A study on phase control of ZnSn(OH)6 for high photocatalytic activity

Abstract

Recently, metal hydroxide photocatalysts attracted considerable attention due to its hydroxyl structure favorable for photocatalytic oxidation of persistent organic pollutants. Particularly, ZnSn(OH)6 (ZHS) has been extensively studied for photocatalysis due to high population of the surface OH groups and the abundance of its elements. Previously published works revealed that the surface OH groups of ZHS yield highly reactive hydroxyl radicals which play an important role in the photocatalytic oxidation of organic pollutants. However, practical application of ZHS is severely limited by the extremely narrow optical absorption range inherited from its intrinsic wide band gap (~5.0 eV). Several ZHS-based heterostructure photocatalysts have been devised to extend the optical absorption range, but the majority of those solutions require high cost materials, such as noble metals, with complex synthetic procedures. Thus, a simple and economic solution is still needed to realize the large-scale application of ZHS photocatalyst.

In this paper, I successfully designed a simple and cheap method to extend the optical absorption range of ZHS. I synthesized a mixed phase ZnSn(OH)6/ZnSnO3 (ZHS/ZTO) photocatalyst by calcining ZHS nanoparticles via rapid thermal annealing (RTA) process. The obtained mixed phase photocatalyst was characterized on x-ray diffraction, UV-vis diffuse reflectance spectroscopy, field emission scanning electron microscopy, and ultraviolet photoelectron spectroscopy. It was demonstrated that the composition ratio of ZHS/ZTO can be simply controlled by the duration of RTA process. The ZHS photocatalyst calcined at 300°C for 20 sec displayed 23.6 and 5.6 times faster photodegradation rate of methyl orange than pure ZHS and pure ZTO under 254nm UV irradiation, respectively. The enhanced photocatalytic activity of the ZHS/ZTO was mainly ascribed to the photosensitization by ZTO and the ZHS/ZTO heterojunction induced electron-hole pair separation. The low cost and facile synthesis of ZHS/ZTO introduced in this work elucidates a promising strategy for practical application of ZHS photocatalyst.| 최근 유기 오염물 제거에 대한 솔루션으로 금속수산화물 기반의 광촉매가 주목받기 시작했다. 금속 수산화물의 수산기 구조는 광촉매 산화 작용을 촉진하여 잔류성 유기 오염 물질을 분해하는데 효과적인 것으로 평가되고 있다. 특히 ZnSn(OH)6 (ZHS)는 값싼 성분과 다량의 표면 수산화 그룹을 갖고있는 금속 수산화물로서 광촉매 응용에 대한 연구가 활발히 진행되고

있다. 기존의 연구 결과에 따르면 ZHS 의 표면 수산화 그룹이 hydroxyl radical 형성을 촉진시키며, 이는 유기 오염물을 제거하는데 효과적인 것으로 밝혀졌다. 그러나 ZHS 의 실질적인 응용은 아직 실현되지 못하고 있는데, 이는 ZHS 의 광흡수 영역이 매우 한정적이기 때문이다. 이를 해결하기 위해 다양한 이종접합 구조의 광촉매가 제안되었다. 그러나 일반적인 이종접합 기반 ZHS 광촉매는 복잡한 합성 과정과 비싼 공정비용으로 인해 산업적 적용에 적합하지 못한 것으로 평가되고 있다. 따라서 효과적인 ZHS 이종접합 구조를 광촉매로 활용하기 위해서는 광흡수 효율 향상과 동시에 그 제작 방법도 간단하고 경제적이여야 한다.

본 연구에서는 ZHS 나노구조체의 광흡수 영역을 확장시킬 수 있는 간단하고 경제적인 방법으로 급속열처리(RTA)에 의한 상변화 제어기술을 제안한다. ZHS 나노입자를 합성한 후 RTA 공정 시간 제어를 통해 ZnSn(OH)6/ZnSnO3 (ZHS/ZTO) 혼합물 기반의 광촉매를 성공적으로 제작하였다. ZHS/ZTO 의 물성을 분석하기 위해 x-ray diffraction, UV-Vis diffuse reflectance, field emission scanning electron microscopy, 및 ultraviolet photoelectron spectroscopy 가 활용되었다. ZHS/ZTO 혼합물의 상분율이 RTA 공정 시간에 따라 성공적으로 제어되었으며, 섭씨 300 도에서 20 초 동안 열처리된 ZHS/ZTO 나노입자가 가장 높은 광촉매 효율을 나타내었다. 광촉매 효율을 평가하기 위해 자외선(254nm) 조사 하에 methyl orange 의 농도를 측정하였다. 최적화된 조성비를 갖는 ZHS/ZTO 의 광촉매 효율은 ZHS 및 ZTO 에 비해 각각 23.6 및 5.6 배 향상되었다. ZHS/ZTO 의 향상된 광촉매 효율은 ZTO 에 의해 확장된 광흡수 영역과 ZHS/ZTO 이종접합을 통해 향상된 전자-전공 분리에 기인한 것으로 판단된다. 본 연구를 통해 제안된 RTA 공정을 통한 ZHS/ZTO 제작은 간단한 방법으로 ZHS 의 광활성도를 증대시킬 수 있는 기술로서 ZHS 광촉매의 실질적인 산업 적용에 기여할 것으로 기대된다.; Recently, metal hydroxide photocatalysts attracted considerable attention due to its hydroxyl structure favorable for photocatalytic oxidation of persistent organic pollutants. Particularly, ZnSn(OH)6 (ZHS) has been extensively studied for photocatalysis due to high population of the surface OH groups and the abundance of its elements. Previously published works revealed that the surface OH groups of ZHS yield highly reactive hydroxyl radicals which play an important role in the photocatalytic oxidation of organic pollutants. However, practical application of ZHS is severely limited by the extremely narrow optical absorption range inherited from its intrinsic wide band gap (~5.0 eV). Several ZHS-based heterostructure photocatalysts have been devised to extend the optical absorption range, but the majority of those solutions require high cost materials, such as noble metals, with complex synthetic procedures. Thus, a simple and economic solution is still needed to realize the large-scale application of ZHS photocatalyst.

In this paper, I successfully designed a simple and cheap method to extend the optical absorption range of ZHS. I synthesized a mixed phase ZnSn(OH)6/ZnSnO3 (ZHS/ZTO) photocatalyst by calcining ZHS nanoparticles via rapid thermal annealing (RTA) process. The obtained mixed phase photocatalyst was characterized on x-ray diffraction, UV-vis diffuse reflectance spectroscopy, field emission scanning electron microscopy, and ultraviolet photoelectron spectroscopy. It was demonstrated that the composition ratio of ZHS/ZTO can be simply controlled by the duration of RTA process. The ZHS photocatalyst calcined at 300°C for 20 sec displayed 23.6 and 5.6 times faster photodegradation rate of methyl orange than pure ZHS and pure ZTO under 254nm UV irradiation, respectively. The enhanced photocatalytic activity of the ZHS/ZTO was mainly ascribed to the photosensitization by ZTO and the ZHS/ZTO heterojunction induced electron-hole pair separation. The low cost and facile synthesis of ZHS/ZTO introduced in this work elucidates a promising strategy for practical application of ZHS photocatalyst.