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De elopmen o nanos uc u ed ca bon-based ca alys s o pho oca aly ic con e sion o CO2 in o added- alue uels Au ho Yi Cai Di ec o F ancisco Balas Nie o Mas e in Nanos uc u ed Ma e ial o Nano echnology Applica ions 2019/2020 De elopmen o nanos uc u ed ca bon-based ca alys s o pho oca aly ic con e sion o CO2 in o added- alue uels Yi Cai Abs ac Nowadays, ene gy sho ages, ca bon esou ce sho ages, and global wa ming a e he majo challengs. The usage o pho oca alysis o ealize he con e sion o CO2 in o added- alue uels can alle ia e he abo e p oblems. As a isible ligh pho oca alys wi hou me al componen s, g-C3N4 has unique elec onic s uc u e, high ca aly ic ac i i y, good chemical and he mal s abili y, which has caused wide a en ion. This a icle mainly summa ized he basic p ope ies and s uc u e o g-C3N4. Besides, we in oduced he commonly used p epa a ion me hods and modi ica ion me hods o g-C3N4, and p ospec s he applica ion o g-C3N4. Keywo ds: g-C3N4; pho oca alysis; CO2 educ ion INDEX De elopmen o nanos uc u ed ca bon-based ca alys s o pho oca aly ic con e sion o CO2 in o added- alue uels ..................................................................................................................... 2 Abs ac ............................................................................................................................................ 2 1 In oduc ion .................................................................................................................................. 4 2 Pho oca aly ic educ ion o CO2 ................................................................................................. 4 2.1 Basic p inciples and eac ion mechanism ....................................................................... 5 2.2 Applica ion o ca bon-based pho oca aly ic ca alys s in CO2 educ ion ...................... 6 2.2.1 G aphene and i s de i a i es ................................................................................. 6 2.2.2 GDY ......................................................................................................................... 7 2.2.3 g-C3N4 ...................................................................................................................... 7 3 Ad an ages and challenges o g-C3N4 ca alys ........................................................................... 8 4 g-C3N4 ca alys and he e ec o i s p epa a ion me hod g-C3N4 on pe o mance ................. 8 4.1 Ha d- empla ing me hod .................................................................................................. 9 4.2 So - empla ing me hod .................................................................................................. 10 4.3 No- empla ing me hod .................................................................................................... 12 5 Modi ica ion o g-C3N4 pho oca aly ic ma e ial in CO2 educ ion ........................................ 14 5.1 Su ace enginee ing ......................................................................................................... 14 5.1.1 Func ional g oup modi ica ion ............................................................................ 15 5.1.2 Su ace de ec s modi ica ion ............................................................................... 16 5.2 Semiconduc o compound .............................................................................................. 16 5.2.1 Type II he e ojunc ion ......................................................................................... 17 5.2.2 Type I and Type III he e ojunc ion ..................................................................... 18 5.2.3 Scho ky junc ion ................................................................................................. 19 5.2.4 Z ype he e ojunc ion ........................................................................................... 