Hematene: A sustainable 2D conductive platform for visible-light-driven photocatalytic ammonia decomposition

Applied Ma e ials Today 34 (2023) 101881 A ailable online 29 July 2023 2352-9407/© 2023 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/). Hema ene: A sus ainable 2D conduc i e pla o m o isible-ligh -d i en pho oca aly ic ammonia decomposi ion Jana Dzíbelo ´ a a , b , 1 , S.M. Hossein Hejazi a , c , 1 , Ve onika ˇ Sedajo ´ a a , Da id Pan´ aˇ cek a , Pe Jakubec a , * , Zdenˇ ek Ba ď u a a , b , Ondˇ ej Malina a , Jose Kaˇ slík a , Jan Filip a , ˇ S ˇ ep´ an Kmen a , c , Michal O yepka a , d , * , Radek Zboˇ il a , c , * a Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced Technology and Resea ch Ins i u e (CATRIN), Palacký Uni e si y Olomouc, ˇ Slech i elů 241/ 27, Olomouc 779 00, Czech Republic b Depa men o Expe imen al Physics, Facul y o Science, Palacký Uni e si y Olomouc, 17. lis opadu 1192/12, Olomouc 779 00, Czech Republic c Nano echnology Cen e, Cen e o Ene gy and En i onmen al Technologies, Vˇ SB–Technical Uni e si y o Os a a, 17. lis opadu 2172/15, Po uba, Os a a 708 00, Czech Republic d IT4Inno a ions, Vˇ SB–Technical Uni e si y o Os a a, 17. lis opadu 2172/15-Po uba, Os a a 708 00, Czech Republic ARTICLE INFO Keywo ds: Hema ene Ammonia Hyd ogen P oduc ion Pho oca aly ic decomposi ion ABSTRACT The eme ging class o 2D non- an de Waals (n- dW) ma e ials, including 2D i on oxides, possesses unique p ope ies and high applicabili y, making hem a ac i e o a ious echnological applica ions. Howe e , he syn hesis o hese ma e ials h ough a scalable and eco- iendly me hod emains a challenge, as mos known chemical ex olia ion p ocesses equi e oxic o ganic sol en s. In his s udy, we epo a g een syn hesis o 2D hema ene ( α -Fe 2 O 3 ) using an ul asound-suppo ed ex olia ion me hod o ea h-abundan i on oxide o e in a pu e aqueous solu ion. The esul ing hema ene shee s, only a ew nanome e s hick, exhibi supe io elec ochemical pe o mance in e ms o cha ge ans e p ocesses, making hem ideal o pho oca aly ic applica ions. By doping a conduc i e hema ene subs a e wi h u henium, we demons a e a syne gis ic e ec o gene a ing elec ons and holes unde isible ligh i adia ion. Using his app oach, we success ully decomposed ammonia in o hyd ogen and ni ogen, highligh ing he po en ial o his no el class o en i onmen ally- iendly pho oca alys s o clean ene gy p oduc ion. O e all, ou wa e -assis ed scalable syn hesis o hema ene o e s a p omising s a egy o p oducing e icien and sus ainable pho oca alys s. 1. In oduc ion The disco e y o wo-dimensional (2D) ma e ials has kindled immense in e es among scien is s as hey exhibi unique chemical, physical, and elec onic p ope ies, compa ed o hei 3D coun e pa s [1]. Thei p ope ies can be exploi ed in a ious applica ions, including elec onics [2,3], senso s [4–6], ene gy s o age [7–9], and spin onics [10,11]. Owing o hei bonding na u e, 2D ma e ials can be so ed in o wo main classes: i) an-de Waals ( dW) 2D ma e ials, such as g a- phene, black phospho us (BP), ansi ion me al dichalcogenides, me al-o ganic amewo ks (MOFs), o MXenes, whe e he s ong chemical bonding is loca ed in-plane and he weak dW bonding ou -o -plane; and ii) non- an-de Waals (n- dW) 2D ma e ials like me al chalcogenides, oxides, ni ides, and phosphides, which exhibi s ong chemical bonding in all h ee di ec ions [12]. The n- dW ma e ials ha e a laye ed s uc u e wi h an o e all hickness o a ound 10 nm and encompass high ene gy su aces [13]. Me hodologies used o he syn hesis o n- dW ma e ials include a sel -assembly app oach [14–16], ex olia ion [17], lamella in e media e ex olia ion [18,19], and opochemical ans o ma ion [12,20,21]. The ex olia ion p ocess has been adop ed as he mos con enien s a egy o he p epa a ion o i on oxide de i ed n- dW 2D ma e ials such as hema ene (2D α -Fe 2 O 3 ), magne ene (2D Fe 3 O 4 ), ilmenene (2D FeTiO 3 ), and ch omi een (2D FeC 2 O 4 ) [22]. All hese ma e ials we e p epa ed om na u al o es in o ganic sol en s ( ypically DMF; N, N-dime hyl o mamide), deploying ul asonica ion, wi h ha being he * Co esponding au ho s a : Regional Cen e o Ad anced Technologies and Ma e ials, Czech Ad anced Technology and Resea ch Ins i u e (CATRIN), Palacký Uni e si y Olomouc, ˇ Slech i elů 241/27, Olomouc 779 00, Czech Republic. E-mail add esses: [email p o ec ed] (P. Jakubec), [email p o ec ed] (M. O yepka), [email p o ec ed] (R. Zboˇ il). 