Preparation And Photothermal Study Of Black Titania For Solar Energy Applications

Recently,with the depletion of fossil energy and the increasingly serious problem of environmental pollution,clean solar energy has attracted much attention.As one of the main forms of efficient utilization of solar energy,photothermal conversion is attractive.The ideal photothermal conversion material should have high photothermal conversion efficiency and it can effectively reduce the thermal radiation loss.At present,ferrous metal composites are the main photothermal conversion materials,but in which there are some problems such as acid corrosion and low photothermal efficiency.To solve the above problems,developing photo-thermal materials based on oxides or nitrides with acid-alkali resistance and high light absorption has become one of the feasible ways.Therefore,black titanium dioxide(Ti O_2-x)with wide spectral absorption has potential applications in photothermal conversion.Black Ti O_2-x is mainly prepared by gas or metal reduction of Ti O₂,which has the problems of poor homogeneity,repeatability and controllability of the reaction.It can also be oxidized by hydrogen peroxide or electrochemical method from low-valent titanium,which is generally cumbersome in subsequent treatment steps.Therefore,it is necessary to develop a new controllable and simple preparation method for black Ti O_2-x.In order to prepare black Ti O_2-x with high photo-thermal properties,a new idea of oxidizing Ti_nO_2n-1 with solid atmosphere creators?SACs?was put forward.The oxidation process was controlled by widely screening and proportionally adjusting the low-cost and wide-source SACs.The main innovations were listed as follows:?1?To control the oxygen vacancy content in black Ti O_2-x,the ratio of strong oxidizing atmosphere was adjusted.The SAC KCl O₄ was used as O source to realize partial or complete oxidation of Ti₄O₇.XRD phase analysis showed that the critical mixing ratio of KCl O₄ is 17.5 mol%,and Ti₄O₇ could be transformed into black Ti O_2-x?x=0.075?.Under the same illumination conditions,photothermal maximum temperatures(T_max)of 15.0 and 17.5 mol%products were 70.7? and 66.8?,respectively,which were higher than 38.7? of white Rutile Ti O₂.Reaching 0.99 T_max,the used time t _0.99of 15 and 17.5 mol%products was 57.0 s and 51.8 s,respectively,much lower than 182.8 s of white Ti O₂ raw materials.The photothermal results show that increasing the ratio of KCl O₄ will decrease the T_max value and increase the t _0.99value,therefore weakening the photothermal performance.?2?To realize the non-metallic doping of Ti_nO_2n-1 at the same time of oxidation,the ratio of weak oxidizing atmosphere was adjusted.The SAC?NH₄?₂SO₄ provided both O source and N/S dopants,whose ratio to Ti_nO_2n-1 affected the structure of black Ti O_2-x products.XPS results showed that except for oxygen entering the lattice,both N and S elements occupied part of the oxygen position in the lattice of Ti O₂.TEM and XRD results showed that the 5 mol%product was core-shell structure with a full absorption characteristic,which was a mixture of conductive Ti₄O₇ and black Ti O_2-xphases.Meanwhile,the 10 mol%product was Rutile Ti O_2-x without shell structure,also showing the wide spectral absorption.Under the same illumination condition for4 min,the T_max of 5 mol%and 10 mol%products were 82.9? and 76.7?,respectively,which were higher than 38.7? of white Ti O₂ raw materials.The t _0.99indices of 5 mol%and 10 mol%products were 62.8 s and 52.1 s,respectively,which were much lower than 182.8 s of white Ti O₂.The photothermal results showed that increasing the ratio of?NH₄?₂SO₄ will also decrease the T_max value and increase the t_0.99 value,which will weaken the photothermal performance.Compared with KCl O₄samples,N and S doping can improve the photothermal properties of the products to a certain extent.?3?To explore the reaction feasibility of the atmosphere system without oxygen element,the ratio of non-oxidizing atmosphere was adjusted.The SAC NH₄X provided both N source and hydrogen halide atmosphere,which induced an abnormal disproportionation reaction of Ti_nO_2n-1.The black Ti O_2-x with the core-shell structure of Ti O₂@Ti O was in-situ prepared.The black product with this reverse-cladding structure was conductive,close to metal,and it exhibits the full spectrum absorption.For the new disproportionation reaction of Ti_nO_2n-1 induced by NH₄X,the general formula of disproportionation reaction was proposed and verified.The reaction mechanism of decomposition-etching-disproportionation-re-hydrolysis was proposed.The intermediate Ti Cl₄ in the form of?NH₄?₂Ti Cl₆ was also experimentally detected.Furthermore,the effects of reaction time,reaction temperature and heating/cooling rate were studied in detail,and the phase diagram was drawn.