20 5.3 Elemen doping ................................................................................................................ 21 5.3.1 Non-me allic elemen doping ............................................................................... 21 5.3.2 Me al doping ......................................................................................................... 23 5.4 Speci ic su ace a ea con ol modi ica ion echnology ................................................. 24 5.4.1 Mesopo ous g-C3N4 .............................................................................................. 25 5.4.2 Nano lakes ............................................................................................................. 27 5.4.3 g-C3N4 wi h di e en mo phologies .................................................................... 28 6 Conclusions ................................................................................................................................. 29 Bibliog aphy .................................................................................................................................. 30 1 In oduc ion Ca bon dioxide (CO2) is he main g eenhouse gas in he a mosphe e, and he inc easing concen a ion o CO2 is one o he mos se ious p oblems which con ibu e o global wa ming and clima e change. Due o he de elopmen o indus y and socie y, ca bon dioxide emissions mainly come om he bu ning o ossil uels in ene gy use. As he demand o ossil esou ces con inues o g ow in he coming decades, i is essen ial o educe ca bon dioxide emission. The main ways o educe ca bon dioxide can be di ided in o emission educ ion and pos - ea men . The pos - ea men o ca bon dioxide can be u he di ided in o deep sea bu ial, chemical adso p ion s o age and chemical con e sion. Compa ed wi h adi ional, high-ene gy-consuming me hods such as cap u e o geological s o age, he esou ce u iliza ion o CO2 is a mo e p omising app oach which can simul aneously alle ia e he g eenhouse e ec and ene gy c isis. Va ious app oaches ha e been de eloped in basic esea ch, such as he mal ca alysis, pho oca alysis, elec oca alysis, pho o-elec ic syne gis ic ca alysis and o ganic ca aly ic con e sion, o con e CO2 molecules in o high alue-added chemicals. Among hem, CO2 con e sion d i en by sola ene gy is an en i onmen ally iendly ea men me hod which has ou s anding ad an ages such as mild eac ion condi ions and so on. The pho oca aly ic educ ion o CO2 is simila o he pho osyn hesis o plan s in na u e. I akes he g eenhouse gas CO2 emi ed by humans and he abundan wa e esou ces on he ea h as aw ma e ials. Then CO2 is educed o a ious ene gy-con aining educing p oduc s h ough he ac ion o ligh ene gy. I is consumed by he exis ing o ms o ene gy u iliza ion again, and inally comple es he ca bon cycle p ocess, ealizing he balanced de elopmen o ene gy demand o human de elopmen and sus ainable use o en i onmen al esou ces. 2 Pho oca aly ic educ ion o CO2 Inspi ed by pho osyn hesis, sola ene gy is used o p oduce hyd oca bons and O2 om CO2 and H2O. In his p ocess, ene gy can be simul aneously gene a ed when CO2 in he a mosphe e can be educed. The e o e, pho oca aly ic educ ion o CO2 is he mos impo an way in a i icial pho osyn hesis. 2.1 Basic p inciples and eac ion mechanism When he pho oca alys is exci ed by adian ligh , he gene a ed cha ge ca ie s will mig a e o he su ace o he pho oca alys and in e ac wi h he CO2 molecules adso bed on he su ace. The p inciple o pho oca aly ic educ ion o CO2 is shown in Fig 11. In pa icula , he eac ion o pho oca aly ic educ ion o CO2 includes CO2 educ ion and H2O oxida ion, in ol ing he clea age o C=O bonds and he o ma ion o C-H bonds. The e o e, he pho o educ ion o CO2 is no a single-elec on eac ion p ocess, bu a p o on-induced mul i-elec on eac ion o p oduce a ious p oduc s, including he ollowing p ocesses: CO2 + 2H+ + 2e−→HCOOH E edox= − 0.20 V（1） CO2 + 2H+ + 2e−→CO + H2O E edox= − 0.12 V（2） CO2 + 4H+ + 4e−→HCHO + H2O E edox= − 0.07 V（3） CO2 + 6H+ + 6e−→CH3OH + H2O E edox= + 