1 These au ho s con ibu ed equally. Con en s lis s a ailable a ScienceDi ec Applied Ma e ials Today jou nal homepage: www.else ie .com/loca e/apm h ps://doi.o g/10.1016/j.apm .2023.101881 Recei ed 31 Ma ch 2023; Recei ed in e ised o m 30 June 2023; Accep ed 13 July 2023 Applied Ma e ials Today 34 (2023) 101881 2 main d awback o hese ex olia ion p ocesses due o he inhe en oxici y o DMF and unwan ed su ace unc ionaliza ion [23]. Because o he DMF oxici y [24,25], he Eu opean Union is adop ing egula ions es ic ing i s use om Decembe 2023 onwa ds [26]; hence he need o he de elopmen o al e na i e ex olia ion p ocesses in non oxic sol en s. Since he amoun o ossil uels is limi ed, alongside he ac ha global ene gy consump ion is es ima ed o inc ease a leas wice by he midcen u y compa ed o he p esen ime [27], he exploi a ion o enewable and sus ainable ene gy sou ces ep esen s one o he mos impo an asks. P e iously published epo s show ha n- dW ma e- ials such as hema ene (2D α -Fe 2 O 3 ), magne ene (2D Fe 3 O 4 ), ilmenene (2D FeTiO 3 ), and ch omi een (2D FeC 2 O 4 ) ha e s ong po en ial in he ield o pho oelec ochemis y, whe e he combina ion o hese allo- opes wi h Ti nano ubes shows excellen esul s [28,29]. N- dW ma- e ials also hold g ea po en ial in a ious elec oca aly ic applica ions a ge ing he ene gy con e sion p ocesses, including wa e spli ing (HER and OER eac ions), CO 2 educ ion eac ion (CRR), ni ogen educ ion and oxida ion eac ions (NRR and NOR), NO x educ ion and NH 3 oxida ion [12]. Up o now, he highes elec oca aly ic ac i i y is achie ed wi h noble me als, which ep esen s he main d awback o he scalable applica ion o hese elec oca alys s on he comme cial le el, mainly due o hei p ice and a ailabili y. Thus, he de elopmen o no el ma e ials wi h enhanced elec oca aly ic and/o pho oca aly ic ac i i y owa ds sus ainable ene gy con e sion is highly desi ed. An ammonia decomposi ion eac ion is a p omising al e na i e o ossil uels ha leads o he elease o hyd ogen and ni ogen, hus o e coming he challenges o hyd ogen s o age [30,31]. Ammonia is an ideal hyd ogen ca ie since i has a high g a ime ic (17.8 w . % H 2 ) and olume ic (121 kg H 2 m −3 in he liquid o m) H 2 densi y and p oduces a high amoun o CO x ee hyd ogen a e decomposi ion [30, 32]. The e o e, a clean p ocess based on enewable ene gy sou ces o con e NH 3 o H 2 (as a uel) and N 2 (as a ha mless gas) is an impo an scien i ic challenge. The aqueous ammonia decomposi ion (1) is an uphill eac ion and he equi ed eac ion ene gy can be p o ided by he sun ligh pho ons using pho oca alys ma e ials [33]. NH3(aq.)→3/2H2(g) + 1/2N2(g)(1) ΔG298K =27 kJ mol−1 Cu en ly, only a ew pho oac i e ma e ials a e ound o be able o decompose he aqueous ammonia solu ion, including TiO 2 [34–38], ZnO [39,40], ZnS [33], C 3 N 4 [41], g aphene, and o he ca bon based ma e- ials [42]. Howe e , all hese ma e ials su e om se e al d awbacks limi ing hei wide applica ion in p ac ice. TiO 2 is a e y popula pho- oac i e ma e ial ha has ela i ely low cos , high chemical s abili y, and good ligh con e sion e iciency. Howe e , he e iciency is s ill ela i ely low because o he as ecombina ion o he pho oinduced cha ge ans e . Ano he handicap is ha TiO 2 , o ins ance, wo ks only unde he UV illumina ion [34,39,40]. The complica ed mul iple-s ep syn hesis o he pho oca alys s using a ious oxic sol en s (e.g., DMF) [40,42] limi s he p ac ical applicabili y o o he pho oca alys s. The e o e, he p epa a ion o pho oca alys s by g een chemis y p in- ciples while elying on cheap and eco- iendly ansi ion me al oxides as success ul N 2 ac i a o s emains one o he majo challenges in he ield o pho oca aly ic decomposi ion o ammonia. In his wo k, we show ha he wo-dimensional hema ene can be syn hesized by a g een en i onmen ally iendly scalable me hod om an ea h abundan aw ma e ial—i on oxide o e specula i e. The hem- a ene shee s we e a ew nanome e s in hickness and ga e a sa is ac o y elec ochemical pe o mance in e ms o he cha ge ans e limi ing p ocesses. Re lec ing hese p ope ies, we exploi ed he combina ion o a conduc i e 2D hema ene subs a e wi h he ca aly ic su ace o RuO 2 nanopa icles o es hem as a pho oca aly ical pla o m sui able o he p oduc ion o hyd ogen unde he isible ligh i adia ion. The ob ained esul s clea ly p o ed ha his nanocomposi e exhibi ed a syne gic e ec in e ms o he gene a ion o elec ons and holes. In his case, he holes oxidized he al eady abso bed ammonia h ough he quasi- me allic u henium oxide and le he elec ons in hema ene o he educ ion o ammonia unde he isible ligh i adia ion. The excellen pho oca aly ic pe o mance o he u henium oxide-loaded hema ene suppo ed by he ac ha he en i e pho oca alys can be p epa ed by en i onmen ally iendly and scalable ways indica es ha he de eloped s a egy is iable o he ab ica ion o a no el class o pho oca alys s sui able o he p oduc ion o hyd ogen. 2. Expe imen al 2.1. Common eagen s Po assium e icyanide (p.a.) was pu chased om Lachema (B no, Czech Republic) and po assium chlo ide (p.a.) was ob ained om Pen a. I on oxide o e specula i e was ob ained om he Mo a ian Museum (B no, Czech Republic). The RuCl 3 .xH 2 O (con en o Ru: 38% min) and ammonium hyd oxide (28 % NH 3 ) we e pu chased om Al a Aesa . The magnesium oxide powde was pu chased om Sigma Ald ich. Deionized wa e (DI) was used o he p epa a ion o all aqueous solu ions and dispe sions. 