typical formic acid treated polymer (denote FAT polymer) synthesis, 2 g (23.8 mmol) dicyandiamide (DCDA) (Fisher Scientific Ltd.) was first dissolved in 40 mL deionized (DI) water under strong agitation at 25 °C. Then a certain amount of formic acid (Sigma-Aldrich) (e.g., stoichiometry ratio of 0, 0.1, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0 formic acid to DCDA) was added into the DCDA solution and the solution was kept at 130 °C for 6 h before drying overnight under violent stirring. The resultant white crystalline precursor was transferred into a lidded high-quality alumina crucible, then placed inside a muffle furnace and heated at a ramp rate of 2 °C min−1, and finally held at 550 °C for 4 h unless otherwise noted. The products were denoted as FAT-0, FAT-0.1, FAT-0.2, FAT-0.5, FAT-0.8, FAT-1.0, FAT-1.5, and FAT-2.0 with respect to the amount of formic acid used. FAT-0 is typical DCDA-derived g-C3N4. DI water, 0.1 m NaOH (Sigma-Aldrich) and HCl (Sigma-Aldrich) were used to wash the produced powders adequately to remove all unreacted and potentially detrimental surface species. The FAT-based electrodes were prepared as below: 50 mg FAT polymer powder was added into a solution composed of 742.5 µL H2O, 247.5 µL 2-propanol (Fisher Scientific Ltd.) and 10 µL Nafion (Fisher Scientific Ltd.) and the mixture was sonicated for 15 min. 100 µL of the resultant suspension was slowly dropped on an FTO glass. After drying under ambient conditions at 70 °C, it was calcined at 400 °C for 1 h in a muffle furnace.
材料制备：在典型的甲酸处理聚合物（缩写为FAT聚合物）合成中，首先将2克（23.8毫摩尔）的二氰胍（DCDA）（Fisher Scientific Ltd.）溶解在40毫升去离子水中，在25°C强烈搅拌下。然后将一定量的甲酸（Sigma-Aldrich）（例如，摩尔比0、0.1、0.2、0.5、0.8、1.0、1.5和2.0甲酸与DCDA）加入DCDA溶液中，并在130°C下保持6小时，在剧烈搅拌下过夜干燥。得到的白色结晶前驱体转移至带盖的高质量氧化铝坩埚中，然后放入隧道炉中，在2°C/min的升温速率下加热，并在550°C下保持4小时，除非另有说明。根据使用的甲酸量，将产物标记为FAT-0、FAT-0.1、FAT-0.2、FAT-0.5、FAT-0.8、FAT-1.0、FAT-1.5和FAT-2.0。FAT-0是典型的DCDA衍生g-C3N4。使用去离子水、0.1M NaOH（Sigma-Aldrich）和盐酸（Sigma-Aldrich）充分清洗产生的粉末，以去除所有未反应和潜在有害的表面物种。根据以下步骤准备FAT基电极：将50毫克FAT聚合物粉末添加到由742.5微升H2O、247.5微升异丙醇（Fisher Scientific Ltd.）和10微升Nafion（Fisher Scientific Ltd.）组成的溶液中，并超声处理15分钟。将100微升的混悬液缓慢滴在FTO玻璃上。在70°C的常温下干燥后，将其在隧道炉中在400°C下焙烧1小时。
Material Characterization: Powder x-ray diffraction (PXRD) measurements were taken using a StadiP diffractometer from Stoe company, a voltage of 40 kV, at 30 mA, using a Cu source with Kα1 = 1.540562 Å and Kα2 = 1.544398 Å. (Company: Stoe. Diffractometer: StadiP. Cu X-ray tube run at 40 kV 30 mA Capillary transmission geometry. Presample Ge (111) monochromator selects K alpha 1 only. Sample rotated in the beam. Dectris “Mythen 1k” silicon strip detector covering 18° 2θ.) Diffuse reflectance spectra were obtained on a Shimadzu UV–Vis 2550 spectrophotometer fitted with an integrating sphere. A standard barium sulfate powder was used as a reference. Absorption spectra were calculated from the reflection measurements via the Kubelka–Munk transformation. ATR-FTIR spectroscopy was collected using a Perkin-Elmer 1605 FT-IR spectrometer in the wavenumber range 500–4000 cm−1 with a resolution of 0.5 cm−1. Raman spectroscopic measurements were performed on a Renishaw InVia Raman Microscope, using a 325 nm excitation laser and a wavenumber range 100–2000 cm−1. Scanning electron microscopy (SEM) images were gained from a JEOL JSM-7401F high-resolution Field Emission SEM operating at 2–3 kV. Due to the low conductivity of the semiconductor materials, an Au coating was sputtered onto the samples to improve the image quality. Specific surface area measurements were taken using the BET method (N2 absorption, TriStar 3000, Micromeritics). XPS measurements were obtained on a Thermoscientific XPS K-alpha surface analysis machine using an Al source. The results of etched samples were carried out on the same XPS equipment. The XPS analysis was performed using CasaXPS software.
