Reductive modification of carbon nitride structure by metals - The influence on structure and photocatalytic hydrogen evolution

  Ci a ion: Alwin, E.; Wojcieszak, R.; Koˇcí, K.; Edelmanno á, M.; Zieli´nski, M.; Sucho a, A.; P˛edzi´nski, T.; Pie owski, M. Reduc i e Modi ica ion o Ca bon Ni ide S uc u e by Me als—The In luence on S uc u e and Pho oca aly ic Hyd ogen E olu ion. Ma e ials 2022, 15, 710. h ps://doi.o g/ 10.3390/ma15030710 Academic Edi o : Lumini¸ a Isac Recei ed: 30 Decembe 2021 Accep ed: 14 Janua y 2022 Published: 18 Janua y 2022 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2022 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). ma e ials A icle Reduc i e Modi ica ion o Ca bon Ni ide S uc u e by Me als—The In luence on S uc u e and Pho oca aly ic Hyd ogen E olu ion Emilia Alwin 1, Robe Wojcieszak 2, Kamila Koˇcí3, Mi osla a Edelmanno á3, Michał Zieli´nski 1, Aga a Sucho a 1, Tomasz P˛edzi´nski 1,4 and Ma iusz Pie owski 1,* 1Facul y o Chemis y, Adam Mickiewicz Uni e si y, Pozna´n, Uniwe sy e u Pozna´nskiego 8, 61-614 Poznan, Poland; [email p o ec ed] (E.A.); [email p o ec ed] (M.Z.); [email p o ec ed] (A.S.); [email p o ec ed] (T.P.) 2Uni . Lille, CNRS, Cen ale Lille, Uni . A ois, UMR 8181-UCCS-Uni éde Ca alyse e Chimie du Solide, F-59000 Lille, F ance; [email p o ec ed] 3Ins i u e o En i onmen al Technology, CEET, VSB-Technical Uni e si y o Os a a, 17. lis opadu 15/2172, 70800 Os a a-Po uba, Czech Republic; [email p o ec ed] (K.K.); mi [email p o ec ed] (M.E.) 4Cen e o Ad anced Technologies, Adam Mickiewicz Uni e si y, Pozna´n, Uniwe sy e u Pozna´nskiego 10, 61-614 Poznan, Poland *Co espondence: [email p o ec ed] Abs ac : P , Ru, and I we e in oduced on o he su ace o g aphi ic ca bon ni ide (g-C 3 N 4 ) using he we imp egna ion me hod. A educ ion o hese pho oca alys s wi h hyd ogen causes se e al changes, such as a signi ican inc ease in he speci ic su ace a ea, a C/N a omic a io, a numbe o de ec s in he c ys alline s uc u e o g-C 3 N 4 , and he con ibu ion o ni ogen bound o he amino and imino g oups. Acco ding o he X- ay pho oelec on spec oscopy esul s, a ansi ion laye is o med a he g-C 3 N 4 /me al nanopa icle in e phase, which con ains me al a a posi i e deg ee o oxida ion bonded o ni ogen. These s uc u al changes signi ican ly enhanced he pho oca aly ic ac i i y in he p oduc ion o hyd ogen h ough he wa e -spli ing eac ion. The ac i i y o he pla inum pho oca alys was 24 imes g ea e han ha o p is ine g-C 3 N 4 . Mo eo e , he enhanced ac i i y was a ibu ed o signi ican ly be e sepa a ion o pho ogene a ed elec on–hole pai s on me al nanopa icles and s uc u al dis o ions o g-C3N4. Keywo ds: me al pho oca alys s; g aphi ic ca bon ni ide; we imp egna ion me hod; pho oca aly ic ac i i y; physicochemical cha ac e iza ion 1. In oduc ion The g owing demand o ene gy, g adual deple ion o ossil uels, and inc easing e o s aimed a p o ec ing he na u al en i onmen ha e s imula ed he sea ch o new g een ene gy sou ces. One o hem can be hyd ogen ob ained om pho oca aly ic eac ions, such as he pho odecomposi ion o me hanol/wa e mix u es. Me hanol ac s as a sac i icial eagen o imp o e he hyd ogen gene a ion yield [ 1 , 2 ]. The di ec mechanism o his p ocess assumes he oxida ion o me hanol and wa e by elec on holes. The eac ion o me hanol wi h elec on holes leads o hyd ogen ions (H + ) and hyd oxyalkyl adicals ( · CH 2 OH) ha unde go u he eac ion o gi e hyd ogen ca ions and elec ons. The e- ac ion o wa e wi h elec on holes leads o he gene a ion o oxygen and p o ons. A he inal s