2.2. Syn hesis o hema ene and loading wi h u henium The hema i e o e specula i e was g ound in o a ine powde using a Re sch Plane a y Ball Mill PM 100. The bulk sample was ball milled o 5 minu es, allowed o cool down o a oid excessi e hea ing ha migh ha e led o undesi able e ec s, e.g., sin e ing, and hen ball milled again o ano he 5 minu es. The ensued ine hema i e powde (2 g) was dispe sed in 180 mL o DI wa e , mixed ho oughly wi h a e ical o- a o (Heidolph) a 45 pm o 96 h, and sonica ed in a ba h sonica o (B anson; powe ou pu : 130 W, equency: 40 kHz) o 48 h. The ex olia ed hema ene shee s we e sepa a ed by sedimen a ion om he unex olia ed specula i e and u he p ocessed o he desi ed concen- a ion using cen i uga ion (cen i uge Sigma 4–16 K, Sigma Labo - zen i ugen GmbH, Os e ode am Ha z, Ge many) a 21 000 c . The unex olia ed powde o hema i e was collec ed and used again o he p epa a ion o hema ene shee s. The u henium loading was done ia he imp egna ion me hod. Fi s , he hema ene powde s we e sepa a ed om he solu ion by cen i u- ga ion and d ied a 80 ◦C o 1 h. A e p epa ing he p ede ined con- cen a ion o RuCl 3 in DI wa e , he measu ed amoun o d ied hema ene was added o he solu ion and s i ed o 3 hou s. Then he emulsion was cen i uged o sepa a e he solid p ecipices om he supe na an . The cen i uga ion was epea ed wice while i was washed wi h DI wa e o emo e he ex a amoun o RuCl 3 . Then, he powde s we e d ied a 80 ◦C o 1 h and calcined a 300 ◦C o 3 h in ai a mosphe e. 2.3. Mic oscopic echniques Scanning elec on mic oscopy (SEM) and a omic o ce mic oscopy (AFM) analyses we e pe o med wi h a FIB-SEM ins umen SCIOS 2 (The mo Fishe ) equipped wi h an in eg a ed AFM Li escope (NenoVi- sion). The sample was analyzed on a Si wa e . The co ela i e p obe and elec on mic oscopy (CPEM) was used o he su ace analysis, allowing simul aneous SEM and AFM da a acquisi ion a he same place in he same coo dina e sys em. Fo SEM imaging, an accele a ing ol age o 2 kV, a beam cu en o 25 pA, and an ETD de ec o was deployed. The sel -sensing Akiyama p obe in he apping mode was used o he AFM measu emen . High- esolu ion TEM (HRTEM) images including STEM- HAADF (high-angle annula da k- ield imaging) analyses o elemen al mapping o he p oduc s we e collec ed wi h an FEI Ti an HRTEM mi- c oscope using an ope a ing ol age o 80 kV. Fo hese analyses, a d ople o dispe sion o he ma e ial in ul apu e H 2 O a a concen a ion o ~0.1 mg mL −1 was deposi ed on o a ca bon-coa ed coppe g id and J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 3 d ied. 2.4. Spec oscopic measu emen s The X- ay pho oelec on spec oscopy (XPS) measu emen s we e ca ied ou by a PHI Ve saP obe II p obe (Physical Elec onics) wi h an Al Ka sou ce (15 kV, 50 W). The measu ed spec a we e decon olu ed using he Mul iPak so wa e (Ul ac-PHI, Inc.). The Raman spec a o all he samples we e eco ded on a DXR Raman mic oscope using a diode lase wi h an exci a ion line o 633 nm. The EPR spec a we e collec ed on a X-band (~9.14–9.17 GHz) spec ome e JEOL JES-X-320 equipped wi h a a iable He empe a u e se -up ES-CT470 appa a us. The expe imen al empe a u e was se o 80 K. The quali y ac o (Q) was kep abo e 6000 o all measu emen s o make he spec a compa able. High pu i y qua z ubes (Sup asil, Wilmad, ≤0.5 OD) we e used as a sample holde . The accu acy o he g- alues was de e mined by compa ing hem wi h a Mn 2+ /MgO s anda d (JEOL s anda d). The mi- c owa e powe was se o 1.0 mW o a oid any powe sa u a ion e ec s. A modula ion wid h o 1 mT and a modula ion equency o 100 kHz we e used. All he EPR spec a we e collec ed wi h a ime cons an o 30 ms and a sweep ime o 4 min. 2.5. S uc u al and phase analysis X- ay di ac ion (XRD) pa e ns o bulk i on o e and ex olia ed hema ene samples we e collec ed on a PANaly ical X’Pe PRO di ac- ome e (i on- il e ed Co K α adia ion: λ =0.178901 nm, 40 kV and 30 mA) in a B agg-B en ano geome y equipped wi h an X’Cele a o de- ec o , p og ammable di e gence, and di ac ion beam an i-sca e ing sli s. Each sample was placed on a ze o-backg ound Si slide, gen ly p essed, and scanned in he 2θ ange om 5◦ o 105◦The phase iden- i ica ion was pe o med using PANaly ical HighSco e Plus so wa e wi h PDF-4+and ICSD da abases. 