Newswise — Nanoscale photocatalysts are small, man-made particles that harvest energy from sunlight to produce liquid fuels and other useful chemicals. But even within the same batch, the particles tend to vary widely in size, shape and surface composition. That makes it hard for researchers to tell what’s really doing the work.
A real-time imaging solution developed at Washington University in St. Louis could help, as reported in a new study in the journal ACS Catalysis.
“The challenge in correlating single-molecule optical images with specific active sites in nanoscale catalysts is that the 10 to 25 nanometer spatial resolution provided by this technique still averages over many atoms on the surface of the catalyst — thus making it difficult to correlate reaction events with the structure of the catalyst,” said Bryce Sadtler, assistant professor of chemistry in Arts & Sciences and co-lead author of the new study.
Sadtler wanted to try imaging catalytic reactions using single-molecule fluorescence ever since he arrived at Washington University in 2014. The project got a jump-start after he was introduced to Matthew Lew, assistant professor in the Preston M. Green Department of Electrical & Systems Engineering in the McKelvey School of Engineering.
“After several discussions with Matt, we agreed that the microscopy hardware and image processing he was developing for super-resolution microscopy could provide a powerful tool to obtain structural information on the nature of the active sites in nanoscale catalysts that was previously unattainable,” Sadtler said.
For the new work reported in ACS Catalysis, the researchers imaged individual chemical reactions taking place on the surface of single tungsten oxide nanowires, a type of nanoscale photocatalyst that Sadtler’s group synthesized for the study.
They used two different chemical reporters that become fluorescent, or light up, in response to different types of reactions on the surface of the nanowires. By analyzing the spatial patterns of where these chemical reactions occur, they were able to elucidate the chemical structure of active sites on the surface of the nanowires.
The researchers found that clusters of oxygen vacancies along the nanowire surface activate adsorbed water molecules during the photocatalytic generation of hydroxyl radicals — an important intermediate in the production of chemical fuels, including hydrogen gas and methanol, from sunlight.
“While previous studies have focused on isolated oxygen vacancies, a type of defect common in metal oxides, the results reveal the significance of a structural feature — clusters of oxygen vacancies — in achieving high photocatalytic activity,” Sadtler said.
“This new insight provides a path toward designing photocatalysts with enhanced activity for sunlight-to-fuel conversion by controlling the distribution of oxygen vacancies.”
The results themselves — and the process used to uncover them — are both exciting to the researchers.
“It is always a dream to directly observe the single catalytic turnovers on the surface of solid catalysts while the catalytic transformation is going on,” said Meikun Shen, a graduate student in chemistry and first author of the new paper. “I can only speak for myself, this is my personal feeling!”
This particular imaging approach provides detailed spatial and temporal information on the catalytic process — something that is usually invisible to scientists like him, Shen explained.
“In this type of experiment, the chemical properties of the catalyst are usually hard to reveal,” Shen said. “We managed to overcome this difficulty by using two different molecules to probe either the activity or the chemical property of the same catalyst. The direct correlation we observed is unique in the research field of heterogeneous catalysis.”