age, he p o ons a e educed by elec ons, which leads o hyd ogen elease. De ails on he mechanism o his eac ion ha e been desc ibed by many au ho s [ 1 , 3 – 6 ]. Gene ally, one o he bes -known pho oca alys s, i ania (TiO 2 ), is used in his eac ion [4,7–9] . How- e e , his pho oca alys has limi a ions, such as a la ge band gap (ana ase: 3.2 eV, u ile: 3.0 eV ) [ 10 ]; hence, i is ac i a ed by ul a iole (UV) adia ion. Ano he d awback o TiO 2 is he as ecombina ion o gene a ed elec ons and holes. Conside ing hese limi a ions, Ma e ials 2022,15, 710. h ps://doi.o g/10.3390/ma15030710 h ps://www.mdpi.com/jou nal/ma e ials Ma e ials 2022,15, 710 2 o 16 new pho oca alys s o sola -ligh wa e spli ing ha a e ee om such disad an ages should be de eloped. G aphi ic ca bon ni ide (g-C 3 N 4 ) is a p omising al e na i e o TiO 2 [ 11 , 12 ]. Fi s , i has a signi ican ly smalle band gap (2.7 eV) han TiO2; hus, i can be ac i a ed by isible ligh . Mo eo e , i shows eliable he mal and chemical esis ance, non oxici y, and wa e esis ance. In addi ion, he p oduc ion by condensa ion o simple ni ogen- ich compounds such as u ea [ 13 , 14 ], hiou ea [ 15 ], melamine [ 16 , 17 ], and dicyandiamide (DCDA) [ 18 ] is easy and cheap. I s addi ional ad an age is ha , depending on he ype o p ecu so and condensa ion condi ions, inal ca bon ni ide can ha e di e en physicochemical p ope - ies [ 19 ]. Since he i s epo s on he pho oca aly ic p ope ies o g-C 3 N 4 [ 20 ], a emp s ha e been made o enhance i s e ec i eness, mainly by he addi ion o he e oa oms [ 21 , 22 ]. Howe e , i has been disco e ed ha be e e ec s can be achie ed by he in oduc ion o noble me als on he ca bon ni ide su ace. Li e al. epo ed a signi ican inc ease in he g-C 3 N 4 ac i i y du ing wa e spli ing unde isible ligh a e he in oduc ion o 2 w .% P [ 23 ]. The posi i e e ec o P on g-C 3 N 4 was explained by he accumula ion o elec- ons on he me al pa icle su ace, which hinde ed he ecombina ion o pho ogene a ed elec on–hole pai s [ 23 ]. Simila conclusions we e d awn by Khan e al. [ 24 ] who claimed ha he P pa icles on g-C3N4ac as elec on ap cen e s. Following hese ideas, in his s udy, he e ec s o in oducing P -g oup me als (P , Ru, and I ) on he g-C 3 N 4 su ace we e in es iga ed. Howe e , ins ead o he commonly used pho o educ ion o chemical educ ion, he pho oca alys s we e syn hesized using he we imp egna ion me hod wi h an excess o sol en . The pho oca alys s we e hen educed wi h hyd ogen o gene a e s uc u al de ec s as a esul o pa ial hyd ogena ion/hyd ogenolysis o C − N bonds, which was expec ed o imp o e he sepa a ion o pho ogene a ed elec on– hole pai s and, consequen ly, he pho oca aly ic ac i i y. 2. Ma e ials and Me hods G aphi ic ca bon ni ide was p epa ed by he py olysis o dicyandiamide (DCDA) (Sigma-Ald ich, Da ms ad , Ge many, 99%) in a semi-closed sys em. In a 50-mL qua z c ucible wi h a co e , 4 g o DCDA p ecu so was added and hea ed a 600 ◦ C in a u nace o 4 h wi h a hea ing a e o 10 ◦ C min −1 unde ambien p essu e in he ai . A e cooling o oom empe a u e in he u nace, he ob ained yellow ma e ial was g ound in o a ine powde in an aga e mo a and labeled as CN. Me allic (P , Ru, and I ) pho oca alys s we e syn hesized using he we imp egna ion me hod. In 25 mL o wa e solu ions o H 2 P Cl 6 , H 2 I Cl 6 , and me hanolic solu ion o Ru 3 (CO) 12 , 2 g o he g aphi ic ca bon ni ide powde (CN) was dispe sed. The me al loading in he pho oca alys s was 0.5 and 1.0 w .