2.6. Elec ochemical measu emen s All he elec ochemical measu emen s in a h ee-elec ode se -up we e pe o med a ambien oom empe a u e (22 ±2 ◦C) using he Me ohm Au olab PGSTAT128N po en ios a (Me ohmAu olab B.V., Ne he lands). The ob ained elec ochemical da a se s we e e alua ed by he NOVA so wa e package ( e sion 1.11.2). The sys em comp ised a glassy ca bon elec ode (GCE) se ing as he wo king elec ode, a pla - inum wi e elec ode used as he coun e elec ode and an Ag/AgCl (3 M KCl) elec ode as he e e ence elec ode. The GCE was modi ied as ollows: a 10 µL d op o a sample dispe sion in DI wa e (concen a ion o ≈1 mg mL −1 ) was coa ed on o he su ace o he GCE elec ode and allowed o d y a ambien empe a u e o o m a hin ilm. Po assium chlo ide (c =1 mol L –1 ) con aining po assium e icyanide (c =5 mmol L –1 ) as a edox p obe se ed as a suppo ing elec oly e o impedance measu emen s. Indi idual se ings o impedance spec oscopy mea- su emen s (EIS) a e gi en in he Figu e legends. 2.7. Pho oca alys cha ac e iza ion and pho odecomposi ion o ammonia The ul a iole - isible di use e lec ance spec a (UV–Vis DRS) o he syn hesized samples we e ob ained by a Speco d 250 plus (Analy ik Jena, Jena, Ge many) spec opho ome e . An in eg a ing sphe e was used o collec he spec um. The magnesium oxide powde se ed as a backg ound e e ence sample. The pho odecomposi ion o NH 3 was conduc ed in a 17.5 mL qua z eac o . A e sono-dispe sion o he pho oca alys in 10 mL o ammo- nium hyd oxide, he ec o was igh ly sealed wi h a ubbe sep um. Since dissol ed oxygen in a solu ion could ac as an elec on sca enge , i was bubbled wi h a gon o 15 min o emo e he oxygen. Then, he sample was illumina ed unde LED (Solis® High-Powe LED om Tho labs) wi h he powe o 3 W⋅m −2 and a wa eleng h o 400‒700 nm. To a oid he he mal decomposi ion o ammonia, he eac ion was conduc ed in a wa e ba h o keep he empe a u e o he solu ion a 24 ◦C du ing he eac ion. The pho oca aly ically e ol ed hyd ogen was de ec ed wi h a gas ch oma og aph GCMS-QP2010 SE (Shimadzu, Kyo o, Japan) and a TCD (The mal conduc i i y de ec o ) using A as ca ie gas. 3. Resul s and discussion 3.1. S uc u al, mo phological, and (elec o)chemical cha ac e iza ion o hema ene Mul i-laye ed hema ene shee s we e p epa ed ia an ex olia ion p ocess om an (ul a)pu e wa e dispe sion o specula i e, as discussed ea lie in he expe imen al sec ion. The mo phology o he hema ene sample was e alua ed by a CPEM echnique (combina ion o SEM and AFM analysis) and a HRTEM analysis, as shown in Fig. 1a– and Fig. S1a–c. I is e iden ha he ex olia ed sample was o a 2D-like na- u e wi h he la e al sizes in uni s o mic ome e s (Fig. 1a–c). Homog- enous dis ibu ion o he indi idual elemen s such as i on o oxygen (Fig. S1b) implied high pu i y o he p epa ed hema ene sample, which was a i med by an EDAX analysis (Fig. S1c), ul ima ely con i ming he dominan p esence o hese elemen s; he appea ance o he coppe elemen ela ed o he g id used o he HRTEM analysis. The AFM analysis (Fig. 1b,c) and he ela ed heigh p o ile (Fig. 1d) e ealed ha he hickness o he ep esen a i e shee was o a ound 5 nm. The hickness o he molecula dynamic simula ed hema ene (001) and (010) planes was es ima ed as 3.98 Å and 3.2 Å, espec i ely, co obo a ing a p e iously published epo [28]. Thus, he hickness o 5 nm ep e- sen ed ≈12 hema ene laye s in he inal p oduc . A highe numbe o laye s alida es he heo y ha he selec ion o a chemical sol en decidedly a ec s he possibili y o ex olia ing he sample by ul asonic i adia ion up o indi idual laye s [43]. As expec ed, he sonochemical p epa a ion s a egy o hema ene om (ul a)pu e wa e did no enable o p o ide a mono o wo-laye ed sys em. To p o e he size and hick- ness o he hema ene ma e ial, CPEM images wi h ela ed heigh p o iles we e collec ed om di e en shee s, and a e depic ed in Fig. S2. The 2D s uc u e o he hema ene sample was also con i med om magne ic da a measu emen s. De ails o hose expe imen s and ela ed discussion a e p o ided in he ESI. The composi ion o he p oduc was also sup- po ed by XPS, as shown in he box (Fig. 1e, ). The ob ained HR-XPS spec um o he Fe 2p egion (Fig. 1e) e ealed wo peaks a a ound 712 and 725 eV, co esponding o Fe 2p 3/2 and Fe 2p 1/2 , espec i ely [28,44]. The sa elli e peak loca ed a a ound 719 eV u he indica ed he Fe 3+ oxida ion s a e [44]. As e iden om he decon olu ion, bo h spin-o bi componen s we e i ed wi h wo peaks. These i ed peaks se ed mo e as a e e ence, indica ing a end based on al eady pub- lished