Photocatalysts ya nanoscale ni chembe ndogo zilizotengenezwa na binadamu ambazo hukusanya nishati kutoka kwa mwangaza wa jua kutengeneza mafuta ya kioevu na kemikali nyingine muhimu. Lakini hata ndani ya kundi moja, chembe hizo huwa na ukubwa, umbo, na muundo wa uso tofauti. Hiyo inafanya kuwa vigumu kwa watafiti kusema nini <unk> ni kweli kufanya kazi. Utafiti wa picha za wakati halisi uliofanywa na Chuo Kikuu cha Washington huko St. Louis, Marekani, ulionyesha kuwa kuna uwezekano wa kuathiri uwezo wa kugundua picha za wakati halisi. "Kuchanganya picha za macho ya molekuli moja na maeneo maalum ya kazi katika catalysts ya nanoscale ni changamoto kwamba azimio la nafasi ya nanometer 10 hadi 25 linalotolewa na mbinu hii bado ni wastani juu ya atomu nyingi kwenye uso wa catalyst, na hivyo kufanya iwe vigumu kulinganisha matukio ya athari na muundo wa catalyst, ""alisema Bryce Sattler, profesa msaidizi wa kemia katika Sanaa na Sayansi na mwandishi mwenza wa utafiti mpya." Sattler alitaka kujaribu picha ya athari za catalytic kwa kutumia fluorescence ya molekuli moja tangu alipofika Chuo Kikuu cha Washington mnamo 2014. Mfano wa mradi huo ulijulikana kwa muda mrefu baada ya Matthew Lew, profesa msaidizi katika Preston M. Green Idara ya umeme na mifumo ya uhandisi katika McKelvey Shule ya Uhandisi. "Baada ya majadiliano kadhaa na Matt, tulikubaliana kwamba vifaa vya microscopy na usindikaji wa picha alikuwa kuendeleza kwa microscopy super-resolution inaweza kutoa chombo nguvu ya kupata taarifa ya muundo juu ya asili ya maeneo ya kazi katika nanoscale catalysts kwamba ilikuwa hapo awali unreachable. """ Katika utafiti wao wa kwanza, watafiti walichunguza athari za kemikali zinazotokea kwenye nyuzi za nanowires za volfram oksidi, aina ya nanocatalyst ambayo kikundi cha Sattler kilitengeneza kwa ajili ya utafiti huo. Kwa mfano, wanasayansi walitumia kemikali mbili tofauti za kuangaza, ambazo huangaza kwa sababu ya athari tofauti za nyuzi za nanowire. Kwa kuchanganua muundo wa nafasi ambapo athari hizi za kemikali hutokea, waliweza kuelezea muundo wa kemikali wa maeneo ya kazi kwenye uso wa nyuzi za nanowires. Watafiti walipata kwamba makundi ya nafasi za oksijeni kwenye uso wa nanowire huamsha molekuli za maji zilizopigwa wakati wa kizazi cha photocatalytic cha hydroxyl radicals - kiunganishi muhimu katika uzalishaji wa mafuta ya kemikali, pamoja na gesi ya hidrojeni na methanol, kutoka kwa mwangaza wa jua. "Wakati utafiti wa awali umezingatia nafasi za oksijeni zilizotengwa, aina ya kasoro ya kawaida katika oksidi za chuma, matokeo yanafunua umuhimu wa kipengele cha muundo - makundi ya nafasi za oksijeni - katika kufikia shughuli ya juu ya photocatalytic, "Sadtler alisema." "Ugunduzi huu mpya hutoa njia ya kubuni photocatalysts na utendaji ulioimarishwa kwa ajili ya mabadiliko ya jua-kwa-mafuta kwa kudhibiti usambazaji wa nafasi za oksijeni. "" Matokeo yenyewe na mchakato kutumika kufunua yao ni wote wa kusisimua kwa watafiti." "Ni daima ndoto ya moja kwa moja kuchunguza moja catalyst turnover juu ya uso wa catalysts imara wakati catalyst mabadiliko ni kwenda juu", alisema Meikun Shen, mwanafunzi wa kuhitimu katika kemia na mwandishi wa kwanza wa karatasi mpya. "Naweza tu kuzungumza kwa ajili yangu mwenyewe, hii ni hisia yangu binafsi!""Hii mbinu maalum ya picha hutoa maelezo ya kina ya nafasi na muda juu ya mchakato wa catalytic - kitu ambacho kwa kawaida haionekani kwa wanasayansi kama yeye, Shen alielezea. "Katika aina hii ya majaribio, mali za kemikali za kichocheo kwa kawaida ni ngumu kufunua", Shen alisema. <unk>Tuliweza kushinda ugumu huu kwa kutumia molekuli mbili tofauti kuchunguza shughuli au mali ya kemikali ya kichocheo hicho hicho. Uhusiano wa moja kwa moja tuliochunguza ni wa kipekee katika uwanja wa utafiti wa catalysis heterogeneous. <unk>