%. The suspension was s i ed o 2 h a oom empe a u e (only o Ru 3 (CO) 12 a 40 ◦ C) and was subsequen ly e apo a ed. The samples we e d ied o e nigh a 80 ◦ C and educed a 300 ◦ C o 4 h a a hea ing a e o 10 ◦C min−1 unde ambien p essu e in he p esence o pu e hyd ogen. Because he educ ion can in luence he g-C 3 N 4 s uc u e, i was educed unde he same condi ions as hose o he pho oca alys s and was designa ed as CN . The pho oca alys s we e cha ac e ized by low- empe a u e N 2 adso p ion, X- ay di ac ion analysis (XRD), elemen al analysis (EA), ansmission elec on mic oscopy (TEM), scanning elec on mic oscopy (SEM), ul a iole - isible di use e lec ance spec- oscopy (UV-Vis), pho oluminescence spec oscopy (PL), and X- ay pho oelec on spec- oscopy (XPS). The speci ic su ace a ea (SSA) was de e mined by he B unaue –Emme –Telle (BET) me hod using a Mic ome i ics ASAP 2010 (Mic ome i ics, No c oss, GA, USA) su ace a ea and po osi y analyze (su ace a eas we e ob ained om N 2 -adso p ion iso he ms collec ed a 77 K). Cumula i e po e olume and po e diame e we e de e mined by Ba e –Joyne – Halenda (BJH) me hod om he deso p ion b anch o iso e m. The me al loading in he pho oca alys s a e educ ion was de e mined using he induc i ely coupled plasma (ICP) me hod. An Agilen 720-ES ICP (San a Cla a, CA, USA) Ma e ials 2022,15, 710 3 o 16 op ical emission spec ome e combined wi h a Vulcan 42S obo was used o de e mine he me al loading. The measu ed me al con en s we e close o hose assumed and amoun ed o 0.86, 0.82, and 0.90 w .% o P , Ru, and I , espec i ely. A Hi achi HT7700 mic oscope (Hi achi, Tokyo, Japan) a an accele a ing ol age o 100 kV was used o eco d TEM images. The pa icle size dis ibu ion his og ams we e calcula ed om 394 pa icles o P , 338 o Ru, and 172 o I using he ImageJ p og am ( .1.53e, 2020) de eloped a Na ional Ins i u es o Heal h and he Labo a o y o Op ical and Compu a ional Ins umen a ion (LOCI, Uni e si y o Wisconsin, Madison, WI, USA). An FEI Helios NanoLab 660 (The mo Fishe Scien i ic, Wal ham, MA, USA) elec on mic oscope was used o ob ain SEM images. The XRD analysis was pe o med in he 2 θ ange be ween 6 ◦ and 40 ◦ on a B uke D8 Ad ance di ac ome e (Bille ica, MA, USA) using CuK α adia ion. B agg’s law nλ= 2dsinθ (whe e n is an in ege , λ is he adia ion wa eleng h ( λ = 1.5418 Å), and θ is he e lec ion angle o he e lex hkl) was used o calcula e he dis ance d hkl o he sample. The c ys alli e size o g-C 3 N 4 was calcula ed using he Sche e o mula D = K λ / β cos θ , whe e D is he c ys alli e size in nm, K is he Sche e cons an (0.94), λ is he adia ion wa eleng h (λ= 1.5418 Å), and βis he ull wid h o he (002) c ys alli e peak a hal maximum. The elemen al analysis was pe o med using a Flash 2000 exhaus gas analyze (The mo Fishe Scien i ic, Wal ham, MA, USA) by combus ion a 900–1000 ◦C. The UV-Vis di use e lec ance spec a we e eco ded on a Jasco (Tokyo, Japan) model V-670 spec opho ome e . A Jasco (Tokyo, Japan) spec o luo ome e model FP-8300 using an exci a ion sou ce o 350 nm was employed o PL spec oscopy. The XPS analysis o he ca bon ni ides was pe o med using a K a os Axis Ul a spec ome e (K a os Analy ical, Manches e , UK). The exci a ion sou ce was a monoch om- a ized aluminum X- ay sou ce (Al K α (1486.6 eV) ope a ed a 10 mA and 15 kV. The cha ge e e encing me hod used was he C (C, H) componen o he C 1s peak o ad en i ious ca bon ixed a 284.5 eV. Spec oscopic da a we e p ocessed by CasaXPS e . 