Who turned Richard over to the holy roman emperor Henry vi?

Leopold V, Duke of Austria. Leopold had good reasons to detest Richard. Apart from some severe insults during the earlier Crusade, Richard had - successfully - conspired to murder one of Leopold's cousins. Leopold later unloaded Richard on Henry VI, because holding a Crusader captive led to Leopold's excommunication by the Pope. Richard complained loudly about the conditions of his captivity, although according to most other sources his housing and other arrangements were quite comfortable.

Contrary to the 'good king'-image created in the Robin Hood stories, Richard in reality was a rather unpleasant character, who despised the English and much rather spent his non-fighting days in his French lands. In all, he spent no more than a few months in England during his reign and it was he, much more than King John, who saw his English subjects as just a source of revenue.