s udies [45,46]; he ansi ion me als in hei 2p s a es should be i ed by mul iple componen s, ep esen ing he mul iple spli ing. The HR-XPS spec um o he O 1 s egion (Fig. 1 ) was i ed in h ee com- ponen s, based on p e ious epo s [47,48], co esponding o he la ice inco po a ed oxygen a highe binding ene gies and he su ace p esen oxygen (in a de ec s a e and as su ace oxida ion [28,47,48]) a lowe binding ene gies, espec i ely. The na u e o he hema ene sample was also e alua ed by means o Raman spec oscopy. Fig. 2a shows he Raman spec a o o iginal specula i e o e in compa ison wi h he hema ene sample. The posi ions o he mos impo an bands we e ound a a ound 224 and 495 cm –1 (A 1g modes) and 249, 290, 409, 611 cm –1 (E g modes). The b oad in ense band isible a a ound 1318 cm –1 was connec ed wi h he sca e ing o wo magnons in he s uc u e o he hema i e o e [49]. As expec ed, he hema ene showed he posi ion o he mos impo an bands, simila ly o specula i e. Ne e heless, he addi ional band was obse ed a 663 cm –1 , which can be assigned o he s uc u al diso de s inside he c ys al la - ice (E u modes) ela ed o he 2D c ys al s uc u e [28,48,50,51]. The c ys alline s uc u e and chemical phase pu i y o bo h he o iginal J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 4 specula i e and he ex olia ed hema ene we e addi ionally e alua ed by XRD measu emen s (Fig. 2b). The collec ed pa e ns illus a e ha bo h samples con ained only he α -Fe 2 O 3 phase o he co undum c ys al s uc u e (JCPDS ca d 01–089–0599, hombohed al s uc u e, space g oup R-3c), which ules ou he p esence o any o he c ys alline pha- ses. The di ac ion pa e n o hema ene (la ice pa ame e s: a =5.035 Å and c =13.745 Å) showed ha he di ac ion peaks we e sligh ly b oadened and sligh ly shi ed in hei posi ion, compa ed o he o iginal specula i e (la ice pa ame e s: a =5.034 Å and c =13.742 Å). These e ec s a e isible in Fig. 2c, p o iding a de ail abou he posi ions o he wo di ac ion peaks, whe e he di ac ion on he (104) plane is ypi- cally dominan o he α -Fe 2 O 3 s uc u e. Such obse a ion is consis en wi h he ex olia ion o hema i e in p e ious s udies [28,52], whe e he b oadening o he lines is ela ed o he educ ion in c ys alli e size [17, 53]. P io o he modi ica ion o he hema ene wi h u henium, he elec ochemical pe o mance o specula i e and i s 2D analogue was e alua ed by means o impedance spec oscopy (EIS). Fig. 3a illus a es he EIS spec a as he Nyquis plo o a ba e glassy ca bon elec ode (GCE) and GCE modi ied wi h he specula i e p ecu so and he hema ene sample. As isible in Fig. 3b, all EIS spec a should be i ed wi h a simple Randles ci cui , which illus a es he non-complica ed elec ochemical beha io o all he es ed ma e ials. In gene al, esis- ance R S loca ed a poin A (see Fig. 3b) was assigned o he solu ion esis ance o he so-called ESR esis ance (known also as he in e nal esis ance), whe eas he diame e o he semici cle R AB =R B – R A ep- esen ed he sum o he elec ode and con ac esis ance and he cha ge ans e esis ance (R c ) [54,55]. A magni ied e sion o he Nyquis plo (inse o Fig. 3a) e ealed ha he cha ge ans e esis ance (R c ) inc eased signi ican ly when he wo king elec ode (R c =40.4 Ω) was modi ied ei he wi h he specula i e (R c =222 Ω) o he hema ene (R c =173 Ω). Such beha io sugges ed he success ul immobiliza ion o bo h ma e ials on he su ace o he wo king elec ode. I is pe ec ly clea ha he diame e o he semici cle was smalle in a o o he hema ene (inse o Fig. 3a), which e lec ed a be e con ac o he hema ene ilm wi h he su ace o he wo king elec ode and hus a lowe impac on he cha ge ans e limi ing p ocesses. The e o e, i can be expec ed ha he elec on ans e was enhanced in ha case. Fig. 1. (a–c) CPEM images o hema ene shee and ela ed heigh p o ile (d); (e) HR-XPS spec um o Fe 2p and ( ) O 1 s egion. Fig. 2. (a) Raman spec a o specula i e and hema ene; (b) XRD pa e ns o specula i e and hema ene; (c) de ail o XRD pa e ns showing he b oadening and shi o hema ene di ac ion peaks compa ed o pa en specula i e. J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 5 3.2. Pho oca alys cha ac e iza ion Fig. 4 shows he HRTEM images o he hema ene and he Ru- hema ene oge he wi h he elemen al analysis. The dominan c ys al plane dis ance in he hema ene was 2.7 Å, which was ela ed o he (104) plane, while he p esence o u henium oxide was con i med by i s dominan c ys al plane dis ance 3.2 Å ela ed o he (110) plane. Bo h s uc u es in Ru-hema ene we e also iden i ied by XRD (Fig. S6). The EDS elemen al analysis o he Ru-hema ene showed a well dispe sed u henium oxide pa icles wi hou any sign o agglome a ion a he su ace o he