2.3.17PR1.1 so wa e (Casa So wa e L d., Teignmou h, UK) using a peak- i ing ou ine wi h Shi ley backg ound and asymme ical Voig unc ions. Pho oca aly ic es s we e pe o med in a s ainless-s eel ba ch-mixed pho o eac o ( olume = 348 mL). Ini ially, helium sa u a ed he eac ion mix u e, con aining 100 mL o 50% me hanol wi h a pho oca alys (0.1 g), o pu ge he ai . A UV LED lamp (365 nm wa eleng h; M365LP1, Tho labs, Be gki chen, Ge many) was he sou ce o i adia ion and was placed on a qua z glass window on he op o he pho o eac o in a ho izon al posi ion. Be o e he s a o he eac ion, a gaseous sample was aken (a ime 0 h) h ough he sep um using a sy inge. All gaseous samples we e analyzed using a gas ch oma og aph (Shimadzu T ace a GC-2010Plus, Kyo o, Japan) equipped wi h a ba ie discha ge ioniza ion de ec o . The eac ion was pe o med o 4 h, and gaseous samples we e ex ac ed a 1, 2, 3, and 4 h o analysis. Each es was conduc ed wice wi h he same pho oca alys o de e mine a leas sho - ime s abili y and eusabili y. 3. Resul s and Discussion 3.1. Pho oca aly ic Ac i i y One o he ac o s limi ing he pho oca aly ic ac i i y o semiconduc o s is he as cha ge ecombina ion. To es ic ecombina ion, an elec on-dona ing subs ance, such as a sac i icial agen , is added. This subs ance supplies he sys em wi h elec ons and simul aneously binds he elec on holes, which esul s in be e cha ge sepa a ion and enhances he e iciency o he wa e educ ion p ocess. In gene al, such sac i icial agen s a e ie hanolamine, me hanol, e hanol, o alipha ic o a oma ic compounds. Me hanol is one o he mos commonly used sol en s o indus ial applica ions; howe e , i can ha e ha m ul e ec s on he en i onmen . The e o e, as a be e op ion, i can be u ilized as a sac i icial agen in he pho oca aly ic decomposi ion o wa e [ 25 ]. Ou ea lie esul s [ 19 ] sugges ed ha g-C 3 N 4 can be an in e es ing al e na i e o he commonly used pho oca alys s in wa e /me hanol decomposi ion. Ma e ials 2022,15, 710 4 o 16 Figu e 1A shows he ac i i y and selec i i y o CN, CN , and me allic pho oca alys s. The ac i i ies and selec i i ies o CN and CN we e e y simila . This indica es ha he educ ion o ca bon ni ide does no signi ican ly a ec i s pho oca aly ic p ope ies. The ac i i y o CN and CN in he pho oca aly ic decomposi ion o he wa e /me hanol mix u e was ela i ely low (22 µ mol H2· g ca . −1 ) (Figu e 1A). The e o e, 0.5 w .% and 1.0 w .% o each o he me als P , Ru, and I we e in oduced on he su ace o CN using he con en ional imp egna ion me hod, ins ead o he commonly used pho o educ ion o chemical educ ion me hods. Me als inc ease he sepa a ion be ween he pho oinduced elec ons and holes; hus, an inc ease in he ac i i y was expec ed. The ac i i ies o all he pho oca alys s inc eased signi ican ly compa ed o hose o CN and CN , and he highes inc ease in he ac i i y ( o 0.5 w .% ca alys s), ha is by app oxima ely 22 imes, was obse ed o P (Figu e 1A). Mo eo e , he selec i i y o hyd ogen conside ably inc eased om 99.4% o CN o 99.8–100.0% o me al-modi ied pho oca alys s. Figu e 1. Pho oca aly ic ac i i y o ca bon ni ide (CN), educed ca bon ni ide (CN ), and me allic pho oca alys s in he decomposi ion o wa e /me hanol solu ion a e 4 h o i adia ion a di e en me al loadings: ( A ) 0.5 w .% and ( B ) 1.0 w .%. Black poin s and blue ba s ep esen pho oca aly ic ac i i y and selec i i y o hyd ogen, espec i ely. In he nex logical s ep, he possibili y o pho oca aly ic ac i i y enhancemen using a highe me al loading o 1.0 w .