0.03 V（4） CO2 + 8H+ + 8e−→CH4 + 2H2O E edox= + 0.17 V（5） The modynamically, CO2 is an ex emely s able molecule. The dissocia ion ene gy o he C=O double bond is as high as 750 kJ⋅mol-1, which is signi ican ly highe han he C-H bond (430 kJ⋅mol-1) and C-C bond (336 kJ⋅mol-1) in he educ ion p oduc , which means ha a la ge amoun o ene gy needs o be injec ed in o he sys em o igge he ac i a ion and con e sion p ocess o CO2. A he same ime, he C (Ⅳ) is in he highes oxida ion s a e in he CO2 molecule, and a ious p oduc s can be ob ained acco ding o he numbe o elec ons ob ained in he eac ion. In he pho oca aly ic eac ion, CO2 can usually be educed o ca bon monoxide (CO), me hane (CH4), o mic acid (HCOOH) o e hanol (CH3OH) and o he subs ances, which wi h he side eac ion o wa e educ ion o gene a e hyd ogen, and i will be signi ican ly educed he selec i i y o he a ge p oduc . Fig. 1. Schema ic illus a ion o eac ion s eps in pho oca aly ic CO2 educ ion wi h H2O F om he cu en esea ch s a us, he pho o educ ion o CO2 s ill aces he di icul ies o low con e sion a e and poo selec i i y. In o de o ob ain a highe e iciency, he ca alys no only needs o ha e a sui able ene gy band s uc u e, bu also needs o op imize i s su ace s uc u e and elec onic s uc u e o imp o e he abili y o ca alys o adso b and ac i a e CO2 which could inhibi he o ma ion o by-p oduc s. The e o e, how o o e come hese obs acles has become an impo an issue in he design o e icien pho oca alys s o CO2 pho oca aly ic educ ion. 2.2 Applica ion o ca bon-based pho oca aly ic ca alys s in CO2 educ ion The e a e many kinds o pho oca alys s which can be used in he pho o educ ion o CO2. They can be di ided in o me al and non-me al pho oca alys s. The o me includes oxides, sul ides, bismu h oxyhalides (BiOX), me al o ganic amewo k compounds (MOFs) and so on. The la e mainly includes ca bon-based ca alys s and h-BN. This a icle will ocus on he speci ic in oduc ion o ca bon-based ca alys s and examples o hei ealiza ion o he pho oca aly ic con e sion o CO2. 2.2.1 G aphene and i s de i a i es G aphene and i s de i a i es (GO, GO) a e a well-known ca alys . Since g aphene was i s epo ed in 2004, i has been widely used in many esea ch ields due o i s excellen mechanical, he mal, op ical and elec onic p ope ies. As shown in Fig 22, he use o g aphene-based ma e ials o CO2 pho o educ ion shows he ollowing ad an ages: (1) Ul a- hin 2D mo phology gi es g aphene a high speci ic su ace a ea and p o ides abundan adso p ion si es o he eac ion subs a e. (2) G aphene is a ze o band gap ma e ial wi h excellen conduc i i y and elec on mobili y. (3) I can o m π-π conjuga e wi h CO2 molecules o p omo e CO2 ac i a ion. Zou3 ob ained a TiO2/g aphene composi e ma e ial ich in Ti3+ si es by using he simul aneous educ ion-hyd olysis echnology. The abundan Ti3+ in TiO2 pa icles can cap u e pho ogene a ed elec ons o inhibi ecombina ion, and syne gize wi h g aphene o pho oca aly ic eac ion o achie e CO2 educ ion and coupling o gene a e CH4 and C2H6. Fig.2. G aphene-based pho oca alys s o CO2 pho o educ ion. 2.2.2 GDY G aphene (GDY) is an allo ope o g aphene. I is a 2D plana s uc u e composed o sp- and sp2-hyb id ca bon, wi h a band gap anging om 0.46 o 1.22 eV4. In e ms o elec ical p ope ies, GDY has a mode a e band gap and high elec on mobili y (104~105 cm2⋅V-1⋅s-1). The abundan C≡C makes he elec on conjuga ion deg ee o GDY high, which is e y bene icial o he adso p ion and ac i a ion o CO2. F om he s uc u al poin o iew, GDY con ains pe iodic a angemen o iangula holes, which can e ec i ely educe i s densi y4-5. Due o he many uncon ollable ac o s in he eac ion o p epa ing 2D GDY, including monome s abili y, side eac ions, monome o ien a ion coupling6, low yield, e c., he e a e only a ew li e a u e epo s7 abou he use o 2D GDY in CO2 pho o educ ion eac ions. 2.2.3 g-C3N4 g-C3N4 is a new ype o non-me allic semiconduc ing polyme ma e ial. I consis s o C and N a oms h ough sp2-hyb idiza ion o o m a la ge π conjuga ed sys em wi h a high deg