hema ene s uc u e as ca alys si es o ammonia oxida ion. This uni o m dispe sion o u henium oxide helped o in- c ease he cha ge sepa a ion and he e o e i s pho oac i i y. The XPS echnique was employed o de e mine he oxida ion s a es o he u henium in he Ru-hema ene sample. As shown in Fig. 5a, he HR-XPS Ru 3d egion was analyzed and decon olu ed o e eal wo oxida ion s a es o u henium. The Ru 4+ peak a a ound 280.5 eV u he p o ed he p esence o RuO 2 species [56–58], which was in ag eemen wi h he EPR obse a ions (Fig. 5d), and was easonable o expec due o he hea ea men o he sample a 300 ◦C. The adjacen Ru 3+ band below 282 eV indica ed he exis ence o he Ru(III) [56,57,59] a oms, which can be u he explained by he co-p esence o he chlo ine in his sample (1.2 a .% coming om he p ecu so ) (Fig. S5). The ca bon con en (C 1 s peak) is in insic o he hema ene i sel and was no ully elimina ed du ing he hea ing p ocess. These XPS obse a ions con i med he p e ious cha ac e iza ion echniques as well as he pho- oca aly ic ac i i y [58–62]. Fig. 5b shows he alence band XPS (VB-XPS) o he hema ene and he Ru-hema ene. The posi ion o he alence band edge o he hema ene did no change a e loading wi h u henium (+1.1 eV), while he densi y o s a es (DOS) showed an in- c ease a a ound +0.5 eV below he Fe mi le el wi hin he band gap. This ail was possibly due o he p esence o he RuO 2 species, c ea ing he e ojunc ion a he su ace o hema ene, which p o ided a mid-band gap ene gy s a e le els a ound +0.5 eV below he hema ene Fe mi le el (Fig. 5c). These ene gy s a e le els we e able o acili a e he pho ogene a ed hole anspo a ion om he hema ene o RuO 2 and oxidize ammonia o N 2 by p o iding o bi al o e laps [63]. On he o he hand, he pho ogene a ed elec ons wen h ough he hema ene and educed he ammonia o hyd ogen (Fig. 5c). This spa ial cha ge sepa- a ion in he place o RuO 2 and he hema ene he e ojunc ion esul ed in highe pho oac i i y o his pho oca alys in compa ison wi h he ba e hema ene. To ob ain mo e de ailed in o ma ion abou he s uc u e and he na u e o he Ru coupling mechanism on hema ene, we ca ied ou an elec on pa amagne ic esonance (EPR) s udy a low empe a u e (T = 80 K). In a nea hema ene, in spi e o assuming he p esence o a la ge numbe o spin-con aining de ec s loca ed on he edges o indi idual lakes, he eco ded spec um in a ozen wa e ma ix was indeed EPR Fig. 3. (a) Nyquis plo o ba e GCE and GCE modi ied wi h specula i e and hema ene; pa ame e s o EIS: equency ange om 100 kHz o 0.1 Hz, he ampli ude o 10 mV and hal -wa e po en ial o 0.24 V; (b) isualiza ion o Nyquis plo and ela ed equi alen ci cui used o he da a e alua ion. Fig. 4. HRTEM image (le ), STEM-HAADF images and elemen al EDS mapping ( igh ) o (a) hema ene and (b) Ru-hema ene. The inse s show he FFT pa e ns o he shown a ea by dashed ed ec angle. J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 6 silen (g een line in Fig. 5d). Since, his ma e ial exhibi ed an i e o- magne ic o de ing a T below 250 K, his phenomenon was no unex- pec ed. Ne e heless, a s a is ical pe cen age o an e ec i e spin momen (d χ II /dB ∕= 0) should be le a 80 K, which aises a ques ion why hese e ec i e spins canno be obse ed a X-band equency. The easoning can be back aced in he as spin-la ice elaxa ion o he Fe 3+ cen e s unde an exchange coupled egime, and in he la ge ze o- ield spli ing e m a ising om he coupling scheme, which ende s he sys em o be mo e likely a non-K ame mul iple . The esul ing EPR spec um o RuCl 3 as he me al-sal sou ce o he deco a ion o he hema ene lakes is shown o compa ison in Fig. 5d (blue line). The hombic esonan line o Ru ca ions in +3 oxida ion s a e showed small g- enso aniso opy, wi h componen s a g x =2.220, g y = 2.094 and g z =1.802. These g- alues a e ypical o Ru 3+ ions, howe e , he e, hey we e sligh ly smalle han in he Ru 3+ complexes coo dina ed o he o ganic amewo ks [64] because o he absence o a s ong ligand ield. In Fig. 5d ( ed spec um), he EPR en elope o Ru-hema ene showed signi ican di e ences compa ed wi h he EPR spec a o he wo p e ious e e ence samples. In he EPR spec um o he composi e Ru-hema ene ma e ial, a e y b oad and asymme ic esonan line became dominan . This line exhibi ed an a e age g- alue o abou ~ 2.7, which was ully consis en wi h clus e ed Fe 3+ ca ions on he laye ed su ace ha became pe u bed by he Ru ca ions in a way ha hei an i e omagne ically coupling in e ac ion, as seen in he nea hema- ene, was s ongly weakened and he e o e EPR de ec able. We assume ha he Ru ions eac ed wi h oxygen on he op o he hema ene su ace o o m small RuO 2 nanopa icles, and by doing so, hey pa ially al e ed he ac ion