% was in es iga ed. The esul s a e shown in Figu e 1B. The mos signi ican change was he inc ease in he selec i i y o hyd ogen. I eached 100% o all he me allic pho oca alys s, while he selec i i y o hyd ogen using ca bon ni ide was 99.4%. The selec i i y o 100% is undoub edly a g ea ad an age when conside ing applica ions o hese pho oca alys s in uel cells. I is ob ious ha e en he p esence o ace amoun s o CO in hyd ogen uel cells can be dange ous [ 26 ]. Along wi h he inc ease in hyd ogen selec i i y, he change in me al loading om 0.5 o 1.0 w .% also inc eased he pho oca aly ic ac i i y, as seen in Figu e 1B. I should be men ioned ha each pho oca aly ic es was conduc ed wice on he same pho oca alys sample and no dec ease in ac i i y o change in selec i i y was obse ed. This p o es he sho - ime s abili y and eusabili y o he pho oca alys s. The signi ican inc ease in he pho oca aly ic ac i i y o he P pho oca alys (24 imes highe han ha o CN ) could be ela ed o he physical and chemical changes in he s uc u e, mo phology, and composi ion o he pho oca alys . Gene ally, he s anda d physicochemical cha ac e iza ion is used in such cases o e eal he smalles changes in he pho oca alys s and unde s and he ac o s esponsible o he inc ease in hei pho oca aly ic ac i i y. 3.2. Su ace A ea and Po osi y o Me al/CN Pho oca alys s In he imp egna ion me hod, he ac i e phase co e s he suppo su ace. Conse- quen ly, he su ace a ea a e deposi ion o he ac i e phase is expec ed o dec ease com- pa ed o ha o he pu e suppo , because he ac i e phase may block o ill he po es in he Ma e ials 2022,15, 710 5 o 16 suppo . A e he deposi ion o he P -g oup me als, he su ace a ea o he pho oca alys inc eased, and he inc ease in he su ace a ea was signi ican —36% o P /CN pho oca a- lys . Simul aneously, he educ ion did no in luence he SSA o g-C 3 N 4 . Fu he mo e, he su ace a ea o he pho oca alys s wi h highe me al loading (1.0 w .%) was sligh ly highe han ha o he pho oca alys s wi h he me al loading o 0.5 w .% (Figu es 2A and S1). This is inconsis en wi h he mechanism o po e blocking by he ac i e phase. Figu e 2. Speci ic su ace a ea o ca bon ni ide and pho oca alys s wi h 1.0 w .% me al loading—( A ). Cumula i e po e olume and a e age po e size o 1.0 w .% pho oca alys s—(B). Wha is he eason o such an inc ease in he su ace a ea o he P -g oup me al pho oca alys s? Fo ac i e phase con en s as low as 0.5 o 1.0 w .%, he inc ease in he su ace a ea canno be assigned o he appea ance o his phase (me al) in he ca aly ic sys em, hus i mus be ela ed o he s uc u al changes in he suppo . An explana ion could be p oposed assuming ha changes in he suppo s uc u e occu ed du ing he pho oca alys ’s educ ion was ca ied ou a 300 ◦ C o 4 h in he low o hyd ogen. In his p ocess, he g aphi ic ca bon ni ide (o ganic polyme ) is exposed o a omic hyd ogen, an agg essi e educing agen , o med on he me al su ace du ing educ ion. Pla inum-g oup me als a e known o hei high ac i i y in hyd ogenolysis and hyd ogena ion eac ions. In he icini y o me al pa icles, hyd ogenolysis and hyd ogena ion o he C − N bonds can occu , leading o he pa ial e ching o g-C 3 N 4 [ 27 ]. Consequen ly, new po es and new s uc u al de ec s a e gene a ed; hence, he SSA will inc ease. Indeed, he o al su ace a ea o he pho oca alys s con aining me als inc eased compa ed o ha o he CN suppo , as shown in Figu es 2A and S1. This inc ease is pa icula ly no iceable o he pho oca alys s wi h 1.0 w .% o P and Ru. Figu e 2B shows he cumula i e po e olume and a e age po e size o 1.0 w .% pho oca alys s. The dec ease in he po e diame e is due o he inc eased con ibu ion o small po es. The po osi y and SSA esul s clea ly indica e ha g-C 3 N 4 mus unde go s uc u al changes du ing he pho oca aly ic educ ion p ocess, which leads o an inc ease in he SSA. Howe e , hese impo an modi ica ions in he s uc u e should also a ec he esul s ob ained using o he cha ac e iza ion me hods. This was con i med by se e al echniques, as discussed below. 