ee o delocaliza ion, gene ally composed o an i- iazine ing s uc u al which has a la ge binding ene gy and mo e s able. Mos o he li e a u e usually uses N- ich molecules such as u ea, cyanamide and melamine as p ecu so s o o m g-C3N4 a high empe a u es8. I has mo e sui able Eg (≈2.77 eV) and CB posi ions (≈-1 V). A he same ime, he spec al abso p ion co e ing he ul a iole and isible egions can mee he needs o CO2 educ ion. 3 Ad an ages and challenges o g-C3N4 ca alys In ecen yea s, g-C3N4 has become a popula ma e ial in he ield o pho oca alysis due o i s special ene gy band s uc u e and easy manu ac u ing ad an ages. I can be exci ed by isible ligh i adia ion, so i has g ea po en ial alue o he u iliza ion and con e sion o sola ene gy; due o he exis ence o quan um con inemen e ec , he posi ion o CB o g-C3N4 mo es up sligh ly, and he educ ion abili y is u he enhanced. I o ms π-π conjuga ion wi h CO2, which signi ican ly p omo es he adso p ion and ac i a ion o CO2. Howe e , g-C3N4 ca alys s ill has some bo lenecks ha limi i s pho oca aly ic ac i i y, such as high pho o-gene a ed elec on-hole pai ecombina ion e iciency, limi ed su ace ca aly ic eac ion ac i e si es and low speci ic su ace a ea. Bu mos impo an ly, g-C3N4 is a e y ideal modi ica ion pla o m, and i s pe o mance can be op imized by con olling he p epa a ion me hod o he ma e ial, as well as he doping, compounding, de ec cons uc ion, su ace unc ional g oup con ol and o he ways. 4 g-C3N4 ca alys and he e ec o i s p epa a ion me hod g-C3N4 on pe o mance In ecen yea s, some new p ope ies o g-C3N4 ha e been g adually disco e ed by scien i ic esea che s, which has u he p omo ed he upsu ge o esea ch on g-C3N4 semiconduc o ma e ials. So a , g-C3N4 wi h a a ie y o mo phologies has been success ully syn hesized, including nano ibe s, nano ubes, nanoshee s, sphe es, e c. These s udies ha e s ongly p omo ed he deep de elopmen o g-C3N4 in he ield o pho oca alysis. Using ni ogen-con aining o ganic molecules as p ecu so s, i is he simples and di ec me hod o p epa e g-C3N4 ma e ials unde high empe a u e (500~600 ℃) py olysis polyme iza ion in ai o ine a mosphe e. Howe e , g-C3N4 p epa ed by his me hod had sho comings such as low speci ic su ace a ea, which g ea ly limi s i s applica ion in he ield o ca alysis. In compa ison, he empla e me hod is cu en ly he main me hod o p epa ing g-C3N4 wi h high speci ic su ace a ea and la ge po e size. Acco ding o he ype o empla e used, i can be di ided in o ha d empla e me hod, so empla e me hod and no empla e me hod. 4.1 Ha d- empla ing me hod The ha d- empla ing me hod is used o design and p epa e g-C3N4 ma e ials wi h a ious s uc u es and mo phologies anging om millime e s o nanome e s. The in oduc ion o di e en mo phologies and highly egula po e s uc u es can e ec i ely adjus he s uc u al p ope ies and su ace in e ac ion o g-C3N49-10, p omo ing he mass ans e and di usion p ocess o he ca alys su ace, helping op imize i s semiconduc o ene gy band s uc u e and ligh abso p ion cha ac e is ics, and imp o ing g-C3N4 pho oca aly ic pe o mance10. Compa ed wi h adi ional me hods, he pene a ion o he p ecu so solu ion in o he po e s uc u e o he empla e will induce he syn hesis o g-C3N4 wi h di e en mo phologies. The illing o he p ecu so solu ion in he empla e hole channel di ec ly a ec s he inal mo phology o he ca bon ni ide p oduc . The e a e h ee main s eps o p epa e g-C3N4 by ha d- empla ing me hod: (1) Imme se he p ecu so in o he empla e. (2) D y and oas a high empe a u e o make he p ecu so in he po es o he empla e polycondensa e o o m g-C3N4. (3) The empla e is emo ed by using HF o NH4HF2 sol en o ob ain g-C3N4 ma e ials wi h a mesopo ous s uc u e11-12. Goe mann13p epa ed mesopo ous g-C3N4 by ha d- empla ing me hod. Using mesopo ous SiO2 sphe es wi h a con ollable nanos uc u e as a ha d empla e, he cyanamide p ecu so was uni o mly dispe sed in he mesopo es o he SiO2 sphe es, and hen he mally polyme ized. The empla e was emo ed wi h NH4HF2 sol en o he eby g ea ly