材料表征：采用Stoe公司的StadiP衍射仪进行粉末X射线衍射（PXRD）测量，电压40 kV，电流30 mA，使用铜源，其Kα1 = 1.540562 Å，Kα2 = 1.544398 Å。采用Shimadzu UV-Vis 2550分光光度计配备积分球进行漫反射光谱测量。采用Kubelka-Munk变换从反射测量中计算吸收光谱。采用Perkin-Elmer 1605 FT-IR光谱仪进行ATR-FTIR光谱测量，波数范围为500–4000 cm⁻¹，分辨率为0.5 cm⁻¹。Raman光谱测量使用325 nm激光，波数范围为100–2000 cm⁻¹，在Renishaw InVia Raman显微镜上完成。扫描电子显微镜（SEM）图像采用2–3 kV的JEOL JSM-7401F高分辨率场发射SEM获得。若样品的低传导率，需在样品表面溅射Au涂层以提高图像质量。BET比表面积测量采用氮气吸附法（N₂吸附，TriStar 3000，Micromeritics）。XPS测量利用Thermoscientific XPS K-alpha表面分析仪，采用铝源。刻蚀样品的结果也是利用同一XPS设备获得的。XPS分析采用CasaXPS软件进行。
光催化分析：在每次光催化分析前，需要进行光催化剂上的辅助剂的光沉积。将一定量的光催化剂与3 wt% 辅助剂（使用H\u2082PtCl\u42前体光沉积）分散在130毫升反应器中的水溶液中，添加适当的电子受体（NaIO\u42a）或给体（TEOA）后，密封反应器并使用氩气排空30分钟，然后使用300 W的氙光源（Newport 66485-300XF-R1）进行照射。在光沉积过程中，定期测量以确定氢气或氧气是否以稳定速率产生，确保光沉积顺利进行。工作条件下的光强度调整为约100 mW/cm²，使用420 nm长通和710 nm短通滤光片（420 nm<λ<710 nm）来排除红外线以避免热效应。在典型的H₂发生反应中，将50毫克含有3 wt% Pt辅助剂的光催化剂均匀分散在含有10 vol% TEOA作为电子给体的50毫升水溶液中，在130毫升顶部照射反应器中。在典型的O₂发生反应中，将20毫克带有3 wt% PdO辅助剂的光催化剂均匀分散在5×10⁻³ M NaIO₂水溶液中作为电子受体，放入130毫升顶部照射反应器中。这些反应条件已经过优化，包括粉末光催化剂、辅助剂和溶液的数量。量子产率测量时使用了100毫克的FAT-1.0粉末。最终结果来自五次试验计算出的平均活性。表观量子产率（AQY）（Φ）通过以下公式计算：
The light intensity measurements were taken by an optical power meter (Newport 1918-R) with an appropriate band pass filter (420, 500, 600, 700, 800 nm, λ ± 10 nm at 10% of peak height, Comar Optics) inserted between a 300 W Xe light source (Newport 66485-300XF-R1) and reactor.