o he Fe-O-Fe supe exchange pa h. The e o e, he obse ed e ec on he esonance spec um, combined wi h clea asymme y in he b oad esonance, sugges s his was a su ace localiza ion o hese de- ec s, which is in ull ag eemen wi h he 2D s uc u e o he hema ene lakes and he TEM analysis (Fig. S1a). Fu he mo e, in he a ea high- ligh ed in blue in he EPR spec um o he Ru-hema ene (Fig. 5d), a weak modula ion o an Fe 3+ en elope a ound 310 mT was obse ed. I was posi ioned a g ~ 2.09, a alue ha is in line wi h he signal o Ru 3+ , and p obably loca ed on he su ace o he RuO 2 nanopa icles, as usually obse ed [65]. Since he Ru loading was e y low, and we could de ec only he Ru 3+ species ia EPR, he signal o Ru was obse ed as weak modula ion in he b oad esonan line o Fe 3+ . Howe e , pa o he Ru 3+ agmen s in he EPR spec um can also be an indica o o a small amoun o esidual RuCl 3 sal in he esul ing ma e ial, which was also obse ed in XPS. 3.3. Pho oca aly ic decomposi ion o HN 3 ia u henium loaded hema ene P io o pho oca aly ical expe imen s, bo h p is ine and Ru- hema ene we e e alua ed by means o UV–VIS spec oscopy o assess he sui abili y o bo h ma e ials o pho odecomposi ion o ammonia. The band edge abso p ion o bo h samples was a ound 729 nm, as shown in Fig. 6a. The e o e, hese ma e ials can abso b bo h UV and a isible pa o he ligh spec um. The eason o he da k ed colo in he samples is also ela ed o his wide abso p ion egion. The amoun o Fig. 5. (a) Decon olu ion o HR-XPS o Ru 3d o bi al o Ru-hema ene pho oca alys ; (b) Valence band XPS (VB-XPS) o hema ene and Ru-hema ene; (c) Schema ic densi y o s a es (DOS) and band edge posi ions o he main elec onic le els o Ru-hema ene ela i e o he ene gy le els o he edox couples o aqueous ammonia (le ) and he p oposed mechanism o ammonia pho odecomposi ion by Ru-hema ene pho oca alys ( igh ). (d) X-band CW EPR spec a o nea hema ene (g een line), RuCl 3 (blue spec um) and EPR en elope o hema ene deco a ed by RuO 2 nanopa icles ( ed cu e), dispe sed in DI H 2 O and eco ded in he ozen solu ion (T =80 K). J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 7 ligh abso p ion (especially in he isible egion) was highly inc eased a e adding u henium. This huge inc ease in isible ligh abso p ion ul ills one o he main condi ions o an ideal pho oca alys o pho o- ca aly ic eac ions. To highligh his unique p ope y o cu en pho o- ca alys s, all o he pho o eac ions in his wo k we e conduc ed unde he isible pa o he ligh (wa eleng h 400–700 nm). The op ical band gap ene gy o he hema ene and he Ru-hema ene calcula ed by he Tauc plo [66] based on he Kubelka-Munk [67] heo y was 1.70 and 1.71 eV, espec i ely (Fig. 6b). I should be no ed ha in spi e o he huge di - e ence in abso bed ligh , he band gap o he pho oca alys s was almos he same. As discussed in de ail, he p esence o RuO 2 was p o ed by he su - ace XPS analysis (Fig. 5a). This oxida ion occu ed p obably du ing he calcina ion s ep a 300 ◦C. Nagaoka and cowo ke s [68] epo ed ha RuO 2 p oduces a ca aly ic su ace o exo he mic adso p ion o NH 3 . This chemiso p ion o ammonia dec eased he o e all he modynamic ene gy needed o i s decomposi ion. Then, he subsequen pho o ab- so p ion by hema ene esul ed in he gene a ion o elec ons and holes. The holes we e able o oxidize he al eady abso bed NH 3 h ough he quasi-me allic u henium oxide and le he elec ons in he hema ene o he educ ion o he ammonia. The con ol expe imen s show ha he ca aly ic ac i i y o hema ene and u henium loaded hema ene o decomposi ion o ammonia unde da k condi ion is almos ze o. Fu he mo e, he pho odecomposi ion o ammonia as ammonium hyd oxide wi hou he p esence o pho o- ca alys a 24 ◦C is ze o. A e he u henium oxide was con i med as a p ope co-ca alys o hema ene o decompose he ammonia, he loading op imiza ion o Ru was pe o med. Fig. 6c shows ha loading he hema ene wi h he op- imum amoun o Ru—0.5 w . %—led o a con inuously inc eased amoun o H 2 . In lowe amoun s o he Ru loading, he cha ge sepa a ion was no e ec i e enough o p oduce hyd ogen, while a highe amoun o Ru was able o p oduce cha ge ecombina ion cen e s a he su ace o he hema ene [68]. All he cha ac e iza ion was done while loading he hema ene wi h 0.5 w .