educing he ene gy gap (~0.7 eV) 35. Fig. 3. (A) Illus a ion o ab ica ion o he S doping me hyl-modi ied g-C3N4. (B) The o ma ion ene gy (E ) o subs i u e possible N a oms by S a oms in g-C3N4 ne wo ks. 5.1.2 Su ace de ec s modi ica ion De ec enginee ing is an in e es ing s a egy o op imize he pho oca aly ic e iciency o semiconduc o ma e ials. As we all know, de ec enginee ing o g-C3N4 can e ec i ely enhance cha ge sepa a ion, op imize he ene gy band s uc u e and ex end he ligh esponse. The e o e, a ious su ace de ec s o g-C3N4, such as ca bon acancies, ni ogen acancies, cyanamide de ec s and s uc u al edge de ec s, ha e been ex ensi ely s udied in ecen yea s o imp o e he pho oca aly ic pe o mance o g-C3N4 in CO2 educ ion. Zhang36 de ised a new s a egy o p epa e g-C3N4 wi h ich po ous s uc u e and he e os uc u e de ec s doped wi h sul u a oms by p ocessing g-C3N4 in he p esence o CH3CN and H2S. In his special gas en i onmen , he o iginal g-C3N4 nanoshee s a e e ched o p oduce nanopo es. In addi ion, due o he incomple e con e sion o H2S, he melon uni is des oyed o o m CS bonds, cyano g oups and S. The p epa ed pho oca alys showed excellen ene gy band s uc u e, ex ended ligh abso p ion and as ca ie anspo , which g ea ly imp o ed he pho oca aly ic pe o mance o H2O decomposi ion. 5.2 Semiconduc o compound Semiconduc o ecombina ion e e s o he usage o semiconduc o ma e ials wi h di e en ene gy le els and ma ching ela i e ene gy band posi ions o ecombine on he su ace o g-C3N4. g-C3N4 mainly plays wo oles. The i s is o p o ide elec ons o he sys em, because g-C3N4 has a wo-dimensional laye ed s uc u e wi h many small in-plane epea ing uni s, and he epea ing uni s ha e a conjuga e s uc u e, so g-C3N4 has one The la ge conjuga e s uc u e acili a es elec on ansmission; he second is o coope a e wi h o he semiconduc o s o o m a he e ojunc ion, p omo ing he sepa a ion and ansmission o pho ogene a ed ca ie s, and he pho oelec ic con e sion e iciency. Acco ding o he he e ojunc ion o med by he ecombina ion o ca bon ni ide and semiconduc o , i can be oughly di ided in o ype II he e ojunc ion, ype I he e ojunc ion, ype III he e ojunc ion, Scho ky junc ion and Z ype he e ojunc ion. 5.2.1 Type II he e ojunc ion Fo he ype II he e ojunc ion, he band edges o he wo semiconduc o s a e a anged s agge ed, causing he conduc ion band elec ons o he high conduc ion band semiconduc o mig a e o he conduc ion band o he low conduc ion band semiconduc o , and he alence band elec ons o he high alence band semiconduc o o he low alence band semiconduc o . The mig a ion o he alence band causes he pho ogene a ed elec ons and holes o accumula e in he low conduc ion band and low alence band, espec i ely, inhibi ing hei ecombina ion. Liu37 syn hesized ZnIn2S4-g-C3N4 nano-laye ed composi e by hyd o he mal me hod. The band s uc u e o ZnIn2S4 (2.34~2.48 eV) and g-C3N4 can be ma ched o o m a ype II he e ojunc ion, and he in e ace be ween he wo close con ac , e ec i ely ans e and sepa a e pho o-gene a ed ca ie s (as shown in Fig 4). The same ype o subs a e can also be used as a basis o cons uc a homogeneous g-C3N4 ype II he e ojunc ion. Fo example, Dong38 used dicyandiamide and u ea as p ecu so s, and ob ained simila subs a es h ough hea ea men o o m g-C3N4/ g-C3N4 me al- ee ype II he e ojunc ion. Unde isible ligh i adia ion, CN-D (wi h dicyandiamide as he p ecu so ) The conduc ion band elec ons o ca bon ni ide ob ained by he sys em mig a e o he conduc ion band o CN-U (ca bon ni ide ob ained by using u ea as he p ecu so sys em), and he alence band holes o he la e mig a e o he alence band o he o me (as shown in he Fig 5), o achie e e ec i e sepa a ion o elec ons and holes. Fig.4. Mechanism o he enhanced pho oca aly ic ac i i y o ZnIn2S4-g-C3N4 composi es Fig.5. Illus a ion o pho ogene a ed elec ons and holes ans e p ocess on ype Ⅱ g-C3N4/ g-C3N4 he e os uc u e unde isible ligh i adia ion 5.2.2 Type I and Type III he e ojunc ion Fo ype I he e ojunc ion, he conduc ion band and alence band posi ions