3.3. Elec on Mic oscopy Analysis Figu e 3shows he TEM images o ca bon ni ide (be o e and a e educ ion) and he pho oca alys s. The a e age size o he me al c ys alli es was de e mined om hese images, and his og ams o pa icle size dis ibu ion we e p epa ed. The pa icle size dis ibu ion is na ow, and he size o he me allic pa icles is app oxima ely 2 nm. Ma e ials 2022,15, 710 6 o 16 Figu e 3. TEM images o CN, CN , and me al pho oca alys s (1.0 w .%). The inse igu es in he TEM images o he me al pho oca alys s show he co esponding pa icle size dis ibu ion. The SEM images did no show any no iceable di e ences in he s uc u e o he suppo s (CN and CN ) and me allic pho oca alys s (Figu e S2). The images o all he pho oca alys s show la ge c ys alline pa icles o g-C 3 N 4 , which ha e a laye ed s uc u e and a e ayed a he edges. 3.4. Powde Di ac ome y o Me al/CN Ca alys s Powde di ac og ams o CN, CN , and he me al pho oca alys s a e shown in Figu e 4A . Each di ac og am shows wo cha ac e is ic e lec ions o ca bon ni ide. The i s e lec ion (2 θ = 27.7 ◦ ) co esponds o he (002) c ys allog aphic plane and indica es he in e laye s acking o a oma ic ings. The second e lec ion (2 θ = 12.8 ◦ ) co esponds o he (210) plane and is ela ed o he in-plane s uc u al packing mo i o i-s- iazine uni s in melon (sepa a ion be ween pa allel melon chains) [ 28 – 31 ]. The in ensi y o he e lec ion a ~27.7 ◦ (002) in he di ac og ams o he pho oca alys s is sligh ly lowe han ha o he suppo , and a he same ime, i s ull wid h a hal maximum (FWHM) inc eases. F om he la e pa ame e , he size o he g-C 3 N 4 c ys alli es was es ima ed using he Sche e o mula. Fo he me allic pho oca alys s, he size o he g-C 3 N 4 c ys alli es was sligh ly smalle han ha o he CN (o CN ) suppo , as shown in Table 1. The numbe o laye s composing he ca bon ni ide c ys alli e was calcula ed by di iding he size o g-C 3 N 4 c ys alli es by he in e plana dis ance d. The numbe o laye s was ~30 o CN and ~29 o he me al pho oca alys s ( ounded up o uni y). This indica es ha he la ge-scale s uc u e o ca bon ni ide unde goes sligh des uc ion. Ma e ials 2022,15, 710 7 o 16 Figu e 4. XRD pa e ns o p is ine CN, CN , and me allic pho oca alys s (1.0 w .%) a di e en 2 θ angles: ( A ) 11 ◦ –31 ◦ , ( B ) 11.5 ◦ –14.5 ◦ , and ( C ) 35 ◦ –50 ◦ . The g ey lines in ( C ) ep esen he XRD pa e n o CN , which was used as a e e ence o pho oca alys s o highligh e lec ions om me als. Table 1. Sizes o he g-C 3 N 4 c ys alli es calcula ed using he Sche e o mula based on he (002) e lex o CN, CN , and 1.0 w .% me al pho oca alys s. Sample Peak Posi ion 2θ (deg) FWHM (deg) In e plana Dis ance (nm) C ys alli e Size (nm) Numbe o Laye s CN 27.616 0.8383 0.323 10.20 31.6 CN 27.602 0.8331 0.323 10.26 31.7 P /CN 27.586 0.9024 0.323 9.47 29.3 Ru/CN 27.806 0.9182 0.321 9.32 29.1 I /CN 27.669 0.9188 0.322 9.31 28.9 Figu e 4B shows he di ac og ams o he suppo and pho oca alys s in he 2 θ angle ange o 11.5 ◦ –14.5 ◦ . The e lec ion co esponding o he plane (210) appea s in his ange, which is ela ed o he in-plane s uc u al packing mo i o hep azine uni s. A e he imp egna ion o ca bon ni ide wi h he me als, a signi ican dec ease in he in ensi y o his e lec ion and i s shi owa ds highe 2 θ a e obse ed. I indica es a sligh dec ease in he dis ance be ween he pa allel melon chains. These changes a e clea ly