通过一台光功率计（Newport 1918-R）进行光强度测量，其间插入一个合适的带通滤波器（420、500、600、700、800 nm，波长 ± 10 nm，在峰值高度的10%处，Comar Optics），该滤波器位于一个300瓦氙灯光源（Newport 66485-300XF-R1）和反应器之间。
Computational Methodology: All calculations have been performed within the DFT framework as implemented in the VASP (Vienna Ab-initio Software Package) code.59 The electronic energy was obtained using PAW (projected augmented wave) potentials60 and plane-wave basis set, respectively, for core and valence electrons, using the Perdew–Burke–Ernzerhof (PBE) functional.61 Corrections were added to account for long-range interactions by semiempirical Grimme D3 dispersion method62 and for nonspherical contributions to the PAW potentials as natively built in the code. All energies are converged within a cutoff of 520 eV and an electronic self-consistent field (SCF) threshold of 10−5 eV. Convergence was determined using the tetrahedron method, implementing Blochl corrected smearing63 and in all cases spin polarization was disabled. The reciprocal lattice matrix was generated using a 5 × 5 × 5 K-points grid, using the Monkhorst-Pack method. The minimum energy structures were found using a built-in DIIS algorithm with a convergence force threshold of 10−3 eV Å−1.
计算方法学：所有计算都是在VASP（Vienna Ab-initio Software Package）代码中实现的DFT框架内完成的。电子能量是使用PAW（投影增强波）电位和平面波基组获得的，分别用于核心和价电子，使用Perdew–Burke–Ernzerhof（PBE）泛函。通过半经验Grimme D3色散方法对长程相互作用进行修正，对PAW电位的非球形贡献使用代码内置的修正。所有能量在截断为520电子伏的截止能级和10电子伏的电子自洽场（SCF）阈值内收敛。通过实现Blochl修正的斑点法确定收敛，所有情况下都禁用自旋极化。倒易晶格矩阵是使用5 × 5 × 5 K点网格生成的，采用Monkhorst-Pack方法。采用内置的DIIS算法找到了最低能量结构，收敛力阈值为10电子伏/埃。
The bandgap for the nondoped system of is calculated as 1.01 eV, which underestimated the experimental value. Such results are to be expected when using a GGA level of theory such as the one employed by the PBE functional54 and are consistent with previous literature.55 Our analysis does not therefore make use of band structure but focuses on the charge distribution, which is expected to be reliable at this level of theory.
对于非掺杂系统，带隙计算得到的数值为1.01电子伏，低估了实验值。这样的结果在使用类似PBE泛函这样的GGA理论水平时是可以预期的，并且与以前的文献一致。因此，我们的分析并没有使用带结构，而是侧重于电荷分布，在这个理论水平上是可靠的。
The unit cell for g-C3N4 was derived from previous literature64 through the Inorganic Crystal Structure Database. Each unit cell is organized in two planar layers accounting for a total of four linear organic chains, each composed of 2 tri-s-triazine units, 2 linker groups, and 2 terminal groups. In the g-C3N4 (FAT-0) model, all 16 groups contain nitrogen, being modeled as NH for linkers and NH2 for terminals. In the oxygen-doped (FAT) models, four such groups are modified to introduce oxygen, replacing the previous linker and terminal groups with O and OH, respectively. In each of the different FAT models different groups are modified, in order to highlight the effect of local oxygen arrangement on the electronic properties of the material.