o semiconduc o 1 a e mo e nega i e and posi i e han hose o semiconduc o 2, espec i ely, as shown in Fig 6a. The elec ons and holes gene a ed du ing ligh i adia ion accumula e in he semiconduc o wi h a na ow band gap h ough mig a ion. In his case, he elec on-hole pai s a e no e ec i ely sepa a ed, bu he pho oca aly ic ac i i y is educed. Fo ype III he e ojunc ion, he conduc ion band edge and alence band edge o semiconduc o 1 a e comple ely highe han he conduc ion band edge o semiconduc o 2 (Fig 6b). The conduc ion band and alence band edge o he wo semiconduc o s a e comple ely s agge ed because hey a e no e ec i e. G ound sepa a ion o elec on-hole pai s canno imp o e hei ac i i y, so he e a e ew s udies on ype I and ype III he e ojunc ions based on g-C3N4. Fig.6. Illus a ion o pho ogene a ed elec ons and holes ans e p ocess on ype Ⅰ and Ⅲ he e os uc u es 5.2.3 Scho ky junc ion Simila o he TiO2/P sys em, noble me als such as P 39-40, Au41 and Pd42 a e gene ally used as p omo e s o o m Scho ky junc ions wi h g-C3N4. Noble me als ha e a high wo k unc ion and can be used as elec on aps o cap u e and s o e pho ogene a ed elec ons. I is e ec i ely sepa a ed om he space o he pho o-gene a ed holes, imp o ing i s hyd ogen p oduc ion ac i i y. The e a e also s udies on using ansi ion me als as co-ca alys s o o m Scho ky junc ions wi h g-C3N4 o imp o e i s ac i i y43-44. Bi e al. used sol o he mal me hod o use melamine and nickel ace ylace ona e as p ecu so s, and loaded Ni on g-C3N4 o o m Scho ky junc ion. The band bending phenomenon was obse ed, which can e ec i ely sepa a e pho ogene a ed ca ie s and g ea ly imp o e he yield o H2. In addi ion o me als, o ganic ca bon ma e ials, g aphene, and ca bon nano ubes can ac as co-ca alys s o accep g-C3N4 pho ogene a ed elec ons due o hei excellen elec ical conduc i i y. Ong45 p epa ed 2D/2D GO/pCN he e ojunc ion ca alys s h ough elec os a ic sel -assembly, which o med an in ima e con ac be ween he wo, which e ec i ely sepa a ed ca ie s a he he e ojunc ion in e ace and p e en ed he ecombina ion o elec on-hole pai s. , E ec i ely imp o e he pe o mance o pho oca aly ic educ ion o CO2. 5.2.4 Z ype he e ojunc ion The Z- ype he e ojunc ion is simila o he ype II he e ojunc ion in he ene gy band s uc u e, bu he elec on hole low di ec ion is di e en , so i s edox pe o mance is di e en . The conduc ion band elec ons o he low conduc ion band semiconduc o combine and annihila e wi h he alence band holes o he low alence band semiconduc o , making he alence band holes o he o me and he conduc ion band elec ons o he la e a e e ec i ely sepa a ed, and he edox e ec is exe ed. The e o e, his he e ojunc ion no only has a wide spec al abso p ion ange, bu also has a high edox capabili y, which e ec i ely sol es he p oblem o educed edox p ope ies caused by ca ie mig a ion in ype II he e ojunc ions. The mos ypical is he Z- ype he e ojunc ion composed o WO3 and g-C3N4. The band gap o WO3 is 2.7~2.8 eV, which is close o he band gap o g-C3N4, and he conduc ion band and alence band sides a e mo e posi i e han g-C3N4, which can o m s agge ed band-edge po en ial ene gy, sa is ying he s uc u e o Z- ype he e ojunc ion46. The esea ch o Chen47 showed ha when g-C3N4 is he main pa , i o med a Z- ype he e ojunc ion wi h WO3, and he alence band holes o g-C3N4 quickly ecombined wi h he conduc ion band elec ons o WO3, esul ing in pho ogene a ed elec ons o g-C3N4 and WO3. Pho o-gene a ed holes accumula e, and i s pho oca aly ic ac i i y o deg ading BF was mo e han doubled compa ed wi h g-C3N4 and WO3 (as shown in Fig 7). Fig.7. Sepa a ion mechanisms o pho ogene a ed elec ons andholes on WO3/g-C3N4 he e ojunc ion 5.3 Elemen doping Doping usually e e s o he pu pose ul inco po a ion o a small amoun o o he elemen s o compounds in o he ma ix o change he c ys al phase s uc u e, elec onic dis ibu ion o su ace s a e. I is a commonly used modi ica ion me hod o p epa ing high-e iciency ca alys s. 5.3.1 Non-me allic elemen doping The doping o non-me als48-51, such