A diode in series with the fan will reduce the voltage being output to the fan. A silicon diode provides a relatively constant voltage drop of about 0.7 V per diode; data sheets for a specific diode specify its voltage drop, for example the 1N4001 silicon diode's voltage drop varies from approximately 0.7 to 0.9 V as the current varies from 0.01 to 1 A. The power rating should be noted and some diodes may require cooling to operate at their rated current. The voltage drop across the diode will fall with temperature, causing the fan to speed up.