no able. Figu e 4C shows he di ac og ams o he pho oca alys s a he 2 θ angle ange (35 ◦ –50 ◦ ) in which he e lec ions om he me allic phases appea . Such e lec ions we e obse ed in di ac og ams o all pho oca alys s— hey a e wide, which implies a high dispe sion o he me al phase. In summa y, he XRD esul s e ealed changes in he s uc u e o g-C 3 N 4 as a esul o he in oduc ion o me als on o he g-C 3 N 4 su ace. In gene al, he changes can be desc ibed as an inc ease in he numbe o de ec s in he g-C 3 N 4 suppo s uc u e (a dec ease in he deg ee o o de ing). 3.5. Elemen al Analysis P is ine CN, CN , and he me al pho oca alys s we e subjec ed o elemen al analysis a e hei educ ion o de e mine he con en s o N, C, and H (Table 2). No no iceable di e ences in he elemen al composi ion o CN and CN we e obse ed; he e o e, Table 2 p esen s only he esul s o CN . In gene al, he con en s o C, N, and H in he pho- oca alys s we e simila o hose in CN . Howe e , one no iceable di e ence is ha he ca bon con en emains unchanged wi h simul aneous educ ion o ni ogen con en . The dec ease in he ni ogen con en can be clea ly obse ed om he C/N a omic a io (Table 2). Depending on he ca alys , he C/N a omic a io is highe han ha o p is ine CN by 4–8%. Ma e ials 2022,15, 710 8 o 16 Apa om he abo e di e ence, he hyd ogen con en in all pho oca alys s, excep P , was highe han ha in CN . Table 2. Elemen al analysis o CN and 1.0 w .% me al pho oca alys s. Sample A .% C/N N C H O CN 45.94 32.13 19.04 2.89 0.70 P /CN 45.44 33.81 18.52 2.23 0.74 Ru/CN 41.70 31.51 24.98 1.81 0.76 I /CN 45.10 33.39 19.67 1.84 0.74 3.6. UV-Vis and PL Spec al Analysis The UV-Vis spec a o he pho oca alys s (Figu e 5A) display a cha ac e is ic band in he ange o 190–450 nm ha co esponds o he a oma ic s uc u es in g-C 3 N 4 . All UV-Vis spec a show wo cha ac e is ic peaks: he i s a ~250 nm a ibu ed o he π→π* ansi ions in a oma ic ings, and he second a ~380 nm a ibu ed o he elec on ansi ions om he nonbonding o bi als o ni ogen a oms o he an ibonding a oma ic o bi als. An inc ease in abso p ion in he isible ange abo e 600 nm was obse ed (known as he U bach ails) o all pho oca alys s [ 32 , 33 ]. Compa ed o CN , me allic pho oca alys s exhibi an appa en ly b oade and s onge abso p ion ail ex ending o 1000 nm, which is a ibu ed o he de ec - ela ed s a es loca ed wi hin he band gap [ 34 ]. The inc eased ligh abso p ion may gene a e a highe numbe o elec on–hole pai s unde isible ligh , esul ing in u he e ec i e abso p ion o adia ion by he pho oca alys s and hei inc eased ac i i y. Figu e 5. UV–Vis di use e lec ance—( A ) and PL spec a—( B ) o CN, CN , and me al pho oca alys s (1.0 w .%). Inse in (A) shows he band gap ene gies o he samples. As shown in Figu e 5A, ca bon ni ides ha e he highes abso p ion, pa icula ly a 365 nm , a which he pho oca aly ic eac ions we e pe o med. This sugges s ha he e mus be ano he ac o de e mining he pho oca alys ac i i y besides he abili y o abso b adia ion. I should be conside ed ha he added me als gene a e addi ional s a es wi hin he band gap, diminishing he elec on–hole ecombina ion a e, which enhances he pho oca aly ic ac i i y. The band gap size was de e mined om he Kubelka–Munk plo , illus a ing he dependence o F(R) 2 on he pho on ene gy. The band gaps o he pho oca alys s we e de e mined as 2.68–2.74 eV and we e all sligh ly g ea e han hose ob ained o CN and CN (2.66 and 2.67 eV) (inse in Figu e 5A). The inc ease in he band gap ene gy was ea lie obse ed o ul a-low loading o Ru clus e s o e g-C 3 N 4 , and i was in e p e ed