as B, S, O, P, F, C, e c., can eplace he C, N, and H elemen s in he 3-s- iazine s uc u al uni , esul ing in he eplacemen o la ice de ec s and e ec i ely supp essing ecombina ion o elec on-hole o imp o e he pho oca aly ic pe o mance o g-C3N4. Zhai e al. applied i s p inciples o s udy he changes caused by S and O doping quan um g-C3N4. I is ound ha S and O doping can eplace N a oms in di e en posi ions in g-C3N4 and cause changes in he su ounding C-N bonds, signi ican ly educing he HOMO-LUMO ene gy gap. The mac oscopic imp o emen o op ical pe o mance was mani es ed in b oadening he ligh esponse ange and inc easing he ligh abso p ion in ensi y, and he highe he doping concen a ion, he mo e ob ious he imp o emen in op ical pe o mance. I can be seen ha non-me al doping is o g ea signi icance o imp o ing pho oca aly ic ac i i y, and has impo an esea ch alue in he ield o pho oca aly ic educ ion o CO2. Li34 doped g-C3N4 wi h S o in es iga e he eac ion mechanism o educing CO2. I is ound ha he doping o S is easie o eplace he N a oms a he edge o g-C3N4 and o m hyb idiza ion wi h C a oms. Since he a omic adius o S a oms ( =100 pm) is la ge han ha o N a oms ( =65 pm), he c ys al s uc u e changes signi ican ly a e doping. Besides, he numbe o ee elec ons inc eases a e ligh exci a ion, and he Fe mi le el mo es up. I exhibi s ypical n- ype doping. Han e al. used P-doped g-C3N4 o p epa e BP@ g-C3N4 ca alys o pho oca aly ic educ ion o CO2. A e P doping, i was easy o eplace he C a oms in g-C3N4, which signi ican ly changed he c ys al s uc u e o g-C3N4 and appea s as n- ype doping. A e he doping, he Fe mi ene gy le el has shi ed signi ican ly, and a new ene gy band was gene a ed on he basis o he o iginal ene gy band s uc u e, which had an impo an in luence on he adjus men o he g-C3N4 ene gy band s uc u e. The educ ion a e o CO2 inc eased om 2.65 μmol·g-1·h-1 o 6.54 μmol·g-1·h-1, which signi ican ly imp o ed he pho oca aly ic pe o mance. Taking O and S doped g-C3N4 as an example, Table 2 showed he HOMO-LUMO band gap binding ene gy o di e en doping si es. I can be seen om Table 2 ha he HOMO-LUMO ene gy gap was signi ican ly educed when he elemen s a e doped, indica ing ha he doping o O and S can signi ican ly imp o e he ene gy le el s uc u e and elec onic dis ibu ion o g-C3N4. In addi ion, di e en elemen s doping a he same si e and he same elemen doping a di e en si es play di e en oles in he egula ion o ene gy le els. The pho oca aly ic educ ion a e O and S doping was shown in Fig 8. Di e en doping si es and di e en doping elemen s had di e en e ec s on he ligh abso p ion p ope ies. Lowe ing he band gap ene gy can e ec i ely inc ease he ligh abso p ion ange, imp o e he ligh abso p ion and ca aly ic ac i i y. When CO2 molecules we e adso bed on he ca alys su ace, mo e ene gy was used o o e come he ene gy ba ie , so ha he educ ion yield o CO2 was imp o ed. Table 2. The o al ene gy, HOMO-LUMO ene gy gap, minimum equency and impu i y o ma ion ene gy o doped s uc u es a di e en si es o O and S S uc u e Ene gy/eV HOMO-LUMO ene gy F equency/cm-1 Fo ma ion gap ene gy/eV (g-C3N4)6 -793.481 1.863 33.209 (g-C3N4)6-N2-O -790.149 0.256 17.174 -0.682 (g-C3N4)6-N3-O -790.330 0.574 26.571 -0.863 (g-C3N4)6-N8-O -790.283 0.371 24.827 -0.816 (g-C3N4)6-N2-S -787.122 0.326 23.748 1.714 (g-C3N4)6-N3-S -787.433 0.684 24.892 1.403 (g-C3N4)6-N8-S -787.621 0.617 25.196 1.215 Fig 8. S, O doped g-C3N4 pho oca aly ic educ ion o CO2 eac ion mechanism 5.3.2 Me al doping A e he me al doped wi h g-C3N4, elec ons a e ans e ed om he me al o he adjacen N o C a oms, changing he elec on densi y o he N o C a oms, and hen a ec ing he elec onic s uc u e and ene gy band posi ion o g-C3N4. Me als such as Cu52, Ag, Au53, Fe54, P 55-56 can be doped in o g-C3N4, causing la ice de ec s. Li57 ound ha he doping o Cu and Mo change he elec on dis ibu ion and ene gy band s uc u e o g-C3N4, and a he same ime changed he eac ion pa h on he su ace o g-C3N4, which p omo ed he adso p ion and ac i a ion o CO2 on he