Regular ArticleIn-situ solid phase thermal transformation of self-assembled melamine phosphotungstates produce efficient visible light photocatalysts
Abstract
Visible-light-driven stacked-layer heterogeneous photocatalyst carbonitride/tungstophosphate (TCN) was constructed via in-situ solid-state thermal transformation using melamine phosphotungstate (MPW). The structural, morphological and optical properties of the samples were investigated. Compared to the MPW hybrids and phosphotungstic acid hydrate, the TCN photocatalysts showed excellent visible light photocatalytic activity. During the thermal transformation, the melamine molecules polymerize to form the defective heptazine structure carbonitride attached to the surface of mixed-valence Keggin units. The interfacial POMs anions-π interactions, ligand-to-metal charge transfer and mix-valence organic-POMs structure makes the electrons fully delocalized over the MPW hybrids, and the TCN photocatalysts obtain the extended light absorption. The Keggin units accept and transfer electrons, so the recombination of photogenerated carriers is suppressed. 13TCN-390 obtains the optimal photocatalytic activity, its photocatalytic degradation efficiency of imidacloprid and rate constant k are 6.38 and 13.50 times than that of CN-390, respectively. The enhanced photocatalytic activity arises from the extended light absorption, suppressed photogenerated carriers’ recombination, surface structure defect and suitable band structure. h+ and 
OH are the main reactive species when the proposed photocatalytic mechanism was done. This study provides a promising construction strategy for polymer/POMs photocatalysts using different organic-POMs hybrids.
Introduction
Polyoxometallates (POMs) are transitional metal oxide clusters with various structures [1], [2]. POMs exhibit unique redox capacity, acid-base and electronic accept-transfer properties, these characteristics make them widely used in the fields of catalysis [3], [4], medicine [5], electronic and magnetics [2], and so on [1], [2], [3], [4], [5], [6], [7]. POMs also have similar properties to semiconductors [3], [4]. For instance in polyoxotungstates, the electrons will transfer from the O2p-based highest occupied molecular orbital (HOMO) to the W5d-based lowest unoccupied molecular orbital (LUMO) under the ultraviolet light irradiation, then the photoexcited e−–h+ will form and transfer to the surface of photocatalysts to participate in the redox reaction [4]. However, because of the solubility limitations of POMs, they are often used as homogeneous photocatalysts. Therefore, a variety of methods have been used to develop heterogeneous POMs-based photocatalysts. For example, the composite photocatalysts are prepared using zeolite [8], metal-organic frameworks (MOFs) [9], [10], TiO2 [11], [12], [13], carbon Nanodots [14], and C3N4 [15], [16], [17], etc.[18]. There are also many POMs-based photocatalysts which can be obtained by constructing POMs-based MOFs [19], [20], [21], self-assembly organic-POMs hybrids [22], [23], [24], [25], [26], insoluble POMs-based metal salts [27], [28], [29], POMs based on Fe [30], Co [31], Mn [27], [32], and so on [33]. As a result of the limitation from the band structure of POMs, most of these photocatalysts can only be evaluated under ultraviolet light or simulated solar light [3], [4], [34], and the research tasks of visible-light-driven POMs-based photocatalyst are still arduous.