as ela ed o he dec eased deg ee o o de ing o he g-C 3 N 4 s uc u e [ 35 ]. Fu he mo e, Maschmeye e al. [ 36 ] demons a ed a ela ionship be ween he band gap Ma e ials 2022,15, 710 9 o 16 inc ease and s uc u al changes in g-C 3 N 4 esul ing om he high empe a u e (550 ◦ C) ea men in hyd ogen. The PL emission in semiconduc o s, such as g-C 3 N 4 , esul s om he ecombina ion o pho oinduced elec on–hole pai s. Hence, based on he PL in ensi y, we can conclude ha he lowe he PL in ensi y, he smalle is he ecombina ion a e o he pho ogene a ed elec ons and holes. The di e ences in band in ensi y be ween CN and CN we e negligible, he eby indica ing ha he educ ion did no al e he pho oluminescen p ope ies o ca bon ni ide. A s ong (g ea e han h ee olds) PL emission quenching o all me al- loaded samples compa ed o ha o CN was obse ed (Figu e 5B). Me allic si es ac as empo a y elec on- apping si es whe e elec ons om he conduc ion band o g-C 3 N 4 a e apped be o e ecombina ion wi h he holes in he alence band [ 37 ]. As a esul , signi ican ly as ecombina ion o elec ons wi h holes is p e en ed and he li e ime o he pho oinduced elec on–hole pai s is ex ended, which con ibu es o an inc ease in he pho oca aly ic ac i i y. 3.7. XPS Analysis X- ay pho oelec on spec oscopy is an ex emely use ul echnique o he ad anced cha ac e iza ion o g-C 3 N 4 ma e ials [ 28 , 38 ]. This echnique allows he de e mina ion o he combina ions in which ni ogen and ca bon a e linked and he quan i ica ion o hei a ios. In ou p e ious wo k [ 28 ], a de ailed discussion on he possibili y o using he XPS echnique in he s udy o g-C 3 N 4 has been p o ided, and indi idual XPS signal assignmen s o speci ic ni ogen we e p oposed. The e is s ill much con o e sy in he li e a u e conce ning hese a ibu ions. In gene al, he N 1s spec um dis inguishes ou signals a ibu ed o py idinic ni ogen in he hep azine ing (398.6 eV; deno ed Py), p ima y amine -NH 2 (399.3 eV; deno ed NH 2 ), seconda y amine -NH- (400.3 eV; deno ed NH), and qua e na y ni ogen o igina ing om he N-(C)3componen (401.4 eV; deno ed Q) [28,38,39]. Figu e 6shows he N 1s and C 1s co e-le el XPS spec a o he CN , P /CN, and Ru/CN samples (1.0 w .%). The spec a o CN and CN we e p ac ically iden ical; he e o e, only CN is included in Figu e 6. In he N 1s spec a o CN and me al pho oca alys s, he signals o igina ing om Py, -NH 2 , NH, and so-called qua e na y (also called e ia y) ni ogen in sp 2 hyb idiza ion could be dis inguished. A i s glance, he di e ences in he spec a o CN and o he pho oca alys s seem minimal. Howe e , close inspec ion o he spec a e eals an ele a ion o lines in he 399–401 eV egion in he P , Ru, and I ca alys s compa ed o CN ( ep esen ed by he a ows in Figu es 6C,E and S3), he eby indica ing an inc ease in he p opo ion o ni ogen bonded o he p ima y and seconda y amine g oups. An inc ease in he p opo ion o amine g oups was obse ed o all me als. Based on he XPS spec a, he pe cen ages o each ni ogen o m we e calcula ed and a e shown in Figu es 7A and S1. The mos signi ican changes in he dis ibu ion o a a ie y o ni ogen we e obse ed o he P /CN pho oca alys . The e was a s iking dec ease in he p opo ion o Py ni ogen (o e 2%), accompanied by a compa able inc ease in he p opo ion o amine ni ogen (Figu e 7A). The amoun o qua e na y ni ogen did no change. This sugges s ha du ing he educ ion o he P /CN pho oca alys , hyd ogenolysis o N − C=N bonds in hep azine g oups occu s and new − NH 2 g oups a e o med a he expense o Py ni ogen. 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