ca alys su ace, e ec i ely educing he ac i a ion ene gy du ing educ ion p ocess. Beenish52 used Cu doped wi h od-shaped g-C3N4 o in es iga e he pe o mance and p oduc dis ibu ion o CO2 educ ion in di e en eac ion sys ems. I was ound ha Cu in he o m o Cu2+ was doped in g-C3N4, and signi ican ly a ec ed he ene gy le el s uc u e o g-C3N4. In di e en eac ion sys ems, di e en main p oduc s a e ob ained by educ ion. In he CO2-H2O eac ion sys em, he main p oduc is CH4, and he eac ion a e was 1.84 imes ha o pu e g-C3N4, In he CO2-CH4 eac ion sys em, he main p oduc Fo CO and H2, he eac ion a e was 1.33 imes ha o pu e g-C3N4. Yu37 used P -doped g-C3N4 o educe CO2 o p epa e hyd oca bons. I is ound ha P exis s in g-C3N4 in he o m o a oms, bu a e he educ ion eac ion occu s, pa o P exis s in he o m o ions. And he di e en doping amoun s o P made he educ ion p oduc s o CO2 di e en selec i i y, and he main p oduc o CO2 educ ion by pu e g-C3N4 was CH3OH. When he doping amoun o P is 5%, he main p oduc s o educing CO2 we e CH4 and HCHO; when he doping amoun is 10%, he main p oduc s was CH4. The eac ion mechanism o me al-doped g-C3N4 applied o pho oca aly ic educ ion o CO2 was shown in Fig 9. The Fe mi le el o g-C3N4 is close o he bo om o CB. The me al doping makes he Fe mi le el mo e, and he dopan o bi al hyb idizes wi h he C o N elec on o bi al o o m a new elec on o bi al, which e ec i ely imp o e he oxida ion- educ ion pe o mance o he ca alys and o e come he ene gy ba ie o pho oca aly ic educ ion o CO2. When he ligh sou ce is illumina ed, he alence band elec ons on he ca alys we e exci ed o he conduc ion band. Then a se ies o educ ion eac ions occu on he conduc ion band, and he holes on he alence band unde go oxida ion eac ions. Fig. 9. Me al-doped g-C3N4 pho oca aly ic educ ion o CO2 eac ion mechanism 5.4 Speci ic su ace a ea con ol modi ica ion echnology Usually g-C3N4 p epa ed om N-con aining p ecu so s (such as u ea, melamine, dicyandiamine, e c.) h ough high- empe a u e polycondensa ion is shown as bulk pa icles o lamella polyme s wi h a small speci ic su ace a ea (<10 m2/g ), g ea ly es ic ing i s applica ion. Rega dless o he doping modi ica ion echnology o he semiconduc o compound modi ica ion, he in e ac ion only occu s on he su ace o g-C3N4. The e o e, e ec i ely egula ing he g-C3N4 nanos uc u e and expanding i s speci ic su ace a ea no only makes he pe o mance o g-C3N4 mo e s able, bu also inc eases eac i e si es, which is an e ec i e way o imp o e he pho oca aly ic ac i i y o g-C3N4. 5.4.1 Mesopo ous g-C3N4 The in oduc ion o nano-scale po ous s uc u e in o he bulk g-C3N4 can signi ican ly inc ease he speci ic su ace a ea o g-C3N4, which is bene icial o inc ease he eac ion con ac a ea and eac i e si es, he eby imp o ing i s ca aly ic pe o mance. Using o de ed silicon-based ma e ials as ha d empla es, po ous ca bon ni ide ma e ials wi h adjus able po e s uc u e and po e size can be syn hesized. I was epo ed ha Zhao e al. use SBA-15 and a new ype o c oss-linked bimodal mesopo ous SBA-15 (CLBM-SBA-15) as a ha d empla e o p epa e mesopo ous g-C3N4. The esul ing g-C3N4 (CLBM- SBA-15) had a mo phology simila o mesopo ous SiO2, and nume ous po es a e o med on he su ace o g-C3N4. In he pho oca aly ic deg ada ion es o me hyl o ange, he ac i i y o mesopo ous g-C3N4 was nea ly 15.3 imes ha o bulk g-C3N4 ( he esul s we e shown in Fig 7). Such high pho oca aly ic pe o mance was due o he ac ha he mesopo ous g-C3N4 has high speci ic su ace a ea, po e olume and ac i e eac ion si es. Shi e al. syn hesized mpg-CN in si u using TEOS as a mesopo ous empla e and cyanamide as a p ecu so , and he speci ic su ace a ea o mpg-CN eached 152 m2/g. I s pho oca aly ic e ec on he deg ada ion o RhB was ema kable. This can be a ibu ed o he high speci ic su ace a ea and high elec on-hole pai sepa a ion e iciency o mpg-CN, which enhanced he dye adso p ion capaci y o mpg-CN. And mpg-CN s ill pe o med well a e being ecycled o 3 imes. 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