Due to their unique electronic and band structure, self-assembly organic-POMs hybrids are getting more attention. There are numerous self-assembly organic-POMs hybrids synthesized using choline [22], diethyl sulfide [23], alkylamine [24], cationic peptide [25], N-containing and pyridinecarboxylate ligands [26], etc. It is worth noting that organic-POMs hybrids have heterogeneous photocatalytic properties and potential for post-treatment. The organic ligands and POMs anions will construct new crystalline, electronic and band structure through a covalent bond or non-covalent bond [7]. The exposed groups from the organic ligands can provide extensive development space for the post-treatment of these hybrids, and these groups can be used to construct polymeric semiconductor by polymerization reaction and heat treatment. Unfortunately, this strategy was ignored by majority of researchers, who focused on the synthesis of new organic-POMs hybrids using various ligands and POMs anions. Recently, our group reported a novel visible-light-driven phosphotungstate/polyimide photocatalyst constructed by the post-functionalization of melamine phosphotungstate (MPW) hybrids based on the melamine and phosphotungstic acid hydrate [6]. It was discovered that the post-treatment of the organic-POMs hybrids is significant in the construction of visible-light-driven heterogeneous photocatalysts. The terminal amino group of melamine ligand in MPW hybrids can undergo polycondensation with the cyclic anhydride to form the polyimide. The light absorption and recyclability of the photocatalysts are significantly enhanced. Theoretically, for MPW, W(VI) in the Keggin unit of POMs can accept electrons to form mixed-valence clusters and maintain the inherent Keggin structure [1], [2], [3]. Thus, because of the existence of the Keggin structure, the hybrids can still retain the ligand-to-metal charge transfer to accept and transfer electrons [4], [34]. The electrons are fully delocalized over the conjugated organic ligands and the Keggin structural units through unusual POMs anion-π interactions, and the optical properties of the organic-POMs hybrids will be enhanced [6]. Based on the above characteristics, it is expected that the organic-POMs hybrids can be treated by using the simple thermal post-treatment to construct a visible-light-driven heterogeneous photocatalyst. There are also several other considerations for carrying out this work, (I) melamine is one of the main precursors for the preparation of carbonitride (CN) through the thermal polymerization, and the CN-based photocatalysts are the research hotspot of current visible-light-driven photocatalysts due to its low cost, abundant sources and suitable band structure [35], [36]. (II) It has been confirmed that the nitriding process will occur during high-temperature calcination of organic-POMs hybrids [37], [38], considering the acid-base property of POMs anions, the formation of defective carbonitrides is highly probable, and these defects may be beneficial to the photocatalytic reaction [39], [40], [41]. (III) There are no systematic investigations on the construction of carbonitride/POMs by in-situ solid phase thermal transformation at relatively low temperature using MPW as a precursor.
Herein, a series of carbonitride/tungstophosphate (TCN) photocatalysts were successfully constructed by in-situ solid phase thermal transformation of MPW hybrids. The Keggin structure can be maintained to perform the functions of accepting and transferring electrons. The melamine molecules of MPW hybrids can polymerize to form the carbonitride with defective heptazine structure. The TCN photocatalysts have excellent visible light photocatalytic degradation capacity. In addition, the proposed photocatalytic mechanism of the optimal photocatalysts is confirmed. However, there are numerous combinations that can be built based on the various species of organic ligands and POMs, this research can provide a new construction strategy for the polymer/POMs photocatalysts using organic-POMs hybrids as a precursor.
Section snippets
Reagents and chemicals
Melamine (99.0%, C3N6H6, MA) and phosphotungstic acid hydrate (AR, H3PW12O40·xH2O, HPW) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Imidacloprid (98.0%) was obtained from the Institute of Plant Protection, Chinese Academy of Agricultural Sciences. Deionized water was used in all the experimental process. All other unspecified chemicals also meet the respective requirements.
Synthesis of the samples
The TCN photocatalysts were synthesized by a two-step strategy. First, the melamine phosphotungstate
Thermal transformation and structure investigation
The TG-DTG measurement of 13MPW was conducted to representatively investigate the thermal transformation process of MPW hybrids. As shown in Fig. 1, during the calcination process in air, there are five distinct endothermic peaks from ambient temperature to 800 °C and the total weight loss was 38.70 wt%, which is consistent with the W content (59.49 wt%) measured by ICP-OES. This is significantly different from the thermal decomposition process of melamine with only one sharp endothermal peak
Conclusions
Although there are many methods for constructing heterogeneous visible light POMs-based photocatalysts [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], the application of post-treatment of organic-inorganic POMs hybrids has been neglected. Based on the results from previous studies, this work demonstrates that the heterogeneous visible-light-driven carbonitride/POMs photocatalysts can be
Acknowledgements
All authors acknowledge Prof. Changgen Feng and Dr. Qiang Gan from Beijing Institute of Technology for their help on the characterization of photocatalysts. All authors would also like to thank Yin Yijun from College of Science, China Agricultural University for his help in theoretical calculation.
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