g-C 3 N 4 的晶胞是从以前的文献64中通过无机晶体结构数据库推导出的。每个晶胞组织成两个平面层，总共有四条线状有机链，每条链由2个三嗪单位、2个连接基团和2个末端基团组成。在g-C 3 N 4 （FAT-0）模型中，所有16个基团都含有氮，连接基团和末端基团被建模为NH和NH 2 。在掺氧（FAT）模型中，四个这样的基团被修改以引入氧，新的连接基团和末端基团分别被替换为O和OH。在不同的FAT模型中，不同的基团被修改，以突出局部氧排列对材料电子性能的影响。
The definition of formation energy contributes to our understanding the energy change between graphitic carbon nitride and its O-modified homologues: being a difference between the energy of the cell and that of its components, it is largely influenced by the stability of the reference systems. In particular, the N2 bond is much stronger than the CN bond in the polymer, resulting in positive formation energy for g-C3N4. On the contrary, the COC and COH bonds in the FAT models appear to correspond to a deeper energy well than the O2 reference molecule, balancing the effect of N and giving a slightly negative (up to −0.11 eV per atom) formation energy to the FAT models. Ultimately, the formation energy is not a measure of the stability of the polymers relative to each other, but a measure relative to the elemental components of a system. However, it is the only way to compare the stabilities of unit cells comprising different numbers of atoms and gives a sensible reference point for the relative stabilities of the FAT polymers, which are shown to be very similar due to the identical bonds they are comprised of, but still dependent on the O substitution due to interchain H-bonds.
形成能的定义有助于我们理解石墨烯状碳氮和其氧改性同系物之间能量变化：它是晶胞能量和构成元素能量之间的差异，很大程度上受参考系统稳定性的影响。特别是在聚合物中，N键比CN键强得多，导致了石墨烯状C 3 N 4 的正形成能。相反，在FAT模型中的COC和COH键似乎对应于比O参考分子更深的能量井，平衡了N的影响，并使FAT模型的形成能略微为负（每原子最多为 -0.11 eV）。最终，形成能不是相对于相互之间聚合物稳定性的度量标准，而是相对于系统的基本组件的度量标准。然而，它是比较包含不同数量原子的晶胞稳定性的唯一方法，并为FAT聚合物的相对稳定性提供了一个明智的参考点，因为它们由相同的键组成，但仍受到氧取代的影响，因为其中链间氢键。%%每个晶胞的形成能已通过下列公式计算：晶格能量与化合物元素能量之差除以原子数。
在这里，E是聚合物晶胞的能量，E和n分别代表聚合物中每种元素原子的能量以及该元素在晶胞中的原子数，N是晶胞中的总原子数，对于g-CN而言为144，对于所有FAT模型为140。考虑的元素状态是石墨对C，以及其在真空中的二元分子对H、N和O。它们的能量是在适当的单元胞内使用相同的参数计算的，这些参数与上述聚合物晶胞的参数相同。
Acknowledgements 致谢
Y.W., Q.R., and Y.L. thank the CSC for Ph.D. funding. M.K.B and J.T. thank the Leverhulme Trust (RPG-2012-582). R.C. and J.T. acknowledge financial support from EPSRC (EP/N009533/1). J.T. thanks the Leverhulme Trust (RPG-2017-122) and Royal Society-Newton Advanced Fellowship grants (NA170422). F.S. thanks Cardiff University School of Chemistry for a fully funded PhD scholarship. Computing facilities for this work were provided by ARCCA at Cardiff University, HPC Wales, and through our membership of the UK's Materials Chemistry Consortium (MCC). The MCC is funded by EPSRC (EP/F067496). The authors are grateful to Dr. Matthew Quesne and Dr. Alberto Roldan for helpful advice and discussions.
Y.W.、Q.R.和Y.L.感谢CSC提供的博士资助。M.K.B和J.T.感谢Leverhulme Trust（RPG-2012-582）。R.C.和J.T.感谢EPSRC（EP/N009533/1）的财政支持。J.T.感谢Leverhulme Trust（RPG-2017-122）和皇家学会-牛顿高级研究奖资助（NA170422）。F.S.感谢卡迪夫大学化学学院提供全额资助的博士奖学金。本研究的计算设施由卡迪夫大学ARCCA、HPC Wales和我们作为英国材料化学联盟（MCC）成员提供。MCC由EPSRC（EP/F067496）资助。作者感谢Matthew Quesne博士和Alberto Roldan博士提供的有益建议和讨论。
Conflict of Interest 利益冲突
The authors declare no conflict of interest.
作者声明没有利益冲突。
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