% o Ru, labelled as Ru-hema ene. To ha e a ai and eliable compa ison be ween he pho oac i i y o he hema ene and he Ru-hema ene, he mass o he pho oca alys had o be op imized agains he pho oac i i y (Fig. 6d and Fig. S4). The eason is ha he amoun o he H 2 p oduc ion du ing he pho oca aly ic e- ac ion is no necessa ily p opo ional o he mass o he pho oca alys due o he limi a ion o ligh abso p ion and di usion condi ions [69, 70]. I is ob ious om Fig. 6d ha he loading o 2 mg o he pho o- ca alys in bo h cases was able o p oduce he op imum amoun o he p oduc . This plo shows ha he Ru-hema ene pho oca alys p oduced 2.5 imes mo e H 2 han he p is ine hema ene unde op imum condi ions. The Ru-hema ene showed an 11% dec ease in ac i i y a e i e pho oca aly ic uns o he o al o 120 h (Fig. 6e). A cons an dec ease in ac i i y a e each un could be co ela ed o a loss in he pho oca alys caused by he a achmen o he eac o walls and e en o he magne s i e . Addi ionally, as he ca alys was no washed a e each un o he pho o eac ion, he su ace o he ca alys could ha e been passi a ed by eac an s, p oduc s o in e media e species. 4. Conclusions A non- an de Waals 2D ma e ial, hema ene, was simply p epa ed ia he ex olia ion o i on oxide o e specula i e, deploying an (ul a)pu e wa e solu ion. Using his s a egy, he p epa ed ma e ial exhibi ed a o able elec ochemical p ope ies in e ms o cha ge ans e and di usion limi ing p ocesses. As a p oo -o -concep , we ha e amply demons a ed ha his pu e n- dW ma e ial can be easily modi ied wi h u henium oxide nanopa icles and can se e as a ca aly ic eco- iendly pla o m o he pho oca aly ic decomposi ion o an aqueous solu ion o ammonia in o de o p oduce hyd ogen ia he isible ligh i adia ion. The ob ained esul s p o ed ha he combina ion o RuO 2 ca aly ic su ace wi h he conduc i e 2D hema ene subs a e exhibi ed a syne gic e ec in e ms o he gene a ion o elec ons and holes. Based on he esul s, he holes we e able o oxidize he al eady abso bed ammonia h ough he quasi-me allic u henium oxide and le he elec ons in he Fig. 6. (a) UV–VIS DRS o hema ene and Ru-hema ene and (b) co esponding Tauc plo based on Kubelka-Munk model o calcula e he op ical band gap o hema ene and Ru-hema ene; (c) Op imizing he loading amoun o Ru co-ca alys o H 2 e olu ion o ammonia pho odecomposi ion. The eac o s we e loaded wi h 8 mg o pho oca alys ; (d) The amoun o e ol ed H 2 om pho odecomposi ion o ammonia wi h di e en loading amoun o p is ine hema ene and Ru-hema ene. The eac o s we e unde illumina ion o 24 hou s; (e) Reusabili y o 2 mg o hema ene and Ru-hema ene o H 2 e olu ion om ammonia pho o decomposi ion. A e each cycle, he solu ion was sonica ed and bubbled wi h A o 15 min. In all cases, he eac o was unde isible LED illumina ion (wa eleng h om 400 o 700 nm, powe 3 W⋅m −2 ). J. Dzíbelo ´ a e al. Applied Ma e ials Today 34 (2023) 101881 8 hema ene o he educ ion o ammonia. Expe imen al da a indica e ha he op imum dosage o u henium was a ound 0.5 w %, yielding he maximum amoun o hyd ogen a e 24 hou s. Wi h espec o he p is ine hema ene, a co-doped ma e ial p o ided a leas 2.5 imes be e pho oca aly ic esponse owa ds hyd ogen e olu ion. Wi hou any cleaning s eps, he Ru-hema ene pho oca alys exhibi ed only 11 % o he pho oca aly ic eac ion dec ease a e i e success ul pho oca aly ic uns, p edisposing i o p ac ical applica ion. The disco e y opens up possibili ies o c ea ing obus and e ec i e pho oca alys s o a wide ange o applica ions using non- an de Waals 2D ma e ials and me al oxide nanopa icles. CRediT au ho ship con ibu ion s a emen Jana Dzíbelo ´ a: In es iga ion, W i ing – o iginal d a , W i ing – e iew & edi ing. S.M. Hossein Hejazi: Concep ualiza ion, In es iga- ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Ve onika ˇ Sedajo ´ a: In es iga ion, W i ing – o iginal d a . Da id Pan´ aˇ cek: In es iga ion, W i ing – o iginal d a . Pe Jakubec: Concep ualiza- ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Zdenˇ ek Ba ď u a: In es iga ion, W i ing – o iginal d a . Ondˇ ej Malina: In es- iga ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Jose Kaˇ slík: In es iga ion. Jan Filip: Concep ualiza ion. ˇ S ˇ ep´ an Kmen : Concep ualiza ion. Michal O yepka: Concep ualiza ion, W i ing – e- iew & edi ing. Radek Zboˇ il: Concep ualiza ion, W i ing – o iginal d a , W i ing – e iew & edi ing. Decla a ion o Compe ing In e es The au ho s decla e ha hey ha e no known compe ing inancial in e es s o pe sonal ela ionships ha could ha e appea ed o in luence he wo k epo ed in his pape . Da a a ailabili y Da a will be made a ailable on eques . Acknowledgmen s R.Z. acknowledges he suppo om he Czech Science Founda ion, p ojec No. 19–27454X. P.J. acknowledges he Resea ch In as uc u e NanoEn iCz, suppo ed by he Minis y o Educa ion, You h and Spo s o he Czech Republic unde P ojec No. LM2018124. We also acknowledge he suppo om ERDF/ESF “Nano4Fu u e” (No. CZ.02.1.01/0.0/0.0/16_019/0000754) and he Technology Agency o he Czech Republic, P og am TREND (GEFSEM, FW01010183). 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